A Conceptual Model of Ground-Water Flow in the 

Eastern Snake River Plain Aquifer at the Idaho 

National Laboratory and Vicinity with Implications for 

Contaminant Transport

Scientific Investigations Report 2006–5122

U.S. Department of the Interior

U.S. Geological Survey

Lost

River

Big

4600

4500

4400

4300

4200

4100

4500

Idaho

National

Laboratory

DOE/ID-22198

Prepared in cooperation with the U.S. Department of Energy
Cover: Map showing distribution of hydrogeologic units present at the water table and 

water-table contours for 1980 in the model area, Idaho National Laboratory and vicinity, 

Idaho.
A Conceptual Model of Ground-Water 

Flow in the Eastern Snake River Plain 

Aquifer at the Idaho National Laboratory and 

Vicinity with Implications for Contaminant 

Transport

By Daniel J. Ackerman, Gordon W. Rattray, Joseph P. Rousseau, Linda C. Davis, and 

Brennon R. Orr

Scientific Investigations Report 2006-5122

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the 

U.S. Department of Energy
U.S. Department of the Interior

Dirk A. Kempthorne, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Ackerman, D.J., Rattray, G.W., Rousseau, J.P., Davis, L.C., and Orr, B.R., 2006, A conceptual model of ground-water 

flow in the eastern Snake River Plain aquifer at the Idaho National Laboratory and vicinity with implications for 

contaminant transport: U.S. Geological Survey Scientific Investigations Report 2006-5122, 62 p. 
iii

Contents

Abstract ................................................................................................... 1

Introduction................................................................................................ 2

Purpose and Scope ................................................................................. 5

Description of Model Area ........................................................................ 6

Regional Geology.................................................................................... 6

Previous Investigations ........................................................................... 9

Conceptual Model of Ground-Water Flow ............................................................ 11

Hydrogeologic Framework and Hydraulic Properties .......................................... 11

Hydrogeologic Units........................................................................... 15

Younger Rocks........................................................................... 19

Hydrogeologic Unit 1 (Thin, Densely Fractured Basalt) ..................... 19

Hydrogeologic Unit 2 (Massive, Less Densely Fractured Basalt) ......... 20

Intermediate-Age Rocks (Hydrogeologic Unit 3) .................................... 21

Older Rocks .................................................................................... 21

Rhyolite Domes .............................................................................. 21

Sediment ....................................................................................... 22

Model Boundaries ................................................................................. 23

Physical Boundaries ........................................................................ 25

Artificial Boundaries ........................................................................ 25

Inflows, Outflows, and Fluxes Across Model Boundaries .................................... 29

Water-Table Boundary........................................................................ 29

Precipitation Recharge ............................................................... 31

Streamflow-Infiltration Recharge ................................................... 31

Big Lost River ..................................................................... 31

Little Lost River .................................................................. 33

Birch Creek ........................................................................ 33

Ground-Water Withdrawals, Wastewater Return Flows, and

Irrigation-Infiltration Recharge ...................................................... 33

Release of Water from Storage ...................................................... 34

Base of the Aquifer ........................................................................... 34

Northwest Mountain-Front Boundary ...................................................... 34

Northeast Boundary........................................................................... 35

Southeast-Flowline Boundary ............................................................... 35

Southwest Boundary ........................................................................ 35

Ground-Water Budget.............................................................................. 36

Ground-Water Budget for the Conceptual Model Area ................................. 36

Comparison and Analysis of the Ground-Water Budget ................................. 36

Ground-Water Movement ........................................................................ 38

Horizontal Hydraulic Gradient ............................................................... 39

Flow Directions in the Central Part of the Idaho National Laboratory .................. 39

Effect of Episodic Recharge from the Big Lost River on Variation of Local Flow

Directions................................................................................. 43

Average Linear Ground-Water Velocities ................................................ 46
iv

Vertical Ground-Water Movement ......................................................... 47

Model Representation of Features Affecting Ground-Water Flow ........................... 47

Structural and Stratigraphic Controls on Ground-Water Flow........................... 47

Hydrologic Controls on Ground-Water Flow ............................................. 48

Representation of Ground-Water Flow Through the Model Area ..................... 48

Implications for Contaminant Transport ............................................................... 50

Summary and Conclusions .............................................................................. 51

Acknowledgments ....................................................................................... 54

References Cited.......................................................................................... 54

Appendix A. A Brief History of Wastewater-Disposal Practices at the Idaho National

Laboratory ............................................................................................. 61

Appendix B. Levels of Information Needed for Constructing Conceptual Models at

Different Scales....................................................................................... 62

Figures

Figure 1. Map showing location of the Idaho National Laboratory, the model area,

and selected facilities, wells, and streamflow-gaging stations........................ 3

Figure 2. Map showing distribution of selected waste contaminants in water from the

eastern Snake River Plain aquifer, Idaho National Laboratory and vicinity,

Idaho .......................................................................................... 4

Figure 3. Map showing direction of regional ground-water flow, discharge areas, and

irrigated acreage, eastern Snake River Plain, Idaho.................................... 7

Figure 4. Cross section and chart showing generalized geology, water-table altitude,

and geologic groups and formations, and generalized stratigraphy of the

eastern Snake River Plain, Idaho ......................................................... 8

Figure 5. Maps showing areas and grid sizes of ground-water models of the eastern

Snake River Plain aquifer at the Idaho National Laboratory (INL) and vicinity,

Idaho, relative to those of the Snake River Plain Regional Aquifer System

Analysis (RASA) model ..................................................................... 10

Figure 6. Cross sections showing composite stratigraphic units that form the eastern

Snake River Plain aquifer, Idaho National Laboratory and vicinity, Idaho ............ 12

Figure 7. Map showing approximate locations of volcanic rift zones and the axial

volcanic high, Idaho National Laboratory and vicinity, Idaho ........................ 13

Figure 8. Map showing locations of volcanic vents and vent corridors, Idaho National

Laboratory and vicinity, Idaho ............................................................ 14

Figure 9. Map showing distribution of hydrogeologic units present at the water table

and water-table contours for 1980 in the model area, Idaho National

Laboratory and vicinity, Idaho ............................................................ 16

Figure 10. Cross section showing distribution of hydrogeologic units along the

direction of regional ground-water flow in the model area, Idaho National

Laboratory and vicinity, Idaho ............................................................ 17

Figure 11. Chart showing correlation among stratigraphic units of the eastern Snake

River Plain, composite stratigraphic units beneath the Idaho National

Laboratory, and hydrogeologic units of the conceptual model, Idaho

National Laboratory and vicinity, Idaho................................................... 18

Figure 12. Maps showing water-table contours, 1995, and areas of abundant sediment

within the model area, Idaho National Laboratory and vicinity, Idaho ............... 23
Figure 13. Map and cross section showing location of the model boundaries, Idaho

National Laboratory and vicinity, Idaho................................................... 24

Figure 14. Map showing depth to the base of the eastern Snake River Plain aquifer in

the model area, Idaho National Laboratory and vicinity, Idaho........................ 26

Figure 15. Relief map showing contact between the intermediate-age rocks of

hydrogeologic unit 3 and the older rocks underlying the base of the eastern

Snake River Plain aquifer beneath the model area, Idaho National

Laboratory and vicinity, Idaho ............................................................ 27

Figure 16. Map showing Regional Aquifer System Analysis model grid used to

approximate the northeast, southeast-flowline, and southwest boundaries

of the conceptual model, Idaho National Laboratory and vicinity, Idaho ............ 28

Figure 17. Map showing generalized ground-water budget components for the model

area, 1980, Idaho National Laboratory and vicinity, Idaho.............................. 30

Figure 18. Hydrographs showing mean annual streamflow at selected

streamflow-gaging stations on the Big Lost River, Idaho, for 1965-2002 ............ 32

Figure 19. Map showing distribution of lithium concentrations in water from wells in

the eastern Snake River Plain aquifer, Idaho National Laboratory and

vicinity, Idaho .............................................................................. 41

Figure 20. Map showing average linear ground-water velocities and flow directions

calculated from the assumed first arrivals of chlorine-36 or the

tritium/helium-3- and chlorofluorocarbon-model ages of water from

selected wells, Idaho National Laboratory and vicinity, Idaho ........................ 42

Figure 21. Map showing temperature of ground water in samples from wells, 

Idaho National Laboratory and vicinity, Idaho .......................................... 44

Figure 22. Maps showing generalized water-level changes at the Idaho National

Laboratory and vicinity, Idaho, for July 1981–July 1985, April–August 1988 to

April–August 1991, March–May 1991 to March–May 1995, and March–May

1995 to March–May 1998 .................................................................. 45

Figure 23. Hydrographs showing water levels in wells Arbor Test, USGS 9, USGS 12,

and USGS 25 for 1948–2003, Idaho National Laboratory, Idaho........................ 46

Figure 24. Cross section showing distribution of hydrogeologic units and the direction

of ground-water flow for the model area, Idaho National Laboratory and

vicinity, Idaho .............................................................................. 49

Tables

Table 1. Summary of surface and subsurface wastewater disposal at selected

facilities, Idaho National Laboratory, Idaho. ............................................. 5

Table 2. Ranges of hydraulic conductivities and porosities for hydrogeologic units

and other rocks of hydrogeologic importance, Idaho National Laboratory

and vicinity, Idaho............................................................................ 19

Table 3. Summary statistics of basalt flow thickness within individual hydrogeologic

units, Idaho National Laboratory and vicinity, Idaho. ................................. 20

Table 4. Summary of conceptual model boundary characteristics and estimated 1980

inflows and outflows, Idaho National Laboratory and vicinity, Idaho. ............... 29

Table 5. Summary of streamflow at two streamflow-gaging stations on the Big Lost

River, Idaho ................................................................................. 33

Table 6. Ground-water budget for the conceptual model area for 1980 and derived

from the Regional Aquifer System Analysis (RASA) model of the Snake River

Plain aquifer, Idaho ........................................................................ 37
vi

Conversion Factors and Datums

Conversion Factors

Multiply By To obtain

acre 4,047 square meter (m2)

acre-foot (acre-ft) 1,233 cubic meter (m3)

cubic foot (ft3) 0.02832 cubic meter (m3) 

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

cubic foot per second per square 

 mile [(ft3/s)/mi2] 

0.01094 cubic meter per second per square

 kilometer [(m3/s)/km2]

curie (Ci) 37,000 megabecquerel (MBq)

foot (ft) 0.3048 meter (m)

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

foot per day (ft/d) .3048 meter per day (m/d)

foot per year (ft/yr) .3048 meter per year (m/yr)

foot squared per day (ft2/d) 0.09290 meter squared per day (m2

/d) 

gallon (gal) 3.785 liter (L)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

million gallons (Mgal) 3,785 cubic meter (m3)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

mile (mi) 1.609 kilometer (km)

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32.

Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

feet of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Datums

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

Horizontal coordinate information is referenced to North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum.
Abstract

Ground-water flow in the west-central part of the 

eastern Snake River Plain aquifer is described in a conceptual 

model that will be used in numerical simulations to evaluate 

contaminant transport at the Idaho National Laboratory (INL) 

and vicinity. The model encompasses an area of 1,940 square 

miles (mi2) and includes most of the 890 mi2 of the INL. A 

50-year history of waste disposal associated with research 

activities at the INL has resulted in measurable concentrations 

of waste contaminants in the aquifer. A thorough 

understanding of the fate and movement of these contaminants 

in the subsurface is needed by the U.S. Department of Energy 

to minimize the effect that contaminated ground water may 

have on the region and to plan effectively for remediation.

Three hydrogeologic units were used to represent 

the complex stratigraphy of the aquifer in the model area. 

Collectively, these hydrogeologic units include at least 

65 basalt-flow groups, 5 andesite-flow groups, and 61 

sedimentary interbeds. Three rhyolite domes in the model 

area extend deep enough to penetrate the aquifer. The rhyolite 

domes are represented in the conceptual model as low 

permeability, vertical pluglike masses, and are not included 

as part of the three primary hydrogeologic units. Broad 

differences in lithology and large variations in hydraulic 

properties allowed the heterogeneous, anisotropic basalt-flow 

groups, andesite-flow groups, and sedimentary interbeds to be 

grouped into three hydrogeologic units that are conceptually 

homogeneous and anisotropic. Younger rocks, primarily 

thin, densely fractured basalt, compose hydrogeologic unit 1; 

younger rocks, primarily of massive, less densely fractured 

basalt, compose hydrogeologic unit 2; and intermediate-age 

rocks, primarily of slightly-to-moderately altered, fractured 

basalt, compose hydrogeologic unit 3. Differences in hydraulic 

properties among adjacent hydrogeologic units result in much 

of the large-scale heterogeneity and anisotropy of the aquifer 

in the model area, and differences in horizontal and vertical 

hydraulic conductivity in individual hydrogeologic units result 

in much of the small-scale heterogeneity and anisotropy of the 

aquifer in the model area.

The inferred three-dimensional geometry of the aquifer 

in the model area is very irregular. Its thickness generally 

increases from north to south and from west to east and is 

greatest south of the INL. The interpreted distribution of 

older rocks that underlie the aquifer indicates large changes in 

saturated thickness across the model area.

The boundaries of the model include physical and 

artificial boundaries, and ground-water flows across the 

boundaries may be temporally constant or variable and 

spatially uniform or nonuniform. Physical boundaries 

include the water-table boundary, base of the aquifer, and 

northwest mountain-front boundary. Artificial boundaries 

include the northeast boundary, southeast-flowline boundary, 

and southwest boundary. Water flows into the model area 

as (1) underflow (1,225 cubic feet per second (ft3/s)) from 

the regional aquifer (northeast boundary—constant and 

nonuniform), (2) underflow (695 ft3/s) from the tributary 

valleys and mountain fronts (northwest boundary—constant 

and nonuniform), (3) precipitation recharge (70 ft3/s) (constant 

and uniform), streamflow-infiltration recharge (95 ft3/s) 

(variable and nonuniform), wastewater return flows (6 

ft3/s) (variable and nonuniform), and irrigation-infiltration 

recharge (24 ft3/s) (variable and nonuniform) across the water 

table (water-table boundary—variable and nonuniform), 

and (4) upward flow across the base of the aquifer (44 ft3/s) 

(uniform and constant). The southeast-flowline boundary is 

represented as a no-flow boundary. Water flows out of the 

model area as underflow (2,037 ft3/s) to the regional aquifer 

(southwest boundary—variable and nonuniform) and as 

ground-water withdrawals (45 ft3/s) (water table boundary—

variable and nonuniform).

Ground-water flow increases progressively in a direction 

downgradient of the northeast boundary. This increased flow 

is the result of tributary-valley and mountain-front underflows 

along the northwest boundary and precipitation recharge 

and streamflow-infiltration recharge across the water-table 

boundary. Ground water flows in all three hydrogeologic 

units beneath the INL. South of the INL, the younger rocks, 

hydrogeologic units 1 and 2, are either not present or are above 

the water table and all flow occurs through the intermediateage rocks, hydrogeologic unit 3.

A Conceptual Model of Ground-Water Flow in the 

Eastern Snake River Plain Aquifer at the Idaho 

National Laboratory and Vicinity with Implications for 

Contaminant Transport

By Daniel J. Ackerman, Gordon W. Rattray, Joseph P. Rousseau, Linda C. Davis, and Brennon R. Orr
The direction of regional ground-water flow is from 

northeast to southwest. Flow directions beneath the INL vary 

locally from southeast to southwest and fluctuate in response 

to episodic recharge from streamflow infiltration. Water-table 

gradients immediately upgradient of the northeast boundary 

are 27 to 60 feet per mile (ft/mi); and southwest of the INL 

gradients are 4 to 30 ft/mi. Beneath the INL gradients are 

much flatter, 1 to 8 ft/mi, and precise definition of flow 

direction is difficult to determine.

Long-term monitoring of contaminant movement in the 

aquifer at the INL indicates that ground-water velocities in the 

thin, fractured basalts of hydrogeologic unit 1, the uppermost 

hydrogeologic unit of the aquifer, range from 4 to 20 feet 

per day (ft/d) south of the Test Reactor Area and the Idaho 

Nuclear Technology and Engineering Center. These velocities 

probably indicate preferential flow along the many interflow 

zones of the thin, fractured basalt flows composing the 

uppermost hydrogeologic unit. Hydraulic conductivities (500 

to 5,000 ft/d) estimated from velocity measurements were 

consistent with those derived from aquifer tests conducted in 

this hydrogeologic unit. Almost two-thirds of the hydraulic 

conductivities derived from aquifer-test measurements in 

hydrogeologic unit 1 were larger than 100 ft/d and about 

one-third were larger than 1,000 ft/d.

Most contaminant movement beneath the INL 

probably takes place in the thin, densely fractured, and 

highly conductive basalts and interbedded sediments of 

hydrogeologic unit 1, which compose most of the upper 200 ft 

of the aquifer beneath most of the INL. This hypothesis is 

based on interpretation of a generalized northeast-to-southwest 

cross section of ground-water flow across the model area 

that depicts the effects of the hydrogeologic framework on 

flow in each of the hydrogeologic units used to represent the 

aquifer. This interpretation indicates that head decreases and 

then increases with depth with thickening and thinning of the 

aquifer in a direction downgradient of the northeast boundary. 

Beneath the INL, the smaller conductivity of the massive, 

less densely fractured basalts and interbedded sediments of 

hydrogeologic unit 2 restricts the downward movement of 

contaminants from hydrogeologic unit 1. The largest changes 

in water-table gradients are upgradient of where the massive 

basalts of hydrogeologic unit 2 are inferred to intersect the 

water table south of the INL. Water probably flows downward 

through hydrogeologic unit 2 into hydrogeologic unit 3 at this 

location, implying deeper circulation of contaminants that 

migrate offsite.

Features of the conceptual model that most affect 

interpretations of contaminant transport are (1) implicit 

representation of infiltration recharge through the unsaturated 

zone, (2) preferential flow along highly conductive interflow 

zones, primarily in the thin, densely fractured basalts of 

hydrogeologic unit 1, implying large horizontal to vertical 

anisotropy, (3) restricted downward movement of flow and 

contaminants in hydrogeologic unit 1 into the less conductive 

basalts of hydrogeologic unit 2 beneath the INL, (4) the 

inferred downward movement and deeper circulation of water 

upgradient of where the massive, less densely fractured basalt 

of hydrogeologic unit 2 intersects the water table southwest 

of the INL, and (5) enhanced dispersion of contaminants 

resulting from the spatial and temporal variability of 

streamflow-infiltration recharge that is in close proximity to 

contaminated ground water.

Introduction

The Idaho National Laboratory (INL), established in 

1949, occupies about 890 mi2 of the eastern Snake River Plain 

(SRP) in southeastern Idaho (fig. 1). The INL was established 

to build, operate, and test nuclear reactors. The scope of work 

at the INL increased during the 1950s to include other nuclearresearch programs, the reprocessing of spent nuclear fuel, and 

the storage and disposal of radioactive and chemical waste.

The INL overlies the west-central part of the eastern 

SRP aquifer, which is a major source of water for agricultural, 

industrial, and domestic uses in southeastern Idaho. 

Radioactive and chemical wastes were discharged to the 

unsaturated zone and aquifer at the INL for over 50 years and 

several waste contaminants are present in the aquifer (fig. 2) 

at concentrations that exceed U.S. Environmental Protection 

Agency maximum contaminant levels (MCLs) for drinking 

water (table 1). The principal sources of contaminants are 

wastewater-disposal sites at Test Area North (TAN), the Test 

Reactor Area (TRA), and the Idaho Nuclear Technology 

and Engineering Center (INTEC). Wastewater-disposal sites 

have included lined evaporation ponds, unlined infiltration 

ponds and ditches, drain fields, and injection wells. Wastes 

buried in shallow pits and trenches at the Radioactive 

Waste Management Complex (RWMC) also are sources of 

contaminants in ground water. Appendix A (at back of report) 

contains a brief history of wastewater-disposal practices at the 

INL.

The presence of contaminants in the eastern SRP aquifer 

has led to public concern about the quality of water in the 

aquifer and the effect that contaminated ground water may 

have on the region. To respond to this concern a thorough 

understanding of the fate and movement of contaminants in 

the subsurface is needed. The U.S. Geological Survey (USGS) 

in cooperation with the U.S. Department of Energy (DOE) is 

developing a three-dimensional numerical model of groundwater flow and contaminant transport in the aquifer at the INL 

and vicinity. The model will assist the DOE to minimize the 

effect that subsurface contaminants may have on the region 

and to plan effectively for remediation. Ground-water flow 

and contaminant transport in the aquifer beneath the INL 

were modeled previously by Robertson (1974) and Goode and 

Konikow (1990b). The new calibrated, numerical flow-andtransport model will build on the work of these earlier models 

and will be based on a refined conceptual model of flow 

beneath the INL presented in this report. New information 

available for developing the current flow-and-transport model 

 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Figure 1. Location of the Idaho National Laboratory, the model area, and selected facilities, wells, and streamflow-gaging 

stations.

ID19-0016_fig01

USGS 12

USGS

30

USGS 11

43° 30 ́

44° 00 ́

44° 30 ́

114° 00 ́ 30 ́ 113° 00 ́ 30 ́ 112° 00 ́

Hwy 1

USGS 9

USGS 25

Arbor

 Test

Craters

of the

Moon

National

Monument

WO2

CH1

INEL 1

USGS 40,41

Site 17

USGS 4 USGS 28

USGS

31

Mud Lake

Camas

Creek

Lost

River

Big Lost River

Sinks and Playas

Big Lost River

Spreading

Areas

Little

Lost

River

Birch

Creek

Mackay

Reservoir

Big

Little Lost

River Sinks

13132520

13132500

13127000

13132513

13119000

TAN

TRA INTEC (CPP 3)

NRF

RWMC

CFA

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

IDAHO

BOISE

EASTERN

SNAKE RIVER

IDAHO PLAIN

NATIONAL

LABORATORY

Twin

Falls

Idaho

Falls

Pocatello

Model

Area

0

0 10

10

20 KILOMETERS

20 MILES

13132513

CFA

INTEC

NRF

RWMC

TRA

TAN

USGS gaging station and number

Monitoring well and No.

Selected facilities at the Idaho

 National Laboratory

 Central Facilities Area

 Idaho Nuclear Technology and 

 Engineering Center

 Naval Reactors Facility

EXPLANATION

USGS 4 Radioactive Waste Management Complex

 Test Area North

 Test Reactor Area

WHITE KNOB

MOUNTAINS

LEMHI

RANGE

LOST

RIVER

RANGE

CONCEPTUAL

MODEL AREA

BITTERROOT

 RANGE

PIONEER

MOUNTAINS

Idaho

National

Laboratory

Atomic

City

Howe

Arco

Mud

Lake

Terreton

Introduction 
Figure 2. Distribution of selected waste contaminants in water from the eastern Snake River Plain aquifer, Idaho National 

Laboratory and vicinity, Idaho.

includes (1) an increased understanding of the hydrogeology 

beneath the INL, (2) an increased understanding of the 

amount and timing of transient recharge to the aquifer beneath 

the INL (including a near record amount of streamflow 

infiltration from the Big Lost River in 1983 and 1984 and a 

period with little or no infiltration between 1987 and 1994), 

(3) additional information about the amount and timing of 

wastewater disposal, (4) more than 30 years of additional 

water-level data for numerical-model calibration, and (5) more 

than 30 years of additional measurements of contaminant 

concentrations in the aquifer. This additional information will 

allow a new flow-and-transport model to be developed using 

more accurate estimates of hydrogeologic properties, such as 

hydraulic conductivity, storativity, and effective porosity and 

to be calibrated over a longer simulated period than that of the 

previous flow-and-transport models.

ID19-0016_fig02

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Chemical data from Beasley, 1995; Bukowski and 

others, 1998; and Bartholomay and others, 2000. 

EASTERN

SNAKE RIVER 

PLAIN

Trichloroethene

0

0 10

10

20 KILOMETERS

20 MILES

WHITE KNOB

MOUNTAINS

LEMHI

RANGE

LOST

RIVER

RANGE

CONCEPTUAL

MODEL AREA

BITTERROOT

 RANGE

PIONEER

MOUNTAINS

Idaho

National

Laboratory

Tritium

Strontium-90 Direction of regional ground-water flow

43° 30 ́

44° 00 ́

44° 30 ́

114° 00 ́ 30 ́ 113° 00 ́ 30 ́ 112° 00 ́

Mud Lake

Camas

Creek

Lost

River

Big Lost River

Sinks and Playas

Big Lost River

Spreading

Areas

Little

Lost

Birch

Creek

Mackay

Reservoir

Big

Little Lost

River Sinks

River

Atomic

City

Howe

Arco

Mud

Lake

Terreton

IDAHO

BOISE

EASTERN

SNAKE RIVER

IDAHO PLAIN

NATIONAL

LABORATORY

Twin

Falls

Idaho

Falls

Pocatello

Model

Area

Model area

Plume of trichloroethene—Concentration

greater than 5 micrograms per liter in 1997

Plume of strontium-90—Concentration

greater than 2 picocuries per liter in 1998

Plume of tritium—Concentration greater 

than 500 picocuries per liter in 1998

Regional ground-water flowline

Well—Water contains chlorine-36 in 1992 

from waste disposal 

Well—Water contains background 

concentration of chlorine-36

EXPLANATION

 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Purpose and Scope

This report describes a conceptual model of ground-water 

flow in the west-central part of the eastern SRP aquifer that 

will be used in numerical simulations to evaluate contaminant 

transport at the INL and vicinity. This part of the aquifer 

contains contaminants that were discharged to the subsurface 

at facilities located at the INL. The conceptual model was 

developed to provide a qualitative description of ground-water 

flow at the INL and vicinity so that hydrogeologic features 

most strongly affecting contaminant movement could be 

identified. The model integrates the current understanding of 

the stratigraphy, hydraulic properties of the rocks, hydrology, 

and ground-water movement in the model area.

The model area (1,940 mi2) is subregional in scale, and 

is intermediate in scale between the regional-scale model of 

the eastern SRP aquifer (10,800 mi2), described in reports of 

the Regional Aquifer System Analysis (RASA) study of the 

aquifer, and local-scale models of the aquifer at INL facilities 

(less than about 10 mi2) (Schafer-Perini, 1993; Magnuson and 

Sondrup, 1998). The conceptual models for the three scales 

include the same basic information but at different levels of 

detail (Appendix B, at back of report). For example, at many 

INL facilities data are sufficient to represent the heterogeneity 

and anisotropy of the aquifer at the scale of individual basalt 

flows or basalt-flow groups. In the current subregional-scale 

model, however, data are not sufficient to resolve individual 

basalt flows and basalt-flow groups and these were grouped 

with other basalt flows having similar hydraulic characteristics 

into homogenous and anisotropic hydrogeologic units.

Data used for developing the conceptual model were 

obtained from numerous reports published during the past 

50 years, including ground-water modeling studies, reports 

generated for the eastern SRP regional aquifer system as part 

of the USGS RASA study, and several recent studies. These 

data, which include petrologic, stratigraphic, geophysical, 

geochemical, hydraulic-property, and hydrologic data were 

acquired primarily from (1) surficial mapping, (2) analyses of 

numerous outcrops and hundreds of core samples, (3) aquifer 

tests, (4) borehole geophysical logs and surface geophysical 

surveys, (5) more than 50 years of water-level measurements 

and water-chemistry analyses from more than three hundred 

wells, and (6) long-term records of streamflow (100 years) and 

wastewater discharge to the aquifer (34 years).

Although a large quantity of data were available 

for developing the conceptual model, there were several 

limitations with these data. These data limitations included 

(1) uneven spatial distribution, both areally and vertically [for 

example, of the 333 boreholes used in this study to define 

the hydrogeologic framework, 300 are within the boundaries 

of the INL, mostly in and near INL facilities (Anderson and 

others, 1997, figs. 2 to 5, table 1)], (2) scaling-compatibility 

issues involving the application of small-scale measurements 

to a large-scale study (for example, the use of core-scale 

measurements to define large-scale hydraulic properties), 

(3) uncertainties arising from partial borehole penetrations and 

different borehole completions that complicate interpretations 

of water-level, water-chemistry, and hydraulic-conductivity 

measurements (for example, of the 114 monitoring wells 

that were used to estimate hydraulic conductivity only 13 

of these penetrate more than 300 ft into the aquifer), and 

(4) discontinuous or nonexistent hydrologic records (for 

example, streamflow records for Little Lost River and Birch 

Creek streamflow onto the INL).

Table 1. Summary of surface and subsurface wastewater disposal at selected facilities, Idaho National Laboratory, Idaho.

[Estimated disposal quantities: Estimated disposal quantities for Test Area North, Test Reactor Area, and Idaho Nuclear Technology and Engineering Center from French and others (1999). Estimated disposal quantities for the Radioactive Waste Management Complex from Mann and Knobel 

(1987) and Becker and others (1998). Contaminant concentrations from Mann and Beasley (1994), Bukowski and others (1998), and Bartholomay 

and others (2000). Abbreviations: MCLs, U.S. Environmental Protection Agency maximum contaminant levels for drinking water; Ci, curies; 137Cs, 

cesium-137; 3H, tritium; 90Sr, strontium-90; TCE, trichloroethene; PCE, tetrachloroethene; DCE, cis- and trans-1,2-dichloroethene; Cr, chromium, 129I, 

iodine-129; CCl4, carbon tetrachloride; gal, gallon]

Facility Disposal

site Years of disposal Estimated disposal quantities

Contaminants with concentrations 

exceeding MCLs in ground water

Test Area North Well, pond 1953 to 1993 61 Ci

717 million gal

137Cs, 3

H, 90Sr, TCE, PCE, DCE

Test Reactor Area Well, ponds 1952 to present 53,879 Ci

5,180 million gal

3

H, Cr

Idaho Nuclear Technology and 

Engineering Center

Well, ponds 1952 to present 22,254 Ci

19,165 million gal

3

H, 90Sr, 129I

Radioactive Waste Management 

Complex

Excavation 

pits, 

trenches

1952 to present 1,532,600 Ci

0.09 million gal

CCl4

Introduction 
Description of Model Area

The model area extends 75 mi from northeast to 

southwest and 35 mi from northwest to southeast and includes 

most of the INL (fig. 2). The model area is bounded on the 

northwest by mountain fronts and valleys tributary to the 

plain and on the southeast by an inferred regional groundwater flowline. The northeast boundary is defined by a steep 

increase in hydraulic gradient. The southwest boundary is 

25 mi downgradient of the southwestern extent of measured 

concentrations of INL-derived contaminants in the aquifer 

(Beasley, 1995, appendix 2). The vertical dimension of the 

model area is represented by the saturated thickness of the 

aquifer, which is estimated to range from less than 600 ft to 

more than 3,000 ft.

The model area is contained in the eastern SRP, which 

is a relatively flat topographic depression, about 200-mi long 

and 50- to 70-mi wide, surrounded by mountains on three 

sides. The altitude of the plain ranges from about 2,900 ft 

near King Hill to about 6,200 ft near the southwestern extent 

of the Yellowstone Plateau (fig. 3), and the surrounding 

mountains reach altitudes of 12,000 ft. The surface of the plain 

is primarily loess and olivine basalt and contains volcanic 

landforms, such as cinder and lava cones, shield volcanoes, 

rhyolite domes that rise as much as 2,500 ft above the plain, 

sand dunes, and a canyon carved by the Snake River that 

ranges from 50- to 550-ft deep (Lindholm, 1996, p. 5).

Climate in the model area is semiarid and mean annual 

precipitation at the INL is about 0.7 ft (Clawson and others, 

1989, tables D-1 and D-2; Goodell, 1988, fig. 5). About 

0.1 to 0.3 ft of snow usually accumulates at the INL during 

winter and snowmelt and runoff from the adjacent mountains, 

typically in late spring and early summer, produce peak 

streamflow. Mean annual air temperature at the Central 

Facilities Area (CFA) at the INL between 1950 and 1988 was 

30oF and the coldest and warmest mean monthly temperatures 

were measured in January and during July-August, 

respectively (Clawson and others, 1989, table B-3).

Ground water in the eastern SRP aquifer flows to the 

southwest and discharges principally through large springs 

and seeps along the Snake River in the Thousand Springs 

area in the southwestern part of the plain (fig. 3). Recharge to 

the eastern SRP aquifer is from infiltration of precipitation, 

underflow from tributary drainages, infiltration of surface 

water diverted for irrigation, and stream and canal losses 

(Garabedian, 1992, p. 11). Land irrigated with ground water 

on the eastern SRP is located along the southeastern and 

southern margins of the plain, from north of Idaho Falls to 

west of Twin Falls, and in the Mud Lake area northeast of 

the INL (fig. 3). In 1980, about 1,760,000 acre-ft of ground 

water was withdrawn from about 4,000 wells across the plain 

to irrigate about 930,000 acres (Garabedian, 1992, p. 19-21). 

Intermittent streamflow onto the plain near the model area is 

from Camas Creek, Birch Creek, Little Lost River, and Big 

Lost River (fig. 1).

Regional Geology

The eastern SRP was formed by the migration of the 

North American Continental Plate westward across a mantleplume hot spot (Pierce and Morgan, 1992, p. 2). The plain 

is underlain by a 1,000- to 2,000-ft thick-layered sequence 

of Quaternary basalt flows and sediment interbeds overlying 

a several-thousand-feet thick sequence of Quaternary and 

late Tertiary volcanic rocks (fig. 4) (Whitehead, 1992, pl. 3). 

Major Quaternary and late Tertiary stratigraphic units in the 

eastern SRP are the: (1) Snake River Group (Qb), which is 

the most extensive rock unit in the eastern SRP and contains 

mainly olivine basalt and sedimentary rocks; (2) Yellowstone 

Group and Plateau Rhyolite (Qsv), containing silicic volcanic 

rocks; (3) upper part of the Idaho Group (includes the 

Bruneau and Glenns Ferry Formations), containing mainly 

olivine basalt (QTb) and sedimentary rocks (QTs); (4) 

Walcott Tuff, Starlight Formation, and Salt Lake Formation 

(QTs), containing mainly sedimentary rocks, basalt, and tuff; 

(5) lower part of the Idaho Group (Tb, includes the Banbury 

Basalt), containing mainly flood-type basalts, silicic rocks, and 

sediment; and (6) Idavada Volcanics (Tsv), containing silicic 

volcanic rocks and sediment. Sedimentary units include flood 

plain and glacial deposits (Qa), windblown deposits (Qw), 

and lake deposits and tuffaceous sedimentary rocks (QTs) 

(Whitehead, 1992, pl. 1).

Several volcanic landforms define the structure and 

topography of the eastern Snake River Plain. An axial volcanic 

high and volcanic rift zones trend parallel and perpendicular to 

the long axis of the plain, respectively. The rift zones appear 

to be extensions of the adjacent Basin and Range structures 

(Kuntz and others, 1992, 1994; Anderson and Liszewski, 

1997). Volcanic vents and fissures are concentrated along the 

axial volcanic high and volcanic rift zones (Anderson and 

others, 1999, p. 13, fig. 7; Hughes and others, 1999, p. 145), 

and are major sources of basaltic rocks on the plain. Rhyolite 

domes also are concentrated within the axial volcanic high. 

Topographic depressions in areas defined by elevated volcanic 

topography, such as the axial volcanic high and volcanic rift 

zones, are areas where abundant sediment accumulated during 

recent times (Bestland and others, 2002, fig. 1; Geslin and 

others, 2002, p. 12-13, fig. 1).

 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Figure 3. Direction of regional ground-water flow, discharge areas, and irrigated acreage, eastern Snake River Plain, Idaho.

ID19-0016_fig03

3600

EXPLANATION

Boundary of Idaho National Laboratory

Irrigated acreage, 1980

 (Goodell, 1988)

Water-table contours, 1980

 (Lindholm and others, 1988)

Regional ground-water flowline, 

 1980 (Ackerman, 1995)

Model area

0

0

10 20 30 40

20 40 60

MILES

KILOMETERS

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Mud Lake

4100

4100

4000

4000

3800

4200

4300

4400

4600

4800

5000

5400

5800

6200

4700

3600

3400

4500

4200

IDAHO

Pocatello

Idaho

Falls

Eastern Snake

River Plain Idaho

National

Laboratory

Boise

Twin

Falls

Model

area King

Hill

Snake River

43°

112°

44°

113°

114°

115°

Thousand

Springs

Area

Yellowstone

Plateau

Introduction 
Figure 4. Generalized geology, water-table altitude, and geologic groups and formations, and generalized stratigraphy 

of the eastern Snake River Plain, Idaho.

ID19-0016_fig04

QTb

Tb (?)

Tb Tsv

Qb

Qsv

Qb

Water table

Qb

Qw Qb

QTs

QTs

Qsv

Qa

A A’

?

Tsv

7,000

ALTITUDE, ABOVE NATIONAL GEODETIC VERTICAL DATUM OF 1929

6,000

5,000

4,000

3,000

2,000

1,000

0

1,000

FEET

QTb

Tb (?) Tsv

Tb Tsv

Qb

Qsv

Qb

Qb

Qw Qb

QTs

QTs

Qsv

Qa

A A’

Modified from Whitehead, 1992, pl. 3.

Water table

VERTICAL SCALE GREATLY EXAGGERATED

0

0 10 20 30 40 50

10 20 30 40 50

KILOMETERS

MILES

Miocene Pliocene Pleistocene Holocene

TERTIARY QUATERNARY

Alluvium Qa

Qw Windblown deposits

Snake River Group Qb

Qa

Upper part of Idaho Group

 (Bruneau Formation) QTs QTb

Qsv

Yellowstone

Group

and

 Plateau Rhyolite

Upper part of Idaho Group,

undifferentiated (sedimentary

rocks, QTs and basalt QTb.

Includes Glenns Ferry

Formation)

QTs

QTb

QTb

Lower part of Idaho Group (Banbury Basalt) Tb QTs

 Idavada

Volcanics Tsv

Walcott Tuff,

Starlight Formation,

and Salt Lake Formation

Age, in

millions

of years

before

present

0.01

0.8

1.7 to 2.2

4.9 to 5.3

Modified from Whitehead, 1992, pl. 1.

IDAHO

Pocatello

Idaho

Falls

Eastern Snake

River Plain

Idaho

National

Boise Laboratory

Twin

Falls

A’

A

Chiefly flood-plain deposits. May contain some 

glacial deposits and colluvium in the uplands. 

Clay, silt, sand, gravel, and boulders.

Chiefly windblown deposits. Includes some lake 

and glacial-flood deposits.

Olivine basalt, dense to vesicular; irregular to 

columnar jointing; thickness of individual 

flows averages about 20-25 feet (Mundorff 

and others, 1964, p.143). Includes beds of 

basalt cinders, rubbly basalt, and interflow 

sedimentary rocks.

Rhyolitic ash-flow tuff, occurs as thick flows 

and blankets of welded tuff with associated 

fine- to coarse-grained ash and pumice beds.

Olivine basalt similar to Qb above.

Subaerial and lake deposits of clay, silt, sand, 

and gravel. Compacted to poorly 

consolidated.

Flood-type basalt, dense; columnar jointing in 

many places. May include some rhyolitic and 

andesitic rocks. Queried where uncertain.

Rhyolitic, latitic, and andesitic rocks, massive 

and dense.

EXPLANATION AND DESCRIPTION OF MAP UNITS

Qa

Qw

Qb

QTb

Qsv

QTs

Tb

Tsv

 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Previous Investigations

The most complete and systematic study of the eastern 

SRP aquifer was conducted as part of the Regional Aquifer 

System Analysis (RASA) program. In this study, Whitehead 

(1992) provided an extensive description of the regional 

hydrogeologic framework, hydrologic properties, and 

geologic controls on ground-water movement. Kjelstrom 

(1995) described streamflow gains and losses and compiled 

regional ground-water budgets. Garabedian (1992) synthesized 

hydrologic data and constructed a regional ground-water flow 

model (fig. 5). These studies provide the regional context for 

describing flow in the model area.

Information from several previous studies was used to 

construct the conceptual model. Anderson and Liszewski 

(1997) combined information from geophysical logs, rock 

cores, and outcrops to describe the stratigraphy of the 

unsaturated zone and aquifer beneath the INL. This study 

provided the basis for determining the hydrogeologic units 

of the eastern SRP aquifer at the INL. Ackerman (1991) 

and Anderson and others (1999) described the distribution 

of hydraulic properties in the eastern SRP aquifer and the 

geologic features that control that distribution. Bennett (1990) 

estimated the amount, distribution, and timing of recharge 

to the eastern SRP aquifer at the INL from the infiltration of 

streamflow from the Big Lost River.

Previous ground-water models also provided information 

used to construct the conceptual model. Spinazola (1994) 

simulated ground-water flow in the Mud Lake area upgradient 

of the INL (fig. 5). Ackerman (1995) developed a regional 

ground-water flow and advective-transport model based on 

studies by Garabedian (1992). Pathline simulations from 

Ackerman’s (1995) model provide a basis for description of 

ground-water flow near the INL. McCarthy and others (1995) 

developed a ground-water flow model for the area of the INL 

to support ground-water remediation for specific INL facilities 

(fig. 5).

The current modeling effort builds on the groundwater flow and contaminant transport modeling results 

of Robertson (1974) and Goode and Konikow (1990b). 

Robertson (1974) simulated two-dimensional steady-state 

and transient ground-water flow and contaminant transport. 

The simulations successfully reproduced the unusually wide 

contaminant plumes present in the upper 200 ft of the aquifer; 

however, to do so required a larger transverse (αT= 450 ft) 

than longitudinal (αL = 300 ft) dispersivity. These values for 

dispersivity were derived using a steady-state flow model and 

no recharge from the Big Lost River (Goode and Konikow, 

1990b, p. 417); no values for dispersivity were reported for the 

transient flow simulations. The αL/ αT ratio derived from this 

model, 0.67, is smaller than the ratios of 24 other published 

pairs of αL/ αT; the other αL/ αT ratios ranged from 1 to 53 with a 

mean of 8.8 (Gelhar and others, 1992, table 1). This small αL/ 

αT ratio, and the large simulated values for αL and αT, indicated 

that some geologic and hydrologic features of the aquifer that 

were important for modeling contaminant transport were not 

represented in Robertson’s (1974) model.

Goode and Konikow (1990b) recognized that groundwater flow directions and velocities varied temporally in 

the shallow flow field near contaminated ground water at 

the INL, and that the large model-derived αT of Robertson 

(1974) could have included these fluctuations. If transient 

variations in the flowfield are ignored or unknown, then larger 

estimates for αT and smaller αL/ αT ratios may be required to 

account for the temporally-variable directions of advective 

flow. Using Robertson’s (1974) steady-state and transient 

models, but modifying the simulated magnitude, timing, 

and spatial distribution of recharge from the Big Lost River, 

Goode and Konikow (1990b) evaluated the effect of transient 

ground-water flow at the INL on estimates of longitudinal 

and transverse dispersivity. Although Goode and Konikow’s 

(1990b) simulations resulted in αT values that were smaller 

than values for αL, their results were inconclusive and they 

were unable to determine the effect that transient variations in 

the flow field had on dispersivities.

Introduction 
Figure 5. Areas and grid sizes of ground-water models of the eastern Snake River Plain aquifer at the Idaho National Laboratory 

(INL) and vicinity, Idaho, relative to those of the Snake River Plain Regional Aquifer System Analysis (RASA) model. 

ID19-0016_fig05

IDAHO

BOISE

RASA Model

Outline

IDAHO

NATIONAL

LABORATORY

Idaho

Falls

Twin Pocatello Falls

Conceptual

Model Area

EXPLANATION EXPLANATION

4400

4400

Sediment—Thickness estimated to compose 

from 500 to 999 feet of the stratigraphic section

Sediment—Thickness estimated to compose 

from 100 to 499 feet of the stratigraphic section

Silicic rocks—Includes rhyolite domes Big 

Southern Butte (A), Middle Butte (B), and East 

Butte (C). Small unnamed dome and Cedar 

Butte andesite not shown

Water-table contour—Shows altitude of water 

table in 1995. Intervals 10 and 100 feet. Vertical 

datum is NGVD 29. Some contours 

approximately located (Bartholomay and 

others, 1997)

Sediment—Thickness estimated to compose more 

than 11 percent of the stratigraphic section

Silicic rocks—Includes rhyolite domes Big Southern 

Butte(A), Middle Butte (B), and East Butte (C). Small 

unnamed dome and Cedar Butte andesite not shown

Water-table contour—Shows altitude of water table 

in 1995. Intervals 10 and 100 feet. Vertical datum is 

NGVD 29. Some contours approximately located 

(Bartholomay and others, 1997)

Idaho

National

Laboratory

Idaho

National

Laboratory

Idaho

National

Laboratory

Idaho

National

Laboratory

0 40 KILOMETERS

40 MILES

20

0 20

Hagerman

Twin Falls

Pocatello

RASA model

outline

Hagerman

Twin Falls

Pocatello

Idaho

Falls

Hagerman

Twin Falls

Pocatello

Idaho

Falls

Hagerman

Twin Falls

Pocatello

Idaho

Falls

Idaho

Falls

RASA model

outline

RASA model

outline

Snake River Plain Regional 

Aquifer System Analysis (RASA) 

(Garabedian, 1992)

INL sub-regional solute-transport 

model (Robertson, 1974). Solutetransport model by Goode and 

Konikow (1990b) used same grid 

area but different grid spacing.

INL facility studies

(McCarthy and others, 1995)

Mud Lake model 

(Spinazola, 1994)

10 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Conceptual Model of Ground-Water 

Flow

Numerous and complex hydrogeologic and hydrologic 

data used to develop the conceptual model are described in 

six sections of this report: (1) “Hydrogeologic Framework 

and Hydraulic Properties,” which defines three hydrogeologic 

units, and the structural features and hydraulic properties of 

these hydrogeologic units that affect ground-water flow in 

the model area; (2) “Model Boundaries,” which defines the 

location of the three physical and three artificial hydrologic 

boundaries of the model; (3) “Inflows, Outflows, and Fluxes 

Across Model Boundaries,” which characterizes the model 

boundaries and describes inflows, outflows, and fluxes 

across the boundaries; (4) “Ground-Water Budget,” which 

summarizes model inflows and outflows and analyzes 

uncertainties of the water-budget components; (5) “GroundWater Movement,” which describes the hydraulic gradient, 

ground-water flow directions, and average linear ground-water 

velocities in the model area; and (6) “Model Representation of 

Features Affecting Ground-Water Flow,” which describes the 

stratigraphic, structural, and hydrologic controls on groundwater flow in the model area. The report concludes with a 

brief section titled “Implications for Contaminant Transport” 

which summarizes those features of the conceptual model 

that most affect interpretations of contaminant transport in the 

aquifer at the INL and vicinity.

Hydrogeologic Framework and Hydraulic 

Properties

The hydrogeologic framework is primarily based on 

broad differences in the lithology and large variations in the 

hydraulic properties of the heterogeneous, anisotropic basaltflow groups, sedimentary interbeds, and other rocks that 

allow these rocks to be grouped into three homogeneous and 

anisotropic hydrogeologic units. In the unsaturated zone and 

aquifer beneath the INL, basalt makes up about 85 percent 

of the volume of rocks and sediment makes up most of the 

remainder (Anderson and Liszewski, 1997, p. 11). These and 

other rocks form at least 178 basalt-flow groups, 6 andesiteflow groups, 103 sedimentary interbeds, and 4 rhyolite domes 

(Anderson and Liszewski, 1997, p. 21, table 4). The aquifer 

includes at least 65 basalt-flow groups, 5 andesite-flow groups, 

61 sedimentary interbeds, and 3 rhyolite domes. Detailed 

surficial mapping and correlation of subsurface stratigraphic 

units among numerous outcrops and 333 wells at and near the 

INL (Kuntz and others, 1994; Anderson and others, 1996a) 

indicated that many of these stratigraphic units are continuous 

across large parts of the model area. Stratigraphic units were 

combined by Anderson and Liszewski (1997, p. 14), based on 

their similar age, into 14 composite stratigraphic units, each 

made up of from 5 to 90 (Anderson and Liszewski, 1997, p. 14 

and table 4) stratigraphic units of similar age (fig. 6).

Structural features in the model area include (1) rhyolite 

domes (Kuntz and others, 1994), (2) sedimentary troughs 

(Gianniny and others, 1997), (3) areas of subsidence, uplift, 

and dipping beds (Anderson and others, 1997, Anderson and 

Liszewski, 1997), (4) volcanic rift zones, which are broad 

belts of focused volcanism that generally trend northwestward 

(fig. 7) and are perpendicular to the direction of regional 

ground-water flow (Kuntz and others, 1992), and (5) vent 

corridors, which are narrow zones in and near volcanic rift 

zones that contain known or inferred volcanic vents, dikes, and 

fissures (fig. 8) (Anderson and others, 1999, p. 13).

Hydraulic properties of the three hydrogeologic units 

of the conceptual model reflect the distribution of major 

rock types, local stratigraphic units, and structural features. 

Areas of high well density at the INTEC, RWMC, TRA, 

and TAN allowed evaluation of local variations in hydraulic 

properties related to the complex geology. In these areas, 

large variations in hydraulic properties across distances of 

hundreds to thousands of feet were measured. For example, 

horizontal hydraulic conductivity in a single vent corridor at 

the INTEC and TRA varies as much as three to five orders of 

magnitude across distances of 500 to 1,000 ft (Anderson and 

others, 1999, p. 27, table 2). These large variations indicate the 

potential complexity of the aquifer at the scale of an individual 

INL facility. Although this small-scale complexity cannot be 

duplicated at the scale of the conceptual model, it can be used 

as a guide for estimating hydraulic properties in the model 

with greater precision than was possible for the RASA study.

Hydraulic properties for the eastern SRP aquifer used 

in the larger scale RASA study were based on specific 

capacity or transmissivity and storativity estimates from 

medium- to high-capacity (greater than 50 gal/min) irrigation 

and industrial supply wells (Mundorff, 1964; Whitehead, 

1992, tables 4 and 5, Garabedian, 1992, tables 3 and 4). 

Transmissivity, horizontal and vertical hydraulic conductivity, 

storativity, and porosity at the INL and vicinity were 

estimated at various scales. Core-scale estimates of hydraulic 

conductivity and porosity are given in Knutson and others 

(1990, 1992). Transmissivity estimates for single-well aquifer 

tests (generally less than 40 gal/min) are given in Ackerman 

(1991, table 2) and Bartholomay and others (1997, table 3). 

Additional single-well aquifer tests, slug tests, and packer tests 

are summarized in Welhan and Wylie (1997) and Welhan and 

others (2002a). Estimates of horizontal and vertical hydraulic 

conductivity and of storativity from multi-well aquifer tests 

are given in Spinazola (1994), Frederick and Johnson (1996), 

and Wood and Norrell (1996).

Conceptual Model of Ground-Water Flow 11
Figure 6. Composite stratigraphic units that form the eastern Snake River Plain aquifer, Idaho National Laboratory and 

vicinity, Idaho.

ID19-0016_fig06

D ́ D ́ ́

3,200

3,400

3,600

3,800

4,000

4,200

4,400

4,600

4,800

5,000

5,200

5,400

ALTITUDE, ABOVE NATIONAL GEODETIC VERTICAL DATUM OF 1929

Modified from Anderson and Liszewski, 1997, 

figures 8, 9, and 12. 

D

FEET

D

Geologic

section

D-D ́-D ́ ́

D ́

Idaho

National

Laboratory

D ́ ́ Composite stratigraphic units 1 to 7— Includes

unaltered basalt and sediment; locally includes 

andesite and rhyolite. Age of sequence 

generally ranges from about 200 to 800 

thousand years and is consistent with that of 

the Snake River Group.

Composite stratigraphic units 8 to 14— Includes 

unaltered to altered basalt and sediment; 

locally includes andesite and rhyolite. Age of 

sequence generally ranges from about 800 

thousand to 1.8 million years and is consistent 

with that of the Bruneau Formation.

Rocks that form the base of the Eastern 

Snake River Plain aquifer— Includes 

altered basalt and sediment; locally may 

include andesite and rhyolite. Age of 

sequence generally ranges from about 1.8 

to 4 million years and is consistent with that 

of the Glenns Ferry Formation.

Geologic contact—Queried where uncertain

EXPLANATION

?

VERTICAL SCALE GREATLY EXAGGERATED

0 2 4 6

0 KILOMETERS

MILES

2 4 6

Water table

?

?

?

?

?

?

?

?

?

?

?

?

? ?

?

?

? ?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

? ?

?

?

?

?

1

2

3

4

5

6

7

8

9

10

1

2

3

5 4

6

7

8

9

10

11

12

1

2

3

4

4

5

6

7

8

9

10

1

2

3

4

5

6

7

8

9

10

1

2 3 9 8

14

13

12

10

11

10

12

13

14

11

Hydraulic conductivity of the basalts of the eastern 

SRP aquifer at the INL and vicinity range from about 0.01 

to 24,000 ft/d (Anderson and others, 1999, fig. 9, table 2), 

more than six orders of magnitude. Almost two-thirds of these 

estimates exceeded 100 ft/d. Hydraulic conductivities were 

estimated by dividing transmissivity by the total lengths of 

open, perforated, or screened intervals. Transmissivities of the 

eastern SRP aquifer range from 1.1 to 760,000 ft2/d, nearly 

six orders of magnitude (Ackerman, 1991, table 3). More than 

60 percent of these estimates are greater than 20,000 ft2/d 

(Ackerman, 1991, fig. 10). Garabedian (1992, tables 19 and 

20) used a model-calibrated range of 0.45 to 9,500 ft/d for 

the hydraulic conductivity of all types of basalt in the RASA 

study.

Estimates of the fractured basalt porosity in the eastern 

SRP aquifer range from 0.05 to 0.27 (Nace and others, 1959, 

p. 58-61, table 9; Barraclough and others, 1967, p. 61 and 

63; Robertson and others, 1974, p. 176; Robertson, 1974, 

p. 13, 1977, p. 44-45; Garabedian, 1992, p. 44-46; Ackerman, 

1995, p. 10 and 22; Bishop, 1991, p. 77; Knutson and others, 

1992, p. 4-21). Estimates of porosity varied greatly because 

they were dependent on methods, scales, and locations used 

to determine them. For example, porosity estimated from 

laboratory measurements of cores from boreholes and outcrops 

at and near INL facilities were different from estimates derived 

from larger-scale aquifer model simulations. Because porosity 

in the eastern SRP basalts derives mainly from interflow zones 

and their associated rubble, fractures, joints, and vesicles 

(Hughes and others, 1999, fig. 12), the larger-scale estimates 

of porosity for these basalts are probably more appropriate for 

the conceptual model.

12  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Figure 7. Approximate locations of volcanic rift zones and the axial volcanic high, Idaho National Laboratory and vicinity, 

Idaho.

ID19-0016_fig07

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Modified from Kuntz and others, 1992, fig. 7.

Mud

Lake

Big

Lost

River

Little

Lost

River

Birch

Creek

Camas

Creek

Big

Southern

Butte

East

Butte

Middle

Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

IDAHO NATIONAL 

LABORATORY BOUNDARY

113°15’ 113°00’ 45’

44°00’

43°30’

45’

30’ 112°15’

Atomic

City

Howe

Arco

Mud

Lake Terreton

EXPLANATION

Approximate area of volcanic rift 

zones and axial volcanic high

0 5 10 MILES

0 5 10 KILOMETERS

Axial Volcanic High

Lava Ridge-Hell’s Half Acre Rift Zone

Circular Butte-Kettle Butte Rift Zone

Howe-East Butte Rift Zone

Arco Rift Zone

Direction of regional

ground-water flow

Conceptual Model of Ground-Water Flow 13
Figure 8. Locations of volcanic vents and vent corridors, Idaho National Laboratory and vicinity, Idaho.

ID19-0016_fig08

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Modified from Anderson and others, 1999, figure 7

Mud

Lake

Big

Lost

River

Little

Lost

River

Birch

Creek

Camas

Creek

Big

Southern

Butte

East

Butte

Middle

Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

113°15’ 113°00’ 45’

44°00’

43°30’

45’

30’ 112°15’

Atomic

City

Howe

Arco

Mud

Lake Terreton

EXPLANATION

Vent corridor – Zone of aligned vents, dikes, and 

fissures; includes areas of warm water, anoxic 

water, and rock alteration in and below the aquifer. 

Approximately located; queried where uncertain

Volcanic vent – Location of one or more surficial 

vents or fissures (from Kuntz and others, 1994)

Volcanic vent – Approximate location of one or more 

concealed vents or fissures (from Anderson and 

Liszewski, 1997)

Silicic rocks—Includes rhyolite domes Big 

Southern Butte, Middle Butte, and East Butte. 

Small unnamed dome and Cedar Butte andesite 

not shown

?

0 5 10 MILES

0 5 10 KILOMETERS

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

IDAHO NATIONAL 

LABORATORY BOUNDARY

Direction of regional

ground-water flow

14  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Interflow zones may be the most important hydraulic 

features of the eastern SRP aquifer because of their high 

permeability and their ability to localize large ground-water 

fluxes (Welhan and others, 2002a, p. 226). Five interflow 

zones identified near INTEC, in the upper 200 ft of the aquifer, 

range in thickness from less than 1-ft thick to more than 

18-ft thick (Jones, 1961, p. 36), and the effective thickness 

of interflow zones may be between 1 and 8.2 ft (Welhan and 

others (2002b, p. 140). Welhan and others (2002a, p. 226-227; 

2002b, p. 137-139) described two types of interflow zones. 

Type-I interflow zones are hosted in rubble at horizontal 

contacts between basalt flows and Type-II interflow zones 

are hosted in fracture networks along the edges of inflating 

lava flows (Welhan and others, 2002b, p. 136). The largest 

concentration of basalt fractures in eastern SRP basalts occurs 

at interflow zones (Welhan and others, 2002a and 2002b). 

Fractures in Type-I interflow zones consist of tension fractures 

and fractured flow tops in the upper vesicular zone. Fracturing 

in Type-II flow zones occur as interconnected networks of 

fissures along the intensely fractured margins of basalt flows 

that can remain partially open after burial by younger lava 

(Welhan and others, 2002a, p. 229).

Hydrogeologic Units

The hydrogeologic units in the model area are 

(1) hydrogeologic unit 1, younger rocks of thin, densely 

fractured basalt (440,000 to 650,000 years old) and 

interbedded sediment, (2) hydrogeologic unit 2, younger rocks 

of massive, less densely fractured basalt (650,000 to 800,000 

years old) and interbedded sediment, and (3) hydrogeologic 

unit 3, intermediate-age rocks of slightly altered, fractured 

basalt (800,000 to 1,800,000 years old) and interbedded 

sediment (figs. 9 and 10). Other rocks of hydrogeologic 

importance in the model area are (1) older rocks of intensely 

altered basalt (older than 1,800,000 years), the top of which 

is interpreted to form the base of the aquifer, (2) rhyolite 

domes that penetrate the younger and intermediate-age rocks, 

and (3) sediment that is distributed throughout the younger, 

intermediate-age, and older rocks.

Hydrogeologic units 1, 2, and 3 correlate with the 

regional stratigraphy and the stratigraphy beneath the INL as 

defined by Anderson and Liszewski (1997) as follows: (1) 

hydrogeologic unit 1 is equivalent to the Snake River Group 

and composite stratigraphic units 4 to 6; (2) hydrogeologic 

unit 2 is equivalent to the Snake River Group and composite 

stratigraphic unit 7; and (3) hydrogeologic unit 3 is equivalent 

to the Bruneau Formation and composite stratigraphic units 

8 to14 (fig. 11). Equivalent groups and formations were 

determined based on broad correlations between cores at 

the INL and outcrops in southern Idaho (Whitehead, 1992; 

Anderson and Liszewski, 1997). In the model area these 

composite stratigraphic units can be saturated (aquifer) or 

unsaturated (fig. 11).

Younger rocks form the uppermost part of the aquifer 

in much of the model area and, based on stratigraphic 

interpretations, intermediate-age rocks make up the largest 

volume of the aquifer in the model area. Interpreted 

distribution of older rocks indicates large changes in saturated 

thickness across the model area that may be related to 

differential subsidence and uplift (Anderson and Liszewski, 

1997, fig. 6). In the southwestern part of the model area the 

hydrogeologic framework is interpreted as having a zone 

of differential subsidence and uplift that affects the dip of 

hydrogeologic units 1 and 2 (fig. 10). This interpretation, 

based on the trend of composite stratigraphic units 4 through 

7 (Anderson and Liszewski, 1997, figs. 21, 23, 25, and 27), is 

uncertain due to the few core and stratigraphic data available 

in the southwestern part of the model area.

Hydraulic conductivities for hydrogeologic units 1, 2, 

and 3 and older rocks were estimated from aquifer tests in 

wells using straddle packers or having perforated or open 

intervals only in the respective hydrogeologic unit or older 

rocks, and hydraulic conductivities of sediment were estimated 

from laboratory determinations of the saturated hydraulic 

conductivity of core samples (table 2). Estimated hydraulic 

conductivity from core-scale tests were not used to estimate 

the hydraulic conductivities of the hydrogeologic units or 

older rocks because they reflect the character of the rock 

matrix and are not representative of the magnitude and spatial 

variability of hydraulic conductivity estimated from field-scale 

tests arising from larger-scale heterogeneities (Welhan and 

others, 2002a, p. 229). These core-scale tests have minimum 

and median values 3 to 4 orders of magnitude less than fieldscale test results (Welhan and others, 2002a, fig. 3).

Conceptual Model of Ground-Water Flow 15
Figure 9. Distribution of hydrogeologic units present at the water table and water-table contours for 1980 in the 

model area, Idaho National Laboratory and vicinity, Idaho.

ID19-0016_fig09

43° 30 ́

44° 00 ́

44° 30 ́

114° 00 ́ 30 ́ 113° 00 ́ 30 ́ 112° 00 ́

Mud Lake

Camas

Creek

Lost

River

Little

Lost

River

Birch

Creek

Mackay

Reservoir

Big

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

0

0 10

10

20 KILOMETERS

20 MILES

EXPLANATION

Hydrogeologic unit 1—Younger rocks consisting of 

densely fractured basalt and interbedded sediment

Hydrogeologic unit 2—Younger rocks consisting of 

massive, less densely fractured basalt and 

interbedded sediment

Hydrogeologic unit 3—Intermediate-age rocks 

consisting of slightly altered fractured basalt, and 

interbedded sediment

Silicic rocks—Includes rhyolite domes Big Southern 

Butte (A), Middle Butte (B), and East Butte (C). Small 

unnamed dome and Cedar Butte andesite not shown

Water-table contour—Shows altitude of water table in 

1980. Interval 50 feet. Datum is NGVD 29. Some 

contours approximately located (modified from 

Lindholm and others, 1988)

1

2

3

4500

WHITE KNOB

MOUNTAINS

LEMHI

RANGE

LOST

RIVER

RANGE

CONCEPTUAL

MODEL AREA

BITTERROOT

 RANGE

PIONEER

MOUNTAINS

1

2

2

3

3

A

B

C

Line of

section

for figures

10 and 24

4700

4600

4500

4400

4300

4200

4100

4500

B

B’

Atomic

City

Howe

Arco

Mud

Lake

Terreton

Idaho

National

Laboratory

16  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Figure 10. Distribution of hydrogeologic units along the direction of regional ground-water flow in the model area, Idaho 

National Laboratory and vicinity, Idaho.

ID19-0016_fig10

1 Hydrogeologic unit 1—Younger rocks 2 3

consisting of densely fractured 

basalt and interbedded sediment

Hydrogeologic unit 2—Younger rocks 

consisting of massive, less densely 

fractured basalt and interbedded 

sediment

Hydrogeologic unit 3—Intermediateage rocks consisting of slightly 

altered fractured basalt and 

interbedded sediment

EXPLANATION

Direction of regional ground-water flow

5,000

5,500

FEET

4,000

2,000

1,000

3,000

4,500

2,500

1,500

3,500

B B’

?

?

ALTITUDE, ABOVE NATIONAL GEODETIC VERTICAL DATUM OF 1929

1

2

3

Base of 

the aquifer

Older rocks

Older rocks Older rocks

Unsaturated zone

Line of section shown in figure 9.

Water table

VERTICAL SCALE GREATLY EXAGGERATED

0 4 8 12 MILES

0 12 KILOMETERS

Conceptual Model of Ground-Water Flow 17
Figure 11. Correlation among stratigraphic units of the eastern Snake River Plain, composite stratigraphic units 

beneath the Idaho National Laboratory, and hydrogeologic units of the conceptual model, Idaho National Laboratory 

and vicinity, Idaho.

ID19-0016_fig011

1Unit thickness is variable across the model area.

2Number of boreholes penetrating the unit within the model area.

3Rhyolite domes are not included in conceptual model hydrogeologic units 1, 2, or 3.

Quaternary

Tertiary

Holocene

Pleistocene

Pliocene

Selected

ages, in

millions

of years

before

present

0.01

0.80

1.7 to 2.2

4.9 to 5.3

Alluvium

Snake

 River

Group

Upper part of

Idaho Group

(Bruneau

Formation)

Upper part of

Idaho Group

(includes Glenns

Ferry Formation)

Eastern Snake

River Plain

stratigraphic units

(Whitehead, 1992)

Hydrogeologic

unit 1—

Younger rocks

Hydrogeologic

unit 2—

Younger rocks

Hydrogeologic

unit 3—

Intermediate-age rocks

Unsaturated zone

Unsaturated zone

0.65

0.44

Unsaturated

zone

Geologic age

0-1,007 ft thick

86 boreholes

41 basalt-flow groups

1 andesite-flow group

26 sedimentary interbeds

2 rhyolite domes

Composite units 8-14

0-409 ft thick

70 boreholes

7 basalt-flow groups

10 sedimentary interbeds

Composite unit 7

0-347 ft thick

118 boreholes

5 basalt-flow groups

8 sedimentary interbeds

1 rholite dome

Composite unit 6

0-329 ft thick

143 boreholes

3 basalt-flow groups

6 sedimentary interbeds

Composite unit 5

0-482 ft thick

172 boreholes

9 basalt-flow groups

4 andesite-flow groups

11 sedimentary interbeds

Composite unit 4

0-305 ft thick

164 boreholes

17 basalt-flow groups

1 andesite-flow group

17 sedimentary interbeds

Composite unit 3

0-321 ft thick

234 boreholes

18 basalt-flow groups

13 sedimentary interbeds

1 rhyolite dome

Composite unit 2

0-284 ft thick1

333 boreholes2

78 basalt-flow groups

12 sedimentary interbeds

Composite unit 1

Composite stratigraphic units

(Anderson and Liszewski, 1997) Conceptual model

hydrogeologic units3

EXPLANATION

Transition from unsaturated to saturated zone occurs within or between 

several different composite stratigraphic units and conceptual model 

hydrogeologic units.

18  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Table 2. Ranges of hydraulic conductivities and porosities for hydrogeologic units and other rocks of hydrogeologic 

importance, Idaho National Laboratory and vicinity, Idaho.

[Conductivities for thin, densely fractured basalt; massive, less densely fractured basalt; intermediate-age rocks; and older rocks were estimated 

from aquifer tests in wells having perforated or open intervals only within the respective hydrogeologic unit or rock. Values were calculated 

from stratigraphic and hydraulic data presented by Mann (1986), Wood and Norrell (1996, table 6-2), Anderson and Liszewski (1997), and 

Anderson and others (1999, table 2). Saturated hydraulic conductivities for sediment were measured in the laboratory on intact core samples 

from the unsaturated zone using either the constant- or falling-head method and were reported by McElroy and Hubbell, 1990, p. 23; Perkins 

and Nimmo, 2000, table 1; Perkins, 2003, table 1; Winfield, 2003, table 1; Winfield, 2005, table 2. Values for porosity were measured in the 

laboratory on core samples and were reported by Nace and others, 1959, p. 58-61, table 9; Barraclough and others, 1967b, p. 61 and 63; Knutson and others, 1992, p. 4-21; Robertson, 1977, p. 21; McElroy and Hubbell, 1990, p. 23; Perkins and Nimmo, 2000, table 1; Perkins, 2003, 

table 1; Winfield, 2003, table 1; Winfield, 2005, table 2. Abbreviations: ft/d, foot per day; 100s, hundreds; <, less than]

Hydrogeologic unit or rock

Property and sample data

Hydraulic conductivity

(ft/d)

Number of 

wells (w) or 

core samples 

(c)

Range or

median

porosity

Number of 

core samples

Smallest Largest

Hydrogeologic unit 1—Younger rocks 

consisting of thin, densely fractured 

basalt

0.01 24,000 67 (w) 0.05 to 0.27 100s

Hydrogeologic unit 2—Younger rocks 

consisting of massive, less densely 

fractured basalt

6.5 1,400 4 (w) 10.11 100s

Hydrogeologic unit 3—Intermediate-age 

rocks consisting of slightly altered 

fractured basalt and interbedded 

sediment

.32 24,000 14 (w) 0.05 to 0.08 20

Older rocks consisting of intensely altered 

fractured basalt and rhyolitic ash-flow 

tuffs

.002 .03 1 (w) <0.09 to 0.19 19

Sediment .000032 240 109 (c) 0.25 to 0.73 100s

1

Median value. 

Younger Rocks

Younger rocks (hydrogeologic units 1 and 2) make up 

the unsaturated zone throughout most of the model area and 

the uppermost part of the aquifer in the central part of the 

model area (figs. 9 and 10). Sediment content is largest in the 

younger rocks in a sedimentary trough, known as the Big Lost 

Trough, in the northern part of the model area. Petrographic 

analyses of cores of the younger rocks from wells at and 

immediately north of the INTEC (Lanphere and others, 1993, 

p. 19-33) indicated that alteration was rare and weak and that 

secondary mineralization consisted of thin coats of carbonate 

minerals. The younger rocks also include four rhyolite domes 

[fig. 9, rhyolite domes A, B, C, and an unnamed dome (not 

shown in fig. 9) between B and C].

Hydrogeologic Unit 1 (Thin, Densely Fractured Basalt)

Hydrogeologic unit 1 comprises 17 basalt-flow groups, 

4 andesite-flow groups, and 25 sedimentary interbeds (fig. 11) 

(Anderson and Liszewski, 1997, p. 16-17, figs. 9-15). These 

thin, densely fractured basalt flows are mostly 14- to 24-ft 

thick (table 3) tube-fed pahoehoe flows having thin, massive 

interiors (Anderson and others, 1999). Mean thickness of the 

basalt flows in hydrogeologic unit 1 is 20 ft.

Hydraulic conductivity and porosity of hydrogeologic 

unit 1 primarily is controlled by void spaces of interconnected 

interflow zones. Other factors that probably modify the 

hydraulic conductivity and porosity of the fractured basalt 

include near-vent deposits, dikes, fissures, and tension cracks 

within volcanic rift zones and vent corridors (Anderson and 

Conceptual Model of Ground-Water Flow 19
others, 1999, p. 27). Secondary mineralization is uncommon 

in these rocks, except where carbonates fill vesicles near the 

land surface (Nace and others, 1975, p. 13).

Results of single-well aquifer tests in 67 wells with 

perforated or open intervals only in hydrogeologic unit 1 

indicated that the hydraulic conductivity of these rocks ranges 

from about 0.01 to 24,000 ft/d (table 2), more than six orders 

of magnitude. Almost two-thirds of these estimates were larger 

than 100 ft/d and about one-third were larger than 1,000 ft/d. 

Estimates larger than 100 ft/d primarily are associated with 

interflow zones of thin pahoehoe flows (Anderson and others, 

1999, p. 27). Many estimates of hydraulic conductivity smaller 

than 100 ft/d may be associated with dikes (Anderson and 

others, 1999, p. 27).

Three previous studies described specific yield, porosity, 

and effective porosity applicable to hydrogeologic unit 1. In 

the RASA study, Garabedian (1992, p. 44-46) used an average 

specific yield of 0.05 for all types of basalt and silicic volcanic 

rocks and 0.20 for all types of sediment for the uppermost 

200 ft of the eastern SRP aquifer. Ackerman (1995, p. 10) used 

a range of effective porosity from 0.10 to 0.25 for simulation 

of advective transport in the uppermost 200 ft of the regional 

aquifer system. The calibration value for this simulation, 0.21, 

was based on the travel time of iodine-129 (129I) from disposal 

well CPP 3 at the INTEC to well USGS 11 near Big Southern 

Butte (fig. 1) (Ackerman, 1995, p. 22). Knutson and others 

(1992, p. 4-21 and fig. 4-10) determined the porosity of 1,504 

core samples from the unsaturated zone of hydrogeologic unit 

1 near the RWMC. The porosities ranged from 0.01 to 0.43 

with 90 percent of values between 0.05 and 0.27.

Hydrogeologic Unit 2 (Massive, Less Densely Fractured Basalt)

Hydrogeologic unit 2 consists of 7 basalt-flow groups, 

10 sedimentary interbeds, and includes basalt-flow group I, 

a major basalt-flow group within composite unit 7 (fig. 11) 

(Anderson and Liszewski, 1997, p. 16, figs. 9-15). These 

massive, less densely fractured basalt flows are mostly 21- to 

37-ft thick (table 3) tube-fed pahoehoe flows having thick, 

massive interiors (Anderson and others, 1999). Mean thickness 

of the basalt flows in hydrogeologic unit 2 is 29 ft (table 3), 

or nearly one-and-one-half times the average thickness of 

the basalt flows composing hydrogeologic unit 1. As a result 

of the larger average thickness of basalt flows, the basalt of 

hydrogeologic unit 2 is probably more massive, less densely 

fractured, and has fewer interflow zones than hydrogeologic 

unit 1.

The massive basalt of hydrogeologic unit 2 includes 

basalt-flow group I, one of the thickest and most extensive 

flow groups near the INL (Anderson, 1991, p. 22; Wetmore 

and others, 1997, p. 50, table 1). The interpreted thicknesses 

of the other basalt-flow groups in hydrogeologic unit 2 were 

based entirely on natural gamma logs and are less certain than 

the interpretations for basalt-flow group I. Basalt-flow group I 

and the overlying HI interbed (a layer of clay and silt) underlie 

all but the northern and extreme southeastern parts of the INL 

(Anderson, 1991, p. 21-22; Anderson and others, 1997, p. 19). 

The thickness of composite stratigraphic unit 7 ranges from 0 

to 409 ft, averages 266 ft in the 15 wells that fully penetrate to 

its base, and is greatest in wells at the TRA near the exposed 

vents of basalt-flow group I. Correlations of composite 

stratigraphic unit 7 were primarily based on natural-gamma 

logs from uncored boreholes because only five coreholes 

within a 3-mi radius of the INTEC are known to penetrate this 

stratigraphic unit.

Hydraulic conductivity and porosity of hydrogeologic 

unit 2 generally are less than hydrogeologic unit 1 because 

of the thick, massive interiors and fewer interconnected 

interflow zones in the massive, less densely fractured basalt. 

Other factors that probably modify the hydraulic conductivity 

and porosity of the fractured basalt include sediment-filled 

interflow zones and near-vent deposits, dikes, fissures, and 

tension cracks in volcanic rift zones and vent corridors 

(Anderson and others, 1999, p. 27). Secondary mineralization 

is uncommon in these rocks, except where carbonates fill 

vesicles near the land surface (Nace and others, 1975, p. 13).

Estimated hydraulic conductivity of hydrogeologic unit 

2 ranged from 6.5 to 1,400 ft/d for single-well aquifer tests 

in four wells with perforated intervals only in hydrogeologic 

unit 2 (table 2). This range of conductivities is similar to 

conductivity ranges estimated for massive basalt of the 

Table 3. Summary statistics of basalt flow thickness within individual hydrogeologic units, Idaho National 

Laboratory and vicinity, Idaho.

[Data compiled from Anderson and others (1996, table 8)]

Hydrogeologic unit

Basalt flow thickness (feet)

Minimum First quartile Median Mean Third quartile Maximum Number of 

flows

Unit 1 2 14 18 20 24 54 1,095

Unit 2 4 21 28 29 37 78 212

Unit 3 2 15 22 23 28 107 441

20 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Columbia Plateau (Anderson and others, 1999, p. 22 and 27), 

results of flowmeter tests in two wells penetrating part of 

unit 2 (Morin and others, 1993, p. 23 and 26), and numerical 

model simulations of aquifer tests that include unit 2 

(Frederick and Johnson, 1996, p. 63, layer 3).

Porosity of the massive basalt of hydrogeologic unit 2 

probably is within the lower end of the range estimated for that 

of the densely fractured basalt of hydrogeologic unit 1, 0.05 

to 0.27. Knutson and others (1992, p. 4-21) reported a median 

porosity of 0.11 for hundreds of nonvesicular basalt cores, a 

measure that may approximate the porosity of massive basalt 

because it does not include the effects of porous interflow 

zones.

Intermediate-Age Rocks (Hydrogeologic Unit 3)

Intermediate-age rocks of hydrogeologic unit 3 underlie 

younger rocks in the model area, except for the area northwest 

of the Big Lost River (Anderson and Liszewski, 1997, fig. 30) 

(fig. 1) where the intermediate-age rocks are not present. The 

intermediate-age rocks constitute the full thickness of the 

aquifer in the northern part of the INL and in the southwestern 

part of the model area (figs. 9 and 10). Hydrogeologic unit 3 

consists of 41 basalt-flow groups, 1 andesite-flow group, and 

26 sedimentary interbeds (fig. 11) (Anderson and Liszewski, 

1997, p. 17-18, figs. 9-15). Sediment content is greatest in 

a sedimentary trough, known as the Big Lost Trough, in the 

northern part of the model area. Average thickness of fractured 

basalt flows in hydrogeologic unit 3 is 23 ft and 50 percent of 

the flows are between 15- and 28-ft thick (table 3).

Basalt flows of hydrogeologic unit 3 are slightlyto-moderately altered (Whitehead, 1992, p. 10, table 3; 

Lanphere and others, 1994, p. 22-39; Fromm and others, 

1994; Anderson and Liszewski, 1997, p. 28). Detailed analysis 

of petrography of cores from wells near TAN indicates that 

intermediate-age rocks near TAN contain secondary porefilling minerals such as calcite (Lanphere and others, 1994, 

p. 22-39). Calcite also is commonly present in the vesicles and 

fractures of these rocks. This alteration and the larger average 

thickness of the basalt flows, compared with the thickness 

of basalt flows in hydrogeologic unit 1 (Whitehead, 1992, p. 

11-13), result in fewer interflow zones and a smaller hydraulic 

conductivity and porosity for hydrogeologic unit 3 than for 

hydrogeologic unit 1.

Results of single-well aquifer tests in 14 wells with 

perforated or open intervals only in hydrogeologic unit 

3 indicated that the hydraulic conductivity of these rocks 

ranges from about 0.32 to 24,000 ft/d (table 2). However, a 

comparison of 24 hydraulic conductivity estimates near the 

INTEC with 68 estimates near TAN indicate that the average 

hydraulic conductivity of intermediate-age rocks near TAN 

is about one order of magnitude smaller than younger rocks 

(hydrogeologic units 1 and 2 undifferentiated) near the 

INTEC (John Welhan, Idaho State Geological Survey, written 

commun., 1999). Welhan calculated a median and geometric 

mean hydraulic conductivity of 500 and 130 ft/d for younger 

rocks near the INTEC and 30 and 20 ft/d for the intermediateage rocks near TAN. In the analysis of the regional aquifer 

system, Garabedian (1992, p. 42) accounted for this difference 

by decreasing the hydraulic conductivity of lower model layers 

by as much as two-thirds.

Porosity of hydrogeologic unit 3 probably is within 

the lower end of the range of that estimated for the densely 

fractured basalt of hydrogeologic unit 1, 0.05 to 0.27. 

Median values of porosity reported for 10 nonvesicular and 

10 vesicular basalt cores from the intermediate-age rocks at 

TAN were about 0.05 and 0.08, respectively (Allan Wylie, 

Idaho Water Resources Research Institute, written commun., 

2000). Median values of porosity reported for hundreds of 

nonvesicular and vesicular basalt cores from the younger 

rocks (hydrogeologic units 1 and 2) at and near the RWMC 

were 0.11 and 0.22, respectively (Knutson and others, 1992, 

p. 4-21). Although these values do not include porosity of 

interflow zones, they indicate that porosity of intermediate-age 

rocks may be smaller than that of younger rocks.

Older Rocks

Older rocks (Tb, Tsv, and QTb in fig. 4) underlie 

intermediate-age rocks throughout most of the model area 

and contain intensely altered basalt (which generally is 

equivalent to the Glenns Ferry Formation), Miocene rhyolitic 

ash-flow tuffs (figs. 4 and 11), and interbedded sediment. 

Only a few estimates of hydraulic conductivity and porosity 

were available for the older rocks. Based on analyses from 

four aquifer tests, Mann (1986, p. 21) reported a range of 

0.002 to 0.03 ft/d for hydraulic conductivity of altered basalt, 

interbedded sediment, and rhyolitic ash-flow tuffs that make 

up the older rocks. Nace and others (1959, table 9) reported 

two measurements of porosity for older basalt, both about 

0.19, at a spring in the discharge area of the eastern SRP 

aquifer. This value is greater than the median porosity, about 

0.09, of 17 cores of altered basalt obtained from the older 

rocks at well WO2 (fig. 1), about 3 mi east of the INTEC 

(Carroll Knutson, EG&G Idaho, Inc., written commun, 1992).

Rhyolite Domes

Rhyolite domes in the model area are clustered in two 

areas (fig. 9; rhyolite domes A, B, C, and an unnamed dome 

between B and C that is not shown in fig. 9). Rhyolite domes 

are vertical plug-like masses (Qsv in fig. 4) interpreted 

to penetrate a large thickness of the younger rocks and 

intermediate-age rocks (Kuntz and Dalrymple, 1979, p. 30-34; 

Spear and King, 1982, p. 396-400; Kuntz and others, 1994; 

Hughes and others, 1999, fig. 16; McCurry and others, 1999, 

p. 170-174). Big Southern Butte is the largest of four rhyolite 

domes. Other domes, Middle Butte, East Butte, and an older 

unnamed dome of small surficial extent between Middle Butte 

and East Butte, are near the southeast boundary of the model 

area.

Conceptual Model of Ground-Water Flow  21
Hydraulic properties of rhyolite domes have not been 

measured; however, based on rock characteristics, water-table 

contours (Bartholomay and others, 1997, fig. 9), and well 

Corehole 1 (CH1 in fig. 1) that penetrates the saturated part 

of the unnamed dome (Kuntz and Dalrymple, 1979, fig. 5; 

Morse and McCurry, 1997, fig. 2; McCurry and others, 1999, 

fig. 3), rhyolite domes probably have low permeability and 

may have hydraulic properties similar to those of the massive 

basalt of hydrogeologic unit 2. A temperature log of Corehole 

1 (Morse and McCurry, 2002, fig. 2) indicates that most 

of the unnamed dome is in hydraulic contact with the cold 

water of the aquifer. However, inferred water-table contour 

deflections around this dome and Middle and East Buttes 

(fig. 12) probably indicates that the hydraulic conductivity of 

these rocks is smaller than that of adjacent fractured basalt. 

The lack of a similar deflection of contours near Big Southern 

Butte probably indicates that this dome is fractured at depth 

or may lie above the water table. If the hydraulic conductivity 

and porosity of rhyolite domes are similar to that of the 

massive basalt of hydrogeologic unit 2, then the conductivity 

probably ranges from about 6.5 to 1,400 ft/d and the porosity 

is probably within the lower end of the 0.05 to 0.27 range of 

porosity estimated for hydrogeologic unit 1. In the RASA 

study, Garabedian (1992, p. 44-46, tables 19 and 20) used a 

model-calibrated value of 0.65 ft/d for hydraulic conductivity 

and a value of 0.05 for specific yield of silicic volcanic rocks.

Sediment

Sediment in the model area is interbedded with basalt 

and deposited in four overlapping depositional environments 

in younger rocks, intermediate-age rocks, and older rocks 

(Whitehead, 1992; Kuntz and others, 1994; Anderson and 

others, 1996a, b). Sediment deposited in the first depositional 

environment consists of thin layers of windblown sediment 

that are present throughout most of the model area (Nace 

and others, 1975, p. 35; Anderson and others, 1996b, p. 3). 

Sediments from the second depositional environment range 

from fluvial, sandy gravel in stream channels to finer-grained 

clayey silt in terminal playas at the distal ends of river systems 

(Nace and others, 1975, p. 19-27; Gianniny and others, 1997, 

p. 31). Mainly thick layers of clay and silty clay lacustrine 

deposits in an area of closed topographic depressions at and 

near Mud Lake (Stearns and others, 1939, p.17, 39; Spinazola, 

1994, p.10) constitute sediment from the third depositional 

environment. Alluvial deposits along and near the mouths 

of tributary valleys and adjacent mountain fronts mainly 

constitute sediment from the fourth depositional environment.

Sedimentary deposits along and near the channel, 

floodplain, sinks, and playas of the Big Lost River are referred 

to as the Big Lost Trough (Gianniny and others, 1997, p. 31). 

The definition of the Big Lost Trough is expanded in this 

report beyond that of Gianniny and others (1997, p. 31) to 

include the coalescing sedimentary deposits along and near 

the channel, floodplain, sinks, and playas of the Big Lost 

River, Little Lost River, Birch Creek, Camas Creek, and Mud 

Lake (fig. 12). Two interpretations of the Big Lost Trough are 

shown in figure 12 to account for differences in methods, data, 

and interpolation techniques used to evaluate the distribution 

of sediment. Figure 12A shows an area, interpreted from drill 

logs (Whitehead, 1992, pl. 5), where sediment was estimated 

to compose from 100 to 999 ft of the stratigraphic section 

(including the older rocks). Figure 12B shows an area where 

sediment was estimated to compose more than 11 percent of 

the stratigraphic section (excluding the older rocks) based on 

interpretations from the geologic map of the INL (Kuntz and 

others, 1994) and natural-gamma logs from wells drilled to 

depths of more than 300 ft below land surface (Anderson and 

others, 1996a, b). Sedimentary deposits compose from less 

than 1 percent to 50 percent of the rocks penetrated by wells 

in the model area. Although sediment is present throughout 

the stratigraphic section of the model area, it is not represented 

with separate hydrogeologic units, but is included in younger 

rocks (hydrogeologic units 1 and 2), intermediate-age rocks 

(hydrogeologic unit 3), and older rocks.

Estimates of hydraulic conductivity and porosity 

measured by or estimated for previous studies reflect the 

wide range of characteristics of the sedimentary interbeds 

beneath the INL. For example, laboratory determinations of 

saturated hydraulic conductivity for 109 core samples from 

selected sedimentary interbeds at and near the RWMC range 

from about 0.000032 to 240 ft/d (McElroy and Hubbell, 

1990; Winfield, 2005). Reported textures of these samples 

range from clay to sand. By comparison, for the RASA study, 

Garabedian (1992, tables 19 and 20) used model-calibrated 

ranges of hydraulic conductivity from 0.0033 to 0.65 ft/d for 

silt and clay, 0.65 to 5,200 ft/d for sand, and 33 to 17,000 ft/d 

for sand and gravel. Robertson (1977, p. 21) reported a 

range in porosity of from 0.25 to 0.45 for selected surficial 

sediments. Laboratory determinations of porosity for 109 core 

samples from selected sedimentary interbeds at and near the 

RWMC and INTEC range from about 0.33 to 0.73 (McElroy 

and Hubbell, 1990, table 8; Perkins and Nimmo, 2000, table 1; 

Perkins, 2003, table 1; Winfield, 2003, table 1; Winfield, 2005, 

table 2). Estimates of effective porosity used for modeling 

contaminant movement through the unsaturated zone at the 

INL were 0.30 and 0.35 (Robertson, 1977, p. 31, 45). In the 

RASA study, Garabedian (1992, p. 44-46) specified a value of 

0.20 for the specific yield of sediments.

Hydraulic properties of interbedded sediment probably 

significantly influence the hydraulic properties of basalt 

in the eastern SRP aquifer. However, because hydraulic 

conductivity and porosity for sediment were available for 

only a few sedimentary interbeds in the model area and were 

not available for complete interflow zones, the influence of 

sediment on hydraulic properties of hydrogeologic units was 

not directly included in the conceptual model. For instance, in 

areas where large amounts of sediment were deposited, such 

22  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
ID19-0016_fig12

43° 30 ́

44° 00 ́

44° 30 ́

114° 00 ́ 30 ́ 113° 00 ́ 30 ́ 112° 00 ́ 114° 00 ́ 30 ́ 113° 00 ́ 30 ́ 112° 00 ́

Mud Lake

Camas

Creek

Lost

River

Little

Lost

RiverRiver

Birch

Creek

Big

Mud Lake

Camas

Creek

Lost

River

Little

Lost

Birch

Creek

Big

Base from U.S. Geological Survey 

digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator 

projection, Zone 12

Datum is North American Datum of 1927

Modified from Whitehead, 1992, pl. 5.

IDAHO

BOISE

EASTERN

SNAKE RIVER

IDAHO PLAIN

NATIONAL

LABORATORY

Twin

Falls

Idaho

Falls

Pocatello

Model

Area

0

0 10

10

20 KILOMETERS

20 MILES

EXPLANATION EXPLANATION

4400

4400

Sediment—Thickness estimated to compose 

from 500 to 999 feet of the stratigraphic section

Sediment—Thickness estimated to compose 

from 100 to 499 feet of the stratigraphic section

Silicic rocks—Includes rhyolite domes Big 

Southern Butte (A), Middle Butte (B), and East 

Butte (C). Small unnamed dome and Cedar 

Butte andesite not shown

Water-table contour—Shows altitude of water 

table in 1995. Intervals 10 and 100 feet. Vertical 

datum is NGVD 29. Some contours 

approximately located (Bartholomay and 

others, 1997)

Sediment—Thickness estimated to compose more 

than 11 percent of the stratigraphic section

Silicic rocks—Includes rhyolite domes Big Southern 

Butte (A), Middle Butte (B), and East Butte (C). Small 

unnamed dome and Cedar Butte andesite not shown

Water-table contour—Shows altitude of water table 

in 1995. Intervals 10 and 100 feet. Vertical datum is 

NGVD 29. Some contours approximately located 

(Bartholomay and others, 1997)

WHITE KNOB

MOUNTAINS

LEMHI

RANGE

LOST

RIVER

RANGE

CONCEPTUAL

MODEL AREA

BITTERROOT

RANGE

PIONEER

MOUNTAINS

WHITE KNOB

MOUNTAINS

LEMHI

RANGE

LOST

RIVER

RANGE

CONCEPTUAL

MODEL AREA

BITTERROOT

RANGE

PIONEER

MOUNTAINS

A

B C

A

B C

A B

4600

4500

4400

4300

4200

4200 4300 4400

4500

4600

Howe

Arco

Howe

Arco

Idaho

National

Laboratory

Idaho

National

Laboratory

Figure 12. Water-table contours, 1995, and areas of abundant sediment within the model area, Idaho National Laboratory 

and vicinity, Idaho. 

Areas interpreted from (A) drill logs and (B) geologic map and natural-gamma logs.

as in the Big Lost Trough, hydraulic conductivity of basalt 

interflow zones probably is greatly reduced because sediment 

fills cracks, joints, fissures, and fractures, reducing the original 

porosity of the basalt and impeding ground-water flow. The 

effects of sedimentary deposits on hydraulic conductivities 

of hydrogeologic units could be estimated with a numerical 

model.

Model Boundaries

The model area, in the west-central part of the eastern 

SRP aquifer, is bounded by three physical and three artificial 

hydrologic boundaries. Physical boundaries are the water-table 

boundary, base of the aquifer, and northwest mountain-front 

boundary. Artificial boundaries are the northeast boundary, 

southeast-flowline boundary, and southwest boundary 

(fig. 13). Artificial boundaries were used for the model 

because using the boundaries of the eastern SRP aquifer 

would produce a model area much larger than that required 

for the purposes of this study and would include more area 

with little available information. Artificial boundaries were 

located (1) to include within the model area the part of the 

eastern SRP aquifer containing detectable concentrations of 

contaminants derived from wastewater discharged at the INL, 

and (2) in areas where information was available to define and 

characterize the boundary and flows across the boundary.

Conceptual Model of Ground-Water Flow  23
Figure 13. Location of the model boundaries, Idaho National Laboratory and vicinity, Idaho.

ID19-0016_fig13

WHITE KNOB

MOUNTAINS

114° 00 ́ 30 ́ 113° 00 ́ 30 ́ 112° 00 ́

43° 30 ́

44° 00 ́

Little

Mud Lake

Camas

Creek

Lost

River

Big Lost River

Sinks and Playas

Big Lost River

Spreading

Areas

Little

Lost

River

Birch

Creek

Mackay

Reservoir

Big

Little Lost

River

Sinks

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

IDAHO

BOISE

EASTERN

SNAKE RIVER

IDAHO PLAIN

NATIONAL

LABORATORY

Twin

Falls

Idaho

Falls

Pocatello

Model

Area

0

0 10

10

20 KILOMETERS

20 MILES

LEMHI

RANGE

LOST

RIVER

RANGE

CONCEPTUAL

MODEL AREA

BITTERROOT

 RANGE

PIONEER

MOUNTAINS

Idaho

National

Laboratory

Atomic

City

Howe

Arco

Mud

Lake

Terreton

B

B ́

Northeast

boundary

Southeast

flowline

boundary

Southwest

boundary

Northwest

mountainfront boundary

B B ́

Southwest

boundary

Unsaturated zone

0

0 10

10

20 KILOMETERS

20 MILES

Northeast

boundary

Base of

the aquifer

Saturated zone

Direction of regional ground-water flow

Water table

5,000

5,500

FEET

4,000

2,000

1,000

3,000

4,500

2,500

1,500

3,500

ALTITUDE, ABOVE

NATIONAL GEODETIC VERTICAL DATUM OF 1929

VERTICAL SCALE GREATLY EXAGGERATED

24  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Physical Boundaries

The water-table boundary is represented by the contact 

between the unsaturated zone and the saturated zone. The 

thickness of the unsaturated zone ranges from about 200 to 

1,000 ft (fig. 10). The altitude of the water table decreases 

about 500 ft across the length of the model area, from 4,600 ft 

at the northeast boundary of the model to about 4,100 ft at 

the southwest boundary of the model (fig. 9). The watertable boundary rises or declines in response to climatic- or 

irrigation-induced changes in recharge to the aquifer.

The base of the aquifer is interpreted as the contact 

between the older rocks and the overlying intermediate-age 

rocks. This contact generally is characterized by an abrupt 

downward increase in intensely altered basalt and interbedded 

sediment (Mann, 1986, p. 4-5; Whitehead, 1992, p. 10; 

Morse and McCurry, 1997 p. 6-7; Anderson and Liszewski, 

1997, p. 28). In seven wells and coreholes, this contact 

also coincides with a change from convective-dominated to 

conductive-dominated heat flow and a decrease in hydraulic 

conductivity and porosity (Mann, 1986, p. 21; Morse and 

McCurry, 1997, p. 6-7; Welhan and Wylie, 1997, p. 99; Morse 

and McCurry, 2002, p. 222; Mazurek and others, 2004). 

Collectively, these factors indicate that water circulation is 

limited in older rocks.

Direct evidence indicating the location of the base of the 

aquifer was limited to data from 13 deep wells and coreholes 

(Mann, 1986, p. 21; Whitehead, 1992, fig. 8 and pl. 6; Morse 

and McCurry, 1997, p. 4-7, fig. 2; Anderson and Liszewski, 

1997, table 3; Mazurek and others, 2004). Morse and McCurry 

(1997, 2002) correlated the base of the aquifer with an 

increase in alteration of basalts and temperature gradient in 

six deep wells where temperature was measured (Blackwell 

and others, 1992). Mazurek and others (2004) made the same 

correlation based on data from a more recently drilled deep 

borehole. Morse and McCurry (1997) estimated depth to the 

base of the aquifer in the central part of the INL to be about 

1,340 ft. Anderson and Liszewski (1997, p. 4), on the basis 

of stratigraphic data from 10 deep wells and coreholes in the 

western half of the INL, suggested that the base most likely 

coincides with the top of a thick and widespread layer of clay, 

silt, sand, and altered basalt that is consistent in age with rocks 

of the Glenns Ferry Formation. They reported a depth to the 

base of the aquifer in the western half of the INL ranging from 

815 to 1,710 ft below land surface and a saturated thickness 

ranging from 445 to 1,200 ft.

Indirect evidence indicating the location of the base of the 

aquifer included regional surface-based electrical-resistivity 

surveys (Whitehead, 1992, pl. 6) and regional gravity surveys 

(Whitehead, 1992, fig. 8). Based on surface-based electricalresistivity soundings and drillhole data, Whitehead (1986, 

sheet 2, section H-H ́) estimated that (1) depth to the base of 

the aquifer could exceed 3,000 ft in the eastern half of the 

INL, and (2) saturated thickness of the aquifer could exceed 

2,500 ft in the eastern half of the INL and 4,000 ft in the 

southwestern part of the model area. Whitehead (1986, sheet 

2) reported saturated thickness using contour intervals of 

500 ft; therefore, the uncertainty of these estimates is at least 

±250 ft. However, because electrical-resistivity surveys tend 

to overestimate basalt thickness (Zohdy, 1974, p. 32), these 

estimates of depth to the base of the aquifer and saturated 

thickness may be high.

Whitehead’s (1986, sheet 2) data were used to calculate 

the depth to the base of the aquifer, rather than data from 

the 13 deep wells and coreholes, because Whitehead’s 

data provide coverage over the entire model area. Using 

Whitehead’s (1986, sheet 2) data for thickness of Quaternary 

basalt and sediment and a digital elevation model, depth to 

the base of the aquifer was calculated using a geographic 

information system. However, depth to the base of the 

aquifer in the area mountainward of the Big Lost River was 

subsequently adjusted to a lesser depth based on stratigraphic 

interpretations that indicate the aquifer is thinner in this area 

(Anderson and Liszewski, 1997, figs. 29, 30). Interpreted 

depth to the base of the aquifer in the model area ranges 

from about 700 to 4,800 ft. Depth is least near the northwest 

mountain-front boundary, increases to the east, and is greatest 

along the southeast-flowline boundary (fig. 14). A shaded 

relief map (fig. 15) of the contact between the intermediateage rocks of hydrogeologic unit 3 and the older rocks that 

form the base of the aquifer indicates that the inferred threedimensional topography along the base of the aquifer is 

irregular.

The northwest mountain-front boundary is defined by 

the edge of the mountain fronts of the Pioneer Mountains, 

Lost River Range, Lemhi Range, and Bitterroot Range and 

the mouths of the Big Lost River, Little Lost River, and Birch 

Creek valleys (fig. 1). This boundary coincides with part of 

the northwest boundary of the eastern SRP as described in the 

RASA study (Garabedian, 1992). Aquifer thickness along the 

northwest mountain-front boundary ranges from about 200 to 

500 ft.

Artificial Boundaries

The northeast boundary is upgradient of the INL in an 

area where the hydraulic gradient increases sharply (figs. 3

and 9). The hydraulic gradient in this area ranges from 27 to 

60 ft/mi and averages about 36 ft/mi (fig. 12A) (Lindholm and 

others, 1988). These steep gradients coincide with changes in 

aquifer transmissivity near Mud Lake where basalt interfingers 

with less transmissive layers of sediment (Crosthwaite, 1973, 

p. 6; Lindholm, 1996, p. 22) and probably are caused by a 

decrease in hydraulic conductivity associated with these finegrained sediments (Nace and others, 1959, p. 151-2; Mundorff 

and others, 1964, p. 133; Lindholm and others, 1988). This 

low-transmissivity zone impedes horizontal flow (Crosthwaite, 

1973, p. 11; Spinazola, 1994, p. 29) and results in a large 

downward vertical component of flow. Aquifer thickness 

along the northeast boundary ranges from about 500 to 800 ft.

Conceptual Model of Ground-Water Flow  25
Figure 14. Depth to the base of the eastern Snake River Plain aquifer in the model area, Idaho National Laboratory and 

vicinity, Idaho.

ID19-0016_fig14

43° 30 ́

44° 00 ́

44° 30 ́

114° 00 ́ 30 ́ 113° 00 ́ 30 ́ 112° 00 ́

Mud Lake

Camas

Creek

Lost

River

Big Lost River

Sinks and Playas

Big Lost River

Spreading

Areas

Little

Lost

River

Birch

Creek

Mackay

Reservoir

Big

Little Lost

River Sinks

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Modified from Whitehead, 1986, sheet 2

IDAHO

BOISE

EASTERN

SNAKE RIVER

IDAHO PLAIN

NATIONAL

LABORATORY

Twin

Falls

Idaho

Falls

Pocatello

Model

Area

0

0 10

10

20 KILOMETERS

20 MILES

EXPLANATION

683-1,000

1,001-2,000

2,001-3,000

3,001-4,000

4,001-4,800

Depth to the base of the aquifer, in 

feet below land surface

WHITE KNOB

MOUNTAINS

LEMHI

RANGE

LOST

RIVER

RANGE

CONCEPTUAL

MODEL AREA

BITTERROOT

 RANGE

PIONEER

MOUNTAINS

Idaho

National

Laboratory

Atomic

City

Howe

Arco

Mud

Lake

Terreton

26  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Figure 15. Contact between the intermediate-age rocks of hydrogeologic unit 3 and the older rocks underlying the base of 

the eastern Snake River Plain aquifer beneath the model area, Idaho National Laboratory and vicinity, Idaho.

86 miles

83 miles

Big Lost River

Little Lost River

Birch Creek

N

BOUNDARY OF IDAHO NATIONAL LABORATORY

MODEL AREA (depth to the base of the eastern Snake River Plain aquifer,

in feet from land surface, is shown in figure 14)

EXPLANATION

IDAHO

Pocatello

Idaho

Falls

Boise

Twin

Falls

Model

area

King

Hill

Idaho

National

Laboratory

Eastern Snake

River Plain

The southeast-flowline boundary is near the central axis 

of the eastern SRP (figs. 3 and 13), and the aquifer thickness 

along this boundary ranges from about 800 to 4,000 ft. The 

boundary is represented as a southwest trending flowline 

derived from pathline analyses (Ackerman, 1995) of the 

steady-state, three-layer RASA model (Garabedian, 1992). 

Ackerman’s model computed three-dimensional pathlines 

that originated as particles introduced near the water table. 

Particles introduced at the water table upgradient of the 

model area followed nearly parallel pathlines along the model 

area’s southeastern boundary (Ackerman, 1995, fig. 8). These 

simulations showed that, although most flow is horizontal and 

contained in the uppermost 500 ft of the aquifer, particles that 

penetrate deeper into the aquifer also follow nearly parallel 

pathlines along this boundary. The sustained parallelism of 

these pathlines implies that little or no lateral divergence of 

flow occurs at depth along the central axis of the eastern SRP 

aquifer (fig. 3) east of the INL. Projections of pathlines to the 

water table indicated that particle pathlines at depth are nearly 

parallel to flowlines oriented perpendicular to water-table 

contours in the central part and along the central axis of the 

eastern SRP aquifer east of the INL (Robertson and others, 

1974; Barraclough and Jensen, 1976; Barraclough and others, 

1981; Lewis and Jensen, 1985; Pittman and others, 1988; Orr 

and Cecil, 1991; Ackerman, 1995, fig. 8; and Bartholomay 

and others, 1995). Therefore, the pathline representing the 

southeast boundary of the model area can be approximated as 

a flowline in a vertical subsurface projection.

The southwest boundary is located so the model includes 

areas of the aquifer downgradient of where contaminants 

were detected and attributed to wastewater discharged at the 

INL (fig. 2). No physical hydrologic features are present that 

could be used as a model boundary in this area. Consequently, 

an artificial boundary was located along a gridline from 

the RASA model (fig. 16) about 25 mi downgradient of the 

southwestern extent of chlorine-36 (36Cl) detections in the 

aquifer (Beasley, 1995, appendix 2). Few wells penetrate the 

aquifer in this area, so the subsurface geology and hydraulic 

properties along the southwest boundary are not well known. 

Aquifer thickness along this boundary ranges from about 500 

to 3,000 ft.

Conceptual Model of Ground-Water Flow  27
Figure 16. Regional Aquifer System Analysis model grid used to approximate the northeast, southeast-flowline, and 

southwest boundaries of the conceptual model, Idaho National Laboratory and vicinity, Idaho.

ID19-0016_fig16

43° 30 ́

43° 00 ́

44° 00 ́

44° 30 ́

114° 00 ́ 113° 30 ́ 113° 00 ́ 112°30 ́ 112° 00 ́

Mud Lake

Camas

Creek

Lost

River

Little

Lost

Birch

Creek

Mackay

Reservoir

Big

River

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

IDAHO

BOISE

EASTERN

SNAKE RIVER

IDAHO PLAIN

NATIONAL

LABORATORY

Twin

Falls

Idaho

Falls

Pocatello

Model

Area

0

0 10

10

20 KILOMETERS

20 MILES

EXPLANATION

Conceptual model area

Row or column in grid from Regional Aquifer System 

Analysis model (Garabedian, 1992)

Trace of cells from Regional Aquifer System Analysis 

model used to approximate the conceptual model area

Grid--Cell with high precipitation-recharge rate in the Regional 

Aquifer System Analysis

Water-table contour--Shows altitude of water table, March 1980 

(modified from Lindholm and others, 1988). Interval 50 feet. 

Datum is NGVD 29

Well or borehole

35

4200

WHITE KNOB

MOUNTAINS

LEMHI

RANGE

LOST

RIVER

RANGE

BITTERROOT

 RANGE

PIONEER

MOUNTAINS

Idaho

National

Laboratory

Atomic

City

Howe

Arco

Mud

Lake

Terreton

ROW

5

10

25

30

COLUMN

35

40

4400

4500

4600

4300

4200

4100

4700

28  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Boundary

Boundary characteristics Estimated 1980 

flow (ft3/s) Type Spatial Temporal

Physical Boundaries

Water-table boundary

Precipitation recharge Inflow Uniform Constant 70

Streamflow-infiltration recharge Inflow Nonuniform Variable 95

Ground-water withdrawals Outflow Nonuniform Variable 45

Industrial wastewater return Inflow Nonuniform Variable 6

Irrigation infiltration Inflow Nonuniform Variable 24

Change in storage Inflow Uniform Variable 80

Base of the aquifer Noflow Uniform Constant 44

Northwest mountain-front boundary Inflow Nonuniform Constant 695

Artificial boundaries

Northeast boundary Inflow Nonuniform Constant 1,225

Southeast-flowline boundary Noflow Uniform Constant 0

Southwest boundary Outflow Nonuniform Variable 2,037

Table 4. Summary of conceptual model boundary characteristics and estimated 1980 inflows and outflows, Idaho 

National Laboratory and vicinity, Idaho.

[Abbreviations: ft3/s, cubic foot per second]

Inflows, Outflows, and Fluxes Across Model 

Boundaries

Inflows, outflows, and fluxes (flow per unit area) across 

the model boundaries (fig. 17 and table 4) were estimated on 

the basis of hydrologic data collected and interpretive studies 

done during the past 50 years and application of Darcy’s Law 

to physical and hydrologic features along these boundaries. 

The past 50 years represent a time when the hydrologic 

conditions of the system were relatively stable compared 

with conditions in the first 50 years of the twentieth century, 

when changes in irrigation practices, climatic variations, 

and surface-water diversions placed greater stresses on the 

system (Garabedian, 1992, p. 31, 50). Inflows and outflows 

across model boundaries were conceptualized as spatially 

uniform or nonuniform, and as temporally constant or variable 

(table 4). These boundary characteristics represent varying 

degrees of simplification that reflect (1) physical features of 

the boundary (for example, the tributary-valleys along the 

northwest mountain-front boundary), (2) hydraulic features 

of the boundary (for example, the water-table gradients along 

the northeast boundary), (3) data limitations that do not justify 

characterization of flow across the boundary in greater detail 

(for example, the base of the aquifer), (4) proximity of flow 

across the boundary to areas of known aquifer contamination 

(for example, industrial wastewater return flows), and (5) 

the magnitude and relative importance of the boundary flux 

to contaminant transport (for example, precipitation and 

streamflow-infiltration recharge).

Streamflow infiltration and underflow from the tributary 

streams and valleys (Big Lost River, Little Lost River, and 

Birch Creek), respectively, provide recharge to the eastern SRP 

aquifer. In the conceptual model, infiltration recharge from the 

tributary streams was treated as flow across the water-table 

boundary, and underflow from the tributary valleys was treated 

as flow across the northwest mountain-front boundary.

Water-Table Boundary

Inflows and outflows across the water-table boundary 

(fig. 13) were treated implicitly1 in the conceptual model and 

comprise five components: (1) diffuse areal precipitation 

recharge; (2) infiltration recharge from streams originating in 

tributary valleys; (3) ground-water withdrawals for irrigation 

and industrial use; (4) wastewater return flows; (5) and 

irrigation-infiltration recharge. Release of water from storage, 

which is an internal flow, is discussed with flows across the 

water-table boundary. Individual flow components may be 

small or zero during some periods, but may be important 

because of their proximity to wastewater discharged in the 

eastern SRP aquifer at the INL.

1

Flow through the unsaturated zone is treated as time-averaged (annual and 

seasonal) net-infiltration recharge to the aquifer. Downward flow in response 

to changes in the saturation-matric potential and saturation-hydraulic 

conductivity relations embodied in the Richards’ equation for unsaturated 

flow is assumed to be included in inflow components of the water budget 

that are areally uniform (precipitation) or areally nonuniform (streamflow 

infiltration, industrial wastewater return flow, and irrigation infiltration) 

and temporally constant (precipitation) or temporally variable (streamflow 

infiltration, industrial wastewater return flow, and irrigation return flow). 

Typical unsaturated-zone flow processes, such as moisture depletion from 

evapotranspiration, downward percolation and lateral redistribution, wetting 

and drying, and perching, are not treated as distinct flow processes.

Conceptual Model of Ground-Water Flow  29
Figure 17. Generalized ground-water budget components for the model area, 1980, Idaho National Laboratory and 

vicinity, Idaho.

ID19-0016_fig17

43° 30 ́

44° 00 ́

44° 30 ́

114° 00 ́ 30 ́ 113° 00 ́ 30 ́ 112° 00 ́

Mud Lake

Camas

Creek

Lost

River

Little

Lost

Birch

Creek

Mackay

Reservoir

Big

River

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

0

0 10

10

20 KILOMETERS

20 MILES

Idaho

National

Laboratory

Atomic

City

Howe

Arco

Mud

Lake

Terreton

4200

4100

4500

4600

4400

4300

WHITE KNOB

MOUNTAINS

LEMHI

RANGE

LOST

RIVER

RANGE

CONCEPTUAL

MODEL AREA

BITTERROOT

 RANGE

PIONEER

MOUNTAINS

EXPLANATION

Regional ground-water flowline

Volumetric flux—Value is rounded to nearest 1 

cubic foot per second. For the Big Lost River, 

the number above the arrow represents 

streamflow-infiltration recharge, and the 

number below the arrow represents underflow

Water-table contour—Shows altitude of water 

table, March 1980 (modified from Lindholm and 

others, 1988). Interval 50 feet. Vertical datum is 

NGVD 29

367

95

4500

Total ground-water

withdrawals

Precipitation

recharge

Change

in storage

95

102

2,037

1,225

0

70

45

80

226

367

Irrigation

infiltration 24

Flow across 

44

base of aquifer

0

Southeastflowline

boundary

Industrial

wastewater

return 6

Southwest

boundary

Northeast

boundary

Mountain-front

underflow

30 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Precipitation Recharge

Although the quantity, temporal variability, and spatial 

distribution of precipitation recharge in the model area are 

not well defined, studies indicate that the net effect of this 

recharge probably is small and is of less importance than that 

of other recharge sources. The effect of precipitation recharge 

in the model area has not been discernable in water-level 

measurements. Mundorff and others (1964, p. 184) estimated 

precipitation recharge for two areas of different soil cover 

in the model area. Precipitation recharge was 0.02 ft/yr in 

the central part of the eastern SRP. Precipitation recharge 

was 0.3 ft/yr near Craters of the Moon National Monument 

(fig. 1), where precipitation is greater and fractured basalt and 

rubble cover the surface. Kjelstrom (1995, p. 11) reported an 

average precipitation recharge of 0.08 ft/yr based on a basinwide water budget for the eastern SRP. Based on 36Cl and 

tritium (3H) profiles and neutron logging, Cecil and others 

(1992, p. 713) estimated that net precipitation recharge at the 

INL ranges from 2 to 5 percent of mean annual precipitation. 

Precipitation at the INL is about 0.7 ft/yr (Clawson and others, 

1989, tables D-1 and D-2; Goodell, 1988, fig. 5), and 2 to 

5 percent of this value would be from 0.01 to 0.04 ft/yr. Using 

a maximum recharge rate of 0.04 ft/yr for the 1,940 mi2 of 

the model area, maximum mean annual precipitation recharge 

across the water table is about 70 ft3/s. A uniform distribution 

of 70 ft3/s of precipitation recharge over the model area would 

result in an infiltration flux of about 0.04 (ft3/s)/mi2. This 

very low areal flux indicates that the effect of precipitation 

recharge on ground-water flow directions and velocities is 

probably very small. In areas where precipitation accumulates 

as runoff into small, closed basins, numerous sedimentary 

interbeds in the thick, unsaturated zone (200-1,000 ft) serve 

to intercept and laterally redistribute downward percolation. 

This redistribution tends to offset effects of local runoff 

accumulation and maintains an approximately uniform and 

constant rate of recharge to the aquifer.

Streamflow-Infiltration Recharge

Streamflow onto the INL is intermittent and nearly all 

streamflow infiltrates through the thick, unsaturated zone 

and recharges the eastern SRP aquifer. Streams tributary to 

the eastern SRP near the INL originate in mountain ranges 

north and west of the model area (fig. 15) and include the Big 

Lost River, the Little Lost River, and Birch Creek. Routine 

streamflow measurements were made by the USGS at several 

streamflow-gaging stations on these streams for more than 

50 years.

Big Lost River

Because of its large flux and proximity to known sources 

and areas of contamination in the aquifer, streamflowinfiltration recharge from the Big Lost River is considered 

more significant to contaminant transport than the other 

components of inflow across the water-table boundary. During 

years with sufficient precipitation, the Big Lost River flows 

onto the eastern SRP near Arco and terminates on the INL 

in a series of four playas southwest of TAN (Kjelstrom and 

Berenbrock, 1996). Water from the river is stored in Mackay 

Reservoir (capacity 44,370 acre-ft; Bennett, 1990) (fig. 1), 

about 40 mi upstream of the INL. In most years, much of the 

water in the river downstream of the reservoir is diverted for 

irrigation or infiltrates through the riverbed before reaching 

Arco (Kjelstrom and Berenbrock, 1996). The average annual 

streamflow of the Big Lost River near Arco for the period of 

record available between 1946 and 2003 was 95 ft3/s (fig. 18, 

gaging station 13132500); (Brennan and others, 2004, p. 196). 

However, streamflow fluctuates greatly in response to shortterm (3 to 8 years) climate cycles. During dry climate cycles, 

streamflow in the Big Lost River near Arco frequently is zero 

(fig. 18). In 1984, however, during a wet climate cycle, the 

mean annual streamflow at the gaging station near Arco was 

488 ft3/s (fig. 18), or more than 20 percent of the estimated 

1980 outflow across the southwest boundary of the model area 

(table 4).

Flow from the Big Lost River is diverted to spreading 

areas southwest of the RWMC near the southwest boundary 

of the INL (fig. 1) to prevent flooding of site facilities. The 

spreading areas were constructed in 1958, first used in 1965, 

and enlarged in 1984 (Bennett, 1990, p. 6). Between 1965 and 

2002, about 47 percent (48 ft3/s, table 5) of the streamflow 

at the INL diversion [(103 ft3/s, table 5), estimated from the 

combined streamflow for gaging stations INL diversion at 

head near Arco (gaging station 13132513, fig. 1) and Big Lost 

River below diversion near Arco (gaging station 13132520, 

fig. 1)] was diverted to the spreading areas. Because the 

spreading areas were used for flood control, during wet 

climate cycles an even larger percentage of streamflow was 

diverted to the spreading areas. For instance, in 1984, the year 

of highest recorded flows, about 83 percent (379 ft3/s, fig. 18, 

and table 5) of streamflow at the INL diversion was diverted 

to the spreading areas. The use of the spreading areas has 

changed the location of much of the streamflow-infiltration 

recharge to the aquifer. Before 1965, the Big Lost River 

recharged mostly in the sinks and playas in the west-central 

part of the INL; currently, when flow extends onto the plain, 

much of it recharges in the spreading areas near the southwest 

boundary of the INL.

Conceptual Model of Ground-Water Flow  31
Figure 18. Mean annual streamflow at selected streamflow-gaging stations on the Big Lost River, Idaho, for 1965-2002.

ID19-0016_fig18

159

45

98

35

149

36

68

36

23

45

64

52

5

0

0

0

1

64

285

379

39

59

10

0

0

0

0

0

0

0

50

5

53

32

31

0

0

0

334

70

233

89

343

135

256

96

55

158

199

89

17

10

11

34

40

238

478

488

121

205

29

3

0

0

0

0

15

0

126

67

179

157

147

24

0

0

564

241

495

297

497

370

449

347

279

453

441

370

193

326

271

350

361

467

591

614

329

438

243

188

190

207

227

169

309

189

443

359

427

366

376

230

173

176

169

12

100

46

190

92

169

44

11

96

121

24

4

1

4

27

126

158

76

61

122

7

0

0

0

0

0

11

0

64

42

102

108

109

0

0

0

4

13132513 Idaho National Laboratory diversion at head near Arco

13132500 Big Lost River near Arco

Estimatedvalues

Average annual streamflow1from 1946 to 2003 = 95 cubic feet per second

13127000 Big Lost River below Mackay Reservoir

13132520 Big Lost River below diversion near Arco

Estimatedvalues

1Calendar year streamflow data for these four gaging stations can be accessed at http://idaho.usgs.gov/.

 0

700600500400300200100

1965

1970

1975

1980

1985

1990

1995

2000

DISCHARGE, IN CUBIC FEET PER SECOND

700600500400300200100 0

DISCHARGE, IN CUBIC FEET PER SECOND

1965

1970

1975

1980

1985

1990

1995

2000

1965

1970

1975

1980

1985

1990

1995

2000

1965

1970

1975

1980

1985

1990

1995

2000

32  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Streamflow-gaging station

Average annual streamflow 

(ft3/s) for 1965 to 2002

Average Maximum

13132513, INL diversion at head 

near Arco 48 379 (1984)

13132520, Big Lost River below 

diversion near Arco 55 190 (1969)

Combined streamflow at gaging 

stations 13132513 and 13132520 103 455 (1984)

Table 5. Summary of streamflow at two streamflow-gaging 

stations on the Big Lost River, Idaho.

[Location of streamflow-gaging stations (for example 13132513) shown in 

figure 1. Average annual streamflow: Calendar year data. Streamflow data 

may be accessed at http://idaho.usgs.gov/. Streamflow data for streamflowgaging stations not on the website are from Bennett, 1990, tables 4 and 6. 

Abbreviations: ft3/s, cubic feet per second; INL, Idaho National Laboratory]

Streamflow onto the INL is spatially concentrated in 

the channel, sinks, playas, and spreading areas (about 5 mi2 

total) (fig. 1). Infiltration losses from the river channel were 

estimated to be from about 1 to 4 (ft3/s)/mi between Arco 

and the Big Lost River Sinks to as much as 28 (ft3/s)/mi 

from the Big Lost River Sinks (Bennett, 1990, p. 24 and 26). 

Infiltration and evaporation rates from the Big Lost River 

playas were estimated to be 0.16 to 0.37 ft/d and 0.01 to 

0.02 ft/d, respectively, and infiltration rates from Big Lost 

River spreading areas A and B were estimated to be 0.7 and 

2.6 ft/d, respectively (Barraclough and others, 1967, p. 24). 

Owing to the much larger rates of infiltration than evaporation, 

all streamflow in the model area was assumed to infiltrate to 

the aquifer.

Large amounts of streamflow during wet climate cycles 

provide a source of episodic streamflow-infiltration recharge 

to the aquifer. Average infiltration flux from streamflow 

is about 20 (ft3/s)/mi2, which is more than two orders of 

magnitude larger than precipitation recharge per unit area. 

However, during 1984, a year of record-high diversions to 

the spreading areas, average infiltration flux at the spreading 

areas was 170 (ft3/s)/mi2 and monthly infiltration fluxes ranged 

from about 80 to about 360 (ft3/s)/mi2 [calculated using mean 

monthly streamflow to the spreading areas (Bennett, 1990, 

table 4) and a combined area for spreading areas A, B, C, and 

D of 2.3 mi2]. Maximum monthly infiltration flux was nearly 

four orders of magnitude larger than precipitation recharge per 

unit area.

Little Lost River

Data were not adequate to estimate infiltration recharge 

from the Little Lost River to the model area; however, most 

streamflow either infiltrates or is diverted for irrigation 

upstream of the INL (Kjelstrom and Berenbrock, 1996, p. 3). 

When the Little Lost River flows onto the eastern SRP, it 

terminates in an area commonly referred to as the Little Lost 

River Sinks, near Howe, just east of the northwest mountainfront boundary of the model area (fig. 1). Stone and others 

(1992, p. 20-21) reported a mean annual streamflow of 77 ft3/s 

for the combined periods of record 1941–81 and 1986–90 at a 

streamflow-gaging station on the Little Lost River about 7 mi 

northwest of Howe (streamflow-gaging station 13119000, 

fig. 1).

Birch Creek

The amount of infiltration from Birch Creek streamflow 

to the eastern SRP aquifer is not well documented because 

streamflow-gaging stations upstream of the INL boundary 

were discontinued or operated intermittently (Kjelstrom 

and Berenbrock, 1996, p. 7-8). Flow from Birch Creek is 

diverted upgradient of the model area to a hydroelectric 

power-generating facility several miles east of Birch Creek. 

Return flows as large as 46 ft3/s from the power facility 

(Ted S. Sorenson, Sorenson Engineering, written commun., 

March 2001) are diverted to the northeast for irrigation use 

for 6 months each year. During the nonirrigation season, this 

water flows onto the northeastern part of the INL, where it 

may infiltrate to the aquifer through diversion channels and 

gravel pits northwest of TAN (Kjelstrom and Berenbrock, 

1996, p. 5). No flow has reached the Birch Creek Sinks near 

TAN since 1969, when accumulation of water in the playas 

prompted construction of channels to divert water away from 

TAN (Kjelstrom and Berenbrock, 1996, p. 5).

Ground-Water Withdrawals, Wastewater Return Flows, 

and Irrigation-Infiltration Recharge

Total ground-water withdrawals and industrial 

wastewater return flows at the INL in 1980 were 45 and 6 ft3/s, 

respectively (fig. 17, table 4). Irrigation-infiltration recharge in 

the model area was estimated at about 24 ft3/s. Ground-water 

withdrawals in the model area include those for industrial use 

at onsite facilities and those for irrigation use offsite. Industrial 

wastewater return flows include wastewater discharged in 

(1) disposal ponds or ditches, where it infiltrates into the 

unsaturated zone and the aquifer, and (2) injection wells 

completed in the aquifer.

Conceptual Model of Ground-Water Flow  33
Industrial ground-water withdrawals and wastewater 

return flows ranged from zero before the INL was established 

to a maximum in 1974 of about 12 ft3/s for withdrawals 

and 7.3 ft3/s for return flows (White, 1975). The volume 

of industrial ground-water withdrawals from the aquifer 

and wastewater discharged in infiltration ponds and wells 

varied over the years in response to the amount of industrial 

activity at INL facilities and changes in wastewater-disposal 

methods (Appendix A). In 1980, total industrial groundwater withdrawals at the INL were about 8.2 ft3

/s, the volume 

of wastewater discharged to ponds and ditches was about 

2.5 ft3/s, and the volume of wastewater discharged to injection 

wells was about 3.4 ft3/s (Batchelder, 1981). Local effects 

of these withdrawals and returns at INL facilities were not 

expected to have long-term effects on aquifer response beneath 

the central part of the INL; however, withdrawals and returns 

at TRA and INTEC could have significant short-term effects 

on contaminant migration. Most contaminants in the aquifer 

beneath the INL originate at these two facilities and more 

than 50 percent of industrial ground-water withdrawals and 

50 percent of industrial wastewater returns were pumped from 

or injected into the eastern SRP aquifer at these two facilities.

Ground-water withdrawals for irrigation increased 

substantially during the 1940s and continued to increase 

until the 1970s (Kjelstrom, 1995, p. 5). For 1980, estimated 

irrigation withdrawals in the model area are 37 ft3/s. Ground 

water used for irrigation in the model area is withdrawn from 

the aquifer primarily near Mud Lake, but smaller quantities 

also are withdrawn near Howe (fig. 1) (Garabedian, 1992, 

pl. 3). Spinazola (1994, p. 33-38) calculated withdrawals 

near Mud Lake based on electrical-power-consumption 

records, irrigation-system characteristics, and hydrologic 

measurements made between 1983 and 1990, and determined 

that ground-water withdrawals for irrigation in the Mud Lake 

area in 1983 (similar to those in 1980) were about 31 ft3/s. 

Based on estimates of irrigated acreage (DeWayne McAndrew, 

U.S. Bureau of Reclamation, written commun., 1996) and the 

volume of water used per acre in the Mud Lake area, an annual 

average of about 6 ft3/s is withdrawn for irrigation near Howe.

Estimates of irrigation-infiltration fluxes for the SRP 

were reported by Goodell (1988) and Spinazola (1994). 

Goodell (1988, p. 48, table 15) estimated average infiltration 

fluxes for ground- and surface-water irrigation for the whole 

SRP to be 0.62 and 4.1 (ft3/s)/mi2, respectively. The infiltration 

flux for surface-water irrigation estimated by Goodell (1988, 

p. 48, table 15) includes conveyance losses between points of 

withdrawal and application that are outside the model area. 

Spinazola (1994, p. 29-33, fig. 38) estimated infiltration fluxes 

at and north of the model area to be 0.15 (ft3/s)/mi2 from 

sprinkler-applied irrigation (primarily from ground-water 

sources) and 0.71 (ft3/s)/mi2 from ditch-applied irrigation 

(from surface-water and diverted ground-water sources). 

Spinazola’s (1994) estimates did not have to be adjusted 

for conveyance losses, which were estimated separately 

from irrigation-infiltration fluxes, and included estimates 

of consumptive use for the location where irrigation was 

applied. Using a geographic-information-system coverage of 

irrigated acreage derived from 1987 aerial photographs and 

field checked in 1992 (DeWayne McAndrew, U.S. Bureau of 

Reclamation, written commun., 1996), about 40 and 26 mi2

of the model area was sprinkler- and ditch-irrigated land, 

respectively. Based on these estimates of irrigated areas and 

Spinazola’s (1994) estimates of irrigation-infiltration fluxes, 

about 24 ft3/s of irrigation-infiltration recharged to the model 

area.

Release of Water from Storage

Release of water from storage in the model area for 1980 

was estimated using measured changes in water levels from 

October 1979 to October 1980 (Garabedian, 1986, pl. 1D). 

Using RASA derived storage coefficients ranging from 0.05 

to 0.2, the calculated net gain to the aquifer from a decrease in 

storage during this period was 60 to 240 ft3/s. Using storage 

coefficients of 0.1 for areas dominated by basalt and 0.2 for 

areas dominated by sediment, the net gain to the aquifer from 

a decrease in storage during this period was 80 ft3/s.

Base of the Aquifer

Because of its low flux and distance from known areas 

of contamination in the aquifer, the base of the aquifer, for 

purposes of contaminant transport modeling, can be treated 

as a no-flow boundary. Upward flow across the base of the 

aquifer was estimated to be about 44 ft3/s based on data from 

a few wells that penetrate the base of the aquifer. Temperature 

data from six deep boreholes indicate that the thermal gradient 

across the base of the aquifer changes from convectivedominated heat flow in the aquifer to conductive-dominated 

heat flow in the rocks below the aquifer (Morse and McCurry, 

2002, p. 215). This change in thermal gradient implies that 

flow across this boundary is small. Mann (1986, p. 22) 

estimated inflow to the aquifer at the INL from underlying 

rocks to be 20 ft3/s on the basis of data from a 10,365-ft-deep 

test hole drilled at the INL. These estimates were based on the 

steepest vertical gradients measured at the INL. Extrapolation 

of that estimate to the 1,940 mi2 of the model area indicates 

an inflow of about 44 ft3/s or a flux of about 0.02 (ft3/s)/

mi2— or about half the flux estimate for precipitation recharge, 

0.04 (ft3/s)/mi2.

Northwest Mountain-Front Boundary

All inflows across the northwest mountain-front 

boundary, conceptualized as nonuniform and constant, consist 

of underflow from alluvial aquifers in the tributary valleys of 

the Big Lost River (367 ft3/s), Little Lost River (226 ft3/s), and 

Birch Creek (102 ft3/s). Because no estimates of underflow 

from the Paleozoic carbonate rocks along the mountain 

fronts were available, and the contribution from mountain 

fronts to underflow across the boundary was assumed to be 

small, the mountain fronts were treated as no-flow sections 

34  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
along the northwest mountain-front boundary. Hydraulic 

gradients abruptly increase near the mouths of the tributary 

valleys along the northwest mountain-front boundary, and 

calculated gradients at the mouths of the Big Lost River, Little 

Lost River, and Birch Creek are 150, 200, and 120 ft/mi, 

respectively. These gradients are relatively constant because 

the variability of the gradients is small compared to the total 

change in gradients. In a study of the water resources in the 

Big Lost River basin, Crosthwaite and others (1970, p. 97–98) 

concluded that the underflow from the Big Lost River basin 

downstream of Arco is nearly constant each year. Mundorff 

and others (1964, p. 185) calculated basin yields for the Big 

Lost River (456 ft3/s), Little Lost River (205 ft3/s), and Birch 

Creek (110 ft3/s). Kjelstrom (1986) calculated mean annual 

basin yields for the Big Lost River, Little Lost River, and 

Birch Creek for water years from 1934 to 1980 of 462, 226, 

and 102 ft3/s, respectively. These values included contributions 

from streamflow, alluvial-valley underflow, and underflow 

from the rocks at the mountain fronts. Subtracting the 95 ft3/s 

estimated average annual streamflow for the Big Lost River 

(Brennan and others, 2004, p. 196) from Kjelstrom’s (1986) 

basin-yield estimate, the underflow from the Big Lost River is 

367 ft3/s.

Northeast Boundary

Underflow across the northeast boundary from the 

regional aquifer, conceptualized as nonuniform and constant, 

was estimated as 1,225 ft3/s for the model area using Darcy’s 

Law, a hydraulic conductivity of 140 ft/d (a small value to 

reflect the presence of the less transmissive sediments), an 

average hydraulic gradient of 36 ft/mi, a boundary length 

of 35 mi, and a saturated thickness of 600 ft. The estimate 

is about 20 percent smaller than the estimate of underflow 

across the boundary (1,500 ft3/s) summed from a flow net of 

the eastern SRP aquifer by Mundorff and others (1964, pl. 4), 

and was used for the conceptual model because it was derived 

from (1) more numerous and better distributed boreholes in 

the area, (2) a longer record of water-level measurements, 

and (3) more numerous measurements of transmissivity and 

hydraulic conductivity for the area (Ackerman, 1991, table 2; 

Spinazola, 1994, p. 41 and fig. 35; Bartholomay and others, 

1997, table 3; Anderson and others, 1999, table 2). The 

average saturated thickness along the northeast boundary of 

the model area was estimated to be about 600 ft on the basis 

of Whitehead’s (1986, sheet 2) interpretation of the thickness 

of basalt and sediment, digital elevation models, and the 

1980 potentiometric surface (Lindholm and others, 1988).

Garabedian (1986, table 13) estimated transmissivity near the 

northeast boundary of 60,000 ft2/d and Norvitch and others 

(1969, fig. 12) estimated 700,000 ft2/d. Using the saturated 

thickness of 600 ft, the corresponding hydraulic conductivity 

ranges from 100 to about 1,000 ft/d. Using the saturated 

thickness of 600 ft and transmissivity values at the northeast 

boundary of 100,000 ft2/d from Mundorff and others (1964, 

pl. 6) and less than 20,000 ft2/d from Newton (1978, table 16), 

the corresponding hydraulic conductivities are less than 200 

ft/d and about 30 ft/d, respectively. Garabedian (1992, pl. 6) 

estimated a range of hydraulic conductivity near the boundary 

of from 55 to 130 ft/d. Anderson and others (1999, table 3) 

reported geometric means and arithmetic means for hydraulic 

conductivity of 140 ft/d and 1,500 ft/d, respectively.

Southeast-Flowline Boundary

The southeast-flowline boundary is treated as a noflow 

boundary. The boundary is represented as a flowline 

projected vertically to depth, and consequently flow across 

this boundary, conceptualized as uniform and constant, is 

presumed to be zero. Water-table maps constructed for the 

eastern SRP aquifer for 1928–30, 1956–58, and 1980 (Stearns 

and others, 1938; Mundorff and others, 1964; and Lindholm 

and others, 1988) as shown by Garabedian (1992, pl. 4) 

indicate that changes in water levels near the INL are spatially 

and temporally uniform. Consequently, flow directions along 

most of the southeast boundary of the model area are relatively 

stable, and flowlines constructed for this area should be 

temporally constant.

Southwest Boundary

Outflow across the southwest boundary (fig. 17) was 

represented as nonuniform and variable underflow to the 

regional aquifer and was estimated at 2,037 ft3/s. This value 

was estimated for the model area using Darcy’s Law and data 

from previous studies, and is similar to the value, 2,000 ft3/s, 

summed for underflow across this boundary from the flow net 

by Mundorff and others (1964, pl. 4).

The true physical length and shape of this boundary are 

not well known and the water-table contours and hydraulic 

gradients in the area are not well defined because head 

definition in this part of the aquifer was limited to water-level 

measurements in only six wells that are from 3 to 10 mi from 

the boundary. The southwest boundary was estimated to be 

about 24-mi long; however, water-table contours indicate that 

flow may not cross some sections of the boundary (Lindholm 

and others, 1988). Whitehead (1986, sheet 2) estimated the 

average saturated thickness of the aquifer in the southwestern 

part of the model area to be about 2,000 ft. Estimates of 

transmissivity in this area range from about 1,700,000 ft2/d 

(Norvitch and others, 1969, p. 37) to about 2,300,000 ft2/d 

(Mundorff and others, 1964, pl. 6). Assuming an aquifer 

thickness of 2,000 ft the corresponding hydraulic conductivity 

ranges from 850 to 1,150 ft/d. Using Darcy’s Law, the smallest 

(850 ft/d) and largest (1,150 ft/d) estimates of hydraulic 

conductivity, a boundary length of about 22 mi (across which 

flow occurs), a hydraulic gradient of 4 ft/mi (fig. 3), and a 

saturated thickness of 2,000 ft, underflow across the southwest 

boundary ranges from 1,731 to 2,343 ft3/s. The average of 

these values, 2,037 ft3/s, was used in the water budget for the 

conceptual model.

Conceptual Model of Ground-Water Flow  35
Ground-Water Budget

A ground-water budget was derived for the conceptual 

model area for calendar year 1980 from estimates of inflows 

and outflows across the model boundaries. A budget residual 

of about 7 percent between inflow and outflow results from 

the uncertainty in the estimates of flow across each boundary 

using data from different sources. These ground-water budget 

components were compared with components calculated for 

the conceptual model area using output from the RASA model 

(table 6) (Garabedian, 1992). Even though the conceptual 

model area covers only about 18 percent of the RASA model 

area (fig. 5), the flows calculated for the conceptual model 

from the RASA model were considered reasonably accurate 

because the RASA model was constrained by measured 

outflows in the Thousand Springs area (fig. 3).

Ground-Water Budget for the Conceptual Model 

Area

Of the inflow components of the ground-water budget 

for the model area, about 8 percent enters at the water table, 

releases from storage account for about 4 percent, upward 

flow across the base of the aquifer contributes about 2 

percent, and about 86 percent enters as underflow along the 

northwest mountain-front and northeast boundaries (table 6). 

Of the 8 percent of the budget that enters the model area at 

the water table, 3 percent is by recharge from precipitation, 

4 percent is from streamflow-infiltration recharge along the 

Big Lost River, and 1 percent is from infiltration of irrigation 

and industrial wastewater return flows. Underflow along the 

northwest mountain-front and northeast boundaries contribute 

31 percent and 55 percent, respectively, of the total inflow into 

the model area. Flow across the southeast-flowline boundary, 

approximated by a pathline projected from depth to the water 

table (Ackerman, 1995), was designated as zero because it 

was represented as a flowline or noflow boundary. At the 

scale of the conceptual model area, this noflow boundary is an 

idealized approximation, but some local flow probably occurs 

across this boundary.

Of the outflow components of the ground-water budget 

for the conceptual model area, about 2 percent is from 

ground-water withdrawals and about 98 percent is underflow 

across the southwest boundary to adjacent parts of the aquifer. 

The ground-water withdrawals are the amount of ground water 

withdrawn for irrigation and industrial uses.

Comparison and Analysis of the Ground-Water 

Budget

For comparison purposes, a ground-water budget for the 

conceptual model area, which lies entirely in the RASA model 

area (fig. 5), was calculated using input and output from the 

transient three-dimensional RASA model simulations (table 6) 

(Garabedian, 1992). The RASA-derived budget was calculated 

for the period 1976–80, a time when hydrologic conditions 

in the eastern SRP aquifer were relatively stable. Flow across 

each conceptual model boundary was calculated by tabulating 

flow values for each regional model cell (4 mi on a side) that 

most closely approximated conceptual model boundaries.

Total inflow and total outflow from the RASA-derived 

ground-water budget exceeded the ground-water budget 

estimates for the conceptual model by 14 and 23 percent, 

respectively (table 6). These percentage differences are 

relatively large. Unlike the conceptual model budget, the 

RASA budget was derived from a transient model simulation 

that, by design, precluded a budget residual because 

differences between inflow and outflow within every grid cell 

were accounted for by changes in storage. Residual in the 

conceptual model budget, 157 ft3/s (table 6), represents the 

equivalent of 6 percent of the inflow and outflow components 

of the RASA-derived budget. To some extent, the 157 ft3/s 

residual is a measure of the cumulative uncertainty of the 

inflow and outflow estimates and may account for some of 

the differences in the inflow (311 ft3/s) and outflow (469 ft3/s) 

estimates of the two water budgets.

Some of the differences in the individual inflow and 

outflow estimates of the two water budgets can be attributed 

to a combination of (1) coarse discretization of the RASA 

model grid, (2) interpolation errors resulting from inflow and 

outflow estimates that are assumed to be uniformly distributed 

within the RASA model grid cells and that accumulate where 

the grid cell faces approximating the conceptual model 

boundaries are offset from the grid cell faces of the RASA 

model (fig. 16), (3) noncompensating summation errors, 

particularly for estimates along the southeast boundary, which 

is oriented parallel to the direction of regional flow and is 

therefore particularly susceptible to this type of error (fig. 16), 

(4) hydrogeologic judgment, and (5) accounting methodology.

The 79 ft3/s difference between the conceptual-model 

and RASA-derived estimates of outflow from groundwater withdrawals reflects accounting limitations owing to 

coarse discretization of the model grid. Most ground-water 

withdrawals for irrigation are in the Mud Lake area, north of 

the northeast boundary of the conceptual model. The grid cells 

in the RASA model that approximate the alignment of the 

northeast boundary of the conceptual model encompass more 

wells than are actually present in the conceptual model area 

(fig. 16) as summed from Spinazola’s (1994) and McAndrew’s 

(U.S. Bureau of Reclamation, written commun., 1996) data.

Interpolation errors were associated with estimates of 

underflow across the south corner of the southwest boundary 

of the conceptual model. Estimates of underflow across this 

boundary, for which data are very limited, are perhaps the least 

reliable of all the water-budget components. These estimates 

were based on application of Darcy’s Law. To account for 

uncertainties in hydraulic conductivities, hydraulic gradients, 

and saturated thickness of the aquifer in the southwestern 

36  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Table 6. Ground-water budget for the conceptual model area for 1980 and derived from the Regional Aquifer System Analysis (RASA) 

model of the Snake River Plain aquifer, Idaho.

[Data from sources listed in “Inflows, Outflows, and Fluxes Across Model Boundaries” section. Flow values are in cubic feet per second (ft3/s). Percentage 

rounded to nearest 1 percent; sum of inflows may not equal sum of outflows because of independent calculation of flows from various data sources, resulting 

in a budget residual. Values derived from RASA (Garabedian, 1992) model simulation output for a transient-state three-dimensional regional flow model for 

1976-80. Symbol: <, less than] 

Inflow

1980 (this report) Derived from RASA model

Flow Percentage of total Flow Percentage of total

Water-table boundary 

Precipitation recharge 70 3 157 6

Big Lost River infiltration recharge 95 4 101 4

Little Lost River infiltration recharge 0 0 17 1

Industrial wastewater return 6 <1 0 0

Irrigation infiltration 24 1 0 0

Change in storage 80 4 145 6

Base of the aquifer 44 2 0 0

Northwest mountain-front boundary 

Big Lost River underflow 367 16 366 15

Little Lost River underflow 226 10 212 8

Birch Creek underflow 102 5 103 4

Mountain-front underflow 0 0 48 2

Northeast boundary 1,225 55 1,207 47

Southeast-flowline boundary 0 0 194 8

Total 2,239 100 2,550 100

Percentage difference, 1980 and RASA inflow 14 

Outflow 

Water-table boundary 

Ground-water withdrawals 124 5

Irrigation 37 2

Industrial 8 <1

Southwest boundary 2,037 98 2,427 95

Total 2,082 100 2,551 100

Budget residual 157 1

Percentage difference between inflow and outflow 7 0

Percentage difference, 1980 and RASA outflow 23

part of the model area, various estimates of these factors were 

used to compute underflow estimates. Underflow estimates 

ranged from 1,731 to 2,343 ft3/s; the average of this range, 

2,037 ft3/s, was used for the conceptual model. This range 

represents a difference of 612 ft3/s, or about 25 percent of 

the 2,427 ft3/s RASA-derived estimate for underflow across 

the southwest boundary. The distribution of flow along the 

22-mi length of the southwest boundary of the conceptual 

model, for each vertical stack of cell faces in the three-layer 

RASA model, ranges from a minimum of 1.2 ft3/s at the north 

corner of this boundary to a maximum of 606 ft3/s at the south 

corner, and averaged 404 ft3/s. Because the alignment of the 

southwest boundary of the conceptual model is coincident 

with a grid line in the RASA model (fig. 16), interpolation 

errors were limited to the south corner of the boundary, where 

interpolation of the 606 ft3/s outflow across the vertical stack 

of cell faces at this corner of the boundary can introduce a 

substantial difference in outflow estimates.

Noncompensating summation errors were the likely 

source of budget differences for the southeast-flowline 

boundary, which was treated as a noflow boundary in 

the conceptual model. Inflows across the southeastConceptual Model of Ground-Water Flow  37
flowline boundary of the conceptual model were based 

on the summation of interpolated flows into and out of a 

stair-stepped array of 19 three-layer stacks of cells consisting 

of 63 cell faces in the RASA model (Garabedian, 1992) that 

approximate the southeast boundary of the conceptual model 

(fig. 16). Along this boundary, 10 three-layer vertical cell 

stacks produce a net outflow and 11 produce a net inflow. 

Flow into or out of a single three-layer vertical stack of cell 

faces along this boundary ranges from a minimum of 9 ft3/s 

to a maximum of 594 ft3/s and averages 125 ft3/s. Summation 

of inflows and outflows across this boundary were susceptible 

to both interpolation errors and summation errors that 

probably are not fully compensating, and these kinds of errors 

could have resulted in the 194 ft3/s difference between the 

conceptual-model inflow estimates and the RASA-derived 

inflow estimates (table 6).

Precipitation-recharge estimates for the RASA-derived 

budget were 87 ft3/s larger than those for the conceptual model 

budget, but most of this difference represents a 59-percent 

allocation (92 ft3/s) of the total precipitation recharge 

(157 ft3/s) in the RASA-derived budget to the southwest corner 

of the conceptual model, an area that represents 18 percent 

(352 mi2) of the conceptual model area (fig. 16). In the RASA 

model, this allocation represents a precipitation-recharge flux 

of 0.26 (ft3/s)/mi2. This flux is more than six times larger than 

estimates for the remainder of the conceptual model area, 0.04 

(ft3/s)/ mi2. The larger precipitation-recharge estimate for this 

area in the RASA model was attributed to the presence of bare 

basalt outcrops and the assumed effects of higher altitude on 

precipitation. However, there are few data to justify this large 

estimate.

Other noteworthy differences in the two groundwater budgets are attributed to differences in accounting 

methodology. These include (1) an irrigation infiltration 

component of 24 ft3/s in the conceptual model budget that was 

accounted for in the surface-water irrigation component of the 

RASA-derived budget, (2) flow across the base of the aquifer 

that was assumed to be zero in the RASA-derived budget but 

was estimated to be as large as 44 ft3/s in the conceptual model 

budget, and (3) northwest mountain-front boundary underflow 

of 48 ft3/s in the RASA-derived budget that was not accounted 

for separately in the conceptual model budget.

Inflows from the northwest mountain-front boundary, 

the northeast boundary, and streamflow infiltration are similar 

in the two budgets. Combined inflows of 2,054 ft3/s for these 

components in the RASA-derived budget compare favorably 

with combined inflows of 2,015 ft3/s for these components in 

the conceptual model budget.

Differences in the two water budgets emphasize the 

probable magnitude of the uncertainty associated with current 

water-budget estimates. This uncertainty is approximately 

±7 percent of the average value of the total inflow (2,395 

ft3/s) and ±10 percent of the average value of the total outflow 

(2,317 ft3/s) of the two budgets. These averages do not include 

any adjustments for the residual flow shown in table 6.

Ground-Water Movement

Water-table maps, based primarily on water levels from 

the upper 200 ft of the of the eastern SRP aquifer, indicate that 

the direction of regional ground-water flow is from northeast 

to southwest (fig. 3) (Garabedian, 1992, fig. 26). Water-table 

maps for the central part of the INL, however, indicate ground 

water in this area flows southward and southeastward. Locally, 

flow directions in the INL vary from southeast to southwest 

in response to episodic recharge from streamflow-infiltration. 

Flow increases progressively in a direction downgradient 

of the northeast boundary because of mountain-front and 

tributary-valley underflows along the northwest boundary 

and precipitation recharge and streamflow-infiltration 

recharge across the water-table boundary. Ground-water flows 

through all three hydrogeologic units beneath the INL. In the 

northern part of the INL and south of the INL, the younger 

rocks (hydrogeologic units 1 and 2) are either absent or are 

above the water table and all flow takes place through the 

slightly altered basalts composing the intermediate-age rocks 

(hydrogeologic unit 3).

Horizontal hydraulic gradients and the direction and 

velocity of horizontal ground-water movement have been 

inferred from water-table maps and interpretations of waterchemistry data and are primarily representative of the upper 

200 ft of the aquifer in the model area. The earliest water-table 

maps for the eastern SRP aquifer were compiled during 

1928–30 (Stearns and others, 1938) and, over the 

past 50 years, tens of thousands of aquifer water-level 

measurements were collected by the USGS. In 2004, for 

example, about 670 water levels were measured at 172 

aquifer wells and 36 perched-water wells in the model area. 

Water-chemistry data were routinely collected by the USGS 

in the model area since 1950. In 2004, for example, water 

samples were collected at 133 aquifer wells, 26 perchedwater wells, and 7 surface-water sites. Water-chemistry data 

referred to in this report include (1) major and trace elements, 

(2) stable isotope ratios, (3) isotopes resulting from water-rock 

interactions, (4) atmospheric tracers, and (5) concentrations or 

ratios of contaminants discharged at facilities.

Water moves horizontally in the aquifer principally 

through porous interflow zones between basalt flows rather 

than in the massive interiors of basalt flows (Whitehead, 1992, 

p. 26). Because interflow zones are more prevalent in the 

densely fractured basalt of hydrogeologic unit 1 than in the 

38  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
massive, less densely fractured basalt of hydrogeologic unit 2, 

the amount of water moving through hydrogeologic unit 1 is 

probably much larger than that moving through hydrogeologic 

unit 2.

Ground water moves vertically in the aquifer principally 

through fracture systems (Mann, 1986, p. 21). Water 

also moves vertically through fine-grained sediment and 

massive basalts, but vertical movement probably is impeded 

substantially by these relatively horizontal, impermeable 

layers. Few data were available to define the vertical hydraulic 

gradients and the vertical movement of ground water in the 

aquifer. The data that were available were (1) measurements 

of hydraulic head at several depths during drilling and testing 

of the 10,365-ft deep INEL 1 test hole (fig. 1), (2) water-level 

measurements from dedicated piezometers located at various 

depths in wells USGS 30 and Hwy 1 (fig. 1), and (3) intraborehole flow measurements of upward or downward groundwater flow in 16 wells located near TAN, the Naval Reactors 

Facility (NRF), the TRA, and the INTEC (fig. 1).

Horizontal Hydraulic Gradient

Horizontal hydraulic gradients vary spatially across 

the model area in response to local changes in hydraulic 

conductivity, aquifer thickness, and recharge. These watertable gradients were defined by measurements of water levels 

in 201 aquifer wells in the model area, of which 175 wells are 

in the INL. A few wells have multiple completions, and of 

the 207 well completions 158 penetrate only the upper 200 ft 

of the aquifer. At a regional scale, water-table gradients have 

remained relatively stable during the past 70 years (see watertable maps in Stearns and others, 1938; Garabedian, 1992, 

plate 4; and Bartholomay and others, 2000, fig. 9). Locally, 

however, the gradients may fluctuate annually in response to 

streamflow-inflitration and irrigation recharge and cyclically 

over 3- to 8-year periods in response to climate variation and 

its effect on the quantity of streamflow-infiltration recharge.

Water-table gradients range from about 1 to 60 ft/mi 

(fig. 17) in the model area and from about 1 to 8 ft/mi beneath 

the INL (fig. 12), and precise definition of flow direction 

beneath the INL is difficult owing to the small gradient. 

Gradients near the northeast boundary of the model area range 

from 27 to 60 ft/mi (fig. 16) (Lindholm and others, 1988). 

These large gradients probably are caused by a decrease in 

hydraulic conductivity associated with fine-grained sediments 

(Nace and others, 1959, p. 151; Mundorff and others, 1964, 

p. 133; Lindholm and others, 1988). The gradient in the 

northern part of the INL is poorly defined, but was estimated 

to be about 1 ft/mi near TAN (figs. 1 and 12) (Anderson and 

Liszewski, 1997, p. 20; Sorenson and others, 2000, p. 340). 

In much of the central part of the INL the gradient is about 

6 to 8 ft/mi, and this larger gradient probably reflects the 

larger amount of sediment and the resulting smaller hydraulic 

conductivity in this area (fig. 12) (Anderson and Liszewski, 

1997, p. 20-21). Water-level contours (fig. 12) indicate a 

gradient of about 2 to 4 ft/mi in most of the southern and 

eastern parts of the INL, except in the area north and east 

of Middle and East Buttes, where gradients are about 4 to 

6 ft/mi. The smaller gradient in the southern and eastern parts 

of the INL may reflect increased transmissivity resulting 

from less sediment and greater aquifer thickness in this area 

(figs. 12 and 14), and the larger gradient north and east of 

Middle and East Buttes probably results from the smaller 

hydraulic conductivity associated with the silicic rocks in 

this area. Gradients southwest of the INL range from about 

4 to 30 ft/mi (fig. 17) (Lindholm and others, 1988). The 

larger gradients may result from structural uplift in this area, 

perhaps related to differential subsidence or faulting, or the 

emplacement of laccoliths and domes within the stratigraphic 

section (Anderson and others, 1997, p. 5 and fig. 3), causing 

the relatively impermeable basalts of hydrogeologic unit 2 

to intersect the water table (figs. 9 and 10). Alternatively, 

Lindholm and others (1988) noted that this large gradient is 

immediately upgradient of, and generally parallel to, a zone 

of smaller transmissivity. This zone of smaller transmissivity 

may be attributed to the healing of fractures or the presence 

of numerous, closely spaced dikes that may be related to 

the nearby and downgradient Great Rift volcanic rift zone 

(Lindholm and others, 1988; Anderson and others, 1999, 

p. 26). In either case, larger gradients would be necessary 

to move water through the low-permeability basalts of 

hydrogeologic unit 2 or the low-permeability rocks associated 

with the rift zone.

Flow Directions in the Central Part of the Idaho 

National Laboratory

Although ground water moves southwestward regionally, 

ground water in the central part of the INL was inferred to 

flow southward and southeastward on the basis of water-table 

contours (fig. 12), hydraulic-gradient analyses from waterlevel triangulations (Michael Rohe, Bechtel, Babcock, and 

Wilcox, Idaho, written commun., 2000), and interpretation of 

water-chemistry data (Sorenson and others, 1996, figs. 2-23; 

Johnson and others, 2000, p. 873; Luo and others, 2000, 

fig. 9b; Busenberg and others, 2001, fig. 25). The southward 

and southeastward flow directions may result from the flow 

directions of tributary valley and regional underflow and 

directional changes in aquifer transmissivity.

Conceptual Model of Ground-Water Flow  39
The variation in flow direction may be attributable to the 

two principal sources of water to the aquifer beneath the INL: 

(1) regional underflow from the northeast (55 percent of total 

estimated inflow for 1980; table 6) and (2) tributary-valley 

underflow (Big Lost River, Little Lost River, and Birch Creek 

valleys) from the northwest (31 percent of total estimated 

inflow for 1980; table 6). Underflow from the Little Lost 

River and Birch Creek valleys (15 percent of total estimated 

inflow for 1980; table 6) enters the eastern SRP aquifer in 

the north-central and northern parts of the INL (fig. 17), 

respectively. These valleys trend southeastward and, given 

the large hydraulic gradients at the mouths of these valleys, 

underflow entering the eastern SRP aquifer from these valleys 

would be expected to flow in the same direction. When the 

tributary valley underflow converges with regional underflow, 

which moves in a southwesterly direction, the combined 

gradients driving these flows may produce flow that moves in 

a southerly direction.

Evidence that tributary-valley and regional underflow 

converge in the central part of the INL is provided by the 

chemical signatures of ground water in this area. Regionalsource and tributary-valley source ground water are 

distinguishable beneath the INL because these ground waters 

originate in silicic and carbonate aquifers, respectively, 

and contain distinctive chemical signatures (Olmstead, 

1962, p. 37-38; Robertson and others, 1974, p. 42-55). 

Concentrations of major ions (potassium, sodium, calcium, 

magnesium, fluoride, bicarbonate), silica, trace elements 

(lithium, boron, strontium), and stable isotope ratios (delta 

oxygen-18, delta carbon-13) in waters from the eastern SRP 

aquifer (Olmstead, 1962, p. 37-38; Robertson and others, 

1974, p. 42-55; Busenberg and others, 2001, figs. 3-5 and 

35-36), indicate that ground water beneath the southeastern 

part of the INL retains the chemical signature of regional 

ground water and ground water beneath the northwestern part 

of the INL retains the chemical signature of ground water from 

the tributary valleys. For example, lithium (Li) concentrations 

beneath the southeastern part of the INL are greater than 

5 μg/L, similar to those in regional ground water, and beneath 

the northwestern part of the INL they are less than 5 μg/L, 

similar to those in tributary-valley ground water (fig. 19). The 

lithium line in figure 19 separates the area of Li concentrations 

in the aquifer attributed to regional ground water (greater than 

5 μg/L) from that attributed to tributary-valley ground water 

(less than 5 μg/L), and the line may indicate the area where 

regional and tributary-valley ground water converge.

Ground water also was interpreted to flow southward 

through the central and northern parts of the INL by 

identifying water-rock interactions that produced 

chemically-evolved isotope concentrations (uranium and 

thorium decay-series disequilibria: uranium, thorium, 

and radium isotopes) (Luo and others, 2000, fig. 9b) and 

distinctive stable isotope ratios (strontium-87/strontium-86) 

(Johnson and others, 2000, fig. 2a). Fast-flow zones were 

interpreted to extend south from the mouths of the Little Lost 

River and Birch Creek and slow-flow zones were interpreted 

to be present in the western and central parts of the INL. These 

fast- and slow-flow zones, however, may not be consistent 

with ground-water flow velocities calculated by Busenberg 

and others (1993, 2001), as shown on figure 20, on the basis 

of model ages of water using atmospheric tracers (3H/3He, 

chlorofluorocarbon) and contaminant ratios (3H/3He). The 

model ages indicated slower flow (2 to 8 ft/d) between the 

mouth of the Little Lost River and the INTEC and faster 

flow (10 to 14 ft/d) in the central part of the INL (fig. 20) 

(Busenberg and others, 1993, p. 30; Busenberg and others, 

2001, fig. 25). Busenberg and others (2001, p. 54) state that 

the results of their study are consistent with the interpretation 

of a fast-flow zone in the central part of the INL by Luo and 

others (2000, fig. 9b) and Johnson and others (2000, fig. 2a). 

Although the results of Busenberg and others (2001, fig. 25) 

support a conclusion that fast- and slow-flow zones exist in the 

aquifer beneath the INL, the fast-flow zone in the central part 

of the INL indicated by Busenberg and others (2001, fig. 25) 

is not located within the fast-flow zones indicated by Luo and 

others (2000, fig. 9b) and Johnson and others (2000, fig. 2a).

The transition from southerly flow in the central part 

of the INL to regional, southwesterly flow also may reflect 

differences in aquifer transmissivity beneath the INL. The 

aquifer is thinner near the mountain fronts, thickens to the 

southeast, and is thickest along the southeast model boundary 

(fig. 14). These aquifer thicknesses and the distribution 

of hydrogeologic units and hydraulic properties described 

in the section, “Hydrogeologic Framework and Hydraulic 

Properties,” (figs. 9 and 12, table 2), indicate that the 

transmissivity of the aquifer probably is (1) smaller in the 

northern and southwestern parts of the INL, (2) larger in the 

central and eastern parts of the INL, and (3) much smaller in 

the northwestern than in the southeastern part of the INL. This 

smaller-to-larger northwest-to-southeast difference in aquifer 

transmissivity indicates that there is less regional underflow 

through the northwestern than through the southeastern part 

of the aquifer. The lesser volume of regional underflow in the 

northwestern part of the INL probably allows the volume and 

direction of tributary-valley underflow to exert a relatively 

large influence on shallow flow directions in the northern and 

central part of the INL.

40 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Figure 19. Distribution of lithium concentrations in water from wells in the eastern Snake River Plain aquifer, Idaho 

National Laboratory and vicinity, Idaho.

Water samples were collected between 1995 and 1997.

ID19-0016_fig19

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Mud

Lake

Big

Lost

River

Little

Lost

River

Birch

Creek

Camas

Creek

Big

Southern

Butte

East

Butte

Middle

Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

IDAHO NATIONAL 

LABORATORY BOUNDARY

113°15’ 113°00’ 45’

44°00’

43°30’

45’

30’ 112°15’

Atomic

City

Howe

Arco

Mud

Lake Terreton

EXPLANATION

Tributary valley inflow and regional underflow—Indicates 4400

source areas of streamflow and underflow from the Birch 

Creek, Little Lost River, and Big Lost River tributary valleys 

and regional ground-water underflow

Lithium line—Line divides areas of lithium (Li) concentration 

of less than and greater than 5 micrograms per liter 

(Busenberg and others, 2000). Approximately located, 

queried where uncertain. Concentrations to the northwest 

and southeast of the line are similar to those in tributaryvalley and regional ground water, respectively

Water-table contour—Shows altitude of water table in 1995. 

Intervals 10 and 100 feet. Datum is NGVD 29. Some contours 

approximately located. (Bartholomay and others, 1997)

Well—Water in well contains lithium (Li) concentration of 

less than 5 micrograms per liter (<5 μg/L)

Well—Water in well contains lithium (Li) concentration of 

more than 5 micrograms per liter (>5 μg/L)

0 5 10 MILES

0 5 10 KILOMETERS

4600

4500

4200

4300

4400

<5 μg/L of Lithium

>5 μg/L of Lithium

Birch

Creek

Little Lost

River

Big Lost

River

Regional

ground-water

underflow

?

?

Conceptual Model of Ground-Water Flow  41
Figure 20. Average linear ground-water velocities and flow directions calculated from the assumed first arrivals 

of chlorine-36 or the tritium/helium-3- and chlorofluorocarbon-model ages of water from selected wells, Idaho 

National Laboratory and vicinity, Idaho.

TAN, Test Area North; NRF, Naval Reactors Facility; TRA, Test Reactor Area; INTEC, Idaho Nuclear Technology and 

Engineering Center.

ID19-0016_fig20

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Modified from Busenberg and others, 2001, fig. 25. 

Ground-water data from Busenberg and others, 1993; 

Cecil and others, 2000; Busenberg and others, 2001

0 5 10 MILES

0 5 10 KILOMETERS

Mud

Lake

Big

Lost

River

Little

Lost

River

Birch

Creek

Camas

Creek

Big

Southern

Butte

East

Butte

Middle

Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

113°15’ 113°00’ 45’

44°00’

43°30’

45’

30’ 112°15’

Atomic

City

Howe

Arco

Mud

Lake Terreton

PSTF

2 ft/d

USGS 102

USGS 97 2 ft/d

3 ft/d

USGS 98

5 ft/d

USGS 99

8 ft/d

USGS 121

5 ft/d

USGS 100

15 ft/d

USGS 1

18 ft/d

USGS 12

5 ft/d USGS 17

10 ft/d

USGS 5

11 ft/d

NPR Test

14 ft/d

USGS 86

4 ft/d

USGS 9

7 ft/d

USGS

125

10 ft/d

Crossroads

Well

10 ft/d

USGS 11

12 ft/d

USGS 8

3 ft/d

USGS 103

4 ft/d

USGS 14

20 ft/d

USGS 124

8 ft/d

USGS 105

6 ft/d

USGS 109

7 ft/d

EXPLANATION

USGS 12

5 ft/d

Ground-water-flow direction based on location of 

well sampled and interpreted source of ground water

Well in the USGS water-level monitoring network and

identifier with calculated ground-water flow velocity, 

in feet per day (ft/d)

TAN

TRA

INTEC

NRF

Idaho

National

Laboratory

42  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Effect of Episodic Recharge from the Big Lost 

River on Variation of Local Flow Directions

Flow directions beneath the southwestern part of the 

INL vary temporally because of gradient reversals produced 

by episodic streamflow-infiltration recharge from the Big 

Lost River (Barraclough and others, 1976, p. 52-56). Flow in 

the Big Lost River is usually largest in the spring and early 

summer when snow melts in the high-altitude river basin and, 

during years with sufficient precipitation, runoff reaches the 

central and southwestern parts of the INL where it infiltrates 

and recharges the aquifer. During wet years, flow in the Big 

Lost River in the central part of the INL was more than three 

times (190 ft3/s) the average annual streamflow (55 ft3/s) and 

flow diverted to the spreading areas was more than seven 

times (379 ft3/s) the average annual streamflow (48 ft3/s) 

(table 5; streamflow-gaging stations 13132520 and 13132513, 

respectively). This large amount of cold surface-water 

recharge probably acts as a persistent heat sink in the aquifer 

and could have depressed ground-water temperatures in these 

areas (fig. 21).

Pulses of streamflow-infiltration recharge have created 

large localized water-level changes and water-table mounds 

(fig. 22) in the low-gradient area beneath the central INL 

(fig. 17). For example, in the early 1980s larger than normal 

streamflow infiltrated and recharged the aquifer during a 

wet climate cycle. The water table rose unevenly beneath the 

INL, and water-table mounds formed near the Big Lost River 

spreading areas and in the area north of the NRF. The water 

table fell unevenly during a dry cycle in the late 1980s and 

early 1990s, when there was no streamflow recharge, and the 

water-table mounds dissipated. Water-table mounds were less 

pronounced during a wet cycle in the late 1990s when the 

water table rose moderately.

Nonuniform water-level changes are typical beneath the 

INL, with the largest changes occurring near or beneath areas 

of episodic recharge. Hydrographs of water levels in well 

USGS 9 near the spreading areas and well USGS 12 north 

of the NRF show water-level rises from 13 to 16 ft during 

and following the wet climate cycle in the early 1980s, and 

declines from 16 to 22 ft during the dry climate cycle of the 

late 1980s and early 1990s (fig. 23). The water level at well 

USGS 9 reached its highest peak of the period 1948–2003 in 

1984 (fig. 23), the second year of a 2-year period of recordhigh diversions to the spreading areas (fig. 18, gaging station 

13132513). The water level in well USGS 9 dropped rapidly 

after diversions to the spreading areas decreased during 

1985. In contrast, the high water levels in well USGS 12 that 

persisted from 1985 through 1987 (fig. 23) reflect the more 

uniform flow in the Big Lost River downstream of the INL 

diversion dam (fig. 18, gaging station 13132520) between 

1982 and 1986. During the wet climate cycle of the early 

1980s, the rise in water levels at wells USGS 25 and Arbor 

Test, in the northern and southeastern parts of the INL (fig. 1), 

was about 7 and 11 ft, respectively (fig. 23). The smaller 

increase in water levels at these wells in response to the wet 

climate cycle probably reflects their greater distance from 

the localized pulses of streamflow-infiltration recharge. A 

similar but smaller rise in water levels at these four wells 

was recorded during the wet climate cycle of the late 1990s 

(fig. 23).

Ground water in and near a water-table mound would 

be expected to flow radially away from the mound and, 

at the INL, water east of the mound would be expected to 

flow southward and southeastward around the mound. An 

example of how local flow directions may vary, and may 

even reverse direction owing to water-table mounds was 

presented by Barraclough and others (1976, p. 52-56 and 

fig. 33). On the basis of water-level measurements in the 

vicinity of the RWMC during 1972, Barraclough and others 

(1976, p. 52-56 and fig. 33) inferred that ground water flowed 

away from and around a water-table mound near the Big Lost 

River spreading areas (fig. 1). They noted that the northeast 

direction of flow indicated by these data was counter to the 

prevailing southwestern regional flow direction, and suggested 

that streamflow-infiltration recharge from the Big Lost River 

spreading areas appeared to be the predominant influence on 

this anomalous flow direction.

The variation in the direction of ground-water flow in 

response to pulses of streamflow-infiltration recharge may 

be large, perhaps tens of degrees. Simulated flow directions 

in a flow-and-transport model (Goode and Konikow, 1990b, 

p. 420, fig. 2) varied by as much as 20 degrees, between 

N. 70° E. to N. 90° E., about 700 ft downgradient of the 

disposal well (CPP 3) at the INTEC in response to variable 

streamflow-infiltration recharge from the Big Lost River. 

Larger, unreported, variations in flow directions may have 

been present at other locations in the model. Flow directions 

west of INTEC, near the Big Lost River, were interpreted to 

range from about N. 80° E. to N. 280° W. during 1976–1998 

based on a study evaluating the temporal fluctuation of 

hydraulic gradients at the INL (Michael Rohe, Bechtel, 

Babcock, and Wilcox, Idaho, written commun., 2000).

Conceptual Model of Ground-Water Flow  43
Figure 21. Temperature of ground water in samples from wells, Idaho National Laboratory and vicinity, Idaho.

ID19-0016_fig21

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Modified from Busenberg and others, 2001, figure 31

14

14

18

14

10

14

10

10

14

Mud

Lake

Big

Lost

River

Little

Lost

River

Birch

Creek

Camas

Creek

Big

Southern

Butte

East

Butte

Middle

Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

IDAHO NATIONAL 

LABORATORY BOUNDARY

113°15’ 113°00’ 45’

44°00’

43°30’

45’

30’ 112°15’

Atomic

City

Howe

Arco

Mud

Lake Terreton

11.3

15.2

11.5

12.0

18

10

18.8 15.5 13.9

 13.4

14.0

14.9

13.4

13.7

11.3

14.3

19.8

11.8

11.6

12.3

12.5

12.3 14.9

13.7

14.9

13.6

14.2

9.6

9.9

12.1 12.1

11.8

11.6

14.5

12.0 13.4

14.8

12.8

14.6

15.4

10.1

16.9

13.9

13.5

13.7

13.2

10.4

(16.7)

(13.9) (12.8)

(14.2)

(12.8)

(13.8)

(14.4)

(10.0)

(13.3)

16.3

11.3

15.6

15.7 12.8

11.1

14.6

14.1

9.5

8.2

7.0

10.0

11.7

 13.3

12.3

11.6

11.8 13.8

14.0

18.8

12.8

12.2

13.2

11.0

11.4

10.0

11.2

EXPLANATION

11.6

Zone of low ground-water temperature

within the eastern Snake River Plain 

aquifer

Line of equal water temperature, 

(1991-96)—Interval 4 degrees Celsius

Well in the USGS water-level monitoring 

network—Number is the ground-water 

temperature in degrees Celsius. 

Temperatures in parentheses are from 

Olmstead (1962).

10

0 5 10 MILES

0 5 10 KILOMETERS

44  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Figure 22. Generalized water-level changes at the Idaho National Laboratory and vicinity, Idaho, for July 1981–July 

1985, April–August 1988 to April–August 1991, March–May 1991 to March–May 1995, and March–May 1995 to March–

May 1998.

ID19-0016_fig22

Base from U.S. Geological Survey digital data, 1:24,000 and 1:100,000

Universal Transverse Mercator projection, Zone 12

Datum is North American Datum of 1927

Modified from Bartholomay and others, 1997, fig. 10. Modified from Bartholomay and others, 2000, fig. 10.

Modified from Pittman and others, 1988, fig. 10. Modified from Bartholomay and others, 1995, fig. 10.

CONCEPTUAL

MODEL AREA

CONCEPTUAL

MODEL AREA

CONCEPTUAL

MODEL AREA

CONCEPTUAL

MODEL AREA

Mud

Lake

Big Lost River

Little

Lost

River

Birch

Creek

Camas

Creek

Mud

Lake

Big Lost River

Little

Lost

River

Birch

Creek

Camas

Creek

Mud

Lake

Big Lost River

Little

Lost

River

Birch

Creek

Camas

Creek

Mud

Lake

Big Lost River

Little

Lost

River

Birch

Creek

Camas

Creek

Big

Southern

Butte

East Middle Butte Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

Big

Southern

Butte

East Middle Butte Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

Big

Southern

Butte

East

Butte Middle

Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

Big

Southern

Butte

East

Butte Middle

Butte

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

Idaho

National

Laboratory

Idaho

National

Laboratory

Idaho

National

Laboratory

Idaho

National

Laboratory

113°15’ 113°00’

44°00’

43°30’

45’

44°00’

43°30’

45’

112°15’ 113°15’ 113°00’ 112°15’

Atomic

City

Howe

Arco

Mud

Lake Terreton

Atomic

City

Howe

Arco

Mud

Lake Terreton

Atomic

City

Howe

Arco

Mud

Lake Terreton

Atomic

City

Howe

Arco

Mud

Lake Terreton

Line of equal water-level change—Interval 2 feet. Negative value 

indicates water-level decline. Hachures indicate water-level 

change is less than the contour value

Well and identifier

EXPLANATION

USGS 9

2

0 5 10 MILES

0 5 10 KILOMETERS

Generalized water-level 

rise, July 1981 to July 1985

Generalized water-level 

decline, April–August 

1988 to April–August 1991

Generalized water-level 

decline, March–May 1991 

to March–May 1995

Generalized water-level rise, 

March–May 1995 to MarchMay 1998

-8

-10

-8

10

8 10

4

2

2

4 6

6

6

4

4

2

-6

4

-4

-4

-6

4

-6

4

4

8

6

6

USGS 9

USGS 12

USGS 25

USGS 25 USGS 25

USGS 9

USGS

12

Arbor

Test

USGS 25

USGS 9

USGS

12

Arbor

Test

Arbor

Test

Arbor

Test

USGS 9

USGS 12

Conceptual Model of Ground-Water Flow  45
Average Linear Ground-Water Velocities

The estimated average linear ground-water velocities 

in the model area range from 2 to 20 ft/d (fig. 20) and were 

determined from concentrations or ratios of atmospheric 

tracers (3H/3He, chlorofluorocarbons) and long-term 

monitoring of contaminant movement in the aquifer (3H, 129I, 36Cl, 3

H/3He). Velocities determined using atmospheric tracers 

were estimated from the 3H/3He- or chlorofluorocarbonmodel age of the water and the probable location of 

recharge (Busenberg and others, 1993, p. 29; Busenberg 

and others, 2001, p. 41, fig. 25). Velocities determined 

using concentrations or ratios of contaminants discharged at 

facilities were calculated based on assumed first-arrival times 

of contaminants at wells downgradient of known contaminant 

input locations (Barraclough and others, 1981, p. 59; Pittman 

and others, 1988, p. 51; Mann and Beasley, 1994, p. 24; Cecil 

and others, 2000, p. 686). Velocities were estimated to be 

2 ft/d near TAN (Busenberg and others, 1993, p. 30; 

Busenberg and others, 2001, fig. 25); 15 to 18 ft/d between 

Mud Lake and Atomic City (Busenberg and others, 1993, 

p. 30); 2 to 14 ft/d in the area north and east of the TRA 

and southeast of Howe (Busenberg and others, 1993, p. 30; 

Busenberg and others, 2001, fig. 25); 4 to 20 ft/d south of 

the TRA and INTEC (Barraclough and others, 1981, p. 59; 

Pittman and others, 1988, p. 51; Mann and Beasley, 1994, 

p. 24; Cecil and others, 2000, p. 686; Busenberg and others, 

2001, fig. 25); and 3 to 4 ft/d south of the Big Lost River and 

near the western boundary of the INL (Busenberg and others, 

1993, p. 30; Busenberg and others, 2001, fig. 25) (fig. 20).

These estimated average linear ground-water velocities 

generally correlate with the ranges and spatial distributions 

of hydraulic conductivity values estimated previously in 

the section, “Hydrogeologic Units” (table 2). High groundwater velocities were estimated for areas where the highly 

conductive, densely fractured basalt (hydrogeologic unit 1) is 

present at the water table, and lower velocities were estimated 

for areas where less conductive, less densely fractured 

basalt (hydrogeologic unit 2) and intermediate-age rocks 

(hydrogeologic unit 3) are present at the water table (fig. 9). 

Using the range of velocities presented above, an average 

hydraulic gradient at the INL of 4 ft/mi, a range of effective 

porosities of 0.1 to 0.2 for fractured basalt and 0.05 to 0.15 

for massive basalt and intermediate-age rocks, calculated 

horizontal hydraulic conductivities range from 300 to 

5,000 ft/d, 200 to 800 ft/d, and 100 to 400 ft/d for the densely 

fractured basalt (hydrogeologic unit 1), less-densely fractured 

basalt (hydrogeologic unit 2), and intermediate-age rocks 

(hydrogeologic unit 3), respectively.

Hydraulic conductivities derived from velocity estimates 

for the area of ground-water contamination south of the 

TRA and INTEC range from 500 to 5,000 ft/d for assumed 

Figure 23. Water levels in wells Arbor Test, USGS 9, USGS 12, and USGS 25 for 1948–2003, Idaho National Laboratory, Idaho.

Water-level data are from the web site http://waterdata.usgs.gov/id/nwis/gwlevels. NGVD 29, National Geodetic Vertical Datum of 

1929. Location of wells shown in figure 1.

ID19-0016_fig23

Arbor Test

660

665

670

675

680

685

690

DEPTH TO WATER, IN FEET BELOW LAND SURFACE

4,474

4,479

4,484

4,489

4,494

4,499

4,504

USGS 9

590

595

600

605

610

615

620 4,410

4,415

4,420

4,425

4,430

4,435

4,440

USGS 12

305

310

315

320

325

330

335

19481952195619601964196819721976198019841988199219962000

4,484

4,489

4,494

4,499

4,504

4,509

4,514

USGS 25

255

260

265

270

275

280

285

19481952195619601964196819721976198019841988199219962000

4,563

4,568

4,573

4,578

4,583

4,588

4,593

ALTITUDE OF WATER LEVEL, IN FEET ABOVE NGVD 29

46  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
2

Hydrologic data on the World Wide Web may be accessed at http://idaho.

usgs.gov/

3

Wells USGS 4, USGS 28, USGS 31, and Site 17 are shown in figure 1. The 

other wells discussed in this sentence are located inside or near the indicated 

facilities and are not shown on map. The facility locations are shown in 

figure 1.

effective porosities of 0.1 and 0.2. These estimates are 

generally consistent with results of aquifer testes conducted 

in hydrogeologic unit 1. The ground-water velocity in 

hydrogeologic unit 1 in the range from 4 to 20 ft/d probably 

reflects preferential flow along the many interflow zones of 

the thin, densely fractured basalts composing the uppermost 

hydrogeologic unit.

High wastewater-discharge rates at disposal wells may 

produce increased average linear ground-water velocities 

downgradient of the wells. Evidence of an increase in 

ground-water velocity in the area between the disposal well 

(CPP 3) at the INTEC and two wells (USGS 40 and USGS 41, 

in fig. 1) less than 1,000 ft to the southwest was provided in 

two tracer studies conducted one year apart (Jones, 1963). 

During the first tracer study, the rate of wastewater discharged 

at CPP 3 was gradually increased from less than 100 gal/min 

to about 700 gal/min. During the second tracer study, the 

discharge rate remained steady at about 970 gal/min (Jones, 

1963, p. 228). The ground-water velocity between the two 

wells calculated in the second study was 2 to 4 times higher 

than that calculated in the first study (Jones, 1963, p. 228). 

The most probable explanation for this increase in velocity 

was the higher wastewater-discharge rate at the disposal well 

during the second study.

Vertical Ground-Water Movement

The small number of deep wells and the few 

measurements of vertical hydraulic head and upward 

or downward ground-water movement provided limited 

information about the vertical gradients and movement of 

ground water in the model area, but did demonstrate that 

the direction of vertical movement is spatially variable. 

The information available regarding vertical hydraulic 

conductivity and head indicates that hydraulic conductivity 

decreases (Mann, 1986, p. 6-14) and hydraulic head increases 

(Jones, 1961, p. 41; Morin and others, 1993, p. 16-19; Mann, 

1986, p. 14) with depth in the southwestern part of the INL. 

However, in the upper part of the aquifer, near TAN, the NRF, 

the TRA, and the INTEC, measurements of ground-water 

movement in wells indicates that water moves both upward 

and downward (Morris and others, 1964, p. 40-42; Morris and 

others, 1965, p. 42-44; Barraclough, Teasdale, and Jensen, 

1967, p. 94-98; Morin and others, 1993, table 1).

Water-level measurements2 at test hole INEL 1 (fig. 1) 

show that the hydraulic head at depth (4,210–10,365 ft below 

land surface) is about 115 ft higher than the head for the upper 

200 ft (395–595 ft below land surface) of the aquifer (Mann, 

1986, p. 14). Water-level measurements in piezometers at 

wells USGS 30 and Hwy 1, in the eastern part of the INL 

and east of the southeast boundary of the model area (fig. 1), 

respectively, also show head increasing with depth. At well 

USGS 30, head increases of 2 and 11 ft were measured at 

depths of 335 and 545 ft below the water table, respectively, 

and at well Hwy 1, head increases of 0.4 and 0.3 ft were 

measured at depths of 100 and 425 ft below the water table, 

respectively. The increased head with depth at wells USGS 30 

and Hwy 1 may be related to the steep hydraulic gradient 

along the northeast boundary of the model area (fig. 3). The 

steep gradient coincides with the interfingering of basalt with 

less transmissive layers of sediment (discussed in the section 

“Artificial Boundaries”). These sediment layers may act as 

confining units and, if the deeper sediment layers extend 

further southwestward than the shallower layers, the heads in 

the northeastern part of the model area could be larger beneath 

the deeper sediment layers than in the overlying rocks.

Variation in the direction of vertical ground-water flow in 

the upper 300 ft of the aquifer is confirmed by measurements 

of intra-borehole flow at 16 wells. Downward flow was 

measured at one well3 (USGS 4) southeast of TAN, one 

well (Site 17) north of the NRF, two wells (MTR Test, TRA 

disposal) at the TRA, and one well (USGS 49) at the INTEC; 

upward flow was measured at two wells (USGS 28, USGS 31) 

southeast of TAN and at five wells (USGS 42, USGS 44, 

USGS 45, USGS 46, USGS 47) at the INTEC; and upward 

and downward flow was measured at four wells (USGS 48, 

USGS 51, USGS 52, USGS 59) at the INTEC (Morris and 

others, 1964, p. 40-42; Morris and others, 1965, p. 42-44; 

Barraclough, Teasdale, and Jensen, 1967, p. 94-98; Morin and 

others, 1993, table 1).

Model Representation of Features Affecting 

Ground-Water Flow

Ground-water flow through the model area is controlled 

by the structural and stratigraphic framework of the aquifer 

and the distribution and timing of hydrologic inputs to and 

outputs from the model area. Although the conceptual model 

simplifies geologic and hydrologic features it retains features 

of the system important for modeling contaminant transport.

Structural and Stratigraphic Controls on GroundWater Flow

Structural features in the model area that may control 

ground-water flow in the aquifer by providing preferential 

pathways for, and impediments to, flow include (1) rhyolite 

domes, (2) a sedimentary trough, and (3) volcanic rift zones 

and vent corridors. Smaller hydraulic conductivities associated 

with rhyolite domes and rocks in the sedimentary trough 

decrease flow velocities and may locally alter flow directions 

where these features are present. Volcanic rift zones and vent 

corridors may produce both porous, rapid flow zones and 

vertical, low permeability structures that impede flow.

Conceptual Model of Ground-Water Flow  47
The conceptual model represents rhyolite domes, areas 

of subsidence, uplift, and dipping beds, and the sedimentary 

trough. It does not represent volcanic rift zones or vent 

corridors. The effects of volcanic rift zones and vent corridors 

on ground-water movement in the eastern SRP aquifer 

are undetermined. Stratigraphic controls on ground-water 

flow in the model area are the horizontal and subhorizontal 

layers of basalt and sediment in the eastern SRP aquifer. In 

the conceptual model, three homogeneous and anisotropic 

hydrogeologic units (fig. 10) represent the stratigraphic layers. 

Hydraulic conductivity of each of the three hydrogeologic 

units reflects the aggregate lithology, thickness, and number of 

basalt flows and sedimentary interbeds in each hydrogeologic 

unit. Stratigraphic layers produce both large- and small-scale 

heterogeneity and horizontal-to-vertical anisotropy in the 

aquifer.

The large-scale heterogeneity and anisotropy in the 

aquifer are represented by the subhorizontal layering and 

differences in hydraulic conductivity among adjacent 

homogeneous and anisotropic hydrogeologic units (interunit). 

The difference in conductivity is particularly prominent 

between hydrogeologic units 1 and 2, with the conductivity 

of hydrogeologic unit 2 estimated to be much smaller than 

hydrogeologic unit 1. Consequently, hydrogeologic unit 2 

acts as an impediment to the downward movement of water 

and contaminants, and wastewater that infiltrates through 

the unsaturated zone or is discharged to the aquifer at the 

INL probably is constrained to move in hydrogeologic unit 1 

beneath the INL.

Small-scale heterogeneity and anisotropy in the aquifer, 

which exists in and between the individual basalt flows 

and sedimentary interbeds (intraunit) that compose the 

hydrogeologic units, are not represented in the conceptual 

model. The intraunit stratigraphy produces (1) preferential 

flow paths and interflow zones that facilitate rapid horizontal 

ground-water movement, and (2) hydraulic barriers, in 

the form of massive interiors of individual basalt flows 

and sedimentary interbeds, that substantially impede 

vertical ground-water movement. Although the small-scale 

heterogeneity and anisotropy are not represented in the 

conceptual model, they could be indirectly represented in 

the numerical model through the choice of values for the 

horizontal and vertical hydraulic conductivities for each of the 

hydrogeologic units.

Hydrologic Controls on Ground-Water Flow

Hydrologic controls on ground-water flow in the 

model area include regional and tributary-valley underflow, 

precipitation and streamflow-infiltration recharge, and 

industrial wastewater return flows. All these hydrologic 

controls are represented in the conceptual model. Regional and 

tributary-valley underflow and precipitation recharge can be 

adequately modeled by steady-state numerical simulations of 

flow; however, streamflow-infiltration recharge from the Big 

Lost River and the variable amounts of industrial wastewater 

return flows requires transient numerical simulations.

The predominant northeast-to-southwest flow direction 

through the model area is primarily controlled by regional 

underflow across the northeast boundary, and the southto-southeast flow direction in the central part of the INL is 

controlled primarily by underflow entering the eastern SRP 

aquifer from the Little Lost River and Birch Creek valleys. 

Regional and tributary-valley underflow entering the model 

area have undergone gradual changes in magnitude since 1890 

in response to climate cycles and changing irrigation practices. 

However, these changes in underflow do not appear to have 

affected the magnitude and direction of ground-water flow 

in the central part of the eastern SRP aquifer. Potentiometric 

maps of the aquifer indicate that the shape of the water table 

and flow directions in the model area have remained relatively 

stable since about 1930 (Garabedian, 1992, p. 21).

In contrast to the gradual changes in underflow 

entering the model area, large fluctuations were measured in 

precipitation, streamflow-infiltration recharge, and industrial 

wastewater return flows. Precipitation and streamflowinfiltration recharge fluctuate annually and in response to 

short-term (3–8 years), wet-dry climate cycles. Recharge 

from precipitation in the model area is areally diffuse 

(about 0.04 (ft3/s)/mi2), and little or no streamflow from the 

Little Lost River or Birch Creek reaches the INL, so these 

sources of recharge do not significantly impact water levels 

or flow directions in the model area. However, because 

streamflow infiltration from the Big Lost River is spatially 

focused, fluctuates considerably in response to short-term 

climate cycles, and can significantly alter water levels and 

flow directions locally in the model area, it has important 

consequences for the movement of water and contaminants. 

Industrial wastewater return flows in the model area have 

varied in volume and location over time. These flows are small 

relative to the volume of water flowing through the model 

area; however, because they occur near contaminated ground 

water they affect the movement of water and contaminants.

Representation of Ground-Water Flow Through 

the Model Area

A semi-quantitative distribution of ground-water flow 

paths is shown on a vertically-exaggerated hydrogeologic 

section of the model area (fig. 24). Flow paths drawn on the 

section were based on flow paths generated with a version 

of the software package FLOWNET (Fetter, 1994, p. 314). 

Inflows were based on steady-state conditions. Input for the 

flow net included (1) a uniform, two-dimensional vertical 

discretization (cells were 4 mi horizontally and 400 ft 

vertically) of hydrogeologic units 1, 2, and 3 along the line 

of section shown in figure 9, (2) inflow across the northeast, 

northwest mountain-front, and water-table boundaries (fig. 17

48  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Figure 24. Distribution of hydrogeologic units and the direction of ground-water flow for the model area, Idaho National 

Laboratory and vicinity, Idaho.

Trace of section is shown in figure 9.

ID19-0016_fig24

Unsaturated zone ?

?

1 Hydrogeologic unit 1—Younger rocks 

consisting of densely fractured 

basalt and interbedded sediment

Hydrogeologic unit 2—Younger rocks 

consisting of massive, less densely 

fractured basalt and interbedded 

sediment. Queried where 

uncertain.

Hydrogeologic unit 3—Intermediateage rocks consisting of slightly 

altered fractured basalt and 

interbedded sediment

EXPLANATION

Generalized flowpath— 

Arrow in direction of flow

Generalized line of equal 

head potential

VERTICAL SCALE GREATLY EXAGGERATED

0 4 8 12 MILES

0 12 KILOMETERS

Water table

5,000

5,500

FEET

4,000

2,000

1,000

3,000

4,500

2,500

1,500

3,500

B B’

ALTITUDE, ABOVE NATIONAL GEODETIC VERTICAL DATUM OF 1929

Older rocks

Older rocks

Radioactive WasteManagement Complex

Big Lost River underflow

Test Reactor Area

Little Lost Riverunderflow

Big Lost Riverplayas

Birch Creek playa

Test Area North

Idaho Nuclear Technology and Engineering Center

Birch Creekunderflow

Idaho NationalLaboratory Boundary

and table 6, (3) the altitude of the water table along the line of 

section shown in figure 9, and (4) estimates of the horizontal 

hydraulic conductivity (Kh) and vertical hydraulic conductivity 

(Kv) for hydrogeologic units 1, 2, and 3, where

Hydrogeologic unit 1 (younger rocks—thin, densely 

fractured basalt)

Kh1 = 5,300 ft/d

Kv1 = 400 ft/d

(estimates from Frederick and Johnson, 1996, p. 59).

Hydrogeologic unit 2 (younger rocks—massive, 

less-densely fractured basalt)

Kh2 = 900 ft/d

Kv2 = 600 ft/d

(estimates from Frederick and Johnson, 1996, p. 59).

Hydrogeologic unit 3 (intermediate-age rocks—slightly 

altered fractured basalt)

Kh3 = 530 ft/d

Kv3 = 40 ft/d

Hydrogeologic unit 3 estimates were assumed to be 

approximately one order of magnitude smaller than those for 

hydrogeologic unit 1 (John Welhan, Idaho State Geological 

Survey, written commun., 1999).

Other assumptions that were used to draw flow paths on 

the hydrogeologic section included:

Inflow across the water-table and the northwest mountainfront boundaries as given in table 6, with inflow from the 

northwest mountain-front boundary represented as inflow 

across the water table,

Uniform, vertical distribution of inflow and outflow 

across the northeast and southwest boundaries of the 

model area, respectively,

Restriction of ground-water withdrawals to the area near 

the northeast boundary of the model area,

Uniform distribution of precipitation recharge and 

changes in aquifer storage, and

Allocation of streamflow-infiltration recharge from the 

Big Lost River to the spreading areas (50 percent) and the 

playas (50 percent).

1.

2.

3.

4.

5.

Conceptual Model of Ground-Water Flow  49
Inflows across the water-table boundary were represented 

on the hydrogeologic section (fig. 24) by superimposing the 

inflows onto the section. These inflows produced a progressive 

increase of flow in the aquifer downgradient of the northeast 

boundary and contributed about 45 percent of the total outflow 

from the system (table 6).

The hydrogeologic section and semi-quantitative flow 

paths (fig. 24) indicate that steady-state flow through the 

aquifer is controlled by the stratigraphic and hydraulic features 

of the aquifer. The intermediate-age rocks of hydrogeologic 

unit 3 compose about 75 percent of the aquifer, and most of 

the water in the aquifer resides in this hydrogeologic unit 

because of its large volume. Younger rocks of hydrogeologic 

units 1 and 2 together essentially form a thin layer on top 

of hydrogeologic unit 3 in the upper and central part of the 

aquifer. Hydrogeologic unit 1, which is similar in thickness 

to hydrogeologic unit 2 and much smaller in thickness than 

hydrogeologic unit 3, has a much larger hydraulic conductivity 

than that of hydrogeologic units 2 and 3; consequently, 

hydrogeologic unit 1 transmits a disproportionately large 

amount of water. The smaller hydraulic conductivity of 

hydrogeologic unit 2 and the large horizontal-to-vertical 

anisotropy of the aquifer (Kh is much larger than Kv) generally 

impedes the downward flow of water from hydrogeologic 

unit 1 (fig. 24). The pyramid-shaped segment of hydrogeologic 

unit 2 that extends above the water table in figure 24 is not 

laterally extensive (fig. 9) and does not significantly impede 

the horizontal movement of water through hydrogeologic 

unit 1.

Water moves up and down in the intermediate-age 

rocks of hydrogeologic unit 3 and, southwest of the INL, 

water in hydrogeologic unit 1 probably moves down across 

hydrogeologic unit 2 into hydrogeologic unit 3 (fig. 24). 

Upward and downward movement of water in hydrogeologic 

unit 3 reflects changes in head with the changing thickness 

and transmissivity of this hydrogeologic unit. Vertical 

flow is downward in areas where the aquifer thickens 

and transmissivity increases because head decreases with 

depth; flow is upward in areas where the aquifer thins 

and transmissivity decreases because head increases with 

depth (fig. 24). Hydrogeologic unit 2 generally acts as an 

impediment to the downward movement of water from 

the fractured basalt of hydrogeologic unit 1 because of the 

smaller hydraulic conductivity of hydrogeologic unit 2. The 

stratigraphic interpretation of Anderson and Liszewski (1997, 

fig. 27) indicates that southwest of the INL hydrogeologic 

units 1 and 2 were uplifted. As a result of uplift, 

hydrogeologic unit 1 is no longer saturated and hydrogeologic 

unit 2 intersects the water table (fig. 24). Hydrogeologic unit 

2 is less transmissive than hydrogeologic unit 1 and thus the 

water-table gradient steepens from about 2 to 3 ft/mi (fig. 9) 

upgradient of where hydrogeologic unit 2 intersects the water 

table to about 25 to 30 ft/mi (Lindholm and others, 1988) 

at and downgradient of this location. The steepening of the 

gradient also may be caused by the addition of underflow 

to the aquifer from the Big Lost River, which significantly 

increases the volume of water moving through this area. 

The steeper gradient, increased aquifer transmissivity, and 

increased volume of water in this area probably facilitate 

the downward flow of water across the younger rocks of 

hydrogeologic unit 2 into the intermediate-age rocks of 

hydrogeologic unit 3.

Implications for Contaminant Transport

The primary objective of developing a conceptual 

model of ground-water flow beneath the INL and vicinity 

is to identify features of the flow system that most affect 

interpretations of contaminant transport in the aquifer. The 

conceptual model simplifies real-world conditions and 

interactions through time and in space, primarily because of 

scaling considerations, computational constraints, and data 

availability. Although hydrogeologic judgment is used in 

determining input to the model to supplement or substitute for 

the lack of data, simplification invariably decreases the realism 

of model results. This decrease is a necessary trade off so that 

the model can represent large-scale, complex systems and 

processes that include the effects of many small-scale features 

and interactions.

Two of the more significant simplifications in the 

conceptual model are (1) representation of individual 

hydrogeologic units as homogeneous anisotropic porous 

media, and (2) implicit representation of infiltration recharge. 

The first simplification largely precludes simulating 

the tortuous character of flow that occurs naturally in 

heterogeneous and anisotropic fractured porous media; and 

the second simplification precludes realistic simulations of 

residual contaminant movement through the 200- to 600-ft 

thick unsaturated zone, a source of contaminants that is likely 

to persist over a long period of time.

The conceptual model implies that most contaminant 

movement beneath the INL takes place in the thin, 

densely fractured basalts and interbedded sediment of 

hydrogeologic unit 1 that compose most of the upper 200 ft 

of the aquifer. This hypothesis is based on (1) definition 

of the hydraulic properties and the inferred distribution of 

the three hydrogeologic units composing the aquifer, and 

(2) interpretation of the generalized northeast-to-southwest 

cross section of ground-water flow across the model area. 

The hypothesis is consistent with conclusions reached by 

earlier investigators who studied contaminant migration and 

aquifer characteristics at the INL and noted that waste plumes 

“...generally remain as relatively thin lenses within about 

250 feet of the water table” (Robertson, 1974, p. 6). The 

current conceptual model still needs to be verified through 

field testing, guided by the results of future flow-and-transport 

modeling.

50 Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
The features of the flow system represented in the 

conceptual model that significantly affect contaminant 

transport in the aquifer at the INL and vicinity are (1) the 

implicit representation of infiltration recharge through the 

unsaturated zone that precludes realistic simulation of flow 

and future contaminant movement through the unsaturated 

zone, (2) preferential flow along highly conductive interflow 

zones, primarily in thin, densely fractured basalts of 

hydrogeologic unit 1, implying large horizontal to vertical 

anisotropy and rapid lateral movement of contaminants in 

the upper 200 ft of the aquifer, (3) the restricted downward 

movement of flow and contaminants in hydrogeologic 

unit 1 into the older, less conductive massive basalts of 

hydrogeologic unit 2 beneath the INL, implying limited 

dilution and dispersion of contaminants near their sources, 

(4) the inferred downward movement and deeper circulation 

of water upgradient of where the massive, less densely 

fractured basalt of hydrogeologic unit 2 intersects the water 

table southwest of the INL, implying enhanced dispersion 

and dilution of contaminants away from their sources and 

at deeper depths in the aquifer, and (5) enhanced dispersion 

of contaminants resulting from the spatial and temporal 

variability of streamflow-infiltration recharge that is in close 

proximity to contaminated ground water.

Summary and Conclusions

Ground-water flow in the west-central part of the eastern 

Snake River Plain (SRP) aquifer is described in a conceptual 

model that will be used in numerical simulations to evaluate 

contaminant transport at the Idaho National Laboratory (INL) 

and vicinity. The conceptual model emphasizes the effects of 

various geologic and hydrologic controls on the transport of 

contaminants at the INL.

The conceptual model encompasses an area of 1,940 mi2

and includes most of the 890 mi2 of the INL. The model 

area extends 75 mi from northeast to southwest and 35 mi 

from northwest to southeast. The subregional scale of the 

conceptual model is intermediate in size between that of the 

Regional Aquifer System Analysis (RASA) of the eastern 

SRP aquifer (10,800 mi2) and that of a local INL facility-scale 

model (less than about 10 mi2).

Three hydrogeologic units were used to represent 

the complex stratigraphy of the aquifer in the model area. 

Collectively these hydrogeologic units include at least 

65 basalt-flow groups, 5 andesite-flow groups, and 61 

sedimentary interbeds. Three rhyolite domes in the model area 

extend deep enough to penetrate the aquifer and are treated as 

low permeability, vertical pluglike masses and are not included 

as part of the three primary hydrogeologic units.

Broad differences in lithology and large variations in 

hydraulic properties allowed the heterogeneous, anisotropic 

basalt-flow groups, andesite-flow groups, and sedimentary 

interbeds to be grouped into three hydrogeologic units that 

are conceptually homogeneous and anisotropic. Younger 

rocks, primarily of thin, densely fractured basalt, compose 

hydrogeologic unit 1; younger rocks, primarily of massive, 

less densely fractured basalt, compose hydrogeologic 

unit 2; and intermediate-age rocks, primarily of slightly-tomoderately altered, fractured basalt, compose hydrogeologic 

unit 3. Differences in hydraulic properties among adjacent 

hydrogeologic units result in much of the large-scale 

heterogeneity and anisotropy of the aquifer in the model 

area, and differences in horizontal and vertical hydraulic 

conductivity within individual hydrogeologic units result in 

much of the small-scale heterogeneity and anisotropy of the 

aquifer in the model area.

Intermediate-age rocks make up the largest volume of 

the aquifer. Hydrogeologic units 1 and 2 are not present in the 

aquifer in the northern and southern parts of the model area; 

in those areas, hydrogeologic unit 3 is inferred to be present at 

the water table. An alternative depiction of the hydrogeologic 

framework includes a large area of the aquifer beneath the 

northern and central parts of the INL where the combined 

sediment content of all three hydrogeologic units exceeds 

11 percent and, in some cases, accounts for 25 to more than 

50 percent of the composite stratigraphic section. The large 

sediment content of these rocks may require incorporation of 

a fourth hydrogeologic unit or modification of the existing 

hydrogeologic units to properly characterize flow and 

contaminant transport in the aquifer.

The aquifer is underlain by older rocks composed of 

intensely altered basalt and interbedded sediment. Based on 

surface-based electrical-resistivity surveys and stratigraphic 

data from boreholes at and near the INL, depth to the base 

of the aquifer in the model area was estimated to range from 

700 to 4,800 ft below land surface. Although stratigraphic 

data from 10 deep boreholes indicated that depth to the base 

of the aquifer ranges from 815 to 1,710 ft, the 700 to 4,800 ft 

estimate was used to define the base of the aquifer because the 

surface-based electrical-resistivity surveys provided coverage 

over the entire model area.

The geometry of the aquifer is an important feature for 

developing models of flow and contaminant transport. Most 

interpretations of this geometry, particularly the thickness 

and distribution of hydrogeologic unit 2, depth to the base 

of the aquifer, and distribution of the hydrogeologic units 

Summary and Conclusions  51
south of the INL were based on very limited data and indirect 

measurements that are subject to considerable uncertainty. 

These data indicate that the three-dimensional geometry of 

the aquifer is very irregular. Its thickness generally increases 

from north to south and from west to east and is greatest 

south of the INL. The interpreted distribution of older rocks 

that underlie the aquifer indicates large changes in saturated 

thickness of the aquifer across the model area. Variations in 

thickness, depth, and attitude of individual hydrogeologic units 

are attributed primarily to differential subsidence and uplift.

Ground-water flow in hydrogeologic unit 1 occurs 

preferentially in the interflow zones of the many thin, 

mainly 14- to 24-ft thick, pahoehoe flows that compose this 

hydrogeologic unit. Aquifer tests conducted in 67 wells 

indicated that the hydraulic conductivity of the densely 

fractured rocks composing hydrogeologic unit 1 varies by 

more than six orders of magnitude, from 0.01 to 24,000 ft/d. 

Almost two-thirds of these estimates are greater than 100 ft/d, 

and about one-third are greater than 1,000 ft/d. The hydraulic 

conductivity of hydrogeologic unit 2 was estimated based on 

comparisons with massive basalts of the Columbia Plateau, 

measurements from flowmeter tests conducted in two wells 

at the Idaho Nuclear Technology and Engineering Center, 

and single-well aquifer tests in four wells having perforated 

intervals only in hydrogeologic unit 2. These estimates ranged 

from 6.5 to 1,400 ft/d. Individual basalt flows composing 

hydrogeologic unit 2 are mainly 21- to 37-ft thick, and the 

hydraulic conductivity of hydrogeologic unit 2 is presumed 

to be much smaller than that of hydrogeologic unit 1 because 

of the massive character of the individual basalt flows 

and the fewer number of interflow zones. Estimates of the 

hydraulic conductivity of the slightly altered, fractured basalt 

and interbedded sediment composing hydrogeologic unit 3 

were based on single-well aquifer tests in 14 wells. These 

test results indicated that the hydraulic conductivity of these 

rocks ranges from about 0.32 to 24,000 ft/d. Although large 

hydraulic conductivities were measured for hydrogeologic 

unit 3, the hydraulic conductivity of hydrogeologic unit 3 

is presumed to be smaller than that of hydrogeologic unit 1 

because hydrogeologic unit 3 probably has fewer interflow 

zones and is slightly-to-moderately altered.

Estimates of porosity vary greatly because they are 

dependent on the methods, scales, and locations used to 

determine them. Core measurements indicated that the 

porosity of hydrogeologic unit 1 ranges from about 0.05 

to 0.27. The porosity of hydrogeologic unit 2 probably is 

less than that of hydrogeologic unit 1 because of the dense, 

massive character and fewer interflow zones associated 

with the rocks that compose hydrogeologic unit 2. For 

similar reasons, and because of alteration, the porosity of 

hydrogeologic unit 3 is likely to be smaller than hydrogeologic 

unit 1.

Three physical and three artificial boundaries define 

the model area. The physical boundaries are the (1) watertable boundary, (2) base of the aquifer, and (3) northwest 

mountain-front boundary. The artificial boundaries are the 

(1) northeast boundary, (2) southeast-flowline boundary, and 

(3) southwest boundary. In the conceptual model ground water 

flow across these boundaries is represented as temporally 

constant or variable with spatially uniform or nonuniform flow 

distributions.

Flow through the unsaturated zone to the water-table 

boundary is represented implicitly in the conceptual model 

as time-averaged net infiltration recharge. Inflow across the 

water-table boundary, represented in the model as variable and 

nonuniform, occurs as (1) diffuse areal precipitation recharge 

(70 ft3/s) (constant and uniform), (2) streamflow-infiltration 

recharge from the Big Lost River (95 ft3/s) (variable and 

nonuniform), (3) wastewater return flows (6 ft3/s) (variable 

and nonuniform), (4) irrigation-infiltration recharge (24 ft3/s), 

and (5) release of water from storage (80 ft3/s) (variable 

and uniform). Flux across the water-table boundary from 

streamflow-infiltration recharge is about 20 (ft3/s)/mi2, or 

more than two orders of magnitude larger than flux from 

precipitation recharge. Flux from streamflow-infiltration 

recharge during years with much larger than average 

streamflow was estimated to be as large as 170 (ft3/s)/mi2 . 

Because of its large flux and proximity to known sources and 

areas of contamination in the aquifer, streamflow-infiltration 

recharge from the Big Lost River is considered much more 

significant to contaminant transport than the other components 

of inflow across the water-table boundary.

The base of the aquifer is represented as a constant-flow, 

uniform boundary. Hydraulic conductivity of the intensely 

altered older rocks near the boundary, 0.002 to 0.03 ft/d, was 

estimated to be three to four orders of magnitude smaller 

than that of the overlying intermediate-age rocks. Inflow 

across this boundary, for that part of the aquifer underlying 

the INL, was previously estimated at 20 ft3/s. This estimate 

was based on data from one 10,365-ft-deep corehole at the 

INL. Extrapolation of the estimate to the 1,940 mi 2 area of the 

conceptual model resulted in an inflow estimate of 44 ft3/s or 

about 2 percent of the total water-budget inflow estimate. 

This inflow estimate represents a flux of about 

0.02 (ft3/s)/mi2, or about half of the flux for precipitation 

recharge, 0.04 (ft3/s)/mi2. Because of its low flux and distance 

from known areas of contamination in the aquifer, the base of 

the aquifer, for purposes of contaminant transport modeling, 

can be treated as a no-flow boundary.

The northwest mountain-front boundary is represented as 

a constant-flow, nonuniform boundary. Inflows to the aquifer 

across this boundary (695 ft3/s) consist of tributary-valley 

underflow from the Big Lost River valley (367 ft3/s), the 

Little Lost River valley (226 ft3/s), and the Birch Creek valley 

(102 ft3/s).

52  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
The northeast boundary of the model area is represented 

as a nonuniform, constant-flow boundary. Inflow into the 

aquifer across this boundary consists of underflow from 

the regional aquifer (1,225 ft3/s) and is the largest inflow 

component of the budget. Estimates of inflow across this 

boundary are considered fairly reliable because there is a 

relative abundance of information available to characterize the 

hydrologic conditions and hydraulic properties of the aquifer 

in this area.

The southeast-flowline boundary is represented as a 

noflow boundary and was defined by a generalized flowline 

derived from the RASA model and three-dimensional pathline 

analyses in an advective transport model.

Most of the outflow (1,731–2,343 ft3/s) from the 

aquifer occurs as underflow across the southwest boundary 

of the study area, which is represented as a variable-flow, 

nonuniform boundary. The variable flow across this boundary 

reflects the character of large, episodic inflows from Big Lost 

River streamflow-infiltration recharge. Head definition for this 

part of the aquifer was limited to water-level measurements 

from six wells that are from 3 to 10 mi from the southwest 

boundary; consequently, outflow estimates across this 

boundary are perhaps the least reliable of all the water-budget 

estimates.

Flow in the aquifer increases progressively in a direction 

downgradient of the northeast boundary. Increased flow is 

the result of mountain-front and tributary-valley underflows 

along the northwest boundary (695 ft3/s), precipitation and 

streamflow-infiltration recharge (165 ft3/s) across the watertable boundary, and release of water from storage (80 ft3

/s). 

Together these additions account for more than 45 percent 

of the outflow across the southwest boundary. Ground-water 

flow beneath the INL occurs in all three hydrogeologic units. 

In the northern part of the INL and south of the INL, where 

the younger rocks of hydrogeologic units 1 and 2 are either 

not present or are above the water table, all flow takes place 

through the slightly-to-moderately altered intermediate-age 

rocks of hydrogeologic unit 3.

Hydraulic gradients were defined by measurements of 

water levels in 201 wells in the model area, 175 of which are 

within the boundaries of the INL. These gradients indicate that 

the regional direction of ground-water flow is from northeast 

to southwest. Hydraulic gradients are largest immediately 

upgradient of the northeast boundary, 27 to 60 ft/mi, and 

southwest of the INL, 4 to 30 ft/mi. Beneath the INL, 

gradients are much smaller, 1 to 8 ft/mi, and precise definition 

of flow direction is difficult to determine. Flow directions 

in the aquifer beneath the INL vary locally from southeast 

to southwest and fluctuate in response to episodic recharge 

from streamflow infiltration. In 1984, streamflow-infiltration 

recharge from the Big Lost River accounted for more than 

20 percent of the estimated 1980 steady-state water budget 

and resulted in water-level rises in the aquifer exceeding 

10 ft in places. These temporary changes in water levels alter 

flow directions locally and, in some extreme cases, can cause 

reversals in flow direction. Lateral spreading of contaminants 

could be strongly affected by episodic recharge and resulting 

temporal fluctuations in water-table gradients.

Long-term monitoring of contaminant movement in the 

aquifer at the INL indicates that ground-water velocities in 

hydrogeologic unit 1 range from 4 to 20 ft/d in areas affected 

by contamination south of the Test Reactor Area and the 

Idaho Nuclear Technology and Engineering Center. These 

velocities were based on presumed first-arrival times of 

contaminants at wells downgradient of known contaminantinput locations (injection wells) at various INL facilities. 

Hydraulic conductivities derived from these velocity estimates 

range from 500 to 5,000 ft/d for assumed effective porosities 

of 0.1 and 0.2. These estimates are generally consistent with 

results of aquifer tests conducted in hydrogeologic unit 1. 

The ground-water velocity in hydrogeologic unit 1 of from 

4 to 20 ft/d probably reflects preferential flow along the 

many interflow zones of the thin, densely fractured basalts 

composing hydrogeologic unit 1. However, because of the 

sometimes uncertain flow directions and gradients that are 

subject to fluctuations, actual flow paths may be longer and 

velocities higher than those estimated from first arrivals of 

contaminants.

Because most wells penetrate only the upper 200 ft of the 

aquifer, there are few data to evaluate how hydraulic gradients 

and flow directions vary with depth. Two boreholes at and 

near the INL are equipped with dedicated piezometers to 

measure head changes at depth. These two boreholes are near 

the northeastern boundary of the INL. At one well 2 and 11 ft 

increases in head at depths of 335 and 545 ft below the water 

table, respectively, were measured, and at another well 0.4 and 

0.3 ft increases in head at depths of 100 and 425 ft below the 

water table, respectively, were measured. The resulting vertical 

head differences probably are a reflection of head losses 

resulting from regional aquifer underflow into the layered 

basalts and interbedded sediments downgradient of the steep 

hydraulic gradient, 27 to 60 ft/mi, which defines the northeast 

boundary of the model area.

A simplified northeast to southwest cross section showing 

the approximate distribution of hydrogeologic units and 

ground-water flow paths through the model area indicated that 

head decreases and then increases with depth with thickening 

and thinning of the aquifer in a direction downgradient of 

the northeast boundary. Downward flow is indicated in areas 

where head decreases with depth, and upward flow in areas 

where head increases with depth. The largest changes in 

vertical gradient are depicted as occurring upgradient of where 

hydrogeologic unit 2 is inferred to intersect the water table 

south of the INL. Downward movement of water across the 

less permeable rocks of hydrogeologic unit 2 probably occurs 

at this location, implying downward movement and deeper 

circulation of contaminants that may move offsite.

Summary and Conclusions  53
The conceptual model implies that most contaminant 

movement beneath the INL probably takes place in the highly 

conductive rocks of hydrogeologic unit 1 that compose most 

of the upper 200 ft of the aquifer. This hypothesis is based 

on (1) definition of the hydraulic properties and the inferred 

distribution of the hydrogeologic units selected to characterize 

flow in the aquifer, and (2) long-term observations of 

contaminant movement in the aquifer, the location of 

contaminant sources, and the history of waste-disposal 

practices at the INL. This hypothesis is consistent with 

conclusions reached by earlier investigators who studied waste 

behavior and aquifer characteristics at the INL and noted that 

waste plumes “...generally remain as relatively thin lenses 

within about 250 feet of the water table.”

The features of the flow system represented in the 

conceptual model that most affect interpretations of 

contaminant transport at the INL and vicinity are the (1) 

implicit representation of infiltration recharge through the 

unsaturated zone, (2) preferential flow along highly conductive 

interflow zones, implying large horizontal to vertical 

anisotropy, (3) restricted downward movement of flow and 

contaminants in hydrogeologic unit 1 into the less conductive 

basalts of hydrogeologic unit 2 beneath the INL, (4) inferred 

downward movement and deeper circulation of water 

upgradient of where the massive, less densely fractured basalt 

of hydrogeologic unit 2 intersects the water table south of the 

INL, and (5) enhanced dispersion of contaminants resulting 

from the spatial and temporal variability of streamflowinfiltration recharge that is in close proximity to contaminated 

ground water.

Uncertainties were associated with all aspects of the 

current conceptual model. Many aspects of this model 

are based on interpretations or extrapolations using very 

limited data or indirect measurements that may be subject 

to considerable error. To understand contaminant transport, 

some uncertainties are considered to be more significant 

than other uncertainties. Sensitivity analysis using numerical 

models can be used to assess the significance of some of 

these uncertainties. Model performance depends on how 

well simulation results compare with historical records 

of the spatial and temporal distribution of contaminants. 

Confidence in a well-documented source inventory is a 

necessary prerequisite. However, three-dimensional field 

definition of contaminant concentrations in the subsurface 

at the INL and vicinity is lacking; so, regardless of how well 

transport simulations match the available field observations, 

independent verification of those aspects of the conceptual 

model that rely on interpretations of vertical flow to describe 

the movement of contaminants will be needed.

Acknowledgments

The authors are grateful for technical reviews of the 

manuscript by Wayne Belcher, Jim Bartolino, and Steve 

Garabedian of the USGS, and for the many helpful comments 

and suggestions made over the years by USGS participants 

of internal program reviews, especially those of Keith Prince, 

Devin Galloway, and Claire Tiedeman.

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book 2, chap. D1, p. 5-66.
Wastewater-disposal sites at Test Area North (TAN), the 

Test Reactor Area (TRA), and the Idaho Nuclear Technology 

and Engineering Center (INTEC) were the principal sources 

of radioactive- and chemical-waste constituents in water 

in the Snake River Plain aquifer. In the past, wastewaterdisposal sites have included lined evaporation ponds, unlined 

infiltration ponds and ditches, drain fields, and disposal wells. 

Waste materials buried at the Radioactive Waste Management 

Complex (RWMC) also are sources of some constituents in 

ground water. A brief history of wastewater-disposal practices 

at various facilities on the INL is provided below. Most 

of this historical information is documented in a report by 

Bartholomay and others (2000).

Test Area North—From 1953 to 1972, low-level 

radioactive, chemical, and sanitary waste­water was discharged 

at TAN (fig. 1) into the Snake River Plain aquifer through 

a 310-ft-deep disposal well. In 1972, the disposal well 

was replaced by a 35-acre infiltration pond. No records 

are available on the amount of radioactivity in wastewater 

discharged at TAN before 1959. During 1959–93, about 61 Ci 

of radioactivity was in wastewater discharged to the disposal 

well and infiltration pond. Of this amount, about 20 Ci was 

discharged to the disposal well in 1968 and 1969 in response 

to problems with an evaporator used to reduce the volume of 

wastewater. No radioactive wastewater was discharged to TAN 

disposal sites since 1993.

Test Reactor Area—Since 1952, wastewater containing 

low-level radioactive contaminants was discharged to 

infiltration and evaporation ponds at the TRA. The wastewater 

was discharged to a series of infiltration ponds until 1993, 

when two lined evaporation ponds replaced the infiltration 

ponds to prevent the wastewater from entering the aquifer. The 

average annual discharge to the infiltration and evaporation 

ponds was about 116 Mgal during 1960–98 (Bartholomay 

and others, 2000, p. 10). Radioactive constituents in the 

wastewater discharged to the ponds included tritium (about 

10,000 Ci), strontium-90 (about 93 Ci), cesium-137 (about 

138 Ci), and cobalt-60 (about 438 Ci). During 1974–79, about 

10 percent of the radioactivity in wastewater discharged to the 

subsurface was attributed to tritium; in 1980, about 50 percent 

was attributed to tritium; and in 1981–85, about 90 percent 

was attributed to tritium (Pittman and others, 1988, p. 22). 

Since 1986, about 97 percent of the radioactivity in wastewater 

discharged at the TRA was attributed to tritium.

The radioactive-waste infiltration ponds also were used 

for disposal of wastewater containing nonradioactive chemical 

wastes until 1962, when a chemical-waste infiltration pond 

was constructed. This infiltration pond was used principally 

for disposal of chemical wastewater from an ion-exchange 

system at the TRA. The average annual discharge to this 

pond was about 17.5 Mgal for the period 1962–98. Sulfate 

and sodium hydrate were the predominant constituents in the 

wastewater.

Nonradioactive cooling-tower wastewater was discharged 

to the radioactive-waste infiltration ponds from 1952 to 1964, 

to the Snake River Plain aquifer through a 1,267-ft-deep 

disposal well from 1964 until 1982, and into two cold-waste 

infiltration ponds from 1982 to present. Until 1972, coolingtower wastewater contained concentrations of dissolved 

chromium used as a rust inhibitor. The average annual 

discharge to the well and the infiltration ponds was about 226 

Mgal during 1964–95 and about 181 Mgal during 1996–98.

Idaho Nuclear Technology and Engineering 

Center—From 1952 to February 1984, most of the low-level 

radioactive, chemical, and sanitary wastewater at the INTEC 

(fig. 1) was discharged to the Snake River Plain aquifer 

through a 600-ft-deep disposal well. The average annual 

discharge of wastewater to the well was about 363 Mgal 

(Pittman and others, 1988, p. 24). Beginning in 1984, two 

infiltration ponds were used for wastewater disposal. An 

average of 442 Mgal of wastewater was discharged annually 

to the well and infiltration ponds during 1962–98. The 

annual discharge to the well and ponds ranged from a low of 

260 Mgal in 1963 to a high of 665 Mgal in 1993.

Tritium has accounted for more than 90 percent of the 

radioactivity in wastewater discharged at the INTEC since 

1970. More than 20,000 Ci of tritium in wastewater was 

discharged to the disposal well and infiltration ponds at the 

INTEC.

Radioactive Waste Management Complex—Solid and 

liquid radioactive and chemical wastes were buried in trenches 

and pits at the Subsurface Disposal Area (SDA) at the RWMC 

(fig. 1) since 1952. These constituents include transuranic 

wastes (buried in trenches until 1970), other radiochemical 

and inorganic chemical constituents, and organic compounds. 

Before 1970, little or no sediment was retained between the 

excavation bottoms and the underlying basalt. Since 1970, 

a layer of sediment was retained in excavations to inhibit 

downward migration of waste constituents.

About 9,600 Ci of plutonium-238, 160,000 Ci of 

plutonium-239, 38,000 Ci of plutonium-240, 960,000 Ci of 

plutonium-241, and 365,000 Ci of americium-241 were buried 

in the SDA during 1952–70 (Becker and others, 1998, table 

4-1). An estimated 88,400 gal of organic waste was buried 

before 1970 (Mann and Knobel, 1987, p. 1). These buried 

wastes included about 24,400 gal of carbon tetrachloride; 

39,000 gal of lubricating oil; and about 25,000 gal of 

other organic compounds, including trichloroethane, 

trichloroethylene, perchloroethylene, toluene, and benzene.

Appendix A. A Brief History of Wastewater-Disposal Practices at the 

Idaho National Laboratory

Appendix A  61
Regional-, subregional-, and local-scale models require 

different levels of descriptive information (table B1). For 

example, all three models require information about the 

distribution of the major rock types that compose the aquifer. 

In the eastern Snake River Plain, basalt is the most abundant 

rock type and has many different forms. For a regional-scale 

model (Garabedian, 1992) it may not be necessary to account 

explicitly for these different forms of basalt. However, for 

a subregional-scale model, it is meaningful to describe the 

general distribution of fractured, massive, and altered basalt. 

For a local- or facility-scale model, it may be necessary to 

describe the distribution of the many different forms of basalt 

and associated volcanic deposits that are present at any one 

place (Anderson and others, 1999).

Table B1. Information needed for constructing conceptual models at regional, subregional, and local scales.

Conceptual model 

component

Regional Aquifer System Analysis, 

regional-scale model

Idaho National Laboratory and vicinity,

subregional-scale model

Idaho National Laboratory facility, 

local-scale model

Model boundaries Extent and saturated thickness of 

aquifer

Northwestern extent of aquifer Extent of perched ground water

Southwestern extent of waste solutes Locations of local hydraulic 

boundaries (flowline or distant 

specified head)

Locations of stable hydraulic boundaries (northeastern 

zone of steep head gradients, southeastern regional 

flowline)

Local ground-water velocity field

Saturated thickness of aquifer Effective saturated thickness of 

aquifer (local confining unit below 

waste solutes)

Hydrogeologic 

framework

Stratigraphy of regional geologic 

formations and groups

Stratigraphy of regional geologic formations and groups

Stratigraphy of composite basalt flows and basalt-flow 

groups

Stratigraphy of individual basalt 

flows, basalt-flow groups, and 

sedimentary interbeds

Distribution of 

hydraulic properties

Distribution of major rock types 

(basalt; silicic rocks; clay, silt, 

sand, and gravel)

Distribution of hydrogeologic units (younger rocks, rocks 

of intermediate age, sediment, silicic rocks, older rocks)

Distribution of hydrogeologic units 

(local impeding layers or confining 

units)

Distribution of major rock types (fractured, dense, and 

altered basalt; silicic rocks; clay, silt, sand, and gravel)

Distribution of major rock types (thin 

or thick pahoehoe basalt flows; 

near-vent volcanic deposits; dikes; 

clay, silt, sand, and gravel)

Distribution of local stratigraphic units Distribution of local stratigraphic 

units

Distribution of subregional geologic settings (rhyolite 

domes; sedimentary troughs; areas of subsidence, uplift, 

and dipping beds; volcanic rift zones and vent corridors)

Distribution of local geologic settings 

(vent corridors, sedimentary 

depositional environments)

Hydrologic fluxes Measured flux (Snake River gains 

and losses, spring flow)

Measured flux (streamflow infiltration from the Big Lost 

River channel, sinks, playas, and spreading areas)

Measured flux (streamflow losses 

in Big Lost River channel, sinks, 

playas, and spreading areas)

Estimated flux (ground-water 

pumpage, irrigation return flow, 

stream and canal losses, tributary 

basin underflow, precipitation 

recharge)

Estimated flux (ground-water pumpage, irrigation 

return flow, ground water from or to adjacent parts of 

the aquifer, tributary-valley underflow, precipitation 

recharge, flow from underlying rocks) 

Estimated flux (ground-water 

pumpage, ground water from or 

to adjacent parts of the aquifer, 

infiltration of precipitation, 

injection and infiltration of 

wastewater, water-system leaks, 

landscape-watering return flow)

Ground-water 

movement

Long-term (20 to 50 years) changes 

in climate and irrigation practices

Episodic and localized streamflow infiltration resulting 

from short-term (3 to 8 years) changes in climate

Thickness and orientation of preferential 

flowpaths (for example - fractures, 

lava tubes, etc.)

Appendix B. Levels of Information Needed for Constructing Conceptual Models 

at Different Scales

62  Conceptual Model of Ground-Water Flow, Eastern Snake River Plain Aquifer, Idaho National Laboratory and Vicinity
Manuscript approved for publication, May 25, 2006

Prepared by the USGS Publishing Network, 

Publishing Service Center, Tacoma, Washington

Bill Gibbs 

Debra Grillo

Donita Parker

Sharon Wahlstrom

For more information concerning the research in this report, contact the 

Director, Idaho Water Science Center 

U.S. Geological Survey, 230 Collins Road 

Boise, Idaho 83702

http://id.water.usgs.gov/
SIR 2006–5122 Ackerman and others

A Conceptual Model of Ground-Water Flow in the Eastern Snake River Plain Aquifer at the 

Idaho National Laboratory and Vicinity with Implications for Contaminant Transport