Technical Basis Document No. 12: 
Biosphere Transport 
Revision 1 
Kurt R. Rautenstrauch, Anthony J. Smith, and Robert Andrews 
Prepared for: 
U.S. Department of Energy 
Office of Civilian Radioactive Waste Management 
Office of Repository Development 
P.O. Box 364629 
North Las Vegas, Nevada 89036-8629 
Prepared by: 
Bechtel SAIC Company, LLC 
1180 Town Center Drive 
Las Vegas, Nevada 89144 
Under Contract Number 
DE-AC28-01RW12101 
QA: NA 
September 2003


1. INTRODUCTION 
This Technical Basis Document summarizes the methods used to model the transport through the 
biosphere of radionuclides that may be released from the repository at Yucca Mountain. It 
includes a description of the reference biosphere, characteristics of the receptor, and the methods 
used to model environmental transport and exposure pathways. This is one in a series of 
Technical Basis Documents that are being prepared to describe components of the Yucca 
Mountain repository system that are important for predicting the likely post-closure performance 
of the repository. The relationship of biosphere transport to the other post-closure performance 
components is illustrated in Figure 1-1. 
This Technical Basis Document was written to respond to open key technical issue (KTI) 
agreements made between the U.S. Nuclear Regulatory Commission (NRC) and the 
U.S. Department of Energy (DOE). The appendices address KTI agreements associated with 
biosphere modeling. This document places the responses to KTI agreements within the context 
of the overall biosphere transport and exposure model, explains their relationship to the postclosure 
safety analyses, and allows for a more complete discussion of the relevance of the 
agreements. The information in this document, along with the associated references, also 
describes the biosphere characteristics and modeling to be used in the postclosure safety analysis 
that will be presented in the license application. 
This Technical Basis Document and appendices are responsive to agreements made between the 
DOE and the NRC during Technical Exchange and Management Meetings on Total System 
Performance Assessment and Integration (Reamer 2001a), and Igneous Activity (Reamer and 
Williams 2000; Crump 2001; Reamer 2001b). Most of the agreements were based on questions 
that NRC staff developed from their review of the site recommendation support documents and 
DOE presentations at the technical exchanges. The agreements, in general, required the DOE to 
present additional information or perform sensitivity or validation activities for models or 
assumptions used in the Yucca Mountain Site Suitability Evaluation (DOE 2002). Since the 
technical exchanges, the DOE has conducted the additional work designed to meet those 
agreements. 
1.1 OBJECTIVE AND SCOPE 
The objectives of this Biosphere Transport Technical Basis Document are to: 
• Synthesize the methods and data used to model transport of radionuclides within the 
biosphere 
• Synthesize the methods and data used to model exposure to the applicable receptor 
• Summarize the results of the biosphere transport models used in the assessment of 
postclosure performance at Yucca Mountain. 
September 2003 1-1 No. 12: Biosphere Transport
Revision 1 
Components of the Post-Closure Technical Basis for the License Application Figure 1-1. 
Transport of radionuclides through the biosphere and evaluation of exposure to a receptor are 
considered in the Environmental Radiation Model for Yucca Mountain Nevada (ERMYN). This 
model calculates the concentrations of radionuclides in environmental media (e.g., soil, food) per 
unit concentration of radionuclides in two sources of contaminants, groundwater and volcanic 
ash. It also calculates the dose to a human receptor per unit concentration of radionuclides in the 
sources. These dose estimates, called biosphere dose conversion factors (BDCFs), are combined 
in the Total System Performance Assessment (TSPA) with estimates of radionuclide 
concentrations in groundwater and ash to perform human radiation dose assessments. 
The biosphere model developed to support the license application has been substantially revised 
since completion of the TSPA for the site recommendation. This model includes additional 
relevant pathways, provides the capability to stochastically sample all input parameters, and 
further enhances the model of biosphere transport (BSC 2003a, Section 6.7.2). 
This document presents a summary and synthesis of the detailed technical information presented 
in analyses and model reports written to describe the modeling of biosphere transport. The 
Biosphere Model Report (BSC 2003a) describes the ERMYN conceptual and mathematical 
models, model validation, and software implementation. Input parameters values developed for 
use in the ERMYN model are described and justified in five analysis reports. 
No. 12: Biosphere Transport September 2003 1-2
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• Inhalation Exposure Input Parameters for the Biosphere Model (BSC 2003b) 
• Characteristics of the Receptor for the Biosphere Model (BSC 2003c) 
• Agricultural and Environmental Input Parameters for the Biosphere Model (BSC 
2003d) 
• Environmental Transport Input Parameters for the Biosphere Model (BSC 2003e) 
• Soil-Related Input Parameters for the Biosphere Model (BSC 2003f). 
The calculation of BDCFs for use in the TSPA is described in the reports Nominal Performance 
Biosphere Dose Conversion Factor Analysis (BSC 2003g) and Disruptive Event Biosphere Dose 
Conversion Factor Analysis (BSC 2003h). 
1.2 SUMMARY OF BIOSPHERE TRANSPORT PROCESSES AND MODELING 
The biosphere transport and exposure pathways included in the biosphere model were selected 
based on potential sources of radionuclides from a repository at Yucca Mountain (groundwater 
and volcanic ash), the site-specific conditions in the Yucca Mountain region, and the lifestyle of 
the people living in Amargosa Valley. There are about 2,000 acres of commercial agricultural 
crops, a dairy, and a fish farm in Amargosa Valley. Because the region is arid, all crops must be 
irrigated. The only source of irrigation water is groundwater, which is not treated prior to use. 
Many of the people living in Amargosa Valley have gardens and some raise livestock; thus, 
many inhabitants consume local produce and animal products. About 40 percent of the residents 
of Amargosa Valley are not employed. Of those employed, the largest group is miners, and only 
about six percent work on farms. Most residents live in mobile homes and use evaporative 
coolers. 
Based on this site-specific information, six environmental media that could result in exposure to 
a human receptor are considered in the biosphere model: groundwater, soil, air, crops, animal 
products, and fish. The model calculates exposure to radionuclides in those media that are 
external to the body and to radionuclides that are inhaled and ingested. Because the two possible 
sources of radionuclides, groundwater and volcanic ash, would result in different transfer 
pathways, separate conceptual and mathematical models have been developed for these two 
exposure scenarios. 
For the groundwater exposure scenario, radionuclides enter the biosphere from wells drilled into 
the aquifer. The model considers use of that groundwater for drinking, irrigating commercial 
and garden crops, watering livestock, raising fish, and running evaporative coolers. Irrigation of 
crops could result in radionuclide transport to topsoil and crops. The soil could be resuspended 
and deposited on crops, inhaled by the receptor, and inadvertently ingested by the receptor and 
farm animals. Radioactive gasses escaping from the soil also may be taken up by crops and 
inhaled by the receptor. The crops may be eaten by the receptor and fed to farm animals. 
Products from those farm animals and fish also may be eaten by the receptor. Based on these 
pathways, the model calculates BDCFs (annual dose per unit concentration of radionuclides in 
groundwater) from external exposure to the soil; from inhaling resuspended soil, radioactive 
September 2003 1-3 No. 12: Biosphere Transport
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gasses and their decay products, and aerosols from evaporative coolers; and from ingesting 
water, crops, fish, animal products, and soil. 
For the volcanic ash exposure scenario, ash from an eruption at the repository could be deposited 
throughout the reference biosphere by initial deposition or by redistribution following a volcanic 
eruption. The model considers transport of radionuclides from the ash to soil, air, crops, and 
animal products. Radionuclide transport to crops may arise from deposition of resuspended 
particles of ash and contaminated soil and from uptake through the roots. Farm animals may be 
fed those crops and may ingest soil containing radionuclides. The receptor could receive a dose 
from external exposure to soil; from inhaling radon decay products and resuspended ash; and 
from ingesting crops, animal products, and soil. Pathways and environmental media related to 
radionuclides in groundwater are not considered in this exposure scenario. Because the dose 
resulting from exposure to volcanic ash could vary depending on the depth of ash in the 
biosphere and could change over time, for this exposure scenario the model calculates BDCFs 
(annual dose per unit concentration of radionuclides in ash) as a function of ash depth and time. 
The mathematical representations of the biosphere transport processes and exposure pathways 
were constructed based on a review of applicable calculations in other environmental radiation 
models. Distributions of the input parameters that may contribute to variation in the BDCFs are 
stochastically sampled in the model. Input parameter values were developed from site-specific 
data and surveys, from reviews of distributions used in other models, data from analog sites, and 
other applicable publications. The selected distributions incorporate the full range of variation 
and uncertainty in environmental parameters and the range of uncertainty about the average of 
parameters that represent dietary and lifestyle characteristics of the receptor. Propagation of that 
uncertainty and variation has resulted in BDCFs that generally vary by less than an order of 
magnitude. 
1.3 ORGANIZATION OF THIS REPORT 
The characteristics of the biosphere and human receptor that have an important influence on 
development of the model and input parameters are described in Section 2. The radionuclide 
transport pathways and exposure pathways included in the two biosphere exposure scenarios 
considered in the ERMYN model, and the manner in which radionuclide decay and ingrowth is 
handled, are described in Section 3. Descriptions of the submodels used to track radionuclide 
transport and calculate exposure for the groundwater and volcanic ash exposure pathways are 
presented in Sections 4 and 5. The results of the ERMYN model are summarized in Section 6. 
Appendices to this report address KTI agreements related to biosphere modeling. 
• Appendix A – Surface-Disturbing Activities (Response to IA 2.11) 
• Appendix B - Mass Loading and Ash Depth (Response to IA 2.14) 
• Appendix C - External Exposure (Response to IA 2.15) 
• Appendix D - Soil-Sorption (Response to TSPAI 3.33) 
• Appendix E - Radionuclide-Specific Biosphere Parameters (Response to TSPAI 3.34) 
• Appendix F - Crop Interception Fraction (Response to TSPAI 3.35) 
• Appendix G - Leaching Coefficients (Response to TSPAI 3.36). 
September 2003 1-4 No. 12: Biosphere Transport
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2. DESCRIPTION OF THE BIOSPHERE AND RECEPTOR 
This section describes the reference biosphere and receptor considered in the postclosure 
performance assessment of the repository. This information was derived from surveys of 
Amargosa Valley and its residents conducted by or for the DOE (e.g., DOE 1997; Horak and 
Carns 1997; CRWMS M&O 2000a) and from information about the region reported by other 
agencies such as the U.S. Bureau of the Census and U.S. Department of Agriculture. The 
information here was used to select the relevant transfer and exposure pathways to include the 
ERMYN model. 
2.1 DESCRIPTION OF THE BIOSPHERE 
During development of the biosphere model, the region surrounding Yucca Mountain was 
characterized to better understand the reference biosphere. The characterization focused on 
north-central Amargosa Valley, the location of the receptor specified in 10 CFR Part 63. 
Information on the geography, climate, infrastructure, communities, and other aspects of the 
region was obtained. That information was used to select the features, events, and processes 
(FEPs) applicable to the model, select the exposure pathways that must be considered for 
inclusion in the model, and develop input parameter values that are representative of the 
reference biosphere. More detailed descriptions of the reference biosphere are presented in the 
Yucca Mountain Site Description (CRWMS M&O 2000b) and the Biosphere Model Report 
(BSC 2003a, Section 6.1.1). 
Communities-The region surrounding Yucca Mountain is sparsely populated. The closest 
residents to the potential repository live in northern Amargosa Valley at the intersection of 
U.S. Highway 95 and Nevada State Route 373, about 20 km south of Yucca Mountain. Most 
people in Amargosa Valley live more than 30 km south of Yucca Mountain (Figure 2-1). At the 
time of the 2000 census, it was estimated that 1,176 people in 429 households resided in the 
approximately 1,300-km2 Amargosa Valley Census County Division (Bureau of the Census 
2001). Other communities and employment centers in south-central Nevada, and the 
approximate highway distance from the intersection of Highway 95 and State Route 373, are 
Beatty (45 km), Pahrump (70 km), Indian Springs (70 km), and Las Vegas (120 km). This 
information was used to characterize the size of the reference biosphere and to define the 
receptor (BSC 2003c, Sections 5.1.1 and 6.1, and the analysis reports listed in Section 1.1). 
Infrastructure-In 2000, there was a small general store, a community center, a senior center, a 
library, a small medical clinic, an elementary school, a restaurant, a hotel-casino, and a motel in 
Amargosa Valley. Most of the major roads in the area are paved. The nearest indoor recreation 
(e.g., movie theatres, other restaurants), larger stores, and hospitals are in Pahrump and Las 
Vegas. Because some Amargosa Valley residents likely leave the area for employment and to 
obtain some goods and services, the model accounts for the receptor spending some time away 
from areas where radionuclides may be present in the environment (BSC 2003c, Sections 5 and 
6). 
All water used for domestic, municipal, and agricultural purposes in Amargosa Valley comes 
from local groundwater wells. There are no public water treatment systems in Amargosa Valley, 
and there is only a small, quasi-municipal water delivery system for which drinking water 
September 2003 2-1 No. 12: Biosphere Transport
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standards could be enforced (State of Nevada 1997). Therefore, it is assumed that water used by 
the receptor is from groundwater wells and that water is not treated prior to use. 
Source: CRWMS M&O 2000a. 
NOTE: Each dot represents one occupied residence. 
South-Central Nevada Census Geography 
2-2 
Figure 2-1. 
No. 12: Biosphere Transport September 2003
Revision 1 
Agriculture-There is a small agricultural industry in Amargosa Valley. Approximately 2,000 
acres are commercially farmed, of which over 90 percent is planted in alfalfa or other hay. 
Commercial crops are irrigated with groundwater, primarily using center pivot and other 
overhead sprinkler systems. Small grains, pistachios, grapes, orchard crops, garlic, and onions 
also are grown commercially. There is a dairy with more than 5,000 cows and there was a fish 
farm in the 1990s that had about 15,000 fish. Many residences have gardens with vegetable plots 
and some have a few cattle, sheep, chickens, and other farm animals (CRWMS M&O 1997, 
Section 3.4; Horak and Carns 1997; USDA 1999a; YMP 1999, Section 3.4). 
The same crops, plus many others, are grown in eastern Washington, which is the analog 
location for the upper bound of the predicted future glacial transition climate. The most common 
crops grown there, wheat and other grains, are not irrigated (USDA 1999b; Washington State 
University Cooperative Extension 2002). 
This information was used to identify some of the important environmental media to include in 
the model (e.g., commercial and garden crops, locally grown feed for livestock, and locally 
raised fish). This information was also used to select representative crops and animal products 
for developing input parameters related to crop and livestock production (e.g., BSC 2003d, 
Appendix A). 
Climate-Currently, the Yucca Mountain region has low precipitation, hot summers, cool 
winters, low relative humidity, and a high rate of evaporation (CRWMS M&O 2000b, 
Section 6.2). Five years of weather data collected at Yucca Mountain meteorological monitoring 
Site 9, located in northern Amargosa Valley, were used to characterize the current climate. 
Average annual precipitation at Site 9 was about 100 mm (BSC 2003d, Table 4.1-2), and average 
monthly temperatures ranged from 6.9ºC in December to 31.0ºC in July (Figure 2-2). Data 
reported by the National Climatic Data Center of the National Oceanic and Atmospheric 
Administration for analog weather stations were used to characterize the predicted future 
climates of the region. The selection of analog weather stations representative of the predicted 
future climates are justified in the Future Climate Analysis (USGS 2001, Sections 6.6.1 
and 6.6.2). Average annual precipitation for the upper bound of the future climate state predicted 
to occur during most of the next 10,000 years (glacial transition, based on conditions in Spokane, 
Washington) is about 400 mm and average monthly temperatures are about 10ºC cooler than 
current conditions (Figure 2-2). These arid to semi-arid climatic conditions were used to 
determine irrigation requirements of crops (BSC 2003d) and the proportion of the year that 
people living in Amargosa Valley would use evaporative coolers (BSC 2003c, Section 6.3.4). 
Soils-The soils on alluvial fans and in stream channels in northern Amargosa Valley generally 
are deep and well to excessively drained. The surface soil layer generally is less than 20 cm 
thick and subsurface soils are up to 150 cm deep. Soil textures are very gravelly with fine sands 
to sandy loams. The soils are calcareous and moderately alkaline. This information is from the 
Natural Resource Conservation Service (Dollarhide 1999; CRWMS M&O 1999) and was used to 
develop soil erosion and leaching rates (BSC 2003f) and to determine growth characteristics and 
irrigation requirements of plants grown in Amargosa Valley (BSC 2003d). 
September 2003 2-3 No. 12: Biosphere Transport
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Source: Based on BSC 2003d, Tables 4.1-2 and 4.1-5. 
Figure 2-2. Average Monthly Temperature for the Current and Predicted Future Climate (upper bound of 
September 2003 
the glacial transition climate) in Amargosa Valley 
2.2 DESCRIPTION OF THE RECEPTOR 
Regulation 10 CFR Part 63 requires that the human receptor considered in dose calculations for 
the license application be the reasonably maximally exposed individual (RMEI). According to 
10 CFR 63.312, the RMEI is a hypothetical person who meets the following criteria: 
a) Lives in the accessible environment above the highest concentration of 
radionuclides in the plume of contamination. 
b) Has a diet and lifestyle representative of the people who now reside in the Town of 
Amargosa Valley, Nevada. DOE must use projections based upon surveys of the 
people residing in the Town of Amargosa Valley, Nevada, to determine their 
current diets and living styles, and use mean values of these factors in the 
assessments conducted for 10 CFR 63.311 and 10 CFR 63.321. 
c) Uses well water with average concentrations of radionuclides based on an annual 
water demand of 3,000 acre-feet. 
d) Drinks 2 liters of water per day from wells drilled into the ground water at the 
location specified in paragraph (a). 
e) Is an adult with metabolic and physiological considerations consistent with present 
knowledge of adults. 
The following information on the diet and lifestyle of the residents of Amargosa Valley was used 
to develop a biosphere model that meets the requirements of 10 CFR Part 63 and to develop 
input parameter values that characterize the RMEI. 
2-4 No. 12: Biosphere Transport
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Diet-Based on a survey of Amargosa Valley residents conducted in 1997 (DOE 1997), it was 
determined that many people in that region consume some locally produced vegetables, fruit, 
grain, meat, poultry, fish, eggs, and milk (Figure 2-3). This information was used to identify the 
ingestion pathways that must be included in the model. Information from the survey on the 
frequency that Amargosa Valley residents ate locally produced foods was combined with 
information from the U.S. Department of Agriculture on daily rates of food intake in the western 
United States to calculate consumption rates of locally produced foods (BSC 2003c, Section 6.4). 
In general, locally produced foods comprise a small portion of the total diet of Amargosa Valley 
residents. For example, it is estimated that residents eat, on average, about 13 kg of locally 
produced fruits (including tomatoes) per year, compared to a total annual consumption rate of 
about 150 kg per year by people in the western United States (BSC 2003c, Tables 6.4-1 and 
6.4-2). 
Source: Based on DOE 1997, Table 2.3.1. 
Figure 2-3. Percent of Amargosa Valley Residents Surveyed that Consumed Any Tap Water and 
September 2003 2-5 
Locally Produced Foods 
Use of Evaporative Coolers-About 73 percent of Amargosa Valley residents surveyed in 1997 
used evaporative coolers and they used them for an average of five months per year (DOE 1997, 
Table 2.4.2; BSC 2003c, Section 6.3.4). Therefore, the model includes exposure to radionuclides 
that may be present in indoor air resulting from the use of evaporative coolers during part of the 
year. 
Gardens-About 46 percent of Amargosa Valley residents surveyed in 1997 had gardens 
(DOE 1997, Table 2.4.2). This and other information from the survey was used to select 
representative crops and identify methods used to grow crops that were considered during 
development of input parameters that characterize irrigation requirements and methods 
(BSC 2003d). 
Employment-About 39 percent of Amargosa Valley residents (16 or more years old) were not 
employed in 2000 (Bureau of the Census 2002, Table P47; BSC 2003c, Table 6.3-1). Of the 
residents who worked, the largest proportion (26 percent) worked in mining and only about 
6 percent (26 of 449) worked in agriculture (Table 2-1). This information was used to determine 
No. 12: Biosphere Transport
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the proportion of the population that worked indoors and outdoors in Amargosa Valley 
(BSC 2003c, Section 6.3.1). Those population proportions are used to calculate exposure times 
(Section 4.7 and 5.5). 
Table 2-1. 
Agriculture 
Mining 
Total 
26 
119 
Construction 7 
33 Retail trade 
49 Transportation and warehousing 
Utilities 
Educational services 
Health care and social assistance 
Arts, entertainment, recreation, accommodation, and food services 
Other services (except public administration) 
Public administration 
Industry of Employed Amargosa Valley Residents 
Industry of Employment 
Number of 
Females 
0 
18 
0 
14 
26 
0 
47 
8 
71 
15 
18 
Number of 
Males 
26 
101 
7 
19 
23 
8
0 
20 
22 
6
0 
232 217 
8 
47 
28 
93 
21 
18 
449 Total 
Source: Bureau of the Census 2002, Table P49. 
NOTES: Data represents employed residents 16 or more years old measured during the week prior to the 
September 2003 2-6 
April 2000 census. 
Commute Time-About 64 percent of Amargosa Valley residents (16 or more years old) who 
worked commuted 10 minutes or more to work one way. About 21 percent commuted 
35 minutes or more one way (Bureau of the Census 2002, Table P31). This information was 
used to determine the proportion of the Amargosa Valley population who would work in areas 
where radionuclides may be present and the amount of time that local workers would commute 
within those areas (BSC 2003c, Section 6.3). 
Housing Type-About 89 percent (375 of 422) of occupied housing units in Amargosa Valley 
during 2000 were mobile homes and about 91 percent (1,043 of 1,142) of the total population 
lived in mobile homes (Bureau of the Census 2002, Tables H30, H31, H33). This information 
was used to select building shielding factors for lightly constructed housing (BSC 2003c, 
Section 6.6), and parameters related to evaporative coolers, house ventilation rates, and 
equilibrium factors for radon decay products indoors (BSC 2003e, Sections 6.5. and 6.6). 
Physiology-The dose conversion factors and dose coefficients (from Eckerman et al. 1988 and 
Eckerman and Ryman 1993) used in the model to convert exposure to committed effective dose 
equivalents and effective dose equivalents are based on a hypothetical ‘average’ adult person 
with the anatomical and physiological characteristics defined in the Report of the Task Group on 
Reference Man by the International Commission on Radiological Protection (ICRP 1975). 
Metabolic models used in the development of the dose conversion factors are also consistent 
with the Reference Man representation of an adult. Breathing rates used in the model were based 
on the more recent biometric results for adult persons used in the respiratory track model 
developed by the International Commission on Radiological Protection (ICRP 1994). 
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3. RADIONUCLIDE TRANSPORT AND EXPOSURE PATHWAYS 
Two human exposure scenarios are included in the ERMYN model: groundwater and volcanic 
ash. These scenarios are considered separately because the initial radionuclide source terms and 
radionuclide transport mechanisms in the biosphere differ between the scenarios. The 
relationship between these human exposure scenarios and the TSPA modeling cases and scenario 
classes (BSC 2002a, Section 4) is fully described in the Biosphere Model Report (BSC 2003a, 
Section 6.1.3). 
For the groundwater exposure scenario, radionuclides potentially released from the repository 
would enter the biosphere from groundwater wells. Human exposure could arise from use of the 
water for domestic and agricultural purposes. BDCFs for the groundwater scenario apply to the 
modeling cases included in the TSPA for the license application (TSPA-LA) that consider 
groundwater releases of radionuclides, including the nominal scenario class, the igneous 
intrusion case of the igneous scenario class, the seismic scenario class, and the human intrusion 
analysis (BSC 2002a, Section 4). 
For the volcanic ash scenario, the source of radionuclides is ash from a volcanic eruption at the 
repository. This scenario applies to the TSPA volcanic eruption modeling case of the igneous 
scenario class. The biosphere model for the volcanic ash scenario and BDCFs generated using 
the model support only the TSPA volcanic eruption modeling case of the igneous scenario class 
(BSC 2002a, Section 4). The remaining disruptive event scenario classes evaluate radionuclide 
releases to groundwater and are supported by the biosphere model for the groundwater scenario. 
Based on these mechanisms of potential radionuclide release into and movement within the 
biosphere and the characteristics of the reference biosphere and receptor described in Section 2, 
conceptual models were developed for the groundwater and volcanic ash exposure scenarios. 
The first step in the process was to identify the FEPs that are applicable to these models. From 
the total list of FEPs to be considered for the TSPA, 31 were identified as applicable to the 
biosphere model (BSC 2003a, Section 6.2). These FEPs represent features of the arid to 
semi-arid environment in the Yucca Mountain area and the possible events and processes leading 
to radionuclide transport in the environment and exposure to the receptor. The environmental 
transport pathways, human exposure pathways, and related environmental media that address the 
FEPs and are applicable to each scenario were then identified and described (BSC 2003a, 
Section 6.3). Environmental transport pathways are the routes by which radionuclides move 
from the source to other environmental media. Human exposure pathways arise when people are 
exposed, internally or externally, to contaminated media. The environmental and exposure 
pathways for the groundwater and volcanic ash exposure scenarios are described in Sections 3.1 
and 3.2, respectively. The radionuclides considered in the biosphere model and the methods 
used to incorporate radionuclide decay and ingrowth are discussed in Section 3.3. 
3.1 GROUNDWATER SCENARIO PATHWAYS 
Under the groundwater exposure scenario, radionuclides could be released into the biosphere 
from groundwater drawn from a well. Human exposure could occur when the local community 
where the receptor resides uses the water for domestic and agricultural purposes. The 
groundwater scenario is used to evaluate the radiological consequences of nominal performance 
September 2003 3-1 No. 12: Biosphere Transport
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of the geologic repository and performance under disrupted conditions (igneous intrusions and 
seismic events) that can lead to radionuclide releases into the groundwater. Use of groundwater 
could result in radionuclide concentrations in the following six environmental media to which 
the receptor may be exposed (BSC 2003a, Section 6.3.1). 
• Groundwater. 
• Irrigated soil. 
• Indoor and outdoor air. 
• Crops consumed by the receptor and farm animals. 
• Animal products consumed by the receptor. 
• Fish raised at a fish farm and consumed by the receptor. 
Migration of radionuclides through the biosphere occurs due to a number of environmental 
transport processes that lead to accumulation of radionuclides in the media (Figure 3-1). The 
environmental transport processes explicitly included in the biosphere model for the groundwater 
scenario are listed in Table 3-1. Exposure could occur when the receptor is exposed to 
radionuclides in environmental media external to the body (i.e., external exposure) or by 
inhalation or ingestion of media into the body. The typical activities that may lead to radiation 
exposure are summarized in Table 3-2. Other environmental media and transport pathways that 
could lead to exposure were considered during development of the model (e.g., external exposure 
to air and water, inhalation of soil particles by farm animals) but were excluded because they 
have a negligible influence on the model results (BSC 2003a, Sections 6.3.3, 7.4.2, 7.4.3). 
Table 3-1. Transport Pathways Explicitly Included for the Groundwater and Volcanic Ash Scenarios 
Transport Pathway Volcanic Groundwater 
  
    
    
  
    
    
    
  
  
    
  
  
September 2003 
Radionuclide accumulation in soil from irrigation with water 
Resuspension of soil 
Deposition of resuspended soil on crops 
Deposition of irrigation water on crops 
Translocation of radionuclides to the edible tissues of crops 
Post-deposition retention by crops (including weathering processes) 
Radionuclide uptake by crops through the roots 
Release of gaseous radionuclides (Radon-222, 14CO2) from the soil 
Absorption of 14CO2 by crops from the atmosphere 
Radionuclide uptake by animals through consumption of feed, water, and soil, 
followed by transfer to animal products 
Radionuclide transfer from water to air via evaporative coolers 
Radionuclide transfer from water to fish (aquatic food). 
3-2 
Source: BSC 2003a, Sections 6.3, 6.4, 6.5. 
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Figure 3-1. Conceptual Representation of the Transport and Exposure Pathways for the Groundwater 
Exposure Scenario 
September 2003 3-3 No. 12: Biosphere Transport
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Environmental 
Medium 
Ingestion Water 
Ingestion Soil 
External Soil 
Inhalation Air 
Ingestion Plants 
Ingestion Animals 
Ingestion Fish 
Table 3-2. Exposure Pathways for the Groundwater Scenario 
Exposure 
Mode 
Water intake 
Examples of Typical Activities 
Drinking water and water-based 
beverages. Water used in food 
preparation. 
Recreational activities, occupational 
activities, gardening, consumption of 
fresh fruits and vegetables. 
Time spent on or near soil containing 
radionuclides. 
Outdoor activities, including soildisturbing 
activities related to work 
and recreation. Domestic activities, 
including sleeping. 
Eating crops. 
Eating animal products. 
Eating fish. 
September 2003 
Exposure Pathway 
Inadvertent soil ingestion 
External radiation exposure 
Breathing resuspended particles, gases 
(222Rn and progeny, plus 14CO2), and 
aerosols from evaporative coolers 
Consumption of locally produced crops (leafy 
vegetables, other vegetables, fruit, and grain) 
Consumption of locally produced animal 
products (meat, poultry, milk, and eggs) 
Consumption of locally produced freshwater 
fish 
3-4 
Source: BSC 2003a, Table 6.3-1. 
In summary, groundwater may be used to irrigate crops, water farm animals, raise fish, run 
evaporative coolers, and for drinking. Irrigation of crops could result in radionuclides in topsoil 
and crops. The soil may be resuspended and deposited on crops, inhaled by the receptor, and 
inadvertently eaten by the receptor and farm animals. Radioactive gasses from the soil also may 
be taken up by crops and inhaled by the receptor. The crops may be eaten by the receptor and 
fed to farm animals. Products from those farm animals and fish also may be eaten by the 
receptor. The receptor could receive a dose from external exposure to the soil; from inhaling 
resuspended soil, radioactive gasses, and aerosols from evaporative coolers; and from ingesting 
water, crops, fish, animal products, and soil. 
3.2 VOLCANIC ASH SCENARIO PATHWAYS 
The biosphere model for the volcanic ash exposure scenario considers the same reference 
biosphere and receptor as the groundwater scenario. The major difference between the models is 
the radionuclide source. For the volcanic scenario, the source is ash on the ground surface; 
radionuclides in groundwater are not considered for this biosphere exposure scenario. The ash 
could come directly from a volcanic eruption or it could be transported into the biosphere by 
aeolian and fluvial processes. On cultivated soils, the ash would mix with surface soil and 
radionuclides could be transferred to crops and animal products. That could result in exposure 
when those crops and products are ingested. On all lands, the volcanic ash could be 
resuspended, causing exposure from inhalation of ash particles. The ash also may be 
inadvertently ingested and may cause external exposure (BSC 2003a, Section 6.3.2). 
Because of the different radionuclide sources, only those environmental media and transport 
processes related to radionuclides in soil are considered in the volcanic ash model; media and 
processes related only to radionuclides in water are excluded (Table 3-1 and Figure 3-2). Thus, 
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only four of the six media listed in Section 3.1 are included in this model (soil, air, plants, and 
animals); groundwater and fish are excluded. The human exposure pathways included in the 
biosphere model for the volcanic ash scenario, and the typical activities that may cause radiation 
exposure are summarized in Table 3-3. Exposure of fish from deposition of ash on ponds is 
excluded for the following reasons. Fish in small ponds are susceptible to suffocation when their 
water is polluted by particulates (e.g., catastrophic loss of fish in Amargosa Valley ponds 
occurred when smoke from a forest fire drifted into the valley, Roe 2002) and, therefore, likely 
would die after an eruption. Even if they survived, fish ponds in Amargosa Valley are cleaned at 
least once every other year. Ash particles probably would be removed and future crops of fish 
would not be exposed to radiation (BSC 2003e, Section 6.4; Roe 2002). In addition, ingestion is 
an unimportant pathway for this scenario for most radionuclides (BSC 2003h, Section 6.2.5); 
therefore, inclusion of fish ingestion would have a negligible contribution to BDCFs. 
In summary, ash from an eruption at the repository could be deposited throughout the reference 
biosphere by initial deposition or by redistribution following a volcanic eruption. Crops may 
take up radionuclides from resuspended ash particles, from roots, and from exhalation of radon 
from the soil. Farm animals may be fed those crops and may ingest soil containing 
radionuclides. The receptor could receive a dose from external exposure to soil; from inhaling 
radon and resuspended ash; and from ingesting crops, animal products, and soil. 
Environmental 
Medium 
Ingestion Soil 
External Soil 
Inhalation Air 
Ingestion Plants 
Ingestion Animals 
Table 3-3. Exposure Pathways for the Volcanic Ash Scenario 
Exposure 
Mode Exposure Pathways 
3-5 
Inadvertent soil ingestion 
External radiation exposure 
Breathing of airborne particulates; 
breathing of gases (222Rn and progeny) 
Consumption of locally produced crops, 
including leafy vegetables, other 
vegetables, fruit, and grain 
Consumption of locally produced animal 
products, including meat, poultry, milk, 
and eggs 
Examples of Typical Activities 
Recreational activities, occupational 
activities, gardening, consumption of 
fresh fruit and vegetables. 
Activities on or near soil containing 
radionuclides. 
Outdoor activities, including soildisturbing 
activities related to work 
and recreation. Domestic activities, 
including sleeping. 
Eating and drinking plant materials. 
Eating and drinking animal products. 
September 2003 
Source: BSC 2003a, Table 6.3-3. 
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Conceptual Representation of the Transport and Exposure Pathways for the Volcanic Ash 
Scenario 
Figure 3-2. 
3.3 RADIONUCLIDE DECAY AND INGROWTH 
The radionuclides to be considered in the TSPA are identified in Radionuclide Screening 
(BSC 2002b, Section 7), which lists 28 radionuclides (called primary radionuclides) representing 
the 17 elements that are major contributors to the postclosure dose calculated by the TSPA. The 
TSPA analysis only considered radionuclides with a half-life of greater than 10 years. Although 
this is reasonable for TSPA calculations because of the long-term physical processes considered 
in that model, the short-lived decay products of the primary radionuclides must be considered in 
the biosphere model dose calculations. 
To include the effects of decay products in the biosphere model, the decay chain of each primary 
radionuclide was defined (BSC 2003a, Section 6.3.5). Within each decay chain, radionuclides 
were classified as long-lived or short-lived (i.e., half-life longer or shorter than 180 days, 
respectively). Long-lived decay products that are not primary radionuclides (e.g., Radium-228 
and Thorium-228) are tracked separately from the primary radionuclides in the model. This is 
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done because these radionuclides may have different environmental transport properties and may 
not be in radioactive equilibrium with their parent radionuclide. 
Short-lived radionuclides in each decay chain are assumed to be in equilibrium with the 
long-lived parent. This assumption is reasonable because the primary radionuclides have long 
half-lives, so equilibrium in an environmental medium would occur after a relatively short period 
of time. This assumption is conservative because the activities of the decay products are highest 
when in equilibrium with the long-lived parent radionuclides (BSC 2003a, Section 5.2). The 
dose contribution of short-lived radionuclides are included in the effective dose coefficients 
(external exposure) and effective dose conversion factors (inhalation and ingestion) developed 
for primary radionuclides (e.g., BSC 2003a, Sections 6.3.5 and 6.4.7.2). 
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4. GROUNDWATER SCENARIO MODEL 
The conceptual and mathematical models developed to calculate BDCFs for the groundwater 
scenario are described in this section. Included are summaries of the structure of the ERMYN 
groundwater model and associated submodels; methods used to calculate radionuclide 
concentrations in the environmental media; methods used to calculate exposure from inhalation, 
ingestion, and external exposure via the three pathways; assumptions upon which the 
calculations are based; and the input parameters used to make the calculations. The Biosphere 
Model Report (BSC 2003a, Sections 5 and 6) presents detailed descriptions. 
The purpose of the biosphere model is to provide to the TSPA the inputs needed to calculate the 
annual dose to a human receptor from any release into the biosphere of radionuclides from the 
repository. Because the biosphere calculations are conducted independently of the TSPA 
calculations of radionuclide concentrations, the biosphere model calculates conversion factors 
that are equal to the annual dose per unit concentration of radionuclides in the source of 
contamination (groundwater or volcanic ash). These concentration factors, BDCFs, are the 
primary output of the biosphere model. 
A BDCF for this scenario is numerically equal to an all-pathway dose that the RMEI would 
receive when exposed to the concentration of a radionuclide in environmental media arising from 
a unit concentration of the radionuclide in groundwater (i.e., Bq/m3). The concentrations of 
radionuclides in groundwater are time dependent and are calculated outside the ERMYN model 
by the TSPA model; thus, the groundwater concentrations are unknown until the TSPA model is 
run. To calculate BDCFs that are independent of time (and thus independent of the TSPA 
model), it is assumed in the biosphere model that radionuclide concentrations in groundwater do 
not change over time. This assumption is valid because the time required for radionuclides 
considered in the TSPA to approach equilibrium concentrations in surface soils is much less than 
the 10,000-year compliance period for the TSPA-LA (BSC 2003a, Sections 5.1 and 7.4.2). 
Based on this assumption, BDCFs can be calculated in the ERMYN model based on unit 
concentrations of radionuclides in groundwater (i.e., 1 Bq/m3), and the annual dose can be 
calculated in the TSPA as the product of the BDCFs and radionuclide concentrations in 
groundwater. 
Based on this assumption, BDCFs are calculated in the ERMYN model as the annual dose per 
unit concentrations of radionuclides in groundwater (i.e., Sv/year per Bq/m3), and the annual 
dose is calculated in the TSPA as the product of the BDCFs and radionuclide concentrations in 
groundwater. The TSPA calculates the annual dose in terms of the total effective dose 
equivalent (10 CFR 63.2) to the specified human receptor. In this context, the annual total 
effective dose equivalent is the sum of the effective dose equivalent for external exposures 
received in one year and the committed effective dose equivalent resulting from one-year intake 
of radionuclides. The commitment period used is 50 years. 
The conceptual and mathematical models for the groundwater scenario are developed in the 
ERMYN as a series of eight submodels, representing five of the environmental media (the source 
of radionuclides, groundwater, is not included as a submodel) and the exposure pathways 
described in Section 3.1. Figure 4-1 shows the interactions among those submodels (i.e., the 
transfer pathways). In addition to the eight submodels, a special submodel is included to 
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calculate Carbon-14 concentrations in surface soil, air, crops, and animal products because the 
transfer mechanisms for this radionuclide are different from the other radionuclides considered. 
The methods used in the ERMYN model to calculate radionuclide concentrations in 
environmental media and exposure rates were selected based on a review of applicable methods 
used in other environmental radiation models (BSC 2003a, Section 6.4 and 7.3). The input 
parameter values used in the model were developed based on site-specific data, such as those 
obtained during the survey of Amargosa Valley residents (DOE 1997), and from reviews of 
values used in other environmental radiation models, data from analog sites, and from other 
applicable publications. All input parameters that could contribute to variation in the model 
results are input as distributions and stochastically sampled in the model. 
Climate change is incorporated into the ERMYN groundwater model by using different values 
for input parameters that are influenced by temperature and precipitation (e.g., irrigation rate and 
evaporative cooler use) and calculating separate sets of BDCFs for each climate state considered 
in the TSPA (BSC 2003g, Section 6.2.2). The future climate for the region around Yucca 
Mountain is predicted to be cooler and wetter than the current climate (USGS 2001). A wetter 
climate may cause the water table to rise and discharge groundwater at springs. The ERMYN 
model applies to the discharge of groundwater from springs if the use of spring water remains the 
same as the use of well water, and there is no mixing of contaminated and uncontaminated water 
(or other processes that would cause the radionuclide concentrations to change). The model does 
not apply to a biosphere with permanent rivers or lakes because they do not occur in Amargosa 
Valley now and are not likely to occur there in the future, and because these features would 
require additional pathways (e.g., water immersion due to swimming and external exposure due 
to contaminated sediments) that are not in the ERMYN model. 
The following sections describe the submodels of the ERMYN model for the groundwater 
exposure scenario. 
4.1 SOIL SUBMODEL 
Radionuclide concentrations in the surface soil (i.e., the soil layer down to the tilling depth, 
which contains the majority of plant roots) are calculated in this submodel. The source of 
radionuclides is groundwater used for crop irrigation. Based on agricultural practices in 
Amargosa Valley, groundwater is the only source of irrigation water considered. Because the 
objective of the postclosure dose assessment is to evaluate the potential dose from the repository, 
continuous irrigation with groundwater for hundreds to thousands of years is assumed. This 
would result in the buildup of radionuclides in irrigated soil until equilibrium conditions are 
reached. Thus, the biosphere model assumes that radionuclides in irrigated soil are at saturation 
concentrations (BSC 2003a, Section 5.5). This assumption is conservative for the period when 
radionulcide concentrations in groundwater are increasing, as would be expected during the 
10,000-year compliance period. However, it is not conservative when applied to the period long 
after closure of the repository when groundwater concentrations are declining, because soil 
concentrations may be underestimated (BSC 2003a, Section 6.4.10.3). 
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Figure 4-1. Relationship among Biosphere Submodels for the Groundwater Scenario 
The equilibrium concentrations of radionuclides in the soil are calculated as a function of 
groundwater concentrations (which are considered constant at 1 Bq/m3); the annual irrigation 
rate for crops; and loss by radionuclide decay, leaching, and erosion (BSC 2003a, Section 6.4.1). 
This calculation is based on the conservation of the mass of radionuclides in the topsoil. The rate 
of increase of radionuclides in topsoil is equal to the rate of addition from irrigation water less 
the rate of loss from radioactive decay, leaching, and erosion. The solution to the resulting 
equation is time dependent, so the ERMYN model uses the time-independent asymptotic 
solution to the rate equations. This is a conservative approach and avoids speculation about 
changes in agricultural practices and land use over the 10,000-year compliance period. 
As discussed in Section 3.3, short-lived radionuclides (half-life less than 180 days) are 
considered to be in equilibrium with the primary radionuclides. Long-lived non-primary 
radionuclides are treated numerically similar to the primary radionuclides, but the rate of their 
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addition to the system is from decay of the primary radionuclide rather than application of 
groundwater. 
The annual irrigation rate, which is used to calculate the addition of radionuclides into the soil, 
was estimated from 26 garden crops, commercial crops, and horticultural plants representative of 
all crop types considered in the ERMYN model (BSC 2003d, Section 6.5). This was done 
because it is likely that a variety of field, garden, and horticultural crops would be grown on a 
plot of land over a long period. Methods developed by the Food and Agriculture Organization of 
the United Nations (Allen et al. 1998; Doorenbos and Pruitt 1977) were used to calculate 
irrigation rates, as described in Agricultural and Environmental Input Parameters for the 
Biosphere Model (BSC 2003d). The average annual irrigation rates estimated for the current 
climate and upper bound of the glacial transition climate are 0.94 and 0.50 m/year, respectively 
(BSC 2003d, Section 6.5). 
Leaching is included in the soil submodel to account for the residence time of radionuclides in 
the surface soil and their removal to deeper soil. The leaching rate is a function of the amount of 
water that percolates below the surface soil (i.e., the overwatering rate), element-specific 
solid-liquid partition coefficients, and other soil properties (e.g., bulk density, soil porosity, and 
soil moisture content at field capacity). In the current arid conditions at Yucca Mountain, 
leaching occurs primarily when irrigation water is added to flush accumulated salts from the 
surface soil to maintain plant productivity. In wetter climates, such as those predicted to occur in 
the future at Yucca Mountain, leaching also occurs when excess precipitation flows through the 
surface soil, primarily during winter. The annual average overwatering rates estimated for the 
current climate and upper bound of the glacial transition climate are 0.079 and 0.067 m/year, 
respectively (BSC 2003d, Section 6.9). The partition coefficients used in the ERMYN model 
were selected based on site-specific soil conditions and range over several orders of magnitude 
(i.e., a lower limit of about 1 × 10-2 L/kg for Technetium to an upper limit of 4.8 × 104 L/kg for 
Americium), as described in Soil-Related Input Parameters for the Biosphere Model (BSC 
2003f, Sections 4.1.2 and 6.3). 
Soil erosion accounts for the loss of radionuclides via wind and water erosion. The upper limit 
of the erosion rate is based on the minimum average loss measured in Nevada for cultivated and 
non-cultivated land and the maximum annual soil loss tolerable before crop yields are reduced. 
The lower limit of erosion loss is estimated from soil influx and an equal loss (no net loss or gain 
in soil), such that the average atmospheric dust level is maintained in association with an 
appropriate dry settling velocity. Erosion rates typical for soils in northern Amargosa Valley 
range from 0.19 to 1.1 kg m-2 y-1 (BSC 2003f, Section 6.4). 
When overhead irrigation is used, radionuclides in irrigation water can be intercepted by crop 
leaves. However, crop weathering by wind and other mechanisms will displace some initially 
intercepted radionuclides onto the soil. Therefore, the model conservatively assumes that all 
radionuclides in the irrigation water, including the fraction calculated to have been intercepted by 
crops, reach the soil (BSC 2003a, Section 5.6). It is also assumed that the loss of radionuclides 
due to crop harvest is compensated by the addition of contaminated animal manure and nonharvested 
plant residue as fertilizer. This assumption is reasonable because the use of animal 
manure to fertilize fields is a common practice in Amargosa Valley (BSC 2003a, Section 5.4). 
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Radionuclide concentrations in the soil calculated in this submodel are used in most of the other 
submodels (Figure 4-1) because most environmental transport and exposure pathways require 
estimates of radionuclide concentrations in the surface soil. 
4.2 AIR SUBMODEL 
The air submodel calculates the concentrations of radionuclides in air resulting from three 
transport pathways: soil resuspension, release of radioactive gases, and the generation of aerosols 
by evaporative coolers. 
4.2.1 Soil Resuspension 
Soil particles may be resuspended by wind or during mechanical disturbances (e.g., tilling of 
fields). These resuspended particles would result in exposure when deposited on plants or 
inhaled by the receptor. Separate calculations are used in the ERMYN model to estimate the 
concentrations of radionuclides in the air around plants and inhaled by the receptor. 
Radionuclide concentrations in the air resulting from resuspension of soil particles that may be 
directly deposited on plants are calculated as the product of activity concentrations in cultivated 
soil and atmospheric mass loading (i.e., the mass concentration of resuspended particles) in the 
environment around plants (BSC 2003a, Section 6.4.2). Mass loading in agricultural fields and 
gardens during the latter part of the growing season is estimated to range from 0.025 to 
0.200 mg/m3, similar to or higher than that measured outdoors in rural, agricultural environments 
(BSC 2003b, Section 6.1.5). Radionuclide concentrations in the air, calculated using this 
method, are used in the plant submodel to estimate dry deposition of radionuclides on plant 
surfaces. 
To account for variation and uncertainty in the characteristics of the RMEI and concentrations of 
radionuclides throughout the biosphere, the ERMYN model uses a micro-environmental 
modeling approach to calculate inhalation exposure (this method is also used to evaluate external 
exposure). For micro-environmental models, the total exposure environment (i.e., the biosphere) 
is divided into segments, or environments, with different concentrations of contaminants. The 
contaminant concentration, time spent in each environment, and intake rates or exposure factors 
(e.g., breathing rates and shielding factors) are determined for each environment, and the total 
dose is calculated as the sum of the doses from all environments (Mage 1985, pp. 409 and 410; 
BSC 2003a, Section 6.4.8). Micro-environmental models are commonly used to evaluate 
exposure to particulate matter and other contaminants (Duan 1982; Mage 1985; Klepeis 1999). 
To evaluate inhalation exposure in an environment, activity concentrations in the air are 
calculated as the product of activity concentrations in cultivated soil, an environment-specific 
enhancement factor, and mass loading in each environment (BSC 2003a, Section 6.4.2). The 
enhancement factor is defined as the ratio of the mass activity concentration of resuspended 
particles to the mass activity concentration in surface soil for a radionuclide. This parameter is 
included because the size distribution of resuspended particles may be different from the particle 
size distribution of the host soil and because the activity concentration per unit mass may be a 
function of particle size. For soil particles contaminated by irrigation water, contaminants would 
be adsorbed onto particles in the form of a thin film on the particle surface. The surface coating 
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would result in a higher activity concentration on smaller particles compared to larger particles 
(because surface area per unit mass is greater for smaller particles). Distributions of 
enhancement factors used in the ERMYN model range from 2.2 to 6.5 for outdoor environments 
and 0.21 to 1.04 for other environments (BSC 2003f, Section 6.5). 
Five environments associated with different human activities are considered in the ERMYN 
model. These mutually exclusive environments represent behavioral and environmental 
combinations for which the receptor would receive a substantially different rate of exposure via 
inhalation or external exposure (BSC 2003c, Section 6.2). The mass loading distributions for 
each environment are representative of the average annual concentration of resuspended particles 
to which the receptor would be exposed. Therefore, triangular distributions that incorporate 
variation and uncertainty in the annual average value, but do not include the entire range of 
concentrations for all activities and situations in Amargosa Valley, are used in the model. 
Distributions of mass loading for these environments are developed in the analysis report 
Inhalation Exposure Input Parameters for the Biosphere Model (BSC 2003b, Section 6.1). 
Active Outdoors–This category includes time spent outdoors in contaminated areas conducting 
activities that would resuspend soil, including dust-generating activities while working 
(e.g., plowing, excavating, livestock operations) and recreating outdoors (e.g., gardening, 
landscaping, riding horses or motorbikes). Mass loading in this environment is estimated to 
range from 1.0 to 10.0 mg/m3. This range was selected based on measurements of personnel 
exposed to resuspended dust during soil-disturbing activities (BSC 2003b, Section 6.1.1). 
Inactive Outdoors–This category includes time spent outdoors in contaminated areas engaged 
in activities that do not resuspend soil (e.g., sitting, swimming, walking, barbecuing, and 
equipment maintenance). This category also includes time spent commuting within the 
contaminated area because the major roads in Amargosa Valley are paved (thus soil is not 
resuspended). The distribution of mass loading in the inactive outdoor environment ranges 
from 0.025 to 0.100 mg/m3 and was developed from measurements taken at static air-quality 
monitoring stations located in rural agricultural settings in arid to semi-arid environments 
(BSC 2003b, Section 6.1.2). 
Active Indoors–This category includes time spent awake indoors in contaminated areas, 
including work time. The mass loading distribution for this environment ranges from 0.060 to 
0.175 mg/m3, and is based on static and personnel exposure measurements taken indoors while 
people were active (BSC 2003b, Section 6.1.3). 
Asleep Indoors–This category includes time spent indoors in contaminated areas sleeping. 
Mass loading in this environment is quite low, with a range of 0.010 to 0.050 mg/m3. This range 
is based on measurements taken indoors while people were sleeping or inactive (BSC 2003b, 
Section 6.1.4). 
Away from Potentially Contaminated Area–This category includes time spent away from 
areas potentially contaminated by groundwater or ash, including time spent commuting to work 
and working outside the contaminated areas. No mass loading estimate is required for this 
distribution. 
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4.2.2 Evaporative Cooler Operation 
The contribution to the inhalation dose from operation of evaporative coolers was added to the 
ERMYN model because a large proportion of the Amargosa Valley population uses evaporative 
coolers (Section 2.2). The concentration of radionuclides in indoor air resulting from the 
operation of evaporative coolers is calculated as a function of the concentration of radionuclides 
in groundwater, the rate at which water evaporates from coolers while in operation, the air flow 
rate, and the fraction of radionuclides in the water that transfer to the air (BSC 2003a, 
Section 6.4.2.2). This method is based on how evaporative coolers operate and the conservation 
of radioactivity (i.e., activity transferred to air is equal to the loss of activity from water). The 
evaporation and air flow rates are estimated from specifications of evaporative cooling units 
typically used in mobile homes. Because there is no information available on the fraction of 
radionuclides in water that transfer to air during the operation of evaporative coolers, a 
distribution of 0.0 to 1.0 is used. A lower bound of 0.0 is reasonable because coolers are 
designed such that most or all minerals dissolved in water precipitate out on the pads or in the 
sump of the cooler (BSC 2003e, Section 6.5). 
4.2.3 Radon Exhalation from Surface Soil 
Airborne concentrations resulting from gaseous release from the soil are considered in the 
ERMYN model for two radionuclides, Carbon-14 and Radon-222. The calculation of Carbon-14 
concentrations in air is described in Section 4.6. Concentrations of radon are calculated in the air 
submodel separately for indoor and outdoor air. The contribution of radon to airborne 
radionuclide concentrations is included in the ERMYN model because inhalation of radon decay 
products is an important contributor to the dose resulting from Radium-226 in groundwater 
(BSC 2003a, Section 7.4.3.1). 
The concentration of radon outdoors is estimated from the amount of radon released from the 
soil. It is calculated as the product of activity concentration of Radium-226 in surface soil and a 
radon release factor of 0.25 kg/m3 (BSC 2003a, Section 6.4.2.3). That release factor is based on 
the global average value of the concentration ratio of Radon-222 activity in air to Radium-226 in 
soil (BSC 2003e, Section 6.6.1). 
The method for calculating the indoor concentration of radon was developed for a single-story 
house built on contaminated soil, assuming steady-state conditions between the rate of radon 
entry into the house and the rate of removal. The main sources of indoor radon are outdoor air 
and the soil beneath the house, which is assumed to have a saturation concentration of radium. 
This assumption is conservative because it is unlikely that all houses would be built on soil 
irrigated long enough to reach a saturation concentration of radium. The indoor radon 
concentration is calculated as a function of the concentration of radon in outdoor air, home 
ventilation rate, interior wall height, flux density of radon from outdoor soil, and fraction of that 
radon that would enter a home (BSC 2003a, Section 6.4.2.3). Values of the ventilation rate and 
wall height are based on conditions in manufactured homes. The ventilation rate differs 
substantially when evaporative coolers are off (0.3 to 2.9 air exchanges per hour) and when they 
are operated (1 to 30 exchanges per hour); therefore, the radon concentration in indoor air is 
calculated separately for periods when coolers are on and off (BSC 2003a, Section 6.4.2.3). 
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Values for input parameters used to calculate radon concentrations are developed in 
Environmental Transport Input Parameters for the Biosphere Model (BSC 2003e, Section 6.6). 
Radon concentrations due to the use of evaporative coolers and household water (e.g., showers) 
are not included in the ERMYN model because those concentrations contribute little to the total 
inhalation and external exposure radon doses (BSC 2003a, Section 7.4.3.1). 
The activity concentrations of airborne radionuclides (as particles, aerosols, and gases) 
calculated in this submodel are used in the inhalation submodel to calculate the dose from 
inhalation exposure. They are also used in the plant submodel to calculate the transfer of 
radionuclides to crops from particle deposition and uptake of carbon via photosynthesis 
(Figure 4-1). 
4.3 PLANT SUBMODEL 
Radionuclide concentrations in plant parts consumed by humans and farm animals are calculated 
in the plant submodel. Three transport pathways are included in this submodel: root uptake, 
water interception, and dust interception. The addition of radionuclides from soil splash during 
irrigation is not included in the ERMYN model because it is equivalent to direct deposition of 
resuspended particles (BSC 2003a, Sections 6.4.3 and 7.4.4.3). In addition, this submodel does 
not include radionuclide decay following harvest because the radionuclides considered are long 
lived and would decay very little during the short time between harvest and consumption of fresh 
produce and forage. 
The ERMYN model includes four types of crops consumed by humans: leafy vegetables, other 
vegetables, fruit, and grain. Leafy vegetables include plants such as lettuce, spinach, and 
cabbage having aboveground, edible portions (i.e., the leaves) that are exposed and eaten with 
little processing. Other vegetables include root crops (e.g., carrots and potatoes) and crops with 
edible parts that are not directly exposed (e.g., peas and beans that grow inside pods). Fruits 
include a variety of products such as berries, grapes, melons, and apples. Grains include 
seed-producing crops such as wheat, corn, and barley. Crops consumed by farm animals are also 
considered in the plant submodel. It is assumed that beef cattle and dairy cows are fed fresh, 
locally produced forage (e.g., alfalfa) and poultry and laying hens are fed locally produced grain 
(BSC 2003a, Section 5.8). 
Many of the input parameters in this submodel vary substantially among crops (e.g., irrigation 
rates and growing season length). To ensure that variation among crops within each crop type is 
included in the parameter distributions, a set of representative crops was selected for each crop 
type. Five to seven representative crops were selected for leafy vegetables, root vegetables, and 
fruits. Crops grown in gardens, orchards, and fields in Amargosa Valley and commonly eaten 
(based on national food consumption data) were selected. Fewer representative crops were 
selected for grains and cattle forage because there is little diversity in the types of field crops that 
are grown in the region. Cumulative distributions were then developed for each parameter that 
incorporate the values for each crop within a crop type and uncertainty about environmental 
factors that influence the parameters, as described in Agricultural and Environmental Input 
Parameters for the Biosphere Model (BSC 2003d, Section 6 and Appendix A). 
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4.3.1 Root Uptake 
The concentration of radionuclides in edible portions of crops due to uptake through roots of 
radionuclides in soil is calculated as the product of activity concentrations in soil (from the soil 
submodel; Section 4.1), a radionuclide-specific soil-to-plant transfer factor for each crop type, 
and the dry-to-wet ratio of foodstuffs within each crop type (BSC 2003a, Section 6.4.3.1). 
The soil-to-plant transfer factor is the ratio of the activity concentration of a radionuclide in dry 
edible parts of plants to the activity concentration in dry soil. Observed values of transfer factors 
vary widely, mainly because of differences among soils, crops, and environmental conditions. 
The values of soil-to-plant transfer factors for each radionuclide and crop type are developed in 
Environmental Transport Input Parameters for the Biosphere Model (BSC 2003e, Section 6.2.1). 
Transfer factors were selected from published technical information, mainly review reports, 
compendia of biosphere parameter values, and dose assessment reports. Selection of these 
factors included consideration of site-specific soil characteristics and the crops typically grown 
in Amargosa Valley. Truncated lognormal probability distribution functions that include all 
relevant transfer factors reported in the literature are used in the ERMYN model (BSC 2003e, 
Section 6.2.1.1.5). Dry-to-wet ratios are based on well-established values for foodstuffs and 
animal feed; the average ratio is 0.9 for grains and from 0.07 to 0.22 for the other crop types 
(BSC 2003d, Section 6.2). 
4.3.2 Water Interception 
The calculation of radionuclide concentrations in foodstuffs resulting from irrigation water 
sprayed on plants incorporates the processes of deposition, interception, translocation, and 
retention (BSC 2003a, Section 6.4.3.2). The activity concentration of radionuclides initially 
sprayed on crops is calculated as the product of concentrations in groundwater, daily irrigation 
rates per crop type, and the fraction of irrigation applied using overhead methods. Average daily 
irrigation rates over the entire growing season were determined by calculating 
evapotranspiration, effective precipitation, and overwatering requirements for representative 
crops. Average values per crop type for the current climate range from 4.6 mm per day for 
grains to 7.6 mm per day for other vegetables. Values for the glacial transition climate are about 
25 to 50 percent lower (BSC 2003d, Section 6.8). The fraction of overhead irrigation is included 
in the model because some crop types, such as fruit trees and some vegetables, may not be 
irrigated using overhead spray methods. Because most field crops in Amargosa Valley are 
irrigated using center pivot or rolling sprinklers, the average fraction of overhead irrigation for 
grains and cattle forage used in the ERMYN model is 0.9 (BSC 2003d, Section 6.3). 
The fraction of activity concentration in irrigation water that is initially intercepted by plants is 
estimated using an empirical formula derived by Hoffman et al. (1989). This formula is used to 
calculate the water interception fraction from crop biomass, the amount and intensity of 
irrigation, and empirical constants that quantify differences in interception of radionuclides with 
different charges and particle sizes. Because there is no information available to calculate 
radionuclide-specific constants for most radionuclides considered in the ERMYN model, and 
because radionuclides in the groundwater may be present in different chemical forms (e.g., have 
different ionic charges) or as suspended particles, one set of empirical constants are used in the 
ERMYN model for all radionuclides. To ensure that interception is not underestimated, the 
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constants used were derived from experiments involving the radionuclide that had the highest 
interception fraction (cationic beryllium). Distributions of the other input parameters required to 
calculate the interception fraction were developed from average values per crop type (biomass 
and irrigation application) and the soil types and irrigation methods in Amargosa Valley 
(irrigation intensity) (BSC 2003d, Sections 6.1, 6.6, and 6.7). Interception fractions calculated 
using this method for the current climate range from about 0.1 to 1.0; averages per crop type 
range from 0.24 for leafy vegetables to 0.51 for grains (BSC 2003a, Section 7.3.3.2 and 
Attachment I [file ERMYN_GW_Pu239verf.gsm]). 
The translocation factor in the water interception calculation quantifies the fraction of 
radionuclides initially intercepted by plant surfaces that is absorbed and translocated to edible 
plant parts. Values used in the ERMYN model were selected based on a review of interception 
fractions reported in the literature and used in other environmental radiation models (BSC 2003e, 
Section 6.2.2.2). A fixed value of 1.0 is used for leafy vegetables and cattle forage because 
radionuclides are deposited directly on the edible plant parts (i.e., the leaves). A distribution 
ranging from 0.05 to 0.30 is used for other crop types. 
The calculation of the retention of intercepted activity includes a loss component to account for 
weathering and other field losses. This component is calculated as a function of weathering half 
life and growing time per crop type. The weathering half life is the time it takes for the activity 
concentration on plants to be reduced by 50 percent. A distribution of 5 to 30 days is used in the 
model (BSC 2003e, Section 6.2.2.3). Growing time was selected from information on the length 
of the growing season for field and garden crops in arid and semiarid regions (BSC 2003d, 
Section 6.4). 
4.3.3 Dust Interception 
The calculation of the fraction of dust intercepted by plants incorporates the processes of 
deposition, initial interception, translocation, and retention (as a function of weathering half life 
and growing season length) and uses the same input parameter distributions for the translocation 
factor, weathering half life, and growing time that are used for water interception. A parameter 
analogous to the fraction of overhead irrigation is not included in the calculation of dust 
interception because all crops would be exposed to resuspended dust. Because the process of 
dust interception differs from water interception, different methods are used to estimate the 
deposition rate and the intercepted fraction of dust. 
The deposition rate of resuspended particles quantifies the combined effects of contaminant 
removal from the atmosphere by several processes (e.g., gravitational settling, diffusion, 
turbulent transport). It is calculated in the ERMYN model as a function of activity 
concentrations in the air around crops (calculated in the air submodel) and the dry deposition 
velocity. The deposition velocity was developed based on wind speeds in Amargosa Valley, 
surface roughness, and the expected particle size distribution of resuspended particles in fields 
and gardens (BSC 2003e, Section 6.2.2.1). 
The fraction of airborne particles intercepted by plant surfaces is calculated as an exponential 
function of dry biomass per crop type and an empirical factor that quantifies differences among 
crop types. Distributions of dry biomass per crop type were developed from published 
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measurements of crop yield, dry-to-wet ratios of crop foodstuffs, and harvest indices (the ratio of 
foodstuff dry biomass to total aboveground dry biomass) (BSC 2003d, Section 6.1). The 
empirical factors used in the model result in higher estimates of dust interception (per unit 
biomass) for leafy vegetables and cattle forage than for other crop types (because the edible parts 
of leafy vegetables and forage are directly exposed). This method results in average interception 
fractions that range from 0.46 for leafy vegetables (the crop type with the lowest biomass) to 
0.96 for grains (highest biomass) (BSC 2003a, Section 7.3.3.3). 
The total activity concentration in plants is calculated as the sum of the concentrations due to 
root uptake, water interception, and dust interception. These estimates are then used in the 
animal submodel to calculate the transfer of radionuclides from feed to farm animals and in the 
ingestion submodel to calculate the dose from eating locally produced crops (Figure 4-1). 
4.4 ANIMAL SUBMODEL 
This submodel is used to calculate the concentration of radionuclides in animal products due to 
ingestion of feed, water, and soil (BSC 2003a, Section 6.4.4). Inhalation of soil particles is not 
included because it contributes very little to the total radionuclide concentration in animal 
products (BSC 2003a, Section 7.3.4.4). 
Four types of animal products are included in the ERMYN model: meat, milk, poultry, and eggs. 
The meat category includes beef, pork, lamb, and game animals. Milk includes that from dairy 
cows, goats, and sheep. Poultry includes chickens, turkey, duck, geese, and game hens, and eggs 
come from laying hens (chickens) and ducks. The parameter distributions that are specific to 
each animal product were developed primarily based on information from cattle and chickens, 
although variation and uncertainty related to other farm animals are considered (BSC 2003a, 
Section 5.9 and 6.4.4). 
Activity concentrations in animal products due to ingestion of the three media considered in this 
submodel (feed, water, and soil) are calculated as the product of the concentration of 
radionuclides in those media, ingestion rates of the media, and animal-intake to animal-product 
transfer coefficients. Activity concentrations in feed and soil are taken from the plant and soil 
submodels, respectively. Consumption rates for each animal product were primarily from 
published consumption rates of feed, water, and soil by beef cattle, dairy cows, and chickens 
(BSC 2003e, Section 6.3.2). Transfer coefficients quantify the fraction of daily intake of a 
radionuclide by animals that is transferred to one kilogram of animal product. These coefficients 
differ among elements, chemical forms, and animal products, and uncertainty is considerable for 
most elements. Truncated lognormal distributions of transfer coefficients are used in the 
ERMYN model. These distributions were selected from published compendia of generic values 
and reports containing recommendations or applications of coefficients in other models. 
Concentrations of radionuclides in animal products calculated with this submodel are used in the 
ingestion submodel to calculate the dose from ingestion of locally produced farm animals, milk, 
and eggs (Figure 4-1). 
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4.5 FISH SUBMODEL 
The ERMYN model includes radionuclide transport through an aquatic food chain because there 
was a fish farm in Amargosa Valley during the 1990s (Section 2.1). Although most of the fish 
were used to stock ponds and lakes elsewhere in Nevada, the farm owner allowed Amargosa 
Valley residents to fish the ponds (BSC 2003e, Section 6.4.2). 
In this submodel, accumulation of radionuclides in fish is caused by the use of groundwater in 
the fishponds. The addition to water of radionuclides from resuspended soil particles also is not 
included because it contributes negligibly to the total activity concentration in the pond water 
(BSC 2003a, Section 6.4.5). 
The activity concentration in fish is calculated as the product of radionuclide concentrations in 
water, a bioaccumulation factor, and a water concentration modifying factor (BSC 2003a, 
Section 6.4.5). The bioaccumulation factor is the ratio of the activity concentration in edible 
portions of fish tissue to that in the water. There is a large amount of uncertainty and variation in 
this factor, so the distributions used in the ERMYN model range over several orders of 
magnitude (BSC 2003e, Section 6.4.3). Bioaccumulation factors have been developed for 
natural systems and include accumulation of radionuclides in fish from food. Fish in Amargosa 
Valley were fed commercial feed that was not produced locally; therefore, the accumulation 
factor is an upper bound of conditions in Amargosa Valley ponds. The modifying factor is 
included because radionuclide concentrations will increase over time as additional water is added 
to the ponds to replace that lost by evaporation. Distributions of the modifying factor were 
developed based on the assumption that activity accumulates in the ponds for up to two years 
(after which fish are harvested and the ponds drained and cleaned) and that water is replaced at a 
rate equal to the annual evaporation rate for the climate (about 2 m for the current climate and 
0.8 m for the glacial transition climate). These distributions range from 2.2 to 6.1 m for the 
current climate (i.e., the radionuclide concentration in pond water is about 2 to 6 times higher 
than in groundwater) and 1.5 to 3.3 m for the glacial transition climate (BSC 2003e, 
Section 6.4.3 and 6.4.5). 
The output of this model is used in the ingestion submodel to calculate the dose from the 
ingestion of locally raised fish (Figure 4-1). 
4.6 CARBON-14 SPECIAL SUBMODEL 
Carbon is an abundant and ubiquitous element and moves through the environment as a gas in 
the form of carbon dioxide. Consequently, some of the environmental transport pathways for 
Carbon-14, a radioactive isotope of carbon, are different from those for other radionuclides that 
are present in solid form. The transport of gaseous carbon in the environment is calculated in the 
Carbon-14 special submodel. This submodel includes calculations of Carbon-14 concentrations 
in soil and air; in crops from root uptake and photosynthesis; and in animal products from the 
ingestion of feed, water, and soil. The bioaccumulation of Carbon-14 in fish is calculated in the 
same manner as other radionuclides, as described in Section 4.5, with one exception. Carbon-14 
can be lost from pond water by emission of 14CO2 to the atmosphere. This additional loss 
mechanism is included in calculations of water concentration modifying factor. 
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The method used to calculate soil concentrations of Carbon-14 is the same as that used for other 
radionuclides (Section 4.1), with two exceptions (BSC 2003a, Section 6.4.6.1). First, an 
additional loss mechanism, gaseous emission loss, is included because Carbon-14 is volatile and 
quickly released from soil as 14CO2. The emission rate used in the ERMYN model is 22 times 
per year (i.e., six percent of Carbon-14 in the soil is lost per day), based on rates measured for 
sandy soils (BSC 2003e, Section 6.7.1). Thus, Carbon-14 concentrations in surface soils will 
reach equilibrium conditions within 1 to 2 months and emission is the dominant mechanism for 
removing Carbon-14 from the soil. Second, Carbon-14 concentrations in the soil are calculated 
separately for each crop type based on crop-type-specific daily irrigation rates (versus a single 
soil concentration for all crops, calculated based on the annual average irrigation rate as is used 
for other radionuclides, Section 4.1). This is done because Carbon-14 is rapidly released from 
the soil, and, therefore, irrigation must be considered locally and only during the growing season. 
Because Carbon-14 is quickly released from the soil as a gas, the activity concentration of this 
radionuclide in air cannot be calculated using the soil resuspension method (Section 4.2). Thus, 
a separate calculation is used that is based on the flux density of gaseous Carbon-14 from soil 
(calculated as the product of soil concentration and emission rate) and the movement or dilution 
of the released gas in a mixing cell of defined dimensions (BSC 2003a, Section 6.4.6.2). 
Because Carbon-14 would only be released from irrigated land, the size of the mixing cell is 
equal to the estimated surface area of irrigated land. The height of the cell is 2 m for human 
environments and 1 m for crops. The mixing rate also differs for human environments and 
crops, with a lower rate for crops (based on a slower wind speed closer to the ground). The 
mixing rate is based on wind speeds measured in northern Amargosa Valley and local terrain 
conditions (BSC 2003e, Section 6.7.2). 
The activity concentration of Carbon-14 in crops due to photosynthesis is calculated as a 
function of the concentration in the air, the fraction of carbon in plants that is derived from this 
pathway, and the ratio of the concentration of stable carbon in plants to stable carbon in air 
(BSC 2003a, Section 6.4.6.3). Similarly, the concentration of Carbon-14 in crops from root 
uptake is calculated as a function of the fraction of concentration in soil, the fraction of soilderived 
carbon in plants, and the ratio of the concentration of stable carbon in plants to stable 
carbon in soil. The fractions of carbon in plants derived from air and soil used in the ERMYN 
model are 0.98 and 0.02, respectively (BSC 2003e, Section 6.7.3). Thus, Carbon-14 
concentrations in crops are due primarily to uptake during photosynthesis. 
The activity concentration of Carbon-14 in animal products is derived from animal feed, soil, and 
drinking water. The transfer of Carbon-14 from these media to animal products is modeled by 
calculating the ratio of total Carbon-14 intake over all ingestion pathways to total carbon intake 
over the same pathways. Multiplication of this ratio by the fraction of stable carbon in the 
animal product provides the required Carbon-14 activity per unit mass of the product 
(BSC 2003a, Section 6.4.6.4). Because there is only a small amount of carbon in the soil and 
water, the primary source of Carbon-14 in animal products is feed. 
The results of these calculations are included with the outputs of the soil, air, plant, and animal 
submodels. 
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4.7 EXTERNAL EXPOSURE SUBMODEL 
This submodel calculates the annual dose (per unit concentration of radionuclides in 
groundwater) to the RMEI from external exposure. The next two submodels (inhalation and 
ingestion) calculate the dose from the other exposure pathways considered in the biosphere 
model (Table 3-2). All doses discussed in this and the following sections refer to annual doses 
per unit radionuclide concentration in groundwater. 
The external exposure submodel is used to calculate the annual dose due to radiation emitted by 
radioactive materials outside the human body. The annual effective dose equivalent is calculated 
for this pathway. Contaminated materials typically considered in calculations of external 
exposure doses include soil, air, and water. The ERMYN model only considers exposure to soil. 
External exposure to air and water are not considered because they contribute little to the annual 
dose (BSC 2003a, Section 7.4.8). For example, the dose due to external exposure to water 
(e.g., during showers, baths, and while swimming) is 1 to 5 orders of magnitude lower 
(depending on the radionuclide) than that from soil exposure, in part because the time exposed to 
water is much lower than time exposed to soil (BSC 2003a, Section 7.4.8.2). Likewise, the dose 
from external exposure to contaminants in air is about 6 orders of magnitude lower than that 
from exposure to soil, because activity concentrations in the air are much lower than those in the 
soil (BSC 2003a, Section 7.4.8.1). External exposure from other media (e.g., building material, 
furniture, and clothing) is possible, but few or no building materials, clothes, or other materials 
are produced in Amargosa Valley using groundwater. Furthermore, it is assumed that the size 
and depth of contaminated soils are infinite (BSC 2003a, Section 5.10), and residents are 
considered to be exposed to contaminated soil at all times while within their community. Thus, 
the soil exposure time is longer than for other media, and therefore it is reasonable to exclude 
exposure from those media in the ERMYN model. 
External exposure to soil is calculated for each primary radionuclide and long-lived decay 
product as the product of a radionuclide-specific effective dose coefficient, the saturation 
concentration of the radionuclide in the soil (from the soil submodel), the average time spent by 
the RMEI in each of five environments, and an environment-specific shielding factor. The total 
annual external exposure for a primary radionuclide is then calculated as the sum of doses for 
that radionuclide and its long-lived decay products (BSC 2003a, Section 6.4.7). 
Effective dose coefficients calculated in the ERMYN model include dose contributions of longlived 
radionuclides and short-lived decay products (Section 3.3; BSC 2003a, Section 6.4.7.2). 
Although radionuclide concentrations calculated in the soil submodel and used here apply only 
to surface soil, dose coefficients for soil contaminated to an infinite depth are used to calculate 
the external exposure dose for the groundwater scenario. This choice of dose coefficients is 
appropriate because the radiation contributing to external exposure may also originate in deep 
soil contaminated due to long-term radionuclide leaching from the surface soil (BSC 2003a, 
Section 5.10). 
As described in Section 4.2, a micro-environmental modeling approach is used to calculate 
external exposure and inhalation doses. The biosphere is divided into five environments (active 
outdoors, inactive outdoors, active indoors, asleep indoors, and away from the contaminated 
area). To account for variation and uncertainty in the characteristics of the receptor population 
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that influence exposure times, the population is divided into four mutually exclusive groups, and 
exposure times are estimated separately for each group. 
The four population groups represent the range of behaviors that would most influence the 
amount of time that people are exposed to radionuclides through the external exposure and 
inhalation pathways. Variation among individuals in these exposure pathways is influenced 
primarily by the amount of time they spend indoors and outdoors within contaminated areas and 
the amount of time they spend away from contaminated areas. For adults, variation among these 
time factors primarily is a function of occupational characteristics, as people working outside the 
contaminated area generally would experience less exposure than people who remain within the 
area, and people who work outdoors would be exposed differently from those who remain 
indoors. Therefore, the categories are based on work location and type of occupation. Estimates 
of the proportion of the adult Amargosa Valley population in each of the following four groups, 
and the average time they spend in each environment, are developed in Characteristics of the 
Receptor for the Biosphere Model (BSC 2003c, Sections 6.3.1 and 6.3.2). Time spent in 
environments was estimated based on information collected during the 2000 census on the 
amount of time people in Amargosa Valley spent commuting and working (Bureau of the Census 
2002) and from information in the U.S. Environmental Protection Agency’s Exposure Factors 
Handbook (EPA 1997). 
Local Outdoor Workers–This group includes residents who work outdoors and disturb (and, 
therefore, resuspend) contaminated soil. Based on an assumption that this group includes all 
adult Amargosa Valley residents identified in the 2000 census (Table 2-1) that worked in 
agriculture, 25 percent of those working in construction, 10 percent of utility workers, and 
10 percent of miners, 5.5 percent of the population is classified as local outdoor workers. These 
workers are estimated to spend an average of 3.1 hours per day active outdoors, 4.0 hours 
inactive outdoors, 6.6 hours active indoors, 8.3 asleep, and 2.0 hours away from contaminated 
areas. 
Commuters–This group includes residents who work outside of contaminated areas. 
Thirty-nine percent of the population is classified as commuters, based on an assumption that all 
employed adult Amargosa Valley residents who commute 10 minutes or more to work are 
employed outside of the contaminated area. This time limit was developed by considering the 
amount of land that could be irrigated by 3,000 acre-feet of water (10 CFR 63.312) and the 
amount of time it would take to drive out of that area. Commuters are estimated to spend an 
average of 0.3 hours per day active outdoors, 1.4 hours inactive outdoors, 6.0 hours active 
indoors, 8.3 asleep, and 8.0 hours away from the contaminated area. 
Local Indoor Workers–Local indoor workers are residents who work indoors (or outdoors in 
enclosed vehicles) in areas contaminated by groundwater or ash. Sixteen percent of the 
population is in this group. This includes employed adults who were not classified as local 
outdoor workers or commuters. Local indoor workers are estimated to spend an average of 
0.3 hours active outdoors, 1.3 hours inactive outdoors, 12.1 hours active indoors, 8.3 asleep, and 
2.0 hours away from contaminated areas. 
Non-workers–Non-workers are residents who are unemployed or are not in the labor force 
(e.g., retired people). Thirty-nine percent of the resident adult population met this criterion in 
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2000. Non-workers are estimated to spend an average of 0.3 hours active outdoors, 1.2 hours 
inactive outdoors, 12.2 hours active indoors, 8.3 asleep, and 2.0 hours away from contaminated 
areas. 
To meet the requirement of 10 CFR 63.312 that average values of lifestyle characteristics be 
used in the TSPA dose assessments, the average exposure time per environment is calculated as 
the average of exposure times per group weighted by the proportion of the population in each 
group (BSC 2003a, Section 6.4.7). These average exposure times are 0.45 hours per day in the 
active outdoor environment, 1.45 inactive outdoors, 9.45 hours active indoors, 8.3 hours asleep 
indoors, and 4.35 hours away from contaminated areas (BSC 2003a, Table 6.10-1). 
The shielding factor is included in the external exposure dose calculation to account for the 
effects of shielding from radiation provided by buildings when the receptor is indoors. Shielding 
factor values used in the ERMYN model range from zero for radionuclides that emit no 
penetrating radiation (e.g., Polonium-212 and Radium-228) to 0.4 for radionuclides with highly 
penetrating radiation (gamma emitters of energy greater than 100 keV) (BSC 2003c, 
Section 6.6). These factors were developed for lightly constructed housing and are applicable to 
Amargosa Valley because most residents live in mobile homes (Section 2.2). Shielding factors 
are only applied to indoor environments (i.e., the shielding factor for outdoor environments 
equals 1.0). 
The output of this submodel, the total annual external exposure for each primary radionuclide, is 
used to calculate the all-pathway BDCFs, as described in Section 4.10. 
4.8 INHALATION EXPOSURE SUBMODEL 
The inhalation submodel is used to calculate exposure from the inhalation of radionuclides in air. 
Inhalation exposure is estimated using radionuclide concentrations in the air (from the air 
submodel), parameters describing conditions of human exposure, and dose conversion factors for 
inhalation exposure that convert radionuclide intake to a committed effective dose equivalent. In 
contrast to external exposure, where emissions arise from outside the human body, inhalation 
and ingestion exposures arise from radiation emitted inside the body, and the exposure continues 
for as long as the radionuclides are in the body. Therefore, inhalation and ingestion doses are 
presented in terms of the committed effective dose equivalent, which represents the effective 
dose equivalent integrated over a 50-year commitment period. The annual committed effective 
dose equivalent, although delivered over the commitment period, is assigned to the one-year 
period of intake. 
This submodel is used to calculate the dose (per unit concentration of radionuclides in 
groundwater) from the inhalation of three types of radionuclides from three sources: 
resuspended soil; aerosols from evaporative coolers; and gaseous emissions from soil, which 
includes exhalation of Radon-222 and gaseous emissions of Carbon-14. The total inhalation 
dose is the sum of the doses resulting from these three inhalation exposure pathways. 
Exposure to the three sources of radionuclides is calculated similarly. Dose due to inhalation of 
each primary radionuclide and long-lived decay product is the product of a radionuclide-specific 
effective dose conversion factor, the concentration of radionuclides in the air within each of the 
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receptor environments (calculated in the air submodel), the time spent by the RMEI in each 
environment, and environment-specific breathing rates. The dose for a primary radionuclide is 
then calculated as the sum of doses for that radionuclide and its long-lived decay products 
(BSC 2003a, Section 6.4.7). As described in Section 3.3, the effective dose conversion factors 
include the contribution to dose from short-lived decay products (BSC 2003a, Sections 6.4.8.1 
and 6.4.8.5). The same exposure times and population proportions described in Section 4.7 for 
the external exposure submodel are used to calculate inhalation exposure. Breathing rates used 
in this calculation are based on the expected level of activity within each environment, and they 
range from 0.39 m3/hour for time spent asleep to 1.57 m3/hour for time spent active outdoors 
(BSC 2003c, Section 6.3.3). 
The calculation of inhalation exposure to radionuclides introduced into the air from evaporative 
coolers includes factors that quantify the proportion of residences with coolers and the 
proportion of the year that coolers are operated. Those factors are also included in the 
calculation of dose from inhalation of Radon-222 because the home ventilation rate influences 
the buildup of Radon-222 indoors (Section 4.2.3). The proportion of homes with coolers 
(average equals 73 percent) is from the 1997 survey of Amargosa Valley residents (DOE 1997). 
The range of the proportion of time coolers are operated is calculated as the average percentage 
of days per year that the daily maximum temperature exceeds 80ºF to 90ºF. This distribution 
ranges from 32 to 46 percent for the current climate, and it ranges from 3 to 14 percent for the 
glacial transition climate (BSC 2003c, Section 6.3.4). The contribution of radionuclides from 
evaporative coolers to inhalation in outdoor environments is not considered in the model because 
the air from coolers would be diluted quickly when blown outside. 
The output of this submodel, the total annual inhalation exposure for each primary radionuclide, 
is used to calculate the all-pathway BDCFs, as described in Section 4.10. 
4.9 INGESTION EXPOSURE SUBMODEL 
The ingestion submodel determines exposure to the receptor from ingesting drinking water, four 
types of crops (leafy vegetables, other vegetables, fruits, and grains), four types of animal 
products (meat, poultry, milk, and eggs), freshwater fish, and soil. Ingestion exposure is 
calculated as the committed effective dose equivalent for the 50-year period resulting from 
one year of intake. 
Ingestion exposure is calculated for each of the 11 media listed above as the product of the 
effective dose conversion factor, activity concentration (from the groundwater source and the 
soil, plant, and animal submodels), and annual consumption rate. The dose contribution of all 
short-lived radionuclides is included in the effective dose conversion factors for the long-lived 
primary radionuclides (Section 3.3). In addition, for ingestion of plants, animal products, and 
soil, the contribution due to radionuclide decay and ingrowth in surface soil is added into the 
primary radionuclides. Total ingestion exposure is calculated as the sum of the dose from the 
11 media. 
The annual consumption rate of water is 2 liters per day, or 730 liters per year, as specified in 
10 CFR 63.312(d). The soil ingestion rate is representative of the amount of soil that adults 
inadvertently ingest (e.g., from dirty hands, from food, while breathing through the mouth) and 
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does not include purposeful soil ingestion. Based on the dry, dusty conditions in Amargosa 
Valley, a range of 50 to 200 mg/day is used in the ERMYN model (BSC 2003c, Section 6.4.3). 
Ingestion rates of locally produced crops and animal products are based on the 1997 survey of a 
sample of Amargosa Valley residents (DOE 1997). During that survey, people were asked how 
often they ate locally produced items. To develop distributions of annual consumption rates, the 
information on frequency of consumption from the survey was combined with information 
obtained from the U.S. Department of Agriculture on the consumption rate of foods by people in 
the western United States (BSC 2003c, Section 6.4.2). Statistical uncertainty associated with 
surveying a sample of the population was incorporated into the distributions of the ingestion-rate 
parameters (BSC 2003c), Section 6.4). 
The output of this submodel, the total annual ingestion dose for each primary radionuclide, is 
used to calculate the all-pathway BDCFs, as described in the next section. 
4.10 BIOSPHERE DOSE CONVERSION FACTORS 
The all-pathway annual dose per unit concentration of each primary radionuclide is expressed as 
the total effective dose equivalent, and is the sum of the annual effective dose equivalent from 
external exposure to each radionuclide and the committed effective dose equivalent from annual 
intake of that radionuclide into the body by inhalation and ingestion (BSC 2003a, 
Section 6.4.10.1). 
The results of all calculations of radionuclide concentrations in environmental media 
(Sections 4.1 through 4.6) and dose for each exposure pathway (Sections 4.7 through 4.9) are 
linearly proportional to the concentration of radionuclides in the groundwater (BSC 2003a, 
Section 6.4.10.2). Therefore, the all-pathway annual dose per unit concentration of each 
radionuclide (i.e., BDCF) calculated in this model is independent of the concentration of 
radionuclides in groundwater estimated in the TSPA. Although the ERMYN model can be used 
to calculate the dose for any predefined concentration, the BDCFs calculated for the TSPA are 
based on a unit concentration of radionuclides in water (i.e., 1 Bq/m3). Therefore, the dose to the 
RMEI from exposure to contaminated groundwater can be calculated in the TSPA as the product 
of the BDCFs and groundwater concentrations estimated in the TSPA for the hypothetical 
location of the well at a specified time. 
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5. ERMYN VOLCANIC SCENARIO MODEL 
This section describes the conceptual and mathematical models used to calculate the transfer of 
radionuclides in the biosphere and exposure to the receptor for the volcanic ash exposure 
scenario. As discussed in Section 3.2, the transport and exposure pathways for this scenario are 
similar to the pathways considered for the groundwater analysis. This discussion focuses on 
differences between the methods used in the groundwater submodels and those used in the 
volcanic ash scenario. 
The source of radionuclides for the volcanic scenario is ash from a volcanic eruption at the 
repository. Ash at the location of the receptor could be deposited during the initial fallout after 
the eruption or it could be transported into the region by fluvial or aeolian processes. These 
redistribution processes are modeled in the TSPA. Groundwater is not considered to contain 
radionuclides, and, therefore, transfer pathways resulting from use of water (e.g., irrigation 
water, on soil and plants, fish farms, ingestion of water by farm animals and the receptor) are not 
considered for the volcanic ash scenario. 
The conceptual and mathematical models for the volcanic scenario are developed in 
seven submodels that represent the four contaminated environmental media (soil, air, plants, and 
animals), and external, inhalation, and ingestion exposure (Figure 5-1). A special submodel for 
Carbon-14 is not included because Carbon-14 is not a primary radionuclide for this scenario 
(BSC 2003a, Section 6.3.2). 
For the groundwater scenario, the dose from all exposure pathways is combined into one set of 
BDCFs for use in the TSPA. Calculation of BDCFs is much more complicated for the volcanic 
scenario because inhalation exposure would decrease over time following an eruption as the 
concentration of radionuclides in the air decreases. Inhalation exposure also would be affected 
by the depth of the tephra deposit, which is not calculated in the biosphere model. To account 
for the effects of time and ash depth, a set of three BDCF components is calculated for the 
volcanic ash scenario. The first component accounts for the exposure pathways that would not 
be affected by time or ash depth (ingestion, inhalation of radon decay products, and external 
exposure). The second accounts for inhalation of particulates following a volcanic eruption 
when airborne concentrations would be increased. This short-term inhalation component is timedependent 
because mass loading would decrease after an eruption. The third component 
accounts for inhalation when mass loading would not be elevated as the result of the volcanic 
eruption. The latter two components are affected by ash thickness. All three components are 
numerically equal to the annual doses from a radionuclide per pathway that the RMEI would 
receive when exposed to radionuclide contamination in environmental media arising from a unit 
areal concentration of the radionuclide in the ash deposited on the ground (BSC 2003a, Section 
6.5.8). 
One set of outputs from the volcanic ash scenario biosphere model is used for current and 
predicted future climate states because the important input parameters (e.g., mass loading, 
(BSC 2003b, Section 6) and the model results (BSC 2003h, Section 6.2.6) are insensitive to the 
changes in climate predicted to occur at Yucca Mountain during the next 10,000 years. 
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Relationship between the Biosphere Submodels for the Volcanic Ash Scenario Figure 5-1. 
The biosphere model for the volcanic ash scenario applies for the period after the initial 
deposition of contaminated ash from an eruption has ceased. Independent of the biosphere 
model, dose factors for the eruptive phase were developed in the Disruptive Event Biosphere 
Dose Conversion Factor Analysis (BSC 2003h, Section 6.3). These factors are used in the TSPA 
to calculate dose accrual to the RMEI during the ash-fall event. Exposure during this period may 
be important because airborne concentrations of radioactive particulate matter may be high 
during an eruption. Only the inhalation of airborne ash particles is considered for this phase. 
The dose factor for each primary radionuclide is numerically equal to the dose resulting from one 
day of intake of the radionuclide (and any short-lived decay products) from inhaling air 
containing a unit activity concentration of the radionuclide. Inhalation exposure depends on the 
exposure time and breathing rate, as described Section 4.8. 
5.1 SOIL SUBMODEL 
The surface soil submodel for the volcanic ash scenario differs from the groundwater scenario 
primarily because a volcanic eruption would spread ash over a large area, while irrigating would 
deposit radionuclides on the relatively small farming area. In addition, radionuclides would not 
accumulate in the surface soil because they are not continuously added to the environment. 
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Radionuclide concentrations in soil are calculated separately in this submodel for cultivated and 
uncultivated soils (BSC 2003a, Section 6.5.1). The calculation for cultivated land is the same as 
that used in the soil submodel for the groundwater scenario. Radionuclides are assumed to be 
mechanically mixed by normal farming operations over the 5- to 30-cm tillage depth. If the 
predicted ash thickness is greater than the tillage depth, the contaminants are conservatively 
assumed to be uniformly distributed within the tillage depth (BSC 2003a, Section 5.12). The 
activity concentrations calculated using this method are used to evaluate radionuclide transport to 
plant foodstuffs and animal products, and for inadvertent soil ingestion (BSC 2003a, 
Section 6.5.1.1). 
On uncultivated lands, volcanic ash would not quickly mix with surface soil, and the proportion 
of resuspended particles comprised of ash (versus clean soil) would depend on the thickness of 
the ash deposit. To account for this, a critical thickness is considered in the submodel and an 
ash-depth function is developed for use in the TSPA. The critical thickness is the layer from 
which particles would be resuspended, which is, at most, a few millimeters (BSC 2003e, 
Section 6.8). For an ash thickness equal to or greater than the critical thickness, only ash would 
be resuspended because clean soil is covered by too much ash to be resuspended. In this case, 
only a portion of the ash (and the activity concentration it contains) would be available for 
resuspension. The portion of the activity concentration in the soil that would be resuspended is 
calculated as the ratio of critical thickness to ash depth. If the deposit is thinner than the critical 
thickness, the resuspended material would be a mix of ash particles and clean soil, and all 
radionuclides in the ash would be available for resuspension. The TSPA calculates ash thickness 
and activity concentration in ash as a function of time; therefore, the loss factors (leaching, 
erosion, and radioactive decay) considered in the soil submodel for the groundwater scenario are 
not included in the calculations of radionuclide concentrations for the volcanic ash scenario. 
5.2 AIR SUBMODEL 
The air submodel calculates concentrations of radionuclides in air resulting from two transport 
pathways: resuspension of ash and soil and exhalation of radon from the mixture of ash and soil 
(BSC 2003a, Section 6.5.2). 
Resuspension of ash into the atmosphere is considered separately for cultivated and uncultivated 
lands. Activity concentrations in the air resulting from resuspension of particles on cultivated 
land is used to calculate surface contamination of crops in the plant submodel and is calculated in 
the same manner as described in Section 4.2. The mode of the mass loading distribution for 
post-volcanic conditions in cultivated fields is two times greater than that used for the 
groundwater scenario (BSC 2003b, Section 6.2.5). 
Activity concentrations in the air resulting from resuspension of particles on uncultivated land 
are used in the inhalation model to calculate inhalation exposure. The concentration of 
radionuclides in the soil available for resuspension is calculated in the soil submodel. To 
evaluate inhalation exposure, the ERMYN model uses a microenvironmental modeling method 
that considers five receptor environments (see Section 4.2), each having a mass loading 
distribution that is representative of typical activities conducted within the environment. To 
allow for the possibility of radionuclides being preferentially resuspended from the surface soil 
and inhaled, an empirical enhancement factor is used to quantify the net airborne activity 
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concentration relative to surface soil activity concentration. Distributions of enhancement 
factors for the volcanic scenario range from 2.8 to 8.4 for the outdoor active environment (about 
20 percent higher than the bounds of the distribution used for the groundwater scenario) and 
0.21 to 1.04 for other environments (BSC 2003f, Section 6.5). 
Mass loading would be higher for some time after a volcanic eruption because there would be 
more unconsolidated, fine particles on the soil surface that would be readily resuspended by 
wind, human activity, or other disturbances. Over time, the ash would erode, become mixed into 
the soil, become buried, or otherwise become stabilized. The ERMYN model, therefore, 
assumes that mass loading would eventually return to levels experienced before the eruption 
(BSC 2003a, Section 5.14). This assumption is based on measurements of mass loading after the 
eruption of Mount St. Helens and other volcanoes (BSC 2003b, Section 6.2 and 6.3). To account 
for this effect, two sets of mass loading distributions and a mass loading time function are 
developed (BSC 2003b, Section 6.3). The first mass loading set is for the period immediately 
after the eruption and before the ash stabilizes and is representative of the average annual 
concentration of resuspended particles the first year following a volcanic eruption. The second 
set is representative of conditions after resuspended particle concentrations have returned to 
pre-eruption levels. This second set is the same as that used for the groundwater scenario. Mass 
loading distributions for the first year following an eruption generally are two to three times 
higher than those for stabilized conditions (BSC 2003b, Section 6.2). The time function 
quantifies the rate at which mass loading is expected to return to pre-eruption levels. Mass 
loading distributions are used to calculate radionuclide concentrations in the air. Radionuclide 
concentrations in the air associated with both sets of mass loading are influenced by ash depth, 
but only the first is time dependent. By separating the time and ash thickness components, 
changes in radionuclide concentrations immediately after the eruption can be evaluated 
(BSC 2003a, Section 6.5.2.1). 
The calculation of radon exhalation for the volcanic ash scenario is simpler than that used in the 
groundwater scenario. Because ash thickness is expected to be relatively thin, it is 
conservatively assumed that all radon created by decay in the ash-soil layer is released to the 
atmosphere, where it decays and contributes to the inhalation dose (BSC 2003a, Section 5.15). 
Unlike the groundwater scenario, indoor radon concentrations are considered to be the same as 
outdoor concentrations because there are no mechanisms for increased infiltration of radon into 
buildings and the initial amount of indoor volcanic ash would be limited. Even if new buildings 
were built on contaminated land, the infiltration of Radon-222 from the ground into indoor 
spaces would be limited because the thin layer of ash would be removed or mixed with surface 
soil during construction. Because it is assumed that all radon from outdoor soil is released, it is 
not necessary to consider an additional source of indoor radon (BSC 2003a, Section 6.5.2.2). 
5.3 PLANT SUBMODEL 
Two transfer pathways are considered in this submodel to calculate radionuclide concentrations 
in plants consumed by humans and farm animals: root uptake from soil and foliar uptake from 
intercepted resuspended matter. Root uptake is calculated using the same methods and transfer 
factors as for the groundwater scenario (Section 4.3). Plant uptake from deposition of 
resuspended soil on plant surfaces also uses the same methods as in the groundwater scenario, 
but atmospheric concentrations are higher because mass loading is likely to be higher following 
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an eruption. The other parameters in this submodel (translocation factors, weathering constants, 
crop growing time, and crop biomass) are the same as those used in the groundwater scenario 
(BSC 2003a, Section 6.5.3). 
5.4 ANIMAL SUBMODEL 
The animal product submodel considers two pathways for the accumulation of radionuclides in 
animal products, ingestion of feed and soil. The methods and input parameters used to calculate 
activity concentrations are the same as those described in Section 4.4. Inhalation of resuspended 
ash was not included because this pathway has a negligible influence on the concentrations of 
radionuclides in animal products (BSC 2003a, Section 7.4.5). 
5.5 EXTERNAL EXPOSURE SUBMODEL 
As with the groundwater scenario, this submodel only considers exposure to radionuclides in 
soil. Air submersion is excluded because of the low contribution to the results of this submodel 
(BSC 2003a, Section 7.4.8). All doses discussed in this and the following sections refer to 
annual effective doses equivalent per unit radionuclide concentration in volcanic ash. 
The methods used to calculate external exposure are the same as those described in Section 4.7, 
but the effective dose coefficients and distributions of population proportions and exposure times 
are different than those used for the groundwater scenario. The effective dose coefficients used 
to calculate external exposure to ash were selected based on the assumption that, regardless of 
the predicted thickness of the ash, all radionuclides would be located on the soil surface. This 
assumption is valid because thin deposits are more likely than thicker deposits and only a small 
portion of the soil in Amargosa Valley is cultivated. It is conservative because it eliminates 
self-shielding within the ash-soil mixture (BSC 2003a, Section 5.16). As with the other exposure 
submodels, the decay products of primary radionuclides are treated as discussed in Section 3.3. 
For the volcanic scenario, ingrowth in the soil of the long-lived decay products of primary 
radionuclides is not considered because it is accounted for in the TSPA as part of the timedependent 
source. 
Population proportions and exposure times differ from those used for the groundwater scenario 
because it is likely that radionuclides would be spread over a larger area following an eruption 
than would occur as a result of using groundwater for irrigation. It is, therefore, assumed that all 
Amargosa Valley residents that commute 35 minutes or less (versus 10 minutes for the 
groundwater scenario) work within areas containing radionuclides (BSC 2003c, Section 5.1). 
Based on this assumption and data on commute time from the 2000 census (Bureau of the 
Census (2002), 13 and 43 percent of the population are classified as commuters and local indoor 
workers, respectively (versus 39 and 16 percent for the groundwater scenario). The only 
difference in exposure times between scenarios is for time spent in the inactive outdoor 
environment, because commute times within the area containing radionuclides are longer for the 
volcanic scenario (BSC 2003c, Sections 6.3.1 and 6.3.2). The average exposure times per 
environment, weighted by the proportion of the population in each group, are 0.45 hours per day 
in the active outdoor environment, 1.59 hours inactive outdoors, 10.87 hours active indoors, 8.3 
hours asleep indoors, and 2.79 hours away from the area containing radionuclides (BSC 2003a, 
Table 6.10-5). 
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5.6 INHALATION EXPOSURE SUBMODEL 
The inhalation dose is calculated as the committed effective dose equivalent for the 50-year 
committed period resulting from annual intake of radionuclides by inhalation. Two sources of 
radionuclides in air are considered, resuspended particles and radon gas, and the inhalation dose 
is the sum of the dose from both sources (BSC 2003a, Section 6.5.6). 
As is done for the groundwater scenario, the inhalation dose is calculated considering specific 
environments associated with human activities and population groups. However, for the 
volcanic scenario, there are two components to the radionuclide concentrations in the air (see 
Section 5.2), one related to post-volcanic, time-dependent mass loading and one related to 
conditions after mass loading concentrations are stabilized. Both components depend on ash 
thickness. Annual inhalation exposure is calculated separately for the first year following a 
volcanic eruption and for the long-term period when airborne concentrations have stabilized 
(BSC 2003a, Section 6.5.6). The total resuspension dose is considered to be the sum of the doses 
during the short-term and long-term periods. The change in dose during the period when mass 
loading is decreasing is modeled as exponential in time with a defined time constant 
(BSC 2003b, Section 6.3). 
5.7 INGESTION EXPOSURE SUBMODEL 
The ingestion exposure pathway for the volcanic scenario is the same as that for the groundwater 
scenario except that only 9 of the 11 media are used: (4 crops, 4 animal products, and soil). 
Ingestion of water and fish are not included because groundwater is not considered to contain 
radionuclides in this scenario. 
5.8 BIOSPHERE DOSE CONVERSION FACTORS 
The calculation of BDCFs, and the use of those BDCFs to calculate the dose in the TSPA for the 
post-eruptive phase, is more complicated for the volcanic scenario than for the groundwater 
scenario. This is because the inhalation dose changes over time (as mass loading decreases with 
time from a maximum concentration the first year following an eruption) and varies depending 
on ash depth (because radionuclide concentrations in the air from resuspension of particles from 
uncultivated soil are a function of ash depth). To incorporate these time and depth functions, 
three BDCF components, a critical ash thickness, and two functions (time and ash thickness) are 
developed for use in the TSPA to calculate the dose for the post-eruptive phase (BSC 2003a, 
Section 6.5.8; BSC 2003h, Section 7). In addition, dose factors are provided to calculate the 
inhalation dose during the eruptive phase (BSC 2003h, Section 6.3). 
The first BDCF component includes the dose contribution (per unit activity concentration of 
radionuclides in soil) from external exposure, inhalation of radon, and ingestion. These exposure 
pathways are not affected by time or ash depth; therefore, their dose contribution can be 
calculated by multiplying the BDCF component for a radionuclide by the areal concentration of 
that radionuclide in the soil at a specified time. 
The second BDCF component addresses the short-term dose contribution from inhaling 
increased concentrations of resuspended particulates following a volcanic eruption. This dose 
contribution decreases through time and is also affected by ash depth, so in the TSPA the BDCFs 
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must be multiplied by the areal activity concentration in the soil, the ash depth function, and the 
mass loading time function. 
The third component of the BDCFs is for the long-term contribution to the dose from inhaling 
resuspended particles at concentrations expected after mass loading has decreased to 
pre-eruption levels (i.e., long-term inhalation dose contribution). This dose contribution is 
affected by ash depth and therefore this BDCF component must be multiplied in the TSPA by the 
areal activity concentrations in soil and the ash depth function. 
The mass loading time function defines the rate of decrease in mass loading after a volcanic 
eruption. It is modeled as an exponential function, and the rate of change is controlled by the 
mass loading decrease constant (BSC 2003b, Section 6.3; BSC 2003h, Section 7.3). That 
decrease constant was developed based on the rate of change in mass loading following Mount 
St. Helens and other volcanic eruptions and the consideration of uncertainty about the influence 
of climate change and ash depth, redistribution, and particle sizes. Because of the uncertainty in 
changes in mass loading over time for deep ash deposits, separate distributions of the mass 
loading decrease constant were developed for deposits less than 10 mm and those equal to or 
greater than 10 mm. The minimum and maximum decrease constants of the distribution for 
shallow deposits would result in mass loading decreasing to five percent of the initial 
concentration within 2 to 15 years. For deeper deposits, mass loading would decrease to five 
percent of the initial concentration within about 3 to 25 years (BSC 2003b, Section 6.3.3). 
The ash depth function is described in Section 5.1 and accounts for the fact that, for an ash 
thickness greater than the critical thickness, only a fraction of the ash on the soil surface (and the 
activity it contains) would be available for resuspension. For ash depths less than the critical 
depth, this function equals one because all radionuclides in the ash/soil are available for 
resuspension. For ash depths deeper than the critical depth, this function is calculated as the 
critical depth divided by ash depth (BSC 2003a, Section 6.5.1; BSC 2003h, Section 7.3). The 
value of the critical thickness is randomly sampled in each biosphere model realization and 
included with the values of the three BDCF components for each radionuclide (one vector) as 
input to the TSPA-LA model. The initial depth of ash deposited at the location of the receptor 
and changes in depth through time due to fluvial and aeolian processes are calculated in the 
TSPA. The TSPA model randomly samples from the vectors containing BDCF components and 
critical thickness to predict dose at a given time for a radionuclide concentration in ash (BSC 
2003a, Section 6.5.8). 
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6. BIOSPHERE MODEL RESULTS 
The BDCFs calculated by the ERMYN model are used in the TSPA to calculate the annual dose 
from a concentration of radionuclides in groundwater or volcanic ash. A single realization 
generated using the ERMYN model consists of a (row) vector of BDCFs, within which 
numerical elements represent the BDCFs for each radionuclide of interest. The sampling of 
inputs to the ERMYN model is fixed for a given realization, thereby capturing the inherent 
correlation among the BDCFs for each radionuclide. This sampling process is repeated to 
generate the required number of realizations for the TSPA. When used by TSPA, this set of row 
vectors is sampled at random within the TSPA code to propagate uncertainty from the biosphere 
calculations into the TSPA dose calculations. 
6.1 GROUNDWATER SCENARIO 
For the groundwater scenario, each realization of the model produces BDCFs for 
28 radionuclides and three climate states (current, monsoon, and glacial transition). Mean 
BDCFs and standard deviations are presented (Table 6-1) for comparison and to convey 
information on the uncertainty in their values propagated through the model. 
The total annual dose is calculated in the TSPA as the sum of the products of 
radionuclide-specific BDCFs and time-dependent activity concentrations of radionuclides in 
groundwater (BSC 2003a, Section 6.4.10). This calculation is based on a linear relationship 
between radionuclide concentrations in groundwater and the BDCF for each radionuclide. 
The BDCFs for the groundwater scenario are developed in Nominal Performance Biosphere 
Dose Conversion Factor Analysis (BSC 2003g). To illustrate variability in BDCF values, 
normalized BDCFs (i.e., BDCF value per realization divided by mean BDCF) were calculated 
for a set of four radionuclides likely to have an important contribution to the dose at 10,000 years 
(Figure 6-1). The shape of the resulting cumulative distribution functions for these radionuclides 
is similar, although there are some differences, especially for Technetium-99 at the upper end of 
the range. These differences are due to variation among radionuclides in the contribution of the 
pathways considered (Table 6-2) and the influence of stochastically sampled input parameters on 
the calculated rate of transport of each radionuclide through the biosphere. The distributions in 
Figure 6-1 show the degree of variability in BDCFs after all uncertainties have been propagated 
through the biosphere model. From the fifth to the ninety-fifth percentile of the distributions, the 
BDCF values extend by a factor of up to two below and above the mean value. 
To illustrate the effect of climate change on the results of the ERMYN model, BDCFs for the 
two extreme climates, current (interglacial) and glacial transition, are summarized in Table 6-1. 
Average BDCFs for the glacial transition climate are 4 to 26 percent lower, and standard 
deviations for most BDCFs are smaller than those for the current climate. This change is due 
primarily to a decrease in irrigation rates for the future climate, reducing the input of 
radionuclides onto cultivated soil. The change in BDCFs due to climate change is small 
compared to the total amount of variation in the BDCFs for each radionuclide. For example, the 
ratios of the standard deviations to means is greater than 0.25 for most radionuclides (Table 6-1). 
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Source: Based on data from DTN: MO0307MWDNPBDC.001 (see Rautenstrauch 2003). 
Figure 6-1. Normalized Distributions of Groundwater Scenario Biosphere Dose Conversion Factors for 
September 2003 6-2 
Selected Radionuclides 
For each radionuclide, the relative importance of exposure pathways is summarized in Tables 6-2 
and 6-3. Pathways excluded from the biosphere model used for the site recommendation 
(CRWMS M&O 2000c), but included in the ERMYN model (e.g., inhalation of radon decay 
products, inhalation of aerosols generated by evaporative coolers), have a large contribution for 
some radionuclides. 
Pathwa