HYDROGEOLOGIC FRAMEWORK OF THE

PUGET SOUND AQUIFER SYSTEM, 

WASHINGTON AND BRITISH COLUMBIA

BRITISH COLUMBIA

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Hydrogeologic Framework of the Puget Sound Aquifer System, 

Washington and British Columbia

By].]. Vaccaro, AJ. Hansen,Jr., and M.A. Jones

REGIONAL AQUIFER-SYSTEM ANALYSIS 

PUGET-WILLAMETTE LOWLAND

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1424-D

1998
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Mark Schaefer, Acting Director

The use of firm, trade, and brand names in this report is for 

identification purposes only and does not constitute 

endorsement by the U.S. Government.

Library of Congress Cataloging in Publications Data

Vaccaro, J.J.

Hydrogeologic framework of the Puget Sound aquifer system, Washington and British Columbia / by J.J. Vaccaro, A.J. 

Hansen, Jr., and M.A. Jones.

77 p. cm.-(Regional aquifer-system analysis Puget-Willamette lowland ; D) (U.S. Geological Survey professional paper; 1424) 

Includes bibliographical references. 

Supt.ofDocs.no.: I 19.16:1424-0

1. Hydrogeology Washington (State) Puget Sound Region. 2. Hydrogeology British Columbia. 3. Aquifers Washington 

(State) Puget Sound Region. 4. Aquifers British Columbia. I. Hansen, Arnold J., 1932- . II. Jones, M.A. (Myrtle Anne), 

1952- . Ill Title. IV. Series. V. Series: U.S. Geological Survey professional paper; 1424.

GB1025.W2V33 1998 97-23883 

551.49'09797'7 dc21 CIP

ISBN 0-607-87047-8

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

Box 25286, Federal Center 

Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The RASA Program represents a systematic effort to study a number of 

the Nation's most important aquifer systems, which, in aggregate, underlie 

much of the country and which represent an important component of the 

Nation's total water supply. In general, the boundaries of these studies are 

identified by the hydrologic extent of each system and, accordingly, transcend the political subdivisions to which investigations have often arbitrarily been limited in the past. The broad objective for each study is to 

assemble geologic, hydrologic, and geochemical information, to analyze and 

develop an understanding of the system, and to develop predictive capabilities that will contribute to the effective management of the system. The use 

of computer simulation is an important element of the RASA studies to 

develop an understanding of the natural, undisturbed hydrologic system 

and the changes brought about in it by human activities and to provide a 

means of predicting the regional effects of future pumping or other stresses. 

The final interpretive results of the RASA Program are presented in a 

series of U.S. Geological Survey Professional Papers that describe the 

geology, hydrology, and geochemistry of each regional aquifer system. Each 

study within the RASA Program is assigned a single Professional Paper 

number beginning with Professional Paper 1400.

Mark Schaefer 

Acting Director

CONTENTS

Abstract....................................................................................

Introduction ..............................................................................

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

Previous Investigations.......................................................

Regional Setting........................................................................

Physical Setting ..................................................................

Cultural Setting ..................................................................

Geologic Setting..................................................................

Hydrogeology............................................................................

Hydrogeologic Units............................................................

Ground-Water Flow System ................................................

Background...................................................................

Water-Level Configurations ..........................................

Hydraulic Characteristics....................................................

Hydraulic Conductivity ................................................

Transmissivity..............................................................

Specific Capacity...........................................................

Recharge.............................................................................

Background...................................................................

Previous Results and Relations Between Water-Budget 

Components............................................................

Estimates of Mean Annual Recharge ............................

Reliability of Estimates ................................................

Water-Quality Characteristics .............................................

Cross-Sectional Numerical Models of Ground-Water Flow ........

Page Page

Dl Model Grid Systems............................................................. 46

1 Boundary Conditions ........................................................... 49

2 Hydraulic Characteristics..................................................... 50

5 Calculated Flow Systems ..................................................... 50

7 Hydrologic Controls....................................................... 51

7 Hydraulic-Head Gradients............................................. 52

9 Ground-Water Flowpaths and Traveltimes .................... 53

10 Evaluation of the Flow Systems .................................... 54

17 Calculated Water-Budget Components ................................ 55

17 Total Ground-Water Flow ............................................. 55

20 Partitioning of Ground-Water Flow ............................... 57

21 Hydrologic Controls on Discharge ................................. 57

22 Discharge to Saltwater Bodies ....................................... 59

26 Vertical Movement of Water ......................................... 59

26 Conceptual Model of the Aquifer System ................................... 60

30 Description of Conceptual Model......................................... 61

30 Glacial-Interglacial Sequence ......................................... 61

30 Surficial Semiconfining Unit.................................. 61

31 Confining Unit...................................................... 61

	Fraser and Puget Aquifers .................................... 67

31 Alluvial Valley Aquifers ................................................. 67

37 Recessional Deposits...................................................... 67

40 Application of Conceptual Model......................................... 67

43 Summary and Conclusions ......................................................... 71

45 Selected References ................................................................... 73

ILLUSTRATIONS

PLATE 1. Cross-sectional model grid systems with boundary conditions and calculated hydraulic-head distributions for detailed and

aggregated aquifer system geometry, Puget Sound aquifer system, Washington and British Columbia In pocket 

FIGURES 1-2. Maps showing:

1. Location of the Puget Sound Lowland and the Willamette Lowland, which make up the Puget-Willamette Lowland

regional aquifer-system study area ........................................................................................................................ 3

2. Location and features of the Puget Sound Lowland, Washington and British Columbia ............................................ 4

3. Graphs showing climate characteristics for selected areas in the Puget Sound Lowland (A) water-year precipitation, 

(B) mean monthly distribution of precipitation, (C) daily minimum and maximum air temperature, and (D) daily 

mean solar and net radiation ........................................................................................................................................ 8

4-5. Diagrams showing:

4. Generalized tectonic features ...................................................................................................................................... 11

5. Framework for the regional correlation of hydrogeologic and stratigraphic units for the Puget Sound Lowland,

(A) Fraser-Whatcom Basin, (B) north-central Puget Sound Lowland, and (C) southern Puget Sound Lowland ... 12

6. Map showing generalized surficial geology of the Puget Sound Lowland and location of three major structural basins or

subareas........................................................................................................................................................................ 15

7. Diagram showing idealized sequence of unconsolidated deposits in the Puget Sound Lowland ......................................... 16

8. Map showing generalized thickness of the unconsolidated deposits composing the Puget Sound aquifer system .............. 18

9. Graph showing linear relations between water-level altitude and land-surface altitude .................................................... 24

10-11. Maps showing:

10. Generalized pattern of ground-water movement for the Puget Sound aquifer system ................................................. 25

11. Location of 26 basins or areas where recharge was estimated by investigators using daily climate data and 7 basins where

recharge, estimated during this study, is compared to streamflow ......................................................................... 34
VI CONTENTS

FIGURES 12. Graph showing relations between mean annual precipitation and recharge for 26 basins or areas ....................................

13. Map showing estimated distribution of mean annual ground-water recharge for the Puget Sound aquifer system ..........

14. Graph showing estimated distribution of annual recharge at two weather stations, representative of small- and largeprecipitation areas, for two different surficial geologic units .........................................................................................

15. Map showing the lateral extents of the surficial regional hydrogeologic units ...................................................................

16. Relation between stratigraphic units and regional hydrogeologic units for the Puget Sound aquifer system, (A) FraserWhatcom Basin, ( B) north-central Puget Sound Lowland, and (C) southern Puget Sound Lowland ............................

17. Idealized section showing examples of extent and names of regional hydrogeologic units for (A) section E-E' and (B) 

section F-F \..................................................................................^

18. Diagrams showing geometry of aquifer and confining units and model grid system with boundary conditions for section

G-G '.Fraser-Whatcom Basin ......................................................................................................... ...............................

19. Diagrams showing model-calculated hydraulic heads and estimated and observed water levels for section G-G ', FraserWhatcom Basin ............................................................................................................................................................

36

38

43

62

63

66

69

70

TABLES

TABLE 1. Availability of well information for the Puget Sound Lowland in the U.S. Geological Survey's National Water Information

System as of 1990 ......................................................................................................................................................... 22

2. Descriptive statistics for:

A. Well information .................................................................................................................................................. 23

B. Well information and regression information ........................................................................................................ 23

3. Summary of published estimates of horizontal hydraulic conductivity ............................................................................ 27

4. Summary of published estimates of ground-water recharge calculated using daily climate data ....................................... 32

5. Information and statistics on the relation between water-budget components ................................................................. 35

6. Estimates of mean annual recharge on the basis of mean annual precipitation, generalized surficial geology, and land-use

and cover categories...................................................................................................................................................... 39

7. Information for comparison of streamflow and estimated recharge for seven basins ......................................................... 41

8. Summary of historical ground-water-quality data ............................................................................................................ 44

9. Relation between model layers and local hydrogeologic units and the potential relations between the hydrogeologic and

stratigraphic units......................................................................................................................................................... 47

10. Information for the model grid systems for six cross-sectional ground-water flow models ................................................. 48

11. Ranges in recharge estimates and hydraulic characteristics for six cross-sectional ground-water flow models ................... 49

12. Model-calculated water-budget components for six cross-sectional ground-water flow models .......................................... 56

13. Model-calculated percentage of recharge that moves downward from above the identified unit to underlying units and moves

upward from below the identified unit to overlying units ............................................................................................. 60
VII

CONVERSTION FACTORS AND VERTICAL DATUM

Multiply By To obtain

inch (in.) 2.54

foot (ft) 0.3048

mile (mi) 1.609

acre 0.4047

square foot (ft 2) 0.0929

square mile (mi 2) 2.590 

acre-foot (acre-ft) 1,233.50

cubic mile (mi 3) 4.168

inch per year (in/yr) 2.54

foot per second (ft/s) 0.3048

foot per foot (ft/ft) 1.0

foot per mile (ft/mi) 0.1894

foot per day (ft/d) 0.3048

foot squared per second (ft2/s) 0.0929

foot squared per day (ft2/d) 0.0929

cubic foot per second (ft3/s) 0.02832

cubic foot per second per mile [(ft3/s)/mi] 0.0176

cubic foot per second per square mile [(ft3/s)/mi2] 0.01093

million gallons per day (Mgal/d) 0.04381

gallon per minute (gal/min) 0.06308

_____gallon per minute per foot [(gal/min]/ft] 0.2070

centimeter

meter

kilometer

hectare

square meter

square kilometer

cubic meter

cubic kilometer

centimeter per year

meter per second

meter per meter

meter per kilometer

meter per day

meter squared per second

meter squared per day

cubic meter per second

cubic meter per second per kilometer

cubic meter per second per square kilometer

cubic meter per second

liter per second

liter per second per meter__________

TEMPERATURE

To correct temperature given in this report in degrees Fahrenheit (°F) to degrees Celsius (°C), use the following equation:

°C = 5/9( °F-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.

REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND Dl

HYDROGEOLOGIC FRAMEWORK OF THE

PUGET SOUND AQUIFER SYSTEM, 

WASHINGTON AND BRITISH COLUMBIA

ByJJ. VACCARO, A.J. HANSEN, JR., and M.A. JONES

ABSTRACT

The Puget-Willamette Lowland is located in western Washington, 

western Oregon, and a small part of southwestern British Columbia, 

Canada. The aquifer systems contained within the Puget-Willamette 

Lowland are within two distinct areas the Puget Sound Lowland and 

the Willamette Lowland. The study area for this report is the Puget 

Sound Lowland, which encompasses 17,616 square miles, 2,556 square 

miles of which is saltwater. The Puget Sound aquifer system underlies 

7300 square miles (41 percent) of the study area of which about 

117 square miles is overlain by surface-water bodies. This report 

describes the hydrogeologic framework of the Puget Sound aquifer 

system.

Alluvial, glacial, and interglacial sediments of Quaternary age 

compose the aquifer system. The lateral extent of the aquifer system 

generally is delineated by the extent of the glacial drift of the youngest 

glaciation, the Fraser Glaciation. Excluding large lakes, of the Quaternary deposits present at land surface, alluvial deposits underlie 

1,570 square miles, fine-grained deposits underlie 3320 square miles, 

and coarse-grained deposits underlie 2,293 square miles. The coarsegrained deposits and alluvial deposits generally compose aquifer 

units, and the fine-grained deposits compose semiconfining to confining units. An alternating pattern of coarse- and fine-grained deposits 

occurs from land surface to depths locally of more than 3,300 feet. Tertiary and older rock units define the lateral and basal boundaries of the 

aquifer system, and together these units are called the basement confining unit.

Ground-water movement is from topographic highs to topographic lows. The direction of ground-water flow is predominantly 

lateral in aquifer units and vertical in the other semiconfining and confining units. Vertical gradients generally are downward, except near 

streams, rivers, and saltwater bodies, where they generally are 

upward. Ground water in the uppermost aquifer unit occurs under 

water-table conditions, and ground water in deeper units becomes 

increasingly confined with depth.

Flow within aquifer units indicates the existence of predominantly 

subregional flow systems. These systems are controlled by topography, the basement confining unit, and the extent of saltwater bodies; 

under the present hydrologic stress, there is no regional flow system.

Horizontal hydraulic conductivity of aquifer units generally 

ranges from about 10 to 700 feet per day, and median values are in the 

range of 15 to 50 feet per day. The vertical hydraulic conductivity of 

the glacial tills is estimated to range from about 0.001 to 0.01 feet per 

day, and the vertical conductivity of the fine-grained clayey interglacial 

units is on the order of 0.0001 feet per day.

Mean annual recharge to the aquifer system was estimated to 

range (using linear regression) spatially from nearly 0 to more than 

70 inches per year and to average about 27 inches per year. The

average annual precipitation, surficial geology, and land use and cover 

are dependent variables in linear regression.

The quality of water in the aquifer system is suitable for most uses. 

Dissolved solids generally are less than 150 milligrams per liter. Dissolved nitrate generally is less than 1 milligram per liter; locally large 

concentrations are attributed to land-use practices. Seawater has 

intruded some aquifers near the coast.

Six cross-sectional numerical models of ground-water flow were 

constructed to provide information to describe the major hydrologic 

and geologic controls on the ground-water flow system, to estimate the 

partitioning of recharge into discharge components, and to develop 

and test a conceptual model of the aquifer system. Information presented in the report and the results of the models together indicated 

that a simplified conceptual model of the aquifer system could be 

developed from a regional perspective. The simplified conceptual 

model allows for a description of the ground-water flow system. The 

conceptual model provides a framework in which regional hydrogeologic units could be identified.

On the basis of the conceptual model, regional glacial and interglacial aquifer, semiconfining, and confining units and alluvial valley 

aquifers were defined for the Puget Sound aquifer system. These 

regional hydrogeologic units consist of nine alluvial valley aquifers, 

the surficial semiconfining unit, the Fraser aquifer, the confining unit, 

the Puget aquifer, and the basement confining unit. Ground water in 

the aquifer system can be adequately described using these regional 

units. The alluvial valley aquifers constrain flow within the underlying 

and adjacent glacial deposits.

INTRODUCTION

The U.S. Geological Survey initiated a Regional 

Aquifer-System Analysis (RASA) Program in 1978 in 

response to congressional concerns about the availability and quality of the Nation's ground water. The purpose of the RASA Program is to aid in the effective 

management of important ground-water resources by 

providing information on the hydrogeology and 

geochemistry of regional aquifer systems, as well as 

providing analytical capabilities necessary to assess 

management alternatives (Sun, 1986). The aquifer systems in the Puget-Willamette Lowland were chosen to 

be studied as part of this program.

Dl
D2 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

To meet the overall RASA Program goals, the major 

objectives of the Puget-Willamette Lowland study were 

to: (1) describe the geologic framework of the regional 

aquifer systems, (2) describe the hydrogeologic characteristics of the regional aquifer systems, (3) describe the 

regional ground-water flow system and its major 

hydrologic controls for each subarea, (4) estimate the 

water budgets for selected areas and use this information to describe the regional water budget, and (5) provide for a synthesis of knowledge of the two regional 

ground-water flow systems. A detailed description of 

the purpose of the study is given by Vaccaro (1992).

The Puget-Willamette Lowland regional aquifer-system study area is located in western Washington, western Oregon, and a small part of southwestern British 

Columbia, Canada (fig. 1). The study area is contained 

within a structural basin that extends from near the 

Fraser River, British Columbia, Canada, at about 

49 degrees, 15 minutes latitude, to just south of Cottage 

Grove, Oregon, at about 44 degrees latitude. The PugetWillamette Lowland study area includes about 

23,290 mi2. The study area is comprised of two distinct 

subareas, the Puget Sound Lowland and the Willamette 

Lowland.

The study area for this report is the Puget Sound 

Lowland (fig. 2), which encompasses 17,616 mi2, about 

2,556 mi2 of which is saltwater. The study area includes 

most of the Puget Sound drainage basin, which includes 

part of the Cascade Range and the Olympic Mountains, 

a small part of the Fraser River Basin in Washington and 

British Columbia, Canada, and small areas both south 

and southwest of the Puget Sound drainage. The part 

of the Puget Sound drainage basin not included is about 

400 mi2 of the upper Skagit River Basin north of Ross 

Lake in Canada (fig. 2).

The lateral extent of the principal aquifer units in the 

Puget Sound Lowland is defined by the area of outcrop 

of Quaternary sediments. These sediments underlie 

7,183 mi2 (41 percent) of the study area; this area does 

not include the extent of the sediments underlying saltwater and large lakes. The sediments, which are highly 

variable because of the area's complex geologic history, 

consist of alluvial, glacial, and interglacial deposits. 

These deposits consist principally of recent alluvium, 

recessional and advance outwash, till, and other glaciofluvial and interglacial sediments. The aquifer system 

contained in these sediments is herein named the Puget 

Sound aquifer system. Pre-Tertiary rock units and Tertiary sedimentary, volcaniclastic, volcanic, and metamorphic rock units overlie the remaining part of the 

study area (7,760 mi2), and these units define both the 

lateral and basal boundaries of the aquifer system.

About 70 percent of the population of Washington 

reside within the study area, mainly in the metropolitan

areas of Bellingham, Everett, Seattle and vicinity, 

Tacoma, and Olympia. A burgeoning population is 

increasing the demand for the available water. The 

population growth is mainly in the lowlands overlying 

the aquifer system, and the growing population not 

only has dramatically increased the demand for water 

but also has initiated conflicts over water supplies. In 

some areas, available water sources are already fully 

appropriated, and some supplies are limited because 

of contamination from human activities and saltwater 

intrusion from the Puget Sound and other saltwater 

bodies. Additionally, the population growth and the 

attendant changes in land use may detrimentally affect 

the quantity and quality of water that recharges the 

ground-water system.

The results of the Puget-Willamette Lowland study 

are presented in the U.S. Geological Survey Professional 

Paper 1424, Chapters A-D. Chapter A (Gannett and 

Caldwell, in press) presents the geologic framework for 

the Willamette Lowland aquifer system. The hydrogeologic framework for the Willamette Lowland aquifer 

system is presented in Chapter B (Woodward and others, in press). Chapter C (Jones, in press) presents the 

geologic framework for the Puget Sound aquifer system. This report, Chapter D, describes the hydrogeologic framework of the Puget Sound aquifer system and 

is based on a synthesis and analysis of background and 

hydrologic information.

PURPOSE AND SCOPE

This report presents the generalized hydrogeologic 

framework of the Puget Sound aquifer system. The 

framework includes a conceptual model of the division 

of the aquifer system into regional hydrogeologic units 

for describing on a regional basis the ground-water 

flow in the Puget Sound aquifer system of the Puget 

Sound Lowland. The conceptual model is based on an 

analysis of historical data and on results of crosssectional numerical models of ground-water flow.

This report first describes background information 

on the regional physical, cultural (including water use), 

and geologic setting. Local hydrogeologic units present 

in the Puget Sound Lowland are then described. The 

ground-water flow system is described on the basis of 

preliminary regional water-level configurations and 

generalized aquifer-system geometry. Next, the report 

presents and synthesizes information on hydraulic 

characteristics. Estimates of mean annual recharge to 

the aquifer system are then presented. This section 

describes methods that have been used to estimate 

recharge, provides results including relations among 

water-budget components, discusses the method used
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D3

123' 119° 117°

BRITISH COLUMBIA

45° \~

43

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972

SCALE 1:2,500,000 

25 50 75 100 MILES

50 75 100 KILOMETERS

FIGURE 1. Location of the Puget Sound Lowland and the Willamette Lowland, which make up the PugetWillamette Lowland regiona1 aquifer-system study area.
D4 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

47° -

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972

FIGURE 2. Location and features of the Puget Sound Lowland, Washington and British Columbia.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D5

to estimate recharge for this study and the results of 

using this method, and discusses the reliability of the 

results. An overview of the water-quality characteristics of the aquifer system is then presented on the basis 

of historical data and previous investigations.

To improve understanding of the ground-water flow 

system and the major controls on the system and to help 

develop a simplified conceptual model for describing 

ground-water flow based on the division of the aquifer 

system into regional hydrogeologic units, numerical 

ground-water flow models were constructed for hydrogeologic sections in six "type" areas. The type areas 

were selected to determine the hydrologic control or 

effects of the configuration of the older basement units 

(the boundaries of the aquifer system) and the topography on the flow system and to analyze the effects of the 

wide range in hydrogeologic settings present in the 

Puget Sound Lowland on ground-water flow. The construction of the cross-sectional flow models is 

described, and aspects of the flow systems along the 

sections are then analyzed on the basis of the results of 

the flow models. The analysis considers the movement 

of water near and under saltwater bodies; topography; 

the configuration of the basement; fine-grained units; 

ranges in hydraulic characteristics; and the hydrologic 

aspects of flow systems and water budgets. The analysis 

compares results that are based on using a detailed definition of local hydrogeologic units (aquifer-system 

geometry) with results that are based on an aggregation 

of those hydrogeologic units. Last, a conceptual model 

for describing regional flow is described, and an example of the application of the model is presented.

For the purposes of this regional analysis and in the 

descriptions used in this report, certain terminology is 

used. The term "local" is used to describe flow or flow 

systems that are affected or controlled by topography 

and include lengths ranging from a few hundreds of 

feet to about 5 to 10 mi and areas ranging up to about 

10 mi2 (encompassing areas that are on the order of 

about 0.1 percent of the total area of the aquifer system). 

The term "regional" is used in two ways. In the first 

way, "regional" is used throughout the report to denote 

(as necessitated by the purpose of this study) the 

regional scale of this study; thus, "regional aquifer system," "regionally representative value," and similar 

terms are used throughout the report. In the second 

way, "regional flow system" is used to denote flow or 

flow systems that contain flowpaths with lengths on the 

order of the average width of the aquifer system, about 

40 mi. What may be a regional flowpath or system in a 

local study probably would be described as a subregional (intermediate) path or system in this study. This 

usage of "regional" generally is used in discussions of 

local in contrast to regional flow systems and paths.

PREVIOUS INVESTIGATIONS

The work of numerous investigators over the last 

100 years has provided the basis for the study of the 

Puget Sound aquifer system. A list of the pertinent 

studies (Jones, 1991) was compiled as part of this investigation to assimilate the available information; many of 

the previous investigations are listed in the "Selected 

References" section of this report. A short summary of 

selected previous investigations related to geology, surface water, and ground water is given in the following 

paragraphs.

Willis (1898) provided the initial geologic overview 

of the sediments in the Puget Sound Lowland; he proposed the glacial origin of the Puget Sound and presented sedimentologic evidence for two continental 

glaciations. These he named the Vashon and Admiralty 

Glaciations, which were separated by the Puyallup 

interglaciation. Bretz (1910, 1911, 1913) surveyed the 

lowland in some detail, summarized the stratigraphic 

relations, included additional evidence for at least two 

continental glaciations (including more pre-Vashon sediments), and further concluded that the arms of Puget 

Sound were glacially modified river channels. Weaver 

(1937) established the geologic framework for further 

studies related to the Quaternary and Tertiary history of 

the area. Two later studies (Hansen and Mackin, 1949; 

Crandell and others, 1958) established that there was 

more than one pre-Vashon Glaciation and that there 

were four glaciations within the southern part of the 

Puget Sound Lowland. Later studies by Armstrong and 

others (1965), Easterbrook and others (1967), Easterbrook 

(1968), Thorson (1980), Booth (1984), and Blunt and others (1987) have greatly added to the understanding of 

the glacial history of the Puget Sound Lowland. More 

detailed information on the previous investigations of 

the geologic setting of the study area is given by Jones 

(in press).

Most of the earlier surface-water investigations in 

the study area analyzed the potential for hydropower 

development. Later investigations were more concerned with streamflow statistics and their relation to 

the construction of dams for water supply. It was not 

until the early 1970's that streamflow characteristics of 

selected streams were first analyzed. In his seminal 

paper, Gladwell (1970) described the relation between 

runoff and precipitation and watershed characteristics 

for western Washington. Starting with the reports of 

Hidaka (1973) for the Puget Sound region and Nassar 

(1973) for the Pacific Slope basins, the low-flow characteristics of streams in the Puget Sound Lowland were 

systematically analyzed. The later reports of Cummans 

(1977), Haushild and LaFrance (1978), and Williams
D6 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

(1987) documented the low-flow characteristics for 

much of the remainder of the Puget Sound Lowland.

Concurrently with these studies, Higgins and Hill 

(1973) described the stream-temperature data collected 

on all streams in Washington prior to 1968; the streamtemperature data were fitted by harmonic curves to 

define the annual time-temperature relation. The magnitude and frequency of flows were described by 

Bodhaine and Thomas (1964). As part of a Federal 

flood-insurance program, many streams in the area 

were mapped to estimate the recurrence intervals and 

stages of floods.

Water-quality and fisheries management issues also 

have spurred various surface-water investigations. For 

example, many counties established surface-waterquality management plans during the mid-1970/s. Fishery studies have mainly addressed instream-flow 

assessment and utilization of streams by native and 

anadromous fish. Williams and others (1975) cataloged 

streams in the Puget Sound region on the basis of 

salmon utilization, and Swift (1976,1979) calculated discharges of selected streams preferred by anadromous 

fish for spawning and rearing in Washington and the 

Puget Sound Lowland, respectively. The methods used 

by Swift were later supplanted by the more comprehensive Instream Flow Incremental Methodology (Bovee, 

1982). This method was used by Embrey (1987) in the 

Stillaguamish River Basin and by Embrey (1991) for the 

Puyallup River Basin. Additionally, Ebbert and others 

(1987) assessed the quality of the water in the lower 

Puyallup River Basin as part of the latter fishery study.

The need to manage surface-water runoff spurred 

several investigations. Prych and Ebbert (1986) evaluated the quantity and quality of runoff based on data 

collected during 1979 to 1982 in three urban catchments 

in Bellevue, Washington. Dinicola (1990) developed 

regional rainfall-runoff relations for King County, 

Washington, during a watershed modeling study of five 

basins with 21 streamflow sites in King County. These 

regional relations were later used in another King 

County study of 11 basins (R.S. Dinicola, U.S. Geological Survey, oral commun., 1992) and were tested in 

studies in Thurston (Berris, 1995) and Pierce Counties 

(Mastin, 1996).

The principal surface-water inflow into Puget Sound 

was described by Williams (1984) in a map report. 

Many of the streamflow characteristics described by 

Williams were later summarized in two reports (Williams and others, 1985a,b). These two reports document 

the streamflow statistics and drainage-basin characteristics for all streams in the Puget Sound region that have 

been measured continuously by the U.S. 

Geological Survey.

Appraisals of the ground-water resources in the 

study area were begun as early as 1902 (Byers, 1902). A 

few years later, Landes (1905) compiled information on 

municipal water supplies, deep wells, and springs of 

Washington State on a county basis. Over the next 

45 years there were few investigations, and the primary 

ground-water evaluations were the localized studies of 

Blair (1929), Piper (1930), Pardee (1931), Piper and 

La Rocque (1938), Robinson and Piper (1942), and Newcomb and others (1949). During the 1950's and 1960's, 

the U.S. Geological Survey, in cooperation with Washington State agencies, completed a series of areal 

ground-water studies that covered much of the Puget 

Sound Lowland. These studies were the first attempts 

to systematically categorize and quantify ground-water 

resources on a large scale. In some instances the same 

areas were studied by several investigators (see for 

example, Garling and others, 1965; Deeter, 1979; Hansen 

and Bolke, 1980). During the 1970's, the U.S. Geological 

Survey completed ground-water studies of nine Indian 

reservations in the Puget Sound Lowland (for example, 

Lum and Walters, 1976; Drost, 1979a,b; Walters and others, 1979; Drost, 1983b; Lum, 1984). Many localized 

studies have been completed by private companies for 

municipal well fields and ground-water contamination 

sites; these studies provide a wealth of site-specific 

information.

The first attempt to quantify the ground-water 

resources of the entire Puget Sound area was completed 

during the 1970's. The Puget Sound Task Force (1970) 

determined that the availability of ground water in the 

Puget Sound Lowland should be based on estimates of 

natural recharge to aquifers by the infiltration of precipitation. The Pacific Northwest River Basins Commission 

(1970) analyzed the Quaternary aquifer system in the 

Puget Sound Lowland as part of a comprehensive study 

of water and related lands of the Pacific Northwest 

region. Foxworthy (1979) provided an appraisal of the 

ground-water resources of both the Puget Sound region 

and the Willamette Valley in Oregon.

Countywide ground-water studies recently have 

been completed for San Juan County (Whiteman and 

others, 1983) and Island County (Sapik and others, 1988). 

During the 1980's and 1990's, the U.S. Geological Survey and other investigators, in cooperation with the 

State of Washington Department of Ecology, began a 

series of ground-water studies as part of the State of 

Washington Ground Water Management Area Program 

(GWMA). Halstead (1986) described the ground-water 

system for most of the part of the Puget Sound Lowland 

that lies within Canada.

Walters (1971) conducted a reconnaissance of seawater intrusion into the coastal aquifers in the Puget 

Sound Lowland during 1966-68; he identified local
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D7

areas that were intruded with seawater. Dion and Sumioka (1984) reanalyzed Walters' work in 1978 to determine if the quantity and occurrence of seawater 

intrusion in the Puget Sound Lowland had increased; 

they concluded that the conditions in 1978 were similar 

to those of 1968.

REGIONAL SETTING

Pertinent background information on the regional 

setting is presented in the following sections. The overall physical setting is first described, including physiographic, climatic, and hydrologic information. Next, 

the regional cultural setting is described; this description includes a discussion of water use in the region. 

Last, the regional geologic setting is presented, providing the background information that is necessary for 

understanding the description of hydrogeologic units.

PHYSICAL SETTING

The Puget Sound Lowland is bordered by the Fraser 

River and the international boundary on the north, the 

Cascade Range on the east, and the Olympic Mountains 

on the west. North of the Olympic Mountains, the 

western boundary is defined by the Canadian-Washington border located in the waters south and east of Vancouver Island, British Columbia. The southern 

boundary of the Puget Sound Lowland is approximately defined by the extent of the Pleistocene glaciation, which is characterized by a series of low hills near 

the divide between the Puget Sound drainage basin and 

the Chehalis River Basin; however, for this study, the 

Puget Sound Lowland was defined to include part of 

the Chehalis River Basin namely the drainages of the 

Black River (136 mi2), Scatter Creek (37 mi2), and the 

Skookumchuck River (101 mi2) to the south and part of 

the drainage of the Satsop River and several small 

creeks to the west and southwest (fig. 2).

The Puget Sound Lowland is an elongated basin that 

extends about 200 mi in a north-south direction; the 

lowlands range from about 15 to 80 mi in width and 

average about 40 mi. Geologic structure (Gower, 1978; 

Cheney, 1987; Jones, in press) appears to separate the 

aquifer system into discrete ground-water basins. 

These basins are drained by rivers that carry water from 

the adjacent mountains to lowlands and then to Puget 

Sound, Hood Canal, and other saltwater bodies that are 

inland arms of the Pacific Ocean.

The topographic features in the Puget Sound Lowland are a result of repeated continental and alpine glaciation that has affected both erosion and deposition. 

The lowlands typically are alluvial river valleys with

glacial outwash and till plains. These lowlands are separated from the bordering mountains by uplands with 

rolling hills and terraces. The transition from the lowlands to the uplands is abrupt in some areas; these areas 

are typified by steep bluffs. Numerous topographic features in the lowlands and uplands parallel the northsouth direction of past glacial advances. Except near 

the southern boundary, the transition from the uplands 

to the mountains is abrupt. Where the Olympic Mountains meet Hood Canal, there is an abrupt transition 

from the lowlands to the mountains.

Altitudes of the valley floors or lowlands range 

from sea level to about 500 ft. The uplands have altitudes ranging from 500 to about 1,500 ft. The mountains generally start at an altitude of about 1,500 ft and 

extend to the crests of the Cascade Range and Olympic 

Mountains. The altitude of the crest of the Cascade 

Range averages about 7,500 ft, but a series of stratovolcanoes in the Cascade Range rise from about 8,000 to 

14,000 ft above sea level. The altitude of the Olympic 

Mountains averages about 3,000 ft, with many peaks 

between about 6,000 and 8,000 ft above sea level.

The Puget Sound Lowland has a mid-latitude 

humid, Pacific Coast marine climate because of the easterly moving air masses from the Pacific Ocean. The 

adjacent mountains provide protection from the southward-moving Canadian cold-air masses and from the 

Pacific Ocean winter storms. As a result, both summer 

and winter temperatures are moderated; there is a distinct winter precipitation season and summer dry season; and altitude and location have a large effect on the 

distribution of precipitation and temperature. Climate 

characteristics for selected areas of the Puget Sound 

Lowland are shown in figure 3.

Within the lowlands, mean annual precipitation varies from about 16 to 19 in/yr in the "rain shadow" 

northeast of the Olympic Mountains in northeastern 

Clallam County to about 53 in/yr near Olympia, Washington (fig. 3A). In the Cascade and Olympic foothills, 

the precipitation ranges from 45 to about 60 in/yr, with 

the larger values being more typical of the Olympic 

foothills. Throughout the mountains, at altitudes 

between about 1,500 and 2,500 ft, precipitation ranges 

from about 60 to more than 100 in/yr, and at higher altitudes, precipitation is generally more than 90 in/yr. 

Excluding saltwater areas, mean annual precipitation 

for the study area (about 15,000 mi2) is about 74 in. or 

about 82,000 ftVs.

About 80 percent of the precipitation falls during the 

winter season (October through March; fig. 3B). Winterseason precipitation generally is rain below altitudes of 

about 1,000 ft, is both rain and snow at altitudes 

between about 1,000 to 1,500 ft, and is snow at higher 

altitudes. The summers are relatively dry; average July
D8 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

10.0

9.0

8.0

-- 7.0

 2 6.0

5.0

4.0

3.0

2.0

1.0

B.

n Olympia (fig. 2)

 PortTownsend (fig. 2)

i, I

1876 1886 1896 1906 1916 1926 1936 1946 1956 1966 1976 1986 

Water year

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. 

Month

1 31 61 121 151 181 211 241 

Day of 1992 water year

331 361

Solar radiation 

Net radiation

1 31 61 91 121 151 181 211 241 271 301 331 361 

Day of 1992 water year

FIGURE 3. Climate characteristics for selected areas in the Puget Sound Lowland (A) water-year precipitation, (B) mean monthly 

distribution of precipitation, (C) daily minimum and maximum air temperature, and (D) daily mean solar and net radiation.

precipitation in the lowlands generally is less than 

about 0.75 in. (fig. 3B).

Mean annual daily maximum air temperatures 

range from about 60 °F in the lowlands to about 47 °F in 

the mountains, and mean annual daily minimum temperatures range from about 40 °F in the lowlands to 

about 31 °F in the mountains (Puget Sound Task Force, 

1970). Generally, summer temperatures in the lowlands

range from 60 to 80 °F, and winter temperatures range 

from 30 to 50 °F (Puget Sound Task Force, 1970). Nighttime, below-freezing temperatures in the lowlands 

occur 20 to 90 days per year, and daytime freezing temperatures are generally limited to 10 to 20 days per year 

and occur during the winter (Puget Sound Task Force, 

1970). Daily minimum and maximum temperatures 

collected during this study for a pasture site just
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D9

southeast of Tacoma, Washington (fig. 3C), indicate the 

typical range and cycle in temperatures over a water 

year (October 1 to September 30) in the lowlands. Summer temperatures in the mountains are cooler than in 

the lowlands because they are moderated by the high 

altitudes; winter minimum temperatures in the mountains generally stay below freezing.

Solar radiation is the major control on evapotranspiration and thus on the regional water budget. Daily 

solar radiation is controlled by the time of year, landsurface slope and aspect, and cloudiness. The daily 

mean solar and net radiation over a water year measured at the pasture site southeast of Tacoma (fig. 3D) 

indicates the typical cycle in the lowlands. The incoming solar radiation provides a basis for the energy balance at the land surface, and net radiation provides a 

measure of that energy balance. The net radiation generally approximates the sum of sensible heat and 

evapotranspiration, and thus, the annual cycle of net 

radiation indicates the cycle and variations in 

evapotranspiration.

The Puget Sound Lowland can be divided into 20 

complete and 2 partial drainages or hydrologic units 

(U.S. Geological Survey, 1976). For the larger rivers, the 

surface-water and ground-water basin boundaries 

probably are equivalent because of bedrock configuration and topography. The median annual surface-water 

runoff is about 51 in. (50,000 ftVs), and median September runoff is about 22 in. (21,600 ftVs); these values represent runoff from about 13,355 mi2 of the Puget Sound 

Lowland (Williams, 1984), excluding Canada and the 

Chehalis River Basin in the Puget Sound Lowland. 

Runoff for the islands in the northern Puget Sound, 

which represent less than 3 percent of the total land area 

of the Puget Sound Lowland, averages about 10 in/yr 

(Puget Sound Task Force, 1970; Dietrich, 1975). However, runoff for most of the Puget Sound Lowland 

ranges from about 20 to 80 in/yr. The runoff in parts of 

the Puget Sound Lowland is controlled by the presence 

of 386 glaciers, which cover about 116 mi2 and contain 

about 13 million acre-ft (3.85 mi3) of water. The glaciers 

help to provide consistent summer flows, especially in 

the Skagit, Nooksack, Puyallup, Elwha, and Nisqually 

River Basins.

CULTURAL SETTING

Native Indian villages were located near rivers in 

the Puget Sound region prior to the first settlement by 

non-Indians in the mid-1800's. By 1845, only a few settlements had been established; these consisted of settlers in a small settlement near Olympia, scattered fur 

trappers, and Hudson Bay Company personnel. The

Territory of Washington was established in 1853, at 

which time the population was less than 5,000 settlers. 

Population increased to about 20,000 in 1870 and to 

about 350,000 in 1890. The population growth occurred 

along rivers in the lowlands and within urban settlements such as Seattle, Olympia, Bremerton, and 

Tacoma. The dominant economic activity at this time 

revolved around timber. The building of the railroads 

and the gold rush to Alaska spurred rapid development 

and increased population growth between about 1895 

and 1910. Many of the port and hydropower systems 

were established during this period. The rail and highway transportation systems allowed products to be 

shipped to market centers in other states and abroad. 

Additional growth in manufacturing resulted in a population in the Puget Sound Lowland of more than 

500,000 by 1930.

With the expansion of the military bases, wartime 

manufacturing, and the increasing development of timber and shipping resources in the 1940's, the population 

of the area expanded greatly. Rapid growth, from about 

800,000 people in 1960 to about 2,200,000 in 1979, 

resulted in the conversion of more forest lands to urban 

and nonurban settlements and to agricultural lands. In 

1967, cropland, urban, rural, and rangeland were the 

dominant land uses in the lowlands; however, forest 

land was the dominant land use mainly in that part of 

the lowlands bordered by the Hood Canal on the west 

and the Puget Sound on the east (fig. 2). The remainder 

of the lands in the Puget Sound Lowland at that time, 

some 80 percent, were still forest lands. Over 75 percent 

of these forest lands were, and still are, dedicated to 

commercial usage.

Population in the Puget Sound Lowland (excluding 

Canada) is expected to increase by more than 1.8 million from 1978 to 2020, for a total of more than 4 million. 

Moreover, while the population of the major cities has 

grown only slightly since the early 1970's, the 

population in the unincorporated parts of the counties 

has nearly doubled from 1970 to 1980. As a result of the 

current growth, croplands, rangelands, and forest lands 

are being converted to commercial, city, and urban-suburban residential lands. The growth also has resulted in 

some diversification of the economic base, from predominantly timber and aerospace to include shipping, 

computer software development, manufacturing, and 

agriculture.

Regional water-use information compiled in three 

studies is presented in the following paragraphs; note 

that hydropower use is not included. However, 

because these studies defined the water-use categories 

differently, a direct comparison of this historical wateruse information cannot always be made. The largest 

water uses in the Puget Sound Lowland are for
DID REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

municipal, agriculture, and industrial purposes; water 

use for livestock generally is small.

In 1965, agricultural water use was about 

202 Mgal/d; municipal use was 219 Mgal/d; and industrial use was about 431 Mgal/d, about 75 percent of 

which was associated with the manufacturing of paper 

(Pacific Northwest River Basins Commission, 1971). 

Nonmunicipal domestic water use was only about 

9 Mgal/d in 1965. Of the total water use in 1965 (about 

861 Mgal/d), about 85 percent was derived from surface-water sources, and the remainder (129 Mgal/d) 

from ground-water sources. On the basis of countywide information for 1975 (Dion and Lum, 1977), 

municipal water use appeared to be increasing and 

industrial water use decreasing for 1965-75. For example, Dion and Lum (1977) estimated that the 1975 

municipal use was about 461 Mgal/d and industrial use 

about 316 Mgal/d.

R.C. Lane (U.S. Geological Survey, written commun., 

1992) has estimated water use in the Puget Sound 

region during 1990 as part of a nationwide assessment 

of water use. Lane estimated a total withdrawal of 

810 Mgal/d for 1990 distributed by the following wateruse categories:

Water-use 

category

Domestic

Irrigation

Industrial

Commercial

Livestock

Quantity 

(million 

gallons per day)

411

163

131

97

8

These estimates were for the part of the Puget Sound 

Lowland within the United States. About 497 Mgal/d 

of the total was from public supplies, of which about 

35 percent (174 Mgal/d) was provided by ground-water 

sources and about 65 percent was provided by surfacewater sources. The remaining 313 Mgal/d was privately supplied 57 percent (178 Mgal/d) from 

ground-water sources and 43 percent (135 Mgal/d) 

from surface-water sources. Whereas Dion and Lum 

(1977) considered industrial use to include most commercial use and considered municipal use to include 

principally domestic use, Lane has separated these 

quantities. However, industrial and commercial water 

use still appears to be declining, as does irrigation 

water use. Although domestic (municipal) water use 

appears to have decreased from 1975, Dion and Lum 

(1977) included other uses in their municipal category.

From 1965 to 1990, there has been an estimated 

reduction in total water use. This may be due to the

reduction of large industrial uses such as the manufacturing of paper resulting from the conversion and 

diversification of the economic base of the region. 

However, for 1990, the estimate of water supplied from 

ground-water sources (352 Mgal/d 545ftVs) was 

nearly three times the quantity supplied from groundwater sources in 1965. Additionally, for 1988, the estimate of water supplied from ground-water sources for 

the part of the Puget Sound Lowland in Canada was 

about 182 Mgal/d (281 ftVs) (B.C. Environment, 1993). 

Thus, about 532 Mgal/d or 826 ftVs of ground water 

was withdrawn for all uses.

As population and industrial growth continues, 

more water is being derived from ground water rather 

than from surface water, which is nearly fully appropriated. Much of the unappropriated surface water is far 

from the locations of need, and most new and smaller 

water-supply systems are using ground water as the 

source. The source of future supplies for the large and 

expanding municipal areas is not defined and is being 

vigorously debated.

GEOLOGIC SETTING

The regional geologic setting of the PugetWillamette Lowland is largely a function of plate tectonics. The western part of the Pacific Northwest is a 

region where the North American continental plate has 

been converging with the oceanic plates of the north 

Pacific Basin (fig. 4) throughout much of Cenozoic time 

(Atwater, 1970; Engebretson and others, 1985). Thus, the 

large-scale regional topography and geology are the 

products of tectonic convergence of the plates.

The Cascade Range is an active magmatic arc 

formed by the continuing subduction of the Juan De 

Fuca plate beneath the North American plate (fig. 4). 

Most of the western edge of the Cascade Range is 

bounded at depth by marine sedimentary and volcanic 

complexes of Eocene through Miocene age that have, in 

part, been accreted to the North American plate (Cady, 

1975; Adams, 1984; Snavely, 1988). The Olympic Mountains, for example, are part of an exotic terrane, a remnant of the upper part of the descending oceanic plate 

that was scraped off the subducting plate, became 

attached to the leading edge of the North American 

plate, and then was uplifted; the mountains lie on the 

trench-slope break and are part of the trench system. 

The Puget-Willamette Lowland is a discontinuous valley that is a forearc basin, which commonly lies between 

the trench and magmatic arc (Dickinson, 1976) (fig. 4).

Pleistocene glaciation modified the regional geologic 

setting of the Puget Sound Lowland, and there have 

been several recognized glaciations and interglaciations
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM Dll

WEST EAST

OLYMPIC MOUNTAINSCOAST RANGE

PUGET-WILLAMETTE 

LOWLAND

CASCADE RANGE

Magmatic arc

Trench-slope 

break

Backarc

fold-thrust belt

JUAN DE FUCA PLATE

NOT TO SCALE

FIGURE 4. Generalized tectonic features. Modified from Dickinson (1976).

(fig. 5). The Cordilleran ice sheet, which originated in 

the Coast Range in British Columbia, advanced into the 

Puget Sound Lowland at least four times, and several 

partial glacial advances have been identified in the 

northern part of the Puget Sound Lowland (Crandell 

and others, 1958; Armstrong and others, 1965; Blunt and 

others, 1987). For example, during the Sumas Stade of 

the Fraser Glaciation (fig. 5), the ice sheet readvanced a 

short distance into the northern Puget Sound Lowland 

about 11,500 years BP (before present) and totally 

retreated by about 9,500 years BP (Blunt and others, 

1987).

The major advances of the ice sheets occupied most 

of the lowland areas and have extended laterally into 

the lower mountain valleys. About 18,000 years BP 

during the Vashon Stade, the last major advance of the 

Cordilleran ice sheet reached the northern limit of the 

Puget Sound Lowland and divided into two lobes 

(Thorson, 1980). The Juan de Fuca Lobe advanced westward about 120 mi along the Strait of Juan de Fuca to a 

Pacific Ocean tidewater terminus, and the Puget Lobe 

advanced southward about 95 mi into the Puget Sound 

Lowland between the Cascade Range and the Olympic 

Mountains. The Puget Lobe reached its maximum 

advance near the southern limit of the Puget Sound 

Lowland about 14,500 years BP, stagnated, and then 

began to retreat (Thorson, 1980; Blunt and others, 1987).

At its maximum extent, the Puget Lobe was about 60 mi 

wide, nearly 4,000 ft thick, and had a volume of about 

3,350 mi3 (Thorson, 1980,1989). The Puget Lobe retreat 

allowed for a series of ice-dammed proglacial lobes to 

form in much of the lowlands. The Juan de Fuca Lobe 

retreated from the Strait of Juan de Fuca by about 

14,500 years BP, allowing for the later draining of the 

proglacial lakes and the incursion of marine waters into 

the Puget Sound Lowland. The Puget Lobe probably 

retreated from its terminal position to near the latitude 

of Seattle, between 13,500 and 14,500 years BP (Thorson, 

1980; Blunt and others, 1987). The Puget Lobe retreated 

about twice as fast as it has advanced (Booth, 1987). In 

turn, during this rapid final retreat of the Puget Lobe, 

the incursion of marine water allowed for floating and 

melting of the glaciers and the deposition of the glaciomarine drift during the Everson Interstade between 

about 11,000 to 13,500 years BP (fig. 5) (Blunt and others, 

1987).

Alpine glacial advances originating in the Cascade 

Range and the Olympic Mountains also extended into 

the Puget Sound Lowland (Booth, 1987; Thorson, 1989). 

These alpine glacial advances generally were out of 

phase with the Cordilleran ice sheet. A good example 

of the alpine glacial advances and their asynchronous 

nature is from the Evans Creek Stade (fig. 5). With the 

onset of the Fraser Glaciation, alpine glaciers advanced
D12 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

A. Fraser-Whatcom Basin

Geologic framework

System

Quaternary

Age (K-A) 1

10 

12

20

60 

80 

>100

Geologic/climate units

Interglacial

Fraser 

Glaciation

Sumas Stade

Everson 

Interstade

Vashon 

Stade

Evans Creek 

Stade 5

Olympia Interglacial

Possession Glaciation

Whidbey Interglacial

Double Bluff Glaciation

Stratigraphy

Alluvium, marine 

deposits

Sumas Drift

Bellingham Drift/^

Everson <f Deming Sand 

"glaciomarine" \_

Drift2 Kulshan Drift^^x^

Fort Langley Formation^

Recessional outwash

Vashon Till

Advance outwash

Esperance Sand Sand 

Member

Quadra Cowichan Head 

Formation Formation^

Possession Drift 

Semiahmoo Drift7

Whidbey Formation

Double Bluff Drift

Pre-Quaternary rock units

1 Radioactive age based on the determination of the potassium 40 argon 40 ratio; 

>, greater than.

2 Also includes glaciofluvial sediments Everson sand (early Everson) and Everson 

gravel (late Everson).

3 Canadian name for Everson "glaciomarine" drift.

* Canadian name for Vashon deposits older than till, although in many locations 

the unit does not include the advance outwash.

5 Deposit of similar age and older than Evans Creek Stade generally not exposed 

in basin, inferred from well-log information and from some exposures in Canada.

6 Canadian name for Olympia Interglacial deposits.

7 Canadian name for pre-Olympia Interglacial deposits.

FIGURE 5. Framework for the regional correlation of hydrogeologic and stratigraphic 

units for the Puget Sound Lowland, (A) Fraser-Whatcom Basin, (B) north-central Puget 

Sound Lowland, and (C) southern Puget Sound Lowland. Location of areas shown in 

figure 6. Modified from D. Molenaar (U.S. Geological Survey, written commun., 1982), 

P.O. Haase (U.S. Geological Survey, written commun., 1988), Blunt and others (1987), 

and Glaster and Coombs (1989). Drift sequences separated by unconformities.

downward from the crest of the Cascade Range during 

this stade; this alpine glaciation was more pronounced 

in the northern Puget Sound Lowland than in the

southern Puget Sound Lowland. The alpine glaciers 

subsequently retreated before the advance of the continental glacier (Armstrong and others, 1965). Alpine
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D13

B. North-central Puget Sound Lowland

Geologic framework

System

Quaternary

Age (K-A)1

10 

12

20

60 

80 

>100

Geologic/climate units

Interglacial

Fraser 

Glaciation

Everson 

Interstade

Vashon 

Stade

Evans Creek 

Stade

Olympia Interglacial

Possession Glaciation

Whidbey Interglacial

Double Bluff Glaciation

?

Stratigraphy

Alluvium, marine 

deposits, lahars

Everson 

"glaciomarine" 

Drift2 ^^

.^^Partridge 

<^ Gravel 

^^^^ (recessional)

Recessional outwash ^^^^^^^

Vashon Till

Advance outwash

Esperance Sand Member

Lawton Clay

/ Pilrhurk Clay

Quadra Formation

Possession Drift

Whidbey Formation

Double Bluff Drift

Pre-Quaternary rock units

1 Radioactive age based on the determination of the potassium 40 argon 40 ratio; 

>, greater than.

2 Also includes glaciofluvial sediments Everson sand (early Everson) and Everson 

gravel (late Everson).

FIGURE 5. Framework for the regional correlation of hydrogeologic and stratigraphic 

units for the Puget Sound Lowland, (A) Fraser-Whatcom Basin, (B) north-central Puget 

Sound Lowland, and (C) southern Puget Sound Lowland. Location of areas shown in 

figure 6. Modified fromD. Molenaar (U.S. Geological Survey, written commun., 1982), 

P.C. Haase (U.S. Geological Survey, written commun., 1988), Blunt and others (1987), 

and Glaster and Coombs (1989). Drift sequences separated by unconformities Continued

glacial deposits are present at land surface mainly at the 

higher altitudes in the mountainous areas, predominantly in the southeastern part of the study area. In 

other areas, the Cordilleran ice sheet has overridden the 

alpine glacial deposits.

The lateral extent of the Puget Sound aquifer system 

is generally delineated by the extent of the drift deposited during the Fraser Glaciation (in particular, the Vashon Drift of the Vashon Stade). Except for the northern

Fraser-Whatcom Basin, most of the nonalluvial Quaternary deposits shown in the generalized surficial geologic map of the Puget Sound Lowland (fig. 6) belong to 

the Vashon Drift, which is relatively undisturbed. The 

Vashon Drift is associated with the advance and retreat 

of the Puget Lobe.

During the past and present interglacial periods, the 

major rivers emanating from the Cascade Range and the 

Olympic Mountains transported large quantities of
D14 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

C. Southern Puget Sound Lowland

Geologic framework

System

Quaternary

Age (K-A)1

10

12 

20

50- 

100

Geologic/climate units

Interglacial

Fraser 

_ Glaciation Vashon 

Stade

Evans Creek Stade

Olympia Interglacial

Upper 

Stade

Salmon 

Springs Interstade 

Glaciation

Lower 

Stade

Puyallup Interglacial

Stuck Glaciation

Alderton Interglacial

Orting Glaciation

Stratigraphy

Alluvium, 

Electron Mudflow 

Osceola Mudflow 

lahars, marine deposits

Alluvium

Steilcoom gravels

., . Recessional outwash

_ ... Moraine deposits

Vashon Till

Advance outwash

^~"~- _ Colvos Sand Member

Alluvium, Evans Creek Drift

\ Skokomish gravels

Kitsap Formation

\ "Penultimate Drift" 

\ Hyden Creek Drift2

Upper Salmon Springs Drift

Sediments, tephra

Wingate Hill Drift 2 

Lower Salmon Springs Drift

Puyallup Formation .°

m

Stuck Drift

Alderton 

Formation

E

Logan o 

Hill £ 

Formation 2 <i>

' i

Orting Drift =

Pre-Quaternary rock units

1 Radioactive age based on the determination of the potassium 40 argon 40 ratio; 

>, greater than.

2 Alpine glacial deposits generally located in mountainous areas of Cascade Range.

FIGURE 5. Framework for the regional correlation of hydrogeologic and stratigraphic 

units for the Puget Sound Lowland, ( A) Fraser-Whatcom Basin, ( B) north-central Puget 

Sound Lowland, and ( C) southern Puget Sound Lowland. Location of areas shown in 

figure 6. Modified fromD. Molenaar (U.S. Geological Survey, written commun., 1982), 

P.O. Haase (U.S. Geological Survey, written commun., 1988), Blunt and others (1987), 

and Glaster and Coombs (1989) Continued

sediments from the mountains to the lowlands. Alluvium was deposited along the large broad alluvial river 

valleys throughout the study area. Alluvial deposits in 

some of those valleys are more than 300 ft thick. The 

orientation of many of these valleys suggests not only 

that they were glacially derived but also that they functioned as massive outwash channels.

As a result of the Holocene-Pleistocene geologic history, the Quaternary deposits consist of nonglacial sediments and one to four regional drift sequences and

additional local sequences in parts of Whatcom County 

and British Columbia, Canada (the Fraser-Whatcom 

Basin); the sequences are generally separated by 

unconformities and by nonglacial fluvial and lacustrine 

sediments.

In a regional context, three types of deposits are 

associated with continental glaciers advance outwash, 

till, and recessional outwash. The lateral and vertical 

extents of each of these types of deposits are best known 

for the Vashon Drift (fig. 5). In turn, the rapid retreat of
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D15

122°

49° -

oAbbotsford 

FRASER-WHATCOM BASIN

WHATCGfV!

Mt Baker ':f

Mountains 

J 

JEFFERSON

-Mt Rainier

EXPLANATION

Quaternary System

I | Alluvial deposits

Fine-grained unconsolidated deposits 

\ Coarse-grained unconsolidated deposits 

Tertiary and older systems

\ | Basement confining unit

SCALE 1:1,500,000 

10 20

Boundary of major structural basin 

or subarea

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972

FIGURE 6. Generalized surficial geology of the Puget Sound Lowland and location of three major structural basins or subareas.
D16 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

the Puget Lobe is suggested by the extent of these types 

of deposits a rather thin till sheet, large quantities of 

advance outwash compared to recessional outwash, 

absence of a significant terminal moraine, and similar 

age of recessional deposits along the path of retreat 

(Thorson, 1989). Although not glacially derived, proglacial deposits also are associated with glaciers. Generally, extensive proglacial units, such as the Colvos Sand 

and the Esperance Sand (fig. 5), have been aggregated 

with the advance outwash deposits in other investigations. Fine-grained, silty-to clayey deposits generally 

are associated with interglacial or interstade periods; 

however, in some areas, the interglacial deposits are 

coarse grained. For the former case, the interglacial 

deposits generally are associated with deposition in 

bays or lakes, and for the latter case, the coarse-grained 

interglacial deposits generally are associated with fluvial depositional environments, typically along the 

periphery of the Quaternary deposits. Pre-Vashon age 

outwash, till, and interglacial type deposits generally 

are covered by deposits from younger glaciations and 

are more difficult to discern; exposures of these deposits 

are found along the coastline, cliffs, and erosional features. An idealized sequence of unconsolidated 

deposits in the Puget Sound Lowland is shown diagrammatically in figure 7.

Thus, at many locations there is a large sequence of 

alternating Quaternary fine-grained and coarse-grained 

deposits; the coarse-grained deposits generally are

aquifer units, and the fine-grained deposits generally 

are semiconfining to confining units. These Quaternary 

deposits compose the Puget Sound aquifer system; the 

lateral extent of these deposits is shown in figure 6. 

Excluding large lakes, of the 7,183 mi2 of unconsolidated deposits present at the land surface in the Puget 

Sound Lowland (fig. 6), alluvial deposits overlie 

1,570 mi2, coarse-grained deposits (mainly recessional 

and advance outwash) overlie 2,293 mi2, and finegrained deposits [mainly glacial till, but also parts of 

other deposits such as the Everson "glaciomarine" Drift 

(fig. 5) and mudflowsl overlie 3,320 mi2.

The interlayered drift and interglacial deposits overlie Tertiary and pre-Tertiary rocks in the Puget Sound 

Lowland. Generally, the pre-Tertiary rock units are the 

predominant units in about the northern one-half of the 

Puget Sound Lowland. These pre-Tertiary rock units 

and Tertiary marine and nonrnarine sediments and 

volcanic rock materials compose the base of the aquifer 

system, which is herein named the basement confining 

unit. The total volume of unconsolidated deposits in 

the study area overlying the basement confining unit is 

about 1,315 mi3; the unconsolidated deposits have an 

average thickness of about 400 ft (Jones, in press).

Because of the partial glacier advances in the northern Puget Sound Lowland and the timing of the retreats 

of the ice lobes, the geologic framework varies across 

the study area. Therefore, the major stratigraphic units 

(fig. 5) for the Puget Sound Lowland are presented by

Recessional outwash

Till

Advance outwash

Proglacial deposits

Interglacial deposits

Undifferentiated 

deposits

Bedrock

FIGURE 7. Idealized sequence of unconsolidated deposits in the 

Puget Sound Lowland.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D17

subareas the southern Puget Sound Lowland, the 

north-central Puget Sound Lowland, and the FraserWhatcom Basin (fig. 6). The second subarea represents 

a large structural basin that includes a combination of 

smaller structural basins (Jones, in press). The FraserWhatcom Basin (fig. 6) is a structural basin underlain by 

several thousands of feet of continental sediments. This 

basin is unique in that it is separated from the remainder of the study area by more than 20 mi of bedrock 

uplands, and there have been more glacier advances in 

the basin than in the remainder of the Puget Sound 

Lowland. The generalized thickness of unconsolidated 

deposits composing the Puget Sound aquifer system 

(fig. 8) shows the relation between the three subareas, 

bedrock, and thickness.

The Puget Sound aquifer system is thus comprised 

of unconsolidated deposits that are locally more than 

3,300 ft thick (Hall and Othberg, 1974; Yount and others, 

1985; Jones, in press) but average about 400 ft thick 

(Jones, in press). The aquifer system consists of materials deposited: (1) by at least four advancing and retreating continental glaciations; (2) by numerous alpine 

glaciers emanating from the adjacent Cascade Range 

and Olympic Mountains and by streams during the 

alpine glaciations; and (3) by streams during interglacial (and glacial) episodes when much of the lowland 

consisted of saltwater embayments and freshwater 

lakes, including the present interglacial period. Each of 

these depositional regimes resulted in complex lithostratigraphic patterns and corresponding geologic units 

that are difficult to correlate on a regional scale. The 

complexity within each depositional sequence was 

compounded by the erosive action of the continental 

glaciers, which largely altered the pre-existing topography and geology. As a result, the classification of 

regional hydrogeologic units and correlation of these 

units with previously named stratigraphic and hydrogeologic units is complex, but it is important for incorporating the results of previous investigations.

Revisions in the chronology of the Pleistocene stratigraphic units proposed by Blunt and others (1987) suggest that previous correlations of stratigraphic units 

might be incorrect. However, hydrogeologic units are 

not necessarily time correlated; that is, stratigraphic and 

hydrogeologic units may be different. Additionally, the 

nomenclature for the stratigraphic units presented in 

figure 5 is representative of what is the most commonly 

accepted usage for major units; note that the nomenclature used for Bainbridge Island (fig. 2) generally follows 

the usage for southern Puget Sound Lowland (fig. 5) 

and not the north-central Puget Sound Lowland. The 

actual number of named units is more than 100, many 

of which have been identified or named during local or 

small-scale geologic investigations. Jones (in press)

describes the relationship between some of these locally 

named units and (1) the major units presented in 

figure 5 and (2) regional units that are based on both 

depositional environment and lithology. This description applies only to the surficial units in the study area.

HYDROGEOLOGY

The following sections provide information on the 

hydrogeology of the Puget Sound aquifer system. This 

information is needed for developing a conceptual 

model to describe regional ground-water flow, for providing a framework for constructing numerical models 

of ground-water flow, and for synthesizing the available information for the aquifer system. Hydrogeologic 

units in the Puget Sound Lowland are first described. 

The ground-water flow system is then described by presenting general aspects of ground-water movement in 

the Puget Sound Lowland and pertinent background 

information. The regional flow system is described on 

the basis of generalized water-level configurations. 

Information on the hydraulic characteristics of the 

hydrogeologic units are presented and summarized; 

conclusions comparing representative regional values 

are discussed. Estimates of recharge and the methods 

used to derive them from both previous investigations 

and this study are described. Last, a summary of the 

historical water-quality characteristics is presented.

HYDROGEOLOGIC UNITS

For the purpose of this report, the Quaternary 

deposits in the Puget Sound aquifer system have been 

divided into aquifers and semiconfining to confining 

(nonaquifer) units. In comparison to confining units, 

semiconfining units generally have much smaller clay 

contents and, therefore, larger vertical hydraulic conductivities that allow for increased movement of water 

through them. In some locations, a confining unit may 

grade into a semiconfining unit either by a change in 

facies or by thinning. Thus, the difference between 

semiconfining and confining units is that the confining 

units generally are comprised predominantly of silt- to 

clay-size materials, whereas the silt- to clay-size materials in semiconfining units are generally less than about 

30 percent of the weight.

The fine-grained, semiconfining to confining units 

generally consist of till, lacustrine, glaciomarine, and 

mudflow deposits (see Jones, in press). The coarsegrained aquifer units generally consist of outwash 

deposited during glacial (including alpine) advances 

and retreats, proglacial deposits, or alluvium deposited 

during glacial interstades. Because the hydrogeologic
D18 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

BFilTISH COLUMRtA

o Abbe o for; 

f" f

CLALLA 

Olympic

Mountains

JEFFERSON

GRAYS 

HARBOR EXPLANATION

Thi« kness, in feet

I | 100 to 300 

I I 301 to 600

601 to 1,500 

[ 1 1,501 to 2,400 

^H Greater than 2,400 

I I Bedrock outcrop 

I | Area of no data

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972

FIGURE 8. Generalized thickness of the unconsolidated deposits composing the Puget Sound aquifer system. Modified from Jones (in press).
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D19

units are mapped in this manner, the hydrogeologic 

units identified herein do not necessarily correspond to 

geologic time-stratigraphic units identified in previous 

reports that discuss the geology of the area. For example, geologic units composing the water-table aquifer in 

one area may be as much as 100,000 years older than the 

units composing the water-table aquifer in another area. 

Relations between regional hydrogeologic units and 

stratigraphic units are presented in Jones (in press) and 

are described later in this report. A summarized 

description of the local hydrogeologic units is provided 

below. Symbols in parenthesis (al) are used in table 9 

later in this report.

1. (al) Recent (Holocene) alluvial deposits, present at 

the land surface, are represented by both small discontinuous deposits of alluvium scattered throughout the Puget Sound Lowland and extensive 

deposits in the broad alluvial valleys (fig. 6). 

Because of the latter, the alluvial deposits are 

treated as a separate hydrogeologic unit. This unit's 

lithology varies from fine grained to coarse grained. 

Where it is predominantly fine grained, lenses of 

sand and gravel found within the fine-grained facies 

function as aquifers. Thus, this unit generally can 

be classified as an aquifer to a semiconfining unit on 

the basis of its overall lithologic character. However, within the Puget Sound Lowland, this unit is 

more commonly coarse grained. Little information 

is available on the change in lithology of the unit 

with depth because of the sparse number of deep 

wells (greater than about 250 ft deep). This unit 

generally is described separately from other interglacial deposits in this report and also is described 

separately in discussion of the sequence of glacial 

and interglacial deposits.

2. (Qr) The uppermost (surficial) deposits of glacial origin generally consist predominantly of the recessional outwash deposited while the glaciers were 

retreating during the last glaciation. These deposits 

are predominantly coarse grained. Locally, the finegrained facies include recessional marine and icecontact deposits. The recessional deposits generally 

are discontinuous within the Puget Sound Lowland, 

but where the deposits are sufficiently extensive, 

coarse grained, and saturated, they are considered 

an aquifer unit.

3. (Qt) An extensive till sheet mantles a large part of the 

Puget Sound Lowland; about 40 percent of the surficial Quaternary deposits are till. This predominantly fine-grained (averaging about 17-percent 

clay) till unit is comprised of unsorted and

unstratified glacial sediments that vary in size from 

clay to boulder, in compaction, and in composition 

throughout the area. The till unit generally is classified as a nonaquifer (semiconfining) unit. However, 

where this unit is saturated, numerous shallow 

domestic wells obtain water from it. Although this 

unit is predominantly composed of the Vashon Till, 

about four other tills (including alpine glacial till) 

have been previously identified (Jones, in press).

4. (aq) Underlying the till unit are predominantly 

coarse-grained deposits. These deposits generally 

are associated with the deposits of the Vashon 

advance and proglacial periods, but in some areas 

these deposits are associated with Sumas, preFraser, and alpine glacial advance deposits. This 

unit was probably nearly continuous throughout the 

area at one time, but subsequent erosion has dissected it in many places. In some locations this unit 

is the surficial unit. Locally, this unit contains laterally mappable fine-grained zones and has been 

divided into a fine-grained (generally silty sand) and 

a coarse-grained unit. Where it has been divided on 

the basis of lithology, it has been classified as a 

sequence of aquifers and semiconfining units. 

Where the unit has not been divided in previous 

investigations, it has been classified as an aquifer 

unit.

5. (nonaq) The fine-grained interglacial or glacial and 

proglacial deposits are classified as nonaquifer units 

that typically function as semiconfining to confining 

units. Regionally, thick, laterally extensive deposits 

that are associated with interglacial periods generally are silt-to-clay units that function as confining 

units. These units may contain lenses of sand or 

gravel that locally function as aquifers. Additionally, in some areas there is a 20- to 100-ft thick distinct sequence of a clay-rich unit underlain by 

coarse-grained deposits, which in turn are underlain 

by another clay-rich unit. Along the periphery of 

the aquifer system, near the Olympic Mountains 

and Cascade Range, the interglacial deposits can be 

coarse grained. In these areas the units have been 

combined, if present, with the overlying coarsegrained deposits into a defined aquifer.

6. (undiv) The deeper undivided deposits generally 

occur about 250 ft or more below land surface. The 

deposits consist of both glacial and interglacial 

materials; laterally extensive, clay-rich units (such 

as the Puyallup Formation; fig. 5) are known to 

occur within the deposits. These deposits generally
D20 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

are undifferentiated, and on a regional basis, the 

aggregated deposits function as an aquifer.

This alternating pattern of coarse- and fine-grained 

deposits occurs from the surface to depths of more than 

3,300 ft in the major structural basins. However, locally, 

information is only available from two locations that 

define the sequence of units 1 through 5 because most 

wells do not penetrate deeply into the system. In some 

areas, till deposits overlie or underlie fine-grained interglacial deposits and generally cannot be differentiated 

from interglacial deposits using well-log information. 

Thus, the tills and the fine-grained interglacial deposits 

in these cases may be grouped as one hydrogeologic 

unit. Conversely, when coarse-grained glacial and 

interglacial units are in direct contact, they also may be 

grouped as one hydrogeologic unit.

The thicknesses of the hydrologic units described 

above are highly variable, both locally and regionally. 

The recent alluvial deposits in the major river valleys 

vary from a few tens of feet in thickness in the upper 

reaches of the rivers to as much as 600 ft in thickness in 

the lower reaches of some river valleys. The small discontinuous deposits of alluvium generally have thicknesses that range from about 5 to 10 ft. The localized, 

uppermost recessional outwash deposits generally 

average about 10 ft in thickness. However, in a few 

areas, there are large, well-defined relic recessional outwash channels filled with coarse-grained deposits; 

these deposits can be up to 150 ft thick but generally 

average about 40 ft. The thickness of the uppermost till 

unit reaches a maximum of more than 125 ft in some 

locations and generally averages between about 20 to 

40 ft. The thickness of the coarse-grained deposits 

below the upper till unit averages between about 20 to 

50 ft. Although the thickness reaches a maximum of 

more than 400 ft in some areas, a maximum of 150 to 

200 ft is more common. The underlying fine-grained 

deposits consist of silt- to clay-sized interglacial sediments or glacial till. Their thickness exceeds 150 ft in 

some areas and averages between about 40 to 65 ft.

The above sequence of glacial and interglacial 

deposits generally comprises the upper 250 ft or so of 

the aquifer system, and the deposits underlying this 

sequence generally are undifferentiated, especially on a 

regional basis. The extent and thickness of the waterbearing units in these undifferentiated deposits largely 

are unknown because water wells commonly penetrate 

only the upper 250 ft of the total thickness (see table 2A 

later in this report). Drillers' logs from a few deep 

exploratory wells drilled for petroleum or water are 

available, but these data are too sparse to adequately 

characterize the deep deposits throughout the Puget 

Sound Lowland. However, in some smaller areas, these 

deeper deposits can be divided into hydrogeologic

units because of the availability of deep well 

information.

Hydrogeologic sections were constructed in six 

"type" areas that generally contain representative variations in the sequences of the hydrogeologic units 

present in the Puget Sound Lowland, as defined on a 

detailed, local basis and include many of the stratigraphic units described for the Puget Sound Lowland. 

The sections were constructed (Jones, in press) on the 

basis of thickness and structure contour maps prepared 

during local studies that divided the aquifer system 

into local hydrogeologic units. These sections show the 

subsurface distribution of the previously described 

hydrogeologic units, including the additional divisions. 

Because of the large size of the study area and the lack 

of data to describe the local hydrogeologic units on a 

regional basis, the "hydrogeologic type sections" were 

then used in analyses to: (1) provide information for 

describing ground-water flow in the Puget Sound Lowland, (2) develop a method to aggregate local units into 

regionally representative hydrogeologic units, (3) test 

hydrologic controls on ground-water flow, and (4) 

develop a conceptual model for describing regional 

ground-water flow based on the division of the aquifer 

system into regional hydrogeologic units. The generalized distribution of the units described above within 

each section are shown and discussed later (plate 1; 

table 9; in section on "Cross-Sectional Numerical Models of Ground-Water Flow"). Additionally, estimated 

correlations between local hydrogeologic and stratigraphic units for each section are given in table 9 later in 

this report.

GROUND-WATER FLOW SYSTEM

Regionally, ground water moves from topographic 

highs to topographic lows, which typically are stream 

drainages or saltwater bodies. That is, ground water 

moves from the glaciated uplands fronting the Olympic 

Mountains and Cascade Range to the surface-water 

drainages and to the saltwater bodies, and ground 

water moves from topographic highs on the peninsulas 

and islands in the Puget Sound Lowland to the 

saltwater bodies.

The direction of ground-water movement generally 

is horizontal in the aquifer units and vertical in the 

semiconfining to confining units. This overall pattern is 

affected by the hydrologic setting. Vertical gradients 

generally are downward, except near discharge locations, typically streams and saltwater bodies where gradients are upward; generally, ground water moves 

downward to deeper units in the high altitude recharge 

areas and upward from the deeper units to shallower
D21

units in the low altitude recharge areas. Ground water 

in the uppermost aquifer unit generally occurs under 

water-table conditions but may also occur under semiconfined conditions. For the glacial deposits, the water 

table may occur in recessional outwash, till, or advance 

outwash (either exposed at the land surface or directly 

underlying the till); the location of the water table is a 

function of the physical setting. Ground water in the 

deeper aquifer units generally occurs under semiconfined or confined conditions. Ground water in the aquifer units that are intermediate in depth between the 

uppermost and deeper aquifer units generally occurs 

both under water-table and confined conditions. The 

wide range in conditions under which ground water 

occurs is due to the highly variable extent of materials 

with similar lithologic characteristics.

Movement of ground water is controlled by topography, geometry of the aquifer system, and the distribution and rate of ground-water recharge and discharge. 

The configuration of the water table in the uppermost 

aquifer unit generally parallels the surface topography, 

and water levels generally are within about 100 ft of 

land surface. The configuration of the potentiometric 

surface of each successively deeper aquifer unit is a 

more subdued replica of land surface. In aquifer units 

near or below sea level, the relation between the potentiometric surface and topography diminishes, and aquifer geometry and discharge locations (stream drainages 

and saltwater bodies) become more important controls.

Water levels in wells are a function of their topographic settings and the location of their open intervals 

within the flow system. In areas where the vertical 

movement of water is downward (generally high altitude areas), shallow wells will have water levels near 

land surface, and deeper wells generally will have 

water levels much below land surface. In areas where 

the vertical movement of water is upward (generally 

low altitude areas), shallow wells will have water levels 

near land surface, and deep wells generally will have 

water levels above either land surface or nearby surface-water bodies. Thus, at higher altitudes the water 

levels tend to decrease with depth, and at lower altitudes (near sea level) water levels tend to increase with 

depth. Generally, the depth to water in the uppermost 

aquifer units will be greater in areas of large 

topographic relief or high land-surface altitudes than in 

areas of small topographic relief or of low land-surface 

altitudes. Water levels in the aquifer units above sea 

level will vary from near sea level to land-surface altitude (maximum of about 800 to 1,000 ft), and water levels in the units below sea level will vary from sea level 

to a maximum altitude of about 250 ft.

Natural seasonal water-level fluctuations in the 

uppermost aquifer generally range from 1 to 10 ft;

fluctuations are related to recharge, aquifer geometry, 

and lithology. Water-level fluctuations in the deeper 

aquifers generally are less than 4 ft. Fluctuations in 

shallow wells tend to show rapid response to recharge 

during the winter precipitation season and in deeper 

wells tend to show a lag of 1 to 3 months. Water levels 

in shallow aquifers generally start to rise in October to 

November, continue rising through the winter, and start 

to decline in late spring to early summer. Water levels 

in the deeper aquifers exhibit a more subdued and 

delayed fluctuation. Ground-water levels also fluctuate 

in response to tides. Tidal effects have been found to 

range up to 7.5 ft and quickly approach 0 ft at distances 

of 500 to 1,000 ft from the shoreline (Lum and Walters, 

1976; Drost, 1979a).

BACKGROUND

Although general aspects of the ground-water flow 

system can be described, a regional assessment of the 

flow system has not been completed. Analysis of 

ground-water flow and availability in the Puget Sound 

Lowland has been completed at a variety of spatial 

scales in several areas (Jones, 1991; Vaccaro, 1992). 

However, the methods, descriptions, and division of the 

aquifer system into hydrogeologic units of previous 

investigators have varied widely. Additionally, the following factors indicate that the aquifer system may consist of several discrete ground-water flow systems 

contained in large ground-water (or structural) basins: 

(1) the configuration of the top of the basement confining unit (Jones, in press); (2) the presence and orientation of geologic structures (Jones, in press); (3) the 

presence and orientation of peninsular land bodies; (4) 

the size, depth, and north-south orientation of Puget 

Sound, Hood Canal, and other saltwater bodies; and (5) 

the numerous large, incised alluvial river valleys. 

Extrapolation of local results and analyses to a regional 

scale is, thus, difficult and problematic. For example, 

local results and analyses concerning ground-water 

recharge, stream-aquifer relations, and hydraulic characteristics may be transferrable on a regional scale, 

whereas descriptions of ground-water flow (horizontal 

and vertical gradients and quantities of water moving) 

may not be transferrable or are lacking. Because of the 

large size of the study area and the factors just discussed, it became apparent early in the study that a simplified, realistic method for describing regional groundwater flow was needed. Therefore, early in this study, 

available well information was analyzed, and results of 

this analysis are presented in the following paragraphs.

Table 1 presents a summary of the availability of 

well information in the U.S. Geological Survey's 

National Water Information System (NWIS) for the
D22 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

TABLE 1. Availability of well information for the Puget Sound

Lowland in the U.S. Geological Survey's National Water

Information System as of 1990

County

Clallam

Island

Jefferson

King

Kitsap

Mason

Pierce

San Juan

Skagit

Snohomish

Thurston

Whatcom

Total

Number of wells

1,797

2,194

289

4,356

2,989

562

3,829

428

399

1,807

4,231

1,586

24,467

Number of wells 

with water-level 

information

1,532

1,971

224

3,492

2,374

380

2,672

334

326

1,453

3,518

1,254

19,530

Number of wells 

with land-surface 

altitude, waterlevel, and welldepth information

1,523

1,652

215

3,407

2,353

364

2,622

305

307

1,418

3,475

1,189

18,830

Puget Sound Lowland as of 1990, and tables 2A and 2B 

present selected descriptive statistics on that well information. Cursory examination of new well-drillers' logs 

that are not in NWIS indicates that the total number of 

wells in the Puget Sound Lowland is at least 30,000. 

The information (tables 1 and ZA) shows that although 

there are numerous wells in the Puget Sound Lowland, 

most of the wells are not deep; for example, 80 percent 

are less than 175 ft deep. Although this indicates that 

the aquifer system is productive (the median discharge 

of 1,099 wells that had well-yield data and that were 

less than or equal to 50 ft deep was 30 gal/min), it also 

shows the paucity of available information for the 

deeper part of the aquifer system. Indeed, very few 

wells penetrate below -200 ft altitude.

The water-level data were examined to determine 

relations between well depth, land-surface altitude, 

depth to water, and generalized locations of recharge 

areas (land-surface altitudes greater than 50 ft) and 

discharge areas (land-surface altitudes equal to or less 

than 50 ft; table 2B). The definition of the generalized 

locations is based on the assumption that areas with 

altitudes of 50 ft or less generally will be near the shorelines or in (or near) the alluvial valleys, both of which 

are discharge areas. The regression information 

(table 2B) indicates that a significant relation exists 

between the water-level altitude and land-surface altitude. This relation is typical for an aquifer system comprised of unconsolidated deposits and located in a 

humid climate. The relation shows the importance of 

topographic control on ground-water flow, at least for

the upper 200 to 300 ft of the aquifer system where most 

of the ground-water flow occurs the "active" part of 

the system. In turn, the topographic control indicate 

potential importance of local flow systems and the 

potential usefulness of constructing, on the basis of well 

depth and incorporating land-surface altitude, generalized water-level maps for describing aspects of regional 

ground-water flow. The information in table 2B also 

shows the nearly one-to-one relation between land-surface and water-level altitudes for wells less than 50 ft 

deep. It was due to this relation and the fact that the 

best estimate of water levels would be land surface due 

to the accuracy of the topography contours at the scale 

(1:100,000) of the working maps used in this study that 

water levels from wells less than 50 ft deep were not 

included for the construction of generalized preliminary water-level configuration maps. Also note that the 

relations between water-level and land-surface altitudes stratified by wells less than 51 ft deep and wells 

greater than 105 ft deep show that the downward vertical head gradient increases with increasing land-surface 

altitude (see fig. 9). When this well information is further stratified by land-surface altitude, the relations for 

altitudes less than 51 ft (table 2B) generally indicate that 

water levels increase with well depth.

WATER-LEVEL CONFIGURATIONS

Generalized water-level configuration maps for the 

Puget Sound aquifer system were constructed at a scale 

of 1:100,000. Existing water-level information (both 

measured during previous studies and reported by well 

drillers) from the NWIS (table 1 and table 2A) was segregated by well depth into two categories shallow 

(more than 50 ft and less than 106 ft) and deep (more 

than 105 ft). Water-level maps then were constructed on 

the basis of those data, and the relations shown in 

table 2B; the water-level configurations for only the 

deep wells are presented in this report (fig. 10). Because 

the water-level information is not segregated by aquifer 

unit (that is, the geologic framework was not accounted 

for), the maps constructed in this manner are not suitable for addressing ground-water availability. Note 

that because of the general lack of wells deeper than 

350 ft, the deep system described here generally represents, depending on location, the lower part of the aquifer unit that underlies the surficial till or the upper part 

of the deeper undivided deposits described in the previous section. Therefore, the water-level map (fig. 10) 

probably represents composite water levels from several local aquifer units or one or two regional aquifer 

units. Additionally, the lack of water-level data limit 

the ability to describe the deep flow system. However, 

the thickest parts of the aquifer system generally
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D23

TABLE 2A. Descriptive statistics for well information 

[values in feet, except as indicated]

Number of 

wells

Mean

Minimum

Maximum

Percentile

10

20

30

40

50

60

70

80

90

Well depth

23,344

121

1

9,530

19

32

49

65

84

105

134

175

250

Well depth 

greater than 

250 feet

2,264

434

251

9,530

264

277

295

310

328

356

396

460

604

Well depth 

greater than 

500 feet

364

984

501

9,530

524

546

575

610

672

750

825

992

1,230

Well information

Landsurface 

altitude

22,910

251

1

4,500

40

80

121

160

200

250

320

400

502

Water-level 

altitude

18,830

205

0

2,992

11

38

75

113

154

198

270

345

458

Depth to water

18,830

49

0

470

3

7

10

16

24

37

56

84

132

Bottom-of-well 

altitude

18,830

141

-2,500

2,965

-57

-16

20

62

102

146

216

298

412

TABLE 2B. Descriptive statistics for well information and regression information

o 

[values in feet, except as indicated; WD, well depth; LSA, land-surface altitude; DtW, depth to water; WL, water-level altitude; R , regression correlation

coefficient squared]

Slope 

Intercept1 

R2

Slope 

Intercept1 

R2

Slope 

Intercept1 

R2

WD vs LSA

0.0251 

113.7 

.0007

WLvsLSA 2

0.951 0.771 

-76.5 0 

.87 .904

WLvsLSA 6

0.723 0.483 

-8.59 0 

.152 .33

DtW vs LSA

0.0376 

39.34 

.0162

WLvsLSA 3

0.993 0.942 

-20.69 0 

.988 .992

WLvsLSA 7

0.9802 0.868 

-47.1 0 

.9088 .952

DtW vs WD

0.288 

16.25 

.29301

WL vs LSA 3

0.998 0.961 

-11.55 0 

.992 .996

WLvsLSA 7

0.860 0.723 

-4.85 0 

.725 .914

WL vs LSA

0.962 

-39.34 

.9154

WLvsLSA 4

0.960

-100.1 

.83

WLvsLSA 8

0.600 

5.45 

.302

0.868 

0 

.951

0.722 

0

.87

0.750 

0 

.807

Intercept = 0 indicates regression constrained such that regression line passes through zero.

wells greater than 105 feet deep. Wells less than 106 feet deep. Wells less than 50 feet deep.

Wells greater than 105 feet deep and land-surface altitude greater than 50 feet.

Land-surface altitude equal to or less than 50 feet. Land-surface altitude greater than 50 feet.

Land-surface altitude less than 51 feet and wells less than 50 feet deep, 

q

Land-surface altitude less than 51 feet and wells greater than 105 feet deep.
D24 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

1,000

800

600

400

200

0

All wells

Wells greater than 105 feet deep

Wells less than 50 feet deep

0 200 400 600 800 1,000

Land-surface altitude, in feet

FIGURE 9. Linear relations between water-level altitude and 

land-surface altitude.

underlie saltwater bodies that may limit flow in such a 

system.

The water-level maps were used to obtain an initial 

understanding of the range of horizontal and vertical 

head gradients present in the aquifer system, and the 

generalized regional direction of ground-water movement. The maps were also used, in conjunction with 

basement structure contours (Jones, in press), to estimate whether geologic structure possibly separates the 

aquifer system into discrete ground-water basins and, 

thus, subregional flow systems.

Although the geologic structures, the configuration 

of the basement confining unit, and the thickness of the 

aquifer system indicate about six distinct structural 

basins (Jones, in press), information provided by the 

water-level configurations cannot be used to infer six 

ground-water basins. However, the water levels in conjunction with the overall geology of the aquifer system 

indicate, on a regional basis, that there are at least three 

ground-water basins whose boundaries generally 

correspond to the subareas (Fraser-Whatcom, northcentral, and southern) shown in figure 6. Additionally, 

within each of these basins there does not appear to be a 

consistent flow system only isolated local to subregional flow systems that contain flowpaths that do not 

traverse a complete basin. This corresponds to results 

from local investigations; for example, Woodward and 

others (1995) in a study of part of King County define a 

regional flow system existing only in the bedrock units.

Additionally, the north-south orientation of the 

major saltwater bodies and the landforms of the Puget 

Sound Lowland, the large depths of the saltwater bodies (the low areas in the Puget Sound Lowland near 

where flowpaths in the deepest parts of the system will 

generally end), and the overall orientation of the waterlevel surfaces together indicate that discrete areas of the 

Puget Sound aquifer system can be analyzed without 

regard to other areas because not all areas are hydraulically connected. Thus, on the basis of overall physical 

setting (for example, see fig. 8) and the information 

shown in figure 10, there does not appear to be a 

regional ground-water flow system.

The water-level configurations based on the shallow 

well information showed that ground-water flow is 

toward streams and saltwater bodies (surface-water features). The flow systems represented by these contours 

generally are similar to those presented in figure 10 and 

typify local flow systems supported by local recharge 

and controlled by topography and surface-water features. Most horizontal head gradients range from 10 to 

100 ft/mi (0.002 to 0.02 ft/ft) and, on a regional basis, 

average about 35 ft/mi (0.007 ft/ft). Average gradients 

are least in coarse, recessional outwash and alluvial 

deposits and are steepest in fine-grained deposits. Gradients are steeper (as much as 500 ft/mi or 0.09 ft/ft) in 

steep terrain areas and near bluffs and cuts; in these 

areas, gradients average about 200 ft/mi (0.04 ft/ft). 

The steeper gradients show both the topographic and 

boundary-condition control at bluffs and cuts where 

units are truncated.

The water-level configuration in the deeper aquifer 

system (fig. 10) shows that the principal direction of 

ground-water flow is toward surface-water features. 

Although topography is still a major control, the waterlevel surfaces generally are flatter than those in the 

shallow aquifer system and are a more subdued replica 

of land surface. As a result, horizontal head gradients 

also are smaller and typically range from 10 to 40 ft/mi 

(0.002 to 0.008 ft/ft) and, on a regional basis, average 

about 25 ft/mi (0.005 ft/ft). Again, steeper gradients 

typically are located in steep terrain areas or near bluffs 

and cuts. Horizontal gradients near saltwater bodies 

range from 10 to 30 ft/mi (0.002 to 0.006 ft/ft) and generally are less than the gradients in the shallow system. 

The estimated gradients are very similar to those 

reported in detailed local studies of individual aquifer 

units (for example, Hart Crowser and Associates, 1986; 

Sapik and others, 1988; Turney and others, 1995; Woodward and others, 1995).

Horizontal head gradients in the alluvial deposits in 

the major river valleys are flatter than most gradients in 

the glacial and interglacial deposits in the adjacent 

uplands; for example, see the water-level contours in
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D25

123 122°

BRITISH COLUMBIA ___ 

~ WASHINGTON

WHATCOM 

Mt Baker

\"% CLALLAM " SNOHOMISH *- " - \-

)

Olympic /

Mounfa/ns.

JEFFERSON

EXPLANATION

Puget Sound 

aquifer system

I I Basement confining 

unit

Water-level contour ^Based 

on information from wells 

that generally are greater than 

105 feet deep. Contour 

interval 100 feet with a supple 

mentary contour of 50 feet 

(dashed). Datum is sea level

Arrow shows direction of 

ground-water flow and 

approximate length of 

flowpath

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972

FIGURE 10. Generalized pattern of ground-water movement for the Puget Sound aquifer system. Lateral hydraulic gradients are indicated

by spacing of generalized water-level contours.
D26 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

the Skagit, Green, and Puyallup River Valleys in 

figure 10. This is attributable to (1) coarse-grained 

materials along the rivers that have good hydraulic connection with the rivers and (2) the relatively flat topography of the top of the alluvium. Although some glacial 

deposits are as permeable as the alluvium, the discontinuous nature of the glacial-interglacial aquifer units 

and topographic control results in steeper gradients.

The vertical head gradients, estimated from well 

data (depth and water level) used to construct the generalized water-level configuration maps, range from 

about 0.005 to 1.6 ft/ft and appear to average about 

0.3 ft/ft. Gradients generally are downward (decreasing water levels with depth) over much of the area and 

upward (increasing water levels with depth) near saltwater bodies and major streams. The steeper gradients 

occur near bluffs in the steep upland areas, and in 

places where aquifer units are separated by clay-rich 

nonaquifer units comprised of interglacial or finegrained glaciomarine drift deposits. The estimated vertical-head differences, which were used to calculate the 

vertical gradients, ranged from about 1 to 200 ft.

The preceding simplified analysis indicates that (1) 

aspects of ground-water flow generally can be 

described by the segregation and subsequent mapping 

of water-level information based on well depth and 

land-surface altitude and (2) a simplified conceptual 

model probably can preserve the important regional 

characteristics of the aquifer system for describing 

ground-water flow in the region. Additionally, to 

improve understanding of the flow system and to 

address ground-water availability, the geologic framework needs to be accounted for by the mapping of 

regional hydrogeologic units. The conceptual model 

presented later in this report and the geologic 

framework of Jones (in press) provide the basis for 

future mapping of these regional units.

HYDRAULIC CHARACTERISTICS

The hydraulic and geometric characteristics that are 

most important for describing ground-water flow and 

for dividing or aggregating hydrogeologic units are 

hydraulic conductivity and transmissivity of aquifer 

units (product of horizontal hydraulic conductivity and 

aquifer thickness). The vertical hydraulic conductivity 

of confining and semiconfining units comprised of clayrich materials and glacial till also is important for 

describing the quantity of water that can flow into and 

between aquifers. Additionally, although not strictly a 

hydraulic characteristic, the specific capacity is an 

important characteristic for identifying the productivity 

of an aquifer.

HYDRAULIC CONDUCTIVITY

Table 3 summarizes estimates of horizontal hydraulic conductivity made in previous studies (references 

listed in table). The numerous depositional environments and large variety of source materials, and thus 

lithologic characteristics, have resulted in a wide range 

of conductivity values. However, the estimates of 

hydraulic conductivity for similar materials generally 

are consistent from study to study, especially with 

respect to the average or median values.

The units that consist principally of sand have average conductivity values of about 15 to 50 ft/d. Coarsegrained equivalents (including gravel facies) of some 

units average about 100 ft/d. Zones within the deeper 

deposits (both divided and undivided) that appear to 

be predominantly sand have conductivities that range 

from 34 to 59 ft/d and average 41 ft/d. Hydraulic conductivity values less than about 15 ft/d indicate a finegrained equivalent of the sand units (probably silty 

sand), and values less than about 10 ft/d indicate that 

the fine-grained equivalent contains more silt and probably has 1- to 10-percent clay content. The medium- to 

coarse-grained alluvium has conductivities that generally range from 35 to 700 ft/d; these values generally 

are larger than the values for the glacial and proglacial 

materials. The conductivity of coarse-grained alluvium 

ranges from about 100 to 700 ft/d, although, when the 

value for the Elwha River is not included, a value of 

about 200 ft/d appears to be a reasonable estimate for 

the average conductivity of the coarse-grained alluvium. The conductivity of fine-grained alluvium probably ranges from about 1 to 15 ft/d. Fine-grained 

alluvium is typically fine sand intermixed with silt and 

clay; unless a sequence is predominantly clay or siltclay, a fine-sand description in drillers' logs can vary 

from nearly uniform homogenous fine sand to a mixture of silt and clay. This fine-grained alluvium typically is present in the lower reaches of the major river 

valleys.

The preceding information indicates that the aquifer 

units, in particular the more productive parts, generally 

have median conductivities in the range of 15 to 50 ft/d. 

Larger values (50 to 200 ft/d) indicate either coarsegrained (gravel) equivalents of glacial deposits or alluvial deposits. Values less than about 10 ft/d indicate 

fine-grained materials (either glacial, interglacial, or 

alluvial).

The glacial till in the study area consists of an 

unsorted, unstratified matrix of sand, silt, and clay with 

clasts of gravel, cobble, and boulders; gravel is the dominant coarse-grained material. The grain-size distribution of the till matrices averages about 55-percent sand, 

28-percent silt, and 17-percent clay (Dethier and others,
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM

TABLE 3. Summary of published estimates of horizontal hydraulic conductivity 

[ , indicates no data]

D27

Sediment type or stratigraphic unit 1

Advance outwash

Advance outwash (fine sand, 

silt)3

Advance outwash and Colvos 

Sand (coarsegrained)

Alluvium, Dungeness River

Alluvium, Elwha River (coarse 

deposits)

Alluvium, Green River

Alluvium, Green-Cedar River

Alluvium, Green- White River 

(fine to medium grained) 4

Alluvium, Nisqually River

Coarse grained

Medium grained

Fine grained

Alluvium, Puyallup River

Alluvium, undivided glacial and 

interglacial

Coarse grained

Medium grained

Fine grained

Alluvium and recessional 

outwash

Colvos Sand

Colvos Sand (coarse grained)

Double Bluff Drift (sand only)

Glacial and interglacial, 

undivided6

Gravel

Reference2

E

H

G

E

P

Q

N

E

0

R

R

R

R

M

P

P

P

H

A 

A 

A 

A 

A 

A

B

C

E

D

M

C

H

Number of 

values

68

73

37

6

62

18

51

10

-

-

-

42

121

93

150

15

-

17

15

74

20

41

4

22

Range (feet per 

day)

0.1-2,990

3.2-6,970

0.02-16.7

127-298

-

20-7,570

1-100

8.5-170

-

-

-

85-475

140-170

22-52

2-15

6.2-915

-

5-25

1.1-1,160.0

1.1-5,174

3.2-501

43.0-320.0

13-1,160

3.7-915

Geometric mean (G), 

average (A), or 

median (M) 

(feet per day)

33 (M)

47 (M)

2.33 (A)

174 (M)

410 (M)

700 (A)

100 (A)

290 (M)

7.5 (M)

~

170 (A)

85 (A)

8.5 (A)

475 (M)

170 (M)

35 (M)

8(M)

36 (M)

5.0 (A) 5 

20 (A)5 

33 (A)5 

33 (A)5 

88 (A)5 

100 (A)5

10 (G)

41 (M)

51 (M)

42 (M)

121 (M)

800 (M)

50 (M)

Interglacial silty clay 0.00007-0.0029 .0014 (A,M)
D28 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

TABLE 3. Summary of published estimates of horizontal hydraulic conductivity Continued

Sediment type or stratigraphic unit 1

Kitsap Formation with coarse 

lens

Quadra Formation (sand only)

Recessional outwash channel

Salmon Springs Drift

Salmon Springs Drift, 

(penultimate drift)

Salmon Springs Drift and 

undivided deep deposits

Salmon Springs Drift and 

undivided deep sands 7

Sand

Sand, fine

Sand and gravel

Sand and gravel, undivided deep

Till

Undivided, deep

Whidbey Formation (sand only)

Reference2

C

D

M

C

H

E

F

C

H

C

C

D

H

D

G

J

K

L

H

D

Number of 

values

34

37

-

67

77

19

20

49

54

37

52

3

166

2

_

-

-

50

134

Range (feet per 

day)

1.5-310

4.5-6,998

-

0.54-180

2.5-5,260

6-201

40-450

.54-150

2.5-4,350

0.4-45

3.6-310

432-3,888

2.9-6,970

8.3-207

0.003-0.08

0.0003-14.2

0.0003-0.03

0.1-0.13

3.7-2,680

1.5-2,160

Geometric mean (G), 

average (A), or 

median (M) 

(feet per day)

26 (M)

81 (M)

242 (M)

14 (M)

59.0 (M)

41 (M)

280 (A)

15 (M)

30 (M)

4.2 (M)

38 (M)

49 (M)

59 (M)

41 (G)

_

.014 (A)

.12 (A)

34 (M)

134 (M)

Stratigraphic units shown in figure 5.

A, Hansen and Bolke (1980); B, Tracy and Dion (1976); C, Dion and others (1988); D, Sapik and others (1988); 

E, Woodward and others (1995); F, Seattle Water Department (1986); G, URS Consultants, Inc. (1992); H, G.L. 

Turney, (U.S. Geological Survey, written commun., 1994); I, METRO (1983); J, Miller and Pessl (1986); K, Olmstead 

(1969); L, Dion and others (1983); M, R.C. Lane, (U.S. Geological Survey, written commun., 1993); N, Lum and others 

(1984); 0, Hart Crowser and Associates (1984); P, Drost (1983a); Q, Walters and others (1979); R, Lum (1984).

O A K

37 slug tests of 9 wells. Presented as ranges; average of the 10 averaged ranges. Average values are for different 

areas.

Range of values between 25th and 75th quartiles. Estimated from transmissivity and aquifer thickness.

1981; Vanderwal, 1985). Although the degree of consolidation of the till varies, it is usually highly consolidated. The hydraulic conductivity of till generally is 

smaller than both the glacial outwash, proglacial, and 

alluvial deposits and larger than the fine-grained interglacial deposits (table 3). However, little information is

available on the hydraulic conductivity of the till. 

Because of this fact, field tests using a permeameter to 

estimate vertical hydraulic conductivity of the till in 

selected areas were conducted during this study. Values obtained by this method are not strictly representative of vertical conductivities but include horizontal
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D29

components. Representative vertical values would be 

smaller than the values described in the following table. 

Results of the field tests showed the following 

attributes:

Number

of 

values

Range 

(feet per day)

Median 

(feet per day)

57 0.0002-53.0 0.12

The range indicates the variability of the till; however, 

the middle 80 percent of the values were in the range of 

about 0.005 to 22 ft/d.

The grain-size characteristics of the till matrix in the 

Puget Sound Lowland (Dethier and others, 1981; 

Vanderwal, 1985, figs. 4 and 16) are similar to those in 

southern New England (Melvin and others, 1992, fig. 4), 

and the consolidation and stoniness of the two tills also 

appear to be similar. Thus, information on hydraulic 

conductivity for southern New England probably can 

be used as an analog for the conductivity of till in the 

Puget Sound Lowland. For more than 250 reported values of till conductivity for southern New England 

(Melvin and others, 1992), the range was 0.00023 to 

96 ft/d, and the median was about 0.3 ft/d. The middle 

80 percent were in the range 0.01 to 5.95 ft/d, similar to 

the range obtained from the permeameter tests. Additionally, the smallest and largest 20 percent of these values were in the range 0.00023 to 0.03 ft/d and 3.97 to 

96 ft/d, respectively, showing that the middle 

60 percent (range, 0.03 to 3.97 ft/d) is essentially the 

same as the 80-percent range. Based on the assumptions that the median is a good estimate for a regional 

value and a vertical conductivity value would be conservatively one to two orders of magnitude smaller than 

a horizontal value, the preceding information suggests 

that the regional effective vertical hydraulic conductivity of the till in the study area is in the range 0.001 to 

0.01 ft/d. This range is less than the upper limit of values presented in table 3 and near the lower quartile of 

the permeameter-derived values and the New England 

values.

Most interglacial deposits consist of well-bedded, 

clay-rich units that are not productive aquifers. However, these interglacial deposits may consist of sand to 

clay with some beds of sand and gravel and may locally 

contain peat. Some of the interglacial deposits appear 

to become more coarse grained and have more lenses of 

sand and gravel at the periphery of the aquifer system. 

For example, the interglacial deposits present along the 

Strait of Juan de Fuca in parts of Clallam County generally are coarse grained (Washington Department of Ecology, 1978); the close proximity to the source of the 

deposits, the Olympic Mountains, can account for the 

coarseness of these deposits.

The conductivities of the fine-grained interglacial 

deposits probably are on the order of 0.00001 to 1.0 ft/d 

(Freeze and Cherry, 1979, and table 3). Investigations of 

the fine-grained (clayey) till in the Interior Plains of 

North America (Grisak and Cherry, 1975; Grisak and 

others, 1976; Keller and others, 1989) show vertical conductivity to range from about 0.000003 to 0.003 ft/d and 

average about 0.0001 ft/d. These investigations suggest 

that the clayey till is a good model for compacted lacustrine sediments. The clayey till is similar to the finegrained interglacial deposits, suggesting that the vertical conductivity of the fine-grained deposits is on the 

order of 0.0001 ft/d. This value is about one order of 

magnitude smaller than the average value and one 

order of magnitude larger than the smallest estimated 

by METRO (1983) for interglacial sediments (table 3).

Although the hydraulic conductivity varies locally, 

on a regional basis the hydraulic conductivity of hydrologic units can be characterized by average and median 

values. Further, excluding till, grain size appears to be 

a reasonable indicator for estimating average conductivity values and for describing hydrogeologic 

sequences or units. The known relations between grain 

size and hydraulic conductivity have been used in 

many investigations, for example, Gutentag and others 

(1984). Till appears to have much smaller conductivities 

than most glacial, proglacial, and alluvial deposits but 

much larger conductivities than the fine-grained interglacial deposits. The small conductivities of most interglacial deposits indicate that they can be considered 

semiconfining to confining units for the conceptual 

model of ground-water flow.

Additionally, analyses of ground-water flow systems in areas where the Everson "glaciomarine" Drift 

exists indicate that the fine-grained part of this unit has 

hydraulic conductivities that are similar to both the till 

and the interglacial deposits (Sapik and others, 1988; S. 

Cox, U.S. Geological Survey, written commun., 1993). 

This drift was deposited during the Everson Interstade 

and is present in (1) the Fraser-Whatcom Basin (fig. 6), 

where it generally is fine-grained, and in (2) the northcentral Puget Sound Lowland, where it varies from fine 

to medium grained. Available information suggests 

that this drift generally functions as a confining unit in 

the Fraser-Whatcom Basin and as a semiconfining unit 

in the remainder of the area where it is present. Outside 

of the Fraser-Whatcom Basin, its range in conductivity 

is probably similar to till, and thus the conductivity of 

the drift is estimated to range from 0.001 to 0.1 ft/d.
D30 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

TRANSMISSIVITY

Reported transmissivity values for the aquifers in 

the Puget Sound Lowland range from 5 to 

3,010,000 ftVd (Cline, 1969; Hansen and Bolke, 1980; 

Drost, 1982; Brown and Caldwell, 1985; Carr and Associates, 1983, 1988; Hart Crowser and Associates, 1986, 

1988a,b, 1989a,b). Values greater than about 2,000 ft2/d 

generally are for thick, coarse-grained deposits located 

in alluvial valleys; well-defined massive recessional 

outwash channels; or deep, below-sea-level aquifer 

units. On a regional basis, reported transmissivity values generally range from 50 to 2,000 ft2/d and average 

about 500 ftVd.

To better understand the range in transmissivity values for the Puget Sound Lowland, all specific-capacity 

information in NWIS was used to calculate transmissivity values by the method of Theis (1963). There were 

5,502 data points with the following attributes:

Attribute Average

Discharge 94

(gallon per

minute)

Well depth (feet) 153

Median

20

120

10th 

decile

8

38

90th 

decile

215

300

Calculated transmissivity values ranged from 1 to 

764,000 ftVd and averaged 2,890 ftVd. However, the 

10-, 50-, and 90-decile values were 38, 430, and 

4,460 ftVd, respectively.

SPECIFIC CAPACITY

The specific-capacity information, which also characterizes the hydraulic conductivity, was analyzed for 

(1) descriptive statistics by calculating statistics of the 

available data and (2) spatial trends by plotting the values on a map. The specific-capacity values ranged from 

0.004 to 1,135 (gal/min)/ft and averaged 

11 (gal/min)/ft, with a median of 2 (gal/min)/ft. The 

10- and 90-decile values are 0.25 and 22 (gal/min)/ft, 

respectively. As described previously, of 1,099 wells 

that were less than 50 ft deep, the median yield was 

30 gal/min, which is 10 gal/min larger than the median 

for all wells. The larger yield suggests that the upper 

part of the aquifer system may be more productive than 

the system on average, probably because of the productive, shallow wells located in the alluvium and the 

recessional outwash channels. Analysis of 837 of the 

1,099 wells that had specific-capacity values showed 

values that ranged from 0.04 to 818 (gal/min)/ft and 

averaged 24 (gal/min)/ft [median of (8 gal/min)/ft],

the latter two values are larger than the system on average. Also note that these values do not appear to be 

largely affected by well discharge because the correlation coefficient between well diameter and specific 

capacity was only 0.21.

The mapped distribution of specific capacity displays some spatial trends. These trends indicate the following well-defined areas of larger-than-average 

specific-capacity values: (1) the large alluvial river valleys (for example, the Skagit, Green, and Puyallup River 

Valleys), (2) the areas with larger extents of recessional 

outwash, and (3) the Fraser-Whatcom Basin (fig. 6). 

These trends indicate the potential for aggregating units 

in the Puget Sound Lowland. For example, the trends 

indicate that, on the basis of hydraulic characteristics, 

alluvial deposits within the large river valleys should 

be separated into a distinct hydrogeologic unit and that 

the alluvial deposits probably compose large separate 

aquifer units in these valleys. Drillers' logs indicate this 

unit exceeds 600 ft in thickness in the Puyallup River 

Valley (Noble, 1990) and 400 ft in the Green River Valley 

(Woodward and others, 1995).

RECHARGE

Ground-water recharge is perhaps the most important control on ground-water availability. An 

understanding of the range in recharge and the controls 

on recharge is important for describing the groundwater flow. The controls on recharge include physical 

characteristics topography, soil characteristics, land 

use and cover, and surficial geology and water-budget 

components at or near the land surface actual evapotranspiration (AET), surface and subsurface runoff, and 

precipitation. Together, the physical characteristics and 

water-budget components, including recharge, 

uniquely define the surface hydrology and are important for describing the hydrology of the area. Therefore, 

a brief background covering recharge estimates and 

methods used to derive recharge estimates from previous investigations is first presented. Limitations in 

these methods also are discussed. Previous estimates 

based on the use of daily climate information are summarized, and information relating to physical characteristics, water-budget components, and relations to 

recharge are then discussed. The estimates for mean 

annual recharge, including the method used and its limitations, to the Puget Sound aquifer system are 

described next. Last, the reliability of the estimates is 

discussed.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D31

BACKGROUND

The most commonly used methods to estimate 

recharge in the Puget Sound Lowland have been the 

monthly water-balance methods of Blaney-Criddle (U.S. 

Department of Agriculture, 1970) and Thornthwaite 

(Thornthwaite, 1948; Thornthwaite and others, 1957). 

Examples of the use of these methods in the Puget 

Sound Lowland are found in Noble and Wallace (1966), 

Richardson and others (1968), Eddy (1975), Drost 

(1979a,b), Grimstad and Carson (1981), Drost (1982), Carr 

and Associates (1983), and Brown and Caldwell (1985). 

These methods generally are based on long-term average values of precipitation, temperature, potential 

evapotranspiration (PET), and AET. Potential ET, AET, 

precipitation, and an assumed soil water-storage capacity provide an estimate of the quantity of water available the surplus for either ground-water recharge or 

surface runoff. Estimates are then made of runoff, and 

the quantity of recharge is calculated as the residual of 

the surplus. Generally, the water surplus has been used 

to define the maximum potential recharge that could 

occur. The results of these methods are sensitive to the 

soil water-storage capacity, the time-period of record 

chosen for the analysis, and the estimation of runoff.

An important limitation on the use of these methods 

is the period of record that is used. That is, the 

variability of the recharge estimate is much smaller than 

the variability of natural recharge because most estimates are derived using averaged climate data, not 

annual or monthly climate data, which display large 

variability. For example, the water-year precipitation at 

three selected weather stations in Washington in the 

Puget Sound Lowland shows the following attributes:

Average Range 

Location (inch per year) (inch per year)

Port Townsend

Orcas Island

Olympia

19

29

53

11-35

16-10

31-80

Range 

difference 

(inch per year)

24

24

49

These attributes indicate the large variability in precipitation (the range difference is similar to the average) in 

the low-lying part of the Puget Sound Lowland. The 

interannual variability in precipitation and, thus, in 

recharge is important because of (1) the numerous shallow water-table wells in the Puget Sound Lowland, 

which are more sensitive to short-term recharge variations, and (2) the need to understand the natural variability of recharge for long-term assessment of groundwater availability. The monthly methods may be more 

useful if several years of monthly data are used to

calculate potential ranges in the water surplus to derive 

improved estimates of recharge, including its potential 

variability; the potential variability in recharge is discussed at the end of this section. However, even in 

humid climates, using monthly climate data can result 

in inaccurate estimates of recharge (Giambelluca and 

Oki, 1987).

Using several years of daily climate data can provide more reliable estimates of mean recharge than can 

the monthly data. The use of daily water-budgeting 

methods to estimate recharge for large areas for 

ground-water investigations is relatively new. The earliest known use in the Puget Sound Lowland was in a 

ground-water study of Island County (Sapik and others, 

1988). This investigation used a daily energy-soil-water 

budget model (H. Bauer, U.S. Geological Survey, oral 

commun., 1992) that was an early precursor to a deep 

percolation model (DPM) developed and documented 

by Bauer and Vaccaro (1987). The DPM calculates, as a 

residual, the quantity of water percolating below the 

active root zone (deep percolation) for any number of 

model cells that an area is divided into. The calculated 

deep percolation is assumed to be ground-water 

recharge. Observed streamflow minus the groundwater contribution (base flow) is used as an input into 

DPM. The DPM was applied to eight basins in southwest King County (Woodward and others, in press) and 

to two areas located within the lower Puyallup River 

Basin (R.C. Lane, U.S. Geological Survey, written commun., 1992).

A second daily water-budgeting method that has 

been used in the Puget Sound Lowland to indirectly calculate ground-water recharge is the Hydrological Simulation Program-FORTRAN (HSPF) watershed model 

(U.S. Environmental Protection Agency, 1984). Dinicola 

(1990) applied the HSPF and model-derived parameters 

to 33 low-altitude basins with drainage areas generally 

less that 20 mi2 in King and Snohomish Counties. Berris 

(1995) and Mastin (1996) applied the model to three 

basins in Thurston County and three basins in Pierce 

County, respectively. The HSPF calculates the quantity 

of precipitation that becomes either subsurface runoff 

or recharge to the deeper ground-water reservoir. The 

latter is assumed to be equal to the quantity of deep percolation and the estimate of recharge. The HSPF also 

calculates base flow.

PREVIOUS RESULTS AND RELATIONS BETWEEN 

WATER-BUDGET COMPONENTS

The results from these previous studies that applied 

HSPF and DPM are summarized for 26 basins or areas. 

The results are shown in table 4, and the location of the 

basins is shown in figure 11. For the 26 basins, the 

ranges in mean annual values were precipitation 25.17
TABLE 4. Summary of published estimates of ground-water recharge calculated using daily climate data

Basin

identification

number

(fig. 11)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Mean annual values, in inches

Basin name

Cottage Lake Creek

Covington Creek

Evans Creek

Jenkins Creek

Little Soos Creek

Lower Bear Creek

North Creek

Quilceda Creek

Scriber Creek

Swamp Creek

Upper Big Soos Creek

Lakota Creek

Hylebos Creek

Joes Creek

Unnamed Creek #1

Unnamed Creek #2

Unnamed Creek #3

Hylebos Creek,

tributary

Big Soos Creek

Lower Puyallup

Valley, north

Area

(square

miles)

26.81

20.6

14.6

16.01

10.61

8.02

27.61

24.22

6.98

23.6

18.6

3.03

5.92

2.58

1.15

.80

2.92

7.77

572.12

31.38

Simulation period

(month-day-year)

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

10-01-84 to 09-30-86

01-01-87 to 12-31-87

01-01-87 to 12-31-87

01-01-87 to 12-31-87

01-01-87 to 12-31-87

01-01-87 to 12-31-87

01-01-87 to 12-31-87

01-01-87 to 12-31-87

01-01-67 to 12-31-87

01-01-64 to 12-31-82

Precipitation

40.45

42.19

40.61

38.64

36.32

40.18

38.22

42.12

33.30

33.36

34.50

33.01

32.82

33.31

32.56

32.68

32.17

33.00

47.80

39.52

Potential 

evapotranspiration

24.33

24.33

24.33

24.33

24.33

24.33

24.33

24.33

24.33

24.33

24.33

28.63

29.08

28.17

29.19

29.05

28.82

28.72

27.31

25.87

Actual 

evapotranspiration

20.25

20.60

19.58

19.15

18.88

19.13

17.89

18.83

14.44

16.25

17.98

14.77

14.62

13.26

17.01

15.56

15.64

16.91

19.21

12.58

Surface and 

shallow subsurface

runoff

8.59

6.13

8.61

4.76

6.43

8.48

11.85

10.38

14.04

10.65

7.70

2.93

8.06

6.86

4.16

3.01

7.59

5.15

8.99

9.40

Stream base

flow

11.38

11.35

11.06

218.90

11.39

8.54

8.41

10.61

4.72

6.43

8.29

2.64

4.07

320.82

322.10

3'454.18

7.35

9.15

14.60

7.60

Recharge

11.88

15.37

13.17

14.63

11.25

12.75

8.70

12.68

74.77

76.55

8.79

15.83

10.62

14.26

11.87

14.49

9.82

11.52

20.61

17.66

Ch

st

-0
Basin

identification

number

(fig. 11)

21

22232425

26

^sn. T

A ABL/rL t. UU/IIHIHliI I/ uj puuMsriKU: vditiimivs u<J yiUUIIiU-Uiruie/ iv^HUiiyv uuti/M/tuteu u/oiiiiy uutf,y KMIIHlilV UUI U \^UIHIill>U/t;u

Mean annual values, in inches

Basin name

Lower Puyallup

Valley, south

Camano Island

Whidbey IslandWoodland CreekPercival Creek

Woodard Creek

lininnla ("19901 and also P. S

Area

(square

miles)

23.63

40.1

170

29.5

8.37.0

Dininnla CTI.S. Gi

Simulation period

(month-day-year)

01-01-64 to 12-31-82

01-01-51 to 12-31-7701-01-51 to 12-31-7703-01-88 to 02-28-9003-01-88 to 02-28-90

03-01-88 to 02-28-90

anlnorinal Riirvpv. writ.fpn nnmm

Precipitation

38.18

29.12

25.1751.760.7

51.5

nn. 1991 IrH?

Potential

evapotranspiration

25.87

21.2521.2524.024.0

24.0

?PF U.S. TCnvi

Actualevapotranspiration

11.9416.4413.3216.9

17.0

17.6

rnnmpntal Prr>1

Surface and

shallow subsurface

runoff

9.59

6060

5.48

21.8

10.2

hpntinn Aorpnr

Stream base

flow

231.67

00

22.220.2

18.7

 vC1984^ RRR. i

Recharge

17.02

612.68611.85

28.821.723.5

3.S. Riiminka. (1

Change in

storage

0

00

14.5

3.16

9.88

T.R.Gpnlnoriral

Source and

technique used to

compute

recharge 1

RCL, DPM

DBS, PDPMDBS, PDPM

SNB, HSPFSNB, HSPF

SNB, HSPF

Rnrvpv.

§s

^oQhHO5

M

0

O

H

a

H"a ci

Q H

written commun., 1991) and Woodward and others (1995); DPM, Bauer and Vaccaro (1987); RCL, R.C. Lane (U.S. Geological Survey, written commun., 1993); DBS, Sapik and others (1988); PDPM, 

Unpublished Fortran program, precursor to DPM (H.H. Bauer, U.S. Geological Survey, written commun., 1985); and SNB, Berris (19%). 

o 

Ground-water divide encompasses much greater area than surface-water divide, and base flow includes ground-water flow that originates outside of the surface-water basin.

QSame as footnote 2, or part of identified stream base flow is actually surface and shallow subsurface runoff. 

Stage-discharge relationship, probably in error due to accumulating sand at gage. 

Includes a 6.33-square-mile lake.

02O

Recharge calculations made under the assumption that runoff is negligible.

03

MO

OS 00
D34 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

123'

BRITISH COLUMBIA

WHATCOM 

Mt Baker

SedroWoolley Skagit

CLALLAM SNOHOMISH

| Olympic /

GRAYS 

 A HARBOR

EXPLANATION

I I Basin or area 

Information shown 

in table 4

:'.T^ Basin Information 

shown in table 7

2 Basin identification

number Information 

shown in tables 4 and 7

SCALE 1:1,500,000 

0 10 20

0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972

FIGURE 11. Location of 26 basins or areas where recharge was estimated by investigators using daily climate data and 7 basins where

recharge, estimated during this study, is compared to stream!low.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D35

to 60.7 in.; runoff 0 to 21.8 in.; AET 11.94 to 20.60 in.; 

base flow (excluding the largest values, see footnote 

table 4) 0 to 22.2 in.; and recharge 4.77 to 28.8 in. The 

large range in the estimates of ground-water recharge is 

due to the large range in the precipitation, AET, and 

runoff water-budget components. Additionally, three 

other factors affect the large range in values shown in 

table 4. (1) Streamflow for basins 4, 14, 15, 16, and 21 

includes a ground-water component that originates 

from outside the basin; (2) streamflow was assumed to 

be zero for the analyses of basins 22 and 23; and (3) 

basins 9 and 10 have the largest fraction of impervious

area.

The information presented in table 4 is summarized 

by descriptive statistics in table 5; correlation matrices 

and regression relations between water-budget components developed on the basis of the mean annual values 

(excluding base flow) of table 4 and annual values for 

basin 19 also are presented in table 5. The statistical 

summary clearly shows the large ranges in values 

described. The correlation matrices show, as expected, 

that the strongest relation exists between precipitation 

and the other water-budget components. However, the 

expected relation between recharge and runoff is not 

stronger because some runoff values include water 

derived from outside of a basin, recharge does not

TABLE 5. Information and statistics on the relation between water-budget components 

[Based on the annual water-budget components from basins in table 4; R2, regression correlation coefficient squared]

Variable

Precipitation (P)

Recharge (R)

Actual evapotranspiration (AET) 

Surface/subsurface runoff (RO) 1 

Base flow (BSF) 2

Mean

38.20

13.95

16.76 

7.72 

13.68

Minimum

25.17

4.77

11.94 

0 

0

Maximum

Values,

60.70

28.80

20.60 

21.80 

54.18

20th 

percentile

Percentiles

50th 

percentile

80th 

percentile

percentage 

of precipitation

in inches per year

32.74

10.14

14.51 

4.40 

5.40

37.25

12.71

16.95

7.88 

10.83

42.16

17.40

19.14 

10.31 

20.74

-

37

44 

20 

36

Correlation matrix. 3 4 Correlation matrix

R P AET RO BSF

R 100 74 3 7 39

P 74 100 39 65 27

AET 3 39 100 9 -2

RO 7 65 9 100 1

BSF 39 27 -2 1 100

R

R 100

P 77

AET -11

KO 21

BSF 31

Regression equations showing relation between water-budget components

R = 0.98 P - .99 RO - 1.00 AET + 1.00 

R = 0.96 P - .98 RO - 1.01 AET +1.81

R = 0.497 P - 5.03 (R = 0, when P = 10.12 in.)d

R = 0.517 P - 5.62 (R = 0, when P = 10.88 in.)

R = 0.542 P - 6.06 (R = 0, when P = 11.18 in., till-covered areas)

R =0.838 P - 9.77 (R = 0, when P = 11.66 in., outwash-covered area)

P AET

77 -11

100 29

29 100

69 4

9 -17

Equation 

number

(1)

4 (2)

(3)

4 (4)

5 (5)

5 (6)

RO BSF

21 31

69 9

4 -17

100 -14

-14 100

R2

0.995 

.995

.540

.589

.727

.921

1Assumed zero for basins 22 and 23 (table 4), Actual values probably 5 to 10 percent of precipitation; recharge is a maximum estimate.

2Basins 4,14,15,16, and 21 (table 4) include base flow from ground-water flow that originates outside of the basin.

Correlation coefficients multiplied by 100.

4Values for basins 9,10, 22, and 23 (table 4) not included in analysis.

5Relation based on 21 years of annual values for basin 19 (table 4).
D36 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

always discharge within a basin, and runoff was 

assumed to be zero for basins 22 and 23.

Equations 1 and 2 (table 5) show the overall relation 

among water-budget components and were derived 

with recharge as the dependent variable; each component contributes to the explained variance in the calculated recharge values. For example, the percentage of 

the variance of recharge (R) explained by precipitation 

(P), surface and subsurface runoff (RO), and AET in 

equation 1 is 54, 28, and 18 percent, respectively. The 

dominant control on recharge is thus the quantity of 

precipitation. This is further shown by the relation 

between mean annual precipitation and recharge presented in figure 12. Therefore, two regression relations 

(equations 3 and 4, table 5) were initially developed 

using only precipitation as the independent variable; 

the first used all 26 mean annual values from table 4, 

and the second used 22 mean annual values. For 

equation 4, basins 9, 10, 22, and 23 in table 4 were not 

used; basins 9 and 10 would have smaller than average 

R because of the large impervious area cover, and 

basins 22 and 23 would have larger than average R 

because RO was assumed to be zero for these areas.

The scatter in the data in figure 12 is due to the variability in water-budget components other than precipitation, which, in turn, are affected by three physical 

factors: surficial geology (fine-grained deposits mainly till, glaciomarine, and mudflow deposits in contrast to coarse-grained deposits mainly 

outwash and alluvial deposits), land use and cover (forest, pasture, residential, urban), and land-surface slope.

For watersheds with similar cover and slope, Dinicola (1990) found that the surficial geology was the primary control on the quantity of recharge. Soils 

generally are not a factor because most soils in the glaciated lowlands are highly permeable, with infiltration 

capacities exceeding most rainfall intensities. Thus, the 

underlying surficial geologic materials control the rate 

at which soil water can move downward. In areas 

underlain by fine-grained deposits, more water will 

move as subsurface flow to streams than in areas underlain by coarse-grained deposits; in the areas underlain 

by coarse-grained deposits, the soil water can more 

freely percolate downward to the water table. Therefore, after incident precipitation is abstracted by the ET 

processes, the dominant control on ground-water 

recharge becomes surficial geology.

Land use and cover control the quantity of AET, 

which affects the quantity of recharge. For example, 

Dinicola (1990) estimated that AET from forests is about 

4 to 5 in. more than AET from grasses; that difference is 

about the same as that described in the literature 

(Pearce and others, 1980). Note that before development, most of the Puget Sound Lowland was covered 

by forests with highly permeable soils and a well-developed forest litter with a large water-retention capacity. 

For the grass-covered areas or areas cleared of forests, 

the additional water not used by AET is available for 

runoff or recharge generally increased runoff in areas 

covered by fine-grained deposits or increased recharge 

in areas covered by coarse-grained deposits. Recharge 

and AET generally are zero for impervious areas, and

32

28

24

a 20

16

S 12

25

23-

Basin identification number 

(table 4)

 Equation 3 (table 5)

 Equation 4 (table 5)

----- Equation 5 (table 5)

---Equation 6 (table 5) 

i i i i i i i i i i

20 25 30 35 40 45 50 55 60 65 

Mean annual precipitation, in inches

FIGURE 12. Relations between mean annual precipitation and recharge for 

26 basins or areas.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D37

the incident precipitation becomes runoff (see for example, basins 9 and 10 in table 4, the two basins with the 

largest percentage of impervious area and plotted as the 

two lowest points in figure 12). In urban and industrial 

areas, most of the runoff is directed to drains and sewers, whereas in residential areas this runoff may be 

directed to manmade ditches, stormwater detention 

ponds, or small streams. For the latter case, some of the 

runoff may percolate downward to the water table. 

Additionally, recharge in residential areas may be 

altered by grading and subsequent importing of finegrained soil. A part of the permeable soils is removed, 

the remaining soils are graded and compacted, reducing the effective infiltration rate, and fine-grained top 

soil is applied. Together, these act to limit the potential 

recharge rates.

Land-surface slope affects the quantity of recharge 

by controlling the quantity of incident solar radiation 

that drives AET and by providing larger downslope 

gravitational control on the movement of soil water. 

Dinicola (1990) showed a decrease in AET for steep hillslopes, especially north-facing ones. Dinicola (1990) 

also showed that for the headwater basins investigated 

in King and Snohomish Counties, a large part of the 

available water in steep topography moved downslope 

as runoff to streams or to outwash-covered areas where 

it infiltrates.

ESTIMATES OF MEAN ANNUAL RECHARGE

Developing regional estimates of recharge is problematic because of the complex and highly variable controlling factors. However, the information presented in 

table 5 and figure 12 suggests that regional estimates of 

mean annual recharge can at least be approximated. 

Although equations 1 and 2 show that each water-budget component explains some of the variance in the estimated recharge, the AET and RO components generally 

are unknown for large areas. Thus, only the precipitation water-budget component can be used in regressions that are applied regionally.

Regional estimates of mean annual recharge then 

can be made using equation 3 or 4. However, work 

completed in this study includes a compilation of surficial geology in terms of hydrogeologic units for the 

Puget Sound Lowland. This compilation allows for 

identification of areas covered by fine-grained or 

coarse-grained deposits (Jones, in press; see also fig. 6), 

and as described previously, the surficial geology is a 

dominant control on recharge. Because of the effect of 

surficial geology, S.S. Sumioka and R.S. Dinicola (U.S. 

Geological Survey, written commun., 1992) used the 21 

annual values of precipitation and recharge for each of 

the DPM-cells for Big Soos Creek (basin 19, table 4) 

delineated by till and outwash to develop regression

relations. The information for Big Soos Creek was used 

because of the longer period of model simulation. 

However, for the same mean annual precipitation, the 

resulting regression relations (equations 5 and 6, table 5) 

would estimate larger values of recharge than either 

equation 3 or 4. Additionally, equation 4 may overestimate recharge for till-covered areas because slow drainage out of both the overlying soils and the till can easily 

be misidentified as base flow instead of subsurface runoff. For example, streamflow data were collected for 

more than 2 water years in three headwater basins 

located in the lowlands as part of an extensive fieldbased study to estimate recharge through till. These 

three basins are completely covered by till, and the 

streamflow-gaging sites were located above the water 

table in the till (H.H. Bauer, U.S. Geological Survey, 

written commun., 1994). H.H. Bauer (U.S. Geological 

Survey, written commun., 1994) showed that the 

streamflow hydrograph, which represents drainage out 

of the soil and till, is similar in shape to other hydrographs that include a contribution from ground water.

Considering the preceding information, preliminary 

estimates of recharge for the three headwater basins 

and the estimated range in the vertical hydraulic conductivity of the till described previously, the regression 

equation that estimated the least quantity of recharge 

for the range in mean annual precipitation for the aquifer system was selected to be applied to fine-grained 

deposits. Mean annual recharge to the aquifer system 

was thus estimated using equation 3 for areas underlain 

by till and other fine-grained deposits (3,320 mi2) and 

equation 6 for areas underlain by outwash and other 

coarse-grained deposits (3,863 mi2). The estimates are 

for all nonbedrock areas. The distribution of precipitation was based on the mean annual precipitation for 

1930-57 for the State of Washington (U.S. Weather 

Bureau, 1965) and the mean annual precipitation for 

1951-80 for the part of the aquifer system that underlies 

British Columbia, Canada (Environment Canada, n.d.). 

Next, to account for land use and cover, the estimates of 

recharge for urban-type areas (more than 90 percent of 

the surface covered by impermeable materials) were set 

to zero, recharge for built-up areas (about 95 percent of 

area covered) was reduced by 75 percent, and recharge 

for residential areas with large population densities 

(about 50 percent of area covered) was reduced by 

50 percent. These areas were identified using digital 

land-use and cover data (U.S. Geological Survey, 1990). 

The resulting spatial distribution of the estimated mean 

annual recharge is shown in figure 13, and the results of 

the analysis are given in table 6.

Several limitations should be noted concerning the 

derived estimates of recharge (table 6; fig. 13). First, 

recharge estimates for the mainly mountainous areas
D38 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

CANAOA_ __ BRETISHCrLUMBIA 

UNITED STATES ""S^"

Mean annual 

recharge, in 

inches

I I Ito6 

I I 7 to 11 

^B 12 to 16 

 17 to 21 

22 to 26 

27 to 36 

37 to 50 

More than 50

I T3 Basement confining 

unit Basal and 

lateral boundary of 

the Puget Sound 

aquifer system

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972

FIGURE 13. Estimated distribution of mean annual ground-water recharge for the Puget Sound aquifer system.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D39

TABLE 6. Estimates of mean annual recharge on the basis of mean annual precipitation, generalized surficial geology, and land-use

and cover categories

[mi2, square miles; in/yr, inches per year, ft3/s, cubic feet per second]

Surficial geology

Fine-grained deposits

Land use and cover

Undeveloped and nonbuilt up

Residential

Built up

Urban

All categories

Area

(mi2 )

2,709

51

471

89

3,320

Recharge1

(in/yr)

20.0

10.5

7.4

0

17.6

Recharge 1 Predpitation

(ft3/s) (in/yr)

3,999

40

255

0

4,294

50.5

38.4

40.0

39.4

48.4

Precipitation

(ft3/s)

10,066

144

1,379

259

11,848

Coarse-grained deposits Undeveloped and nonbuilt up 3,314 40.1 9,800 59.6 14,542

Residential 51 18.6 70 41.3 154

Built up 355 13.2 346 43.3 1,131

Urban 143 0 0 46.8 494

All categories 3,863 35.9 10,216 57.4 16,321

Bedrock Undeveloped and nonbuilt up 7,699

Residential 3

Built up 29

Urban 92

All categories 7,823

93.1

42.8

44.6

74.6

92.7

52,829

10

97

503

53,439

Total2 15,006 13.1 14,510 73.8 81,608

Recharge not estimated for bedrock areas and areas covered by large-surface water bodies, see figure 13. 

2Recharge totals include a value of 0 for bedrock area.

receiving more than about 60 in/yr of precipitation may 

have a larger error than estimates for areas with less 

than 60 in/yr of precipitation because of the lack of precipitation data-collection sites (and thus, the data used 

to derive the mean annual precipitation contours). 

Next, because of the large variability in the hydraulic 

characteristics of the fine-grained deposits, the estimates for fine-grained covered areas may have a large 

error associated with them. Additionally, implicit in the 

linear relation for fine-grained deposits (equation 3) is 

the assumption that the effective infiltration rate 

increases with increasing precipitation. Within the lowlying parts of the study area, this rate varies from about 

0.0008 ft/d at 17 in/yr of precipitation to about 

0.006 ft/d at 60 in/yr. However, the recharge rate 

probably would approximate a constant value above 

some annual precipitation quantity because this 

increasing rate would exceed the maximum infiltration 

capacity of fine-grained deposits. Thus, rather than a 

linear relation such as defined by equation 3, a nonlinear relation that would approach some constant value 

of recharge above some mean annual precipitation 

quantity may better define the recharge-precipitation

relation for fine-grained deposits. In contrast, both the 

overlying soils and the outwash deposits have infiltration capacities that range from 6 to 20 in/hr. These 

capacities are much higher than precipitation intensities 

in the Puget Sound Lowland. Thus, at larger values of 

mean annual precipitation with attendant increased 

cloudiness and decreased temperatures (decreased 

AET), the percentage of the precipitation that becomes 

recharge should increase. This again implies a nonlinear relation, but one that displays gradual upward curvature. Last, for the urban-type areas where recharge 

was set to zero, no attempt was made to estimate potential recharge due to leaking waterlines, sewer lines, 

storm drains, infiltration in small open spaces, and 

other factors that might contribute to recharge. 

Recharge is not zero in these areas, but estimating these 

quantities was beyond the scope of this study

It should be noted that not all of the geologic settings found in the Puget Sound Lowland were represented in the 26 basins, for example, the setting where 

till directly overlies fine-grained interglacial deposits. 

Although this geologic setting is not common in the 

Puget Sound Lowland, it is known to occur (Lum and
D40 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

Walters, 1976). For this setting, the equations using precipitation as the dependent variable would yield larger 

estimates of recharge than would actually occur; estimated recharge would be greater than actual because of 

the low permeability of the fine-grained, predominately 

clay deposits underlying the till. Most areas with this 

geologic setting were too small for analysis at a regional 

scale. Thus, it should be noted that for localized investigations of areas with this type of setting, the regional 

estimates of mean annual recharge (fig. 13) developed 

during this study would probably need to be reduced 

substantially (Lum and Walters, 1976).

Mean annual recharge for the area overlying the 

Puget Sound aquifer system was estimated to be about 

27in/yr or 14,510 ft^/s (about 10,500,000 acre-ft of 

water 51 percent of precipitation). These estimates 

include about a 2-in/yr (1,050-ftVs) reduction in 

recharge due to land-use and cover effects described 

previously. For areas underlain by fine-grained deposits, recharge was estimated to be about 17.6 in/yr 

(36 percent of precipitation), and for areas underlain by 

coarse-grained deposits, recharge was about 35.9 in/yr 

(63 percent of precipitation). Estimated values ranged 

from zero in urban areas to more than 70 in/yr in some 

forest areas. Additionally, the total recharge is much 

larger than the estimated ground-water withdrawals in 

the Puget Sound Lowland, which was estimated for 

1990 to be 826 ftVs on the basis of information previously presented in the "Cultural Setting" section.

For comparison, the Puget Sound Task Force (1970) 

estimated annual recharge "to the aquifer system in the 

lowlands of the study area" (the Puget Sound drainage 

basin) to be "conservatively" 800,000 acre-ft, and the 

Pacific Northwest River Basins Commission (1970) estimated annual recharge to the glacial and post-glacial 

deposits to be 5,700,000 acre-ft. Neither of these estimates accounted for recharge to the Quaternary deposits in the mountainous areas where the estimated 

recharge rates are the largest. These values can be compared with this study's estimate of 9,350,000 acre-ft or 

12,915 ftVs (reduced by the quantity of 

recharge 1,595 ftVs estimated to occur in the part of 

the Puget Sound Lowland in Canada). The 12,915 ftVs 

of mean annual recharge represents 60 percent and 

26 percent of the median September and annual streamflow for the Puget Sound drainage basin in Washington.

RELIABILITY OF ESTIMATES

The reliability of the estimates of recharge (fig. 13 

and table 6) can be assessed by comparing them with 

values from selected additional studies, to streamflow 

in selected basins, and to seasonal water-level changes. 

Additionally, because the estimates are based on mean

annual values of precipitation, the potential interannual 

variability of recharge due to the interannual variability 

of precipitation also is of interest. These aspects are discussed in the following paragraphs.

In a study of the ground-water resources for 59 mi2 

of Pierce County, Drost (1982) used a water-budget 

method to estimate recharge to be about 86ftVs or 

20 in/yr. The area-weighted estimate of recharge 

derived for this study (fig. 13) for the same 59-mi2 area 

was 84 ftVs or about 19 in/yr. The estimates from 

Drost (1982) and from this study are about 39 and 

38 percent of mean annual precipitation, respectively. 

Hart Crowser and Associates, Inc. (1986) estimated 

recharge for five areas in a study for the city of Tacoma; 

these estimates were presented as a range in values for 

each area and appeared to account for the effects of 

impervious areas. Recharge was estimated to range 

from 10 to 17 in/yr (26 to 44 percent of mean annual 

precipitation) in four low-lying areas and to range from 

38 to 60 in/yr (63 to 75 percent of mean annual precipitation) in an upland area. The estimates of recharge 

(fig. 13) for these same small and large precipitation 

areas (low-lying and upland) ranged from 6 to 25 in/yr 

and 41 to 60 in/yr, respectively. These results indicate 

that the recharge values derived in this study are in 

general agreement with values estimated by other 

investigators.

Estimates of recharge for seven small lowland basins 

(fig. 7) ranging in area from 5.01 to 39.3 mi2 were compared with observed mean annual streamflow and 

median 7-day, low-flow values (table 7). The information in table 7 indicates that these basins include a wide 

range of physical and hydrologic characteristics present 

in lowland areas of the Puget Sound Lowland. Also, 

note that, as with streamflow in most basins in the 

Puget Sound Lowland, in these basins the mean annual 

flows are nearly equivalent to both the mean April 

flows and the mean daily discharge that is exceeded 

25 percent of the time. Therefore, these basins were 

assumed to be representative basins. Except for the 

case where streamflow in a basin is derived from 

ground water that originates outside the basin, mean 

annual recharge for a basin should be more than the 

median 7-day, low-flow discharge and less than the 

mean annual flow. Mean annual recharge was obtained 

from area-weighted values shown in figure 13.

Comparison of values of the estimated mean annual 

recharge and the observed streamflow for the basins 

(table 7) shows that recharge ranges from about 51 to 

97 percent of the mean annual flow, and the median 7- 

day, low-flow discharge ranges from about 14 to 

64 percent of the recharge. Although for every basin the 

estimated recharge is less than the mean annual flow 

and greater than the median 7-day, low flow, there is a
TABLE 7. Information for comparison of streamflow and estimated recharge for seven basins 

[mi2, square miles; ft 3/s, cubic feet per second; in/yr, inches per year; (ft3/s)/mi2, cubic feet per second per square mile]

Basin 

identification 

number1

27

28

29

30

31

32

33

Mean annual streamflow

Basin name

Dogfish Creek

Huge Creek

Burley Creek

Big Beef Creek

Quilceda Creek

Dewatto River

Goldsborough Creek

Area

(mi2 )

5.01

6.47

10.7

13.8

15.4

18.4

39.3

(ft3/s)

8.7

11.2

28.3

37.9

25.2

70.6

116.1

(in/yr)

23.6

23.5

35.9

37.3

22.3

52.1

40.1

Mean 

annual 

precipitation

(in/yr)

35.98

52.5

55.5

55.2

43.5

59.5

87.8

Mean annual recharge 2

(ft3/s)

5.3

10.6

21.2

24.7

17.6

35.6

113

(in/yr)

14.3

22.4

26.8

24.3

15.5

26.3

39

Mean 

- annual unit 

recharge

[(ft3/s)/mi2]

1.06

1.64

1.97

1.79

1.14

1.94

2.87

Median 

unit, 7-day, 

low flow

[(ft3/s)/mi2]

0.58

.65

1.27

.25

.27

.67

.52

Mean 

annual unit - 

streamflow

Kft3/s)/mi2]

1.74

1.73

2.64

2.75

1.64

3.84

2.95

Su

(p

Coarse

33

17

50

27

50

12

19

1Location of basin is presented by reference number in figure 11.

2Area-weighted average is based on distribution of recharge shown in figure 13.

3Coarse, coarse-grained deposits, generally alluvium or outwash; fine, fine-grained deposits, generally till; bedrock, bedrock/basement confining unit and may include some urba
D42 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

large range in percentages. The large range in percentages is partly due to the fact that some of the recharge 

will move vertically to deeper hydrogeologic units and 

that some may move laterally outside a basin so that 

some quantity of the recharge will not be available to 

supply water to a stream. In turn, the quantity that may 

or may not be available is controlled by the geologic 

and physical setting of a basin; these factors affect both 

the ground-water flow and surface runoff. In addition, 

the information presented in table 7 shows that the relation between ground-water and surface-water flow in 

the Puget Sound Lowland is highly variable and that 

recharge is only one of several factors that need to be 

considered in an analysis of basin runoff (also see 

table 4). The latter aspect is shown by the range in the 

median unit 7-day low-flow values (table 7). The smallest and largest values occur for Big Beef and Burley 

Creek Basins, respectively, which are similar in area and 

precipitation input. Although the estimated unit 

recharge values for these two basins are similar, the 

geologic and physical settings are different and, thus, 

affect the low-flow values. For example, because Burley 

Creek is in a low-lying basin located near saltwater, it 

obtains additional discharge, which has traveled over 

longer flowpaths, from the deeper ground-water 

system.

Water-level changes in the shallow deposits at or 

near the land surface generally can be grouped on the 

basis of three lithologic sequences: (1) coarse-grained 

deposits overlying till, (2) till, and (3) coarse-grained 

deposits directly underlying till. Seasonal water-level 

changes due to recharge from precipitation in these 

three sequences in the Seattle-Tacoma area typically are 

about 6 to 8 ft (Walters and Kimmel, 1968), 2 to 8 ft 

(Walters and Kimmel, 1968; Dion and others, 1983), and 1 

to 5 ft (Walters and Kimmel, 1968; Drost, 1982; Carr and 

Associates, 1983), respectively. Using the estimated 

maximum water-level changes (8, 8, and 5ft, respectively) and estimated specific yields of 0.25, 0.10, and 

0.20 for the three sequences or units, respectively, gives 

24.0, 9.6, and 12.0 in. as the quantity of water needed to 

account for the change in maximum water levels. The 

last value is based on the assumption that the waterlevel change in the deposits underlying till occurs 

under water-table conditions. Also, note that if the 

water-level changes in deposits underlying till are 

assumed to be supported mainly by water that has 

moved through the till, then the latter two values of 

recharge should be similar. Using a mean annual precipitation of 40 in/yr yields 14.85 in/yr of recharge for 

areas underlain by till (equation 3, table 5) and 

23.75 in/yr of recharge for areas underlain by outwash 

deposits (equation 6, table 5). These values are in general agreement with those estimated using maximum

observed water-level changes. Although the response 

of water levels to recharge from precipitation is more 

complex than just described, the recharge still needs to 

be sufficient to support observed water-level changes. 

For example, an estimated recharge value of 5 to 

10 in/yr could not support an 8-ft water-level change in 

the coarse-grained deposits and would indicate that the 

estimated value was too small.

The estimated regional recharge distribution (fig. 13) 

is assumed to represent a long-term average. As discussed previously, the interannual variability in these 

estimates can be expected to be generally similar to the 

interannual variability of precipitation. Therefore, to 

understand the variability of recharge in the Puget 

Sound Lowland, equations 3 and 6 (table 5) were used 

to estimate annual recharge for fine-grained and coarsegrained deposits, respectively, at two weather stations. 

These weather stations have a long period of record and 

are representative of the range in precipitation within 

the low-lying parts of the Puget Sound Lowland. The 

small-precipitation site (average water-year precipitation of 19 in/yr) is near Port Townsend, and the largeprecipitation site (average water-year precipitation of 

53 in/yr) is near Olympia. Other precipitation information for these sites is shown in figures 3A and 3B and at 

the beginning of this section.

Recharge at the Port Townsend site ranged from 0.15 

to 12.48 in/yr for fine-grained deposits and from 0 to 

19.56 in/yr for coarse-grained deposits (fig. 14). At the 

Olympia site, annual recharge ranged from 10.7 to 

35.5 in/yr for fine-grained deposits and from 16.7 to 

56.9 in/yr for coarse-grained deposits (fig. 14). The 

information presented in figure 14 thus indicates both 

the spatial and temporal variability in recharge and, 

potentially, in ground-water availability that may occur 

in the study area. The information also indicates the 

importance for detailed analyses of recharge for local 

studies and shows that the estimates of recharge for any 

location (fig. 13) are long-term averages of widely varying values.

The information shown in figure 14 indicates additional limitations and aspects concerning the relations 

given in table 5. First, note that, at about 13.5 in/yr of 

precipitation, the regression relation for coarse-grained 

deposits gives smaller quantities of recharge than does 

the relation for fine-grained deposits. This difference 

shows the problem of using relations for mean annual 

values to estimate annual recharge values. The problem 

is due to the fact that annual precipitation values may 

be outside of the range in values used to develop the 

regression relations (tables 4 and 5). This is the case for 

many of the precipitation values at the Port Townsend 

site. This particular problem is mainly related to small 

precipitation values (the limitations for large
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D43

---- Fine-grained deposits

1876 1886 1896 1906 1916 1926 1936 1946 1956 1966 1976 1986 

Water year

FIGURE 14. Estimated distribution of annual recharge at two

weather stations, representative of small- and large-precipitation

areas, for two different surficial geologic units.

precipitation values were described previously). The 

major aspect suggested by this limitation is the need to 

estimate recharge in a small-precipitation area for both 

types of deposits using daily water-budgeting methods 

for a long simulation period. This would provide additional information for (1) developing more accurate 

relations and (2) improving the understanding of the 

interannual variability in recharge in this climatic setting. However, although annual precipitation within 

the Puget Sound Lowland may be much smaller than 

the smallest value used to develop the relations, small 

values may not be a major limitation because the mean 

annual precipitation is more than about 16 in/yr everywhere in the Puget Sound Lowland and is more than 

27 in/yr in most of the Puget Sound Lowland.

The relation between the interannual variability of 

precipitation and recharge and the subsequent response 

of water levels because of recharge variations was recognized early in the Puget Sound Lowland by Walters 

and Kimmel (1968) and Anderson (1968). Walters and 

Kimmel (1968) show that the timing and magnitude of 

the winter peak in water levels is related to both differences and trends in annual precipitation. Walters and 

Kimmel (1968) further show that seasonal water-level 

changes can have annual differences of at least 4 ft; for 

1945-50, there was a positive trend in precipitation 

(fig. 4) and a corresponding trend in the annual water 

levels in selected wells of about 4 ft. Annual precipitation at Tacoma varied by as much as 20 in. during this

period, and equation 3 (table 5) indicates recharge could 

have varied by 10 in., or using a specific yield of 0.20, 

water levels could have varied by about 4 ft. Anderson 

(1968) shows the effects of variations in precipitation on 

water levels for 1964 and 1965; the winter peak in the 

water level for 1965 was about 4 ft higher than in 1964 

when precipitation was substantially less than 1965.

WATER-QUALITY CHARACTERISTICS

Previous assessment of ground-water quality in the 

Puget Sound Lowland has ranged from site-specific 

investigations of contaminants in ground water or soils 

(Tracey and Dion, 1976; Hart Crowser and Associates, 

1984; Lum and Turney, 1985; Turney and Goerlitz, 1988; 

and URS Consultants, Inc., 1992) to statewide and 

nationwide studies (VanDenburgh and Santos, 1965; 

Foxworthy, 1979; Molenaar and others, 1980; Lum and 

Turney, 1982). Specific investigations that have 

included assessment of seawater intrusion and general 

quality of the ground water for counties and other large 

areas have been carried out since the early 1970's 

(Walters, 1971; Drost, 1982, 1985; Whiteman and others, 

1983; Dion and Sumioka, 1984; Jones, 1985; Dion and others, 1988, 1994; Woodward and others, in press). Additionally, an assessment of the ground-water quality for 

the Puget Sound region (Turney, 1986) was completed as 

part of a statewide assessment. Ground-water-quality 

investigations of the part of the aquifer system in British 

Columbia have been completed by Kwong (1986), 

Kohut and others (1989), Liebscher and others (1992). A 

summary of the historical ground-water-quality data 

for selected constituents for the part of the study area 

within the State of Washington is given in table 8; both 

the average and median values of the analyses are presented in table 8. The data are based on 5,131 analyses 

that were in NWIS as of October 1992.

The dominant water types in the upper aquifer units 

are calcium bicarbonate, magnesium bicarbonate, or 

calcium magnesium bicarbonate. Deeper aquifer units, 

in addition, contain ground water of a sodium bicarbonate or a sodium chloride type. Generally, sodium 

bicarbonate water has been attributed to proximity to 

consolidated bedrock, to longer residence time in the 

ground-water system, and to the presence of finegrained marine deposits of recent age (Whiteman and 

others, 1983; Dion and others, 1988; G.L. Turney, U.S. 

Geological Survey, written commun., 1994). The sodium 

bicarbonate water also contains larger concentrations of 

dissolved solids. Sodium chloride water generally is 

associated with mixing of native ground water with 

seawater; this water also has larger concentrations of 

magnesium.
D44 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

TABLE 8. Summary of historical ground-water-quality data

[Values in milligrams per liter unless otherwise indicated; cols/100 mL, colonies per 100 milliliters of water; ug/L; micrograms per liter]

Constituent

Specific conductance (microsiemens per centimeter at 25 degrees Celsius)

pH (standard units)

Hardness (as CaCOg)

Alkalinity, total (as CaCO 3 )

Dissolved solids, calculated (sum of constituents)

Calcium, dissolved

Magnesium, dissolved

Sodium, dissolved

Sodium-adsorption ratio

Potassium, dissolved

Sulfate, dissolved

Chloride, dissolved

Silica, dissolved (as SiO 2 )

Nitrate plus nitrite (as N), dissolved

Iron, dissolved ( ug/L)

Manganese, dissolved ( ug/L)

Bacteria, fecal-coliform (cols/100 mL)

Average

365

7.3

81

81

164

17

8.2

23

1.2

2.4

9.2

55

30

1.4

920

130

1.2

Median

180

7.3

60

65

112

13

5.9

6.5

.4

1.8

5.0

4.7

29

.10

38

17

1.0

Number of sampling 

sites

14,414

1,905

1,960

1,113

1,668

1,871

1,871

1,735

1,699

1,722

1,789

^,371

1,713

1,117

1,314

1,207

1,072

1Most of the samples collected as part of seawater-intrusion investigations.

Although large concentrations of chloride are typically found in ground water near the shoreline, S.E. Cox 

(U.S. Geological Survey, written commun., 1993) found 

large concentrations of chloride in water at depth in 

deposits several miles inland from the shoreline in the 

Fraser-Whatcom Basin in Whatcom County. It appears 

that this water is contained in glaciomarine drift. After 

the drift was deposited, its subsequent burial at shallow 

depths during the Sumas Stade may have been so rapid 

that the contained saltwater was not flushed out. 

Within the part of the study area in British Columbia, 

Halstead (1986) mapped large concentrations of chloride in ground water in both the glaciomarine drift and 

some of the alluvial deposits. The water-level configurations (fig. 10) indicate that the chloride in the alluvial 

deposits is derived from water that has moved through 

the glaciomarine drift into the alluvial deposits.

Ground water in the study area is generally soft to 

moderately hard, as indicated by the median hardness 

value of 60 mg/L (table 8). The hardness is determined 

by the calcium and magnesium concentrations in the 

ground water; in areas with larger concentrations of 

these constituents, the water may vary from hard to 

very hard, such as in parts of San Juan and Island Counties (Turney, 1986). Because sodium is not a dominant 

cation in ground water in the Puget Sound Lowland, 

the sodium-adsorption ratio (SAR) is generally less than 

2 and has a median value of 0.4. In areas with larger 

sodium concentrations or a sodium chloride water type, 

the SAR has been found to be as large as 30; these areas 

are usually associated with both seawater intrusion and 

larger concentrations of dissolved solids (and larger 

specific-conductance values). Dissolved solids generally are in the range of 100 to 150 mg/L; larger values in 

the range of 300 to 500 mg/L are found in the ground 

water in low-lying areas adjacent to Puget Sound and 

are commonly related to seawater intrusion. Generally, 

large dissolved-solids concentrations are associated 

with a sodium chloride water type, and small concentrations are associated with calcium bicarbonate and 

calcium magnesium bicarbonate types.

Elevated concentrations of iron and manganese as 

large as 2,500 and 650 fig/L, respectively, have been 

found in the glacial aquifers in the Puget Sound Lowland. Water-sample analyses in some of the coastal 

areas also have shown that manganese can be more 

abundant than iron, which is atypical of glacial aquifers 

(Dion and others, 1988). The large differences between
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D45

the average (which is affected by extreme values) and 

median values for both dissolved iron and manganese 

indicate that quite a few sites had elevated concentrations.

Concentrations of dissolved nitrate plus nitrite 

(herein referred to as nitrate) generally are small in the 

ground water in the study area; most reported values 

are less than 1.0 mg/L expressed as nitrogen, and the 

median nitrate concentration is about 0.10 mg/L. The 

order of magnitude difference between the average and 

median nitrate indicates that locally large concentrations of nitrate have been observed. Moderate to large 

concentrations of nitrate have been found in Thurston, 

Pierce, Skagit, and Whatcom Counties. Larger concentrations in parts of Thurston and Pierce Counties may 

be derived from septic-waste disposal systems in the 

extensively developed, unsewered residential areas. 

The larger concentrations found in Skagit and Whatcom 

Counties, both of which have large agricultural areas, 

may be derived from feedlot wastes or fertilizers. The 

sampled wells with larger concentrations of nitrate generally occur in areas overlain by coarse-grained deposits and, thus, may be attributed to the rapid movement 

of water with large nitrate concentrations to the water 

table. The vertical distribution of tritium concentrations obtained from soil water in unsaturated till shows 

pre-1964 water still contained within the till in some 

areas (H.H. Bauer, U.S. Geological Survey, written commun., 1994), indicating that large concentrations of 

nitrate are not found in aquifers covered by till or finegrained deposits because water with large concentrations of nitrate may not yet have reached the water 

table.

Although Turney (1986) reported a maximum value 

of nitrate of 9.3 mg/L in Whatcom County, a 1992 study 

of part of Whatcom County has observed values of 

more than 40 mg/L (S.E. Cox, U.S. Geological Survey, 

written commun., 1993). In this study of Whatcom 

County, samples collected from closely spaced wells 

open to different depths in the aquifer system showed 

that the largest concentrations were found in the shallow wells and that there was a rapid decrease in nitrate 

concentrations with depth, indicating that the large concentrations were probably due to local sources at the 

land surface. This part of the aquifer system in Whatcom County is contained in the structural Fraser-Whatcom Basin, which includes the Fraser Lowland of 

British Columbia. Kwong (1986), Kohut and others 

(1989), and Liebscher and others (1992) also found elevated concentrations of nitrate in part of the Puget 

Sound Lowland within British Columbia. About onethird of the major aquifer unit (the Abbotsford aquifer) 

studied by Kohut and others (1989) and Liebscher and 

others (1992) extends into Whatcom County. Liebscher

and others (1992) observed nitrate concentrations in the 

ground water that were as much as 41 mg/L and concluded that large nitrate concentrations generally are 

site specific, fluctuate seasonally, and are due to landuse activities; additionally, 12 of 23 pesticides that were 

sampled for were detected in the ground water.

Seawater has intruded some aquifers near the coast, 

most commonly in the island and peninsular areas of 

the Puget Sound Lowland and near large coastal pumping centers. Aquifer units that are below sea level contain larger concentrations of chloride and show 

seasonal fluctuations in concentrations in selected locations (Sapik and others, 1988). A chloride concentration 

value of more than 100 mg/L has been used as an indicator of seawater intrusion in the Puget Sound Lowland 

(Walters, 1971; Dion and Sumioka, 1984; Dion and others, 

1988; Sapik and others, 1988). Seawater intrusion has 

been most prevalent in San Juan and Island Counties. 

For example, in a study of San Juan County, Whiteman 

and others (1983) showed that of 279 wells sampled in 

1981, 9 percent had chloride concentrations indicative 

of seawater intrusion. A similar study of Island County 

showed that about 20 percent of 330 wells sampled 

were in areas intruded by seawater (Sapik and others, 

1988). Seawater intrusion is most prevalent in these 

counties because of the small recharge quantities 

(fig. 13), low topographic relief, and the small extent of 

the Puget Sound aquifer system (fig. 3).

In summary, the quality of the ground water in the 

Puget Sound aquifer system is good and suitable for 

most uses. Larger concentrations of selected constituents can be attributed to locally derived sources. The 

large quantity of recharge that enters the aquifer system, large values of hydraulic conductivity, and short 

flowpaths generally result in a nearly vertically homogeneous system with respect to general constituent concentrations. Proximity to saltwater and selected types 

of bedrock can locally affect the chemistry, and thus, the 

quality of the flowing ground water. The potentially 

important water-quality problems are related to seawater intrusion and locally derived sources of nitrogen. 

However, site-specific ground-water contamination has 

occurred and may continue to occur because of anthropogenic sources. Although rarely a health problem, 

larger concentrations of iron and manganese in parts of 

the glacial aquifers may be considered a significant 

aesthetic problem.

GROSS-SECTIONAL NUMERICAL 

MODELS OF GROUND-WATER FLOW

Six cross-sectional numerical models of groundwater flow were constructed to provide (1) an improved
D46 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

understanding of both the ground-water flow system 

(hydraulic-head distribution and water budgets) and 

the major controls on the flow system and (2) information for developing a simplified regional conceptual 

model of the aquifer system. The grid systems and 

hydraulic characteristics of the models were varied by 

aggregating hydrogeologic units (simplifying the aquifer system geometry) to test conceptual models of the 

aquifer system. The models were constructed for 

hydrogeologic sections in "type" areas; traces of the sections are shown on plate 1. To the extent possible, the 

sections were constructed so that they lie along flowpaths as delineated by the generalized water-level configurations (fig. 10) and by local studies (Sapik and 

others, 1988; G.L. Turney, U.S. Geological Survey, written 

commun., 1994; Woodward and others, 1995). The relations between model layers (plate 1) and the local 

hydrogeologic units discussed previously are given in 

table 9.

Ground-water flow was simulated using the finitedifference model MODFLOW (McDonald and Harbaugh, 1988) that was modified by Hansen (1993). The 

models were operated under steady-state conditions 

with the top active cell as a water table and all other 

cells fully convertible to a water table. The modelcalculated heads and water-budget components represent long-term mean annual values.

The following sections describe (1) the construction 

of the cross-sectional models (including the model grid, 

boundary conditions, and hydraulic characteristics), 

calculated ground-water flow, and calculated waterbudget components and (2) evaluation of the results of 

the models and other pertinent information for developing a simplified conceptual model of the aquifer system. The latter is discussed in conjunction with the 

description of the flow systems and water budgets.

MODEL GRID SYSTEMS

The model grid systems for the six models for both 

the detailed and aggregated aquifer-system geometry 

are shown on plate 1, and general information for the 

grid systems is given in table 10. The shortest section 

(D-D', Bainbridge Island-2) is about 2.3 mi long, and 

the longest section (£-£', South King County) is about 

19.5 mi long. Where applicable, sections initially were 

extended offshore to account for the saltwater interface.

The vertical discretization of the aquifer system was, 

with one exception, based on the number of mapped 

hydrogeologic units in each section. Because of lack of 

information on the subsurface distribution of units 

below the small topographic high situated in the Green 

River Valley in section E-E' and the outcrop of coarsegrained materials at the eastern base of this high, the 

alluvial unit was assumed to exist throughout the valley 

and, thus, was defined as one unit. Each unit was discretized and simulated as a separate model layer; the 

deeper undifferentiated deposits generally were arbitrarily discretized with two layers. Where both layers 

of undifferentiated deposits exist, the upper layer was 

defined by a relatively constant thickness. As a result, 

the models were constructed with 6 to 10 layers 

(table 10). The vertical discretization for the model cells 

of each layer was based on the average thickness of the 

mapped unit for the appropriate cell length and location. Horizontal discretization for the model cells was 

based on the extent of the mapped units over the length 

of a section. The longitudinal cell size was uniform 

within each cross section but varied from 1,100 to 

1,800 ft among sections. The transverse dimension or 

width size perpendicular to the section was set equal to 

the longitudinal dimension (cells, thus, were square in 

the horizontal dimension). The cell size used in the 

models allowed for simulation of local flow systems.

The models initially were operated to determine the 

reasonableness of both the boundary conditions (discussed in the next section) and the initial discretization 

of saltwater boundaries. The location of the freshwatersaltwater interface for each model, where appropriate, 

also was calculated using the Ghyben-Herzberg equation (Hubbert, 1940) under the assumption that the 

hydraulic-head distribution in the freshwater part of 

the flow system is hydrostatic. Although the position of 

the interface estimated using the Ghyben-Herzberg 

equation is not exact due to the effects of vertical head 

gradients in the freshwater and vertical and horizontal 

head gradients in the saltwater, and acknowledging 

that the interface is affected by variations in tides, currents, recharge, and pumpage, it is still reasonable to 

use this information to guide the discretization of the 

aquifer system, especially considering the purpose of 

constructing the models and the potentially large width 

of the saltwater transition zone in the Puget Sound 

Lowland (Sapik and others, 1988; R.C. Lane, U.S. Geological Survey, written commun., 1993). Together, the 

results from the preliminary model simulations and the 

interface calculated using the Ghyben-Herzberg equation were then used to define the freshwater-saltwater 

boundary used in the subsequent simulations. Additionally, for sections A-A and B-B'', the model results of 

Sapik and others (1988) were used as the principal guide 

for defining the freshwater-saltwater boundary.

The model grid systems were later modified to test a 

conceptual model of the regional aquifer system. Several layers were aggregated (combined) into a single 

layer for each of the models for this test. Calculated 

heads, hydraulic gradients, water-budget components,
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D47

TABLE 9. Relation between model layers and local hydrogeologic units and the potential relations between hydrogeologic and

stratigraphic units

Section 

(plate 1) Layer

A-A', B-B' 1

1

2

3

4

5

6

7

8

9

C-C', D-D' 1

2

3

4

5,6

E-E' 1

2

3

4

5

6

7

8,9

F-F 1

2

3

4

5

6

7,8

2 Hydrogeologic unit

Or

Qt

aql

nonaql

aq2

nonaq2

aq3

nonaqS

aq4

nonaq4

bcu

Qr

aql

nonaql

aq2

undiv

bcu

al

Or

Qt

aql

nonaql

aq2

nonaq2

undiv

bcu

al

Qr

Qt

aql

nonaql

aq2

undiv

bcu

0

Stratigraphic unit

Everson "glaciomarine" Drift recessional outwash

Vashon till

Advance outwash Esperance Sand Member

Lawton Clay/Pilchuck Clay/Quadra Formation

Quadra Formation

Possession Drift

Lower Possession Drift upper Whidbey Formation

Whidbey Formation

Double Bluff Drift

?

Bedrock, basement confining unit

Vashon till

Advance outwash Esperance/Colvos Sand Member

Kitsap Formation

Upper Salmon Springs Drift

Salmon Springs Drift and older deposits

Bedrock, basement confining unit

Alluvium, Green River

Recessional outwash

Vashon till

Advance outwash or advance outwash Colvos

Sand Member

Advance outwash or Kitsap Formation

Advance outwash Colvos Sand Member or Salmon

Springs Drift

Kitsap Formation or Puyallup Formation

Salmon Springs Drift and older deposits or Stuck

Drift and older deposits

Bedrock, basement confining unit

Alluvium, Nisqually River

Recessional outwash

Vashon till

Advance outwash Colvos Sand Member

Kitsap Formation (Salmon Springs till?)

Upper Salmon Springs Drift

Salmon Springs Drift and older deposits

Bedrock, basement confining unit

Layers shown on plate 1, detailed geometry. ^Jnits described in "Hydrogeologic Units." See figure 5.
TABLE 10. Information for the model grid systems for six cross-sectional ground-water flow models 

[T, topographic high, ground-water divide; T*, T and also terminates at bedrock; B, bedrock outcrop]

Section

(plate 1)

A-A'

B-B'

C-C'

D-D'

E-E'

F-F'

Cross section name

Whidbey Island

Whidbey-Camano

Island

Bainbridge Island-1

Bainbridge Island-2

South King County

Thurston County

Length

(miles).

6.25

15.62

4.79

2.29

19.25

14.66

Section

origin

T*

T

T*

T

T

B

Number of

layers

9

10

6

6

9

8

Number of

cells

113

359

90

56

357

166

Length of

cells1

(feet)

1,650

1,650

1,100

1,100

1,320

1,800

Maximum 

landSUTIclCG

altitude of

cells

(feet)

250

485

330

395

560

415

Length of 

section

underlying

saltwater

(miles)

1.56

6.25

1.46

.21

1.0

1.36

Minimum/maximum 

thickness

Shallow

system2

(feet)

5/115

10/280

9/82

5/54

5/320

10/210

De

syst

(fe

25/4

176/

35/9

45/1

35/8

5/9

*Width of cell was defined to be same as length, that is, horizontal dimension were the same a square. 

2Thicknesses for upper (shallow) aquifer system where well data allowed identification of hydrogeologic units. 

3Thicknesses are for deeper aquifer system that was not differentiated.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D49

locations of discharge, and flowpaths then were compared for these two formulations of the aquifer-system 

geometry. The comparison was done to determine if the 

calculated flow system for the aggregated or simplified 

geometry retained the principal features of the more 

complex natural system and, thus, could be used to 

help develop a conceptual model of the aquifer system. 

For the aggregation, excluding the alluvial unit 

present in the alluvial valley in section E-E', all the layers above the first major (areally extensive) finegrained, generally clay unit were combined into one 

layer. The fine-grained layer was chosen so that it generally corresponded to the first major interglacial unit, 

generally the Kitsap or Whidbey Formations (fig. 5; 

table 9); these units generally are located near sea level 

where they abut saltwater bodies. The fine-grained unit 

was retained as a layer, as were the layers representing 

the deeper undifferentiated deposits; the fine-grained 

unit and the deeper deposits were described previously 

in the "Hydrogeologic Units" section the fine-grained 

unit as "nonaq" and the deeper deposits as "undiv" (also 

see table 9). In addition, to test what was possibly the 

maximum aggregation of units, all units above the finegrained (nonaq) unit that lie below the sea-level nonaq 

unit were combined into a single layer for sections A-A 

and B-B'. The aggregated or simplified layers also are 

shown on plate 1.

BOUNDARY CONDITIONS

The boundary conditions for the cells at land surface 

(water-table cells) for all models were input mean 

annual recharge rates. Total flux to each of these cells is 

calculated by the model as the input rate (table 11) multiplied by the area of the cell. Excluding sections A-A 

and B-B', the recharge rates were estimated on the basis 

of estimates of recharge presented in figure 13. For sections A-A and B-B', estimates of recharge for a threedimensional model of the ground-water flow system of 

Island County (Sapik and others, 1988) were used as 

model input. The boundary condition for the contact

between the Quaternary deposits and the basement 

confining unit was simulated as no flow. Additionally, 

the south end of section D-D', a ground-water divide, 

was simulated as no flow.

The boundary condition for each cell at the downstream, terminal end or ends of each section within saltwater was defined as either a head-dependent function 

(HDF) or a no-flow boundary. The HDF was used to 

simulate seepage faces or spring discharge. The controlling head for the cells with a HDF in direct contact 

with saltwater was defined as (set equal to) an equivalent freshwater head using the rewritten GhybenHerzberg relation:

Hf = 0.021 xD (7)

where //, is the controlling head (equivalent freshwater 

head referenced to sea level); D is the depth, in feet, of 

the center of the cell below sea level; and 0.021 is the 

average difference in specific gravity between the saltwater of Puget Sound (1.021) and freshwater (1.0). The 

head-dependent boundary condition was used to 

calculate the quantity of water discharging from the 

ground-water system to saltwater. The extension of the 

sections, where applicable, to include deposits underlying saltwater allows for an improved simulation of that 

discharge. Under the assumption of a sharp interface 

and movement of water parallel to it, no-flow boundaries were used for selected interface cells for the 

sections with an extensive interface; for example, see 

sections C-C and F-F on plate 1.

For all layers that terminate at a bluff or in a canyon, 

a head-dependent boundary condition also was used. 

These boundaries were simulated using either the drain 

or the general head boundary (GHB) module of MODFLOW; the GHB's were only used in simulation of flow 

for sections C-C' and E-E'. In the mathematical formulation of MODFLOW, ground water in drain cells can 

only discharge water from the aquifer system, and 

ground water in GHB cells can both discharge and 

recharge the system. The altitude chosen for the controlling head for these boundaries was the bottom

TABLE 11. Ranges in recharge estimates and hydraulic characteristics for six cross-sectional ground-water flow models

Section

A-A'

B-B'

C-C'

D-D'

E-E'

F-F

Cross section name

Whidbey Island

Whidbey-Camano Island

Bainbridge Island-1

Bainbridge Island-2

South King County

Thurston County

Recharge 

(inches per year)

6.09

5.76

13.86

16.32

13.63

15.60

-10.27

-14.80

- 14.28

- 17.70

-34.07

-30.17

Horizontal hydraulic 

conductivity 

(feet per day)

0.01

0.01

0.52

0.02

0.16

6.05

- 302.4

- 216.0

- 10.37

- 60.48

- 66.53

- 1,469

Vertical conductance 

(feet per day per foot)

2.2xlO-5

4.9xlO'6

1.7xlO'6

6.0xlO'7

4.3xlO'5

7.8X10'5

-7.8X10-4

-2.3X10-4

-3.5X10-4

-5.2xlO'4

-1.4xlO'2

-7.8xlO'2
D50 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

altitude of the cell or, in the case where only part of the 

layers crop out, the bottom altitude of the outcrop. 

Although not strictly a boundary condition, cells with 

surface-water bodies, such as streams and lakes 

(excluding large rivers), also were identified as drain 

cells.

To differentiate between discharge occurring below 

sea level from that occurring above sea level through 

seepage faces, springs, or streams (together these three 

are herein referred to as "seeps"), the discharge from 

cells that were below sea level was simulated using the 

river module. Although water can both discharge and 

recharge the aquifer system in the formulation of the 

river module, water only discharged in the cross-sectional models because the controlling altitude was 

always less than the calculated heads.

The ET module of the ground-water flow model was 

used in all of the cross-sectional models. The component of ET calculated is the ET from the ground water, 

which approximates PET minus evaporation and plant 

transpiration from the unsaturated zone; that is, PETAET (see table 4). The physical setting along sections 

E-E' and F-F' indicated that ET probably was an important component of the water budget. For section E-E', 

ground-water levels are near land surface in the broad 

flat alluvial valley. For section F-F', the northern part of 

the section lies along flat topography in a river delta; in 

this area, ground-water levels are essentially at land 

surface. The ET module was used for the other four 

sections to test whether ET also might be an important 

component. All cells at land surface for these other four 

sections were initially identified as ET cells. Later, only 

those cells for which ET may occur (as estimated by preliminary simulations) were assigned an ET rate.

Of the six sections, only one (E-E') contains a major 

stream (the Green River) located in an alluvial valley. 

Discharge to the river was simulated using the GHB 

module of the ground-water flow model. Additionally, 

section E-E' contained a small creek within the alluvial 

valley; discharge to this creek also was simulated using 

the GHB module. Discharge to these cells was printed 

out separately for each simulation to identify that discharge. The locations of the drain, GHB, ET, and river 

cells for each model are shown on plate 1.

HYDRAULIC CHARACTERISTICS

Horizontal hydraulic conductivity for each unit was 

initially estimated from available specific-capacity data 

in the area of a section and the regional compilation of 

values completed in this study (table 3). For sections AA' and B-B' (plate 1 and table 10), the initial hydraulicconductivity values used were those of Sapik and others

(1988), who estimated values on the basis of a threedimensional, ground-water flow model.

The flow model uses vertical conductance as the 

hydraulic characteristic that controls the vertical movement of water between layers. Vertical conductance 

between the cells of two layers is approximated by the 

effective vertical hydraulic conductivity of the cells 

divided by the sum of one-half the thickness of the cells. 

Initial estimates of the effective vertical hydraulic conductivity were computed as 0.1 times the harmonic 

mean of horizontal-conductivity values of the cells of 

the two layers in contact.

Initial drain, GHB, and river conductances were chosen as two times the initial estimate of transmissivity of 

the cell (equal to the lateral hydraulic conductivity multiplied by the saturated thickness) (Sapik and others, 

1988). The ranges in the final lateral hydraulic conductivity and the vertical conductance for each model, 

together with the ranges in the estimates of recharge, 

are presented in table 11.

CALCULATED FLOW SYSTEMS

The calculated hydraulic heads for each cross-sectional model (plate 1) show the large variability in 

ground-water flow systems present within the study 

area. The variability in flow systems is exhibited in 

modeling results using both the detailed and aggregated geometry of the aquifer system. This section 

describes the following three aspects of these flow systems (assumed to be representative of those present in 

the Puget Sound Lowland) based on the results of the 

models: (1) hydrologic controls on the ground-water 

flow systems, (2) horizontal and vertical head gradients 

present in the flow systems, and (3) flowpaths. On the 

basis of these three aspects for both the detailed and 

aggregated aquifer-system geometries, some general 

conclusions are described.

Four major hydrologic controls are described: (1) 

topography, (2) the presence of saltwater bodies, (3) 

fine-grained units, and (4) the configuration of the 

geometry of the basement confining unit. Hydraulichead gradients are described because gradients (1) indicate the direction of ground-water flow and (2) are an 

integrated function of aquifer-system geometry, 

recharge, hydraulic characteristics, stream-aquifer 

hydraulic connection, and pumpage. Additionally, gradients provide a measure for comparison of aquifer systems. Flowpaths then are briefly described on the basis 

of the result of models and generalized paths estimated 

using the water-level configurations (fig. 10). Flowpaths indicate (1) the path taken by ground water as it 

moves from recharge areas to discharge areas,
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D51

(2) relative residence time of water along a section, (3) 

types of flow systems local in contrast to regional, and 

(4) potential ground-water contributing area.

HYDROLOGIC CONTROLS

The dominant hydrologic control on each flow system under both aquifer-system geometries is topography this includes the overall topographic setting or 

factors such as altitude and topographic relief. The 

relation between land-surface altitude and water-level 

altitude described previously (table 2B) suggests strong 

hydrologic control. The relation between relief (differences in land-surface altitude over selected horizontal 

dimensions) and hydrologic control is described in the 

following paragraphs. The type of control, in turn, is 

affected by the particular topographic setting; this 

aspect is best shown by the flow system for section EE', South King County (plate 1). The eastern part of this 

section starts at a ground-water divide in the uplands at 

an altitude of about 500 ft and slopes downward to a 

valley at an altitude of about 80 ft; this part of the section is bounded on the west by a bluff. Except near the 

bluff, irrespective of the subsurface conductivity distribution, the flow system in the glacial and interglacial 

deposits in this area is typified by small vertical gradients and predominantly horizontal ground-water flow. 

However, near the bluff, vertical ground-water flow 

predominates in the upper part of the system. The 

western part of the section is defined by an upland 

about 5 mi wide that is bounded on both sides by steep 

bluffs. The flow system in this area is typified by predominantly vertical ground-water flow. As for the eastern area, the presence of bluffs (large topographic relief) 

provides for strong topographic control.

Within the flat central part of section E-E' is a broad 

alluvial valley. Along this feature, the lateral gradients 

are small, and ground-water flow is upward throughout most of the vertical section; however, downward 

flow is calculated below the topographic high located 

near the center of the valley (The small lateral gradients and upward flow in the alluvial valleys were 

described earlier on the basis of the generalized waterlevel configurations.) Topography provides minimal 

control on the flow system in the alluvial valley deposits, especially when compared with the other type of 

deposits.

The effective length and altitude change between a 

topographic high or ground-water divide and the principal discharge location for the aquifer system (saltwater bodies or major river valley) appears to define the 

topographic control on the flow system. On the basis of 

the horizontal grid dimensions for all sections (table 10), 

a topographic high needs to extend over at least 2,000 to

5,000 ft and have vertical relief on the order of 100 ft or 

more to provide strong control on the ground-water 

flow system.

The next important control on the flow system is the 

presence of saltwater bodies; this includes the configuration of the shoreline and the depth of the saltwater 

body. All flow systems terminate at a freshwater-saltwater boundary except where they terminate at the 

major rivers situated in the broad alluvial valleys, at 

bluffs in the upper units, or at bedrock. Thus, the 

hydraulic-head distribution within each section is 

partly controlled by the saltwater boundary (for example, see section C-C', plate 1). Topographic control 

interacts with the saltwater control by affecting the type 

of flow system present near saltwater. For example, if 

there is steep topography near saltwater, the gradients 

generally will be large (see section E-E', plate 1); 

whereas if the topography is relatively flat near saltwater, head gradients generally will be small (see section 

F-F'f plate 1). The latter example is always true for the 

alluvial deposits in the large alluvial valleys near the 

coastline.

Depending on the particular location, the other 

important hydrologic controls on the flow system are 

(1) the presence, extent, and location of the fine-grained 

(clay dominated), generally interglacial units, and (2) 

the configuration or geometry of the basement confining unit. The fine-grained units generally have conductivities several orders of magnitude smaller than the 

other units (table 3), and the tops of these units generally are at an altitude of -50 to 200 ft. If a fine-grained 

unit is present above sea level, it will not only impede 

the vertical movement of water to the deeper units, but 

also the top altitude of this unit at the terminus of a 

flowpath will be a major control on the upper part of 

the flow system. If flowpaths are relatively short, on the 

order of 1 to 4 mi, much of the recharge will discharge 

above sea level, leaving smaller quantities of water 

available to move to deeper units, and saltwater will 

provide the greatest control on the deeper system. Saltwater also may not be in direct hydraulic connection 

with the upper system, that part of the system overlying 

these fine-grained deposits. For example, layer 2 (aql) 

at the east end of section A-A' (detailed geometry on 

plate 1) typifies the above setting where layer 3 

(nonaql) (plate 1; table 9) provides the above-sea-level 

control. For the case where the top of the fine-grained 

unit is below sea level, this allows for (1) saltwater control for the upper flow system, which may restrict horizontal freshwater flow; and (2) as much as 100 ft of 

additional hydraulic-head difference in the upper part 

of the flow system, which in turn, can provide for more 

vertical flow. An example of this setting is shown by 

layer 3 (aql) at the east end of section B-B' (detailed
D52 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

geometry on plate 1; table 9). The fine-grained units are 

present as alternating sequences within the glacial and 

interglacial deposits, and thus the previous discussions 

apply to these deposits and not to the alluvial deposits.

The geometry of the basement confining unit for 

each section (see plate 1) provides a wide range of control on the flow system. The most obvious control is on 

the total thickness of the unconsolidated deposits; 

where the top of the basement confining unit is deep, 

the unconsolidated deposits are thick, and thus, the 

ability of the aquifer system to store and transmit water 

is increased; but where the top of the basement confining unit is shallow, the aquifer system is thin and is 

truncated where the basement confining unit crops out. 

Another control of the basement confining unit is by the 

geometry of the bedrock-bounded valleys in the 

uplands and mountainous areas. The location and 

extent of these valleys control the amount of unconsolidated deposits in them; thus, the extent of the aquifer 

system in these valleys is controlled by the basement 

confining unit.

The model results for the two Bainbridge Island sections, C-C' and D-D', show two aspects of the control of 

the basement confining unit. For the flow system along 

section C-C (plate 1), the basement confining unit does 

not affect the flow system, at least under the modeling 

strategies used because of the presence of saltwater 

boundaries and a deep top of basement confining unit. 

For the flow system along section D-D' (plate 1), the 

basement confining unit provides a boundary that truncates the flow system, which results in ground water 

flowing upward. (The truncation of a flow system by 

the basement confining unit also is shown for section 

A-A; see plate 1). The calculated vertical hydraulichead distributions for sections B-B', £-£', and F-F 

(plate 1) show the transition of control from the basement confining unit to saltwater boundaries.

Interpretation of the hydrologic control of the basement confining unit and other factors on the basis of 

modeling results needs to be done carefully. For example, on the west end of section E-E' (plate 1), the basement confining unit has a high situated below a 

topographic high (upland), and it appears that the 

ground-water divide in the deeper system in this area, 

at least in part, is caused by the geometry of the basement confining unit. However, results of additional 

model simulations for this section indicate that the 

divide is controlled by topography. For these simulations (not shown), the deepest part of the basement confining unit below the alluvial valley was assumed to 

extend westward to the end of the section; that is, the 

model grid system was reconstructed such that the 

basement confining unit high was eliminated, and the 

unconsolidated layers (units) were extended to the

western end. Using this hypothesized geometry, the 

model-calculated flow system was nearly identical to 

that shown on plate 1, and the location of the groundwater divide in the deeper system did not change, 

showing how the topographic effects can propagate 

deep into the flow system.

HYDRAULIC-HEAD GRADIENTS

Horizontal and vertical hydraulic-head gradients in 

the aquifer system are described on the basis of results 

of the model for section E-E' because of the range in 

aquifer-system geometries in this section. Gradients at 

the eastern, upgradient end of this section are small, 

predominantly horizontal and vertically downward. 

Horizontal gradients average about 0.008 ft/ft in this 

part of the section, and the vertical gradients range 

from about 0.18 ft/ft near the eastern part to about 

0.02 ft/ft in the central part of the section near the 

alluvial valley. These gradients appear to be typical for 

this type of setting a recharge area with relatively 

small topographic relief, reasonable lateral continuity of 

units, long distance to major discharge area (alluvial 

valley), and flowpaths on the order of 5 to 7 mi. In the 

western end of the section, vertical gradients generally 

are larger than in the eastern part, averaging about 

0.36 ft/ft, and horizontal gradients range from about 

0.002 ft/ft in the upper system away from the bluffs to 

about 0.09 ft/ft near the bluffs. These gradients in the 

western end of the section appear to be typical of this 

type of setting relatively large topographic relief, a 

confining fine-grained unit that crops out above sea 

level on the bluff, and a short distance to the major discharge areas (alluvial valley and saltwater body).

Large or steep gradients near bluffs, described previously in the "Ground-Water Flow System" section, are 

clearly shown on plate 1; the magnitude of these gradients generally will correspond to the altitude difference 

between the top of a bluff (several 1,000 ft inland) to the 

base level of the discharge area, either sea level or the 

floor of the broad alluvial valleys. Locally, the largest 

vertical hydraulic-head gradients occur between the 

upper system and the first major fine-grained unit 

under both the detailed and aggregated section 

geometries.

The hydraulic-head gradients in the alluvial valley 

deposits generally are smaller than the gradients in the 

other deposits. Horizontal gradients in the alluvial valley are on the order of 0.0007 ft/ft, and vertical gradients are about 0.01 ft/ft; these small gradients indicate 

the effects of relatively flat topography and large 

hydraulic conductivity. Small gradients also are exhibited in areas with thick, coarse-grained recessional
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D53

outwash deposits such as south-central and western 

Pierce County (fig. 10).

GROUND-WATER FLOWPATHS AND TRAVELTIMES

Under the current pumping conditions, lengths of 

ground-water flowpaths range from about 1,000 ft in 

the upper parts of the aquifer system to about 15 mi in 

the deeper parts of the flow system (plate 1 and fig. 10). 

The hydraulic-head distributions on plate 1 together 

with the water levels shown in figure 10 (ignoring very 

localized flowpaths on the order of several hundreds of 

feet) indicate that most flowpaths generally range from 

2 to 7 mi; maximum lengths of flowpaths within the 

Puget Sound aquifer system are on the order of 20 mi in 

southeast Pierce County (see fig. 10). Flowpaths within 

the alluvial valley deposits generally are on the order of 

0.5 to 4 mi.

Ages of seven ground-water samples collected in 

1990 were determined from tritium concentrations to 

estimate residence times in the upper part of the flow 

system and to assess the relation between the residence 

times and potential flowpath traveltimes calculated 

from information from model simulation. Five of the 

sample points were on or near section £-£' (see plate 1); 

the following analysis will discuss information for these 

points.

Large concentrations of tritium in ground water 

indicate that the water is less than about 40 years old; it 

has entered and moved in the aquifer system after 

about the beginning of 1953. Tritium concentrations 

less than about 0.1 (in tritium units) would indicate that 

the water is older than 50 years. The estimates of 

ground-water ages are based on a monthly concentration of tritium (fallout) (H.H. Bauer, U.S. Geological 

Survey, written commun., 1994), the decay of tritium, 

monthly precipitation quantities, and a piston-flow analog (Marshall and Holmes, 1979; Plummer and others, 

1993); thus, the estimates do not account for possible 

mixing of water of different ages. Using this information, the ages of the ground water for each sample were 

estimated as follows:

Sample point 

(plate 1)

Pi

p2

P3

P4

P5

Tritium concentration 

(tritium units)

2.6

16.0

6.8

10.1

.1

Age 

(years)

37-38

30-34, 16-19, 13-14

35-38, 8-9

36-38, 33-34, 9-12

older than 50

The range in ages is due to the variability of both the tritium concentrations and the monthly precipitation. As 

shown in the preceding table, the water at four of the 

five sample points is less than 40 years old.

With a known or estimated flowpath, an effective 

velocity can also be calculated for each of the points for 

each of the age groups. However, for this analysis, 

velocities were estimated using the tritium information 

for only points p4 and p5 because the flowpaths could 

be estimated with some degree of reliability the paths 

generally are vertical from the land surface to the well 

intake. Particle tracking, described in the following 

paragraphs, showed that there are many potential flowpaths to points pi, p2, and p3, and thus, ground-water 

velocities were not calculated for these points.

Flowpaths were calculated using the particle-tracking program MODPATH (Pollock, 1989). Particles were 

tracked both forward and backward. For forward 

tracking, three particles per cell were placed along a line 

that spanned the potential recharge area for points pi, 

p2, and p3; this line extended from the eastern end of 

the section to the first major bluff, for a total of 26 cells 

(see plate 1). The particles were placed at the upper surface of these 26 cells, and the paths and traveltimes 

were calculated. For the backward tracking, flowpaths 

were calculated by placing particles in each of the five 

cells from which the wells that were sampled withdrew 

water. To estimate variations in flowpaths and traveltimes, several schemes of particle placement and number of particles were used in the backtracking. The 

traveltimes and velocities discussed in the following 

paragraphs do not account for movement through the 

unsaturated zone.

Although the overall ground-water flow is both 

upward and horizontal in the alluvial valley, the 

location of sample point p4 is near a locally downward 

flow area where the flow is controlled by a topographic 

high. To obtain a maximum range in velocities, ages of 

38 and 9 years were used in the calculations. Assuming 

no mixing and that the sampled parcel of water at point 

p4 moved from the land surface downward to a depth 

of 134 ft (the depth of the well), the range in effective 

velocities was from 0.01 to 0.04 ft/d. Backward tracking resulted in all particles following the same flowpath 

with a traveltime of about 62 years; for a depth of 134 ft, 

62 years corresponds to a velocity of about 0.006 ft/d. 

For point p5, the effective maximum velocity (based on 

an age of 50 years) from the land surface to the bottom 

of the well would be about 0.017 ft/d. Particle backtracking resulted in a large range in traveltimes, from 30 

to 335 years. Backtracking 27 particles showed 

67 percent of the particles having traveltimes between 

about 30 and 80 years, and backtracking 8 particles 

resulted in 75 percent of the particles having traveltimes between 30 and 70 years. Using 30 years and 

80 years gave a potential range in velocity of 0.011 to 

0.028 ft/d. Thus for sample points p4 and p5, the 

model results and age dating qualitatively support the
D54 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

conceptual flow model and the overall residence times 

in these two locations.

The age of the water at sample point pi was about 

37 to 38 years. Backtracking gave traveltimes that 

ranged between 6 and 790 years; the large range was 

due to the different flowpaths that the particles followed. Although the particles followed different flowpaths, the end of each flowpath was in the same cell. 

For the forward tracking, from the top of the water 

table, additional potential flowpaths and traveltimes 

were calculated. The age dating suggests that the sampled water followed one of the shorter possible flowpaths. The age of the sampled water at point p2 probably ranged from 13 to 34 years. Backtracking gave traveltimes that ranged from about 855 to 1,065 years. 

Similar to point pi, some backtracked flowpaths were 

different, but the end of the paths were the same. Each 

of these paths had the particles moving quickly to a 

water-table cell and then long paths that backtracked 

upgradient along a section of the water table. Forward 

tracking showed that the particles in any of these watertable cells could move to point p2. Traveltimes for forward tracking ranged from 5 to 870 years, depending on 

the water-table cell that a particle originated in. Again, 

the tritium-dated age of the sampled water at point p2 

suggests that it followed one of the shorter flowpaths 

calculated with MODPATH. The relatively narrow 

ranges in ages for the samples as compared to the particle traveltime further suggest that the assumption of 

piston flow with little mixing may be reasonable.

The analysis for point p3 is slightly different because 

of the thickness of the unit that the well is completed in. 

A particle at the bottom of the unit can follow a much 

different path than a particle at the top of the unit. 

There were two estimated age ranges for the sample 

from point p3 35 to 38 and 8 to 9 years. Particles backtracked showed a wide range in flowpaths and thus 

traveltimes. The traveltimes ranged from 15 to 

900 years. Particles originating near the upper part of 

the unit had the shortest flowpaths and traveltimes. 

Forward-tracked particles had an even larger range in 

potential flowpaths and traveltimes. However, similar 

to backtracking, several short paths had traveltimes 

within the two age ranges estimated from the tritium 

information. This suggests that the sampled water followed one of the shorter possible flowpaths and that 

complete mixing over the thickness of the aquifer unit 

was not achieved at this location in the flow system.

The ages based on the tritium information and the 

flowpaths and traveltimes calculated by MODPATH 

indicate the overall reasonableness of the flow system 

calculated by the flow model; the ages were within the 

range of the calculated traveltimes. The analysis also 

indicates that the shorter calculated flowpaths were

more representative than the longer paths of the tritium-estimated ground-water age. The shorter flowpaths also were conceptually more simple and smooth. 

The paths tracked forward displayed a broader range in 

lengths and traveltimes but provide a good representation of how water moves from the top of the water table 

to some point in the system. Additionally, had extensive mixing of ground water occurred, the flowpaths 

calculated with MODPATH suggest that the resulting 

age in most cases should have been older than the sampled water. Again, this indicates less mixing along the 

short flowpaths.

EVALUATION OF THE FLOW SYSTEMS

The flow systems calculated using both the detailed 

and aggregated system geometries generally are similar 

to each other and to the system suggested by figure 10. 

Thus, the results of model simulations indicate the 

potential usefulness of the regional water-level configurations (fig. 10), and they also allow for a more detailed 

description of the regional flow system and of its controls based on the local sectional information. Differences between calculated flow systems for the two 

geometries are most pronounced in areas with large 

downward gradients in the upper part of the aquifer 

system. In these areas, using the aggregated geometry, 

the model calculated a predominantly horizontal flow 

system because there is only one layer (unit) present. 

However, the similarities indicate that (1) the major 

controls previously described are represented under 

both geometries, (2) the fine-grained unit is an important control, and (3) the detailed geometry does not necessarily need to be defined to describe, on a regional 

basis, the ground-water flow system. Indeed, the calculated flow system (including the location and depth of 

the saltwater interface) for section B-B' under both 

aquifer-system geometries generally is similar to that 

calculated by Sapik and others (1988, plate 4) using a 

detailed three-dimensional model of ground-water 

flow.

A large part of the deposits present in the Puget 

Sound Lowland below the first major fine-grained 

(clayey) unit occurs below sea level. Division of the 

deeper system into layers or zones for the construction 

of the models, although somewhat arbitrary, was based 

on an assumed practical depth for future ground-water 

exploration, an assumption of small gradients, and the 

concept that the movement of water decreases with 

depth in the system. In some areas, this division would 

have natural boundaries that correspond to deeply buried, extensive, fine-grained units such as the Puyallup 

Formation (fig. 5C). On the basis of the model results, 

the head distribution in these deeper deposits is not as
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D55

affected by the presence of intercalated fine-grained 

deposits as those deposits above the clay unit. The 

deeper parts of these deposits also are typified by a 

slower and less-active flow system. Large vertical-head 

changes at depth (the most likely effect of areally extensive fine-grained deposits) have not been identified. 

However, there are factors that are not conducive for 

these types of head changes, including the type of flow 

system present, the hydrologic controls on this deeper 

system, and the lack of stresses.

The surficial (generally till) fine-grained layer was 

combined with the upper aquifer layer for the model 

simulations using the aggregated geometry Although 

this unit provides strong control to the ground-water 

system by limiting recharge (and, thus, ground-water 

availability), its hydraulic characteristics do not appear 

to provide strong control on the flow systems. This is 

because on a regional basis (1) the unit varies areally 

from unsaturated to seasonally saturated to saturated,

(2) its hydraulic conductivity generally is much larger 

than that of the fine-grained silty-to-clayey units, and

(3) its thickness generally is small in comparison to the 

total thickness of the units aggregated in the upper 

layer. However, the surficial unit probably should be 

assessed as a separate unit on a regional basis to 

address ground-water availability or contaminant 

transport because (1) it exerts strong control on 

recharge, (2) its conductivity is much smaller than the 

underlying aquifers, and (3) where it is least seasonally 

saturated, it can affect the flow system in the underlying aquifer.

Thus, the presence and hydrologic control of the 

fine-grained units probably result in a flow system in 

the glacial and interglacial deposits typified by (1) an 

upper system that is complex, has both small and large 

hydraulic-head gradients, and is principally controlled 

by topography and discharge altitude (sea level or 

major alluvial valley); and (2) a lower system that is less 

complex, has small head gradients, is dominated by 

horizontal flow, and is principally controlled by saltwater and the basement confining unit. Variations to these 

overall patterns will be due to extension of the topographic control to deeper units, lengths of flowpaths, 

and local aquifer-system geometry.

The major alluvial valleys are glaciofluvial carved 

channels that have been filled with large amounts of 

alluvial deposits. These valleys generally are continuous topographic, low-altitude features compared to the 

surrounding glacial-interglacial deposits and represent 

major ground-water discharge areas (see for example 

section E-E', plate 1). These valleys provide regional 

hydrologic control to the flow systems in the glacial and 

interglacial sequence of deposits; the alluvial deposits 

generally are distinct and mappable from the glacialinterglacial deposits and may be in contact with as 

many as four local glacial-interglacial hydrologic units. 

As discussed previously, the combination of topographic control, geometry of the basement confining 

unit, configuration of the saltwater shoreline, large 

depths of the saltwater bodies (several hundreds of 

feet), and the location and size of the major broad alluvial valleys results in a regional aquifer system dominated by short flowpaths and isolated flow systems; 

that is, there is no regional flow system, only an adjacent series of somewhat isolated subregional flow systems. Because of these factors and the control of the 

fine-grained deposits within the glacial and interglacial 

sequence of hydrogeologic units, the aquifer system has 

a decreased ability to transmit water to the deeper parts 

of the system and there generally are only small contributing areas of recharge. Together, these factors potentially interact to limit ground-water availability and 

increase the potential for saltwater intrusion in some 

areas.

CALCULATED WATER-BUDGET COMPONENTS

The water-budget components for each of the six 

cross-section models are presented in table 12. The 

water budget for each section, and thus the quantity of 

water moving through each section, is a function of the 

recharge, length of section (flowpath) or size of 

contributing area, discharge locations, and aquifer-system geometry. This section of the report will briefly 

describe these functions on the basis of model results. 

The total ground-water flow (in terms of unit flow 

quantities per mile of section) and the potential hydrologic importance of this measure are first described. 

The partitioning of the total flow into the discharge 

components is then described. Next, the unit recharge 

values are compared to streamflow information previously presented (table 7) and to low-flow values for the 

Puget Sound region. Physical factors affecting the partitioning of the total flow (recharge) to discharge components are then discussed. Next, the partitioning of 

flow to saltwater discharge for the model sections and 

for the Puget Sound Lowland is discussed. Last, the 

quantity of ground water moving into the deeper part 

of the aquifer system is described; the quantity is of 

interest to water-resource planners because the upper 

part of the system is more vulnerable to contamination, 

is already being utilized in many areas, and pumping 

from it can more readily affect streamflow.

TOTAL GROUND-WATER FLOW

The total ground-water flow in each section ranges 

from 0.12 to 0.54 (ft3 /s)/mi (table 12). The largest flows
TABLE 12. Model-calculated water-budget components for six cross-sectional ground-water flow models

g g ^2

[ft /s cubic feet per second; (ft /s)/mi, cubic feet per second per mile; (ft /s)/mi , cubic feet per second per square mile; , no discharge]

Section 

(plate 1) Cross section name

A-A' Whidbey Island

B-B' Whidbey-Camano Island

C-C' Bainbridge Island-1

D-D' Bainbridge Island-2

E-E' South King County

F-F' Thurston County

1D, detailed aquifer-system geometry; S, 

2Recharge divided by section length. 

3Recharee divided bv section area.

Aquifer 

system 

geometry1

D

S

D

S

D

S

D

S

D

S

D

S

Recharge

(ft3/s)

0.75 

.75

2.22

2.22

.72

.72

.54 

.54

6.91 

6.91

7.60 

7.86

Total 

groundwater flow

Kft3/s)/ 

mi]2

0.12 

.12

.14

.14

.15

.15

.24 

.24

.36 

.36

.53

.54

Mean unit 

recharge

I(ft3/|)/ 

mi]3

0.38 

.38

.45

.45

.69

.69

1.13 

1.13

1.44 

1.44

1.56 

1.57

Discharge (ft3/s)

Seeps/ 

springs/ 

streams Rivers

0.49 

.49

.94

1.41

.24

.31

.12 

.20

2.37 1.78 

1.67 2.18

55.91 

66.28

Evapotranspiration

-

0.10

.25

.21

_.

2.24 

2.56

1.00 

1.01

Saltwater

0.26 

.26

1.18

.81

.23

.20

.42 

.34

.52 

.50

.69 

.57

Unit discharge 

Seeps/ 

springs/ 

streams

0.25 

.25

.19

.29

.23

.30

.25 

.42

.49 

.35

1.21 

1.26

Rivers 

-

 - 

0.37 

.45 

simplified aquifer-system geometry (see plate 1).

4Discharge divided by section area.

5About 20 percent of discharge is to McCalister Spring.

6About 17 percent of discharge is to McCallister Spring.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D57

are in sections £-£' and F-F', both of which are long 

sections located in areas with relatively large quantities 

of recharge (fig. 13, tables 11 and 12). Section B-B' also 

is long (15.62 mi, table 10), but because it is located in an 

area with small quantities of recharge, the total flow is 

small, 0.14 (ft3 /s)/mi (fig. 13, tables 11 and 12). For the 

case of section D-D', the average recharge (about 

17in/yr) is about 7in/yr larger than the average for 

section B-B', but because the section is short (2.29 mi), 

the total flow [0.24 (ftVs)/mi] is intermediate in the calculated range. Therefore, to sustain large flows along a 

flowpath (which can provide for ground-water availability because it is indicative of, or a measure of, a 

larger ground-water contributing area), a long flowpath 

and large quantities of recharge are needed.

The importance of the total ground-water flow as a 

hydrologic measure of the integration of flowpath 

length and recharge quantities can be further shown by 

several examples. As shown previously, section D-D' is 

in an area with larger recharge quantities than section 

B-B', but the actual recharge to section D-D' is much 

less than that to section B-B'. In contrast, the unit 

recharge of section B-B' is much less than that to section 

D-D'. Thus, recharge rates and unit recharge would 

suggest different hydrologic budgets when these two 

sections are compared discharge to seeps and saltwater for section D-D' is less than that for section B-B', 

whereas unit discharge to seeps and saltwater for 

section D-D' is greater than that for section B-B'. However, for assessing the quantity of water, the total flow 

(a unit measure) indicates the actual quantity of water 

that moves through a section or flowpath. Thus, 

although section B-B' has much greater recharge than 

section D-D', there is about 0.10 (ft3 /s)/mi less water 

moving through it (table 12); that is, excluding storage, 

there generally would be less water available for capture by wells when the total flow is small.

PARTITIONING OF GROUND-WATER FLOW

Under the various physical settings and aquifer-system geometries, the part of the total flow that discharges to seeps, to evapotranspiration, to major rivers, 

and to saltwater bodies can be characterized on the 

basis of the model results (table 12). The relations 

between the partitioning allow for an initial assessment 

of the effects of the physical settings and aquifer-system 

geometries present in the sections. The percentage of 

total flow that discharges to seeps ranges from about 22 

to 81 percent and that which discharges to saltwater 

ranges from about 7 to 78 percent. About 24 to 

26 percent of the ground-water flow in section £-£' discharges to a major river. The large range in these percentages, as much as 71 percent, is due to the complex

interaction of the hydrologic controls described in the 

previous section and further shows the large variability 

of flow systems in the Puget Sound Lowland.

The total ground-water flows just described correspond to unit recharge values under steady-state conditions that range from 0.38 to 1.57 (ft3/s)/mi2 (table 12). 

The mean annual unit recharge values should be within 

the range of the median 7-day unit low-flow values 

(7du) for the seven basins (table 7) discussed previously 

and less than the mean annual unit streamflow values 

for those basins because flow in both the sections and 

the basins was assumed to be representative of the flow 

for much of the low-lying parts of the Puget Sound 

Lowland. Additionally, the unit recharge values also 

should be in the range of the observed 7du for the Puget 

Sound Lowland. As described previously, low-flow 

values relate to basin area, recharge, surficial geology, 

and physical setting. In turn, recharge supports the low 

flows (and annual flows), and the relation between 

recharge and low flows indicates both the variability in 

those relations and the areal representativeness of the 

recharge for the low-lying areas. The recharge values 

are both smaller and larger than the median 7du, and 

smaller than the mean annual unit flows of the selected 

basins (table 7). These values also are larger than the 

25th percentile and smaller than the 80th percentile of 

the 7du for the Puget Sound region (Hidaka, 1973). The 

mean unit recharge for sections A-A' through D-D' are 

between the 25th and 50th 7du percentiles, and values 

for £-£' and F-F' are between the 75th and 80th 7du 

percentiles. The preceding comparison indicates that 

not only is the recharge representative but also that the 

partitioning (and its variability) of the recharge into discharge components is representative for the area. For 

this partitioning, a part of the total flow for all sections 

discharges to seeps and to saltwater (table 12), and thus, 

a part of the total flow is not available to support 

streamflow.

HYDROLOGIC CONTROLS ON DISCHARGE

Topographic relief is a major hydrologic control and 

partly controls the part of the total flow that supports 

streamflow. In turn, drainage networks are related to 

topography. Areas with relatively small topographic 

relief and small quantities of precipitation "the water 

input to an area which provides the major driving force 

in the development of morphology" (Beven and others, 

1988; p. 353) are not conducive for establishment of 

well-developed drainage networks. This is especially 

true of the young landscape in much of the Puget 

Sound Lowland and also in areas with large quantities 

of precipitation and thick recessional outwash deposits 

(small runoff and erosion); for example, south-central
D58 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

Pierce County (figs. 6 and 13). Therefore, if short flowpaths are present in these types of areas, much of the 

ground-water flow should discharge to seeps and saltwater. For example, about 65 percent of the recharge in 

section A-A discharges to seeps (mainly along bluffs), 

and 35 percent discharges to saltwater; this indicates 

that ground water may provide only small quantities of 

water to streams in this type of physical setting. With 

long flowpaths, variable relief, and large quantities of 

precipitation, ground water can provide a larger part of 

the flow in a stream. The physical setting of the outflow 

location of a basin under the latter conditions can also 

affect ground-water flow to a stream. For example, Burley Creek (table 7, fig. 11) has variable relief and large 

quantities of precipitation. However, because the outflow location is at saltwater in an area of small relief, 

flowpaths do not need to be as long, and most of the 

ground-water recharge discharges to the stream. For 

this setting, subregional flow also contributes to streamflow.

The quantities of ground water available to discharge to saltwater under longer flowpaths, variable 

relief, and large quantities of precipitation depend on 

the presence or absence of topographic features such as 

broad alluvial valleys or wide (2 to 5 mi) topographic 

highs, which control the flow system. Such features 

tend to limit discharge to saltwater. For example, only 

about 8 percent of the recharge along section E-E' 

discharges to saltwater because of the presence and 

subsequent hydrologic control of these features (plate 1, 

table 12). Additionally, section F-F, which has a similar 

section length and recharge quantities as those of section E-E', discharges about 9 percent of its recharge to 

saltwater. The similarity in percentage of water discharging to saltwater is because the flat alluvial valley 

along section F-F is near the terminus of the section, 

and the valley's trend is colinear with the flowpath; the 

valley provides both local hydrologic control (plate 1) 

and regional control (fig. 10). Therefore, even with long 

flowpaths, the topographic control results in less than 

10 percent of the recharge discharging to saltwater. A 

further comparison of how the physical setting may 

affect discharge is provided by the model results from 

section F-F' and by information for Burley Creek. The 

overall physical setting and the unit recharge value for 

section F-F' are similar to those for the Burley Creek 

Basin (fig. 11, table 7), but there are more relief and 

topographic features in section F-F'. Thus, the unit discharge for seeps for section F-F' (table 12) should be 

similar to the 7du for Burley Creek (table 7). Comparison of these values shows that the large 7du for Burley 

Creek is similar to the unit discharge to seeps for section 

F-F'.

The water budgets for each model (table 12) generally are similar under both the detailed and simplified 

aquifer-system geometries, including sections A-A' and 

B-B', for which the aggregation of units was most 

severely tested. The differences in the water budgets 

are due to the effects of a predominant flow system that 

is calculated for the upper part of the aquifer system 

using the aggregated geometry. This flow generally 

allows for increased discharge and decreased vertical 

movement of water to the deeper system. However, the 

differences are much less than the total range in estimated discharge values for the sections and typically 

are less than the probable error associated with values 

estimated using standard field methods. Indeed, except 

for major springs and streamflow, discharge to seeps 

and saltwater is difficult to measure and generally is 

estimated with water-budget techniques.

The largest calculated difference in unit discharge to 

saltwater between the aquifer-system geometries 

occurred for the shortest section, D-D' 

[0.17 (ft3 /s)/mi2], where the effects of a lateral flow system calculated using the simplified geometry are the 

greatest. Note that section D-D' also has the largest 

unit discharge to saltwater because of aquifer-system 

geometry. Sections B-B' and E-E' have the most complex aquifer-system geometries, variable hydrologic 

controls, and subsequent complex flow systems. The 

results of the models for these two sections indicate 

about a 31- and 2-percent difference in discharge to saltwater, respectively, between the simulations using the 

two different geometries. Again, these differences 

would be less than the potential error in estimates 

derived using field and other techniques, and the 31- 

percent difference occurred for the most severe test of 

aggregation of units.

Therefore, irrespective of aquifer-system geometry, 

the model's results indicate the range in values of 

recharge and discharge that probably can be expected in 

the Puget Sound Lowland. Additionally, the range in 

and partitioning of discharge to seeps, saltwater bodies, 

and rivers may be considered representative of the 

region. Although major rivers were only present in sections E-E' and F-F', the water-level configurations 

(table 2) and the overall configuration of the major alluvial valleys (fig. 6) indicate that the partitioning of discharge to rivers is representative. The partitioning is 

strongly affected by the hydrologic controls previously 

described. Of particular interest is the fact that, excluding section D-D', the unit discharge (table 12) to saltwater bodies is similar for most sections.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D59

DISCHARGE TO SALTWATER BODIES

The fresh ground-water discharge to saltwater bodies is important because it may locally affect the characteristics of the Puget Sound (Puget Sound Water Quality 

Authority, 1992), the major saltwater body in the study 

area. Therefore, using the results of the models, which 

are assumed to be regionally representative, estimates 

of the projected range in this discharge are described in 

the following paragraphs. Excluding section D-D', the 

model results indicate that about 7 to a maximum of 

53 percent of the recharge in the Puget Sound Lowland 

discharges to saltwater. On the basis of these percentages and the regional estimate of recharge (14,510 ft3 /s), 

from 1,015 to 7,690 ft3 /s of ground water may discharge 

to saltwater. Unit discharge to saltwater bodies ranges 

from about 0.10 to 0.24 (ft3 /s)/mi2. Using these unit 

values and a total area of 7,300 mi2, estimated discharge 

would range from about 730 to 1,752 ft3 /s. The modelcalculated range in discharge per mile of shoreline varies from about 0.04 to 0.37 (ft3 /s)/mi. Using a total 

shoreline length of 2,250 mi, discharge to saltwater 

bodies would range from about 90 to 832 ft3 /s. Except 

for the last values, these estimated ranges probably are 

too large because much of the recharge in the upland 

and mountainous areas of the Puget Sound Lowland 

(where recharge is greatest, see fig. 13) discharges 

within local flow systems. That is, the total or effective 

area contributing discharge to saltwater bodies is less 

than that used for the previous estimates, and thus, the 

recharge value and the total area used to estimate these 

ranges should be smaller.

On the basis of generalized water-level configuration (fig. 10) and assuming that (1) area and recharge 

vary linearly (that is, a reduction in area is equivalent to 

the same reduction in recharge) and (2) water that 

moves to the major alluvial valleys does not discharge 

to saltwater, a reduction in area of 80 percent was estimated. This reduction results in an area of about 

1,460 mi2 and a mean annual recharge of about 

2,902 ftVs. These area and recharge values correspond 

to ranges in saltwater discharge values of 146 to 350 and 

203 to 1,538 ftVs, respectively; again, the latter range 

may be too large. Together, the ranges suggest that discharge to saltwater bodies ranges from about 100 to 

1,000 ft3 /s.

VERTICAL MOVEMENT OF WATER

The upper part of the aquifer system generally consists of all the units above the first major fine-grained 

unit and generally corresponds to a water table or semiconfined system. The deeper part of the aquifer system 

is the remaining part of the Quaternary deposits below 

the fine-grained aggregation unit. On the basis of

model results for both geometries (plate 1), the percentage of the total recharge that moves either downward to 

the units underlying or upward to the units overlying 

the fine-grained unit was calculated. In addition, water 

moving only through the fine-grained aggregation unit 

was calculated. Percentage of total recharge is discussed so that information from sections can be compared and because of the large range in recharge 

estimates for both the models and the Puget Sound 

Lowland. Sections A-A and B-B' do not have the latter 

values for the simplified geometry because a deeper 

fine-grained unit was used in the testing; however, values were calculated for the detailed geometry.

The calculated percentage of recharge that moves 

from the upper part of the aquifer system into the 

deeper part varies widely, from 1 percent for section AA under the simplified geometry to 80 percent for section F-F' under the detailed geometry (table 13). The 

large range in values further indicates the variability of 

flow systems in the Puget Sound Lowland. The largest 

values (80 percent, section F-F' and 78 percent, section 

D-D') are due to different hydrologic controls the first 

is due to the absence of the fine-grained aggregation 

unit (nonaq) over part of section F-F', and the second is 

due to the overall aquifer-system geometry of section 

D-D'. The control for the first case is also represented in 

the much smaller quantity that moves through nonaq 

(table 13). Except for section E-E', a smaller percentage 

of recharge moves to the deeper system under the simplified geometry than under the detailed geometry 

because increased lateral flow is calculated in the upper 

system. The increased lateral flow for section E-E' corresponds to lower calculated heads in the long, eastern 

part of the section, and thus, less water moves to 

streams (table 12), with a corresponding increase in 

water that moves to the deeper system.

The percentage of recharge that moves upward from 

the deeper system underlying the aggregation unit to 

the shallow system also varies considerably, but with a 

smaller range of values than that moving downward 

(table 13). The percentage ranged from 0 for section AA to 43 percent for section E-E', both values being for 

the simplified geometry Only sections E-E' and F-F' 

had a relatively large percentage of water moving 

upward because of the hydrologic control of topographic features the alluvial valley for section E-E' 

and the flat topography near the Nisqually River, which 

is located near the shoreline at the end of a long flowpath, for section F-F' (plate 1).

The percentage of recharge that moved through the 

fine-grained aggregation unit is dependent upon the 

lateral extent of the unit; if the unit is present throughout a section, then all of the water must move through 

it. The differences in the downward percentages
D60 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

TABLE 13. Model-calculated percentage of recharge that moves downward from the identified unit to 

underlying units and moves upward from below the identified unit to overlying units

Percentage of recharge

Section 

(plate 1)

A-A'

B-R

C-C"

D-D'

E-E'

F-F'

Aquifersystem 

geometry1

D

S

D

D

S

D

D

S

D

S

D

S

D

S

Unit name 2

nonaq2

nonaq2

nonaql

nonaq2

nonaq2

nonaql

nonaql

nonaql

nonaql

nonaql

nonaq2

nonaq2

nonaql

nonaql

Layer 

number3

5

2

3

6

2

4

3

2

3

2

7

2

5

2

Downward to 

underlying 

units

18

1

60

14

5

46

28

27

78

64

41

50

80

69

Upward to 

overlying units

2

0

0

5

1

15

1

.5

5

6

33

43

31

26

Downward to 

underlying 

unit4

18

1

60

14

5

30

28

27

78

64

39

47

26

16

Upward to 

overlying 

units4

2

0

0

5

1

15

1

.5

5

6

0

0

1

1

1D, detailed aquifer-system geometry; S, aggregated aquifer-system geometry (see plate 1).

2Unit names for each section are shown on plate 1.

3Layer number for each section is shown on plate 1.

40nly that part of the recharge that has moved through identified unit name/layer number.

indicate the overall effects of the continuity of units. 

The percentage moving upward through the finegrained unit generally is small under both geometries. 

The small values indicate the hydrologic control of the 

aquifer-system geometry and location of discharge 

areas. The largest percentage (15) that moved upward 

through the aggregation unit occurred for section B-B' 

(table 13); the value is larger than the other sections 

because the water in part of the section must move 

through the unit to the shallow system in order to discharge.

The differences between the percentage of recharge 

that moved either upward or downward for the two 

geometries ranged from 0.5 to 17 percent. These differences generally are related to the effects of the increased 

lateral flow calculated with the simplified geometry and 

to the correspondence of the increased flow to the particular aquifer-system geometry of a section. Excluding 

sections A-A' and B-B' where the maximum aggregation was tested, the quantity of water that moved to the 

deeper system changed by about 4 to 18 percent. However, the overall flow systems calculated under the 

simplified geometry still reasonably represent the overall flow system in a section. The maximum aggregation 

used in sections A-A and B-B' does not allow for as 

much vertical movement of water to the deeper system

as the other aggregations. Thus, the results described in 

this and the previous section suggest that the aggregation used for sections C-C through F-F' is sufficient for 

describing ground-water flow in the region.

CONCEPTUAL MODEL OF THE 

AQUIFER SYSTEM

The conceptual model, based on the pertinent information previously presented and the results of the 

cross-sectional models of ground-water flow, includes 

the characteristics of the aquifer system necessary to 

describe ground-water flow on a regional basis. These 

characteristics are based on the major hydrologic controls on the ground-water system, including topography, the configuration of the basement confining unit, 

the presence and extent of fine-grained deposits, the 

configuration of saltwater bodies, the presence and 

extent of large alluvial valleys, the distribution of 

hydraulic heads within the aquifer system, and the 

hydraulic characteristics of the Quaternary deposits. 

The conceptual model is assumed to be applicable to 

any area within the Puget Sound Lowland. Because of 

the varying hydrogeologic settings within the model 

sections, the aggregation of units used to develop the
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D61

regional conceptual model of ground-water flow for the 

Puget Sound aquifer system, although simplified, is 

regionally representative.

The model is applied to an area located in the FraserWhatcom Basin that was chosen because it has a unique 

geologic setting and because no information from the 

basin was used to develop the conceptual model.

DESCRIPTION OF CONCEPTUAL MODEL

The Puget Sound aquifer system comprises highly 

variable sequences of glacial, interglacial, and alluvial 

deposits. These deposits comprise four types of 

units glacial and interglacial aquifers, semiconfining 

units, confining units, and alluvial valley aquifers. 

Although recessional outwash channel-fill aquifers are 

locally important sources of water, they are not defined 

as separate aquifers for this regional analysis.

The simplified conceptual model of the Puget Sound 

aquifer system for use in describing ground-water flow 

in the region thus consists of the nine alluvial valley 

aquifers that may or may not contain other types of 

deposits, a surficial semiconfining unit, and a sequence 

of two coarse-grained aquifers separated by a finegrained confining unit (the Fraser aquifer, the confining 

unit, and the Puget aquifer). The basement confining 

unit forms the basal and lateral boundaries of the aquifer system. The deposits that comprise the aquifer units 

vary from silty sand to coarse gravel and may locally 

contain extensive deposits or zones that are more or less 

productive.

The lateral extents of the surficial regional units are 

shown in figure 15, and the naming conventions for the 

units and the relations between stratigraphic units and 

regional hydrogeologic units are shown in figure 16. 

Examples of the regional hydrogeologic units are presented in figure 17 for two of the six sections (sections 

E-E' and F-F') described previously (plate 1). Note that 

as previously described, there is no regional flow system in these units. The Puget Sound aquifer system 

thus contains numerous subregional flow systems that 

generally are isolated from each other under the present 

conditions of hydrologic stress. These flow systems are 

contained in 13 regional hydrogeologic units (four glacial and interglacial and nine alluvial). These units are 

briefly described in the following sections.

GLACIAL-INTERGLACIAL SEQUENCE

Both locally and regionally, the aquifer system 

within the glacial and interglacial deposits may be considered a sequence of aquifers and semiconfining to 

confining units. The areal extent of the aquifers in the 

sequence of deposits is determined by erosional and

depositional processes resulting from the advances and 

retreats of the glaciers and the formation of and deposition in the alluvial valleys. Topography, the configuration of the basement confining unit, and the presence of 

saltwater bodies further act to divide and truncate the 

flow systems in the aquifers.

Recessional deposits present at land surface can be 

categorized into four groups (1) locally nonextensive, 

discontinuous deposits that, depending on the physical 

setting, may or may not be saturated; (2) horizontally 

and vertically extensive deposits that in some areas are 

in direct contact with advance outwash; (3) thick recessional channel-fill deposits that in some areas also are in 

direct contact with the underlying advance outwash; 

and (4) glaciomarine deposits. The first and last groups 

generally do not function as aquifers, though the first 

group can locally provide water for domestic use. The 

second and third groups function as aquifers. Each of 

these groups has been included in a regional unit 

described in the following sections.

Surficial Semiconfining Unit

The fine-grained (generally till) deposits present at 

land surface generally have much larger hydraulic conductivity than the fine-grained (generally lacustrine) 

interglacial sediments; however, their conductivity generally is one to three orders of magnitude smaller than 

the coarse-grained deposits. Available data indicate 

that the till can act as either a semiconfining unit or a 

low-yield aquifer unit. Although these deposits do not 

appear to largely affect flow systems, they limit groundwater recharge and affect ground-water availability, 

contaminant transport, and surface-water runoff. 

Therefore, these deposits have been separated out from 

the other glacial and interglacial deposits as a distinct 

(semiconfining) unit. Where the Everson "glaciomarine" Drift is at land surface, excluding the Fraser-Whatcom Basin, it is considered to be part of this regional 

semiconfining unit. The locally nonextensive recessional outwash deposits also are considered part of this 

unit. Last, extensive mudflow deposits along the White 

River in Pierce and King Counties are considered part 

of the surficial semiconfining unit. Thus, excluding the 

Fraser-Whatcom Basin, the surficial semiconfining unit 

includes most of the surficial fine-grained deposits 

shown in figure 6.

Confining Unit

On a regional basis, only the shallowest, finegrained, clayey deposits that are areally extensive 

appear to largely affect the ground-water flow system; 

these deposits generally are located near sea level
D62 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

WHATCOiV

-. Mt Baker . / « 

tinenam

% Olympic /

°<-fjL

$/ THURS^GN

EXPLANATION

| | Alluvial valley aquifers

[ J Surficial semiconfining unit

^^| Fraser aquifer

HH Confining unit

| . I Basement confining unit

_£' Trace of hydrogeologic section 

Sections shown in figure 17

Base modified from U.S. Geological Survey 

digital data, 1:2,000,000, 1972

FIGURE 15. The lateral extents of the surficial regional hydrogeologic units. Names of alluvial valley aquifers identified by names of rivers.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D63

A. Fraser-Whatcom Basin

Geologic framework^

System

Quaternary

Age (K-A)

10 

12

20

60 

80 

>100

Geologic/climate units2

Interglacial

Fraser 

Glaciation

Sumas Stade

Everson 

Interstade

Vashon 

Stade

Evans Creek 

Stade 7

Olympia Interglacial

Possession Glaciation

Whidbey Interglacial

Double Bluff Glaciation

Stratigraphy

Alluvium, marine 

deposits^

Sumas Drift

Bellingham Driftx^

Everson <f Deming Sand 

"glaciomarine" \^

Drift4 Kulshan Drift^^^

Fort Langley Formation^

Recessional outwash

Vashon Till

Advance outwash

Esperance Sand 

Member

Quadra 

Sand6

Quadra Cowichan Head 

Formation Formation^

Possession Drift 

Semiahmoo Drift^

Whidbey Formation

Double Bluff Drift

Pre-Quaternary rock units

Hydrologic framework

Name of unit

Fraser River aquifer 

Nooksack River aquifer

Fraser aquifer

Confining unit

Puget aquifer

Upper Puget zone

Lower Puget zone

AQUIFER SYSTEM PUGET SOUND

Basement confining units

1 Modified from D. Molenaar (U.S. Geological Survey, written commun., 1982), P.C. Haase (U.S. Geological Survey, 

written commun., 1988), Blunt and others (1987), and Galster and Coombs (1989).

2 Drift sequences are generally separated by unconformities.

3 Marine deposits are considered part of aquifer system where saturated with freshwater.

4 Also includes glaciaofluvial sediments Everson sand (early Everson) and Everson gravel (late Everson).

5 Canadian name for Everson glaciomarine drift.

6 Canadian name for Vashon deposits older than till, although in many locations the unit does not include the 

advance outwash.

^ Deposits of similar age and older than Evans Creek Strade generally not exposed in the basin, inferred from 

well-log information and from some exposures in Canada.

8 Canadian name for Olympia Interglacial deposits

9 Canadian name for pre-Olympia Interglacial deposits.

FIGURE 16. Relation between stratigraphic units and regional hydrogeologic units for the Puget Sound aquifer system, (A) FraserWhatcom Basin, (B) north-central Puget Sound Lowland, and (C) southern Puget Sound Lowland. Location of areas shown in figure 6.
D64 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

B. North-central Puget Sound Lowland

Geologic framework^

System

Quaternary

Age (K-A)

10 

12

20

60 

80 

>100

Geologic/climate units^

Interglacial

Fraser 

Glaciation

Everson 

Interstade

Vashon 

Stade

Evans Creek 

Stade

Olympia Interglacial

Possession Glaciation

Whidbey Interglacial

Double Bluff Glaciation

?

Stratigraphy

Alluvium, marine 

deposits-^, lahars

Everson 

"glaciomarine" 

Drift4 ^^

^^^Partridge 

<^ Gravel 

^^" ^^^ (recessional)

Recessional outwash ^~"^--->^_

Vashon Till

Advance outwash

Esperance Sand Member

Lawton Clay

/ Pjlnhnnkriay

Quadra Formation

Possession Drift

Whidbey Formation

Double Bluff Drift

Pre-Quaternary rock units

Hydrologic framework

Name of unit

Skagit-Stillaguamish River aquifer 

Snoqualmie River aquifer

\ Surficial semiconfining unit

Fraser aquifer

Confining unit

< Puget aquifer

Upper Puget zone

Lower Puget zone

SOUND AQUIFER SYSTEM PUGET 

Basement confining unit

1 Modified from D. Molenaar (U.S. Geological Survey, written cummun., 1982), P.C. Haase (U.S. Geological Survey, 

written commun., 1988), Blunt and others (1987), and Galster and Coombs (1989).

2 Drift sequences are generally separated by unconformaties.

3 Marine deposits are considered part of aquifer system where saturated with freshwater.

4 Also includes glaciaofluvial sediments Everson sand (early Everson) and Everson gravel (late Everson).

FIGURE 16. Relation between stratigraphic units and regional italic hydrogeologic units for the Puget Sound aquifer system, (A) FraserWhatcom Basin, (B) north-central Puget Sound Lowland, and (C) southern Puget Sound Lowland. Location of areas shown in

figure 6 Continued.

where they abut saltwater bodies. These fine-grained 

deposits are considered a regional confining unit and 

are called the confining unit. In the Fraser-Whatcom 

Basin, the Everson "glaciomarine" Drift is considered 

the confining unit. Although the till at the land surface 

is part of the surficial semiconfining unit where it 

directly overlies the coarse-grained outwash or

proglacial deposits, it may be considered part of the 

confining unit where it is in direct contact with the finegrained, clayey deposits. For example, the Vashon till 

crops out in the Fraser-Whatcom Basin where it has not 

been either eroded or covered by Everson "glaciomarine" Drift. Where it crops out in the low-lying parts of 

this basin, it abuts the Everson Drift, and where it is
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D65

C. Southern Puget Sound Lowland

Geologic framework^

System

Quaternary

Age (K-A)1

10

12 

20

BO100

Geologic/climate units^

Interglacial

Fraser 

Glaciation Vashon 

Stade

Evans Creek Stade

Olympia Interglacial

Upper 

Stade

Salmon 

Springs Interstade 

Glaciation

Lower 

Stade

Puyallup Interglacial

Stuck Glaciation

Alderton Interglacial

Orting Glaciation

Stratigraphy

Alluvium, 

Electron Mudflow^ 

Osceola Mudflow3 

lahars, marine deposits3

Alluvium

Vashon 

Drift

"""" -- .____

Alluvium,

Steilcoom gravels

Recessional outwash

Moraine deposits

Vashon Till4

Advance outwash

Colvos Sand Member

Evans Creek Drift5

\ Skokomish gravels

Kitsap Formation

\ "Penultimate Drift" 

Hyden Creek Drift5

Upper Salmon Springs Drift

Sediments, tephra

Wingate Hill Drift5 

Lower Salmon Springs Drift

Puyallup Formation

Stuck Drift

Alderton 

Formation

Log an 

Hill 

Formation-*

Orting Drift

Pre-Quaternary rock units

Formation Lily Creek 

Hydro logic framework

Name of unit

Nisqually River aquifer. Green River aquifer, 

Puyallup River aquifer, Snoqualmie River aquifer, 

Skokomish River aquifer, Chehalis River aquifer

/Surficial C. _

unit J Fraspr p quifer

Confining unit

Puget aquifer

Upper Puget zone

ILower Puget zone

PER SYSTEM

PUGET AQUI SOUND 

Basement confining unit

1 Modified from D. Molenaar (U.S. Geological Survey, written commun., 1982), P.C. Haase (U.S. Geological Survey, 

written commun., 1988), Blunt and others (1987), and Galster and Coombs (1989).

2 Drift sequences are generally separated by unconformities.

3 Mudflows and lahars are part of alluvial valley aquifers where confined in channels and not principal unit in channel, 

otherwise, considered part of surficial semiconfining unit; marine deposits are considered part of aquifer system 

where saturated with freshwater.

4 Vashon Till makes up the surficial semiconfining unit where it crops out at land surface or is covered by only a thin 

veneer of younger unsaturated deposits.

5 Alpine glacial deposits, generally located in mountainous areas of Cascade Range.

FIGURE 16. Relation between stratigraphic units and regional italic hydrogeologic units for the Puget Sound aquifer system, (A) FraserWhatcom Basin, (B) north-central Puget Sound Lowland, and (C) southern Puget Sound Lowland. Location of areas shown in

figure 6 Continued.

buried, it underlies the Everson Drift (and perhaps 

some Vashon recessional outwash). Thus, for this 

regional analysis, the Vashon till is considered part of 

the confining unit in this basin. Although in other areas

the Vashon till is known to be locally in contact with the 

fine-grained confining unit, it was not considered part 

of the confining unit because of the regional scale of this 

study. For local investigations, it probably would be
D66 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

Surficial

semiconfining

unit

£'

Surficial

semiconfining

unit

Surficial

semiconfining

unit

Trace of section shown on plate 1 and in figure 15

Vertical scale greatly exaggerated

3 KILOMETERS

Surficial

semiconfining

unit

Trace of section shown on plate 1 and in figure 15

Vertical scale greatly exaggerated

0 1 2 3 KILOMETERS

FIGURE 17. Examples of extent and names of regional hydrogeologic units for (A) section E-E' and (B) section F-F'.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D67

combined with the confining unit. Additionally, the 

older, buried till that is in contact with the overlying 

confining unit has not been included as part of the 

confining unit. However, as described previously in 

"Hydrogeologic Units and Sections," this unit may or 

may not be combined with the confining unit in local 

investigations.

Fraser and Puget Aquifers

For this regional study, the aquifer units consist of 

all deposits not included in either the surficial semiconfining unit or the confining unit. The aquifer units thus 

consist of glacial outwash, proglacial, interglacial, and 

locally, alluvial and mudflow deposits. Where present, 

the aquifer unit that overlies the confining unit is called 

the Fraser aquifer, and the aquifer unit that underlies 

the confining unit is called the Puget aquifer. Locally, 

the Puget aquifer may be further informally divided 

into the upper Puget zone and the lower Puget zone. 

The glacial and interglacial deposits thus comprise two 

aquifer units, one semiconfining unit, and one confining 

unit. Water in the Fraser aquifer occurs under watertable to semiconfined conditions, and water in the 

Puget aquifer generally occurs under confined conditions. Generally, the Fraser aquifer corresponds to the 

regional water-table aquifer.

ALLUVIAL VALLEY AQUIFERS

The alluvial valley aquifers are comprised of extensive alluvial deposits in the major river valleys. These 

valleys traverse the glacial deposits in the study area, 

and the rivers in the valleys discharge to saltwater bodies; some of these valleys originate in the bedrock 

uplands. These alluvial valley aquifers truncate and 

control the flow systems in the glacial and interglacial 

deposits, and they typically are linear-type features; 

that is, their length is much greater than their width. 

Each of these alluvial aquifers closely follows the associated river valley and is considered a separate aquifer 

unit for this regional study.

In the selected parts of the upper reaches of the rivers in the bedrock-controlled valleys, glacial (recessional, till, and advance), interglacial, and mudflow 

deposits are considered part of the alluvial valley aquifer units; this is due to the generally small extent of 

these deposits in comparison to the alluvial deposits 

and, in many instances, the similarity of hydraulic characteristics of the alluvial deposits and of the continental 

and alpine glacial deposits. Where the mudflow deposits are extensive, such as along the White River, they 

have not been included in the alluvial aquifers. Less 

extensive and also discontinuous alluvial deposits are 

not separated as alluvial valley aquifers. Generally,

these deposits have been included in the surficial semiconfining unit where they overlie till or other finegrained deposits or in the Fraser aquifer where they 

overlie coarse-grained deposits. On the basis of the 

extent of the alluvium in the major river valleys, nine 

alluvial valley aquifers have been identified and named 

(figs. 15 and 16). Note that in some areas the Nooksack, Nisqually, and Skokomish Rivers the alluvial 

deposits are laterally extensive but do not appear to be 

vertically extensive.

RECESSIONAL DEPOSITS

Well-defined, deep recessional outwash channels 

filled with coarse-grained glacial deposits are productive aquifers. However, except for one major channel, 

the South Tacoma channel in Pierce County, the channels generally are valleys that are now occupied by 

streams and are located both in the bedrock-controlled 

upper reaches of rivers and in most of the valleys in the 

southern part of the study area. Therefore, they are not 

defined as separate aquifer units, but depending on 

their locations, they are considered part of either the 

alluvial valley aquifers or the Fraser aquifer. For this 

study, the South Tacoma channel also is not defined as a 

separate aquifer unit but is considered a highly productive part of the Fraser aquifer. Additionally, nonchannelized extensive recessional outwash deposits are 

considered part of the Fraser aquifer. These deposits 

are most prevalent in the southern part of the study 

area. For this area, the underlying till (if present) also is 

considered part of the Fraser aquifer.

APPLICATION OF CONCEPTUAL MODEL

The conceptual model was applied to a selected area 

in the Fraser-Whatcom Basin by using available welllog information and maps showing the extents of surficial hydrogeologic units (S.E. Cox, U.S. Geological Survey, written commun., 1993; Jones, in press; fig. 15) to 

map the regional hydrogeologic units presented in 

figure 16 for the Fraser-Whatcom Basin. The units were 

mapped within a section that follows an approximate 

ground-water flow line estimated from the generalized 

water-level configurations (fig. 10). A cross-sectional 

ground-water flow model then was constructed for the 

section. The model was constructed on regionally representative values that accorded with the conceptual 

model. For example, the hydraulic characteristics of the 

units were estimated from effective average or median 

values, on the basis of information described previously 

both in the "Hydraulic Characteristics'7 section and in 

the development of the conceptual model, and because 

the upper part of the aquifer system in the basin
D68 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

appears to be more coarse grained than other areas 

within the Puget Sound Lowland, the effective values 

used in the model were larger for this part of the system. The results of the model then were compared with 

the estimated generalized water-level configurations 

shown in figure 10, with measured water levels in wells, 

and with estimated water-level configurations obtained 

from maps constructed during a multiyear study of the 

ground-water flow system in part of Whatcom County 

and British Columbia, Canada (S.E. Cox, U.S. Geological Survey, written commun., 1993). Model-calculated 

water budgets also were compared to streamflow in the 

Nooksack River.

The Fraser aquifer exists in the basin and has been 

called the Abbotsford aquifer in part of British Columbia (Liebscher and others, 1992) and the Sumas-Abbotsford aquifer in part of both Whatcom County and 

British Columbia (S.E. Cox, U.S. Geological Survey, 

written commun., 1993). The Fraser aquifer provides a 

large part of the water used for municipal, domestic, 

and agricultural uses. Most of the wells within the 

basin are highly productive and shallow. Additionally, 

water also is obtained from the Nooksack River aquifer. 

Throughout much of the Fraser-Whatcom Basin, 

ground-water levels are near land surface. In many 

areas, ditches and agricultural drains intercept the 

water table and discharge water from the aquifer system to local streams and the Nooksack River, which 

traverses part of the basin (fig. 2).

A large part of the deposits in the Fraser-Whatcom 

Basin exhibit larger than average hydraulic conductivities for the aquifer system. The larger values are due to 

the fact that surficially (excluding the Everson 

"glaciomarine" Drift), and to some extent with depth, 

the uppermost deposits consist principally of coarsegrained outwash and alluvial deposits (Jones, in press). 

Additionally, much of the surficial deposits were not 

overridden by the last incursion of continental glaciers 

during the Sumas Stade.

The constructed section G-G' (fig. 18) extends over 

9.59 mi, from about 4 mi south of the Fraser River in 

British Columbia to the Nooksack River in Whatcom 

County. The Nooksack River aquifer (fig. 16) is comprised of Holocene alluvial deposits and is present at 

the southern or downgradient end of the section 

(fig. 18). The Nooksack River aquifer was combined 

with the Fraser aquifer for modeling purposes because 

the Nooksack River aquifer in this location of small 

topographic relief is relatively thin and grades laterally 

into the Fraser aquifer. The Fraser aquifer in this area is 

comprised of the Sumas Drift, and the confining unit is 

comprised of the divided units (Bellingham Drift and 

Kulshan Drift) that compose the Everson "glaciomarine" 

Drift, recessional outwash, and the Vashon till (figs. 16

and 18). The Puget aquifer is comprised of the remaining stratigraphic units that compose the undifferentiated glacial and interglacial deposits underlying the 

confining unit (figs. 16 and 18). The basement confining 

unit within the basin consists of consolidated continental sediments, principally sandstone, probably the most 

permeable of the units that compose the basement confining unit. Therefore, to test potential flow to and from 

the basement confining unit, the upper 250 ft of the 

sandstone was included as a model layer.

The cross-sectional numerical model of groundwater flow was constructed using 25 square cells in the 

horizontal dimension; the cells have lengths and widths 

of 2,025 ft (fig. 18). Excluding the sandstone, vertical 

discretization was based on the average thickness over 

a cell for each of the mapped units (fig. 18). The boundary condition for each of the uppermost cells was a constant rate of recharge (average of 26.7 in/yr) that was 

obtained from the distribution shown in figure 13. The 

boundary condition for the contact between the upper 

part of the basement confining unit and the remainder 

of the basement confining unit was defined as no flow. 

Although the upgradient (north) end of the section was 

placed at an estimated ground-water divide on the basis 

of shallow water-level data, the lateral boundary condition for the vertical column of cells was identified as a 

head-dependent boundary and was simulated using the 

GHB module of the ground-water flow model. Similarly, the lateral boundary condition for the southern 

end of the section, which extends beyond the Nooksack 

River, also was identified as a head-dependent boundary and was simulated using the GHB module; the section was extended three cells beyond the river to 

account for the contribution of flow to the river originating from the other side. The river module was used 

to simulate flow to the Nooksack River. The ET module 

was used to simulate ET from the uppermost cells 

where the water levels are near land surface (see fig. 18) 

such that plants may extract ground water on the basis 

of unused PET (PET - AET). The drain module was 

used for 12 cells where water can discharge from the 

Fraser aquifer; these cells represent agricultural drains, 

ditches, seeps, springs, and streams. Note that discharge from the aquifer system to these drain cells goes 

to the Nooksack River, and thus, the total contribution 

of ground water to the river along the section is the discharge to both the river and drain cells. The model grid 

system with boundary conditions is shown in figure 18.

Horizontal hydraulic conductivities were estimated 

as previously described, and these values correspond to 

basinwide estimates provided by S.E. Cox (U.S. Geological Survey, written commun., 1993). Except for the 

Fraser aquifer, conductivities in each layer were nearly 

uniform. Conductivity of the sandstone (the basement
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D69

Inset map

100-

Sea level 

100- 

200- 

300- 

400- 

500 

600

EXPLANATION

Model cell Number identifies layer 

Drain boundary cell 

River boundary cell 

General head boundary 

Evapotraspiration boundary cell 

Aquifer

'-'^ ' / Bedrock Basement confining unit 

 Sea level

Confining unit

Basement confining unit

Arbitrary boundary

Vertical scale greatly exaggerated 3 MILES

0 1 3 KILOMETERS

G

G G' Trace of section Shown on inset map

FIGURE 18. Geometry of aquifer and confining units and model grid system with boundary conditions for section G-G1 , FraserWhatcom Basin.

confining unit) was estimated to be about 17-ft/d; Cox 

(U.S. Geological Survey, written commun., 1993) estimated the sandstone conductivities to range from about 

0.01 to 75 ft/d. The 17-ft/d value is similar to Cox's 

75th-percentile value (the larger estimate was chosen to 

test for maximum potential flow in the basement confining unit). Vertical conductance was estimated as previously described for the other constructed groundwater flow models.

A comparison of the calculated hydraulic heads for 

the Fraser aquifer to the estimated water levels shown 

in figure 10 and to water levels measured and mapped 

by S.E. Cox (U.S. Geological Survey, written commun., 

1993) shows that the calculated heads are in reasonable 

agreement with the previously mapped heads (fig. 19).
D70 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

Map view

G'

50 75

n i i 100 120 150

EXPLANATION

Water levels, in feet

 Model calculated

 Estimated figure 10

 Estimated S.E. Cox (U.S. 

Geological Survey, written 

commun. 1993)

100 Water level for the above 

three groups

Plane view

EXPLANATION

60 Line of equal hydraulic head

Contour interval is variable

G G' Trace of section Shown in 

figure 18

1 00 Location of well along section 

j G-G' with water level Length 

of line indicates well depth

3 MILES

3 KILOMETERS

Vertical scale greatly exaggerated

FIGURE 19. Model-calculated hydraulic heads and estimated and observed water levels for section G-G,

Fraser-Whatcom Basin.

For 22 cells representing the Fraser aquifer, the mean 

absolute difference between the model calculated heads 

and Cox's mapped heads is 12.5 ft or about 8 percent of 

the total hydraulic-head change along this 8.44 mi part 

of the section.

Calculated lateral gradients range from 0.0004 to 

0.01 ft/ft for the Fraser aquifer and from 0.0002 to 

0.004 ft/ft for the Puget aquifer; average lateral gradients are 0.003 and 0.002 ft/ft for the Fraser and Puget 

aquifer, respectively. The model calculated a 60-ft 

smaller total lateral head change in the Puget aquifer 

than in the Fraser aquifer. The maximum lateral gradients occur at a terrace abutting the Nooksack River. The 

average calculated lateral gradients are smaller than the 

regional estimates described previously; the smaller

values correspond to the large hydraulic conductivities 

(on the order of 100 to 200ft/d) estimated for the 

deposits of the Fraser aquifer along the section. 

Calculated downward vertical gradients from the 

Fraser aquifer to the Puget aquifer ranged from about 

0.02 to 0.16 ft/ft. The values were largest at the upgradient end of the section and smallest at the transition 

from downward-to-upward vertical gradients (this 

transition occurred where the terrace abuts the river). 

Upward gradients, ranging from about 0.01 to 0.06 ft/ft, 

occurred at the five most-downgradient cells below the 

alluvial valley. The range and direction of the vertical 

gradients are similar to those discussed previously in 

the report.
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D71

Model results indicate that of the average annual 

recharge (7.23 ft3 /s) to layer 1, 80 percent (5.82ft3 /s) 

discharges to the Nooksack River either directly or indirectly through drains, and 20 percent (1.41 fi~/s) discharges through evapotranspiration. An additional 

0.22 ftVs enters the system through the lateral boundaries (for a total recharge of 7.45 ftVs) and discharges to 

drains and the river. No water exited the system from 

the lateral boundaries. Total discharge to the river is 

thus 6.04 ftVs (2.49 ftVs directly to the river and 

3.55 ftVs through drains).

Model results indicate that about 44 percent of the 

recharge (3.18 ftVs) to layer 1 (Fraser aquifer) moves 

into layer 2 (the confining unit), and eventually back to 

layer 1 ultimately to discharge. The remaining 

56 percent of the recharge (4.05 ftVs) discharges to 

drains and as evapotranspiration. Of the water that 

moves into layer 2, only about 13 percent (0.41 ftVs) 

moves into layer 3 (the Puget aquifer), indicating that 

the upper part of the flow system is the most active part 

and that recharge is discharged predominantly in local 

flow systems (even on this spatial scale). Less than onehalf of 1 percent (0.025 ft3 /s) of the total recharge was 

calculated to occur as lateral inflow into layer 4 (the 

basement confining unit) from the GHB cells. Additionally, only about 0.28 ftVs (4 percent of the recharge to 

layer 1) was calculated as moving from layer 3 into 

layer 4 and back to layer 3. Therefore, even when large 

conductivities are used in the model for layer 4 (the 

basement confining unit), the flow contribution from 

and the quantity of water moving in the upper part of 

the basement confining unit are estimated to be 

minimal.

On the basis of U.S. Geological Survey's gaged discharge of the Nooksack River, the Nooksack River gains 

water from the point where it enters the lowlands until 

it discharges to saltwater at an annual average rate that 

ranges from about 2 to 10 (ft3 /s)/mi2 and averages 

about 3 (ft3 /s)/mi2. The latter value can be compared 

to the average annual unit recharge rate of 

1.96 (ft3/s)/mi2 or, after accounting for evapotranspiration, 1.64 (ft3 /s)/mi2. Although these values are 

smaller than the average unit streamflow gain (indicating that estimates of recharge may be too small), a part 

of the gain also may be derived from direct surface runoff and, thus, would not be included in the recharge 

estimates.

The calculated discharge to the river from the section is 6.04 ft3 /s, which corresponds to 15.7 (ft3 /s)/mi 

of river. Streamflow data indicate that discharge to the 

river ranges from about 6 to 27 (ft3 /s)/mi of river and 

averages about 9 (ft3 /s)/mi of river. The calculated 

contribution per mile of river is much larger than the 

observed average contribution. However, this

difference probably is due to the fact that the modeled 

area-to-river mile ratio (3.68 mi2 /0.38 mi or 9.6 mi) for 

the section is three times larger than the area-to-rivermile ratio (3.2 mi) based on the location of the gaging 

stations. This aspect highlights the potential for 

increased flow from a basin with long flowpaths with a 

large contributing area such that larger quantities of 

recharge are intercepted. Additionally, the large range 

in observed discharge per mile of river further shows 

the effects of interannual variability in precipitation 

and, thus, in recharge in this basin. An example of this 

variability is that water-year precipitation near Bellingham (located in the western part of the Fraser-Whatcom 

Basin, fig. 2) has ranged from about 22 to 48 in/yr.

SUMMARY AND CONCLUSIONS

The aquifer systems in the Puget-Willamette Lowland were studied by the U.S. Geological Survey as part 

of a national Regional Aquifer-System Analysis Program. The Puget-Willamette Lowland is located in 

western Washington, western Oregon, and a small part 

of southwestern British Columbia, Canada, and consists 

of two distinct areas the Puget Sound Lowland and 

the Willamette Lowland. The study area for this report 

is the Puget Sound Lowland, which encompasses 

17,616 mi2, about 2,556 mi2 of which is saltwater.

The Puget Sound Lowland is an elongated basin that 

extends over about 200 mi in a north-south direction; 

the width of the lowlands ranges from 15 to 80 mi and 

averages about 40 mi. The Puget Sound Lowland is a 

discontinuous valley that is part of a larger forearc 

basin, the topography and geology of which are products of tectonic convergence of oceanic and continental 

plates. Pleistocene glaciation has modified the regional 

geologic setting of the Puget Sound Lowland. There 

have been at least four recognized continental glaciations and several alpine glacial advances. As a result of 

the glaciations, the Quaternary deposits locally consist 

of one to several regional drift sequences that generally 

are separated by unconformities and by nonglacial fluvial and lacustrine sediments. The drift sequences are 

overlain by large thicknesses of alluvial sediments in 

the major broad alluvial valleys in the area.

Excluding large lakes, the Puget Sound aquifer system underlies 7,183 mi2 of the Puget Sound Lowland. 

Alluvial, glacial, and interglacial sediments of Quaternary age compose the Puget Sound aquifer system; 

these deposits consist principally of recent alluvial, 

recessional and advance outwash, till, and other glaciofluvial and interglacial sediments. Tertiary and older 

sedimentary, volcaniclastic, volcanic, and metamorphic 

rock units underlie these sediments and form the lateral
D72 REGIONAL AQUIFER-SYSTEM ANALYSIS PUGET-WILLAMETTE LOWLAND

and basal boundaries of the aquifer system; together, 

these rock units are called the basement confining unit. 

The basement confining units crops out over about 

7,760 mi2 of the Puget Sound Lowland.

The areal extent of the Puget Sound aquifer system 

is generally delineated by the extent of the drift of the 

youngest glaciation, the Fraser Glaciation, and in particular, the Vashon Stade. The Vashon Drift is relatively 

undisturbed and close to the land surface in much of 

the Puget Sound Lowland. Of the 7,183 mi2 of unconsolidated deposits present at the land surface in the 

Puget Sound Lowland, alluvial deposits overlie 

1,570 mi2; fine-grained deposits (mainly glacial till but 

including other deposits such as glaciomarine drift and 

mudflows) overlie 3,320 mi2; and coarse-grained deposits (mainly recessional and advance outwash) overlie 

2,293 mi2.

The Quaternary deposits were divided into regional 

hydrologic units aquifers, semiconfining units, and 

confining units. The aquifer units generally consist of 

coarse-grained outwash deposited during glacial 

advances and retreats, proglacial deposits, and fluvial 

sediments deposited during glacial interstades. The 

semiconfining and confining units generally consist of 

fine-grained till, glaciomarine, mudflow, and lake 

deposits. This alternating sequence of coarse- and finegrained deposits occurs from land surface to depths of 

more than 3,300 ft and averages about 400 ft thick. Individual units range in thickness from 10 to more than 

400 ft.

On the basis of the information presented in the 

report, it was estimated that there is no significant 

regional flow system in the Puget Sound aquifer system 

under the present conditions of hydrologic stress. The 

Puget Sound aquifer system contains subregional flow 

systems that are generally isolated from each other.

Ground-water movement generally is from topographic highs to topographic lows, the latter of which 

typically are stream drainages or saltwater bodies. The 

direction of ground-water movement is predominantly 

horizontal in the aquifer units and vertical in the semiconfining and confining units. Vertical gradients generally are downward, except near streams and saltwater 

bodies where gradients are upward. Ground water in 

the uppermost aquifer unit generally occurs under 

water-table conditions, and ground water in the deeper 

units is confined.

Horizontal hydraulic-head gradients generally 

range from 10 to 70 ft/mi (0.002 to 0.013 ft/ft) and average about 30 ft/mi (0.006 ft/ft). The gradients are flattest in coarse recessional outwash and alluvial deposits 

and are steepest in fine-grained deposits and in all 

deposits located just upgradient of cliffs and cuts. Vertical hydraulic-head gradients generally range from

about 0.004 to 1.6 ft/ft and appear to average about 

0.3 ft/ft. Vertical head differences range from about 1 to 

200 ft over at least the upper 600 ft of the aquifer 

system.

Horizontal hydraulic conductivity for the aquifer 

units generally ranges from about 10 to 700 ft/d, and 

median values generally are in the range of 15 to 

50 ft/d. Values larger than 50 ft/d generally are associated with coarse-grained outwash or alluvial deposits. 

Values less than about 10 ft/d indicate fine-grained 

deposits. Regional effective vertical hydraulic 

conductivity of the till is estimated to range between 

0.001 and 0.01 ft/d, and vertical hydraulic conductivity 

of fine-grained (clayey) interglacial deposits is on the 

order of 0.0001 ft/d. Vertical hydraulic conductivity of 

fine-grained glaciomarine drift probably ranges from 

0.001 to 0.1 ft/d.

Mean annual recharge from precipitation was estimated to be about 27 in/yr or 14,510 ftVs. This quantity is 51 percent of the 53 in. of average annual 

precipitation that falls on the aquifer system. Recharge 

was estimated from the results of previous studies, and 

the estimates were derived using linear regression 

based on average annual precipitation, surficial geology, and land use and cover. Recharge was estimated to 

range spatially from about 0 to more than 70 in/yr. The 

potential range in annual recharge is larger than the 

mean annual value. For example, if an area has an estimated mean annual recharge value of 20 in/yr, then the 

range in annual recharge value can be on the order of 

20 in.

The quality of water in the Puget Sound aquifer system is suitable for most purposes. The dominant water 

types are calcium bicarbonate, magnesium bicarbonate, 

calcium magnesium bicarbonate, and in some areas, 

sodium bicarbonate or sodium chloride. The water is 

generally soft to moderately hard. Dissolved solids 

generally are in the range of 100 to 150 mg/L. Large 

concentrations of iron and manganese are common in 

the glacial aquifers of the Puget Sound aquifer system. 

Concentrations of dissolved nitrate generally are small, 

less than 1.0 mg/L, expressed as nitrogen, except in 

some areas of the shallow parts of the aquifer system 

where land-use practices have resulted in large concentrations. Seawater has intruded some aquifers near the 

coast, as indicated by chloride concentrations of more 

than 100 mg/L.

Cross-sectional numerical models of ground-water 

flow constructed for selected areas used both detailed 

and simplified (aggregated units) aquifer-system geometries. The aggregated geometry was based on the 

aggregation of multiple units into one unit for the part 

of the aquifer system that lies above the first major 

areally extensive, fine-grained (clayey) unit that is
HYDROGEOLOGIC FRAMEWORK OF THE PUGET SOUND AQUIFER SYSTEM D73

located near sea level where the unit is near saltwater. 

The results of the models showed the importance of 

topography, saltwater bodies, the configuration of the 

basement confining unit, and the presence of the first 

clay as varying hydrologic controls on the flow system 

and, thus, on ground-water movement. In addition, the 

geometry, recharge, and hydraulic characteristics of the 

aquifer system also control ground-water movement. 

Calculated hydraulic-head distributions and flow systems generally were similar under both detailed and 

simplified aquifer-system geometries. Increased 

horizontal flow was calculated using the simplified 

geometry, generally resulting in less discharge to saltwater bodies and decreased heads in the deeper system. 

Calculated differences in the water budgets between the 

two geometries generally are less than the potential 

error in field measurements of the water-budget components. On the basis of model results, it was estimated 

that between about 100 and 1,000 ftVs of the total 

ground-water recharge (14,510 ftVs) discharges directly 

to saltwater bodies.

A simplified conceptual model of the Puget Sound 

aquifer system was described. This model includes the 

important regional characteristics of the aquifer system 

that are necessary to describe ground-water flow in the 

Puget Sound Lowland. The conceptual model of the 

aquifer system consists of four types of units glacial 

and interglacial aquifers, semiconfining units, confining 

units, and alluvial valley aquifers. On the basis of 

regional characteristics of these units, the aquifer system was divided into nine alluvial valley aquifers, the 

surficial semiconfining unit, the Fraser aquifer, a confining unit, and the Puget aquifer. Additionally, in areas 

with large thicknesses of unconsolidated deposits, the 

Puget aquifer may be informally divided into the upper 

Puget zone and the lower Puget zone. The basement 

confining unit forms the lateral and basal boundaries of 

the aquifer system.

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