U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5227

Prepared in cooperation with the Maine Geological Survey

Simulation of Groundwater Conditions and 

Streamflow Depletion to Evaluate Water 

Availability in a Freeport, Maine, Watershed
Cover. Map of modeled area for Freeport aquifer groundwater-flow model, showing stream network, deep aquifer area (darker color), and 

shallow aquifer area (light color) in Freeport, Maine.
Simulation of Groundwater Conditions 

and Streamflow Depletion to Evaluate 

Water Availability in a Freeport, Maine, 

Watershed

By Martha G. Nielsen and Daniel B. Locke

Prepared in cooperation with the Maine Geological Survey

Scientific Investigations Report 2011–5227

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Nielsen, M.G., and Locke, D.B., 2012, Simulation of groundwater conditions and streamflow depletion to evaluate 

water availability in a Freeport, Maine, watershed: U.S. Geological Survey Scientific Investigations Report 2011–

5227, 72 p., at http://pubs.usgs.gov/sir/2011/5227/.
iii

Acknowledgments

Richard Knowlton of AquaAmerica and AquaMaine was generous in sharing information on 

water use and hydrogeologic data. Robert Gerber, of Ransom Environmental Consultants, kindly 

shared many unpublished reports and drilling records on the geology and hydrology of the 

Freeport aquifer.
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v

Contents

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

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

Purpose and Scope ..............................................................................................................................4

Description of Study Area ...................................................................................................................4

Hydrogeologic Framework...........................................................................................................................4

Geology...................................................................................................................................................4

Surficial Geology and Mapped Soils ........................................................................................5

Glacial Geology at Depth............................................................................................................5

Bedrock .........................................................................................................................................5

Groundwater Resources .....................................................................................................................7

Hydraulic Properties .................................................................................................................10

Groundwater Flow .....................................................................................................................11

Recharge............................................................................................................................11

Groundwater Levels .........................................................................................................11

Surface-Water Resources ................................................................................................................13

Streamflow Measurements in Harvey and Merrill Brooks.................................................14

Conceptual Model of the Groundwater Flow System...................................................................14

Water Use and Withdrawals......................................................................................................................16

Reported Withdrawals from the Freeport Aquifer ........................................................................16

Estimated Withdrawals from Groundwater....................................................................................16

Simulation of Groundwater Flow and Discharge to Streams...............................................................17

Steady-State Numerical Groundwater-Flow Model .....................................................................17

Spatial Discretization of the Model ........................................................................................18

Boundary Conditions.................................................................................................................18

Stresses.......................................................................................................................................18

Hydraulic Properties .................................................................................................................21

Model Calibration Using Parameter Estimation and Observations............................................21

Observations...............................................................................................................................24

Parameters..................................................................................................................................26

Changes to the Conceptual Model .........................................................................................26

Model Fit to Observations ........................................................................................................30

Simulated Groundwater Levels and Flow Under Steady-State Conditions .....................32

Model Sensitivity Analysis and Parameter Uncertainty .....................................................32

Limitations of the Model ...........................................................................................................40

Model-Calculated Water Budget for Harvey and Merrill Brooks and the Buried 

Freeport Aquifer ....................................................................................................................40

Evaluation of Streamflow Depletion in Harvey Brook ...........................................................................42

Calculation of Instream Flow Requirements for Harvey Brook...................................................45

Streamflow Depletion Estimates Based on STRMDEPL08 ..........................................................45

Simulation of Streamflow Depletion Based on the Steady-State GroundwaterFlow Model.............................................................................................................................48

Comparison of Methods Used to Evaluate Streamflow Depletion ....................................50

Comparison of Streamflow Depletion Estimates to Instream Flow Requirements.........50
vi

Figures

1. Map showing location of watersheds in the Freeport aquifer study area, Maine............3

2. Map showing buried valley extent, showing altitude of bedrock in wells and 

interpolated bedrock surface altitude, Freeport, Maine........................................................6

3. Map showing interpreted surficial geology of the Freeport aquifer study area................8

4. Geologic cross sections A–A’, B–B’, and C–C’ in the Freeport aquifer study area...........9

5. Map showing groundwater level and streamflow measurement sites in the 

Freeport aquifer study area ......................................................................................................12

6. Graphs showing A, streamflow measurements in Harvey and Merrill Brooks, and 

B, precipitation in Portland, Maine, May–September 2009.................................................15

7. Graph showing estimates of monthly mean streamflow at the Harvey and Merrill 

Brook sites based on data from long-term index sites in southern Maine and 

New Hampshire ..........................................................................................................................15

8. Model grid and boundary conditions for the Freeport aquifer groundwater-flow 

model ............................................................................................................................................19

9. Vertical cross section of the Freeport aquifer groundwater-flow model, row 65............20

10. Map showing recharge rates applied to the numerical model of the Freeport aquifer 

study area ....................................................................................................................................22

11. Maps showing calibrated horizontal hydraulic conductivities (Kh) used in the 

Freeport aquifer groundwater-flow model.............................................................................23

12. Maps showing calibrated vertical hydraulic conductivities (Kv) used in the 

Freeport aquifer groundwater-flow model.............................................................................27

13. Map showing drain hydraulic conductivity values (Kdr) used in calibrated model..........28

14. Map showing groundwater level and streamflow measurement observations in the 

Freeport aquifer study area, Maine.........................................................................................29

15. Graphs showing relation between observed and model-calculated values for 

A, heads, and B, early summer streamflow, for the Freeport aquifer groundwaterflow model....................................................................................................................................31

16. Scatterplot showing weighted residuals and unweighted simulated values for 

heads in the Freeport aquifer groundwater-flow model......................................................32

17. Map showing spatial distribution of weighted residuals across the domain of the 

Freeport aquifer groundwater-flow model.............................................................................33

18. Boxplot showing ninety-five percent confidence intervals on steady-state 

streamflow simulation for the five streamflow sites in the Freeport aquifer 

study area ....................................................................................................................................35

19. Map showing steady-state simulated groundwater levels in layer 1, Freeport 

aquifer model area .....................................................................................................................36

20. Map showing steady-state simulated groundwater levels in layer 3, Freeport 

aquifer model area .....................................................................................................................37

21. Map showing steady-state simulated streamflow (groundwater discharge) from 

drain cells and drain observation zones in the Freeport aquifer model area...................38

Suggestions for Improving Methods of Study for Water Availability..................................................52

Summary and Conclusions.........................................................................................................................53

References Cited..........................................................................................................................................55

Appendix 1. List of Wells and Observations Used in the Freeport Aquifer Study..........................59

Appendix 2. Details of Groundwater Model Calibration ....................................................................67
vii

22. Boxplots showing parameter estimates, 95-percent confidence intervals, and 

reasonable ranges for A, hydraulic conductivity and B, recharge parameters for the 

Freeport aquifer groundwater-flow model.............................................................................39

23. Graph showing steady-state simulated inflows and outflows for three subareas 

within the Freeport aquifer groundwater-flow system.........................................................42

24. Graph showing simulated water budget components under various hydrologic 

scenarios, including no pumping, decreased recharge, and increases in pumping 

for the Freeport aquifer groundwater-flow model ................................................................43

25. Graphs showing monthly median flow estimates, pumping, and instream flow 

requirements for A, Merrill Brook, and B, Harvey Brook, Freeport, Maine.....................46

26. Graph showing Harvey Brook median flow estimates and 90-percent prediction 

intervals........................................................................................................................................47

27. Map showing steady-state calculated streamflow depletion in each drain cell in 

the Freeport aquifer groundwater-flow model ......................................................................49

28. Boxplot showing ninety-five-percent confidence intervals for simulated streamflow 

depletion caused by pumping for the Harvey and Merrill Brook sites ..............................50

29. Boxplot showing steady-state streamflow projections from the groundwater model 

for Harvey and Merrill Brooks for pumping and recharge scenarios, showing the 

95-percent confidence intervals ..............................................................................................51

30. Graph showing summer streamflows, pumping, instream flow requirements and 

projections of streamflow depletion in Harvey Brook at site #1, Freeport, Maine..........52

Tables

1. Hydraulic properties of hydrogeologic units in the Freeport Aquifer study area............10

2. Streamflow site information, Harvey and Merrill Brook, 2009 ............................................13

3. Water users in Harvey and Merrill Brook watersheds, as reported by 

HarrisInfosource, and personal reconnaissance .................................................................17

4. Estimated total withdrawals from groundwater by water use category in the 

Harvey and Merrill Brook watersheds, 2009..........................................................................17

5. Hydrogeologic units and associated calibrated hydraulic conductivities .......................24

6. Observations used in calibration of the Freeport aquifer groundwater flow model, 

with variance and weights for optimization used for each observation. ..........................25

7. Drawdown observed in layer 3 wells during pumping tests, and model-calculated 

steady-state drawdown from pumping in the AquaMaine production wells...................34

8. Steady-state model calculated and observed early summer streamflows in the 

Freeport aquifer groundwater model ......................................................................................34

9. Steady-state model calculated annual water budget for the Freeport aquifer 

groundwater-flow model and three subareas.......................................................................41

10. Simulated water budgets for the Freeport aquifer groundwater-flow model and 

subareas under alternative hydrologic conditions ...............................................................44

11. Sensitivity analysis of STRMDEPL08 streamflow depletion evaluations for the 

Freeport aquifer, based on reasonable ranges for unknown variables ............................48

12. Model-calculated streamflow depletion at each streamflow measurement site ...........48

13. Model-calculated streamflow and streamflow depletion in Harvey Brook and 

Merrill Brook under various pumping and recharge scenarios .........................................51
viii

Conversion Factors, Datum, and Acronyms

Inch/Pound to SI

Multiply By To obtain

Length

inch (in) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

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

Flow rate

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

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

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

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

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

million gallons per year (Mgal/yr) 3,785 cubic meter per year (m3/yr)

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

Hydraulic conductivity

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

Transmissivity*

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

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

 °C=(°F–32)/1.8

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

(NAVD 88).

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

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

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

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

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

Acronyms

MGS Maine Geological Survey

USGS U.S. Geological Survey

WRPC (Maine) Water Resources Planning Committee
Abstract

In order to evaluate water availability in the State of 

Maine, the U.S. Geological Survey (USGS) and the Maine 

Geological Survey began a cooperative investigation to 

provide the first rigorous evaluation of watersheds deemed 

“at risk” because of the combination of instream flow requirements and proportionally large water withdrawals. The study 

area for this investigation includes the Harvey and Merrill 

Brook watersheds and the Freeport aquifer in the towns of 

Freeport, Pownal, and Yarmouth, Maine. A numerical groundwater-flow model was used to evaluate groundwater withdrawals, groundwater-surface-water interactions, and the effect of 

water-management practices on streamflow. The water budget 

illustrates the effect that groundwater withdrawals have on 

streamflow and the movement of water within the system.

Streamflow measurements were made following standard USGS techniques, from May through September 2009 at 

one site in the Merrill Brook watershed and four sites in the 

Harvey Brook watershed. A record-extension technique was 

applied to estimate long-term monthly streamflows at each of 

the five sites.

The conceptual model of the groundwater system consists 

of a deep, confined aquifer (the Freeport aquifer) in a buried 

valley that trends through the middle of the study area, covered by a discontinuous confining unit, and topped by a thin 

upper saturated zone that is a mixture of sandy units, till, and 

weathered clay. Harvey and Merrill Brooks flow southward 

through the study area, and receive groundwater discharge 

from the upper saturated zone and from the deep aquifer 

through previously unknown discontinuities in the confining 

unit. The Freeport aquifer gets most of its recharge from local 

seepage around the edges of the confining unit, the remainder 

is received as inflow from the north within the buried valley. 

Groundwater withdrawals from the Freeport aquifer in 

the study area were obtained from the local water utility and 

estimated for other categories. Overall, the public-supply 

withdrawals (105.5 million gallons per year (Mgal/yr)) were 

much greater than those for any other category, being 

almost 7 times greater than all domestic well withdrawals 

(15.3 Mgal/yr). Industrial withdrawals in the study area 

(2.0 Mgal/yr) are mostly by a company that withdraws 

from an aquifer at the edge of the Merrill Brook watershed. 

Commercial withdrawals are very small (1.0 Mgal/yr), and no 

irrigation or other agricultural withdrawals were identified in 

this study area.

A three-dimensional, steady-state groundwater-flow 

model was developed to evaluate stream-aquifer interactions 

and streamflow depletion from pumping, to help refine the 

conceptual model, and to predict changes in streamflow resulting from changes in pumping and recharge. Groundwater levels and flow in the Freeport aquifer study area were simulated 

with the three-dimensional, finite-difference groundwater-flow 

modeling code, MODFLOW-2005. Study area hydrology was 

simulated with a 3-layer model, under steady-state conditions. 

The groundwater model was used to evaluate changes 

that could occur in the water budgets of three parts of the 

local hydrologic system (the Harvey Brook watershed, the 

Merrill Brook watershed, and the buried aquifer from which 

pumping occurs) under several different climatic and pumping 

scenarios. The scenarios were (1) no pumping well withdrawals; (2) current (2009) pumping, but simulated drought 

conditions (20-percent reduction in recharge); (3) current 

(2009) recharge, but a 50-percent increase in pumping well 

withdrawals for public supply; and (4) drought conditions and 

increased pumping combined. In simulated drought situations, 

the overall recharge to the buried valley is about 15 percent 

less and the total amount of streamflow in the model area is 

reduced by about 19 percent. Without pumping, infiltration to 

the buried valley aquifer around the confining unit decreased 

by a small amount (0.05 million gallons per day (Mgal/d)), 

and discharge to the streams increased by about 8 percent 

(0.3 Mgal/d). A 50-percent increase in pumping resulted in a 

simulated decrease in streamflow discharge of about 4 percent 

(0.14 Mgal/d).

Simulation of Groundwater Conditions and 

Streamflow Depletion to Evaluate Water Availability 

in a Freeport, Maine, Watershed

By Martha G. Nielsen1 and Daniel B. Locke2

1

U.S. Geological Survey.

2

Maine Geological Survey.
2 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Streamflow depletion in Harvey Brook was evaluated by 

use of the numerical groundwater-flow model and an analytical model. The analytical model estimated negligible depletion 

from Harvey Brook under current (2009) pumping conditions, 

whereas the numerical model estimated that flow to Harvey 

Brook decreased 0.38 cubic feet per second (ft3/s) because of 

the pumping well withdrawals. A sensitivity analysis of the 

analytical model method showed that conducting a cursory 

evaluation using an analytical model of streamflow depletion 

using available information may result in a very wide range 

in results, depending on how well the hydraulic conductivity 

variables and aquifer geometry of the system are known, and 

how well the aquifer fits the assumptions of the model. Using 

the analytical model to evaluate the streamflow depletion with 

an incomplete understanding of the hydrologic system gave 

results that seem unlikely to reflect actual streamflow depletion in the Freeport aquifer study area.

In contrast, the groundwater-flow model was a more 

robust method of evaluating the amount of streamflow depletion that results from withdrawals in the Freeport aquifer, 

and could be used to evaluate streamflow depletion in both 

streams. Simulations of streamflow without pumping for 

each measurement site were compared to the calibratedmodel streamflow (with pumping), the difference in the total 

being streamflow depletion. Simulations without pumping 

resulted in a simulated increase in the steady-state flow rate of 

0.38 ft3/s in Harvey Brook and 0.01 ft3/s in Merrill Brook. This 

translates into a streamflow-depletion amount equal to about 

8.5 percent of the steady-state base flow in Harvey Brook, 

and an unmeasurable amount of depletion in Merrill Brook. If 

pumping was increased by 50 percent and recharge reduced 

by 20 percent, the amount of streamflow depletion in Harvey 

Brook could reach 1.41 ft3/s.

Introduction

In 2007, the State of Maine established the Maine Water 

Resources Planning Committee (WRPC), whose mandate is 

to plan for the sustainable use of water resources in the State. 

The WRPC gathers water-resources data, analyzes watershed 

withdrawals relative to instream flow requirements, and 

provides guidance and tools for State and local governments 

with respect to water availability and managing water 

resources. The State uses estimates of natural monthly flows, 

sometimes combined with site-specific geomorphic analysis, 

to evaluate streamflow requirements in support of aquatic 

habitat when a proportionally large withdrawal is identified in 

a watershed. The instream flow requirements are specific to six 

time periods—winter (January 1 to March 15), spring (March 

16 to May 15), early summer (May 16 to June 30), summer 

(July 1 to September 15), fall (September 16 to November 15), 

and early winter (November 16 to December 31)—each of 

which is based on estimates of median monthly streamflows. 

Although information on large water withdrawals is well 

coordinated among State agencies, there is no consistently 

applied method for tracking the sum of water withdrawals 

in a watershed, and assessing the potential effects of those 

withdrawals on streamflows. In addition, to better implement 

the instream flow requirements, further insight into the effects 

of groundwater withdrawals on the overall water budget and 

natural streamflows in watersheds with high withdrawals is 

needed in the state.

The WRPC, through the lead of the Maine Geological 

Survey (MGS), desires to conduct additional investigations 

into watersheds deemed “at risk” because of the combination 

of instream flow requirements and proportionally large water 

withdrawals. These additional investigations are intended 

to provide in-depth analyses of the hydrologic systems and 

water budgets to determine first, whether certain watersheds 

are indeed approaching their withdrawal limit and second, 

to better understand the potential effect of withdrawals on 

aquatic base flows and the overall hydrologic system. The 

MGS identified two adjacent watersheds in the Freeport, 

Maine, area (fig. 1) as having a permitted withdrawal from 

pumping wells for public water supply in combination with 

flows required to meet instream flow requirements that are 

quite large in comparison to the total annual runoff. These 

watersheds (Harvey and Merrill Brook) and the Freeport 

aquifer, from which water is withdrawn, serve as the study 

area to illustrate the issues just identified. This study area 

provides an example of the hydrologic processes and effects 

that pumping well withdrawals can have on a small (less than 

10 square miles (mi2)) watershed.

In 2009, the U.S. Geological Survey (USGS) and the 

MGS cooperatively began the first rigorous evaluation (or 

pilot study) of the hydrologic effects of withdrawals in “watersheds at risk.” The study has three goals related to evaluating 

water availability in watersheds with large withdrawals: 

• Provide a blueprint for estimating total withdrawals in 

a watershed, both reported and unreported; 

• Evaluate the current and future potential watermanagement scenarios in a watershed to assist waterresource managers in making decisions about watersupply issues; and 

• Evaluate the effect of withdrawals on streamflows and 

streamflow depletion in light of the State requirements 

to maintain instream flows for aquatic habitat protection. 

The use of a numerical groundwater-flow model to evaluate groundwater withdrawals allows water to be accounted 

for as it flows through the groundwater system to the surfacewater system, and provides a method to evaluate the effect 

of water-management practices on streamflow. The Freeport 

aquifer groundwater-flow model (hereafter referred to as “the 

groundwater-flow model” or “the model”) was also used to 

help refine the conceptual model of groundwater flow in the 

study area, because the possible source(s) of groundwater to 

the withdrawal wells was poorly understood at the outset.
Introduction 3

!

!

!

295

MAINE

Study area

Bangor

Augusta

Portland

Merrill Brook

Harvey Brook

Freeport

Pownal

Yarmouth

North Yarmouth Merrill Brook

Harvey Brook

Cousins River

Harraseeket River

Freeport

n

70°10' 70°08' 70°06'

43°52'

43°50'

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

Universal Transverse Mercator projection, Zone 19N

Central meridian -69°

Shaded relief from U.S. Geological Survey National Elevation

Dataset 10-meter digital elevation model

North American Datum of 1983 (NAD 83) 

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Study area watershed boundary

Figure 1. Location of watersheds in the Freeport aquifer study area, Maine.
4 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Purpose and Scope

This report documents the simulation of streamflow 

conditions and groundwater depletion for two adjacent small 

watersheds (Harvey Brook and Merrill Brook) in southeastern 

Maine. Specifically, the report describes the determination of 

total water use and withdrawals, the use and calibration of a 

steady-state groundwater-flow model of the Freeport aquifer, 

and its use in evaluating the effect of groundwater withdrawals 

on streamflow. The data collected to construct and calibrate 

the groundwater-flow model are presented. Simulation results 

for varying water withdrawal and climatic scenarios on the 

water budgets for the Freeport aquifer, Harvey Brook, and 

Merrill Brook are described. The parameter estimation used 

for model calibration, model sensitivities and limitations, and 

prediction uncertainties also are reported for the model. In 

addition to the groundwater-flow model, the report describes 

the use of a program, STRMDEPL08, which uses analytical 

solutions to evaluate streamflow depletion in Harvey Brook. 

The report presents a summary of the effect of withdrawals on 

streamflows in the study area, and on the overall movement of 

water through the hydrologic system. Finally, suggestions for 

conducting future studies are presented.

Description of Study Area

The study area includes the Harvey Brook and Merrill 

Brook watersheds and the Freeport aquifer, which lies within 

the Harvey and Merrill Brook watersheds beneath the towns of 

Freeport, Pownal, and Yarmouth, Maine (fig. 1). The aquifer 

was mapped initially as a significant sand and gravel aquifer 

by the Maine Geological Survey (Neil and Locke, 1999a, b). 

Studies commissioned by the AquaMaine–Freeport Division 

water utility, which is the primary user of water in the Freeport 

aquifer, refined the mapped extent of the aquifer, showing that 

it lies in a buried river valley (fig. 2) that does not specifically 

coincide with the surficial aquifer mapping, which was based 

largely on the extent of sandy soils in the area. The northern 

and southern limits of the aquifer have not been identified, 

but well data suggest that the buried valley extends southward 

under the Atlantic Ocean. The northern extent of the buried 

valley could extend outside the study area toward the Durham 

southwest bend of the Androscoggin River (Gerber, 1979). 

The Freeport aquifer, as referred to herein, constitutes the 

buried valley aquifer, often called the Harvey Brook aquifer 

in consulting reports, and is the subject of the modeling effort 

in this report along with the surficial sediments overlying the 

aquifer (fig. 2). 

The study area has rolling topography, dissected by 

streams in valleys having slopes that range from shallow and 

gradual to steep. The relief is largely controlled by bedrock 

ridges, and valleys created by glacial action. Land surface 

altitude in the study area ranges from 0 to 220 feet (ft).

Harvey Brook and Merrill Brook join and become the 

Cousins River, which flows into the ocean approximately 

2.5 miles (mi) southwest of their confluence. The Cousins 

River is tidal along its entire length. Above the confluence 

at the Cousins River, the Harvey Brook watershed is 4.0 mi2

and the Merrill Brook watershed is 4.6 mi2, although the 

watershed areas for which data are compiled upstream from 

the streamflow measurement sites are slightly smaller (3.18 

and 3.81 mi2, respectively). 

The mean annual precipitation in the Freeport area for 

the 1961–1990 period is 45.9 inches (in.) (Oregon State 

University, 2010; Natural Resources Conservation Service, 

1998). The closest long-term temperature station is in 

Portland, Maine, which is 15.5 mi southwest of Freeport 

along the coast. The average annual temperature for the 

Portland station is 45.7°F (National Weather Service, 2010), 

which is expected to be the same in the Freeport area, given 

their similar altitude, distance from the Atlantic Ocean, and 

proximity to each other. The land use in and around the study 

area is primarily rural residential, with the exception 

of the town centers of Freeport and Yarmouth, and the U.S. 

1/Interstate-295 corridor, which is dominated by commercial 

development. The rural residential areas are largely forested, 

with interspersed hayfields along roadways and small areas 

of unbroken forest between adjacent roadways and residential 

corridors. The largest area of unbroken forest in the study 

area is in the center of the Harvey Brook watershed. The 

populations of Freeport and Yarmouth in 2000 were 7,800 

and 8,360, respectively (U.S. Census Bureau, 2000), with 

population densities of 225 to 626 persons/mi2.

Hydrogeologic Framework

The highly productive portion of the Freeport aquifer 

lies within the buried bedrock valley (fig. 2), which trends 

roughly north-south in the Freeport area (Gerber, 1979, 1985). 

The aquifer consists of layered sand, fine sand, silt, and 

gravel. Above these productive sediments, a silt and clay layer 

(known as the Presumpscot Formation (Bloom, 1960, 1963)) 

forms a discontinuous confining unit between the deep aquifer 

and sandy deposits at the surface. The Presumpscot Formation 

is thickest in the middle of the buried valley, but thins considerably at the valley edges, and is absent in parts. The bedrock 

hillsides adjacent to the buried valley are covered with a 

heterogeneous distribution of till, sand, and clay.

Geology

The geologic units in the study area include fractured 

crystalline bedrock and stratified, unconsolidated glacial 

deposits that are draped over the bedrock. The glacial deposits include till (in moraines and as a blanket deposit), stratified marine sand and gravel, marine silt and clay, beach and 

nearshore sand and gravel deposits, and eolian sand deposits 

(Marvinney, 1999b; Retelle, 1999a, b; Weddle, 1999; Weddle 

and Retelle, 1995). More recent sediments include Holocene 

stream alluvium and Holocene wetlands.
Hydrogeologic Framework 5

Surficial Geology and Mapped Soils

After the last glacial maximum, the melting glacier 

retreated northward past coastal Maine, leaving a number of 

deposits as the retreat occurred. The retreat was accompanied 

by a marine transgression onto the depressed land surface, 

so that sediments carried by the melting glacier were deposited in a shallow marine environment (Weddle and Retelle, 

1995). Unsorted sediment from the bottom of the glacier was 

deposited as till over the bedrock surface. Submarine meltwater conduits transported coarse sediment out toward the sea, 

where it was deposited in submarine fans and deltas. In calmwater areas, finer grained sediments (silt and clay) fell out of 

suspension onto the submarine surface, forming a widespread 

silt and clay layer known as the Presumpscot Formation 

(Bloom, 1960, 1963). As the glacier retreated farther inland, 

the land surface rebounded, exposing the marine sediments 

first to wave action, and then to subaerial erosion and deposition. During this phase, the top layer of marine sediments was 

reworked by wave action, leaving widespread nearshore sandy 

deposits over the silt and clay (Weddle and Retelle, 1995). 

Wind and water further reworked these sediments, exposing 

till uplands, creating eolian sand deposits, and filling in stream 

valleys with alluvial deposits. 

The surficial materials and surficial geology maps show 

areas of thin drift, where the depth to bedrock is very shallow, 

and locations of bedrock outcrop. The surficial geology map 

for the Freeport quadrangle does not differentiate sandy areas 

from silt and clay areas within one of the surficial geologic 

units, which required the use of additional sources and some 

field investigation for this study. The interpreted surficial 

geology in the study area is shown in figure 3; this map 

represents a combination of surficial geology mapped by the 

Maine Geological Survey (Marvinney, 1999a, b; Retelle, 

1999a, b), soils (Natural Resources Conservation Service, 

2006), and field observations. The stratigraphically uppermost 

units are the eolian sands and nearshore marine deposits, 

which generally overlie the silt and clay of the Presumpscot 

Formation. (Sandy deposits overlying the silt and clay of 

the Presumpscot Formation have often been identified as 

an upper nearshore sand facies of the formation, but have 

sometimes been found to unconformably overlie the silt and 

clay (Weddle and Retelle, 1995); figure 3 shows the sandy 

deposits as a separate unit.) The largest area of eolian sand lies 

in the middle of the Harvey Brook watershed. To the north 

and south along Harvey Brook, and to the east of these eolian 

sands, sandy soils (interpreted as being the same as mapped 

nearshore marine sands in the north part of the study area) are 

widespread. An area of soils described as consisting of sand, 

gravel, and boulders is exposed at the surface on the western 

side of Harvey Brook. This bouldery, sandy unit can be found 

exposed in gullies leading into the valley of Harvey Brook, 

and has been observed to be a zone in which runoff from 

rainfall events is absorbed into the subsurface. 

The Presumpscot Formation silt and clay lies stratigraphically beneath the eolian sands and sandy nearshore sediments. 

Gerber and Hebson (1996) describe the Presumpscot Formation as follows: 

“The Presumpscot Formation clay-silt is typically 

composed of about 10 ft of desiccated brown and 

olive clay-silt overlying a softer “blue” or gray siltclay. The desiccated zone is fissured into a subangular block pattern, more dense and closely spaced 

at the ground surface and diminishing at depth. The 

softer gray clay lies below the position of the permanent water table.”

Till is the stratigraphically lowest glacial unit in the study 

area, and directly overlies the bedrock. The till is sandy, dense, 

and is often shown as being about 5 ft in thickness in drillers 

logs in the area. In addition, There are many areas in which 

the glacial deposits are very thin, and these have been mapped 

as thin till or undifferentiated thin glacial deposits (fig. 3). 

Holocene alluvium and wetlands can be found in many of the 

stream valleys in the study area.

Glacial Geology at Depth

The available surficial geology maps do not show the 

overall thickness and stratigraphy of the entire suite of glacial 

deposits in the study area. The Freeport aquifer lies beneath 

an area that is mapped as Presumpscot Formation and sandy 

eolian and nearshore deposits at the surface. Well logs from 

(1) the Maine Geological Survey’s well driller’s database, 

(2) seismic refraction lines, (3) well and boring logs from 

numerous studies commissioned by the AquaMaine–Freeport 

Division water utility (Weston Geophysical (1973), Maine 

Water Company (1978), Hydro Group (1984, 1988, 1991, 

1992, 1995, 1997), Caswell, Eichler, and Hill (1993, 1994), 

Gerber (1979, 1980, 1985, 1995), Gerber-Jacques Whitford 

(1996), Stratex (1999, 2001a, b, 2002, 2003), Drumlin 

Environmental (2000), Sevee & Maher (2002), Reynolds 

(2000), Earth Tech (2004), URS Corporation (2002), and 

R.E. Chapman Company (2003), and (4) logs of two wells 

drilled for this study collectively indicate that a buried valley 

lies beneath the Harvey Brook watershed and extends for an 

indeterminate distance to the north and south of the study area 

(fig. 2). The sediments at depth consist of layered coarse sand, 

fine gravel, fine sand, and silt, which indicates that this buried 

valley probably once served as a conduit for sediment-laden 

glacial meltwater flowing under a retreating glacier. All the 

wells and borings with data used for this study are listed in 

appendix 1.

Bedrock

Bedrock in the study area consists of the Hutchins Corner 

Formation, a biotite-quartz-plagioclase granofels metamorphic 

rock (Berry and Hussey, 1998). The bedrock surface was 

interpolated for the study from several sources of data. 

Drilling records for domestic (primarily bedrock) wells were 

obtained for the study area from the Maine Geological Survey, 
6 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Harraseeket River

E

E

E

E

E

E

E

E

E

E

E

E

EE

E

EE

E

EE

EE

EE

E

E

E

E

E

E

EE

E

E

E

E E

E

E

E

E

E E

E E

E

E E

E

E

E

E E

E

E

E

E

EE

EE

EE

E

E

E

E

E

E

EE

E

E

E

E E

E

E

EEE EE

EE

EE

E

EE

E

EE

EE E

EE

E

E

EEEE

E

EE EE

Freeport

Pownal

Yarmouth

North 

Yarmouth Merrill Brook

Harvey Brook

-15

-58

-37

-36

-30 -79

-16

0

5

9

9

1

76

13

55

59

31

95

77

80

54

82

51

31 -8

-6

44

-5

15

38

30

36

43

45

-3

-9

23

15 67

-95

-99

-57

-29

-24

-40

-25

-25

-36 -31

-32

-12

-56

-17

-16

-24

-48 -57

-79

-31

-48

-41

-77

-76

-118

-113

70°10' 70°8'

43°52'

43°50'

43°48' ?

? ?

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

Universal Transverse Mercator projection, Zone 19N

Central meridian -69°

Shaded relief from U.S. Geological Survey National Elevation

Dataset 10-meter digital elevation model

North American Datum of 1983 (NAD 83) 

Geology modified from Maine Geological Survey

Surficial Geology maps 1:24,000

0 0.5 1 MILE

0 0.5 1 KILOMETER

Area of inset

EXPLANATION

Buried valley extent

Well or boring in buried valley 

showing altitude of bedrock—

In feet

E Bedrock outcrop

? Area of uncertain extent

Active model area

295

Cousins River

38

Figure 2. Buried valley extent, showing altitude of bedrock in wells and interpolated bedrock surface altitude, Freeport, Maine.
Hydrogeologic Framework 7

and typically include depth to bedrock, total well depth, 

and well yield. Seismic profiles from Weston Geophysical 

(1973) and Caswell, Eichler, and Hill (1994), and the Maine 

Geological Survey (Neil and Locke, 1999a, b, 2002) indicate 

depth to bedrock in many areas. Additional well logs from 

consulting reports (listed previously) were also consulted to 

determine the bedrock surface at depth. 

Groundwater Resources

Groundwater available to wells occurs in four hydrogeologic units in the Freeport area. Although water within the 

fractured crystalline bedrock underlying all the surficial units 

is utilized as a water source for domestic wells, the low yields 

typically obtained from these wells limit this source to small, 

domestic uses. Where it overlies the bedrock in upland areas, 

water-containing till is used as a water supply for dug wells. 

The deep sand and gravel deposits in the buried valley under 

Harvey Brook (fig. 2) compose the Freeport aquifer. This 

aquifer consists of layered fine and silty fine sand containing 

lenses of coarse clean sand, with the most transmissive layers 

being about 20–30 ft thick. Finally, sandy deposits at the surface can hold significant amounts of water and provide water 

to dug wells and springs that are used for domestic water 

supplies. Stratigraphically separating the surficial sands from 

the deep sand and gravel deposits, the Presumpscot Formation acts as a discontinuous confining layer above the Freeport 

aquifer. 

Hilltops with thin till cover and undifferentiated thin 

glacial deposits present opportunities for recharge to the 

bedrock aquifer, and to the buried sand and gravel deposit 

underneath the confining layer. Figure 4 shows three cross 

sections across the study area, two from west to east (A–A’ 

and B–B’), and one from south to north along the buried 

valley (C–C’). Each cross section shows the following, from 

top to bottom: (1) land surface; (2) the top of the confining 

Presumpscot Formation, interpolated across the study area 

from well logs and surface exposures; (3) the bottom of 

the Presumpscot Formation (the line indicating the base of 

Altitude of bedrock

—In feet

High: 220

Low: -95

EXPLANATION

Figure 2. Buried valley extent, showing altitude of 

bedrock in wells and interpolated bedrock surface 

altitude, Freeport, Maine.—Continued
8 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

E

E

E

E

E

E

E

E

E

E E

E E

E

E

E E

E

E

E

E

EE

EE E EE

E

E

E

EE

E

E

E

C'

C

A

A'

B'

B

Freeport

Pownal

Yarmouth

North 

Yarmouth

Harvey Brook

Merrill Brook

70°10' 70°08'

43°52'

43°50'

0 0.5 1MILE

0 0.5 1 2 KILOMETERS

Geology modified from Maine Geological Survey

Surficial Geology maps 1:24,000

Geology mapped by C.L. Marvinney (1999), T. Weddle (1999),

and M. Retelle (1999), and modified by M. Nielsen in 2009 

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

Universal Transverse Mercator projection, Zone 19N

Central meridian -69°

North American Datum of 1983 (NAD 83) 

EXPLANATION

Till

Thin till—Less than 10 feet to bedrock

Gravel pit

Mapped marine nearshore

Sand at surface—Nearshore marine

Mapped marine shoreline

Eolian sand—Mostly above clay

Upland area

Valley bottom

Sand, gravel, boulders

Silty alluvium (Holocene)

Thin drift—Undifferentiated

Wetland

Active model area

Cross-section trace

Bedrock outcrop

Presumpscot Formation

E

A A'

295

Figure 3. Interpreted surficial geology of the Freeport aquifer study area.
Hydrogeologic Framework 9

ACTIVE MODEL

AREA

ACTIVE MODEL

AREA

ACTIVE MODEL

AREA

Harvey

Brook

Harvey

Brook Merrill

Brook

Harvey

Brook

Harvey

Brook Cousins

River

ACTIVE MODEL

AREA

ACTIVE MODEL

AREA

ACTIVE MODEL

AREA

C

SOUTH NORTH

A

WEST EAST

B

C'

A'

B'

WEST EAST

-100

-50

NAVD 88

50

100

150

200

250

FEET

FEET

FEET

Altitude, in feet

Distance, in feet

Distance, in feet

Distance, in feet

Altitude, in feet Altitude, in feet

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500

-100

-50

NAVD 88

50

100

150

0 500 1,000 1,500 2,000 2,500 3,000

-150

-100

-50

NAVD 88

50

100

150

200

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

VERTICAL EXAGGERATION x4.0

VERTICAL EXAGGERATION x5.3

VERTICAL EXAGGERATION x6.2

Surficial units

Presumpscot Formation

Sand and gravel

Till

Bedrock

EXPLANATION

Figure 4. Geologic cross sections A–A’, B–B’, and C–C’ in the Freeport aquifer study area. Extent of the active 

model area is between the active model area lines indicated in each cross section.
10 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

the confining layer shown in figure 4 represents an average 

thickness of the Presumpscot Formation from available well 

logs); (4) a layer representing a uniform 10-ft thick layer of 

till above the bedrock, which is present at the bottom of most 

well logs in the study area; and (5) the bedrock. The surface 

representing the top of the bedrock is interpolated from 

well data.

Harvey and Merrill Brooks flow in valleys incised into 

the surficial materials. Merrill Brook flows in a basin that 

is dominated by fine-grained soils (till, silt, and clay). The 

Harvey Brook valley is more deeply incised into the surficial 

units. The steep valley sides of Harvey Brook are sandy at 

the top and transition into the silt/clay of the Presumpscot 

Formation near the bottom. Holocene alluvium, consisting 

of fine sand and silt, fills the bottom of the stream valleys 

(not shown in fig. 4). Outcrops of layered silt/clay are often 

exposed on the outside meander curve of the streambed, and in 

places within the bottom of the streambed.

Hydraulic Properties

Published estimates of the hydraulic properties (transmissivity or hydraulic conductivity) of the hydrogeologic units 

in the study area are available from several studies (table 1). 

These estimates are based on calibrated groundwater modeling 

studies, pump tests, grain-size analysis, and other sources. 

Few published studies directly address the hydraulic 

conductivity (K) of the uppermost sandy units in the 

study area. Tepper and others (1990) reported a hydraulic 

conductivity of a shallow, very fine-medium sandy deposit 

with some silt, which is very similar to the nearshore sandy 

deposits in the Freeport aquifer study area, to be 2 to 5 feet per 

day (ft/d). Other published values of hydraulic conductivities 

of a fine sand or silty sand range from about 0.5 to 5 ft/d 

(Freeze and Cherry, 1979). The eolian sands (fig. 3) are better 

sorted and more coarsely grained, and would be expected 

to have a somewhat higher K value than the other fine-tomedium-grained sands.

Table 1. Hydraulic properties of hydrogeologic units in the Freeport Aquifer study area.

[ft/d, foot per day; Fm., formation]

Hydrogeologic unit, 

location

Horizontal hydraulic 

conductivity, 

in ft/d

Vertical hydraulic 

conductivity, 

in ft/d

Source

Very fine to medium sand, with some silt 

(similar to marine nearshore sands), 

Fryeburg, Maine

2 to 5 Tepper and others (1990)

Presumpscot Fm. silt/clay (unweathered), 

central Maine

6.2 × 10-3 (mean of 

32 measurements)

1.4 × 10-4 to 5 × 10-6

(range of three methods)

Brainerd and others (1996)

Presumpscot Fm. silt/clay, Saco, Maine 2.7 × 10-5 Nielsen and others (1995)

Presumpscot Fm. silt/clay (several sites) 1.2 × 10-4 to 5 × 10-4 Gerber and Hebson (1996)

Stratified outwash sands (similar to the 

Freeport aquifer), Oxford, Maine

15 to 80 Morrissey (1983)

Freeport aquifer sand and gravel, 

Freeport, Maine

29 to 300 (range from 

8 pump tests)

Maine Water Company (1978); Gerber 

(1995); Hydro Group (1988, 1991, 1995, 

1997); Stratex (2003); Earth Tech (2004)

Till, Bald Mountain, Maine 0.045 to 0.91 Gerber and Hebson (1996)

Till, Fryeburg, Maine 4 Morrissey (1983)

Streambed hydraulic conductivity, 

Fryeburg, Maine

2 Morrissey (1983)

Streambed hydraulic conductivity, 

Saco River, Maine

2 to 5 Tepper and others (1990)
Hydrogeologic Framework 11

Because of its regional importance as a confining unit, 

the Presumpscot Formation has been studied thoroughly. 

In addition to the sandy zone at its top, the Presumpscot 

Formation often has a weathered, desiccated zone about 10 ft 

in thickness above the softer, massive blue-gray clay and silt 

(Gerber and Hebson, 1996). This zone, which occurs in the 

unsaturated zone above the level of the local water table, is 

characterized by extensive fissuring and an olive-brown color. 

According to Gerber and Hebson (1996), this layer can have 

a hydraulic conductivity 50 times greater than the underlying 

saturated, unweathered silt/clay. Brainerd and others (1996) 

measured the horizontal and vertical hydraulic conductivity 

of the unweathered Presumpscot silt/clay at a site in central 

Maine using isotopic analysis and groundwater age dating. 

The reported horizontal K of the Presumpscot Formation was 

6.2 × 10-3 ft/day and the vertical hydraulic conductivity of 

unweathered Presumpscot Formation was 1.4 × 10-4 to 5 × 10-6

ft/day. Nielsen and others (1995) reported a vertical K of the 

Presumpscot Formation of 2.7 × 10-5 ft/d, measured in an area 

south of Portland, Maine.

There have been numerous evaluations of the Freeport 

aquifer, particularly in the area within 1⁄2 mi of the pumping 

wells. Pump tests indicate a heterogeneous, stratified aquifer, 

with transmissivities that ranged from 650 to 13,000 square 

feet per day (ft2/d), corresponding to hydraulic conductivities 

ranging from 29 to 300 ft/d (Maine Water Company, 1978; 

Gerber, 1995; Hydro Group, 1984, 1988, 1991, 1995, 1997; 

Stratex, 2003; Earth Tech, 2004). 

The till in the study area has not been studied directly, 

but evidence from a small number of studies in Maine and 

other sources (Morrissey, 1983; Gerber and Hebson, 1996; 

Freeze and Cherry, 1979) indicates that the K of the till could 

be 0.5 to 10 ft/d.

Groundwater Flow

This section describes the flow of groundwater in 

the study area and, together with the description of the 

hydrogeologic units, forms the basis for the conceptual model 

described later.

Recharge

Recharge to the Freeport aquifer has not been measured 

directly, nor have the numerous consulting reports addressed 

specific recharge rates for the study area. The theoretical 

maximum amount of shallow recharge to sandy soils is about 

25 inches per year (in/yr), based on the Lyford and Cohen 

(1988) method, or about 54 percent of precipitation. The sandy 

nearshore marine units and the eolian sands could, therefore, 

have recharge rates in the 22–25 in/yr range. Morrissey (1983) 

found that in a valley-fill sand and gravel aquifer, recharge 

from runoff from adjacent uplands was an important part of 

the water budget, and could be as much as 60 percent of the 

precipitation in upland areas. Recharge to till in other areas 

of Maine has been calculated or estimated to be 7.5 in/yr in 

Oxford County (Morrissey, 1983); 5 to 5.5 in/yr in Washington 

County (Gerber and Hebson, 1996); 3.5 to 8 in/yr in 

Woodland, Maine (Gerber and Hebson, 1996); and 7 in/yr 

in the Bald Mountain area of Aroostook County (Fontaine, 

1989). Published values of recharge into the Presumpscot 

Formation also have been summarized by Gerber and 

Hebson (1996). Recharge into the unweathered, saturated silt/

clay ranges from less than 0.5 to 1.9 in/yr, whereas recharge 

into the weathered, fissured zone of the silt/clay could be as 

much as 12 in/yr (Gerber and Hebson, 1996). The presence of 

unsewered suburban housing developments could add to the 

total amount of recharge entering the unsaturated zone in some 

locations. Most houses in the study area use deep bedrock 

wells for their water supply, which is largely returned to the 

subsurface by way of individual septic systems. Although this 

process does not change the overall recharge rate, it does move 

water from the bedrock aquifer into the unsaturated zone, 

effectively increasing the local recharge rate to the uppermost 

hydrogeologic units. 

Groundwater Levels

Water levels have been measured in the Freeport aquifer 

and in the overlying surficial units since the 1970s. Most water 

levels were measured at observation wells in the vicinity of 

the pumping center (fig. 5); these measurements date from the 

early 1990s through about 2008. Other water-level data were 

compiled from sand-and-gravel aquifer maps (Neil and Locke, 

1999a, b, 2002), and included many one-time measurements 

without a specific collection date. Some water levels from 

exploratory borings also were available (Gerber, 1979; Hydro 

Group, 1992; Reynolds, 2000; Stratex, 2002, 2003; Earth 

Tech, 2004), but only represent single measurements taken 

shortly after drilling (usually after 24 hours, to let the water 

level stabilize). Longer term monitoring well water-level data 

from AquaMaine were available for as many as 15 wells, from 

1992 through 2008 (Richard Knowlton, AquaMaine, written 

commun., 2009). The Maine Geological Survey conducted 

a new series of shallow seismic profiles in the study area to 

help determine the depth to both the bedrock and water table 

surface during 2008–09, and two new wells were drilled into 

the Freeport aquifer in 2009. In summary, water-level data are 

unevenly distributed in time and space, and many of these data 

are not very precise. 

Figure 5 shows the geographic distribution of the water 

level measurement points. The aquifer (surficial or confined) 

in which the measurements were made is also indicated. 

There are not enough data points to construct a representative 

water table map or potentiometric surface map of the 

confined aquifer.

Water levels in the shallow, unconfined units generally 

follow the topography of the study area, and are highest in the 

upper sandy units and on the hill underlain by thin drift on the 

eastern side of the study area. In the unconfined units, measured water levels ranged from 1 to 29 ft below land surface. 
12 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

!(

!(!(

!(

!(

!( !(!(

!( !(!( !(!( !( !(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!( !(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

!(

#

#

#

#

# Freeport

Pownal

Yarmouth

North 

Yarmouth

Merrill Brook

Harvey Brook #4

Harvey Brook #3

Harvey Brook #2

Harvey Brook #1

Harvey Brook

Merrill Brook

Pumping

Area

70°10' 70°8'

43°52'

43°50'

0 0.5 1MILE

0 0.5 1 KILOMETER

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

Universal Transverse Mercator projection, Zone 19N

Central meridian -69°

Shaded relief from U.S. Geological Survey National Elevation

Dataset 10-meter digital elevation model

North American Datum of 1983 (NAD 83) 

EXPLANATION

Active model area

Streamflow site watershed

# Streamflow site and identifier

Groundwater level points

!( Confined, wells and borings

!( Unconfined, wells and borings

!( Unconfined, seismic

Buried valley extent

295

Figure 5. Groundwater level and streamflow measurement sites in the Freeport aquifer study area.
Hydrogeologic Framework 13

The Freeport aquifer is under confined conditions, 

whereas above the confining layer the saturated materials are 

unconfined. The saturated thickness of the unconfined water 

table aquifer is quite thin in some places, and the surficial 

sediments are not saturated everywhere above the bedrock 

surface, particularly where these surficial sediments are very 

thin. Before pumping began in 1989, monitoring wells located 

near Harvey Brook and screened in the confined Freeport 

aquifer had water levels that were often above land surface. 

After the pumping wells were developed, a cone of depression 

formed within a half mile of the pumping wells. Consequently, 

water levels are no longer consistently above land surface in 

observation wells within this zone. In the confined Freeport 

aquifer wells, measured water-level depths range from 0 to 

over 50 ft below land surface (the latter from an isolated well 

drilled in 1985) for measurements made since 2001. Groundwater levels in the confined aquifer are higher to the north and 

lower toward the south. 

Several springs are located around the edge of the eolian 

sand deposit, at the top of the silt/clay confining layer. These 

springs flow where topographic valleys intersect the water 

table in the eolian sand, providing a conduit for shallow 

groundwater above the Presumpscot Formation to exit the 

unconfined sandy aquifer. Smaller springs and numerous seeps 

were observed at the upper boundary of the exposed Presumpscot Formation in the Harvey Brook stream valley as well.

Water levels measured in several bedrock wells in the 

study area were 40–80 ft below land surface, and generally 

are much lower than water levels in surficial unit wells. This 

suggests a downward hydraulic gradient between the Freeport 

aquifer and the bedrock.

In the areas outside the immediate vicinity of the pumping wells and away from the Harvey Brook valley, the hydraulic gradients appear to be from the upper unconfined aquifer 

downwards toward the confined aquifer and (or) the fractured 

bedrock, and from upslope areas toward downslope areas. 

In the Harvey Brook valley, wells drilled into the confined 

aquifer near the stream had heads above land surface before 

pumping, indicating an upward gradient toward the stream. 

Head measurements in one well near the stream (MW 01-01) 

is still above land surface on occasion. Close to the pumping 

wells, heads in the confined aquifer are below land surface. 

Surface-Water Resources

The primary surface-water bodies in the study area are 

Harvey Brook and Merrill Brook, which have watersheds of 

4.0 and 4.6 mi2, respectively (figs. 1 and 5). As no data for 

streamflow had previously been collected, streamflow measurements were made from May to September 2009 at 

5 locations in the watersheds: 1 in the Merrill Brook watershed, and 4 in the Harvey Brook watershed (fig. 5, table 2). 

The four measurement locations in the Harvey Brook watershed were established to determine the seasonal streamflow 

patterns, as well as the distribution of groundwater inflow or 

outflow along several reaches of the stream within a mile of 

the pumping wells.

Table 2. Streamflow site information, Harvey and Merrill Brook, 2009.

[mi2, square mile; ft3/s, cubic foot per second; bl, below; nr, near, Cem, cemetery]

Site Station name Station number Drainage area, 

in mi2

Number of 

measurements

Lowest flow, 

in ft3/s, date

Highest flow, 

in ft3/s, date

Harvey Brook #1 Harvey Brook bl RR crossing 

near Freeport, Maine

01060026 3.992 16 2.22, 9/17/2009 11.47, 7/10/2009

Harvey Brook #2 Harvey Brook nr Webster 

Cem near Freeport, Maine

01060024 3.91 14 2.06, 8/20/2009 9.17, 7/10/2009

Harvey Brook #3 Harvey Brook near Webster 

Road near Freeport, Maine

01060022 3.488 13 1.79, 9/17/2009 9.41, 7/1/2010

Harvey Brook #4 Harvey Brook near 

Pownal, Maine

01060020 2.582 14 0.79, 8/20/2009 3.42, 7/1/2009

Merrill Brook Merrill Brook bl RR crossing 

near Freeport, Maine

01060030 3.92 15 0.68, 9/16/2009 20.43, 6/30/2009
14 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Streamflow Measurements in Harvey and 

Merrill Brooks

Streamflow measurements representing a range of flows 

were obtained at each station so that long-term mean monthly 

flow estimates could be computed for each site by use of the 

MOVE.1 regression method (Hirsch, 1982). The beginning of 

the 2009 summer season was unusually rainy, which provided 

an occasion to measure high flows that would not normally 

occur during the summer months. Later in the summer, flows 

returned to more typical base-flow conditions. This wide range 

in flows allowed for a robust correlation between the measured 

flows and index stations used to estimate monthly flows. All 

of the measurements were made following standard USGS 

techniques, using a pygmy meter. Figure 6 shows the streamflow measurements made at the 5 sites (from 13 to 16 measurements per site), from May to September 2009, along with 

the daily precipitation record for the National Weather Service 

station in Portland, Maine, 15.5 mi away. The measurements 

at the Merrill Brook station exhibit a much wider range in 

flow than those at the Harvey Brook station. During the late 

August to September period, the lowest measured flow at the 

Harvey Brook #1 site was 2.22 ft3/s, whereas flow during the 

same time period at the Merrill Brook site was 0.68 ft3/s. The 

Merrill Brook watershed is underlain primarily by till and clay, 

which converts precipitation into runoff more quickly than the 

Harvey Brook watershed. The latter has abundant sandy soil, 

and therefore, the opportunity for groundwater recharge and 

more consistent groundwater discharge during dry periods.

Seventeen index stations in western Maine and southern 

New Hampshire were tested for correlations with streamflows 

collected at the Harvey and Merrill Brook sites. The best correlations for the MOVE.1 regression were obtained for index 

stations in western Maine (Swift River, station no. 01055000) 

and southeastern New Hampshire (Bearcamp River, station 

no. 01064801). Correlation coefficients (r2) for the logs of flow 

at the five Freeport stations and index stations ranged from 

0.74 to 0.80.

As indicated in figure 7, the seasonal patterns of estimated monthly flow differ markedly in Harvey Brook and 

Merrill Brook. Merrill Brook has a higher spring peak and 

lower summer base flow than all of the Harvey Brook stations 

except the most upstream site (Harvey Brook #4), indicative 

of the differences in geology between the two watersheds. The 

amount of groundwater discharge between the Harvey Brook 

stations is indicated by the change in flow from one station to 

the other, which can be seen in both the individual measurements (fig. 6) and the monthly mean flow estimates (fig. 7). 

The difference between the farthest upstream station (Harvey 

Brook #4) and the next one downstream (Harvey Brook #3) 

is quite large, as flow increases by 100 to 300 percent over 

a short distance (figs. 6 and 7). There is much less inflow 

from groundwater between the Harvey Brook #3 and Harvey 

Brook #2 stations, and the difference in streamflow between 

the two stations is minimal. The groundwater inflow increases 

again (slightly) between Harvey Brook #2 and Harvey Brook 

#1 (fig. 7). Mean monthly flow during the summer low-flow 

period (July–September) at the Harvey Brook #4 station is 

estimated to be about 1 ft3/s, and is about 1.5 ft3/s in Merrill 

Brook. The mean monthly flow during the summer months for 

the downstream Harvey Brook stations ranges from about 2.6 

to 3.5 ft3/s. 

Conceptual Model of the Groundwater Flow 

System

Within the Freeport aquifer study area, the groundwater 

system can be divided into two individual, but connected 

aquifers. The first is a shallow unconfined aquifer, which 

includes all the surficial saturated sediments above the 

Presumpscot Formation, or, where the Presumpscot Formation 

is absent, above the bedrock surface. The second is the deep, 

confined Freeport aquifer that fills the bottom of the buried 

valley in the Harvey Brook watershed (and extends beyond 

to the north and south). The Presumpscot Formation acts as 

a confining unit for the deep aquifer, but does not completely 

separate it from the shallow aquifer, because it appears to be 

absent in many areas along the eastern and western edges of 

the Harvey Brook valley; the formation extends across most 

of the Merrill Brook watershed. Groundwater levels in the 

confined aquifer are higher to the north and lower toward the 

south, indicating an overall north-south flow direction within 

the aquifer. 

The upper unconfined aquifer is quite thin in places, and 

these upper sediments are unsaturated in some areas. In the 

Harvey Brook watershed, the upper unconfined aquifer is primarily composed of sandy soils and sediments (eolian sands 

and nearshore marine sands), whereas in the Merrill Brook 

watershed, the soils and sediments above the bedrock surface 

are composed of till and (or) the Presumpscot Formation. The 

upper unconfined aquifer discharges to streams in both watersheds, and to springs in the Harvey Brook watershed. Infiltrating precipitation provides recharge to the unconfined system. 

Because sediments in the Merrill Brook watershed are less 

permeable than those in the Harvey Brook watershed, more 

precipitation is converted to runoff (resulting in more flashy 

streamflow), and less is converted to recharge.

The hydraulic gradient on the edges of the Harvey Brook 

watershed is downward, so where the Presumpscot Formation 

is absent, groundwater can flow around its edges and recharge 

the Freeport aquifer below. Because the buried valley extends 

an unknown distance to the north, and the gradient within the 

buried valley aquifer is toward the south, it is expected that 

some water enters the Freeport aquifer from the north. The 

gradient across the Presumpscot Formation is upward in the 

center of the Harvey Brook valley beneath the stream, except 

where the potentiometric surface has been lowered near the 

pumping wells. This gradient allows groundwater from the 

Freeport aquifer to take advantage of any discontinuities or 

fractures in the Presumpscot Formation and flow upward 
Hydrogeologic Framework 15

0.5

5

50

Streamflow, in cubic feet per second

0

1

2

3

4

May June July August September

Daily precipitation, in inches

2009

A

B

Merrill Brook

Harvey Brook #1

Harvey Brook #2

Harvey Brook #3

Harvey Brook #4

EXPLANATION

EXPLANATION

Daily precipitation

Figure 6. A, Streamflow measurements in Harvey and Merrill Brooks, and B, precipitation in 

Portland, Maine, May–September 2009.

Month

Harvey Brook #1

Harvey Brook #2

Harvey Brook #3

Harvey Brook #4

Merrill Brook

EXPLANATION

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

0.5

5

50

Streamflow, in cubic feet per second

Figure 7. Estimates of monthly mean streamflow at the 

Harvey and Merrill Brook sites based on data from longterm index sites in southern Maine and New Hampshire.
16 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

toward Harvey Brook. Boiling springs observed in the bottom 

of Harvey Brook near Harvey Brook #1 support this scenario. 

Runoff from the western uplands can also recharge both 

the upper and lower aquifers. As Morrissey (1983) has shown, 

valley-fill aquifers receive runoff from adjacent uplands, 

which is converted into recharge as it reaches the valley bottom. Morrissey indicated that as much as 60 percent of the 

upland runoff may recharge the valley aquifer. In the case 

of the Freeport aquifer study area, the incised gullies on the 

western edge of the Harvey Brook valley are most likely to 

provide this sort of recharge, which would be divided between 

the upper and deep aquifers, depending on how much flowed 

under the edge of the Presumpscot Formation. 

Heads in the bedrock wells are generally much lower 

than heads in the surficial units. Although there may be some 

water exchange between the unconsolidated aquifers and fractured bedrock, the hydraulic conductivity of fractured crystalline bedrock is very low in comparison to the buried valley 

aquifer and the sandy surficial sediments, and it is not thought 

to contribute substantially to the flow system in the study area. 

Correspondence from consultants conducting a long-term 

(20-day) pumping test to homeowners with bedrock wells in 

the area confirms that most bedrock has little hydraulic connection with the buried valley aquifer.

Water Use and Withdrawals

The accounting of all known water withdrawals from 

the study area can be useful for evaluating current and future 

water needs, and as a comparison against water availability. 

Although the State does require water withdrawal reporting 

for many types of large water users, there is no single agency 

that tracks individual water withdrawals for all types of 

users. Also, nonconsumptive users and users above certain 

thresholds (which vary according to the size of the affected 

water body) are exempt from reporting. Therefore, data do 

not currently exist that can be used as a complete inventory 

of withdrawals in a watershed. This accounting of water 

withdrawals in the Freeport aquifer study area will provide 

an example of how this could be accomplished in other 

watersheds in the State. 

Water withdrawals for human use include industrial, 

commercial, agricultural (irrigation and other agricultural 

uses), public supply withdrawals, and domestic withdrawals 

from private wells. Large water withdrawals in Maine are 

governed by several State laws and implemented by multiple 

agencies. Maine’s Instream Flows and Lake and Pond Water 

Level Rules (Chapter 587) apply to most water withdrawals, 

and are intended to protect natural aquatic life and other 

designated uses in Maine waters. Community water systems 

are subject to additional rules, and all must report their 

withdrawals to the State. All non-agricultural withdrawals 

above certain thresholds, beginning at 50,000 gallons per 

day (gal/d), are regulated according to one statute or another. 

Under Chapter 587 rules, withdrawals are not regulated if they 

do not affect river or stream flows by a certain percentage, 

which varies by the U.S. Environmental Protection Agency 

water-quality classification, or if they do not affect water 

levels in a lake or pond by a certain amount. The law is 

aimed at protecting the natural resource by requiring that 

flows be maintained, but does not require reporting of water 

withdrawals for most water users. Therefore, water-withdrawal 

data are not always available for a given watershed, and 

estimates based on water-use coefficients must be applied to 

account for all potential water withdrawals. 

The only water user in this study area required to report 

water withdrawals is the AquaMaine–Freeport Division 

water utility. Estimates of withdrawals for rural domestic use, 

agricultural use, and commercial/industrial users that are not 

connected to the public water supply were made using wateruse coefficients.

Reported Withdrawals from the Freeport Aquifer

Monthly water withdrawal data from the Freeport aquifer 

by AquaMaine were obtained for 2005–08. The average 

monthly withdrawals follow a seasonal pattern, with the 

minimum occurring in February (6.8 million gallons (Mgal)) 

and the maximum occurring in August (11.5 Mgal). The yearly 

total withdrawals ranged from 99.6 Mgal in 2005 to 110 Mgal 

in 2007. The yearly average withdrawal for these 4 years was 

105.5 Mgal. 

Estimated Withdrawals from Groundwater

A commercially available business database (Harris

Infosource) was used to locate any industrial water users, or 

commercial establishments with 20 or more employees in the 

Harvey and Merrill Brook watersheds. Aerial photos collected 

in 2008 were used to search for any potential irrigation (agriculture or golf courses) in the watersheds. The aerial photos 

also were used to verify the locations of the establishments 

identified in the HarrisInfosource database. Residences in the 

watersheds also were mapped using the 2008 aerial photos, 

and a map showing the extent of the public water supply was 

used to determine which of the houses used public supply and 

which had private wells. Water-use estimates were derived 

using coefficients from the IWR-MAIN water-use model, primarily from 1995, and published in Horn (2000). The coefficients are based on the Standard Industrial Classification (SIC) 

codes for each facility and the number of employees.

Three commercial and two industrial water users were 

identified in the watershed outside the service area of the 

public water utility (table 3). One commercial establishment 

operates a seasonal tourist destination for which employment data are not available, so their total water-use estimates 

are based on an estimated number of employees. Together, 

these commercial and industrial water users withdrew a 

total of 3.03 million gallons per year (Mgal/yr) from the two 
Simulation of Groundwater Flow and Discharge to Streams 17

Table 3. Water users in Harvey and Merrill Brook watersheds, as reported by HarrisInfosource, and personal reconnaissance.

[SIC, Standard Industrial Classification; gal/d, gallon per day; Mgal/yr, million gallons per year; Mfg, Manufacturer]

Water user Line of business SIC code

Water use rate per employee 

(from Horn, 2000), 

in gal/d

Estimated water use, 

in Mgal/yr

1 Mfg of malt beverages 2082 2,691 1.96

2 Office furniture, except wood 2522 30 .022

3 Retail hardware 5251 58 .064

4 School 8211 116 .85

5 Tour operator 7990 106 .13

watersheds. Three of these, including the largest (withdrawing an estimated 1.96 Mgal/yr) are located in the northeastern 

corner of the Merrill Brook watershed, and withdraw from a 

separate, small aquifer northeast of the Freeport aquifer (outside the model area). 

There were 291 houses identified in the watershed served 

by individual domestic wells (dug wells or bedrock wells). 

Domestic water users in Maine used, on average, 60 gal/d 

per person (U.S. Geological Survey water use compilation 

for Maine, unpub. data, 2005). Census data for Cumberland 

County, Maine, indicates an average household size of 2.4 persons for Freeport, Pownal, and Yarmouth. In total, domestic 

withdrawals from the bedrock or till aquifers was estimated to 

be 15.3 Mgal/yr. Of this amount, estimates of the percent that 

percolates back into the subsurface through individual septic 

systems range from 84 to 96 percent (Ralf Topper, Colorado 

Geological Survey, written commun., 2007). 

Overall, public-supply withdrawals were much greater 

than any other category, being almost 7 times greater than all 

domestic well withdrawals, most of which eventually returned 

to the subsurface locally (table 4). The next largest withdrawal 

category, industrial, consists mostly of withdrawals made 

by one company from an aquifer at the edge of the Merrill 

Brook watershed that is not connected to the Freeport aquifer. 

Commercial withdrawals are very small (less than 1 Mgal/yr), 

and there were no irrigation or other agricultural withdrawals 

identified in this study area.

Simulation of Groundwater Flow and 

Discharge to Streams

The groundwater modeling component of the study was 

used to further develop the conceptual model of groundwater 

flow and the interaction of groundwater with streamflow in the 

Freeport aquifer study area. A steady-state groundwater model 

of the Freeport aquifer was constructed using the three-dimensional, finite-difference groundwater-flow modeling code, 

MODFLOW-2005 (Harbaugh, 2005). This model was used to 

simulate flow in the unconsolidated glacial deposits above the 

bedrock surface in the Freeport aquifer study area. 

Steady-State Numerical Groundwater-Flow 

Model

The aquifer system is represented by a 3-layer model. 

The bottom layer represents the Freeport aquifer in the buried 

valley, above the bedrock surface. The middle layer contains 

the Presumpscot Formation confining layer, and coarser materials where the Presumpscot Formation is absent. The upper 

layer represents the shallow unconfined aquifer. 

The steady-state model was constructed using available 

water-level data at the wells indicated in figure 5, using the 

estimated average June–August streamflows in the Harvey and 

Merrill Brook watersheds. The model was constructed using 

Table 4. Estimated total withdrawals from groundwater by water 

use category in the Harvey and Merrill Brook watersheds, 2009.

[Mgal/yr, million gallons per year]

Water use category

Estimated 2009 

water withdrawal, 

in Mgal/yr

Percent of total

Public water supply 105.5 85

Domestic (homeowner) 

water supply

15.3 12

Commercial 1.0 1

Industrial 2.0 2

Irrigation 0 0

Other agricultural 0 0

Total: 123.8 100
18 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

all the available water-level data from the unconsolidated 

units, which were taken at all times of the year. Therefore, the 

streamflow targets were selected to represent an average baseflow condition for most of the year, rather than just the lowest 

streamflow period. The June–August period was selected to 

fulfill this need, as it represents a mixture of the lowest annual 

water flows and a somewhat higher base-flow period (June), 

and represents a period when groundwater discharge dominates the streamflow. 

The part of the study area selected for inclusion in the 

groundwater model was based on the conceptual model of the 

groundwater system, the topography of the bedrock surface, 

the land-surface topography, and previous groundwater models 

of the study area (Stratex, 1999, 2003). The western boundary 

of the model area (fig. 2) is parallel to the edge of the buried 

valley, and does not include the western uplands because the 

soils there are extremely thin. The southern boundary was 

chosen as an arbitrary boundary across the buried valley, 

downstream from the primary area of interest (Harvey Brook 

and the AquaMaine pumping wells). The northern and northeastern boundaries coincide with the surface-water divide. The 

southeastern boundary is generally parallel to the edge of the 

buried valley and spans the Merrill Brook watershed (along 

lines intended to parallel the groundwater flow paths) to the 

surface-water divide of the Harvey Brook watershed. Streamflow in Merrill Brook was apportioned on a per-square-mile 

basis, because the soils and geology were similar in the areas 

within and outside the model area.

Parameter estimation (also referred to as optimization) 

was used in the calibration phase. Model variables such as 

recharge, streambed conductance, and horizontal and vertical 

hydraulic conductivity were used as parameters in the model. 

Head and streamflow measurements were designated as the 

calibration targets, or observations. Insensitive or highly correlated parameters were set and adjusted using a trial and 

error process. 

Spatial Discretization of the Model

The model area was discretized into a grid of 148 rows 

and 96 columns of cells, with uniform 164-ft (50-m) spacing. 

The grid was rotated to the northwest at an angle of 30 degrees 

to coincide with the primary orientation of the buried valley 

along the Harvey Brook valley (fig. 8). The top of the upper 

layer (layer 1) corresponds to the land surface. The top of 

the second layer was assigned to the top of the Presumpscot 

Formation (interpolated from well logs and surface exposures) 

where it was present, and an altitude 10 ft below land surface 

where it was absent. The upper two layers extend throughout 

the entire model. The bottom layer (layer 3), representing 

the buried valley aquifer, extends as far as the buried valley 

(figs. 8 and 9). The top of layer 3 corresponds to the bottom 

of the silt/clay layer, and the bottom of layer 3 represents the 

bedrock surface. The bedrock surface is the bottom of layer 2 

where layer 3 is not present. 

Boundary Conditions

The horizontal boundaries of the active model were 

mostly defined as no-flow boundaries, with the exception of 

areas near the north and south ends of layer 3, which were 

modeled as head-dependent flux boundaries (fig. 8). The bottom of the model, representing the bedrock surface, also was 

treated as a no-flow boundary. The no-flow boundaries on the 

northern and western edges of the model corresponded to the 

surface-water divide and areas where the surface geologic 

units were mostly very thin and relatively impermeable (clayrich soils near the surface), except at the western edge of the 

model, where the model boundary is parallel to the edge of 

the buried valley. Head measurements in layer 3 within and 

outside of the model area were used to define the head-dependent flux boundary to the north. To the south, a head of zero 

(representing sea level, the ultimate discharge point of water in 

the aquifer) was used to define the head-dependent boundary.

All layers in the model were simulated as being confined. 

This use of a fixed-transmissivity approach in the upper two 

layers was necessary because of numerical instabilities during 

model convergence when the upper layers were very thin 

(<10 ft) in relation to the overall slope and scale of the model 

cells (164 ft).

Discharge to streams was simulated using the Drain 

Package (Harbaugh, 2005) in MODFLOW. In this package, 

stream discharge is modeled as head-dependent flow across 

the stream bottom, which is one-directional—flow can only 

go from groundwater to the drains. Figure 8 shows the location of drain cells in the model, which generally emulate the 

surface-water network. The streambed altitudes were based on 

land-surface altitudes, and adjusted so that the stream bottom 

altitude was realistic, as the land-surface altitudes are averaged 

over a larger area than the actual stream itself, and the streams 

are incised into the land surface by 3–5 ft in most cases. 

The thickness of each streambed was arbitrarily set at 1.6 ft 

(0.5 m). The widths of the streambed in the drain cells ranged 

from 9.8 ft (3 m) in the Cousins River Estuary to 0.8 ft in the 

smallest tributaries. The drain hydraulic conductivity values 

were set as parameters in the model, and six drain conductance 

zones were established that were very similar in extent to the 

layer 1 K zones. During calibration, additional drain cells were 

added in valley-bottom areas in the contact zone between the 

Presumpscot Formation and surficial sandy sediments, where 

groundwater seeps were common. 

Stresses

The stresses applied to the groundwater system in the 

Freeport aquifer groundwater model include recharge and 

pumping; evapotranspiration was not modeled. Recharge was 

applied as a constant flux to the top active cell in the model. 

Recharge rates were based on (1) calculations for the study 

area using the method of Lyford and Cohen (1988), and (2) 

literature rates for the hydrogeologic units in the study area 

using values published for Maine when possible, as described 
Simulation of Groundwater Flow and Discharge to Streams 19

"

"

EXPLANATION

Model zones

Inactive

Layers 1 and 2—Active

Layers 1, 2, and 3—Active

Drain cells—Layer 1

Head-dependent flux boundary—

Layer 3

" Pumping well—Layer 3

96

30

90

40

20

10

60

50

80

70

130

140

100

110

120

148

0

10

30

90

20

60

80

40

50

70

1

Model row

Model column

Model row 65 (fig. 9)

Figure 8. Model grid and boundary conditions for the Freeport aquifer groundwater-flow model.
20 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed Figure 9. Vertical cross section of the Freeport aquifer groundwater-flow model, row 65. -50

0

50

250200150100

Altitude relative to NAVD 88, in feet

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96

Model column

Increased recharge used to simulate upland runoff infiltration

Layer 1—InactiveLayer 1Layer 2—InactiveLayer 2Layer 3—InactiveLayer 3Bedrock—Not modeled

EXPLANATION
Simulation of Groundwater Flow and Discharge to Streams 21

earlier. Zones of equal recharge were laid out based on the 

surficial geology and soil characteristics. Recharge rates were 

treated as model parameters during the optimization phase of 

model calibration and were adjusted within a range of reasonable values. The recharge rates used in the calibrated model 

are shown in figure 10. They range from 0.75 in/yr in the 

unweathered clay of the Presumpscot Formation to 

25 in/yr in several areas of sandy soils, which is near the maximum reasonable value calculated using Lyford and Cohen 

(1988). Recharge to the model was increased in the upper 

layer at the model edge to account for runoff entering from 

uplands to the west (fig. 9). To account for inflow to the model 

from upland runoff on the western edge of the model, an 

increased recharge rate of 50 in/yr was used in one thin zone 

running along a portion of the western edge of the model. 

As discussed previously, the presence of the unsewered houses in the study area has the potential to effectively 

increase the recharge rate to the uppermost hydrogeologic 

layers by moving water from the bedrock aquifer to septic 

systems. If all 291 of the unsewered houses in the Freeport 

watersheds withdrew water from bedrock rather than the 

surficial units, the spatially-averaged recharge rate could be 

increased by 0.11 in/yr. This increase was considered generally 

negligible, and was grouped with the overall recharge rates.

Pumping was simulated for the AquaMaine water utility 

wells in layer 3 on the basis of long-term pumping records 

supplied by the water utility. The pumping wells are 250 ft 

apart (fig. 8). Average simulated withdrawals were 191 and 

18 gallons per minute (gal/min). The well with the larger 

pumping rate is about 275 ft from Harvey Brook.

Hydraulic Properties

Hydraulic properties used in the model included streambed hydraulic conductivity, horizontal hydraulic conductivity 

in the hydrogeologic units, anisotropy, and vertical hydraulic conductivity, which were distributed on the basis of the 

surficial geology and soils (Natural Resources Conservation 

Service, 2006) and refined during the calibration process. 

Horizontal hydraulic conductivity in the uppermost layer 

ranged from 0.56 to 13.6 ft/d (fig. 11A, table 5). Layer 2 

was divided into two zones to coincide with the presence or 

absence of the Presumpscot Formation (fig. 11B, table 5). 

Horizontal hydraulic conductivity was 3.3 × 10-5 ft/d in zone 1, 

which corresponds to areas where the Presumpscot Formation 

was present and saturated, and 9.0 ft/d in zone 2, which represents a mixture of till and the upper fractured and weathered 

zone of the Presumpscot Formation. Hydraulic conductivity 

of the lowest model layer ranged from 18.1 to 167 ft/d on the 

basis of the calibration process. All the hydraulic properties 

associated with the zones in layers 1 through 3 were treated 

as parameters during the optimization phase of the calibration process (appendix 2 and table 6). Reasonable ranges were 

based on literature values and the results of past pumping test 

within the study area (table 1). The hydraulic conductivity of 

the head-dependent flux boundaries on the north and south 

(fig. 5) was set to a relatively high value (246 ft/d) to facilitate 

the movement of water in and out of these boundaries. This 

value does fall within the reasonable range for the buried aquifer, as determined by pumping tests (table 1).

The calibrated vertical hydraulic conductivities (Kv) 

ranged from 3 × 10-6 ft/d in the Presumpscot Formation confining unit to 16.4 ft/d in the buried valley sediments (fig. 12).

Simulated hydraulic conductivity values for the drains 

(Kdr) ranged from 0.017 to 5 ft/d (fig. 13), with the lowest values being in the Merrill Brook watershed, which is underlain 

by shallow till and clayey soils. The streams with very low 

(0.017 and 0.02 ft/d) hydraulic conductivities are very small, 

and the 1.6 ft uniform streambed thickness used in the model 

is unreasonably large for these drain cells. Because the overall 

streambed leakance is inversely proportional to streambed 

thickness, the actual hydraulic conductivity of the streambed 

in these areas is probably substantially greater than what was 

used in the model. The upper range of Kdr values obtained is 

comparable to the Kdr values used in other Maine groundwater 

studies (table 1), and most values are within reasonable ranges 

for the hydraulic conductivities of the geologic materials in 

which they are situated. The model was somewhat sensitive to 

the drain conductance representing Harvey Brook and highly 

sensitive to the drain conductance representing Merrill Brook.

Model Calibration Using Parameter Estimation 

and Observations

Methods outlined in Hill and Tiedeman (2007) were 

followed during calibration using the UCODE_2005 software 

package (Poeter and others, 2008). These methods allow for 

the explicit accounting for uncertainty in both the water levels 

and streamflows used as calibration targets, documenting the 

model sensitivity to model variables, and sensitivity to data 

used in the model. During the prediction phase, these methods 

assist in documenting the uncertainty in the model predictions. 

Statistics used to quantify the fit of the model to the 

observed values (the ability of the model to reproduce the 

observations) are used as the dependent variables in the 

parameter estimation, whereas the model parameters are 

treated as independent variables through a series of linear and 

nonlinear regression calculations. The optimization of the 

parameter values is conducted through an iterative process, 

during which the user uses the output of each iteration to 

determine which parameters can be estimated and whether 

trial-and-error changes to other parameters, or changes to 

the conceptual model, are needed. During the optimization 

process, the sensitivity of the model to each parameter value 

is determined for each optimization iteration, and only 

parameters that exceed a threshold sensitivity (and are not 

highly correlated with each other) are estimated. Hill and 

Tiedeman (2007) provide further details on the process of 

parameter estimation.
22 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

EXPLANATION

Recharge—In inches per year

0.750

5.04

7.1

10.75

24

25

50

96

30

90

40

20

10

60

50

80

70

130

140

100

110

120

148

0

10

30

90

20

60

80

40

50

70

1

Model row

Model column

Figure 10. Recharge rates applied to the numerical model of the Freeport aquifer study area.
Simulation of Groundwater Flow and Discharge to Streams 23

A. Layer 1 B. Layer 2 C. Layer 3

Active model area

0.560.691.610.713.6

0.0000339.0

EXPLANATION

Calibrated horizontal hydraulic conductivity (Kh)—In feet per day

Active model area

18.1150167

Active model area

Figure 11. Calibrated horizontal hydraulic conductivities (Kh) used in the Freeport aquifer groundwater-flow model.
24 Report Title A. Layer 1 B. Layer 2 C. Layer 3

Active model area

0.161.643.28

16.4

Active model area

EXPLANATION

Calibrated vertical hydraulic conductivity (Kv)—In feet per day

Active model area

0.0000030.982.46

9.84

Figure 12. Calibrated vertical hydraulic conductivities (Kv) used in the Freeport aquifer groundwater-flow model.
Simulation of Groundwater Flow and Discharge to Streams 25

EXPLANATION

Drain cell streambed hydraulic conductivity 

(Kdr) values—In feet per day

0.017

0.02

1.5

4.5

5

Active model area

96

30

90

40

20

10

60

50

80

70

130

140

100

110

120

148

0

10

30

90

20

60

80

40

50

70

1

Model row

Model column

Figure 13. Drain hydraulic conductivity values (Kdr) used in calibrated model.
26 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Observations

For the Freeport aquifer numerical model, 54 head 

observations and 5 flow observations were used (table 6). 

The head observations included water-level measurements 

from the wells, borings, and seismic line locations shown 

in figure 14. As stated earlier, the water-level measurements 

came from data collected over a long time span (1978–2009). 

For the wells with a long period of measurements, the value 

used for the observation is the mean of all the measurements. 

The distribution of head measurements in the model area was 

best in the central and northern parts of the model, but there 

were areas in the southern part of the model where there was 

no information at all on water levels. To stabilize the model, 

11 estimated water-level observations were added in the 

uplands, primarily in the southern part of the model 

(fig. 14, table 6). These observations, measured in feet below 

land surface, were based on the average depth to water in all 

of the “till” wells in the statewide Maine USGS groundwaterlevel monitoring network. Overall, 30 of the head observations 

were in layer 1, 2 were in layer 2, and 22 were in layer 3. 

The variance used to weight the observations for 

water levels in wells (table 6) included a combination of 

measurement, estimation, and altitude errors. Because of the 

long time period during which measurements were made (and 

several wells were measured once during that time span), the 

variance represented climatic differences among years of data 

collection and uncertainty about the water level itself. The 

uncertainty in the water levels with repeated measurements 

over time was based on the standard deviation about the 

mean of the total number of measurements. The uncertainty 

in the water-level measurements for wells or borings with 

few, or only one, measurement, was set to ±3.28 ft (1 m). 

The uncertainty in water-level measurements for the seismic 

lines was also set at ±3.28 ft. The uncertainty in water-level 

measurements for the estimated water levels in till areas was 

±6.5 ft (2 m). The errors from uncertainty in the altitude of 

the measuring point varied, depending on the source of the 

altitude data. Wells with precisely measured altitudes were 

assigned an uncertainty of ±0.5 to 1 ft. Wells, borings, and 

seismic lines with altitudes read from a topographic map were 

given an altitude error of ±6.4 ft, as were the water levels 

estimated in till (table 6). 

The flow observations consisted of the sum of the 

drain cells in the subbasins of each of the five streamflow 

measurement sites (fig. 14). The streamflow observations 

were the 3-month average (May–July) of the statistical 

estimate of monthly flows based on the individual streamflow 

measurements, as described earlier. A rigorous analysis of 

the errors associated with these estimates was not performed, 

and a standard deviation of ±5 percent was used to calculate 

the variance. The streamflow observations also are presented 

in table 6. Additional details on the use of observations and 

weighting can be found in Hill and Tiedeman (2007).

Thirteen of the wells used for the observations had data 

for time periods when the pumping was not active but within 

the longer timeframe during which pumping has been in 

progress (since the early 1990s). Although these data were 

not used during the optimization phase, in the final stages 

of calibration, the drawdown in those wells (the difference 

between pumping and non-pumping heads) was compared 

to the drawdown predicted by the model and used to further 

refine the model. 

Table 5. Hydrogeologic units and associated calibrated hydraulic conductivities.

[Kh, horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity; ft/d, foot per day; Fm., formation; NA, not applicable]

Unit description Layer Calibrated Kh

, 

in ft/d

Calibrated Kv, 

in ft/d

Surficial sandy deposits above Presumpscot Fm. 1 13.6 3.28

Surficial sandy deposits, eolian 1 13.6 3.28

Till and weathered Presumpscot Fm., undivided 1 .69 1.64

Weathered Presumpscot Fm. silt/clay 1 10.7 1.64

Unweathered Presumpscot Fm. silt/clay 1 .56 .16

Holocene alluvium and fine sand 1 1.6 3.28

Till and weathered Presumpscot Fm., undivided 2 9.0 0.98

Unweathered Presumpscot Fm. silt/clay 2 .00003 .000003

Vertical flow zones in Presumpscot Fm. silt/clay 

(2 separate zones)

2 NA 9.84, 2.46

Fine sand, buried aquifer 3 18.1 16.4

Coarse sand unit A 3 150 16.4

Coarse sand unit B 3 167 16.4
Simulation of Groundwater Flow and Discharge to Streams 27

Table 6. Observations used in calibration of the Freeport aquifer groundwater flow model, with variance and weights for 

optimization used for each observation.

[Observation locations shown in figure 14; additional data on observations can be found in appendix 1. ft, foot; ft3/s, cubic foot per second; m, meter; 

m3/d, cubic meter per day; σ, sigma; confidence intervals and weights based on metric units used in the model]

Observation name Observation 

number Model layer Measured value, in ft or ft3/s

Measured value, 

in m or m3/d

Total variance 

from all sources 

(σ2)

Weight, 

1/σ2

Wells and borings, measured values of head in ft and m

Boring LLB_FP 1 1 83.0 25.3 1.302 0.768

Boring LLB_RR 2 1 7.0 2.1 2.500 0.768

Well_GolDUG 3 1 107.0 32.6 1.302 0.768

Well_MGS_Dug1 4 1 154.0 46.9 2.000 0.500

Well_RogDUG 5 1 63.5 19.4 1.302 0.768

Merill 1 6 3 92.0 28.0 2.603 0.384

MGS S&Ga 7 2 116.5 35.5 3.380 0.296

MGS S&Gb 8 2 124.7 38 3.380 0.296

RGGI_85_2 9 3 36.1 11 3.380 0.500

Well_01_1 10 3 29.0 8.8 0.300 3.333

Well_83_1 11 3 24.1 7.3 0.076 13.158

Well_83_3A 12 3 21.8 6.6 1.230 0.794

Well_83_3B 13 3 17.8 5.4 2.450 0.408

Well_83_3C 14 3 22.8 7.0 1.290 0.775

Well_89_7 15 3 24.7 7.5 0.311 3.215

Well_89_8 16 3 29.2 8.9 0.556 1.799

Well_89_9 17 3 30.3 9.2 0.496 2.016

Well_91_1 18 3 31.8 9.7 0.592 1.689

Well_91_2 19 3 31.6 9.6 0.460 2.174

Well_91_3 20 3 33.9 10.3 0.401 2.494

well_95_1 21 3 25.1 7.6 0.500 2.000

Well_95_2 22 3 30.6 9.3 0.500 2.000

Well_95_3 23 3 19.3 5.9 0.236 4.237

Well_95_4 24 3 24.3 7.4 0.072 13.889

Well_96_1 25 3 16.2 5.0 0.204 4.902

Well_Suneast_T6 26 3 10.6 3.2 2.000 0.500

Well_TwnBR 27 3 20.3 6.2 1.302 0.768

Seismic lines, measured values of head in ft and m

Seis_NPW8 28 1 128.0 39.0 1.302 0.768

Seis_YAR1 29 1 100.0 30.5 1.302 0.768

Seis_YAR13 30 1 46.0 14.0 1.302 0.768

Seis_YMH1 31 1 49.0 14.9 1.302 0.768

Seis_YMH15 32 1 45.0 13.7 1.302 0.768

Seis_YMH16 33 1 43.0 13.1 1.302 0.768

Seis_YMH17 34 1 52.0 15.9 1.302 0.768

Seis_YMH18 35 1 46.0 14.0 1.302 0.768

Seis_YMH19 36 1 52.0 15.9 1.302 0.768

Seis_YMH2 37 1 88.0 26.8 1.302 0.768

Seis_YMH20 38 1 54.0 16.5 1.302 0.768

Seis_YMH3 39 1 58.0 17.7 1.302 0.768

Seis_YMH4 40 1 151.0 46.0 1.302 0.768

Seis_YMH5 41 1 119.0 36.3 1.302 0.768

Seis_YMH7 42 1 119.0 36.3 1.302 0.768

Seis_YMH8 43 1 79.0 24.1 1.302 0.768
28 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Parameters

As stated earlier, the model was calibrated using a 

combination of parameter estimation and trial-and-error 

adjustments to the model variables. Of the initial 33 

parameters used in the model, 12 were determined by use of 

parameter estimation, including three recharge parameters, 

nine horizontal K parameters, and one drain K parameter. As 

the model variables were adjusted and tested for model fit, 

alternatives to the original conceptual model were also tested 

to determine if they helped improve model fit. For the Freeport 

aquifer model, the final set of parameters used is listed in 

appendix 2, along with their composite scaled sensitivities 

and significant correlations between parameters. Generally, 

parameters with composite scaled sensitivities greater than 0.5 

can be estimated (Hill and Tiedeman, 2007), as long as they 

are not highly correlated with other estimated parameters. 

Changes to the Conceptual Model

During the calibration of the Freeport aquifer groundwater model, it became obvious that the initial conceptual 

model of a confined aquifer underneath a homogeneous and 

continuous confining unit could not reproduce the observed 

water levels and measured streamflows used in the calibration, 

while keeping the model variables within reasonable ranges. 

Therefore, some substantial changes to the conceptual model 

were necessary to finalize the model calibration.

The increases in observed flow from Harvey Brook #4 

site to site #3 and again from Harvey Brook #2 to Harvey 

Brook #1 could not be accounted for by adjusting the 

geometry, hydraulic conductivity, or streambed K of the units 

in layer 1 without creating additional, unacceptable errors in 

the head observations. Because drilling logs in the central part 

of the stream valley consistently show 50 ft or more of silt/

clay above the confined aquifer, it was initially thought that the 

silt/clay layer was uniform and relatively homogeneous. After 

discussions with geologists at the Maine Geological Survey, 

however, it was determined that the Presumpscot Formation 

silt/clay layer could have some sort of preferential pathway 

(vertical fractures, perhaps) that allow for vertical movement 

of water from the confined aquifer up into the streambed. 

This is supported by the observation of several springs in the 

Harvey Brook streambed downstream from Harvey Brook 

#1. Adding zones of increased vertical hydraulic conductivity 

through the silt/clay in layer 2 allowed the fluxes and heads 

in the model to agree with the observed data while keeping 

other model variables within realistic ranges. The model was 

Table 6. Observations used in calibration of the Freeport aquifer groundwater flow model, with variance and weights for 

optimization used for each observation.—Continued

[Observation locations shown in figure 14; additional data on observations can be found in appendix 1. ft, foot; ft3/s, cubic foot per second; m, meter; 

m3/d, cubic meter per day; σ, sigma; confidence intervals and weights based on metric units used in the model]

Observation name Observation 

number Model layer Measured value, in ft or ft3/s

Measured value, 

in m or m3/d

Total variance 

from all sources 

(σ2)

Weight, 

1/σ2

Water level estimates in till, measured values of head in ft and m

Syn_Till1 44 1 39.7 12.1 3.384 0.296

Syn_Till10 45 1 137.1 41.8 3.384 0.296

Syn_Till11 46 1 200.1 61 3.384 0.296

Syn_Till2 47 1 54.8 16.7 3.384 0.296

Syn_Till3 48 1 59.4 18.1 3.384 0.296

Syn_Till4 49 1 56.4 17.2 3.384 0.296

Syn_Till5 50 2 25.6 7.8 3.384 0.296

Syn_Till6 51 1 31.0 9.45 3.384 0.296

Syn_Till7 52 2 76.8 23.4 3.384 0.296

Syn_Till8 53 1 67.9 20.7 3.384 0.296

Syn_Till9 54 1 135.2 41.2 3.384 0.296

Streamflow discharge, measured values of discharge in ft3/s and m3/d

DROB_Har1_1 55 1 -0.62 -1,513 1,489 1.75 × 10-4

DROB_Har2_1 56 1 -0.29 -708 326 7.98 × 10-4

DROB_Har3_1 57 1 -2.04 -5,001 16,277 1.60 × 10-5

DROB_Har4_1 58 1 -1.33 -3,242 6,838 3.81 × 10-5

DROB_Meril_1 59 1 -0.79 -1,944 2,459 1.06 × 10-4
Simulation of Groundwater Flow and Discharge to Streams 29

!

!

!.

!.!.

!.

!.

!.!.!.

!.!.!. !.!. !.!.

!.

!.!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!. !.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

!.

#

#

#

#

#

6

3

1

2

4

8

7

9

5

53

46

45

54

52

51

50

49

48

47

44

A1

A3

A3

A2

A5

A8

A9

A4

A6

36

32

33

35

34

38

40

42

43

39

31

37

41

26

28

30

29

17

10

20

Freeport

Pownal

Yarmouth

North 

Yarmouth

58

56

55 59

Harvey Brook

Merrill Brook

Cousins River

70°10' 70°8'0

43°52'

43°50'

!

! !.

!.

!.

!.

!.

!. !.

!.

!.

!.

!. !.

!.

!.

# 21

22

16

24

27

11

23

25

14

12

13

15

18

19

57

A10

A11

Area

of inset

0 250 500 FEET

0 100 200 METERS

0 0.5 1 2 KILOMETERS

Base from U.S. Geological Survey digital data, 1:24,000 0 0.5 1 MILE

Universal Transverse Mercator projection, Zone 19N

Central meridian -69°

Shaded relief from U.S. Geological Survey National Elevation

Dataset 10-meter digital elevation model

North American Datum of 1983 (NAD 83) 

295

EXPLANATION

Active model area boundary

Buried valley extent

# Streamflow site and identifier

Groundwater level points

!. Confined, wells and borings

!. Unconfined, wells and borings

!. Unconfined, spring

!. Unconfined, seismic

!. Unconfined, synthetic

! Pumping well

Figure 14. Groundwater level and streamflow measurement observations in the Freeport aquifer study area, Maine. Points labeled 

with an “A” were not included in the model. Additional information on labeled points is provided in table 6 and appendix 1.
30 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

not sensitive enough to the specific Kv values in these zones 

to estimate them, although the model was very sensitive to 

their existence. More detailed geologic mapping, streamflow 

data collection, or drilling could provide data that would lead 

to further modifications and improvements to the conceptual 

model of the flow system. 

Model Fit to Observations

The overall model fit is assessed by evaluating 

observations and their simulated equivalents. The simulated 

water levels for the head observations at 27 wells and borings, 

16 seismic lines, and 11 points of estimated water levels are 

graphed against the observed values in figure 15A. The match 

between the observed and simulated heads appears to be fairly 

good, especially in layer 3. The mean absolute difference 

between the observed and simulated heads is 6.7 ft in layer 

1 and 3.7 ft in layer 3. These differences represent 4 percent 

of the total head change across the whole model. Weighted 

residuals represent the difference between observed and 

simulated, adjusted for the weight applied to each observation 

during calibration. Ideally, the weighted residuals would be 

randomly distributed, both in magnitude and location. A plot 

of the weighted residuals and unweighted simulated values 

(fig. 16) shows that there is a slightly greater range in negative 

residuals than positive residuals, but little in the way of pattern 

from low to high heads. Spatially (fig. 17), the weighted 

residuals in layer 3 do not have any particular pattern. 

Although the weighted residuals in layer 1 in the north-central 

part of the model are relatively large in magnitude, they are 

both positive and negative. The large weighted residuals 

correspond to an area that has a thin saturated zone above the 

confining layer, and small-scale heterogeneities not modeled 

in this layer can have large effects on the heads. Some of the 

difficulty in fitting the heads in layer 1 was not unexpected due 

to the limited dataset of shallow water levels. Furthermore, 

some points were collected in very steep terrain, where water 

levels can vary greatly over short distances. Small-scale 

adjustments to these areas in the absence of data to support 

them were not warranted during the model calibration process, 

although they could have improved the model fit. 

Although the absolute differences between the simulated 

and observed water levels in the estimated till water-level 

points is quite high in many cases, the low weights of those 

points translate into relatively low weighted residuals. This 

should not be interpreted as indicating the model is “correct” 

in these areas, only that the model error in those areas is considered less important to the goals of the model purpose than 

errors in other areas. 

The head data in layer 3 were somewhat less difficult 

to fit for two reasons: (1) the target water level for many of 

these wells represents the average over a long-term record, 

and (2) most of the wells were clustered in one area around 

the pumping well. Differences between the observed and 

model-calculated heads in layer 3 also may partially be a result 

of vertical gradients within the aquifer not being represented 

by the model, which only uses 1 layer to represent this aquifer. 

None of the observations in layer 3 are screened across the 

entire aquifer, which is quite heterogeneous vertically. The 

observation in layer 3 that was hardest to fit was boring 85-2. 

The uncertainty in this observation is quite high, because 

the water level was taken 25 years ago when the boring was 

drilled and no observation well was left in place. The aquifer 

is known to be heterogeneous in this area and the water level 

was measured during a time of drought. 

Drawdown data from a 20-day pump test also were used 

to calibrate the model parameters in layer 3. Observed rebound 

in 13 wells when the pumping well was turned off for 2 days 

was compared to model-simulated steady-state differences in 

water levels at these wells with the pumping well active and 

turned off. Measured drawdown from pumping (the inverse 

of rebound) ranged from -1.1 to -32.5 ft (table 7). The simulated equivalent drawdowns ranged from -2.2 to -25.6 ft for 

the calibrated model. Most of the simulated drawdowns were 

somewhat greater than observed, which results from (1) the 

vertical averaging of the hydraulic conductivity field in the 

model, as compared to the lenses of higher-K sediments that 

some of these wells intersect, and (2) the simulation of steadystate conditions as compared to the relatively short-term 

drawdowns in the observations. 

The observed and simulated streamflows in the reaches 

above the streamflow measurement sites shown in table 8 and 

figure 15B indicate that the fit of the streamflow data is very 

good, even though the weights assigned to the streamflow 

measurements were low. Because the streamflow observations 

represent discharge from hundreds of individual model cells, 

the exact spatial distribution of drain flows on a cell-by-cell 

basis is averaged out. The pattern generally fits well with 

observations in the field. 

The calibration of the model using the UCODE_2005 

software creates data that can be used to estimate the 

individual 95-percent confidence intervals on the streamflow 

simulated at the five streamflow measurement locations, 

shown in figure 18 (Hill and Tiedeman, 2007; Poeter and 

others, 2008). These calculated confidence intervals take into 

account all of the uncertainty in the estimated parameters 

and the uncertainty in the model variables set by trial and 

error, and the uncertainty in the observations, but they do not 

account for uncertainty in the possible variations in conceptual 

models that could have been used. The simulated streamflow 

for Harvey Brook #3 was the most uncertain, which reflects 

its influence by many factors, including recharge in several 

areas of the model, drain cell conductances, Kh in both layer 

1 and layer 3, and Kv in layer 2. Because it is less influenced 

by discharge up through layer 2, the simulated streamflow 

for Harvey Brook #2 has the lowest amount of uncertainty 

calculated for this model.
Simulation of Groundwater Flow and Discharge to Streams 31

Harvey Brook #1

Harvey Brook #2

Harvey Brook #3

Harvey Brook #4

Merrill Brook #1

B

A

Layer 1

Layer 2

Layer 3

Line of equality

EXPLANATION

Discharge to streams 

Line of equality

EXPLANATION

Model calculated head, in feet above NAVD 88 Model calculated streamflow, in cubic feet per second

Observed head, in feet above NAVD 88

Observed streamflow, in cubic feet per second

2.5

2.0

1.5

1.0

0.5

0

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120 140 160 180 200

0 0.5 1.0 1.5 2.0 2.5

Figure 15. Relation between observed and model-calculated values for A, heads, 

and B, early summer streamflow, for the Freeport aquifer groundwater-flow model.
32 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Simulated Groundwater Levels and Flow Under 

Steady-State Conditions

The simulated steady-state heads in model layers 1 and 

3 are represented in figures 19 and 20, respectively. The layer 

1 heads follow the terrain, with higher heads on the hills and 

valley edges, and lower heads in the stream valleys. Heads 

are highest in the upland hills along the northeast edge of the 

model, and decline to the west toward Harvey Brook. The 

representation of head in the Merrill Brook watershed follows 

the land surface contours quite well. Simulated groundwater 

flow is from areas of higher head to lower head, which in this 

case is from upland areas downhill toward the stream network. 

Simulated flow from the upper saturated zones in layer 1 

through layer 2 and into layer 3 occurs where heads in layer 1 

are higher than those in layer 3. This process occurs primarily 

along the western edge of the model and about halfway down 

the slope on the eastern edge of the model, where simulated 

recharge flows around the edge of the confining layer and into 

layer 3. 

The modeled heads in layer 3 (fig. 20) are highest at 

the northern edge of the model and on the eastern and western edges of the aquifer, where recharge enters from upland 

areas. The simulated pumping wells create a cone of depression in the center of the model, and heads decline gradually 

from there toward the southern edge of the model. Simulated 

discharge upwards to the streams through the vertical conductance zones in layer 2 creates a fairly flat hydraulic gradient in 

the southern half of the model in layer 3. Recharge to layer 3 

is greatest where the heads are highest at the western edge of 

the model within a half mile of the pumping wells. 

The distribution of simulated steady-state discharge of 

groundwater from drain cells in layer 1 is shown in figure 21. 

The spatial pattern of simulated discharge shows that the 

areas of greatest discharge in the Harvey Brook #4 watershed 

occur along the eastern tributary of Harvey Brook. Springs 

are known to occur in this area, which is adjacent to the eolian 

sand deposits overlying Presumpscot Formation silt and clay. 

The highest simulated stream discharge in the Harvey Brook 

#3 watershed area occurs along the main stem of Harvey 

Brook, which is consistent with field observations. The eastern 

tributary of the stream in that watershed has somewhat higher 

simulated streamflows than expected, however, which probably is reflective of uncertainties in the distribution and thickness of the Presumpscot Formation in that area. Simulated 

stream discharge in the Harvey Brook #2 watershed areas is 

fairly constant along Harvey Brook. The cone of depression 

caused by the modeled pumping wells in layer 3 (fig. 20) 

creates an area of lower simulated discharge, but it is possible 

that streamflow may actually move from the stream down into 

the lower aquifer. Because the model used the drain package 

(which only allows for one-way water movement, from the 

aquifer to the drain cell), this possibility could not be evaluated with the model. Stream discharge along the segment of 

Harvey Brook in the Harvey Brook #4 watershed area is quite 

high, which is consistent with the field observations of springs 

in that area. The simulated discharge for most of the Merrill 

Brook watershed area is quite low, except in the westernmost 

tributary where it is slightly higher. No field data were available to compare against this finding.

The construction and calibration of the numerical 

groundwater model of the Freeport aquifer area provided 

important insights into the flow of water within the groundwater system and the interaction of the groundwater system with 

the streams. Prior to the current modeling exercise, the source 

of groundwater discharge to Harvey Brook was unknown, 

and the pathway of recharge to the confined aquifer was 

also unknown. 

Model Sensitivity Analysis and Parameter 

Uncertainty

All the heads in layer 1 were sensitive to Merrill Brook 

streambed K, to recharge in the sandy units, and to the K of 

the sandy units. The areas north of the pumping wells were 

also sensitive to the K of poorly-drained clayey deposits 

(till and weathered Presumpscot Formation sediments/

rock). The layer 1 heads south of the pumping well also were 

sensitive to recharge in the areas having thin deposits over 

bedrock. Layer 3 heads were most sensitive to the hydraulic 

conductivity zones in layer 3, and to recharge in the enhanced 

recharge area to the west and on the highest-altitude areas 

in the east-central part of the model, but not to the general 

head boundaries. This finding indicates that locally-derived 

recharge is much more important than surface water inflow 

from the north in feeding the Freeport aquifer. None of the 

Weighted residuals on simulated head

-6

-4

-2

0

2

4

6

0 50 100 150 200 250

Unweighted simulated head, in feet

Figure 16. Weighted residuals and unweighted simulated values 

for heads in the Freeport aquifer groundwater-flow model.
Simulation of Groundwater Flow and Discharge to Streams 33

!

!!

!!

!

!

!

# #

#

##

##

#

#

# #

#

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

#

#

#

#

#

#

#

#

#

#

#

#

#

#

#

EXPLANATION

Active model area

Layer 3 

Layers 1 and 2—Weighted residuals

# Less than -3.0

# -3.0 to -2.1

# -2.0 to -1.1

# -1.0 to 0

! 0.01 to 1.0

! 1.1 to 2.0

! 2.1 to 3.0

! Greater than 3.0

Layer 3—Weighted residuals

# Less than -3.0

# -3.0 to -2.1

# -2.0 to -1.1

# -1.0 to 0

! 0.01 to 1.0

! 1.1 to 2.0

! 2.1 to 3.0

! Greater than 3.0

96

30

90

40

20

10

60

50

80

70

130

140

100

110

120

148

0

10

30

90

20

60

80

40

50

70

1

Model row

Model column

Figure 17. Spatial distribution of weighted residuals across the domain of the Freeport aquifer groundwater-flow model.
34 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Table 7. Drawdown observed in layer 3 wells during pumping tests, and model-calculated steady-state 

drawdown from pumping in the AquaMaine production wells.

[Wells identified in figure 14; ft, foot]

Well name Well number 

(figure 14, table 6)

Measured short-term drawdown from pumping wells, 

in ft

Predicted steady-state drawdown from pumping wells, 

in ft

Difference

Well #95-3 23 -10.3 -16.0 5.7

Well #95-4 24 -5.8 -10.1 4.3

Well #96-1 25 -13.2 -18.7 5.5

Well #83-3A 12 -13.0 -13.4 .5

Well #83-3B 13 -17.0 -13.1 -3.8

Well #83-3C 14 -10.1 -12.8 2.7

Well #88-18" A10 -32.5 -25.6 -6.9

Well #89-7 15 -7.9 -11.7 3.9

Well #89-8 16 -5.0 -9.8 4.8

Well #89-9 17 -3.9 -3.2 -.7

Well #91-1 18 -3.1 -12.1 9.0

Well #91-2 19 -1.1 -11.9 10.8

Well #91-3 20 -2.2 -4.4 2.2

Table 8. Steady-state model calculated and observed early summer streamflows in the Freeport aquifer groundwater model.

[ft3/s, cubic foot per second]

Stream discharge site

Measured 

incremental flow, 

in ft3/s

Measured 

total flow, 

in ft3/s

Model-calculated 

incremental flow, 

in ft3/s

Model-calculated 

total flow, 

in ft3/s

Difference 

(model calculated 

minus observed)

Harvey Brook #1 0.618 4.277 0.543 4.203 -0.075

Harvey Brook #2 .289 3.658 .275 3.660 -.014

Harvey Brook #3 2.044 3.369 2.048 3.384 .0043

Harvey Brook #4 1.325 1.325 1.336 1.336 .0109

Merrill Brook #1 .794 .794 .750 .750 -.0444
Simulation of Groundwater Flow and Discharge to Streams 35

heads in the model were very sensitive to the recharge to the 

Presumpscot Formation, or to Kh, or Kv in the Presumpscot 

Formation areas, or to any Kv parameters, including the Kv

of the vertical conductance zones through the clay in layer 

2 (although the model was very sensitive to their existence). 

The streamflows were all sensitive to recharge in the till and 

weathered Presumpscot Formation areas, and recharge in 

sandy areas south of the pumping well, although the Harvey 

Brook #2 site was particularly sensitive to recharge of the 

sandy units. This is likely because the site gets a relatively 

larger proportion of its discharge from the upper sand unit 

than the other Harvey Brook sites. Of the drain conductance 

parameters, the drain conductance in the Merrill Brook area 

had the greatest effect on the drain discharge, even in the 

Harvey Brook watersheds, because this parameter had some 

control on the position of the groundwater divide between 

the watersheds. Several of the streamflow observations 

were sensitive to the Kh parameters in layer 3, because these 

controlled the overall movement of water through layer 3 and 

toward the vertical conductance zones in the layer 2 clay. Also 

important to this flow was the recharge from upland runoff to 

the west, to which the Harvey Brook sites were all sensitive. 

Additional details of specific parameter sensitivities and 

95-percent confidence intervals of the parameter estimates are 

presented in appendix 2.

The model may also be sensitive to how the hydrogeology of the system was discretized into the model components. 

For example, the model was very sensitive to the presence of 

the vertical conductance zones in layer 2, and to the recharge 

zone with the highest recharge rate, representing upland runoff 

from outside the model boundary. The model was not very 

sensitive to the head-dependent boundaries at the northern and 

southern ends of the model. The drain cells are very important 

in the model. The bottom no-flow boundary to the model, 

representing the bedrock surface, may affect the model outcome, although this prediction was not tested. An alternative 

would have been to add a layer of low-hydraulic conductivity 

material (representing the shallow bedrock) to the bottom of 

the model. This addition might have affected the movement 

of recharge from the uplands to the buried valley aquifer, but 

probably not by a large amount.

The optimized values of the estimated parameters in the 

Freeport aquifer model were evaluated against known reasonable ranges and the degree of confidence of the final optimized 

value. The estimated hydraulic conductivity parameters and 

recharge parameters are shown in figure 22, along with the calculated 95-percent confidence intervals and reasonable ranges. 

Ideally, the parameter confidence intervals would be small, 

and would fall entirely within the reasonable ranges. However, 

a large degree of uncertainty in most parameter values is quite 

common (Mary Hill, U.S. Geological Survey, written commun., 2011). Most of the estimated values fall within the reasonable ranges, except for two hydraulic conductivity parameters. The estimates for one hydraulic conductivity parameter 

in layer 1 and the hydraulic conductivity of the area outside 

the Presumpscot Formation in layer 2 fall outside what was 

thought to be a reasonable range. This result may indicate that 

the reasonable ranges are incorrect, which is possible given 

the lack of stratigraphic certainty in the model area. Simulations that are sensitive to parameters having small confidence 

intervals are more likely to be robust than simulations that are 

sensitive to parameters having large confidence intervals.

Upper limit

Predicted value

Lower limit

EXPLANATION

0

0.5

1.0

1.5

2.0

2.5

3.0

Streamflow, in cubic feet per second

Harvey Brook

#4

Harvey Brook

#3

Harvey Brook 

#2

Harvey Brook 

#1

Merrill

Brook

Streamflow site

Figure 18. Ninety-five percent confidence 

intervals on steady-state streamflow simulation 

for the five streamflow sites in the Freeport 

aquifer study area.
36 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

!

!

!

!

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!

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#

#

#

#

#

Merrill Brook

Harvey Brook #4

Harvey Brook #3

Harvey Brook #2

Harvey Brook #1

90

80

70

110

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120

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EXPLANATION

Active model area

Layer 1 head contour—Contour 

interval 10 feet. Datum is 

NAVD 88

Layer 1 steady-state heads—

Value

#0 Streamflow site and identifier

! Layer 1 observation

High: 193

Low: 0.4

96

30

90

40

20

10

60

50

80

70

130

140

100

110

120

148

0

10

30

90

20

60

80

40

50

70

1

Model row

Model column

Figure 19. Steady-state simulated groundwater levels in layer 1, Freeport aquifer model area. Contour interval is 

10 feet.
Simulation of Groundwater Flow and Discharge to Streams 37

!

!!

!

!

!

!

! !!

!! ! !

!

!

!

!

!

!

!.

!.

5

75

70

65

60

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85

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115

100

20

60

15

80

55

45

95 90

!.

EXPLANATION

Active model area

Layer 3 head contour—Contour 

interval 5 feet. Datum is

NAVD 88 

Layer 3 steady-state heads—

Value

Production well

! Layer 3 observation

Layer 3 boundary

High: 117.2

Low: 3.2

96

30

90

40

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10

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50

80

70

130

140

100

110

120

148

0

10

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1

Model row

Model column

Figure 20. Steady-state simulated groundwater levels in layer 3, Freeport aquifer model area. Contour interval is 

10 feet.
38 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Figure 21. Steady-state simulated streamflow (groundwater discharge) from drain cells and drain observation zones 

in the Freeport aquifer model area.

%%

Harvey Brook #4

Merrill Brook

Harvey Brook #3

Harvey Brook #2

Harvey Brook #1

Steady-state discharge from drain

cells—In cubic feet per day

Active model area

Drain observation zones

% Pumping well

EXPLANATION

< 50.00

50.01 to 150

150.1 to 300

300.1 to 475

475.1 to 675

675.1 to 900

900.1 to 1,300

1,301 to 1,670

1,671 to 2,250

2,251 to 3,500

3,501 to 10,000

10,010 to 18,500

96

30

90

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148

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Model row

Model column
Simulation of Groundwater Flow and Discharge to Streams 39

Figure 22. Parameter estimates, 95-percent confidence intervals, and reasonable 

ranges for A, hydraulic conductivity and B, recharge parameters for the Freeport aquifer 

groundwater-flow model.

A

B

0.01

0.10

1.00

10.00

100.00

1,000.00

0

5

10

15

20

25

30

35

40

Estimated hydraulic conductivity parameter

Hydraulic conductivity, in feet per day Recharge, in inches per year

Maximum

Reasonable range

Minimum

Upper limit

Optimal value

Lower limit

EXPLANATION

Estimated recharge parameter

KDR_Merill

HK_Lay1_A

HK_Lay1_B

HK_Lay1_C

HK_Lay1_CD

HK_Lay1_D

HK_Lay2_TL

HK_Lay3_Sa

HK_Lay3_K2

HK_Lay3_K3

RCH_Till

RCH_SandyN

RCH_SandyS
40 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Limitations of the Model

The Freeport aquifer groundwater-flow model provides a generalized simulation of the processes involved in 

groundwater recharge, flow, and discharge in the study area. 

The model is developed using observations of head and flow. 

Generally, models can simulate processes represented by 

observations used in the calibration, but are not well suited 

for the simulation of other processes. Therefore, this model is 

most appropriate for the simulation of flow and groundwater 

levels, but may not adequately simulate chemical transport. As 

with any numerical simulation of a natural system, the model 

incorporates simplifications and assumptions about the natural 

system that impose limitations in the use and interpretation of 

the model results. Some of the simplifications inherent in the 

model include a simplification of the surface-water system, 

a lack of incorporating the unsaturated zone, and the role of 

evapotranspiration in the water budget. 

The stream network is simplified from reality, and the 

modeled streambed (drain) conductances incorporate very 

simplified representations of the width of each stream segment. The discretization of the stream network also assumes 

a straight-line segment in each model cell, which may differ 

substantially from reality. The water-level altitudes in each 

model cell are interpolated from the topography in a way that 

may not represent each individual stream segment accurately. Finally, the conceptualization of the streamflow did 

not incorporate any potential losing reaches in Harvey Brook. 

Future modeling efforts that involve streamflow depletion and 

significant groundwater-surface water interactions may be 

improved using other packages in MODFLOW that allow for 

two-way movement of water between the aquifer and streams. 

Doing so would allow for losing reaches of streams as well as 

gaining reaches.

The boundaries were chosen for the model assuming 

that the surface-water divide coincides with the groundwater 

divide between watersheds. Limitations in the amount of field 

data—both in terms of the spatial and temporal distribution 

of measurements of groundwater levels and in terms of the 

data on the stratigraphy of the system—also translate into 

limitations in the model itself. More groundwater level data, 

particularly at the edges of the model, would help to constrain 

the boundary conditions and recharge. Additional stratigraphic 

data would have helped to make the movement of water more 

certain, particularly in the southern part of the model. There 

is some evidence of a certain amount of model error because 

although the model is sensitive to the recharge parameters, 

one of the recharge parameter estimates had to be constrained 

within the reasonable limits. This would not occur if the model 

construction perfectly matched physical reality. Similarly, 

some of the K parameters were estimated outside their reasonable ranges. 

Finally, bedrock was considered to be a minor part of the 

overall water budget and was not modeled. However, adding a 

layer of shallow bedrock to the model may have reduced some 

of the model error.

Model-Calculated Water Budget for Harvey and 

Merrill Brooks and the Buried Freeport Aquifer

The simulated average annual water budget for three 

subareas of the Freeport aquifer study area (the Harvey Brook 

watershed in layer 1—Subarea 1; the modeled portion of the 

Merrill Brook watershed—Subarea 2; and the buried valley 

aquifer—Subarea 3) were calculated using the MODFLOW 

supplemental software ZONEBUDGET (Harbaugh, 1990). 

This software is used to calculate internal flows between different zones of the model, which are summarized in table 9 

along with the water budget of the entire model domain. 

Within this domain, recharge to surficial materials in layer 1 

accounted for 91 percent of the total inflows (4.12 Mgal/d), 

with inflow from head dependent boundary cells accounting for the remainder. Outflows from the model domain went 

to drain cells representing streams (90 percent), flux cells 

representing pumping wells (7 percent) and head-dependent 

boundary cells (2 percent). The model-calculated discrepancy 

between inflows and outflows was less than 0.001 percent. Of 

the total amount of recharge and other inflows entering the 

entire model domain, about 37 percent reaches the confined 

aquifer (Subarea 3). Simulated inflows and outflows beyond 

the boundaries of the Harvey and Merrill Brook watersheds 

(in the area around the Cousins River) are not included within 

Subareas 1 and 2.

The Harvey Brook watershed (Subarea 1), with its 

diverse geology and stream hydrology, has a more complex 

simulated water budget (table 9 and fig. 23) than the Merrill 

Brook watershed (Subarea 2). Subarea 1 receives more 

simulated recharge per unit land area than Subarea 2 because 

of its more permeable soils, and it also receives input in the 

form of upward groundwater seepage from layer 3, which 

is immediately discharged to the streams (fig. 23). This flow 

constitutes 27 percent of the total simulated inflows to the 

watershed, with the remainder consisting of recharge from 

precipitation (68 percent), horizontal subsurface flow from 

the Merrill Brook watershed (4 percent) and from other 

upland areas (1 percent). Simulated outflows from the Harvey 

Brook watershed are to streams (78 percent of the total) 

and downward infiltration to layers 2 and 3 (22 percent). 

The total simulated streamflow generated within the Harvey 

Brook watershed (2.7 Mgal/d) consists of shallow-generated 

streamflow from above the clay (1.8 Mgal/d) and the upward 

seepage from below the clay (0.9 Mgal/d) (table 9). 

Of the two watersheds, Merrill Brook has the simpler 

simulated water budget, because of the shallow depth to bedrock and widespread distribution of fine-grained materials. All 

simulated inflows to the Merrill Brook watershed (Subarea 2) 

come from recharge (table 9 and fig. 23); simulated outflows 

consist of discharge to streams (64 percent), infiltration to 

layer 3 (16 percent), horizontal subsurface flow to the Harvey 

Brook watershed (18 percent), and horizontal subsurface flow 

to other adjacent uplands (2 percent). 

Subarea 3, the buried valley aquifer represented by 

layer 3, receives most of its input from downward seepage 
Simulation of Groundwater Flow and Discharge to Streams 41

Table 9. Steady-state model calculated annual water budget for the Freeport aquifer groundwater-flow model and three 

subareas.

[Mgal/d, million gallons per day; Subarea 1 is Harvey Brook watershed; Subarea 2 is modeled portion of the Merrill Brook watershed; 

Subarea 3 is layer 3, buried valley deposits]

Hydrologic budget component Rate of flow, in Mgal/d

Entire model domain Subarea 1 Subarea 2 Subarea 3

Inflow

Recharge to surficial units 3.77 2.34 0.77

Shallow subsurface flow from adjacent uplands .05

Shallow subsurface flow from Merrill Brook watershed .13

Flow up from layer 3 .93

Infiltration down through layer 2 (till) 1.17

From head-dependent boundaries 0.36 .36

Total inflow 4.12 3.45 .77 1.52

Outflow

Groundwater discharge to streams 3.72 2.69 .49

To head-dependent boundaries .10 .10

Infiltration to layer 3 .77 .13

Discharge upwards below streams 1.12

Shallow subsurface flow to adjacent uplands .02

Shallow subsurface flow to Harvey Brook watershed .13

Water-supply withdrawals .30 .30

Total outflow 4.12 3.45 .76 1.52

Budget error (inflow minus outflow) 0.000 0.001 0.000 0.000

through layer 2, and 98 percent of that flow occurs within 

what is simulated as the till zone of layer 2 (primarily within 

the Harvey Brook watershed). The remaining inflow to layer 3 

is from the head-dependent boundary on the north end of the 

study area. Outflows from layer 3 are divided between the 

water-supply well withdrawals (20 percent), upward flow to 

streams in layer 1 (74 percent), and outflow from the headdependent boundary on the south end of the model (7 percent). 

The groundwater model was used to evaluate changes 

that could occur to the water budgets of the 3 subareas 

under four simulated climatic and pumping scenarios: (1) no 

pumping well withdrawals; (2) current (2009) pumping, but 

simulated drought conditions (equaling a 20-percent reduction 

in recharge); (3) current (2009) recharge, with a 50 percent 

increase in pumping well withdrawals for public supply; and 

(4) drought conditions and increased pumping combined. The 

water budgets for the model under these scenarios are presented in figure 24 and table 10. 

The effect of removing the pumping wells (0.3 Mgal/d) is 

primarily seen in the movement of water between layers 1 and 

3 (tables 9 and 10). Hydraulic head in layer 3 is increased, the 

hydraulic gradient at the edges of the aquifer is reduced, and 

infiltration from layer 1 to layer 3 is reduced by about 4 percent (from 1.17 to 1.12 Mgal/d). Beneath the Harvey Brook 

stream, however, the head in layer 3 is increased relative to the 

head in layer 1, and upward flow to the stream increases by 

about 23 percent (from 0.93 to 1.15 Mgal/d). 

Simulated changes to the system from the drought scenario (a 20-percent decrease in total recharge, 0.76 Mgal/d, 

applied to each of the recharge zones) also affect the movement of water across the confining layer. Simulated head 

decreases in layer 3 as the infiltration downward from 

layer 1 to layer 3 in Harvey and Merrill Brook watersheds 

(Subareas 1 and 2) is reduced by 0.13 Mgal/d (13 and 17 percent, respectively), which is a nearly 15-percent reduction 

in downward recharge to Subarea 3 (table 10; fig. 24). This 

decrease however, is somewhat balanced by an increase 

in flow (0.03 Mgal/d) from the northern head-dependent 

boundary and a nearly 11-percent decrease in upward flow 

from layer 3 to the streams (0.13 Mgal/d). Simulated stream 

discharge from the whole model is reduced by 0.71 Mgal/d 

(19 percent), most of which comes from a reduction in the 

streamflow coming from recharge to layer 1.
42 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

A simulated 50-percent (0.15 Mgal/d) increase in 

pumping using the original recharge values resulted in slight 

changes to the hydrologic system and water budget. Modeled 

streamflow is reduced by 4 percent (0.14 Mgal/d), due primarily to decreased upward flow to Harvey Brook from layer 3 

(about 0.11 Mgal/d, or 12 percent) as heads are reduced by the 

pumping increase.

Reducing the recharge combined with increasing pumping in the model resulted in the largest changes to the water 

budgets. The reduction in recharge (by 0.76 Mgal/d) and subsequent downward flow to layer 3 was balanced somewhat by 

an increase in flow of 0.04 Mgal/d from the north end of the 

model as head in layer 3 was lowered. Streamflow throughout 

the model was reduced by 0.85 Mgal/d (23 percent), and by 

0.66 Mgal/d (24 percent) in the Harvey Brook watershed. 

The simulation of the movement of water between 

different parts of the modeled hydrologic system illustrates 

the complexity of hydrologic systems in Maine. Although the 

base-case total water withdrawals from the system are only 

7.2 percent of the entire water budget, those withdrawals do 

affect the hydrology of the system as a whole, and affect the 

rates of flow in many areas of the model. Adding potential 

drought conditions to the simulations significantly altered the 

overall water budget, but it also altered the movement of 

water within the system, between layers and across the 

confining unit.

Evaluation of Streamflow Depletion in 

Harvey Brook

Direct instream withdrawals of water are simple to 

measure, and the effect on streamflows downstream of the 

withdrawal is readily understood. Groundwater withdrawals, 

on the other hand, can affect streamflows by varying amounts 

along a stream, and these affects can be difficult to measure. 

Surface-water or groundwater withdrawals may remove the 

same amount of water from the hydrologic system, but the 

effect of groundwater withdrawals on streamflow are distributed over space and time. A reduction in streamflow resulting from a groundwater withdrawal is known as streamflow 

Inflows

Outflows

Flow rate, in million gallons per day

-4

-3

-2

-1

0

1

2

3

4

Subarea 1Subarea 2Subarea 3

EXPLANATION

Æ

Æ

Æ

Æ

Æ

Æ

Æ

Æ

Æ

Æ

Æ

Æ

Æ

To Harvey Brook watershed

To adjacent uplands

To wells

To head dependent boundary

Discharge up below streams

Infiltration to layer 3

To streams/drains

From Merrill Brook watershed

From uplands outside southwest divide

Up from layer 3

From head dependent boundary

From infiltration through till

From recharge

Inflow

Outflow

Figure 23. Steady-state simulated inflows 

and outflows for three subareas within the 

Freeport aquifer groundwater-flow system.
Evaluation of Streamflow Depletion in Harvey Brook 43

Flow rate, in million gallons per day

1 aer abuS

2 aer abuS

3 aer abuS

1 aer abuS

2 aer abuS

3 aer abuS

1 aer abuS

2 aer abuS

3 aer abuS

1 aer abuS

2 aer abuS

3 aer abuS

1 aer abuS

2 aer abuS

3 aer abuS

Base case No pumping Decreased

recharge

Increased

pumping

Decreased

recharge and

increased pumping

Inflows

Outflows

-1-2-3-4

43210

EXPLANATION

ÆÆÆÆÆÆÆÆÆÆÆÆÆ

From Merrill Brook watershedFrom uplands outside southwest divideUp from layer 3From rechargeFrom head dependent boundaryFrom infiltration through tillTo streams/drainsInfiltration to layer 3To adjacent uplandsTo Harvey Brook watershedDischarge up below streamsTo head dependent boundaryTo wells

InflowOutflow

Figure 24. Simulated water budget components under various hydrologic scenarios, including no pumping, decreased recharge, and increases in pumping for the Freeport aquifer groundwater-flow model.
44 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed Table 10. Simulated water budgets for the Freeport aquifer groundwater-flow model and subareas under alternative hydrologic conditions. [Mgal/d, million gallons per day; Subarea 1 = Harvey Brook watershed; Subarea 2 = modeled portion of the Merrill Brook watershed; Subarea 3 = layer 3, buried valley deposits] Hydrologic budget component Rate of flow, in Mgal/d No well scenario Less recharge scenario More pumping scenario Less recharge and more pumping scenario Entire model domain Sub- area 1 Sub- area 2 Sub- area 3 Entire model domain Sub- area 1 Sub- area 2 Sub- area 3 Entire model domain Sub- area 1 Sub- area 2 Sub- area 3 Entire model domain Sub- area 1 Sub- area 2 Sub- area 3 Inflows Recharge to surficial units 3.77 2.34 0.77 3.01 1.87 0.61 3.77 2.34 0.77 3.01 1.87 0.61 Shallow subsurface flow from adjacent uplands 0.06 0.04 0.05 0.04 Shallow subsurface flow from Merrill Brook watershed 0.14 0.08 0.13 0.08 Flow upwards from layer 3 1.15 0.82 0.82 0.71 Infiltration down through layer 2 (till) 1.12 1.00 1.19 1.01 From head-dependent boundaries 0.35 0.35 0.39 0.39 0.36 0.36 0.40 0.40 Total inflows: 4.12 3.68 0.77 1.47 3.41 2.82 0.61 1.39 4.12 3.34 0.77 1.55 3.41 2.70 0.61 1.41 Outflows Groundwater discharge to streams 4.02 2.93 0.49 3.01 2.15 0.41 3.58 2.56 0.49 2.87 2.03 0.41 To head-dependent boundaries 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 Infiltration down through layer 2 (till) 0.75 0.12 0.66 0.10 0.78 0.13 0.67 0.11 Discharge upwards below streams 1.37 0.99 1.00 0.87 Shallow subsurface flow to adjacent uplands 0.02 0.01 0.02 0.01 Shallow subsurface flow to Merrill Brook watershed 0.14 0.08 0.13 0.08 Water-supply withdrawals 0.00 0.30 0.30 0.45 0.45 0.45 0.45 Total outflows: 4.12 3.68 0.76 1.47 3.41 2.82 0.61 1.39 4.12 3.34 0.76 1.55 3.41 2.70 0.61 1.41 Budget error (inflow minus outflow) -0.00 -0.00 -0.00 0.00 0.00 -0.00 -0.00 0.00 -0.00 -0.00 -0.00 0.00 0.00 -0.00 -0.00 0.00
Evaluation of Streamflow Depletion in Harvey Brook 45

depletion. Estimating this depletion at a particular location 

requires knowing how that withdrawal propagates through 

time and space across the river or stream. If one were to pick a 

stream location far enough downstream from the withdrawal, 

the total amount of overall depletion would equal the amount 

of withdrawal. If the withdrawal remained constant over the 

whole year, the effect of the withdrawal would be constant 

through the seasons. If the withdrawal varied with time, 

however, it would likely have variable effects on streamflow 

depletion that are difficult to determine. 

Several methods exist for evaluating potential streamflow 

depletion, and two of them were applied in this study: the 

groundwater model and an analytical solution. The analytical 

solution was provided by use of the program STRMDEPL08. 

The following sections illustrate the effect of streamflow 

depletion on natural flows in the Freeport aquifer study area 

and the use and limitations of the two methods on evaluating 

streamflow depletion.

Calculation of Instream Flow Requirements for 

Harvey Brook

The instream flow requirements (seasonal aquatic base 

flows) for the State of Maine are designed to protect the 

aquatic health of riverine ecosystems, and vary with the 

seasons. The State regulations (Chapter 587 rules: Instream 

Flows and Lake and Pond Water Levels, part of the Maine 

Sustainable Water Use Program) define six different time 

periods during the year for which instream flows are calculated 

based on various median monthly flows. The most stringent of 

these requirements occurs during the summer (July 1 through 

September 15), when the instream flow requirement level is 

the August median flow. 

Chapter 587 states that without site-specific flow data, the 

median monthly flows can be determined by using statewide 

flow equations developed by Dudley (2004), which best apply 

to watersheds larger than 10 mi2. Although these equations 

are not recommended for watersheds as small as those in the 

Freeport study area (Dudley, 2004), the State requirements 

indicate that these calculations can provide an interim estimate 

that may be refined with the collection of additional data. The 

seasonal instream flow requirements for the State, as defined 

in Chapter 587, are shown in figure 25 for Harvey and 

Merrill Brooks, along with the calculated median monthly 

flows from which these instream flows are based. The total 

amount of pumping from the water-supply wells in the Harvey 

Brook watershed is also shown. 

Site-specific streamflow data for Harvey and Merrill 

Brooks were used to perform a record extension using the 

MOVE.1 method (described earlier), which is an acceptable 

alternative method of determining median monthly flows to 

use for the instream flow requirements (Ch. 587 rules). The 

median monthly flows estimated using the record extension 

are shown in figure 25 with the flows calculated using the 

statewide equations. The equation-based calculations and the 

site-specific data agree closely for Merrill Brook, except in the 

winter (January–March), when the site-specific data indicate a 

lower natural median monthly flow than the statewide equations predict (fig. 25A). The statewide equations did not perform as well for Harvey Brook (fig. 25B), because of the large 

aquifer and more complex hydrologic system. The equations 

over-predict the natural flows for December through April, and 

predict a natural flow that is almost double what site-specific 

data would indicate for the April high-flow period. The statewide equations predict the summer low-flow period fairly well 

for both streams, although the equations indicate a somewhat 

greater natural median flow in August and September than 

what was measured. 

Because the statewide equations (Dudley, 2004) are 

predictions based on a statistical analysis of many other 

watersheds, the PRESS statistic can be used to calculate 

prediction intervals for each month as well as the individual 

monthly predicted value (Riggs, 1968). The prediction 

intervals indicate the level of certainty surrounding the 

individual monthly predicted value. For the Harvey Brook 

watershed, the 90-percent prediction intervals were calculated 

for each median monthly flow (fig. 26). The flows bounded 

by the prediction interval would be expected to contain the 

actual (measured) flow for a given stream 90 percent of the 

time. The measured flows for Harvey Brook fall well within 

the 90-percent prediction intervals for the months of May 

through November. The flows from December to February are 

at the lower edge of the 90-percent interval, and the measured 

flows for March and April are well below what was predicted, 

as well as what would be allowed for the maintenance of 

instream flows if site-specific data were not collected.

There is a clear benefit to using site-specific data against 

which to evaluate withdrawals and streamflow requirements, 

as it avoids potentially under- or over-estimating the natural 

instream flows. Because the goal of the State requirement is to 

protect natural flows but allow pumping if possible, understanding the site-specific hydrologic regime would be very 

helpful in evaluating whether any particular flow was above or 

below the threshold.

Streamflow Depletion Estimates Based on 

STRMDEPL08

The analytical model STRMDEPL08 (Reeves, 2008) also 

was used to analyze streamflow depletion in Harvey Brook. 

STRMDEPL08’s analytical solutions assume that aquifers 

are homogeneous, isotropic, and infinite; because these 

conditions were not met for the Freeport aquifer, results are 

generalizations only. However, the results can be informative 

in determining what additional steps may be necessary to 

proceed with studying a particular situation. The usefulness of 

the analysis is limited by the degree to which the hydrology of 

the situation under analysis is understood and how well it fits 

the assumptions. The Freeport aquifer was modeled as a leaky 

aquifer underlying an aquitard, where the stream partially 
46 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

B

Cubic feet per second

0

5

10

15

20

25

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

Instream flow requirements

Pumping

Dudley method median flows

MOVE.1 median flows

EXPLANATION

Month

A

0

5

10

15

20

25

Instream flow requirements

Dudley method median flows

MOVE.1 median flows

EXPLANATION

Figure 25. Monthly median flow estimates, pumping, and instream flow requirements for 

A, Merrill Brook, and B, Harvey Brook, Freeport, Maine.
Evaluation of Streamflow Depletion in Harvey Brook 47

penetrates the aquitard but not the aquifer (Reeves, 2008). 

The pumping rate for this evaluation was constant, although 

the program is designed to accommodate pumping rates that 

vary with time. The effect of multiple wells or hydrologic 

boundaries that are parallel to the stream can be evaluated 

using the principal of superposition.

Input data for the evaluation included the following: 

aquifer transmissivity (6,000 ft2/d), aquitard thickness (50 ft), 

vertical distance from stream bottom to bottom of aquitard 

(40 ft), vertical K of the aquitard (5.0 × 10-4 ft/d), stream width 

(15 ft), storativity of the aquifer (0.0001), specific yield of 

the aquitard (0.05), streambed conductance (5 ft/d), pumping 

rate (355 gal/min, which is the maximum for the Freeport 

aquifer wells), and distance to the stream (100 ft). Streamflow 

depletion calculated by STRMDEPL08 after 365 days of 

pumping was 2.3 × 10-4 ft3/s, which would not be detectable 

using any available measuring method. This evaluation 

assumed a uniform, relatively thick and impermeable aquitard, 

which the groundwater modeling exercise showed to be a 

flawed assumption.

In order to understand the relative importance of the 

input variables on the evaluation, and to determine which 

variables need the most scrutiny when applying this method, 

a sensitivity analysis was conducted, based on the potential 

uncertainty in the input variables in the Freeport aquifer study 

area. Reasonable ranges for each input variable (transmissivity 

of the aquifer, thickness of aquitard, vertical K of the aquitard, 

specific yield of the aquitard, and specific storage of the aquifer) were tested, as well as the effect of changing the pumping 

rate or changing the distance between the pumping well and 

the stream (table 11). This sensitivity analysis was run using 

the input data listed previously, and changing one variable at a 

time. The evaluation was run for 1 year with constant pumping, and converged to a steady-state solution for most combinations of variables tested within that time period. 

Based on data that represents a set of reasonable ranges 

for the unknown variables in the study area, the evaluation of 

streamflow depletion is most sensitive to the transmissivity of 

the aquifer, the thickness of the confining unit, and the vertical 

K of the confining unit. In fact, the solution is particularly 

sensitive to the vertical K of the confining unit (within the 

reasonable range for this variable) as the difference between 

the lowest and highest tested value exceeds 400 percent. The 

solution also is highly sensitive to the transmissivity of the 

aquifer and thickness of the aquitard, but the potential range 

in these values is less for the Freeport aquifer because there 

is more site-specific information for these variables. Some of 

the unknown variables for the Freeport aquifer, such as the 

storage terms (the specific storage of the aquifer and specific 

yield of the aquitard) had very little effect on the evaluation of 

streamflow depletion, assuming a reasonable range of values for the study area (table 11). Doubling the pumping rate 

doubled streamflow depletion, which would be the expected 

result. However, the overall solution was not very sensitive to 

the distance between the well and stream, in terms of the total 

amount of streamflow depletion. The distance was positively 

related, however, to the amount of time required to reach 

maximum depletion.

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

Month

0

5

10

15

20

25

30

35

Streamflow, in cubic feet per second

90-percent prediction intervals

for Dudley method median flows

Dudley method median flows

MOVE.1 median flows

EXPLANATION

Figure 26. Harvey Brook median flow estimates and 90-percent prediction intervals.
48 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

These results illustrate that conducting a cursory evaluation of streamflow depletion using available information may 

result in a very wide range in results, depending on how well 

the hydraulic conductivity variables and aquifer geometry 

are known. Without using a range in the possible variables, it 

would be impossible to use an analytical solution to give an 

estimate of streamflow depletion at a given location that can 

be known with any degree of certainty. However, the tool can 

be used to help understand which variables are most important 

to refine in order to increase the certainty in the estimate. In 

this case, the analysis shows that the vertical conductivity of 

the confining unit is crucial. This also was confirmed during 

the calibration of the groundwater-flow model.

Simulation of Streamflow Depletion Based on 

the Steady-State Groundwater-Flow Model

The groundwater-flow model was used as an alternate, 

more robust method to evaluate streamflow depletion. Current 

streamflow depletion was calculated as the difference between 

streamflow (discharge to drain cells) with no pumping (no 

simulated withdrawals from the calibrated model) and streamflow with pumping (the base-case calibrated model) (table 12). 

Streamflow depletion was 0.38 ft3/s, or about 8.5 percent of 

the base flow in Harvey Brook, and 0.01 ft3/s in Merrill Brook. 

Streamflow depletion in the uppermost subbasin of the 

Harvey Brook watershed (Harvey Brook site #4) is negligible; 

most of the depletion occurs within the lower three stream 

segments, increasing from 0.23 ft3/s at Harvey Brook site #3 

to 0.38 ft3/s at Harvey Brook site #1. Most of the change in 

streamflow occurs above Harvey Brook site #3, and about half 

that amount occurs between Harvey Brook sites #1 and #2. 

Only a small amount of the total depletion occurs in the upper 

reaches (recharge areas) of the subbasins (fig. 27). Some of the 

effect on pumping occurs outside the measured subbasins, and 

downstream of the measured sites.

The 95-percent confidence intervals on streamflow 

depletion estimates were calculated using UCODE_2005 

(Poeter and others, 2008), and are shown in figure 28. 

Table 11. Sensitivity analysis of STRMDEPL08 streamflow depletion evaluations for the Freeport aquifer, based on reasonable 

ranges for unknown variables.

[ft3/d, cubic foot per day; max, maximum; min, minimum; ft2, square foot per day; d, day; K, hydraulic conductivity; gal/min, gallon per minute]

Variable Reasonable range tested

Streamflow depletion after 

1 year of pumping, 

in ft3/d

Percent difference 

(max-min/min)

Low High Low result High result

Transmissivity of the aquifer, in ft2/d 4,000 13,000 2.83 × 10-4 1.57 × 10-4 45

Specific storage of the aquifer .00001 .001 2.28 × 10-4 2.19 × 10-4 4

Thickness of the aquitard, in ft 30 80 3.63 × 10-4 1.66 × 10-4 54

Vertical K of the aquitard, in ft/d .00028 .005 2.31 × 10-4 1.22 × 10-3 428

Specific yield of the aquitard .01 .1 2.48 × 10-4 2.29 × 10-4 8

Distance from well to stream, in ft 25 1,000 2.32 × 10-4 2.25 × 10-4 3

Pumping rate, in gal/min 178 355 1.18 × 10-4 2.32 × 10-4 97

Table 12. Model-calculated streamflow depletion at each streamflow measurement site.

[ft3/s, cubic foot per second]

Streamflow site

Model-calculated total steadystate streamflow with pumping, 

in ft3/s

Predicted total steady-state 

streamflow without pumping, 

in ft3/s

Incremental streamflow 

depletion (difference), 

in ft3/s

Cumulative streamflow 

depletion, 

in ft3/s

Harvey Brook #4 1.34 1.34 0.00 0.00

Harvey Brook #3 3.39 3.62 .23 .23

Harvey Brook #2 3.67 3.92 .02 .25

Harvey Brook #1 4.21 4.59 .13 .38

Merrill Brook .75 .76 .01 .01
Evaluation of Streamflow Depletion in Harvey Brook 49

Figure 27. Steady-state calculated streamflow depletion in each drain cell in the Freeport aquifer groundwaterflow model.

%

%

Harvey Brook #4

Merrill Brook

Harvey Brook #3

Harvey Brook #2

Harvey Brook #1

EXPLANATION

Drain observation zones

Steady-state streamflow depletion—

In cubic feet per day

< 5.00

5.01 to 15

15.1 to 30

30.1 to 50

50.1 to 100

101 to 150

151 to 250

251 to 475

476 to 850

851 to 1,500

1,510 to 2,400

2,410 to 4,060

Active model area

% Pumping well

96

30

90

40

20

10

60

50

80

70

130

140

100

110

120

148

0

10

30

90

20

60

80

40

50

70

1

Model row

Model column
50 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

These confidence intervals take all the expressly identified 

uncertainty in parameter values and observations into account 

(as previously discussed). For the depletion simulations in the 

two stream segments with the most depletion (Harvey Brook 

#3 and Harvey Brook #1), most of the range in the confidence 

intervals fall above the line of zero streamflow depletion. 

The widest confidence band, for the segment above Harvey 

Brook site #3, reflects the combined inflows to that segment 

from layers 1 and 3, and the combined uncertainty from many 

different parameters that relate to groundwater flow in that 

area. The narrowest confidence band is for the segment above 

Harvey Brook site #2, which reflects the dominance of layer 1 

(and fewer parameters) in the streamflow generation in 

that segment. 

The 95-percent confidence intervals for streamflows and 

streamflow depletion also can be used to help illustrate the 

projected streamflows in Harvey and Merrill Brooks under the 

different pumping and recharge scenarios discussed earlier. 

The steady-state simulated drain discharge (streamflow) 

projected for Harvey and Merrill Brooks is compared to the 

prediction of streamflow with no pumping, with increased 

pumping (by 50 percent), with a 20 percent reduction in 

recharge, and increased pumping combined with decreased 

recharge (fig. 29, table 13). The streamflow in Harvey Brook 

is projected to decrease from 4.21 to 3.18 ft3/s (about a 

25 percent drop) if pumping were increased and recharge 

was somewhat lower than it is currently. Of that reduction, 

some is from increased streamflow depletion, but most of the 

change is caused by the reduction in recharge. The change 

in streamflow for Merrill Brook would be less pronounced 

under that scenario, because it is not affected by pumping 

(although it is affected by recharge as much as Harvey Brook). 

Overall, Harvey Brook streamflow at site #1 could decrease to 

3.37 ft3/s under a drought, without any change in pumping.

Comparison of Methods Used to Evaluate 

Streamflow Depletion

The estimates of streamflow depletion from the groundwater-flow model and from the STRMDEPL08 runs are 

substantially different. The STRMDEPL08 runs, which were 

done using the best available data from the pump tests, drilling 

records, and consultant reports from the water utility, indicated 

that negligible amounts of streamflow depletion are caused by 

the pumping of the water-supply wells in the buried aquifer, 

although the result is very sensitive to the chosen values for 

the hydraulic conductivity variables in the analysis. Conversely, the calibrated groundwater-flow model indicated that 

streamflow depletion in Harvey Brook is about 8.5 percent of 

the natural, unaltered flow. Even when the 95-percent confidence intervals are taken into consideration, it is clear that 

at least some measurable amount of streamflow depletion 

likely occurs, and it may be larger than indicated here. Using 

the STRMDEPL08 tool to evaluate the streamflow depletion 

with an incomplete understanding of the hydrologic system 

gave results that seem unlikely to reflect the actual streamflow 

depletion situation in the Freeport aquifer study area.

Comparison of Streamflow Depletion Estimates 

to Instream Flow Requirements

To conduct a comparison of the instream flow 

requirements to the site-specific data (including the modeled 

streamflow depletion) in Harvey Brook, the summer time 

period (June 1 to August 31) was chosen. As described earlier, 

measurements were made to estimate the median monthly 

flows using the MOVE.1 regression technique. However, with 

the withdrawals being active at the time, the measurements do 

not represent the natural flow, but rather, the flows altered by 

streamflow depletion. Therefore, in order to use site-specific 

Figure 28. Ninety-five-percent 

confidence intervals for simulated 

streamflow depletion caused by pumping 

for the Harvey and Merrill Brook sites.

Streamflow depletion, in cubic feet per second

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Upper limit

Predicted value

Lower limit

EXPLANATION

Harvey Brook 

#4

Harvey Brook 

#3

Harvey Brook 

#2

Harvey Brook 

#1

Merrill 

Brook

Streamflow site
Evaluation of Streamflow Depletion in Harvey Brook 51

Figure 29. Steady-state streamflow projections from the groundwater model for Harvey and Merrill Brooks for pumping and 

recharge scenarios, showing the 95-percent confidence intervals. Streamflow, in cubic feet per second

0

1

2

3

4

5

6

Model 

calibrated 

No 

pumping

Increased 

pumping

Decreased 

recharge

Decreased 

recharge and

increased pumping 

Harvey Brook upper 95-percent confidence limit

Harvey Brook #1

Harvey Brook lower 95-percent confidence limit

Merrill Brook upper 95-percent confidence limit

Merrill Brook

Merrill Brook lower 95-percent confidence limit

EXPLANATION

Streamflow site

Table 13. Model-calculated streamflow and streamflow depletion in Harvey Brook and Merrill Brook under 

various pumping and recharge scenarios.

[ft3/s, cubic foot per second]

Scenario Merrill Brook Harvey Brook #1

Total amount of model-calculated streamflow (in ft3/s)

Model-calculated steady-state with existing pumping 0.75 4.21

Predicted steady-state without pumping .76 4.59

Predicted with pumping increased by 50 percent .75 4.01

Predicted with 20 percent less recharge .63 3.37

Predicted with increased pumping and 20 percent less recharge .63 3.18

Incremental streamflow depletion1

Model-calculated steady-state with existing pumping .01 .38

Predicted with pumping increased by 50 percent .01 .58

Predicted with 20 percent less recharge .13 1.22

Predicted with increased pumping and 20 percent less recharge .13 1.41

1

Difference between no pumping and the other scenarios (in ft3/s).
52 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

measurements to accurately determine monthly flow amounts, 

there must be an independent evaluation of streamflow 

depletion that can be used to adjust the measured flows. (At 

minimum, the total amount of the withdrawal could be added 

to the measured flows for a conservative evaluation of flow.) 

The groundwater-flow model of the Freeport aquifer has 

shown that for the watersheds in the Freeport study area, the 

streamflow depletion is about 0.38 ft3/s for Harvey Brook, 

but essentially zero for Merrill Brook. Figure 30 illustrates 

the instream flow requirements for Harvey Brook at site #1 

and the median monthly flows for June, July, and August 

as determined by Dudley’s (2004) statewide equations. The 

graph also shows the median monthly flows as determined by 

the site-specific measurements (the MOVE.1 median flows), 

and what would be the “natural” flows if pumping were 

not occurring in the aquifer. The “natural” median monthly 

flow for August agrees closely with what is predicted by the 

statewide equations, although the June and July flows are 

underestimated using those equations (fig. 30). As the instream 

flow requirement is set by the August flows, it appears that the 

measured flows in Harvey Brook are less than what would be 

allowed, and would be even lower if pumping were increased 

(fig. 30). From table 13, the difference between the steadystate “natural” flows (without pumping, 4.59 ft3

/s) and the 

flows with pumping for Harvey Brook at site #1 if a drought 

occurred (a 20-percent reduction in recharge, 3.37 ft3/s) is 

1.22 ft3/s, and the predicted amount of depletion with 

increased pumping and a drought together is 1.41 ft3/s. 

Applying this amount of modeled depletion to the measured 

median monthly flows indicates that the resulting streamflow 

could fall well below what the 90-percent confidence limits for 

the statewide equation would predict (fig. 30).

Suggestions for Improving Methods of 

Study for Water Availability

Using the Freeport aquifer study as a pilot for future 

studies of water availability in “watersheds at risk” in Maine 

provides an opportunity to evaluate the procedures used 

herein to study water availability. Improvements to numerical 

groundwater modeling to reduce the uncertainty in the results 

could include (1) conducting a synoptic groundwater-level 

survey in which groundwater levels are measured in as many 

wells as possible during a short time frame (one week or 

less); (2) use a different package in MODFLOW, such as 

the streamflow-routing package (SRF) that allows flow into 

and out of cells, instead of the drain (DRN) package which 

only allows for gaining stream reaches; (3) include a layer 

of shallow bedrock underneath the unconsolidated aquifer 

materials, to decrease numeric instability and allow for a small 

amount of groundwater exchange between the bedrock and 

unconsolidated materials; and (4) divide the principal aquifer 

into additional vertical layers, particularly if there is evidence 

of vertical layering within the sediments. 

Figure 30. Summer streamflows, pumping, instream flow requirements and projections of streamflow depletion in Harvey Brook 

at site #1, Freeport, Maine.

0

1

2

3

4

5

6

Streamflow, in cubic feet per second

June July August

EXPLANATION

Instream flow requirements

Pumping

90-percent upper bound

Dudley method median flows

90-percent lower bound

MOVE.1 median flows

Natural flows

Increased pumping

Increased pumping, decreased recharge 

Month
Summary and Conclusions 53

Summary and Conclusions

In 2009, the U.S. Geological Survey and the Maine 

Geological Survey began a cooperative investigation of the 

Freeport aquifer and two small watersheds (Harvey Brook 

and Merrill Brook), in the towns of Freeport, Pownal, and 

Yarmouth, Maine, to provide the first rigorous evaluation of 

the hydrologic effect of withdrawals in watersheds at risk of 

overdevelopment in the state. 

The Freeport aquifer (sometimes referred to as the 

Harvey Brook aquifer) is a productive, deep, confined sand 

and gravel deposit in a buried valley that trends north-south 

under the Harvey and Merrill Brook watersheds. The aquifer 

is used as a water supply for the AquaMaine Freeport Division 

water utility. Above the Freeport aquifer, a silt and clay layer 

(known as the Presumpscot Formation) forms a discontinuous 

confining unit below an upper saturated zone consisting of 

a mixture of sandy units, till, and weathered clay. Harvey 

Brook and Merrill Brook flow in valleys incised into the 

upper surficial materials. Merrill Brook flows in a watershed 

dominated by fine-grained soils (till, silt, and clay). The 

Harvey Brook watershed contains a mixture of sandy units and 

fine-grained soils. 

Water-level measurements from shallow, unconfined 

units and water-level measurements from the Freeport aquifer 

were collected sporadically from the 1970s to 2009. Water 

levels in the shallow units generally follow the topography of 

the study area, and are highest in the upper sandy units and on 

the hill underlain by thin drift on the eastern side of the study 

area. Shallow groundwater in the sandy surficial units has 

been observed to flow towards the streams, especially Harvey 

Brook. The Presumpscot Formation, which is composed of silt 

and clay, separates the shallow units from the underlying 

Freeport aquifer. Wells screened in the confined aquifer had 

water levels that were often above land surface before development. Pumping by the water utility created a cone of depression within about a half-mile radius of the pumping wells. 

Streamflow was measured at five locations on Harvey 

and Merrill Brooks from May to September 2009. Estimates 

of long-term monthly flows in both brooks by record extension 

were based on 13 to 16 streamflow measurements. 

Water-utility withdrawals from the study area 

amount to about 106 Mgal/yr; domestic well withdrawals, 

about 15 Mgal/yr; and industrial withdrawals, about 

2.5 Mgal/yr. Commercial withdrawals are less than 1 Mgal/yr, 

and no irrigation or other agricultural withdrawals were identified in the study area. 

A steady-state groundwater-flow model of the Freeport 

aquifer system was constructed to help understand groundwater and surface-water interactions. Calibration targets included 

water-level data and the estimated average June to August 

streamflows in Harvey and Merrill Brooks. 

The model was discretized into 3 layers, all above the 

bedrock surface, 148 rows, and 96 columns. The layers 

included an upper saturated zone consisting of a mix of sandy 

units, till, and weathered clay; a second layer that includes 

the confining silt and clay of the Presumpscot Formation; and 

a bottom layer representing the confined Freeport aquifer. 

Lateral model boundaries were based on the topography of 

the bedrock surface, the land-surface topography, and previous groundwater models of the study area. No-flow boundaries were established as the base of the model and along most 

edges, with the exception of areas on the north and south 

end in layer 3 (the bottom layer), which were modeled as 

head-dependent flux boundaries. All layers in the model were 

simulated as being confined. Streams were simulated by use of 

drain cells. The stresses applied to the model include recharge 

and pumping. Recharge was applied as a constant flux to the 

top active cell in the model. Distributed recharge rates ranged 

from 0.75 in/yr in the unweathered clay of the Presumpscot 

Formation, to 25 in/yr in several areas of sandy soils. One 

zone, an area of focused recharge from runoff from upland 

areas, was assigned a recharge rate of 50 in/yr.

Pumping was simulated for two wells in layer 3. Average 

pumping rates assigned were 191 and 18 gal/min. Evapotranspiration was not modeled. 

Hydraulic properties in the model were based on surficial 

geology, soil properties, drilling records, and pumping tests. 

The model was calibrated using a combination of parameter estimation and trial-and-error adjustments to the model 

variables. The assigned horizontal hydraulic conductivity 

values ranged from 3.3 × 10-5 ft/d in the layer 2 confining unit 

to 167 ft/d in layer 3. The vertical hydraulic conductivities 

ranged from 3 × 10-6 to 16.4 ft/d. 

The mean absolute difference between the observed and 

simulated heads was 6.7 ft in layer 1 and 3.7 ft in layer 3. 

The fit of simulated streamflow to corresponding data was 

very good. 

The construction and calibration of the numerical groundwater model of the Freeport aquifer area provided significant 

insights into the flow of water within the groundwater system and the interaction of the groundwater system with the 

streams. In particular, the process resulted in the conclusion 

that previously unidentified discontinuities or vertical fractures 

in the confining unit allow groundwater from the confined 

aquifer to discharge upward into Harvey Brook.

Simulated average annual water budgets for various 

components of the Freeport aquifer study area were calculated 

using the MODFLOW supplemental software ZONEBUDGET. The program was used to calculate inflows and outflows 

to 3 subareas of the model: the Harvey Brook watershed area 

(Subarea 1), the area of the Merrill Brook watershed within 

the model domain (Subarea 2), and the buried valley aquifer 

(layer 3; Subarea 3). 

Inflows to the Harvey Brook watershed (Subarea 1) 

include distributed recharge from precipitation (68 percent of 

the total), upward seepage of groundwater from layer 3, which 

is directly discharged to the streams (27 percent), horizontal 

subsurface flow from the Merrill Brook watershed (4 percent) 

and other upland areas (1 percent). Outflows from the Harvey 

Brook watershed are to streams (78 percent) and infiltration downward to layers 2 and 3 (22 percent). Streamflow 
54 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

generated within the Harvey Brook watershed (2.7 Mgal/d) 

consists of shallow-generated streamflow above the clay 

(1.8 Mgal/d) and the upward seepage beneath the clay 

(0.9 Mgal/d).

All inflows to the Merrill Brook watershed (Subarea 2) 

come from distributed recharge; outflows include discharge to 

streams (64 percent), downward infiltration to layer 3 (16 percent), horizontal subsurface flow to the Harvey Brook watershed (18 percent), and horizontal subsurface flow to other 

adjacent uplands (2 percent). 

The buried valley aquifer (Subarea 3) gets most of 

its recharge from seepage downward through layer 2, and 

98 percent of that flow occurs within the till zone of layer 2 

(primarily within the Harvey Brook watershed). The remaining 

inflow to layer 3 is head-dependent boundary flux. Outflows 

from layer 3 include withdrawals (20 percent), discharge to 

streams in layer 1 (74 percent), and head-dependent boundary 

flux (7 percent).

The groundwater model was further used to evaluate 

changes that could occur in the water budgets of these three 

subareas under several climatic and pumping scenarios. 

The scenarios were: (1) no pumping from the water-supply 

wells; (2) current pumping, but simulated drought conditions 

(20 percent reduction in recharge); (3) current recharge, but a 

50-percent increase in pumping from the water-supply wells; 

and (4) drought conditions and increased pumping combined.

The changes to the water budget that result from removing the pumping well (0.3 Mgal/d) primarily effect the movement of water downwards and upwards between the watersheds in layer 1 and the confined aquifer in layer 3. Under the 

Harvey Brook stream, the head in layer 3 is increased relative 

to the head in layer 1, and additional flow upwards to the 

stream occurs (0.22 Mgal/d, a 23 percent increase).

Simulated changes to the system from the drought 

scenario also affect the movement of water across the 

confining layer by reducing vertical recharge to layer 3 by 

about 15 percent. The total streamflow discharge is reduced 

by 0.71 Mgal/d (19 percent), most of which comes from 

a reduction in the streamflow (discharge to drain cells) 

originating from layer 1 (not from flow upward from layer 3).

When the recharge was left alone and pumping increased 

by 50 percent (by 0.15 Mgal/d), the simulated changes are 

primarily in the amount of streamflow in Harvey Brook that 

is generated by upward flow from layer 3 (a decrease of about 

0.11 Mgal/d, or 12 percent).

A simulated decrease in recharge combined with 

increased pumping resulted in a reduction in recharge to 

layer 3 (0.76 Mgal/d), a reduction in streamflow of about 

23 percent, and an increase in flow of 0.04 Mgal/d from the 

northern specified head boundary. 

The simulation of the movement of water between 

different parts of the modeled hydrologic system illustrates 

the complexity of hydrologic systems in Maine. Although the 

base-case total water withdrawals from the system are only 

7.2 percent of the entire water budget of the model, those 

withdrawals do affect the hydrology of the system as a whole, 

as well the rates of flow in many areas. Adding potential 

drought conditions to the simulations substantially altered both 

the overall water budget and the movement of water within the 

system, between layers and across the confining unit.

This study is intended to provide insight into the effect of 

withdrawals on streamflows under a certain set of conditions 

(that is, the aquifer geometry presented by the Freeport aquifer 

study area), and to help understand streamflow depletion in 

light of the State requirements to maintain instream flows for 

habitat protection. The analysis illustrates the effect of streamflow depletion on natural flows in the Freeport study area, in 

the context of the State instream flow requirements, and also 

illustrates the use and limitations of two different methods of 

evaluating streamflow depletion.

The instream flow requirements (seasonal aquatic base 

flows) for the State of Maine are designed to protect the 

aquatic health of riverine ecosystems, and vary by season. The 

most stringent of these requirements applies to the summer 

period from July 1 through September 15, when the instreamflow-requirement level equals the August median flow. 

Site-specific streamflow data from five stations were used to 

estimate median monthly flows in Merrill and Harvey Brooks 

by use of the MOVE.1 algorithm. These measured median 

monthly flows agree fairly well with estimates from statewide 

equations during the summer months, but the statewide 

equations significantly overpredict streamflows in the spring 

(thereby overpredicting the amount of streamflow necessary 

to protect habitat). The PRESS statistic was used to calculate 

90-percent prediction intervals for the Harvey Brook median 

flows estimated with the statewide equations. The measured 

monthly flows fall well within the 90-percent prediction 

intervals for the summer months, are just at the lower edge 

of the prediction interval for the winter months, and fall well 

below the 90-percent prediction interval for March and April. 

These results indicate the benefit of using measured data for 

predicting flow thresholds, as it avoids potentially under- or 

over-estimating the natural instream flows.

Several methods exist for the evaluation of potential 

streamflow depletion, and two of them were applied in this 

study: an analytical solution and the groundwater-flow model. 

The analytical solution, STRMDEPL08, was used to evaluate streamflow depletion caused by withdrawal from a leaky 

aquifer underlying an aquitard, where the stream partially 

penetrates the aquitard but not the aquifer below. Projected 

streamflow depletion after 365 days of pumping was estimated 

to be 2.3 × 10-4 ft3/s, which would not be detectable using any 

available measuring method. 

A sensitivity analysis indicated that the transmissivity 

of the aquifer, the thickness of the aquitard, and the vertical 

conductivity of the aquitard have the greatest effect on stream 

depletion by this method. The results of the sensitivity analysis 

illustrate that conducting a cursory evaluation of streamflow 

depletion using available information may result in a very 

wide range in results, depending on how well the hydraulic 

conductivity variables and aquifer geometry are known. Using 

the STRMDEPL08 tool to evaluate the streamflow depletion 
References Cited 55

with an incomplete understanding of the hydrologic system 

yielded results that seem unlikely to reflect the actual streamflow depletion situation in the Freeport aquifer study area.

The groundwater-flow model was used as another, more 

robust method to evaluate the amount of streamflow depletion that results from withdrawals in the buried aquifer. The 

pumping well was turned off in the base-case model, and the 

resulting streamflow (without pumping) for each measurement 

site was compared to the base-case streamflow (with pumping), the difference being streamflow depletion. Depletion was 

0.38 ft3/s in Harvey Brook (8.5 percent) and 0.01 ft3/s in 

Merrill Brook (unmeasureable). 

Although results from the two simulations are widely 

different, it is clear that at least some measurable amount of 

streamflow depletion occurs. Because of the sensitivity of 

streamflow depletion amounts on model parameters that were 

estimated and had wide ranges, depletion may be larger than 

calculated by these two methods. If pumping were increased 

and a drought occurred (simulated by a 20-percent reduction 

in recharge), the total amount of streamflow in Harvey Brook 

could decrease by 1.41 ft3/s. 

The process of evaluating water use, instream flow 

requirements, and streamflow depletion in the Freeport aquifer 

study area as a pilot study area has shown some areas in 

which future studies could be improved. These areas include 

(1) measuring water levels in wells during a synoptic waterlevel survey to avoid seasonal differences in water-level 

measurements used as model calibration targets, (2) using 

the streamflow routing (SRF) package in MODFLOW, 

instead of the drain (DRN) package, which would allow 

for losing stream reaches, (3) including one or more layers 

of shallow bedrock underneath the unconsolidated aquifer 

materials to decrease numeric instability and allow for a small 

amount of groundwater exchange between the bedrock and 

unconsolidated materials; and (4) divide the principal aquifer 

into additional vertical layers, particularly if there is evidence 

of vertical layering within the sediments.

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156 p.

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Suneast development: Portland, Maine, Maine Water 

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Quadrangle, Maine: Geological Survey Open-File 

no. 99–93, Augusta, Maine, scale 1:24,000.

Morrissey, D.J., 1983, Hydrology of the Little Androscoggin 

River Valley aquifer, Oxford County, Maine: U.S. 

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Survey Techniques and Methods 6–A11, revised February 

2008, 283 p.

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variously paged.

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calculate streamflow depletion by nearby pumping wells: 

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Quadrangle, Maine: Geological Survey Open-File 

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Quadrangle, Maine: Geological Survey Open-File 

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at Webster Road wellfield in Freeport: Freeport, Maine, 

Richard T. Reynolds, P.E., variously paged.

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U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 4, chap. A1, 39 p. 

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Cumberland Center, Maine, Sevee & Maher Engineers, Inc., 

variously paged.

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THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 1 59

Appendix 1. List of Wells and Observations 

Used in the Freeport Aquifer Study

Tables

1–1. Wells used in Freeport aquifer study, for water levels, bedrock surface altitude, or 

other stratigraphic data.............................................................................................................60

1–2. Steady-state model calculated and observed average annual water levels and 

measured water levels in wells, borings, seismic profiles, and estimated water-level 

points in the Freeport aquifer groundwater model ...............................................................64
60 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Table 1–1. Wells used in Freeport aquifer study, for water levels, bedrock surface altitude, or other 

stratigraphic data.—Continued

[Wells used for water-level data are shown in figure 14 and table 6, and are cross referenced by map number. “C”-number 

refers to the USGS site-file component number. Site types: GW-TH, groundwater test hole; GW, groundwater; SP, spring; 

--, not shown on map]

Project well name Map number Site type (C802) USGS site name (C12)

LLB_FP 1 GW-TH ME-CW 2126—LLB FP boring

LLB_RR Tunnel boring 2 GW-TH ME-CW 2072—LLB RR tunnel boring

Well_GolDUG 3 SP ME-CW 2149—spring

Well_MGS_Dug1 4 GW ME-CW 2177

Well_RogDUG 5 GW ME-CW 2091

Merill 1 6 GW ME-CW 2172—Merrill Rd. well

MGS S&Ga 7 GW ME-CW 2169

MGS S&Gb 8 GW ME-CW 2174

RGGI 85-2 9 GW-TH ME-CW 2142—RGGI boring 83-1

MW 01-1 10 GW ME-CW 2080—MW 01-1

MW TOW 83-1 11 GW ME-CW 2110—MW TOW 83-1

MW 83-3A 12 GW ME-CW 2103—MW 83-3A

MW 83-3B 13 GW ME-CW 2098—MW 83-3B

MW 83-3C 14 GW ME-CW 2107—MW 83-3C

MW 89-7 15 GW ME-CW 2094—MW 89-7

MW 89-8 16 GW ME-CW 2117—MW 89-8

MW 89-9 17 GW ME-CW 2124—MW 89-9

MW 91-1 18 GW ME-CW 2097—MW 91-1

MW 91-2 19 GW ME-CW 2099—MW 91-2

MW 91-3 20 GW ME-CW 2122—MW 91-3

Boring 95-1 21 GW ME-CW 2111—Boring 95-1

MW 95-2 22 GW ME-CW 2115—MW 95-2

MW 95-3 23 GW ME-CW 2101—MW 95-3

MW 95-4 24 GW ME-CW 2116—MW 95-4

MW 96-1 25 GW ME-CW 2105—MW 96-1

Suneast T-6 26 GW ME-CW 2062—Suneast TW T-6

Towns 73WR 27 GW ME-CW 2114

Yarmouth LF SB90 A1 GW-TH ME-CW 2051—Yarmouth LF SB90

Municip PW 88-1 A10 GW ME-CW 2096—PW 88-1

PW 03-1 A11 GW ME-CW 2102—PW 03-1

Dug well MGS map A2 GW ME-CW 2180

Dr S&G A4 GW ME-CW 2168

Elmwood 1 A6 GW ME-CW 2190—Elmwood Rd. well

Dr S&G A7 GW ME-CW 2189

Test Boring A8 GW-TH ME-CW 2157

MGS-07532 -- GW ME-CW 2049—MGS-07532

MGS-62871 -- GW ME-CW 2050—MGS-62871

MGS-84316 -- GW ME-CW 2052—MGS-84316

MGS-91490 -- GW ME-CW 2053—MGS-91490

MGS-04874 -- GW ME-CW 2054—MGS-04874
Appendix 1 61

Table 1–1. Wells used in Freeport aquifer study, for water levels, bedrock surface altitude, or other 

stratigraphic data.—Continued

[Wells used for water-level data are shown in figure 14 and table 6, and are cross referenced by map number. “C”-number 

refers to the USGS site-file component number. Site types: GW-TH, groundwater test hole; GW, groundwater; SP, spring; 

--, not shown on map]

Project well name Map number Site type (C802) USGS site name (C12)

MGS-64767 -- GW ME-CW 2055—MGS-64767

MGS-09224 -- GW ME-CW 2056—MGS-09224

MGS-60059 -- GW ME-CW 2057—MGS-60059

MGS-04871 -- GW ME-CW 2058—MGS-04871

MGS-06624 -- GW ME-CW 2059—MGS-06624

MGS-08087 -- GW ME-CW 2060—MGS-08087

MGS-55577 -- GW ME-CW 2061—MGS-55577

MGS-51612 -- GW ME-CW 2063—MGS-51612

MGS-06623 -- GW ME-CW 2064—MGS-06623

MGS-73687 -- GW ME-CW 2065—MGS-73687

MGS-10240 -- GW ME-CW 2066—MGS-10240

MGS-55513 -- GW ME-CW 2067—MGS-55513

MGS-07565 -- GW ME-CW 2068—MGS-07565

MGS-55440 -- GW ME-CW 2069—MGS-55440

MGS-62149 -- GW ME-CW 2070—MGS-62149

MGS-65380 -- GW ME-CW 2071—MGS-65380

BR -- GW ME-CW 2073

MGS-62146 -- GW ME-CW 2074—MGS-62146

MGS-66301 -- GW ME-CW 2075—MGS-66301

MGS-71795 -- GW ME-CW 2076—MGS-71795

MGS-62145 -- GW ME-CW 2077—MGS-62145

MGS-60538 -- GW ME-CW 2078—MGS-60538

MGS-06044 -- GW ME-CW 2079—MGS-06044

MGS-76243 -- GW ME-CW 2081—MGS-76243

MGS-56491 -- GW ME-CW 2082—MGS-56491

MGS-08630 -- GW ME-CW 2083—MGS-08630

MGS-122361 -- GW ME-CW 2084—MGS-122361

BR -- GW ME-CW 2085

MGS-71719 -- GW ME-CW 2086—MGS-71719

MGS-55621 -- GW ME-CW 2087—MGS-55621

MGS-07497 -- GW ME-CW 2088—MGS-07497

MGS-07400 -- GW ME-CW 2089—MGS-07400

MGS-64771 -- GW ME-CW 2090—MGS-64771

LLBean B115 -- GW-TH ME-CW 2092—LLB B115

MGS-51536 -- GW ME-CW 2093—MGS-51536

LLBean B110 -- GW-TH ME-CW 2095—LLB B110

MGS-07399 -- GW ME-CW 2100—MGS-07399

Sutton BR -- GW ME-CW 2104

TW 8-in (97-1) -- GW ME-CW 2106—TW 97-1

MGS-55630 -- GW ME-CW 2108—MGS-55630
62 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Table 1–1. Wells used in Freeport aquifer study, for water levels, bedrock surface altitude, or other 

stratigraphic data.—Continued

[Wells used for water-level data are shown in figure 14 and table 6, and are cross referenced by map number. “C”-number 

refers to the USGS site-file component number. Site types: GW-TH, groundwater test hole; GW, groundwater; SP, spring; 

--, not shown on map]

Project well name Map number Site type (C802) USGS site name (C12)

LLBean B104 -- GW-TH ME-CW 2109—LLB B104

LLBean B101 -- GW-TH ME-CW 2112—LLB B101

MW 83-2 -- GW ME-CW 2113—MW 83-2

MGS-07292 -- GW ME-CW 2118—MGS-07292

MGS-08466 -- GW ME-CW 2119—MGS-08466

MGS-07724 -- GW ME-CW 2120—MGS-07724

Batch 90WR -- GW ME-CW 2121

MGS-78537 -- GW ME-CW 2123—MGS-78537

MGS-84441 -- GW ME-CW 2125—MGS-84441

Weeks 89WR -- GW ME-CW 2127

MGS-08976 -- GW ME-CW 2128—MGS-08976

BR -- GW ME-CW 2129

Washburn -- GW ME-CW 2130

MGS-52271 -- GW ME-CW 2131—MGS-52271

MGS-08743 -- GW ME-CW 2132—MGS-08743

MGS-53568 -- GW ME-CW 2133—MGS-53568

MGS-100343 -- GW ME-CW 2134—MGS-100343

BR -- GW ME-CW 2135

BR -- GW ME-CW 2136

MGS-100342 -- GW ME-CW 2137—MGS-100342

MGS-07398 -- GW ME-CW 2138—MGS-07398

MGS-128657 -- GW ME-CW 2139—MGS-128657

MGS-101075 -- GW ME-CW 2140—MGS-101075

MGS-07513 -- GW ME-CW 2141—MGS-07513

MGS-60016 -- GW ME-CW 2143—MGS-60016

MGS-05053 -- GW ME-CW 2144—MGS-05053

MGS-61559 -- GW ME-CW 2145—MGS-61559

MGS-09932 -- GW ME-CW 2146—MGS-09932

MGS-67014 -- GW ME-CW 2147—MGS-67014

MGS-61812 -- GW ME-CW 2148—MGS-61812

MGS-06635 -- GW ME-CW 2150—MGS-06635

MGS-10001 -- GW ME-CW 2151—MGS-10001

BR -- GW ME-CW 2152

MGS-60046 -- GW ME-CW 2153—MGS-60046

BR -- GW ME-CW 2154

MGS-80936 -- GW ME-CW 2155—MGS-80936

BR -- GW ME-CW 2156

MGS-58053 -- GW ME-CW 2158—MGS-58053

MGS-81983 -- GW ME-CW 2159—MGS-81983

MGS-99680 -- GW ME-CW 2160—MGS-99680
Appendix 1 63

Table 1–1. Wells used in Freeport aquifer study, for water levels, bedrock surface altitude, or other 

stratigraphic data.—Continued

[Wells used for water-level data are shown in figure 14 and table 6, and are cross referenced by map number. “C”-number 

refers to the USGS site-file component number. Site types: GW-TH, groundwater test hole; GW, groundwater; SP, spring; 

--, not shown on map]

Project well name Map number Site type (C802) USGS site name (C12)

MGS-103038 -- GW ME-CW 2161—MGS-103038

MGS-67043 -- GW ME-CW 2162—MGS-67043

MGS-08261 -- GW ME-CW 2163—MGS-08261

MGS-78105 -- GW ME-CW 2164—MGS-78105

MGS-08624 -- GW ME-CW 2165—MGS-08624

MGS-08114 -- GW ME-CW 2166—MGS-08114

MGS-58368 -- GW ME-CW 2167—MGS-58368

MGS-61081 -- GW ME-CW 2170—MGS-61081

MGS-67572 -- GW ME-CW 2171—MGS-67572

MGS-78104 -- GW ME-CW 2173—MGS-78104

MGS-08763 -- GW ME-CW 2175—MGS-08763

MGS-80412 -- GW ME-CW 2176—MGS-80412

Dug -- GW ME-CW 2178

MGS-82000 -- GW ME-CW 2179—MGS-82000

MGS-84486 -- GW ME-CW 2181—MGS-84486

BR -- GW ME-CW 2182

MGS-08858 -- GW ME-CW 2183—MGS-08858

MGS-08213 -- GW ME-CW 2184—MGS-08213

BR -- GW ME-CW 2185

MGS-08076 -- GW ME-CW 2186—MGS-08076

MGS-87240 -- GW ME-CW 2187—MGS-87240

BR -- GW ME-CW 2188

MGS-09209 -- GW ME-CW 2191—MGS-09209

MGS-09928 -- GW ME-CW 2192—MGS-09928

MGS-66959 -- GW ME-CW 2193—MGS-66959

MGS-08357 -- GW ME-CW 2194—MGS-08357

MGS-07563 -- GW ME-CW 2195—MGS-07563

Dr S&G -- GW ME-CW 2196

MGS-07514 -- GW ME-CW 2197—MGS-07514

Dug -- GW ME-CW 2198

MGS-08358 -- GW ME-CW 2199—MGS-08358

MGS-110758 -- GW ME-CW 2200—MGS-110758

MGS-98995 -- GW ME-CW 2201—MGS-98995

MGS-58390 -- GW ME-CW 2202—MGS-58390

MGS-81960 -- GW ME-CW 2203—MGS-81960

MGS-05933 -- GW ME-CW 2204—MGS-05933

MGS-05945 -- GW ME-CW 2205—MGS-05945

BR -- GW ME-CW 2206
64 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Table 1–2. Steady-state model calculated and observed average annual water levels and measured water levels in wells, borings, 

seismic profiles, and estimated water-level points in the Freeport aquifer groundwater model.—Continued

[NAVD 88, North American Vertical Datum of 1988; ft, foot]

Well identifier Group

Observation 

number

Model location

Model 

calculated 

water level 

(in feet above 

NAVD 88)

Measured water level

Difference 

(model 

minus 

observed)

Layer Row Column

Measured 

or average 

water level 

(in feet above 

NAVD 88)

90 percent 

confidence 

interval, 

in ft

Boring_LLB_FP Layer1_So 1 1 88 72 81.8 83.0 5.2 -1.2

Boring_LLB_RR Layer1_So 2 1 109 52 24.3 7.0 7.3 17.3

Well_GdrupDUG Layer1_North 3 1 61 63 98.3 107.0 5.2 -8.7

Well_MGS_Dug1 Layer1_North 4 1 26 82 159.1 154.0 6.5 5.1

Well_RogDUG Layer1_So 5 1 88 44 55.5 63.5 5.2 -8.0

Well_MGS_L3New Layer3 6 3 22 65 92.4 92.0 7.4 0.4

Well_MGS_SG_a Layer3 7 2 42 94 126.3 116.5 8.4 9.9

Well_ MGS_SG_b Layer3 8 2 35 88 123.6 124.7 8.4 -1.1

Boring_85_2 Layer3 9 3 56 47 59.0 36.1 8.4 22.9

Well_01_1 Layer3 10 3 92 41 21.4 29.0 2.5 -7.6

Well_83_1 Layer3 11 3 81 46 25.3 24.1 1.3 1.2

Well_83_3A Layer3 12 3 80 42 22.4 21.8 5.1 0.7

Well_83_3B Layer3 13 3 80 42 22.4 17.8 7.2 4.6

Well_83_3C Layer3 14 3 79 42 23.6 22.8 5.2 0.7

Well_89_7 Layer3 15 3 80 40 26.1 24.7 2.6 1.4

Well_89_8 Layer3 16 3 77 43 26.9 29.2 3.4 -2.3

Well_89_9 Layer3 17 3 73 43 33.6 30.3 3.2 3.3

Well_91_1 Layer3 18 3 80 40 24.9 31.8 3.5 -6.9

Well_91_2 Layer3 19 3 80 40 25.3 31.6 3.1 -6.3

Well_91_3 Layer3 20 3 74 41 33.7 33.9 2.9 -0.2

well_95_1 Layer3 21 3 78 42 25.1 25.1 3.2 0.1

Well_95_2 Layer3 22 3 77 42 27.5 30.6 3.2 -3.1

Well_95_3 Layer3 23 3 81 43 18.8 19.3 2.2 -0.5

Well_95_4 Layer3 24 3 78 44 26.0 24.3 1.2 1.7

Well_96_1 Layer3 25 3 80 43 16.3 16.2 2.1 0.1

Well_Suneast_T6 Layer3 26 3 115 37 9.2 10.6 6.5 -1.3

Well_TownsBR Layer3 27 3 80 47 25.8 20.3 5.2 5.4

Seis_NPW8 Layer1_North 28 1 9 58 128.1 128.0 5.2 0.2

Seis_YAR1 Layer1_North 29 1 53 60 104.9 100.0 5.2 4.9

Seis_YAR13 Layer1_North 30 1 89 46 55.0 46.0 5.2 9.0
Appendix 1 65

Table 1–2. Steady-state model calculated and observed average annual water levels and measured water levels in wells, borings, 

seismic profiles, and estimated water-level points in the Freeport aquifer groundwater model.—Continued

[NAVD 88, North American Vertical Datum of 1988; ft, foot]

Well identifier Group

Observation 

number

Model location

Model 

calculated 

water level 

(in feet above 

NAVD 88)

Measured water level

Difference 

(model 

minus 

observed)

Layer Row Column

Measured 

or average 

water level 

(in feet above 

NAVD 88)

90 percent 

confidence 

interval, 

in ft

Seis_YMH1 Layer1_North 31 1 70 48 56.0 49.0 5.2 7.0

Seis_YMH15 Layer1_So 32 1 101 44 44.0 45.0 5.2 -1.0

Seis_YMH16 Layer1_So 33 1 103 46 43.2 43.0 5.2 0.1

Seis_YMH17 Layer1_So 34 1 97 47 48.0 52.0 5.2 -4.1

Seis_YMH18 Layer1_So 35 1 100 46 45.9 46.0 5.2 -0.1

Seis_YMH19 Layer1_So 36 1 98 45 47.2 52.0 5.2 -4.8

Seis_YMH2 Layer1_North 37 1 67 48 78.5 88.0 5.2 -9.5

Seis_YMH20 Layer1_So 38 1 92 47 51.9 54.0 5.2 -2.1

Seis_YMH3 Layer1_North 39 1 65 57 68.0 58.0 5.2 10.0

Seis_YMH4 Layer1_North 40 1 49 78 137.7 151.0 5.2 -13.3

Seis_YMH5 Layer1_North 41 1 55 62 107.0 119.0 5.2 -12.0

Seis_YMH7 Layer1_North 42 1 32 71 120.3 119.0 5.2 1.3

Seis_YMH8 Layer1_North 43 1 55 50 98.1 87.0 5.2 11.1

Syn_Till1 Estimated 44 1 114 20 35.3 39.7 8.4 -4.4

Syn_Till10 Estimated 45 1 9 42 118.3 137.1 8.4 -18.9

Syn_Till11 Estimated 46 1 63 81 190.9 200.1 8.4 -9.2

Syn_Till2 Estimated 47 1 136 35 48.2 54.8 8.4 -6.5

Syn_Till3 Estimated 48 1 141 26 55.8 59.4 8.4 -3.6

Syn_Till4 Estimated 49 1 122 14 55.8 56.4 8.4 -0.7

Syn_Till5 Estimated 50 2 137 11 29.9 25.6 8.4 4.3

Syn_Till6 Estimated 51 1 114 36 27.8 31.0 8.4 -3.2

Syn_Till7 Estimated 52 2 99 71 80.4 76.8 8.4 3.6

Syn_Till8 Estimated 53 1 82 57 74.7 67.9 8.4 6.8

Syn_Till9 Estimated 54 1 75 73 159.0 135.2 8.4 23.8
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 2 67

Appendix 2. Details of Groundwater Model 

Calibration

Parameter Estimation and Model Calibration................................................................................68

Model Sensitivity Analysis ................................................................................................................71

Tables

2–1. Hydrogeologic units and corresponding model parameters for hydraulic properties, 

with their calibrated hydraulic conductivities .......................................................................68

2–2. Streambed hydraulic conductivity model parameters and calibrated hydraulic 

properties.....................................................................................................................................69

2–3. Recharge model parameters and calibrated recharge values...........................................69

2–4. Model parameters and final composite scaled sensitivities and correlations used to 

determine which parameters to estimate using parameter estimation............................70

2–5. Dimensionless scaled sensitivities for observations, by parameter, for head 

observation groups as a measure of model sensitivity in the Freeport aquifer 

groundwater flow model, Freeport, Maine.............................................................................72

2–6. Dimensionless scaled sensitivities for observations, by parameter, for streamflow 

observations as a measure of model sensitivity in the Freeport aquifer groundwater 

flow model, Freeport, Maine.....................................................................................................72
68 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

This appendix presents additional details about the calibration and sensitivity analysis of the Freeport aquifer groundwater flow model. Details about the parameters and their 

final values and sensitivities are presented, along with a table 

showing the complete list of simulated compared to observed 

values for the observations. In addition, detailed information 

on the sensitivity of the model, including the streamflow simulations, to the various parameters is discussed. 

Parameter Estimation and Model Calibration

The groundwater flow model of the Freeport aquifer was 

calibrated using a combination of parameter estimation and 

trial-and-error adjustments to the model variables. Methods 

outlined in Hill and Tiedeman (2007) were followed during 

calibration, which uses a regression method as programmed 

in the UCODE_2005 software package (Poeter and others, 

2008). The regression method compares alternative parameter 

values to the model fit, defined as the best match between 

observations and their simulated equivalents. The match is 

quantified using an objective function (Hill and Tiedeman, 

2007; Poeter and others, 2008), and the software tests many 

variations of parameter values to minimize the overall objective function. This method allows for (1) the explicit accounting of uncertainty in both the water levels and streamflows 

used as calibration targets, (2) the documentation of model 

sensitivity to model variables and to data used in the model. 

During the prediction phase, this method assists the user in 

documenting the uncertainty in the model predictions. 

Model variables including hydraulic properties (horizontal and vertical hydraulic conductivity, table 2–1), streambed 

conductance (table 2–2), and recharge (table 2–3) were used 

as parameters in the model. Head and streamflow measurements were used for the calibration targets, or observations. 

Statistics indicating the fit of the model to the observed values 

(the ability of the model to reproduce the observations) were 

used as the dependent variables in the parameter estimation, 

whereas the model parameters are treated as independent 

variables through a series of linear and nonlinear regression 

calculations. The optimization of the parameter values was 

conducted through an iterative process in which the output of 

each iteration was used to determine which parameters could 

be estimated and whether trial-and-error changes to other 

parameters, or changes to the conceptual model were needed. 

Appendix 2. Details of Groundwater Model Calibration

Table 2–1. Hydrogeologic units and corresponding model parameters for hydraulic properties, with their calibrated hydraulic 

conductivities.

[Model parameters shown in bold were set by parameter estimation. Kv, vertical hydraulic conductivity; Kh, horizontal hydraulic conductivity; ft/d, foot per 

day; Fm., Formation; NA, not applicable]

Unit description Layer Kh

 parameter 

name

Calibrated 

Kh, 

in ft/d

Kv parameter name

Calibrated 

Kv, 

in ft/d

Surficial sandy deposits above Presumpscot Fm. 1 HK_Lay1_A 13.6 VK_Sandy 3.28

Surficial sandy deposits, eolian 1 HK_Lay1_A 13.6 VK_Sandy 3.28

Till and weathered Presumpscot Fm., undivided 1 HK_Lay1_C .69 VK_Till 1.64

Weathered Presumpscot Fm. silt/clay 1 HK_Lay1_CD 10.7 VK_Till 1.64

Unweathered Presumpscot Fm. silt/clay 1 HK_Lay1_D .56 VK_ClayL1 .16

Holocene alluvium and fine sand 1 HK_Lay1_B1 1.6 VK_Sandy 3.28

Till and weathered Presumpscot Fm., undivided 2 HK_Lay2_TL 9.0 VK_Till2 .98

Unweathered Presumpscot Fm. silt/clay 2 HK_Lay2_CL .00003 VK_Clay_L2 .000003

Vertical flow zones in Presumpscot Fm. silt/clay 2 NA NA VK_C_FracN, VK_C_FracS 9.84, 2.46

Fine sand, buried aquifer 3 HK_Lay3_Sa 18.1 VK_Lay3 16.4

Coarse sand unit A 3 HK_Lay3_K2 150 VK_Lay3 16.4

Coarse sand unit B 3 HK_Lay3_K3 167 VK_Lay3 16.4

General head boundary, north 3 GHB_ParN 241 NA NA

General head boundary, south 3 GHB_ParS 241 NA NA

1

The model was sensitive to this parameter, but it was highly correlated with a recharge parameter, and was set by hand.
Appendix 2 69

During the optimization process, the sensitivity of the model 

to each parameter value was determined for each optimization 

iteration, and only parameters that exceeded a threshold sensitivity (and were not highly correlated with each other) were 

estimated. Insensitive or highly correlated parameters were 

set and adjusted manually using a trial and error process. Hill 

and Tiedeman (2007) provide further details on the process of 

parameter estimation.

Twelve of the 33 parameters set up for the model were 

determined by parameter estimation, and the others were set 

manually (table 2–4). The final set of parameters is listed 

in table 2–4, along with their composite scaled sensitivities 

and significant correlations between parameters. Generally, 

parameters with composite scaled sensitivities greater than 0.5 

can be estimated (Hill and Tiedeman, 2007), provided they are 

not highly correlated with other estimated parameters. This 

guidance was followed for all parameters except RCH_DOM, 

which was set by hand because the estimation routine yielded 

values above the reasonable maximum recharge for the area. 

The Kv values were set by hand at the beginning of the 

calibration phase (with the exception of two enhanced vertical 

conductance zones in layer 2, discussed next) based on reasonable values from literature, and only those that displayed a 

minimal amount of sensitivity were adjusted during calibration 

using trial-and-error methods. This resulted in some Kv values 

being larger than the corresponding Kh values for the same 

unit, but because of the insensitivity of the model to those Kv

parameters, this inconsistency was considered acceptable. The 

model was not very sensitive to the Kv parameters, and none 

were set by parameter estimation.

Table 2–2. Streambed hydraulic conductivity model parameters and calibrated hydraulic properties.

[Values in bold were determined using parameter estimation; ft/d, foot per day; Kdr, streambed hydraulic conductivity]

Streambed hydraulic conductance zones Kdr parameter 

name

Calibrated Kdr, 

in ft/d

Harvey Brook valley bottom areas, south part of the valley. KDR_Har1 1.5

Headwaters of Harvey Brook watershed. Mixed surficial geology, primarily sandy with 

some weathered clay areas.

KDR_HarUp 4.5

Upland areas in Harvey Brook watershed, where streams are intermittent. KDR_HarMix .02

Harvey Brook valley adjacent to eolian sands. KDR_HarSan 5

Merrill Brook watershed uplands and some areas near stream. KDR_Merrill .017

Cousins River valley bottom areas, and other areas with saturated Presumpscot Fm. 

near surface. Cousins River valley has significant estuarine marsh sediments as well.

KDR_ClyBot 1.5

Table 2–3. Recharge model parameters and calibrated recharge values.

[Values in bold were determined using parameter estimation; in/yr, inch per year]

Recharge zones

Recharge 

parameter 

name

Calibrated 

recharge, 

in in/yr

Uplands with till and (or) weathered Presumpscot Fm. soils. RCH_Till 5.037

Cousins River valley bottom areas, and other areas with saturated Presumpscot Fm near surface. RCH_Clay .75

Area of high slope at western edge of model, where runoff from upland areas can enter subsurface 

through short losing stream reaches.

RCH_HSL1 50

Areas of eolian sands, and Harvey Brook valley adjacent to those areas. RCH_DOM2 24

Areas where surficial units above bedrock are 10 ft thick or less. RCH_Thin2 7.1

Upland areas in Harvey Brook watershed, where streams are intermittent, soils primarily sandy. RCH_SandyN 10.75

Upland areas on either side of Harvey Brook and to the south of the eolian sand deposits where 

nearshore marine sandy units are present.

RCH_SandyS 25

1

The model was sensitive to this parameter, but parameter estimation yielded values outside the reasonable range, so it was set by hand.

2

The model was sensitive to this parameter, but it was highly correlated with a hydraulic conductivity parameter, and was set by hand.
70 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed

Table 2–4. Model parameters and final composite scaled sensitivities and correlations used 

to determine which parameters to estimate using parameter estimation.

[--, none significant]

Parameter name Composite scaled 

sensitivity Correlations with other parameters

Set using 

parameter 

estimation?

RCH_SandyS 5.58 -- Yes

HK_Lay3_Sa 2.63 -- Yes

RCH_DOM 2.54 -- No

RCH_Thin 2.49 HK_Lay1_C No

KDR_Merill 2.28 -- Yes

RCH_Till 2.26 -- Yes

RCH_SandyN 2.15 HK_Lay1_B Yes

HK_Lay1_C 2.15 RCH_Thin Yes

HK_Lay1_A 1.63 -- Yes

HK_Lay1_B 1.51 RCH_SandyN No

HK_Lay3_K2 1.27 -- Yes

HK_Lay2_TL 1.15 -- Yes

HK_Lay1_CD 1.14 -- Yes

RCH_HSL .97 -- No

HK_Lay3_K3 .95 -- Yes

HK_Lay1_D .78 -- Yes

KDR_HarUp .34 -- No

VK_C_FracS .29 VK_C_FracN; VK_Lay3 No

KDR_Har1 .29 VK_Lay3; VK_C_FracN No

KDR_HarSan .27 -- No

KDR_HarMix .20 -- No

VK_C_FracN .15 VK_C_FracS; VK_Lay3; KDR_Har1 No

KDR_ClyBot .15 -- No

VK_Lay3 6.62 × 10-2 VK_C_FracS; VK_C_FracN; KDR_Har1 No

VK_Sandy 6.23 × 10-2 -- No

VK_ClayL1 3.73 × 10-2 -- No

VK_TillL2 2.91 × 10-2 -- No

RCH_Clay 2.62 × 10-2 -- No

VK_Till 1.79 × 10-2 -- No

VK_ClayL2 1.63 × 10-2 -- No

HK_Lay2_Cl 1.54 × 10-2 -- No

GHB_ParN 5.25 × 10-3 -- No

GHB_ParS 3.30 × 10-4 -- No
Appendix 2 71

Model Sensitivity Analysis

The sensitivity of the model results to the numerous 

model variables and parameters was tested in UCODE_2005. 

The UCODE_2005 output can report the sensitivity of the 

observations to the optimized parameters and the parameters 

set by hand. Dimensionless scaled sensitivities calculated 

by UCODE_2005 indicate how much an observation would 

change given a 1-percent change in the parameter value, 

scaled by the error in the observation (Hill and Tiedeman, 

2007, p. 49). The sensitivity values for every parameter/variable are reported for each observation; these observations can 

be grouped according to their use and position in the model, as 

observations representing similar parts of the model geometry 

are sensitive to similar parameters. Although the dimensionless scaled sensitivity is a “fit-independent” statistic, the analysis was run using the optimal parameter values for the model. 

The parameters of greatest importance to the simulation have 

the largest absolute values (Hill and Tiedeman, 2007, p. 49). 

The sensitivity of the model results to the input variables 

is summarized in tables 2–5 and 2–6. This summary presents 

the average of the absolute values of the dimensionless scaled 

sensitivities for parameters in three groups of head observations and the individual streamflow observations. The observation groups in table 2–5 are Layer 1&2 north (observations in 

wells, borings, and seismic lines in the surficial units above 

the confining unit and north of the pumping wells), Layer 

1&2 south (observations in wells, borings, and seismic lines 

in the surficial units south of the pumping wells), and Layer 

3 (all head observations in layer 3). Table 2–6 contains the 

sensitivities for the streamflow observations. In both tables, 

parameters that had minimal sensitivities to the observations 

were not listed. The heads in both zones of the surficial units 

(Layer 1&2 north and Layer 1&2 south) were sensitive to the 

KDR_Merrill, RCH_SandyS, HK_Lay1_A, and HK_Lay1_B 

parameters. The northern surficial heads were also quite 

sensitive to the recharge parameters RCH_DOM and RCH_

SandyN, as well as the HK_Lay1_CD hydraulic conductivity 

parameter. The southern surficial heads were sensitive to the 

RCH_Thin, RCH_Till, and HK_Lay1_C parameters. Layer 3 

heads were most sensitive to the hydraulic conductivity zones 

in layer 3 (HK_Lay3_Sa, HK_Lay3_K2, and HK_Lay3_K3), 

and the recharge parameters RCH_HSL and RCH_SandyS, 

and to a lesser extent the recharge parameters RCH_Till 

and RCH_DOM. None of the heads were very sensitive to 

recharge, Kh, or Kv in the clay units, to the Kh of the general 

head boundaries, to the Kv parameters in the till area, or to any 

other Kv parameters, including the Kv of the vertical conductance zones through the clay in layer 2. The streamflows 

were all sensitive to the recharge parameters RCH_Till and 

RCH_SandyS, although the Harvey Brook #2 site was particularly sensitive to the RCH_SandyS parameter. This outcome is 

likely because the site gets a larger proportion of its discharge 

from the upper sand unit than from the other Harvey Brook 

sites. Of the drain conductance parameters, KDR_Merrill had 

the greatest effect on the streamflows, even in the Harvey 

Brook watersheds, because this parameter had some control on 

the position of the groundwater divide between the watersheds. Several of the streamflow observations were sensitive 

to the Kh parameters in layer 3, as these controlled the overall 

movement of water through layer 3 and toward the vertical 

conductance zones in the layer 2 clay. Also important in this 

movement of water was the recharge parameter RCH_HSL, to 

which the Harvey Brook sites were all sensitive. Parameters 

that the streamflow observations were insensitive to included 

the following: recharge; Kh, or Kv in the clay units (with the 

exception of the vertical conductance zone VK_C_FracS 

through the clay in layer 2); the Kh of the general head boundaries; and the Kv

 parameters in the till area, or any other Kv

parameters. Streamflows were also relatively insensitive to the 

KDR_HarUp, KDR_ClyBot, and KDR_HarSan streamflow 

conductance parameters.
72 Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed Table 2–5. Dimensionless scaled sensitivities for observations, by parameter, for head observation groups as a measure of model sensitivity in the Freeport aquifer groundwater flow model, Freeport, Maine. [Parameters not listed have negligible sensitivity. Bold values are the highest 8 for each observation group] Head observation group Number of observations in group Average dimensionless scaled sensitivities (absolute values) per observation in each group

HK_Lay3_Sa

HK_Lay2_TL

KDR_Merill

KDR_HarSan

RCH_Till

RCH_HSL

RCH_DOM

RCH_Thin

RCH_SandyN

RCH_SandyS

HK_Lay3_K2

HK_Lay1_A

HK_Lay1_B

HK_Lay1_C

HK_Lay1_CD

HK_Lay1_D

HK_Lay3_K3

Layer 1&2 north 12 0.78 0.33 0.46 0.12 0.43 0.03 2.68 0.38 1.75 1.37 0.05 1.79 0.86 0.22 1.64 0.04 0.02Layer 1&2 south 9 0.09 0.07 3.61 0.01 0.57 0.02 0.06 2.00 0.00 7.23 0.01 1.32 2.15 1.53 0.01 0.56 0.00Layer 3 22 2.76 0.10 0.23 0.31 0.42 1.15 0.30 0.09 0.09 1.57 1.01 0.19 0.24 0.01 0.25 0.01 1.27Synthetic till 11 1.00 1.36 1.13 0.03 4.15 0.04 0.86 0.58 0.40 2.92 0.02 1.09 0.19 2.46 0.18 1.13 0.02

Table 2–6. Dimensionless scaled sensitivities for observations, by parameter, for streamflow observations as a measure of model sensitivity in the Freeport aquifer groundwater flow model, Freeport, Maine.[Parameters not listed have negligible sensitivity. Bold values are the highest 8 for each observation]

Streamflow 

observation Station

Individual dimensionless scaled sensitivities (absolute values)

HK_Lay3_Sa

KDR_Merill

KDR_HarMix

KDR_Har1

RCH_Till

RCH_HSL

RCH_DOM

RCH_Thin

VK_C_FracS

RCH_SandyN

RCH_SandyS

HK_Lay3_K2

HK_Lay1_A

HK_Lay1_B

HK_Lay1_C

HK_Lay3_K3

Harvey Brook 1 1 2.91 1.46 0.02 1.85 1.12 1.84 0.31 0.14 1.74 0.02 9.51 0.70 0.93 0.77 0.03 1.79

Harvey Brook 2 2 1.71 5.78 0.72 0.31 0.77 2.08 0.19 0.04 0.34 0.01 20.41 0.25 0.44 5.58 0.18 0.84Harvey Brook 3 3 0.69 1.13 0.01 0.17 0.72 3.42 7.75 0.12 0.18 0.75 5.32 6.15 0.42 0.11 0.14 1.45Harvey Brook 4 4 2.95 0.18 0.00 0.01 2.44 0.42 8.75 1.83 0.01 9.25 1.20 0.75 0.38 0.26 0.15 0.04Merrill Brook 5 0.81 6.55 0.21 0.05 2.47 0.04 0.42 3.04 0.05 0.00 8.92 0.01 0.86 1.76 1.04 0.05
Prepared by the Pembroke, Raleigh, and Rolla Publishing 

Service Centers.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Maine Water Science Center

196 Whitten Road

Augusta, ME 04330

dc_me@usgs.gov

or visit our Web site at:

http://me.water.usgs.gov
Nielsen and Locke—Groundwater Conditions and Streamflow Depletion to Evaluate Water Availability in a Freeport, Maine, Watershed—SIR 2011–5227