U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2016–5026

Prepared in cooperation with the Bureau of Reclamation

Simulation of Groundwater Storage Changes in the 

Eastern Pasco Basin, Washington
Cover: Irrigated agricultural fields in Ringold Coulee, 24 miles north of Pasco, Washington.

View to the north includes Pliocene Upper Ringold Formation lacustrine sediments exposed at the head 

scarp (back cover) and debris from the May 2006 landslide that covered a portion of State Route 170.

Photograph by Charles Heywood, U.S. Geological Survey, June 10, 2014.
Simulation of Groundwater Storage 

Changes in the Eastern Pasco Basin, 

Washington

By Charles E. Heywood, Sue C. Kahle, Theresa D. Olsen, James D. Patterson, and 

Erick Burns

Prepared in cooperation with the Bureau of Reclamation

Scientific Investigations Report 2016–5026

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Heywood, C.E., Kahle, S.C., Olsen, T.D., Patterson, J.D., and Burns, Erick, 2016, Simulation of groundwater storage 

changes in the eastern Pasco Basin, Washington: U.S. Geological Survey Scientific Investigations Report 2016–5026, 

44 p., 1 pl., http://dx.doi.org/10.3133/sir20165026.

ISSN 2328-0328 (online)
iii

Contents

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

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Previous Investigations........................................................................................................................2

Methods of Investigation.....................................................................................................................8

Well Data.......................................................................................................................................8

Hydrogeology................................................................................................................................8

Hydrogeologic Framework...........................................................................................................................8

Geologic Setting....................................................................................................................................8

Hydrogeologic Units...........................................................................................................................10

Basalt Hydrogeologic Unit (CRBG) .........................................................................................12

Sedimentary Hydrogeologic Units ..........................................................................................12

Hydraulic Properties .................................................................................................................14

Hydrologic Setting ..............................................................................................................................15

Recharge.....................................................................................................................................15

Rivers .........................................................................................................................................15

Agricultural Drains.....................................................................................................................15

Springs.........................................................................................................................................15

Historical Groundwater Levels................................................................................................15

Simulation of Groundwater Flow...............................................................................................................19

Modeling Strategy ..............................................................................................................................19

Numerical Method..............................................................................................................................20

Spatial Discretization .........................................................................................................................20

Time Discretization .............................................................................................................................20

Boundary Conditions..........................................................................................................................20

Specified Flow Boundaries ......................................................................................................20

Head-Dependent Flow Boundaries ........................................................................................20

Hydraulic Property Distribution........................................................................................................23

Horizontal Flow Barriers...........................................................................................................23

Water-Level Measurements .............................................................................................................25

Model Calibration................................................................................................................................25

Parameter Sensitivities.............................................................................................................26

Simulated Water Levels.....................................................................................................................26

Simulated Groundwater Budget.......................................................................................................29

Simulated Storage Changes in the 508-14 Area ...................................................................33

Limitations and Appropriate Use of the Model..............................................................................36

Scenarios ......................................................................................................................................................36

Scenario 1—Landslide Abatement..................................................................................................37

Scenario 2—Withdrawal from Pasco Gravels ..............................................................................38

Scenario 3—Withdrawal from Upper Ringold Formation............................................................40

Scenario 4—Withdrawal from Ringold Formation and Saddle Mountains and 

Wanapum Basalts .................................................................................................................41
iv

Contents—Continued

Plate

1. Hydrologic unit map, hydrogeologic sections, and locations of project wells,

eastern Pasco Basin, Washington. download at http://dx.doi.org/10.3133/sir20165026

Summary........................................................................................................................................................42

Acknowledgments.......................................................................................................................................42

References Cited..........................................................................................................................................42

Figures

1. Map showing location of the active model domain and 508-14 area, eastern 

Pasco Basin, Washington ...........................................................................................................3

2. Map showing simplified National Land Cover of the eastern Pasco Basin,

Washington, 2006..........................................................................................................................4

3. Map showing locations of rivers, springs, water-delivery infrastructure, buried

drains, and pending water-right applications, eastern Pasco Basin, Washington...........5

4. Map showing locations of selected hydrogeologic studies of the eastern Pasco

Basin and selected adjacent areas, Washington ...................................................................6

5. Map showing surficial geology of the eastern Pasco Basin, Washington.........................9

6. Map showing top altitude of basalt surface, eastern Pasco Basin, Washington ...........13

7. Map showing extent and thickness of combined overburden units, eastern 

Pasco Basin, Washington .........................................................................................................14

8. Map showing water-level altitudes in wells, 1939–45 and hydrograph of well at

location B, eastern Pasco Basin, Washington. ....................................................................18

9. Map showing finite-difference grid for the groundwater model with river and 

drain cells, general-head boundary cells, and horizontal-flow barriers, eastern 

Pasco Basin, Washington .........................................................................................................21

10. Map showing distribution of recharge specified in the Pasco Basin model 

domain for 2007, eastern Pasco Basin, Washington ............................................................22

11. Map showing SOil WATer balance (SOWAT) model-estimated groundwater

withdrawals for 2007 and locations of simulated withdrawal wells, eastern 

Pasco Basin, Washington .........................................................................................................24

12. Graph showing simulated total annual groundwater withdrawals, eastern Pasco

Basin, Washington......................................................................................................................25

13. Graph showing measured and simulated water levels in the eastern Pasco 

Basin, Washington......................................................................................................................28

14. Map showing simulated water-level altitudes, eastern Pasco Basin, 

Washington, 2013........................................................................................................................30

15. Map showing simulated changes in water-level altitude, eastern Pasco Basin,

Washington, 1920–2013..............................................................................................................31

16. Graph showing simulated net flows into the groundwater model domain, eastern

Pasco Basin, Washington, 1920–2013.....................................................................................32
v

Figures—Continued

17. Graph showing simulated changes in groundwater storage in the model domain

and areas in the 508-14 area, eastern Pasco Basin, Washington,1920–2013 ..................34

18. Map showing subareas with summarized simulated change in groundwater 

storage in the active model domain and 508-14 area, eastern Pasco Basin, 

Washington..................................................................................................................................35

19. Map showing locations of hypothetical wells and simulated drawdown after 

7 years of withdrawals at 2 million gallons per day near White Bluffs, eastern 

Pasco Basin, Washington .........................................................................................................37

20. Hydrograph showing simulated water-level altitudes in the upper Ringold

Formation near the middle of the hypothetical well field near White Bluffs, 

eastern Pasco Basin, Washington...........................................................................................38

21. Digital imagery showing extent of the Pasco gravels, locations of wells 

screened in the Pasco gravels and upper Ringold Formation, and the simulated 

drawdown after 7 years of an additional 1 million gallons per day withdrawal 

from the upper Ringold Formation, eastern Pasco Basin, Washington ............................39

22. Hydrograph showing simulated water-level altitudes where an additional 1 million

gallon per day withdrawal was specified from a well screened in the Pasco

gravels, eastern Pasco Basin, Washington ...........................................................................39

23. Hydrograph showing simulated water-level altitudes for the upper and middle

Ringold Formations, and the Saddle Mountains and Wanapum Basalts where an

additional 1 million gallon per day withdrawal was specified from a well screened

in the upper Ringold Formation, eastern Pasco Basin, Washington .................................40

24. Hydrograph showing simulated water-level altitudes for the upper and middle

Ringold Formations, and Saddle Mountains and Wanapum Basalts where an

additional 1 million gallon per day withdrawal was specified from a well screened

through the upper and middle Ringold Formation and Saddle Mountains Basalt,

eastern Pasco Basin, Washington...........................................................................................41

Tables

1. Comparison of overburden units described in selected hydrogeologic studies in

or near the eastern Pasco Basin, Washington........................................................................7

2. Geologic and hydrogeologic units of the eastern Pasco Basin, Washington..................11

3. Previous estimates of horizontal hydraulic conductivity, storage coefficient, and

specific yield in the overburden deposits near the eastern Pasco Basin, 

Washington..................................................................................................................................16

4. Parameter values and sensitivities for the calibrated groundwater-flow model for

eastern Pasco Basin, Washington...........................................................................................27

5. Groundwater budget of the 508-14 area, eastern Pasco Basin, Washington,

1900–1920 and 2013 ....................................................................................................................32

6. Net increase of stored groundwater in subareas of the 508-14 area, eastern

Pasco Basin, Washington, 1920–2013.....................................................................................33

7. Selected physical and hydrologic data for the project wells in the eastern Pasco

Basin study area, Washington. download at http://dx.doi.org/10.3133/sir20165026
vi

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m)

acre 0.4047 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

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

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

)

Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3

/yr)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

cubic foot per day (ft3

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

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

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

International System of Units to Inch/Pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

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

°C = (°F – 32) / 1.8.
vii

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

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

Abbreviations

CBP Columbia Basin Project

CRBG Columbia River Basalt Group

DEM Digital Elevation Model

Ecology Washington Department of Ecology

HFB Horizontal Flow Barrier

MNW2 Multi-Node Well package of MODFLOW

MODFLOW USGS modular three-dimensional finite-difference groundwater-flow model

NWIS National Water Information System

Reclamation Bureau of Reclamation

RMSE root-mean-square error 

SOWAT model SOil WATer balance model

USGS U.S. Geological Survey
viii

Well-Numbering System

In Washington, wells are assigned numbers that identify their location in a township, range, 

section, and 40-acre tract. For example, well number 09N/30E-08B02D1 indicates that the well is 

in township 9 north of the Willamette Base Line, and range 30 east of the Willamette Meridian. 

The numbers immediately following the hyphen indicate the section (8) in the township, and the 

letter following the section (B) gives the 40-acre tract of the section. The two-digit sequence 

number (02) following the letter indicates that the well was the second one inventoried in 

that 40-acre tract. The “D” following the sequence number indicates that the well has been 

deepened. In the illustrations of this report, wells are identified individually using only the 

section and 40-acre tract, such as 08B02D1. The townships and ranges are shown on the map 

borders.

Willamette Meridian

Willamette Base Line

ABCD

E F G H

JKLM

N P Q R

R. 30 E.

T.

09

 N.

123456

7 12

13

8

18

19 24

30 25

31 32 33 34 35 36

WA S H I N G T O N

09N/30E-08B02D1

SECTION 08

Well numbering system in Washington.
Simulation of Groundwater Storage Changes in the 

Eastern Pasco Basin, Washington 

By Charles E. Heywood, Sue C. Kahle, Theresa D. Olsen, James D. Patterson, and Erick Burns

Abstract

The Miocene Columbia River Basalt Group and younger 

sedimentary deposits of lacustrine, fluvial, eolian, and 

cataclysmic-flood origins compose the aquifer system of the 

Pasco Basin in eastern Washington. Irrigation return flow 

and canal leakage from the Columbia Basin Project have 

caused groundwater levels to rise substantially in some areas, 

contributing to landslides along the Columbia River. Water 

resource managers are considering extraction of additional 

stored groundwater to supply increasing demand and possibly 

mitigate problems caused by the increased water levels. To 

help address these concerns, the transient groundwater model 

of the Pasco Basin documented in this report was developed 

to quantify the changes in groundwater flow and storage. The 

MODFLOW model uses a 1-kilometer finite-difference grid 

and is constrained by logs and water levels from 846 wells in 

the study area. Eight model layers represent five sedimentary 

hydrogeologic units and underlying basalt formations. 

Head‐dependent flux boundaries represent the Columbia 

and Snake Rivers to the west and south, respectively, 

underflow to and (or) from adjacent areas to the northeast, 

and discharge to agricultural drains, springs, and groundwater 

withdrawal wells. Specified flux boundaries represent 

recharge from infiltrated precipitation and anthropogenic 

sources, including irrigation return flow and leakage from 

water-distribution canals. The model was calibrated with 

the parameter‐estimation code PEST++ to groundwater 

levels measured from 1907 through 2013 and measured 

discharge to springs and estimated discharge to agricultural 

drains. Increased recharge since pre-development resulted 

in a 6.8 million acre-feet increase in storage in the 508-14 

administrative area of the Pasco Basin. Four groundwatermanagement scenarios simulate the 7-year drawdown resulting 

from withdrawals in different locations. Withdrawals of 

2 million gallons per day (Mgal/d) from a hypothetical well 

field in the upper Ringold Formation along the Columbia 

River could generate 30–70 feet of drawdown, which may 

reduce landslide susceptibility along the White Bluffs. 

Drawdowns resulting from a 1 Mgal/d withdrawal from wells 

screened in either Pasco gravels, upper Ringold Formation, or 

both Ringold Formation and underlying basalt are simulated in 

the other three scenarios, and differ because of the contrasting 

hydraulic conductivities within the screened intervals.

Introduction

Since 1952 water diverted from the Columbia River 

has been used to irrigate parts of the Bureau of Reclamation 

(Reclamation) Columbia Basin Project (CBP) in eastern 

Washington. As a result of the large volumes of surface‐water 

irrigation, groundwater levels in the Pasco Basin sediments 

generally have risen and caused various problems, such as 

landslides along the Columbia River, septic system failures, 

and loss of agricultural lands because of ponding. As demands 

for water use increase, State and local water resource managers 

are increasingly looking to the additional groundwater in 

storage as a potential source of water for development. The 

development of these groundwater resources not only could 

provide additional water for beneficial use, but also could 

potentially mitigate some of the adverse consequences of the 

rise in water levels.

There is a need to better understand the groundwaterflow system before and after the start of surface-water 

irrigation in eastern Washington, and to quantify the increases 

in groundwater fluxes and storage attributed to excess 

irrigation. Under Washington State law, subject to existing 

rights, all natural groundwater and all “artificially stored” 

groundwater that has been abandoned or forfeited are available 

for appropriation (Washington Administrative Code [WAC] 

508-14-030). The sedimentary deposits underlying the Pasco 

Basin have large quantities of groundwater in “artificial” storage 

that could be made available for a Declaration of Claim, but the 

storage volume attributed to the CBP irrigation is unknown.

State and local water resource managers are considering 

extraction of the additional stored groundwater to supply 

increasing demand, as well as possibly mitigating problems 

caused by rising water levels. To manage future groundwater 

allocations, an assessment of increased groundwater storage 

attributed to irrigation return flow from CBP irrigation 

is needed.
2 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

Purpose and Scope

The “508-14 area” is the area in the Pasco Basin 

designated by Chapter 508-14 of the Washington 

Administrative Code (WAC) in which the Washington State 

Department of Ecology (Ecology) regulates groundwater 

withdrawals. In 2002, Washington State legislation (Substitute 

House Bill 2874) recognized Reclamation ownership of 

artificially stored groundwater and authorized Ecology to enter 

into a joint management program with Reclamation to issue 

permits for groundwater withdrawals from the 508-14 area 

of Franklin County. Since 2002, Ecology and Reclamation 

have actively pursued development of a 508-14 groundwater 

management program to permit withdrawals of commingled 

naturally occurring and artificially stored 508-14 groundwater. 

The U.S. Geological Survey (USGS), in cooperation with 

the Bureau of Reclamation conducted this study to quantify 

natural and artificially stored groundwater in the 508-14 area. 

This report documents the groundwater-flow model that was 

developed to quantify the changes of groundwater storage in 

eastern Pasco Basin, Washington.

The four primary objectives of this study are to:

1. Define the hydrogeology of the study area,

2. Determine groundwater flow directions and flows 

through the aquifer system,

3. Quantify the effect of anthropogenic recharge and 

groundwater withdrawals on storage in the groundwater 

system, and

4. Simulate the effects of four groundwatermanagement scenarios.

Description of Study Area

The Pasco Basin study area includes the 508-14 

area in Adams, Franklin, and Grant Counties (fig. 1), and 

encompasses 2,192 mi2

 in the Columbia Plateau Province. 

Altitudes in the area range from 340 ft along the Columbia 

River near Pasco to 2,700 ft in the Saddle Mountains, with 

an average altitude of about 900 ft. The climate is arid to 

semi-arid, with mean annual precipitation ranging from 7 to 

10 in. and occurring primarily during the winter. Average 

temperatures range from the upper 20s °F in December and 

January to more than 90 °F in July and August.

The 2010 population of the study area was about 40,000, 

at which time the largest city was Othello, Washington, 

with a population of 7,400. The three most common types 

of land cover in the study area during 2006 (fig. 2) were 

planted/cultivated crops (51 percent), followed by shrubland 

(38 percent) and developed (5 percent). Use of groundwater 

resources in the Pasco Basin began in the late 1800s, when 

groundwater was withdrawn for agriculture, stock watering, 

and domestic use. In the 1950s, the CBP began to deliver 

water diverted from the Columbia River at Grand Coulee 

Dam for large-scale agricultural development. Subsequently, 

groundwater levels generally have risen in the shallow 

basin‐fill sediments.

Although groundwater withdrawals have increased since 

the 1950s, surface-water irrigation systems supply most of the 

agricultural water demand in the study area. Water obtained 

from the Columbia River is distributed through a system of 

canals and pipes, or buried drains (fig. 3), and excess irrigation 

water that is not recycled through drains and wasteways 

recharges the groundwater system. As of 2014, there were 

140 pending water-right applications in the study area (John J. 

Covert, Washington State Department of Ecology, written 

commun., December 2014) (fig. 3).

Previous Investigations

Regional and site-specific hydrogeologic investigations 

have reported on the hydrogeologic framework and hydrologic 

characteristics of the Pasco Basin. Selected studies are 

discussed here; the locations of these studies are shown in 

figure 4, and a comparison chart of the overburden units 

described is included in table 1.

Some of the earliest hydrogeologic studies in the 

Pasco Basin were associated with the development of the 

Hanford Nuclear Reservation (Hanford Site), and later 

studies characterized radionuclide contamination in and 

around the Hanford Site. Investigations in the early and 

mid-1970s by Newcomb and others (1972) and Tanaka 

and others (1974) (fig. 4) characterized the hydrogeologic 

properties of the overburden and the underlying basalt, 

including the transmissivity of each hydrogeologic unit. 

These investigations used well tests to determine hydraulic 

characteristics, mapped hydrogeologic-unit thicknesses, and 

documented water-level changes caused by artificial recharge. 

In the mid-1980s through late 1990s, Drost and 

Whiteman (1986); Drost and others (1989a, 1989b, 1990, 

1993, 1997), and Ebbert and others (1996) completed a 

series of investigations to characterize the surficial geology, 

structure, top altitudes and thicknesses of hydrogeologic 

units, water levels and quality, and various components 

of groundwater recharge and discharge in the area of this 

study. Some of these investigations encompassed the entire 

Columbia Plateau (Drost and Whiteman, 1986; Drost and 

others, 1990), whereas the remainder focused on the Pasco 

Basin (Drost and others, 1989a, 1989b, 1993, 1997; Ebbert 

and others, 1996). Drost and others (1997) developed and 

interpreted contoured potentiometric-surface maps, and 

summarized the water-level change from pre-development to 

1986. The present study area contains about two-thirds of the 

900 mi2

 Drost and others study area (fig. 4), in which Drost 

and others estimated a groundwater-storage increase of about 

5 million acre-ft between 1950 and 1986. 
Introduction 3

tac15-0988_fig01

WASHINGTON STATE

Figure 

location

Columbia River

White

 Bluffs

Hanford

Reservation

Saddle Mountains

GRANT

COUNTY

ADAMS

COUNTY

FRANKLIN

COUNTY

Snake River

0 5 10 15 20 2.5 MILES

0 5 10 15 20 2.5 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Base map from National Geographic, ESRI, DeLorme, Copyright © Esri. All rights reserved.

Washington State Plane South, FIPS zone 4602, units in feet. 

Horizontal datum is North American Datum of 1983.

EXPLANATION

Boundary of 508-14 area

Boundary of active model domain

Figure 1. Location of the active model domain and 508-14 area, eastern Pasco Basin, Washington.
4 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig02

GRANT

COUNTY

ADAMS

COUNTY

FRANKLIN

COUNTY

Columbia River

Snake River

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

46°

00'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Boundary of active model 

domain

Simplified 2006 National Land Cover

Open water

Developed

Barren

Forest

Shrubland

Herbaceous

Planted/cultivated

Wetlands

Figure 2. Simplified National Land Cover of the eastern Pasco Basin, Washington, 2006.
Introduction 5

tac15-0988_fig03

Savage Island landslide spring

WB5 Wasteway Springs

Ringold Springs

Baxter Springs

Rankin Springs

Block 15 landslide

Columbia River

Snake River

Crab Creek

Ice Harbor

Dam

Priest Rapids

Dam

Wanapum

Dam

White Bluffs

Lake Wallula

GRANT

COUNTY

ADAMS

COUNTY

FRANKLIN

COUNTY

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Base map from National Geographic, ESRI, DeLorme, Copyright © Esri. All rights reserved.

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Buried drains

Canal, wasteway, open drain, main waterway

Pending water-right application

Spring

Figure 3. Locations of rivers, springs, water-delivery infrastructure, buried drains, and pending water-right applications, 

eastern Pasco Basin, Washington.
6 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig04

0 10 20 30 40 MILES

0 10 20 30 40 KILOMETERS

120°00' 118°00'

48°

00'

46°

00'

44°

00'

Base map from National Geographic, ESRI, DeLorme, Copyright © Esri. All rights reserved.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Area of hydrogeologic study, citations

Parts of Pasco Basin, Drost and others (1997)

Pasco Basin, Lindsey (1996)

Hanford Reservation, Newcomb and others (1972)

Columbia Basin Project, Tanaka and others (1974)

Hanford Reservation, U.S. Department of Energy (2002)

Columbia Basin Groundwater Management Area, 

Lindsey and others (2007)

Yakima Basin, Vaccaro and others (2009); 

Ely and others (2011)

Columbia Plateau Regional Aquifer System (model area), 

Ely and others (2014); Vaccaro and others (2015)

508-14 area

Figure 4. Locations of selected hydrogeologic studies of the eastern Pasco Basin and selected adjacent areas, Washington.
Introduction 

7

Table 1. Comparison of overburden units described in selected hydrogeologic studies in or near the eastern Pasco Basin, Washington.[No vertical scale. Abbreviations: FM, formation; ka, thousand years ago; Ma, million years ago; –, not a separately recognized unit]

Newcomb and others (1972) Tanaka and others (1974) Lindsey (1996) Drost and others (1997) U.S. Department of Energy (2002) Lindsey and others (2007) This study (2014)

HANFORD SITE FRANKLIN AND GRANT COUNTIES (COLUMBIA PLATEAU) PASCO BASIN EAST PASCO BASIN HANFORD SITE FRANKLIN AND GRANT COUNTIES (COLUMBIA PLATEAU) EAST PASCO BASIN

AGE EPOCH FORMATION FORMATION HYDROGEOLOGIC SUB-UNIT FORMATION MEMBER / SUB-UNIT HYDROGEOLOGIC UNIT LITHOFACIES SUB-UNIT UNIT (not regionally continuous and interbedded) UNIT

0 ka

HOLOCENE

COLLUVIUM – –

–

–

UNNAMED ALLUVIUM,

COLLUVIUM, LANDSLIDE, 

AND EOLIAN DEPOSITS

PASCO

GRAVELS

ALLUVIUM

GLACIOFLUVIAL

DEPOSITS

Gravel and

sand

EOLIAN AND ALLUVIUM HOLOCENE DEPOSITS

Fine-grained, massive,

well sorted

Fine-grained, weakly

laminated, poorly

sorted

Medium-grained, cross

bedded, well-sorted

Coarse-grained,

rounded

Coarse-grained,

angular, basaltic

Coarse- to fine-grained,

massive, matrix

supported

13 ka Tephra

3.4 Ma

PLEISTOCENE GLACIOFLUVIATILE AND FLUVIATILE DEPOSITS HANFORD FORMATION

Touchet

beds and

Missoula

gravels

TOUCHET

BEDS AND

CONFINING BED

HANFORD

FORMATION

Interbedded sand-tosilt-dominated

PLEISTOCENE

CATACLYSMIC

FLOOD

DEPOSITSED SILT,

SAND, AND

GRAVEL

Pre-Missoula

(PlioPleistocene)

Sand dominated

PASCO GRAVELS

AQUIFER

(GENERALLY

UNCONFINED)

Gravel dominated

COLD CREEK

UNIT

Fine-grained,

laminated to massive

Coarse- to fine-grained, 

carbonate cemented

Coarse-grained,

multilithic

Coarse-grained,

angular, basaltic

Coarse-grained,

rounded, basaltic

PLIOCENE

RINGOLD

FORMATION

RINGOLD

FORMATION

Sand and silt

RINGOLD

FORMATION

Savage Island UPPER RINGOLD FORMATION

RINGOLD FORMATION

PLIO-PLIESTOCENE

CALICHE

UPPER

RINGOLD

Taylor Flats

MIDDLE RINGOLD

FORMATION

RINGOLD FM (MEMBER OF

SAVAGE ISLAND)

—?—

Conglomerate

Wooded

Island Unit E RINGOLD FM (MEMBER OF

TAYLOR FLATS)

MIDDLE

RINGOLD

MIOCENE

Unit CUnit B LOWER RINGOLD

 FORMATION RINGOLD FM

(MEMBER OF

WOODED ISLAND)

LOWER

RINGOLD

Clay

Unit DUnit A

8.5 Ma

Snipes Mountain

Conglomerate

BASAL RINGOLD 

FORMATION

BASAL

RINGOLD
8 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

Bauer and Vaccaro (1990) developed a model to quantify 

groundwater recharge to the Columbia Plateau Regional 

Aquifer System during pre-development and 1975 land-use 

conditions. A rigorous error analysis was not done, and they 

estimated a maximum uncertainty of 25 percent for their 

recharge calculations.

Lindsey (1996) described the stratigraphy of the 

Miocene-Pliocene Ringold Formation and associated 

deposits of the ancestral Columbia River system in the 

Pasco Basin (fig. 4). The U.S. Department of Energy (2002) 

standardized the stratigraphic nomenclature for post-RingoldFormation sediments (table 1) in the Central Pasco Basin and 

described their lithology and depositional setting. Lindsey 

and others (2007) described the geologic framework of the 

sedimentary aquifer system in the Columbia Basin Ground 

Water Management area of Adams, Franklin, and Grant 

Counties (fig. 4). 

Recent studies of the hydrogeologic framework, water 

budget, groundwater conditions, and post-development trends 

in the regional Columbia Plateau Regional Aquifer System 

encompass the current study area and focus on the basalt 

aquifers (Snyder and Haynes, 2010; Kahle and others, 2011; 

Burns and others, 2012; Vaccaro and others, 2015). These 

studies provided data used for the construction of a threedimensional numerical model of groundwater flow in the 

Columbia Plateau Regional Aquifer System (Ely and others, 

2014), which was used to specify recharge, groundwater 

withdrawal, and groundwater underflow fluxes in the model 

documented by this report.

Methods of Investigation

Well Data

Construction, geophysical log, and water-level data from 

846 wells in the project area were compiled from 811 sites 

retrieved from the USGS National Water Information System 

(NWIS) database and 35 wells from the Ecology well-log 

database. The three criteria for selecting wells from NWIS 

were: (1) sites located in the study area were previously visited 

by USGS personnel, (2) complete construction information 

and well logs were available, and (3) multiple water-level 

measurements were available.

The 35 additional wells compiled from the Ecology welllog database were used in areas where NWIS well data were 

not available using the three well-selection criteria. These 

wells were not field-located and were assigned approximate 

latitude and longitude coordinates using the public land 

survey locations (township, range, section, and quarter-quarter 

section), well addresses, and (or) parcel numbers available 

on the drillers’ logs for each well. The locations of all 846 

project wells are shown on plate 1, and selected physical and 

hydrologic data for these wells are provided in table 7.

Table 7. Selected physical and hydrologic data for the project 

wells in the eastern Pasco Basin study area, Washington.

[Table 7 is a Microsoft© Excel file and can be downloaded at http://dx.doi.

org/10.3133/sir2065026]

Hydrogeology

A map of the surficial hydrogeology of the study area was 

made by grouping surficial geologic units of similar lithology 

and extent into hydrogeologic units, and was further refined 

with lithologic information obtained from drillers’ logs of area 

water wells (fig. 5 and pl. 1). A digital tabulation of borehole 

hydrogeologic unit assignments facilitated creation of seven 

hydrogeologic cross sections and correlation of hydrogeologic 

units at depth (pl. 1). Raster representations of the top altitudes 

(surfaces) and extents of the hydrogeologic units described in 

this report were generated using a grid with square cells 100-ft 

on a side in a geographic information system. These surfaces 

were used to construct the digital hydrogeologic framework 

for the USGS modular three-dimensional finite-difference 

groundwater-flow (MODFLOW) model. Unit-top altitudes 

at project wells were interpolated through the extent of the 

hydrogeologic units using the Australian National University 

Digital Elevation Model (ANUDEM) procedure (Hutchinson, 

1989). Each hydrogeologic unit surface was constrained by 

the National Elevation Dataset 100-ft land surface Digital 

Elevation Model (DEM) where the unit outcropped.

The interpolated hydrogeologic unit surfaces and 

thicknesses were compared to previous hydrogeologic-unit 

maps, sections, and well interpretations to honor previous data 

interpretations wherever feasible. Hydrogeologic-framework 

uncertainties are greatest in areas where the surficial geology 

changes abruptly, or the project wells do not provide sufficient 

subsurface lithologic data.

Hydrogeologic Framework

Geologic Setting

The study area lies in the eastern half of the Pasco Basin, 

which is a structural and topographic low area in south-central 

Washington. Most locally significant groundwater occurs in 

three major stratigraphic units that underlie the eastern Pasco 

Basin. In ascending order, these units are (1) the Columbia 

River Basalt Group (CRBG), (2) the Ringold Formation, and 

(3) the sediments deposited by catastrophic flooding (Drost 

and others, 1997). A map of the surficial geology of the study 

area (fig. 5) was simplified from the digital geologic map 

database of Washington (Washington Division of Geology and 

Earth Resources, 2005).
Hydrogeologic Framework 9

tac15-0988_fig05

Columbia River

GRANT

COUNTY

ADAMS

COUNTY

FRANKLIN

COUNTY

HANFORD

RESERVATION

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Surficial geologic unit

Folds

Faults

Qa

Qd

Qls

Ql

Qfg

Qfs

PLMc(r)

PLMcg(r)

Basalt

wtr

Alluvium

Dune sand, active and stabilized

Mass-wasting deposits, gravel

Loess

Outburst flood deposits, gravel

Outburst flood deposits, sand and silt

Continental sedimentary deposits or rocks

Continental sedimentary deposits or rocks, conglomerate

Basalt, Columbia River Basalt Group, undifferentiated

Water

Q fg

wtr

Q fs

Basalt

wtr Q fs

Q fg

Q d

PLMc(r) Q a

Q l

Q d wtr Q fs

Basalt

Basalt

Q l

Q fg Basalt wtr

PLMc(r)

Q fg Q a

Q l

Q d Q a

Q fs Q fg

Basalt

Q a

wtr

Basalt

Q l Q ls PLMc(r) Q ls

Q a

Q l

PLMc(r)

Q fs

Q fg

Q fs

Basalt

PLMc(r) PLMc(r) wtr Q d

Basalt

Q a

Q d

Basalt

Q a

Q ls

Q fg

PLMcg(r) Q d

wtr Q a

Q l

Q fg

Q a

Q fs

PLMc(r)

PLMc(r)

Q d

Basalt

Q fg PLMcg(r)

Q l

Q d

Q fg

PLMcg(r)

Basalt

Q a

Figure 5. Surficial geology of the eastern Pasco Basin, Washington.
10 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

The geologic and hydrogeologic setting of the area 

described by Drost and others (1997) is summarized here. 

During the Tertiary period, basalts intermittently flooded 

into the Pasco Basin, creating a basalt-layer sequence greater 

than 15,000 ft thick. Between eruptions, particularly those 

producing the younger flows, minor amounts of sediment 

(Ellensburg Formation) were interbedded with the basalts. 

Northwest- to west-trending folds were formed during late 

stages of the flood basalt volcanism. At that time, the greatest 

deformation was west of the study area in the Yakima fold 

belt that extends eastward into the study area in the Saddle 

Mountains, within which the Saddle Mountains Fault (Lidke, 

2002) is a major structural feature. 

Regional folding and subsidence of the basalts in the 

Late Miocene resulted in the deposition of fluvial sediments in 

the Pasco Basin by ancestral rivers flowing into and through 

the basin. These sediments formed the Ringold Formation 

of Miocene - Pliocene age, which consists of four textural 

sub‐units referred to as the basal, lower, middle, and upper 

Ringold Formation (table 1). 

The basal Ringold Formation, which is a coarse gravel 

conglomerate, typically is less than 20 m thick and resulted 

from the deposition of channel bedload on braided plains. 

This deposition is limited to the farthest lateral extent of the 

shallow, shifting channels. There also were occasional silt 

overbank deposits near the channel from individual flood 

events. This conglomerate deposition pre-dates the northsouth regional shortening of the basalts that formed the basin 

topography (Brown and Brown, 1965; Lindsey, 1996).

As structural deformation began to obstruct the river 

systems, a shallow lake formed in the new basin. The gradual 

formation of Ringold Lake slowly changed the depositional 

setting. Silt and clay were deposited above the basal Ringold 

Formation and the basalt bedrock (outside the former channel 

boundaries) throughout the basin. Blue clay that was deposited 

and preserved in an anaerobic environment and sporadic sandy 

gravels deposited during flood events typically were identified 

by Drost (1997) as the lower Ringold Formation (Brown and 

Brown, 1965). 

As a new channel began to form and provide an outlet 

for the basin, Ringold Lake slowly drained. The river once 

again re-channelized into a shifting braided system. Frequent 

flood events continued to produce fine-grained silt and clay 

overbank deposits throughout the basin, similar in texture 

and composition to those in the lower Ringold Formation. 

Conglomerate in the middle Ringold Formation formed in 

river channels and continued to thicken as the basin subsided 

(6.5–5.5 Ma). Although the quantity of gravel deposited 

generally decreases with distance from a channel swath, 

channel migration and repeated floods of various sizes resulted 

in conglomerate, well-sorted sands, and overbank deposits that 

routinely inter-finger, pinch out, or form lenses (Brown and 

Brown, 1965; Lindsey, 1996; Drost and others, 1997). 

About 5.5 Ma, the basin began to fill again, forming a 

shallow lake. Sands, silts, and clays filled the basin with 30 to 

more than 230 m of fine-grained laminated sediments in the 

upper Ringold Formation. These fine-grained sediments are 

similar in composition and hydraulic characteristics to those in 

the lower Ringold Formation. After Ringold Lake drained, a 

calcium-carbonate (CaCO3

) precipitate formed that cemented 

the fine-grained sedimentary surface with a thick caliche layer. 

Subsequent to the formation of the caliche layer, severe dust 

storms left thick eolian deposits in low-lying areas (Lindsey, 

1996; Drost and others, 1997).

The Pasco gravels are unconsolidated sandy gravels 

deposited during the Missoula floods that overly the Ringold 

Formation. The contact between the Ringold Formation and 

the Pasco gravels is an irregular disconformity, caused by 

high-energy erosion during the initial stages of flooding. A 

temporary lake again filled the basin due to a downstream 

restriction in the Columbia River Gorge. The Missoula flood 

deposits consist of very poorly sorted sediments ranging from 

silts to boulders. The coarsest material was deposited near the 

river channel. Sedimentation farther from the channel consists 

of fine-grained slack-water deposits. The size of the lake was 

directly related to the volume of floodwater. The varying size 

of flood pulses created inter-fingering between the coarse- and 

fine-grained deposits (Lindsey, 1996).

Holocene post-Missoula-flood deposits consist of 

eolian loess, slope wash, eolian dune sand, alluvium, talus, 

landslide/debris flow, and volcanic ash. Both the fine-grained, 

well‐sorted wind deposits and the poorly sorted, massive 

landslide and debris-flow deposits typically are unweathered 

and unconsolidated. These silts, sands, and gravels generally 

are less than 5 m thick (U.S. Department of Energy, 2002).

Hydrogeologic Units

All geologic units described in the section, “Geologic 

Setting,” function either as aquifers or confining units in some 

parts of the study area. The sediments above the basalt were 

separated into hydrogeologic units based on their textures and 

position, following the hydro-stratigraphic classification of 

Drost and others (1997) (table 2). Fine- and coarse-grained 

sedimentary units alternate in sequence from the Pasco 

gravels downward through the basal Ringold Formation. 

The coarse-grained units (Pasco gravels, middle and basal 

Ringold Formation) generally form productive aquifers where 

saturated. The fine-grained units (upper and lower Ringold 

Formation) have smaller hydraulic conductivity and generally 

function as confining units that produce small quantities 

of water to wells that may be completed in them. Due to 

their storativity, however, the fine-grained units may yield 

substantial quantities of water over time in areas where water 

levels have declined in surrounding coarse-grained units. All 

these units constitute the alluvial aquifer system.
Hydrogeologic Framework 11

Table 2. Geologic and hydrogeologic units of the eastern Pasco Basin, Washington.[Surficial geologic units included in hydrogeologic unit: Qa, alluvium; Qaf, alluvial fan deposits; Qd, dune sand; Qfg, outburst flood deposits, gravel; Qfs, outburst flood deposits, sand; Ql, loess; Qls, masswasting deposits; Msdr, continental sedimentary deposits or rocks; Mv(GRB), Grand Ronde Basalt; MV(SMB), Saddle Mountains Basalt; Mv(WB), Wanapum Basalt; PLMc(r), Ringold Formation, continental sedimentary deposits or rocks; PLMcg(r), Ringold Formation, continental sedimentary deposits or rocks, conglomerate. Abbreviations: ft, foot; <, less than; >, greater than; –, data not available]

Simplified geologic 

unit

Hydrogeologic 

unit name

Hydrogeologic 

unit symbol

Surficial geologic units 

included in 

hydrogeologic unit

Description and occurrence

Median or representative 

values of horizontal 

hydraulic conductivity (ft/d) 

(Drost and others, 1997)

Number of 

project wells 

open to unit

Groundwater 

model layer

Hanford Formation

and overlying

Holocene deposits

Pasco gravels PSCO Mostly Qfg, Qfs, Qa; some Qd, Ql

Composed mostly of sand and gravel(Pasco gravels), but includes sand and silt(Touchet Beds) in places. Occurs at landsurface. Thickness generally 20–150 ft; maximum thickness >200 ft.

880 157 1

Ringold Formation

Upper Ringold

Formation UPRG

PLMc(r),

overlying Qfs,

Qd, Ql

Composed mostly of silt, sand, and clayand generally occurs at land surface exceptwhere overlain by PSCO. Thickness generally >100 ft; maximum thickness nearly 500 ft.

2.7 135 2

Middle Ringold

Formation MDRG PLMcg(r) 

Includes sand, gravel, and silt, commonlycemented; locally includes beds of silt andclay. Generally overlain by Pasco gravels orupper Ringold Formation. Present in centraland southern parts of study area. Averagethickness is about 75 ft; maximum thickness 

>300 ft.

180 46 3

Lower Ringold

Formation LRRG Not exposed at land surface

Composed of silt and clay with localgravels. Overlain by Pasco gravels, upperRingold Formation, or middle RingoldFormation. Present primarily within a severalmile-wide zone along the Columbia River.Thickness generally <100 ft; maximum thickness >250 ft.

4.1 8 4

Basal Ringold

Formation BSRG Not exposed at land surface Composed of sand and gravel and overlain by lower Ringold Formation. Present in small area along the Columbia River. Thickness <20 ft. – 1 5

 Columbia

River Basalt

Group

Saddle

 Mountains

Basalt

Basalt CRBG

Mv(SMB), Mv(WB), 

Mv(GRB) [minor 

Msdr, Otsdr, and thin 

overlying Ql, Qd, 

Qls, Qfg, Qfs, Qaf]

Basalt and interbedded sediment that occurs atland surface or is overlain by sediments. 2.3

379

6

Wanapum

Basalt

At depth. Almost everywhere overlain by Saddle Mountains Basalt. Present throughout study area.

11 7

Grande Ronde

Basalt

At depth. Overlain by Wanapum Basalt. Present throughout study area.

– 8
12 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

Basalt Hydrogeologic Unit (CRBG)

The Basalt hydrogeologic unit underlies the entire 

basin and occurs at land surface over about 40 percent of the 

study area, primarily in its high-altitude northern and eastern 

margins (pl. 1). The basalts form a complex series of aquifers 

and confining units at depth in the study area, in which most 

groundwater movement occurs in zones between basalt flows 

containing features such as flow breccia, rubble, and vesicles 

(Kahle and others, 2011). The principal basalt aquifer units 

in the study area are, from youngest to oldest, the Saddle 

Mountains Basalt, the Wanapum Basalt, and the Grande Ronde 

Basalt. Where it exists in the study area, the Saddle Mountains 

Basalt unit occurs at land surface or is overlain by younger 

sediment. The Wanapum Basalt unit generally is overlain by 

the Saddle Mountains Basalt and is present throughout most of 

the study area, except along Crab Creek (fig. 3). The Grande 

Ronde unit is present throughout the study area. The top 

altitude of the CBRG (fig. 6), which is based on the geologic 

map, project well logs, hydrogeologic sections, and the basalt 

altitude map of Drost and others (1997), was used for defining 

the base of the overburden and top altitude of the Saddle 

Mountains Basalt hydrogeologic unit. Manually drawn 100-ft 

contours of the basalt surface in the study area extent were 

combined with DEM land-surface altitudes where the basalt 

outcrops to generate a digital representation of basalt-surface 

altitude (fig. 6). The altitude of the basalt ranges from more 

than 2,600 ft in the Saddle Mountains to nearly -50 ft near 

the Columbia River. The top altitudes of the older and deeper 

Wanapum and Grande Ronde Basalt units were obtained from 

Burns and others (2011).

Of the project wells for which logs are available, 379 

are completed in the basalt unit (table 2), where overlying 

sedimentary units do not occur, are insufficiently saturated, 

or are unable to yield sufficient quantities of water to wells. 

Four wells (16N/23E-12G01 through 16N/25E-06M01) that 

are depicted on the western end of section C–C′ (pl. 1) are 

completed in basalt where overlying sediment either does 

not occur or is thin. In contrast, wells 13N/28E-09L01 and 

13N/28E-11E01, which are depicted on the western side of 

section E–E′ (pl. 1), penetrated more than 500 ft of overlying 

fine-grained sediment (upper Ringold and lower Ringold 

Formation) before being completed in the basalt.

Sedimentary Hydrogeologic Units

Sedimentary hydrogeologic units, also referred to as 

overburden include all sediment that overlies the basalt, and 

primarily are part of the Ringold Formation. The thickness 

of this overburden (fig. 7) was calculated by subtracting the 

basalt-surface altitude (fig. 6) from the land-surface altitude. 

Overburden thickness is greatest (more than 840 ft thick) 

in the center of the basin along the course of the Columbia 

River (fig. 7).

Pasco Gravels Unit (PSCO)

The Pasco Gravels unit is composed primarily of sand 

and gravel (the Pasco gravels), but includes sand and silt 

(Touchet Beds) in places. This unit occurs at land surface over 

about 32 percent of the study area (pl. 1). Lithologic data from 

157 project wells completed in the Pasco gravels (table 2) 

indicate that Pasco gravels generally are 20–150 ft thick. The 

thickest occurrence (more than 200 ft) of the unit is along the 

Wahluke Slope (section D–D′, pl. 1).

Upper Ringold Formation (UPRG)

The upper Ringold Formation is composed primarily of 

silt, sand, and clay, and generally occurs at land surface except 

where overlain by the Pasco gravels. The unit occurs at land 

surface over about 26 percent of the study area (pl. 1). The 

upper Ringold commonly is about 200 ft thick, but is more 

than 600 ft thick on the western end of section E–E′ (pl. 1). 

Despite being primarily fine-grained, 135 project wells are 

completed in the unit (table 2) and provide usable quantities 

of water.

Middle Ringold Formation (MDRG)

The middle Ringold Formation includes sand, gravel, and 

silt (commonly cemented), with interbeds of silt and clay in 

places. This unit is overlain by Pasco gravels or upper Ringold 

Formation, and is not known to occur at land surface in the 

study area. It occurs only in the central and southern parts of 

the study area (sections A–A′, D–D′, F–F′, and G–G′; pl. 1). 

Thickness averages about 75 ft, but it is more than 300 ft thick 

beneath the Wahluke Slope (section A-A′, pl. 1). Forty-six 

of the project wells are completed in the middle Ringold 

Formation (table 2).

Lower Ringold Formation (LRRG)

The lower Ringold Formation is composed of silt and 

clay with gravel lenses, and is similar to and difficult to 

distinguish from the upper Ringold Formation using well logs. 

It does not occur at land surface and is overlain either by the 

Pasco gravels, upper Ringold Formation, or middle Ringold 

Formation. It occurs primarily in a several-mile-wide zone 

along the Columbia River where thicknesses generally are 

less than 100 ft, but occasionally approach 250 ft. Only eight 

project wells are completed in this unit (table 2).

Basal Ringold Formation (BSRG)

The basal Ringold unit is composed of sand and gravel 

and is overlain by the lower Ringold Formation. It occurs only 

in a small area along the Columbia River and generally is less 

than 20 ft thick (section E–E′, pl. 1). Only one project well is 

completed in this unit (table 2).
Hydrogeologic Framework 13

tac15-0988_fig06

GRANT

COUNTY ADAMS

COUNTY

FRANKLIN

COUNTY

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Top of basalt, in feet above North 

American Vertical Datum of 1988

Boundary of active model domain

High : > 2,600

Low : < -50

Figure 6. Top altitude of basalt surface, eastern Pasco Basin, Washington.
14 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig07

GRANT

COUNTY

FRANKLIN

COUNTY

ADAMS

COUNTY

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

Boundary of active model 

domain

EXPLANATION

Overburden thickness, 

in feet

0

> 0 – 100

100.1 – 200

200.1 – 500

> 500

Figure 7. Extent and thickness of combined overburden units, eastern Pasco Basin, Washington.

Hydraulic Properties

Previous studies of hydrogeologic units in the Pasco 

Basin have measured or estimated values of horizontal 

hydraulic conductivity (K) and storage coefficient (S) or 

specific yield Sy). Values of these properties compiled 

from reference documents are summarized in table 3. 

Whereas some previous studies reported storage coefficient 

values (S) for confined aquifers, in this study the storage 

properties of confined hydrogeologic units were specified 

with specific storage (Ss), which, when multiplied by the 

hydrogeologic-unit thickness (b), is transformed to a storage 

coefficient (S = Ss × b). 
Hydrogeologic Framework 15

Hydrologic Setting

Recharge

Prior to the 1950s, deep infiltration of precipitation was 

the primary source of groundwater recharge in the study area. 

Since the early 1950s, irrigation return flows have become 

a substantial source of additional groundwater recharge 

beneath agricultural areas in the eastern Pasco Basin. The 

spatial distribution and temporal variation of recharge from 

precipitation and irrigation return flow to the Columbia 

Plateau Regional Aquifer System have been quantified using 

a monthly SOil WATer (SOWAT) balance model (Kahle and 

others, 2011). The SOWAT model uses simple relations among 

climatic, soils, land cover, and irrigation data to compute 

irrigation requirements and surplus moisture available for 

recharge. The SOWAT model incorporated evapotranspiration 

estimates derived from remotely sensed land-surface 

temperature data that were combined with other spatially 

distributed datasets including precipitation, soil moisture 

storage, and irrigation practices (Kahle and others, 2011). 

Recharge from Canal Leakage

An extensive canal system (fig. 3) transports water 

diverted from the Columbia River to supply irrigation 

requirements in the Pasco Basin. Drost and others (1997) 

estimated that the recharge resulting from seepage losses 

from these canals was about 170,000 acre-ft/yr in the area of 

his study, which encompassed about 30 percent of the area in 

this study. 

Rivers

Because the Columbia and Snake Rivers occupy the 

lowest-altitude regions in the study area, most groundwater 

that flows through the overburden sediments and shallow 

basalt flows in the Pasco Basin ultimately discharges to the 

Columbia or Snake Rivers. In local areas where the river stage 

is greater than the adjacent aquifer head, water from these 

rivers also may infiltrate the aquifer system. Infiltration from 

the river to the aquifer can occur upstream of the dams on the 

Columbia and Snake Rivers, and increased discharge back 

to these rivers likely occurs downstream of the dams (fig. 3). 

The completion of McNary Dam (not shown in fig. 3) in 1954 

created Lake Walula on the Columbia River and a consequent 

rise in the groundwater base level at the southern end of the 

study area. Farther upstream on the Columbia River, the 

Wanapum and Priest Rapids Dams were completed in 1963 

and 1961, respectively. On the Snake River, Ice Harbor Dam 

was completed in 1962.

Agricultural Drains

To control shallow groundwater levels, about 936 mi of 

agricultural drains were buried in the South Columbia Basin 

Irrigation District in Franklin County (fig. 3) from 1964 

through 1988, with installation peaking in 1976 (Drost and 

others, 1997). About 76,000 acre-ft/yr (9,063,819 ft3/d) of 

water estimated to be collected from these drains is routed 

to irrigation wasteways in the area studied (fig. 4; Drost and 

others, 1997).

Springs

Groundwater discharges to six springs along the 

White Bluffs along the Columbia River (fig. 3). The 

mean annual discharge at these springs ranges from 

about 290 acre-ft/yr (34,560 ft3/d) at Rankin Springs to 

46,366 acre-ft/yr (5,529,600 ft3/d) at Ringold Springs; 

the total measured discharge rate from the springs was 

about 49,000 acre-ft/yr (5,883,840 ft3/d) during 1986–87 

(Drost, 1997). 

Historical Groundwater Levels

Water levels measured during 1939–45 in wells 

screened in both overburden sediments and underlying 

basalt aquifers (fig. 8) may be considered representative of 

the pre-development conditions that were present before 

the construction of surface-water delivery infrastructure or 

substantial groundwater withdrawals. During that period, 

the horizontal directions of groundwater flow generally 

were perpendicular to water-level altitude contours shown 

in figure 8. Groundwater flowed southwestward from 

high-altitude areas in the northeastern part of the study area 

toward the Columbia and Snake Rivers. Because water-level 

gradients could differ between the overburden and basalt 

aquifers, the directions of pre-development groundwater 

flow in both overburden and basalt aquifers might not be 

perpendicular to the contours in figure 8 for all areas.

Water levels in the basalt and overburden aquifers began to 

rise substantially following the development of surface‐water 

delivery infrastructure and associated agricultural irrigation 

in the early 1950s. A hydrograph depicting a 270-ft waterlevel rise (fig. 8B) from 1957 to 1970 exemplifies the change 

in water levels measured beneath irrigated areas where both 

basalt and the upper Ringold Formation occur.
16 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington Table 3. Previous estimates of horizontal hydraulic conductivity, storage coefficient, and specific yield in the overburden deposits near the eastern Pasco Basin, Washington.

[Name of unit: Shaded cells indicate terms as referred to in source document.–, not applicable]

Name of unit Basin or area

Horizontal hydraulic conductivity, in feet per day

Source Storage coefficient Specific yield Source Minimum Median Mean Maximum Model derived

Alluvium Pasco/Benton 10 – 280 1,000 – Zimmerman (1983) – 0.2 U.S. Geological Survey 

(1975)

Alluvium, fluvialgravels, coarse sands

Hanford 6 – – 100,000 – Vermeul and others (2001) – 0.2 Bolke and Skrivan (1981)

Alluvium, coarse tomedium grained (somefines and caliche)

Hanford/Black Rock – – 0.85 – – Didricksen (2004) – –

Overburden aquifer(all sedimentary units overlying the Columbia River Basalt Group)

Columbia Plateau 0.04 43 99 1,100 – Hansen and others (1994); 

Vaccaro, (1999)

– –

Columbia Plateau 0.02 240 – 150,000 – Whiteman and others (1994) 0.0002 0.2 Vaccaro (1999)Columbia Plateau 0.04 43 – 1,123 – Bauer and Hansen (2000) – –

West Pasco Basin 2 – – 80 – Singleton and others (2005) – –

Wind-blown deposits(loess)

Columbia Plateau – – – – 5 Lum and others (1990) – –

Columbia Plateau 1 – – 10 – Vaccaro (1999); 

Vaccaro and others (2009)

– –

Plio-Pleistocene Unit(Hanford and RingoldFormations)

Hanford 0.02 – – 57 – Khaleel and Freeman (1995) – –

Sediments Horse Heaven Hills – – 24,000 – – Davies-Smith and others (1989) – –

Slack water deposits(Touchet beds)

Pasco/Benton 8 – – 11 9 Drost and others (1997) – 0.08 Drost and others (1997)

Badger Coulee 7.9 – – 10.8 – Bureau of Reclamation (1984) – 0.1 Prych (1983)

Soil buried horizon,caliche, gravel

Hanford 47 – – 620 – Vermeul and others (2001) – –

Soils, Palouse Hanford 0.2 – – 0.4 – Khaleel and Freeman (1995) – –

Glaciofluvial deposits (Pasco gravel)

Pasco/Benton 48 880 – 73,000 880 Drost and others (1997) – –

Kennewick/Finley – 1,200 – – – Meyers and others (1985) – –

Pasco Greenbelt and 

Smith Canyon

– 1,600 – – – Drost and others (1997) 0.06–0.2 – Tanaka and others (1974)

Badger Coulee – 320 – – – Drost and others (1997) 0.03–0.2 – Newcomb and others (1972)

Richland/West Richland – 670 – – – Drost and others (1997) 0.03–0.07 0.15–0.2 Bierschenk (1957)

Pasco/Quincy Basins 1,700 7,100 3,096 8,900 – Tanaka and others (1974) 0.03–0.07 0.15–0.2 Drost and others (1997)

Hanford Formation – 0.0625 Vermeul and others (2001)

Hanford 3 – – 3,300,000 – Wurstner and others (1995);

Thorne and Newcomer (1992)

– 0.18 Drost and others (1993)

Ringold Formation Hanford 0.38 0.64 Newcomb and others (1972) – 0.196 Vermeul and others (2001)

Hanford/Black Rock – 2.6 – – – Didricksen (2004) 0.06 – Tanaka (1974)Pasco/Quincy Basins 5 60 49 80 Tanaka and others (1974) – –
Hydrogeologic Framework 17

Table 3. Previous estimates of horizontal hydraulic conductivity, storage coefficient, and specific yield in the overburden deposits near the eastern Pasco Basin, Washington.—Continued

Name of unit Basin or area

Horizontal hydraulic conductivity, in feet per day

Source Storage coefficient Specific yield Source Minimum Median Mean Maximum Model derived

Ringold Formation, upper Pasco/Benton 2 25 – 210 5 Drost and others (1997) – –

Hanford – – – – 0.02 Vermeul and others, 2001 – –

Hanford 0.001 – – 0.3 – Wurstner and others (1995); 

Thorne and Newcomer (1992)

– –

East Pasco 1.5 – 5.1 11 – Drost and others (1997) – 0.07–0.21 Drost and others, 1997

Ringold Formation, middle

Pasco/Benton 200 – – 250 – Bergeron and others (1986) – –Pasco/Benton 8 180 – 5,000 200 Drost and others (1997) – –Pasco/Benton – 41 – – – Newcomb and others (1972); Graham 

(1981); Meyers and others (1985)

– –

Hanford 0.1 – – 5,500 – Vermeul and others (2001) – –Hanford 0.03 – – 280 – Vermeul and others (2001) 0.0002 Newcomb (1972)

Hanford 0.03 – – 0.3 – Vermeul and others (2001) – –

Hanford 0.3 – – 656 – Wurstner and others (1995); 

Thorne and Newcomer (1992)

– 0.1 Vaccaro and others (2009)

Hanford 0.001 – – 0.3 – Wurstner and others (1995); 

Thorne and Newcomer (1992)

0.0007–0.06 – Drost and others (2009)

Hanford 0.3 – – 660 – Wurstner and others (1995); 

Thorne and Newcomer (1992)

– –

Richland/ West 

Richland

– 720 – – – Drost and others (1997) – –

Franklin County – 110 – – – Drost and others (1997) – –

Ringold Formation,middle, gravel facies

Hanford 2,000 – – 6,400 – Vermeul and others (2003) – –

Ringold Formation,middle, sand facies

Hanford – – 1,300 – – Vermeul and others (2003) – –

Ringold Formation,middle, silt facies

Hanford – – 2 – – Vermeul and others (2003) – –

Ringold Formation,lower

Pasco/Benton 2 46 – 230 4 Drost and others (1997) 0.002–0.05 0.02–0.21 Drost and others (1997)Hanford 0.001 – – 0.3 – Wurstner and others (1995); 

Thorne and Newcomer (1992)

– –

East Pasco 2.2 – 4.1 6 – Drost and others (1997) – –

Ringold Formation,basal

Pasco/Benton 200 – – 250 – Bergeron and others (1986) – –Hanford 0.3 – – 660 – Wurstner and others (1995); 

Thorne and Newcomer (1992)

– –

Ringold, basal, sand/gravel

Hanford 0.03 – – 700 – Vermeul and others (2001) – –

Conglomerate Pasco/Qunicy Basins 20 – – 60 – Tanaka and others (1974) – –

Clay Pasco/Qunicy Basins – – – 1 – Tanaka and others (1974) – –
18 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig08

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800

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1,250

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1,000

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650

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1,000

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500

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1,250

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Columbia River

Snake River

Crab Creek

0 10 20 MILES

0 10 20 KILOMETERS

120°00' 119°30' 119°00'

47°

30'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

!

Boundary of active model domain

Water-level contour—Shows altitude of 

water level. Contour interval is variable. 

Datum is North American Vertical 

Datum of 1988

Well with water-level measurement

400

A

Figure 8. Water-level altitudes in wells, 1939–45 (A) and hydrograph of well at location B shown in figure 21 (B), eastern 

Pasco Basin, Washington. 
Simulation of Groundwater Flow 19

tac15-0988_fig08b

750

800

850

900

950

1,000

1,050

1,100

1955 1960 1965 1970

Year

Water-level altitude, in feet above North American Vertical Datum of 1988

B

Figure 8.—Continued

Simulation of Groundwater Flow

The groundwater model is based on a simple conceptual 

model in which natural precipitation and irrigation return 

flow recharge the aquifer system by vertical flow through 

sedimentary overburden (where sediments occur) and into 

basalt layers. Regional groundwater flow through basalt 

aquifer layers occurs from areas to the north and east of the 

model domain and generally discharges to the Columbia 

or Snake Rivers, although flow through the deepest basalt 

layers may discharge farther away. The model was designed 

to address a primary project objective—quantification of 

the spatial distribution of additional groundwater in storage 

resulting from agricultural development. The model also can 

be used to address secondary project objectives, which include 

assessments of various pumping scenarios on groundwater 

levels, and, in particular, the pumping required to sufficiently 

lower groundwater levels in areas where high water levels 

may have increased the susceptibility to landslides.

Modeling Strategy

Recharge and groundwater withdrawals are major 

components of the groundwater budget, and although 

reasonable estimates of their magnitude and distribution are 

available from the previous SOWAT model, the location, 

depth, and magnitude of actual groundwater withdrawals 

remain substantially uncertain. Because records of actual 

groundwater withdrawals are not available, the SOWAT 

estimated withdrawals were assigned to aquifer intervals 

according to available well-screen data. In groundwater 

systems, the groundwater flux and hydraulic conductivity 

are correlated. Because of the necessity of specifying 

uncertain groundwater-withdrawal fluxes, relatively uniform 

hydraulic‐conductivity distributions were simulated to avoid 

overfitting the water-level measurements during calibration.
20 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

Numerical Method

Simulation of a substantially rising water table, such 

as that which occurred in the Pasco Basin between 1950 

and 1986, involves rewetting nonlinearities and associated 

numerical instabilities with most versions of MODFLOW. 

The Newton formulation of the unconfined groundwater-flow 

equation available in MODFLOW-NWT (Niswonger and 

others, 2011) facilitates simulation of these systems, and, 

therefore, was selected to simulate groundwater flow in the 

eastern Pasco Basin.

Spatial Discretization

The model grid contains 87 rows and 105 columns of 

square finite-difference cells that are 1 km on a side (fig. 9). 

The grid was positioned so that the model cells correspond 

with the rasterized SOWAT model recharge and groundwaterwithdrawal data used to specify historical flows into and out 

of the model domain. The active model domain is north of 

the Columbia and Snake Rivers, and encompasses 2,192 mi2

, 

or about 62 percent of the area covered by the rectangular 

finite-difference grid. Eight model layers (table 2) represent 

the five hydrogeologic units in the sedimentary overburden 

and the three basalt units underlying them. The thicknesses 

of individual model layers vary spatially to represent the 

thickness of the Pasco Gravel, upper Ringold Formation, 

middle Ringold Formation, lower Ringold Formation, basal 

Ringold Formation, Saddle Mountains Basalt, and Wanapum 

Basalt. The upper 100 ft of the Grande Ronde Basalt is 

represented by the lowest model layer. Although the eight 

model layers generally correspond to these hydrogeologic 

units, the sedimentary hydrogeologic units do not occur 

in all areas of the model domain. In order to simulate 

hydraulic connection between hydrogeologic units where a 

stratigraphically intervening hydrogeologic unit is not present, 

the absent units were assigned a 1-ft layer thickness so that all 

layers are represented everywhere in the active model domain.

Time Discretization

A steady-state stress period was used to simulate 

groundwater conditions in the Pasco Basin prior to 1920. 

Ninety-four annual stress periods encompass the time interval 

from January 1, 1920, through December 31, 2013, that 

was used for calibration to measured water levels. Seven 

additional annual stress periods from January 1, 2014, 

through December 31, 2020, span the time interval of the 

four groundwater-management scenarios that are described 

in section, “Scenarios.” Because of the uncertainty in the 

specification of boundary conditions representing recharge and 

groundwater withdrawals, additional numerical accuracy from 

multiple time steps in each stress period was not warranted, 

and a single time step was used for each annual stress period 

to simulate water levels and flows.

Boundary Conditions

Specified- and head-dependent flow boundary 

conditions were assigned along the spatial boundaries of 

the groundwater‐flow system. Within the model domain, 

head‐dependent flow boundaries were used to simulate rivers, 

drains, springs and groundwater-withdrawal wells.

Specified Flow Boundaries

Time-varying specified inflow through the top of the 

model simulated recharge to the water table. Except for the 

lateral parts of the model domain that simulate groundwater 

underflow with head-dependent flow boundaries (see section, 

“Head-Dependent Flow Boundaries”), the lateral and bottom 

surfaces of the model domain are no-flow boundaries.

Recharge

The annual recharge specified in the regional model 

of Ely and others (2014), which includes both natural and 

anthropogenic components, was used for the Pasco Basin 

model documented in this report. Although the regional model 

of Ely and others (2015) uses larger (3-km2) finite‐difference 

cells, recharge for that model was computed with 1-km2 raster 

arrays that are compatible with the 1-km2 finite-difference 

cells of this model. Ely and others (2015) estimated natural 

recharge using gridded historical estimates of annual 

precipitation for 1895–2007. Pre-development recharge was 

estimated as the average natural recharge for this period. 

Irrigation recharge and irrigation pumping were estimated 

using a remote-sensing based SOWAT model for 1985–2007. 

Pre-1985 irrigation recharge and pumping were estimated 

using previously published compilation maps and the history 

of large-scale irrigation projects. The historical recharge 

estimated for 2007 (fig. 10) was specified for all subsequent 

years of the simulation. Because 2007 precipitation was 

similar to the mean annual precipitation (Kahle and others, 

2011), extrapolation of the 2007 simulated recharge was a 

reasonable representation for subsequent years.

Head-Dependent Flow Boundaries

Groundwater flows to rivers, agricultural drains, natural 

springs, and across the northern and eastern model perimeter 

depend on the hydraulic heads of those features and the 

aquifer groundwater levels, and therefore, are simulated 

as head‐dependent boundaries. Groundwater withdrawals 

simulated with the Multi-Node Well (MNW2) package 

of MODFLOW also are considered head-dependent flow 

boundaries because, although the total flow from each 

withdrawal well is specified, the distribution of each specified 

flow among model layers depends on the heads in cells 

connected to the well.
Simulation of Groundwater Flow 21

tac15-0988_fig09

GRANT

COUNTY ADAMS

COUNTY

FRANKLIN

COUNTY

Columbia River

Snake River

Crab Creek

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

River cells

Drain cells

General-head boundary cells

Horizontal-flow barrier

Figure 9. Finite-difference grid for the groundwater model with river and drain cells, general-head boundary cells, and 

horizontal-flow barriers, eastern Pasco Basin, Washington.
22 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig10

GRANT

COUNTY ADAMS

COUNTY

FRANKLIN

COUNTY

Columbia River

Snake River

Crab Creek

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

0.1 – 1.0

1.1 – 2.0

2.1 – 3.0

3.1 – 4.0

4.1 – 5.0

5.1 – 6.0

6.1 – 7.0

7.1 – 8.0

8.1 – 9.0

9.1 – 10.0

10.1 – 11.0

11.1 – 12.0

2007 recharge, in inches

Figure 10. Distribution of recharge specified in the Pasco Basin model domain for 2007, eastern Pasco Basin, Washington.
Simulation of Groundwater Flow 23

Groundwater Underflow

The flow of groundwater in the CRBG through the 

northern and eastern perimeter of the study area was simulated 

at the locations in figure 9 with the General Head Boundary 

(GHB) package of MODFLOW. The GHBs were connected to 

model layer 8, which represents the top 100 ft of the Grande 

Ronde Basalt. Water levels specified for each of the GHBs 

were those simulated in layer 34 during the steady-state, 

pre-development stress period of the revised Columbia Plateau 

Regional Aquifer System (CPRAS) model (Ely and others, 

2014) at the location corresponding to the GHB.

Groundwater Withdrawals

The groundwater withdrawals estimated with the SOWAT 

model were simulated with the MNW2 package. Assignment 

of the SOWAT model-estimated groundwater withdrawals to 

appropriate depths likely introduces the greatest errors in the 

model because records of the actual location and magnitude 

of withdrawals are not available. For each stress period, the 

groundwater withdrawal estimated by the SOWAT model for 

each model cell was assigned to one or more project wells 

(which have known screen-interval information, and are 

shown as “additional wells used in model” in figure 11) if 

a project well occurred in the cell. For cells with no project 

well, screen-interval information from a project well in 

a neighboring cell was used. If no project well was in a 

neighboring cell, the withdrawal was assigned across all 

aquifers above the total depth of a neighboring non-project 

well (fig. 11). Multiple variations of this type of assignment 

scheme were considered, all of which involve uncertainty 

in the depth and location of the withdrawal. Some structural 

model error, therefore, was a byproduct of the specification of 

these specified groundwater withdrawals.

The magnitude of groundwater withdrawals from the 

study area was negligible until the 1930s, when it increased 

to about 5,000 acre-ft/yr by 1945 (fig. 12). Withdrawal rates 

increased ten-fold over a 25-year period from the early 1960s 

through the late 1980s—from about 25,000 acre-ft/yr in 1963 

to more than 250,000 acre-ft/yr by the late 1980s.

Drains

Agricultural drains and springs were simulated with the 

Drain (DRN) package of MODFLOW. The total length of 

buried agricultural drains (fig. 3) in each model cell was used 

to specify hydraulic conductance from the model cell to each 

drain boundary. The drain-boundary altitudes were specified 

as the minimum DEM land-surface altitude in the model cell 

containing the drain. For cells in which the minimum DEM 

land-surface altitude was less than that of the top model layer, 

the drain boundary was connected to the underlying model 

layer that corresponded to the minimum DEM altitude.

Discharge to six springs (fig. 3) also was simulated with 

the MODFLOW DRN package. The simulated springs include 

Ringold Springs, Baxter Springs, WB5 Wasteway Springs, 

Savage Island Landslide Spring, Rankin Springs, and the 

Block 15 landslide. Because simulated drainflow is sensitive 

to the specified drain altitude, and the altitude of the springs 

was uncertain, simulated flow to most springs was not useful 

to model calibration, and the spring-flow observations were 

assigned a small weight in the calibration regression. 

Rivers

The Columbia and Snake Rivers bound the western, 

southern, and southeastern sides of the model domain, and 

were simulated with the River (RIV) package of MODFLOW 

(fig. 9). The part of Crab Creek in the model domain also was 

simulated with the RIV package. The parts of the Columbia 

and Snake Rivers that border the model domain were 

simulated without dams prior to 1962. For simplicity, stage 

changes on segments of these rivers due to construction of 

the Wanapum, Priest Rapids, McNary, and Ice Harbor Dams 

(fig. 3) were simulated to occur in 1962. The river stage, 

depth, and width at appropriate model cells were interpolated 

between actual measurement points along each river with the 

program RIVGRID (Leake and Claar, 1999). 

Hydraulic Property Distribution

Parameters representing horizontal and vertical hydraulic 

conductivities, specific storage, and specific yield were 

assigned to each of the eight simulated hydrogeologic units. 

Heterogeneity of horizontal hydraulic conductivity in the 

Pasco gravels and Wanapum Basalt units was simulated by 

division of their spatial extents into three and two zones, 

respectively. The upper Ringold Formation was divided into 

six zones to enable simulation of possible heterogeneity of 

specific yield in that formation. Hydraulic conductivity of 

the other hydrogeologic units and specific storage in each 

hydrogeologic unit was uniform throughout the model domain.

Horizontal Flow Barriers

The Saddle Mountains Fault (Lidke, 2002) was simulated 

with horizontal flow barriers (HFBs; Hsieh and Freckleton, 

1993) in all model layers at the locations shown in figure 9. 

The decreased hydraulic conductance between adjacent 

model cells is specified with a hydraulic characteristic for 

each HFB that is dimensionally equivalent to the hydraulic 

conductivity of the fault zone (in the direction perpendicular to 

the fault plane) divided by the width of the fault zone. A single 

hydraulic characteristic for HFBs representing the Saddle 

Mountains Fault barriers was parameterized and estimated 

during model calibration.
24 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig11

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GRANT

COUNTY ADAMS

COUNTY

FRANKLIN

COUNTY

Columbia River

Snake River

Crab Creek

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

0 – 1

2 – 10

11 – 20

21 – 40

41 – 80

81 – 160

161 – 320

321 – 876

Annual groundwater withdrawals, 

in acre-feet per year per square kilometer

! Project well ! Additional wells used in model

Simulated withdrawal wells

Figure 11. SOil WATer balance (SOWAT) model-estimated groundwater withdrawals for 2007 and locations of simulated 

withdrawal wells, eastern Pasco Basin, Washington.
Simulation of Groundwater Flow 25

tac15-0988_fig12

0

50

100

150

200

250

300

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Year

Simulated total annual withdrawals, in thousands of acre-feet per year

Figure 12. Simulated total annual groundwater 

withdrawals, eastern Pasco Basin, Washington

Water-Level Measurements

The model-calibration dataset included 7,559 water 

levels measured in the 726 project wells with screened 

interval information. Although only a single measurement was 

recorded from some wells, multiple measurements from most 

of the project wells constituted water-level hydrographs of as 

much as 46 years in duration (well 17N/26E-34D01 has the 

longest record). Almost one-half of the measurements (3,289) 

were made in wells with screens that penetrated more than 

one hydrogeologic unit. For these multi-layer measurements, 

the proportion of the well screen connected to each model 

layer was specified in the Head-Observation (HOB) package 

of MODFLOW, which calculates an aggregate simulatedequivalent water level from these specified proportions. An 

additional 99 water-level measurements that were used to 

construct the map of pre-development water levels (fig. 8) also 

were specified to represent the 1939 water-table altitude in the 

HOB package.

Model Calibration

The model was calibrated with the parameter estimation 

code PEST++ (Welter and others, 2012), which adjusted 

the model parameters within specified reasonable limits 

to minimize the objective function (eq. 1) that quantifies 

differences between simulated water levels and flow rates and 

their measured values. The calibration uses 7,858 water‐level 

measurements, which were classified either as heads or 

the head change from the initial head measured in each 

observation well. About 200 water levels were measured 

during the period corresponding to the steady-state stress 

period (1900–1920); the remaining water levels were 

measured after 1920. Because the assessment of change in 

groundwater storage is a model objective, it was important to 

accurately simulate changes in water levels. Accordingly, the 

regression equation (eq. 1) uses 826 water-level measurements 

as heads (the earliest water-level observation from each 

project well, plus the pre-development observations) and 

7,032 water-level measurements as head changes, or the 

water-level change from the earliest water level measured in 

each well. The measured flow rates from six springs during 

1986–87 and the estimated flow through agricultural drains 

reported by Drost (1997) also were used for calibration.
26 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

ω ω

ω ω

ω ω

k k k k

k

j j j j

j

i i i

H H'

D D'

F F'

( ) −

+ − ( )

+ −

=

=

∑

∑

( )

( )

b

b

1

826 2

1

7032 2

i

i

( ) ( ) b

=

∑

1

7 2

. (1)

In equation 1, symbols in the first summation are associated 

with water levels,

where

826 is the number of measurements used as 

water‐level heads, 

Hk is the measured water level for observation k, 

H′k is the simulated-equivalent water level for 

observation k,

ωk is the weight applied to water-level 

measurement Hk and its simulated 

equivalent H′k , and 

b is the vector of 37 model parameters. 

Symbols in the second summation are associated with 

water‐level measurements used as “drawdowns,”

where

Dj is the measured water-level drawdown for 

observation j, 

D′j is the simulated-equivalent water-level 

drawdown for observation j, and

ωj is the weight applied to water-level 

measurement Dj and its simulated 

equivalent D′

j

.

Symbols in the third summation are associated with measured 

or estimated flows,

where

Fi is the flow for observation i, 

F′i is the simulated-equivalent flow for 

observation i, and

ωi is the weight applied to water-level 

measurement Fi and its simulated 

equivalent F′i .

The weight (ω) assigned to water-level or flow 

observations should represent the measurement error, 

and typically is computed as the inverse of the total 

observation-error variance. Substantial differences in 

measurement errors of the water levels were not determined, 

and, therefore, were assumed to be equivalent, which 

resulted in uniform weighting among the water-level and 

drawdown‐observations. 

The calibrated values of 37 model parameters 

representing aquifer hydraulic conductivity, storage, and 

boundary and fault conductances are tabulated with their 

corresponding sensitivities (table 4). The calibrated values 

of hydraulic conductivity, storage coefficient, and specific 

yield are all within the range of values measured or estimated 

for their corresponding hydrogeologic units by previous 

studies (table 3).

Parameter Sensitivities

The composite parameter sensitivity represents the 

total amount of information provided by the water-level, 

drawdown, and flow observations for the estimation of 

the model parameter (Hill and Tiedeman, 2007). PEST++ 

computes a composite sensitivity of each model parameter 

with respect to all the weighted simulated heads, drawdowns, 

and flows. The composite sensitivities of parameters included 

in the regression computed at the final parameter values are 

summarized in table 4. Generally, parameters with sensitivities 

greater than about 0.1 were more accurately estimated and 

had correspondingly small confidence intervals. Parameters 

with composite-scaled sensitivities less than about 0.1 were 

more difficult to estimate and (or) had larger confidence 

intervals than parameters with larger sensitivities. Simulated 

water levels were sensitive to parameters representing the 

horizontal hydraulic conductivity of the Pasco gravels (kpas), 

the horizontal and vertical hydraulic conductivities of the 

Wanapum Basalt (k_wanapum1, vk_7), the specific yield of 

the Pasco gravels and most of the Ringold Formation (sy

), 

specific storage (ss), conductance to boundaries representing 

agricultural drains (drain_1970), and groundwater underflow 

across the northern and eastern model perimeter (ghb).

Simulated Water Levels

The distribution of the measured versus simulated 

water-level altitudes (fig. 13) is not random about a 1:1 line; 

a cluster of simulated water levels below this line in the range 

of measured water levels between 300 and 500 ft indicates 

that the model overpredicts water-level altitudes in some areas 

within this range. Conversely, a greater number of simulated 

water levels above this line for measured water levels 

more than 1,000 ft indicates that the model underpredicts 

water levels in some areas with high water levels. Some 

model bias is indicated by this deviation from a random 

residual distribution.
Simulation of Groundwater Flow 27

Table 4. Parameter values and sensitivities for the calibrated groundwater-flow model for eastern Pasco Basin, 

Washington.

[Abbreviations: ft, foot; ft/d, foot per day; ft2/d, foot squared per day]

Parameter type Units Parameter name Calibrated value Composite sensitivity

Horizontal hydraulic conductivity ft/d k_not 12.0 0.0165

k_pas_gra2 328.6 0.2520

k_pas35678 519.6 0.1694

k_pas_gra4 4,245.9 0.2595

k_upp_ring 9.2 0.0855

k_mid_ring 13.6 0.0892

k_low_ring 11.7 0.0360

k_bas_ring 10.0 0.0020

k_wanapum1 6.5 0.3127

k_wanapum2 98.8 0.0748

k_basalt 4.2 0.1283

Vertical hydraulic conductivity ft/d vk_coarse 4.49E-01 0.0009

vk_1345 9.04E-03 0.1000

vk_2 2.88E-03 0.6740

vk_6 3.92E-02 0.0555

vk_7 2.85E-03 0.8076

vk_8 1.41E-01 0.0093

Specific yield dimensionless sy_pgravel 0.20 0.1676

sy_up_rin1 0.12 0.1553

sy_up_rin2 0.20 0.0617

sy_up_rin3 0.20 0.0951

sy_up_rin4 0.20 0.1246

sy_up_rin5 0.10 0.1011

sy_up_rin6 0.19 0.0643

sy_md_ring 0.20 0.1982

sy_lo_ring 0.20 0.1285

sy_12345 0.17 0.0291

sy_678 0.01 0.5442

Specific storage ft-1 ss 1.00E-06 0.1501

Fault hydraulic

characteristic

day-1 hfb 1.00E-05 0.0404

Boundary conductance ft2

/d ringspring 769.5 0.0021

other_sprg 10.0 0.0155

drain_1970 1,125.2 0.1589

columbia_r 10.0 0.0001

snake_rivr 10.0 0.0001

crab_creek 0.1 0.0176

ghb 1,000.0 0.2145
28 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig13

Simulated water-level altitude, in feet

300 400 500 600 700 800 900 1,000 1,100 1,200 1,300

1,200

1,250

1,150

1,100

1,050

1,000

950

900

850

800

750

700

650

600

550

500

450

400

350

300

250

200

Measured water-level altitude, in feet above North American Vertical Datum of 1988

Slope = 1

Figure 13. Measured and simulated water levels in the eastern Pasco Basin, Washington.
Simulation of Groundwater Flow 29

To quantify the fit of simulated water levels to the 

7,858 water-level measurements used for model calibration, 

summary statistics were calculated on the water-level residuals 

(measured minus simulated water-level altitudes). The mean 

magnitude of the water-level residuals was 84 ft. The rootmean-square error (RMSE) was calculated as the square root 

of the mean of the unweighted squared residuals:

RMSE

H H' k k

k =

( ) −

=

∑

1

7 858 2

7858

,

, (2)

where:

Hk is the measured water level for observation k, 

and

H′

k is the simulated-equivalent water level for 

observation k.

The RMSE and the standard deviation of the residuals 

were 105 and 101 ft, respectively. The normalized RMSE was 

0.1 and was obtained by dividing the RMSE of the residuals 

by the 1,051-ft range of measured water levels. The mean 

error, calculated as the mean of the residuals (measured minus 

simulated water levels) was -26.7 ft, indicating that overall 

simulated water levels are higher than measured water levels. 

The mean absolute error (mean of the absolute value of the 

residuals) was 84 ft.

The general directions of simulated groundwater flow 

during 2013 may be inferred from contours of simulated 

water-level altitudes for 2013 (fig. 14). In Grant County north 

of the Saddle Mountains, groundwater generally flows from 

high-altitude areas and discharges into Crab Creek. South of 

the Saddle Mountains, water flows from a central groundwater 

mound toward the Columbia River. In the northeastern part of 

the study area, groundwater generally flows from high‐altitude 

areas in Adams County toward the southwest and the 

Columbia River. In Franklin County, groundwater flows south 

or southwest toward the Snake or Columbia Rivers.

The simulated rise in groundwater levels since 1920 

in areas of the eastern Pasco Basin primarily is a function 

of the hydrogeologic units present and the magnitude of 

groundwater recharge in the area. Although groundwater 

withdrawals also affect water levels, their effect is difficult 

to discern because groundwater withdrawals are simulated in 

areas of high recharge. East of the Columbia River in Franklin 

County, water levels rose by more than 350 ft by 2013 in two 

distinct mounds where the upper Ringold Formation is at or 

near land surface both north and south of Ringold Coulee 

(fig. 15), and simulated recharge is 5–10 in/yr (fig. 10). 

Similar mounding of recharged groundwater occurs where the 

upper Ringold Formation is at or near land surface in Adams 

County and Grant County north of the Saddle Mountains, 

and where the upper Ringold Formation subcrops beneath 

the Pasco gravels unit in Grant County south of the Saddle 

Mountains and in southern Franklin County (fig. 15). The high 

groundwater levels in these mounds partly result from the 

relatively low horizontal hydraulic conductivity of the upper 

Ringold sediments, which inhibits flow toward surrounding 

areas with lower groundwater levels. Water levels have not 

risen substantially in the Pasco gravels because their high 

hydraulic conductivity permits greater flow toward areas of 

groundwater discharge. Simulated water levels rose in basalt 

aquifers beneath cultivated areas (fig. 2) in Grant County north 

of the Saddle Mountains and eastern Franklin County where 

overburden sediments are not simulated. In the northeastern 

part of the model domain in Adams County where overburden 

sediments are not simulated (fig. 15), simulated water levels 

generally have risen by 50–100 ft, but also have declined 

by about 50 ft in some areas with low recharge (fig. 10), 

presumably from groundwater withdrawals. 

Simulated Groundwater Budget

The net flows (inflow minus outflow from the model 

domain) of groundwater-model components were calculated 

for each model stress period (fig. 16). Prior to 1946, variations 

in annual precipitation resulted in relatively modest changes 

in storage and net discharge to rivers. Note that increased 

groundwater storage is an outflow from the groundwater‐flow 

system and is negatively signed; increasing negatively 

signed absolute values of storage in figure 16 indicate 

increases in the quantity of stored groundwater. Increased 

anthropogenic recharge from the late 1940s through the 

mid-1960s substantially increased groundwater storage in the 

aquifer system, and discharge to rivers approximately tripled 

by 1963. Beginning in 1964, outflow to agricultural drains 

began to control rising water levels and associated increases 

in groundwater storage. Increasing groundwater withdrawals 

(pumpage) after 1964 further abated contributions to storage, 

and together with outflow to drains, somewhat decreased the 

outflow to rivers. Net changes to groundwater storage since 

1984 have resulted primarily from variations in groundwater 

recharge caused by changes in annual precipitation.

The simulated groundwater budget in the 508-14 area 

calculated for the initial steady-state stress period (which 

represents average “pre-development” conditions from 1900 

to 1920) and the last transient stress period (which represents 

2013) are summarized in table 5. During 1900–1920, which 

is simulated with steady-state conditions, vertical recharge, 

lateral underflow, and some seepage from rivers into the 

508-14 area were balanced by discharge to rivers and springs 

or lateral underflow out through the 508-14 boundary. By 

2013, substantially more recharge into the area (compared to 

1900–1920) resulted in increased discharge to rivers, springs, 

and drains, and increased net underflow out of the 508-14 

area. The distribution of both recharge and groundwater 

withdrawals causes both increases and decreases in 

groundwater storage in different areas, which are summarized 

as outflows to and inflows from storage, respectively, 

in table 5. During 2013, a 9,000 acre-ft net decrease in 

groundwater storage was simulated for the 508-14 area.
30 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig14

700

400

400

1,000

1,500

1,000

1,100

1,200

600

400

700

1,100

700

600

500

1,000

900

1,100

600

1,100

400

700

500

600

900

700

900

900

1,200

500

900

1,100

1,200

500

1,000

1,000

1,000

700

1,000

1,000

1,000

800

1,100

1,100

1,200

700

1,200

400

500

800

600

1,200

600

1,100

1,500

900

600

900

900

800

900

1,000

1,300 1,200

1,100

700

1,100 1,000

900

800

1,000

500

1,100

900

1,000

600

800

700

GRANT

COUNTY ADAMS

COUNTY

FRANKLIN

COUNTY

Columbia River

Snake River

Crab Creek

Saddle Mountains

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Simulated water-level contour—

Shows altitude of simulated water 

level, 2013. Contour interval 100 feet. 

Datum is North American Vertical 

Datum of 1988

400

500

600

700

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

1,700

Figure 14. Simulated water-level altitudes, eastern Pasco Basin, Washington, 2013.
Simulation of Groundwater Flow 31

tac15-0988_fig15

200

0

0

50

250

0

0

-50

-50

50

200

50

50

50

150

250

300 100

300

50

350

0

250

150

150

350

200

50

0

50

50

550

150

350

250

-50

100

150

100

200

150

100

0

150

200

100

100

250

100

300

200

250

250

50

50

0

250

50

50

250

50

50

550

50

300

350

250

50

200

0

300

150

100

200

-100

100

200

250

400

250

100

0

400

150

100

250

100

0

350

200

100

0

150

500

300

200

150

0

50

100

350

350

250

450

250

300

300

250

-50

100

300

300

-50

200

150

150 200

200

250

100

200

50

50

250

0

250

200

50

GRANT

COUNTY ADAMS

COUNTY

FRANKLIN

COUNTY

Columbia River

Snake River

Crab Creek

Saddle Mountains

Ringold

Coulee

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Extent of Pasco gravels

Extent of upper Ringold Formation at land surface

Exent of upper Ringold Formation beneath Pasco gravels

Line of equal water-level change. Interval, 50 feet 

Boundary of active model domain 

50

Figure 15. Simulated changes in water-level altitude, eastern Pasco Basin, Washington, 1920–2013.
32 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig16

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

-60

-40

-20

0

20

40

60

80

Year

Simulated net flow, in millions of cubic feet per day

Recharge

Storage

Drains

Boundary underflow

Pumpage

River

EXPLANATION

Figure 16. Simulated net flows into the groundwater model domain, eastern Pasco Basin, Washington, 1920–2013.

Table 5. Groundwater budget of the 508-14 area, eastern Pasco 

Basin, Washington, 1900–1920 and 2013.

[–, not applicable]

1900–1920

(steady-state)

(thousands of 

acre-feet per year)

2013

(thousands of 

acre-feet per year)

Inflow

Recharge 27 346

River leakage 5 2

Underflow 41 34

Storage – 18

Total 73 400

Outflow

River leakage 40 105

Springs and drains 7 79

Underflow 25 60

Net withdrawals – 147

Storage – 9

Total 73 400
Simulation of Groundwater Flow 33

Simulated Storage Changes in the 508-14 Area

The cumulative simulated change in storage through 

2014 was summarized for the entire active model domain and 

five subareas (fig. 17) that compose the “508-14 area” (fig. 1) 

in which Ecology regulates groundwater withdrawals. Note 

that cumulative increases in groundwater storage are depicted 

as a positive quantity in figure 17, (in contrast to the negative 

rates of flow from the groundwater system into storage 

depicted in figure 16) To evaluate the effect of approving 

pending applications for groundwater‐withdrawal permits, 

anthropogenic additions to storage (referred to as “artificially 

stored” groundwater by Ecology) must be quantified. The 

changes to groundwater storage in five subareas (fig. 18) of the 

508-14 area are depicted with the total for the model domain 

(fig. 17), and net increases through 2013 are summarized 

(table 6). The additional groundwater in storage since the predevelopment period in the 508-14 area reached a maximum of 

7.4 million acre-ft by 1985 and has since decreased by about 

0.5 million acre-ft to a quantity of about 6.8 million acre-ft in 

2013 (fig. 17). Whereas simulated storage changes stabilized 

or decreased after the late 1970s in most subareas, storage 

was simulated as continuing to increase in the west subarea 

through 2013. Simulated storage increases in the northwest 

subarea reached maximum levels in 1963, and decreased after 

drains were installed in 1964. Simulation of the installation of 

agricultural drains also decreased the rate of storage increases 

in the northeast, south, and central subareas. 

Table 6. Net increase of stored groundwater in subareas of the 508-14 area, eastern Pasco Basin, 

Washington, 1920–2013.

[Values in thousands of acre-feet]

Hydrogeologic

unit

Stored groundwater in the 508-14 subarea

Northwest Central West Northeast South

Pasco 33 23 383 38 191

Upper Ringold 30 560 735 920 1,058

Middle Ringold 3 5 744 12 413

Lower Ringold 3 592 89 12 290

Basal Ringold 3 5 4 12 10

Basalt 201 99 83 147 131

Total 273 1,284 2,038 1,141 2,093
34 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

tac15-0988_fig17

1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

-0.5

0.5

1.5

2.5

3.5

4.5

5.5

6.5

7.5

8.5

Year

Simulated changes in groundwater storage, in millions of acre-feet

Active model domain

508-14 area

Northwest subarea

Northeast subarea

West subarea

Central subarea

South subarea

EXPLANATION

Figure 17. Simulated changes in groundwater storage in the model domain and areas in the 508-14 area, eastern 

Pasco Basin, Washington,1920–2013.
Simulation of Groundwater Flow 35

tac15-0988_fig18

GRANT

COUNTY ADAMS

COUNTY

FRANKLIN

COUNTY

Columbia River

Snake River

Crab Creek

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

120°00' 119°30' 119°00'

47°

00'

46°

30'

Elevation data from the National Elevation Dataset.

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Storage change, in thousand acre-feet 

by subarea

Northwest (273)

Northeast (1,141)

West (2,038)

Central (1,284)

South (2,093)

Boundary of 508-14 Area (6,830)

Boundary of active model domain (7,345)

273

2,038

1,141

1,284

2,093

Figure 18. Subareas with summarized simulated change in groundwater storage in the active model domain and 508-14 area, 

eastern Pasco Basin, Washington.
36 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

Limitations and Appropriate Use of the Model

The hydrogeologic framework developed for this 

study is based on available geologic mapping, well drillers’ 

descriptions of material encountered during drilling, and 

limited site visits to exposed material along bluffs. Correlation 

of units at depth required interpolating and extrapolating 

the available data through areas with sparse or no data. The 

hydrogeologic framework developed for this study is regional 

in nature, and local conditions may vary.

The absence of historical withdrawal data from wells 

results in the greatest limitation of the groundwater model. 

Although the best available land- and water-use data were 

used, the method that specified the spatial distribution of 

withdrawals both areally and by aquifer (that is, assigned 

model layer) is uncertain and introduces error in the headdependent flow boundaries that simulated withdrawal wells 

because the actual historical withdrawals are unknown. The 

measured water levels used for model calibration respond 

to actual groundwater withdrawals, but because the location 

and magnitude of withdrawals specified in the model is at 

best an approximation of the actual withdrawals, water levels 

simulated at observation-well locations cannot reproduce 

measured water levels with high fidelity. Although the model 

may not simulate water levels measured at a particular location 

with high accuracy, simulated changes in water levels, such 

as those resulting from increased recharge over many years, 

should be more reliable. Changes in water levels resulting 

from an additional simulated stress also may be simulated 

more accurately than the water-level altitudes.

Because groundwater flux and hydraulic conductivity are 

correlated, and because uncertain groundwater-withdrawal 

fluxes were specified, relatively homogeneous hydraulicconductivity distributions were simulated to avoid overfitting 

the water-level measurements during model calibration. 

The values of hydraulic properties in the model, such 

as hydraulic conductivity, specific storage, and specific 

yield, represent large-scale, or “average,” values for each 

simulated hydrogeologic unit. Because each of these units 

is heterogeneous, values of these properties determined by 

analysis of aquifer-pumping or slug tests may differ from the 

model-simulated values.

The simulated values of specific yield were calibrated to 

measured changes in rising water levels that occurred from 

about 1952 through the mid-1990s as water filled effective 

pore space in overburden sediments. Fluid retention as pore 

space drains can cause a hysteretic effect, resulting in a 

smaller specific yield during a sediment dewatering phase than 

during sediment wetting (Maqsoud and others, 2004). Because 

the possible specific retention of overburden sediments 

was not modeled, actual water-level declines resulting 

from a particular groundwater withdrawal may be slightly 

greater than that simulated with the model. Because specific 

retention is probably less than about 1 percent, however, this 

effect would be difficult to discern from effects due to other 

model uncertainties.

Scenarios

Four simulations were constructed to quantify the 

groundwater-level drawdown under hypothetical pumping 

scenarios. Each scenario simulates a constant withdrawal rate 

from groundwater withdrawal wells for a 7-year period from 

early 2014 through 2020. Scenario 1 investigates using a well 

field to reduce the high water levels thought to contribute to 

landslides along the White Bluffs area along the Columbia 

River (fig. 1). The remaining three scenarios use the model 

to simulate the drawdown that might be caused by permitting 

an additional groundwater-withdrawal application at two 

locations where (1) a thick sequence of the Pasco gravels is 

present about 5 mi northwest of Pasco and (2) the upper and 

middle Ringold Formations overly the Saddle Mountains 

Basalt about 13 mi northwest of Pasco. Two scenarios are 

simulated at the second location—one in which the well is 

screened only in the upper Ringold Formation, and a second in 

which the well is screened throughout the Ringold Formation 

and into the underlying Saddle Mountains Basalt. Each of 

these three permit-application scenarios simulate an additional 

1 Mgal/d withdrawal from a single cell, which could be 

provided by one or more wells in that cell. One Mgal/d is 

typical of the annualized withdrawal rate of currently pending 

permit applications (John J. Covert, Washington State 

Department of Ecology, written commun., July 31, 2015).
Scenarios 37

Scenario 1—Landslide Abatement

A field of seven hypothetical groundwater withdrawal 

wells screened in the upper Ringold Formation (model 

layer 2) each withdraw 285,714 gal/d, for a total of 2 Mgal/d 

(267,361 ft3/d). Four simulated withdrawal wells are 

positioned 3,281 ft (1,000 m) apart along a north-south 

trending line and three wells are positioned 4,640 ft (1,414 m) 

apart along a northwest to southeast trending line (fig. 19). 

The simulated well positions generally are between 300 and 

3,000 ft northeast of the edge of the upper escarpment of the 

White Bluffs. Withdrawals were equally distributed among the 

seven wells for 7 years.

0

Columbia River

-5

-45

-20

-10

-30

-40

-55

-45 -50

-25

-35

-70

-65

-60

-15

White Bluffs

escarpment

EXPLANATION

Line of equal simulated drawdown—Interval, 5 feet

Hypothetical well

-60

0 1 2 3 MILES

0 1 2 3 KILOMETERS

119°25' 119°20'

46°

40'

46°

35'

2013 USDA NAIP (National Agricultural Imagery Program) imagery

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

Figure 19. Locations of hypothetical wells and simulated drawdown after 7 years of withdrawals at 2 million gallons per day near 

White Bluffs, eastern Pasco Basin, Washington.
38 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

Drawdowns after 7 years along the edge of the White Bluffs 

escarpment, where the cliffs can be unstable and prone to landslides 

(fig. 19), range from 30 to 70 ft within a distance of about 3,000 ft 

from a well. The simulated hydrograph for a hypothetical observation 

well located near the central pumping well (fig. 20) indicates the 

water-level rise resulting from irrigation since 1947, and 78 ft of 

drawdown resulting from the hypothetical well field withdrawals that 

begin in 2014. Drawdown of about 60 ft, or 76 percent of the 7-year 

drawdown, occurs after 4 years. Although the model simulates the 

drawdown resulting from the specified withdrawals, low hydraulic 

conductivity of the upper Ringold Formation sediments may limit the 

withdrawal rates achievable from actual wells.

1945 1955 1965 1975 1985 1995 2005 2015 2025

500

550

600

650

Water-level altitude, in feet 

above North American Vertical Datum of 1988

Year

Figure 20. Simulated water-level altitudes in the upper Ringold 

Formation near the middle of the hypothetical well field near 

White Bluffs, eastern Pasco Basin, Washington.

Scenario 2—Withdrawal from Pasco 

Gravels

The relatively small drawdown that might 

result from an additional withdrawal from the Pasco 

gravels, which have high hydraulic conductivity, 

was simulated with a 7-year withdrawal at 1 Mgal/d 

about 5 mi northwest of Pasco, Washington 

(location A in fig. 21), where the Pasco gravels are 

about 200 ft thick. 

The screen of the hypothetical well at this 

location is in the Pasco gravels, which supply all 

of the specified withdrawal. The hydrograph of 

simulated water-level altitudes for the model cell 

of the additional simulated withdrawal depicts a 

water-level rise of about 63 ft from 1948 to 1976, 

followed by a water-level decline of about 1 ft/yr 

from 1976 to 2014 because of decreased recharge 

and regional groundwater withdrawals (fig. 22). 

When the additional withdrawal is simulated in 

2015, the rate of water-level decline accelerates, 

declining 3 ft in the year of well activation, but 

resumes the previous rate of about 1 ft/yr in 

subsequent years. The additional water-level 

decline created by this withdrawal from the Pasco 

gravels is barely discernible. 
Scenarios 39

tac15-0988_fig21

-5

-10

-15

B

A

Columbia River

0 1 2 3 MILES

0 1 2 3 KILOMETERS

119°10' 119°00'

46°

25'

46°

20'

2013 USDA NAIP (National Agricultural Imagery Program) imagery

Washington State Plane South, FIPS zone 4602, units in feet.

Horizontal datum is North American Datum of 1983.

EXPLANATION

Extent of Pasco gravels

-5 Line of equal simulated drawdown—Interval 5 feet

Figure 21. Extent of the Pasco gravels, locations of wells screened in the Pasco gravels (location A) and upper Ringold 

Formation (location B), and the simulated drawdown after 7 years of an additional 1 million gallons per day withdrawal from the 

upper Ringold Formation, eastern Pasco Basin, Washington.

1945 1955 1965 1975 1985 1995 2005 2015

400

410

420

430

440

450

460

470

480

490

500

Year

Water-level altitude, in feet above 

North American Vertical Datum of 1988

Figure 22. Simulated water-level altitudes where an 

additional 1 million gallon per day withdrawal was specified 

from a well screened in the Pasco gravels, eastern Pasco 

Basin, Washington. Well at location A in figure 21.
40 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

Scenario 3—Withdrawal from Upper Ringold 

Formation

In contrast to scenario 2, the drawdown that may 

result in overburden sediments where the high-conductivity 

Pasco gravels are not present was simulated by specifying 

an additional 1 Mgal/d withdrawal from the upper Ringold 

Formation 13 mi north-northwest of Pasco (location B in 

fig. 21). The simulated thicknesses of the upper Ringold 

Formation, middle Ringold Formation, and Saddle Mountains 

Basalt units at this location are 315, 118, and 300 ft, 

respectively. The hydrograph of simulated water-level 

altitudes where this withdrawal is imposed depicts about 38 ft 

of drawdown at the top and in the middle of the upper Ringold 

Formation, but negligible drawdown in the underlying middle 

Ringold Formation and Saddle Mountains Basalts units 

(fig. 23). The 7-year drawdown caused by this simulated 

withdrawal decreases to 5 ft at a distance of 1–13⁄4 mi from the 

withdrawal well (fig. 21). 

450

500

550

600

650

700

750

800

850

900

Year

Water-level altitude, in feet above North American Vertical Datum of 1988

1945 1955 1965 1975 1985 1995 2005 2015

Top of upper Ringold Formation (layer 1)

Middle of upper Ringold Formation (layer 2)

Middle Ringold Formation

Saddle Mountains Basalt

Wanapum Basalt 

EXPLANATION

Figure 23. Simulated water-level altitudes for the upper and middle Ringold Formations, and the Saddle Mountains and 

Wanapum Basalts where an additional 1 million gallon per day withdrawal was specified from a well screened in the upper 

Ringold Formation, eastern Pasco Basin, Washington. Well at location B in figure 21.
Scenarios 41

Scenario 4—Withdrawal from Ringold 

Formation and Saddle Mountains and 

Wanapum Basalts

A more likely scenario than scenario 3 where Pasco 

gravels are not present is for an additional withdrawal well 

to be screened through the Ringold Formation and into the 

underlying basalt formation. This was simulated by specifying 

an additional 1 Mgal/d withdrawal from a well screened in 

the upper and middle Ringold Formations and the Saddle 

Mountains Basalt at the same location as in scenario 3 

(location B in fig. 21). In this scenario, the simulated 7-year 

drawdown at the top and in the middle of the upper Ringold 

Formation is 26 ft, or about 12 ft less than in scenario 3 

(fig. 24). About 12 ft of drawdown also occur in the middle 

Ringold Formation and Saddle Mountains Basalt because 

the withdrawal well also is screened in those formations in 

this scenario. Simulated flows from the upper and middle 

Ringold Formations and Saddle Mountains Basalt provide 

about 64, 21, and 15 percent of the 1 Mgal/d total withdrawal 

specified from this multi-aquifer well. Despite the lower 

hydraulic conductivity of the upper Ringold Formation, 

its large thickness at this location results in the relatively 

large proportion of water withdrawn from the hypothetical 

production well. The simulated hydrographs (fig. 24) indicate 

the downward vertical hydraulic gradient at the location of the 

hypothetical well. 

Year

1945 1955 1965 1975 1985 1995 2005 2015

450

500

550

600

650

700

750

800

850

900

Water-level altitude, in feet above North American Vertical Datum of 1988

Top of upper Ringold Formation (layer 1)

Middle of upper Ringold Formation (layer 2)

Middle Ringold Formation

Saddle Mountains Basalt

Wanapum Basalt 

EXPLANATION

Figure 24. Simulated water-level altitudes for the upper and middle Ringold Formations, and Saddle Mountains and 

Wanapum Basalts where an additional 1 million gallon per day withdrawal was specified from a well screened through the 

upper and middle Ringold Formation and Saddle Mountains Basalt, eastern Pasco Basin, Washington. Well at location B in 

figure 21.
42 Simulation of Groundwater Storage Changes in the Eastern Pasco Basin, Washington

Summary

The Miocene Columbia River Basalt Group and younger 

sedimentary deposits of lacustrine, fluvial, eolian, and 

cataclysmic-flood origin compose the aquifer system of the 

eastern Pasco Basin in Washington. Irrigation return flow and 

canal leakage from the Columbia Basin Project have recharged 

this aquifer system under parts of the eastern Pasco Basin 

since 1952. Groundwater levels in the sedimentary overburden 

have risen substantially in some areas of the eastern Pasco 

Basin, and may contribute to landslides along the Columbia 

River and septic system failures, among other problems. 

State and local water-resource managers are considering 

extracting the additional stored groundwater to supply 

increasing demand, as well as possibly mitigating problems 

caused by the increased water levels. The objectives of this 

study were to (1) define the area hydrogeology, (2) determine 

rates and directions of groundwater flow through the aquifer 

system, (3) quantify the effect of anthropogenic recharge 

and groundwater withdrawals on groundwater storage, and 

(4) simulate the effects of groundwater-management scenarios.

To address the objectives of this study, the U.S. 

Geological Survey developed a transient groundwater 

model of the eastern Pasco Basin that quantifies changes in 

groundwater flow and storage. The model uses a 1-kilometer 

finite difference grid with MODFLOW-NWT, and is 

constrained by logs and water levels from 846 wells in the 

study area. Eight model layers represent five overburden 

hydrogeologic units and underlying basalt formations. 

Head-dependent flux boundaries represent the Columbia and 

Snake Rivers to the west and south, respectively, discharge 

through groundwater withdrawal wells, underflow to and 

(or) from adjacent areas to the northeast, and discharge to 

agricultural drains and springs. Specified flux boundaries 

represent recharge from precipitation and anthropogenic 

sources, including irrigation return flow and leakage from 

water-distribution canals. The model was calibrated with 

the parameter-estimation code PEST++ to groundwater 

levels observed from 1907 through 2013 and discharge to 

springs and agricultural drains observed or estimated from 

1986 to 1987. A simulated increase of 6.8 million acre-feet 

in groundwater storage in the 508-14 administrative area 

since pre-development results from anthropogenic sources 

of recharge into the aquifer system. Four hypothetical 

groundwater-withdrawal simulations estimate the water‐level 

drawdowns that might result from various pumping scenarios. 

Scenario 1 simulates a 2 million gallon per day (Mgal/d) 

groundwater withdrawal from a field of seven wells screened 

in the upper Ringold Formation on the White Bluffs along 

the Columbia River. Simulated drawdowns ranging from 

30 to 70 feet along the White Bluffs near the well field 

after 7 years may reduce the susceptibility of this area to 

landslides. Scenarios 2–4 simulate the drawdowns resulting 

from an additional groundwater withdrawal of 1 Mgal/d in 

wells screened in either the Pasco gravels, the upper Ringold 

Formation, or both the Ringold Formation and underlying 

basalt. Whereas withdrawals from the Pasco gravels generate 

negligible drawdown, withdrawals from the Ringold 

Formation generate more significant drawdown, although the 

water produced from wells screened only in sediments with 

low hydraulic conductivity may be limited.

Acknowledgments

The authors would like to thank the many people who 

have contributed to our understanding of the hydrogeology 

of the Pasco Basin, and the personnel of the Bureau of 

Reclamation and Washington State Department of Ecology 

involved in the 508-14 Area Groundwater Management 

Program. U.S. Geological Survey colleagues Devin Galloway, 

Andrew Long, and Michelle Sneed provided thoughtful and 

helpful reviews of the draft manuscript.

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For more information concerning the research in this report, contact the

 Director, Washington Water Science Center

U.S. Geological Survey

934 Broadway, Suite 300

Tacoma, Washington 98402 

http://wa.water.usgs.gov
Heywood and others—Simulation of Groudwater Storage Changes in the Eastern Pasco Basin, Washington—Scientific Investigations Report 2016–5026

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20165026