NUREG-1538

Preliminary Performance-Based 

Analyses Relevant to Dose-Based 

Performance Measures for a 

Proposed Geologic Repository at 

Yucca Mountain 

Center for Nuclear Waste Regulatory Analyses 

U.S. Nuclear Regulatory Commission 

Office of Nuclear Material Safety and Safeguards 1 AEG" 

Washington, DC 20555-0001
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NUREG-1538

Preliminary Performance-Based 

Analyses Relevant to Dose-Based 

Performance Measures for a 

Proposed Geologic Repository at 

Yucca Mountain 

Manuscript Completed: August 1999 

Date Published: October 2001 

Edited by 

T. J. McCartin, M. P. Lee 

Division of Waste Management 

Office of Nuclear Material Safety and Safeguards 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001 

Center for Nuclear Waste Regulatory Analyses 

6220 Culebra Road 

San Antonio, TX 78228-5155
NUREG-1538 has been reproduced from 

the best available copy.

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Internet: bookstore.gpo.gov Phone: (202) 512-1800 Fax: (202) 512-2250 

Mail: Stop SSOP, Washington, DC 20402-0001 

ISBN 0-16-050978-5
ABSTRACT

The National Academy of Sciences' (NAS') 

Committee on Technical Bases for Yucca 

Mountain (Nevada) Standards recommended 

that standards for the disposal of high-level 

radioactive waste (HLW) at Yucca Mountain 

should: (1) set a limit on the risk to 

individuals; (2) use the critical group 

approach, as defined by the International 

Commission on Radiological Protection; 

(3) define the critical group using present 

knowledge and cautious, reasonable 

assumptions; (4) use a time period for 

conducting compliance assessment that 

includes the period of greatest risk; and 

(5) use a stylized calculation to assess whether 

the repository's performance would be 

substantially degraded as a consequence of a 

postulated intrusion. The staff and its 

technical assistance contractor-the Center 

for Nuclear Waste Regulatory Analyses-have 

undertaken a series of focused technical 

analyses to better understand the NAS 

recommendations and their implementation 

within the U.S. Nuclear Regulatory 

Commission's HLW performance assessment 

program. Overall, the staff believes that it will 

be able to integrate a dose-based approach 

into its regulatory framework and supporting 

guidance. The staff was able to: (1) tentatively

identify characteristics for two potential 

receptor groups-"farming" and "residential" 

(non-farming)-using information for 

lifestyles and water-use practices presently 

occurring in the Yucca Mountain area and in 

other, similar, environments; (2) ascertain the 

potential for reducing radionuclide 

concentrations in ground water caused by 

dispersive transport processes (reduction of 1 

to 75 times) and borehole mixing in a pumping 

water well (reduction of 1 to 50 times); 

(3) describe an approach to implement a dose 

calculation for the residential [mean dose of 

30 millirem (mrem)] and farming (mean dose 

of 10 mrem) receptor groups; (4) describe an 

approach to implement a dose calculation for 

direct disruption of the repository from 

volcanic activity (probability weighted dose of 

about 1 mrem); (5) describe an approach to a 

stylized calculation for human intrusion; and 

(6) evaluate the time dependence of 

radiological hazard of HLW by comparison 

with naturally concentrated uranium in an ore 

body (after 10,000 years the relative 

radiological hazard of HLW has decreased by 

99.9 percent and is within less than an order of 

magnitude of the hazard of the hypothetical 

ore body).

111 NUREG- 1538
CONTENTS 

Page 

A bstract ...................................................................... iii 

Executive Summary ............................................................ xv 

Forew ord ..................................................................... xxi 

English/M etric Conversion Factors ............................................... xxiii 

Acknowledgm ents ............................................................. xxv 

Acronyms and Abbreviations .................................................... xxvii 

1. Introduction .............................................................. 1- 1 

MR Lee/NMSS; J.P Kotra/NMSS; and TJ. McCartin/NMSS 

1.1 Background ......................................................... 1- 1 

1.2 Regulatory Considerations for Dose-Based Performance Measures .......... 1-2 

1.3 Approach and Content of this NUREG ................................. 1-3 

1.4 References .......................................................... 1-4 

2. Considerations in the Definition of a Receptor Group ........................... 2-1 

2.1 Current Lifestyles and Water-Use Practices in the Yucca Mountain Area ..... 2-1 

M.P Lee/NMSS; G. W Wittmeyer/CNWRA; N.M. ColemanINMSS; and 

R.B. Codell/NMSS 

2.1.1 Introduction ................................................... 2- 1 

2.1.2 Approach ..................................................... 2-2 

2.1.2.1 Local Population and Lifestyles .......................... 2-2 

2.1.2.2 History of Inhabitation of the Amargosa Desert Region 

and the Ash Meadows Area ............................. 2-5 

2.1.2.3 Ground-Water Availability .............................. 2-8 

2.1.2.4 Water Supply and Consumption .......................... 2-12 

2.1.2.5 Water Well Surveys .................................... 2-13 

2.1.2.6 Water Well Properties .................................. 2-14 

2.1.2.7 Ground-Water Utilization (End Use) ..................... 2-14 

2.1.2.8 W ell Costs ............................................ 2-18 

2.1.3 Other Issues Related to Near-Term Ground-Water Use .............. 2-18 

2.1.4 Sum m ary ..................................................... 2-20 

2.1.5 References .................................................... 2-21 

2.2 Use of Ground Water in the Arid and Semi-Arid Western United ........... 2-25 

States: Possible Implications for the Yucca Mountain Area 

G. W Wittmeyer/CNWRA; M.P Miklas/CNWRA; R. V Klar/CNWRA; 

D. Williams 1; and D. Balin2 

2.2.1 Introduction .................................................... 2-25 

2.2.2 Scope of Study and Methods of Analysis ........................... 2-25 

2.2.3 Exploitation of Deep Ground Water .............................. 2-26 

llndependent consultant from Oakland, California. 2

1ndependent consultant from San Antonio, Texas. 

v NUREG-1538
CONTENTS continued 

Page 

2.2.3.1 Exploitation of Deep Ground Water in Arizona ............ 2-27 

2.2.3.2 Exploitation of Deep Ground Water in Nevada ............. 2-30 

2.2.3.3 Exploitation of Deep Ground Water in New Mexico ........ 2-30 

2.2.3.4 Exploitation of Deep Ground Water in Texas .............. 2-35 

2.2.3.5 Estimated Well Construction Costs in the Yucca Mountain 

and Amargosa Desert Areas ............................ 2-35 

2.2.4 Sum m ary ..................................................... 2-39 

2.2.5 References .................................................... 2-40 

2.3 Definition of Hypothetical Receptor Groups Used in this NUREG .......... 2-41 

All Contributors 

2.3.1 Receptor Group Characteristics .................................. 2-41 

2.3.2 Limitations to the NUREG Definition ............................ 2-42 

2.3.3 References .................................................... 2-42 

3. Ground-Water Pathway Analyses ............................................. 3-1 

3.1 Analysis of Radionuclide Dilution in Ground Water ....................... 3-1 

R. G. Baca/CNWRA3 " G. W. Wittmeyer/CNWRA; and R. W Rice4 

3.1.1 Introduction ................................................... 3- 1 

3.1.2 A nalysis ...................................................... 3- 2 

3.1.2.1 Conceptual Models of Ground-Water Flow ................ 3-2 

3.1.2.2 Computer Simulations for Lateral Flow Model ............. 3-9 

3.1.3 Assumptions and Limitations .................................... 3-21 

3.1.3.1 Conceptual M odels .................................... 3-21 

3.1.3.2 Hydraulic and Transport Properties ...................... 3-23 

3.1.3.3 Radionuclide Source Term .............................. 3-23 

3.1.4 Summary of Analysis Results .................................... 3-24 

3.1.4.1 Ground-Water Flow Paths and Particle Travel Times ........ 3-24 

3.1.4.2 Ground-Water Fluxes .................................. 3-25 

3.1.5 References .................................................... 3-25 

3.2 Initial Assessment of Dilution Effects Induced by Water Well Pumping in the 

Yucca M ountain Region .............................................. 3-28 

R. W. FedorsICNWRA; and G. W Wittmeyer/CNWRA 

3.2.1 Introduction ................................................... 3- 28 

3.2.1.1 Geosphere Release Pathways Considered in a TSPA ........ 3-30 

3.2.1.2 Literature Review ..................................... 3-30 

3.2.1.3 Methods Used to Conduct this Study ..................... 3-32 

3.2.1.4 Limitations of this Study ................................ 3-32 

3

Now with the Sandia National Laboratories (Albuquerque, New Mexico). 

"4Independent consultant from El Paso, Texas.

NUREG- 1538 vi
CONTENTS continued 

Page 

3.2.2 Hydrogeology of the Amargosa Desert Region ..................... 3-33 

3.2.2.1 Structure and Depositional History ....................... 3-33 

3.2.2.2 Basin-Scale Ground-Water Flow ......................... 3-33 

3.2.3 Well Construction and Water Use in the Amargosa Farms Area ....... 3-34 

3.2.3.1 Number and Distribution of Wells ........................ 3-34 

3.2.3.2 Statistical Analysis of Well Construction Practices .......... 3-35 

3.2.3.3 Estimation of Water Use ................................ 3-35 

3.2.4 3-D Capture Zone Analysis and Plume Delineation ................. 3-38 

3.2.4.1 Determination of Flow Field and Capture Zone ............ 3-42 

3.2.4.2 Radionuclide Plume Shape and Location .................. 3-46 

3.2.4.3 Borehole Dilution Factors Based on Volumetric Flux ........ 3-53 

3.2.4.4 Borehole Dilution Factors Based on Dispersive Transport ... 3-63 

3.2.5 References .................................................... 3-68 

3.3 Annual Individual Dose Estimates ...................................... 3-70 

TJ. McCartin/NMSS; and R.B. Codell/NMSS 

3.3.1 Introduction ................................................... 3- 70 

3.3.2 Simulation Approach ........................................... 3-71 

3.3.3 Results and Conclusions ........................................ 3-72 

3.3.4 References .................................................... 3- 77 

3.4 Overall Observations and Implications .................................. 3-77 

All Contributors 

3.4.1 Dilution Resulting from Natural Mixing ........................... 3-77 

3.4.2 W ell Head Pumping ............................................ 3-79 

3.4.3 Peak D ose .................................................... 3-80 

3.4.4 References .................................................... 3- 80 

4. Consequences of Human Intrusion ........................................... 4-1 

R.G. Wescott/NMSS; and TI. McCartin/NMSS 

4.1 Introduction ........................................................ 4- 1 

4.2 Specification of Intrusion Scenarios ..................................... 4-1 

4.3 Description of Modeling Approach ..................................... 4-2 

4.4 Assumptions and Limitations .......................................... 4-3 

4.5 Results and Conclusions .............................................. 4-4 

4.6 R eferences .......................................................... 4- 5 

vii NUREG-1538
CONTENTS continued 

Page 

5. Example Calculation of Expected Annual Dose, as a Function of Time, from 

Extrusive Volcanic Events at Yucca Mountain .................................. 5-1 

M.S. Jarzemba/CNWRA;5 PA. LaPlante/CNWRA; and TJ. McCartin/NMSS 

5.1 Introduction ........................................................ 5- 1 

5.2 A pproach ........................................................... 5- 1 

5.3 Calculation of the Expected Dose to a Hypothetical Receptor Group ........ 5-4 

5.4 Results and Conclusions .............................................. 5-5 

5.5 R eferences .......................................................... 5- 8 

6. Analysis of Relative Hazard of HLW Over Long Time Periods .................... 6-1 

R.D. Mantuefel/CNWRA; 6 M.S. Jarzemba/CNWRA; and R.G. Baca/CNWRA 

6.1 Introduction ........................................................ 6- 1 

6.2 D iscussion .......................................................... 6- 1 

6.3 Description of Modeling Approach ..................................... 6-2 

6.3.1 Ore Body and Geologic Repository ............................... 6-2 

6.3.2 Radioactivity as a Function of Time ............................... 6-2 

6.3.3 Radiological Hazard as a Function of Time ........................ 6-5 

6.4 Assumptions and Limitations .......................................... 6-13 

6.5 Summary and Conclusions ............................................ 6-13 

6.6 R eferences .......................................................... 6- 14 

FIGURES 

2-1 Simplified Geologic Map of the Yucca Mountain Area ..................... 2-3 

2-2 Index Map of the Yucca Mountain Area ................................. 2-4 

2-3 Simplified Geologic Cross-Section of the Basins within the Yucca 

M ountain A rea ...................................................... 2-9 

2-4 Ground-Water Sub-Basins in the Yucca Mountain Area ................... 2-10 

2-5 Landsat Image of the Yucca Mountain and the Amargosa Desert Region ............................................. See Last Page of NUREG 

2-6 Locations of Water Wells in Arizona with Depths to Water Greater Than or 

Equal to 150 M eters .................................................. 2-29 

2-7 Locations of Water Wells in Nevada with Depths to Water Greater Than or 

Equal to 150 M eters .................................................. 2-33 

5

Now with the Los Alamos National Laboratory (Los Alamos, New Mexico). 

6

Now with the Department of Mechanical Engineering at the University of Texas (San Antonio).

NUREG-1538 viii
FIGURES continued 

Page 

2-8 Locations of Water Wells in New Mexico with Depths to Water Greater 

Than or Equal to 150 M eters .......................................... 2-34 

2-9 Locations of Water Wells in the Trans-Pecos Region of Texas with Depths to 

Water Greater Than or Equal to 150 Meters ............................. 2-38 

3-1 Location of Lateral Flow Model, Material Zones, Well Locations, 

and Location of the Proposed Repository Footprint ....................... 3-4 

3-2 Vertical Cross-Section Through Boreholes USW H-5 and USW H-4 ....... 3-7 

3-3 Hydraulic Head Fields for the Lateral Flow Model for Two Cases, 

with the Bow Ridge Fault Assumed to be (a) the Preferential Flow 

Pathway and (b) the Flow Barrier ...................................... 3-11 

3-4 Darcy Flux Vector Plots for Lateral Flow Model for Two Cases, with 

the Bow Ridge Fault Assumed to be (a) the Preferential Flow Pathway 

and (b) the Flow Barrier .............................................. 3-12 

3-5 Pathlines and Particle Travel Times for Lateral Flow Model for Two 

Cases, with the Bow Ridge Fault Assumed to be (a) the Preferential Flow 

Pathway and (b) the Flow Barrier ...................................... 3-14 

3-6 Radionuclide Plume Distributions for Lateral Flow Model for Two Cases, 

with the Bow Ridge Fault Assumed to be (a) the Preferential Flow Pathway 

and (b) the Flow Barrier .................... ......................... 3-15 

3-7 Hydraulic Head Fields for Vertical Cross-Section Model ................... 3-17 

3-8 Darcy Flux Vector Plots for Vertical Cross-Section Flow Model ............. 3-18 

3-9 Pathlines and Particle Travel Times for Vertical Cross-Section Flow Model ... 3-20 

3-10 Radionuclide Plume Distributions for Vertical Cross-Section Model 

with Contour Levels in Terms of Dilution Factors ......................... 3-22 

3-11 Lower Amargosa Desert Region South of the Proposed Yucca Mountain 

Repository Site, including the Amargosa Valley and Amargosa Farms 

A reas .............................................................. 3- 29 

3-12 The Distribution of Domestic and Quasi-Municipal Wells Based on Range 

and Township, from Well Drillers' Logs ................................. 3-37 

3-13 Distribution of Annual Water Use by Type and by Range and Township, 

for Commercial, Irrigation, and Quasi-Municipal Wells for the Year 1996 .... 3-40 

3-14 Distribution of Water Use by Type for the Year 1996 ...................... 3-41 

3-15 Relative Comparison Between Cross-Sections of Contaminant Plume, 

Irrigation Well Capture Area, and Domestic Well Capture Area ............ 3-42 

3-16 Plot Illustrating the Effect of Well Penetration Depth on a Small Irrigation 

Well's Capture Zone Width and Thickness ............................... 3-45 

3-17 Effect of Combinations of Transmissivity and Hydraulic Head Gradient on 

a Large Irrigation Well's Capture Zone Width and Thickness ............... 3-47

NUREG- 1538 ix
FIGURES continued 

Page 

3-18 Effect of Combinations of Transmissivity and Hydraulic Head Gradient on a 

Domestic Well's Capture Zone Width and Thickness ...................... 3-48 

3-19 Plot Illustrating the Effect of Pumping Rate on the Capture Zone's Width 

and Thickness ....................................................... 3- 49 

3-20 Effect of Pumping Rate on the Flux-Based Borehole Dilution Factor for 

Plumes of Thickness 10 and 25 Meters .................................. 3-54 

3-21 Effect of Screen Position for Domestic-Sized Wells on the Flux-Based 

Borehole Dilution Factor for Plumes of Thickness 10 and 25 Meters ......... 3-55 

3-22 Effect of Transmissivity on the Flux-Based Borehole Dilution Factor for 

Plumes of Thickness 10 and 25 Meters .................................. 3-57 

3-23 Effect of the Regional Gradient on the Flux-Based Borehole Dilution Factor 

for a Domestic-Sized Well and Plumes of Thickness 10 and 25 Meters ........ 3-58 

3-24 Effect of Pumping Rate on Flux-Based Borehole Dilution Factors for 

Irrigation Wells and a 25-Meter-Thick Plume with No Vertical Dispersion .... 3-59 

3-25 Effect of Pumping Rate on Borehole Dilution Factors for Irrigation-Sized 

Wells and a Plume with 3-D Dispersion ................................. 3-60 

3-26 Effect of Transmissivity on Borehole Dilution Factors for Irrigation-Sized 

Wells and a Plume with 3-D Dispersion ................................. 3-61 

3-27 Effect of Regional Hydraulic Gradient on Borehole Dilution Factors for 

Irrigation-Sized Wells and a Plume with 3-D Dispersion ................... 3-62 

3-28 Effect of Pumping Rate for Domestic Wells on Transport Dispersion-Based 

Borehole Dilution Factors for Two Different Plume Configurations .......... 3-64 

3-29 Effect of Transmissivity for Domestic Wells on Transport Dispersion-Based 

Borehole Dilution Factors for Two Different Plume Configurations .......... 3-65 

3-30 Effect of Pumping Rate for Irrigation Wells on Transport Dispersion-Based 

Borehole Dilution Factors for Four Different Plume Configurations ......... 3-66 

3-31 Effect of Transmissivity for Large Irrigation Wells on Transport Dispersion Based Borehole Dilution Factors for Two Different Plume Configurations .... 3-67 

3-32 Uncertainty Analysis of Dose for 100 Vectors at a Point 5 Kilometers 

Down-Gradient from the Proposed Geologic Repository at Yucca Mountain 3-73 

3-33 Uncertainty Analysis of Dose for 100 Vectors at a Point 30 Kilometers 

Down-Gradient from the Proposed Geologic Repository at Yucca Mountain 3-74 

3-34 Annual Individual Dose from all Radionuclides TEDE and for Selected 

Radionuclides for the Drinking Water Pathway at a Point 5 Kilometers 

Down-Gradient from the Proposed Geologic Repository at Yucca Mountain 3-75 

3-35 Annual Individual Dose from all Radionuclides TEDE and for Selected 

Radionuclides for the Drinking Water Pathway at a Point 30 Kilometers 

Down-Gradient from the Proposed Geologic Repository at Yucca Mountain 3-76 

3-36 Plume Dispersion Between Repository Footprint and Pumping Well ......... 3-78

NUREG- 1538 X
FIGURES continued 

Page 

5-1 Diagram Showing the Exposure Scenario Selected for the Extrusive 

Volcanic Event Calculation ............................................ 5-2 

5-2 Average Annual Conditional Dose Resulting from Extrusive 

Volcanic Events Occurring at Specific Times ............................. 5-6 

5-3 Expected Annual Dose as a Function of Time from Extrusive Volcanic Events 5-7 

6-1 Major Steps in Extraction and Use of Uranium, Along with Major Byproducts 6-3 

6-2 Radioactivity of SNF ................................................. 6-4 

6-3 Radionuclide Inventory of the Hypothetical Ore Body ..................... 6-6 

6-4 Comparison of Radioactivity between the Spent Fuel Repository and 

the Hypothetical Ore Body ............................................ 6-7 

6-5 Drinking Water Dose Model ........................................... 6-8 

6-6 Relative Radiological Hazard of SNF Calculated as the Ratio of Doses 

from Drinking Ground Water Contaminated by the Spent Fuel Repository 

and by the Hypothetical Ore Body ...................................... 6-11 

6-7 Comparison of Hazards Accounting for Uncertainties in Radionuclide 

Solubilities and Release Rates ......................................... 6-12 

TABLES 

2-1 Comparative 1990 Population Information on California, Nevada, and the 

Yucca M ountain Area ................................................ 2-6 

2-2 Estimated Level of Water Consumption for Selected Communities in the 

Yucca M ountain Area ................................................ 2-12 

2-3 Water Well Production for Selected NTS Wells ........................... 2-15 

2-4 Summary of 1993 Water Well Discharge Information for NTS ............... 2-16 

2-5 Summary of Pumping Test Data for Wells J-12 and J-13, during the 

Period 1960-69 ..................................................... 2- 17 

2-6 Water Production from Wells J-12 and J-13, during the Period 1983-85 .... 2-17 

2-7 Ground-Water Pumping Inventories in the Amargosa Desert and Pahrump 

Valley Hydrographic Basins, During the Period 1985-95 ................... 2-19 

2-8 Use of Water in Wells with Depth to Water Greater than 150 Meters, 

in A rizona .......................................................... 2- 28 

2-9 Percentage Water Use for Wells in Arizona .............................. 2-31 

2-10 Arizona Wells with Depth to Water in Excess of 240 Meters as a Function 

of W ell Head Elevation ............................................... 2-31 

2-11 Use of Water in Wells with Depth to Water Greater than 150 Meters, 

in N evada ........................................................... 2- 32

xi NUREG-1538
TABLES continued 

Page 

2-12 Use of Water in Wells with Depth to Water Greater than 150 Meters, in 

N ew M exico ......................................................... 2- 32 

2-13 Use of Water in Wells with Depth to Water Greater than 150 Meters, 

in Texas ............................................................ 2- 36 

2-14 Use of Water in Wells with Depth to Water Greater than 150 Meters, in the 

Trans-Pecos Region of Texas ........................................... 2-37 

2-15 Unit Water Costs for Four Yucca Mountain-Amargosa Desert Area 

W ells ............................................................... 2- 40 

2-16 Use of Water in Wells with Depth to Water Greater than 240 Meters, in 

Arizona, Southern Nevada, New Mexico, and the Trans-Pecos Region 

of Texas ............................................................ 2- 40 

3-1 Estimates of Hydraulic Conductivities Computed Using Autocalibration 

Technique .......................................................... 3- 5 

3-2 Assumed Hydraulic Conductivities for the Vertical Cross-Section Model, 

Based on Data Presented by Whitfield et aL (1985) ........................ 3-9 

3-3 Distribution of Wells by Water Use Across Townships T 15 S. Through 

T 17 S. Using Well Drillers' Logs ....................................... 3-36 

3-4 Statistics for Well Construction Practices and Water Level Positions for 

Wells Recorded in the GWSI Database in Amargosa Valley and 

Am argosa Farms Areas ............................................... 3-36 

3-5 Annual Estimates of Water Use by Type ................................. 3-39 

3-6 Summary Statistics of Individual Irrigation Users on an Annual Basis ........ 3-39 

3-7 Plume Configuration and Point Dilution Factors at 20 Kilometers from 

the Proposed Repository for a Range of Dispersivity Values ................ 3-52 

3-8 Plume Configuration and Point Dilution Factors at 30 Kilometers from 

the Proposed Repository for a Range of Dispersivity Values ................ 3-52 

3-9 Plume Configuration in Terms of Width at 20 and 30 Kilometers from 

the Proposed Repository and Point Dilution Factors for a Source Area Width 

of 500 Meters with No Vertical Dispersion ............................... 3-53 

4-1 Peak Annual Dose from a Damaged Waste Package ....................... 4-5 

4-2 Peak Annual Dose from a Perforated Waste Package ...................... 4-5 

6-1 Solubilities and Dose Conversion Factors for the 43 Radionuclides Used 

in the Relative Hazards Scoping Calculation ............................. 6-10

NUREG- 1538 xii
APPENDICES 

Page 

A. Overview of Farming and Ranching Activities in the Yucca Mountain Area ......... A-1 

N.A. Eisenberg/NMSS1

, M.P Lee/NMSS; N.M. Coleman/NMSS; and C. Glenn/NMSS 

B. Lifestyles and Water-Use Practices in and around the Nevada Test Site, 

before its Establishment: A Preliminary Evaluation ............................. B-1 

M.RP LeeINMSS; L.H. McKague/CNWRA; and P.T Prestholt/NMSS1. 

C. Detailed Descriptions of Deep Wells .......................................... C-1 

G. W WittmeyerlCNWRA; M.RP Miklas/CNWPA; R. V Klar/CNWRA; 

D. Williams2; and D. Balin3 

D. W ell Construction Costs .................................................... D -1 

D. Williams 

E. Detailed Water Use Table for the Years 1983 and 1985-96 ....................... E-1 

D. Williams; and D. Balin 

E Table of Borehole Dilution Factors ........................................... F-1 

D. Williams; and D. Balin 

G. Capture Zone Delineation Table ............................................. G-1 

D. Williams; and D. Balin 

H. Latin Hypercube Sampled Input Parameters Used in the Analysis of Dose ......... H-1 

TJ. McCartin/NMSS; and R.B. CodellINMSS 

I. Staff Participating in the Preliminary Performance-Based Analyses ................ I-1 

1

Now retired. 

2

Independent consultant from Oakland, California. 

3 1ndependent consultant from San Antonio, Texas.

°,°i NUREG- 1538
EXECUTIVE SUMMARY

1. INTRODUCTION 

In 1992, Congress directed the U.S. 

Environmental Protection Agency (EPA), at 

Section 801 of the Energy Policy Act of 1992 

(EnPA), Public Law 102-486, to contract with 

the National Academy of Sciences (NAS) to 

advise EPA on the appropriate technical basis 

for public health and safety standards 

governing the proposed geologic repository at 

Yucca Mountain, Nevada. On August 1, 1995, 

the NAS Committee on Technical Bases for 

Yucca Mountain Standards issued its report, 

Technical Bases for Yucca Mountain Standards. 

In its report, the NAS recommended: (a) the 

standard should set a limit on the risk to 

individuals; (b) the critical group approach, as 

defined by the International Commission on 

Radiological Protection, should be adopted; 

(c) the critical group should be defined using 

present knowledge, and cautious and 

reasonable assumptions; (d) the compliance 

assessment should be conducted over a 

timeframe that includes the period of greatest 

risk; and (e) a stylized calculation should be 

used to assess whether the repository's 

performance would be substantially degraded 

as a consequence of a postulated intrusion. 

The NAS recommended an approach for 

evaluating the performance of a geologic 

repository, that is significantly different from 

previous evaluations performed as part of the 

high-level waste (HLW) performance 

assessment program at the U.S. Nuclear 

Regulatory Commission (NRC). In this 

NUREG, the staff and its technical assistance 

contractor-the Center for Nuclear Waste 

Regulatory Analyses-have undertaken a 

series of focused technical analyses, to better 

understand the NAS recommendations and 

their implementation within NRC's HLW 

program (e.g., modifications and 

enhancements to NRC's total-system 

performance assessment computer code). 

These analyses focused on: (a) evaluation of 

lifestyles and water-use practices to be used 

for identification of potential receptor groups; 

(b) estimation of receptor doses from the

ground-water pathway; (c) evaluation of the 

consequences from an assumed intrusion 

scenario; (d) evaluation of the risk for 

low-probability events; and (e) evaluation of 

the relative radiological hazard of HLW over 

time. 

2. IDENTIFICATION OF 

HYPOTHETICAL RECEPTOR 

GROUPS 

The staff examined a range of land and water

use practices occurring in the Yucca Mountain 

area and in other, similar, environments, as 

part of the process of establishing reasonable 

bounds on estimates of these practices. 

Current use of ground water in the Yucca 

Mountain area is affected by both 

physiographic factors and institutional 

controls. Because the climate is arid to 

semi-arid, agriculture is restricted to areas 

where the depth to water is shallow enough to 

make irrigation economically feasible (i.e., 

Amargosa Desert, Pahrump, and Oasis 

Valleys). Agriculture also tends to be confined 

to areas where the relief is gentle enough to 

make center-pivot or furrow irrigation 

methods practical and where the soil 

conditions are suitable, such as near the distal 

margin of alluvial fan deposits. North of U.S. 

Highway 95, much of the land comprising the 

Nevada Test Site (including Yucca Mountain) 

is controlled by either the U.S. Department of 

Energy, the U.S. Bureau of Reclamation, or 

the U.S. Air Force, which has precluded 

agricultural development and ranching. 

Although the withdrawal of this land from 

private development may have affected land

use patterns, the great depth to ground water 

and limitations to soil irrigability, throughout 

the controlled area, suggest that farming is not 

economically feasible with current technology. 

Assuming current technology, future 

agricultural development will most likely 

continue to be centered in the Amargosa 

Desert area. 

Data from Arizona, southern Nevada, New 

Mexico, and the Trans-Pecos region of Texas 

were examined to provide additional insights

xv NUREG- 1538
on the use of ground water from depths 

greater than the depths that currently exist in 

the Amargosa Desert area. The data indicate 

that ground water is pumped from aquifers in 

arid to semi-arid regions, where the depth to 

water is as great as that in Jackass Flats to the 

east of Yucca Mountain. Of 115 wells that 

pump from aquifers with depths to water 

equal to or greater than 240 meters; only 2 

(1.7 percent) are used for irrigation; 38 (33 

percent) are used to supply stock water; and 

63 (54.8 percent) are used for public and 

domestic water supply. Based on local 

practices in Amargosa Valley and on data 

gathered in the well survey, it appears that 

pumping ground water for irrigation from 

depths greater than 240 meters is a rare 

practice (1.7 percent). However, data from 

the water well survey does suggest that existing 

communities will construct wells that pump 

from great depths to water if more easily 

exploited water supplies are insufficient or 

unreliable. Although current lifestyles in the 

Yucca Mountain area do not support the idea 

that very small communities (25 to 100 

persons) or individual homeowners will 

construct wells to pump fresh water from great 

depths, the survey data do suggest that a 

residential-type receptor group would be more 

likely to withdraw ground water from greater 

depths than a farming-type receptor group. 

However, the residential receptor group would 

also be expected to withdraw significantly less 

water than the farming receptor group. 

For the purposes of this NUREG, the staff 

identified and considered the characteristics 

for two potential (hypothetical) receptor 

groups-"farming" and "residential" 

(non-farming)-in the Yucca Mountain area. 

"Farming" Receptor Group: Because of 

topographic, meteorologic, pedologic, and 

hydrologic considerations, this hypothetical 

receptor group is a farming community 

located in an topographically closed basin at 

a distance no closer than 20 kilometers to 

the site. The farming community is able to 

obtain its water from relatively shallow wells 

located in near-surface, local alluvial 

aquifers (i.e., generally 30 to 100 meters 

deep). This is an important consideration

because development and operating costs 

for such wells are, in large measure, related 

to the depth of the water table. Thus, for 

reasons of practicality, this hypothetical 

receptor group would establish its irrigation 

wells in areas where the water table is near 

the surface. The exposure pathways 

considered for the hypothetical farming 

receptor group include ingestion (of 

contaminated water, crops, and animal 

products); inhalation (from resuspension of 

contaminated soil); and direct exposure. 

"Residential" Receptor Group: This 

hypothetical receptor group is a residential 

community located on one of the volcanic 

mesas or mountainous uplands that flank 

the topographically-closed basins, but may 

also be located in higher-elevation valleys, 

which are characterized by thin sequences of 

unsaturated alluvial sediments. The 

residential community is located between 5 

and 20 kilometers from the site where the 

depth to the water table ranges from 

approximately 300 to 100 meters, 

respectively, at the two locations. Because 

freshwater demands for a residential 

community are substantially less than those 

for a farming community, the residential 

community could be located closer to the 

site than a farming community and could 

obtain its water from wells located in the 

deeper tuff or carbonate (regional) aquifers 

found at depths of 300 meters or more. The 

exposure pathway considered for the 

hypothetical residential receptor group is the 

ingestion of contaminated water 

3. GROUND-WATER PATHWAY 

Radionuclides released from a potential 

repository at Yucca Mountain can be expected 

to migrate to the water table and be carried 

downgradient toward discharge areas, most 

likely at some point in the Amargosa-Death 

Valley area. Thus, the ground-water pathway 

is the most likely exposure pathway. Exposure 

to humans is assumed to occur through the use 

of well water that is contaminated with 

radionuclides. Determination of the 

concentration of radionuclides in well water 

depends on: (a) the location of the discharge 

point; (b) the degree of plume dispersion; and

NUREG-1538 xvi
(c) pumping of the well itself. The staff 

performed a series of analyses to determine 

the importance of these three factors. 

Ground-water flow beneath Yucca Mountain 

would initially be in volcanic tuff, generally 

dipping down-gradient toward the 

Amargosa-Death Valley area. Several tens of 

kilometers from the proposed repository site, 

the water table laterally intersects valley-fill 

alluvial deposits. The areal extent of a 

radionuclide plume would increase with 

distance from the repository because of 

processes of molecular diffusion and 

mechanical mixing. However, staff analyses 

indicate that passive ground-water mixing at 

the Yucca Mountain site is not likely to 

produce very large dilution factors. In the 

immediate vicinity of the proposed repository, 

dilution is limited because of the directional 

characteristics of the flow and the magnitudes 

of the Darcy fluxes (i.e., the tendency for 

contaminant plumes to remain on or close to 

the water table surface). As the radionuclide 

plumes travel away from the proposed 

repository, they have a greater chance of 

spreading and becoming diluted both laterally 

and vertically as a result of movement through 

or around large-scale structural features such 

as faults. Depending on their hydraulic 

properties, faults in the tuff aquifer could play 

a major role in determining the rate and 

direction of plume spread. At substantial 

distances, radionuclide plumes traveling 

through the alluvium are expected to be 

further mixed with uncontaminated waters, 

but the dilution at locations such as the 

Amargosa Desert, is unlikely to increase by 

many orders of magnitude. However, mixing 

resulting from well pumping becomes a 

significant dilution process as natural mixing 

decreases. 

Dose calculations were made for hypothetical 

farming and residential receptor groups. The 

hypothetical farming receptor group is 

assumed to be located 30 kilometers from the 

proposed repository site and withdraws large 

amounts of water (30,400 cubic meters per 

day) to support the water needs of an 

agricultural community. The hypothetical 

residential receptor group is assumed to be

located 5 kilometers from the proposed 

repository site and withdraws 3800 cubic 

meters per year of water to support household 

uses of water. Thus, the farming receptor 

group has more borehole dilution and a longer 

transport path that reduces radionuclide 

concentrations and delays the time of the peak 

dose relative to values for the hypothetical 

residential group. However, the hypothetical 

farming receptor group is assumed to have 

more exposure pathways (e.g., ingestion of 

contaminated water, crops, and animal 

products) than the hypothetical residential 

receptor group (exposure pathway from 

ingesting contaminated water only) which 

tends to offset differences in doses between 

the two hypothetical groups. 

Although assumptions concerning location and 

lifestyle were quite different for the two 

hypothetical receptor groups, conclusions 

regarding the cause of the estimated peak 

dose were similar. Peak doses were the result 

of a relatively small number of long-lived, 

mobile radionuclides (e.g., technetium-99, 

neptunium-237, and iodine-129) that are the 

first radionuclides to arrive at the hypothetical 

receptor location. Despite the differences in 

the time of the peaks for the two locations 

(2500 years for the 5-kilometer location versus 

15,000 years for 30-kilometer location), the 

estimated mean doses were somewhat similar 

[30 millirem (mrem) for the 5-kilometer 

location versus 10 mrem for the 30-kilometer 

location]. It should be noted that dose curves 

for both locations are a reflection of the 

conservative assumption that all waste 

packages fail and release radionuclides at 1000 

years. 

4. HUMAN INTRUSION 

The NAS recommended the use of a stylized 

calculation to evaluate the resilience of the 

repository to a postulated intrusion. The 

objective of this calculation is to determine if 

an intrusion into the repository would degrade 

the ability of the repository to comply with the 

overall performance objective (i.e., individual 

dose or risk limit for the critical group). 

Because the probability of such a scenario 

would be highly speculative, the NAS 

recommended that the calculation not be

xvii NUREG-1538
included in the overall performance 

assessment of the site, but considered 

separately using the same critical group 

assumptions as the overall performance 

assessment. The staff evaluated the 

consequences of human intrusion by 

considering the following two scenarios: (a) a 

single borehole intersecting the emplacement 

drift and damaging the waste package; and (b) 

a single borehole intersecting the 

emplacement drift, drilling completely through 

a waste package, and continuing through the 

unsaturated zone to the water table. 

The mean peak doses from the postulated 

intrusion scenarios was between 1 to 4 

microrems. Based on these analyses, the staff 

concludes that the consequences of a 

postulated human intrusion event can be 

readily evaluated with current performance 

assessment techniques, and repository 

performance is not overly sensitive to 

assumptions regarding the specifics of the 

intrusion (i.e., borehole diameter, catchment 

area, and timing of the drilling event). 

5. RISK FROM LOW-PROBABILITY 

EVENTS 

The NAS recommended that individual risk be 

used for standards for a potential geologic 

repository at Yucca Mountain. Estimation of 

risk needs to consider likely pathways for 

radiological exposure (e.g., ground-water 

pathway) as well as less likely exposure 

pathways from low-probability events. 

Estimation of the individual dose or risk from 

an extrusive volcanic event is used to provide 

insights on approaches for estimating 

individual dose for low-probability events and 

the relationship, if any, between the time 

period of the analysis and the dose calculation. 

The exposure scenario for an extrusive 

volcanic (igneous) event consists of four 

components in the following progression: (i) 

magma enters the repository and becomes 

contaminated with spent nuclear fuel particles; 

(ii) tephra forms from the contaminated 

magma and is released from the repository; 

(iii) an eruption column and plume form and 

transport contaminated tephra to locations

downwind from the event; and (iv) 

radionuclide-contaminated ash collects on the 

earth's surface, potentially exposing receptor 

groups. The expected dose or "average risk" 

to an individual is a function of the dose and 

the probability of receiving that dose. In 

calculating the expected annual dose at a 

particular time T, the probability that an 

individual has an exposure at time T is the 

probability that the event has occurred at any 

time previous to T (i.e., exposure from a 

volcanic event occurs during the year of the 

event and in each subsequent year because of 

the residual surface contamination). Thus, at 

1000 years, there is a 1 chance in 10,000 that a 

volcanic event has occurred in the initial 1000 

years (for the current analysis, it is assumed 

that a volcanic cone forms within the center of 

the repository block with an annual probability 

of 10-1b). 

The expected annual dose reaches a peak of 

approximately 1 mrem around 1000 years after 

repository closure (the expected annual dose 

is determined by multiplying the consequence 

or dose times the probability that the dose has 

occurred). This early peak in the expected 

annual dose is caused by a rapid decrease in 

the dose, over the initial 1000 years, from 

radioactive decay of key radionuclides (e.g., americium-241, which has a radioactive 

half-life of 432 years, is the largest contributor 

to peak dose). 

6. RADIOLOGICAL HAZARD OF 

HLW 

The NAS recommended that compliance with 

HLW standards should be over the time 

period of maximum risk up to one-million 

years. The purpose of this recommendation 

was to focus the analysis of repository 

performance on the time at which future 

populations might be at maximum risk. The 

health hazard of radioactive waste depends on 

two primary factors: (a) the inherent 

radiotoxicity of the material; and (b) the 

accessibility of the material to possible human 

intake or exposure. To evaluate the health 

hazard, a comparison is made between the 

radiological hazard of a geologic repository 

containing spent nuclear fuel and a 

hypothetical ore body. The analysis was done

NUREG- 1538 xviUi
for a 100-million-year time period and 

assumed both the repository and ore body 

were located in the unsaturated zone at Yucca 

Mountain. 

A primary difference between a geologic 

repository and the hypothetical ore body 

referred to in this analysis is that the 

repository-destined waste has a significant 

man-made radionuclide inventory as 

compared with the naturally occurring 

radionuclides of the ore body. The hazard of 

the geologic repository and the hypothetical 

ore body will decrease, over time, at different 

rates, because of differences in the inventories 

(e.g., spent nuclear fuel contains significant

amounts of short-lived radionuclides that are 

not present in the ore body). 

The relative radiological hazard of the spent 

fuel repository is initially about 4 orders of 

magnitude greater than that of the 

hypothetical ore body. The hazard diminishes 

most rapidly over the first few hundred to a 

few thousand years. By 10,000 years the 

relative hazard will have decreased by 99.9 

percent and be within less than an order of 

magnitude of the hypothetical ore body. A 

time period of interest, for regulation of a 

proposed repository, of 10,000 years, would, 

therefore, focus attention on the time period 

when the waste has a significant man-made 

hazard component that is readily discernable 

from an equivalent hypothetical ore body.

xix NUREG- 1538
FOREWORD

Since its inception in the mid-1970s, the U.S. 

Nuclear Regulatory Commission's (NRC's) 

geologic repository program for the disposal 

of spent nuclear fuel and other high-level 

radioactive waste has relied on the evolving 

science of performance assessment to 

accomplish several objectives. Accordingly, 

the staff has sought to develop, maintain, and 

enhance its performance assessment 

capability, in the form of Iterative 

Performance Assessments (IPAs). Experience 

developing IPAs has enabled the staff to 

evaluate total-system performance 

assessments submitted in support of the U.S. 

Department of Energy's (DOE's) 

Pre-licensing activities (e.g., site 

characterization and the forthcoming site 

suitability determination), and to prepare to 

review a potential DOE license application 

and decision on a possible request for 

construction authorization. 

The NRC staff has also relied on its evolving 

IPA capability during early interactions with 

the U.S. Environmental Protection Agency 

(EPA) staff as EPA developed its site-specific 

radiation protection standards for a potential 

geologic repository at Yucca Mountain, 

Nevada, pursuant to the Energy Policy Act of 

1992. In light of the 1995 National Academy 

of Sciences' findings and recommendations for 

those standards, the NRC staff and its 

technical assistance contractor - the Center 

for Nuclear Waste Regulatory Analyses 

undertook a limited series of analyses, using 

its IPA capability to better understand what 

technical challenges may be faced in 

implementing and evaluating health-based 

(dose) performance measures at Yucca 

Mountain. EPA published its final site-specific 

radiation standards on June 13, 2001 (66 FR 

23074). 

Most of the principal analyses described in 

this NUREG were conducted between 1995 

and 1996, with the key results published and 

made publicly available as short summaries in

Sagar (1997), as NUREG/CR-6513.1 Two 

additional analyses and one field investigation 

were subsequently conducted during the 

period 1996-98.2 Documentation in support 

of the completed work was later placed in 

NRC's Public Document Room (PDR), but 

did not receive widespread distribution. As a 

consequence, a decision was made to include 

this additional documentation in this 

NUREG, after limited editorial review and 

modification. Despite the editorial treatment, 

this additional technical documentation is 

substantially the same as the original versions 

found in the PDR. Lastly, the results of two 

analyses first published in NUREG/CR-6513 

(human intrusion and extrusive volcanism) 

were updated in this NUREG owing to the 

availability of more recent data and/or newer 

analytical approaches. 

No proprietary or unpublished data sources 

were used in these analyses - only data 

currently in the public domain have been 

cited. The calculations, figures, and 

conclusions presented herein result from the 

continuing exercise of the staff capability to 

review a performance assessment for a 

potential geologic repository at Yucca 

Mountain. In this regard, the approaches, 

assumptions, and conclusion described in this 

NUREG should in no way be construed to 

express the views, preferences, or positions of 

the staff regarding what the eventual form of 

the site-specific regulatory framework for 

Yucca Mountain should be. 

Finally, more site characterization information 

now exists than was available at the time these 

analyses were first conducted and 

documented. Thus, given the iterative nature 

of performance assessments, this NUREG 

serves to document, as a snapshot in time, the 

staff's capability for reviewing a performance 

assessment for the proposed Yucca Mountain 

site. As DOE proceeds with characterizing 

1See Chapter 9, entitled "Activities Related to the 

Development of the EPA Yucca Mountain Standard." 

2As noted in text.

xxi NUREG- 1538
the Yucca Mountain site and its environs, the 

staff expects that additional information will 

be forthcoming that will contribute to an

improved understanding of combined systems 

and events and processes that are key to 

repository performance.

NUREG-1538 xxii
ENGLISH/METRIC CONVERSION FACTORS

The preferred system of measurement today is 

the "System Internationale (SI)," or the 

metric system. However, for some physical 

quantities, many scientists and engineers 

prefer the familiar and continue to use the 

English system (inch-pound units). With few 

exceptions, all units of measure cited in this 

NUREG are usually in the metric system.

The following table provides the appropriate 

conversion factors to allow the user to switch 

between the English and SI systems of 

measure. Not all units nor methods of 

conversion are shown. Unit abbreviations are 

shown in parentheses. All conversion factors 

are approximate.

NUREG- 1538

Multiply Inch-Pound Units By To Obtain SI Units 

Length 

inch (in) 2.54 centimeter (cm) 

feet (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

Area 

square mile (mi 2) 2.590 square kilometers (km2) 

acre 4046.873 square meters (m2 ) 

0.40468 hectacres 

Flow 

cubic feet per second (ft3/sec) 0.02832 cubic meters per second (m3/sec) 

U.S. gallons per minute (gal/min) 0.06309 liters per second (L/sec) 

Transmissivity 

foot squared per day (ft2/day) 0.0929 meter squared per day (m 2

/day) 

Temperature 

degree Fahrenheit ('F) 'C = (*F - 32)/1.8 degree Celsius (°C) 

Radiation Dose 

millirem (mrem) 0.01 millisievert (mSv)

x..°
ACKNOWLEDGMENTS

The authors and editors gratefully 

acknowledge the many contributions made by 

other Nuclear Material Safety and Safeguards 

(NMSS), Nuclear Regulatory Research, 

General Counsel (OGC), and the Center for 

Nuclear Waste Regulatory Analyses 

(CNWRA) management and staff, which have 

been incorporated into the text of this 

NUREG. In this regard, thanks are also owed 

the following staff for their thoughtful review 

comments and suggestions on various sections 

of this NUREG: 

A. Armstrong/CNWRA 

N. Jensen/OGC 

K. McConnell/NMSS 

B. Sagar/CNWRA 

M. Weber/NMSS 

J. Winterle/CNWRA 

Special thanks are also due to Ellen Kraus, 

who provided editorial guidance. 

The authors and editors are especially 

appreciate the conversations and assistance 

(including limited review comments, in some 

instances) provided generously by the 

following individuals: T. Gallagher and C. 

Hickenbottom, of the State of Nevada

Division of Water Resources/Department of 

Conservation and Natural Resources, for 

information and data on water use in Nye 

County, Nevada; D.A. Singer and J.H. 

DeYoung, Jr., of the U.S. Geological Survey 

(Menlo Park, California, and Reston, Virginia, 

respectively), for information on issues related 

to natural resources and exploratory drilling; 

R.L. Kimble, of Science Applications 

International Corporations, Inc. (Las Vegas), 

for information and data on the 

socioeconomics of the Yucca Mountain area; 

and TD. Mayer, of the U.S. Fish and Wildlife 

Service (Portland, Oregon) for information on 

the regional hydrology of southern Amargosa 

Valley. 

Finally, thanks are also due to Beth St. 

George, of the U.S. Fish and Wildlife Service, 

for information on the Ash Meadows National 

Wildlife Refuge, and K.G. Garey, of 

Amargosa Valley, for his views on farming and 

ranching in Amargosa and Pahrump Valleys, 

Nevada. 

The views expressed in this report are those of 

the contributing authors and editors, not those 

of the individuals representing the 

aforementioned organizations.

NUREG- 1538
ACRONYMS AND ABBREVIATIONS

AEC Atomic Energy Commission 

AEM analytical element methods 

AFB Air Force Base 

CA California °C degrees Celsius 

CCDF complementary cumulative distribution function 

CNWRA Center for Nuclear Waste Regulatory Analyses (in San Antonio, Texas) 

CRMP Community Radiation Monitoring Program 

DCFs dose conversion factors 

DOE U.S. Department of Energy 

EnPA Energy Policy Act of 1992 

EPA U.S. Environmental Protection Agency 

ET evapotranspiration 

'F degrees Fahrenheit 

GTCC greater than class-C (radioactive waste) 

GWSI Ground-Water Site Inventory 

HLW high-level radioactive wastes (including spent nuclear fuel) 

ICRP International Commission on Radiation Protection 

IPA iterative performance assessment 

MPC multi purpose canister 

msl mean sea level 

MTU metric ton (equivalent) of uranium 

NAS National Academy of Sciences 

NMSS Office of Nuclear Material Safety and Safeguards 

NRC U.S. Nuclear Regulatory Commission 

NTS Nevada Test Site 

NV Nevada 

NWPA Nuclear Waste Policy Act of 1982, as amended 

OGC Office of the General Counsel 

OCRWM Office of Civilian Radioactive Waste Management 

PDR public document room 

RES Office of Nuclear Regulatory Research 

SCP site characterization plan 

SNF spent nuclear fuel 

TDNs total digestible nutrients 

TEDE total effective dose equivalent 

TSPA total system performance assessment 

U.S. United States 

USGS U.S. Geological Survey 

UTM Universal Transverse Mercator

xxvii NUREG-1538
1 -D one-dimensional 

2-D two-dimensional 

3-D three-dimensional

NUREG- 1538 °°°ii
1. INTRODUCTION

In August 1995, the National Academy of 

Sciences (NAS) issued its findings and recom

mendations on a revised environmental 

standard for the disposal of high-level 

radioactive waste (HLW)1 specific to Yucca 

Mountain, Nevada (National Research Council, 

1995). Important differences exist between these 

findings and recommendations and prior U.S. 

Environmental Protection Agency (EPA) 

standards for HLW, as well as the existing U.S. 

Nuclear Regulatory Commission's (NRC's) 

geologic disposal regulations at 10 CFR Part 60 

(NRC, 1983 and 1986). NRC has used its 

Iterative Performance Assessment (IPA) 

capability to evaluate the NAS findings and 

recommendations and has identified a number 

of key issues relevant to the implementation of 

dose-based performance measures at Yucca 

Mountain. The staff and its technical assistance 

contractor, the Center for Nuclear Waste 

Regulatory Analyses, have thus undertaken a 

series of focused technical analyses, described in 

this NUREG, to better understand these issues. 

1.1 Background 

Since the late 1970s, EPA has been engaged in 

setting standards to protect the public and 

environment from the potential hazards 

associated with the geologic disposal of HLW. 

Pursuant to its legislative authority and 

responsibilities, EPA issued its HLW 

standards in the form of 40 CFR Part 191, in 

September 1985 (see EPA, 1985; 50 FR 

38066). However, in July 1987, the U.S. Court 

of Appeals for the First Circuit in Boston 

vacated Subpart B of the HLW standards and 

remanded the rule to EPA for further 

consideration (see EPA, 1993; 58 FR 7924).2 

1As used in this document, HLW includes spent nuclear fuel 

(SNF) and transuranic wastes, unless otherwise specifically 

stated. 

2Principal among these was Subpart B, the individual and 

ground water protection requirements. The Court requested 

further notice and comment on these provisions as well as 

their inter-relationship to the Safe Drinking Water Act. In 

October 1992, the Waste Isolation Pilot Project (WIPP) Land 

Withdrawal Act (WIPP LWA-Public Law 102-579) was 

enacted which reinstated all of Part 191 except for those 

provisions that were the subject of the remand by the First 

Circuit. Moreover, the WIPP LWA also required issuance of 

new standards to address those that were the subject of the 

judicial remand and exempted the Yucca Mountain site from 

the Part 191 standards. Final disposal standards in 40 CFR 

Part 191 were issued in December 1993.

After the 1987 court decision, EPA attempted 

to revise its environmental standards (see U.S. 

General Accounting Office, 1993). However, 

before EPA could complete its work, Congress 

enacted the Energy Policy Act of 1992 

(EnPA-Public Law 102-486). Through 

EnPA, Congress directed EPA to promulgate 

new environmental standards specific to a 

potential geologic repository for HLW at 

Yucca Mountain. EnPA stipulates that EPAs 

new standards are to be based on, and 

consistent with, the NAS findings and 

recommendations. After the final standards 

are promulgated, NRC is required by EnPA to 

modify its regulations to be consistent with the 

new EPA standards. 

Section 801(a)(2) of EnPA directed the NAS 

to provide EPA with recommendations on the 

following issues: 

" Whether health-based standards based on 

doses to individual members of the public 

from releases to the accessible 

environment ... will provide a reasonable 

standard for protection of the health and 

safety of the general public? 

" Whether it is reasonable to assume that a 

system of post-closure oversight of the 

repository can be developed, based on 

active institutional controls, that will 

prevent an unreasonable risk of breaching 

the repository's engineered or geologic 

barriers or increasing the exposure of 

individual members of the public to 

radiation beyond allowable limits? 

" Whether it is possible to make 

scientifically supportable predictions of 

the probability that the repository's 

engineered or geologic barriers will be 

breached as a result of human intrusion, 

over a period of 10,000 years? 

The previously applicable EPA environmental 

standards and the current NRC implementing 

regulations are found, respectively, at 40 CFR 

Part 191 (EPA, 1985) and Part 60 (NRC, 1983 

and 1986). These regulations are somewhat 

different from proposals for a health-based

1-1 NUREG- 1538
1 Introduction

standard only (e.g., limiting individual dose or 

risk), as suggested by the EnPA. Previously, 

Part 191 had established containment 

requirements that limited the releases of 

radioactive material to the accessible 

environment, weighted by a factor approxi

mately proportional to radiotoxicity, and 

integrated over a period of time (a span of 

10,000 years is the current regulatory 

requirement) after permanent closure of the 

geologic repository. For its part, Part 60 

incorporates Part 191 as the overall 

performance requirement for a geologic 

repository. The requirements in 10 CFR 

60.112 had set an overall system performance 

objective that amounted to meeting EPAs 

containment requirements, whereas certain 

other sections (10 CFR 60.113) set forth 

additional subsystem performance objectives.3 

Forthcoming revisions to both EPAs site

specific standards and NRC's conforming 

regulations notwithstanding, NRC regulations 

will continue to require compliance with 

applicable EPA environmental standards as 

the overall system performance objective for 

the repository and that demonstration of 

compliance with that objective will continue to 

necessitate a quantitative performance 

assessment to estimate post-closure 

performance of the repository system (see 

Kotra et al., 1998). 

1.2 Regulatory Considerations for 

Dose-Based Performance Measures 

As noted above, the NAS has published its 

findings and recommendations for site-specific 

environmental standards for Yucca Mountain 

(see National Research Council, 1995). 

Among the NAS findings and 

recommendations was a key recommendation 

that the revised standards limit individual risk 

to a member of the public. Specifically, the 

NAS has recommended that the level of 

3

At their time of inclusion in Part 60, these subsystem perform ance objectives were expected to support NRC's ability to find, with reasonable assurance, that the EPA standards would be 

met. In developing a Yucca Mountain-specific disposal regulation, the NAS asked NRC to reconsider the role of 

numerical subsystem performance objectives. In its findings and recommendations, the NAS indicated the potential for 

such requirements to lead to the sub-optimization of 

repository design (National Research Council, 1995; p. 126).

protection provided by new environmental 

standards for Yucca Mountain should be 

comparable to that level of risk that may be 

acceptable to society at large (Op cit., pp. 

47-57), given that society currently tolerates 

certain involuntary risks [e.g., in the range of 10-5 to 10-6 per year-see Smith (1995, p. 

B-2) and NAS (1995, p. 50)]. To demonstrate 

that a geologic repository can be designed to 

provide such protection, the NAS 

recommended that assessments of individual 

risks be conducted for specific target 

populations, in the Yucca Mountain vicinity, 

using the approach specified by the 

International Commission on Radiological 

Protection (ICRP) (1985)-(e.g., "critical 

groups"-see Smith, 1995).4 As EPA considers 

this particular recommendation, the staff have 

been exploring how its existing performance 

assessment review capability, to be used to 

evaluate DOE's demonstration of compliance 

with NRC's regulations, might be revised to 

accommodate such an exposure scenario. 

Many of the considerations for a dose-based 

model are similar to those required for other 

models used in performance assessments. For 

example, an assessment of dose will be a 

function of both the calculation methodology 

itself (i.e., the calculation of an annual 

individual dose for an adult male) and certain 

modeling assumptions (e.g., exposure 

scenario)-although the two may be hard to 

distinguish at times [see Neel ("Dose

Assessment Module") in Wescott et al. 

(1995)]. Some of the issues that may need to 

be considered and treated in a dose-based 

calculation include the following: 

" Who is exposed (and where are they 

located)? 

"• What is the time period of concern ? 

"* What are the exposure pathways? 

For the purposes of the United States' (U.S.') 

waste management program, answers to these 

questions have not been decided, although the 

4The ICRP defines risk to a critical group in terms of dose. The 

term "dose" generically refers to the quantity of radiation 

absorbed by body organs or tissue. NRC defines "dose" for its 

regulatory purposes at 10 CFR Part 20. In its recommenda

tions, the NAS adapts the ICRP terminology to its proposed risk-based framework.

NUREG- 1538 1-2
1 Introduction

NAS made recommendations relevant to many 

of them. It is expected that answers to most, if 

not all, all of these will be developed as EPA 

and NRC promulgate their respective 

standards and regulations, consistent with the 

NAS findings and recommendations. 

However, anticipating that some type of 

dose-based standard might be adopted in the 

future, the staff has begun to evaluate the 

mechanics of implementing such an approach 

in its performance assessment program and 

thus provide some insights into the issues cited 

above. 

1.3 Approach and Content of this 

NUREG 

The staff have begun to evaluate how to 

implement a dose-based HLW standard as 

part of its overall IPA effort. The staff's earlier 

in-house efforts to date have been docu

mented in LaPlante et al. (1995); Manteufel 

and Baca (1995); Neel ("Dose-Assessment 

Module"), in Wescott et al. (1995); and 

Mohanty and McCartin (2000). The analyses 

described in this NUREG are a continuation 

of these efforts. 5 

In assessing the NAS findings and 

recommendations, the staff (with the support 

of its technical assistance contractor-see the 

last appendix of this NUREG) undertook a 

series of short-duration analyses, beginning in 

1995. The areas selected for initial study by the 

staff were as follows: 

Potential Receptor Groups: The NAS 

recommended that the dose calculations 

be performed for specific populations, 

using the "critical group approach" 

specified by the ICRP Based on the 

staff's earlier work, the most likely 

exposure scenario for a receptor 

population in the Yucca Mountain vicinity 

would be through some ground-water/ 

food-ingestion pathway [see Neel ("Dose

Assessment Module") in Wescott et al. 

(1995)]. In the first series of analyses, 

described in Section 2, the staff applied 

the tenants of the ICRP "critical group 

51nitial results from some of the analyses described in this 

NUREG were first documented in 1996 and later published in 

1997, in NUREG/CR-6513 (Sagar, 1997).

approach" to better understand the 

lifestyle and habits of existing 

communities in the Yucca Mountain area. 

The staff was able to identify two 

hypothetical (potential) receptor groups 

based on a review of information on 

population demographics and water 

availability (e.g., drilling practices) of 

current residents in the Yucca Mountain 

area. As an expansion of this analysis, the 

staff also performed a regional evaluation 

of deep well drilling practices in the arid 

southwest U.S., to better understand how 

some communities exploit fresh-water 

resources found in deep (non-alluvial) 

aquifers. 

Ground-Water Pathway: The calculation of 

peak dose and the definition of the 

site-specific critical group requires 

consideration of the behavior of the 

radionuclide contaminant plume as it 

migrates away from the geologic 

repository. In Section 3, the staff 

conducted a preliminary simulation of 

radionuclide dilution in the saturated 

zone beneath Yucca Mountain, and 

analyzed the role of dilution/mixing 

caused by a pumping water well. 

As noted above, the NAS recommended 

that the compliance period should include 

the time when greatest risk occurs. 

Consistent with this NAS recommen

dation, the staff undertook an analysis to 

calculate peak dose. Specifically, the staff 

was interested in determining: (a) the 

difficulty of performing the calculation, 

itself, given NRC's existing IPA capability; 

(b) which radionuclides contribute 

significantly to peak dose; and (c) the 

relative importance of site-specific 

assumptions and parameters in dose

based calculations of the ground-water 

pathway. 

Human Intrusion: The NAS study 

commented that it is not possible to make 

scientifically supportable predictions of 

the probability that engineered or 

geologic barriers will be breached over 

1-3 NUREG-1538
1 Introduction

the next 10,000 years. Therefore, the NAS 

recommended that human intrusion 

should not be included in a probabilistic 

performance assessment. Instead, the 

NAS recommended that a consequence 

analysis of an assumed, stylized human 

intrusion scenario should be required and 

that the resulting risk should not exceed 

the limit adopted for undisturbed 

performance. The staff's analysis in 

Section 4 examines the feasibility of 

performing such a calculation. 

Disruptive Events: The NAS recommen

dation for a risk calculation conducted 

over the timeframe of greatest risk has 

implications for the treatment of 

disruptive events in a performance 

assessment. Evaluation of certain 

disruptive events can affect dose 

estimates by raising the significance of 

alternative exposure pathways (e.g., 

inhalation and direct pathways from the 

release of radionuclides via extrusive 

volcanism). Additionally, long compliance 

periods increase the likelihood that 

low-probability events will occur during 

the time period of regulatory interest. 

Section 5 presents the staff analysis of a 

volcanic scenario, to better understand 

parameters and assumptions associated 

with estimating dose from an extrusive 

volcanic event, and the effect of the 

timeframe of the analysis on the 

evaluation of low-probability events. 

Hazards Comparison Over Time: As noted 

earlier, one of the NAS' recommenda

tions was that the assessment of 

compliance with the HLW standard 

should be conducted over a timeframe 

that includes the period when greatest 

risk occurs. In Section 6, the staff did a 

comparative analysis, over time, of the 

relative radiological hazard between a 

geologic repository containing only SNF, 

and a hypothetical uranium ore body 

containing only naturally occurring 

radionuclides. 

These particular topic areas were selected for 

study because they: (a) would likely provide

preliminary insights into the NAS recommen

dations; and (b) would focus on the major 

implementation issues that the staff would 

need to consider as it continues to modify and 

enhance its IPA capability. 

Finally, it should be noted that as part of the 

Commission's Strategic Plan and Rebaselining 

Initiative, the Commission directed the staff to, 

among other things, pursue the development 

of site-specific regulations for the disposal of 

HLW at Yucca Mountain, consistent with the 

findings and recommendations of the NAS 

(see Kotra et al., 1998). Because revision to 

the Commission's geologic disposal 

regulations is currently underway, at this time, 

the approaches, assumptions, and conclusion 

described in this report should in no way be 

construed to express the views, preferences, or 

positions of the staff regarding what the 

eventual form of the site-specific regulation 

should be. 

1.4 References 

International Commission on Radiological 

Protection, "Radiological Protection 

Principles for the Disposal of Solid 

Radioactive Waste," Pergamon Press, Oxford, 

1985. [ICRP Publication 46.] 

Kotra, J. P, et al., "Strategy for Development 

of NRC Regulations for the Proposed 

Repository at Yucca Mountain, Nevada," in 

American Nuclear Society, High-Level 

Radioactive Waste Management: Proceedings of 

the Eighth International Conference, May 11

14, 1998, Las Vegas, Nevada, pp. 361-363 

[1998]. 

LaPlante, PA., S.J. Maheras, and M.S. 

Jarzemba, "Initial Analysis of Selected 

Site-Specific Dose Assessment Parameters and 

Exposure Pathways Applicable to a 

Groundwater Release Scenario at Yucca 

Mountain," San Antonio, Texas, Center for 

Nuclear Waste Regulatory Analyses 

[CNWRA], CNWRA 95-018, September 

1995. [Prepared for NRC.] 

Mantueufel, R.D., and R.G. Baca, "Iterative 

Performance Assessment Phase 3: Status of 

Activities," San Antonio, Texas, Center for

NUREG- 1538 1-4
1 Introduction

Nuclear Waste Regulatory Analyses, CNWRA 

95-007, April 1995. 

Mohanty, S., and T.J. McCartin 

(coordinators), "NRC Sensitivity and 

Uncertainty Analyses for a Proposed HLW 

Repository at Yucca Mountain, Nevada, Using 

TPA [Computer Code Version] 3.1-Volume 

1: Conceptual Models and Data," U.S. 

Nuclear Regulatory Commission, 

NUREG-1668, February 2001. 

National Research Council, "Technical Bases 

for Yucca Mountain Standards," Washington, 

D.C., National Academy Press, Commission 

on Geosciences, Environment, and Resources, 

July 1995. 

Sagar, B. (ed.), "NRC High-Level Radioactive 

Waste Program Annual Progress Report: 

Fiscal Year 1996," U.S. Nuclear Regulatory 

Commission, NUREG/CR-6513, No. 1, 

January 1997. [Prepared by the Center for 

Nuclear Waste Regulatory Analyses.] 

Smith, G.M., "Compilation and Analysis of 

Information on Critical Groups (Draft)," 

Oxfordshire, United Kingdom, Intera 

Information Technologies, August 1995. 

[Prepared for the Electric Power Research 

Institute.] 

U.S. Environmental Protection Agency, 

"Environmental Standards for the 

Management of Spent Nuclear Fuel,

High-Level and Transuranic Wastes [Final 

Rule]," Federal Register, vol. 50, no. 182, 

September 19, 1985, pp. 38066-38089. 

U.S. Environmental Protection Agency, 

"Environmental Radiation Protection 

Standards for the Management and Disposal 

of Spent Nuclear Fuel, High-Level and 

Transuranic Radioactive Wastes [Proposed 

Rule]," Federal Register, vol. 58, no. 26, 

February 10, 1993, pp. 7924-7936. 

U.S. General Accounting Office, "EPAs 

Development of Radiation Protection 

Standards," Washington, D.C., 

GAO/RCED-93-126, June 3, 1993. 

U.S. Nuclear Regulatory Commission, 

"Disposal of High-Level Radioactive Wastes 

in Geologic Repositories [Final Rule]," Federal 

Register, vol. 48, no. 120, June 21, 1983, pp. 

28194-28229. 

U.S. Nuclear Regulatory Commission, 

"Disposal of High-Level Radioactive Wastes 

in Geologic Repositories [Conforming 

Amendments]," Federal Register, vol. 51, no. 

118, June 19, 1986, pp. 22286-22301. 

Wescott, R.G., et al. (eds.), "NRC Iterative 

Performance Assessment Phase 2: 

Development of Capabilities for Review of a 

Performance Assessment for a High-Level 

Waste Repository," U.S. Nuclear Regulatory 

Commission, NUREG-1464, October 1995.

1-5 NUREG- 1538
2. CONSIDERATIONS IN THE DEFINITION 

OF A RECEPTOR GROUP

In its 1995 findings and recommendations, the 

NAS proposed that dose calculations be 

performed, for specific populations, to avoid 

unlimited speculation about the behavior of 

future human society. Specifically, in 

performing the requisite calculations, the NAS 

recommended consideration of the local 

biosphere, using the "critical group approach" 

specified by the ICRP and employing "cautious but reasonable assumptions." The 

ICRP has generally defined the critical group 

to be a relatively homogenous group of people 

whose location and habits are such that they 

are representative of those individuals 

expected to receive the highest doses as a 

result of radionuclide releases.1 

With this guidance in mind, the initial step in 

defining a critical group is to identify a set of 

receptor groups from which those individuals 

expected to receive the highest dose would be 

thus designated as the "critical group." Staff 

initially identified the following types of 

information that might provide informed 

judgments on what factors might have 

influenced the location and behavior of the 

current population in the greater Yucca 

Mountain area: 

Distribution and characteristics of existing 

communities in the Yucca Mountain area. 

" Regional geography and climate of 

southern Nevada. 

"* Surface and subsurface geohydrology. 

" Current sources of water supply for 

existing communities, focusing on water 

well properties and water use patterns. 

" Relative costs of drilling and pumping 

freshwater from shallow and deep wells. 

Following a review of the aforementioned, the 

staff prepared analyses for the following two 

population groups: 

1See ICRP (1977, 1985).

" Current residents in the area: Section 2.1 

presents information on lifestyles, habits, 

and locations of possible receptor groups 

in the Yucca Mountain area. 

" Other southwest communities: Section 2.2 

describes ground-water and land-use 

practices in other arid communities 

located in Arizona, Nevada, New Mexico, 

and Texas. 

Finally, Section 2.3 presents the definitions for 

the hypothetical receptor groups used in the 

analyses described elsewhere in this NUREG 

for calculations of individual dose. 

2.1 Current Lifestyles and Water-Use 

Practices in the Yucca Mountain 

Area 

2.1.1 Introduction 

Yucca Mountain has been proposed as a deep 

geologic repository for HLW because, in part, 

of the regional hydrogeologic regime. 

Moisture fluxes within the 700-meter thick 

unsaturated zone at Yucca Mountain are 

generally believed to be small, because of the 

region's arid climate and the low permeability 

of the tuff units comprising the mountain. Low 

moisture fluxes may reduce the rate of waste 

package corrosion, subsequent dissolution of 

the exposed waste form, and transport of 

radionuclides to the accessible environments. 

However, it is likely, given time, that some 

water will eventually contact the waste form 

and transport dissolved radionuclides out of 

the waste package. Radionuclides not sorbed 

by the zeolitic bedded tuffs that underlie the 

repository [e.g., technetium (Tc), iodine (I), 

neptunium (Np)], or diffused from 

fluid-conducting fractures into the rock matrix 

within welded tuff units, will enter the water 

table, which, based on current engineering 

designs, lies 250 to 300 meters below the 

repository. Hydrogeologic studies (Czarnecki 

and Waddell, 1984; TRW Environmental 

Safety Systems, Inc., 1995a) indicate that 

radionuclides that enter the saturated zone 

beneath Yucca Mountain will generally flow

2-1 NUREG- 1538
2 Considerations in the Definition of a Receptor Group 

(down-gradient) to the south-southeast into 

western Jackass Flats within the welded tuff 

aquifer and then south-southwest into the 

Amargosa Desert, where the water table lies 

within an alluvial aquifer. Accordingly, the 

staff believes that it is appropriate to examine 

the range of land- and water-use practices 

occurring in the Yucca Mountain area and in 

other, similar, environments, as part of the 

process of establishing reasonable bounds on 

estimates of land- and water-use practices of 

potential receptor groups. 

2.1.2 Approach 

Based on the staff's earlier work, the most 

likely exposure scenario for a specific 

population in the Yucca Mountain vicinity 

would be through the ground-water/ 

food-ingestion pathway [see Neel 

("Dose-Assessment Module") in Wescott et al. 

(1995)]. In the staff's view, based on 

information previously reviewed (DOE, 1986, 

1988, and 1996), definition of a receptor group 

for the Yucca Mountain region will likely be 

based in large part on the group's accessibility 

to ground water. Thus, consistent with the 

ICRP's guidance, the staff has undertaken a 

limited review of some types of site-specific 

information that, in whole or in part, might aid 

in definition of hypothetical receptor groups in 

the Yucca Mountain area. 

2.1.2.1 Local Population2 and Lifestyles 

Yucca Mountain is located in Nye County, 

which is in southern Nevada and is 

approximately 160 kilometers northwest of 

Las Vegas. The proposed repository site 

straddles the southeastern boundary of the 

Nellis Air Force Base (AFB) bombing range, 

and land administered by the Bureau of Land 

Management (BLM). NTS is generally located 

on lands managed by the U.S. Department of 

Energy (DOE) and BLM, whereas the 

extreme northern portions of NTS lie within, 

2

Population information for portions of Clark (Nevada-NV) 

and Inyo (California-CA) Counties was provided because 

these counties share the same ground-water basin and are 

believed to have lifestyles similar to those of residents of 

southern Nye County.

or abut, the Nellis AFB Range. Most if not all 

NTS property (3500 square kilometers) has 

been withdrawn from the public domain for 

about a half century. [NTS was created in 1951 

by the then Atomic Energy Commission 

(AEC) from the pre-existing Nellis AFB 

(established 1941) and the Desert National 

Wildlife Refuge (established 1935).] 

Physiographically, the area is considered to be 

part of the Basin and Range Province that is 

characterized by topographically-closed 

(intermontane) basins separated by ranges, 

hills, and mesas, with internal drainage into 

the basins. Within the Basin and Range, 

mountain ranges trend north-northeast; 

however the trend of these features is 

generally north-northwest in the Yucca 

Mountain area. The basin floors range in 

altitude from 600 to 1400 meters mean sea 

level (msl); the mountains reach altitudes of 

up to 3700 meters. Basins are filled with 

alluvial fan deposits and account for about 50 

percent of the exposed geology in the study 

area, whereas the mountain 

ranges-principally volcanic and carbonate 

sequences-account for the remainder 

(Thordarson and Robinson, 1971; p. 14). See 

Figure 2-1. Most major population centers in 

the study area occur in the basins (valleys). 

See Figure 2-2. 

NTS lies within the most arid part of Nevada, 

which is the most arid State in the 

conterminous U.S. (Geraghty et al., 1973; 

Plate 3)-the present climate in the NTS area 

is classified as a mid-latitude desert. 

Temperatures range from about 13 degrees 

Celsius (°C) in January to over 49 *C in July. 

The annual precipitation is less than 15 

centimeters-the precipitation is usually lower 

in the basins and higher in the ranges, hills, 

and mesas. The major agricultural crop in the 

area is alfalfa. Certain meteorologic/ 

geographic conditions (e.g., altitude, length of 

growing season, the "rain-shadow" effect, 

accessibility to water, soil types) control the 

extent and type of the agriculture in this 

portion of the State (see summary in 

Appendix A) and, therefore, have limited

NUREG-1538 2-2
2 Considerations in the Definition of a Receptor Group

1170 00' 1160 30' 1150 30'

370 30' 

370 00-' O 9 

Lathrop Wells. t Mercu *lIndianf 

Corner 4" iprngs Pi 

A?77argosa 2- Cactu s 

oRat * springs 

360 30' Stoe i Amargosa Crystal 

Valley eWe•ll. 

Devil's *Johnrti 

Hole 

Death Valley 

Junction 

nPahrump 

Pau Valley 

360 00' 

0 10 20 MILES i I I I n 

EXPLANATION 0 20 40 KILOMETERS 

•1 Valley-Fill, Basin-Type Deposits 

Undifferentiated Rocks 

(Includes Volcanic, Carbonate, and Clastic Rocks, and Granite) 

City, Town, or Community 

A Proposed Geologic Repository at Yucca Mountain 

Figure 2-1. Simplified geologic map of the Yucca Mountain area. Adopted from Laczniak et aL (1996, pp. 3, 9). 

Thordarson and Robinson (1971, p. 14) estimate that 50 percent of the rocks outcropping within 160 

kilometers of the Yucca Mountain area are valley-fill deposits and have the highest density of wells. The 

mesas and ranges are undifferentiated rocks (principally volcanic rocks and carbonates) which account 

for about 45 percent of the outcrop area within 160 kilometers of the site.

NUREG- 1538 2-3
2 Considerations in the Definition of a Receptor Group

380°

EXPLANATION

1150 

- 380

LINCOLN

-- - - - -

I I 

I I 

i I |Ii~i I 

I 

NYE 

Ii 

"UCCA 

". M DUNTAIN 

I BEATTY, A 

", RIYOLITE . 

E MERCURY.

E LATHROIý WELLS. 

* CORIqER 

AMARGOSA VAL*EY. CRYSTAL 

FURNACE• a \. 

CREEK IN%-" ". JOHNNIE 

RANCH 

VALT_'Y \. eFA 

JUNCTION \* 

fSHOSHONE 

I TECOPA 

1170 1160

I IINI I AN I~ 

INDIAN SPRINGS 

CACTUS-\ .% 

SPRINGS *,i

CLARK 

HRUMP 

LAS VEGAS 

-360 

1150 I 

0 10 20 MILES 

i 2 I 4 I 0 20 40 KILOMETERS

OUTLINE OF NEVADA TEST SITE 

.-.....--- OUTLINE OFTHE NELLIS AIR FORCE BOMBING AND GUNNERY RANGE 

(INCLUDES PORTIONS OF UNSPECIFIED WILDLIFE MANAGEMENT AREAS) 

OUTLINE OF THE DEATH VALLEY NATIONAL PARK (DVNP) 

THE ASH MEADOWS NATIONAL WILDLIFE REFUGE (INCLUDES DEVILS HOLE/DVNP) 

A PROPOSED GEOLOGIC REPOSITORY AT YUCCA MOUNTAIN 

0 CITY, TOWN, OR COMMUNITY

Figure 2-2. Index Map of the Yucca Mountain Area

NUREG- 1538

-I 

I. .

1170 

\TONOPAH

1160

ESMERALDA

370_

INYO 

STOVEPIP 

WELLS

36°-

NEVADA " 

TEST SITE

-- 370

2-4
agriculture to about 3 percent of the county, 3 

and thus the size of the local population 

engaged in this activity. In the past, the size of 

the population in the area has been most 

influenced by the level of activity at the three 

largest local employers-NTS, Nellis AFB 

(Clark County), and the Town of Beatty. 

However, over the past decade, tourism and 

leisure, together, have fast become a major 

industry in the State, particularly in the Las 

Vegas area (Clark County), and many local 

residents now earn a living in this growing 

area of the economy. 

Information on the population of Nye County 

(and adjacent Clark and Inyo Counties) is 

available principally from the Bureau of the 

Census (U.S. Department of Commerce, 

1994b). Nye County is largely rural (less than 

1 person per square kilometer) and occupies 

approximately 16 percent of the State (in 

terms of area). Based on population estimates 

listed in Table 2-la, Nye County currently 

accounts for about 1.5 percent of the State's 

population. (By contrast, Inyo County contains 

less than 0.1 percent of California's 

population.) Most of the population 

throughout the area is uneven and confined to 

discrete rural locations. Several locations have 

populations in excess of 1000 people; however, 

many communities are in fact collections of 

isolated farms and ranches, mining 

settlements, or commercial (trade) centers. 

Table 2-lb shows the principal communities 

in the study area. Approximately 13,000 

people live within 100 kilometers of the site. 

About 70 percent of the local population live 

in five major communities-the Amargosa 

Desert area, Beatty, Indian Springs, Lathrop 

Wells, and Pahrump. (This number increases 

to 90 percent when Tonopah is included.) A 

significant percentage of the local population 

may be transient-reportedly ranging from 20 

to 25 percent (DOE, 1986; p. 3-102). As 

discussed below, most if not all these 

3In 1987, the Bureau of Census reported that 136 farms were 

operating on about 150,000 hectares in Nye County (U.S. 

Department of Commerce, 1994b; p. 169). Over the past 10 

years, there has been about a 10 percent reduction (from about 

162,000 hectares in 1978) in the amount of county acreage 

dedicated to agriculture.

2 Considerations in the Definition of a Receptor Group 

communities acquire their water supply from 

the Death Valley ground-water system. 

2.1.2.2 History of Inhabitation of the 

Amargosa Desert Region and the Ash 

Meadows Area 4 

In the arid Southwest and elsewhere, Native 

Americans and settlers have historically 

chosen to establish camps, homesteads, 

outposts, and eventually permanent 

communities where there are reliable water 

supplies. In southern Nevada, for example, to 

the south of Yucca Mountain, the springs in 

Ash Meadows have provided fresh water for 

agriculture, ranching, and domestic purposes. 

The grasslands of Ash Meadows, framed by 

the leather-leaf ash trees, provided a lush 

oasis in the otherwise sere Mojave Desert. 

Prior to the great western migration, Southern 

Paiute and Shoshone Native Americans 

camped at the spring waters while foraging 

and hunting in the highlands of Ash Meadows 

(McCracken, 1992). Explorers, gold 

prospectors (California-bound forty-niners), 

and settlers subsequently sought out and used 

the Ash Meadows spring waters. Eventually, 

these pioneers cultivated alfalfa hay to support 

small cattle operations and grew various truck 

crops-including potatoes, tomatoes, melons, 

apples, pears, peaches, and more recently 

pistachios. 

In earlier days, baths were routinely taken by 

some in Devil's Hole and more than one 

pioneers' child enjoyed fishing in Ash 

Meadows springs for "minews" (or minnows 

that were probably one of several species of 

Desert pupfish) and feeding them to their cats 

(Op cit.). Nonetheless, before the early 1950s, 

there were never great numbers of permanent. 

inhabitants in either Ash Meadows or the 

Amargosa Desert area. 

Although local soils are not rich, on average, 

the presence of exploitable ground water 

resources, coupled with a long, hot growing 

4

For editorial reasons, this discussion, which originally appeared 

in Witmeyer etal. (1996), and now repeated in Section 2.2, has 

been placed in this section of the NUREG. For additional 

discussion of the history of this area, the reader is referred to 

Appendices A and B.

2-5 NUREG-1538
2 Considerations in the Definition of a Receptor Group

Table 2-1. Comparative 1990 Population Information on California, Nevada, 

and the Yucca Mountain Area 

a. Comparison of Populations in Clark and Nye Counties (Nevada) 

and Inyo County (California) 

1990 Population 

Population Area Density 

County/State Estimation a (km 2) (per km 2) 

Clark County 741,459 20,489 36 

Nye County 17,781 47,000 < 1 

(All Nevada) (1,201,833) (284,396) (4) 

Inyo County 18,281 26,397 < 1 

(All California) (29,760,021) (403,968) (74) 

b. Population Centers in the Yucca Mountain Area 

1990 Distance From 

Population Yucca Mountain 

Community Estimation a (kin) 

Amargosa Desert area (NV) b 2300 c 20-70 

Ash Meadows (NV) b 55 

Beatty (NV) 1623 29 

Cactus Springs (NV) d 72 

Crystal (NV) d 41 

Death Valley Junction (CA) 15 e 60 

Furnace Creek Ranch (CA) 800 f 56 

Indian Springs (NV) 1164 g 75 

Johnnie (NV) d 58 

Lathrop Wells Corner (NV) 30 b 23 

Mercury (NV) 3 0 0 c, h 45 

Pahrump (NV) 7424 82 

Rhyolite (NV) d 35 

Stovepipe Wells (CA) d 67 

Shoshone (CA) 140 f 112 

Tonopah (NV) 3616 145 

Tecopa (NV) 200 f 125

1u99 Bureau or Ctensus estimates ()u.6. Department or Commerce, 194a) unless otherwise noted. 

Includes the Communities of Ash Meadows, Amargosa Farms, Amargosa Valley, and Lathrop Wells. 

DOE (1986, p. 3-75) estimate. 

Not believed to be occupied by permanent residents. 

U.S. Geological Survey (USGS) (1970) citing Bureau of Census estimates. 

Semi-permanent residents. Population increases by an additional 2000 during the tourist season. DOE (1988, p. 

3-120) estimates.

g Could be as high as 8400 if Indian Springs AFB personnel are included. 

h About 3000 personnel are currently employed at NTS (see TRW Environmental Safety Systems, Inc., 1995a; p. 4-3).

NUREG-1538

b 

C 

d 

e 

f

2-6
season, ultimately allowed permanent 

inhabitation of this otherwise inhospitable 

portion of Nevada. Farming has generally 

produced a wide variety of fruits, vegetables, 

and alfalfa hay (see Table A-1 in Appendix 

A), but not in quantities or diversity to be 

judged truly sufficient. Consequently, 

permanent inhabitation has been tenuous in 

the region. Mineral production (specialized 

clays and borax) and rail transportation (the 

Tonopah and Tidewater, and the Las Vegas and 

Tonopah railroads-see Appendix B) were 

early catalysts to the continuing human 

presence before establishment of NTS and 

Nellis AFB. Numerous jobs and local 

development of habitations and associated 

amenities that NTS generated in the early 

1950s have continued through the 1990s. 

NTS employment opportunities encouraged 

an increase in the population of the Amargosa 

Desert area. The community of Amargosa 

Valley5 (as specified by tax boundaries 

established by the Nye County Board of 

Commissioners), includes about 1300 square 

kilometers with a population of 909 for an 

average of about 0.7 persons/square kilometer. 

The low average population per square 

kilometer in Amargosa Valley is because most 

of the tax district is uninhabited except for the 

area of Amargosa Valley known as Amargosa 

Farms. There are approximately 60 industrial/ 

commercial establishments supported by 

about 350 housing units averaging 2.6 

individuals per unit (TRW Environmental 

Safety Systems, Inc., 1995b). 

In 1994, housing units in the community of 

Amargosa Valley increased from 352 to 

365-attributed to single-family units. This 

represented a 3.7 percent increase in annual 

growth, with an attendant population growth 

of about 33 (assuming 2.5 individuals per new 

house). Additionally, Amargosa Valley now 

5Elsewhere in this NUREG, the terms "Amargosa Valley" and 

"Amargosa Farms area" were used to refer collectively to the 

set of communities of Ash Meadows, Amargosa Farms, 

Amargosa Valley, and Lathrop Wells, which all generally lie 

within the Amargosa Valley hydrographic basin. However, in 

this section of the NUREG, the term "Amargosa Farms" refers 

to the southern portion of the Amargosa Valley bounded 

approximately by the California-Nevada border, Nevada State 

Highway 373, and Amargosa Farm Road because residences 

and farming activities in the valley are concentrated in this 

triangular parcel of land.

2 Considerations in the Definition of a Receptor Group 

has a school with about 150 children enrolled 

and present on a normal school day. 

There is a relatively new dairy at the southern 

end of Amargosa Desert Valley. Locally grown 

alfalfa hay provides about 10 percent of the 

fodder for the dairy herd. In 1994, 2376 dairy 

cows were reported at the Amargosa Valley 

operation and over 2.4 square kilometers 

nearby were planted in alfalfa, yielding about 

2500 metric tons of alfalfa hay in 1994 (TRW 

Environmental Safety Systems, Inc., 1995a; pp. 

A-21-A-22). 

Currently, the nearest private water well to 

the proposed geologic repository at Yucca 

Mountain can be found at the Cind-R-Lite 

mining facility, at the Lathrop Wells volcanic 

cinder cone, about 10 kilometers north of the 

Amargosa Farms area. Recent information 

suggests that only two employees work at the 

facility (TRW Environmental Safety Systems, 

Inc., 1995a; p. 4-4), and it is not clear 

whether water is used solely for processing 

cinders or for human consumption as well. 

The next nearest non-NTS commercial activity 

where ground water is pumped is at the 

junction of U.S. Highway 95 and Nevada State 

Highway 373 at the site known as Lathrop 

Wells. At least four individuals reside there 

continuously and others live there during the 

weekdays. Additionally, there is a 91-unit 

recreational vehicle park that was completed 

in late 1995. At present, the Lathrop Wells 

location does not include families with 

children. The nearest area with a 

concentration of families with children is 

found about 32 kilometers to the south of 

Yucca Mountain near the school and 

community center at Amargosa Farm Road 

(known as Amargosa Farms-Op cit., p. 3-3). 

Today, Ash Meadows has been classified as a 

National Wildlife Refuge and commercial 

development is prohibited. In 1980, Preferred 

Equities of Pahrump (Nevada) purchased 

about 68.9 square kilometers in Ash Meadows 

and moved forward with plans to subdivide its 

holdings there, anticipating eventual 

development of 33,600 residential lots and a 

community population of 50,000 (McCracken, 

1992; p. 96). On learning of the plans, 

environmental activists took legal action to 

2-7 NUREG-1538
2 Considerations in the Definition of a Receptor Group 

block further development. Eventually, the 

Nature Conservancy purchased 51 square kilometers of land and water rights owned by Preferred Equities in Ash Meadows. In 1984, 

the U.S. Fish and Wildlife Service purchased 

the Conservancy's Ash Meadows interests and 

established the Ash Meadows National 

Wildlife Refuge, to conserve threatened and 

endangered species found there, to promote 

all native wildlife, and to provide the public 

with recreational opportunities that would not 

threaten the wildlife. (It should be stressed 

that current knowledge of the regional ground-water flow system in the Yucca 

Mountain area suggests that water discharging 

at Ash Meadows from the Paleozoic carbonate 

aquifer is part of the Ash Meadows 

ground-water basin, whereas ground water, 

flowing beneath Yucca Mountain, being 

pumped in the Amargosa Farms region, is 

from the Alkali Flat-Furnace Creek Ranch 

ground-water basin. Hence, it is unlikely that 

radionuclides that reach the saturated zone 

beneath the proposed repository would be 

discharged at Ash Meadows.) 

2.1.2.3 Ground-Water Availability6 

There are no perennial or intermittent 

streams in or near the immediate Yucca 

Mountain area; however, short reaches of the 

Amargosa River near Beatty (NV) and south 

of Amargosa Farms to Shoshone (CA) are 

perennial to intermittent. The only major 

occurrences of surface water are the 

concentrations of springs found in Oasis 

Valley, Ash Meadows, the Amargosa Desert, 

Death Valley, and NTS. 

There are many ephemeral stream channels 

present in the area, including those associated 

with the drainage systems of Fortymile Wash 

and the Amargosa River. Runoff occurs 

irregularly, in response to both summer 

convective storms and winter frontal storms, 

with stream discharge rates varying greatly in 

6

Numerous investigations of the geohydrology of the Yucca 

Mountain region have been conducted. The results of these 

investigations have been summarized in various site 

characterization studies. To name a few, these studies include 

DOE (1986, 1988); Carr and Yount (1988); Bedingeretal. (1989); the National Research Council (1992); Wittmeyer and 

Turner (1995); and Laczniak etal. (1996).

magnitude. For example, the estimated 

discharge for the 100-year flood along 

Fortymile Wash, just below its confluence with 

Drillhole Wash, is 340 cubic meters/second 

(DOE, 1988; p. 3-21). Data on rainfall, runoff, and evaporation are inadequate to 

determine rainfall-runoff-recharge 

relationships (Op cit., p. 3-9). 

Because there are no substantial sources of 

surface water supply, most water used in the 

area is ground water. Yucca Mountain lies 

within the Death Valley ground-water flow 

system, which is composed of a series of 

topographically-closed basins that are 

hydraulically connected at depth by the 

highly-conductive Paleozoic carbonate aquifer. 

Tertiary- and Quaternary-age alluvium and 

playa lake deposits fill the basins, and locally 

overlie Tertiary-age volcanic rocks or 

Paleozoic-age carbonate, clastic, and 

metamorphic rocks. The flow system 

originates primarily from the infiltration of 

precipitation in the mountainous areas 

through the alluvium to fracture and joint sets 

or solution cavities in the Paleozoic carbonate 

rocks that underlie the basins and intervening 

highlands (Figure 2-3). Ground-water flow is 

to the south and west from the respective 

basins toward the regional topographic 

depression at Death Valley or smaller 

depressions in Sarcobatus Flats, Oasis Valley, Ash Meadows, and the Amargosa Desert 

(Thomas, 1964; p. 308). 

Although about 30 individual hydrographic 

basins comprise the Death Valley 

ground-water system, the system itself can 

generally be divided into three major 

ground-water basins: (a) Oasis Valley; (b) Ash 

Meadows; and (c) Alkali Flat-Furnace Creek 

Ranch (see Figure 2-4). The Oasis Valley 

ground-water basin discharges in a series of 

springs that are forced to the surface by a 

region of low-permeability rocks 

down-gradient of Beatty. The Ash Meadows 

ground-water basin comprises the eastern half 

of the Death Valley System and discharges 

along a 14-kilometer spring line formed by the 

juxtaposition of low-permeability Tertiary-age 

lake deposits on the western downthrown side 

of a north-trending normal fault, against the

NUREG-1538 2-8
2 Considerations in the Definition of a Receptor Group

Basin-fill deposits Low-permeability bedrock 

Playa that receives ground-water , Permeable bedrock

S> Direction of ground-water 

movement 

SFault-Arrows show relative 

vertical movement

discharge 

Dry playa 

Phreatophytes-Plants with roots 

that extend to water table

[Modified from Eakin and others, 19761

Description

Single, undrained Basin has no surface flow across boundary; all ground-water discharge is ultimately by evapotranspiratlon.Typified 

closed basin by valleys with wet playas and phreatophyte stands in shallow ground-water areas. Ground-water gradient is toward 

center, or low part, of valley. 

Terminal sink Basin has surface and (or) subsurface inflow across boundaries. All ground-water discharge ultimately is by 

basin evapotranspiration. Most sinks have playas that are large in proportion to the size of the area. Valley can contain 

a large volume of saline ground water. 

Drained, closed Basin has no surfaceflow acrossboundary. Almost all ground-water discharge is by subsurface outflow; the deep water 

basin table prevents evapotranspiration. 

Partly drained, Basin has no surface inflow or outflow. Valley has moist playa and stand of phreatophytes. Area of ground-water 

closed basin discharge can be small in comparison to undrained basins of similar size. Ground-water gradients may indicate 

subsurface outflow, if wells are strategically located. 

Connected basin Basin system has surface and (or) subsurface outflow and (or) inflow that links several individual basins. Ground water 

flow system discharges to playas and (or) springs or streams. Surface inflow and outflow is by ephemeral and perennial 

streams. 

Figure 2-3. Simplified geologic cross-section of the basins within the Yucca Mountain area. Cross-section is 

representative of the relationships depicted by the simplified geologic map shown in Figure 2-1. Basin 

types are differentiated on the basis of differences in ground-water flow. Taken from Planert and Willaims 

(1995, p. B7) citing Eakin et aL (1976).

NUREG-1538

Basin type

M

2-9
2 Considerations in the Definition of a Receptor Group

NTONOPAH

ESMERALDA 

RHVOLITE BEATTE 

STOVEPIPE \ 

WELLS . * \.AM) 

FURNACE 

CREEK 

RANCH INYO 

Ju 

0 10 20 MILES 

i 20 4 KLE 0 20 40 KILOMETERS

DEATH VALLEY 

GROUND-WATER 

SYSTEM

EXPLANATION 

OUTLINE OF DEATH VALLEY GROUND-WATER SYSTEM (DASHED WERE INFERRED): ® OASIS VALLEY SUBBASIN ® ALKALI FLAT-FURNACE CREEK RANCH SUBBASIN 

® ASH MEADOWS SUBBASIN 

OUTLINE OF PAHRUMP VALLEY HYDROGRAPHIC AREA DESIGNATED BY RUSH (1968)

W• GENERAL DIRECTION OF GROUND-WATER FLOW 

Figure 2-4. Ground-water sub-basins in the Yucca Mountain area. Taken from Rush (1968) and DOE (1998, p. 3-3), 

citing Rush (1970); Blankennagel and Weir (1973); Winograd and Thordarson (1975); Dudley and Larson 

(1976); Waddell (1982); and Waddell et aL (1984).

NUREG- 1538 2-10
2 Considerations in the Definition of a Receptor Group

high-permeability Paleozoic carbonate aquifer. 

Yucca Mountain lies within the Alkali 

Flat-Furnace Creek Ranch basin, which is 

recharged in the uplands of Pahute Mesa and 

discharges by evapotranspiration (ET) at 

Franklin Lake Playa (aka Alkali Flat) and 

through a series of springs in the Furnace 

Creek Ranch area of Death Valley. In general, 

the regional ground-water flow patterns do not 

coincide with topographic basins. 

The regional hydrostratigraphic units within 

the Alkali Flat-Furnace Creek Ranch basin 

are: (a) the valley fill aquifer; (b) volcanic rock 

aquifers and aquitards; (c) upper carbonate 

aquifer, which occurs primarily in the Yucca 

Flat area; (d) upper clastic aquitard; (e) lower 

carbonate aquifer; and (f) the lower clastic 

aquitard (see Winograd and Thordarson, 

1975). Yucca Mountain and the Pahute Mesa 

uplands consist of layered volcanic rocks of 

Tertiary age. In the vicinity of Yucca 

Mountain, the water table occurs within these 

volcanic rocks, including perched ground 

water. Deeper within the volcanic rocks, flow 

conditions may be confined or semi-confined. 

A much deeper, confined system exists within 

the lower carbonate aquifer that underlies the 

volcanic rocks. The uppermost aquifer 

underlying the proposed geologic repository 

footprint is principally a tuff aquifer and 

occurs at great depth [e.g., 380 to 860 

meters-see Wittwer et al. (1995, p. 24)]. This 

aquifer becomes much shallower to the south 

where it is believed to laterally intersect 

alluvial fan basins. In the Amargosa Desert 

and Pahrump Valley areas, the depth to the 

water table in the alluvial aquifer is reported 

to be no more than 30 meters (USGS, 1977; 

pp. 61-62), although local variations in water 

table depth do exist. Overall, the relative 

elevation of the ground-water surface in the 

area is influenced by geologic structure and 

stratigraphy, individual aquifer properties, 

proximity to recharge and discharge areas, and 

the extent of previous and current 

development (i.e., pumping). 

Schoff and Moore (1964, p. 45) noted that the 

ground water from NTS is a relatively small 

contributor to the total discharge in the 

Amargosa Desert-no more than 71⁄2 percent

2-

[citing the water chemistry work of Eakin et al. 

(1963, pp. 2-24)]. The remaining ground 

water used in the Amargosa Desert area 

apparently comes from a combination of 

interbasin flows as well as upwelling from the 

deep Paleozoic carbonate aquifer, as 

suggested by geochemical signatures and 

observed vertical gradients in the region (see 

Winograd and Thordarson, 1975). There also 

may be areas of local recharge from mountain 

runoff into the alluvial-fan deposits, as occurs 

in other alluvial basins. 

Regionally, the alluvial fan and lower 

carbonate aquifers are the most important 

sources of ground water; the welded tuff 

aquifer is only locally important in Jackass 

Flats. Local and regional ground-water flow is 

controlled largely by the major structural 

features of the site as well as the hydro

stratigraphy. Fracture flow is predominant 

within the carbonate sedimentary rocks and 

welded tuff and lava flow volcanic sequences. 

Matrix flow is assumed to be predominant 

within the bedded, non-welded tuff and 

alluvial fan aquifers. The rates of flow can be 

quite variable, depending on the types of 

aquifers present, the degree of fracturing and 

secondary solution features within the 

carbonate aquifers, and the hydraulic 

gradients present in a given area. For example, 

the quantity of water moving through the 

Paleozoic carbonate aquifer beneath Yucca 

Flat is estimated at about 0.013 cubic meters/ 

second, whereas the quantity of water moving 

beneath Pahute Mesa (volcanic aquifer) is 

estimated at about 0.31 cubic meters/second 

(Corchary and Dinwiddie, 1974, p. 25). 

Estimates for the valley-fill aquifers in the 

area can be twice as much (see Eakin et al.; 

1976, pp. G-32-G-33). 

Most recharge to the Alkali Flat-Furnace 

Creek Ranch subbasin is thought to occur in 

areas of higher elevation and correspondingly 

higher precipitation, located north of Yucca 

Mountain. A major proportion of recharge 

occurs in the winter and spring because of 

lower temperatures and ET rates. Recharge 

rates in lower elevation areas of the Amargosa 

Desert are unknown but are estimated to be 

very small. Czarnecki (1985) estimated the 

average areal recharge to be 0.7 millimeters/ 

11 NUREG- 1538
2 Considerations in the Definition of a Receptor Group 

year for a zone of precipitation that includes 

Yucca Mountain. 7 

There is a general lack of hydraulic property 

data in the region, although some field data 

exist in the vicinity of Yucca Mountain, NTS, 

and elsewhere. Available hydraulic property 

data are summarized in Bedinger et al. (1985, 

Table 1; p. A18). Regionally, Winograd and 

Thordarson (1975) have estimated average 

flow rates to range from 2 to 201 meters/year. 

The most transmissive unit in the region is the 

lower carbonate aquifer, which has transmiss

ivity values as large as 10,000 square meters/ 

day. All other aquifers have transmissivities 

that are more than an order of magnitude 

smaller (DOE, 1988; p. 3-68). Transmissivity 

for the valley-fill aquifer ranges from about 10 

to 400 square meters/day. 

2.1.2.4 Water Supply and Consumption 

One factor that can aid in characterizing 

receptor groups is the manner in which they 

acquire their water supply. Almost all water 

used in Nye County is provided through either 

public or private wells, although some springs 

meet (limited) local needs. Some smaller 

communities, individual residences, and 

businesses rely entirely on private (commer

cial) water suppliers (see DOE, 1988; pp. 

3-118-3-119). Only a few communities 

(Beatty, Mercury, Tonopah, and portions of 

Pahrump) have centralized public water 

systems (DOE, 1986; p. 3-75). 

DOE previously published information on 

water consumption in the Yucca Mountain 

7

Other theories about regional flow in the subbasin were presented and documented by Czarnecki (1987, 1989) and 

Czarnecki and wilson (1989). The previously accepted conceptual model of the regional ground-water system assumed that flow beneath the central Amargosa Desert 

ultimately discharges from two major areas: Furnace Creek 

Ranch and Franklin Lake Playa. In the baseline model of 

Czarnecki (1985), zero recharge was assumed to be occurring over most of the subregion south of Yucca Mountain. New 

data were obtained by Czarnecki (1989) which led to an alternate conceptual model of subregional flow. Potentiometric 

data were obtained from mining property boreholes in the 

Greenwater Range (between Death Valley and the Amargosa Desert). These data show a water-table altitude in that area of 

about 875 meters, providing evidence of significant ground

water recharge and the probable presence of a ground-water flow divide beneath this range. The data suggest that flow 

divides beneath the Greenwater and Funeral Mountains may 

isolate the water table aquifer in the Amargosa Desert area from the flow system in Death Valley.

area in the following: the 1986 Environmental 

Assessment; the 1988 Site Characterization Plan 

(SCP); and, most recently, in the 1996 Final 

Environmental Impact Statement (EIS) for the 

Nevada Test Site and Off-Site Locations in the 

State of Nevada (see, respectively, DOE, 1986, 

1988, and 1996). Using limited consumption 

data for only 5216 Nye County residents, DOE 

estimated that the per-capita consumption of 

water county-wide was 2473 cubic meters/day 

per 1000 people (DOE, 1986; p. 3-74).8 

Extrapolating this figure to the 1990 

population produces a consumption rate esti

mate of approximately 43,972 cubic meters/ 

day county-wide. DOE also provided informa

tion on estimated water consumption for 

selected communities in the Yucca Mountain 

area (see Table 2-2). This information was 

first reported in the Environmental Assessment 

(DOE, 1986; p. 3-75) and subsequently 

updated in the SCP (DOE, 1988; p. 3-135) 

and the Final NTS EIS (DOE, 1996; p. 4-132).

8Estimate based on consumption rate of 12,900 cubic 

meters/day per 5216 persons.

NUREG-1538

Table 2-2. Estimated Level of Water 

Consumption for Selected Communities in the 

Yucca Mountain Area. From DOE 

(1988, p. 3-315).

Estimated Water 

Consumption 

Community (m 3

/day) 

Amargosa Valley 6000 

Ash Meadowsa 150 

Beatty 3000 

Crystala 230 

Death Valley Junction 64 

Indian Springsa 3000 

Indian Springs AFBa 1500 

Johnniea 26 

Mercurya 1100 

Pahrumpa 9400 

Rhyolite 11

a Not within Alkali Flat-Furnace Creek Ranch ground

water basin.

2-12
In general, water quality throughout the area 

is adequate for personal consumption, stock 

and agricultural purposes except for areas in 

the southern Amargosa Desert where 

evaporites from lacustrine deposits cause the 

ground water to have total dissolved solids 

concentrations in excess of 10,000 parts/ 

million (Winograd and Thordarson, 1974). As 

can be seen in Figure 2-5 (see end plate), 

there are small clusters of wells near the 

communities of Amargosa Valley and Death 

Valley Junction, located at highway crossroads. 

Until recently, some ground water was used 

for processing specialty clays and zeolites 

mined from lacustrine deposits in southern 

Amargosa Desert and in Ash Meadows. 

Agricultural development is currently confined 

to the southern Amargosa Desert area where 

depths to water range from 10 to 40 meters 

(Kilroy, 1991) and the topography is suitable 

for center pivot irrigation. 

It should be noted that since 1973, irrigation 

pumping in the Ash Meadows area has been 

controlled to prevent the lowering of ground

water levels in Devils Hole and subsequent 

threats to the long-term survival of a unique 

species of desert pupfish [Cyprionodon 

diabolis-see Dudley and Larson (1976)]. As 

noted earlier, in 1984, the Ash Meadows 

National Wildlife Refuge was established to 

protect flora and fauna unique to the area. 

The U.S. Supreme Court has subsequently 

ruled that maintenance of water levels in 

Devils Hole has precedence over other water 

uses in the area (Also see Appendix A). 

2.1.2.5 Water Well Surveys 9 

The arid, desert-like climate in the Yucca 

Mountain area results in few sources of 

surface water supply although there have 

proven to be extensive ground-water resources 

in this portion of the Basin and Range. To 

better understand how the ground-water 

resources have been used in the past, the staff 

9

Because of time and resource limitations, not all available sources of ground-water information were examined. For 

example, the Nevada State Engineer's Office of Division of 

Water Resources (in Carson City) has extensive information on current ground-water availability and use, including 

pumping inventories by hydrographic basin. Some of this 

information was studied for the purposes of the analyses 

described in Sections 2.2 and 3.3.

2 Considerations in the Definition of a Receptor Group 

reviewed previously published information on 

water well surveys. 

The USGS performed one of the first 

comprehensive surveys of ground-water 

availability (as well as ground-water use) in 

the Yucca Mountain area in 1971 (see 

Thordarson and Robinson, 1971). To assist 

AEC officials in assessing damage claims to 

water wells and springs possibly resulting from 

nuclear explosions, the USGS reviewed 

published and unpublished geologic and 

ground-water data within a 161-kilometer 

radius of NTS. The census covered 81,367 

square kilometers in portions of six counties in 

southern California and Nevada. The types of 

information collected for the census included 

aquifer type, water-table depth, yield, and 

end-use. This census identified 6032 water 

wells (and 754 springs and seeps). Most of the 

wells (98 percent) were located in alluvial fan 

deposits. In addition to being the most 

extensive surficial geology in the region, 

alluvial basins are usually the most accessible 

topographically (occupying physiographic low 

terrain) and have the highest water-yielding 

capacity of the aquifer types identified [see 

Bedinger et al. (1989; Figure 1, p. Al)]. 

To a lesser extent, water wells are also 

reported in carbonate and volcanic rocks, but 

these aquifers generally tend to have lower 

water yields in the study area. Almost 54 

percent of the wells were reported to be less 

than 60-meters deep; almost 84 percent were 

reported at depths less than 150-meters deep. 

Of the wells identified, 60 percent were 

reported to yield at least 380 liters/minute. In 

terms of end use, most of the wells in the study 

area were reported to be used for domestic 

purposes (56 percent); followed by irrigation 

(17 percent); municipal and commercial (6 

percent each); and stock supply (2 percent). 

(Nine percent of the wells in the 1971 study 

area were reportedly no longer in use.) In a 

1988 survey of wells in the Amargosa Desert 

area, for purposes of characterization of the 

Yucca Mountain site, DOE reported that 50 

percent of the wells in the area (199 of 397) 

were used for domestic use and 41 percent 

(164 of 397) were used for agricultural 

irrigation (see DOE, 1988; p. 3-119).

2-13 NUREG- 1538
2 Considerations in the Definition of a Receptor Group 

The greatest depths to water in wells currently 

being pumped to supply agricultural or 

domestic needs in the immediate Yucca 

Mountain region occur in the community of 

Amargosa Valley where depths to water range 

from 100 to 120 meters. Well J-13 in Jackass 

Flats on NTS pumps water from a depth of 

280 meters to supply drilling and tunneling 

operations for the Yucca Mountain Project; 

however, this may be considered an except

ional practice for this region. Depth to water 

increases monotonically from approximately 

100 to more than 300 meters along a trajectory 

extending from the community of Amargosa 

Valley in the south and terminating in the 

north approximately 10 kilometers from the 

perimeter drift of the proposed Yucca 

Mountain repository (see Figure 2-5). 

2.1.2.6 Water Well Properties 

There are about 170 boreholes at NTS 

(Fernandez, 1991)10 but only a few are 

reported to be pumped periodically to obtain 

hydrologic information. In its 1988 SCP, DOE 

provided information on depth to the water 

table and production statistics for selected 

wells at NTS (Table 2-3). Some of these data 

were updated in 1993 (Table 2-4). Two of the 

wells (J-12 and J-13) are approximately 7 

kilometers southeast of the Yucca Mountain 

site and have been used regularly during site 

characterization and provide water to NTS 

facilities (see Tables 2-5 and 2-6). Czarnecki 

(1992) simulated water level declines in the 

Yucca Mountain area in response to future 

ground-water withdrawals from these wells. 

This work is important because it represents a 

10-year forecast of how future ground-water 

levels will be affected by human activities (as 

discussed later in Section 3.2 of this NUREG). 

This work also supported a DOE request to 

the State of Nevada for a permit to pump up 

to 5.7 x 10-3 cubic meters/second from J-13, 

for the purposes of characterization work. 

Eight different pumping scenarios were 

analyzed, using a transient version of 

1

0Fernandez, J.A., "Overall Approach and Performance 

Calculations [Strategies to Seal Shafts, Ramps, and 

Underground Openings]," Unpublished Presentation by the 

Sandia National Laboratories to the U.S. Technical Review 

Board/Panel on Structural Geology and Geoengineering, 

Seattle, Washington, November 12-13, 1991.

Czarnecki's 1985 model and various 

combinations of withdrawal rates. Four of the 

scenarios involved pumping from a single well, 

whereas both wells were pumped in the other 

four scenarios. For each withdrawal rate, 

simulations were made with aquifer-specific 

yields11 set at 0.001, 0.005, and 0.01. Czarnecki 

(1992) considered a specific yield of 0.01 to be 

the value that is best supported by available 

data. The most extreme scenario represented 

the combined maximum pumping capacities 

from both wells (8.771 x 10- cubic meters/ 

second) and assumed an aquifer specific yield 

of 0.001. This resulted in a drawdown of over 

12.2 meters at both wells J-12 and J-13 at 

the end of the 10-year period. Under the same 

pumping conditions, and assuming a specific 

yield of 0.01, simulated drawdowns at J-13 

and J-12 were about 3.0 meters and varied 

from 1.8 to 2.4 meters at the Yucca Mountain 

site. The simulated drawdown for the 

anticipated withdrawal rate of 5.7 x 10-3 cubic 

meters/second from well J-13 (based on a 

10-year pumping period and a specific yield of 

0.01) was 0.29 meters at J-13. The drawdown 

at the Yucca Mountain site would be about 0.2 

meters. If one of the scenarios analyzed by 

Czarnecki (1992) is representative of actual 

ground-water withdrawals during site charac

terization, it can be used to test how well the 

regional model represents the flow system. 

2.1.2.7 Ground-Water Utilization (End-Use) 

Another criterion that might be used to aid in 

the identification of potential receptor groups 

in the Yucca Mountain area is the manner in 

which ground-water supplies are used locally. 

Although most of the wells in the study area 

are used for municipal, domestic, or ranching 

(livestock) needs, by volume (end-use), most 

ground water in southern Nevada (excluding 

Las Vegas) is applied chiefly to agricultural 

irrigation and mining (Solley et al., 1993). 

Since the early 1980s, the State Engineer has 

maintained ground-water pumping records 

that also reveal how ground water is being 

ttVolume of water an unconfined aquifer releases from storage 

per unit surface area of aquifer per unit decline in the water 

table.

NUREG-1538 2-14
2 Considerations in the Definition of a Receptor Group

Pumping Total Withdrawals 

Aquifer Rate In 1985 

Well Type (L/min) (m 3) 

Alkali Flat-Furnace Creek Subbasin 

8 Tuff 1500 241,000 

19 Tuff 1360 433,200 

U20a-2 Tuff 1290 76,400 

Ash Meadows Subbasin 

UE-it Not reported 1020 21,200 

2 Carbonate (?) 625 78,100 

A Alluvial 610 148,300 

5b alluvial 910 231,300 

5c Alluvial 1230 231,300 

UE-5c Not reported 1320 16,400 

C Carbonate (?) 1020 99,000 

C-1 Carbonate (?) 1060 99,000 

4 Not reported 2460 158,300 

UE-15d Not reported 1020 not reported 

UE-16d Tuff 734 59,100 

Army Well - 1 Carbonate (?) 2010 192,900

NUREG- 1538

Table 2-3. Water Well Production for Selected NTS Wells. 

Adopted from DOE (1988, pp. 3-120, 3-124).

2-15
2 Considerations in the Definition of a Receptor Group

NUREG- 1538

Table 2-4. Summary of 1993 Water Well and Discharge Information for NTS. Adopted from 

DOE (1996, p. 4-133), unless otherwise noted. 

Well Static Water Pump Depth Annual 

Aquifer Depth Level Setting Yield Pumping a 

Well Type (M) (M) (M) (m 3/min) (106 M3

) 

Army Well 1 Carbonate 593.14 210.31 289.86 2.01 0.4178 

Well 5C Alluvial 361.80 211.23 238.96 1.23 0.2393 

Well 5B Alluvial 274.32 208.48 --- 1.02 0.1126 

Well 4 Alluvial 450.80 286.82 387.40 2.46 0.2856 

Well 4a Volcanic ............- 0.4172 

Well C Carbonate 518.46 470.61 473.35 1.02 0.2390 

Well C1 Carbonate 520.29 471.83 484.94 1.06 0.0357 

Well 8 Volcanic 1673.35 327.05 374.29 1.51 0.1185 

UE-16D Carbonate 914.40 230.12 330.10 0.73 0.1813 

UE- 15d b, c Volcanic 5940 203.6 505.7 --- 0 

J-12 Volcanic 347.17 225.25 250.55 3.09 0.0945 

J-13 Volcanic 1063.14 283.16 350.82 2.57 0.1584 

UE-5c d Alluvial ... 0.0278 

UE-19c b Volcanic 0.0269 

UE-20a d Volcanic 0.1058 

TOTAL 2.4606

a Well yields calculated from controlled pump tests are typically within one order of magnitude of well drillers' estimates. 

b Well no longer in use. 

c Claassen (1973, pp. 104-105). 

d Construction well water.

2-16
2 Considerations in the Definition of a Receptor Group

Table 2-5. Summary of Pumping Test Data for Wells J-12 and J-13 (in the Topopah Spring 

Member of the Paintbrush Tuff), during the Period 1960-69. 

Adopted from DOE (1988, p. 3-226) citing Claassen (1973). 

Distance From Depth to Aquifer Specific Capacity Pumping Rate 

Well Site (km) (M) [(m3/d)/m] (m 3/d) 

J-12 12 180-347.2 220-2800 2000-4500 

J -13 8 207.3-449.6 310-540 2800-3800 

Table 2-6. Water Production from Wells J-12 and J-13, during the Period 1983-85. 

Adopted from DOE (1988, p. 3-226) citing Witherill (1986). 

Production 

Well (M 3

) 

1983 1984 1985 

J-12 96,460 98,700 94,900 

J-13 159,500 152,700 143,100

2-17 NUREG- 1538
2 Considerations in the Definition of a Receptor Group 

used within the respective hydrographic 

basins. In the Amargosa Desert and Pahrump 

Valley hydrographic basins, the pumping 

inventories are shown in Table 2-7. For 

example, during the 1985 -95 period, this 

table shows that in the Amargosa Desert 

hydrographic basin, irrigation averaged about 

78 percent of the pumping inventory, followed 

by mining/industrial usage-about 21 percent. 

Domestic/quasi-municipal usage has 

accounted for little more than 1 percent of the 

ground-water pumping during this period. By 

contrast, in the Pahrump Valley hydrographic 

basin, irrigation averaged only about 69 

percent of the pumping inventory for the same 

period of time, followed by mixed 

public/commercial usage by Central Nevada 

[Water] Utilities-about 22 percent. 

Domestic/quasi-municipal usage from other 

sources of ground-water supply has accounted 

for about 10 percent of the pumping inventory 

during this period. A little less than 4 percent 

of the ground-water pumping inventory in the 

Pahrump Valley hydrographic basin during this 

period can be attributed to the Champion Golf 

Course. 

2.1.2.8 Well Costs 

The costs of producing (e.g., extracting) the 

water could provide an additional criterion for 

distinguishing between potential receptor 

groups in the Yucca Mountain area. Inasmuch 

as the water table below NTS usually occurs at 

great depth (see DOE, 1988; pp. 3-157 

3-159), extraction and operating costs could 

be prohibitive for certain scenarios, thereby 

creating disincentives to agricultural, 

residential, or even commercial development 

at particular locations within the site. Because 

of its accessibility (e.g., depth), there is an 

economic argument that the costs of drilling, 

developing, and pumping ground water from 

the (deep) carbonate ground-water resource 

within (below) the site are higher in 

comparison with the costs of using (shallow) 

alluvial ground-water resources found beyond 

NTS boundaries. For example, pumping costs 

will tend to increase because of the additional 

electricity needed to lift the water from the 

greater depths, even when the price of 

electricity drops or remains constant (see 

Schefter, 1984).

Previously, the State Engineers Office (State 

of Nevada, 1982; p. 11) made the following 

estimates with regard to the development (and 

production) of new ground-water resources in 

southern Nevada, which tend to support this 

argument: 

Capital Cost Total 

Aquifer Yield Per Well Production Cost 

Type (gpm) (1982 Dollars) (Dollars/Acre-foot) 

Alluvial 300- $85,000- $25-$30 

2500 $500,000 

Carbonate < 5000 $2,000,000 $115 

(In Section 2.2 of this NUREG, an analysis 

was undertaken that evaluated the capital 

costs of deep well construction in semi-arid 

portions of the U.S. outside of the Yucca 

Mountain area.12 The cost estimates 

developed are believed to be generally 

consistent with the State's 1982 cited figures.) 

Additional economic information about the 

marginal (preferred) use of ground water, as a 

resource (see Science Applications 

International Corporation, 1991; and 

Frederick et al., 1996), may provide further 

insights in defining potential receptor groups. 

2.1.3 Other Issues Related to Near-Term 

Ground-Water Use 

Over the past decade, Nevada has experienced 

about a 50 percent increase in population. 13 

This high rate of growth has led to concerns 

about the general availability of water and in 

particular, the potential for overdrafting of 

regional ground-water systems. Surface water 

has been fully appropriated for many years 

and most priority water rights in the major 

river basins were established well before 1900 

(Moody et al., 1986; p. 328). The principal 

1

2

Arizona, Nevada, New Mexico, and the Trans-Pecos region of 

Texas. 

1

3

Nye County has had one of the highest population growth 

rates in the State (115 percent), followed by Clark County (82 

percent), and the City of Las Vegas (79.5 percent). During the 

1980-92 period, Las Vegas was the second fastest growing 

municipality in the conterminous U.S. (U.S. Department of 

Commerce, 1994b; p. xxvii).

NUREG- 1538 2-18
Table 2-7. Ground-Water Pumping Inventories in the Amargosa Desert and Pahrump Valley 

Hydrographic Basins, During the Period 1985-95. Based on data 

from the State Engineers Office cited in Buqo (1997, pp. 29, 34). 

Inventoried Pumping by Year (106 M3

) 

Category 1995 1994 1993 1992 1991 1990 1989 1988 1987 1986 1885 

Amargosa Desert Hydrographic Basin 

Irrigation a 15.2 12.3 10.7 7.1 6.1 6.1 1.9 3.7 7 8.1 10 

Mining/Industrial b 3.2 3.1 3.1 2.8 1.3 3.4 2.7 1.2 0.37 0.7 1.7 

Quasi-Municipal/Domestic 0.123 0.123 0.123 0.123 0.123 0.154 0.154 0.154 0.154 0.154 0.28 

Commercial 0.012 0.012 0.012 0.012 0.012 0.012 0.012 0.012 0.012 10.012 0.025 

TOTAL 18.54 15.54 13.94 10.04 7.54 9.67 4.77 5.07 7.54 8.97 12.01 

Pahrump Valley Hydrographic Basin 

Irrigation 17.5 21.1 17.3 20.1 23.8 19 18.3 17.9 18.3 19 23.6 

Quasi-Municipal/Domestic 6.8 6 5.2 4.6 4.1 3.7 3.7 3.4 2.3 2.1 1.9 

Central Nevada [Water] 3.4 2.2 1.9 1.8 1.3 1.6 1.3 1.5 1.6 1.4 1.5 

Utilities c 

Commercial d 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 

TOTAL 29.2 30.8 25.9 28 30.7 25.8 24.8 24.3 23.7 24 28.5 

a Includes unpermitted irrigation. 

b Includes pumping inventories from the St. Joe Bullfrog, American Borate, and IMV Floridin mining/mineral processing operations. 

c A publicly owned water utility. Inventory likely to include commercial and domestic consumption not listed elsewhere in this table. 

z d Pumping for the Champion Golf Course.

00 

U. 

00

C.') 

C.' 

0 

0 

0 

0 

1 

*1 0

to I
2 Considerations in the Definition of a Receptor Group 

source of water for the greater Las Vegas area 

is an allocation from the Colorado River 

(vis-a-vis the Colorado River Compact-see 

California Department of Water Resources, 

1998; pp. 9-28-9-29), which accounts for 

about half of the State's allocation (Moody et 

al., 1986; p. 323). However, this allocation is 

finite and is believed to be insufficient to meet 

the region's growing needs beyond the end of 

the century (Water Resources Management, 

Inc., 1992). It is believed, therefore, that there 

will be growing pressure to develop the 

available regional ground-water resources, 

including those in southern Nye County (see 

Basse, 1990). 

Ground-water use in many southern Nye 

County communities (e.g., Amargosa Desert, 

Pahrump Valley, Indian Springs, Oasis Valley) 

is already over-appropriated and there are 

restrictions in place to protect the affected 

aquifers (see DOE, 1988; pp. 134-135; and 

Appendix A, this NUREG). Consequently, as 

is the case in much of the arid southwestern 

U.S. (see Reisner, 1993), it is not clear 

whether there will be sufficient water 

resources to support additional development 

in southern Nevada. Information about the 

ground-water resources is incomplete and 

appears to be limited to the first 100 meters 

for the alluvial ground-water reservoirs (Eakin 

et al., 1976); information about the 

consolidated rock-reservoirs (e.g., carbonates 

and volcanics) is believed to be even more 

incomplete. Complicating these matters are 

the complexities associated with developing 

realistic estimates of the water budgets 

(recharge, discharge, ET) for the respective 

hydrographic basins in the area (see 

D'Agnese et al., 1997; pp. 43-56). For these 

reasons, it is reasonable to assume that finite 

ground-water resources and expanding needs 

in areas like Las Vegas will cause continued 

ground-water mining and overdrafts in the 

region (Coleman, 1993). 

2.1.4 Summary 

Current use of ground water in the Yucca 

Mountain area is affected by both 

physiographic factors and institutional 

controls. Because the climate is arid to semiarid, agriculture is restricted to areas where 

the depth to water is shallow enough to make 

irrigation economically feasible (i.e., 

Amargosa Desert, Pahrump, and Oasis 

Valleys). Agriculture also tends to be confined 

to areas where the relief is gentle enough to 

make center-pivot or furrow irrigation 

methods technically feasible and where the 

soil texture is suitable, such as near the distal 

margin of alluvial fan deposits. North of 

Highway 95 much of the land in the vicinity of 

Yucca Mountain is controlled by either the 

NTS, the U.S. Bureau of Reclamation, or the 

U.S. Air Force, which has precluded potential 

development. Although the withdrawal of this 

land from private development may have 

affected land use patterns, the great depth to 

ground water and limitations to soil 

irrigability, 14 throughout the controlled area, 

suggests that farming would not be 

economically feasible with current technology. 

As noted in Appendix A, none of the residents 

in this area appears to be living a "subsistence" lifestyle. No resident is 

understood to subsist solely off the food 

produced from his/her garden or ranch. Most 

residents still purchase the majority of their 

food stuffs at local grocery stores and use the 

locally grown produce/meat-poultry to 

supplement their diets. Most, if not all 

residents therefore appear to need 

"infrastructure" to subsist and run their 

households-bottled gas to cook their meals; 

roads to get to work and to town (to purchase 

food and to obtain services); and in particular, 

electricity to pump water from their wells. 

Present water use in the Amargosa Valley area 

is much greater than what could be sustained 

by the in-flow of ground water coming into the 

Alkali Flat-Furnace Creek Ranch 

1

4

In this regard, it is noted that the soil classification system in 

Nevada is based on both judgments of the feasibility of 

performing initial site improvements such as drainage, 

removing stones, leveling, ripping hardpan (i.e., calich6), or removing soluble salts and alkali, and on continuing site 

limitations for use requiring special management, such as the 

control of salt build-up (State of Nevada, 1974). Most of the 

soils in the Yucca Mountain area are gravelly and coarse textured with low inherent fertility and low waterholding 

capacity. Consequently, they have been classified as having 

properties that "...preclude their use for irrigated 

agriculture..." or "...have severe limitations that reduce [the] 

choice of crops or require special conservation practices or both...." (Op cit.)

NUREG-1538 2-20
ground-water basin underlying Yucca 

Mountain. Ground water from NTS is 

believed to be a relatively small contributor to 

the total discharge in the Amargosa Desert 

area-no more than 71/ percent. The 

remaining ground water used in the Amargosa 

Desert area apparently comes from a 

combination of interbasin flows as well as 

upwelling from the deep Paleozoic carbonate 

aquifer, as suggested by geochemical 

signatures and observed vertical gradients in 

the region. There also may be areas of local 

recharge from mountain runoff into the 

alluvial-fan deposits, as occurs in other alluvial 

basins in Death Valley. It is reasonable to 

assume, therefore, that under a hypothetical 

farming-type receptor group scenario 

(30-kilometer location), based on current 

water use, all (incoming) ground water 

entering the basin would be taken up by the 

irrigation wells pumping in the area. 

Based on current technology, agricultural 

activity will most likely continue to be limited 

to the Amargosa Desert vicinity. Geochemical 

facies in various wells in the region show that 

the ground water is highly heterogeneous, 

depending on such factors as the location, 

screened well depth, and pumping (see 

Wittmeyer and Turner, 1995). There is reason 

to believe, therefore, that there is only limited 

mixing between Alkali Flat-Furnace Creek 

Ranch ground waters and those from the other 

hydrologic basins in the Death Valley 

ground-water system (see Figure 2-2). The 

net effect of this absence of mixing, under 

natural conditions, is to suggest that 

radionuclide concentrations may vary greatly 

from irrigation well to irrigation well. 

As regards the use of specific criteria for the 

selection of a potential critical group in the 

Yucca Mountain area, the staff has the 

following observations based on the 

aforementioned review: 

Information on current lifestyles

existing population distribution, 

employment centers, agricultural/ 

ranching practices-is useful in limiting 

speculation on potential receptor groups.

2 Considerations in the Definition of a Receptor Group 

" The availability and accessibility to 

ground water-such as aquifer source 

type and depth-to-aquifer-are useful 

parameters to consider in the selection of 

potential receptor groups. 

" Farming in the area is highly influenced 

by unique geographic and meteorologic 

factors that were not considered to any 

significant extent in this review. This or 

similar types of information may be used 

to further define the attributes of receptor 

groups for the Yucca Mountain area. 

2.1.5 References 

Basse, B., "Water Resources in Southern 

Nevada," Denver, Colorado, Adrian Brown 

Consultants, Inc., 1990. [Prepared for the 

CNWRA.] 

Bedinger, M.S., WH. Langer, and J.E. Reed, 

"Synthesis of Hydraulic Properties of Rocks in 

the Basin and Range Province, Southwestern 

United States," U.S. Geological Survey Water 

Supply Paper 2310, 1985. 

Bedinger, M.S., et al., "Studies of Geology and 

Hydrology in the Basin and Range Province, 

Southwestern United States, for Isolation of 

High-Level Radioactive Waste-Basis for 

Characterization and Evaluation," U.S. 

Geological Survey Professional Paper 

1370-A, 1989. 

Blankennagel, R.K., and J.E. Weir, Jr., 

"Geohydrology of the Eastern Part of Pahute 

Mesa, Nevada Test Site, Nye County, 

Nevada," U.S. Geological Survey Professional 

Paper 712-B, 1973. 

Buqo, TS., "Baseline Water Supply and 

Demand Evaluation of Southern Nye County, 

Nevada," Blue Diamond, Nevada, [1997]. 

[Independent consultants' report prepared for 

the Nye County Nuclear Waste Repository 

Office (Nye County, Nevada)]. 

California Department of Water Resources, 

"California Water Plan Update," Sacramento, 

Department of Water Resources Bulletin 

160-98, 2 vols., November 1998. 

Carr, M.D. and J.C. Yount (eds.), "Geologic 

and Hydrologic Investigations of a Potential

2-21 NUREG- 1538
2 Considerations in the Definition of a Receptor Group 

Nuclear Waste Disposal Site at Yucca 

Mountain, Southern Nevada," U.S. Geological 

Survey Bulletin 1790, 1988. 

Claassen, H.C., "Water Quality and Physical 

Characteristics of Nevada Test Site Water 

Supply Wells," Lakewood, Colorado, U.S. 

Geological Survey, USGS-474-158 

[NTS-242], 1973. [Non-serial report 

prepared for the AEC.] 

Coleman, N.M., "Groundwater Impacts of 

Foreseeable Human Activities on a HLW 

Repository," in American Nuclear Society/ 

American Society of Civil Engineers, 

Proceedings of the Fourth Annual International 

Conference: High-Level Radioactive Waste 

Management, April 26-30, 1993, Las Vegas, 

Nevada, 1:261-264 [1993]. 

Corchary, G.S., and G.A. Dinwiddie, "Field 

Trip to [the] Nevada Test Site," Denver, 

Colorado, U.S. Geological Survey [1974]. 

[NTS field guide with limited distribution 

prepared for the AEC.] 

Czarnecki, J.B., "Simulated Effects of 

Increased Recharge on the Ground-Water 

Flow System of Yucca Mountain and Vicinity, 

Nevada-California," U.S. Geological Survey, 

Water Resources Investigations, 

WRI-84-4344, 1985. 

Czarnecki, J.B., "Should the Furnace Creek 

Ranch-Franklin Lake Playa Ground-Water 

Subbasin Simply Be the Franklin Lake Playa 

Ground-Water Subbasin? [Abstract]," EOS 

Transactions, American Geophysical Union, 

68:1292 [1987]. 

Czarnecki, J.B., "Characterization of the 

Subregional Ground-Water Flow System at 

Yucca Mountain and Vicinity, Nevada

California," Radioactive Waste Management 

and the Nuclear Fuel Cycle, 13:51-61 [1989]. 

Czarnecki, J.B., "Simulated Water-Level 

Declines Caused by Withdrawals from Wells 

J-13 and J-12 Near Yucca Mountain, 

Nevada," U.S. Geological Survey, Open-File 

Report 91-478, 1992.

Czarnecki, J.B. and R.K. Waddell, 

"Finite-Element Simulation of Ground-Water 

Flow in the Vicinity of Yucca Mountain, 

Nevada-California," U.S. Geological Survey, 

Water-Resources Investigations Report, 

WRI-84-4349, 1984. 

Czarnecki, J.B. and WE. Wilson, "Site 

Characterization and Conceptual Models of 

the Regional Ground-Water Flow System, 

Yucca Mountain and Vicinity, Nevada

California [Abstract]," in Post, R.G. (ed.), 

Waste Management '89: Proceedings of the 

Symposium on Waste Management, February 

26-March 2, 1989, Tucson, Arizona, 1:473 

[1989]. 

D'Agnese, FA., et al., "Hydrogeologic 

Evaluation and Numerical Simulation of the 

Death Valley Regional Ground-water Flow 

System, Nevada and California, Using 

Geoscientific Information Systems," U.S. 

Geological Survey, Open-File Report 

96-4300, 1997. 

Dudley, WW, Jr., and J.D. Larson, "Effect of 

Irrigation on Desert Pupfish Habitats in Ash 

Meadows, Nye County, Nevada," U.S. 

Geological Survey Professional Paper 927, 

1976. 

Eakin, TE., S.L. Schoff, and P. Cohen, 

"Regional Hydrology of a Part of Southern 

Nevada," U.S. Geological Survey, Trace 

Elements Investigations Report TEI-833, 

1963. [Prepared for the AEC.] 

Eakin, TE., D. Price, and J.R. Harrill, 

"Summary Appraisals of the Nation's 

Ground-Water Resources," U.S. Geological 

Survey Professional Paper 813-G, 1976. 

Frederick, K.D., J. Vandenberg, and T 

Hanson, "Economic Values of Freshwater in 

the United States," Washington, D.C., 

Resources for the Future Discussion Paper 

97-03, October 1996. [Prepared for the 

Electric Power Research Institute.] 

Geraghty, J.J., et al., "Water Atlas of the 

United States," Port Washington, New York, 

Water Information Center, 1973. 

International Commission on Radiological 

Protection, "Recommendations of the ICRP,"

NUREG- 1538 2-22
Annals of the ICRP, Vol. 1, No. 3 [1977]. 

[ICRP Publication 26] 

International Commission on Radiological 

Protection, "Radiological Protection 

Principles for the Disposal of Solid 

Radioactive Waste," Pergamon Press, Oxford, 

1985. [ICRP Publication 46.] 

Kilroy, K.C., "Ground-Water Conditions in 

Amargosa Desert, Nevada-California, 

1952-87," U.S. Geological Survey, 

Water-Resources Investigations Report 

89-4101, 1991. 

Laczniak, R.J., et al., "Summary of 

Hydrogeologic Controls on Ground-Water 

Flow at the Nevada Test Site, Nye County, 

Nevada," U.S. Geological Survey, Water

Resources Investigations Report 96-4109, 

1996. 

McCracken, R.D., A History of Amargosa 

Valley, Nevada, Tonopah, Nevada, Nye County 

Press, 1992. 

Moody, D.W, et al. (compilers), "Nevada" in 

"National Water Summary 1985-Hydrologic 

Events and Surface-Water Resources," U.S. 

Geological Survey, Water-Supply Paper 2300, 

1986. 

National Research Council, "Ground Water at 

Yucca Mountain: How High Can It Rise?

Final Report of the Panel on Coupled 

Hydrologic Tectonic/Hydrothermal Systems at 

Yucca Mountain: Board on Radioactive Waste 

Management; Commission on Geosciences, 

Environment, and Resources," Washington, 

D.C., National Academy Press, April 1992. 

Planert, M., and J.S. Williams, "Ground Water 

Atlas of the United States. Segment 1: 

California, Nevada," U.S. Geological Survey, 

Hydrologic Investigations Atlas 730-B, 1995. 

Reisner, M., Cadillac Desert-The American 

West and Its Disappearing Water, New York, 

Penguin Books, 1986. [Revised 1993.] 

Rush, FE., "Index Map of Hydrographic 

Areas," Carson City, Division of Water 

Resources/Department of Conservation and

2 Considerations in the Definition of a Receptor Group 

Natural Resources, Information Series Report 

6, 1968. 

Rush, FE., "Regional Ground-Water Systems 

in the Nevada Test Site Area, Nye, Lincoln, 

and Clark Counties, Nevada," Carson City, 

Division of Water Resources/Department of 

Conservation and Natural Resources, Water 

Resources Reconnaissance Series Report 54, 

1970. 

Schefter, J.E., "Declining Ground-Water 

Levels and Increasing Pumping Costs: Floyd 

County, Texas-A Case Study," in "National 

Water Summary 1984-Hydrologic Events, 

Selected Water Quality Trends, and 

Ground-Water Resources," U.S. Geological 

Survey, Water-Supply Paper 2275, 1984. 

Schoff, S.L. and J.E. Moore, "Chemistry and 

Movement of Ground Water, Nevada Test 

Site," U.S. Geological Survey, Trace Elements 

Investigations Report TEI-838, 1964. 

[Prepared for the AEC.] 

Science Applications International 

Corporation, Inc., "Water Resource 

Assessment of Yucca Mountain, Nevada," Las 

Vegas, Nevada, U.S. Department of Energy, 

Study Plan 8.3.1.9.2.2, Revision 0, June 1991. 

[Prepared for DOE's Office of Civilian 

Radioactive Waste Management (OCRWM).] 

Solley, WB., R.R. Pierce, and H.A. Perlman, 

"Estimated Water Use in the United States in 

1990," U.S. Geological Survey Circular 1081, 

1993. 

State of Nevada and University of Nevada of 

Nevada [Reno], "Water for Nevada: Report 8. 

Forecasts for the Future-Agriculture," 

Carson City, Division of Water Resources/ 

Department of Conservation and Resources 

and Division and Agricultural and Resource 

Economics/Max C. Fleischmann College of 

Agriculture, January 1974. [Contains a 

separate map plate entitled "Irrigability 

Classification of Soils," (dated 1973) that 

shows the distribution of soil irrigability 

classes within the state and provides a legend 

describing the soils.] 

Thomas, H.E., "Water Resources," in Nevada 

Bureau of Mines, "Mineral and Water

2-23 NUREG-1538
2 Considerations in the Definition of a Receptor Group 

Resources of Nevada," University of Nevada 

(Reno), Mackay School of Mines, Bulletin 65, 

1964. [Prepared in cooperation with the 

USGS. Reprinted 1974.] 

TRW Environmental Safety Systems, Inc., 

"Yucca Mountain Site Characterization 

Project: Summary of Socioeconomic Data 

Analyses Conducted in Support of 

Radiological Monitoring Program during 

Calendar Year 1994," Las Vegas, Nevada, 

Science Applications International 

Corporation, Inc., Document No. 

DE-AC01-91RW00134, June 19 95a. 

[Prepared for OCRWM. The most recent 

update is dated June 1998, but was not 

considered in this analysis] 

TRW Environmental Safety Systems Inc., 

"Total System Performance Assessment

1995: An Evaluation of the Potential Yucca 

Mountain Repository," Las Vegas, Nevada, 

Document No. B00000000-01717-2200

00136 (Rev. 01), November 1995b. 

Thordarson, W and B.P. Robinson, "Wells and 

Springs in California and Nevada within 100 

Miles of the Point 37 Deg. 15 Min. N., 116 

Deg. 25 Min. W, on Nevada Test Site," U.S. 

Geological Survey, USGS-474-85, 1971. 

[Prepared for the AEC.] 

U.S. Department of Commerce, "County and 

City Data Book-1994 (12th edition)," 

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Bureau of Census, 1994a. 

U.S. Department of Commerce, "1992 Census 

of Agriculture: State and County Data [for California and Nevada]," Economics and 

Statistics Administration/Bureau of Census, 

Volume 1, Geographic Area Series, 1994b. 

U.S. Department of Energy, "Environmental 

Assessment: Yucca Mountain Site, Nevada 

Research and Development Area, Nevada

Volume I," Office of Civilian Radioactive 

Waste Management, Nevada, 

DOE/RW-0073, May 1986. 

U.S. Department of Energy, "Site 

Characterization Plan, Yucca Mountain Site, 

Nevada Research and Development Area,

Nevada," Office of Civilian Radioactive Waste 

Management, Nevada, DOE/RW-0199, 9 

vols., December 1988. 

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Environmental Impact Statement for the 

Nevada Test Site and Off-Site Locations in the 

State of Nevada," Nevada Operations Office, DOE/EIS 0243, 3 vols. (and appendices), August 1996. 

U.S. Geological Survey, "The National Atlas 

of the United States," Washington, D.C., 1970. 

U.S. Geological Survey, "Ground-Water 

Levels in the United States-1971-74: 

Southwestern States," Washington, D.C., Water Supply Paper 2162, 1977. 

Waddell, R.K., "Two-Dimensional, Steady State Model of Ground-Water Flow, Nevada 

Test Site and Vicinity, Nevada-California," 

U.S. Geological Survey, Water-Resources 

Investigations Report 82-4085, 1982. 

Waddell, R.K., J.H. Robison, and R.K. 

Blankennagel, "Hydrology of Yucca Mountain 

and Vicinity, Nevada-California-Investigative 

Results Through Mid-1983," U.S. Geological 

Survey, Water-Resources Investigations 

Report 84-4267, 1984. 

Water Resources Management, Inc. [WRMI], "WRMI Process-Water Supply Planning for 

the Las Vegas Region," Columbia, Maryland, May 1992. [Prepared for the Las Vegas Valley Water District.] 

Wescott, R.G., et al. (eds.), "NRC Iterative 

Performance Assessment Phase 2: 

Development of Capabilities for Review of a 

Performance Assessment for a High-Level 

Waste Repository," U.S. Nuclear Regulatory 

Commission, NUREG- 1464, October 1995. 

Winograd, I.J. and W Thordarson, 

"Hydrogeologic and Hydrochemical 

Framework, South-Central Great Basin, Nevada-California, with Special Reference to 

the Nevada Test Site (Hydrology of Nuclear 

Test Sites)," U.S. Geological Survey, Professional Paper 712-C, 1975. 

Witherill, J.V, U.S. Department of Energy/ 

Nevada Test Site, untitled letter to M.D.

NUREG- 1538 2-24
Tuebner, Science Applications International 

Corporation [General Subject: Water Supply 

Information for the Nuclear Test Site"], 

June 4, 1986. 

Wittmeyer, G.W. and D.R. Turner, 

"Conceptual and Mathematical Models of the 

Death Valley Regional Groundwater Flow 

System," San Antonio, Texas, Center for 

Nuclear Waste Regulatory Analyses, CNWRA 

95-109, September 1995. 

Wittmeyer, G.W, et al., "Use of Groundwater 

in the Arid and Semi-Arid Western United 

States: Implications for [the] Yucca Mountain 

Area," San Antonio, Texas, Center for Nuclear 

Waste Regulatory Analyses, letter report, 

August 1996. 

Wittwer, C., et al., "Preliminary Development 

of the LBL/USGS Three-Dimensional Site

Scale Model of Yucca Mountain, Nevada," 

Berkeley, California, Lawrence Berkeley 

Laboratory, LBL-37356/UC-814, June 1995. 

[Prepared for DOE.] 

2.2 Use of Ground Water in the Arid and 

Semi-Arid Western United States: 

Possible Implications for the Yucca 

Mountain Area 15 

2.2.1 Introduction 

Assessment of water and land-use practices in 

the arid to semi-arid western U.S. can be used 

to help define a hypothetical receptor group 

for the Yucca Mountain area. Unlike Section 

2.1 of this NUREG, which examined water

and land-use practices in the area immediately 

around the proposed repository, this section 

examines such practices on a broader, 

state-wide scale, to determine what additional 

inferences, if any, may be made. Inasmuch as 

most of the land in the immediate Yucca 

Mountain area was withdrawn from private 

sector development, it is difficult to determine 

15This analysis was first published as Wittmeyer et al. (1996). 

Although it can be found in NRC's Public Document Room 

(PDR), Wittmeyer et al. has not received wide-spread 

distribution thus far and as a consequence, a decision was 

made to include it in this NUREG. However, for the purposes 

of publication in this format, it has undergone a limited 

editorial review. Despite this review, the analysis is 

substantially the same as the earlier version placed in the 

PDR.

2 Considerations in the Definition of a Receptor Group 

if current water- and land-use practices are 

solely the result of obvious topographic, 

hydrogeologic, climatologic, and pedologic 

factors, or the result of certain institutional 

factors, such as the establishment of NTS and 

Nellis AFB. To evaluate the type of water use 

practices that might have existed in the vicinity 

of Yucca Mountain in the absence of NTS and 

Nellis AFB, the staff have conducted a review 

of water- and land-use practices from similar 

arid regions for comparison with the Yucca 

Mountain area. 

2.2.2 Scope of Study and Methods of 

Analysis 

The scope of this study has been limited to 

gathering data from Arizona, New Mexico, 

southern Nevada, and the Trans-Pecos region 

of west Texas on ground-water utilization 

practices potentially relevant to the Yucca 

Mountain area. The climate of this region of 

the southwestern U.S. generally ranges from 

arid to semi-arid, except in higher elevations, 

where orogenic effects result in a more humid, 

temperate climate. Based on the Kbppen

Geiger climate classification scheme,16 the 

climate at Yucca Mountain is arid. Hence, 

certain water-use practices, such as pumping 

ground water for cattle grazed on marginal, 

semi-arid lands, should perhaps not be 

extrapolated to the immediate Yucca 

Mountain area. 

The tables and maps in this analysis present 

data on wells with water levels deeper than 

150 meters. The basic data were obtained 

from the Ground-Water Site Inventory (GWSI) 

database for Arizona, New Mexico, and 

Nevada. (GWSI is part of the National Water 

16

1n the K6ppen-Geiger system of climate classification, the 

annual precipitation boundary between humid and arid/ 

semi-arid precipitation/climate regimes is determined for 

places where winter is the wet season (like Yucca Mountain 

vicinity) by the following equation: 

R(HumidlSub-humid boundary) = .44 (T - 32) 

where R is the average annual boundary precipitation in 

inches and T is the average annual temperature, in Fahrenheit 

degrees (TF). 

For Yucca Mountain which has an average annual 

temperature of about 61 * F (or 16' C), the boundary 

between a humid and sub-humid climate would be 12.8 inches 

(325 millimeters) precipitation annually. 

continued on next page

2-25 NUREG-1538
2 Considerations in the Definition of a Receptor Group 

Information System developed and maintained 

by the USGS.) Because the data for each State 

were obtained from that State's USGS Water 

Resources Division Office, the format and 

content of the data varied somewhat. Data for 

Texas were obtained directly from the Texas 

Water Development Board. Additional water 

well and depth-to-water data were obtained 

from various reports prepared by State and 

Federal agencies. Anecdotal information 

about the occurrence of wells with great 

depths to water was obtained during telephone 

conversations with USGS personnel, 

ground-water supply consultants, and local 

well drillers. 

Although the majority of wells studied have 

depths to the water table ranging from 150 to 

240 meters, other locations in the arid U.S. 

pump ground water from depths greater than 

240 meters. Because of the higher capital (i.e., 

developmental) and operating costs, the 

practice of pumping water from these "very 

deep wells" is less common, and is generally 

due to site-specific conditions and local water 

supply needs. Nonetheless, because it is 

possible (but not necessarily practical) to 

construct deep water wells in the NTS area, as 

a matter of background, the staff has 

evaluated these very deep well pumping 

practices. This evaluation can be found in 

Appendix C 

2.2.3 Exploitation of Deep Ground Water 

Outside of a few, very deep (greater than 240 

meters) wells drilled on NTS to supply water 

16 continued 

The boundary in the Koeppen-Geiger climate classification 

between semi-arid and arid climates is one-half the 

precipitation value obtained for the humid/sub-humid 

boundary. The formula is 

R(Semi-arid/Arid boundary) = .22 (T - 32) 

Thus, the precipitation boundary for an arid climate 

classification at Yucca Mountain would be 6.4 inches (163 

millimeters) annually. 

The approximate average annual precipitation at Yucca 

Mountain is 5.9 inches (150 millimeters). Yucca Mountain 

classifies as an arid climate according to the K6eppen-Geiger 

climate classification scheme because its precipitation is 

below 6.4 inches, the boundary between arid/semi-arid 

climates at sites that have winter-dominated precipitation and 

an average annual temperature of 16' F.

to remote locations, and several wells that 

supply water to mining operations in Crater 

Flat (e.g., Sterling Mine) and the Greenwater 

Range (U.S. Borax), most domestic and 

agricultural wells in the Yucca Mountain area 

are located where the water table is from 10

to 100-meters deep. This is in accord with the 

assumption that water wells tend to be drilled 

where the depth to water is shallowest, to limit 

development and production costs. Clearly, 

the occurrence of valuable mineral deposits 

may make the construction of a 500-meter

deep well economically feasible for a mining 

company, whereas a farmer might be unwilling 

to expend funds on such a well to irrigate 

alfalfa. The extremes to which a commercial 

enterprise will go to secure a water supply 

depends wholly on the marginal value that a 

unit of water has in producing a commodity, 

such as an ounce of gold or a bale of alfalfa. 

At the Sterling Mine, which is on the eastern 

flank of Bare Mountain at an altitude of 

approximately 1150 meters, water is currently 

trucked in from borehole VH-2 in Crater 

Flat to supply heap leach operations. For this 

case, it is evident that the cost of transporting 

water a distance of some 8 kilometers is less 

than pumping expenses in conjunction with the 

amortized cost of constructing a well at the 

mine. It must be noted that VH-2 was 

constructed as part of the Yucca Mountain site 

characterization program, before being ceded 

to the Sterling Mine, so costs borne by the 

mining company do not reflect actual costs of 

borehole construction. 

Because water used to supply basic human 

needs tends to have a higher marginal utility 

than water used for mining and agriculture, 

domestic (household) users are more willing 

to pay for water than nonagricultural, 

commercial, and industrial users, who in turn 

have a greater willingness to pay for water 

than agricultural users (Science Applications 

International Corporation, 1991). Although 

domestic users tend to be willing to pay for 

water, they generally use far less water than 

industrial and agricultural users. Based on this 

pattern of consumptive behavior alone, one 

would expect domestic water use to 

predominate where water is expensive and 

agricultural use to predominate where water is

NUREG- 1538 2-26
inexpensive. Of course other physiographic 

factors influence the development of a region: 

(a) climate and native vegetation; (b) 

topography; and (c) soil fertility. For this 

study, it is assumed the two primary factors 

affecting ground-water use are depth to the 

water table and climate. Depth to ground 

water directly affects the cost of obtaining 

fresh water, whereas climate influences 

agricultural practices. 

Current well drilling and pump technology 

allows water to be pumped from extreme 

depths (in excess of 1000 meters); however, its 

potability and, thus, suitability for watering 

stock or irrigating cropland, generally 

decreases with depth. The power that must be 

delivered to a pump is proportional to the 

product of pump discharge and total lift, 

whereas the capital cost of a well depends on 

the rated power of the motor and capacity of 

the pump; diameter, length, and composition 

of the casing and well screen; and the method 

used to drill the well. As an illustration of 

pumping costs, consider two cases: (a) a 

domestic well that annually supplies 617 cubic 

meters (0.5 acre-feet-about 150 gallons/ 

person per day for a household of 3 persons) 

of water to a home from a depth of 30 meters; 

and (b) an agricultural well used to supply 1.52 

meters of water to 64.7 hectares of cropland 

during the growing season, from a depth of 7.6 

meters. Assuming a composite well efficiency 

of 60 percent, annual operating expenses 

(excluding maintenance and amortized capital 

costs) for the domestic well, based on 

electricity costs of 100 mil/kWh would be 

$8.40.17 Assuming 60 percent well efficiency 

and electricity costs of 100 mil/kWh, annual 

operating expenses for the agricultural well 

would be $3391.04. If the pump lift for both 

the domestic and agricultural wells were 

increased to 240 meters, annual energy costs 

1

7

[Volume Pumped (617 M3 ) x Pump Lift (30 m) x Unit 

Weight of Water (9800 N/m3

)] + [Efficiency (60 percent

0.60)] + [Conversion from Joules to Kilowatt-hours (3.6 x 106 

J/kWh) x Unit Cost of Electricity (0.10 Dollars/kWh)] 

$8.40.

2 Considerations in the Definition of a Receptor Group 

would rise to $67.20 and $32,553.98, 

respectively. This example illustrates the effect 

of depth to water on variable pumping 

expenses and highlights the costs of irrigating 

cropland where the depth to water is large. 

2.2.3.1 Exploitation of Deep Ground Water 

in Arizona 

Records for 1172 wells with depths to water 

equal to or greater than 150 meters were 

obtained from the GWSI database for 

Arizona. A general breakdown of these 

occurrences by depth to water and, where 

known, use of water, is given in Table 2-8. 

Note that the entries in the Total Column 

exceed the row total because some well 

records have no entry under the water use 

data field. The locations of water wells in 

Arizona with depths to water equal to greater 

than 150 meters are shown in Figure 2-6. 

Many of the wells with depths to water from 

150 to 180 meters are in the major agricultural 

areas of Pinal County and southern Maricopa 

County; however, these depths to water reflect 

extensive ground-water overdraft that 

occurred over the past 30 to 60 years, rather 

than natural pre-development hydrogeologic 

conditions. Since agricultural development 

began, water levels have declined 137 meters 

in the Stansfield Basin in western Pinal 

County, 128 meters southeast of Chandler, 

and more than 61 meters in the Eloy Basin in 

central Pinal County (Anderson, 1995). These 

depths to water do not reflect pre-development 

conditions; hence it was believed that water 

use practices in these areas could not be used 

to infer water-use practices that would have 

prevailed in the absence of current 

governmental controls in the relatively 

undeveloped welded tuff of the immediate 

Yucca Mountain vicinity. Therefore, analysis 

of the data instead focused on the occurrence 

of clusters of wells with depths to water in 

excess of 240 meters. Table 2-9 indicates the 

percentage of wells in a specified depth to 

water range that are used for a given purpose.

2-27 NUREG- 1538
ý4 

z 

00 

00

Depth Water to Specified Use Total with ____________ Specified______ (in) Irrigation Public Stock Industrial Domestic Commercial Unused Total Specified 

(m______ Use 

150 - 180 2825 76b 50 5 80 0 184 717 493 

180 - 210 110a 29c 2 8 d 4 26 1 51 262 198 

210 - 240 25e 11 12 5 13 0 22 96 66 

240 - 270 2 7 4 3 4 0 4 31 20 

270 - 300 0 6 3 0 2 0 5 17 11 

300 - 330 0 3 1 0 1 0 2 8 5 

330 -360 0 3 2 0 3 0 1 9 8 

360 -390 0 6 0 0 1 0 3 10 7 

390 -420 0 2 1 0 1 0 4 8 4 

420 -450 0 3 1 0 0 0 1 5 4 

450 - 480 0 2 0 0 1 0 1 4 3 

> 480 0 1 0 0 0 0 2 5 3 

TOTALS 419 149 102 17 132 1 280 1172 822 

a Primarily Pinal, Maricopa, and La Paz Counties. 

b Phoenix and Chandler areas. c Tucson, Casa Grande, Chandler, Phoenix, Flagstaff, and Kingman areas. d Casa Grande area. 

e Cave Creek Carefree area.

Table 2-8. Use of Water in Wells with Depth to Water Greater than 150 Meters, in Arizona

0~ 

0 

0ra 

51 

0•
2 Considerations in the Definition of a Receptor Group

0

0 Depths less than 150 meters 

o Depths greater than or equal to 

150 and less than 180 meters 

0 Depths greater than or equal to 

180 and less than 210 meters 

o Depths greater than or equal to 

210 and less than 240 meters

* Depths greater than or equal to 

240 and less than 270 meters 

o Depths greater than or equal to 

270 and less than 300 meters 

X Depths greater than or equal to 

300 meters

Figure 2-6. Locations of water wells in Arizona with depths to water greater than or equal to 150 meters.

2-29 NUREG-1538
2 Considerations in the Definition of a Receptor Group 

Table 2-9 clearly shows that as the depth to 

water increases, the percentage of wells used 

for agriculture decreases, whereas the 

percentage of wells used for public supply, 

stock water, and domestic supply increases. 

This pattern in part reflects the higher value 

placed on water for drinking-and hygiene

uses that consume very little water compared 

with irrigated agriculture. For wells with 

depths to water in excess of 240 meters, public 

and domestic use accounts for 65 percent, 

stock water for 19 percent, and irrigation use 

for 3 percent. Of the 97 wells in Arizona with 

depths to water greater than 240 meters, 53 

are located north of the 35th parallel and east 

of the 113th meridian on the highlands of the 

Colorado Plateau; 29 are north of the 35th 

parallel and west of the 113th meridian in the 

transition zone between the Colorado Plateau 

and eastern Mojave Desert; and the remaining 

15 wells are scattered throughout a wide area 

south of the 35th parallel. Table 2-10, which 

gives a breakdown of wells with depth to water 

in excess of 240 meters as a function of 

land-surface elevation, indicates that 63 

percent of these wells are located in regions 

where the elevation exceeds 1500 meters. For 

approximately 40 of these 97 deep wells, 

detailed descriptions of locations and 

climatologic and topographic conditions are 

presented in Section C- 1 of Appendix C. 

2.2.3.2 Exploitation of Deep Ground Water 

in Nevada 

Records for 58 wells with depths to water 

equal to or greater than 150 meters were 

obtained from the USGS GWSI database for 

Nevada. Of the 58 wells, 49 lie in the southern 

half of Nevada, within the counties of Nye (4 

wells); Lincoln (6 wells); and Clark (38 wells); 

and one well in these data is in Death Valley 

National Monument, Inyo County, California. 

Of the 16 wells in Nevada with depths to water 

equal to or greater than 240 meters, 13 are in 

the southern Nevada counties of Nye (1 well); 

Lincoln (5 wells); and Clark (7 wells). A 

general breakdown by water use for all 58 

deep wells in Nevada is listed in Table 2-11. 

The locations of water wells in Nevada with 

depths to water in excess of 150 meters are

shown in Figure 2-7. Detailed descriptions of 

water wells with depths to water equal to or 

greater than 240 meters are given in Section 

C-2 of Appendix C. In the alluvial aquifer in 

the Amargosa Farms region, there is presently 

an estimated to be about a 40 percent 

ground-water overdraft (DOE, 1988; p. 

3-121). 

2.2.3.3 Exploitation of Deep Ground Water 

in New Mexico 

Records for 94 wells with depths to water 

equal to or greater than 150 meters were 

obtained from the USGS GWSI database for 

New Mexico. Additional data on water wells 

were extracted from Orr (1987). Of the 94 

wells, 83 are located in the northwestern 

quadrant of New Mexico in the counties of 

San Juan, Rio Arriba, Taos, McKinley, Sandoval, Santa Fe, Cibola, and Bernalillo. 

Elevations in this region range from 1500 

meters msl near Four Corners and along the 

Rio Grande River, near Albuquerque, to 4011 

meters msl at Wheeler Peak in the Sangre de 

Cristo Mountains. Although no references on 

the climate of northwestern New Mexico were 

available at the time this report was prepared, 

it is estimated the climate varies from cool 

semi-arid at the lower elevations to cool 

humid at the higher elevations. This wide 

range in climatic conditions is typical of 

so-called highland climates where enclosed 

valleys, plateaus, and exposed peaks in a 

highland region are very different climatically 

(Trewartha, 1954). A general breakdown by 

water use for all 94 deep wells in New Mexico 

is listed in Table 2-12. The locations of water 

wells in New Mexico with depths to water in 

excess of 150 meters are shown in Figure 2-8. 

Twenty-seven of these 34 wells occur in 

clusters in the northwestern quadrant of New 

Mexico in McKinley, Cibola, Sandoval, and 

Bernalillo Counties. Because the data received 

from the USGS for New Mexico did not 

include well head elevation in the well records, 

a table similar to Table 2-10 for Arizona was 

not prepared. Detailed descriptions of wells 

with depths to water equal or greater than 240 

meters are given in Section C-3 of Appendix 

C.

NUREG- 1538 2-30
2 Considerations in the Definition of a Receptor Group

Depth to Specified Use (percent) Water (in) Irrigation Public Stock Industrial Domestic Commercial 

150- 180 57 15 10 1 16 0 

180-210 56 15 14 2 15 1 

210-240 38 17 18 8 20 0 

240-270 10 35 20 15 20 0 

270-300 0 55 27 0 18 0 

300 - 330 0 60 20 0 20 0 

330-360 0 38 25 0 38 0 

360-390 0 86 0 0 14 0 

390-420 0 50 25 0 25 0 

420-450 0 75 25 0 0 0 

450-480 0 66 0 0 33 0 

> 480 0 100 0 0 0 0 

> 240 3 52 19 3 13 0

Land Surface 

Elevation 

(m) Number of Wells 

0-300 1 

300-600 5 

600- 900 12 

900 - 1200 9 

1200 - 1500 9 

1500 - 1800 21 

1800 - 2100 27 

2100 - 2400 13

NUREG- 1538

Table 2-9. Percentage Water Use for Wells in Arizona

Table 2-10. Arizona Wells with Depth to Water in Excess of 240 Meters 

as a Function of Well Head Elevation

2-31
Table 2-11. Use of Water in Wells with Depth to Water Greater than 150 Meters, in Nevada

z 

C-1 

00 

00 

(J

Depth to Number of Wells by Specified Use Total with Water Specified (in) Irrigation Public Stock Industrial Domestic Commercial Unused Total Use 

150-180 1 3 2 1 4 0 6 21 11 

180-210 0 0 3 2 5 0 6 16 10 

210-240 0 0 1 0 2 0 2 5 3 

240-270 0 0 3 0 1 0 7 12 4 

270-300 0 0 1 0 0 0 2 3 1 

300-420 0 0 0 0 0 0 1 1 0 

TOTALS 1 3 10 3 12 0 24 58 29 

Table 2-12. Use of Water in Wells with Depth to Water Greater than 150 Meters, in New Mexico 

Depth to Number of Wells by Specified Use Total with 

Water Specified 

(M) Irrigation Public Stock Industrial Domestic Commercial Unused Total Use 

150- 180 1 9 2 2 0 0 6 22 14 

180-210 1 7 3 2 0 1 6 20 14 

210-240 0 4 4 1 0 0 8 18 9 

240-270 0 1 1 3 1 2 4 12 8 

270-300 0 2 5 0 0 1a 2 11 8 

300- 330 0 1 1 0 1 0 6 9 3 

330-360 0 1 0 0 0 0 0 1 1 

360-480 0 0 0 1 0 0 0 1 1 

> 480 0 0 0 1 0 0 0 1 1 

TOTALS 2 25 16 9 2 4 32 94 58 

a Institutional water use.

S', 

;3 

0
2 Considerations in the Definition of a Receptor Group

Stoney

Douglas.

0

0 Depths less than 150 meters 0 Depths greater than or equal to 

240 and less than 270 meters 

o Depths greater than or equal to 

150 and less than 180 meters 0 Depths greater than or equal to 

270 and less than 300 meters 

0 Depths greater than or equal to 

180 and less than 210 meters K Depths greater than or equal to 

300 meters 

o Depths greater than or equal to 

210 and less than 240 meters 

Figure 2-7. Locations of water wells in Nevada with depths to water greater than or equal to 150 meters.

2-33 NUREG- 1538
2 Considerations in the Definition of a Receptor Group

0 Depths less than 150 meters 

0 Depths greater than or equal to 

150 and less than 180 meters 

V Depths greater than or equal to 

180 and less than 210 meters 

o Depths greater than or equal to 

210 and less than 240 meters

0 Depths greater than or equal to 

240 and less than 270 meters 

o Depths greater than or equal to 

270 and less than 300 meters 

B Depths greater than or equal to 

300 meters

Figure 2-8. Locations of water wells in New Mexico with depths to water greater than or equal to 150 meters.

NUREG-1538 2-34
2.2.3.4 Exploitation of Deep Ground Water 

in Texas 

Records for 220 wells with depths to water 

equal to or greater than 150 meters were 

obtained from the Texas Water Development 

Board water well database. Additional data for 

the Trans-Pecos region were extracted from 

Rees (1987). Of the 220 wells, 168 are located 

in the Trans-Pecos (area in Texas west of the 

Pecos River) and in the adjoining counties of 

Upton, Crockett, and Val Verde, immediately 

to the east of the Pecos River. The Trans

Pecos region as a whole is arid to semi-arid, 

with mean annual precipitation of 

approximately 30 centimeters, although there 

are isolated mountain ranges where 

precipitation may exceed 40 centimeters/year, 

such as the Chisos Mountains, where average 

annual precipitation at the Chisos Basin is 

42.4 centimeters/year (Bomar, 1983). The 

remaining 52 wells are located in the Llano 

Estacado and Panhandle regions of the High 

Western Plains great physiographic province, 

where mean annual precipitation is 45 

centimeters (Op cit) and the climate is 

semi-arid steppe. General exploitations by 

water use of all 220 wells and the 168 wells in 

the Trans-Pecos region are listed in Tables 

2-13 and 2-14, respectively. 

The locations of water wells in the Trans-Pecos 

region of Texas are shown in Figure 2-9. Well 

construction practices for low-discharge, high

lift stock and domestic wells used in the 

Trans-Pecos region and New Mexico may have 

implications for water use within 10 kilometers 

of the proposed repository where depths to 

water exceed 240 meters. Typical stock and 

domestic wells only need to be capable of 

pumping 1.26 x 10-4 to 3.79 x 10-4 cubic 

meters/second. For example, the Pate Altuda 

Ranch near Alpine (Brewster County), Texas, 

has three wells that each pump 1.26 X 10-4 

cubic meters/second from depths to water of 

335, 396, and 430 meters. 18 One of the Pate 

Altuda wells is pumped with a 3.7-kilowatt 

(5-horse power) submersible pump, whereas 

the other two use pump jacks. 19 (Pump jacks 

18Personal communication, W Skinner, Independent Well 

Driller (Alpine, Texas), July 1996. 

191bid.

2 Considerations in the Definition of a Receptor Group 

or sucker rods are commonly used to extract 

heavy oils from reservoirs that have 

de-pressurized; however, using pump jacks for 

deep water wells is uncommon in most of the 

U.S.) According to local drillers in Brewster 

County, pump jacks are regularly used for 

high-lift, low-discharge water wells. Because 

several of the deep water wells in the 

Trans-Pecos region were originally drilled as 

oil or gas tests, the use of pump jacks may 

reflect the drillers' experience with, and 

preference for, oilfield technology, rather than 

a decision based on economic or technical 

considerations. Windmills are also used for 

high-lift, low-discharge wells in the Trans

Pecos region. A typical windmill may employ a 

6-meter A ermotor turbine mounted on a 

18-meter-high tower.20 

2.2.3.5 Estimated Well Construction Costs in 

the Yucca Mountain and Amargosa 

Desert Areas 

As illustrated in Section 2.2.3, pumping costs 

vary in direct proportion to the depth to water. 

Capital costs of a well depend on the total 

borehole depth, well diameter, and rated 

capacity of the pump. A detailed well 

construction cost study was conducted to 

estimate capital costs for four wells typical of 

the Yucca Mountain and Amargosa Desert 

regions. Wells 1 and 2 are based on the actual 

design of Wells J- 13 and J- 12, respectively 

(Young, 1972). Well 3 is based on a generic 

design for a well that would be used to supply 

water to a quarter-section, center-pivot 

irrigation plot in the Amargosa Farms area. 

Well 4 is based on a generic design of a well 

used to supply domestic or public water to the 

community of Amargosa Valley. Detailed 

construction and completion costs are shown 

for each of these four wells in Appendix D, 21 

which will be referred to in the following 

section. 

20

lbid. 

21Most drilling engineers in the U.S. still prefer the use of 

inch-pound units (the so-called English system), when 

describing water well characteristics. Therefore, for ease of 

comparison with existing engineering practice, in this regard, 

the English system has been used in this section. Conversion 

factors can be found in the front of this NUREG. Also, all 

cost estimates are in 1996 dollars.

2-35 NUREG-1538
Table 2-13. Use of Water in Wells with Depth to Water Greater than 150 Meters, in Texas 

Depth to Number of Wells by Specified Use Total with 

Water Specified 

(M) Irrigation Public Stock Industrial Domestic Commercial Unused Total Use 

150 - 180 9 6 38 4 11 1 17 87 69 

180-210 2 3 20 6 8 0 14 53 39 

210 -240 0 0 9 4 3 0 7 24 23 

240 -270 0 0 7 5 2 0 4 19 14 

270-300 0 3 4 2 1 0 8 18 10 

300-330 0 0 1 2 1 0 2 6 4 

330 -360 0 0 2 0 1 0 1 4 3 

360 -390 0 0 0 4 0 0 1 5 4 

390-420 0 0 0 1 0 0 0 1 1 

420-450 0 0 0 0 0 0 0 0 0 

450-480 0 0 0 0 1 0 0 1 1 

>480 1 0 1 0 0 0 0 2 2 

TOTALS 12 12 82 28 28 1 54 220 163

z 

00 00

0 

0 

0 

0 

0 

0
Table 2-14. Use of Water in Wells with Depth to Water Greater than 150 Meters, in the Trans-Pecos Region of Texas

Depth to Number of Wells by Specified Use Total with 

Water Specified 

(M) Irrigation Public Stock Industrial Domestic Commercial Unused Total Use 

150- 180 0 3 36 0 11 1 13 64 51 

180-210 0 2 20 5 8 0 11 46 35 

210-240 0 0 9 0 3 0 7 20 19 

240-270 0 0 7 0 2 0 3 13 9 

270-300 0 3 4 2 1 0 8 14 10 

300-330 0 0 1 1 1 0 2 5 3 

330- 360 0 0 2 0 1 0 1 4 4 

> 360 0 0 1 0 1 0 0 2 2 

TOTALS 0 8 80 8 28 1 45 168 133

0 I0

0 

0 

0 

0 

0 

0 

0 

0
0 Depths less than 150 meters 

o Depths greater than or equal to 

150 and less than 180 meters 

4 Depths greater than or equal to 

180 and less than 210 meters 

o Depths greater than or equal to 

210 and less than 240 meters

* Depths greater than or equal to 

240 and less than 270 meters 

o Depths greater than or equal to 

270 and less than 300 meters 

)K Depths greater than or equal to 

300 meters

Figure 2- 9. Locations of water wells in the Trans-Pecos region of Texas with depths of water greater than or equal to 150 meters.

z 

00

00

0 

0 

0 

0 

0 

0

0 250 km
Using current construction practices, the total 

cost of installing a well similar to J-13 (Well 

1) is $1,117,670. This large expenditure is 

primarily caused by the great depth of Well 

J- 13 (1066 meters/3500 feet), directly 

reflected in the costs of drilling, casing, 

screening, and installing the gravel pack (Table 

D-1). Because J-12 (Well 2) is only 274 

meters (900 feet) deep, its estimated cost is 

$229,145 (Table D-2). The generic Amargosa 

Farms irrigation well (Well 3), which is 97 

meters deep, pumps from a depth of 30 meters 

and has a rated pump capacity of 

approximately 0.15 cubic meters/second (2400 

gallons/minute) and costs $167,745 (Table 

D-3). The Amargosa Valley domestic well 

(Well 4), which has a total depth of 183 meters 

(600 feet), a depth to water of 91 meters (300 

feet), and a rated pump capacity of only 

0.00063 cubic meters/second (10 gallons/ 

minute) costs $161,470 (Table D-4).22,

23 

Examples of unit and total pumping costs for 

each of the four archetypal Yucca Mountain 

and Amargosa Desert region wells are 

illustrated in Table 2-15. 

Construction methods used for water wells in 

the Trans-Pecos region of Texas and in New 

Mexico suggest that the installation of a 

small-diameter, high-lift, low-capacity 

domestic well powered by either a pump jack 

or a windmill is possible for the area near 

Yucca Mountain, where the depth to water is 

approximately 300 meters. Such a well may be 

economically feasible for drinking water only. 

Construction cost estimates for such a well, 

pumped by a submersible turbine, are shown 

in Table D-5. As shown in Table D-6, 

substituting an appropriately-sized windmill 

costs approximately $10,000 more than the 

submersible, although unit pumping costs for 

the windmill would be minimal. A comparable 

pump jack would probably cost somewhat less 

than the submersible. 

22Estimated construction costs are based on an informal bid 

received from a commercial water well drilling company 

unfamiliar with local hydrogeology and local well construction 

practices. The staff believe that actual construction costs for a 

well with the specifications listed in Appendix D may be 

substantially less. 23

well 4 is designed to supply a small community of 40 persons, 

assuming daily per capita water use of 0.57 cubic meters (150 

gallons) and a pump utilization capacity of 50 percent.

2 Considerations in the Definition of a Receptor Group 

2.2.4 Summary 

Data from Arizona, southern Nevada, New 

Mexico, and the Trans-Pecos region of Texas 

indicate that ground water is pumped from 

aquifers in arid to semi-arid regions, where 

the depth to water is as great as that in Jackass 

Flats to the east of Yucca Mountain. 

Table 2-16 lists the number of wells with 

depths to water greater than 240 meters by 

State and type of water use. Of 115 wells that 

pump from aquifers with depths to water 

equal to or greater than 240 meters, only 2 

(1.7 percent) are used for irrigation, 38 (33 

percent) are used to supply stock water, and 

63 (54.8 percent) are used for public and 

domestic water supply. Based on local 

practices in Amargosa Valley and on the data 

gathered in the well survey, it appears that 

pumping ground water for irrigation from 

depths greater than 240 meters is a rare 

practice (1.7 percent). 

There is evidence that small ranching 

operations existed in the portions of the Yucca 

Mountain area prior to establishment of the 

NTS (see Table B-1 in Appendix B). 

Although there are the remains of a corral at 

Cane Spring, which lies approximately 30 

kilometers east of Yucca Mountain at an 

elevation of 1241 meters msl, most relict 

corrals are located near Captain Jack, 

Tippipah, White Rock, and Topopah springs, 

which lie approximately 40 kilometers 

north-northeast of Yucca Mountain at 

elevations ranging from 1530 to 1737 meters 

msl. Although not unequivocal, the historic 

data appear to suggest that cattle from these 

ranches were grazed on Timber Mountain, 

Pahute Mesa, and Rainier Mesa, where the 

mean annual precipitation of 320 millimeters 

(Wang et al., 1993) is sufficient to support 

grasslands suitable for grazing. Although feral 

burros in Crater Flat west of Yucca Mountain 

are apparently able to sustain themselves on 

the meager grasses that grow in the much 

dryer lowlands of Jackass and Crater Flats, it 

seems unlikely that less hardy cattle could find 

sufficient forage in the immediate vicinity of 

Yucca Mountain.

2-39 NUREG- 1538
2 Considerations in the Definition of a Receptor Group

Table 2-15. Unit Water Costs for Four Yucca Mountain -Amargosa Desert Area Wells 

Well 1 Well 2 Well 3 Well 4 

Unit Pumping Cost ($/m 3) a 0.128 0.102 0.0136 0.0413 

Annual Consumption (m 3

) 6 22 ,00 0b 62 2 ,0 0 0b 771,000c 8300d 

Amortized Capital Cost ($) e 117,687 24,127 17,669 17,012 

Unit Amortized Capital Cost ($/m3) 0.189 0.0387 0.0229 2.05 

Total Unit Cost ($/m 3

) 0.317 0.141 0.0365 2.091 

TOTAL ANNUAL COST f ($) 197,174 87,515 28,141 17,355

ow n , .• co s oun:u. vulume rumpedu ty i ma ) x rump Lift k_3U m) x unit weight of water (9800 N/m') +- .- Eticiency (0.60) - Conversion from Joules to Kilowatt-hours (3.6 106 J/kWh) x Unit Cost of Electricity (0.10 Dollars/kWh) = $8.40. 

Population of 3000, daily per-capita water use 0.57 m3

. 

Alfalfa Farm, 1.52 m of water per season, on 50.7 ha. 

Population of 40, daily per-capita water use 0.57 m3. 

Economic lifetime of 30 years with an interest rate of 10 percent. 

Does not include distribution and maintenance costs.

b 

C 

d 

e 

f

Data from the water well survey suggest that 

existing communities will construct wells that 

pump from great depths to water if more 

easily exploited water supplies are insufficient 

or unreliable. Note that approximately 

one-third of the public supply wells listed 

earlier in Table 2-16 are found in the 

Flagstaff (Arizona) area, which originally 

relied on water pumped from shallow water 

table aquifers. Although current practices in 

the Yucca Mountain area do not support the 

idea that very small communities (25 to 100 

persons) or individual homeowners will 

construct wells to pump fresh water from great 

depths (Appendix A), the survey data do

suggest that a residential receptor group 

would be more likely to withdraw ground 

water from greater depths than a farming 

receptor group. However, the residential 

receptor group would also be expected to 

withdraw significantly less water that the 

farming receptor group. 

2.2.5 References 

Anderson, TW, "Summary of the Southwest 

Alluvial Basins, Regional Aquifer-Systems 

Analysis, South-Central Arizona and Parts of 

Adjacent States," U.S. Geological Survey 

Professional Paper 1406-A, 1995.

NUREG- 1538

Table 2-16. Use of Water in Wells with Depth to Water Greater than 240 Meters, in Arizona, Southern Nevada, New Mexico, and the Trans-Pecos Region of Texas

State! Number of Wells by Specified Use 

Region Irrigation Public Stock Industrial Domestic Commercial Total 

Arizona 2 33 12 3 13 0 63 

Nevada 0 0 4 0 1 0 5 

New Mexico 0 5 7 4 2 2 20 

Trans-Pecos Texas 0 3 15 3 6 0 27 

TOTALS 2 41 38 10 22 2 115 

(Percent Total) (1.7) (35.7) (33.0) (8.7) (19.1) (1.7)

2-40
Bomar, G.W, Texas Weather, Austin, 

University of Texas Press, 1983. 

Orr, B.R., "Water Resources of the Zuni 

Tribal Lands, McKinley and Cibola Counties, 

New Mexico," U.S. Geological Survey Water 

Supply Paper 2227, 1987. 

Rees, R.W, "Records of Wells, Water Levels, 

Pumpage, and Chemical Analyses from 

Selected Wells in Parts of the Trans-Pecos 

Region, Texas," Austin, Texas, Texas Water 

Development Board Report 301, 1987. 

Science Applications International 

Corporation, "Water Resource Assessment of 

Yucca Mountain, Nevada (Study Plan for 

Study 8.3.1.9.2.2), "Las Vegas, Nevada, 

Revision 0, June 1991. [Prepared for 

OCRWM.] 

Trewartha, G.T., An Introduction to Climate, 

New York, McGraw-Hill Book Company, Inc., 

1954. 

U.S. Department of Energy, "Chapter 3, 

Hydrology" in "Site Characterization Plan, 

Yucca Mountain Site, Nevada Research and 

Development Area, Nevada," Office of 

Civilian Radioactive Waste Management, 

DOE/RW-0199, Vol. II, Part A, December 

1988. 

Wang, J.S.Y., et al., "Geohydrologic Data and 

Models of Rainier Mesa and Their 

Implications to Yucca Mountain", in 

American Nuclear Society/American Society 

of Civil Engineers, Proceedings of the Fourth 

Annual International Conference: High-Level 

Radioactive Waste Management, April 26-30, 

1993, Las Vegas, Nevada, 2:675- 681 [1993]. 

Wittmeyer, G.W, et al., "Use of Groundwater 

in the Arid and Semi-Arid Western United 

States: Implications for [the] Yucca Mountain 

Area," San Antonio, Texas, Center for Nuclear 

Waste Regulatory Analyses, letter report, 

August 1996. 

Young, R.A., "Water Supply for the Nuclear 

Rocket Development Station at the U.S. 

Atomic Energy Commission's Nevada Test

2 Considerations in the Definition of a Receptor Group 

Site," U.S. Geological Survey Water-Supply 

Paper 1938, 1972. 

2.3 Definition of Hypothetical Receptor 

Groups Used in this NUREG 

2.3.1 Receptor Group Characteristics 

As discussed earlier, an ICRP-type critical 

group is to be based on consideration of 

current population locations and lifestyles. 

Based on the information reviewed and 

summarized in Sections 2.1 and 2.2 (as well as 

Appendices A through D), as possible 

measures of these parameters, the staff 

identified characteristics for two potential 

hypothetical receptor groups--"farming" and 

"residential" (non-farming) in the Yucca 

Mountain area: 

"Farming" Receptor Group: Because of 

topographic, meteorologic, pedologic, and 

hydrologic considerations, this hypothetical 

receptor group is a farming community 

located in an topographically-closed basin 

at a distance no closer than 20 kilometers to 

the site. The farming community is able to 

obtain its water from relatively shallow wells 

located in near-surface, local alluvial 

aquifers (i.e., generally 30 to 100 meters 

deep) in so-called "discharge zones" 

(Mifflin, 1968). This is an important 

consideration because development and 

operating costs for such wells are, in large 

measure, related to the depth of the water 

table. Thus, for reasons of practicality (and 

cost), this hypothetical receptor group would 

establish its irrigation wells in areas where 

the water table is near the surface. The 

exposure pathways considered for the 

farming hypothetical receptor group include 

ingestion (of contaminated water, crops, 

and animal products); inhalation (from 

resuspension of contaminated soil); and 

direct exposure. 

"Residential" Receptor Group: This 

hypothetical receptor group is a residential 

community located on one of the volcanic 

mesas or mountainous uplands that flank 

the topographically-closed basins, but may 

also be located in higher-elevation valleys, 

which are characterized by thin sequences of 

unsaturated alluvial sediments. The

2-41 NUREG-1538
2 Considerations in the Definition of a Receptor Group 

residential community is located between 5 

and 20 kilometers from the site where the 

depth to the water table ranges from 

approximately 300 to 100 meters, 

respectively, at the two locations. Because 

freshwater demands for a residential 

community are substantially less than those 

for a farming community, the residential 

community could be located closer to the 

site than a farming community and could 

obtain its water from wells located in the 

deeper tuff or carbonate (regional) aquifers 

found at depths of 300 meters or more. 24 

The exposure pathway considered for the 

hypothetical residential receptor group is the 

ingestion of contaminated water 

2.3.2 Limitations to the NUREG Definition 

The two hypothetical receptor groups 

described in this analysis were identified for 

the purpose of supporting the analyses 

described elsewhere in this NUREG. Not all 

potential sources of information were 

examined. Other information that might be 

useful to examine would include: 

"Socio-economic information on current 

lifestyles (habits) and population density 

within the Yucca Mountain area, such as 

the type currently being collected by DOE 

[e.g., Black et al. (1995) and University of 

Nevada (1997)]. 

" Information on the pedogenic 

classification and distribution of soils was 

not evaluated. This type of information 

has been previously collected by the Soil 

Conservation Service (U.S. Department 

of Agriculture) and the State Engineers 

Office, and may be useful in identifying 

the location of potential receptor groups. 

If a farming scenario is contemplated, a 

determination will need to be made 

whether there were suitable conditions at 

a particular site (such as those in some 

portions of the central Amargosa Desert 

area), sufficient to support agriculture. A 

24 1t should also be noted that shallow wells may also not 

practicable at this location because the alluvial sediments 

constitute "recharge zones" (Mifflin, 1968) that drain into 

basins found at lower elevations, and thus have little ground 

water in storage.

preliminary review of published soil 

information (State of Nevada/University 

of Nevada, 1974) already suggests that a 

significant percentage of the land in the 

greater Yucca Mountain area have 

limitations to their irrigability. (Also see 

Appendix A.) This type of information, in 

conjunction with other types of practical 

information related to farming

topography/slope; extent of and depth to 

the "hardpan;" crop types; length of 

growing season; number of freeze-free 

days; altitude; and the like-may also be 

useful to evaluate. 

Finally, the staff has not attempted to 

speculate on what type of activities might take 

place at NTS if the site were opened to private 

development, at this time.25 DOE recently 

evaluated alternative land-use issues at NTS in 

its Final Environmental Impact Statement for 

the Nevada Test Site and Off-Site Locations in 

the State of Nevada (DOE, 1996). However, 

because there is pre-existing contamination at 

the site, ground-water availability and quality 

have been noted to figure prominently in 

possible alternative (non-nuclear) land-use 

decisions (Op cit., pp. 4-106-4-135). Although 

there are environmental restoration programs 

currently in place at NTS, there are no plans 

to open the site to public or private 

development at this time. 

2.3.3 References 

Black, S.C., WM. Glines, and Y.E. Townsend 

(eds.), "U.S. Department of Energy Nevada 

Operations Office Annual Site Environmental 

Report-1994," Las Vegas, Nevada, Reynolds 

Electrical & Engineering Co., Inc., Document 

No. DE-AC08-94NV11432, September 

1995. [Prepared for OCRWM.] 

Mifflin, M.D., "Delineation of Ground-Water 

Flow Systems in Nevada," Las Vegas, Desert 

Research Institute Technical Report Series 

H-W, 1968. 

U.S. Department of Energy, "Final 

Environmental Impact Statement for the 

25

The staff performed a limited examination of lifestyle and 

water-use practices within the current boundaries of the NTS 

before its withdraw from public use. This evaluation can be 

found in Appendix B.

NUREG- 1538 2-42
Nevada Test Site and Off-Site Locations in the 

State of Nevada," Nevada Operations Office, 

DOE/EIS 0243, 3 vols. (and appendices), 

August 1996. 

University of Nevada, "Identifying and 

Characterizing the Critical Group: Results of 

a Pilot Study of Amargosa Valley," Las Vegas, 

Nevada, Cannon Center for Survey Research, 

1997. [Preliminary data/ results of the survey 

were made by D.A. Swanson (DOE) in an 

unpublished presentation entitled "The 

'Biosphere' Food Survey," during a DOE/NRC

2 Considerations in the Definition of a Receptor Group 

Technical Exchange on Total-System 

Performance Assessments held in Las Vegas, 

Nevada, on November 6, 1997.] 

State of Nevada and University of Nevada of 

Nevada [Reno], "Water for Nevada: Report 8. 

Forecasts for the Future-Agriculture," 

Carson City, Division of Water Resources/ 

Department of Conservation and Resources 

and Division and Agricultural and Resource 

Economics/Max C. Fleischmann College of 

Agriculture, January 1974.

2-43 NUREG-1538
3. GROUND-WATER PATHWAY ANALYSES I

Based on the staff's earlier work, the most 

likely exposure scenario for a receptor 

population in the Yucca Mountain vicinity 

would be exposure at some point down

gradient from the site, through a ground

water/food-ingestion pathway. When 

evaluating this type of exposure scenario, the 

following types of questions come to mind: 

"* How and where would radionuclides travel 

in the aquifer? 

"* "hat are the potential concentrations of 

radionulides in ground water used by 

possible receptor groups? 

"* What is the estimation of risk (dose) to 

possible receptor groups over long time 

periods? 

To address these concerns, the staff undertook 

a series of analyses, in this section of the 

NUREG, that evaluated dose from a 

ground-water exposure pathway. These 

analyses addressed the following: 

Radionuclide Dilution: During transport in the 

saturated zone, from an area immediately 

beneath Yucca Mountain, to two hypothetical 

receptor group locations (farmer and 

residential). See Section 3.1. 

Well Head Dilution/Mixing: Of radionuclides in 

ground water, caused by a pumping water well. 

See Section 3.2. 

Peak Dose: For two hypothetical receptor 

groups (farmer and residential) to determine 

when the greatest risk occurs. See Section 3.3. 

3.1 Analysis of Radionuclide Dilution in 

Ground Water 

3.1.1 Introduction 

Because of its importance to determining 

dose, dilution of radionuclides in ground water 

1The figures and tables shown in this section present the results 

from the demonstration of the continuing staff capability to 

review a total-system performance assessment (TSPA). These 

figures and tables, like the demonstration, are limited by the 

use of simplifying assumptions and sparse data.

is likely to be a central issue in future 

performance assessments of the Yucca 

Mountain site. For example, if mixing of a 

contaminant stream with ground-water flow in 

the tuff aquifer dilutes the concentration by a 

factor of 100, then the dose (and associated 

radiologic risk) would be reduced by the same 

factor (assuming no dilution effects from 

water well pumping). Dilution of radionuclides 

released into the ground water occurs as a 

result of fluid mixing along the flow path 

between the source point(s) and the location 

of the potential receptor group(s). Mixing a 

dissolved contaminant (i.e., hydrodynamic 

dispersion) is, in general, strongly related to 

variations in both the magnitude of the fluid 

velocity and flow direction. These variations 

are principally caused by small- and 

large-scale heterogeneities in the geologic 

media (Waldrop et al., 1985; Gelhar, 1993; 

Fetter, 1993). Large-scale features, such as 

faults, may in some instances induce flow 

variations and thereby enhance natural mixing, 

whereas in other cases, they may produce 

highly channelized flow with limited mixing. 

In this scoping analysis, ground-water flow and 

transport models were used to study dilution 

characteristics of the proposed repository site 

for two basic purposes: 

" To gain insight into site-specific factors 

that may affect ground-water mixing and 

attendant dilution of dissolved 

radionuclides in the Yucca Mountain 

area; and 

" To determine if there are any 

methodology issues that may impact 

implementation of a dose-based standard, 

as proposed by the NAS (National 

Research Council, 1995). 

The analysis presented herein was limited to 

evaluation of concentration variations during 

passive transport considering only a few 

variations in the assumed hydraulic properties 

and boundary conditions. In addition, the 

geohydrologic system was treated as an 

equivalent porous continuum, and no attempt 

was made to account for flow and transport

3-1 NUREG-1538
3 Ground-Water Pathway Analyses

through discrete fractures or to include matrix 

diffusion effects. Additionally, mixing induced 

by water well pumping was not considered in 

this analysis; the dilution effects from well 

pumping are addressed in Section 3.2. 

Because of the simplifications made and 

incompleteness of site characterization, the 

calculations presented in this section should 

not be viewed as an evaluation of regulatory 

compliance with existing or future HLW 

disposal standards. 

3.1.2 Analysis 

To assess ground-water dilution and its 

dependence on the hydrogeologic 

characteristics of the Yucca Mountain setting, 

a series of two-dimensional (2-D) computer 

simulations of ground-water flow and 

radionuclide transport were performed. 

Computer models were applied to compute 

four quantities: (a) hydraulic head 

distributions; (b) flow paths; (c) particle 

travel times; and (d) radionuclide plume 

distributions. Dilution of 99Tc was modeled 

because it is important to dose and reflects the 

dilution behavior of important radionuclides 

with relatively large inventories, long 

half-lives, and non-sorbing characteristics. 

Numerical calculations and graphical display 

of these four quantities were used to gain 

insight into the nature of the hydrogeologic 

processes that may control the degree of 

dilution at the Yucca Mountain site. Although 

available field data for the Yucca Mountain 

site are used, this scoping analysis did not 

consider uncertainties associated with the 

conceptualizations of ground-water flow or the 

spatial variability of hydraulic properties. 

Two computer codes were used in performing 

the scoping analysis: MAGNUM-2D, a 

saturated flow model (England et al., 1985) 

and CHAINT, a multi-component transport 

model (Kline et al., 1985). Hydraulic head 

distributions simulated with the 

MAGNUM-2D code were post-processed to 

provide visualizations of the flow paths (i.e., 

streamlines) and particle travel times.

Dilution factors 2 calculated with the Cf-fAINT 

code were contoured to depict plume 

spreading and dilution patterns. First, a 2-D 

representation of planar flow from the 

proposed repository site down-gradient to the 

Amargosa Desert (i.e., the potential location 

of a farmer/rancher for a hypothetical 

receptor group, as proposed for this analysis in 

Section 2.3) was considered to assess the 

extent of hydrodynamic dispersion that may 

occur as the hypothetical 99Tc plumes move 

through relatively long and heterogeneous 

flow paths. A second 2-D representation of a 

vertical cross-section through the proposed 

repository site was also considered. The 

purpose of this case was to examine mixing 

processes immediately beneath the site that 

may occur as a result of channelized flow 

through the complex geometry of the 

hydrostratigraphic units and fault zones. Of 

particular interest was the extent to which 

structures such as fracture zones and faults 

control flow patterns, mixing, and dilution. 

3.1.2.1 Conceptual Models of Ground-Water 

Flow 

Conceptualizations of lateral and vertical flow 

used in this scoping analysis drew largely on 

information from previous DOE modeling 

studies (Czarnecki and Waddell, 1984; Wilson 

et al., 1994) and existing field data. The lateral 

flow model consisted of a 580-square

kilometer flow tube extending from the 

repository site south to the Amargosa Desert 

area. The vertical flow model approximates 

the cross-section through the repository from 

borehole USW H-5 and extending through 

USW H-4; this cross-section, which 

encompasses about 3 square kilometers, is 

especially relevant because it appears to be 

aligned with the general direction of 

ground-water flow beneath the proposed 

repository site. Both the lateral and vertical 

flow conceptual models are defined in terms 

of: 

* Geometry of the hydrostratigraphic units; 

2

Dilution factor refers to the ratio of an initial concentration 

and a later concentration (e.g., ratio of concentration in the 

saturated zone directly below the repository footprint and the 

concentration in the saturated zone 20 kilometers down

gradient from the site).

NUREG- 1538 3-2
3 Ground-Water Pathway Analyses

" Contrasting values of saturated hydraulic 

conductivity; 

"* Variability of effective porosity; 

"* Location of distinct fault zones; and 

" Hydraulic head gradient and flow 

boundary conditions. 

The specific aspects of the two conceptual 

models are summarized in the following 

sections. 

3.1.2.1.1 Conceptual Model of Lateral Flow 

in the Yucca Mountain Region 

In developing a 2-D lateral flow model, 

computer simulation results previously 

published by the USGS (Czarnecki and 

Waddell, 1984) were examined and used. 

Czarnecki and Waddell applied a vertically 

integrated, steady-state model to simulate the 

regional flow system. These authors present a 

plot of the ground-water flux vectors that were 

computed from the hydraulic head field [see 

Plate 2 in Czarnecki and Waddell (1984)]. A 

subdomain of the Czarnecki and Waddell 

regional flow model was selected by tracing 

selected streamlines west and east of the 

proposed repository. Locations of the upper 

and lower boundaries of this streamtube were 

taken coincident with head contours of 800 

meters and 675 meters, respectively, as 

estimated from available field measurements. 

The streamtube, which is shown in Figure 

3-1, was divided into seven distinct material 

types or zones; each of these zones is 

designated by a number (see circled numbers). 

Boundaries for the seven zones (designated by 

dashed lines) were determined by inspecting 

available hydrostratigraphic cross-sections 

(Gillson et al., 1995; Roberson et al., 1995) and 

hydraulic head contours. 

Estimates of the horizontal hydraulic 

conductivities K,, and K/y were obtained by a 

manual calibration procedure in which the 

hydraulic conductivities of the seven zones 

were adjusted until the MAGNUM-2D code 

produced a reasonable fit with measured 

hydraulic heads (Robison, 1984; and Ervin et 

al., 1993). These initial estimates were

subsequently checked and adjusted using an 

autocalibration algorithm that employed an 

indirect inverse procedure based on either 

maximum likelihood or statistically robust 

M-estimator theory to estimate model 

parameters (Wittmeyer, 1990; Wittmeyer and 

Neuman, 1992; and Carrera and Neuman, 

1986a,b,c). Within the selected flow domain 

(see Figure 3-1), there were 146 locations at 

which estimates of hydraulic head data were 

available. Of these measurements, 22 were in 

the general vicinity of Yucca Mountain; 5 just 

west of the community of Lathrop Wells; and 

the remaining 119 in the greater Amargosa 

Desert. Within each of the seven zones, the 

hydraulic conductivity was assumed uniform 

and isotropic. 

Because the lateral flow model used two 

Dirichlet and two no-flow boundary 

conditions, the values of hydraulic conductivity 

in the seven major zones are not uniquely 

identifiable in the absence of prior estimates 

of either areal flux or hydraulic conductivity 

(Carrera and Neuman, 1986b). Inasmuch as 

areal recharge within this region is minimal, 

fixing at least one hydraulic conductivity value 

was judged the best option. Accordingly, Zone 

1, located at the southern end of the 

streamtube model, was assigned a fixed 

hydraulic conductivity value of 1.7 x 10-5 

meters/second, consistent with the estimate of 

Czarnecki (1985). To assess the fit between 

predicted and observed values, the hydraulic 

head residual was calculated for each well 

location. The head residuals ranged from 

-38.4 to 62.3 meters, with an average head 

residual of 1.4 meters and an average absolute 

head residual of 6.3 meters. The head residual 

value is equal to the head value predicted by 

the streamtube model minus the measured 

head value. An improved fit could have been 

achieved by increasing the number of zones; 

however, the selected zonation was considered 

adequate for this 2-D scoping analysis. The 

seven zones and assumed hydraulic properties 

are summarized in Table 3-1. 

After completing the calibration of the flow 

model, an additional zone representing the 

Bow Ridge fault was added to the conceptual

3-3 NUREG- 1538
3 Ground-Water Pathway Analyses

0

6

0 0

I I 1 0 t 0 a * I

10 20 

Horizontal Distance (km)

Figure 3-1. Location of lateral flow model, material zones (circled numbers), well locations, and location of the 

proposed repository footprint. The symbol 0 designates a private water well location whereas the 

symbol o designates a DOE water well location.

NUREG-1538

60

50

40

y 

x

30

E 

Co 

0 

a" 

U) 

a)

20 

10 

0

0 30

3-4
3 Ground-Water Pathway Analyses

Table 3-1. Estimates of Hydraulic Conductivities Computed 

Using Autocalibration Technique 

Hydraulic Conductivity 

Zone (m/sec) 

K .. Kyy 

1 - Alluvium 1.7 x 10-5 1.7 x 10-5 

2 - Alluvium 8.6 x 10-6 8.6 x 10-6 

3 - Alluvium 4.5 x 10-6 4.5 x 10-6 

4 - Composite (Alluvium/Tuff) 1.7 X 10-5 1.7 x 10-5 

5 - Tuff Aquifer 1.1 X 10-5 1.1 X 10-5 

6 - Solitario Canyon 7.4 x 10-7 7.4 X 10-7 

7 - Drill Hole Wash 2.0 x 10-9 2.0 x 10-9

model, with hydraulic properties assigned to 

represent two contrasting cases: 

Preferential flow along the fault and 

partial barrier to flow across the fault 

(i.e., Kx and Ky set to 10-7 and i0-5 

meters/second, respectively); and 

Barrier to ground-water flow (i.e., K., and 

16 set to 10-8 meters/second). 

These two cases produced distinct flow paths, 

particle travel times, and plume dilution 

patterns in the vicinity of the proposed 

repository. 

In addition to the hydraulic conductivities 

discussed previously, calculation of 

ground-water velocity and plume dilution 

required estimates of "effective porosity" (i.e., 

the portion of the total porosity participating 

in the transmission of water). At the time this 

analysis was performed, there are no field data 

for effective porosities of the tuff formations 

or the alluvium at the Yucca Mountain site. 

Fracture porosity, estimated using the cubic 

law (Snow, 1969) and observed fracture 

porosities, is one surrogate for effective 

porosity. Erickson and Waddell (1985) 

estimated fracture porosities of productive 

zones in the tuff aquifer to range from about 

10-4 to 10-3; this range was estimated using 

transmissivity data for fracture zones from 

borehole USW H-4. In an unconfined system,

the specific yield is another surrogate 

parameter for effective porosity (Domenico 

and Schwartz, 1990). The USGS obtained 

specific yield data for the tuff aquifer (Geldon, 

1995) in boreholes east of the proposed 

repository site and for the alluvium (Walker 

and Eakin, 1963) in the vicinity of the 

Amargosa Desert. In the calculations 

presented, the specific yield data were used to 

provide representative estimates of effective 

porosity 4. 

The C-Well Complex (Geldon, 1995) of 

boreholes (i.e., UE-25c #1, UE-25c #2, and 

UE-25c #3), located on the east flank of the 

Yucca Mountain site, penetrates the saturated 

Calico Hills aquifer, Upper Prow Pass 

confining unit, the Prow Pass-Upper Bullfrog 

aquifer, the Middle Bullfrog confining unit, 

the Bullfrog aquifer, the Lower Bullfrog 

confining unit, and the Tram aquifer. Geldon 

analyzed two well interference tests conducted 

in the Calico Hills and Prow Pass-Upper 

Bullfrog aquifers using the Neuman-type curve 

method (1975) for an unconfined, anisotropic 

aquifer. Type curve analysis of heads 

measured in UE-25c #1 with pumping in 

UE-25C #2 indicated that the specific yield 

for the unconfined Calico Hills aquifer is 0.003 

(Op cit.). The Prow Pass- Upper Bullfrog 

aquifer may either be confined or unconfined 

in UE-25C #1. If unconfined, the Prow 

Pass-Upper Bullfrog aquifer has a specific 

yield of 0.004(Op cit.). For a field test where

3-5 NUREG-1538
3 Ground-Water Pathway Analyses

UE-25c #3 was pumped with heads monitored 

in UE-25c #2, the specific yield for the com

posite column was estimated to be 0.07 (Op 

cit.). Thus, for those portions of the planar 

flow model in which the upper 100 to 300 

meters of the saturated zone are ontained in 

the fractured volcanics, effective porosity was 

assumed to be bounded by 0.003 < ( •0.07. 

For the remaining zones of the model domain, 

effective porosities for the alluvium were 

inferred from specific yield estimates made by 

Walker and Eakin (1963). These authors 

estimated the average specific yield to be 0.17 

from textural descriptions from drillers' logs 

for 57 wells in the Amargosa Desert. Walker 

and Eakin also noted that the variation in 

physical conditions throughout the Amargosa 

Desert would suggest that the specific yield 

ranges from about 0.10 to 0.20. Accordingly, 

the effective porosity for these portions of the 

flow domain model was assumed to be 

bounded by 0.10 < ( < 0.20. It is important to 

acknowledge that the effective porosities of 

alluvium can be much larger than 0.20-e.g., 

0.30 to 0.40 (Freeze and Cherry, 1979; 

Domenico and Schwartz, 1990)]. The 

significance of this observation is that larger 

values of effective porosities result in longer 

particle travel times. 

3.1.2.1.2 Conceptual Model for Vertical 

Flow beneath the Yucca Mountain 

Site 

To develop a 2-D representation of flow in 

the tuff aquifer beneath the proposed 

repository site, the geologic cross-section 

developed by the USGS (Scott and Bonk, 

1984) was used (see Figure 3-2). This 

cross-section clearly illustrates the 

heterogeneous nature and complex geometry 

of the strata beneath Yucca Mountain, which 

are expected to influence mixing and dilution. 

The cross- section depicts the slightly 

east-dipping hydrostratigraphic units of the 

tuff aquifer, as well as the Ghost Dance and 

Bow Ridge fault zones. To simplify the 

generation of the computational grid, various 

secondary faults in this cross-section were not 

explicitly modeled. This northwest to 

southeast cross-section, which passes through

boreholes USW H-5 and USW H-4, is 

particularly relevant because it is oriented 

along the principal direction of ground-water 

flow and through the center of the proposed 

repository site. 

The location of the upper boundary of the 

conceptual model was obtained by 

interpolation of available borehole data. Both 

the upper and lower boundaries of the model 

domain were treated as no-flow boundaries. 

The hydraulic heads at the inflow and outflow 

boundaries were set to impose an average 

hydraulic gradient of 3.4 x 10-3. This gradient 

was estimated from the steady-state hydraulic 

head field calculated for the planar flow 

model. The hydraulic conductivities assigned 

to the individual hydrostratigraphic units were 

largely drawn from the field data for borehole 

USW H-4 presented in Whitfield et al. 

(1985). A hydraulic conductivity profile for 

USW H-5 was not available because the field 

test results were apparently too difficult to 

interpret (Robison and Craig, 1991) possibly 

because of the hydraulic influence of high

angle fractures near the borehole. The 

effective porosities assigned to the units were 

consistent with those used in the planar flow 

model. 

The hydraulic conductivity profile measured in 

borehole USW H-4 (Whitfield et al., 1985) 

was used to assign properties to individual 

hydrostratigraphic units. Whitfield et al., 

report pump test data for 19 individual 

hydrostratigraphic units. For simplicity, 

certain adjacent flow zones with similar 

hydraulic conductivities were lumped together. 

This produced a simpler hydrostratigraphic 

model consisting of 11 major zones. To 

account for anisotropic characteristics of these 

strata, an anisotropy ratio (i.e., ratio of 

vertical hydraulic conductivity K. to the 

horizontal hydraulic conductivity K,) of 1 to 5 

was assumed. This assumption had the effect 

of emphasizing channelized flow along the 

hydrostratigraphic units. The Bow Ridge fault 

was represented as an anisotropic feature in 

the conceptual model. The hydraulic 

conductivity values assumed for the 

hydrostratigraphic model are summarized in 

Table 3-2.

NUREG-1538 3-6
3 Ground-Water Pathway Analyses

Figure 3-2. Vertical cross-section through boreholes USW H-5 and USW H-4 

(Scott and Bonk, 1984). 

Symbol Unit 

Q~hc Alluvium and Colluvium 

Tmrn Ranier Mesa Member of Timber Mountain Tuff, nonwelded 

Tpcw Tiva Canyon Member of Paintbrush Tuff, welded 

Tptw Topopah Spring Member of Paintbrush Tuff, welded 

Tcbw Bullfrog Member, welded 

Tctw Tram Member, welded 

BF Zone of west-dipping strata containing abundant breccia and faults 

TCpw Brow Pass Member of Crater Flat Tuff, welded 

n Nonwelded tuff

3-7 NUREG-1538
z I Paintbrush Canyon Fault 

0X

Figure 3-2. continued

z 

00

0

I0

Elevation 

(M) 

1500 

-1000 

-500 

-0 

-- 500
3 Ground-Water Pathway Analyses

Hydraulic Conductivity 

Zone (misec) 

K__ KYY 

1 - Calico Hills (CH) 1.0 x 10-6 2.0 x 10-7 

2 - Prow Pass #1 (PP1) 1.0 X 10-5 2.0 X 10-6 

3 - Prow Pass #2 (PP2) 2.0 x 10-6 4.0 x 10-7 

4 - Bull Frog #1 (BF1) 2.0 x 10-5 4.0 X 10-6 

5 - Bull Frog/Tram (BF/TR) 4.0 x 10-6 8.0 x 10-7 

6 - Tram #1 (TR1) 2.0 x 10-5 4.0 x 10-6 

7 - Tram #2 (TR2) 2.0 x 10-6 4.0 x 10-7 

8 - Tram #3 (TR3) 2.0 x 10-5 4.0 x 10-6 

9 - Tram #4 (TR4) 1.0 X 10-6 2.0 X 10-7 

10 - Lithic Ridge #1 (LR1) 2.0 x 10-5 4.0 x 10-6 

11 - Lithic Ridge #2 (LR2) 2.0 x 10-6 4.0 x 10-7 

12 - Bow Ridge Fault 1.0 x 10-7 5.0 X 10-7

3.1.2.2 Computer Simulations for Lateral 

Flow Model 

Steady-state representations of the 

potentiometric field for the lateral flow model 

were generated for two cases in which the Bow 

Ridge fault was treated as a preferential flow 

pathway and a flow barrier. The hydraulic 

head fields calculated (with the 

MAGNUM-2D computer code) for both cases 

were post-processed to obtain head contours, 

flow vectors, Darcy fluxes, streamlines, and 

cumulative particle travel times. Flow paths, 

for particles released at locations along a line 

tangential to the lower boundary of the 

proposed repository footprint, were plotted to 

provide a visualization of ground-water flow 

patterns. Along each flow path (or 

streamline), the particle travel time was 

calculated and summed to give an indication 

of the impact of velocity variations. 

The formation and movement of hypothetical 

99Tc plumes were computed (with the 

CHAINT computer code) for 104 years using a 

longitudinal mass dispersivity aL of 200

meters and transverse mass dispersivity a T of 

10 meters. The longitudinal dispersivity value 

was selected by examining dispersivity data 

plotted in Gelhar (1993), which displays the 

relation between aL and the scale of 

observation; the selected value is about 

one-fifth of that used by DOE (Wilson et al., 

1994) in radionuclide transport simulations for 

the proposed repository site. The transverse 

dispersivity was computed as a T = QL /20, 

following Fetter (1993). 

A relatively fine grid, consisting of more than 

6000 elements, was used to represent the flow 

domain. This fine grid was used to minimize 

numerical dispersion. The following sections 

present and interpret the significance of 

subregional flow path and particle travel time, 

and plumes and dilution factors. 

3.1.2.2.1 Hydraulic Heads, Flow Vectors, 

and Darcy Fluxes 

Patterns of subregional ground-water flow are 

determined by the combined effects of the 

hydraulic boundary conditions, geometry of 

flow domain, contrasts in hydraulic properties,

NUREG-1538

Table 3-2. Assumed Hydraulic Conductivities for the Vertical Cross

Section Model, Based on Data Presented by Whitfield et al. (1985). 

Abbreviations are for stratigraphic units depicted in Figure 3-7.

3-9
3 Ground-Water Pathway Analyses

and structural features such as fault zones. 

Some of these effects are illustrated in the 

contour plot of the hydraulic head field shown 

in Figure 3-3; the dashed and heavy solid 

lines designate the material zone boundaries 

within the flow domain. In both cases (the 

preferential flow case and the barrier to flow 

case, for properties of the Bow Ridge fault), 

the hydraulic heads exhibit large gradients to 

the west (Solitario Canyon) and north (Drill 

Hole Wash) of the repository and then 

transition to more gradual variations in the 

tuff (Zone 5), composite (Zone 4), and 

alluvial regions (Zones 1, 2, and 3). In 

comparing the head fields for the two cases, 

the hydraulic characteristics assigned to the 

Bow Ridge fault only appear to have local and 

relatively small effects. 

Flow vector plots for the two cases are shown 

in Figure 3-4, along with the tabulated ranges 

for the Darcy fluxes; note that the flux 

magnitude is indicated by the arrow length. 

The principal differences in calculated results 

are confined to a small region between the 

proposed repository and Bow Ridge fault. For 

the preferential flow pathway case, 

ground-water flow occurs along and through 

the fault. In contrast, the flow barrier case 

shows that flow is routed around the fault. 

Particularly noteworthy was the fact that 

between the two cases the range of flux 

magnitudes was not substantially different, 

except in the Bow Ridge fault zone where 

differences were expected. Also presented in 

the figure are the Darcy fluxes (maximum and 

minimum values) computed for each zone. In 

the zone below the proposed repository 

footprint, the calculated fluxes for both cases 

range from about 0.5 to 1.9 meters/year. The 

largest flux magnitude is 3.7 meters/year in the 

tuff aquifer (i.e., Zone 5). 

3.1.2.2.2 Pathlines, Particle Travel Times, 

and Dilution Factors 

Flow paths calculated for particles released 

along the border of the repository provide 

insight into the subregional flow patterns. As 

can be seen in Figure 3-5, the streamlines for 

the preferential flow pathway case refract as

they pass through the Bow Ridge fault 

whereas, for the flow barrier case, the 

streamlines flow around the fault. The 

isopleths (heavy dashed lines) of constant 

particle travel time (also shown in this figure) 

add additional detail to the contrasting effects 

of the fault. These isopleths depict the relative 

rate of travel of particles moving passively 

with the ground water. It is clear from these 

isopleths that patterns of ground-water flow in 

the vicinity of the fault are quite distinct for 

the two cases considered. However, these 

distinct flow patterns appear to have relatively 

small local effects on lateral mixing and 

almost no observable influence on the 

larger-scale transport; this is more clearly 

shown in the subsequent figure. 

The impact of local and subregional flow 

patterns on contaminant movement is 

illustrated in the contour plot of 99Tc (see 

Figure 3-6); the isopleths are quantified in 

terms of dilution factors instead of 

radionuclide concentrations. Plume 

representations for a snapshot in time at 104 

years after release are shown in the figure. 

The contour plots suggest that local mixing 

and dilution in the vicinity of the repository 

are relatively small-i.e., dilution factors are 

about 2. Significant in both cases is that 

predicted dilution factors in the Amargosa 

Desert area are about the same. 

3.1.2.2.3 Computer Simulations for Vertical 

Flow Model 

A steady-state hydraulic head field was 

generated with the MAGNUM-2D computer 

code, using the boundary conditions and 

hydraulic properties described previously. The 

calculated hydraulic head field was contoured 

as well as post-processed to obtain flow 

vectors, Darcy fluxes, streamlines, and 

cumulative particle travel times. Flow paths 

for particles released at selected locations 

along the Ghost Dance fault were computed to 

provide a visualization. Along each flow path, 

the particle travel time was calculated and 

summed to give an indication of the influence 

of velocity variations.

NUREG-1538 3-10
50 

40

10

0

0 5 10 15 20 25 

Horizontal Distance (kin) 

A

0 5 10 15 20 25 

Horizontal Distance (kin) 

B

Figure 3-3. Hydraulic head fields for the lateral flow model for two cases, with the Bow Ridge fault assumed to be (a) the preferential 

flow pathway and (b) the flow barrier.

50 

40 

E 

• 30 

C) 

Ca 

2

• E 20

E 

- 30 

'- 2 

(I, 

i 

'-E 20

10

0-

:z 

I0

0F

t• I
3 Ground-Water Pathway Analyses

Figure 3 -4. Darcy flux vector plots for lateral flow model for two cases, with the Bow Ridge fault assumed to be (a) the preferential flow pathway and (b) the flow barrier. Darcy velocity values are provided in the 

accompanying table. Material types in table refer to circled numbers in figure. 

A-The Preferential Flow Pathway B--The Flow Barrier 

Darcy Velocities Darcy Velocities Material (m/yr) Material (m/yr) Type Minimum Maximum Type Minimum Maximum 

1 0.42 0.68 1 0.41 0.67 

2 0.43 0.66 2 0.41 0.65 

3 0.022 0.61 3 0.02 0.6 

4 0.027 0.8 4 0.023 0.81 

5 0.013 3.7 5 0.0085 3.8 

6 0.17 2.2 6 0.19 2.3 

7 0.00016 0.0055 7 0.0002 0.0056 

8 0.48 1.9 8 0.55 1.9 

9 0.031 1.8 9 0.0033 0.018

NUREG- 1538 3-12
50

40 

E 

." 30 

a( 

0 

"-E 20 

10 

0

0 5 10 15 20 25 

Horizontal Distance (km)

0 5 10 15 20 25 

Horizontal Distance (km)

A B

Figure 3-4. continued

50

40

E 

.• 30 

U) 

C.) 

fl) 

-F 20 

10 

0

z 

0 

00

N 

N
0 5 10 15 

Horizontal Distance (km) 

A

Z 

C 

C 

00

20 25

50

40 -1

0 5 10 15 20 25 

Horizontal Distance (kin) 

B

Figure 3-5. Pathlines and particle travel times for lateral flow model for two cases, with the Bow Ridge fault assumed to be (a) the preferential flow pathway and (b) the flow barrier.

50 

40 

? 30 

0 2 

10 

0

C N 0 

Ct 

N 

'-U 

a

'C 

a 

'C 

Ct

30 

C, 

C._ Q 

2 

0 2
50 50 

40

E 

CZ 

(a 

U.) -0 

0 20

10 

0

0 5 10 15 20 25 

Horizontal Distance (kin) 

A

0 

1 0 5 10 15 20 25 

Horizontal Distance (km) 

B

Figure 3-6. Radionuclide plume distributions for lateral flow model for two cases, with Bow Ridge fault assumed to be (a) the preferential flow pathway and (b) 

the flow barrier. Contour levels are in terms of dilution factors.

40 

30 30 

20

E) 

0 

0 

'C)

10 

0

Z 

00

I
3 Ground-Water Pathway Analyses

The formation and movement of the 

hypothetical 99Tc plumes were computed with 

the CHAINT computer code, using a 

longitudinal dispersivity aL of 30 meters and 

transverse dispersivity a T of 3 meters. The 

mass dispersivities were chosen to be smaller 

than those used in the lateral flow model 

because of the shorter length of the flow 

domain (i.e., smaller scale of observation). 

The longitudinal dispersivity was chosen to be 

consistent with the value used in IPA Phase 2 

(Wescott et al., 1995) whereas the transverse 

dispersivity was taken as a T = aL /10; a 

slightly larger transverse-to-longitudinal

dispersivity ratio was assumed, to reduce 

gridding requirements. 

A relatively fine grid, consisting of more than 

8000 elements, was used to represent the flow 

domain. This fine grid was used to minimize 

numerical dispersion. The following sections 

present and interpret the significance of: (a) 

local flow path and particle travel times; and 

(b) plumes and dilution factors. A range of 

computer simulations was made for this 

conceptualization; however, only selected 

cases are presented. 

3.1.2.2.3.1 Hydraulic Heads, Flow Vectors, 

and Darcy Fluxes 

Patterns of vertical ground-water flow in the 

tuff aquifer are determined by the combined 

effects of the hydraulic boundary conditions, 

geometry of the strata (e.g., dipping layers), 

and the presence of discontinuities associated 

with fault zones. These effects are illustrated 

in the contour plot of the hydraulic head field 

shown in Figure 3-7. Although the boundary 

conditions are uniform, the contours indicate 

the head field becomes reoriented. In some 

locations, the plot suggests that the hydraulic 

head fields adjusted to move water along the 

most conductive hydrostratigraphic units. In 

contrast, the hydraulic head field in the lower 

permeability units is oriented in such a 

manner as to gradually move water up, toward 

more conductive units. Very high local 

gradients develop in the vicinity of the Bow 

Ridge fault, indicating that this feature acts as 

a partial flow barrier.

NUREG-1538

Additional insights into the vertical flow field 

were obtained by computing and plotting the 

velocity vectors (see Figure 3 -8). The 

presence of the Ghost Dance fault appears to 

have little or no effect on the flow field, 

whereas the Bow Ridge fault zone produces a 

distinct downward flow field. Downstream of 

the Bow Ridge fault, the flow field becomes 

upward trending, negating the effects of the 

downward-dipping hydrostratigraphic units. 

Also presented in Figure 3-8 are the 

calculated ranges of Darcy fluxes computed 

for each unit (see accompanying table). In the 

vicinity of the water table, the maximum fluxes 

are estimated to be about 1.3 meters/year in 

the Prow Pass unit and about 1.5 meters/year 

in the Bullfrog unit. The largest flux (i.e., 2.9 

meters/year) occurs in the Lithic Ridge unit 

located about 500 meters below the water 

table. 

3.1.2.2.3.2 Flow Paths, Particle Travel Times, 

and Dilution Factors 

The flow paths calculated for particles 

released along the Ghost Dance fault confirm 

interpretations drawn from flow vectors. As 

can be seen in Figure 3-9, the streamlines 

near the water table in the Prow Pass unit are 

horizontal and then dip down, avoiding flow 

within the lower-permeability Calico Hills 

unit. These streamlines dip down, as they cross 

the Bow Ridge fault zone, but return to levels 

very near the water-table surface. Also shown 

in Figure 3-9 are isopleths (heavy dashed 

lines) of constant particle travel time. These 

depict the relative rate of particle travel 

through the ground-water system. It is clear 

from these isopleths that ground-Water 

movement is highly nonuniform in both the 

Prow Pass and Bullfrog units. This pattern of 

flow suggests that contaminants entering the 

aquifer would be transported primarily along 

the surface of the water table, with vertical 

mixing only occurring in areas where there are 

large changes in flow direction-i.e., the Bow 

Ridge fault. 

The previously inferred trends of contaminant 

movement are clearly illustrated in the 

3-16
a H-5 Ghost Dance Fault H-4 Bow Ridge Fault 

.0 Water Table RIdg a 

H Pass 2 ( Prow Pass 1 (PP1) "co Hills (CH) PP1 

2)-152- Bullfrog (BF) PP1 PP 

fro [Tram (BF Tra 1.. PP2 TR1 B 

-304 BF BF/T 

3ýTram 2( BUMT TR2 A 0 TT3 

Z 456- Tram 4 (TR4) Tram 3 (TR3) TR2 TR LR1 TR4 

a) -608- 4 OLithic Ridge 1 (LR1) LRI Lithic Ridge 2 (LR2) LR2 

S-760. 

o 0 880 1760 2640 3520 4400 

Horizontal Distance (m) 

E 

a) 

-' Water Table 

-152

S-304

0B D -4608

C749 748 

S-760-• , 

C Q 0 880 1760 2640 3520 4400 

Horizontal Distance (m) 

r• l~io~zr, .--'7 Ild~zlJ h~A f.I~ld far uprftiQi pracl.=cpfinni mndl. •tr~tior!~nhirT nnmpncl~ttnr• t~akpn frnm Whitfield it nI (1 Q•;'\

M A "lip hoot] fi-Ids For vertical P se-section model Strati rahic nomenclature taken from Whitfield et aL (1985N
3 Ground-Water Pathway Analyses

Figure 3- 8. Darcy flux vector plots for vertical cross-section flow model. Darcy velocity values are provided in the 

accompanying table.

NUREG-1538

Darcy Velocities 

Stratigraphic (m/yr) 

Unit Minimum Maximum 

Calico Hills 0.03 0.79 

Prow Pass 1 0.015 1.3 

Prow Pass 2 0.018 0.27 

Bullfrog 0.087 1.5 

Bullfrog/Tram 0.066 0.38 

Tram 1 0.28 1.2 

Tram 2 0.029 0.23 

Tram 3 0.045 2.2 

Tram 4 0.011 0.32 

Lithic Ridge 1 0.041 2.9 

Lithic Ridge 2 0.027 0.83 

Bow Ridge 0.018 0.5

3-18
0 880 1760 2640 3520 

Horizontal Distance (m)

Figure 3-8. continued

P 

U) 

U)

Z 

0

4400

(jj 

C) 

0
Contaminant Source 

(Ghost Dance Fault)

Horizontal Distance (m)

Figure 3-9. Pathlines (a) and particle travel times (b) for vertical cross-section flow model.

~rj C.I 

0 

00

E 

0 

a) 

I

U) 

0

A

a) "Z 0

-152

• -304

o -456

) -608

-o 760 

W) 

5 0 880 1760 2640 3520 

Horizontal Distance (m)

B

4400

I
3 Ground-Water Pathway Analyses

contour plot of the 99Tc plume shown in Figure 

3-10; the isopleths in this case are expressed 

in terms of dilution factors. Plume 

representations for two snapshots in time (i.e., 

times of 200 and 1000 years after release) are 

shown in the figure. The computer simulation 

results suggest there is relatively little local 

mixing and dilution (i.e., dilution factor of 

about 2) near the contaminant source. The 

plot for 1000 years clearly illustrates two 

important points: (a) the contaminant plumes 

remain relatively undiluted near the water

table surface; and (b) structural features such 

as the Bow Ridge fault can indeed produce 

significant vertical spreading of the 

contaminant plume. 

3.1.3 Assumptions and Limitations 

3.1.3.1 Conceptual Models 

The reliability of the dilution factor estimates 

presented here depend, to a very large degree, 

on the appropriateness of ground-water flow 

conceptualizations implemented in the 

numerical models. Much direct and indirect 

evidence (e.g., hydrostratigraphy, head 

gradients, and temperature profiles) suggests a 

relatively complex three-dimensional (3- D) 

flow system in the tuff aquifer. For example, 

the general lateral ground-water flow in the 

tuff aquifer, which appears to primarily occur 

through interconnected shear fracture zones 

(Geldon, 1993), can be interrupted by upward 

flow (or upwelling) in the vicinity of faults

e.g., upward flow along splays of the Solitario 

Canyon fault (Wilson et al., 1994). In 

developing conceptual models for this scoping 

analysis, a number of simplifying assumptions 

were made regarding: 

"* Dimensionality of the conceptual model; 

" Hydraulic conductivity and mass 

dispersivity tensors; 

" Heterogeneity and spatial variability of 

hydraulic properties; and 

"* Hydraulic boundary conditions. 

As discussed previously, 2-D conceptual 

models were adopted to simplify the modeling

task. The reduced dimensionality of the 

conceptual models used in this analysis is 

significant in that mixing processes in the third 

dimension are neglected, which results in 

underestimating the degree of dilution. This 

limitation may be particularly significant in the 

lateral flow model, where the plume was 

assumed to be confined to a 10-meter mixing 

depth, because of the 2-D assumption. In the 

actuality, vertical mixing of the plume would 

occur over the long flow path length (i.e., 

more than 30 kilometers), dispersing the 

plume over much greater depths and 

enhancing dilution. In the case of the vertical 

flow model, the 2-D assumption is probably 

less significant because of the short path 

length of the flow domain. 

In the 2-D models, the tensorial nature of 

hydraulic conductivity and mass dispersivity 

was simplified by assuming the principal 

directions were aligned with the coordinate 

axes. Although this assumption is convenient 

(i.e., the cross terms of the tensors become 

zero) and commonly employed, it reduces the 

ability of the models to capture important 

directional characteristics of the flow field. 

These simplifications, however, are typically 

conservative with respect to dilution, because 

certain aspects of hydrodynamic dispersion are 

neglected. Further conservatism was 

introduced by choosing mass dispersivities (aL 

and a T) which are expected to be on the low 

side relative to values reported in the 

literature (Waldrop et aL, 1985). 

Available site data indicate that the actual 

ground-water system exhibits much 

heterogeneity and spatial variability. 

Hydraulic conductivities of relatively large 

spatial regions and individual 

hydrostratigraphic units were assumed 

uniform in both the lateral and vertical flow 

models. This assumption of homogeneous 

regions and strata is consistent with 

simplifications made in recent DOE 3-D 

ground-water flow analyses (Wilson et al., 

1994; Arnold and Barr, 1996); however, it is 

significant to analysis of mixing processes (as 

well as to flow paths and particle travel times). 

Effects of heterogeneity were indirectly taken 

into account through the use of mass

3-21 NUREG- 1538
Contaminant Source 

(Ghost Dance Fault)

A •

Bow Ridge Fault H-4

z 

C) 

00

E 

.0 

-152

O -304 

6-456 

m -608 

Z -760

0 880 1760 2640 3520

Figure 3-10. Radionuclide plume distributions for vertical cross-section model with contour levels in terms of dilution factors. 

200 years after release (a); 1000 years after release (b).

Horizontal Distance (M)

4400

E 

a) 

0 

a) 

a) ID 

.1, 

0

-152

-304 

-456 

-608 

-760

44B 

4400

Horizontal Distance (m)
3 Ground-Water Pathway Analyses

dispersivities, but nevertheless the 

homogeneity assumption probably leads to an 

underestimation of dilution. 

The current sparsity of data required making 

certain assumptions regarding the hydraulic 

boundary conditions for both the lateral and 

vertical flow models. The hydraulic heads 

assigned to the inflow and outflow boundaries 

of the lateral flow model are consistent with 

field data; however, the side boundaries were 

assumed to be no-flow boundaries. The 

no-flow assumption may be tenuous in the 

vicinity of the Amargosa Desert, where water 

well pumping probably affects flow patterns as 

may interbasin transfers. In the case of the 

vertical flow model, the uniform head profile 

assigned to the inflow boundary (at USW 

H-5) is probably not accurate because this is 

a region of likely upward flow; the true head 

profile at this boundary probably exhibits 

distinct vertical gradients. How significantly 

these types of assumptions affect the dilution 

effects is uncertain at this time. 

One of the fundamental assumptions made is 

that the hydraulics of the ground-water system 

can be modeled as an equivalent porous 

medium. This assumption is probably quite 

defensible for the alluvial aquifer (lower 

portion of the lateral flow model) but 

potentially weak for the tuff aquifer (upper 

portion of lateral flow model). Dual porosity 

or dual permeability models (National 

Research Council, 1996) may provide more 

realistic representations of hydraulic and 

transport behavior of fracture zones in the tuff 

formations. For at least one borehole at Yucca 

Mountain, the dual porosity approach has 

been shown to yield a better interpretation of 

pump test data (Moench, 1984). 

3.1.3.2 Hydraulic and Tl-ansport Properties 

At present, there are many published data sets 

on the hydraulic and transport properties of 

the unsaturated zone at Yucca Mountain. 

These data sets-documented in Flint and 

Flint (1990); Wittwer et al. (1995); Rautman et 

al. (1995); Schenker et al. (1995); and Flint et 

al. (1996)-have been used in various 

subsystem performance assessments (i.e.,

Arnold et aL, 1995; Ho et al., 1995; Arnold and 

Barr, 1996) as well as TSPAs (i.e., Wilson et al. 

1994; TRW Environmental Safety Systems, 

Inc., 1995). In contrast to the considerable 

data published for the unsaturated zone, the 

amount of field data available for the 

saturated zone is limited, particularly for 

parameters necessary for dilution calculations. 

These data are not only limited in amount but 

also in spatial coverage. 

In the conduct of this scoping analysis, past 

and recent USGS reports on field testing 

conducted in the tuff aquifer were reviewed to 

compile necessary data. Other borehole data 

for the alluvial aquifer were also examined. 

Particularly important was identifying data for 

estimation of: 

"* Hydraulic heads and gradients; 

"* Hydraulic conductivities; 

"* Effective porosities; and 

"• Mass dispersivities. 

Most hydraulic head data (from which head 

gradients may be calculated) available for the 

Yucca Mountain site are in terms of composite 

heads (i.e., vertically averaged heads). As 

such, these data do not provide a means of 

estimating vertical head gradients. Hydraulic 

conductivity profiles such as those measured 

in USW H-4 (Whitfield et al., 1985) provide a 

good indication of the range of values for the 

horizontal component K. of the conductivity 

tensor. At present, there are no data to 

estimate the vertical component Kzz. There are 

no field data for effective porosities or mass 

dispersivities, but the USGS is currently 

evaluating tracer tests in the C-Well Complex 

(Geldon, 1995) expected to yield such data. 

At the subregional scale, there are again 

composite head data but no known data for 

hydraulic conductivity, effective porosity, or 

mass dispersivities. 

3.1.3.3 Radionuclide Source Term 

The calculation of the radionuclide release 

from a repository generally requires the 

application of a detailed source term and 

release model (Sagar et al., 1992) that takes 

into account such factors as the engineered

3-23 NUREG- 1538
3 Ground-Water Pathway Analyses

barrier design, thermohydrologic conditions, 

near-field chemistry, and drift-scale flow 

conditions. For the purposes of this scoping 

analysis, a simple approach for calculating the 

99Tc release to ground water was adopted. In 

this approach, the release was computed 

assuming a fractional release rate of 105/year, 

a mixing depth of 10 meters, and 50 percent of 

the waste packages failed. This simple 

calculation of the source term assumed no 

dilution of 99

Tc in the unsaturated zone and 

that the radionuclide instantaneously reached 

the saturated zone. For the lateral flow case, 

three separate source locations on the 

periphery of the repository footprint were 

assumed for the purpose of creating distinct 

plumes. For the vertical cross-section model, 

source locations adjacent to the Ghost Dance 

fault were assumed; these source zones 

spanned a distance of about 120 meters on 

each side of the fault. No uncertainties or 

parameter variations in the radionuclide 

source term were examined. These 

assumptions are significant when interpreting 

the calculational results. 

3.1.4 Summary of Analysis Results 

3.1.4.1 Ground-Water Flow Paths and 

Particle Travel Times 

Computer visualizations of flow paths for the 

two conceptualizations of 2-D ground-water 

flow provided a preliminary understanding of 

flow patterns. For example, the simulations of 

lateral flow in regional ground-water flow 

systems indicated that, depending on the 

hydraulic characteristics of faults (such as the 

Bow Ridge fault), streamlines depict either 

flow across and along the fault zone or, 

alternatively, flow completely around it. Such 

observations are consistent with the hydraulic 

head field generally orienting itself to move 

water along the most conductive components 

of the hydrogeologic system. Similarly, the 

simulations for the vertical flow conceptual 

model (based on the cross-section through 

USW H-5 and USW H-4) suggested that the 

streamlines in the aquifer beneath the 

proposed repository generally follow 

hydrogeologic units with higher hydraulic 

conductivity. This trend was only altered by

the presence of fault zones that caused 

refraction and spreading of streamlines. 

Particle travel time (tp) calculations for both 

the vertical flow and lateral flow models 

highlighted the sensitivity to location of the 

particle release point and hydraulic 

conductivities and effective porosity values. 

For example, release points on the 

northeastern boundary of the proposed 

repository footprint appear to follow the 

longer flow paths, whereas those on the 

southern boundary trace out more direct paths 

with shorter particle travel times. Order

of-magnitude estimates of particle travel times 

were calculated for the assumed upper bound 

values (see Section 3.1.2.1.1) for effective 

porosities: 

" Lateral flow model (from edge of the 

proposed repository to the Amargosa 

Desert): tp - 104 years; and 

" Vertical flow model (from Ghost Dance 

fault to Bow Ridge fault): tp - 500 years. 

It is important to note that the calculation of 

particle travel times is very sensitive to 

effective porosity (0 values. For instance, if 

the lower bound values of 4) (see Section 

3.1.2.1.1) are assumed, the lateral flow model 

produces particle travel times of about 3000 to 

5000 years for a path length of about 30 

kilometers (to the Amargosa Desert area); for 

equivalent 4) assumptions, the vertical flow 

model yields particle travel times of about 25 

years for a path length of about 3 kilometers. 

The relatively short particle travel times in the 

vertical flow model are consistent with 

conditions of fracture flow as opposed to 

matrix flow. The expectation of fracture flow is 

supported by field data (Geldon, 1993) that 

indicate the primary ground-water flow in the 

tuff aquifer occurs in fracture zones (e.g., 

shear fractures). It is noteworthy to mention 

that although some field data were used in 

these calculations, the deterministic analyses 

presented have not considered parameter 

uncertainties (e.g., spatial variability of 

hydraulic properties), flow in discrete 

fractures, or possible implications of matrix 

diffusion effects.

NUREG-1538 3-24
3 Ground-Water Pathway Analyses

3.1.4.2 Ground-Water Fluxes 

The ratio of moisture flux through the 

unsaturated zone (quz) to the saturated zone 

ground-water flux (qsz) is a rough indicator of 

the bulk mixing and dilution that can 

potentially occur in the aquifer immediately 

beneath the proposed repository footprint. As 

shown in TRW Environmental Safety Systems, 

Inc. (1995), a bulk mass balance can be used 

to derive an approximate expression for the 

dilution factor DF: 

q

DF = qu- gf (3-1) 

where gf is a geometric factor computed by 

dividing the cross-sectional flow area in the 

aquifer by the effective flow area of the 

proposed repository footprint. The factor is gf 

equals 0.1 assuming a cross-sectional flow area 

in the aquifer of 40,000 square meters (i.e., a 4 

kilometer width and a mixing depth of 10 

meters) and an effective flow area of the 

repository footprint of 410,000 square 

kilometers (i.e., 10,000 waste packages and a 

catchment area of 41 square meters/waste 

package). In NRC's IPA Phase 2 study 

(Wescott et al., 1995), the maximum un

saturated zone flux values assumed for the 

current and pluvial climates were 5 X 10-3 

and 10-2 meters/year, respectively. The flow 

simulations using the MAGNUM-2D 

computer code indicate ground-water fluxes 

(below the proposed repository) ranging from 

about 0.5 to approximately 2.0 meters/year. 

Using the above equation, a rough estimate of 

the range of dilution factors yields 5 < DF < 

20. The lower-bound value is consistent with 

the more detailed transport calculations. 

3.1.5 References 

Arnold, B.W, et al., "Unsaturated-Zone 

Fast-Path Flow Calculations for Yucca 

Mountain Groundwater Travel Time Analyses 

(GWTT-94)," Albuquerque, New Mexico, 

Sandia National Laboratories, SAND95

0857, September 1995. [Prepared for DOE.] 

Arnold, B.W, and G.E. Barr, "Numerical 

Modeling for Saturated-Zone Groundwater 

Travel Time Analysis at Yucca Mountain," 

American Nuclear Society/American Society

of Civil Engineers, Proceedings of the Seventh 

Annual International Conference: High-Level 

Radioactive Waste Management, April 29

May 3, 1996, Las Vegas, Nevada, pp. 187-189 

[1996]. 

Czarnecki, J.B., "Simulated Effects of 

Increased Recharge on the Ground-Water 

Flow System of Yucca Mountain and Vicinity, 

Nevada-California," U.S. Geological Survey, 

Water Resources Investigations, 

WRI-84-4344, 1985. 

Czarnecki, J.B. and R.K. Waddell, 

"Finite-Element Simulation of Ground-Water 

Flow in the Vicinity of Yucca Mountain, 

Nevada-California," U.S. Geological Survey, 

Water-Resources Investigations Report, 

WRI-84-4349, 1984. 

Carrera, J., and S.P. Neuman, "Estimation of 

Aquifer Parameters under Transient and 

Steady-State Conditions: 1. Maximum 

Likelihood Method Incorporating Prior 

Information," Water Resources Research, 

22(2):199-210 [1986a]. 

Carrera, J., and S.P. Neuman, "Estimation of 

Aquifer Parameters under Transient and 

Steady-State Conditions: 2. Uniqueness, 

Stability, and Solution Algorithms," Water 

Resources Research, 22(2):211-227 [1986b]. 

Carrera, J., and S.P. Neuman, "Estimation of 

Aquifer Parameters under Transient and 

Steady-State Conditions: 3. Application to 

Synthetic and Field Data," Water Resources 

Research, 22(2):228-242 [1986c]. 

Domenico, P.A., and EW Schwartz, Physical 

and Chemical Hydrogeology, New York, John 

Wiley and Sons, 1990. 

England, R.L., et al., "MAGNUM-2D 

Computer Code: Users' Guide," Richland, 

Washington, Rockwell International, 

RHO-BW-CR-143P, January 1985. 

Erickson, J.R., and R.K. Waddell, 

"Identification and Characterization of 

Hydrologic Properties of Fractured Tuff Using 

Hydraulic and Tracer Tests-Test Well USW 

H-4, Yucca Mountain, Nevada," U.S. 

Geological Survey, Water-Resources 

Investigations Report WRI- 85-4066, 1985.

3-25 NUREG- 1538
3 Ground-Water Pathway Analyses

Ervin, E.M., R.R. Luckey, and D.J. Burkhard, 

"Summary of Revised Potentiometric-Surface 

Map for Yucca Mountain and Vicinity, 

Nevada," American Nuclear Society/ 

American Society of Civil Engineers, 

Proceedings of the Fourth International 

Conference: High-Level Radioactive Waste 

Management, April 26-30, 1993, Las Vegas, 

Nevada, 2:1554-1558 [1993]. 

Fetter, C.W, Contaminant Hydrogeology, New 

York, MacMillan Publishing Company, 1993. 

Flint, L.E. and A.L. Flint, "Preliminary 

Permeability and Water-Retention Data for 

Nonwelded and Bedded Tuff Samples," U.S. 

Geological Survey, Open-File Report, 

USGS/OFR-90-569, 1990. 

Flint, L.E., et al. "Physical and Hydrologic 

Properties of Rock Outcrop Samples at Yucca 

Mountain, Nevada," U.S. Geological Survey, 

Open-File Report, USGS/OFR-95-280, 

1996. 

Freeze, R.A. and J.A. Cherry, Groundwater, 

Englewoood Cliffs, New Jersey, Prentice Hall, 

Inc., 1979. 

Geldon, A.L., "Preliminary Hydrogeologic 

Assessment of Boreholes UE-25c #1, 

UE-25c #2, and UE-25c #3, Yucca 

Mountain, Nye County, Nevada," U.S. 

Geological Survey, Water-Resources 

Investigations Report, WRI-92-4016, 1993. 

Geldon, A.L., "Results and Interpretation of 

Preliminary Aquifer Tests in Boreholes 

UE-25c #1, UE-25c #2, and UE-25c #3, 

Yucca Mountain, Nevada," U.S. Geological 

Survey, Water-Resources Investigations 

Report, WRI-94-4177, 1995. 

Gelhar, L.W, Stochastic Subsurface Hydrology, 

Englewood Cliffs, New Jersey, Prentice Hall, 

Inc., 1993. 

Gillson, R.G., III, et al., 'A Computer 

Hydrogeologic Model of the Nevada Test Site 

and Surrounding Region [Abstract]," 

Geological Society of America, 1995 Annual 

Meeting: Abstracts with Program, November

6-9, 1995, New Orleans, Louisiana, p. A-187 

[1995]. 

Ho, C.K., S.J. Altman, and B.W Arnold, 

'Alternative Conceptual Models and Codes 

for Unsaturated Flow in Fractured Tuff: 

Preliminary Assessments for GWTT-95," 

Albuquerque, New Mexico, Sandia National 

Laboratories, SAND95-1546, September 

1995. [Prepared for DOE.] 

Kline, N.W, R.L. England, and R.G. Baca, 

"CHAINT Computer Code: Users' Guide," 

Richland, Washington, Rockwell International, 

RHO-BW-CR-144P, December 1985. 

Moench, A.F, "Double Porosity Models for a 

Fissured Groundwater Reservoir with 

Fracture Skin," Water Resource Research, 

20(7):831-846 [1984]. 

National Research Council, "Technical Bases 

for Yucca Mountain Standards," Washington, 

D.C., National Academy Press, Commission 

on Geosciences, Environment, and Resources, 

July 1995. 

National Research Council, "Rock Fractures 

and Fluid Flow-Contemporary Under

standing and Applications," Washington, D.C., 

National Academy Press, U.S. National 

Committee for Rock Mechanics, 1996. 

Neuman, S.P., 'Analysis of Pumping Test Data 

from Anisotropic Unconfined Aquifers 

Considering Delayed Gravity Response," 

Water Resources Research, 11(2):329-342 

[1975]. 

Rautman, C.A., et al., "Physical and 

Hydrologic Properties of Rock Outcrop 

Samples from a Nonwelded Tuff Transition, 

Yucca Mountain, Nevada," U.S. Geological 

Survey, Water-Resources Investigation Report, 

WRI-95-4061, 1995. 

Robison, J.H., "Ground-Water-Level Data and 

Preliminary Potentiometric Surface Map of 

Yucca Mountain and Vicinity, Nye County, 

Nevada," U.S. Geological Survey, Water

Resources Investigations Report, 

WRI-84-4197, 1984. 

Robison, J.H., and R.W Craig, "Geohydrology 

of Rocks Penetrated by Test Well USW H-S,

NUREG- 1538 3-26
3 Ground-Water Pathway Analyses

Yucca Mountain, Nye Country, Nevada," U.S. 

Geological Survey, Water-Resources 

Investigations Report, WRI-88-4168, 1991. 

Roberson, K.E., E.H. Price, and S.J. 

Lawrence, "Simplification of the Geologic 

Framework of the Nevada Test Site and 

Surrounding Region for a Hydrogeologic 

Computer Model," Geological Society of 

America, 1995 Annual Meeting: Abstracts with 

Program, November 6-9, 1995, New Orleans, 

Louisiana, A-187 [1995]. 

Sagar, B., et al., "SOTEC: A Source Term 

Code for High-Level Geologic Repositories

Users' Manual (Version 1.0)," San Antonio, 

Texas, Center for Nuclear Waste Regulatory 

Analyses, CNWRA 92-009, July 1992. 

[Prepared for NRC.] 

Schenker, A.R., et al., "Stochastic 

Hydrogeologic Properties Development for 

Total-System Performance Assessments," 

Albuquerque, New Mexico, Sandia National 

Laboratories, SAND94-0244, September 

1995. [Prepared for DOE.] 

Scott, R.B. and J. Bonk, "Preliminary 

Geologic Map of Yucca Mountain, Nye 

County, Nevada, with Geologic Sections," U.S. 

Geological Survey, Open File Report 84-494, 

1984. 

Snow, D.T., "Anisotropic Permeability of 

Fractured Media," Water Resources Research, 

5(6):1273-1289 [1996]. 

TRW Environmental Safety Systems Inc., 

"Total System Performance Assessment

1995: An Evaluation of the Potential Yucca 

Mountain Repository," Las Vegas, Nevada, 

Document No. BOOOOOOOO-01717-2200

00136 (Rev. 01), November 1995. 

Waldrop, WR., et al., 'A Review of Field-Scale 

Physical Solute Transport Processes in 

Saturated and Unsaturated Porous Media," 

Palo Alto, California, Electric Power Research 

Institute, EPRI EA-4190, August 1985. 

[Prepared by the Tennessee Valley Authority

and the Massachusetts Institute of 

Technology.] 

Walker, G.E., and TE. Eakin, "Geology and 

Ground-Water of Amargosa Desert, Nevada

California," Carson City, Nevada, State of 

Nevada Department of Conservation and 

Natural Resources, Ground-Water Resources 

Reconnaissance Series Report 14, 1963. 

Wescott, R.G., et al. (eds.), "NRC Iterative 

Performance Assessment Phase 2: 

Development of Capabilities for Review of a 

Performance Assessment for a High-Level 

Waste Repository," U.S. Nuclear Regulatory 

Commission, NUREG-1464, October 1995. 

Whitfield, M.S., et al., "Geohydrology of 

Rocks Penetrated by Test Well USW H-4, 

Yucca Mountain, Nye County, Nevada," U.S. 

Geological Survey, Water-Resources 

Investigations Report, WRI-85-4030, 1985. 

Wilson, M.L., et al., "Total-System 

Performance Assessment for Yucca 

Mountain- SNL Second Iteration 

(TSPA-1993)," Albuquerque, New Mexico, 

Sandia National Laboratory, SAND93-2675, 

2 vols., April 1994. 

Wittmeyer, G.W "Robust Estimation of 

Parameters in Nonlinear Subsurface Flow 

Models Using Adjoint State Methods," 

Tucson, Arizona, University of Arizona, Ph.D. 

Dissertation, 1990. 

Wittmeyer, G.W, and S.R Neuman, 

"Monte-Carlo Experiments with Robust 

Estimation of Aquifer Model Parameters," 

Advances in Water Resources, 14:252 272 

[1992]. 

Wittwer, C., et al., "Preliminary Development 

of the LBL/USGS Three-Dimensional 

Site-Scale Model of Yucca Mountain, 

Nevada," Berkeley, California, Lawrence 

Berkeley Laboratory, LBL-37356/UC-814, 

June 1995. [Prepared for DOE.]

3-27 NUREG- 1538
3 Ground-Water Pathway Analyses

3.2 Initial Assessment of Dilution Effects 

Induced by Water Well Pumping in 

the Yucca Mountain Region 

3.2.1 Introduction 

The ground-water flow and transport 

modeling studies described in Section 3.1 

provide estimates of the degree to which 

radionuclide concentrations may be reduced 

during passive transport in the saturated zone 

from the area immediately beneath Yucca 

Mountain to potential receptor locations. 

These preliminary studies suggest that 

radionuclide concentrations at the water table 

experience relatively little dilution during 

saturated zone transport. Kessler and 

McGuire (1996) note that dispersive transport 

processes are relatively ineffective at reducing 

contaminant concentrations in a steady-state ground-water flow regime. However, if there 

are large temporal variations in the magnitude and direction of the ground-water velocity field, then mixing and attendant dilution 

during transport may be significant (Bellin et 

al., 1996). 

Current conceptual models of the Yucca 

Mountain saturated ground-water system would suggest that the flow regime in the 

immediate Yucca Mountain region is 

relatively unperturbed by fluctuations in the 

magnitude and location of recharge and 

discharge. However, increased pumping for 

irrigated agriculture in the Amargosa Farms 

area 4 (see Figure 3-11) over the past 30 years 

may have had some effect on the ground-water flow regime south of Yucca Mountain. 

3

This analysis was first published as Fedors and Wittmeyer (1998). Although it can be found in NRC's PDR, Fedors and 

Wittmeyer has not received wide-spread distribution thus far and as a consequence, a decision was made to include it in this 

NUREG. However, for the purposes of publication in this format, it has undergone a limited editorial review. Despite this review, the analysis is substantially the same as the earlier 

version placed in the PDR. 4Elsewhere in this NUREG, the terms 'Amargosa Valley" and 'Amargosa Farms area" were used to refer collectively to the 

set of communities of Ash Meadows, Amargosa Farms, Amargosa Valley, and Lathrop Wells, which all generally lie within the Amargosa Valley hydrographic basin. However, in 

this section of the NUREG, the term 'Anmargosa Farms" refers 

to the southern portion of the Amargosa Valley bounded 

approximately by the California-Nevada border, Nevada State 

Highway 373, and Amargosa Farm Road because residences 

and farming activities in the valley are concentrated in this 

triangular parcel of land.

Nonetheless, in this section it is assumed that 

pumping has no effect on the ground-water 

flow regime between Yucca Mountain and 

potential receptor locations. If the primary 

effect of pumping on the flow regime is 

enhanced mixing or more rapid transport, the 

assumption of steady-state flow conditions, if 

not realistic, is at least conservative from the 

standpoint of radionuclide dose. 

Even if the transient effects of well pumping 

on in situ radionuclide concentrations may be 

neglected, the radionuclide concentrations of 

water pumped from a well may differ 

significantly from in situ concentrations. 

Differences between in situ and borehole 

concentrations arise because a pumped well 

may capture water from different parts of the 

contaminant plume and the water in the 

borehole, therefore, reflects the average concentration of the portions of the 

radionuclide plume captured by the well. For 

small, low-discharge pumps extracting water 

from large, well-mixed plumes, the effects of 

borehole mixing may be negligible; however, if 

the plume is small relative to the discharge 

volume of the pumping, borehole mixing may be significant. 

Dilution factors can be defined in a number of 

ways. Each of the three definitions mentioned 

in this section are based on a particular approach to addressing dilution. The first 

approach addresses dilution that results from 

dispersion of a solute during transport for 

which the dilution factor is calculated as the 

ratio of concentration at the source area to 

that at the receptor location. The second 

approach addresses dilution from mixing and 

the corresponding dilution factor is calculated 

as the mass release rate divided by the largest flux of water into which the solute may be 

mixed and used by a hypothetical receptor 

group. The third approach addresses dilution 

caused by the intersection of the capture zone 

of a pumping well with the plume 

configuration at the withdrawal location. In 

this third case, the dilution factor is calculated 

as the ratio of the plume area intercepted by the capture area and the entire capture area. 

The third approach describes borehole

NUREG- 1538 3-28
3 Ground-Water Pathway Analyses

Figure 3-11. Lower Amargosa Desert Region south of the proposed Yucca Mountain repository site (R), including 

the Amargosa Valley and Amargosa Farms areas. Scale is 1:250,000.

3-29 NUREG-1538
3 Ground-Water Pathway Analyses

dilution from the geometric standpoint and it 

may be linearly combined with the first 

approach for a total borehole dilution factor. 

Usage of the first two approaches is described 

further below. 

Kessler and McGuire (1996) and Baca et al. 

(this report) 5 used the first approach to 

calculate point dilution factors (P - DFs) where 

point refers to concentration at a single point. 

Under assumptions of steady-state flow, 

estimated dilution factors, caused by 

dispersive mixing along the saturated zone 

transport pathway from the proposed Yucca 

Mountain repository to locations 20 to 30 

kilometers to the south, have ranged from 5 to 

50 (Baca et al., this report) and from 4 to 44 

(Kessler and McGuire, 1996). In both 

analyses, the reported dilution factors were 

determined by solving the advection

dispersion equation. Baca et al. contoured the 

P-DF, while Kessler and McGuire tabulated 

P-DFs based on centerline concentration. In 

DOE's TSPA-1993 (Wilson et al., 1994); 

TSPA-1995 (TRW Environmental Safety 

Systems, Inc., 1995); and NRC's IPA Phase 2 

effort (Wescott et al., 1995), it was assumed 

that additional dilution occurs at the receptor 

location because of mixing clean and 

contaminated water in the borehole and, in 

the case of TSPA- 1995, from mixing of waters 

from ground-water basins influent to the 

central region of the Amargosa Desert. 

In NRC's ongoing IPA effort (Mohanty and 

McCartin, 2000), it is assumed that a borehole 

dilution factor can be estimated using a single 

well that is pumped at a rate sufficient to 

supply all water needs for the hypothetical 

receptor group in question. For example, for 

12 quarter- section, center-pivot irrigation 

plots under cultivation with alfalfa, the 

equivalent annual well discharge 6 is 9,300,000 

cubic meters. If a hypothetical receptor group 

consists of a residential community of 500 

persons located 5 kilometers south of Yucca 

Mountain, the equivalent annual well 

5

Described in Section 3.1. 

6(12 plots x 126 acres/plot x 8 feet of water/yr) + (8.107 x 

10 -4 M3 /acre-ft).

discharge 7 would be 103,700 cubic meters. 

Borehole dilution factors can be computed 

directly for a potential receptor group, if the 

volume of contaminated water captured by the 

pumping well is known. For example, if, the 

volume of contaminated water captured by an 

irrigation well is 930,000 cubic meters, the 

dilution factor is 10. However, to determine a 

dose, one must compute the radionuclide 

concentration in the borehole and, hence, 

must either know the concentration of 

radionuclides in the contaminated water or the 

total mass of radionuclides captured by the 

well. Inherent in this approach is the 

assumption that the entire radionuclide plume 

is captured and that there is no well-to-well 

variation in the concentration. This analysis 

addresses the validity of this assumption 

considering the concept of borehole dilution 

as well as the distribution of pumping well 

locations and pumping-rate magnitudes. 

3.2.1.1 Geosphere Release Pathways 

Considered in a TSPA 

Farming in the Amargosa Farms area is 

partially related to the accessibility to well 

water. The combination of soil conditions and 

large depths to the water table restrict 

farming-based population growth to the area 

immediately south of the community of 

Amargosa Valley. The water table gradually 

approaches the land surface toward the 

southern reaches of the Amargosa Farms area. 

Based on the IPA Phase 2 analysis [See Neel 

("Dose Assessment Module") in Wescott et al. 

(1995)], exposure scenarios are assumed to 

occur through a combination of drinking water 

and ingestion of locally-raised produce and 

livestock. The lengths of the ground-water 

flow paths from Yucca Mountain to domestic 

and commercial wells and irrigation wells are 

approximately 25 and 30 kilometers, 

respectively. 

3.2.1.2 Literature Review 

In ground-water hydrology, the term borehole 

dilution is used to describe several phenomena 

including: (a) contaminant sampling biases 

resulting from improper monitoring well 

7(150 gal/person-day) x (500 persons) x (365.25 days/yr) x (3.785 x 10-3 m3 /gal).

NUREG- 1538 3-30
3 Ground-Water Pathway Analyses

construction; (b) the effectiveness of "pump 

and treat" remediation systems; and (c) 

capture zone analysis. Borehole dilution is 

used to explain one to two order-of-magnitude 

differences in values between concentrations 

measured in sampling wells and 

concentrations measured in the aquifer. 

Borehole dilution is also the name of a 

procedure used to estimate permeabilities or 

the seepage velocity in a single well bore 

through analysis of the dilution rate after 

release of a solute in the wellbore. Borehole 

dilution, as used in this scoping analysis, refers 

to dilution of the resident contaminant 

concentrations in a wellbore from pumping a 

well that captures both contaminated and 

uncontaminated portions of the aquifer. 

Six factors that may significantly affect the 

borehole concentration are: (a) well pumping 

rate and well distribution in the well field; (b) 

regional hydraulic gradient; (c) transmissivity; 

(d) hydrostratigraphy and anisotropy; (e) well 

penetration depth and length of screen; and 

(f) vertical and horizontal contaminant plume 

distribution. Analytical solutions for flow can 

incorporate the effects of well pumping rates, 

well design, and regional gradients under 

certain restrictions for a sensitivity analysis. 

Complex numerical models are generally 

required to analyze the effects of 

heterogeneity in the hydraulic properties and 

simulate complex plume configurations, 

especially if 3-D effects are considered to be 

important. An increase in the spacing of the 

wells may increase the capture zone 

horizontally but may decrease the capture 

zone vertically and may introduce gaps in the 

capture zone between wells through which 

contaminants may escape. An increase in the 

regional hydraulic gradient will act to decrease 

the capture area. An increase in horizontal to 

vertical anisotropy will increase the capture 

zone horizontally but decrease it vertically. 

Analytic solutions (Grubb, 1993; Faybishenko 

et al., 1995; Schafer, 1996) and analytic element 

methods-AEMs (Strack, 1989; Haitjema, 

1995) have been published for estimating 

capture zones for partially penetrating wells in 

steady-state 3-D flow fields. Sensitivity 

analyses of effects that include vertical

movement of water or solute in a 

heterogeneous domain require the use of 

numerical models. A good illustration of the 

factors that affect capture zone size and shape 

is found in Bair and Lahm (1996). Bair and 

Lahm used a finite difference method to 

determine the steady-state flow field, and 

particle tracking, to delineate the size and 

shape of the capture zone. They determined 

the magnitude of changes to the capture zone 

area because of perturbations in the regional 

gradient, well penetration, pumping rates, well 

configuration, and degree of hydraulic 

conductivity anisotropy in the context of an 

idealized pump and treat design. 

Three published articles on numerical 

simulation of 3-D flow in and around a 

wellbore contain pertinent information for 

refined modeling in the vicinity of a single 

well. Chiang et al. (1995) simulated 3-D flow 

and advective solute transport in the vicinity of 

a partially penetrating well, to understand the 

order-of-magnitude difference in contaminant 

concentrations between well samples and 

point aquifer samples. The concentration 

profile in the aquifer was known. The well 

bore was modeled as separate elements with a 

permeability in the range of that predicted for 

laminar flow in a tube. They noted that their 

transient simulation results asymptotically 

approached the simple, mass-balance-based 

result that assumes a flat water table. 

Akindunni et al. (1995) simulated 3-D flow 

near a well for various screen and plume 

positions. They approximated the well using a 

Neumann boundary condition at the edge of 

the domain at which the discharge was equally 

apportioned to the nodes along the screened 

length of the well. They compared vertically

averaged values of concentration for both the 

wellbore and the aquifer. In the transient 

simulations, concentrations differed 

significantly in the well and aquifer. 

Concentrations in the wellbore were higher or 

lower than the vertically averaged aquifer 

value, depending on the relative position of 

the plume depth and screened interval. 

However, over long times, the concentration 

in the wellbore asymptotically approached the 

vertically averaged aquifer value. In addition 

to screen position and plume position, they

NUREG- 1538 3-31
3 Ground-Water Pathway Analyses

also investigated the dependence on screen 

length and anisotropy. Again, initial 

concentrations differed significantly, but 

long-time concentrations appeared to 

approach the vertically averaged aquifer value. 

As expected, simulations with large anisotropy 

ratios for hydraulic conductivity exhibited less 

vertical mixing than the isotropic case. 

Reilly et al. (1989) also modeled the wellbore 

as a column of hydraulically connected cells; 

however, their focus was on wellbore flow in a 

monitoring well with implications for sampling 

bias and cross-contamination. In a monitoring 

well, cross-contamination will act to dilute the 

plume. Of note was their conclusion that 

greater than half the aquifer-to-wellbore flow 

occurred in the top 10 percent of the screened 

length, whereas greater than half the 

wellbore-to-aquifer flow occurred in the 

bottom 10 percent of the screened length. 

Hence, solute plumes approaching the top of 

the screened portion will enter the wellbore, 

whereas plumes approaching the bottom will 

tend to flow around the well. This finding may 

be pertinent for a hypothetical farming-type 

receptor group, when irrigation wells are shut 

down, but is probably irrelevant during periods 

of pumping. 

3.2.1.3 Methods Used to Conduct this Study 

Wellbore design and pumping practices in the 

Yucca Mountain region may have a significant 

effect both on the capture of a potential plume 

and, from another perspective, on the 

radionuclide concentration of the water 

pumped from the wells. Existing databases 

were analyzed to characterize the location, 

design, and production of wells. An important 

feature of the wells in the Amargosa Farms 

area is that they partially penetrate the alluvial 

aquifer thickness. The first wells encountered 

in a path of a simulated plume released from 

the proposed repository site are low-pumping

rate domestic, commercial, and quasi

municipal wells at a distance of approximately 

25 kilometers. Large-pumping-rate irrigation 

wells capable of lowering the water table over 

several square kilometers are located at a 

distance of approximately 30 kilometers from 

Yucca Mountain.

NUREG- 1538

The AEM is used to model 3-D flow in the 

vicinity of a partially penetrating well. Particle 

tracking is used to delineate a capture area for 

different well designs, pumping rates, and 

regional flow characteristics. The capture area 

is determined at an up-gradient point from the 

well location where the flow is essentially 

one-dimensional (1 -D); for example, no 

longer 3-D. Also, the cross-sectional area of a 

plume entering the Amargosa Farms area is 

approximated by using 2-D and 3-D 

solutions to the advection-dispersion equation. 

Geometric arguments are used to estimate 

dilution factors caused by the portion of the 

plume captured. For dilution factors based on 

dispersive transport, numerical integration is 

used to estimate a representative 

concentration for the portion of the plume 

captured. 

3.2.1.4 Limitations of this Study 

The geometric borehole dilution factors 

reported here account only for borehole 

dilution from pumping. Dilution caused by 

mixing with clean water, either underneath the 

repository footprint or at the northern portion 

of Fortymile Wash, or from any interbasin 

transfers, is not included. Dilution factors 

calculated using different approaches may 

neither be directly compared nor linearly 

combined except under certain restrictions. 

For this scoping analysis, comparison of the 

streamtubes described in Section 3.1 (ambient 

dilution) with the geometries of the capture 

zone and plume configuration are not 

possible, since they are derived from different 

phenomena. 

Three significant assumptions are used in this 

scoping analysis, in part because of the paucity 

of data for the ground water in the alluvial 

sediments of Amargosa Farms area. Material 

properties are assumed to be homogeneous 

and isotropic; the flow field is assumed to be 

uniform; and steady-state pumping rate and 

contaminant transport are assumed to 

represent the effects of borehole dilution. The 

latter assumption specifically implies that the 

transient effects of irrigation pumping can be 

approximated by an annual pumping discharge 

volume. 

-32
3 Ground-Water Pathway Analyses

3.2.2 Hydrojeology of the Amargosa Desert 

Region• 

The Amargosa Desert region is a northwest

trending, triangular-shaped alluvial basin 

bounded on the north by Bare Mountain, 

Yucca Mountain, and the Specter Range; on 

the east by the Resting Spring Range; and on 

the west by the Funeral Range and Black 

Mountains. Elevations on the valley floor 

range from 975-meter msl at the Amargosa 

River narrows near Beatty and 720 meters msl 

at the proximal edge of the fan formed by 

Fortymile Wash, as it discharges from Jackass 

Flats, to less than 610 meters msl at Franklin 

Lake playa south of the Amargosa Farms area. 

3.2.2.1 Structure and Depositional History 

The Amargosa Desert is an alluvial valley that 

resulted from large-scale block faulting in the 

Basin and Range Province (Bedinger et al., 

1989; Plume, 1996 ). Sediments deposited in 

depressions created by Tertiary to Quaternary 

block faulting can be classified as alluvial fan, 

lake bed, and fluvial deposits. In general, the 

coarsest materials (gravels and boulders) were 

deposited near the mountains, and the finer 

materials (silts and clays) were deposited in 

the central part of the basin. The distribution 

of sediment is generally associated with 

distance from the mountains. Alluvial fans 

with steep gradients and coarse sediments 

flatten and coalesce basinward, interfingering 

with the lake bed deposits. Within the alluvial 

fans there is a complex interfingering and 

interbedding of fine and coarse sediments 

caused shifting of fluvial processes across the 

top of the fan. The finer grained, distal 

portions of the fans merge laterally and 

interlayer with the lake deposits. The lake bed 

deposits can include beach sand and gravel 

lenses, silts and clay layers, and evaporites 

from playa-type environments. The fluvial 

deposits of recent times consist of sand and 

gravel lenses along present or ancestral 

streams. These exhibit a greater degree of 

sorting than the alluvial fan deposits. 

Repeated upheaval events led to a complex 

interbedding and interlayering of the proximal 

8AIso see Sections 2.1.2.2, 2.1.2.3, and Appendix A.

and distal facies of the alluvial basin 

sediments. The repeated upheavals, together 

with the lateral and down-gradient transitions 

within the alluvial fan and grading into the 

lake bed or playa deposits, have strong 

implications for flow and transport on a 

basin-wide scale. 

The Amargosa Farms area is in the distal 

portion in terms of sediment facies of an 

alluvial basin where lowland fans and lake 

beds would comprise much, but not all, of the 

stratigraphic section. Geologic lithologies and 

maps are described in Walker and Eakin 

(1963); Denny and Drewes (1965); Naff 

(1973); Swadley and Carr (1980); Swadley 

(1983); Fischer (1992); and Burchfiel (1966). 

Recent maps of the central Amargosa Desert 

region have followed the lithologic 

characterization of Hoover et al. (1981). Local 

features pertinent to the hydrogeology include 

the presence of tuffaceous beds (ash fall); 

limestone horizons; perched water systems 

(especially where the Funeral Mountain 

fanglomerates overlie lake sediments); 

common occurrence of calich6; and 

cementation of sand and gravel units. The high 

east-west hydraulic gradient, in the otherwise 

north-south regional gradient, between 

Amargosa Farms and Ash Meadows, is 

thought to be caused by low-permeability 

lake-bed sediments faulted into juxtaposition 

with the conductive Paleozoic carbonates of 

Ash Meadows. 

The thickness of the alluvial sediments in the 

Amargosa Farms area is not well-known. 

Bedinger et al. (1989) report the basin-fill as 

greater than 1300 meters, possibly as thick as 

2000 meters for basins in the Death Valley 

Region. Oatfield and Czarnecki (1991) used 

geophysical data to estimate the thickness of 

the alluvial valley fill sediments in the range 

800 to 1100 meters for the Amargosa Farms 

area. Laczniak et al. (1996) infer depths up to 

1140 meters on their east-west cross-section 

across the Amargosa Farms area. 

3.2.2.2 Basin-Scale Ground-Water Flow 

Hydrographically, Amargosa Desert is part of 

the Death Valley ground-water flow system, 

which is a series of topographically-closed

3-33 NUREG-1538
3 Ground-Water Pathway Analyses

basins connected at depth by the Paleozoic 

carbonate aquifer. As noted earlier, the Death 

Valley ground-water system can be further 

subdivided into three basins: (a) the Alkali 

Flat-Furnace Creek Ranch sub-basin; (b) the 

Ash Meadows sub-basin; and (c) the Oasis 

Valley sub-basin. Amargosa Farms is in the 

southern portion of the Alkali Flat-Furnace 

Creek sub-basin and adjacent to the Ash 

Meadows sub-basin (DOE, 1988; D'Agnese et 

al., 1997). The Ash Meadows sub-basin, which 

drains the eastern and northeastern basins of 

the Death Valley regional flow system, is not 

believed to be influent to Alkali Flat-Furnace 

Creek Ranch sub-basin in the vicinity of the 

primary agricultural pumping area. 

The diverse mix of geochemical signatures in 

the Amargosa Desert region suggests that the 

ground water comes from a combination 

interbasin flow, upwelling from the deep 

Paleozoic carbonate aquifer, and intrabasin 

flow from the northwest and from the north 

(Winograd and Thordarson, 1975). Because of 

high ET rates for the Amargosa Desert, most 

of the recharge occurs through the ephemeral 

stream channels (Osterkamp et al., 1994; 

Savard, 1995). Since the stream channels in 

the Amargosa Farms portion of the Amargosa 

Desert rarely have flow, the recharge 

estimates of Osterkamp et al. are about 0.5 

percent of precipitation. Precipitation is 

generally between 100 and 200 millimeters for 

the Amargosa River basin (Op cit.). 

The ground-water contribution from the 

proposed Yucca Mountain repository area is a 

small portion of the southward flow along 

Fortymile Wash. The contribution from the 

Ash Meadows springs area to the Amargosa 

Farms area may be minimal. The Ash 

Meadows springs line and high gradient 

toward the Amargosa Farms area are a 

reflection of the hydraulic conductivity 

contrast across a gravity fault that abuts the 

carbonates of Ash Meadows on the east side 

with the confining playa deposits on the west 

side (Naff, 1973).

3.2.3 Well Construction and Water Use in 

the Amargosa Farms Area 

Four sources of information were used to 

characterize well construction and water use in 

the Amargosa Farms area. The well permit 

database, well drillers' logs, and annual water 

use estimates were obtained from the State 

Engineer's Office in the Nevada Division of 

Water Resources (Bauer and Cartier, 1995; 

State of Nevada, 1997a,b,c). A fourth source 

was the GWSI portion of the National Water 

Information System developed and maintained 

by the USGS (see Mathey, 1989). The well 

permit tables, well drillers' logs, and annual 

water use tables are recorded by location using 

the standard range, township, section, quarter 

section, and possibly quarter-quarter section 

coordinate system. The tables are organized 

by hydrographic basin, with the Amargosa 

Desert being defined as Hydrographic Basin 

230. The Amargosa Farms area of the 

Amargosa Desert includes townships (T.) 15, 

16, and 17 south (S.) and ranges (R.) 48 and 

49 east (E.), as well as the western half of R. 

50 E. 

The GWSI database uses both the township

range coordinate system as well as the 

longitude-latitude coordinate system. The 

wells in Amargosa Farms and Amargosa 

Valley are taken as those bounded by 116' 21' 

34" to 1160 37' 15" west longitude and 36* 40' 

10" to 360 20' 53" north latitude. For 

graphical purposes, township-range 

coordinates and latitude and longitude 

coordinates are converted to Universal 

Transverse Mercator (UTM) section 11 

coordinates using the 1927 North American 

Datum. The former conversion is made 

directly to UTM by assuming a well is in the 

middle of the smallest reported area (e.g., 

quarter section). The latter conversion is 

made using a USGS-supplied conversion 

program. 

3.2.3.1 Number and Distribution of Wells 

A division of wells into two categories based 

on water use is made here for the purpose of 

presentation of separate results for different 

receptor pathways. Domestic and quasiNUREG-1538 3-34
3 Ground-Water Pathway Analyses

municipal wells can be characterized as having 

low but continuous pumping rates throughout 

the year. Irrigation wells and commercial and 

industrial wells constitute the large pumping 

rate category. Although irrigation wells 

operate intermittently through the growing 

season, they are approximated in this study as 

a continuously pumping well at the annual rate 

estimated from the annual volume pumped. 

There are no municipal wells in the Amargosa 

Farms area. Instead, quasi-municipal wells 

and domestic wells support direct human use. 

In addition, a portion of the irrigation wells 

(well drillers' logs) and industrial wells (Buqo, 

1997) may also supply water for direct human 

use. Five percent of the total irrigation wells 

recorded in the well drillers' logs also listed 

domestic use. Dependent on the State 

Engineers concurrence, the water use category 

associated with a permit may be changed at a 

later date. 

At the time this analysis was conducted, there 

were 508 wells recorded in the State of 

Nevada's well drillers' logs, which date back to 

at least 1921. Many of these wells are no 

longer in operation. The GWSI database 

contains 224 well records for approximately 

the same area of central Amargosa Desert. 

The well permit database contained 185 

certificated or permitted water rights entries. 

The estimated water use tables from the State 

Engineer tracked as many as 72 entries in one 

year (1996) and a combined 126 different 

entries over the span 1983-96. Individual 

domestic wells are not recorded in the State 

water use tables, nor were quasi-municipal 

wells, before 1996 for Hydrographic Basin 230. 

The distribution of wells spatially and across 

water use categories is illustrated in Table 3-3 

by township and Figure 3-12 by township and 

range. DOE (1988) identified nine quasi

municipal wells, five commercial wells, and 

three industrial wells that were active at the 

time the SCP was prepared. Changes in water 

use category may occur on permitted or 

certificated water rights. A majority (70 

percent) of all wells were drilled in T 16 S., 

with domestic wells concentrated in T 16 S. 

and R. 48-49 E. Locations of sections where

14 or more (up to 40) domestic wells have 

been drilled, according to the well drillers' 

logs, are marked in Figure 3-12. 

3.2.3.2 Statistical Analysis of Well 

Construction Practices 

The GWSI database (Mathey, 1989) also 

contains information on well construction. Of 

the 227 wells from the Amargosa Farms region 

listed in the database, 188 records included 

water table depth; 113 included screen 

positions, and 15 records included specific 

discharge data. Although 18 wells had 

multiple screened portions; a majority of the 

screened portions are closely spaced. This is 

reflected in the fact that there is only a 2 

percent (1-meter) difference between the 

average of the sum of the screened portions 

and the average of the length of the combined 

screened portion. Table 3-4 is a statistical 

summary of relevant well characteristics. Of 

note are the averages of 11- and 62-meter 

depths from the water table to the top and 

bottom of the screened portions, respectively. 

3.2.3.3 Estimation of Water Use 

For the Amargosa Desert basin, the State has 

estimated the perennial yield to be about 30 x 

106 cubic meters (Buqo, 1997). Committed 

water use, which includes both certificated and 

permitted water use, is over 51 x 106 cubic 

meters. This situation makes it unlikely that 

new permits will be granted by the State 

Engineer. In the past few years, proceedings 

for water users to demonstrate beneficial use 

have led to thousands of acre-feet of forfeiture 

for well permits. These proceedings may have 

had an. impact on the number of water users 

reported in the basin during the mid-1990s 

(Op cit., pp. 30-31). 

Based on information provided by the State 

Engineer, on a volume basis, the water 

pumped in the Amargosa Farms area is 

predominantly used for irrigation and mining 

(see Table 2-7). The bulk of the mining

related water use is in the playa area, which 

lies south of the farming area. The Saint Joe 

Bullfrog gold mine is also a large-volume 

water user, as reported in the tables for the 

3-35 NUREG- 1538
3 Ground-Water Pathway Analyses

NUREG-1538

Table 3-3. Distribution of Wells by Water Use Across Townships T. 15 S. Through T. 17 

S. Using Well Drillers' Logs. There are 34 log entries classified as other. 

Figure 3-12 identifies the locations of the townships and ranges. 

Industrial/ Quasi Township Domestic Irrigation Commercial Municipal 

T.15S. 12 5 2 1 

T.16S. 207 120 1 3 

T.17 S. 55 65 1 1

Table 3-4. Statistics for Well Construction Practices and Water Level Positions for Wells 

Recorded in the GWSI Database in the Amargosa Valley and Amargosa Farms Areas

Well Standard 

Characteristic Average Deviation Number Minimum Maximum 

Distance from Water 11 13.0 113 0.00 66.0 

Level to Top of Screen 

(m) 

Distance from Water 62 36.7 113 1.7 219 

Level to Bottom of 

Screen (m) 

Distance from Water 35 23.1 113 1.2 124 

Level to Screen 

Centerline (m) 

Total Screen Length (m) 52 33.2 113 0.9 191 

Distance from Top to 53 33.1 113 0.9 191 

Bottom of Screens (m) 

Depth of Well (m) 83 42.6 172 0.9 229 

Wellbore Diameter (m) 0.31 0.08 112 0.032 0.41 

Specific Discharge 32.3 33.4 15 2.34 104 

(m 2

/hr) I I I I

3-36
3 Ground-Water Pathway Analyses

R. 50 E.

T. 15 S.

4 T. 16 S.

5 T. 17 S.

E HIGH-DENSITY OF DOMESTIC WELLS LOCATION 

Figure 3-12. The distribution of domestic and quasi-municipal wells based on Range and Township from well 

drillers' logs. The number of wells in each range and township includes those listed for dual usage, 

domestic, and irrigation. Locations of sections (1 square mile/2.56 square kilometers) with 14 or more 

domestic wells are highlighted.

NUREG-1538

-lj- R. 48 E. R. 49 E.

2

U:

105

3-37
3 Ground-Water Pathway Analyses

Amargosa Desert, but it is not located in the 

Amargosa Farms region. 

Historically, ground-water pumping for 

irrigation increased significantly in the late 

1950s (D'Agnese, 1994; Buqo, 1997). 

Irrigation use was 3700 cubic meters by 1962, 

by category, although data for 1984 were not 

recorded. Table 3-5 is the annual summary of 

water use, with 1.15 x 105 cubic meters by 

1967, and 9 x 106 cubic meters in 1973. Kilroy 

(1991) reports rapid declines in the water 

table during the 1970s and less severe declines 

in the 1980s. The declines are about 6 to 9 

meters in three different areas of Amargosa 

Farms, with the largest being a northeast

trending trough near the Nevada-California 

border in T. 16 S., R. 48 E. Since 1983, the 

State Engineer has tabulated water use for 

individual commercial, mining, irrigation, and 

quasi-municipal users and summarized annual 

use by both the Amargosa Desert total and the 

Amargosa Farms portion total. The annual 

totals increased significantly from 1993 to 

1996, because of large increases in irrigation 

use, with the largest volume being 16 x 106 

cubic meters in 1995. 

Individual domestic water use is not recorded 

in the State Engineers' tables, and individual 

records for quasi-municipal water users did 

not start until 1996. Annual estimates were 

lumped together for the domestic and 

quasi-municipal/commercial use for each year, 

although there is some re-categorization 

occurring in 1996. About 1233 cubic meters 

(approximately 1 acre-foot) annual usage is 

assumed for every household, although this 

may be an over-estimate (Buqo, 1997). [For 

the purposes of the SCP, DOE used 2482 cubic 

meters annual consumption (around 1800 

gallons/day) citing State Engineer estimates. 

(See Section A-4 of Appendix A). One acre

foot is about 895 gallons/day or about 3.4 cubic 

meters/day.] 

Individual records for each irrigation user are 

tabulated (see Appendix E) for the years 1983, 

and 1985 -96, and pertinent summaries are 

included in Table 3-6. For individual 

irrigation use, the maximum average daily 

pumping discharge for any particular user is

3960 cubic meters. The average for all years 

for an individual irrigation user is 828 cubic 

meters per day and the range in any particular 

year is 348 to 1300 cubic meters per day. The 

number of irrigation users for any year ranged 

from 15 in 1991 to a high of 55 in 1996. Most 

of the ground-water pumping occurs in T. 16 

S., R. 48-49 E., and T. 17 S., R. 49 E. Figure 

3-13 shows the distribution of ground-water 

pumping for the year 1996 by township and 

range based on the individual records (no 

domestic wells are recorded). Figure 3-14 

also shows the pumping distribution for 1996 

relative to the streamtube model boundaries 

used in Baca et al. (this report). In combi

nation, Figures 3-13 and 3-14 illustrate the 

fact that large-capacity irrigation wells are 

generally located to the south of the 

community of Amargosa Valley where the 

depth to water is between 10 to 40 meters. 

In summary, the typical pumping rates range 

from 300 to 2000 cubic meters/day for 

irrigation wells and 3 to 6.8 cubic meters/day 

for wells supplying water for domestic use. 

Although the Amargosa Desert hydrographic 

basin is generally believed to be over

appropriated (Appendix A), actual usage has 

remained less than 65 percent of the estimated 

perennial yield. Ground-water pumping in the 

Amargosa Farms portion of the Amargosa 

Desert has led to an estimated decline in the 

water table locally of up to 10 meters (Kilroy, 

1991). 

3.2.4 3-D Capture Zone Analysis and 

Plume Delineation 

The approach used here to estimate borehole 

dilution factors in the Amargosa Farms area is 

to separate them into two components; one, 

the factor caused by volumetric-flux within a 

borehole; and two, the factor caused by 

dispersion during transport. The factor caused 

by volumetric flux is a comparison of the 

cross-sectional areas of a capture zone of a 

pumping well to the intercepted portion of a 

contaminant plume. In all cases, the areas 

discussed here refer to the cross-sectional area 

normal to the principal direction of regional 

flow. The second component of borehole 

dilution is the effect caused by dispersion

NUREG-1538 3-38
3 Ground-Water Pathway Analyses

International 

Minerals Venture Quasi

Floridan/American Municipal! Amargosa Basin 230 

Year Irrigation Borate Domestic Commercial Farms Total Total 

1996 13.60 1.26 0.06 0.25 15.20 30.37 

1995 15.20 0.96 0.12 0.01 16.30 32.60 

1994 12.30 0.88 0.12 0.01 13.30 26.62 

1993 10.68 1.24 0.12 0.01 12.10 24.16 

1992 7.05 0.81 0.12 0.01 7.99 15.98 

1991 6.09 0.56 0.12 0.01 6.79 13.57 

1990 6.11 1.09 0.12 0.01 7.37 14.71 

1989 1.90 1.74 0.12 0.01 3.84 7.62 

1988 3.67 1.23 0.12 0.01 5.07 10.11 

1987 7.03 0.37 0.12 0.01 7.57 15.11 

1986 8.09 0.68 0.15 0.01 8.93 17.86 

1985 10.45 1.17 0.28 0.03 11.90 23.83 

1983 11.23 1.54 0.28 0.03 11.72 24.80

Table 3-6. Summary Statistics of Individual Irrigation Users on an Annual Basis. Based 

on data from Mathey (1989). 

Average Number of Minimum Maximum 

Year (m3/day) Users (m3lday) (m3lday) 

1996 772 55 3.4 2707 

1995 886 51 6.8 2928 

1994 771 44 3.4 3960 

1993 711 41 3.4 3960 

1992 645 30 3.4 3368 

1991 1116 15 67.7 3960 

1990 645 26 16.9 2675 

1989 348 16 16.9 1354 

1988 503 20 8.5 2370 

1987 900 20 8.5 2912 

1986 1300 17 8.5 2928 

1985 1134 25 76.9 2928 

1983 1083 26 16.9 2116 

OVERALL 828

NUREG-1538

Table 3-5. Annual Estimates of Water Use by Type (in cubic meters). 

Based on data from Mathey (1989).

3-39
3 Ground-Water Pathway Analyses

R. 48 E.

0

R. 49 E. R. 50 E.

0 0.5 T. 15S. 

0 8.0 

0.0 

2.0 1.0 0 

0.0 34.0 0 T 16 S. 

4313.0 1304.0 0 

330.8 -• 7.5 8 "\ 501.3 

264.2 10.0 0.53 

"1157.0 : 2.00 T. 17 S. 

ZZ 0.00 

EXPLANATION 

COMMERCIAL WELL 0.53 

IRRIGATION WELL 68.00 T. 18 S. 

QUASI-MUNICIPAL WELL 0.00 

Figure 3-13. Distribution of annual water use (cubic meters) by type and by Range and Township for commercial, 

irrigation, and quasi-municipal wells for the year 1996.

NUREG-1538 3-40
3 Ground-Water Pathway Analyses

0 

0 

0 L0 

0 

I 

I 0 

I 4 

I I 014 10 "" 

00 

o I! 

0 

mo i! 

"•I i 

0I I *. I 

0 El 

/ % o I 

0 

E. l , COoMRIU UIIPLW L 

ooI "- " I. 

o0j" IR -GAI ..W L 

S•....I 

l % . I 

1 ...- - - - 0 

0 UA-"MU 

o 540000 550000 

EXPLANATION 

D] COMMERCIAL/MUNICIPAL WELL 

0 IRRIGATION WELL 

K>QUASI-MUNICIPAL WELL 

Figure 3-14. Distribution of water use by type for the year 1996. The symbol size for each category is scaled to the 

magnitude of ground-water pumping volume. Data are from the State of Nevada (1976b) and are 

converted to UTM Section II coordinates so as to correspond with the streamtube model (dashed lines) 

of Baca et aL (Section 3.1 of this report).

3-41 NUREG- 1538
3 Ground-Water Pathway Analyses

during transport. It is calculated as the ratio of 

the source concentration to the areal average 

concentration of the portion of the plume that 

is captured by a pumping well. 

Different configurations for the intersection of 

the plume and the capture area are possible. 

For domestic wells, the capture area is 

generally much smaller than the cross

sectional area of a plume that has undergone 

transverse spreading caused by macro

dispersion during transport along a 20- to 

30-kilometer pathway (Figure 3-15). Hence, 

there would likely be little borehole dilution 

even if the well were aligned along the center 

of the plume, and any borehole dilution that 

did occur would be caused by vertical 

gradients in the plume concentration. For a 

2-D plume of prescribed thickness, the 

location of the plume relative to the capture 

area affects the dilution factor. For irrigation 

wells, or any high discharge wells, the capture 

area is generally thicker than the plume. Also, 

the capture area may be wider or narrower 

than the contaminant plume, depending on the 

problem. In all cases, the well is assumed to be 

in the transverse center of the plume, which is 

the conservative assumption. 

The effects of the regional gradient, 

transmissivity, pumping rate, and screen

position and length on the area of the capture 

zone can be described in qualitative terms. An 

increase in transmissivity or the regional 

gradient will decrease the width of the capture 

area. An increase in the pumping rate will 

increase the capture area. An increase in the 

depth of a partially penetrating well will 

increase the vertical capture area, but 

decrease the horizontal capture area. The 

position and distribution of the plume in 

relation to the capture zone will control the 

dilution of the solute in the well bore. 

At present, there are few published data for 

the hydraulic properties, well construction, 

and pumping in the greater Amargosa Desert 

area. Moreover, the size, location, and shape 

of a plume are uncertain and usually must be 

obtained from large-scale transport modeling. 

Because of the relative paucity of site-specific 

data, the focus of this scoping analysis is 

relating dilution trends to generic well design 

and plume configuration. 

3.2.4.1 Determination of Flow Field and 

Capture Zone 

The ground-water flow simulation program, 

GFLOW (Haitjema, 1995), which is based on 

AEM, was used to estimate the size and shape 

of capture zones for individual wells. GFLOW

Figure 3-15. Relative comparison between cross-sections of contaminant plume (P), irrigation well (I) capture 

area, and domestic well (D) capture area.

NUREG-1538 3-42
3 Ground-Water Pathway Analyses

is designed to simulate partially penetrating 

wells in a uniform regional gradient. The 3-D 

effects of the partially penetrating well are 

superimposed on the 2-D regional flow field. 

At some distance from the well, the vertical 

components from pumping become negligible. 

Forward or backward particle tracking is used 

in GFLOW to determine a capture area at 

some distant, up-gradient point where vertical 

flux components become insignificant. This 

capture area is a vertical plane normal to the 

direction of regional flow. 

3.2.4.1.1 Description of AEM 

AEM provides a composite analytic solution 

that satisfies the differential equation in an 

unbounded domain. Delineation of 

streamlines is more precise than with standard 

numerical methods, since both the head and 

the velocities are known at every point, rather 

than solely at computational nodes. Combined 

2-D and 3-D modeling is accomplished by 

superposition of 3-D effects on the general 

2-D solution. For example, near a partially 

penetrating well, a 3-D solution is used. 

However, at a location sufficiently far from 

the well, the vertical flow components are 

negligible and a 2-D approximation to the 

well may be superimposed on the solution. 

AEM is not well suited for complex flow 

problems in which material property 

heterogeneity is large. The equations for flow 

in AEM are written in terms of discharge 

potentials instead of hydraulic head. The 

discharge potential is defined differently for 

confined, unconfined, 1-D flow, 2-D flow, or 

for any analytic element. An advantage of the 

AEM is that the solution to the equation for 

flow written in terms of the discharge potential 

is not dependent on whether the problem 

domain being solved is confined or 

unconfined. Once the strength of the potential 

is known for each analytic element, the head 

or ground-water discharge may be determined 

at any point in the flow domain. The solution 

for the partially penetrating well is based on 

work by both Muskat and Polubarinova

Kochina (Haitjema, 1995) for the 

representation of the strength distribution

along a line sink (point sinks along a line) 

while constraining the discharge to a fixed 

value. 

3.2.4.1.2 Ranges for Parameter Values 

Four parameters are varied to test their effects 

on the capture area, including: (a) pumping 

rate; (b) well screen position and length; (c) 

regional gradient; and (d) hydraulic 

conductivity or transmissivity. The pumping 

rates range from those typical of domestic 

wells to those typical of irrigation wells. A 

reasonable range to use for wells that pump 

water for domestic use is 1 to 75 cubic 

meters/day. As noted in Section 3.2.3.3, DOE 

previously estimated 6.8 cubic meters/day 

(1800 gallons/day or 2 acre-feet/year) 

consumption for a single household citing the 

State Engineer. Buqo's (1997) analysis 

suggests that this value may have been too 

high and that a more realistic estimate of use 

is around 3.4 cubic meters/day (892 gallons/ 

day or 1 acre-foot/year) per household. 9 The 

high end of the domestic range corresponds to 

a quasi- municipal well or to multiple domes'

tic wells (10 to 20 family wells) modeled as a 

single well. For example, the first wells in a 

potential plume's path are multiple domestic, 

quasi-municipal, and small commercial wells 

near the junction of Highways 95 and 29 at 

9

Buqo does not explicitly explain how he determined the per 

capita fresh water consumption for Amargosa Valley. The 1 

acre-foot/year per household estimate might be explained by 

looking to page 31 of his report, where, assuming 2.8 

people/household x 319 gallons/day per capita, a value of 892 

gallons/day per household or 1 acre-foot/year per household, 

can be derived. The staff believes that Buqo's estimate of 

future water consumption was determined following a review 

of current water use rates for communities in the Amargosa 

Valley, Beatty, and Pahrump Valley areas. This supposition is 

based on consideration of the following: 

(i) Buqo (Op cit., p. 28) makes reference to the current 

"assumption" made by the Nevada Division of Water 

Resources, that domestic water users, including those 

currently living in the Amargosa Valley area,.consume only 

1 acre-foot/year per household. The Nevada Division of 

Water Resources uses the 1 acre-foot/year per household 

to estimate the domestic and quasi-municipal annual water 

use for their pumping inventories as reported in the table 

on page 29 (Op cit.); the source data for this table were 

used in this NUREG. However, it is clear that the 

numbers in these tables are not tied into population, as 

only three values appear in the tables from 1983 to 1995 

(e.g., 230, 125, and 100 acre-feet). 

continued on nextpage

NUREG- 1538 3-43
3 Ground-Water Pathway Analyses

Amargosa Valley. For irrigation wells, 

pumping may be as high as 4000 cubic 

meters/day; however, a more typical large irrigation pumping rate is 2116 cubic 

meters/day. The average pumping rate from 

1983-96 was about 800 cubic meters/day, whereas the lowest was 300 cubic meters/day 

for any particular year. 

The average screened length of the wells in 

the Amargosa Farms area (top to bottom) is 

53 meters; the maximum screen length is 190 

meters (Table 3-4). The typical screen 

position starts 11 meters below the static water 

level at the time of well construction. Hence, 

the typical well modeled here will be screened 

from the water table to 60 meters below the 

water table. 

The range of regional hydraulic gradients 

considered is 0.001 to 0.01. Bedinger et al. 

(1989) list a value of 0.003 for generic basin 

fill environments in the Death Valley region. Estimates for the Amargosa Farms area are 

made from water table maps by Nichols and 

Akers (1985); DOE (1988); and Kilroy (1991), 

and fall within the 0.001 to 0.01 range. Most 

estimates are in the 0.001 to 0.005 range; the 

0.01 values are from the east-west gradients immediately south and east of Amargosa Valley and may reflect the abrupt decrease in 

transmissivity across the northern end of the 

continued 

(ii) Buqo (Op cit., p. 25) estimates the per capita water 

consumption of Beatty to be 182 gallons/day (0.57 acre-foot/year per household for 2.8 people per household). However, using metered data, actual water 

consumption in Beatty was reported to be 393 acre foot/year in 1995 (Op cit., p. 22) instead of the 896 

acre-foot/year expected using the DOE estimate of 1800 

gallons/day per household (as cited in the SCP). The 

actual metered amount of 393 acre-foot/year translates to 

0.57 acre-foot/year per household. 

(iii) Using both metered data and data from other 

sources, actual per capita water consumption in Pahrump Valley was estimated to be 486 gallons/day per capita (Op cit., p. 37). Buqo notes, but does not properly reference, 

that this "...value is somewhat higher than the 390 

gallons/day per capita water use reported in recent years for Las Vegas...." Nonetheless, in developing this estimate, 

Buqo does note that "...the higher demand rate in 

Pahrump Valley reflects in part the disproportionate area of golf courses, tree farms, and parks... relative to the 

number of residents...." (Op cit.). 

Finally, using the reported per capita consumption rate of 210 

gallons/day in Basse (1990, p. 8; Table 2), for Nye County, multiplied by 2.8 people/household yields 588 gallons/day per household or 0.66 acre-feet/year per household.

so-called Gravity fault, which has been inferred 

along the Ash Meadows spring line. 

The range of transmissivities reported for 

basin-fill alluvium in the Death Valley Region is 10 to 400 square meters/day (Winograd and 

Thordarson, 1975; DOE, 1988; Plume, 1996). 

Since Amargosa Farms is in the area of 

sediment facies of lower fans and lowland 

sediments, rather than the coarser sediments 

of the upper and middle fan deposits, the 

saturated hydraulic conductivities should 

encompass a wide range and be highly 

heterogeneous relative to other basin-fill. 

Plume (1996) recently estimated a range of 

0.006 to 43 meters/day for saturated hydraulic conductivity, whereas DOE (1988) had earlier 

reported a range of 0.21 to 2.9 meters/day. 

The transmissivity (L2

/T) can be expressed as 

a product of the saturated hydraulic 

conductivity (L/T) and the saturated thickness 

(L) of the aquifer.10 The aquifer thickness is 

assumed to be 1000 meters for all modeling scenarios. 

3.2.4.1.3 Sensitivity Analysis for Capture 

Zone 

The effects of reasonable variations in 

transmissivity, regional gradient, and pumping 

rate for all well types are presented in this 

section. In addition, the effects of screen 

position and length for domestic wells are 

presented. Because of their large discharge 

rates and small degree of well penetration 

relative to the aquifer thickness, the effects of 

screen position and length are negligible for 

irrigation wells. The capture area is 

determined at an upgradient point from the 

well location where the flow is essentially 1-D, for example, no longer 3-D. At this 

upgradient point, the width and thickness are 

at a maximum for the capture area. A table of 

the widths and depths of the capture area 

results is included in Appendix F 

The effect of a partially penetrating well 

compared with that of a fully penetrating well 

is shown in Figure 3-16 for a small irrigation well pumping at 300 cubic meters/day. The 

maximum screen length of 190 meters is 

marked as maximum on the figure. The 

10

L and T refer to physical dimensions of length and time, respectively.

NUREG-1538 3-44
3 Ground-Water Pathway Analyses

400 

TYPICAL 

600 MAXIMUM 

E 

(jD 

CO) 

"- - INCREASING WELL I- H PENETRATION DEPTH 

03. 

800 

1000 

400 600 800 1000 1200 1400 

Capture Width (m) 

Figure 3-16. Plot illustrating the effect of well penetration depth (60, 190, 500, and 1000 meters) on a small 

irrigation well's capture zone width and thickness. A pumping rate of 300 cubic meters/day and 

regional gradient of 0.005 are used. The "maximum" denotes the maximum well penetration depth, 

and "typical" denotes the typical well penetration depth for the Amargosa Farms region.

3-45 NUREG-1538
3 Ground-Water Pathway Analyses

capture width of the fully penetrating well is 

about 44 percent of that for the typical 

partially penetrating well. 

Figure 3-17 represents the capture zone 

width and thickness for combinations of 

regional gradients and transmissivities for a 

large pumping rate well of 2116 cubic 

meters/day. The combination of a regional 

gradient of 0.001 and transmissivity of 200 

square meters/day (the lowest represented 

here) leads to a capture width of about 5600 

meters, which captures nearly the entire width 

of a streamtube (Baca et al., this report) that 

brackets the repository. Conversely, a larger 

gradient (0.005) and higher transmissivity (400 

square meters/day) lead to a much smaller 

capture area, 1800 meters wide by 720 meters 

deep. A similar trend also occurs for low

discharge, domestic wells (Figure 3-18). 

Maximum capture areas are created either by the smallest regional gradient (0.001) or the 

lowest transmissivity (10 square meters/day) 

for capture thicknesses up to 200 meters. 

Since the Darcy velocity is a function of the 

hydraulic conductivity and hydraulic gradient, 

Figures 3-17 and 3-18 also illustrate the 

effect of Darcy velocity on capture width and 

thickness. 

The effect of pumping rate on the capture 

area is presented in Figure 3-19. A gradient 

of 0.005 and transmissivity of 100 square 

meters/day are used for all pumping rates. Of 

significance for borehole dilution is that all 

wells in the low pump-rate range (less than 75 

cubic meters/day) will have capture areas that 

would be much less than the plume area, 

based on 3-D advection-dispersion equation 

modeling. 

3.2.4.2 Radionuclide Plume Shape and 

Location 

The potential release and subsequent 

movement of radionuclides from a potential 

Yucca Mountain repository is likely to follow a 

path generally southeast to Fortymile Wash 

and then continue south-to-southwest toward 

the Amargosa Valley and Amargosa Farms 

areas. The shape of the plume at a 

30-kilometer distance from the proposed 

repository, in particular the amount of vertical

dispersion that leads to an increase in the 

plume thickness, is yet another unknown. 

Vertical dispersion may be limited by the 

possible presence of confining horizons (Naff, 

1973) in the lake bed facies of the basin-fill 

sediments. 

Given the uncertainty of the plume con

figuration, two scenarios were analyzed. The 

first scenario was a plume modeled for 3-D 

dispersion. The second scenario is a plume for 

which no vertical dispersion is incorporated. 

Both scenarios are simulated to a steady-state 

solution, to assess a wide range of dimensions 

for the plume when it reachs a well. 

Dispersion, adsorption, and radioactive decay 

of the radionuclides will occur along this 

transport path. Adsorption and decay depend 

on the particular radionuclide. However, most 

of the radionuclides of concern in the far field 

(e.g., 237Np, 129j, 99Tc) have half-lives greater 

than 10,000 years. Adsorption also depends 

on the surface mineralogy of the porous media 

as well as the chemistry of the ground water. 

There are no site-specific data for adsorption 

in terms of distribution coefficients for the 

valley fill sediments. Considering these points, 

the conservative approach of neglecting both 

decay and adsorption is adopted. 

To evaluate dilution caused by both vertical 

and horizontal capture of clean water by a 

pumping well, an estimate of the shape of a 

potential plume is needed. Specifically, the 

configuration of the cross-sectional area 

perpendicular to the direction of flow is 

needed. Analytic solutions to the advection

dispersion equation were previously used to 

describe the plume shape at down-gradient 

points from the two recent TSPAs conducted 

for Yucca Mountain (TRW Environmental 

Safety Systems, Inc., 1995; Kessler and 

McGuire, 1996). The advection-dispersion 

equation for 3-D dispersion and 1-D flow is: 

aC- a 2 C±D 2C+Da 2 C VOC C=D,02 +y Dy -2 + D, - C 

at x 2 ay2 z 2 a 

(3-2) 

where C is the concentration, Dx, D4, and Dz 

are the dispersion coefficients in the 

coordinate directions, Vis the seepage velocity 

in the principal direction of flow, and t is time.

NUREG-1538 3-46
3 Ground-Water Pathway Analyses

700 

800

E 

0/) 

60 

Co 

t

I

0~

900 

1000 

1000 2000 3000 4000 5000 6000

Capture Width (m) 

Figure 3-17. Effect of combinations of transmissivity (200,300, and 400 square meters/day) and hydraulic 

head gradient (0.001, 0.002, 0.003, and 0.005) on a large irrigation well's capture zone width and 

thickness. A pumping rate of 300 cubic meters/day is used.

3-47 NUREG-1538
100 200 300 400

Capture Width (m)

Effect of combinations of transmissivity (50, 100,200,300, and 400 square meters/day) and 

hydraulic head gradient (0.001, 0.0025, 0.005, and 0.01) on a domestic well capture zone's width 

and thickness. A pumping rate of 3 cubic meters/day and the screened portion is 60 meters long, 

starting from the water table.

NUREG- 1538

3 Ground-Water Pathway Analyses

0 

50 

E 100 

F) 

C'

I.a

C" 150 

0

200 

250

INCREASING TRANSMISSIVITY 

OR REGIONAL GRADIENT

0

Figure 3 - 18.

I i I

I I I I I I

3-48
3 Ground-Water Pathway Analyses

5000 1 I 

4000 

WIDTH 

3000 

E 

CO 

2000 

1000 THICKNESS 

1000 

0 

0 500 1000 1500 2000 

Pumping Rate (m3

/d) 

Figure 3-19. Plot illustrating the effect of pumping rate (range: 1 to 2000 cubic meters/day) on the capture 

zone's width and thickness. A transmissivity of 100 square meters/day and regional gradient of 0.005 

are used.

3-49 NUREG- 1538
3 Ground-Water Pathway Analyses

3.2.4.2.1 Transport Parameters 

The initial source size, seepage velocity, and 

the dispersivities all control the plume 

configuration after 30 kilometers of 

advective-dispersive transport. Kessler and 

McGuire (1996) noted the inverse relationship 

between source size and mean concentration 

reductions. They also found that a doubling of 

the source thickness led to an increase of 17 

percent in the plume width at 25 kilometers. 

Similarly, a 60-percent increase in the source 

width led to an increase of 6 percent in the 

plume width at 25 kilometers. For the 

purposes of this scoping analysis, the source 

size will be held constant at 500 by 25 meters 

for the 3 -D dispersion plumes and 500 meters 

wide for the 2-D dispersion plumes. 

Since transport simulations were run to steady 

state to determine maximum plume 

dimensions, a reasonable value of the seepage 

velocity along the flow path from the 

repository to potential receptor locations is 

needed. Seepage velocity is related to the 

Darcy flux by porosity. The Darcy flux for the 

transport analysis need not be the same as that 

for the capture zone analysis since the former 

represents the porous media and hydraulic 

head gradients from the repository to a 

potential receptor location approximately 30 

kilometers to the south. Seepage velocity for 

transport was chosen to represent the mean 

pathway velocity from the tuff through the 

alluvium. Baca et al. (this report) report 

calculated ranges of Darcy flux of 0.01 to 3.7 

meters/year for the saturated tuff aquifer and 

0.4 to 0.7 meters/year for the alluvium. 

Assuming a porosity of 0.3 for the alluvium, 

the seepage velocity would be in the range of 

1.3 to 2.3 meters/year. Kessler and McGuire 

(1996) used a seepage velocity of 1.76 x 10-6 

meters/second (55 meters/year) although it is 

not clear whether site-specific information

gradient, hydraulic conductivity, porosity-was 

used to obtain this estimate. The value of 2.4 

meters/year used here for seepage velocity is 

closer to that approximated from the Darcy 

flux values reported by Baca et al. 

The value of the concentration at the source is 

chosen to approximate a mass-release rate of

10 Ci/year, which is taken as an upper bound 

for mass-release rates, as delineated by the 

99

Tc example in Mohanty et al. (1997). 

Assuming that dispersion from the constant 

concentration boundary is negligible, the 

constant source concentration corresponding 

to a 10-Ci/year release rate is 1.11 x 10-6 

Ci/liter for a source size of 500 by 25 meters 

and a Darcy velocity corresponding to a 

seepage velocity of 2.4 meters/year with a 

porosity of 0.3. 

Simulation of 3-D dispersion requires values 

for the longitudinal, horizontal transverse, and 

vertical transverse dispersivities. Generally, 

dispersivities are considered to be scale

dependent (Gelhar et al., 1992). TSPA- 1995 

(TRW Environmental Safety Systems, Inc., 

1995) assumed relatively large transverse 

dispersivities that resulted in exceptionally 

large plumes (especially in the vertical 

direction) and correspondingly large dilution 

factors (103 to 105). Kessler and McGuire 

(1996) recognized that there is a limit to the 

heterogeneity scale that a plume would 

encounter, although they nonetheless used a 

vertical transverse dispersivity equal to the 

horizontal dispersivity. This seems unlikely in 

light of the lithologic layering in the alluvial 

basin sediments that would result in 

differences between the vertical transverse 

and horizontal dispersivities. Contaminant 

plumes generally exhibit limited vertical 

spreading (Gelhar et al., 1992). Thus, small 

vertical transverse dispersivities values are 

likely. In a literature review of measured 

dispersivity values and ratios, Gelhar et al. 

note that horizontal-to-vertical transverse 

dispersivity ratios are often 1 to 2 orders of 

magnitude different. Furthermore, the 

measured vertical dispersivity values were all 

reported, in Gelhar et al., to be less than 1 

meter, generally, in the range 0.06 to 0.3 

meters for scales ranging from 20 meters to 10 

kilometers. In addition, the vertical transverse 

dispersivity values exhibited no scale 

dependency. The longitudinal and horizontal 

transverse dispersivity are scale-dependent, 

with their ratio equal to one order of 

magnitude. For the constant concentration 

source, the longitudinal dispersivity and the 

velocity do not affect the mean plume

NUREG- 1538 3-50
3 Ground-Water Pathway Analyses

concentration in steady-state transport. Plume 

size is controlled by the transverse 

dispersivities. 

In this scoping analysis, the location of the 

radionuclide source area is the same as that 

assumed by Kessler and McGuire (1996). A 

patch source area aligned perpendicular to the 

flow direction is located 5 to 7 kilometers from 

the repository, as described in Kessler and 

McGuire, as opposed to locating the source 

area at the repository. The conceptual model 

consists of a release from the repository 

reaching the 5 to 7 kilometer location from 

where it is modeled as a patch source, to 

obtain a plume configuration 15 to 25 

kilometers farther along Fortymile Wash 

toward the Amargosa Farms area. Noting the 

variations in the flow path lengths; the 

quasi-municipal and domestic wells are about 

20 kilometers from the repository. The 

majority of irrigation wells are first 

encountered approximately 25 kilometers 

from the accessible environment, or 30 

kilometers from the repository. 

3.2.4.2.2 Plume Dimensions for 3-D 

Dispersion from Constant 

Concentration Source 

The analytic solution to Equation (3-3) for 

the constant concentration patch source, as 

described in Wexler (1992) is:

C(x,y,z, CO x exp x V2 2 C -t x P[ r-r2exp T-+ 2 A 

S[eic(Y~,9) 2 TD; -Y) . erfc 2exLTDý (Y2 -Y))] A~~ 

[erfc((Z'j~2 erfc ( (Z2 -z) ) dI 

(3-3) 

where Co is the concentration at the source, r 

is a dummy variable of integration for time, , 

is the decay coefficient, exp is the natural 

exponential, and erfc is the complementary

error function. The dispersion coefficients in 

the x, y, and z directions are defined as the 

products of the seepage velocity and the 

dispersivities in the x, y, and z directions, 

respectively. This equation is the solution to 

the 3-D solute transport equation for a 

vertical patch source aligned normal to the 

principal direction of flow, where the patch 

dimensions are defined byy2-yl and z2-zl. 

The solution to the advection-dispersion 

equation is valid for a 1 -D uniform flow field 

and 3-D dispersion for a constant concen

tration source in an aquifer of infinite depth 

and lateral extent. Adsorption and radioactive 

decay of the solute are incorporated into the 

solution, but were not used in this study. 

Tables 3-7 and 3-8 contrast plume width and 

thickness for various sets of dispersivity values 

at 20 and 30 kilometers, respectively, from the 

repository. The longitudinal dispersivity value 

is reported in the tables, but its magnitude is 

not a controlling factor for the results. The 

plume width and thickness are delineated at a 

threshold concentration of approximately 

Co x 10-4. The P-DF is also included in 

Tables 3-7 and 3-8. These values will be used 

as a reference point for the dispersion-based 

dilution factors that are estimated later in 

Section 3.2.4.4. At 30 kilometers, a reduction 

of the transverse dispersivities by 80 percent 

leads to almost a 50-percent reduction in 

plume width and thickness when the ratio of 

the horizontal and vertical transverse 

dispersivities is kept at an order of magnitude. 

The percentages are approximately the same 

for the 15-kilometers results. Similarly, a 

50-percent reduction in the transverse 

dispersivities leads to almost a 25-percent 

reduction in plume width and thickness at 30 

kilometers. 

3.2.4.2.3 Plume Dimensions Neglecting 

Vertical Dispersion for Constant 

Concentration Source

From the literature (Bedient et al., 1994), it is 

evident that existing plumes (caused either by 

accidental contamination or by deliberate 

injection of tracers for experimental 

purposes), are often confined to a thin layer 

near the water table. Exceptions occur in areas 

of high infiltration. Thus, the conservative case

3-51 NUREG-1538
3 Ground-Water Pathway Analyses

ax:ay:az Thickness Width 

(m) (M) (M) P-DF = Co/ICc 

20:2:0.2 330 2200 6 

50:5:0.5 480 3100 13 

100:20:2 830 5200 48 

100:10:1 640 4000 25 

100:10:0.1 250 4300 9

ax.ay.az Thickness Width 

(M) (M) (M) P-DF = Co/Cc 

20:2:0.2 410 2600 9 

50:5:0.5 580 3700 21 

100:20:2 970 5800 80 

100:10:1 780 4800 41 

100:10:0.1 290 5200 14

is to assume no vertical dispersion so the 

plume remains the same thickness as the 

source area, but is dispersed laterally. This 

conceptual model for plume movement can be 

modeled using the following solution for 2-D 

dispersion for a line source of specified width 

and constant concentration (Wexler, 1992): 

C(x,y,t) = C ex ( ) £ -e + x

4 ,_D-x y2Dx 0 - "_ x " xr 

[_f D - er)c)i] dr (2Jý ( 7

The solution to Equation (3-4) is 

implemented in Wexler's STRIPI computer 

program. The solution for the line source can 

be extended to any source thickness. In light 

of the arguments presented in the previous 

section, a reasonable selection of sets of 

dispersivities is 20:2, 50:5, and 100:10 for the 

longitudinal and transverse directions (Table 

3-9). Although these dispersivities are not 

strictly comparable to the dispersivities used 

for the 3-D analysis (i.e., no transverse 

vertical dispersivity for the 2-D analysis), the 

resulting plume widths were somewhat similar 

for the 2-D and 3-D analyses. The 2-D 

analysis, without vertical dispersion, resulted 

in increases in plume width of 6 to 16 percent 

at 20 kilometers from the repository, and 10 to 

21 percent at 30 kilometers from the proposed 

repository, compared to plume widths in the 

3- D analysis for corresponding dispersivities.

NUREG- 1538

Table 3-7. Plume Configuration and Point Dilution Factors at 20 

Kilometers from the Proposed Repository (15 Kilometers from Source 

Area) for a Range of Dispersivity Values. C, is the centerline 

concentration. The source area is 25-meters thick by 500-meters wide.

Table 3-8. Plume Configuration and Point Dilution Factors at 30 

Kilometers from the Proposed Repository (25 Kilometers from Source 

Area) for a Range of Dispersivity Values

3-52
3 Ground-Water Pathway Analyses

Table 3-9. Plume Configuration in Terms of Width at 20 and 30 Kilometers from the 

Proposed Repository (15 and 25 Kilometers from the Source Area) and Point Dilution Factors 

for a Source Area Width of 500 Meters and No Vertical Dispersion 

Width at 20 Width at 30 

ax:ay Kilometers P-DF = Co/Cc at Kilometers P-DF = Co0 /C at 

(M) (m) 20 Kilometers (M) 30 Kilometers 

20:2 2,330 1.5 2860 1.8 

50:5 3,410 2.1 4230 2.6 

100:10 4,640 2.8 5800 3.6

3.2.4.3 Borehole Dilution Factors Based on 

Volumetric Flux 

Volumetric flux-based borehole dilution factors 

(F-BDFs) are determined by comparison of 

the plume and capture zone configurations 

(Figure 3-15). The ratio of the cross-sectional 

area of the capture zone to the cross-sectional 

area of the portion of the plume that intersects 

the capture area in the plane perpendicular to 

the principal direction of flow is the dilution 

factor caused by borehole mixing, based on 

volumetric flux comparisons. In other words, 

the F-BDF is the ratio of the capture and the 

intersection area. No credit is taken for the 

distribution of the concentration across the 

plume in the calculation of the F-BDF. All 

plumes in this section are modeled from a 

constant concentration source. 

Generally, the plumes are wider than the 

capture zone but not as thick. Four plume 

scenarios are chosen to represent a range of 

conditions. The first and second scenarios are 

10-meter- and 25-meter-thick plumes for 

which no vertical dispersion has occurred. The 

width of the plume depends on the horizontal 

transverse dispersivity that is used. For 

domestic wells, it does not matter what 

horizontal transverse dispersivity is chosen 

since all plumes are wider than all domestic 

well capture zones. The third and fourth 

scenarios incorporate vertical dispersion with 

dispersivity ratios of 20:2:0.2 and 100:10:0.1. 

The F-BDFs for the third and fourth 

scenarios are presented for the high-volume 

irrigation well pumps.

3.2.4.3.1 Domestic Wells 

The plume configuration that results from 

3-D dispersion from a constant concentration 

source (see Tables 3-7 and 3-8) will 

generally be larger than the capture area of a 

single domestic well, a closely spaced 

collection of domestic wells, or a 

quasi-municipal well for wells typical of the 

Amargosa Farms area. Hence, with the 

assumption of a uniform plume concentration, 

there will be no borehole dilution. Only for 

the smallest vertical transverse dispersivity 

values (less than 0.2) and for the largest 

pumping volumes from a closely spaced 

collection of domestic and quasi-municipal 

wells will there be vertical gradients that are 

strong enough to capture clean water and 

provide borehole dilution. 

For a typical domestic well that pumps about 

6.8 cubic meters/day (1800 gallons/day), the 

F-BDF decreases from 10 to 4, when the 

plume thickness increases from 10 to 25 

meters at the 30-kilometer distance (Figure 

3-20). The difference in the factors increases 

as the pumping rate increases. The F-BDFs 

for the 10-meter plume range between 7 and 

26 for pumping rates in the range of domestic 

and quasi-municipal wells. Similarly, the 

F-BDFs for the 25-meter plume range 

between 3 and 10. 

The position of the screened portion of the 

well does not have a significant effect on 

domestic wells for the 25-meter-thick plume 

until the screened portions are lower than 

three standard deviations from the average 

screen position (Figure 3-21). The limited 

effect of screen position is caused by a

3-53 NUREG- 1538
3 Ground-Water Pathway Analyses

30 

20 

PLUME THICKNESS 

U

LL 

LL 

PLUMETHICKNESS 

10 

0 I I I I II 

0 20 40 60 80 

Pumping Rate (m3/d) 

Figure 3-20. Effect of pumping rate (range: 1 to 75 cubic meters/day) on the flux-based borehole dilution factor 

for plumes of thickness 10 and 25 meters (no vertical dispersion). The regional gradient is 0.005 

and the transmissivity is 100 square meters/day for all cases. 

NUREG- 1538 3-54
3 Ground-Water Pathway Analyses

25 35 45 55 65 75

Centerline of Screen Position (m)

Effect of screen position for domestic-sized wells on the flux-based borehole dilution factor for 

plumes of thickness 10 and 25 meters (no vertical dispersion). All screen lengths are 60 meters, the 

regional gradient is 0.005, and the transmissivity is 100 square meters/day for all cases.

NUREG-1538

50 

40 

30

LL 

LL

20 

10 

0

PLUME THICKNESS 

OF 25 METERS 

PLUME THICKNESS 

OF 10 METERS

I I I I I I I I I

Figure 3-21.

3-55
3 Ground-Water Pathway Analyses

combination of the center of mass of the 

plume being near the water table, as well as 

the small impact on the capture area caused 

by different screen position and lengths. 

Within about two standard deviations from the 

average position of the screen, the F-BDFs 

do not vary by more than a factor of 2. In all 

scenarios, the plume is assumed to be at the 

water table. The borehole dilution factors 

exhibited greater sensitivity to the screen 

position by a variation of the F-BDF of 8 to 

26 for approximately two standard deviations 

from the average position of the screen. 

The effects of transmissivity and regional 

gradient on F-BDFs, for the 10- and 

25-meter-thick plumes with no vertical 

dispersion, are not significant until the 

smallest values of transmissivity and gradient 

are used (Figures 3-22 and 3-23). For 

transmissivities greater than 50 square 

meters/day, the F-BDFs are in the range of 7 

to 10 for the 10-meter-thick plume and 3 to 4 

for the 25-meter-thick plume. A regional 

gradient of 0.001 leads to a F-BDF of 13 for 

the plume thickness of 10 meters, whereas for 

the larger gradients, the F-BDFs range from 

7 to 10. The F-BDFs for the 25-meter-thick 

plume are between 3 and 5 for a regional 

gradient range of 0.001 to 0.01. 

3.2.4.3.2 Irrigation Wells and Plumes with 

No Vertical Dispersion 

The F-BDFs were calculated for irrigation 

wells using the scenario of a 25-meter-thick 

plume with no vertical dispersion. In this 

scenario, the large vertical gradients and deep 

capture for the wells lead to large amounts of 

clean water mixing in the borehole with the 

contaminated water from the plume. 

Depending on the capture zone width and the 

plume width, some horizontal mixing of clean 

and contaminated water may occur. The width 

of the plume depends on the transverse 

dispersivity. 

Figure 3-24 shows the F-BDFs for a well 

pumping rate of 300 to 2000 cubic meters/day 

for plumes using three different dispersivity 

values. Since the plume width decreases as the 

dispersivity decreases, the F-BDF increases 

as the dispersivity decreases. This effect is not

present at the low pumping rates for the 

particular flow field parameters chosen for 

this comparison. The F-BDFs range from 19 

to 49 for all dispersivity combinations. It must 

be re-emphasized that the F-BDF only 

reflects the effects of contaminant 

concentration reduction in the borehole and 

not the effects of dispersion on the resident or 

aquifer contaminant concentrations. This 

explains the otherwise counter-intuitive 

observation that, for high capacity wells, the 

F-BDF increases as the transverse 

dispersivity decreases. 

3.2.4.3.3 Irrigation Wells and Plumes with 

Vertical Dispersion 

The F-BDFs are calculated for irrigation 

wells using the scenario of a plume where 

3-D dispersion from a constant concentration 

source occurs. The effect of dispersion on the 

concentration distribution across the plume is 

not considered here; only its effect on the 

overall the shape of the plume is considered in 

the dilution factors. Generally, the capture 

zones are thicker and narrower than the thin 

but wide plumes. Depending on the 

dispersivity values used for the plume and the 

pumping rate and hydraulic properties used 

for the capture zone, the capture zones may be 

wider than the plume. Only for low pumping 

rates are the plumes thicker than the capture 

zone; this occurrence leads to no volumetric

based borehole dilution. 

Plume shapes using dispersivities of 100:10:0.1 

meters and 20:2:0.2 meters are compared to 

capture areas, to calculate F-BDFs. The 

plume for the 100:10:0.1-meter scenario is 

wider but thinner than the plume for the 

20:2:0.2-meter scenario. Figures 3-25 to 3-27 

show the effects of pumping rate, 

transmissivity, and regional gradient on the 

F-BDFs, which generally range from 1 to 5, 

regardless of dispersivity values used. For the 

pumping rate (Figure 3-25) and the regional 

gradient (Figure 3-27) curves, the two 

dispersivity sets intersect because of the 

interplay between the thickness of the plume 

(the 20:2:0.2-meter plume is thicker) and the 

point where the entire plume is captured (the 

100:10:0.1-meter plume is larger in area).

NUREG- 1538 3-56
3 Ground-Water Pathway Analyses

20

15 HS\ LUMET OF 10

10 --

0

Figure 3- 22.

I I IIIII 

HICKNESS 

METERS 

PLUME THICKNESS 

OF 25 METERS 

I I IIIII

100 200 

Transmissivity (m2

/d)

300 400

Effect of transmissivity (10, 50, 100, and 400 square meters/day) on the flux-based borehole 

dilution factor for plumes of thickness 10 and 25 meters (no vertical dispersion). The regional 

gradient is 0.005 and the pumping rate is 3 cubic meters/day for all cases.

NUREG-1538

LL 

0 

Ca rL

5 

0

3-57
0.002 0.004 0.006 0.008 0.010

Regional Gradient

Effect of the regional gradient (0.001, 0.0025, 0.005, and 0.01) on the flux-based borehole dilution 

factor for a domestic-sized well and plumes of thickness 10 and 25 meters (no vertical dispersion). 

The transmissivity is 100 square meters/day and the pumping rate is 3 cubic meters/day for all cases.

NUREG- 1538

3 Ground-Water Pathway Analyses

15 

10

LL 

m

I I I I I I I I I I

PLUME THICKNESS 

OF 10 METERS

PLUME THICKNESS 

OF 25 METERS

I I I I I I I I I

5 

0 

0.000

Figure 3- 23.

3-58
3 Ground-Water Pathway Analyses

50

40 

20:2 METERS 

50:5 METERS 

L. . ....... . 

LL 30 /'- 100:10 METERS 

LI • 

20 

10 I I I I I I 

0 500 1000 1500 2000 

Pump Rate (m3/d) 

Figure 3-24. Effect of pumping rate on flux-based borehole dilution factors for irrigation wells and a 

25-meter-thick plume with no vertical dispersion. Three curves are plotted for different sets of 

dispersivity values.

3-59 NUREG-1538
500 1000 1500 2000

Pump Rate (m3/d)

Effect of pumping rate on borehole dilution factors for irrigation-sized wells and a plume with 3-D 

dispersion. Curves are plotted for two sets of dispersivity values.

NUREG- 1538

3 Ground-Water Pathway Analyses

5

4

LL 

0t U-l 3

2

1

0

Figure 3- 25.

3-60
3 Ground-Water Pathway Analyses

0 100 200 300 400 500

Transmissivity (m3/d)

Effect of transmissivity (range: 50 to 400 square meters/day) on borehole dilution factors for 

irrigation-sized wells and a plume with 3-D dispersion. Curves are plotted for two sets of 

dispersivity values.

NUREG- 1538

4

3

LL 

0 

LL

2

1

Figure 3- 26.

3-61
0.002 0.004 0.006

Regional Gradient

Effect of regional hydraulic gradient (range: 0.001 to 0.0005) on borehole dilution factors for 

irrigation-sized wells and a plume with 3-D dispersion. Curves are plotted for two sets of 

dispersivity values.

NUREG- 1538

3 Ground-Water Pathway Analyses

7

6 

5

U

LL

4

3 

2 

1

0.000

Figure 3-27.

3-62
3 Ground-Water Pathway Analyses

In summary, the effect of the plume size has 

the largest effect on the F-BDE The values 

of the dilution factors are tabulated in 

Appendix F 

The plume width may increase or decrease for 

diverging/converging flow fields, respectively, 

but the volumetric flux does not change. 

3.2.4.4 Borehole Dilution Factors Based on 

Dispersive Transport 

The F-BDFs estimated in the previous 

section do not account for the concentration 

distribution of a migrating plume. Kessler and 

McGuire (1996) accounted for dispersion 

during plume migration by assuming the 

dilution factor was the ratio of the source 

concentration to the centerline concentration. 

Implicit in their assumption is that the plume 

has a uniform concentration equal to the 

centerline value that they justify as a 

conservative choice in terms of eventual dose 

to a receptor group. This section will address 

the effect on borehole dilution of a 

concentration distribution within a plume. 

The transport dispersion-based borehole dilution 

factor (T-BDF) was calculated by integrating 

the concentration distribution across the area 

of the portion of the plume that is captured by 

a pumping well. Portions of the plume not 

captured by the well do not contribute 

radionuclide mass to the well. The T-BDF 

was estimated by numerical integration of the 

concentration distribution in the area of the 

plume that was captured. The total borehole 

dilution factor can be estimated by linear 

combination of the F-BDF and T-BDE The 

effect of domestic and irrigation wells on 

T-BDFs varies significantly because of the 

thickness of the capture area, and will be 

presented separately. 

3.2.4.4.1 Domestic Wells 

Figures 3-27 and 3-28 illustrate the effect of 

the concentration distribution within a plume 

on the T-BDF for two different plume 

configurations; a thin plume (25 meters) with 

no vertical dispersion and a 3-D dispersion

plume. The T-BDF for the thin plume is 

nearly constant and its value is close to that of 

the P-DF (1.8) for pumping rates in the range 

of domestic and quasi-municipal wells (Figure 

3-27). The T-BDF for the plume with 3-D 

dispersion vary from 9 to 18, increasing as the 

pumping rate increases. The larger values of 

T-BDF indicate the significance of pumping 

from less concentrated portions of the plume 

as compared with the centerline. T-BDF is 

inversely proportional to the transmissivity 

(Figure 3-28) with values ranging from 12 to 

9 as transmissivity increases for the 3 -D 

dispersion plume. Smaller transmissivity 

values lead to larger capture areas thus 

drawing water from portions of the plume with 

lower concentration. The effect of hydraulic 

gradient is similar to that of transmissivity. 

3.2.4.4.2 Irrigation Wells 

Figures 3-29 and 3-30 illustrate the effect of 

the concentration distribution on borehole 

dilution for irrigation wells. For the plume 

configuration with 3-D dispersion, the 

T-BDFs are as much as 5 times larger 

(Figures 3-29 and 3-31) than those for the 

domestic wells, because of the large thickness 

of the irrigation capture area drawing in 

portions of the plume with low concentrations. 

Unlike a domestic well, the T-BDF, for a thin 

plume (i.e., 25 meters) increases as the 

pumping rate increases. The straight line 

increase in T-BDF for the plume with 3-D 

dispersion and dispersivity ratio of 100:10:0.1 

meters reflects the large size of the plume 

relative to the capture areas (Figure 3-31). 

The plateau in the curve for the 3-D plume 

with dispersivity ratio of 20:2:0.2 meters at the 

larger pumping rates is caused by the entire 

plume being captured. 

For transmissivity increases from 50 to 400 

square meters/day, the T-BDF decreases 

from 48 to 18 for the 3-D plume with 

dispersivity ratio of 20:2:0.2 meters, and from 

43 to 30, for the 3-D plume with dispersivity 

ratio of 100:10:0.1 meters. Effects caused by 

hydraulic gradient are similar to those of the 

transmissivity (see Appendix F).

NUREG- 1538 3-63
20 40 60 80 

Pumping Rate (m3

/d)

Effect of pumping rate (range: 1 to 75 cubic meters/day) for domestic wells on transport dispersion-based borehole dilution factors for two different plume configurations. A thin plume with no vertical dispersion and a 3-D dispersion plume both with dispersivity ratios as noted in the 

plot.

NUREG- 1538

3 Ground-Water Pathway Analyses

20

15

U10

5

0

0

Figure 3-28.

3-64
3 Ground-Water Pathway Analyses

20 

15 

U__ PLUME WITH 3-D DISPERSION; 

a F- 10 20:2:0.2 METERS - I 

5 

PLUME OF THICKNESS 25 METERS; 

20:2 METERS 

0 I I I I I I I 

0 100 200 300 400 

Transmissivity (m2

/d) 

Figure 3-29. Effect of transmissivity (range: 10 to 400 square meters/day) for domestic wells (Q = 3 cubic 

meters/day) on transport dispersion-based borehole dilution factors for two different plume 

configurations. A thin plume configuration with no vertical dispersion and a 3-D dispersion plume, 

both with dispersivity ratios as noted in the plot.

3-65 NUREG-1538
3 Ground-Water Pathway Analyses

i I 

PLUME WITH 3-D DISPERSION; 

20:2:0.2 METERS A

PLUME WITH 3-D DISPERSION; 

100:10:0.1 METERS

PLUME OF THICKNESS 25 METERS; 

PICKNESS 25 M 202 METERS

500 1000 1500 2000

Pumping Rate (m3

/d)

Effect of pumping rate (range: 300 to 2000 cubic meters/day) for irrigation wells on transport dispersion-based borehole dilution factors for four different plume configurations. Two thin plume 

configurations with no vertical dispersion and two 3-D dispersion plumes, all with dispersivity ratios 

as noted in the plot.

NUREG- 1538

80

60 IU

0 mLL 40 [-

0

V

20 I0

0

Figure 3-30.

I

I I

3-66
3 Ground-Water Pathway Analyses

200 300

Transmissivity (m2

/d)

Effect of transmissivity (range: 50 to 400 square meters/day) for large irrigation wells (Q =2116 

cubic meters/day) on transport dispersion-based borehole dilution factors for two different plume 

configurations. Two 3-D dispersion plumes with dispersivity ratios as noted in the plot.

NUREG-1538

60

40 ILL m 

0

I I I I I I I I I

100:10:0.1 METERS

20:2:0.2 METERS

I I I I I I I I I

20 [-

0

0 100 400

Figure 3-31.

500

3-67
3 Ground-Water Pathway Analyses

3.2.5 References 

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Contaminant Movement Near Pumping Wells: 

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Movement through Shallow Unsaturated 

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NUREG-1538 3-68
3 Ground-Water Pathway Analyses

Hoover, D.L., WC. Swadley, and A.J. Gordon, 

"Correlation Characteristics of Surficial 

Deposits with a Description of Surficial 

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81-512, 1981. 

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Alto, California, Electric Power Research 

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Nichols WD., and J.R Akers, "Water-Level 

Declines in the Amargosa Valley Area, Nye 

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the Great Basin Region of Nevada, Utah, and 

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[1996]. 

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3-69 NUREG- 1538
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Desert," Carson City, Department of 

Conservation and Natural Resources/Division 

of Water Resources, 1997b. 

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Hydrographic Basin 230, Amargosa Desert," 

Carson City, Department of Conservation 

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Resources, 1997c. 

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Swadley, WC., "Map Showing Surficial 

Geology of the Lathrop Wells Quadrangle, 

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1361, 1983. 

Swadley, WC., and WJ. Carr, "Geologic Map 

of the Quaternary and Tertiary Deposits of the 

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December 1988. 

Walker, G.E., and TE. Eakin, "Geology and 

Ground-Water of Amargosa Desert, Nevada

California," Carson City, Nevada, State of 

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Reconnaissance Series Report 14, 1963. 

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Performance Assessment Phase 2: 

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Waste Repository," U.S. Nuclear Regulatory 

Commission, NUREG-1464, October 1995. 

Wexler, E.J., 'Analytical Solutions for One-, 

Two-, and Three-Dimensional Solute 

Transport in Ground-Water Systems with 

Uniform Flow," in "Techniques of Water

Resources Investigations of the U.S. 

Geological Survey," U.S. Geological Survey, 

Chapter B7, Book 3 (Applications of 

Hydraulics), 1992. 

Wilson, M.L., et al., "Total-System Per

formance Assessment for Yucca Mountain

SNL Second Iteration (TSPA-1993)," 

Albuquerque, New Mexico, Sandia National 

Laboratory, SAND93-2675, 2 vols., April 

1994. 

Winograd I. J. and W Thordarson, 

"Hydrogeologic and Hydrochemical 

Framework, South-Central Great Basin, 

Nevada-California, with Special Reference to 

the Nevada Test Site (Hydrology of Nuclear 

Test Sites)," U.S. Geological Survey, 

Professional Paper 712-C, 1975. 

3.3 Annual Individual Dose Estimates 

3.3.1 Introduction 

In its 1995 report, the NAS recommended that 

future standards for Yucca Mountain limit 

individual risk to the average member of the 

exposed critical group and that compliance 

should be evaluated at the time and place 

where greatest risk occurs (up to the limit of 

geologic predictability of the site, which NAS 

asserts to be 1 million years) following 

repository closure. Moreover, no scientific 

basis was found for limiting the compliance 

period to 10,000 years as done under existing 

regulations. Thus, to gain some insight into 

potential implementation issues associated 

with this particular recommendation, as well 

as to better understand the relative 

importance of site-specific assumptions and 

parameters in dose-based calculations at the 

Yucca Mountain site, the staff performed 

some preliminary calculations to estimate 

annual individual dose.

NUREG- 1538 3-70
3 Ground-Water Pathway Analyses

3.3.2 Simulation Approach 

Evaluation of annual individual dose requires 

specification of an exposure scenario that 

defines the geosphere and biosphere pathways 

for the transport of radionuclides released 

from a geologic repository to a human 

receptor in the biosphere. Simulation of 

radionuclide release and transport in the 

geosphere were conducted based on models 

developed within NRC's IPA program. Some 

modifications to the computational modules11 

were necessary to allow investigation of 

sensitivities of the dose calculation, improve 

calculational efficiencies, and allow the 

calculation to go beyond a 10,000-year 

performance period, to the time of peak dose. 

Two exposure pathways were developed for 

release of radionuclides to ground water. The 

first exposure pathway is consistent with the 

distance used for integrated release calcula

tions performed in previous IPA efforts 

(Wescott et al., 1995). It assumes that a 

hypothetical receptor group exists at a distance 

of 5 kilometers down-gradient of the proposed 

repository. Radioactive exposure results from 

ingestion of contaminated water (water 

withdrawal by a well for a residential 

community intercepts radionuclides in the 

saturated zone). The second exposure pathway 

assumes a hypothetical farming/ranching-type 

receptor group is located approximately 30 

kilometers downgradient from Yucca 

Mountain (based on the conclusions reached in 

Section 2.3). Radioactive exposure pathways at 

this location are: ingestion (of contaminated 

water, crops and animal products); inhalation 

(from resuspension of contaminated soil); and 

direct exposure. 

To better understand and quantify the 

variation in dose estimates caused by 

uncertainties in the geosphere models (i.e., 

source-term release, hydrologic flow, and 

radionuclide transport), the NRC staff 

performed probabilistic analyses for the two 

hypothetical receptor locations. Important 

1 iThe modifications in question were performed principally to 

the FLOWMOD (McCartin et at., 1995) and the SOTEC 

(Sagar et al., 1992) computational modules. These codes 

model the leaching and dissolution of radionuclides within a 

waste package and determine their concentrations at a distant 

receptor location.

attributes and assumptions of these analyses 

were as follows. 

Simplifications of the flow and transport 

models include a steady-state flow system; 

no thermal effects; fracture retardation 

assumed to be a fraction of the matrix 

retardation (a range of 0 to 10 percent 

was used for the uncertainty analyses); 

assumed waste package container lifetime 

of 1000 years; disruptive scenarios not 

considered; and a source term based on 

leach rate, solubility, and amount of water 

contacting the waste. 

" A continuous transport path is assumed to 

exist from the saturated zone, below the 

proposed repository footprint, to the two 

hypothetical receptor locations-5 

kilometers (in fractured tuff) and 30 

kilometers (27 kilometers in the fractured 

tuff and 3 kilometers in porous alluvium). 

" All releases from the proposed repository 

eventually pass the 5-kilometer receptor 

location and are uniformly mixed in the 

annual volume of water pumped by the 

hypothetical receptor group at the 

30-kilometer location (at a rate of 3800 

cubic meters/day for the 5-kilometer 

location and 30,400 cubic meters/day for 

the 30-kilometer location).12 Water usage 

was based on broad assumptions 

regarding pumping rates required to 

intercept the entire contaminant plume 

(applied to the 5-kilometer location) and 

water usage consistent with a farming 

community (applied to the 30-kilometer 

location). 

" The hypothetical receptor group at 5 

kilometers uses untreated ground water 

for drinking water only. 

" The hypothetical receptor group at 30 

kilometers uses untreated ground water 

for drinking water, the irrigation of 

agricultural crops, and the watering of 

livestock. [The average member of this 

12These production estimates are consistent with the IPA Phase 

2 analysis and are believed to be sufficient pumping rates, at 

the two respective locations, to entrain all radionuclides in a 

contaminant plume that might emanate from a potential 

geologic repository at Yucca Mountain. 

3-71 NUREG-1538
3 Ground-Water Pathway Analyses

group is assumed to supply half (50 

percent) of his/her food needs and all 

his/her water (including milk) from 

his/her farm/ranch. 13

] 

For both hypothetical receptor group 

locations, certain parameters were sampled 

independently for each of the seven repository 

sub-areas, 14 whereas others were sampled only 

once and applied to the seven sub-areas. 

Parameters sampled for each repository 

sub-area were: (a) the assumed capture area 

to convert infiltration rate to a water flux for 

each waste package; (b) the fraction of the 

fuel that is wetted by the infiltrating water at 

any given time; and (c) the leach rate of the 

waste form. Parameters sampled only once, 

that applied to all the seven repository 

sub-areas, were: (a) radionuclide solubilities; 

(b) infiltration; (c) matrix distribution 

coefficients for each hydrostratigraphic unit; 

(d) matrix permeabilities for each 

hydrostratigraphic layer; and (e) fraction of 

matrix retardation applied to fracture. 

Parameter ranges for the parameters sampled 

in the uncertainty analysis are presented in 

Appendix H. 

3.3.3 Results and Conclusions 

The annual individual dose (expressed as 

TEDE-total effective dose equivalent), as a 

function of time, is presented in Figures 3-32 

and 3-33 for the 5- and the 30-kilometer 

locations, respectively. Doses for 100 sampled 

vectors were calculated versus time, and the 

results were grouped into 100-year intervals. 

The values in each interval were then ranked, 

and the 10th, 50th, and 90th values in the rank 

(100 being the largest) were plotted to provide 

the curves denoted as 10th percentile, median, 

and 90th percentile, respectively. The curve 

marked mean is the arithmetic average of all 

1

3

The reader is referred to Appendix A, in which it is noted that 

the magnitude of personal, home-garden consumption could 

be as high as 30 percent, compared with 50 percent assumed 

for this particular analysis. 

14For the purposes of the IPA Phase 2 analysis, the proposed 

geologic repository was divided into seven sub-areas, based 

on their proximity to individual boreholes. The hydrogeologic 

units below each of the seven sub-areas were assumed to 

correspond to the associated borehole stratigraphy at those 

points, thus producing seven different hydrogeologic 

sequences over the entire repository (see McCartin et al., 

1995; p. 4 -15).

the values in each interval. Note that over a 

large range of time for both curves, the 10th 

percentile value was essentially zero, and is 

coincident with the time axis (ordinate). 

Although the maximum annual individual dose 

estimates for both locations occur at different 

times, the time of the maximum dose occurs 

shortly after initial arrival of radionuclides at 

the two sample locations. This result is 

primarily because of the (conservative) 

assumption that all waste package containers 

fail at 1000 years, which results in large 

releases of long-lived, soluble, and mobile 

radionuclides (e.g., 99Tc, 237Np, and 1291) 

arriving at the receptor location at generally 

overlapping times. After the initial peak, the 

dose-versus-time curve is complicated by both 

short-duration spikes (e.g., spikes that appear 

between 3000 and 8000 years in Figure 3-32) 

and secondary peaks that are significantly less 

than the initial peak (e.g., secondary peaks at 

50,000 and 700,000 years in Figure 3-32, and 

700,000 years in Figure 3-33). This variation 

in dose is a result of varying arrival times for 

individual radionuclides caused by: (a) varying 

flow velocities and retardation; (b) 

representation of the unsaturated zone 

stratigraphy (i.e., the repository was divided 

into seven sub-areas, each with unique 

stratigraphy, and thus a distinct travel time); 

and (c) differences in transport velocities used 

for the fracture flow component versus the 

matrix flow component of flow within a 

specific stratigraphic sequence (e.g., releases 

that are transported through the porous matrix 

will arrive at the receptor location after 

releases that move through the fractures). 

Deterministic simulations, using mean values 

of the parameter ranges used in the 

uncertainty analysis, were performed for the 

two hypothetical receptor locations, to identify 

the contribution of individual radionuclides to 

dose. 15 Figures 3-34 and 3-35 present the 

doses for specific radionuclides for the two 

receptor locations as well as the total dose 

15

The deterministic simulation was used for identifying the 

contribution of individual radionuclides because of limitations 

in NRC's IPA computer code. A better representation of 

overall performance is presented by the probabilistic results 

presented in Figures 3-32 and 3-33.

NUREG-1538 3-72
150.0 l ' 

MEAN 

MEDIAN 

90TH PERCENTILE 

...-- - - - -10TH PERCENTILE 

100.0 

• CD 

I-a 

0 

50.0 

I ", .... 

0.0 

z 1000 10,000 100,000 1,000,000 

Time (years) 

Figure 3-32. Uncertainty analysis of dose for 100 vectors at a point 5 kilometers down-gradient from the 

proposed geologic repository at Yucca Mountain 

00
0.... MEAN 

- -.... MEDIAN • 30.0 90TH PERCENTILE 

--------. 10TH PERCENTILE 

E 

E0 

a) 20V.0 

. 

10.0 1 

0.0 

1000 10,000 100,000 1,000,000 

Time (years)

Figure 3-33. Uncertainty analysis of dose for 100 vectors (all dose pathways) at a point 30 kilometers down-gradient from the 

proposed geologic repository at Yucca Mountain.
30 1 1 1 1 1 1 1 1 

PEAK DOSE 

--...---.- TEDE 24.0 

............... 14c 0.93 

" 94

Nb 3.1 24," "" 9Tc 13.0 

1291 5.9 

.. a = U 237Np 3.9 

- - 79

Se 1.3 

I 

18 I ' I II 

E v / I 

0 o I 

' I) I 

12. 

6/ 

II : 

I / * I I a I 

I "" 

0I 

i" I "i" 

,. ': --- / 00 l - " - 1 \ 0 "-• 10' 

":It IT -•.. . . .- ; Vi_ \ = 

"'" *.. . ....... .. " 

1010,000 100U,000 1,U00,000 

Time (years) 

Figure 3-34. Annual individual dose from all radionuclides TEDE and for selected radionuclides for the drinking water 

pathway at a point 5 kilometers down-gradient from the proposed geologic repository at Yucca Mountain. 

U Mean parameter values were used, including fracture retardation. Time of peak dose is 17,196 years.
10,000 100,000

0 

00

1,000,000

Time (years)

Annual individual dose from all radionuclides TEDE and for selected radionuclides for the drinking water pathway at a point 30 kilometers down-gradient from the proposed geologic repository at Yucca Mountain. Mean parameter values were used, including fracture retardation. Time of peak dose is 19,370 years.

30

0 

1 0

24

18

E 

E a) 

U) Co 

0 0

12 

6 

0

1000

Figure 3-35.
3 Ground-Water Pathway Analyses

from all radionuclides for the pathways 

considered, which is denoted in the legend as 

TEDE. A cursory examination of Figures 

3-34 and 3-35 reveals that the magnitudes 

for the peak dose for the two receptor 

locations are quite similar [14 millirem 

(mrem) versus 24 mrem]. This similarity in 

results is caused by several significant 

differences in key assumptions (cited above) 

that have counterbalancing effects on the 

results. For example, the dose at the 

5-kilometer location should be larger than the 

dose at the 30-kilometer location because of 

differences in the assumed dilution volumes 

(about 3800 cubic meters/day versus 30,400 

cubic meters/day), which reduce concentrations 

at the location farther away. If drinking water 

dose were the only ingestion pathway, then the 

dose at the 30-kilometer location might have 

been an order of magnitude or more lower 

than the dose at the 5-kilometer location. 

However, the hypothetical farming 

community, at the 30-kilometer location 

includes, additional ingestion pathways from 

animal products and crops that counterbalance 

the decrease in dose caused by the larger 

dilution volume associated with agricultural 

activity. Additionally, the difference in 

ingestion pathways result in different 

contributions from specific radionuclides 

responsible for the peak. At the 5-kilometer 

location, the peak dose is significantly 

influenced by arrival of 99Tc and 1291 from 

more than one sub-area, as evidenced by the 

multiple peaks for 99Tc. At the 30-kilometer 

location, the peak dose is influenced more by 

arrival of Niobium-94. However, at both 

locations, it is long-lived, soluble, and mobile 

radionuclides that are the key contributors to 

dose. 

3.3.4 References 

McCartin T.J., et al., "Flow and Transport 

Module [FLOWMOD]," in R.G. Wescott et al. 

(eds.), "NRC Iterative Performance 

Assessment Phase 2: Development of 

Capabilities for Review of a Performance 

Assessment for a High-Level Waste 

Repository," U.S. Nuclear Regulatory 

Commission, NUREG-1464, October 1995.

Sagar, B., et al., "SOTEC: A Source Term 

Code for High-Level Geologic Repositories

Users' Manual (Version 1.0)," San Antonio, 

Texas, Center for Nuclear Waste Regulatory 

Analyses, CNWRA 92-009, July 1992. 

[Prepared for NRC.] 

Wescott, R.G. et al. (eds.), "NRC Iterative 

Performance Assessment Phase 2: 

Development of Capabilities for Review of a 

Performance Assessment for a High-Level 

Waste Repository," U.S. Nuclear Regulatory 

Commission, NUREG-1464, October 1995. 

3.4 Overall Observations and 

Implications 

In summary, the radionuclides released from a 

potential repository at Yucca Mountain can be 

expected to migrate to the water table and be 

carried down-gradient toward discharge areas, 

most likely, at some point, in the Amargosa

Death Valley area. The ground-water pathway 

is the most likely exposure pathway. Exposure 

to humans is assumed to occur through the use 

of well water that is contaminated with 

radionuclides. Determination of the 

concentration of radionuclides in well water 

depends on: (a) the degree of plume 

dispersion; (b) pumping of the well itself; and 

(c) the location of the discharge point. 

3.4.1 Dilution Resulting from Natural 

Mixing 

Ground-water flow beneath Yucca Mountain 

would initially be in volcanic tuff, generally 

dipping down-gradient toward the Amargosa

Death Valley area. Several tens of kilometers 

from the proposed repository site, the water 

table laterally intersects valley-fill alluvial 

deposits. Because the volume of water 

recharging the Death Valley ground-water 

basin is small relative to the volume of ground 

water already within the saturated zone of the 

ground-water basin, the radioactive discharges 

from the repository would be expected to 

consist initially of a shallow, laminar-like layer 

on the surface of the water table, as shown in 

Figure 3-36. The thickness of the radionuclide 

plume would increase with distance from the 

repository because of processes of molecular 

diffusion and mechanical mixing, especially 

where the water table crossed bedding planes,

3-77 NUREG- 1538
YUCCA MOUNTAIN 

C) 

00 GEOLOGIC CO•NTACT-- x , WASH 

"-..-...- -.... PUMPING / . •" """" 1WELL/ 

PROPOSED ".-..- ' '"W REPOSITOR ALLVIU 

uJ HORIZON 

0) Z 

00 GROUNDWATER/""" ' 

SW GROUNDWATER CONTAMI NATED ----- SO.FLOW DIRECTION GROUNDWATER 

,• ~~~PLUME--.... 

Figure 3-36. Plume dispersion between repository footprint and pumping well. Illustration shows rainwater percolating through the unsaturated zone to the 

repository horizon. Water seeps into emplacement drifts, accelerating corrosion of waste package canisters and exposing their radioactive 

contents. This water now becomes the vehicle for dissolving and transporting exposed radionuclides out of the repository horizon into the 

unsaturated zone below and, finally, to the water table, where it forms a plume. Regional ground-water flow in the saturated zone initially dilutes 

the concentration of radionuclides forming the plume through processes of advection, dispersion, and diffusion. Between the proposed repository and possible receptor locations, plume dilution within the water table is achieved principally through advective and dispersive transport. (Diffusion can be important, but only with low Darcy velocities.) Illustration shows plume dispersion can be achieved as the water table passes over bedding planes (A) or between different geologic media (A'). Percolating ground water from intermittent streams can also contribute to 

plume dilution owing to the increased influx of water reaching the water table (B). Mechanical mixing of the plume can occur within fractured or porous materials found along faults or joint sets (C). Lastly, additional dilution of the contaminant plume is achieved when ground water is drawn 

from a pumping well (D) that intercepts the plume and mixes it with fresh water held in storage by the aquifer.
3 Ground-Water Pathway Analyses

faults, joint systems, or breccia zones, since 

the water table is much more horizontally 

aligned than are the dipping stratigraphy and 

structure at the site. Local recharge along 

ephemeral streams, like Fortymile Wash, 

would also affect the position and shape of the 

initial thin plume. 

The overall finding of this analysis is that 

passive ground-water mixing at the Yucca 

Mountain site is not likely to produce very 

large dilution factors. In the immediate 

vicinity of the proposed repository, dilution is 

limited because of the directional 

characteristics of the flow and magnitudes of 

the Darcy fluxes (i.e., tendency for 

contaminant plumes to remain on or close to 

the water table surface). As the radionuclide 

plumes travel away from the proposed 

repository, they tend to have a greater chance 

of spreading and becoming diluted both 

laterally and vertically as a result of movement 

through or around large-scale structural 

features such as faults. Depending on their 

large-scale hydraulic properties, faults in the 

tuff aquifer could play a major role in 

determining the rate and direction of plume 

spread. At substantial distances, radionuclide 

plumes traveling through the alluvium are 

expected to be further mixed with 

uncontaminated waters, but the dilution at 

locations such as the Amargosa Desert, is 

unlikely to increase by many orders of 

magnitude. However, mixing resulting from 

well pumping becomes a more significant 

dilution process as natural mixing decreases. 

3.4.2 Well Head Pumping 

The effect of borehole dilution was examined 

by separating the overall dilution factor into 

two components: volumetric flux-based 

dilution in the borehole and dispersion 

transport-based components. The method 

used to estimate F-BDF-theflux-based 

dilution factor-in the Amargosa Farms region 

is to compare the capture area of a pumping 

well to the cross-sectional area of the portion 

of the plume that is captured. Borehole 

dilution factors presented in this NUREG are 

calculated using the cross-sectional areas 

normal to the principal direction of regional

flow. The method used to estimate the 

component of borehole dilution caused by 

dispersion during transport is to calculate an 

average concentration for the portion of the 

plume captured by a pumping well. The F

BDF and T-BDF have been kept separate to 

better understand the influence of well 

pumping on concentration estimates. 

Different configurations for the plume and the 

capture area were evaluated. For domestic 

residential wells, the capture area is generally 

much smaller than the cross-sectional area of 

a plume that has undergone horizontal and 

vertical transverse spreading caused by 

macro-dispersion during transport along a 20

to 30-kilometer pathway as shown in Figure 

3-15. Thus, as expected, F-BDF was 

minimal when the domestic well was aligned 

with the center of the plume. Any borehole 

dilution that might occur would be solely 

caused by vertical gradients in the plume 

concentration and would be reflected in the 

T-BDF. For irrigation wells, or any high

discharge wells, the capture area is generally 

thicker than the plume and could also be 

wider than the plume, depending on the 

specifics of the pumping scenario. 

Several conclusions can be drawn from this 

analysis. First, as defined in this study, F

BDFs for individual wells are relatively small, 

ranging from 1 to 5 for an irrigation well 

extracting contaminant from a plume that 

exhibits 3-D dispersion; from 19 to 49 for an 

irrigation well extracting contaminant from a 

thin plume that does not disperse vertically; 

and from 4 to 10 for a domestic well [i.e., 6.8 

cubic meters/day (1800 gallons/day or 2 

acre-feet/year)], extracting contaminant from 

a thin plume that does not disperse vertically. 

However, care should be taken when 

comparing F-BDF for different contaminant 

plume configurations, since actual borehole 

concentrations depend on the mass of 

radionuclides captured and the volume of 

water pumped, not just the area of the plume 

that is captured. On the one hand, a 

high-capacity (agricultural) well may capture 

the entire mass of radionuclides in a large 

plume, have an apparent dilution factor of 

only 1, yet still produce a low borehole 

concentration because the large plume would

3-79 NUREG-1538
3 Ground-Water Pathway Analyses

have a corresponding low mean resident 

concentration. On the other hand, a 

low-capacity domestic well may capture the 

entire mass of radionuclides in a small plume, 

have a dilution factor of 10, yet produce a high borehole concentration because the plume has 

a very high mean resident concentration. 

A second, and perhaps obvious, conclusion can 

be drawn from this study. Specifically, for a 

thin/wide plume of specified dimensions, a 

low-capacity well, screened over a thick 

section of the aquifer, may produce a higher 

dilution factor than a larger-capacity well 

screened over a shorter vertical interval. 

Indeed, extremes in the individual borehole 

concentrations within a receptor group will be 

greater if the contaminant plume is thin and 

borehole construction practices are varied, 

than if the plume is very thick and borehole 

construction practices are uniform. These 

results suggest that attention should be paid to 

understanding vertical spreading in the 

saturated zone along the presumed transport pathway. Indirect field evidence (Gelhar et 

al., 1992; Bedient et al., 1994) suggests minimal vertical spreading in alluvial aquifers; however, vertical spreading may be substantial 

in the fractured tuff aquifer, especially where 

flow crosses normal faults across which there 

is significant offset in the conductive and 

non-conductive strata. 

Further work on borehole dilution would 

benefit from better delineation of the plume 

entering the alluvial aquifer and from large scale modeling of the effects of multiple-well 

systems. This analysis has shown that the

plume configuration is an important 

component. Additionally, modeling multiple well systems would better define the pumping effect on ground-water flow patterns in the 

Amargosa Farms area. 

3.4.3 Peak Dose 

In summary, the NAS recommendation for an 

individual risk standard appears implement

able because: (a) estimating dose for long 

time-periods (i.e., tens to hundreds of 

thousands of years) and a variety of exposure 

pathways (e.g., ingestion of contaminated 

water, crops, and animal products) is 

computationally feasible; (b) preliminary 

indications are that a relatively small number 

of long-lived, soluble, and mobile radionuclides 

will be most important to performance; and 

(c) although assumptions concerning the 

receptor group location and lifestyle are 

important in determining appropriate 

exposure pathways and dilution volumes used 

in the performance calculation, peak doses did 

not vary significantly between the two 

hypothetical receptor groups analyzed. 

3.4.4 References 

Bedient, PB., H.S. Rifai, and C.J. Newell, 

Ground Water Contamination Transport and 

Remediation, Englewood Cliffs, New Jersey, PTR Prentice Hall, 1994. 

Gelhar, L.W, C. Welty, and K.R. Rehfeldt, 'A 

Critical Review of Data on Field-Scale 

Dispersion in Aquifers," Water Resources 

Research, 28(7):1955-1974 [1992].

NUREG- 1538 3-80