Analysis of Steady-State Flow and Advective Transport 

in the Eastern Snake River Plain Aquifer System, Idaho

By Daniel J. Ackerman

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 94-4257

Idaho Falls, Idaho 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For additional information write to: Copies of this report can be purchased from:

	U.S. Geological Survey

Project Chief Earth Science Information Center

U.S. Geological Survey Open-File Reports Section

INEL, MS 4148 Box 25286, MS 517

P.O. Box 2230 Denver Federal Center

Idaho Falls, ID 83403 Denver, CO 80225
CONTENTS

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

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

Purpose and scope .................................................................... 2

Geohydrology ............................................................................ 4

Conceptual models.................................................................... 4

Flow........................................................................... 6

Advective transport............................................................... 6

Numerical models. .................................................................... 6

Flow........................................................................... 8

Advective transport............................................................... 8

Model input................................................................. 9

Calibration................................................................. 10

Analysis of steady-state flow and transport .................................................... 10

Compartmentalization ................................................................ 13

Flow directions...................................................................... 15

Traveltime ......................................................................... 18

Limitations of model...................................................................... 18

Discretization....................................................................... 18

Assumptions........................................................................ 22

Summary............................................................................... 22

References.............................................................................. 24

FIGURES

1. Map showing location of the study area .................................................... 3

2. Graph showing ground-water budget for the eastern Snake River Plain regional aquifer system,

1980 water year........................................................................ 5

3. Map showing finite-difference grid and boundary of the eastern Snake River Plain regional aquifer

system ............................................................................... 7

4. Diagram showing simulated flow from well CPP 3 past well USGS 11............................11

5. Graph showing sensitivity of advective traveltime to calibration criteria for porosity ................. 12

Figures 6-9. Maps showing:

6. Compartments of flow in the eastern Snake River Plain regional aquifer system.............. 14

7. Sensitivity of interpretations regarding fate of flow to assumptions regarding influence

of weak sinks................................................................... 16

8. Selected pathlines for regional and intermediate flow and configuration of the simulated

water table, eastern Snake River Plain regional aquifer system............................ 17

9. Traveltimes for simulated flow originating at the water table in the eastern Snake 

River Plain regional aquifer system and discharging to the Snake River 

downstream from Milner Dam ..................................................... 19

10. Graphs showing sensitivity of simulated traveltime to uniform changes in porosity ................. 20

11. Graphs showing sensitivity of simulated traveltime to influence of weak sinks .................... 21
CONVERSION FACTORS AND VERTICAL DATUM

Multiply

foot (ft)

mile (mi)

square mile (mi2)

inch per year

cubic foot per second (ft3/s)

picocurie per liter

By

0.3048

1.609

2.590

25.4

0.02832

0.037

To Obtain

meter

kilometer

kilometer

millimeter per year

cubic meter per second

becquerel per liter

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 a geodetic datum derived 

from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum 

of 1929.
Analysis of Steady-State Flow and Advective 

Transport in the Eastern Snake River Plain 

Aquifer System, Idaho

By Daniel J. Ackerman

Abstract

Quantitative estimates of ground-water 

flow directions and traveltimes for advective 

flow were developed for the regional aquifer 

system of the eastern Snake River Plain, Idaho. 

The work included: (1) descriptions of 

compartments in the aquifer that function as 

intermediate and regional flow systems, (2) 

descriptions of pathlines for flow originating at 

or near the water table, and (3) quantitative 

estimates of traveltimes for advective transport 

originating at or near the water table.

A particle-tracking postprocessing 

program was used to compute pathlines on the 

basis of output from an existing threedimensional steady-state flow model. The flow 

model uses 1980 conditions to approximate 

average annual conditions for 1950-80.

The advective transport model required 

additional information about the nature of flow 

across model boundaries, aquifer thickness, 

and porosity. Porosity of two types of basalt 

strata has been reported for more than 1,500 

individual cores from test holes, wells, and 

outcrops near the south side of the Idaho 

National Engineering Laboratory. The central 

80 percent of samples had porosities of 0.08 to 

0.25, the central 50 percent of samples, 0.11 to 

0.21.

Calibration of the model involved choosing 

a value for porosity that yielded the best 

solution. Two radiologic contaminants, 

iodine-129 and tritium, both introduced to the

flow system about 40 years ago, are relatively 

conservative tracers. Iodine-129 was 

considered to be more useful because of a 

lower analytical detection limit, longer halflife, and longer flow path. The calibration 

value for porosity was 0.21.

Most flow in the aquifer is contained within 

a regional-scale compartment and follows 

paths that discharge to the Snake River 

downstream from Milner Dam. Two 

intermediate-scale compartments exist along 

the southeast side of the aquifer and near Mud 

Lake. One intermediate-scale compartment 

along the southeast side of the aquifer 

discharges to the Snake River near American 

Falls Reservoir and covers an area of nearly 

1,000 square miles. This compartment, which 

receives recharge from an area of intensive 

surface-water irrigation, is apparently fairly 

stable. The other intermediate-scale compartment near Mud Lake covers an area of 300 

square miles. The stability and size of this 

compartment are uncertain, but are assumed to 

be in a state of change.

Traveltimes for advective flow from the 

water table to discharge points in the regional 

compartment ranged from 12 to 350 years for 

80 percent of the particles; in the intermediatescale flow compartment near American Falls 

Reservoir, from 7 to 60 years for 80 percent of 

the particles; and in the intermediate-scale 

compartment near Mud Lake, from 25 to 100 

years for 80 percent of the particles.

Traveltimes are sensitive to porosity and 

assumptions regarding the importance of the
strength of internal sinks, which represent 

ground-water pumpage. A decrease in porosity 

results in shorter traveltimes but not a uniform 

decrease in traveltime, because the porosity 

and thickness is different in each model layer. 

Most flow was horizontal and occurred in the 

top 500 feet of the aquifer.

An important limitation of the model is the 

assumption of steady-state flow. The most 

recent trend in the flow system has been a 

decrease in recharge since 1987 because of an 

extended drought and changes in land use. A 

decrease in flow through the system will result 

in longer traveltimes than those predicted for a 

greater flow. Because the interpretation of the 

model was limited to flow on a larger scale, 

and did not consider individual wells or well 

fields, the interpretations were not seriously 

limited by the discretization of well discharge.

The interpretations made from this model 

also were limited by the discretization of the 

major discharge areas. Near discharge areas, 

pathlines might not be representative at the 

resolution of the grid. Most improvement in 

the estimates of ground-water flow directions 

and traveltimes for advective flow could be 

gained by better estimates of recharge from 

surface-water irrigation.

INTRODUCTION

The regional aquifer system of the eastern 

Snake River Plain (hereafter referred to as the 

aquifer) (fig. 1) is an important component of the 

hydrologic system in eastern Idaho. The aquifer 

was thought to be the largest unified ground-water 

reservoir on the North American continent (Nace, 

1958, p. 136) but is probably second to the 

Floridian aquifer in the southeastern United States 

(Heath, 1984, p. 30). Flow in the aquifer is from 

major recharge areas in the northeastern part of the 

plain to discharge areas in the southwestern part. 

About 3,600 mi2 , or almost one-third of the eastern 

Snake River Plain, was irrigated in 1979. Of the 

irrigated land, about 55 percent was irrigated with 

surface water, 40 percent with ground water, and 

5 percent with surface and ground water

(Garabedian, 1992, p. 6). A comprehensive 

analysis of the occurrence and movement of water 

in the aquifer was presented by Garabedian (1992). 

The analysis included a description of the recharge 

and discharge, the hydraulic properties, and a 

numerical model of the aquifer.

To date (1994), no quantitative estimates of 

ground-water flow directions and traveltimes exist 

for the aquifer as a whole. Estimates of flow paths 

and traveltimes are needed to aid in the management and protection of the water resources of the 

eastern Snake River Plain. Although solute transport has been modeled for a small part of the 

aquifer (Robertson, 1974; Lewis and Goldstein, 

1982) at the Idaho National Engineering 

Laboratory (fig. 1), an analysis of transport has not 

been conducted for most of the regional flow 

system.

The applicability of existing ground-water flow 

models can be extended with recently developed 

software and minimal additional information. A 

model of the aquifer developed by Garabedian 

(1992) was revisited to address environmental and 

management problems, better understand the 

aquifer, and assess needs for future modeling. 

Hypotheses concerning flow paths, traveltimes, 

and compartmentalization of the aquifer were 

tested with the MODPATH advective transport 

particle-tracking package (Pollock, 1989).

This work is an extension of the work described 

by Garabedian (1992) and incorporates information he provided on the conceptual model and 

thickness of the aquifer. Additional information on 

porosity was needed for the traveltime estimates. 

The results of this report were compared with field 

data describing traveltimes for relatively conservative tracers introduced to the flow system and 

monitored for nearly 40 years.

Purpose and Scope

The purposes of this report are to: (1) describe 

compartments in the aquifer that function as 

intermediate and regional flow systems, (2) 

describe pathlines for flow originating at or near 

the water table, and (3) quantify traveltimes for 

advective transport originating at or near the water 

table. The model constructed for this study and
CANADA

48°

2

e

(D

2

I

X

\

\'V.

IDAHO

0

I

50

i

100 15

i i

150 MILES

50 100 150 KILOMETERS

46

Boundary of P WYOMING

Snake River

Basin

Idaho National 

neering Laboratory

42

111"

Figure 1. Location of the study area.
described in this report will aid those concerned 

with the management and protection of the aquifer. 

The model will serve as a tool to further our understanding of the aquifer and will aid in assessing the 

needs for future flow and transport studies of the 

aquifer.

Because Garabedian's (1992) report is the only 

published quantitative study of the eastern Snake 

River Plain and has the only documented numerical model, this study, which uses that model, was 

limited by the scope, assumptions, and resolution 

of that report. Specifically, the numerical model of 

flow used for this report is exactly as presented by 

Garabedian (1992). Because the MODPATH 

postprocessing package available for use at the 

time of this report was limited to steady-state 

analysis, this report describes the period 1950-80 

when flow conditions were relatively stable. 

Pathlines and traveltimes for advective transport 

will evolve from those given in the report as the 

system changes from 1980 conditions.

The study is further restricted to the aquifer that 

underlies the main part of the eastern Snake River 

Plain (Kjelstrom, 1992b, p. 49, fig. 39), and to flow 

at intermediate and regional scales. Specifically, 

the main part of the eastern Snake River Plain 

excludes the area south of the Snake River west of 

113 degrees longitude. Traveltimes identified in 

this report consider only advective transport and 

are generally a conservative estimate of actual 

transport. No consideration is given to the processes of dispersion or to the physical, chemical, or 

biological transformation of solutes. For consistency in interpretation, pathlines used for analysis 

in this report were started within 20 ft of the water 

table.

GEOHYDROLOGY

The eastern Snake River Plain is a 10,000-mi2 

northeast-trending structural basin about 170 mi 

long and 60 mi wide. The regional aquifer underlying the plain consists of a layered sequence of 

basaltic-lava flows and cinder beds intercalated 

with sedimentary deposits, mainly of fluvial and 

lacustrine origin. Individual basalt flows typically 

are 20 to 25 ft thick and 50 to 100 mi2 in areal 

extent. The geology and hydrology of the eastern

Snake River Plain aquifer system describe a watertable aquifer of large areal extent.

The hydrology of the eastern Snake River Plain 

is dominated by the interplay of the Snake River 

and the regional ground-water system. Recharge to 

the aquifer is primarily from infiltration of irrigation water diverted from the Snake River and its 

tributary drainage basins (fig. 2). Almost all 

ground-water discharge is to springs along the 

Snake River or to the Snake River. Of 65 springs in 

the United States that have an average discharge of 

more than 100 ft3/s, 11 springs or groups of springs 

are along the Snake River from Milner Dam to 

King Hill (Meinzer, 1927, p. 42-51).

Changes in water use and management have 

had a significant effect on the ground- and surfacewater systems. Application of surface water for 

irrigation increased recharge to and storage in the 

aquifer (Kjelstrom, 1992b, p. 1). During more than 

100 years of irrigation, annual recharge to the 

aquifer increased by about 70 percent (Lindholm, 

1993, p. 2). Storage in the aquifer increased until 

about 1951, decreased from 1952 to 1964, and has 

decreased since 1976 because of increased 

pumpage of ground water, changes in irrigation 

practice, and persistent drought (Kjelstrom, 1992b, 

p. 1). The average ground-water discharge, which 

is mainly from spring flow, along the north side of 

the Snake River between Milner Dam and King 

Hill for the 1993 water year was estimated to be 

5,090 ft3/s (L.C. Kjelstrom, U.S. Geological 

Survey, written commun. 1994), roughly equivalent to the discharge in 1917 (Kjelstrom, 1992a).

For additional discussions of the geology of the 

eastern Snake River Plain and the hydrology of the 

aquifer, the reader is referred to reports by 

Garabedian (1992), Whitehead (1992) and 

Lindholm (1993).

Conceptual Models

The conceptual model of flow in the aquifer and 

details of numerical simulations given by 

Garabedian (1992, p. 29-50) are summarized here 

and in the section of this report entitled "Numerical 

Models". The approach used by Garabedian was to 

develop a numerical model of the aquifer to refine 

and extend concepts of regional ground-water
8,000

4,000 -

u o -4,000

-8,000

-12,000

Surface-water 

irrigation infiltration

Snake River gains

Tributary drainage-basin underflow

Ground-water 

pumpage (net)

INFLOW

Direct 

precipitation Tributary-stream and canal losses

OUTFLOW

Change in aquifer 

storage

Rgure 2. Ground-water budget for the eastern Snake River Rain regional aquifer system, 1980 water year. From Garabedian (1992, table 15)
flow. After initial conceptualization of the hydroogy, the study progressed in stages from a twodimensional steady-state parameter estimation 

model to three-dimensional steady-state and 

transient models. Estimates of recharge developed 

in the study, estimates and measurements of 

discharge, and water-level measurements in more 

than 800 wells were used to calibrate the transmissivity distribution, aquifer leakance, conductance 

of riverbeds and spring outlets, and storage 

coefficient. For each stage of the model development and calibration the results of simulation were 

compared with the knowledge of the aquifer. The 

final stage in model development was to document 

the sensitivity of model response to changes in 

transmissivity, storage, leakance, recharge, 

riverbed or spring-outlet conductance, groundwater pumpage, and tributary drainage-basin 

underflow.

Flow

The direction of ground-water flow in the 

aquifer is generally from the northeast to the 

southwest, from areas of recharge to areas of 

discharge (Garabedian, 1992, p. 21). On a regional 

scale, ground water moves horizontally thorough 

interflow zones, cinder beds, and fractures in 

basalt. For most of the aquifer, flow is virtually 

two-dimensional (Garabedian, 1992, p. 61). Large 

vertical head gradients exist in major discharge or 

recharge areas and along the margins of the flow 

system. Most recharge is from infiltration of 

surface water diverted for irrigation and leakage 

from the Snake River and its tributaries. Underflow 

from tributary basins and, to a lesser degree, 

precipitation, also recharge the aquifer. Aquifer 

discharge is largely spring flow to the Snake River 

and water pumped for irrigation. Major springs are 

along the Snake River from Milner Dam to King 

Hill (river reaches 1 to 4) (fig. 3) and immediately 

north and east of American Falls Reservoir (river 

reach 7). A ground-water budget for the 1980 

water year (fig. 2) shows the magnitude of 

individual categories of inflow and outlfow.

Hydrologic conditions in the aquifer were 

stable for the period 1950-80 relative to the period 

1880-1950. Garabedian (1992, p. 31) assumed that 

1980 conditions approximated average annual

conditions for 1950-80 and represented a steadystate condition. The aquifer was nearly at steady 

state in 1980 with fluxes nearly equal to the 

average flux for the period 1950-80. The change in 

storage (138 ftVs), calculated from water-level 

changes in the aquifer, was only slightly smaller 

than the residual in the budget (220 ftVs) that 

Garabedian (1992, table 15) compiled for the 

ground-water system.

Advective Transport

Quantitative estimates of transport require 

information on the average linear ground-water 

velocity. Specifically, additional information is 

needed about the nature of flow across model 

boundaries, aquifer thickness, and porosity. All 

flow into or out of the aquifer, except well 

discharge, was assumed to occur as flow across the 

boundary of the aquifer. Well discharge was 

considered to be distributed throughout a given 

aquifer block as discretized for this analysis. The 

thickness of the aquifer in the conceptual model for 

advective transport follows that used in the flow 

model (Garabedian, 1992, p. 38, fig. 23, pi. 5). The 

porosity of the aquifer was considered to be the 

effective porosity of the dominant rock type, 

basalt.

Regional flow in the aquifer is, in some places, 

intercepted by large-capacity wells. Except in the 

case of local contamination, most individual wells 

probably do not remove an appreciable amount of 

any contaminant moving through the aquifer. The 

conceptual model of advective transport did not 

consider the effects of wells or well fields (weak 

sinks) that do not capture a large part of regional 

flow.

Because the model of flow approximates 

average annual conditions for 1950-80 with the 

conditions in 1980, advective transport also 

approximates average annual movement for 

1950-80.

NUMERICAL MODELS

Two numerical models were used to simulate 

steady-state advective transport in the aquifer. An 

existing model simulates three-dimensional
EXPLANATION

RIVER BLOCK AND RIVER-REACH NUMBER 

ACTIVE PART OF THE MODEL

BOUNDARY OF EASTERN SNAKE RIVER PLAIN

King Hill

20 40 60 MILES

20 40 60 KILOMETERS

figure 3. Finite-difference grid and boundary of the eastern Snake River Rain regional aquifer system. After Garabedian (1992, fig. 21)
steady-state flow, and an advective transport model 

developed as part of this study computes threedimensional pathlines. The numerical models of 

flow and advective transport used in this study 

were developed from the conceptual models and 

solved using computer model codes (McDonald 

and Harbaugh, 1988; Pollock, 1989), which are in 

the public domain. For further information please 

see the cited reference to the model code or 

software. Data sets for the models used in this 

report are available from the U. S. Geological 

Survey, District Office, Boise.

Three results were obtained from numerical 

simulations of advective transport: endpoints of 

flow, direction of movement, and traveltime. 

Model evaluation consisted of two parts, 

calibration and sensitivity testing. Calibration of 

the transport model involved choosing a value for 

the least known or defined parameter, porosity, that 

yielded the best solution. The best solution was 

judged to be that which best simulated the 

observed traveltime of the most conservative 

tracer. Sensitivity testing followed the calibration 

and involved documenting the sensitivity of the 

results to variations in assumptions regarding the 

strength of internal sinks and porosity or thickness.

Flow

The model code and input data sets for the flow 

model were the same as those used by Garabedian 

(1992). The model used simulated steady-state 

flow for 1950-80 with four layers and constant 

transmissivity. Boundary conditions were 

simulated as specified flux and head-dependent 

flux using the recharge, well, and river packages 

and as no flow. Garabedian (1992, p. 31) included 

small changes in storage as part of the recharge 

term in the steady-state model calibration. For the 

flow model used in this report, change in storage 

was not included. The input data for recharge were 

those used by Garabedian (1992, p. 60, table 24) 

for the simulation of hypothetical continuation of 

1980 conditions. The transmissivity distribution 

used in the model of the aquifer was calculated 

from hydraulic conductivities and thickness of rock 

types distributed by zones across the aquifer. 

Isotropic conditions were assumed for horizontal 

flow because calibration could not be improved

with regional horizontal anisotropy. Horizontal 

hydraulic conductivity decreased with depth as a 

result of change in character of rock units in the 

aquifer (Doherty and others, 1979, p. 3; Mann, 

1986, p. 21; Garabedian, 1992, p. 10,42). 

Hydraulic conductivity values were identical at a 

given location for layers 1 and 2. Values of 

hydraulic conductivity for layers 3 and 4 were 

multiplied by two-thirds and one-third, 

respectively. Vertical conductance values were 

calculated assuming vertical to horizontal 

anisotropies of 0.01 to 0.1 depending on rock type. 

The discretization of the aquifer used in this study 

(fig. 3) was identical to that used by Garabedian 

(1992, p. 38-42, fig. 21). The grid is 21 rows by 51 

columns and each cell is 4 mi on a side. Coordinate 

translation programs given by Newton (1985, p. 

5-6) were used to translate from the latitude and 

longitude of a location to the grid coordinates. The 

computer code was the version reported in 

McDonald and Harbaugh (1988) and has had no 

computational changes from the version used by 

Garabedian (A.W. Harbaugh, oral commun., 

1992).

Advective Transport

Advective transport in the aquifer was modeled 

using a particle-tracking postprocessing package, 

MODPATH and MODPATH-PLOT (Pollock, 

1989), developed for use with the USGS modular 

flow model. The particle tracking postprocessing 

program was used to compute pathlines on the 

basis of output from simulations using 

Garabedian's (1992) three-dimensional groundwater flow model. Because the model used a semianalytical particle-tracking method for steady-state 

flow, there was no requirement for discrete time 

steps. The velocity within a block of the discretized 

aquifer was assumed to change uniformly across 

the volume, and simple linear interpolation was 

used to compute the principal velocity vectors 

within the model cells that represented the flow 

field. This assumption is a consequence of the 

method of calculating intercell flow rates in the 

flow model. Porosity is not necessary for the 

computations of pathlines of particles but was 

necessary for the calculation of traveltimes.
The advection model is a simple means of evaluating the advective transport characteristics of the 

aquifer (Pollock, 1989, p. 2). The advection model 

cannot be used to compute solute concentrations in 

ground water because it does not account for the 

effect of mixing by dispersion or for the physical, 

chemical, and biological transformation of solutes. 

The advection model is a convenient intermediate 

step between ground-water flow models and 

advection-dispersion solute transport models.

Model Input

Aquifer thickness was not an input variable for 

Garabedian's (1992) work but was used as an 

intermediate step in the creation of transmissivity 

arrays for the flow model. Layers 1 and 2 were 

defined (Garabedian, 1992, p. 38) as the first 200 

and 300 ft, respectively, of aquifer thickness. 

Garabedian (1992, pi. 5) presented the distribution 

of thickness for layers 3 and 4. These values were 

transferred to an input array for use in the 

advective transport model. Layer 3 is 500 ft or less 

in thickness and is present in the central part of the 

eastern Snake River Plain. Layer 4 ranges in 

thickness from 0 to more than 3,000 ft and is 

present over a slightly smaller area than layer 3.

The nature of simulated flow across model 

boundaries is a consequence of the conceptual 

model of flow and of the software requirements 

(Pollock, 1989, p. 22-28) for computing the 

velocity vector field. All flux was distributed 

evenly across cell faces or as an internal sink. 

Specified flux modeled using the well package 

represented pumpage from wells, flow from 

streams and canals, irrigation-return flow in the 

Mud Lake area, and underflow from tributary 

basins. Flux representing inflow from stream and 

canal losses and from irrigation-return flow was 

represented as crossing the upper face of the top 

model cell. Flux representing tributary-basin 

underflow was apportioned uniformly among 

vertical cell faces representing the horizontal 

extent of the aquifer. Specified flux modeled with 

the recharge package and flux to and from river 

blocks (head-dependent flux) also were 

represented as crossing the upper face of the top 

model cell. Pumpage from wells was represented

as an internal sink and was uniformly distributed 

throughout the volume of a model cell.

Porosity of the aquifer is a calibration variable 

in the modeling of traveltimes by advective 

transport. Porosity of two types of basalt strata has 

been determined for more than 1,500 individual 

cores from test holes, wells, and outcrops near the 

south side of the Idaho National Engineering 

Laboratory (Knutson and others, 1990, 1992). 

Porosity values were determined for large core 

samples using volumetric/gravimetric methods 

(Knutson and others, 1990, p. D-17) and for 

samples of the core using a helium porosimeter 

(Knutson and others, 1990, p. D-43). Values from 

the two methods for several sample sets had 

correlation coefficients of 0.98, indicating that 

effective porosity was very similar to total porosity 

(C.F. Knutson, EG&G Idaho, written commun., 

1994). The median values of porosity were 0.11 for 

nonvesicular samples and 0.22 for vesicular 

samples (Knutson and others, 1992, p. 4-21). A 

cumulative distribution curve for all samples 

showed that the central 80 percent of samples had 

porosities of 0.08 to 0.25, the central 50 percent of 

samples, 0.11 to 0.21 (Knutson and others, 1992, 

fig. 4-10). From the description of more than 

3,600 ft of core, Knutson and others (1992, p. 3-8) 

estimated that approximately 57 percent of the core 

examined was nonvesicular basalt and 43 percent 

was vesicular. Bishop (1991) analyzed 71 core 

samples of a vesicular basalt block taken from a 

site in the same area as the core samples reported 

by Knutson and others (1990, 1992). The median 

and mean effective porosities were both 0.23 for 

the 71 cores analyzed by Bishop (1991, p. 77).

Barraclough and others (1967, p. 61) quoting 

George H. Chase (U.S. Geological Survey, unpub. 

data) reported an average porosity of 0.18 for 100 

samples of basalt from the Idaho National 

Engineering Laboratory. Robertson (1969, p. 13) 

also quoting Chase, gave the same value and also 

gives a value of 0.18 for a single sample of basalt.

Unsubstantiated estimates of porosity used 

previously are as follows: solute transport 

modeling for the aquifer at the Idaho National 

Engineering Laboratory, 0.10 (Robertson, 1974, 

p. 13); solute transport modeling for waste seepage 

at the Idaho National Engineering Laboratory,
0.10 (Robertson, 1977, p. 25, 31,45); materials 

balances in the aquifer, 0.06-0.10 (Barraclough and 

others, 1967, p. 63), 0.07 (Robertson and others, 

1974, p. 176); general aquifer descriptions, 

0.05-0.15 (Robertson and others, 1974, p. 176), 

0.05-0.15 (Barraclough and others, 1967, p. 63).

The choice for a range of probable values for 

porosity of the aquifer was 0.10 to 0.25. This range 

is similar to the range of observed values and was 

only slightly higher than the range of values used 

for other studies or suggested by other studies. The 

presence of interflow zones, clinker beds, and large 

fractures, which were not present in core samples, 

would tend to increase the porosity as determined 

from the analysis of samples.

Calibration

The two model variables that influenced 

traveltime calculated from the model output were 

the porosity and the thickness. Because thickness 

was an integral part of the flow model, it was not 

chosen as a calibration variable. Values of porosity 

used in calibration were constrained by prior 

information about basalt porosity.

Two radiologic contaminants, iodine-129 and 

tritium, both introduced to the flow system about 

40 years ago, were available to act as relatively 

conservative tracers. The half-life for iodine-129 is 

15.7 million years and that of tritium is 12.4 years. 

The contaminants were introduced directly through 

a 590-ft-deep injection well (CPP 3) at the Idaho 

Chemical Processing Plant at the Idaho National 

Engineering Laboratory starting in 1953 (Mann 

and Cecil, 1990, p. 13). Tritium was first detected 

in wells at the south boundary of the Idaho 

National Engineering Laboratory in 1983 (Mann 

and Cecil, 1990, p. 27). The method detection limit 

typically used for the analysis of tritium is 

500 picocuries per liter. Iodine-129 was detected in 

well USGS 11 at a concentration just above 

background after no more than 38 years (Mann and 

Beasley, 1994, p. 24). Accelerator mass 

spectroscopy was used to detect iodine-129 at a 

concentration of (1.0±0.1)xlO~5 picocuries per liter 

in water from well USGS 11 (Mann and Beasley, 

1994, p. 9, table 2). The traveltime for iodine-129 

was considered to be more useful for comparison

with predicted traveltimes because of a lower 

analytical detection limit, longer half-life, and 

longer flow path.

The absolute value of the difference between 

simulated and estimated traveltimes for the 

radiologic contaminants was chosen as the 

calibration criterion. A calibration target of 

±20 percent of the traveltime was set.

An example of how the traveltime for a particle 

introduced at a location corresponding to the 

injection well was calculated for various porosities 

is shown on figure 4, a projection of the threedimensional path of a particle to the top of layer 1. 

The pathline of the particle representing tracer 

movement did not cross the bottom of layer 1, a 

depth of 200 ft below the water table. The 

calibration value for porosity was 0.21 for 

iodine-129 and 0.25 for tritium (fig. 5). Values of 

porosity between 0.17 and 0.25 were within the 

calibration target for iodine-129. Values of 

porosity between 0.20 and 0.30 were within the 

calibration target for tritium. The earliest arrival 

for tritium is less likely to be detected given the 

larger method detection limit and the decreased 

concentration because of radioactive decay. To the 

nearest two significant figures, the calibration 

value for porosity was 0.21; this value allowed the 

model to simulate the traveltime for iodine-129 

(fig. 5). Values of 0.15 and 0.25 were chosen as 

representative of the probable range of porosity 

and were used for sensitivity analysis. Values of 

porosity yielding traveltimes at the upper end of 

the range of the tritium calibration targets, 0.25 to 

0.30, were outside the range of probable values for 

basalt porosity.

ANALYSIS OF STEADY-STATE FLOW 

AND TRANSPORT

The results of simulations of particle travel can 

be analyzed to develop or extend knowledge of the 

compartmentalization of the aquifer, flow paths in 

the aquifer, and traveltime for flow in the aquifer. 

The knowledge of compartmentalization, paths, 

and traveltimes is valuable for interpretation of 

studies involving the movement of natural tracers, 

the movement of contaminants, and the chemical 

evolution of water in the flow system.

10
Injection well 

CPP 3

South boundary of

the Idaho National

Engineering Laboratory

Well USGS 11 

traveltime 37.7 years

6 

Figure 4. Simulated flow from well CPP 3 past well USGS 11. [Nodes on pathline are at 1 year intervals
25

0.05 0.10 0.15 0.20 0.25 0.30

POROSITY 

Figure 5. Sensitivity of advective traveltime to calibration criteria for porosity.

0.35

12
The pathlines represent a calculation that 

integrates all the knowledge used to construct the 

flow model, including the location and magnitude 

of inflow and outflow, the spatial distribution of 

head and hydraulic parameters, the size and shape 

of the region of flow, and the equation of flow. In 

essence, the model is a boundary value problem 

(Freeze and Cherry, 1979, p. 67). In this model the 

simulated distribution of head was not directly 

used to construct the pathlines but was used in the 

calibration of the transmissivity distribution for the 

flow model.

Compartmentalization

An analysis of Compartmentalization of the 

aquifer is illustrated in figure 6. Four particles, 

equally spaced areally, were started at a depth of 

0.9 of the cell height (20 ft below the water table) 

for each cell. The particles were sorted according 

to the location of the endpoint of flow, and the 

starting points were posted on a map of the model 

grid (fig. 6). Most pathlines that didn't end at 

strong sinks (rivers and springs) in the model river 

blocks ended in two cells with relatively strong 

sinks near Mud Lake. Most particles followed 

pathlines of regional or intermediate scale, 

although a few were of local scale. The pathlines 

simulated the movement of a solute introduced to 

the flow system at or near the water table.

Division of the flow system into intermediateand regional-scale compartments facilitates the 

discussion of the fate of contaminants that have 

been or might be introduced to a system. Also, the 

discussion of the geochemical evolution of waters 

is usually framed within the concept of a flow 

system from recharge area to discharge area. The 

existence of flow paths confined within a 

compartment has important implications for the 

fate of contaminants. Generally, a contaminant 

introduced in a compartment will be discharged 

within that compartment, limiting the area of 

concern. But, given that flow is three dimensional, 

a regional compartment may exist at depth at a 

given location within the areal projection of an 

intermediate-scale compartment. This report 

considers the case where a contaminant is 

introduced near the water table. The interpretation 

of compartments of the flow system as simulated

by Garabedian's (1992) flow model is qualified by 

considering the sensitivity of pathline analysis to 

assumptions regarding the effects of weak sinks.

Most flow in the aquifer is contained within the 

regional-scale compartment and follows paths that 

discharge to the Snake River downstream from 

Milner Dam (reaches 1 to 4). Intermediate-scale 

compartments exist at two locations along the 

southeast side of the aquifer and near Mud Lake. 

The remaining particles, about 1 percent of all 

particles, follow short paths to river nodes 

representing the Snake River downstream from 

American Falls Reservoir (reach 6) or to sinks at 

the top of the cell and are labeled as miscellaneous 

local flow.

The American Falls compartment, an intermediate scale compartment along the southeast 

side of the aquifer, discharges to the Snake River 

near American Falls Reservoir (reach 7) and covers 

an area of 1,000 mi2 (fig. 6). The American Falls 

compartment receives recharge from an area of 

intensive surface-water irrigation. Garabedian 

(1992, pi. 8) estimated that recharge from surfacewater irrigation and precipitation ranged from 10 to 

100 inches per year for most of the compartment. 

The flow system in the American Falls compartment is apparently fairly stable, because the mean 

annual ground-water discharge to this reach of the 

Snake River has been consistent since 1912 

(Garabedian, 1992, p. 19).

The Mud Lake compartment, an intermediatescale compartment near Mud Lake, covers an area 

of 300 mi2 . Two processes appear to be 

contributing to the existence of the flow system 

within the Mud Lake compartment. A band of 

lower permeability sediments and basalt that 

underlie an area from northwest of Mud Lake to 

the Snake River impedes horizontal ground-water 

movement and contributes to conditions that 

produce upward flow (Spinazola, 1994, p. 29). 

Garabedian (1992) and Spinazola (1994) described 

the distribution of ground-water pumpage, 

recharge from irrigation, and recharge from stream, 

lake, and canal losses near Mud Lake. In this area 

some ground water is pumped or flows from wells 

to canals and streams for use in areas removed 

from the ground-water pumpage center. The net 

result is increased recharge downgradient from the

13
EXPLANATION

PARTICLE STARTING POINTS - sorted according 

to discharge location

Regional compartment discharging to the Snake River 

downstream from Milner Dam

American Falls compartment 

Mud Lake compartment 

Miscellaneous local flow

60 KILOM

Figure 6 . Compartments of flow in the eastern Snake River Rain regional aquifer system.
discharge area. The stability or size of the flow 

system within this compartment is uncertain. Mud 

Lake has been in existence since before 1900 but 

increased dramatically in size between 1908 and 

1923 (Stearns and others, 1939, p. 95). The 

existence of Mud Lake before irrigation 

development may be evidence of a stable discharge 

area reflecting intermediate-scale flow, but the 

increase in size is certainly an indication that the 

compartment is in a state of change.

For the case of a steady-state flow field, the 

pathlines and end points of particles are not 

affected by porosity and thickness. End points of 

flow are sensitive to the assumptions regarding the 

importance of the strength of internal sinks which, 

in this model, represent ground-water pumpage. 

When a model is constructed to represent regional 

flow, the intricate details of local flow, such as the 

influence of local wells or local well fields, can not 

be unambiguously represented. The MODPATH 

software allows selection of the criteria for 

terminating pathlines when they reach a cell where 

discharge to internal sinks is larger than a specified 

fraction of the total inflow to a cell. To test the 

sensitivity of model results to the assumption that 

weak sinks were not important, endpoints of flow 

and traveltimes were calculated for scenarios 

where weak sinks of four different strengths were 

allowed to stop particles. The strengths were 

fractions greater than 0.8, 0.6, 0.4, and 0.2 of the 

total inflow to a cell. The case where the strength 

of a weak sink is greater than 0.2 of the total inflow 

to a cell is the most restrictive, as this represents 

the case where a smaller internal sink (representing 

well discharge) would stop particle movement for 

pathlines that enter that cell in the model. If the 

ratio of weak sink flux to total flux is 0.2 or greater, 

then the size of the compartment near Mud Lake is 

larger, 380 mi2, and more miscellaneous local flow 

is indicated. As illustrated in figure 7, interpretations regarding the compartments of flow are 

sensitive to assumptions regarding the importance 

of the strength of internal sinks. The sensitivity of 

the model to assumptions regarding weak sinks 

indicates that the size of the Mud Lake compartment might be interpreted as possibly being 

somewhat larger. If the ratio of weak sink flux to 

total inflow is 0.8 or greater, then the discharge 

area increases from one to three cells and the flow

system increases to 400 mi2 . If the ratio is 0.6 or 

greater (fig. 7), the area increases to 510 mi2 .

Sensitivity analysis (fig. 7) also indicated the 

possible existence of a third intermediate- to localscale compartment north of the Snake River 

upstream from Milner Dam (reach 5). The compartment may constitute an area of 90 mi2 and 

discharge to three cells. This compartment is 

similar to the Mud Lake compartment in that 

ground-water usage is substantial and no surfacewater irrigation takes place immediately upgradient (Garabedian, 1992, pis. 8 and 9). Of the 

areas where ground-water usage is substantial, this 

area was the first to be developed (Garabedian, 

1992, pi. 9).

Flow Directions

A projection of the pathlines of particles that 

represent the locations of major inflows to the 

aquifer is shown on figure 8. The particles were 

started at the center of the top face of a cell and at a 

depth of 0.9 of the cell height. The starting 

locations corresponded to all cells where inflows 

were greater than 75 ft3/s (63 inches per year per 

cell) and represented 35 percent of the total inflow. 

The pathlines are a projection to the top layer 

(water table) of the three-dimensional path. A 

description of flow paths in a three-dimensional 

flow system is difficult to construct and portray in 

two dimensions. The recharge from intensive 

surface-water irrigation along the southeast side of 

the aquifer, the American Falls compartment, 

resulted in a concentration of pathlines. This 

concentration of pathlines illustrates the increased 

ground-water flow per unit area in this compartent. Conversely, the relative paucity of pathlines on 

the northwest side of the aquifer indicates less 

ground-water flow.

Along the edges of the American Falls 

compartment, the pathlines for regional flow cross 

those for intermediate flow. Crossing pathlines 

depict regional flow passing beneath intermediate 

flow. At the end of the flow system, the pathlines 

converge at the discharge area. Because the pathines are an approximation of most flow, pathline 

density is greatest near cells that represent the river 

reaches with the greatest gain from the aquifer.

15
EXPLANATION

PARTICLE STARTING POINTS - sorted according 

to discharge location

Regional compartment discharging to the Snake River 

downstream from Milner Dam

American Falls compartment 

Mud Lake compartment 

Miscellaneous local flow

60 KILO

Figure 7. Sensitivity of interpretations regarding fate of flow to assumptions regarding influence of weak sinks. [We

of total inflow to a cell stop particle movement]
EXPLANATION

i.OOO SIMULATED POTENT1OMETRIC CONTOUR -- Shows altitude of 

water table, 1980. Contour intervals 200 and 500 feet. 

Datum is sea level

PATH LINES FOR PARTICLES STARTING AT THE WATER TABLE 

(Sorted according to discharge location)

_____ Regional compartment discharging to

Snake River downstream from Mflner Dam

American Falls compartment discharging 

along the east side of the aquifer

Mud Lake compartment discharging 

near Mud Lake

5,000^

60 KI

Figure 8. Selected pathlines for regional and intermediate flow and configuration of the simulated water table, e

regional aquifer system. [Pathlines are projected to the surface of the aquifer]
Traveltime

The results of an analysis of traveltime for 

advective flow in the regional compartment are 

illustrated on figure 9. This simplistic representation of traveltime for flow in the aquifer was 

constructed using the existing conceptual and 

numerical models. The illustration was constructed 

by posting the starting points sorted by intervals of 

traveltime. The model was calibrated for average 

flow conditions for the period 1950-80, very nearly 

the time period for the calibration of traveltime: 

1953-83 for tritium and 1953-91 for iodine-129. 

Inasmuch as flow conditions in the aquifer differ 

from the 1950-80 average because of changes in 

climate or land use, the interpretation of traveltime 

is more or less representative.

Traveltimes for advective flow from the water 

table to discharge points in the regional compartment ranged from 12 to 350 years for 80 percent of 

the particles. For discharge in the American Falls 

compartment, 80 percent of the particles had 

traveltimes between 7 and 60 years. For discharge 

in the Mud Lake compartment 80 percent of the 

particles had traveltimes between 25 and 100 

years.

Traveltimes are sensitive to porosity and 

assumptions regarding the importance of the 

strength of internal sinks, which represent groundwater pumpage. The distribution of traveltimes for 

all particles starting near the water table is 

presented in cumulative distribution graphs (figs. 

10 and 11).

Traveltime is a function of the product of 

thickness and porosity

bnA (1)

where t is the traveltime, b is the aquifer thickness, 

n is the porosity, A is the area, and Q is the 

volumetric flow rate. Because thickness also is 

directly related to traveltime, the sensitivity to 

values of thickness will be similar to that of 

porosity. Traveltimes were sensitive to both the 

value of porosity and the ratio of porosity between 

the upper and lower parts of the aquifer. A 

decrease in porosity resulted in shorter traveltimes,

but not a uniform decrease in traveltime because 

the porosity and thickness was different in each 

model layer. The distribution of traveltimes was 

not as sensitive to variation in thickness, especially 

the thickness of lower layers, because most flow 

was horizontal and occurred in the top two layers.

Assumptions regarding the importance of the 

strength of internal sinks may affect the calculation 

of the endpoints of pathlines for particles and 

therefore the traveltime. As more and more 

particles are stopped when they reach a cell with a 

stronger sink, the shift in the distribution of 

traveltimes indicates a decrease in traveltimes. The 

model is not uniformly sensitive to a change in the 

ratio that governs which cells have a weak sink 

strong enough to simulate capturing a part of 

regional flow. This nonuniformity is most likely a 

consequence of the areal distribution and rate of 

ground-water pumpage.

LIMITATIONS OF MODEL

The model developed for this study has 

limitations that must be understood if it is to be 

used effectively. These limitations concern the 

underlying assumptions of the particle-tracking 

package, the discretization of the aquifer system, 

and the assumptions regarding uncertainty in 

model parameters and boundary conditions. The 

limitations because of the underlying assumptions 

of the method are covered in Pollock (1989, p. 19).

Discretization

An important limitation of this model is the 

assumption of steady-state flow. The most recent 

trend in the flow system has been a decrease in 

recharge since 1987 because of an extended 

drought and changes in land use (Kjelstrom, 

1992a). A decrease in flow through the system 

would result in traveltimes longer than those 

predicted for a greater flow.

Conceptualization of well discharge as internal 

sinks resulted in the creation of many weak sinks 

for the discretization of this model. Discretization 

resulted in some ambiguity near the Mud Lake 

intermediate-scale discharge area. The interpretation of the size of the flow system compartment 

might be underestimated because of the

18
EXPLANATION

TRAVELTIME FOR PARTICLES ORIGINATING AT THE WATER TABLE

 * Less than 50 years

*** 50 to 100 years 

100 to 200 years

 '» 200 to 300 years

*** Greater than 300 years

60 K

Rgure 9. Traveltimes for simulated flow originating at the water table in the eastern Snake River Rain reg

and discharging to the Snake River downstream from Milner Dam.
500

450

400 ~

350 -

300 -

Porosity 0.15 

Porosity 0.21 

- Porosity 0.25

I . . , , I , , . , I . . . . I . , , , I , . , , I , , , , I

50 -

0 "-1 l

10 20 30 40 50 60 70 80 90 

PERCENT OF OBSERVATIONS EXCEEDING A GIVEN TRAVELTIME

Figure 10. Sensitivity of simulated traveltime to uniform changes in porosity.

100

20
500

450

400

350

300

250

200

150

100 -

50 -

No Weak sink active

Weak sinks greater than 0.8

- - - Weak sinks greater than 0.6

------ Weak sinks greater than 0.4

- Weak sinks greater than 0.2

0 10 20 30 40 50 60 70 80 90 100 

PERCENT OF OBSERVATIONS EXCEEDING A GIVEN TRAVELTIME

Rgure 11. Sensitivity of simulated traveltime to influence of weak sinks. [Weak sinks stop particles 

when they are greater than a given proportion of total inflow to a model cell]

21
assumption that discharge to wells is uniformly 

distributed throughout the cell. Because the 

interpretation of the model was limited to flow on a 

larger scale and did not consider individual wells 

or well fields, the interpretations were not seriously 

limited by the discretization of well discharge.

Interpretations made from this model also were 

limited by the discretization of the major discharge 

areas along the Snake River. Discharge to the 

Snake River is known and calibrated only for an 

entire reach length, a distance covering several 

model cells. Near discharge areas, pathlines are 

representative at a scale slightly coarser than the 

resolution of the grid.

Assumptions

The most important assumption of this study is 

the use of the existing flow model and the 

assumptions inherent in that model (Garabedian, 

1992, p.31). The flow model is assumed to be the 

best existing description of the regional flow 

system. The flow model was most sensitive to 

changes in transmissivity and recharge, sensitive to 

riverbed or spring-outlet conductances near major 

discharge areas, and relatively insensitive to 

changes in boundary flux and ground-water 

pumpage (Garabedian, 1992, p. 59). The calibration of transmissivity is related to the distribution 

of head measurements of the aquifer; the calibration of recharge to the quality of the data used in 

the estimates of recharge. Garabedian (1992, p. 59, 

63) suggests that improvement can be gained by 

better estimates of surface-water diversions, 

irrigation-return flow, streamflow, and groundwater pumpage. Most of these improvements are 

related to estimates of recharge from surface-water 

irrigation.

The model is most sensitive to the estimate of 

the largest inflow. Like most models, this flow 

model is not a unique approximation of the flow 

system. The results of this study are a representation of how, where, and how fast water moves in an 

idealized system approximated by the flow model.

The value for porosity of aquifer materials used 

for simulation in this study might not be transferable to other models with different scales, 

discretizations, and assumptions. The calibration of

traveltime in this study is possibly a calibration of a 

parameter made up of the product of porosity, 0.21, 

and thickness, 200 ft, of layer 1.

The calibration was based on the assumption 

that iodine-129 is relatively conservative. Recent 

data for chlorine-36, which has a half-life of 

301,000 years and a lower detection limit, indicate 

that iodine-129 is slightly attenuated (L.D. Cecil, 

U.S. Geological Survey, written commun., 1994). 

Preliminary results from the analysis of archived 

samples indicate that traveltimes to well USGS 11 

may be a few years shorter for chlorine-36. The 

shorter travel time for chorine-36 would indicate 

that the aquifer porosity may be slightly less than 

the value of 0.21 used to calibrate the advective 

transport model (fig. 5).

SUMMARY

Quantitative estimates of ground-water flow 

directions and traveltimes for advective flow were 

developed for the regional aquifer system of the 

eastern Snake River Plain, Idaho. The work 

included: (1) descriptions of compartments in the 

aquifer that contain separate regional or 

intermediate flow systems, (2) descriptions of 

pathlines for flow originating at or near the water 

table, and (3) quantitative estimates of traveltimes 

for advective transport originating at or near the 

water table.

Two numerical models were necessary to 

simulate steady-state advective transport in the 

aquifer. An existing model simulates threedimensional steady-state flow, and an advective 

transport model developed in this study computes 

three-dimensional pathlines.

Advective transport in the aquifer was modeled 

using a particle-tracking postprocessing package 

developed for use with the USGS modular flow 

model. The particle tracking postprocessing 

program was used to compute pathlines on the 

basis of output from flow model simulations. No 

consideration was given to the processes of dispersion or to the physical, chemical, and biological 

transformation of solutes. The model of flow uses 

1980 conditions to approximate average annual 

conditions for 1950-80.

22
The advective transport model required 

additional information about the nature of flow 

across model boundaries, aquifer thickness, and 

porosity. Aquifer thickness was not an input 

variable for the flow model but was documented in 

the descriptions of an intermediate step in the 

creation of transmissivity arrays. In the advective 

transport model, all flux was distributed evenly 

across cell faces or as an internal sink. Some data 

exist that serve as a guide for reasonable input 

values for porosity. Porosity of two types of basalt 

strata has been determined for more than 1,500 

individual cores from test holes, wells, and 

outcrops near the south side of the Idaho National 

Engineering Laboratory. The central 80 percent of 

samples had porosities of 0.08 to 0.25, the central 

50 percent of samples, 0.11 to 0.21.

The calibration of this model involved choosing 

a value for the least known or defined parameter, 

porosity, which yielded the best solution. Two 

radiologic contaminants, iodine-129 and tritium, 

both introduced to the flow system about 40 years 

ago, were available to act as relatively conservative 

tracers. The traveltime for iodine-129 was 

considered to be more useful for comparison with 

predicted traveltimes, because of a lower analytical 

detection limit, longer half-life, and longer flow 

path. The absolute value of the difference between 

simulated and estimated traveltimes for the 

radiologic contaminants was chosen as the 

calibration criterion. To the nearest two significant 

figures the calibration value for porosity was 0.21, 

the value that allowed the model to simulate the 

traveltime for iodine-129. Values of 0.15 and 0.25 

were chosen as representative of the probable 

range of porosity and were used for sensitivity 

analysis. This range is similar to the range of 

observed values and is only slightly higher than the 

range of values used for other studies or suggested 

by other studies.

The results of simulations of particle travel can 

be analyzed to develop or extend knowledge of the 

compartmentalization of the aquifer, flow paths in 

the aquifer, and traveltime for flow in the aquifer. 

Most flow in the aquifer follows regional paths that 

discharge to the Snake River downstream from 

Milner Dam. Intermediate-scale compartments 

exist along the southeast side of the aquifer and 

near Mud Lake.

The American Falls compartment along the 

southeast side of the aquifer discharges to the 

Snake River near American Falls Reservoir and 

covers an area of nearly 1,000 mi2 . The recharge 

from intensive surface-water irrigation along the 

southeast side of the aquifer results in a concentration of ground-water flow. The flow system within 

the compartment is apparently fairly stable because 

the mean annual ground-water discharge to this 

reach of the Snake River has been consistent since 

1912.

The Mud Lake compartment covers an area of 

300 mi2 . The stability or size of the flow system 

within this compartment is uncertain. The 

existence of Mud Lake before irrigation development may be evidence of a stable discharge area 

reflecting intermediate-scale flow, but the changes 

in size since 1908 indicate that the size of the flow 

system is in a state of change.

To test the sensitivity of model results to 

assumptions regarding weak sinks, endpoints of 

flow and traveltimes were calculated for scenarios 

where weak sinks of four different strengths were 

allowed to stop particles. Interpretations regarding 

the compartments were sensitive to assumptions 

regarding the importance of the strength of internal 

sinks. The effects of allowing weak sinks to stop 

particles were that the size of the Mud Lake 

compartment is larger and more miscellaneous 

local flow is indicated. At the end of the flow 

system, the pathlines converge at the discharge 

area. Because the pathlines are an approximation 

of most flow, the density is greatest near the cells 

representing the river reaches with the greatest gain 

from the aquifer.

Traveltimes for advective flow from the water 

table to regional discharge areas in the regional 

compartment ranged from 12 to 350 years for 80 

percent of the particles. For discharge in the 

American Falls compartment, 80 percent of the 

particles had traveltimes between 7 and 60 years. 

For discharge in the Mud Lake compartment, 80 

percent of the particles had traveltimes between 25 

and 100 years.

Traveltimes are sensitive to porosity and 

assumptions regarding the importance of the 

strength of internal sinks that represent ground23
water pumpage. A decrease in porosity resulted in 

shorter traveltimes but not a uniform decrease in 

traveltime because the porosity and thickness was 

different in each model layer. Most flow was 

horizontal and occurred in the top two layers that 

simulate the top 500 ft of the aquifer. Assumptions 

regarding the importance of the strength of internal 

sinks may affect the calculation of the endpoints of 

pathlines for particles and therefore the traveltime. 

As more and more particles are stopped when they 

reach a cell with a stronger weak sink, the shift in 

the distribution of traveltimes indicates a decrease 

in traveltimes. The model is not uniformly 

sensitive to a change in the ratio that governs 

which cells have a weak sink strong enough to 

simulate capturing a part of regional flow. This 

nonuniformity is most likely a consequence of the 

areal distribution and rate of ground-water 

pumpage.

In addition to the limitations because of the 

underlying assumptions of the method, there are 

limitations because of discretization effects, 

uncertainty in input data, and other assumptions. 

An important limitation of the model is the 

assumption of steady-state flow. The most recent 

trend in the flow system has been a decrease in 

recharge since 1987, because of an extended 

drought and changes in land use. A decrease in 

flow through the system would result in traveltimes 

longer than those predicted for a greater flow. The 

conceptualization of well discharge as internal 

sinks resulted in the creation of many weak sinks 

for the discretization of this model. Because the 

interpretation of the model was limited to flow on a 

larger scale, and did not consider individual wells 

or well fields, the interpretations were not seriously 

limited by the discretization of well discharge.

The interpretations made from this model also 

were limited by the discretization of the major 

discharge areas. Discharge to the Snake River is 

known and calibrated only for an entire reach 

length, a distance covering several model cells. 

Near discharge areas, pathlines might not be 

representative at the resolution of the grid. The 

most important assumption of this study is the use 

of the existing flow model and the assumptions 

inherent in that model. The flow model is assumed 

to be the best existing description of the regional 

flow system. The flow model was most sensitive to

changes in transmissivity and recharge, sensitive to 

riverbed or spring-outlet conductances near major 

discharge areas, and relatively insensitive to 

changes in boundary flux and ground-water 

pumpage. Most improvement in the estimates of 

ground-water flow directions and traveltimes for 

advective flow could be gained by better estimates 

of recharge from surface-water irrigation.

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Jensen, R.G., 1967, Hydrology of the National 

Reactor Testing Station, Idaho, 1966: U.S. 

Geological Survey Open-File Report 

(IDO-22049), 95 p.

Bishop, C.W., 1991, Hydraulic properties of vesicular 

basalt: Tucson, University of Arizona, unpublished 

M.S. thesis, 115 p.

Doherty, D.J., McBroome, L.A., and Kuntz, M.A, 1979, 

Preliminary geological interpretation and 

lithologic log of the exploratory geothermal test 

well (INEL-1), Idaho National Engineering 

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Freeze, R.A., and Cherry, J.A., 1979, Groundwater: 

Englewood Cliffs, N.J., Prentice-Hall, 604 p.

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simulation of the regional aquifer system, eastern 

Snake River Plain, Idaho: U.S. Geological Survey 

Professional Paper 1408-F, 102 p., 10 pis.

Heath, R.C., 1984, Ground-water regions of the United 

States: U.S. Geological Survey Water-Supply 

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to the Snake River, Milner Dam to King Hill, 

Idaho: U.S. Geological Survey Open-File Report 

92-147 (Water Fact Sheet), [2] p.

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M.A, and Fishel, M.L, 1992, 3D RWMC vadose 

zone modeling (including FY-89 to FY-90 basalt 

characterization results): EG&G Idaho, Inc., 

Report EGG-ERD-10246 [variously paged].

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Knutson, C.F., McCormick, K.A., Smith, R.P., Hackett, 

W.R., O'Brien, J.P., and Crocker, J.C., 1990, 

FY-89 RWMC vadose zone basalt 

characterization: EG&G Idaho, Inc., Report 

EGG-WM-8949 [variously paged].

Lewis, B.D., and Goldstein, F.J., 1982, Evaluation of a 

predictive ground-water solute-transport model at 

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