Simulation of Ground-Water Flow and Potential

Contaminant Transport at Area 6 Landfill,

Naval Air Station Whidbey Island,

Island County, Washington

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 01-4252

Prepared in cooperation with

Department of the Navy

Engineering Field Activity, Northwest

Naval Facilities Engineering Command

Crescent

Harbor

City of

Oak Harbor

Strrait of Juan de

Fuca

Dugualla

Bay

Oak

Harbor

AULT FIELD

SEAPLANE BASE

State

Highway 20

Area 6 Landfill

Area 6

Model boundary

science for a changing world
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01-4252

Simulation of Ground-Water Flow and

Potential Contaminant Transport at Area 6

Landfill, Naval Air Station Whidbey Island,

Island County, Washington

Prepared in cooperation with

DEPARTMENT OF THE NAVY

ENGINEERING FIELD ACTIVITY NORTHWEST

NAVAL FACILITIES ENGINEERING COMMAND

By F. William Simonds

Tacoma, Washington

2002
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information write to:

District Chief

U.S. Geological Survey

1201 Pacific Avenue – Suite 600

Tacoma, Washington 98402

http://wa.water.usgs.gov

Copies of this report can be purchased

from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286

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

endorsement by the U.S. Government.

District Chief

U.S. Geological Survey

1201 Pacific Avenue – Suite 600

Tacoma, Washington 98402

http://wa.water.usgs.gov
Contents iii

CONTENTS

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

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

Background ................................................................................................................................................. 3

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

Description of the Study Area..................................................................................................................... 5

Previous Investigations ............................................................................................................................... 5

Acknowledgments....................................................................................................................................... 6

Hydrogeology of the Ground-Water Flow System .............................................................................................. 6

Geologic Framework and Hydrologic Units............................................................................................... 6

Hydraulic Properties of Aquifers and Confining Units............................................................................... 15

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

Steady-State Simulation of the Ground-Water Flow System............................................................................... 16

Modeling Approach .................................................................................................................................... 16

Description of Model .................................................................................................................................. 21

Boundary Conditions ......................................................................................................................... 21

Calibration of Model to Pre-Remediation Conditions ................................................................................ 22

Sensitivity Analysis for the Calibrated Model................................................................................... 25

Steady-State Simulation of Post-Remediation Conditions .................................................................................. 30

Evaluation of the Effects of Boundary Conditions on Model Results........................................................ 35

Sensitivity of Water Levels to Boundary Conditions ........................................................................ 35

Sensitivity of Fluxes to Boundary Conditions ................................................................................... 37

Sensitivity of Potential Contaminant Migration to Boundary Conditions......................................... 39

Discussion of Ground-Water Flow Simulation.................................................................................................... 41

Flow Within and Between Aquifers............................................................................................................ 41

Potential Contaminant Migration................................................................................................................ 43

Model Limitations....................................................................................................................................... 43

Summary and Conclusions................................................................................................................................... 44

References Cited .................................................................................................................................................. 45
iv Contents

FIGURES

Figure 1. Map showing location of the Whidbey Island Naval Air Station and Area 6 landfill study 

area, Island County, Washington...................................................................................................... 2

Figure 2. Map showing location of Area 6 landfill and surrounding features, including selected wells, 

hazardous waste storage area, and contaminant plumes................................................................... 4

Figure 3. Map showing generalized hydrogeologic section showing distribution of aquifer and 

confining units and the numbered layers used in the ground-water model...................................... 7

Figure 4. Map showing ground-water flow in the shallow aquifer, July 1989................................................ 8

Figure 5. Map showing thickness of Vashon advance outwash deposits in Area 6, including Vashon 

till where present .............................................................................................................................. 9

Figure 6. Map showing thickness of subunit 1 of the Whidbey Formation that defines the upper 

confining unit, or layer 3 of the model............................................................................................. 11

Figure 7. Map showing ground-water flow in the intermediate aquifer, November 1991 .............................. 12

Figure 8. Map showing thickness of subunit 2 of the Whidbey Formation that defines the 

intermediate aquifer, or layer 4 of the model ................................................................................... 13

Figure 9. Map showing thickness of subunit 3 of the Whidbey Formation that defines the lower 

confining unit, or layer 5 of the model............................................................................................. 14

Figure 10. Map showing distribution of recharge values used in the ground-water model............................... 17

Figure 11. Map showing finite-difference grid for the numerical three-dimensional model of 

ground-water flow at the Area 6 landfill .......................................................................................... 18

Figure 12. Hydrographs showing Water levels for selected wells screened in the shallow aquifer for the 

period April 1994 to December 1998............................................................................................... 20

Figure 13. Map showing model boundaries and pre-remediation water levels for the shallow aquifer............ 23

Figure 14. Map showing model boundaries and pre-remediation water levels for the intermediate aquifer .... 24

Figure 15a. Map showing simulated head distribution in layer 1, the upper portion of the shallow aquifer,

for the pre-remediation period.......................................................................................................... 26

Figure 15b. Graphs showing comparison of simulated and observed heads in wells with screens in the

shallow aquifer for the pre-remediation period and a summary of error.......................................... 27

Figure 16a. Map showing simulated head distribution in layer 4, the intermediate aquifer, for the 

pre-remediation period ..................................................................................................................... 28

Figure 16b. Graph showing comparison of simulated and observed heads in wells with screens in the 

intermediate aquifer for the pre-remediation period and a summary of error.................................. 29

Figure 17a. Map showing simulated head distribution in layer 1, the upper portion of the shallow 

aquifer, for the post-remediation period........................................................................................... 32

Figure 17b. Graph showing comparison of simulated and observed heads in wells with screens in the 

shallow aquifer for the pre-remediation period and a summary of error.......................................... 33

Figure 18a. Map showing simulated head distribution in layer 4, the intermediate aquifer, for the 

post-remediation period.................................................................................................................... 34

Figure 18b. Graph showing comparison of simulated and observed heads in wells with screens in the 

intermediate aquifer for the pre-remediation period and a summary of error.................................. 35

Figure 19a. Map showing simulated head distribution in layer 1, the upper portion of the shallow aquifer,

for the post-remediation pumping simulation with specified head boundary conditions................. 36

Figure 19b. Graph showing comparison of simulated and observed heads in wells with screens in the 

shallow aquifer for the post-remediation period and a summary of error........................................ 37
Contents v

Figure 20a. Map showing simulated head distribution in layer 4, the intermediate aquifer, for the 

post-remediation pumping simulation with specified head boundary conditions ............................ 38

Figure 20b. Graph showing comparison of simulated and observed heads in wells with screens in the 

intermediate aquifer for the post-remediation period and a summary of error ................................ 39

Figure 21. Diagram showing comparison of fluxes for model runs using no-flow and specified-head 

boundary conditions for all cells within the area of primary concern.............................................. 40

Figure 22. Map showing flow paths of particles placed in the vicinity of the contaminant source area 

indicated by elevated soil gas readings............................................................................................. 42
vi Contents

TABLES

Table 1. Precipitation data for Whidbey Island and nearby areas, 1994–98 .................................................. 19

Table 2. Hydrologic parameters used in the steady-state model ..................................................................... 27

Table 3. Results of sensitivity analysis ........................................................................................................... 30

Table 4. Flow rates in extraction wells ........................................................................................................... 31

Table 5. Model-derived volumetric ground-water budget for three scenarios ............................................... 43

Table 6. Summary of well data ....................................................................................................................... 48
Conversion Factors vii

CONVERSION FACTORS AND VERTICAL DATUM

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

°F=1.8 °C+32

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of

1929)--a geodetic datum derived from a general adjustment of the first-order level nets of both the

United States and Canada, formerly called Sea Level Datum of 1929.

Multiply By To obtain

Length

inch (in) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 4,047 square meter

acre 0.004047 square kilometer

square mile (mi2) 2.590 square kilometer 

Volume

gallon (gal) 3.785 liter

Flow rate

foot per day (ft/d) 0.3048 meter per day

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

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

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

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

inch per year (in/yr) 25.4 millimeter per year
Abstract 1

Simulation of Ground-Water Flow and Potential 

Contaminant Transport at Area 6 Landfill,

Naval Air Station Whidbey Island,

Island County, Washington

By F. William Simonds

ABSTRACT

A three-dimensional finite-difference 

steady-state ground-water flow model was 

developed to simulate hydraulic conditions at the 

Area 6 Landfill, Naval Air Station Whidbey 

Island, near Oak Harbor, Washington. 

Remediation efforts were started in 1995 in an 

attempt to contain trichloroethene and other 

contaminants in the ground water. The model was 

developed as a tool to test the effectiveness of the 

pump-and-treat remediation efforts as well as 

alternative remediation strategies. The model 

utilized stratigraphic data from approximately 

76 Navy and 19 private wells to define the 

geometry of the shallow, intermediate, and deep 

aquifers and the intervening confining layers.

Initial aquifer parameters and recharge 

estimates from aquifer tests and published 

remedial investigation reports were used in the 

model and then adjusted until simulated water 

levels closely matched observed water-level data 

collected prior to the onset of remediation in 1995. 

The calibrated model was then modified to depict 

the remedial pump-and-treat system, in which 

contaminated ground water is extracted, treated, 

and returned to the ground surface for infiltration. 

The water levels simulated by the modified model 

were compared with observed water levels for 

the 1998 calendar year, during which time the 

pump-and-treat system was in nearly continuous 

operation and the ground-water system had 

equilibrated to steady-state conditions. Although 

artificial boundaries were used in the model, the 

choice of model boundary conditions was 

determined not to have a significant effect on flow 

simulation in the area of primary concern 

surrounding the western contaminant plume and 

extraction wells. Particle tracking results indicate 

that the model can effectively simulate the 

advective transport of contaminants from the 

source area to the pumping wells and thus be used 

to test alternative remedial pumping strategies.

INTRODUCTION

The United States Naval Air Station (NAS) at 

Whidbey Island, Washington, was commissioned in 

1942 and served as one of the most important facilities 

in the Northwest for operation and maintenance of 

patrol squadrons during World War II. Located on the 

largest island in the Puget Sound lowland (fig. 1), the 

base continues to serve as a strategic air base and 

training facility. From 1942 to about 1992, municipal 

and industrial wastes generated at the base facilities 

were disposed of in onsite landfills, open trenches, or 

by burning. Although considered acceptable at the 

time, past waste disposal practices have produced 

potential health and environmental concerns through 

the release of hazardous contaminants into the soil and 

ground water.

Within Area 6, a 260-acre tract in the 

southeastern corner of Ault Field (fig. 2), there are two 

known waste-disposal areas: a hazardous-waste storage 

area and a 40-acre landfill. The hazardous-waste 

storage area is a 4.5-acre tract located to the northwest 

of the former landfill where an “acid” pit (used for 

disposal of acids, caustics, and solvents), an oilysludge pit, waste oil tanks, and a solvent/caustics waste 

tank were all located. 
2 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Crescent

 Harbor

City of Oak Harbor

0

0 1

1

2

2

3 MILES

3 KILOMETERS 

Strait of

Juan de Fuca Dugualla

Bay

Oak

Harbor

AULT FIELD

SEAPLANE BASE

Area 6

Area of primary

concern

Oak Harbor

Landfill

Naval Air Station

Whidbey Island

Seattle

Tacoma

WASHINGTON

Area 6

Landfill STATE HIGHWAY 20

Area modeled

NAS 

Whidbey

Island

Port

Townsend

Seattle

Puget Sound

SR

20

SR

20

Coupeville

Wh i d b e y

I s l a n d

Crescent

 Harbor

City of Oak Harbor

0

0 1

1

2

2

3 MILES

3 KILOMETERS 

Strait of

Juan de Fuca Dugualla

Bay

Oak

Harbor

AULT FIELD

SEAPLANE BASE

Area 6

Area of primary

concern

Oak Harbor

Landfill

Naval Air Station

Whidbey Island

Seattle

Tacoma

WASHINGTON

Area 6

Landfill STATE HIGHWAY 20

Area modeled

NAS 

Whidbey

Island

Port

Townsend

Seattle

Puget Sound

SR

20

SR

20

Coupeville

Wh i d b e y

I s l a n d

Crescent

 Harbor

City of Oak Harbor

0

0 1

1

2

2

3 MILES

3 KILOMETERS 

Strait of

Juan de Fuca Dugualla

Bay

Oak

Harbor

AULT FIELD

SEAPLANE BASE

Area 6

Area of primary

concern

Oak Harbor

Landfill

Naval Air Station

Whidbey Island

Seattle

Tacoma

WASHINGTON

Area 6

Landfill STATE HIGHWAY 20

Area modeled

NAS 

Whidbey

Island

Port

Townsend

Seattle

Puget Sound

SR

20

SR

20

Coupeville

Wh i d b e y

I s l a n d

Figure 1. Location of the Whidbey Island Naval Air Station and Area 6 landfill study area, Island County, 

Washington.

(From URS Consultants, 1993a.)
Introduction 3

The 40-acre landfill at Area 6 was used primarily 

to dispose of Navy household municipal waste from 

1969–92. The landfill accepted solid waste, asbestos, 

wood, rubble, animal remains, construction debris, and 

hazardous liquids or sludge. The base of the landfill 

was not lined. Disposal at the landfill began at the 

northern end and progressed southward. An estimated 

maximum of 2.2 million gallons of liquids and sludge 

containing hazardous wastes was reportedly disposed 

of in the northern two-thirds of the landfill from 

1969–83.

Immediately southwest of Area 6, the City of 

Oak Harbor operated a 70-acre landfill in a former 

borrow pit from 1953-82 (fig. 2). Approximately 

129 tons of dry-cleaning wastes were reportedly 

disposed of in the Oak Harbor landfill, along with 

domestic wastes, demolition materials, and sewage 

sludge (Ecology and Environment, 1988). A sewagesludge disposal area was located immediately south of 

Navy monitoring wells PW-4, 6-S-29, and 6-S-19, and 

a mixed municipal-waste disposal area is located about 

300 feet south of Navy wells PW-5, PW-6, and PW-7. 

Although the site has not been fully characterized, the 

quality of ground water beneath the landfill has been 

monitored regularly at four locations during the 1990’s 

(R. Knudson, City of Oak Harbor, written 

communication, 1998).

The Navy started remediation efforts in 1995 to 

contain trichloroethene (TCE) and other contaminants 

in the ground water at the Area 6 Landfill by capping 

the landfill and installing an extraction, treatment and 

recharge system (referred to as a pump-and-treat 

system). In 1997, the U.S. Geological Survey began a 

study in cooperation with the Navy to develop a tool 

that could test the effectiveness of the pump-and-treat 

remediation efforts and alternative remediation 

strategies.

Background

Ground water beneath Area 6 has been sampled 

multiple times since 1988, confirming that the shallow 

aquifer is currently contaminated with trichloroethene 

(TCE), 1,1,1-trichloroethane (TCA), and other volatile 

organic compounds (VOCs) that are probable 

degradation products of TCE and TCA. Two somewhat 

distinct contaminant plumes were identified at the site, 

one that begins beneath the former hazardous-waste 

storage area and spreads southward along the western 

site boundary, and a second that begins beneath the 

former landfill and also spreads southward (fig. 2). 

The western plume is primarily TCA, and TCE and the 

degradation products cis-1, 2-dichloroethene (cisDCE), 

1,1-dichloroethane (DCA), and 1,1-dichloroethene 

(DCE). The eastern plume is primarily landfill leachate 

with TCA and the degradation products DCA and vinyl 

chloride (VC).

In 1989, the Washington Department of Health 

sampled 13 public wells located within a 1-mile radius 

of Area 6 and the City of Oak Harbor landfill and found 

no detectable VOCs. Again in 1991, the Department of 

Health sampled one public well and six private wells in 

the vicinity of Area 6 and found no evidence of 

contamination in any of the wells. Six private watersupply wells located within a 1-mile radius of Area 6 

were sampled again in 1994, and no VOCs were 

detected. As a precautionary measure, however, the 

Navy began a program offering voluntary water 

hookups to the public water-supply system for 

landowners that could potentially be affected, and 

many wells were subsequently abandoned.

The remedial action selected for treating 

contaminated ground water at Area 6 included capping 

the landfill and installing an extraction, treatment, and 

recharge system (URS Consultants, 1993b). The goals 

of the remedial action include preventing landfill 

leachate from reaching ground water, preventing 

further spread of VOCs, and reducing concentrations of 

contaminants in the shallow aquifer, with the ultimate 

goal of meeting State and Federal drinking water 

standards at specified points of compliance. 

The ground-water extraction and treatment 

system began operation on June 27, 1995; the landfill 

cap was completed in 1996. From the outset, the 

pump-and-treat system encountered numerous 

operational difficulties due to fouling of extraction and 

treatment equipment and the subsurface drains and 

reinjection wells by bacteria and iron precipitates 

(Personal Communication, Sonia Murphy, U.S. Navy, 

Poulsbo, Washington, 1999; Hart Crowser, 1999a). 

Most of the recharge-related difficulties were overcome 

by discharging treated water into an intermittent stream 

located in a swale just northeast of the landfill and 

allowing it to infiltrate to the water table; most of the 

infiltration occurs in the vicinity of monitoring well 6-

S-26. Fouling problems in extraction wells and 

treatment equipment still remain, despite additional 

recommended maintenance procedures (Hart Crowser, 

1999b).
4 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 2. Location of Area 6 landfill and surrounding features, including selected wells, hazardous waste 

storage area, and contaminant plumes.

0

0

1500 FEET

500 METERS

Swale

P-6

N6-38

N6-37

Ault Field Road

State

Highway 20

Goldie Road 

T.33 N. 

R.1 E.

EXPLANATION

Wetland

Area 6 Landfill

Boundary of area of primary concern

Retention pond

Location of hydrogeologic

 section shown in figure 3

Western plume - predominantly A A' (TCA)1,1,1-trichloroethane,

 (TCE)trichloroethene and

 degradation products 

Eastern plume - predominantly

 (TCA)1,1,1-trichloroethane,

 (DCA)1,1-dichloroethane, and

 (VC)vinyl chloride

Monitoring well

Pumping well

Shallow aquifer well

Naval Air Station Boundary

Former 4.5-acre

hazardous waste

storage area and

other contaminant

sources

A'

A

Road

Oak Harbor Landfill

MW-9

MW-6

MW-5

MW-8

MW-10

MW-7

MW-14

MW-11

MW-1

MW-3B

MW-1

PW-9

PW-8

PW-2

PW-4

PW-6

PW-3

PW-1

PW-5 PW-7

PW-4

6-S-27

6-S-28

6-S-17

6-S-16

6-S-6

6-S-12

6-S-24

6-S-25

6-S-13

6-S-4

6-S-23

6-S-14

6-S-11 6-S-15 6-S-9

6-S-2

6-S-7

6-S-26

6-S-10 6-S-21

6-S-22

6-S-19 6-S-29

6-S-3

6-S-25
Introduction 5

Purpose and Scope

The purpose of this study was to develop a tool 

that could be used at some later time to evaluate the 

effectiveness of the current remediation system in 

containing the western contaminant plume and to 

compare alternative remediation strategies. A threedimensional finite-difference, steady-state groundwater flow model for the Area 6 landfill and vicinity 

was developed to provide a detailed depiction of the 

hydrologic system with the flexibility to be modified to 

simulate a variety of stresses on the system. This report 

describes the three-dimensional ground-water flow 

model and how it was developed, calibrated, and tested. 

The model was developed specifically for the Area 6 

landfill site and is focused primarily on the former 

hazardous-waste storage area, western contaminant 

plume, and extraction wells. The source area for the 

eastern plume from the landfill itself is not well known 

enough to realistically simulate the extent of 

contamination in that area. The sensitivity of the flow 

regime, within the area of primary concern, to 

boundary conditions, hydrologic parameters, and other 

assumptions also was evaluated. The Navy intends to 

use the model to test the effects of operating fewer 

extraction wells, to optimize the location of new 

monitoring wells for a proposed monitored natural 

attenuation remedy (Dinicola, 2000), and to conduct 

more detailed modeling of the contaminant plume. The 

model differs from previous models constructed for the 

site in that it incorporates stratigraphic data from more 

Navy and private wells and is a three-dimensional 

representation of the hydraulic flow regime. 

The model input files used in this study can be 

obtained on CD-ROM from the USGS District Office 

in Tacoma, Wash.

Description of the Study Area

NAS Whidbey is located on Whidbey Island in 

Island County, Washington, on the north end of Puget 

Sound and the eastern end of the Strait of Juan de Fuca 

(fig. 1). The island is from 1 to 10 miles wide and 

almost 40 miles long. The topography of Whidbey 

Island is characterized by rolling uplands 100-300 feet 

above sea level, with steep bluffs along the coast. NAS 

Whidbey is located on the northern part of the island, 

just north of the City of Oak Harbor (population about 

15,000). Forests cover the largest percentage of the 

island, and urban and agricultural areas cover the 

remainder. Land use in the vicinity of Area 6, NAS 

Whidbey is primarily residential with small 

commercial areas and open forested or cleared tracts. 

The former City of Oak Harbor Landfill is immediately 

southwest of Area 6. 

Whidbey Island has a temperate marine climate 

characterized by warm, dry summers and cool, wet 

winters. Mean annual temperature is about 50°F; 

January is the coolest month and August is the 

warmest. Mean annual precipitation ranges from 

18 inches in the northern part of the island to 34 inches 

in the southern part; that for NAS Whidbey is about 

20 inches. Snowfall averages less than 8 inches per 

year (in/yr), and rainstorms are generally not intense. 

There are no perennial streams draining the study area, 

and surface water from extreme storm runoff drains to 

the north into wetlands near the runways at Ault Field 

(Dinicola and others, 2000).

Previous Investigations

The Navy was prompted to begin remedial 

actions at the site by a concern that contaminants could 

be readily transported in the underlying ground water 

from the base and into nearby public and private wells 

on neighboring properties. An Initial Assessment Study 

(IAS) of all Naval Air Station facilities was conducted 

in 1984 to identify potential threats to human health or 

the environment caused by past hazardous-materials 

handling and disposal practices (Naval Energy and 

Environmental Support Activity, 1984). The IAS report 

identified a total of 35 sites as potential sources of 

contamination that were grouped into 11 areas for 

further investigation and possible remedial action. 

Monitoring wells were drilled and a confirmation study 

was conducted in 1987 to verify and characterize the 

extent of the contamination (SCS Engineers, 1988). In 

February 1990, the Ault Field facility was included as a 

Superfund site on the EPA’s National Priorities List. In 

response to EPA’s listing and continued concerns about 

the migration of VOCs in ground water, a Record of 

Decision was signed in 1993 that committed the Navy 

to construct a ground-water extraction and treatment 

system at Area 6 (URS Consultants, 1993a). A 

Remedial Investigations report was also prepared in 

1993 describing the results of extensive 

characterization of the site and available information 

on distribution of contamination (URS Consultants, 

1993b). 
6 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, County, Washington

The ground-water extraction, treatment, and recharge 

system was installed and began operation in 1995. The 

system currently extracts, treats, and recharges 

approximately 275,000 gallons per day.

Four regional ground-water models were 

developed by the U.S. Geological Survey (USGS) for 

Island County to evaluate the potential for seawater 

intrusion into public water-supply wells on Whidbey 

and Camano Islands (Sapik and others, 1987). These 

models provided the regional framework for later 

ground-water models developed specifically for the 

Area 6 landfill facility. 

To aid in design and operation of the pump-andtreat system, a two-dimensional numerical flow model 

for Area 6 was constructed using FLOWPATH version 

4 (IT Corporation, 1993; 1996; 1997). The purpose of 

the two-dimensional model was to determine the 

optimal locations for installation of extraction and 

monitoring wells. Later versions of the model were 

used to evaluate different remediation strategies by 

varying pumping rates and locations of treated effluent 

recharge sites (IT Corporation, 1996, 1997).

When bacteria and iron precipitate buildup began 

clogging treated effluent reinjection wells, the USGS 

was asked to evaluate the effect on the regional groundwater system if treated water was not recharged to the 

aquifer. The USGS model developed for northern 

Whidbey Island (Sapik and others, 1987) was modified 

to simulate the effect of pumping ground water from 

wells at the Naval Air Station, Area 6 landfill facility 

(E.A. Prych, written communication, 1997). Although 

the simulation showed only minor regional effects, the 

need for a more detailed three-dimensional model 

utilizing refined local stratigraphy and more accurate 

hydraulic parameters became apparent.

Hydraulic conductivity estimates made during 

the initial remedial investigation ranged over two 

orders of magnitude (URS Consultants, 1993b). 

Multiple well aquifer tests at four of the extraction 

wells were conducted in June 1998 to obtain more 

reliable aquifer characteristics (Foster Wheeler 

Environmental Corporation, 1998b). The results of the 

1998 aquifer tests yielded hydraulic conductivity 

values slightly higher than those used in the twodimensional ground-water model. The twodimensional FLOWPATH model was updated later in 

1998 to account for the new aquifer-test data and to 

correct several errors in the previous model (Foster 

Wheeler Environmental Corporation, 1998a).

Acknowledgments

The author gratefully acknowledges the 

cooperation and support of Sonia Murphy, Douglas 

Thelin, and John Gordon of the Naval Facilities 

Engineering Command, Engineering Field Activity 

Northwest. Richard Wice of the International 

Technology Corporation and Tom Goodlin of the 

Foster Wheeler Environmental Corporation provided 

geologic information, water-level data and well 

construction specifications. Henry Bauer, Sue Kahle, 

and Marijke Van Heeswijk were invaluable resources 

and provided feedback throughout the study. The report 

greatly benefited from editorial reviews by James 

Lyles, Ginger Renslow, and John Clemens and 

illustration support by Robert Crist.

HYDROGEOLOGY OF THE GROUNDWATER FLOW SYSTEM

Island County lies within the Puget Sound 

lowland, a topographic and structural depression 

between the Cascade Range on the east and the 

Olympic Mountains on the west. Whidbey Island is 

composed of unconsolidated Pleistocene glacial and 

interglacial deposits overlying Tertiary and older 

bedrock (Easterbrook and Anderson, 1968). The 

unconsolidated deposits range in thickness from a few 

hundred to 3,000 feet thick and represent deposits from 

at least three glaciations (Sapik and others, 1987). 

Surficial deposits consist of unconsolidated sand and 

gravel with local exposures of more densely compacted 

glacial till and widely scattered glacial erratic boulders.

Geologic Framework and Hydrologic Units

Ground water in the vicinity of NAS Whidbey 

generally occurs within a series of aquifers composed 

of permeable sand-and-gravel layers deposited by 

glacial melt water, separated by finer grained glacial 

silt-and-clay or interglacial fluvial and lacustrine 

deposits (URS Consultants, 1993b). These subsurface 

materials have been locally characterized into six 

hydrogeologic units (URS Consultants, 1993b; Sapik 

and others, 1987). These units are the Vashon till, 

Vashon advance outwash, and four subunits of the 

Whidbey Formation. 
Hydrogeology of the Ground- Water Flow System 7

Figure 3. Generalized hydrogeologic section showing distribution of aquifer and confining units and the numbered layers used 

in the ground-water model.

(Modified from Dinicola, 2000.) See figure 2 for trace of section. 

Intermediate aquifer

Deep or sea-level aquifer

Shallow aquifer

Upper confining unit

Lower confining unit Clay, sand, silt

Medium to coarse sandy

gravely or gravely sand

Clay, sand, silt

Sand,silt with some clay

Medium to coarse sand

with some gravel

Till

Silty sand

3 Subunit 1

4

5

6

2

1

Subunit 2

Subunit 3

Subunit 4

Vashon till

Model

layers

North

250

200

150

100

50

 50

100

SEA

ALTITUDE, IN FEET 

LEVEL

6-S-26

6-S-10

6-S-22

6-S-15

6-S-14 6-S-23 6-S-25

6-S-13

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

South

 ~5x Vertical exaggeration

Vashon

advance

outwash

Whidbey

Formation

0 500 1000 FEET

0 100 200 300 METERS

Intermediate aquifer

Deep or sea-level aquifer

Shallow aquifer

Upper confining unit

Lower confining unit Clay, sand, silt

Medium to coarse sandy

gravely or gravely sand

Clay, sand, silt

Sand,silt with some clay

Medium to coarse sand

with some gravel

Till

Silty sand

3 Subunit 1

4

5

6

2

1

Subunit 2

Subunit 3

Subunit 4

Vashon till

Model

layers

North

250

200

150

100

50

 50

100

SEA

ALTITUDE, IN FEET 

LEVEL

6-S-26

6-S-10

6-S-22

6-S-15

6-S-14 6-S-23 6-S-25

6-S-13

BEND IN

SECTION

BEND IN

SECTION

BEND IN

SECTION

South

 ~5x Vertical exaggeration

Vashon

advance

outwash

Whidbey

Formation

0 500 1000 FEET

0 100 200 300 METERS

Beneath Area 6, the upper 200 feet of sediments 

contain three principal water-bearing units, referred to 

as the shallow, intermediate, and deep, or sea level, 

aquifers (fig. 3). 

The shallow aquifer is contained within the 

Vashon advance outwash sediments. It is a major 

water-bearing zone in the region and extends 

throughout all of Area 6. The shallow aquifer contains 

the largest contaminant concentrations and is the 

primary focus of this and previous investigations. 

Although some water can be found perched above the 

glacial till in small portions of Area 6, the shallow 

aquifer is generally unconfined, with water levels 

ranging from about 20 to 145 feet below the ground 

surface. Ground-water flow directions are toward the 

southwest in the northern part of the area and toward 

the southeast in the southern part, forming a generally 

accurate path from north to south (fig. 4). North of the 

study area, a poorly defined flow divide separates a 

component of flow to the northwest towards Ault Field. 

Stratigraphy and thickness data are available from 44 

Navy and various private wells that fully penetrate the 

Vashon advance outwash (see table 6 at end of report). 

The Vashon advance outwash is a medium to coarse 

sandy gravel or gravely sand that becomes finer grained 

with depth. Including Vashon till where present, the 

thickness of Vashon advance outwash ranges from 21 

feet to 176 feet thick within the study area (fig. 5). 

However, the saturated portion of the deposit ranges 

from 13 feet to 61 feet thick and averages 30 feet thick. 

The horizontal hydraulic conductivity is estimated to 

be 87 feet per day (ft/d) for the upper portion of the 

unit, and 70 ft/d for the lower, finer grained portion of 

the unit. 
8 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 4. Ground-water flow in the shallow aquifer, July 1989. 

Altitudes are in feet above sea level (water-level contours and flow paths from URS Consultants, 1993b). Contour 

interval is 1 foot.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

48° 

19'

30"

48° 

18'

30"

48° 

19'

122° 39' 122°38'30" 122°38'

T.

33

N.

87

86

85

84

83

82

81

80

79

78

77

90

91

89

88

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

48° 

19'

30"

48° 

18'

30"

48° 

19'

122° 39' 122°38'30" 122°38'

T.

33

N. 

R.1 E.

87

86

85

84

83

82

81

80

79

78

77

90

91

89

88
Hydrogeology of the Ground- Water Flow System 9

150

150

140

130

120

90

90

11

1000

120

90

100

110

60

70

70

80

60

70

30

30

40

50

60

30

40

50

80

140

150

160

150

170

130

120

110

100 90

120

80

50

50

40

60

40 60

70

110

100

90

120

80

80

130

140

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

6-S-10

6-S-7

P-2

P-3

P-4

P-1

Dickson

6-S-20

A381

6-S-22

PW-1

PW-6

MW-8

MW-7

PW-7

Rodewald

MW-10

6-S-21

6-S-17

6-S-29

6-S-19

MW-1

MW-9

6-S-13

6-S-27

6-S-28

6-S-12

6-S-18

MW-11

MW-15

PW-5

PW-4

6-S-11

6-S-15

MW-12

PW-3

MW-3b

MW-6

MW-13

6-S-16

6-S-2

6-S-14

MW-14

N6-37

N6-38

6-S-8

6-S-5

PW-2 PW-8

PW-9

6-S-1

6-S-3

6-S-24

6-S-9

6-S-23

MW-2

MW-4

6-S-4

P-8

P-7

P-6

6-S-25

P-5

6-S-6

MW-5

MW-3a

DOE

6-D-4

OakHarbor

6-D-1

6-D-2

6-D-3

6-I-1

Hilberdink

6-I-8

6-I-5

6-I-6

6-I-7

M&Eenterprizes

6-S-26

6-I-2

6-I-4

6-I-3

Figure 5. Thickness of Vashon advance outwash deposits in Area 6, including Vashon till where present.

Contour interval is 10 feet. Wells that provide stratigraphic information for the shallow aquifer are shown in 

green, and the deep aquifer are shown in red. (Stratigraphic data from URS Consultants,1993b, and 

summarized in Appendix A.)
10 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, County, Washington

The uppermost subunit 1 in the Whidbey 

Formation is a confining unit consisting of dark green 

clay and fine sands and silts with minor peat and wood 

material that immediately underlies the shallow 

aquifer. The confining unit is generally about 10 feet 

thick, but can vary from 1 foot to 40 feet thick within 

the study area (fig. 6). Because of the fine-grained 

nature of the material, the hydraulic conductivity for 

this unit is estimated to be 0.0002 foot per day.

The intermediate aquifer is defined as subunit 2 

of the Whidbey Formation. Although not a major 

water-bearing zone in the region, it extends throughout 

northern Whidbey Island. Only very low 

concentrations of contaminants have been detected 

within this aquifer beneath Area 6. The intermediate 

aquifer is generally confined, and water levels are 

generally 5 to 20 feet lower than those in the shallow 

aquifer. Ground-water flow directions in the 

intermediate aquifer are poorly defined, but are 

generally from the northwest to the southeast (fig. 7). 

Limited stratigraphic and thickness data are available 

from 13 Navy and two private wells that fully penetrate 

subunit 2 of the Whidbey Formation (see table 6). The 

subunit consists of sand and silts or fine sand and may 

contain thin, discontinuous, dark-gray silty clay layers. 

Thickness of the unit varies from 4 feet to 69 feet thick 

within the study area (fig. 8). The horizontal hydraulic 

conductivity for this unit is estimated to be 10 ft/d.

Subunit 3 of the Whidbey Formation is a 

confining unit consisting of dark-colored clay, sand, 

and silts with gravel or silty clay layers and woody 

material, and immediately underlies the intermediate 

aquifer. The confining unit varies from 5 feet to 

54 feet thick within the study area (fig. 9). Because 

of the fine-grained nature of the material, the 

hydraulic conductivity for this unit is estimated 

to be 0.0002 ft/d.

The deep, or sea level, aquifer is defined by 

subunit 4 of the Whidbey Formation. This unit is a 

continuous water-bearing zone in the region. No 

contaminants have been detected in this aquifer 

beneath Area 6, with the exception of those 

resulting from a now abandoned, poorly constructed 

monitoring well. The aquifer is confined, with water 

levels ranging from 11 to 17 feet above sea level. 

Ground-water flow direction in the deep aquifer is not 

well known but is generally towards the southwest. 

Very limited stratigraphic and thickness data are 

available from 4 Navy wells and 1 private well that 

intercepts the deep aquifer (see table 6). Only one deep 

drill hole, just northwest of the study area, penetrates 

the entire section of glacial deposits and is finished in 

bedrock at a depth of 531 feet below land surface. 

Subunit 4 of the Whidbey Formation consists of 

medium to coarse sand with gravel. The thickness of 

subunit 4 is estimated to be about 240 feet, based on the 

one deep drill hole. However, the aquifer thickness 

may be as much as 450 feet because the underlying 

Double Bluff Formation consists of similar sand and 

gravel glacial outwash material and the aquifer could 

continue into it.
Hydrogeology of the Ground- Water Flow System 11

Figure 6. Thickness of subunit 1 of the Whidbey Formation that defines the upper confining unit, or layer 3 of 

the model.

Contour interval is 5 feet. Wells that provide stratigraphic information for the upper confining unit are shown in 

black, green, and red. (Stratigraphic data from URS Consultants,1993b, and summarized in Appendix A.),

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

35

30

40

25

15

20

15

30

15

15

20

25

15

5

10

20

10

5

25

15

20

10

5

10

10

10

10

10

6-S-10

6-S-7

P-2

P-3

P-4

P-1

Dickson

6-S-20

A381

6-S-22

PW-1

PW-6

MW-8

MW-7

PW-7

Rodewald

MW-10

6-S-21

6-S-17

6-S-29

6-S-19

MW-1

MW-9

6-S-13

6-S-27

6-S-28

6-S-12

6-S-18

MW-11

MW-15

PW-5

PW-4

6-S-11 6-S-15

MW-12

PW-3

MW-3b

MW-6

MW-13

6-S-16

6-S-2

6-S-14

MW-14

N6-37

N6-38

6-S-8

6-S-5

PW-2

PW-8

PW-9

6-S-1

6-S-3

6-S-24

6-S-9

6-S-23

MW-2

MW-4

6-S-4

P-8

P-7

P-6

6-S-25

P-5

6-S-6

MW-5

MW-3a

DOE

6-D-4

OakHarbor

6-D-1

6-D-2

6-D-3

6-I-1

Hilberdink

6-I-8

6-I-5

6-I-6

6-I-7

M&Eenterprizes

6-S-26

6-I-2

6-I-4

6-I-3
12 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 7. Ground-water flow in the intermediate aquifer, November 1991.

Altitudes are in feet above sea level (water-level contours and flow paths from URS Consultants, 1993b). Contour 

interval is 5 feet.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

N

R.1 E.

75

70

65

60

70

65

60
Hydrogeology of the Ground- Water Flow System 13

Figure 8. Thickness of subunit 2 of the Whidbey Formation that defines the intermediate aquifer, or layer 4 of 

the model.

Contour interval is 5 feet. Wells that provide stratigraphic information for the intermediate aquifer are shown in 

green and red. (Stratigraphic data from URS Consultants,1993b, and summarized in Appendix A.)

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T

33

N

R.1 E.

60

65

60

45 35

40

45

50

55

45

40

35

30

30

30

30

30

30

30

30

15

25

25

20

20

15

10

5

45

40

35

50

55

25

25

10

30

6-S-10

6-S-7

P-2

P-3 P-4

P-1

Dickson

6-S-20

A381

6-S-22

PW-1

PW-6

MW-8

MW-7

PW-7

Rodewald

MW-10

6-S-21

6-S-17

6-S-29

6-S-19

MW-1

MW-9

6-S-13

6-S-27

6-S-28

6-S-12

6-S-18

MW-11

MW-15

PW-5

PW-4

6-S-11 6-S-15

MW-12

PW-3

MW-3b

MW-6

MW-13

6-S-16

6-S-2

6-S-14

MW-14

N6-37

N6-38

6-S-8

6-S-5

PW-2 PW-8

PW-9

6-S-1

6-S-3

6-S-24

6-S-9

6-S-23

MW-2

MW-4

6-S-4

P-8

P-7

P-6

6-S-25

P-5

6-S-6

MW-5

MW-3a

DOE

6-D-4

OakHarbor

6-D-1

6-D-2

6-D-3

6-I-1

Hilberdink

6-I-8

6-I-5

6-I-6

6-I-7

M&Eenterprizes

6-S-26

6-I-2

6-I-4

6-I-3
14 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 9. Thickness of subunit 3 of the Whidbey Formation that defines the lower confining unit, or layer 5 of the 

model.

Contour interval is 5 feet. Wells that provide stratigraphic information for the lower confining unit are shown in red. 

(Stratigraphic data from URS Consultants, 1993b, and summarized in Appendix A.)

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

45

45

40

35

50

45

30

25

20

35

40

40

30

25

20

1510

5

15

10

15

20

25

30

35

40

10

10

20

25

6-S-10

6-S-7

P-2

P-3 P-4

P-1

Dickson

6-S-20

A381

6-S-22

PW-1

PW-6 MW-8

MW-7

PW-7

Rodewald

MW-10

6-S-21

6-S-17

6-S-29

6-S-19

MW-1

MW-9

6-S-13

6-S-27

6-S-28

6-S-12

6-S-18

MW-11

MW-15

PW-5

PW-4

6-S-11 6-S-15

MW-12

PW-3

MW-3b

MW-6

MW-13

6-S-16

6-S-2

6-S-14

MW-14

N6-37

N6-38

6-S-8

6-S-5

PW-2 PW-8

PW-9

6-S-1

6-S-3

6-S-24

6-S-9

6-S-23

MW-2

MW-4

6-S-4

P-8

P-7

P-6

6-S-25

P-5

6-S-6

MW-5

MW-3a

DOE

6-D-4

OakHarbor

6-D-1

6-D-2

6-D-3

6-I-1

Hilberdink

6-I-8

6-I-5

6-I-6

6-I-7

M&Eenterprizes

6-S-26

6-I-2

6-I-4

6-I-3
Hydrogeology of the Ground- Water Flow System 15

Hydraulic Properties of Aquifers and 

Confining Units

The hydraulic conductivity of the aquifers and 

confining units depends on the density and viscosity of 

the fluid and the grain size, shape, sorting, and packing 

of the matrix material (Freeze and Cherry, 1979). In 

this study, no seawater is involved and density/viscosity 

effects of the contaminants are assumed to be 

negligible, so the fluid (water) of the regional flow 

system is assumed to have a constant density and 

viscosity. Earlier published hydraulic conductivity data 

for the shallow, intermediate, and deep aquifers 

beneath the Area 6 landfill are derived from single-well 

pumping tests, slug tests and associated grain-size 

analyses, and laboratory tests (URS Consultants, 

1993b). The resulting hydraulic conductivities varied 

widely, so in 1998 Navy contractors conducted 

additional multiple-well pumping tests at extraction 

wells PW-1, PW-3, PW-5, and PW- 9 to better define 

the values. Each test used one extraction well and 6 to 

10 nearby observation wells (Foster Wheeler 

Environmental Corporation, 1998b). The multiple-well 

pumping tests yielded horizontal hydraulic 

conductivity values for the shallow aquifer that ranged 

from 47 to 126 ft/day (Foster Wheeler Corporation, 

1998b). The average hydraulic conductivity for all the 

wells in the test was 87 ft/day, a value slightly higher 

than the values (40 to 80 ft/day) used in the previous 

modeling study (IT Corporation, 1997). The hydraulic 

conductivity data from the multiple-well pumping tests 

were considered more reliable and were used as initial 

estimates in this study.

Horizontal hydraulic conductivities are generally 

greater than vertical hydraulic conductivities because 

of the horizontal stratification of coarse and fine matrix 

materials in the glacial sediments. This vertical 

anisotropy in hydraulic conductivity occurs in both 

aquifers and confining units. However, confining units 

have such low hydraulic conductivities that the 

horizontal component of flow is considered to be 

negligible. Thus, a horizontal-to-vertical hydraulic 

conductivity ratio of 1:1 was assumed for the confining 

units. For aquifer materials, more reliable hydraulic 

conductivity data are available. A Neuman analysis 

conducted on one of the multiple-well pump tests 

(PW- 3) allowed for direct computation of vertical 

hydraulic conductivity in the shallow aquifer (Foster 

Wheeler Corporation, 1998b; Neuman, 1975). Test 

results indicated that ratios of horizontal to vertical 

hydraulic conductivities for the aquifers were on the 

order of 4:1 to 8:1, or about 5:1 on average (Foster 

Wheeler Environmental Corporation, 1998b).

The ratio of horizontal to vertical hydraulic 

conductivity is important in determining the leakance 

between aquifer and confining layers. Leakance is a 

function of the layer thickness and the hydraulic 

conductivity of each layer bounding an interface 

between layers. Leakance is generally calculated by the 

equation: 

(1)

where

Recharge

Virtually all of the naturally occurring groundwater recharge on Whidbey Island occurs by direct 

vertical infiltration of precipitation. Very few 

intermittent streams occur on the island and these flow 

only during heavy rainfall events. Based on 10 to 

40 years of precipitation records, Whidbey Island 

receives an average annual precipitation of 18 to 

34 in/yr; approximately 20 in/yr falls at the study area 

(Sapik and others, 1987). The amount of that 

precipitation that recharges the aquifer can be 

estimated on the basis of the type of land use and 

associated root-zone depths, as well as on geological 

considerations such as the presence of glacial till at the 

surface. Previous recharge estimates by Sapik and 

others (1987) range from 4 to 8 in/yr for forested land, 

6 to 10 in/yr for agricultural land, and 8 to 12 in/yr for 

barren rangeland. Previous recharge estimates by IT 

Corporation (1997), based on soil types, ranged from 

3.5 to 9.0 in/yr.

Zu is thickness of the upper layer;

Ku is conductivity of the upper layer;

Z1 is thickness of the lower layer; and

K1 is vertical hydraulic conductivity of the lower 

layer.

Leakage 1

Zu

2 Ku × --------------- Z12 K1 ×

+ ---------------

= -------------------------------------
16 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, County, Washington

In this study, the presence of glacial till is 

assumed to play an important role in governing the 

amount of precipitation recharging the aquifer system. 

In areas where highly compacted till with fine particle 

size mantle the land surface, recharge to the aquifer is 

limited because of the decreased infiltration capacity of 

the till (Bauer and Mastin, 1996). Thus, in this study, 

till-mantled areas are assumed to have a lower recharge 

value (7.0 in./yr) than areas where till is not present 

(10.0 in./yr) (fig. 10). The wetland area to the north of 

the Area 6 landfill is assumed to be an area of high 

recharge due to the fact that little or no surface flow 

leaves the wetland. Other wetland areas within the 

study area represent localized areas of perched water; 

they are wet only during heavy rain events and do not 

appear to influence ground-water flow directions in the 

shallow aquifer.

Remediation activities have changed the 

distribution of recharge across Area 6. The landfill was 

capped with an impermeable barrier in 1996, beneath 

which recharge was essentially eliminated. All 

precipitation falling on the landfill cap was redirected 

toward a retention pond (see fig. 10) on the north side 

of the landfill where it could infiltrate. On June 27, 

1995, the treatment plant was started and ground water, 

extracted through pumping wells, was piped to an airstripping tower where VOCs could be volatilized and 

removed. Treated ground water was returned to the 

shallow aquifer, initially through injection wells, but 

bacteria and iron precipitates caused clogging 

problems in the injection wells forcing treated water to 

be directed to the retention pond. Eventually treated 

ground water was directed to the intermittent stream 

north and east of the landfill, where it could infiltrate 

naturally to the shallow aquifer. All of the treated water 

usually infiltrates before reaching the wetlands. 

STEADY-STATE SIMULATION OF THE 

GROUND-WATER FLOW SYSTEM

A three-dimensional, steady-state ground-water 

flow model of the Area 6 landfill was developed as a 

tool to test the effectiveness of the present pump-andtreat system and to investigate alternative remediation 

strategies. Specifically, the numerical model is 

designed to evaluate whether water traveling down 

gradient from a known former hazardous-waste storage 

area is being captured by the existing pumping wells. 

Other important uses of the model include evaluating 

the effectiveness of different remediation strategies, 

such as the effects of operating fewer extraction wells, 

and helping to optimize the location of new monitoring 

wells for a proposed monitored natural attenuation 

remedy (Dinicola, 2000). 

Modeling Approach

The ground-water flow system was simulated 

using Groundwater Modeling System (GMS version 

2.1), a commercially available graphical user interface 

that supports MODFLOW versions 1638 and 1323 

(McDonald and Harbaugh, 1988), MODPATH version 

3.0 (Pollock, 1994), and other analysis codes. The area 

modeled in this study extends outside of the Naval Air 

Station Area 6 boundary but is centered on the western 

contaminant plume on the west side of the Area 6 

landfill (fig. 2). The finite-difference grid used in the 

model (fig. 11) was designed to provide highest 

resolution for the area of primary concern, which 

includes the former hazardous waste storage area, 

western contaminant plume, extraction wells, and area 

6 landfill. Care was taken to locate model boundaries in 

such a way as to minimize the influence of boundary 

conditions on the area of primary concern, and to limit 

the grid size for computational efficiency. Artificial 

model boundaries had to be used because natural 

hydrologic boundaries were too far away to be included 

in this highly localized modeling study. The limitations 

of the artificial boundaries were tested by specifying 

different boundary conditions and determining the 

effect on the area of primary concern.

Two time periods were selected for simulation, 

a calibration period that represented steady-state 

conditions prior to the onset of remediation activities, 

and a post-remediation period that represented steadystate conditions after water levels had stabilized in 

response to implementation of the pump-and-treat 

system. No significant commercial or residential 

pumping stresses are known to exist in the immediate 

vicinity of Area 6 during either time period; most 

domestic wells near the base boundary have been 

abandoned and both NAS Whidbey and the City of Oak 

Harbor use surface water as their primary water supply.
Steady-State Simulation of the Ground-Water Flow System 17

Figure 10. Distribution of recharge values used in the ground-water model.

Areas where till is present were assigned the low recharge value and areas where it is absent, the high recharge 

value.

200

210

120

120

160

140

180

180

160

140

120

100

100

100

110

100

100

100

140

160

180

120

120

40

60

80

80

200

NAS Boundary

Landfill

Cap

Intermittent

Stream

Retention

Pond

Model

Boundary

Wet

Lands

122 38'30"

T.

33

N. 

R.1 E. Base map modified from U.S. Geological Survey

Oak Harbor, 1:24,000, 1973

SR

20

SR

20

PORTER

GOLDIE

ROAD

AULT FIELD ROAD

ROAD TO

R

PEDO

NAS Boundary

ROAD

HARBOR

OAK

High recharge (10.0 inches per year)

Low recharge (7.0 inches per year)

Altitude above sea level,

 contour interval 20 feet 

0

0

1000 2000 FEET

200 400 600 METERS

EXPLANATION

60

NATIONAL GEODETIC VERTICAL DATUM OF 1929
18 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 11. Finite-difference grid for the numerical three-dimensional model of ground-water flow at the Area 6 

landfill.

The red rectangle represents the area of primary concern, with a dense grid of 25-by-25-foot cells.

SR

20

SR

20

Landfill cap

Intermittent stream

Retention pond

Model boundary

NAS Boundary

NAS Boundary

PORTER AVE

GOLDIE

ROAD

FIELD AULT

ROAD

ROAD TO

R

PEDO

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.
Steady-State Simulation of the Ground-Water Flow System 19

Water levels in approximately 52 wells were 

measured three times prior to the onset of remedial 

activities (IT Corporation, 1995). These water levels, 

measured on 5/19/94, 1/31/95- 2/1/95, and 6/19/95–

6/20/95, were the only data available to constrain the 

pre-remediation steady-state condition. Although the 

pre-remediation water levels showed a slight decrease 

of about 1 foot, the change was likely related to belownormal precipitation (table 1) and recharge in 1994 

(fig. 12). For each well, measured water levels were 

averaged to obtain a representative water level 

indicative of the pre- remediation, steady-state 

condition.

The 52 wells measured during the preremediation period also were measured on a monthly 

basis during the post-remediation period along with 

additional new monitoring wells, a selection of which 

are shown on figure 12. In order to avoid changes in 

ground-water storage due to pumping and climatic 

affects, only water level data from 1998 were used to 

constrain the simulation of the post-remediation period. 

These monthly data were averaged to obtain a 

representative water level indicative of the postremediation steady-state condition. The unsteady-state 

transient response to the pump-and-treat system is 

evident in figure 12 between June 1995 and about 

December 1997 where water levels were equilibrating 

to a new steady-state condition. In addition to the new 

pumping stress on the system, available precipitation 

records (table 1) show a slight increase from 1994 to 

1998 for the nearby climatological stations at 

Coupeville and Port Townsend, as well as for the 

Olympic Mountains and San Juan Islands to the 

northeast as a whole (table 1; NOAA, 1994, 1995, 

1996, 1997, 1998). Thus, the transient ground-waterlevel response seen from June 1995 to about December 

1997 is due to the superimposed effects of the pumpand-treat system and increased precipitation. The water 

levels in well 6-S-2 (an upgradient well not 

substantially affected by remediation activities) 

indicate an increase in ground-water storage from late 

1995 through mid 1997, followed by a period of less 

change. It is noteworthy that after the increase in 

storage, the water level in 6-S-2 did not recede to the 

pre-remediation level; this observation is further 

discussed in the section entitled “Simulation of postremediation conditions.

The model was calibrated using initial hydraulic 

parameters derived from published reports. During 

calibration, the initial hydraulic parameters were 

adjusted iteratively by trial and error until simulated 

water levels most closely matched the averaged 

pre-remediation water levels. As part of the calibration 

processes a sensitivity analysis was performed to 

determine which parameters had the greatest affect on 

the model outcome. 

Table 1. Precipitation data for Whidbey Island and nearby areas, 

1994–98

Precipitation

(inches per year)1

Year Coupeville Port Townsend NE Olympics and

San Juan Division

1998 21.86 (0.84) 23.00 (1.98) 109.82 (12.75)

1997 26.39 (5.37) 21.77 (0.75) 125.40 (28.33)

1996 .no data 21.39 (0.44) 109.04 (11.97)

1995 .no data 23.7 (2.68) 110.87 (13.80)

1994 .no data 13.93 (-7.09) 104.31 (7.24)

1Departure from normal for period of record shown in parentheses; data from 

National Oceanic and Atmospheric Administration, 1994–98.
20 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station, Whidbey Island, Island County, Washington

JUNE

AUG

OCT

DEC

FEB

APRIL

JUNE

AUG

OCT

DEC

FEB

APRIL

JUNE

AUG

OCT

DEC

FEB

DATE

1994 1996 1997 

1995

APRIL

JUNE

AUG

OCT

DEC

FEB

APRIL

707580859095

ALTITUDE, IN FEET ABOVE SEA LEVEL

PRE-REMEDIATION REMEDIATION

Selected wells

6-S-2

6-S-36-S-4

6-S-66-S-76-S-8

6-S-106-S-116-S-12

6-S-156-S-196-S-20

6-S-216-S-226-S-23

Non-pumping

interval

EXPLANATION

Figure 12. Water levels for selected wells screened in the shallow aquifer for the period April 1994 to December 1998.Gray bars at the base of the graph indicate periods when the pump-and-treat system was not in operation.
Steady-State Simulation of the Ground-Water Flow System 21

The calibrated model was then modified to 

simulate the existing remedial extraction and treatment 

system by adding nine extraction wells that withdraw 

water at average rates of flow published in quarterly 

reports (1995, 1996, 1997, 1998, and 1999). In the 

model, treated water was returned to the system by 

adding the extracted volume to recharge cells located 

along the intermittent stream.

To test the effects of assumed boundary 

conditions on simulated flow within the area of primary 

concern, scenarios were run using two different types 

of boundary conditions. In both scenarios, water-level 

distributions, fluxes through the area of primary 

concern, and particle flow paths in the shallow aquifer 

were compared.

Description of Model

The modeled area is overlain by a rectangular 

grid with cells that ranged in size from 25 ft to 300 ft 

on a side (fig. 11). The smallest cells (25 ft by 25 ft) 

were centered on the area of primary concern, which 

includes contaminant source areas, extraction wells, 

and the Area 6 landfill. In the horizontal dimensions, 

the grid was made up of 189 rows and 130 columns and 

covered a total area of about 2.5 square miles. The grid 

was oriented in the north-south direction, roughly 

parallel to the primary ground-water flow direction. 

Aquifer materials are assumed to be homogeneous and 

isotropic and thus, there is no preferred alignment of 

the transmissivity tensor. 

In the vertical dimension, the modeled area was 

conceptualized as a ground-water flow system with six 

layers (see fig. 3) based on subsurface geology from 

lithologic logs from approximately 76 Navy wells and 

19 private wells (URS Team, 1998). The unsaturated 

zone, consisting of surficial gravels, Vashon till, and 

the upper part of the Vashon advance outwash, was 

included as part of layer 1 in the model; however, 

ground-water flow calculations were performed only 

on the saturated portions of the model. Layer 2 consists 

of generally finer grained sediments in the lower 

portion of the Vashon advance outwash. Because the 

contact between layers 1 and 2 is not well defined on 

the available lithologic logs, layer 2 was 

conceptualized as a 10-foot-thick layer with a lower 

hydraulic conductivity than layer 1. Together, layer 1 

and layer 2 comprise the unconfined, shallow aquifer. 

Layer 3 is a confining layer defined as subunit 1 of the 

Whidbey Formation. Horizontal flow through layer 3 

was assumed to be insignificant due to the low 

hydraulic conductivity and presumably small 

horizontal gradients of hydraulic head. Layer 4 

represents the intermediate aquifer and is equivalent to 

subunit 2 of the Whidbey Formation. Layer 5 is the 

lower confining unit and is equivalent to subunit 3 of 

the Whidbey Formation. Horizontal flow through 

layer 5 was also assumed to be insignificant due to the 

low hydraulic conductivity and presumably small 

horizontal gradients of hydraulic head. Layer 6 is the 

bottom or deepest layer in the model and represents the 

deep aquifer, or subunit 4 of the Whidbey Formation. 

Because the depth to bedrock, type of material, and 

actual thickness of the deep aquifer are unknown, the 

deep aquifer was assigned an estimated thickness of 

250 feet based on one well in the area that penetrates to 

bedrock

Boundary Conditions

Conditions along the perimeter of the model are 

important, as they may affect the results of the 

simulation. Ideally, the location of model boundaries 

should correspond to actual hydrologic boundaries. 

However, when hydrologic boundaries cannot be 

represented realistically in the model, it is important 

that prescribed boundaries be located far enough away 

that they do not have an effect on the simulated 

conditions in the area of primary concern. Of the 

various types of boundaries typically used in groundwater flow models (Franke and others, 1987), the 

following types were applicable for use in this study. 

One type is the streamline or stream-surface (no-flow) 

boundary, used where the flow is parallel to a boundary 

and no component of flow crosses the boundary. Here a 

specified flux of zero was assigned at the boundary. 

Another type is a specified-head boundary, where head 

is specified and the flow into or out of the model is 

allowed to adjust accordingly. The bottom of the model 

was specified as a constant-head boundary, used to 

allow flow out the bottom of the model while 

maintaining the same head value at all points. The top 

of the model was specified a free-surface boundary, 

represented by the water table, and allowed to rise and 

fall as needed in response to recharge and flow through 

the model.
22 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, County, Washington

In this study, care was taken to locate lateral 

model boundaries perpendicular to previously 

published ground-water-level contours in the shallow 

aquifer (layers 1 and 2) so that a no-flow boundary 

would be a reasonable assumption on the east and west 

sides of the model (fig. 13). The northern boundary of 

the model was chosen to coincide with a postulated 

hydrologic flow divide so that a no-flow assumption 

would also be reasonable for layers 1 and 2 (see fig. 4). 

The southern boundary was aligned parallel to groundwater-level contours so that heads could be specified on 

the basis of projections of observed water-level data. 

The topography of the study area and conceptual 

ground-water flow suggests that a wetland area just 

southeast of the model boundary, and east of Oak 

Harbor, could be a discharge area for the shallow 

aquifer. In order to represent this concept, head values 

near land surface were specified along the southeast 

corner of the model for layers 1 and 2 to simulate 

discharge to the wetland.

Limited observed water-level data for the 

intermediate aquifer (layer 4) indicates that groundwater flow is generally toward the southeast corner of 

the study area. To simulate flow in the intermediate 

aquifer, a combination of specified-head and no-flow 

boundaries were specified along the perimeter of layer 

4 (fig. 14). The head values were specified along the 

western and a portion of the eastern boundaries in such 

a way as to create flow directions in the corresponding 

model layer 4 that approximate the observed data. 

The lower boundary of the model was defined as 

the bottom of the deep aquifer (layer 6). Although very 

few observational data are available for the deep 

aquifer, data from 1991 and 1992 show a ground-water 

flow direction to the southwest at a gradient of 0.00015 

to 0.006 foot per foot (URS, 1993b). For the purposes 

of this modeling study the observed gradient was 

assumed to be insignificant, thus, all cells in layer 6 

were assigned a uniform constant head set at 14 feet, 

equivalent to the average water level observed in wells 

that penetrate the deep aquifer. All confining units 

(layers 3 and 5) were assigned no-flow lateral 

boundaries because any significant flow in these units 

is assumed to be vertical.

Calibration of Model to Pre-Remediation 

Conditions 

The model was calibrated to the conditions 

existing before installation of the landfill cap and 

operation of remedial extraction wells in June 1995. 

The ground-water system was assumed to be at steady 

state, based on the absence of any pronounced trends in 

water levels for the period May 1994 through June 

1995 (see fig. 12). The model was calibrated by 

running MODFLOW simulations and comparing the 

simulated head values with observed pre-remediation 

water-level data. Values for recharge, hydraulic 

conductivity, and specified head cells at model 

boundaries were systematically adjusted until 

simulated heads most closely matched pre-remediation 

water levels and flow directions. Special care was taken 

to keep values for hydraulic conductivity, recharge, and 

specified heads within reported limits and not introduce 

values unsupported by the available data.

Initial values for hydraulic conductivity were 

established using values from the previous modeling 

study by IT Corporation (1997) and the results of 

recent aquifer tests (Foster Wheeler Environmental 

Corp., 1998b). Hydraulic conductivities for the shallow 

aquifer were initially applied along northeast to 

southwest trending zones of 90 ft/d, 85 ft/d, and 75 ft/d. 

A horizontal to vertical conductivity ratio of 10 to 1 

was used initially in the calculation of leakance. 

Recharge values initially were set in zones of high 

(10 in/yr), medium (8 in/yr), and low (7in/yr) recharge, 

similar to the previous modeling study (IT Corporation, 

1997). The observed pre-remediation water levels are 

from published measurements made on May 19, 1994, 

January 31 to February 1, 1995, and June 19 to 20, 

1995 (IT Corporation, 1995). This data set includes 39 

wells with screens in the shallow aquifer and 7 wells 

screened in the intermediate aquifer. Water-level 

measurements for each well were averaged to obtain a 

time-averaged representation of the pre-remediation 

water level.
Steady-State Simulation of the Ground-Water Flow System 23

Figure 13. Model boundaries and pre-remediation water levels for the shallow aquifer. 

Model boundaries shown are for the no-flow boundary condition; only cells along the southern boundary have

specified heads (diamonds). Pumping wells are shown as red circles and monitoring wells are shown in black. 

Contour interval is 1 foot.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

86

84

82

80

78

90

88
24 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 14. Model boundaries and pre-remediation water levels for the intermediate aquifer.

Model boundaries shown are for the no-flow boundary condition; only cells along the west and southeast 

boundaries have specified heads (diamonds). Pumping wells are shown as red circles and monitoring wells are 

shown as black. Contour interval is 5 feet.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

75

70

65

60

70

65

60
Steady-State Simulation of the Ground-Water Flow System 25

After each trial simulation, layer-wide 

adjustments were made to hydraulic parameters until 

simulated head distributions in all the aquifers were 

reasonably consistent with the observed data set 

(figure 15a, figure 16a). The following changes were 

made to the model during the calibration process. 

(1) Zones of hydraulic conductivity were removed and 

all cells in a given layer were assigned the same value 

which was adjusted to obtain a best fit with the 

observed data. Each modification to the hydraulic 

conductivity in a particular model layer was 

accompanied by a re-computation of the leakance value 

for each cell in that layer. (2) The ratio of horizontal to 

vertical conductivity used in the leakance calculation 

was changed from 10:1 to 5:1. (3) Recharge was 

simplified into two zones of 7 in/yr and 10 in/yr. 

(4) Specified head values at boundary cells were 

changed to match the observed head data and adjust 

flow directions. Final hydraulic parameters, shown in 

table 2, were selected to minimize the mean error of 

computed verses observed values (see error summary, 

(fig. 15b). For the intermediate aquifer, the observed 

data set is sparse and although flow paths appeared 

reasonable (fig. 16a), the simulated heads did not 

match as well as did those for the shallow aquifer 

(fig. 16b). 

Sensitivity Analysis for the Calibrated Model

The sensitivity of model-computed values to 

changes in model-input parameters was investigated by 

using multiple model runs with systematically varying 

model-input parameters. The model-computed values 

used in the analyses were the computed heads for 

shallow aquifer wells 6-S-21 and 6-S-27, the computed 

gradient between wells 6-S-21 and 6-S-27, the mean 

error of all wells in the shallow aquifer, and the mean 

error in all wells in the intermediate aquifer. Modelinput parameters that were varied include recharge, 

hydraulic conductivity of the shallow aquifer, and 

leakance through the shallow aquifer (layer 2) and the 

upper confining layer (layer 3). In each of the model 

runs, a multiplier was applied to the model-input 

parameter so that the value for each cell in the layer 

was increased or decreased by the same amount. The 

results of the sensitivity analyses are summarized in 

table 3. Computed heads and the residual of computedminus-observed heads are shown for comparison.

The sensitivity analyses (table 3) show that 

simulated changes in recharge and hydraulic 

conductivity cause the greatest changes in modelcomputed heads in both the shallow and intermediate 

aquifers. Changing the leakance through the shallow 

aquifer and upper confining unit also caused large 

changes in computed heads, especially in the 

intermediate aquifer. In all the tests conducted, none 

was able to significantly alter the gradient between 

6-S-21 and 6-S-27. In all the simulations the computed 

gradient was slightly less than the observed gradient. 

Because the model is focused on ground-water flow in 

the shallow aquifer, the calibrated parameters were 

adjusted to minimize errors in the shallow aquifer 

rather than in the intermediate aquifer. 
26 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 15a. Simulated head distribution in layer 1, the upper portion of the shallow aquifer, for the preremediation period.

Locations of wells with observational data (black circles) are shown with calibration targets. Green bars indicate 

simulated values within 2 feet of the observed and yellow bars are within 4 feet. Colored bars above center 

indicate simulated values greater than observed; bars below center indicate simulated values less than 

observed. Specified head cells are indicated with diamonds. Brown areas indicate dry cells. Pumping wells are 

shown as red circles and all other monitoring wells are shown as white. Contour interval is 1 foot.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

63

66

66

63

69

69

72

75 72

75

78

78

81

81

84

84

87

87

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.
Steady-State Simulation of the Ground-Water Flow System 27

Figure 15b. Comparison of simulated and observed heads in wells with screens in the 

shallow aquifer for the pre-remediation period and a summary of error.

80.0

90.0

70.0

5.0

2.5

0

-2.5

-5.0

70 74 78 82 86 90 72 76 80 84 88

70 74 78 82 86 90 72 76 80 84 88

COMPUTED RESIDUAL

ERROR SUMMARY

Data set name:

Observed parameter:

Mean error:

Mean absolute error:

Root mean square error:

prepumping-sim_Heads

shallow-aquifer wells

0.08

1.22

1.50

OBSERVED WATER LEVEL, IN FEET

WATER LEVEL, IN FEET

Table 2. Hydrologic parameters used in the steady-state model

[Kh, horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity; ft/d, feet per day; in/yr, inches 

per year; n/a, no value assigned]

Recharge

(in/yr)

Kh

(ft/d) Kh:Kv Porosity1

Layer type

Layer 1 7.0 and 10.0 87 5:1 0.25 Unconfined

Layer 2 n/a 70 5:1 0.25 Confined/unconfined

Layer 3 n/a 0.0002 1:1 0.10 Confined

Layer 4 n/a 10 5:1 0.20 Confined

Layer 5 n/a 0.0002 1:1 0.10 Confined

Layer 6 Constant head elevation set at 14 feet above sea level

1Porosity values were used in MODPATH particle tracking simulations only.
28 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 16a. Simulated head distribution in layer 4, the intermediate aquifer, for the pre-remediation period.

Locations of wells with observational data (black circles) are shown with calibration targets. Green bars indicate 

simulated values within 2 feet of the observed, yellow bars are within 4 feet and red are greater than 4 feet. 

Colored bars above center indicate simulated values greater than observed; bars below center indicate 

simulated values less than observed. Specified head cells are indicated with diamonds. Pumping wells are 

shown as red circles and all other monitoring wells are shown as white. Contour interval is 1 foot.

SR

20

SR

20

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

PORTER AVE

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

78

75

72

69

66

63

60

78

81

75

72

69

66

63

65

61

60
Steady-State Simulation of the Ground-Water Flow System 29

Figure 16b. Comparison of simulated and observed heads in wells with screens in the 

intermediate aquifer for the pre-remediation period and a summary of error.

80.0

75.0

70.0

65.0

5.0

-5.0

-10.0

65.0 67.5 70.0 72.5 75.0 77.5

65.0 67.5 70.0 72.5 75.0 77.5

ERROR SUMMARY

Data set name:

Observed parameter:

Mean error:

Mean absolute error:

Root mean square error:

prepumping-sim_Heads

intermediate-aquifer wells

-1.13

 2.26

 2.77

0

COMPUTED RESIDUAL

WATER LEVEL, IN FEET

OBSERVED WATER LEVEL, IN FEET
30 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, County, Washington

STEADY-STATE SIMULATION OF POSTREMEDIATION CONDITIONS

Using the hydraulic parameters determined for 

the steady-state, pre-remediation condition (table 2), 

the model was then modified to simulate the steadystate conditions resulting from the effects of the pumpand-treat system and landfill cap. Nine extraction wells 

were added to the model to simulate post-remedial 

conditions. To simulate the reinfiltration of treated 

water, the volume of water extracted from the wells 

was returned to the aquifer by applying additional 

recharge to those cells along the trace of the 

intermittent stream (see fig. 11 for location on model 

grid). Recharge for the area beneath the landfill cap 

was adjusted to zero and the volume of water that 

would have recharged the aquifer from precipitation 

falling on the landfill cap was redirected to the 

retention pond; evapotranspiration was not simulated. 

Additional modifications included adding a series of 

drains to represent engineered runway drainage ways in 

the wetland area north of Ault Field Road. The drains 

were added to simulate possible ground-water 

discharge to the land surface should water-table 

altitudes rise above land-surface altitudes in response 

to the reintroduction of treated water and redirection of 

landfill precipitation.

Table 3. Results of sensitivity analysis

[Changes in model-computed values caused by changes in model-input parameters, using the pre-remediation condition]

Head, 6-S-21

(observed 85.6)

(feet)

Head, 6-S-27

(observed 78.3)

(feet)

Computed 

gradient

(observed 

0.00317)

Model input 

parameter

Change in

parameter

ComputedResidualComputedResidualComputedResidual

Mean error

in shallow

aquifer

Mean error 

in

intermediate

aquifer

Calibrated

parameters

None 85.2 -0.39 79.3 1.02 0.00195 -0.00122 0.008 -1.13

Recharge 20 percent

decrease

82.0 -3.6 76.8 -1.5 .00172 -.00145 -2.76 -1.9

20 percent

increase

88.2 2.6 81.6 3.3 .00218 -.00099 2.68 -0.42

Hydraulic 

conductivity in the

shallow

aquifer

20 percent

decrease

88.3 2.7 81.7 3.4 .00218 -.00099 2.77 -0.4

20 percent

increase

82.9 -2.7 77. 577.5 -.08 .00178 -.00139 -1.94 -1.68

Leakage in

layers 2 and 3

20 percent

decrease

85.5 -0.06 79.6 1.3 .00195 -.00122 0.34 -1.94

20 percent

increase

84.9 -0.7 79.1 0.8 .00191 -.00126 -0.22 -0.44

100 percent

increase

83.9 -1.7 78.3 -.005 .00185 -.00132 -1.09 1.57

1,000 percent

increase

81.0 -4.6 75.8 -2.5 .00172 -.00145 -3.92 6.07
Steady-State Simulation of Post-Remediation Conditions 31

The time period for which post-remediation 

water levels were assumed to have been in steady-state 

is from January 1, 1998 to December 31, 1998. For this 

time period, data on flow rates for the extraction wells 

were taken from published quarterly reports (Foster 

Wheeler Environmental Corporation, 1998c-f). Actual 

rates varied because iron and bacteria gradually fouled 

the pump intakes causing electrical systems to fail. The 

extraction wells generally were quickly treated with 

biocide solutions to clear pump intakes and restore 

flow rates. For modeling purposes, an average flow rate 

was estimated using production data from January 

1997 to January 1999 for each extraction well (table 4). 

The flow rates used in the model slightly overestimate 

the published average daily flow through the treatment 

plant for the same period (Foster Wheeler 

Environmental Corporation, 1998c-f). In the model, 

water was extracted from layers 1 and 2 in proportion 

to the length of screened interval in each layer (table 4). 

Because layer 2 is a somewhat arbitrary designation 

designed to simulate a poorly defined trend of finer 

particles with depth, the slight difference in hydraulic 

conductivity between the two layers was not factored 

into the proportioning of extracted water from layers 1 

and 2. 

Simulated heads and observed water levels were 

compared for the shallow aquifer (fig. 17a) and for the 

intermediate aquifer (fig. 18a). The data set of observed 

water levels comes from quarterly reports containing 

monthly measurements for the period January 1, 1998, 

through December 30, 1998 (Foster Wheeler 

Environmental Corporation, 1998c-f). The data set 

includes 52 wells screened in the shallow aquifer and 7 

wells screened in the intermediate aquifer. The monthly 

water-level measurements were averaged for the year 

to obtain a water level representative of the steady-state 

remedial pumping condition. The twelve-month period 

for 1998 was chosen because the pump-and-treat 

system had been in continuous operation except for 

March 9–30, when system maintenance was 

performed, and June 3 to July 13, when pump tests 

were being conducted (fig. 12). Also, water levels 

appeared to be relatively stable during this period and 

not affected by changes in ground-water storage.

Table 4. Flow rates in extraction wells

[gal/min, gallons per minute; gal/d, gallons per day; ft3/d, cubic feet per day; data from Foster Wheeler Environmental 

Corporation, 1998c-f]

Amount of extracted water1

Pumping

wells

Average flow rate Layer 1 Layer 2

(gal/min) (gal/d) (ft3/d) (ft3/d) (Percent) (ft3/d) (Percent)

PW-1 30 43,200 5,776 4,332 (75) 1,444 (25)

PW-2 0 0 0 0 0

PW-3 30 43,200 5,776 4,332 (75) 1,444 (25)

PW-4 22 31,680 4,236 3,008 (71) 1,228 (29)

PW-5 22 3,1680 4,236 3,008 (71) 1,228 (29)

PW-6 23 33,120 4,428 3,321 (75) 1,107 (25)

PW-7 20 28,800 3,851 2,580 (67) 1,271 (33)

PW-8 10 14,400 1,925 962.5 (50) 962.5 (50)

PW-9 20 28,800 3,851 1,925.5 (50) 1,925.5 (50)

Cumulative total 254,880 34,079

Published total,

 January 1997 -

 January 1999

252,000 33,692

1Parentheses indicate percent proportioned: the percentage of water extracted from each layer in proportion to the 

length of screened interval in that layer.
32 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 17a. Simulated head distribution in layer 1, the upper portion of the shallow aquifer, for the postremediation period.

Locations of wells with observational data (black circles) are shown with calibration targets. Green bars indicate 

simulated values within 2 feet of the observed, yellow bars are within 4 feet and red are greater than 4 feet. 

Colored bars above center indicate simulated values greater than observed; bars below center indicate 

simulated values less than observed. Green circles indicate cells with assigned drain attributes, and specified 

head cells are indicated with diamonds. Pumping wells are shown as red circles and all other monitoring wells 

are shown as white. Contour interval is 1 foot.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

63

66

69

72

75

78

81

84

87

90

91

90

78

81

75

63

69

66

72
Steady-State Simulation of Post-Remediation Conditions 33

The simulated water levels for the shallow 

aquifer matched the observed data set generally within 

2 feet. Although simulated water levels in several wells 

were as much as 2 to 4 feet too low, most wells were 

about a foot too low. Overall the simulated water levels 

matched the observed data with a mean error of -

1.27 feet (fig. 17b). One reason for the lower simulated 

water levels throughout the model was that more 

ground water appeared to be in storage during the postremediation simulation period than during the preremediation period (fig. 12). The same recharge 

estimate was used for simulations of both periods, so 

it appears that there may have been somewhat more 

recharge prior to and during the post-remediation 

period. Simulated water levels in the intermediate 

aquifer did not match as well. Although the mean error 

for the intermediate aquifer was -1.87 feet (fig. 18b), 

simulated water levels in several wells were as much as 

6 feet too low.

Simulated head distributions in the shallow 

aquifer for the steady-state remedial pumping 

condition (fig. 17a) differ from those of the steady-state 

pre-remediation condition (fig. 15a) in that a prominent 

ground-water mound develops in the vicinity of the 

intermittent stream during remedial pumping 

conditions. A less obvious but anticipated result is the 

deflection of contours that represents drawdown 

around pumping wells. The effects of the pump-andtreat system are evident in figure 12 where wells in the 

vicinity of the ground-water mound show a significant 

water-level increase. The formation of the groundwater mound has the effect of redirecting a portion of 

ground-water flow toward the northern boundary of the 

model. Because the northern boundary is a specified 

no-flow boundary, ground-water flow is redirected 

parallel to the northern edge of the model. This does 

not occur in reality, and thus indicates that the northern 

no-flow boundary assumption may not be valid for the 

remedial pumping condition. Because the groundwater mound likely causes some amount of flow to 

cross the northern boundary of the model, the effect of 

that flow on the area of primary concern was evaluated.

100.0

90.0

80.0

70.0

2.5

0

-2.5

-5.0

72 74 76 78 80 82 84 86 88 90 92 94

72 74 76 78 80 82 84 86 88 90 92 94

pumpingsimulation_Heads

shallow-aquifer wells

-1.27

 1.38

 1.50

COMPUTED RESIDUAL

WATER LEVEL, IN FEET

OBSERVED WATER LEVEL, IN FEET

ERROR SUMMARY

Data set name:

Observed parameter:

Mean error:

Mean absolute error:

Root mean square error:

Figure 17b. Comparison of simulated and observed heads in wells with screens in the 

shallow aquifer for the pre-remediation period and a summary of error.
34 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 18a. Simulated head distribution in layer 4, the intermediate aquifer, for the post-remediation period. 

Locations of wells with observational data (black circles) are shown with calibration targets. Green bars indicate 

simulated values within 2 feet of the observed, yellow bars are within 4 feet and red are greater than 4 feet. 

Colored bars above center indicate simulated values greater than observed; bars below center indicate simulated 

values less than observed. Specified head cells are indicated with diamonds. Pumping wells are shown as red 

circles and all other monitoring wells are shown as white. Contour interval is 1 foot.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

69

66

66

63

63

69

72

72

75

75

78

60

60

60

60

78

60
Steady-State Simulation of Post-Remediation Conditions 35

Evaluation of the Effects of Boundary 

Conditions on Model Results

The assumed validity of no-flow boundaries 

along the east, north, and west sides of the model was 

evaluated by comparing results from two steady-state 

remedial pumping simulations, each with different 

boundary conditions for the shallow and intermediate 

aquifers. In the first simulation, as described in the 

previous section of this report, the shallow aquifer had 

no-flow boundaries along the east, north, and west 

sides and specified heads along the south boundary of 

the model (fig. 17a). The intermediate aquifer had noflow boundaries along the northeast and south sides 

and heads were specified along the west and southeast 

boundaries of the model (fig. 18a). In the second 

simulation, all no-flow boundaries in the shallow and 

intermediate aquifers were replaced with specified 

heads fixed at pre-remediation water levels. The 

differences in water levels, fluxes through the area of 

primary concern, and potential contaminant migration 

between model runs were compared in order to test the 

validity of the no-flow boundary assumption and 

determine the sensitivity of the choice of boundary 

conditions on the area of primary concern.

Sensitivity of Water Levels to Boundary Conditions

A simulation was run using specified heads fixed 

at pre-remediation water levels at boundary cells for 

both the shallow (fig 19a) and the intermediate 

(fig. 20a) aquifers. The resulting distributions of 

computed head are generally similar to the distributions 

of heads using no-flow boundaries (figs. 17a, 18a). The 

primary difference between the two simulations is that 

for the shallow aquifer, water-level contours are 

dissimilar near the northeastern boundary. Another 

difference is that, for the shallow aquifer, computed 

water levels in the specified-head boundary condition 

scenario are 1 to 2 feet lower than in the no-flow 

boundary condition scenario. For the shallow aquifer, 

this results in a greater error compared to the observed 

water-level data set (fig. 19b). 

80.0

75.0

70.0

65.0

5.0

0

-5.0

-10.0

65.0 67.5 70.0 72.5 75.0 77.5

65.0 67.5 70.0 72.5 75.0 77.5

pumpingsimulation_Heads

intermediate-aquifer wells

-1.87

 2.74

 3.39

COMPUTED RESIDUAL

OBSERVED WATER LEVEL, IN FEET

WATER LEVEL, IN FEET

ERROR SUMMARY

Data set name:

Observed parameter:

Mean error:

Mean absolute error:

Root mean square error:

Figure 18b. Comparison of simulated and observed heads in wells with screens in the 

intermediate aquifer for the pre-remediation period and a summary of error.
36 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 19a. Simulated head distribution in layer 1, the upper portion of the shallow aquifer, for the postremediation pumping simulation with specified head boundary conditions.

Locations of wells with observational data (black circles) are shown with calibration targets. Green bars indicate 

simulated values within 2 feet of the observed, yellow bars are within 4 feet and red are greater than 4 feet. 

Colored bars above center indicate simulated values greater than observed; bars below center indicate 

simulated values less than observed. Green circles indicate cells with assigned drain attributes, and specified 

head cells are indicated with diamonds. Pumping wells are shown as red circles and other monitoring wells are 

shown as white. Contour interval is 1 foot.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

78

75

72

69

69

66

66

63

81

84

87

87

88

84

63

81

72

75

78
Steady-State Simulation of Post-Remediation Conditions 37

Differences between the two simulations are less 

pronounced for the intermediate aquifer although the 

specified-head boundary condition scenario produced a 

slightly greater error compared to the observed waterlevel data set (fig. 20b).

The lower simulated water levels in both of the 

remedial pumping scenarios is, in part, related to 

variability of precipitation and ground-water recharge 

that is not accounted for in this model. Because 

ground-water recharge was calibrated to the slightly 

drier 1994–95 conditions, it is reasonable to expect that 

simulated water levels for 1998 (representing relatively 

wet 1996–98 conditions) might be consistently lower 

than observed water levels. The reason for the greater 

error for the specified-head simulation is that with 

specified-head boundary cells water is allowed to exit 

the model, thus lowering water levels.

Sensitivity of Fluxes to Boundary Conditions

When treated water is discharged to the 

intermittent stream, a ground-water mound develops 

that revises flow at the northern boundary and creates a 

new ground-water divide farther south. Thus, flow 

across the northern boundary is induced. In the no-flow 

boundary scenario, water cannot escape and is forced 

to flow parallel to the boundary. In the specified-head 

boundary scenario, water is allowed to enter and exit 

the model as needed. To determine what effect the 

choice of boundary conditions had on the modeling 

results, the fluxes across model boundaries were 

examined. For each scenario net fluxes through each 

layer were computed for the entire model and for the 

area of primary concern. Flux through the area of 

primary concern was determined by computing the flux 

through the dense grid of 25-by-25 foot cells in the 

model. A comparison of fluxes through the model and 

the area of primary concern for each boundary scenario 

are shown on figure 21.

The flux analysis suggests that the fluxes 

throughout the model, especially downward fluxes, are 

relatively insensitive to alternative boundary 

conditions. Lateral fluxes through model layers are 

greater when specified head boundaries are used. The 

proximity of the northern model boundary to the 

ground-water mound creates a minor problem in that 

some flow is induced across the model boundary.

100.0

75.0

50.0

5.0

0

-5.0

70 75 80 85 90 95

70 75 80 85 90 95

pumpsim-specifiedheads_Heads

shallow-aquifer wells

-2.30

 2.33

 2.51

COMPUTED RESIDUAL

ERROR SUMMARY

Data set name:

Observed parameter:

Mean error:

Mean absolute error:

Root mean square error:

OBSERVED WATER LEVEL, IN FEET

WATER LEVEL, IN FEET

Figure 19b. Comparison of simulated and observed heads in wells with screens in the 

shallow aquifer for the post-remediation period and a summary of error.
38 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Figure 20a. Simulated head distribution in layer 4, the intermediate aquifer, for the post-remediation pumping 

simulation with specified head boundary conditions.

Locations of wells with observational data (black circles) are shown with calibration targets. Green bars indicate 

simulated values within 2 feet of the observed, yellow bars are within 4 feet and red are greater than 4 feet. 

Colored bars above center indicate simulated values greater than observed; bars below center indicate 

simulated values less than observed. Specified head cells are indicated with diamonds. Pumping wells are 

shown as red circles and all other monitoring wells are shown as white. Contour interval is 1 foot.

SR

20

SR

20

PORTER AVE

GOLDIE

ROAD

AULT FIELD

ROAD

ROAD TO

R

PEDO

NAS Boundary

NAS Boundary

ROAD

HARBOR

OAK

0

0

1000 2000 FEET

200 400 600 METERS

T.

33

N. 

R.1 E.

81

78

69

66

66

63

60

75

72

60

78

69

60

60

63

65

75

72

60

61
Steady-State Simulation of Post-Remediation Conditions 39

The flux out of the model through cells along the 

northern boundary in the specified head scenario was 

on the order of 3,850 cubic ft/d for the shallow aquifer. 

Although this induced flux is a small portion of the 

total flow through the model (0.03 percent), it may 

represent a significant portion (13 percent) of the 

amount of treated water being returned to the groundwater system. Within the area of primary concern, 

fluxes through the 25-by-25 foot cells were very 

similar for both scenarios (fig. 21). Therefore, it is 

reasonable to conclude that although the original noflow boundary condition assumption may be violated 

for the remedial pumping condition, the effect on 

fluxes in the area of primary concern is insignificant.

Sensitivity of Potential Contaminant Migration to 

Boundary Conditions

To determine if simulated contaminant migration 

is affected by specified conditions at the model 

boundaries, the paths of particles were compared for 

each alternative boundary condition. A MODPATH 

simulation was developed in which an array of particles 

was placed in 306 model cells in the vicinity of the 

hazardous waste storage area. Using hydrologic 

parameters from table 2 and pumping rates from 

table 3, two particle-tracking simulations were 

performed using the post-remediation steady-state 

model with each boundary condition. Particle tracking 

is based on computed ground-water velocities and thus 

requires that porosities be specified for the aquifer and 

confining unit materials in the model. Porosity values 

of 0.25 for the shallow aquifer, 0.20 for the 

intermediate aquifer, and 0.10 for confining units were 

estimated based on the grain size of the materials in 

each layer and input into the MODPATH simulation 

(table 2). Particles were placed in the center of cells in 

layer 1 and allowed to pass through weak sinks as they 

moved advectively with ground water. In both postremediation steady-state simulations with different 

boundary scenarios the effects of dispersion were not 

accounted for, all other parameters were held constant.

75.0

70.0

80.0

65.0

5.0

0

-10.0

-5.0

65.0 67.5 70.0 72.5 75.0 77.5

65.0 67.5 70.0 72.5 75.0 77.5

pumpsim-specifiedheads_Heads

intermediate-aquifer wells

-2.24

 2.89

 3.58

ERROR SUMMARY

Data set name:

Observed parameter:

Mean error:

Mean absolute error:

Root mean square error: COMPUTED RESIDUAL

OBSERVED WATER LEVEL, IN FEET

WATER LEVEL, IN FEET

Figure 20b. Comparison of simulated and observed heads in wells with screens in the 

intermediate aquifer for the post-remediation period and a summary of error.
40 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station, Whidbey Island, Island County, Washington

Figure 21. Comparison of fluxes for model runs using no-flow and specified-head boundary conditions for all cells within the area of pri

LAYER 1

61,006

118,387

RECHARGE AND

TREATED WATER

2,849

34,633

19,899

NO-FLOW BOUNDARY SCENARIO

SPECIFIED HEAD BOUNDARY SCENARIO

FLUX THROUGH ALL CELLS IN THE MODEL,

IN CUBIC FEET PER DAY

Wells

Drains

Specified

Head

LAYER 2

12,767

39,240

8,999

Wells

Specified

Head

LAYER 4

28,504

17,659 1,922

LAYER 6

28,504

LAYER 1

54,637

107,662

0

45,507

19,899

Wells

Drains

Specified

Head 12,381 Specified Head

7,025 Specified Head LAYER 2

12,191

40,472

8999

Wells

Specified

Head

LAYER 4

28,375

3,158

LAYER 6

28,375

19,342 Specified Head Specified Head

RECHARGE AND

TREATED WATER

-319

LAYER 1

9,698

23,30946,419

-15,004

RECHARGE

2,412

-19,899

NO-FLOW BOUNDARY SCENARIO

SPECIFIED HEAD BOUNDARY SCENARIO

FLUX THROUGH CELLS WITHIN AREA OF P

IN CUBIC FEET PER DAY

58

LAYER 2

9,698

5,172

-14,111

2,295

3,052

LAYER 4

73

5,386

-1,187

-1,724

0

LAYER 6

0

5,386

0

0

0

1,222

LAYER 

-14,132

457

LAYER 

-13,753

3,190

LAYER 

-1,136

0

LAYER 

Wells

Wells

-8,999
Discussion of Ground-Water Flow Simulation 41

The results of the MODPATH simulations 

(fig. 22) show how the particles move with ground 

water. In both scenarios all particles remained in layers 

1 and 2 and were effectively captured by three of the 

extraction wells (PW1, PW3, and PW5). Particles 

followed a north to south trajectory that was nearly 

identical for both specified-head and no-flow boundary 

scenarios. Maximum travel time for particles to reach 

PW-5 in the specified-head boundary scenario was 

2,079 days (5.7 years) and slightly longer, 2,279 days 

(6.2 years) for the no-flow boundary scenario. The 

difference in travel time may be due to the slightly 

lower hydraulic gradient when using the specifiedhead boundaries. The similarity of particle tracks 

further suggests that choice of boundary condition does 

not greatly affect simulated ground-water flow within 

the area of primary concern.

DISCUSSION OF GROUND-WATER FLOW 

SIMULATION

The two steady-state pumping simulations 

modeled in this study, no-flow and specified-head 

boundary conditions, represent conceptual end 

members of a spectrum of possible model boundary 

configurations. In reality, neither head nor flux is 

constant at those boundaries, both vary in time and 

space. Designating these conditions at the model 

boundaries is not an ideal situation, but in some cases it 

is a necessary alternative to extending the model to real 

hydrologic boundaries that are either poorly defined or 

far from the area of primary concern. The solutions 

obtained for the two simulations modeled in this study 

appear to be insensitive to the boundary conditions. 

The fact that both pumping scenarios yielded similar 

output in terms of flow direction, head distribution, and 

particle tracking lends credence to the modeling 

approach used in this study. Although the model may 

misrepresent the flow system near the model 

boundaries, the calibrated model provides a reasonable 

approximation of the three-dimensional ground-water 

flow system in the vicinity of the contamination plume 

and extraction wells.

Flow Within and Between Aquifers

The downward component of ground-water flow, 

governed by the leakance parameter, is controlled 

primarily by the vertical hydraulic conductivity of the 

confining units. In the two steady- state simulations, 

downward flow between the layers was nearly identical 

with 53 percent of the recharge moving through the 

shallow aquifer from layer 1 to layer 2 and 11percent of 

the recharge passing through the upper confining unit, 

layer 3, into the intermediate aquifer, layer 4. Vertical 

downward movement of water through the upper 

confining layer is estimated at a rate of 0.0002 ft/d (see 

table 2). Based on average thickness and vertical 

hydraulic conductivity, travel time of the contaminant 

by advective transport through the upper confining 

layer is estimated to be 137 years, but could take 

between 14 years where the layer is thinnest and 550 

years where it is thickest. Travel times for particles 

advected from the source area to the extraction wells 

suggest that ground water is moving at a horizontal 

velocity of about 420 feet per year (ft/yr). This estimate 

is within the range of velocities (320 to 640 ft/yr) 

computed for the same area using hydraulic gradient, 

hydraulic conductivity, and porosity (Dinicola, 2000). 

The volumetric water budgets for each scenario 

listed in table 5 are derived from MODFLOW output 

files and include all layers in the model. Fluxes at 

constant-head cells are similar in the pre-remediation 

and no-flow remediation scenarios, but when specifiedhead cells are used as a boundary condition, additional 

water is allowed to enter and exit the model. Drains 

were used in the no-flow and specified-head 

remediation scenarios, however head distributions were 

generally lower with the specified-head boundary 

condition so the drains were not activated. Recharge 

from precipitation was held constant for all the 

simulations. In the no-flow and specified-head 

remediation scenarios, water extracted from wells was 

added to the recharge from precipitation. The recharge 

input for the specified-head boundary condition is less 

than the no-flow scenario because specified-head cells 

do not accept recharge. Total volumetric budgets for 

no-flow and specified- head boundary condition 

scenarios differ by about 10 percent.
42 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station, Whidbey Island, Island County, Washington

Figure 22. Flow paths of particles placed in the vicinity of the contaminant source area indicated by elevated soil gas readings.Particle tracks on the left are from the post-remediation pumping simulation with no-flow boundary conditions; particle tracks on the righremediation pumping simulation with specified-head boundary conditions. Pumping wells are shown as red circle symbols.

00

1000 FEET

200 METERS

Swale

Ault Field 

Road

State Highway 20

Goldie Road 

Oak Harbor Landfill

PW

9

PW

8

PW

2

PW

4

PW

6

PW

3

PW

1

PW

5

PW

7

Area 6 Wetland

Landfill

Storm-water

detention basin

Approximate

Western Plume

Approximate

Southern

Plume

Naval Air Station boundary T. 33 N.

R.1 E.

No-Flow Boundary S

Pumping

Well-3

Pumping

Well-1

Pumping

Well-5

00

400 FEET

100 METERS
Discussion of Ground-Water Flow Simulation 43

Potential Contaminant Migration

The advective migration of contaminants from 

the hazardous-waste storage area can be modeled using 

particle tracking. Flow paths of particles do not seem to 

be greatly influenced by the boundary assumptions 

used in the model, as indicated by the nearly identical 

particle tracks (see fig. 22); only travel times were 

affected slightly. In both scenarios particles remained 

in layer 1 except in the immediate vicinity of extraction 

wells PW-1, PW-3, and PW-5, where approximately 

25 percent of the extracted water from each well is 

withdrawn from layer 2. In the simulations no particles 

entered the upper confining unit (layer 3) or the 

intermediate aquifer (layer 4). The model simulations 

indicate that all particles were effectively captured by 

three of the nine extraction wells (PW1, PW3, and 

PW5). Analytical data show that contaminant 

concentrations are highest along an elongate area 

encompassing PW1, PW3, and PW5, suggesting that 

these wells extract the majority of contamination 

originating from the hazardous waste storage area 

(Dinicola, 2000). The other extraction wells do not 

appear to have an influence on the western 

contamination plume, however they do appear to 

capture VC contaminants and landfill leachate from the 

eastern plume. The effectiveness of the pump-and-treat 

system in containing landfill leachate was not 

addressed in this model. Simple modifications to the 

existing model would allow this and other evaluations 

to be made.

Model Limitations

The purpose of the model was to develop a tool 

that could be used to evaluate the effectiveness of the 

current remediation system and to compare alternative 

remediation strategies. To that end the model is useful, 

but for the area of primary concern only, including the 

hazardous waste storage area, landfill, extraction wells, 

and monitoring wells. The ability of the model to 

realistically simulate ground-water flow in the vicinity 

of the model boundaries and in the intermediate aquifer 

is less certain. It is recognized that the assumption of 

steady-state conditions may not be perfectly valid for 

the simulations because of changing climatic 

conditions and variable extraction rates. However, 

every attempt was made to calibrate the model to the 

period of time when precipitation and pumping 

conditions were most stable. 

Table 5. Model-derived volumetric ground-water budget for three 

scenarios

[Cumulative volumes shown in cubic feet for a period of 1 day]

Remediation

PreremediationNo-flow

boundary

Specific-head

boundary

IN:

CONSTANT HEAD = 18,731 18,569 45,391 

WELLS = 0 0 0

DRAINS = 0 0 0

RECHARGE = 89,535 123,570 112,850

TOTAL IN = 108,270 142140 158,240

OUT:

CONSTANT HEAD = 108,270 103,290 124,160

WELLS = 0 34,079 3,4079

DRAINS = 0 4,769.8 0

RECHARGE = 0 0 0

TOTAL OUT = 108,270 142,140 158,240
44 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, County, Washington

Another important limitation is that the steady-state 

model cannot simulate the period(s) of transition from 

different pumping scenarios. Variability of 

precipitation and infiltration make recharge probably 

the most uncertain parameter used in the model. The 

tendency of the model to underestimate heads greater 

than 85 feet and overestimate heads less than 85 feet 

suggests that hydraulic conductivity may have some 

zonal variation within individual layers and may not be 

homogenous, as assumed in the model. Zonal 

variations in hydraulic conductivity, in combination 

with the poorly constrained recharge estimates, create 

the potential for a non-unique solution. However, these 

parameters were constrained to reasonable ranges and 

adjusted so that simulated results achieved a best fit 

with observational data.

The contaminant migration simulations did not 

account for the effects of dispersion or physical and 

chemical retardation of the contaminant. The particle 

tracking model was based on advective transport only. 

The long, narrow shape of the western contaminant 

plume suggests that lateral dispersion is not a dominant 

transport process at Area 6. Dinicola (2000) concludes 

that dispersion does not have a substantial effect on 

contaminant transport in the shallow aquifer because of 

the relatively homogenous aquifer materials and high 

transmissivities. 

SUMMARY AND CONCLUSIONS

A three-dimensional, steady-state, ground-water 

flow model was developed to test the effectiveness of 

remediation efforts to contain trichloroethene and other 

contaminants in the ground water at the Whidbey 

Island Naval Air Station, Area 6 landfill facility. The 

model was calibrated by adjusting model input 

parameters until a best fit was achieved between 

simulated and observed water levels prior to 

installation of a pump-and-treat remediation system in 

July 1995. The calibrated model was subsequently 

modified to simulate the steady-state pumping 

condition of the pump-and-treat remediation in 1998. 

Simulated water levels compared favorably to observed 

average water levels measured in 1998. The effect on 

simulated water levels within the area of primary 

concern was evaluated using two boundary condition 

scenarios (1) mostly no-flow and (2) all specified-head 

boundary conditions. Simulated water levels and fluxes 

near the perimeter of the model domain are sensitive to 

different boundary conditions, although the fluxes, 

water levels, and potential contaminant migration 

within the specific area of primary concern are 

insensitive to different boundary conditions. The 

sensitivity of the model to boundary conditions is 

pronounced in the northern part of the modeled area 

and is caused by the southward migration of the 

northern hydraulic boundary due to recharge of treated 

water through a nearby intermittent stream. 

Flux analysis for the area of primary concern 

surrounding the western contamination plume, 

extraction wells, and observation wells within the 

landfill facility indicates that the model is reasonably 

calibrated within the area of primary concern. 

However, the calibration is less certain near the model 

boundaries. The advective transport of contaminants 

from the hazardous-waste storage area was simulated 

using particle tracking. Particles originating in the 

hazardous-waste storage area had nearly identical 

trajectories in both pumping simulations, indicating 

that advective flow in the vicinity of the contamination 

plume is unaffected by the choice of boundary 

conditions used in the model. Although the simulated 

water levels do not fit the observational data precisely, 

the model can be used to test the effectiveness of the 

existing pump-and-treat system and simulate the 

effects of various pumping strategies that involve 

equivalent or lesser pumping rates once the system 

reaches a new steady-state for the new pumping rate. It 

is less suitable for simulating increased ground-water 

withdrawal due to the close proximity of the model 

boundaries to Area 6. Advective transport of groundwater contamination can be simulated and interactions 

between the shallow, intermediate, and deep flow 

systems can be evaluated. However, model calibration 

is less certain for the intermediate and deep aquifers 

due to a lack of data.
References Cited 45

REFERENCES CITED

Bauer, H.H., and Mastin, M.C., 1996, Recharge from 

precipitation in three small glacial-till mantled 

catchments in the Puget Sound lowland, Washington: 

U.S. Geological Survey Water-Resources Investigations 

Report 96-4219, 119 p.

Dinicola, R. S., Cox, S. E., and Bradley, P. M., 2000, 

Natural Attenuation of Chlorinated Volatile Organic 

Compounds in Ground Water at Area 6, Naval 

Air Station Whidbey Island, Washington: U. S. 

Geological Survey Water-Resources Investigations 

Report 00-4060, 86 p.

Easterbrook, D.J. and Anderson, H.W. Jr., 1968, Pleistocene 

stratigraphy and groundwater Resources of Island 

County: Washington Department of Water Resources 

Bulletin 25, p. 1-34.

Ecology and Environment, 1988, Site inspection report for 

Oak Harbor landfill, Oak Harbor, Washington.

1998a, Update to FLOWPATH Groundwater Model, 

November 1998, NAS, Whidbey Island: Prepared for 

Engineering Field Activity Northwest, Naval Facilities 

Engineering Command, Poulsbo, WA.

———1998b, Pumping Test Report, October 1998 NAS, 

Whidbey Island: Prepared for Engineering Field 

Activity Northwest, Naval Facilities Engineering 

Command, Poulsbo, WA.

———1998c, Quarterly Technical Report operating period 

January-March 1998 Operations of Interim Action 

Groundwater Extraction, Treatment, and Recharge 

System: Area 6 Landfill NAS, Whidbey Island, 

Washington: Prepared for Engineering Field Activity 

Northwest, Naval Facilities Engineering Command, 

Poulsbo, WA.

———1998d, Quarterly Technical Report operating period 

April-June 1998 Operations of Interim Action 

Groundwater Extraction, Treatment, and Recharge 

System: Area 6 Landfill NAS, Whidbey Island, 

Washington: Prepared for Engineering Field Activity 

Northwest, Naval Facilities Engineering Command, 

Poulsbo, WA.

———1998e, Quarterly Technical Report operating period 

July-September 1998 Operations of Interim Action 

Groundwater Extraction, Treatment, and Recharge 

System: Area 6 Landfill NAS, Whidbey Island, 

Washington: Prepared for Engineering Field Activity 

Northwest, Naval Facilities Engineering Command, 

Poulsbo, WA.

———1998f, Quarterly Technical Report operating period 

October-December 1998 Operations of Interim Action 

Groundwater Extraction, Treatment, and Recharge 

System: Area 6 Landfill NAS, Whidbey Island, 

Washington: Prepared for Engineering Field Activity 

Northwest, Naval Facilities Engineering Command, 

Poulsbo, WA.

Franke, L.O., Reilly, T.E., and Bennett, G.D., 1987, 

Definition of Boundary and Initial Conditions in the 

Analysis of Saturated Ground-water Flow Systems-An 

Introduction, Techniques of Water- Resources 

Investigations of the United States Geological Survey, 

Chapter B5, 14 p.

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

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

Hart Crowser, 1999a, Biofouling discussion paper, NAS, 

Whidbey Island, Operable Unit 1, Area 6 Landfill: 

Prepared by Hart Crowser Earth and Environmental 

Technologies, Seattle, Washington for Engineering 

Field Activity, Northwest, Naval Facilities Engineering 

Command, Silverdale, Wash., 25 p.

———1999b, Biofouling technical memorandum, NAS, 

Whidbey Island, Operable Unit 1, Area 6 Landfill: 

Prepared by Hart Crowser Earth and Environmental 

Technologies, Seattle, Washington for Engineering 

Field Activity, Northwest, Naval Facilities Engineering 

Command, Silverdale, Wash., 22 p.

IT Corporation, 1993, Baseline Groundwater Quality Interim 

Action Groundwater Extraction, Treatment, and 

Recharge System, Area 6 Landfill, NAS, Whidbey 

Island, Oak Harbor Washington: Prepared for 

Engineering Field Activity Northwest, Naval Facilities 

Engineering Command, Poulsbo, WA.

———1995, System Startup and Operations December 

1994 through July 1995 Interim Action Groundwater 

Extraction, Treatment, and Recharge System, Area 6 

Landfill, NAS, Whidbey Island, Oak Harbor 

Washington: Prepared for Engineering Field Activity 

Northwest, Naval Facilities Engineering Command, 

Poulsbo, WA.

———1996, FLOWPATH Groundwater Model Scenarios, 

Groundwater Extraction, Treatment and Recharge 

System: Prepared for Engineering Field Activity 

Northwest, Naval Facilities Engineering Command, 

Poulsbo, WA.

———1997, FLOWPATH Groundwater Model Summary 

Report Interim Action Groundwater Extraction, 

Treatment, and Recharge System, Area 6 Landfill, 

NAS, Whidbey Island, Oak Harbor Washington: 

Prepared for Engineering Field Activity Northwest, 

Naval Facilities Engineering Command, Poulsbo, WA.

McDonald, M.G. and Harbaugh, A.W., 1988, A Modular 

Three –Dimensional Finite Difference Groundwater 

Flow Model: Techniques of Water-Resources 

Investigations of the U. S. Geological Survey, U.S. 

Geological Survey Open-File Report 83-875.

National Oceanic and Atmospheric Administration, 1994, 

Climatological Data Annual Summary: Washington, 

V. 98, no. 13, p. 3.
46 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, County, Washington

National Oceanic and Atmospheric Administration, 1995, 

Climatological Data Annual Summary: Washington, 

V. 99, no. 13, p. 3.

National Oceanic and Atmospheric Administration, 1996, 

Climatological Data Annual Summary: Washington, 

V. 100, no. 13, p. 3.

———1997, Climatological Data Annual Summary: 

Washington, V. 101, no. 13, p. 3.

———1998, Climatological Data Annual Summary: 

Washington, V. 102, no. 13, p. 3.

Naval Energy and Environmental Support Activity, 1984, 

Initial Assessment Study of Naval Air Station, Whidbey 

Island, Washington.

Neuman, S.P., 1975, Analysis of pumping test data from 

anisotrophic unconfined aquifers considering delayed 

gravity response: Water Resources Research V. 11, 

no. 329, 42p.

Pollock, D.W., 1994, User’s Guide for 

MODPATH.MODPATH-PLOT, Version 3: a Particle 

Tracking Post-Processing Package for MODFLOW, the 

U.S. Geological Survey finite difference groundwater 

flow model: U.S. Geological Survey Open-File Report 

94-464, 250p.

Sapik, D.B., G.C. Bortleson, B.W. Drost, M.A. Jones, and 

E.A. Prych, 1987, Groundwater Resources and 

simulation of flow in aquifers containing freshwater 

and sea water, Island County, Washington: U.S. 

Geological Survey Water-Resources Investigations 

Report 87-4182, 67 p.

SCS Engineers, 1988, Current Situation Report, NAS 

Whidbey Island, Washington.

URS Consultants, 1993a, Final Record of Decision for 

Operable Unit 1, NAS Whidbey Island, Oak Harbor, 

Washington: Prepared by URS Consultants, Seattle 

Washington for Engineering Field Activity, Northwest, 

Naval Facilities Engineering Command, Poulsbo, 

Wash. 72 p.

———1993b, Final Remedial Investigation for Operable 

Unit 1, NAS Whidbey Island, Oak Harbor, Washington.

URS Team, 1998, Draft monitoring well closure 

management plan: Prepared for EFANW, NFEC, 

Poulsbo, WA.
Table 6. Summary of well data 47

TABLE 6. SUMMARY OF WELL DATA
48 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Local

well

number

Study

well

number

LatitudeLongitude

Measuring

point

altitude

Original

ground

surface

altitude

Measuring

point

above

land

surface

Well

depth

from

measuring

point

Well

depth

from

land

surface

Altitude

at

bottom

of hole

Top

of

screen

33N/01E-26A05 PW-1 481920 1223824 170.8 170.8 0 122 122 48.8 87.5

33N/01E-26J14 PW-2 481858 1223816 212.8 212.8 0 168.5 168.5 44.3 132.5

33N/01E-26H10 PW-3 481913 1223826 198.4 198.4 0 150 150 48.4 108

33N/01E-26J15 PW-4 481855 1223816 209 209 0 165 165 44 128

33N/01E-26J16 PW-5 481900 1223825 197.5 197.5 0 158 158 39.5 120

33N/01E-26J16 PW-6 481900 1223822 201.5 201.5 0 160 160 41.5 116.5

33N/01E-26J17 PW-7 481900 1223819 209.3 209.3 0 165 165 44.3 133

33N/01E-26J18 PW-8 481858 1223811 217.8 217.8 0 166 166 51.8 145

33N/01E-26J19 PW-9 481855 1223809 205.5 205.5 0 150 150 55.5 130

33N/01E-25L01 6-S-1 481907 1223745 173 170.9 2.1 112.1 110 60.9 89.1

33N/01E-25C01 6-S-2 481928 1223740 183.6 182.1 1.5 113.5 112 70.1 95

33N/01E-25N02 6-S-3 481854 1223803 202.9 201.2 1.7 144.7 143 58.2 124.7

33N/01E-26J01 6-S-4 481900 1223817 213 210.2 2.8 149.3 146.5 63.7 132.3

33N/01E-25F03 6-S-5 481917 1223742 183.6 181.5 2.1 109.1 107 74.5 97.1

33N/01E-26H01 6-S-6 481909 1223826 197.5 195.5 2 134 132 63.5 114

33N/01E-26A01 6-S-7 481932 1223825 96.9 95.4 1.5 61.5 60 35.4 30

33N/01E-25D01 6-S-8 481932 1223801 163.7 161.8 1.9 99.9 98 63.8 74.9

33N/01E-26H06 6-S-9 481917 1223811 174.2 177.9 -3.7 108.3 112 65.9 91.3

33N/01E-26A02 6-S-10 481924 1223821 152.3 148.9 3.4 163.4 160 -11.1 93.4

33N/01E-26G01 6-S-11 481915 1223829 190.8 188.3 2.5 143 140.5 47.8 132.5

33N-01E-26G02 6-S-12 481907 1223830 193.1 190.4 2.7 150.7 148 42.4 137.2

33N/01E-26J02 6-S-13 481901 1223826 197.8 194.7 3.1 159.6 156.5 38.2 148.1

33N/01E-26H03 6-S-14 481910 1223821 211.5 204.5 7 164 157 47.5 152

33N/01E-26H04 6-S-15 481914 1223820 200.6 186.5 14.1 155.6 141.5 45 136.6

33N/01E-25M01 6-S-16 481906 1223800 195.7 191.9 3.8 141.8 138 53.9 119.8

33N/01E-25M02 6-S-17 481901 1223802 206.1 195.2 10.9 148.9 138 57.2 127.9

33N/01E-26G03 6-S-18 481913 1223845 142.7 140.8 1.9 79.9 78 62.8 61.4

33N/01E-26R01 6-S-19 481854 1223809 219.4 216.3 3.1 172.1 169 47.3 146.6

33N/01E-26B01 6-S-20 481928 1223833 109.2 106.2 3 63.5 60.5 45.7 22

33N/01E-26A03 6-S-21 481924 1223826 157.7 155.1 2.6 109.1 106.5 48.6 66.1

33N/01E-26A04 6-S-22 481921 1223821 173.5 170.7 2.8 125.3 122.5 48.2 112.8

33N/01E-26H05 6-S-23 481910 1223821 211.7 204 7.7 143.7 136 68 128.7

33N/01E-26G04 6-S-24 481907 1223830 192.6 190.1 2.5 122.5 120 70.1 108

33N/01E-26J03 6-S-25 481900 1223826 197.9 195.5 2.4 128.9 126.5 69 117.4

33N/01E-26A05 6-S-26 481929 1223819 128.5 125.7 2.8 79.3 76.5 49.2 65.8

Table 6. Summary of well data

[All units except latitude//longitude are in feet; <, less than; nd, no data; Layer 1, Vashon till and advance outwash; Layer 2, Vashon advance outash; Layer 3, 

Whidbey unit 1; Layer 4, Whidbey unit 2; Layer 5, Whidbey unit 3; Layer 6, Whidbey unit 4]
Table 6. Summary of well data 49

Table 6. Summary of well data—Continued

Local

well

number

Study

well

number

Bottom

of

screen

Top of

Layer 1

Top of

Layer 2

Top of

Layer 3

Top of

Layer 4

Top of

Layer 5

Top of

Layer 6

Top of

Double

Bluff

Formation

Top of

Bedrock

33N/01E-26A05 PW-1 117.5 nd 170.8 49.3 nd nd nd nd nd

33N/01E-26J14 PW-2 167.5 212.8 202.8 45.8 nd nd nd nd nd

33N/01E-26H10 PW-3 148 198.4 168.4 48.9 nd nd nd nd nd

33N/01E-26J15 PW-4 163 209 149 45 nd nd nd nd nd

33N/01E-26J16 PW-5 155 197.5 167.5 41.5 nd nd nd nd nd

33N/01E-26J16 PW-6 156.5 nd 201.5 42.5 nd nd nd nd nd

33N/01E-26J17 PW-7 163 nd 209.3 45.3 nd nd nd nd nd

33N/01E-26J18 PW-8 155 nd nd nd nd nd nd nd nd

33N/01E-26J19 PW-9 150 nd nd nd nd nd nd nd nd

33N/01E-25L01 6-S-1 99.1 170.9 153.9 <60.9 nd nd nd nd nd

33N/01E-25C01 6-S-2 105 182.1 162.1 71.1 nd nd nd nd nd

33N/01E-25N02 6-S-3 134.7 201.2 191.2 64.2 nd nd nd nd nd

33N/01E-26J01 6-S-4 142.3 210.2 153.2 <63.7 nd nd nd nd nd

33N/01E-25F03 6-S-5 107.1 181.5 119.5 <74.5 nd nd nd nd nd

33N/01E-26H01 6-S-6 124 195.5 185.5 <63.5 nd nd nd nd nd

33N/01E-26A01 6-S-7 40 95.4 69.4 45.4 nd nd nd nd nd

33N/01E-25D01 6-S-8 84.9 161.8 146.8 <63.8 nd nd nd nd nd

33N/01E-26H06 6-S-9 101.3 177.9 162.9 <65.9 nd nd nd nd nd

33N/01E-26A02 6-S-10 103.4 148.9 138.9 46.9 38.9 -10.1 nd nd nd

33N/01E-26G01 6-S-11 142.5 188.3 168.3 47.3 nd nd nd nd nd

33N-01E-26G02 6-S-12 147.2 190.4 161.4 44.4 nd nd nd nd nd

33N/01E-26J02 6-S-13 158.1 194.7 165.2 39.2 nd nd nd nd nd

33N/01E-26H03 6-S-14 162 204.5 184.5 49 nd nd nd nd nd

33N/01E-26H04 6-S-15 146.6 186.5 166.5 53.5 nd nd nd nd nd

33N/01E-25M01 6-S-16 129.8 191.9 171.9 64.9 nd nd nd nd nd

33N/01E-25M02 6-S-17 147.9 nd 195.2 58.2 nd nd nd nd nd

33N/01E-26G03 6-S-18 71.4 140.8 125.8 <62.8 nd nd nd nd nd

33N/01E-26R01 6-S-19 166.6 nd 216.3 51.3 nd nd nd nd nd

33N/01E-26B01 6-S-20 62 106.2 96.2 46.2 nd nd nd nd nd

33N/01E-26A03 6-S-21 106.1 155.1 152.6 49.1 nd nd nd nd nd

33N/01E-26A04 6-S-22 122.8 nd 170.7 49.7 nd nd nd nd nd

33N/01E-26H05 6-S-23 138.7 nd 204 <68 nd nd nd nd nd

33N/01E-26G04 6-S-24 118 nd 190.1 <70 nd nd nd nd nd

33N/01E-26J03 6-S-25 127.4 195.5 175.5 <69 nd nd nd nd nd

33N/)1E-26A05 6-S-26 75.8 nd 125.7 51.2 nd nd nd nd nd
50 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Table 6. Summary of well data—Continued

Local

well

number

Study

well

number

LatitudeLongitude

Measuring

point

altitude

Original

ground

surface

altitude

Measuring

point

above

land

surface

Well

depth

from

measuring

point

Well

depth

from

land

surface

Altitude

at

bottom

of hole

Top

of

screen

33N/01E-26J08 6-S-27 481856 1223826 198.6 198.6 0 130 130 68.6 120

33N/01E-26J09 6-S-28 481856 1223825 198.6 198.6 0 155 155 43.6 146

33N/01E-26J10 6-S-29 481854 1223814 213.1 213.1 0 164 164 49.1 144

33N/01E-26Q02 MW-1 481845 1223834 152.8 152.8 0 131 131 21.8 121

33N/01E-26R02 MW-2 481847 1223822 187.8 187.8 0 99 99 88.8 90

33N/01E-26R03 MW-3A 481845 1223815 177.9 177.9 0 22 22 155.9 15

33N/01E-26R04 MW-3B 481845 1223815 178.2 178.2 0 120 120 58.2 109

33N/01E-26R05 MW-4 481854 1223816 209.6 209.6 0 138 138 71.6 129

33N/01E-26J11 MW-5 481858 1223821 207.1 207.1 0 140 140 67.1 127

33N/01E-26Q03 MW-6 481852 1223827 188.9 188.9 0 139 139 49.9 124

33N/01E-26H09 MW-7 481917 1223826 199.5 197.4 2.1 152.1 150 47.4 118.4

33N/01E-26J12 MW-8 481904 1223822 205.9 203.9 2 164 162 41.9 122

33N/01E-25M03 MW-9 481857 1223806 212.5 210.5 2 154.5 152.5 58 132

33N/01E-26J13 MW-10 481906 1223817 216.2 208.6 7.6 169.6 162 46.6 121

33N/01E-25E01 MW11 481917 1223800 172.8 170.6 2.2 112.2 110 60.6 83

33N/01E-25D02 MW-12 481924 1223754 182.8 180.8 2 122 120 60.8 98

33N/01E-25C03 MW-13 481921 1223746 194.4 192.4 2 123 121 71.4 101

33N/01E-25E02 MW-14 481915 1223750 183 180.9 2.1 111.1 109 71.9 93

33N/01E-25E03 MW-15 481917 1223801 172.2 172.2 0 109 109 63.2 89

33N/01E-26A09 N6-37 481921 1223822 172.3 170.9 1.4 96 94.6 76.3 85.5

33N/01E-26A10 N6-38 481923 1223825 163.9 163.3 0.6 98 97.4 65.9 79.5

33N/01E-26A06 6-I-1 481921 1223821 174 171.3 2.7 181.2 178.5 -7.2 163

33N/01E-26G05 6-I-2 481915 1223830 190.7 188.5 2.2 179.2 177 11.5 162

33N/01E-26H02 6-I-3 481909 1223826 198.6 196.2 2.4 187.9 185.5 10.7 166

33N/01E-26J04 6-I-4 481900 1223826 194.8 194.8 0 193.5 193.5 1.3 172.5

33N/01E-26B02 6-I-5 481928 1223833 108.7 106 2.7 95.7 93 13 81.5

33N/01E-26B03 6-I-6 481921 1223842 139.3 136.9 2.4 137.4 135 1.9 120

33N/01E-26G06 6-I-7 481913 1223845 142.8 140.3 2.5 140.5 138 2.3 124

33N/01E-26A07 6-I-8 481924 1223826 157 154.6 2.4 161.9 159.5 -4.9 147

33N/01E-26J05 6-D-1 481924 1223821 211.9 210.2 1.7 261.7 260 -49.8 241

33N/01E-25L02 6-D-2 481921 1223807 172.9 172.9 0 220 220 -47.1 206

33N/01E-25C02 6-D-3 481900 1223752 183 181.7 1.3 202.8 201.5 -19.8 189.5

33N/01E-26A08 6-D-4 481900 1223752 171.1 171.1 0 220 220 -48.9 193

33N/01E-23Q01 P-1 481935 1223830 97 97 0 20 20 77 5

33N/01E-23R01 P-2 481936 1223824 96.1 96.1 0 20 20 76.1 5
Table 6. Summary of well data 51

Table 6. Summary of well data—Continued

Local

well

number

Study

well

number

Bottom

of

screen

Top of

Layer 1

Top of

Layer 2

Top of

Layer 3

Top of

Layer 4

Top of

Layer 5

Top of

Layer 6

Top of

Double

Bluff

Formation

Top of

Bedrock

33N/01E-26J08 6-S-27 130 nd nd nd nd nd nd nd nd

33N/01E-26J09 6-S-28 166 nd nd nd nd nd nd nd nd

33N/01E-26J10 6-S-29 164 nd 213.1 49.1 nd nd nd nd nd

33N/01E-26Q02 MW-1 126 152.8 144.8 <21.8 nd nd nd nd nd

33N/01E-26R02 MW-2 95 nd 187.8 <88 nd nd nd nd nd

33N/01E-26R03 MW-3A 20 nd 177.9 <155.9 nd nd nd nd nd

33N/01E-26R04 MW-3B 114 nd 178.2 <58.2 nd nd nd nd nd

33N/01E-26R05 MW-4 134 nd 209.6 <71.6 nd nd nd nd nd

33N/01E-26J11 MW-5 132 207.1 202.1 <67.1 nd nd nd nd nd

33N/01E-26Q03 MW-6 129 nd 188.9 <49.9 nd nd nd nd nd

33N/01E-26H09 MW-7 148.4 nd 197.4 47.9 nd nd nd nd nd

33N/01E-26J12 MW-8 162 nd 203.9 42.4 nd nd nd nd nd

33N/01E-25M03 MW-9 152 nd 210.5 58.5 nd nd nd nd nd

33N/01E-26J13 MW-10 161 nd 208.6 47.1 nd nd nd nd nd

33N/01E-25E01 MW-11 108 170.6 150.6 61.6 nd nd nd nd nd

33N/01E-25D02 MW-12 118 180.8 148.8 62.8 nd nd nd nd nd

33N/01E-25C03 MW-13 121 192.4 142.4 72.4 nd nd nd nd nd

33N/01E-25E02 MW-14 108 180.9 160.9 72.9 nd nd nd nd nd

33N/01E-25E03 MW-15 109 nd nd nd nd nd nd nd nd

33N/01E-26A09 N6-37 95.5 nd 170.9 <74.9 nd nd nd nd nd

33N/01E-26A10 N6-38 89.5 163.3 150.8 73.8 nd nd nd nd nd

33N/01E-26A06 6-I-1 177 171.3 162.3 47.3 41.3 14.3 <-7.2 nd nd

33N01I-26G05 6-I-2 172 188.5 160.5 46.5 38.5 13.5 nd nd nd

33N/01E-26H02 6-I-3 176 196.2 176.2 45.2 41.2 14.7 nd nd nd

33N/01E-26J04 6-I-4 182.5 194.8 176.8 41.8 34.8 9.8 nd nd nd

33N/01E-26B02 6-I-5 91.5 106 91 45 36 14 nd nd nd

33N/01E-26B03 6-I-6 130 136.9 116.9 41.9 35.4 3.9 nd nd nd

33N/01E-26G06 6-I-7 134 125.3 34.3 33.3 3.8 nd nd nd nd

33N/01E-26A07 6-I-8 157 nd 154.6 20.6 14.6 -2.9 nd nd nd

33N/01E-26J05 6-D-1 251 210.2 153.2 34.2 30.2 20.2 -27.8 nd nd

33N/01E-25L02 6-D-2 216 172.9 156.4 60.9 50.9 24.9 -10.1 nd nd

33N/01E-25C02 6-D-3 199.5 181.7 166.7 58.7 46.7 22.7 6.7 nd nd

33N/01E-26A08 6-D-4 203 nd 171.1 13.1 12.1 -2.9 -18.9 nd nd

33N/01E-23Q01 P-1 20 nd 77 nd nd nd nd nd nd

33N/01E-23R01 P-2 20 nd 76.1 nd nd nd nd nd nd
52 Ground-Water Flow and Potential Contaminant Transport, Naval Air Station Whidbey Island, Island County, Washington

Table 6. Summary of well data—Continued

Local

well

number

Study

well

number

LatitudeLongitude

Measuring

point

altitude

Original

ground

surface

altitude

Measuring

point

above

land

surface

Well

depth

from

measuring

point

Well

depth

from

land

surface

Altitude

at

bottom

of hole

Top

of

screen

33N/01E-23Q02 P-3 481935 1223828 96.7 96.7 0 20 20 76.7 5

33N/01E-23R02 P-4 481935 1223824 96.7 96.7 0 20 20 76.7 5

33N/01E-26J06 P-5 481855 1223808 204.6 202.3 2.3 140.3 138 64.3 128

33N/01E-26J07 P-6 481855 1223809 205.3 203 2.3 142.3 140 63 129

33N/01E-26H07 P-7 481913 1223825 204.7 201.9 2.8 142.8 140 61.9 130

33N/01E-26H08 P-8 481914 1223826 201.6 199.2 2.4 142.4 140 59.2 125

33N/01E-26N05 Hilberdink 481848 1223916 100 100 0 120 120 -20 115

33N/01E-35G03 A381 481822 1223835 nd nd nd 16 nd nd 11

33N/01E-36C Rodewald 481834 1223735 nd nd nd 39 nd nd 33

33N/01E-N02 Dickson 481847 1223908 109 109 0 67 67 42 62

33N/01E-26Q01 Oak Harbor

M&E

481848 1223837 140 140 0 214 214 -74 184

33N/01E-26M01 Enterprises 481901 1223916 140 140 0 128 128 12 nd

33N/01E-26D05 DOE 481933 1223913 117 117 0 682 682 -565 nd

Local

well

number

Study

well

number

Bottom

of

screen

Top of

Layer 1

Top of

Layer 2

Top of

Layer 3

Top of

Layer 4

Top of

Layer 5

Top of

Layer 6

Top of

Double

Bluff

Formation

Top of

Bedrock

33N/01E-23Q02 P-3 20 nd 76.7 nd nd nd nd nd nd

33N/01E-23R02 P-4 20 nd 76.7 nd nd nd nd nd nd

33N/01E-26J06 P-5 138 202.3 <64.3 nd nd nd nd nd nd

33N/01E-26J07 P-6 139 203 173 <63 nd nd nd nd nd

33N/01E-26H07 P-7 140 201.9 166.9 <61.9 nd nd nd nd nd

33N/01E-26H08 P-8 135 199.2 169.2 <59.3 nd nd nd nd nd

33N/01E-26N05 Hilberdink 120 nd 100 14 -12 nd nd nd nd

33N/01E-35G03 A381 16 nd nd nd nd nd nd nd nd

33N/01E-36C Rodewald 38 nd nd nd nd nd nd nd nd

33N/01E-N02 Dickson 67 109 76 42 nd nd nd nd nd

33N/01E-26Q01 Oak Harbor

M&E

214 nd 140 85 53 23 -6 nd nd

33N/01E-26M01 Enterprises nd nd 117 37 -3 -73 -81 -323 -531

33N/01E-26D05 DOE 117.5 nd 170.8 49.3 nd nd nd nd nd