U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5142

Prepared in cooperation with the Town of Newtown, Connecticut

Hydrogeology and Numerical Simulation 

of the Unconsolidated Glacial Aquifer in the 

Pootatuck River Basin, Newtown, Connecticut
Cover. Pootatuck River looking downstream from streamgage at Sandy Hook, CT. Photo by Jason Pollender, U.S. Geological Survey, 

April 2010.
Hydrogeology and Numerical Simulation 

of the Unconsolidated Glacial Aquifer 

in the Pootatuck River Basin, 

Newtown, Connecticut

By Carl S. Carlson, Remo A. Mondazzi, David M. Bjerklie, and Craig J. Brown

Prepared in cooperation with the Town of Newtown, Connecticut

Scientific Investigations Report 2010–5142

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: 2010

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Carlson, C.S., Mondazzi, R.A., Bjerklie, D.M., and Brown, C.J., 2010, Hydrogeology and numerical simulation of the 

unconsolidated glacial aquifer in the Pootatuck River Basin, Newtown, Connecticut: U.S. Geological Survey Scientific 

Investigations Report 2010–5142, 84 p., at http://pubs.usgs.gov/sir/2010/5142/.
iii

Contents

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

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

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

Location and Description ....................................................................................................................2

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

Hydrogeology of the Pootatuck River Valley.............................................................................................2

Physiography and Geology .................................................................................................................2

Description of Hydrologic Units .........................................................................................................4

Data Collection and Analysis.......................................................................................................................4

Methods of Data Collection ................................................................................................................4

Streamflow....................................................................................................................................7

Observation Wells and Streambed Piezometers....................................................................7

Groundwater-Level Monitoring .................................................................................................7

Temperature Monitoring.............................................................................................................7

Aquifer Test...................................................................................................................................7

Water-Quality and Stable-Isotope Analysis..........................................................................10

Fairfield Hills Well Field......................................................................................................................10

Site Description..........................................................................................................................10

Pumping Information.................................................................................................................10

Aquifer Tests at Fairfield Hills Well Field ...............................................................................10

Groundwater-Level Monitoring ......................................................................................12

Surface-Water Monitoring..............................................................................................12

Temperature Monitoring..................................................................................................14

Water-Quality and Isotope Analysis..............................................................................18

Analysis of Aquifer Tests at Fairfield Hills Well Field...........................................................24

United Water Well Field.....................................................................................................................26

Site Description..........................................................................................................................26

Pumping Information.................................................................................................................26

Induced Infiltration Test at United Water Well Field............................................................26

Groundwater and Surface-Water-Level Monitoring ..................................................27

Temperature Monitoring..................................................................................................30

Historical Aquifer Tests.............................................................................................................30

Analysis of Historical Aquifer Test at United Water Well Field..........................................31

Development of the Groundwater-Flow Models ....................................................................................33

Development of a Conceptual Model of the Pootatuck River Basin..........................................33

Development of Numerical Models.................................................................................................33

Model Discretization and Boundaries....................................................................................36

Model Calibration ......................................................................................................................36

Well-Field-Scale Models ...................................................................................................................37

Fairfield Hills Well-Field-Scale Model....................................................................................37

Simulation of Aquifer Test ...............................................................................................40

Model Calibration and Hydraulic Properties................................................................40
iv

Steady-State Model .........................................................................................................41

United Water Well-Field-Scale Model ...................................................................................41

Simulation of Reverse Aquifer Test................................................................................46

Model Calibration and Hydraulic Properties................................................................46

Limitations of Well-Field-Scale Models .................................................................................50

Basin-Scale Model.............................................................................................................................50

Layer Distribution.......................................................................................................................50

Hydrologic Boundaries .............................................................................................................51

Hydrologic Stresses ..................................................................................................................51

Recharge............................................................................................................................51

Pumping..............................................................................................................................51

Hydraulic Properties .................................................................................................................51

Basin-Scale, Steady-State Model Calibration...............................................................................54

Water-Level Data.......................................................................................................................54

Streamflow Data ........................................................................................................................56

Model Limitations.......................................................................................................................56

Sensitivity Analysis of Groundwater-Flow Models .......................................................................56

Streambed Conductance..........................................................................................................56

Hydraulic Conductivity..............................................................................................................56

Recharge.....................................................................................................................................57

Delineation of Areas Contributing Recharge to Production Wells......................................................57

Simulated Groundwater Levels and Drawdown............................................................................57

Delineation of Groundwater-Recharge Areas ...............................................................................57

Sensitivity Analysis of Groundwater Recharge Areas .................................................................59

Streambed Conductance..........................................................................................................59

Hydraulic Conductivity..............................................................................................................64

Recharge.....................................................................................................................................64

Simulation of the Interaction between Groundwater and Surface Water.........................................64

Sensitivity Analysis of Simulated Groundwater Levels................................................................64

Sensitivity Analysis of Stream Base Flow in the Pootatuck River..............................................66

Simulated Water Budgets .................................................................................................................66

Simulated Effect of Future Recharge Conditions ...................................................................................70

Base Flows in the Main Stem Pootatuck River—Future Scenarios ..........................................70

Water Budgets in Subbasins of Pootatuck River—Future Scenarios .......................................70

Summary........................................................................................................................................................79

Acknowledgments.......................................................................................................................................80

References Cited..........................................................................................................................................80

Appendix. Spatially-Distributed Recharge...........................................................................................83
v

Figures

1. Map showing location of study area, Pootatuck River Basin, Newtown, Connecticut....3

2. Block diagram showing a typical glaciated bedrock valley in the northeastern

United States with coarse-grained valley-fill deposits ..........................................................4

3. Map showing bedrock surface underlying the Pootatuck River Basin, Newtown, 

Connecticut....................................................................................................................................5

4. Map showing surficial geologic materials in the Pootatuck River Basin, Newtown,

Connecticut, generalized for this study ....................................................................................6

5. Map showing location of observation wells, piezometers, and local streamflowmeasuring sites near Fairfield Hills well field, Pootatuck River Basin, Newtown, 

Connecticut....................................................................................................................................8

6. Map showing location of observation wells, piezometers, and local

streamflow-measuring sites near United Water well field, Pootatuck River Basin,

Newtown, Connecticut................................................................................................................9

7. Hydrograph of water levels in Well NT-15, Pootatuck River Basin, Newtown, 

Connecticut..................................................................................................................................11

8. Hydrograph of stream stage recorded at the Berkshire streamgage, water levels in

Wells NT-107 and NT-91, and precipitation recorded in the onsite rain gage,

Fairfield Hills well field, Pootatuck River Basin, Newtown, Connecticut..........................13

9. Graph showing streamflow measured upstream and downstream from Well 3,

Pootatuck River Basin, Newtown, Connecticut ....................................................................14

10. Graph showing streamflow measured upstream and downstream from Well 7,

Pootatuck River Basin, Newtown, Connecticut ....................................................................14

11. Graphs showing water-level measurements of surface water and groundwater in

A, upstream; B, middle; and C, downstream piezometers near Well 3, Pootatuck

River Basin, Newtown, Connecticut........................................................................................16

12. Graphs showing water-level measurements of surface water and groundwater in

A, upstream; B, middle; and C, downstream piezometers near Well 7, Pootatuck

River Basin, Newtown, Connecticut........................................................................................17

13. Graphs showing groundwater temperatures in streambed piezometers SBP7ds

(downstream) and SBP7us (upstream) from Well 7, Pootatuck River Basin,

Newtown, Connecticut..............................................................................................................18

14. Graphs showing groundwater temperatures in streambed piezometers SBP3ds

(downstream) and SBP3us (upstream) from Well 3, Pootatuck River Basin,

Newtown, Connecticut..............................................................................................................19

15. Graphs showing relation between sulfate: chloride mass ratios and A, depth below

ground surface and B, isotope values of delta hydrogen-2 (δ2H) in groundwater and

surface water in the Pootatuck River Basin, Newtown, Connecticut...............................21

16. Graph showing isotope values of delta oxygen-18 as a function of delta hydrogen-2

for samples of groundwater and surface water in the Pootatuck River Basin,

Newtown, Connecticut..............................................................................................................22

17. Graph showing pretest water levels in observation wells A, NT-106 and B, NT-107 near 

Well 3, Fairfield Hills well field, Pootatuck River Basin, Newtown, Connecticut.............25

18. Graph showing pumping rate in Well 2, United Water well field, Pootatuck River Basin, 

Newtown, Connecticut..............................................................................................................27

19. Graph showing water levels in observation well TW-6, United Water well field, 

Pootatuck River Basin, Newtown, Connecticut ....................................................................28
vi

20. Graph showing continuous groundwater levels and stream stage at piezometer

SBP2C, United Water well field, Pootatuck River Basin, Newtown, Connecticut...........29

21. Graphs showing stage in the Pootatuck River near at local stream flow-measuring

sites Well 2 during aquifer test, Pootatuck River Basin, Newtown, Connecticut............30

22. Graphs showing temperature recorded in streambed piezometers at location B

near Well 2 A, P2Blb (left bank) and B, P2Brb (right bank), Pootatuck River Basin,

Newtown, Connecticut..............................................................................................................31

23. Graphs showing groundwater temperature recorded in streambed piezometers at 

location D near Well 2 A, P2Dlb (left bank) and B, P2Drb (right bank), Pootatuck River 

Basin, Newtown, Connecticut..................................................................................................32

24. Map with transects showing extent of well-field- and basin-scale models in the

Pootatuck River Basin, Newtown, Connecticut ....................................................................34

25. Cross sections showing model layers, Pootatuck River Basin, Newtown,

Connecticut..................................................................................................................................36

26. Map showing Fairfield Hills well-field-scale model, Pootatuck River Basin,

Newtown, Connecticut..............................................................................................................38

27. Map showing hydraulic conductivity in the Fairfield Hills well-field-scale model,

Pootatuck River Basin, Newtown, Connecticut ....................................................................39

28. Graphs showing simulated and observed water levels during Fairfield Hills aquifer

test at Well 3, Pootatuck River Basin, Newtown, Connecticut...........................................42

29. Graphs showing simulated and observed water levels during Fairfield Hills aquifer

test at Well 7, Pootatuck River Basin, Newtown, Connecticut...........................................43

30. Scatterplots showing comparison of simulated and observed water levels at

Fairfield Hills and United Water well fields, Pootatuck River Basin, Newtown,

Connecticut; A, simulated residuals compared to simulated water levels, and B,

observed water levels................................................................................................................45

31. Map showing United Water well-field-scale model, Pootatuck River Basin,

Newtown, Connecticut..............................................................................................................47

32. Map showing hydraulic conductivity in the United Water well-field-scale model,

Pootatuck River Basin, Newtown, Connecticut ....................................................................48

33. Graphs showing simulated and observed water levels during United Water aquifer

test, Pootatuck River Basin, Newtown, Connecticut............................................................49

34. Map showing simulated hydraulic conductivity of the basin-scale model,

Pootatuck River Basin, Newtown, Connecticut ....................................................................52

35. Map showing areal distribution of recharge used in the basin-scale

groundwater-flow model, Pootatuck River Basin, Newtown, Connecticut......................53

36. Scatterplots showing comparison of simulated and observed water levels in the

basin-scale model A, simulated residuals compared to simulated water levels and

B, observed compared to simulated water levels, Pootatuck River Basin, Newtown,

Connecticut..................................................................................................................................55

37. Map showing drawdown caused by simulated production wells in the Pootatuck

River Basin, Newtown, Connecticut........................................................................................58

38. Map showing simulated areas contributing recharge to production wells in the

well-field-scale models, Pootatuck River Basin, Newtown, Connecticut ........................60

39. Map showing simulated areas contributing recharge to production wells using Level A

mapping regulations in the well-field-scale models, Pootatuck River Basin, Newtown,

Connecticut..................................................................................................................................61

40. Map showing combination of simulated recharge areas resulting from the sensitivity

simulations, Pootatuck River Basin, Newtown, Connecticut..............................................65
vii

Tables

1. Construction details of production wells, Fairfield Hills well field, Pootatuck River 

Basin, Newtown, Connecticut..................................................................................................11

2. Pumping rates for Wells 3 and 7 during Fairfield Hills aquifer test, Pootatuck River 

Basin, Newtown, Connecticut..................................................................................................12

3. Stage and discharge measurements at local streamflow-measuring sites, Pootatuck 

River near Fairfield Hills well field, Newtown, Connecticut................................................15

4. General chemical characteristics and concentrations of chloride and sulfate, 

sulfate:chloride mass ratios, and stable isotope values for δ2H and δ18O in the 

Pootatuck River Basin, Newtown, Connecticut ....................................................................20

5. Results of mixing models estimated using isotope values for δ2H and δ18O in river and 

groundwater samples in the Pootatuck River Basin, Newtown, Connecticut.................23

6. Observed and corrected drawdowns in observation wells, Fairfield Hills well field, 

Pootatuck River Basin, Newtown, Connecticut ....................................................................24

7. Transmissivity and hydraulic conductivity calculated from aquifer tests, Pootatuck 

River Basin, Newtown, Connecticut........................................................................................26

8. Construction details of production wells, United Water well field, Pootatuck River 

Basin, Newtown, Connecticut..................................................................................................26

9. Calibrated hydraulic parameters of the Fairfield Hills well-field-scale model, Pootatuck 

River Basin, Newtown, Connecticut........................................................................................41

10. Simulated and observed water levels in wells at the Fairfield Hills well field, Pootatuck 

River Basin, Newtown, Connecticut........................................................................................44

11. Simulated and observed base flow, Fairfield Hills well field, Pootatuck River Basin, 

Newtown, Connecticut..............................................................................................................46

12. Calibrated hydraulic parameters of the United Water well-field-scale model, Pootatuck 

River Basin, Newtown, Connecticut........................................................................................49

13. Simulated and observed water levels in wells at the United Water well field, Pootatuck 

River Basin, Newtown, Connecticut........................................................................................50

14. Registered pumping rates and distance from stream for all production wells in the 

basin-scale model, Pootatuck River Basin, Newtown, Connecticut .................................54

15. Source of water to production wells, Pootatuck River Basin, Newtown, Connecticut..59

16. Results of the sensitivity analyses on recharge areas for the well-field-scale models, 

Pootatuck River Basin, Newtown, Connecticut ....................................................................62

17. Results of sensitivity analysis on water levels in the basin-scale model, Pootatuck 

River Basin, Newtown, Connecticut........................................................................................67

41. Scatterplot showing comparison of simulated and observed water levels in the

basin-scale model sensitivity analysis, Pootatuck River Basin, Newtown,

Connecticut..................................................................................................................................67

42. Graph showing cumulative downstream base flow in the Pootatuck River in

basin-scale model sensitivity analysis, Newtown, Connecticut ........................................68

43. Map showing model subbasins in the basin-scale model, Pootatuck River Basin,

Newtown, Connecticut..............................................................................................................69

44. Map showing difference in recharge in commercial land-use scenario as

compared to the calibrated model, Pootatuck River Basin, Newtown, Connecticut......71

45. Graph showing cumulative downstream base flow in the Pootatuck River for the

basin-scale model future-recharge analysis, Newtown, Connecticut..............................72
viii

18. Annual water budget for modeled subbasins under pumping and nonpumping 

conditions, Pootatuck River Basin, Connecticut. ..................................................................73

19. Cumulative base flow in future pumping scenarios, Pootatuck River Basin, Newtown, 

Connecticut..................................................................................................................................77

20. Base flow from individual subbasins in future pumping scenarios, Pootatuck River 

Basin, Newtown, Connecticut. ................................................................................................78

21. Coefficients and statistics for regression equation to determine groundwater 

recharge, Pootatuck River Basin, Newtown, Connecticut..................................................83
ix

Conversion Factors and Datum

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)

gallon (gal) 0.003785 cubic meter (m3

)

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

)

cubic foot (ft3

) 0.02832 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)

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

/d)

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

/s)

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

Hydraulic conductivity

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

Transmissivity*

2 foot squared per day (ft /d) 0.09290 meter squared per day (m /d) 2

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

 °F= (1.8×°C)+32

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 National Geodetic Vertical Datum of 1929 

(NGVD29).

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.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Abstract

A study of the groundwater and stream-aquifer interaction 

in the Pootatuck River Basin, Newtown, Connecticut, was 

conducted to analyze the effect of production wells on the 

groundwater levels and streamflow in the Pootatuck River as 

part of a cooperative program between the U.S. Geological 

Survey and Newtown, Connecticut. This study will help 

address concerns about the increasing competition for 

water for human uses and protection of aquatic habitat. The 

groundwater-flow model developed in the study was designed 

for use as a tool to assist planners in assessing the effects of 

potential future development, which will change the 

amount and distribution of recharge available to the 

groundwater system. 

Several different techniques were used to investigate 

the interconnection between the stream and the aquifer. 

Temperature, groundwater levels, stream stage, and stableisotope data collected during aquifer tests at the principal 

production wells in the Pootatuck River Basin, as well as 

groundwater-flow simulations of the system, indicate that 

more than half of the water pumped from the wells comes 

from the Pootatuck River. This finding potentially has a 

large effect on approaches for protecting the water quality 

of the pumped water. Increases in the amount of impervious 

surface from future development will reduce and redistribute 

recharge to the groundwater system. The simulation of future 

development scenarios showed a decrease in the simulated 

base flow in the main stem of the Pootatuck River and in all 

of the 26 simulated subbasins, with some of the subbasins 

showing a decrease of more than 20 percent when new 

development had 85 percent impervious area.

The groundwater-flow model and particle tracking were 

used to determine areas that contribute recharge to the five 

production wells available for use in the Pootatuck River 

Basin. These areas included narrow portions of the aquifer that 

extended beyond the immediate upgradient areas, probably 

because of deeper groundwater-flow paths. 

Introduction

The Pootatuck aquifer, located entirely in Newtown, 

Connecticut, was designated by the U.S. Environmental 

Protection Agency as a Sole Source Aquifer and in 2007 

supplied water for about 3,000 residents of Newtown. 

Protection of water quality in production wells is a major 

concern of Newtown and the Connecticut Department of 

Environmental Protection (CTDEP). Also they are concerned 

about potential competition for water between increasing 

groundwater withdrawals and instream-flow requirements 

to sustain aquatic habitats, particularly during base flows 

when all the flow is groundwater discharging to streams. In 

order to manage the aquifer and river as a single system, it is 

important to quantitatively account for effects of withdrawals 

by simulating interactions between groundwater and 

surface water. 

Newtown and CTDEP also are concerned about the possible effect that land-use change from development might have 

on recharge to the Pootatuck aquifer and thereby the base flow 

in the Pootatuck River, and the increased potential for spills 

and leaks that could contaminate the aquifer and groundwater supplies. Recent studies by the U.S. Geological Survey 

(USGS) in the nearby Pomperaug River Basin have shown 

that land-use change does affect recharge, and that the change 

can be quantified (Bjerklie and others, in press). Newtown and 

the public are interested in developing a scientific understanding of the aquifer and its interaction with the Pootatuck River 

so that appropriate land- and water-use management strategies 

can be implemented. In response to these concerns, in 2006, 

the USGS and Newtown entered into a cooperative agreement 

to study the hydrology of the area. An important product from 

this study is a basin-wide model that incorporates an estimate 

of areally-distributed recharge. This model can be used to 

assess the effect of decreased groundwater recharge to the 

aquifer resulting from future development.

The Connecticut Aquifer Protection Program (State of 

Connecticut, 1991), managed by the CTDEP, requires water 

suppliers to delineate aquifer-protection areas (referred to 

Hydrogeology and Numerical Simulation 

of the Unconsolidated Glacial Aquifer in the 

Pootatuck River Basin, Newtown, Connecticut

By Carl S. Carlson, Remo A. Mondazzi, David M. Bjerklie, and Craig J. Brown
2 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

as “Level A mapping”) for all wells that obtain water from 

stratified glacial deposits and that provide water to more than 

1,000 people. Level A mapping makes use of groundwaterflow models to delineate the contributing area to the wells 

under steady-state hydrogeologic conditions. Data collected 

for the USGS/Newtown cooperative study could be used to 

satisfy the Level A mapping requirement. The aquifer tests 

performed for this study provided data for model calibration 

to estimate stream inflow (induced infiltration) from the 

Pootatuck River and for Level A mapping of the well fields 

under CTDEP regulations.

Because the Pootatuck River and the aquifer are 

interconnected, an important element of this study was 

using multiple methods of investigating the stream-aquifer 

interaction near production wells. Several techniques were 

used, including temperature and stable-isotope analysis as well 

as the more typical streamflow and water-level measurements. 

These techniques helped quantify the stream inflow and can 

be used in other areas in New England where production wells 

are adjacent to streams. 

Purpose and Scope

This report describes the hydrogeologic analysis and 

computer modeling that was done in the Pootatuck River 

Basin in western Connecticut. Groundwater-flow models were 

used to delineate areas from which groundwater recharge 

moves to production wells and to provide information on the 

interaction between the production wells and the Pootatuck 

River, including estimates of stream inflow from the Pootatuck 

River to the aquifer at the Fairfield Hills and United Water 

well fields in Newtown, Connecticut. The report also describes 

simulated effects of possible land-use change on the distribution and amount of recharge available to the groundwater 

system and the associated effect on base flow in streams. 

Three aquifer tests were performed as part of this 

study—two simultaneous tests at the Fairfield Hills well 

field and another at the United Water well field in Newtown, 

Connecticut (fig. 1). Groundwater and surface-water data from 

the aquifer tests were used to calibrate the groundwater-flow 

models and to assess the interaction between the groundwater 

withdrawals and surface water. 

Location and Description

The Pootatuck River Basin is located in southwestern 

Connecticut (fig. 1). The Pootatuck River flows northward 

in a narrow valley in the highlands of western Connecticut. 

The headwaters of the Pootatuck River are in the towns of 

Easton and Monroe. The Pootatuck River, which drains a total 

of 26.1 mi2, flows through Newtown and discharges into the 

Housatonic River. A general description of the hydrology of 

the lower Housatonic River Basin (the major drainage basin 

in which the Pootatuck River is located) is provided in the 

Connecticut Department of Environmental Protection Water 

Resources Inventory of Connecticut series (Wilson and 

others, 1974).

Previous Investigations

The hydrogeology of the area was first described by 

Wilson and others (1974) as part of a regional study of the 

hydrology of the lower Housatonic River Basin. Surficial 

geology of the area was described by McKeegan (1986) and 

by Stone and others (1992, 1998), and the bedrock geology 

was described by Rodgers (1985).

Haeni (1978) conducted an investigation of the 

hydrogeology of the Pootatuck River aquifer that compiled 

information on the distribution and hydraulic properties of the 

stratified glacial deposits, the altitude of bedrock in the valley, 

streamflow characteristics, and water quality of surface water 

and groundwater. In addition, Haeni (1978) used a numerical 

model based on available data, test drilling, seismic-refraction 

profiling, and the stream-aquifer connection. The report 

presented information on the quantity of water available 

from the sand and gravel aquifer, predictions on the effects of 

withdrawing that quantity on the stream-aquifer system, and 

the quality of groundwater and surface water. 

Grady and Weaver (1988) conducted a study of the 

Pootatuck River Valley as part of a regional groundwaterquality appraisal. Information was presented on the saturated 

thickness, the areal distribution of average horizontal hydraulic conductivity, the altitude of the water table, and water 

quality of the sand and gravel aquifer.

Hydrogeology of the Pootatuck River 

Valley

Physiography and Geology

The Pootatuck River Basin is dominated by a narrow 

bedrock valley, which is less than a mile wide in most places, 

within extensive upland areas and is typical of glaciated valleys in the northeastern part of the United States (fig. 2). The 

valley walls are formed by till-blanketed bedrock that reaches 

elevations of over 650 ft near the confluence of the Pootatuck 

River with the Housatonic River. Highlands to the east and 

west of the valley are underlain by Paleozoic-age crystalline 

(metamorphic) bedrock that is predominantly dioritic gneiss 

of the Brookfield Gneiss (Rodgers, 1985). A bedrock-surface 

contour map (fig. 3) for the Pootatuck Basin was constructed 

using data from test-well borings, domestic-well-completion 

reports, locations of bedrock outcrops, a seismic-refraction 

survey (Haeni, 1978), and a map of stratified glacial deposits 

(McKeegan, 1986). Because this bedrock-surface contour 

map was used as the bottom of the modeled aquifer, in some 

areas the bedrock surface is shown as somewhat lower than 
Hydrogeology of the Pootatuck River Valley 3

Newtown

Monroe

Redding

Southbury

Bethel

Easton

Brookfield

73°20' 73°16'

41°24'

41°20'

0 1 2 MILES

0 1 2 KILOMETERS

CONNECTICUT

NT-15

01203510

012035055

EXPLANATION

Pootatuck River Basin 

and boundary Wells, gages, and stations

Select roads

Towns and boundaries

Flow direction 

Primary Route

Secondary Route

Domestic well

Production well

Continuous-record 

streamgage and identifier

Long-term groundwater-level 

monitoring well

United Water Well Field

Fairfield Hills Well Field

Base from Connecticut Department of Environmental Protection 

Digital Line Graphs, 2005

Pootatuck 

River

Pomperaug Basin

Pootatuck Basin

Monroe

Monroe

Housatonic River

Interstate Highway

Figure 1. Location of study area, Pootatuck River Basin, Newtown, Connecticut.
4 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

it actually is to ensure that the overlying stratified glacial 

deposits are at least 15-ft thick. Also, in the upland areas, the 

bedrock-surface map represents land surface.

The Pootatuck River is underlain by three principal 

hydrogeologic units: bedrock, glacial till, and stratified glacial 

deposits. Surficial materials have been mapped at a regional 

scale (Stone and others, 1992, 1998) and include (1) glacial 

till, which was laid down directly by ice on top of bedrock and 

is the surficial material on the valley sides and in the uplands; 

(2) glacial meltwater deposits, which consist of stratified sands 

and gravels that partially fill the bedrock valley to an altitude 

of approximately 300 ft; and (3) small areas of flood plain 

alluvium, swamp deposits, and fine-grained sediments. The till 

and glacial meltwater deposits were laid down during advance 

and retreat of Pleistocene continental glaciers, particularly during the last (late-Wisconsinan) glaciation. 

Description of Hydrologic Units

The principal hydrologic units in the basin are coarsegrained stratified glacial deposits (sands and gravels); till, 

which is thick in some areas (more than 10–15 ft thick); 

swamp deposits; and bedrock. The coarse-grained sand and 

gravel deposits have the highest hydraulic conductivity and 

form the most productive aquifer that is used at the Fairfield 

Hills and United Water well fields (fig. 4). The bedrock 

aquifer provides water only to individual domestic wells. The 

Pootatuck River streambed is generally composed of alluvium 

overlying sand and gravel that provides a good hydraulic 

connection between the stream and underlying glacial aquifer. 

The average vertical hydraulic conductivity of the streambed 

sediments was calculated (Haeni, 1978) to be 1.9 ft/d based on 

laboratory-determined vertical hydraulic conductivity values 

of undisturbed samples that ranged from 1.3 to 3.9 ft/d . 

Data Collection and Analysis

Methods of Data Collection

The Pootatuck River and the underlying aquifer are interconnected. One of the objectives of this study was to explore 

multiple ways of determining that interconnection. Therefore, 

the data collected to study the hydrogeology of the Pootatuck 

River Basin included (1) monitoring streamflow, (2) installation of observation wells and streambed piezometers, (3) 

groundwater-level monitoring, (4) temperature monitoring of 

groundwater and surface water, (5) hydraulic (aquifer) testing 

of unconsolidated glacial deposits, and (6) water-quality and 

stable-isotope analysis of groundwater and surface water.

Figure 2. A typical glaciated bedrock valley in the northeastern United States with coarse-grained valley-fill deposits.
Data Collection and Analysis 5

Figure 3. Bedrock surface underlying the Pootatuck River Basin, Newtown, Connecticut.

Monroe

73°20' 73°16'

41°24'

41°20'

EXPLANATION

Pootatuck River Basin 

and boundary

Wells

Altitude of bedrock

surface in feet above 

NGVD29—contour interval 

is variable

Towns and boundaries

Production well

Monroe

450

600

600

400

350

350

300350

400

200

700

500

100

550

250

400

550

450

400

700

700

550

600

250

600

550

500

550

500

600

600

400

500

400

550

600

400

550

400

600

700

500

500

300

200

500

700

700

550

550

550

550

550

400

400

450

500

450

Redding

Easton

Monroe

Bethel

Southbury

Brookfield

Newtown

0 1 2 MILES

0 1 2 KILOMETERS

Pootatuck 

River

Housatonic River

Base from Connecticut Department of Environmental Protection 

Digital Line Graphs, 2005
6 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

73°20' 73°16'

41°24'

41°20'

NT-15

012035055

01203510

Well 3

Well 7

Well 8

Well 1

Well 2

0 1 2 MILES

0 1 2 KILOMETERS

Production well and identifier

Pootatuck Basin boundary

Continuous-record streamgage 

and identifier

Long-term groundwater-level 

monitoring well and identifier

EXPLANATION

Coarse-grained stratified glacial deposits, 

divisions are those used in groundwater model

Swamp deposits

Surficial Geology

Till

Thick till

Pootatuck 

River

Housatonic River

Base from Connecticut Department of Environmental Protection 

Digital Line Graphs, 2005. Surficial geology modified from

Stone and others, 1992

Figure 4. Surficial geologic materials in the Pootatuck River Basin, Newtown, Connecticut, generalized for this study. (Modified 

from Stone and others, 1992)
Data Collection and Analysis 7

Streamflow

USGS streamgage 01203510—Pootatuck River at Sandy 

Hook—was reactivated for this study (previously in operation from 1965 through 1973), and a second streamgage, 

012035055—Pootatuck River at Berkshire—was installed 

near Well 3 of the Fairfield Hills well field (fig. 1). These 

streamgages provided data for the basin as a whole and were 

used to calibrate the groundwater-flow models. During the 

course of this study, data from the Sandy Hook streamgage 

were used to verify that prior streamflow data could be used to 

represent current conditions. 

Temporary staff gages were installed in the Pootatuck 

River at local streamflow-measuring sites upstream and downstream from the anticipated area of influence of each production well (figs. 5 and 6). Streamflow measurements were made 

at these sites before, during, and after the aquifer tests, and 

a rating curve for each site was developed to facilitate determining streamflow during the test. Stage measurements were 

recorded frequently throughout the test. However, poor site 

conditions, such as a cobble streambed and backwater from 

a beaver pond, and the short distance between the measuring 

sites precluded accurately quantifying streamflow gain or loss 

over each reach.

Observation Wells and Streambed Piezometers

Five 2-in. inside diameter (ID) observation wells with 

2- or 3-ft screens had been installed at the Fairfield Hills well 

field for previous studies; these observation wells (NT-85, 

NT-86, NT-90, NT-91 and NT-94, fig. 5), are all near Well 7. 

Because there were no existing observation wells near Well 3, 

in September 2006, five 2-in. ID observation wells with 10-ft 

screens were drilled by Soiltesting, Inc., using a hollow-stem 

auger (Wells NT-106 to NT-110). Wells NT-108 and NT-109 

are at different depths at almost the same location (fig. 5). 

These wells were used to determine the depth to bedrock, 

altitude of the water table, and distribution of geologic units 

in the vicinity of Fairfield Hills Well 3. At the United Water 

well field, six 2-in. ID observation wells had been installed 

near Well 2 for previous studies (TW-1 to TW-6); however, 

only three were available for use in the current study (TW-2, 

TW-4, and TW-6, fig. 6). At the start of the current study, all 

observation wells were developed through a combination of 

surging and pumping and were tested for hydraulic connection 

with the aquifer by introducing a slug of water into each well 

and verifying that the water level returned to static conditions.

Three 1.25-in. ID streambed piezometers were installed 

in the Pootatuck River in 2006 near each of Fairfield Hills 

Wells 3 and 7 (SBP3us, SBP3ms, and SBP3ds, and SBP7us, 

SBP7ms, and SBP7ds; us, ms, and ds are upstream, middle, 

and downstream, respectively, fig. 5). Six piezometers were 

installed near United Water Well 2 (SBP2A, SBP2Blb, 

SBP2Brb, SBP2C, SBP2Dlb and SBP2Drb; rb and lb are right 

and left bank, respectively, fig. 6). The piezometers were used 

to monitor groundwater and surface-water levels, measure 

groundwater and surface-water temperatures, and to aid in 

quantifying stream inflow. The piezometers were installed by 

hand, using a post driver and sledge hammer, until the bottom 

of the 0.25-ft screen was 6 ft below the streambed. Prior 

to making water-level measurements, all piezometers were 

developed by continuously surging and pumping until the 

water flowed clear. The streambed piezometers and the wells 

were used as observation wells during the aquifer test period. 

Groundwater-Level Monitoring

Two methods were used to monitor water levels: 

(1) periodic, manual water-level measurements were made 

using an electric tape, and (2) continuous water-level 

measurements were made with pressure transducers. From 

January 1 to September 5, 2007, water levels from outside 

the area of influence of the production wells were measured 

continuously in Well NT-15, which is part of the USGS longterm, groundwater-level-monitoring network (fig. 1). 

Temperature Monitoring

Differences between temperatures of water in the 

stream and in surrounding sediments can indicate whether 

water flows to or from streams (Stonestrom and Constantz, 

2003). Groundwater temperature was monitored in two 

stream piezometers at each of the production-well locations. 

Hobo U12, four-channel temperature loggers with water/soil 

temperature sensors were secured to 0.25-in. plastic tubing 

placed inside the piezometers and set at 0, 2, 4, and 6 ft below 

the streambed, following procedures described in Stonestrom 

and Constantz (2003). Logging rates were set at 1-hour 

intervals prior to the aquifer tests and at 1-minute intervals 

during the tests. Temperature of the discharge water from each 

production well also was monitored manually using a glass 

thermometer. Several times a day, samples from an in-line 

faucet were collected in a sample bottle and the temperature 

was immediately recorded.

Aquifer Test

The purpose of an aquifer test is to determine properties 

of the aquifer by collecting groundwater-level data under controlled pumping conditions and analyzing them to determine 

the transmissivity and storage coefficients of the aquifer. A 

typical aquifer test has three parts: (1) a period of no pumping that allows water levels to recover to nonpumping (static) 

levels, (2) the pump(s) run until the water level in the vicinity 

of the production well has stabilized, and (3) a recovery period 

in which water levels are monitored until they approach static 

levels. Ideally, the pumps should run at a constant rate and 

there should be no rainfall before or during the test. Because 

data from the aquifer tests in the present study will be used for 

Level A mapping, additional guidelines and regulations were 

specified by CTDEP (State of Connecticut, 1991). However, 
8 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Well 3

NT-90, NT-91

NT-110

NT-106

NT-107

NT-109

012035055

NT-108

Pootatuck River

Well 7

Well 8

NT-85, NT-86

NT-94

TILL

SBP3ds

SBP3ms

SBP3us

280

300

300

300

320

320

340

360

360

260

380 400

420

440

380

320

280

280 280

320

300

280

300

320

260

320

300

260

260

380

420

420

380

320

320

380

400

300

360

320

400

360

340

280

300

300

340

300

360

360

380

320

320

320

340

360

360

320

320

340

340

320

400

73°16'30" 73°16'

41°24'30"

41°24'

300

280

300

280

EXPLANATION

Alluvium

Coarse sand and gravel

Till

Land-surface contour, interval 20 feet, NGVD29

Surficial geology

Staff gage at streamflow-measuring site

and identifier

Continuous-record streamgage, and identifier

Piezometer, and identifier

Production well, and identifier

Observation well, and identifier

SBP7ds

SBP7ms

SBP7us

Geology modified from Stone and others (1992)

Land surface modified from town of Newtown

Geographic Information System

320

0 650 1,300 FEET

0 150 300 METERS

Figure 5. Location of observation wells, piezometers, and local streamflow-measuring sites near Fairfield Hills well field, 

Pootatuck River Basin, Newtown, Connecticut.
Data Collection and Analysis 9

380

360

340

400

420

440

440

380

380

380

380

360

360

360

360

400

420

400

400

400

360

360

360

340

340

400

340

340

340

380

73°16'30" 73°16'20" 73°16'10"

41°22'40"

41°22'30"

41°22'20"

380

380

360

340

380

360

340

SBP2A

0 290 580 FEET

0 75 150 METERS

TW-1 

TW-5

TW-6

SBP2D

SBP2C

SBP2B

SBP2A

Well 1 TW-2 

Well 2

TW-4

TW-3 

Geology modified from Stone and others (1992)

Land surface modified from town of Newtown

Geographic Information System

Surficial geology

EXPLANATION

Alluvium

Coarse sand and gravel

Land-surface contour, interval 20 feet, NGVD29

Staff gage at streamflow-measuring site,

and identifier

Continuous-record streamgage, and identifier

Piezometer, and identifier

Production well, and identifier

Observation well, and identifier

380

Figure 6. Location of observation wells, piezometers, and local streamflow-measuring sites near United Water well field, 

Pootatuck River Basin, Newtown, Connecticut.
10 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

it was not possible to conduct the aquifer tests following these 

guidelines because of constraints imposed by the needs of the 

water-supply system and by uncontrollable natural events. 

Calculations of transmissivity were done for the well 

fields using the distance-drawdown method described by 

Cooper and Jacob (1946) using spreadsheets developed by 

the USGS (Halford and Kuniansky, 2002). The Cooper-Jacob 

distance-drawdown method is typically used for confined 

conditions but can be applied to unconfined aquifers after 

drawdown data are corrected for partial penetration as 

described by Cooper and Jacob (1946) and Kruseman and 

DeRidder (1990). Further analysis was done with a numerical 

model to fully incorporate boundary conditions and test a 

range of storage coefficients.

Water-Quality and Stable-Isotope Analysis

Water quality and stable isotopes were used to quantify 

the amount of water moving from the Pootatuck River into 

the aquifer in the vicinity of the production wells. During 

aquifer tests on Fairfield Hills Wells 3 and 7, water from 

the production wells, three observation wells, and the 

Pootatuck River was monitored for field parameters, including 

temperature, pH, and specific conductance, and water-quality 

samples were collected. Observation wells NT-85 and NT-86 

are between Well 7 and the river, and observation well NT-94 

is upgradient from Well 7 and represents ambient groundwater 

(fig. 5). The samples were analyzed for chloride and sulfate. In 

addition, the normalized stable-isotope ratios of oxygen-18 to 

oxygen-16 (delta oxygen-18 or δ18O) and hydrogen-2 

(also known as deuterium) to hydrogen-1 (delta hydrogen-2 or 

δ2H) were determined. Surface-water samples were collected 

from the Pootatuck River adjacent to Well 7 by filling the 

sample bottles below the water surface. Groundwater samples 

were collected from Wells 3 and 7 and observation wells 

NT-85, NT-86, and NT-94. Prior to groundwater-sample 

collection, wells were purged with a peristaltic pump and field 

parameters were monitored until stable. (The amount of water 

removed did not affect the water levels used for the aquifertest analysis.)

The USGS National Water-Quality Laboratory in Denver, 

Colorado, analyzed the water samples for chloride and 

sulfate by ion chromatography, as described by Fishman and 

Friedman (1989). Analysis of the normalized stable-isotope 

ratios of oxygen and hydrogen found in water was done at the 

USGS Reston Stable-Isotope Laboratory in Reston, Virginia; 

δ2H analysis was done using a hydrogen equilibration method 

(Coplen and others, 1991), and δ18O analysis was done using 

a CO2 equilibration technique (Epstein and Mayeda, 1953). 

The δ2H and δ18O isotopic results are reported in per mil 

relative to Vienna Standard Mean Ocean Water (VSMOW) 

and normalized (Coplen, 1994) on scales such that the oxygen 

and hydrogen isotopic values of Standard Light Antarctic 

Precipitation are -55.5 per mil and -428 per mil, respectively. 

Fairfield Hills Well Field

Site Description

The Fairfield Hills system was formerly known as the 

Fairfield Hills Hospital system and was owned and operated 

by the Connecticut Department of Mental Retardation. The 

hospital facilities were closed in 1997 and Newtown acquired 

the property and production wells. Three wells identified as 

Well 3, Well 7, and Well 8 (fig. 5) withdraw water from a 

stratified glacial aquifer that is more than 100-ft thick in places 

(table 1). Well 8 serves as a backup and typically is pumped 

only once per quarter for water-quality samples in order 

to maintain an active status as required by the Connecticut 

Department of Public Health. 

Pumping Information

The groundwater pumping rates registered as a diversion 

with CTDEP for the production wells are shown in table 1. 

Substantial improvements were made to Well 3 in 2006 and 

to Well 7 in 2001. Improvements to Well 8 (the reserve well) 

are planned for some time in the future. Prior to the improvements, Wells 3 and 7 operated together as an on-demand system—the wells turn on automatically when the storage drops 

below a specified level—producing 120,000 to 150,000 gal/d 

(Jason O’Brian, Aquarion Operating Services, oral commun., 

2006); after the improvements, Wells 3 and 7 began operating 

independently and together produce approximately the same 

amount of water as before. 

Aquifer Tests at Fairfield Hills Well Field

Aquifer tests were conducted simultaneously on 

Wells 3 and 7 to determine the transmissivity and boundary 

conditions of the aquifer, and to estimate stream inflow 

from the Pootatuck River. Wells 3 and 7 were selected as the 

production wells because they are the primary source of water 

at the Fairfield Hills well field. 

Water levels in Well NT-15, a USGS long-term observation well, were monitored before and during the test to see if 

there were any background trends in the water level (fig. 7). 

Although there was a downward trend in the water level prior 

to and during the aquifer test, other influences that occurred 

during the test (discussed later) were deemed to have a greater 

effect on the water-level data collected during the test. 

The production wells were shut down on July 23, 2007, at 

09:49 a.m. for 72 hours to allow for water-level recovery. The 

pumping part of the test was conducted from July 26, 2007, at 

11:00 a.m. to August 3, 2007, at 08:00 a.m. when the pumps 

were turned off for the aquifer-test recovery period, which 

continued until August 6, 2007, at 10:00 a.m. At that time, the 

pumps were returned to normal system operation. 
Data Collection and Analysis 11

Table 1. Construction details of production wells, Fairfield Hills well field, Pootatuck River Basin, Newtown, Connecticut.

[LSD, land-surface datum; Registered pumping rate, maximum permissible pumping rate registered with the Connecticut Department of Environmental Protection; Mgal/d, million gallons per day; gal/min, gallons per minute]

Well Date of construction

Reported depth 

below LSD, 

in feet

Casing 

diameter, 

in inches

Screen 

length, 

in feet

Registered pumping rate

Material screened

Mgal/d gal/min

3 1932 

(refurbished 2006)

64 10 outer 

8 inner

33 0.39 270 Sand and gravel

7 1949 

(refurbished 2001)

89 12 20 0.67 467 Sand, some gravel

8 1949 110 12 20 0.77 533 Sand, some gravel

3/1/2007 4/30/2007 6/30/2007 8/30/2007 10/30/2007

pumping start

pumping stop

DATE

12

8

4

0

DEPTH TO WATER, IN FEET BELOW LAND SURFACE DATUM

NT-15

pumping start

pumping high rate start

pumping stop

Fairfield

Hills

United 

Water

Figure 7. Hydrograph of water levels in Well NT-15, Pootatuck River Basin, Newtown, Connecticut. Location of Well NT-15 shown in 

figure 1.
12 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Following CTDEP Level A mapping regulations (State 

of Connecticut, 1991), the pumping rate was set at the highest 

feasible constant rate. All water pumped from the well field 

was discharged to the Pootatuck River downstream from the 

well-field area and the Berkshire streamgage (012035055). 

Wells 3 and 7 are not equipped to record continuous pumping 

rate; both wells record total gallons pumped on mechanical 

meters, which were read periodically by USGS personnel. 

Well 3 has a mechanical meter that can be timed to determine 

the approximate pumping rate in gallons per minute and 

Well 7 has a digital display of pumping rate. During the 

pumping part of the aquifer test, the pumping rate of Well 3 

averaged 113 gal/min (based on total volume pumped and 

total elapsed time) and ranged from 100 to 150 gal/min. The 

pumping rate of Well 7 averaged 460 gal/min and ranged from 

438 to 465 gal/min (table 2). Precipitation was monitored in 

a cup-type rain gage installed at the well field prior to and 

throughout the test. Amounts were recorded each morning 

when field personnel arrived onsite although precipitation may 

have started the previous day. 

On July 19, 2007, 7 days prior to the aquifer test, 

0.6 in. of rain was measured in the onsite rain gage, on 

July 21, 2007, 1.5 in. of rain was measured, and on July 24, 

2007, the day after the pumps were turned off, 1.3 in. of rain 

was measured. Because flow in the Pootatuck River continued 

to be below the maximum streamflow allowed for Level A 

mapping, the tests were run as scheduled. On July 29, 2007, 

3 days after pumping started, 0.41 in. of rain was recorded. 

The test continued because this amount of precipitation was 

within the regulations set by CTDEP; however, the rain 

affected the stage of the Pootatuck River and the water level 

in observation wells (fig. 8).

Groundwater-Level Monitoring

Water levels were measured continuously with transducers in the 10 observation wells, in the middle piezometer 

near Well 7, and in Well 3. Water levels in the remaining five 

streambed piezometers and Well 7 (fig. 5) were measured 

manually because access limitations restricted the installation 

of pressure transducers.

Surface-Water Monitoring

Stream stage was measured next to each of the piezometers and at the four staff gages (fig. 5). During the aquifer 

test, streamflow in the Pootatuck River was measured at the 

upstream and downstream staff gages starting 5 days before 

the aquifer test and during the test. No measurements were 

made after pumping stopped. Flow measurements were made 

at least once each day at each location. Most of the streamflow measurements indicated an increase in flow between the 

upstream and downstream sites even when Wells 3 and 7 were 

pumping; however, because of the error of the measurements 

(discussed earlier), this result is only one of several possibilities. An increase in streamflow may be because of bank or 

wetland storage of stream water caused by the rain events 

Table 2. Pumping rates for Wells 3 and 7 during Fairfield Hills 

aquifer test, Pootatuck River Basin, Newtown, Connecticut.

[gal/min, gallons per minute]

Well 3 Well 7

Date/time

Pumping 

rate, 

in gal/min

Date/time

Pumping 

rate, 

in gal/min

7/26/2007 11:00 Pump on 7/26/2007 11:00 Pump on

7/26/2007 12:18 130 7/26/2007 11:20 457

7/26/2007 12:20 125 7/26/2007 11:22 456

7/26/2007 12:22 140 7/26/2007 11:35 459

7/26/2007 13:04 120 7/26/2007 11:45 460

7/26/2007 13:06 110 7/26/2007 11:55 453

7/26/2007 13:07 125 7/26/2007 12:07 462

7/26/2007 16:04 125 7/26/2007 12:19 459

7/27/2007 06:44 130 7/26/2007 12:39 454

7/27/2007 06:46 100 7/26/2007 12:50 438

7/27/2007 06:47 125 7/26/2007 13:04 458

7/27/2007 10:07 125 7/26/2007 13:13 455

7/27/2007 10:10 110 7/26/2007 13:32 448

7/27/2007 14:02 125 7/26/2007 13:55 454

7/27/2007 14:04 120 7/26/2007 14:30 460

7/28/2007 09:04 120 7/26/2007 17:00 459

7/28/2007 09:08 125 7/27/2007 08:29 462

7/28/2007 09:20 125 7/27/2007 14:17 461

7/29/2007 07:01 120 7/28/2007 07:55 456

7/29/2007 07:05 120 7/29/2007 07:46 463

7/29/2007 10:07 120 7/30/2007 11:28 465

7/30/2007 12:54 120 7/30/2007 13:13 460

7/31/2007 10:45 115 7/31/2007 07:02 458

8/01/2007 09:47 150 7/31/2007 09:35 459

8/01/2007 09:51 110 7/31/2007 10:25 461

8/01/2007 09:54 110 8/01/2007 08:19 463

8/02/2007 09:34 110 8/01/2007 10:39 461

8/03/2007 07:32 100 8/02/2007 09:00 463

8/03/2007 07:34 105 8/03/2007 07:00 462

8/03/2007 08:00 Pump off 8/03/2007 08:00 Pump off
Data Collection and Analysis 13

both before and during the test and groundwater inflow to the 

stream from the east (opposite side from the well). However, 

the magnitude of the increase was less when Wells 3 and 7 

were pumping (figs. 9 and 10). The flow measurements for 

the Pootatuck River at the staff gages near Wells 3 and 7 are 

shown on table 3 and figures 9 and 10.

The relative position of the water levels measured inside 

(groundwater level) and outside (stream stage) the streambed 

piezometers (head gradient) determined whether groundwater was discharging to the stream or the stream water was 

infiltrated into the groundwater (stream inflow). In figure 11, 

the groundwater level is higher than the stream stage in each 

streambed piezometer near Well 3 indicating the groundwater 

is discharging to the stream, but the magnitude of the head 

gradient is smaller—or zero—during the pumping phase of the 

test. This is a similar pattern to that observed in the streamflow measurements. The downstream piezometer shows some 

stream inflow (fig. 11C). 

The head gradients in the piezometers near Well 7 indicate that during pumping, water moves from the stream into 

the aquifer (fig. 12). (Note: no measurement was made immediately after the pump was turned off on August 3, 2007. It is 

likely that the groundwater and surface-water levels would 

have been higher. Similarly, the water-level measurements 

5.0

4.5

4.0

3.5

3.0

2.5

2.0

0

1

2

3

4

no data

DEPTH TO WATER BELOW MEASURING POINT, IN FEET

DATE

PRECIPITATION, IN INCHES AND STREAM STAGE, IN FEET

Period of precipitation

and total amount recorded

Stream stage

Date tic, day begins at

00:00 hours

One whole day, 

00:00 to 24:00

NT-91

NT-107

EXPLANATION

7/22/2007 7/24/2007 7/26/2007 7/28/2007 7/30/2007 8/01/2007 8/03/2007

NT-107

NT-91

Stream stage

1.3 inches

0.41 inches

Figure 8. Hydrograph of stream stage recorded at the Berkshire streamgage, water levels in Wells NT-107 and 

NT-91, and precipitation recorded in the onsite rain gage, Fairfield Hills well field, Pootatuck River Basin, Newtown, 

Connecticut. Location of Wells NT-107 and NT-91 shown in figure 5.
14 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

first. Groundwater temperature measured in the downstream 

piezometer near Well 7 indicated a good connection between 

the surface water and the groundwater. The temperature 

2 ft below the streambed is very similar to the surface-water 

temperature (0 ft below the streambed) except there is a delay 

in the fluctuation of the groundwater temperature (fig. 13). 

The temperatures measured 4 and 6 ft below the streambed 

showed fluctuations that were progressively more subdued 

and with increasing delay. There was little temperature change 

at this location when the well was pumping. In the upstream 

piezometer (fig. 13), the groundwater temperature became 

more similar to that of the surface water when the well was 

pumping, again with a delay that increased with depth. This 

indicates that surface water was drawn into the aquifer by the 

production well. 

may have been higher immediately after the rain event on 

July 29.) Although the changes in head gradients observed in 

the piezometers near Well 7 indicate stream inflow and the 

streamflow measurements do not, the dynamic pattern of the 

two sets of data is similar (figs. 10 and 12). Because of the 

probable errors in the streamflow data, mentioned previously, 

the head-gradient data are considered to be more reliable; thus, 

it is assumed there is stream inflow near Well 7.

Temperature Monitoring

Groundwater temperature was measured in all streambed 

piezometers at varying intervals prior to the aquifer test and 

in all piezometers except SBP7ms, which was instrumented 

with a pressure transducer, during the aquifer tests. The data 

from the piezometers near Well 7 are clearer, and are discussed 

5

10

15

20

25

30

35

0

5

10

15

20

25

30

35

2007

Upstream

Downstream

Pump on Pump off Pump on

Well 3

MEASURED STREAMFLOW, 

IN CUBIC FEET PER SECOND 

Rain

recorded

Rain

recorded

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4

July August

2

4

6

8

10

12

14

16

18

20

Upstream

Downstream

Rain

recorded

0

2

4

6

8

10

12

14

16

18

20

Rain

recorded

Pump on Pump off Pump on

Well 7

MEASURED STREAMFLOW, 

IN CUBIC FEET PER SECOND 

2007

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4

July August

Figure 9. Streamflow measured upstream and downstream from Well 3, Pootatuck River Basin, Newtown, Connecticut. Location 

of Well 3 shown in figure 5.

Figure 10. Streamflow measured upstream and downstream from Well 7, Pootatuck River Basin, Newtown, Connecticut.

Location of Well 7 shown in figure 5.
Data Collection and Analysis 15

Table 3. Stage and discharge measurements at local streamflow-measuring sites, Pootatuck River near Fairfield Hills well field, 

Newtown, Connecticut.

[ft3/s, cubic feet per second; ft, feet; -, indicates a flow loss; --, data not available]

Production Well 7 Production Well 3

Upstream Downstream Upstream Downstream

Date Discharge, 

in ft3/s 

Stage, 

in ft

Discharge, 

in ft3/s

Stage, 

in ft

Discharge 

difference, 

in ft3/s

Date Discharge, 

in ft3/s 

Stage, 

in ft

Discharge, 

in ft3/s 

Stage, 

in ft

Discharge 

difference, 

in ft3/s

7/03/07 3.12 14.87 2.88 21.54 -0.24 7/03/07 4.77 12.64 4.65 11.42 -0.12

7/17/07 2.97 14.85 2.31 21.51 -0.66 7/17/07 3.60 12.59 3.74 11.38 0.14

7/17/07 2.27 14.85 2.55 21.52 0.28 7/17/07 3.96 12.59 3.64 11.38 -0.32

7/19/07 5.35 14.96 5.09 21.62 -0.26 7/19/07 7.43 12.73 7.71 11.51 0.28

7/21/07 9.15 15.06 9.12 21.72 -0.03 7/21/07 10.8 12.80 11.5 11.62 0.70

7/22/07 6.40 15.00 6.06 21.66 -0.34 7/22/07 7.63 12.74 8.28 11.51 0.65

7/23/07 8.04 15.06 11.5 21.80 3.46 7/23/07 26.3 13.10 34.2 12.14 7.90

7/24/07 17.8 15.24 16.0 21.88 -1.8 7/24/07 21.2 13.00 20.7 11.86 -0.50

7/25/07 10.7 15.08 11.0 21.73 0.39 7/25/07 11.0 12.82 11.6 11.64 0.60

7/26/07 7.20 15.02 8.32 21.67 1.12 7/25/07 10.6 12.82 11.9 11.64 1.30

7/26/07 7.26 15.02 7.98 21.67 0.72 7/25/07 10.7 12.82 11.7 11.64 1.00

7/26/07 6.82 15.01 8.09 21.67 1.27 7/26/07 8.54 12.76 8.67 11.56 0.13

7/26/07 6.41 15.00 6.99 21.66 0.58 7/26/07 8.40 12.76 9.05 11.56 0.65

7/26/07 6.25 14.99 6.81 21.65 0.56 7/26/07 8.20 12.76 8.76 11.56 0.56

7/27/07 5.37 14.96 5.54 21.62 0.17 7/26/07 7.57 12.74 8.33 11.54 0.76

7/27/07 5.28 14.96 5.62 21.62 0.34 7/26/07 7.89 12.74 8.47 11.54 0.58

7/28/07 4.65 14.93 4.78 21.58 0.13 7/27/07 6.37 12.70 6.80 11.50 0.43

7/28/07 4.53 14.93 4.70 21.58 0.17 7/27/07 6.50 12.69 7.00 11.50 0.5

7/29/07 12.7 15.13 13.0 21.78 0.31 7/28/07 -- -- 5.73 11.45 --

7/30/07 8.25 15.04 9.15 21.69 0.90 7/28/07 -- -- 5.66 11.45 --

7/31/07 6.34 15.00 7.04 21.64 0.70 7/29/07 15.3 12.88 15.0 11.72 -0.31

8/02/07 3.75 14.91 4.12 21.57 0.37 7/30/07 10.2 12.78 10.4 11.60 0.21

8/03/07 3.43 14.90 3.79 21.56 0.36 7/31/07 6.98 12.70 7.46 11.51 0.48

8/01/07 5.74 12.67 6.07 11.46 0.33

8/02/07 5.98 12.66 5.79 11.46 -0.19

8/03/07 4.66 12.64 4.79 11.43 0.13
16 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

A

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

B

Groundwater

Surface water

1.7

1.9

2.1

2.3

2.5

2.7

DATE

C

SBP3us

SBP3ms

SBP3ds

DEPTH TO WATER, IN FEET BELOW LAND SURFACE DATUM 

7/19/2007 7/21/2007 7/23/2007 7/25/2007 7/27/2007 7/29/2007 7/31/2007 8/02/2007 8/04/2007 8/06/2007

EXPLANATION

Pump off Pump on

Figure 11. Water-level measurements of surface water and groundwater in A, upstream; B, middle; and C, downstream 

piezometers near Well 3, Pootatuck River Basin, Newtown, Connecticut. Location of piezometers shown in figure 5.
Data Collection and Analysis 17

Groundwater

Surface water

EXPLANATION

2.5

2.6

2.7

2.8

2.9

3.0

3.1

A

2.10

2.15

2.20

2.25

2.30

2.35

2.40

2.45

2.50

2.55

2.60

B

2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

C

SBP7us

SBP7ms

SBP7ds

DEPTH TO WATER, IN FEET BELOW LAND SURFACE DATUM 

DATE

7/19/2007 7/21/2007 7/23/2007 7/25/2007 7/27/2007 7/29/2007 7/31/2007 8/02/2007 8/04/2007 8/06/2007

Pump off Pump on

Figure 12. Water-level measurements of surface water and groundwater in A, upstream; B, middle; and C, downstream 

piezometers near Well 7, Pootatuck River Basin, Newtown, Connecticut. Location of piezometers shown in figure 5.
18 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

In the downstream piezometer near Well 3, the water 

temperature at 2 and 6 ft below the streambed became more 

similar to the temperature of the surface water when the well 

was pumping, with the delay in the beginning of the change 

increasing with depth (fig. 14). (There was a problem with 

data at 4 ft below the streambed; it was not used in the analysis.) This implies movement of surface water into the aquifer. 

The upstream piezometer showed a similar pattern for the 

temperature 2 ft below the streambed (fig. 14).

Water-Quality and Isotope Analysis

Differences between surface-water and groundwater 

concentrations of chloride and sulfate and the stable-isotope 

ratios δ18O and δ2H were used to help estimate their mixing in production Wells 3 and 7 during the aquifer tests. The 

specific conductance and concentrations of chloride and 

sulfate generally were higher in samples from Well 3 and 

from the Pootatuck River than in those from Well 7 (table 

4). The sulfate:chloride mass ratio generally was higher with 

increasing depth (fig. 15A). Concentrations of chloride typically are higher in shallow groundwater because of anthropogenic sources such as road salt and septic leachate; sulfate 

in deep groundwater can be higher in some Connecticut 

bedrock aquifers because of weathering of aquifer minerals such as gypsum or pyrite. The sample from Well NT-94, 

which is upgradient from and close to Well 7, had the highest 

sulfate:chloride ratio and was assumed to represent ambient 

groundwater. The ratios of samples from Well 3 are more similar to those in the Pootatuck River, although this well is about 

0.5 mi from NT-94, and it is possible that ambient groundwater chemistry may differ between the two locations.

The δ18O and δ2H values of surface water and groundwater reflect the δ18O and δ2

H values in local precipitation (Craig, 

1961; Clark and Fritz, 1997). The δ2H and δ18O values in river 

water are enriched by evaporation during summer months 

because the lighter isotopes of hydrogen and oxygen evaporate 

more easily than the heavy isotopes of hydrogen and oxygen 

(Gat, 2008). The slope of the evaporation line is a function of 

humidity as well as temperature (Clarke and Fritz, 1997).

6/01/2007 6/13/2007 6/26/2007

10

12

14

16

18

20

22

24

26

TEMPERATURE, IN DEGREES CELSIUS

pump on pump off

SBP7ds

DATE

8

10

12

14

16

18

20

22

24

26

pump off pump on

SBP7us

6 feet below streambed

4 feet below streambed

2 feet below streambed

0 feet below streambed

7/08/2007 7/21/2007 8/02/2007 8/15/2007

6/01/2007 6/13/2007 6/26/2007 7/08/2007 7/21/2007 8/02/2007 8/15/2007

EXPLANATION

Figure 13. Groundwater temperatures in streambed piezometers SBP7ds (downstream) and SBP7us (upstream) 

from Well 7, Pootatuck River Basin, Newtown, Connecticut. Location of piezometers shown in figure 5.
Data Collection and Analysis 19

meteoric water lines that differ from the global line because 

of varying climatic and geographic parameters), has a slope 

of about 8 (Craig, 1961; Clark and Fritz, 1997). Precipitation 

that runs off to streams or that quickly recharges groundwater 

reflects the δ18O and δ2H of the local precipitation. Transpiration does not involve fractionation (Zimmerman and others, 

1967; Förstel, 1982), although some vegetation such as permanent grasses can increase evaporative loss seasonally (Darling 

and Bath, 1988). The sample from NT-94, which is upgradient from Well 7, is relatively depleted in the heavy isotopes 

oxygen-18 and hydrogen-2 (deuterium) and reflects a greater 

contribution from ambient groundwater (fig. 16). The δ18O and 

δ2H values for NT-85 and NT-86 are relatively enriched and 

appear to show more influence from the river (fig. 16). A line 

drawn through these data points has a slope of 9.08 and is very 

similar to a local MWL (LMWL) that is based on groundwater 

and surface-water samples in part of the Pomperaug River 

Basin, which is about 9 mi to the north-northeast (Brown and 

others, 2009). 

During the sampling period, the δ2H and δ18O values 

in river water contrasted with δ2H and δ18O values in 

groundwater samples, especially in the sample from NT-94, 

which represents ambient groundwater (table 4 and figs. 15B 

and 16). The sulfate:chloride mass ratios in water samples 

increase and δ2H values decrease with depth, apparently 

because of the increased effects of ambient groundwater and 

decreased effects of surface water on sample chemistry 

(fig. 15B). The chemistry of samples from the production 

wells during the aquifer tests is more similar to the chemistry 

of the Pootatuck River than to ambient groundwater 

represented by NT-94. The chemistry of samples from Well 3 

is very similar to that of the river samples; the sulfate:chloride 

ratio of Well 3 is only 0.1 higher than the ratio of the river; the 

sulfate:chloride ratio of Well 7 is about 0.65 higher.

The δ18O and δ2H values were plotted to help determine stream inflow to groundwater and the resulting effect 

on groundwater chemistry. The global meteoric water line 

(MWL), which represents a global average of δ18O and δ2H 

in precipitation (and an average of many local or regional 

10

12

14

16

18

20

22

24

pump on

pump off 

SBP3ds

10

12

14

16

18

20

22

24

pump on

pump off

SBP3us

TEMPERATURE, IN DEGREES CELSIUS

DATE

6 feet below streambed

4 feet below streambed

2 feet below streambed

0 feet below streambed

EXPLANATION

6/01/2007 6/13/2007 6/26/2007 7/08/2007 7/21/2007 8/02/2007 8/15/2007

6/01/2007 6/13/2007 6/26/2007 7/08/2007 7/21/2007 8/02/2007 8/15/2007

Figure 14. Groundwater temperatures in streambed piezometers SBP3ds (downstream) and SBP3us (upstream) 

from Well 3, Pootatuck River Basin, Newtown, Connecticut. There was a problem with data at 4 feet below 

streambed. Location of piezometers shown in figure 5.
20 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

through each of the points representing sampled Pootatuck 

River water; however, for simplicity, only the “envelope” area 

created by these lines is shown (fig. 16). 

δ2H and δ18O values in samples from wells near the river, 

including Wells 3 and 7, likely reflect stream inflow from the 

river. Because the mixing lines show little or no change in 

δ18O, only δ2H was used in mixing calculations. The fraction 

of surface water in the mixture is calculated using the following expression:

frw = (Ym–Ygw)/(Ysw–Ygw) (1)

where

f

rw is the fraction of river water, and

Ym, Ygw, and Ysw denote the δ2H in the mixture, 

groundwater, and surface water, 

respectively.

A comparison of samples collected from the production wells 

at the end of the aquifer test with the three surrogate evaporation lines indicates that some of the pumped water originates 

from the river (table 5). 

Water that has evaporated from surface water or that 

has mixed with evaporated water plots below the MWL, 

typically along a divergent line with a lesser slope of 5 or 

6 for most natural waters (Craig, 1957). The mean δ2H and 

δ18O values from the Pootatuck River and samples from both 

production wells at the end of the aquifer test likely plot 

along an evaporation water line (EWL). In this study, there 

were no data for samples that have undergone different stages 

of evaporation to complete the EWL typically observed; 

therefore, EWLs inferred from water samples collected in 

two other studies, in Connecticut and adjacent New York, 

were used as surrogate EWLs for this analysis (fig. 16). An 

EWL with a slope of 6.08 was based mainly on groundwater 

samples affected by nearby streams in the Pomperaug River 

Basin (Brown and others, 2009); and an EWL with a slope 

of 5.75 was based on lake samples collected in Wawarsing, 

New York (C.J. Brown, U.S. Geological Survey, written 

commun., 2009). In addition, laboratory studies show that 

the heavy isotope enrichment ratio δ2H /δ18O at ordinary 

temperatures consistently follows a slope of about 5 (Craig, 

1961). Therefore, EWLs with slopes of 6.08, 5.75, and 5 

were drawn on figure 16 to estimate mixing of surface water 

with groundwater. All three lines could have been drawn 

Table 4. General chemical characteristics and concentrations of chloride and sulfate, sulfate:chloride mass ratios, and stable isotope 

values for δ2H and δ18O in the Pootatuck River Basin, Newtown, Connecticut.

[ft, feet; LSD, land-surface datum; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; SO4, sulfate; Cl, 

chloride; δ2H, stable isotope ratio of hydrogen-2 to hydrogen-1; δ18O, stable isotope ratio of oxygen-18 to oxygen-16; na, not applicable; --, no data]

Site

Sample 

depth, 

in ft 

below LSD

Date Time

Temperature, 

in °C

Specific conductance, 

in μS/cm

pH Chloride, 

in mg/L

Sulfate, 

in mg/L

SO4:Cl 

ratio

δ2H, 

in per mil

δ18O, 

in per mil

Well 7 89 7/26/2007 12:12 11.5 106 7.04 7.7 8.6 1.12 -45.3 -7.6

Pootatuck 

River

na 7/26/2007 12:35 17.1 191 7.61 22 9.5 0.43 -43.5 -6.8

Well 7 89 7/26/2007 13:30 11.4 110 7.01 -- -- -- -- --

Well 3 58.5 7/26/2007 14:20 12.0 282 7.00 20 10 0.50 -42.9 -7.2

Well 3 58.5 7/28/2007 14:00 -- 264 6.81 -- -- -- -- --

Well 7 89 7/29/2007 15:00 -- 113 6.54 8.6 9.3 1.08 -45.1 -7.5

Well 7 89 8/02/2007 14:39 10.7 91.1 7.23 9.0 9.4 1.04 -45.7 -7.5

NT-85 79.7 8/02/2007 16:12 10.9 110 8.26 7.7 11 1.43 -45.4 -7.6

NT-86 21.2 8/02/2007 15:35 12.5 128 7.51 10 9.2 0.92 -45.0 -7.5

Pootatuck 

River

na 8/02/2007 13:40 21.6 203 7.43 26 10 0.39 -43.0 -6.9

Well 3 58.5 8/02/2007 15:00 -- 283 7.08 26 13 0.49 -44.5 -7.2

NT-94 47.5 8/06/2007 14:38 9.9 45.0 6.67 1.7 8.3 4.88 -46.6 -7.7
Data Collection and Analysis 21

Figure 15. Relation between sulfate: chloride mass ratios and A, depth below ground surface and B, isotope values 

of delta hydrogen-2 (δ2H) in groundwater and surface water in the Pootatuck River Basin, Newtown, Connecticut. Data 

collected between July 26 and August 6, 2007. Location of wells sampled shown in figure 5.

0

20

40

60

80

100

DEPTH, IN FEET BELOW LAND SURFACE DATUM

0 1 2 3 4 5

SULFATE: CHLORIDE MASS RATIO

Pootatuck River

NT-94

Well 3

NT-86

NT-85

Well 7

NT-86

NT-94

Well 3

Shallow monitoring well and identifier

Well 7

Well 3

Deep monitoring well and identifier

Pootatuck River

Well 7

-48

-47

-46

-45

-44

-43

-42

0 1 2 3 4 5

Well 3

NT-94

Well 3

Well 7

Well 7

NT-86

SULFATE: CHLORIDE MASS RATIO

NT-85

Pootatuck River

A

B

DELTA HYDROGEN-2 (δ2H), IN PER MIL
22 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

NT-85

NT-94

NT-86

Well 7

Well 3

Well 3

Well 7 Well 7

Pootatuck River

Hypothetical range of

100% river water

Pootatuck River

Pootatuck River

Average

-8.0 -7.8 -7.6 -7.4 -7.2 -7.0 -6.8 -6.6

-47

-48

-49

-50

-46

-45

-44

-43

-42

Pomperaug River meteoric water line

Pootatuck River meteoric water line

(100% ground water)

A

y = 9.077x + 23.78

Hypothetical mixing end 

point mixing pathway

A B

C

D

Evaporation line

(100% river water)

Woodbury, CT

Evaporation line

(100% river water)

Wawarsing, NY

Evaporation line

(100% river water)

y = 5.75x + b

y = 5.0 + b

y = 6.08x + b

7/26/07

7/26/07

7/26/07

7/29/07

8/2/07

8/2/07

8/2/07

NT-86

NT-94

Well 3

Shallow monitoring well and identifier

Well 7

Well 3

Deep monitoring well and identifier

Pootatuck River

Well 7

DELTA HYDROGEN-2 (

δ2H), IN PER MIL

DELTA OXYGEN-18 (δ18O), IN PER MIL

Figure 16. Isotope values of delta oxygen-18 as a function of delta hydrogen-2 for samples of groundwater and surface 

water in the Pootatuck River Basin, Newtown, Connecticut. Data collected between July 26 and August 2, 2007. Location of 

wells sampled shown in figure 5.
Data Collection and Analysis 23

Table 5. Results of mixing models estimated using isotope values for δ2H and δ18O in river and groundwater samples in the Pootatuck River Basin, Newtown, Connecticut.[δ2H, stable isotope normalized ratio of hydrogen-2 to hydrogen-1; δ18O, stable isotope normalized ratio of oxygen-18 to oxygen-16; LMWL, local meteoric water line; EWL, evaporation water line; CT, Connecticut; NY, New York; Stable isotope values in per mil]

Mixing points Mixing points Using Woodbury, CT EWL Using Wawarsing, NY EWL Using Gat (2008) slope of 5 EWL

Y

m

1 Y

gw

1 Y

rw

1 Y

rw

1 Y

rw

1

Sample 

(mixure) Date δ2H δ18O

Theoretical 

point on LMWL 

(end member 1) δ2H δ18O

Theoretical 

point on EWL 

(end member 2) δ2H δ18O

Fraction 

river water, 

f

rw

1

, 

in percent δ2H δ18O

Fraction 

river water, 

f

rw

1

, 

in percent δ2H δ18O

Fraction 

river water, 

f

rw

1

, 

in percent

Mixing model A Well 7 7/29/2007 -45.1 -7.5 Well 7 -44.9 -7.5 Well 7 -47.2 -7.5 7 -46.6 -7.5 9 -46.0 -7.5 14Mixing model B Well 7 8/02/2007 -45.7 -7.5 32 44 69Mixing model C Well 3 7/26/2007 -42.9 -7.2 Well 3 -42.2 -7.2 Well 3 -45.6 -7.2 20 -45.0 -7.2 24 -44.6 -7.2 29Mixing model D Well 3 8/02/2007 -44.5 -7.2 68 81 97

1f

rw = (Ym–Ygw)/(Yrw–Ygw), where Ym, Ygw, and Yrw represent the values of δ2H in the mixture, groundwater, and river water, respectively.
24 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Three samples were collected at Well 7. A shift in δ2H 

was accompanied by a shift in δ18O from the start of pumping 

to a second sample collected on July 29, 2007, after 3 days 

of pumping (fig. 16). This shift may result from analytical 

uncertainty, which is 2 per mil for δ2H and 0.2 per mil for δ18O 

at the 2-sigma level. A water-mixing calculation using δ2H in 

the second Well 7 sample shows a range in contribution of 7 

to 14 percent river water, using the three surrogate evaporation 

lines defined above to determine the end member (Mixing 

model A, table 5). A similar calculation for the third sample 

from Well 7, collected on August 2, 2007, near the end of 

pumping, showed a range in contribution of 32 to 69 percent 

river water (Mixing model B, table 5).

Two samples were collected at Well 3. At the beginning 

of the test between 20 and 29 percent of the sample was 

estimated to have been derived from the river (table 5). The 

second sample from Well 3 collected near the end of pumping 

on August 2, 2007, showed a range in contribution of 68 to 

97 percent river water. Note that these estimates are semiquantitative because of the analytical error of δ2

H and δ18O, 

the LMWL was interpolated, and the evaporation lines 

were estimated.

The water-quality and stable-isotope analysis 

corroborates the stream inflow shown by the streamflow 

and piezometer data near Well 7; however, the water-quality 

and stable-isotope analysis contradicts the streamflow and 

piezometer data near Well 3. It was not possible to determine 

the reason for this discrepancy within the scope of this project; 

more stable-isotope samples would be necessary to reach a 

definitive conclusion of stream inflow at Well 3. 

Analysis of Aquifer Tests at Fairfield Hills Well 

Field

Prepumping data from observation wells were 

plotted to establish if corrections to the drawdown data were 

necessary to account for the effects of natural water-level 

trends. Water levels from the observation wells near Well 7 

fluctuated with the pumping, so it could not be determined 

whether there was a trend. Well 3 was undergoing adjustments 

related to recent refurbishing and was not in use prior to the 

aquifer test. Although water levels in observation wells near 

Well 3 showed a slight downward trend in the weeks before 

the aquifer test (fig. 17), the effect of the rain events recorded 

on July 19, 21, 24, and 29 had a greater effect on the data, so 

no correction was made for the antecedent water-level trend. 

The small, regular groundwater-level fluctuations prior to 

the test are attributed to nearby pumping at a manufacturing 

facility (Frederick Hurley, Newtown, oral commun., 2007) and 

were not considered in the analysis because they were relatively consistent. Well 3 was not able to maintain a constant 

pumping rate; therefore the results of the test are estimates. 

The large water level fluctuation on July 19, 2007, was 

because of a precipitation event. The large fluctuation on 

July 23, 2007, was because of both the shut down of the production wells and a precipitation event that began on that date. 

(The precipitation was recorded on the following day, 

July 24, 2007.) 

The distance-drawdown method for calculating 

transmissivity assumes the aquifer is confined and the 

observation wells fully penetrate the aquifer. Because the 

aquifer at Wells 3 and 7 is unconfined, drawdown data 

collected during the aquifer tests were corrected for partial 

penetration using the following equation (Cooper and Jacob, 

1946; Kruseman and DeRidder, 1990):

s′ = s – (s2/2b) (2)

where

s′ is the corrected drawdown displacement,

s is the observed drawdown, and

b is the saturated thickness of the aquifer.

The observed and corrected maximum drawdowns are shown 

in table 6. Table 7 shows transmissivity and hydraulic conductivity values calculated for the aquifer in the vicinity of 

Wells 3 and 7.

Table 6. Observed and corrected drawdowns in observation 

wells, Fairfield Hills well field, Pootatuck River Basin, Newtown, 

Connecticut.

[ft, feet]

Well identifier Observed drawdown, 

in ft

Corrected drawdown1, 

in ft

NT-85 3.03 2.97

NT-86 1.86 1.77

NT-90 3.29 3.23

NT-91 0.23 0.23

NT-94 7.74 7.02

NT-106 1.05 1.03

NT-107 0.65 0.64

NT-108 1.41 1.39

NT-109 1.91 1.86

NT-110 0.32 0.32

1

Drawdowns corrected for partial penetration using method described by 

Cooper and Jacob (1946) and Kruseman and DeRidder (1990).
Data Collection and Analysis 25

Figure 17. Pretest water levels in observation wells A, NT-106 and B, NT-107 near Well 3, Fairfield Hills well field, Pootatuck River 

Basin, Newtown, Connecticut. Location of observation wells NT-106 and NT-107 shown in figure 5.

A

B

6.2

5.7

5.2

4.7

4.2

3.7

3.2

DEPTH TO WATER, IN FEET BELOW MEASURING POINT

Well NT-106 – 200 feet from production well

well field shut down

07/02/07 07/08/07 07/14/07 07/20/07 07/26/07

DATE

5

4

3

2

DEPTH TO WATER , IN FEET BELOW MEASURING POINT

Well NT-107 – 215 feet from production well (opposite side of Pootatuck River)

well field shut down

07/02/07 07/08/07 07/14/07 07/20/07 07/26/07
26 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

from average gallons per minute, total gallons pumped, and 

the production times. The reported range of total monthly 

volume of groundwater withdrawal for Well 1 for the years 

1997 to 2001 was 0 to 1.8 Mgal and for Well 2 was 9.0 to 

19.6 Mgal. 

Induced Infiltration Test at United Water Well 

Field

A short-duration, single production-well recovery test 

was conducted on Well 2, the primary source of water at the 

United Water well field, because lack of available storage and 

the need to provide fire protection and uninterrupted potable 

water to customers of United Water precluded shutting down 

the production wells for 3 days to allow groundwater-level 

recovery. The maximum time the system could be shut down 

was approximately 9.5 hours during late-night hours when 

there was minimal demand for water. Thus, it was not possible 

to run a typical aquifer test on the United Water well field. 

Because of the system limitations, the CTDEP approved a 

plan to collect data to verify stream inflow from the Pootatuck 

River by conducting a short-duration, reverse aquifer test. The 

aquifer properties were calculated from data collected during 

historical aquifer tests.

Instead of the required minimum 3-day shutdown of the 

well field followed by a 5-day pumping period, the pumping 

rate of Well 2 was increased on August 27, 2007, to the maximum rate that the system could sustain; prior to August 27, 

2007, both Wells 1 and 2 were pumping. The target pumping 

rate of 750 gal/min, the registered rate, could not be achieved, 

so the rate was set at approximately 650 gal/min. A hydrograph of pumping rate prior to, during, and after the aquifer 

test is shown in figure 18. During this period of increased 

pumping, more water was pumped than was needed to supply 

the customers of United Water. The excess was discharged 

from a hydrant to a tributary that joins the Pootatuck River 

downstream from the well field. This discharge may have 

caused a backwater condition, which will be discussed later. 

After 8 days of pumping at the higher rate, Well 2 was 

shut down on September 4, 2007, at 10:06 p.m. Monitoring 

of water-level recovery and stream stage continued until 

September 5, 2007, at 07:38 a.m. when available system 

storage was depleted and Well 2 was reactivated. 

United Water Well Field

Site Description

The United Water well field consists of two wells 245 ft 

apart in stratified glacial deposits, which are 50 to 85 ft thick. 

The identification numbers of these wells is confusing because 

the United Water numbers—Newtown Wells 1 and 2—are 

the reverse of those in CTDEP records. The United Water 

numbers will be used in this report (fig. 6). Information on the 

wells is provided in table 8.

Pumping Information

Well 2 is the primary production well with a registered 

diversion of 1.08 Mgal/d (750 gal/min). Well 1 is the reserve 

well, with a registered diversion of 0.50 Mgal/d (350 gal/min), 

and operates with Well 2 during periods of high water 

demand. The system operates as an on-demand system. 

Wells 1 and 2 share a digital meter, which measures total 

thousands of gallons pumped; this meter is read daily to 

produce monthly production reports. In addition, the operating 

hours for each well are tallied. Pumping rates are calculated 

Table 7. Transmissivity and hydraulic conductivity calculated 

from aquifer tests, Pootatuck River Basin, Newtown, Connecticut.

[ft2/d, feet squared per day; ft/d, feet per day]

Production

well

Saturated 

thickness,

in feet

Transmissivity1,

in ft2/d

Hydraulic conductivity, 

in ft/d

Fairfield Hills well field

Well 3 55 3,200 58

Well 7 85 4,700 55

United Water well field2

Well 2 80 20,000 250

1

Transmissivity calculated from distance-drawdown method (Cooper and 

Jacob, 1946).

2

Data from aquifer test run in June 1981 (Kevin Moran, United Water 

Company, written commun., 2006).

Table 8. Construction details of production wells, United Water well field, Pootatuck River Basin, Newtown, Connecticut.

[LSD, land-surface datum; Registered pumping rate, maximum permissible pumping rate registered with the Connecticut Department of Environmental Protection; Mgal/d, million gallons per day; gal/min, gallons per minute]

Well Date of construction

Reported depth 

below LSD, 

in feet

Casing diameter, 

in inches

Screen length, 

in feet

Registered pumping rate

Material screened

Mgal/d gal/min

1 1980s 50 16 10 0.50 350 Coarse sand and gravel

2 1980s 83 16 20 1.08 750 Coarse sand and gravel
Data Collection and Analysis 27

Figure 18. Pumping rate in Well 2, United Water well field, Pootatuck River Basin, Newtown, Connecticut. Location of Well 2 shown in figure 6.

0

800700600500400300200100

MINUTE PER GALLONS IN RATE, PUPMING

DATE

Test period No data

08/22/07 08/26/07 08/24/07 08/28/07 09/01/07 09/03/07 08/30/07 09/07/07 09/05/07 09/11/07 09/09/07
28 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Figure 19. Water levels in observation well TW-6, United Water well field, Pootatuck 

River Basin, Newtown, Connecticut. Location of observation well TW-6 shown in figure 6.

8/14/2007 8/21/2007 8/28/2007 9/4/2007 9/11/2007

DATE

18

16

14

12

10

8

DEPTH TO WATER , IN FEET BELOW MEASURING POINT

TW-6

increase pumping rate

pump off

pump on

a pressure transducer (fig. 20). When Well 2 was turned off, 

the water level in surface water and groundwater rose, but the 

infiltration continued, most likely because the recovery period 

was just 9.5 hours. 

Because the infiltration test had to be run after dark, it 

was not possible to make streamflow measurements during the 

test; only stage data were recorded. The stage measurements 

for the Pootatuck River upstream and downstream from the 

production well are shown in figure 21. During the period of 

increased pumping, the stage at the upstream and downstream 

sites steadily decreased with a greater decrease at the downstream site. Immediately before the wells were shut down, 

measurements made at 3:30 and 7:50 p.m. indicate a drop in 

the stage at both sites (fig. 21). The discharge line from the 

hydrant was shut down at 7:45 p.m., and it is possible that 

the stream stage dropped with the cessation of the backwater 

condition caused by the discharge from the hydrant. No stage 

measurements were made immediately after the pumps were 

turned off, but measurements made between 5:45 and 

9:00 a.m. on September 5 indicate a rise in stage at the 

upstream and downstream sites, presumably a delayed 

response to cessation of pumping. This confirms that the 

pumping caused some streamflow depletion within the reach 

between the streamflow measurement locations. It also indicates that the upstream measuring site was not outside the 

area of influence of the production well. It was not possible to 

measure further upstream because of a wetland.

Groundwater levels in the three available observation 

wells were monitored during the test. Water levels also were 

monitored inside and outside the six streambed piezometers 

installed in the Pootatuck River and at the two staff gages 

upstream and downstream from the production well (fig. 6). 

No precipitation was recorded from August 27 to September 5, 

2007; 7 days prior to and throughout the test. 

Groundwater and Surface-Water-Level Monitoring

Continuous water levels were recorded in observation 

wells TW-2, TW-4, and TW-6 as well as streambed piezometer 

SBP2C (fig. 6). The water levels recorded in observation well 

TW-6 and piezometer SBP2C are shown in figures 19 and 20, 

respectively.

Water levels in Well TW-6, located 5 ft from Well 2, 

clearly shows the effects of pumping on the groundwater 

system, including (1) normal pumping of Wells 1 and 2 prior 

to August 27, 2007, (2) the change to pumping only Well 2 at 

a relatively constant rate, (3) the pump turn off in Well 2 on 

September 4, 2007, and (4) the return to normal pumping 

(fig. 19). Groundwater levels and stream stage were measured at the streambed piezometers to determine whether the 

groundwater was discharging to the stream or the stream water 

was being infiltrated into the groundwater. The head gradient 

at each piezometer indicated stream inflow to the groundwater 

system. The middle piezometer, SBP2C was equipped with 
Data Collection and Analysis 29 Figure 20. Continuous groundwater levels and stream stage at piezometer SBP2C, United Water well field, Pootatuck River Basin, Newtown, Connecticut. Location of piezometer shown in figure 6.

3.003.203.403.603.804.004.204.403.003.203.403.603.804.004.604.805.00

GroundwaterStream stage

EXPLANATION

WATER LEVEL, IN FEET

Missingdata

DATE

08/21/07 08/25/07 08/23/07 08/27/07 08/31/07 09/02/07 08/29/07 09/06/07 09/04/07 09/08/07

SBP2C
30 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut Figure 21. Stage in the Pootatuck River at local stream flow-measuring sites near Well 2 during aquifer test, Pootatuck River Basin, Newtown, Connecticut. Location of sites shown in figure 6. DATE 8/23/2007 8/25/2007 8/27/2007 8/29/2007 8/31/2007 9/2/2007 9/4/2007 9/6/2007 9/8/2007 10.8 10.9 11.0 11.1 11.2 11.3 11.4 10.6 10.7

Downstream stageUpwnstream stage

Normal operations

Normal operations Increased pumping rate at Well 2

STAGE, IN FEET ABOVE NGVD29

Test period,

no pumping
Data Collection and Analysis 31

Temperature Monitoring

Groundwater temperature was measured in all streambed 

piezometers at varying intervals prior to the aquifer test and in 

all piezometers except SBP2C, which was instrumented with 

a pressure transducer, during the aquifer test. Temperatures 

recorded in piezometers SBP2Blb, SBP2Brb, SBP2Dlb and 

SBP2Drb are shown in figures 22 and 23. When Well 2 was 

pumping, the temperatures of the groundwater at 2, 4, and 

6 ft below the streambed were relatively constant and similar 

to each other and approximately equal to the average temperature of the stream (0 ft below streambed). The temperature 2 

ft below the streambed fluctuates with the same period as the 

stream water indicating stream inflow. The temperature pattern 

appears to have a delay of almost a day, possibly indicating 

that the streambed is retarding the flow of water. When 

Well 2 was turned off, the temperature 2 ft below the streambed in the piezometer near the right (near) bank of the stream 

at both locations became less similar to the average temperature of the surface water. 

Historical Aquifer Tests

Several aquifer tests have been conducted at the United 

Water well field in the past (Kevin Moran, United Water 

Figure 22. Temperature recorded in streambed piezometers at location B near Well 2 A, P2Blb (left bank) and 

B, P2Brb (right bank), Pootatuck River Basin, Newtown, Connecticut. Location of piezometers shown in figure 6.

9/2/2007 9/3/2007 9/4/2007 9/5/2007 9/6/2007

10

12

14

16

18

20

22

24

26

P2Blb

6 feet below streambed

4 feet below streambed

2 feet below streambed

0 feet below streambed

pump off

pump on

9/2/2007 9/3/2007 9/4/2007 9/5/2007 9/6/2007

5

8

11

14

17

20

23

26

29

32

P2Brb

pump off

pump on

A

B

TEMPERATURE, IN DEGREES CELSIUS

DATE

EXPLANATION
32 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Company, written commun., 2006). An aquifer test was conducted on two 10-in. diameter test wells installed at the current 

locations of production Wells 1 and 2 at the United Water well 

field in November 1979 by the Stephen B. Church Company 

to determine the safe yield of the aquifer. Six 2.5-in.-diameter 

observation wells (TW-1, TW-2, TW-3, TW-4, TW-5, and 

TW-6, fig. 6) were installed to monitor groundwater levels 

during the aquifer test. Another aquifer test was conducted at 

the United Water well field in June 1981 by the Layne New 

York Company, Inc., for an aquifer mapping study using a 

new, 16-in.-diameter primary production Well 2 (at the same 

location as the 10-in. well 2). 

Analysis of Historical Aquifer Test at United 

Water Well Field

The short-duration, reverse aquifer test conducted 

on Well 2 at the United Water well field did not allow for 

calculation of transmissivity of the aquifer. Therefore, data 

from the June 1981 test, with Well 2 pumping at an average 

of 703 gal/min, were used to calculate transmissivity and 

hydraulic conductivity for use in the groundwater-flow 

model of the current study (table 7). Data from the test in 

November 1979 were not used for typical aquifer test analysis 

because both wells were pumping and there was considerable 

interference between them.

0

5

10

15

20

25

30

35

40

pump off

pump on

09/02/07 09/03/07 09/04/07 09/05/07 09/06/07

09/02/07 09/03/07 09/04/07 09/05/07 09/06/07

DATE

12

13

14

15

16

17

18

19

20

21

22

23

24

6 feet below streambed

4 feet below streambed

2 feet below streambed

0 feet below streambed

pump off 

pump on

TEMPERATURE, IN DEGREES CELSIUS

A

B

EXPLANATION

P2Dlb

P2Drb

Figure 23. Temperature recorded in streambed piezometers at location D near Well 2 A, P2Dlb (left bank) and 

B, P2Drb (right bank), Pootatuck River Basin, Newtown, Connecticut. Location of piezometers shown in figure 6.
Development of the Groundwater-Flow Models 33

Development of the Groundwater-Flow 

Models

Numerical models synthesize existing hydrogeologic 

information into an internally consistent, mathematical representation of a real system or process and thus are useful tools 

for testing and improving conceptual models or hypotheses 

of groundwater-flow systems (Konikow and Reilly, 1999). 

Three-dimensional, numerical groundwater-flow models were 

developed for the Pootatuck River Basin for the purpose of (1) 

synthesizing the available hydrogeologic data including aquifer tests conducted in Newtown, Connecticut; (2) determining 

areas contributing recharge to five simulated production wells 

in Newtown for steady-state conditions; and (3) assessing the 

effect of three potential future scenarios on the distribution 

and amount of recharge available to the groundwater system 

and the associated effect on base flow in streams.

Development of a Conceptual Model of the 

Pootatuck River Basin

The conceptualization of how and where water enters, 

moves through, and leaves the aquifer is critical to the 

development of an accurate flow model (Reilly, 2001). 

Groundwater flow in the Pootatuck River Basin is in an 

aquifer system that consists of bedrock formations as well 

as the unconsolidated material that overlies the bedrock. The 

production wells in Newtown draw water from the productive 

unconsolidated stratified glacial deposits that lie in the center 

of the bedrock valley.

Water levels in shallow water-table aquifers often are 

a subdued replica of the land surface; therefore, the lateral 

boundaries of the aquifer system can be approximated by the 

surface-water basin divide of the Pootatuck River (fig. 24). 

In some basins, differences may exist between the surfacewater and groundwater basin divides, especially where there 

are large changes in aquifer hydraulic properties or large 

groundwater withdrawals near an aquifer boundary. However, 

these effects were assumed not to be significant in the 

Pootatuck River Basin because the production wells are not 

near the watershed divide.

The lower boundary of the aquifer system was set as 

an imaginary surface (at about 80 ft below the bottom of the 

valley-fill material) that is assumed to separate water that 

discharges to the Pootatuck River from water that leaves the 

basin and discharges elsewhere or is stagnant. Under this 

modeling schematic, the aquifer system includes some water 

in the bedrock and all water in the overlying glacial deposits. 

Any further characterization of the deeper, regional flow system can be considered only in a hypothetical sense (Starn and 

others, 2000). The thickness of this layer in the uplands was 

varied during the modeling process; however, it is the interface between the bedrock and the overlying sediments that is 

important and not the thickness of the bedrock layer.

Precipitation is the source of all water in the basin. Some 

of the precipitation is lost to evaporation and transpiration 

by plants; the remainder infiltrates the ground and recharges 

the groundwater system at the water table. There is no lateral 

inflow to the groundwater system in the basin-scale model 

because the model boundaries coincide with the basin boundaries; however, only a part of the boundaries of the wellfield-scale models coincides with the basin boundaries. (The 

boundaries of the well-field-scale models will be discussed 

later.) The groundwater leaves the basin through groundwater 

outflow across the downgradient model boundary, groundwater evapotranspiration, groundwater discharge to streams, or 

by production well withdrawals. 

Development of Numerical Models

The numerical models developed for simulating 

groundwater flow in the Pootatuck River Basin used the 

USGS three-dimensional, finite-difference groundwater-flow 

computer program MODFLOW-2000 (Harbaugh and others, 

2000). A USGS particle-tracking program (MODPATH) 

developed by Pollock (1994) was used with MODFLOW to 

determine the areas contributing groundwater recharge to the 

simulated production wells.

Groundwater-flow models for the Pootatuck River 

Basin were developed at the well-field scale and basin scale 

(fig. 24). The well-field-scale models, one at each of the 

Fairfield Hills and United Water well fields, were run in 

transient mode to simulate the conditions during aquifer tests. 

These models used data from the aquifer tests to determine 

hydraulic conductivity values for the aquifer materials in the 

vicinity of the production wells. All water withdrawn from 

simulated wells was removed from the groundwater system. 

This was somewhat different from the conditions of the 

aquifer tests because some, if not all, well withdrawals were 

discharged to the river downstream from the well fields. 

The well-field-scale models also were run in steady-state 

mode to determine areas contributing recharge to the production wells. The basin-scale model was run in steady state to 

determine the effect of future development on the distribution 

and amount of recharge available to the groundwater system 

and the associated effect on base flow in streams, that is, the 

water budget. All simulated well withdrawals were removed 

from the groundwater system in the steady-state models. This 

is not representative of long-term average annual conditions 

because some of the water withdrawn from the production 

wells is distributed to customers who then discharge it back to 

the groundwater system through onsite septic systems. Thus, 

these models represent a condition with less recharge to the 

groundwater system than may actually occur.

The development of the well-field-scale and basin-scale 

models was an iterative process: (1) a preliminary basin-scale 

model was developed using land surface as the starting water 

levels; (2) the well-field-scale models were developed from 

the preliminary model and were calibrated using aquifer-test 
34 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Newtown

Monroe

Redding

Bethel

Southbury

Easton

Brookfield

73°20' 73°16'

41°24'

41°20'

012035055

01203510

EXPLANATION

Cell containing simulated stream

Model cells and areas

Inactive area

Active basin-scale-model area Production well, and identifier

Town boundary

Continuous-record streamgage, 

 and identifier

United Water well-field-scale model area

Fairfield Hills well-field-scale model area

A A'

B B'

C C'

A A’ Line of section

Well 3

Well 7 Well 8

Well 1

Well 2

0 1 2 MILES

0 1 2 KILOMETERS

Base from Connecticut Department of Environmental Protection 

Digital Line Graphs, 2005

Figure 24. Extent of well-field- and basin-scale models in the Pootatuck River Basin, Newtown, Connecticut.
Development of the Groundwater-Flow Models 35

data; (3) values from the well-field-scale models were used 

as input data for the basin-scale model, which was then 

calibrated; (4) any changes to modeled input values from the 

basin-scale model were reincorporated into each well-fieldscale model as were the values from the basin-scale model of 

water level at the boundaries; and (5) the latest results of each 

well-field-scale model were examined to determine if they 

were a reasonable representation of the hydrologic conditions. 

Steps 3 through 5 were repeated until the well-field-scale and 

basin-scale models were internally consistent and calibrated. 

Because the models were developed iteratively, this 

report discusses the three models together. This first section 

gives information common to all three models, followed by a 

discussion of the setup and calibration of each of them. The 

next major sections give the results of model simulations—the 

delineation of areas contributing recharge to production wells, 

the interaction between groundwater and surface water, and 

the effect of future recharge conditions.

Model Discretization and Boundaries

A finite-difference model grid consists of a series of 

orthogonal model cells in which user-specified inputs of 

hydraulic parameters, model stresses, and boundary conditions 

are varied spatially. A detailed discussion of grid discretization, boundary conditions, and the use of finite-difference 

equations to simulate groundwater flow is presented in 

McDonald and Harbaugh (1988).

The Pootatuck River Basin includes large areas of upland 

till and bedrock (fig. 4). Modeling investigations in areas that 

include upland till and bedrock areas surrounding an aquifer 

comprised of stratified glacial deposits typically focus just 

on the aquifer materials that fill the valley. These models do 

not explicitly simulate the upland areas but rather specify 

the recharge that enters the aquifer from the upland areas 

as enhanced recharge at the contact of the till and stratified 

glacial deposits (Dickerman and others, 1997; Barlow and 

Dickerman, 2001; DeSimone and others 2002; Granato and 

others, 2003; Friesz, 2004). This approach is used to avoid 

the numerical instabilities that result from simulating steeply 

sloping, thinly saturated deposits, such as the upland till and 

bedrock areas of higher elevation that surround the stratified 

glacial deposits of the lowland Pootatuck River Basin. The 

drawback to this approach, however, is that the upland till 

and bedrock areas are not part of the active modeled area, and 

application of the recharge areally and throughout the thickness of the aquifer is somewhat arbitrary. 

A different approach to model layers was used in an 

investigation in the Upper San Pedro Basin, Arizona and 

Mexico (Pool and Dickinson, 2007). It included a spatiallycontinuous active layer that represented both the bedrock that 

underlies the valley-fill sand and gravel deposits and the bedrock and till of the surrounding upland hillsides. The valley-fill 

sand and gravel deposits were simulated with overlying model 

layers that covered a smaller spatial extent. This approach 

was used in both the basin-scale and well-field-scale models 

of the Pootatuck River Basin; the stratified glacial deposits 

were simulated as layers 1 and 2, and both the upland till and 

bedrock areas and the bedrock beneath layer 2 were simulated 

as layer 3. The benefit of this approach is it allows for simulation of the upland till and bedrock areas and a more realistic 

representation of flow from these upland areas to the lowland 

stratified glacial deposits (fig. 25).

All water enters and leaves the groundwater-flow 

model through the hydrologic boundaries and the boundary 

conditions control that flow. Where possible, the lateral 

boundaries coincide with the basin divide and, therefore, 

are no-flow boundaries. The upper boundary of the models 

is the water table, which is a free-surface boundary that 

receives areally-variable recharge from precipitation. The 

lower boundary of the model is bedrock that underlies the 

entire study area. Streams were modeled as “head-dependent 

flow boundaries” using the Streamflow-Routing Package 

(Prudic, 1989) of MODFLOW-2000. This package simulates 

the hydraulic interaction between the aquifer and adjoining 

streams and tracks the amount of water in each modeled 

stream reach. Modeled streams were divided into reaches, 

corresponding to individual model cells and segments, which 

are groups of reaches that are connected in downstream order 

(Prudic, 1989). 

In the model, water may flow either from the aquifer to 

the stream (groundwater discharge) or from the stream into 

the aquifer (stream inflow). The movement of water across 

the streambed is calculated by multiplying the specified 

streambed conductance by the difference between the stream 

stage and the water level in the underlying aquifer. During 

the simulation, streams may go dry when stream inflow to 

the aquifer exceeds inflow to the stream reach from upstream 

reaches. Because the simulation is of groundwater only, flows 

in modeled streams represent only base flow, the groundwater 

component of streamflow. The component of streamflow that 

results from direct or storm runoff is not included in the 

model simulations.

Model Calibration

Model calibration is the process by which modifications 

are made to the initial model-input parameters to make the 

model output more closely match observed water levels and 

base flows (Reilly and Harbaugh, 2004). Parameters adjusted 

during calibration of the models include hydraulic conductivity of the aquifer and of the streambed and specific yield.

The hydraulic property values reported by Haeni 

(1978) were used as a guide for the initial values used in 

the groundwater-flow model. These values were for general 

categories of glacial materials and were not subdivided 

into the individual geologic units denoted on the surficial 

materials map (Stone and others, 1992). Therefore, assigning 

a different hydraulic value to each geologic unit could not 

be justified. During calibration, simulated water levels were 

used to combine adjacent individual geologic units of similar 

description into larger, more generalized areas of uniform 
36 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

1:1 with figure 24

2:1 with figure 24

A A'

2:1 with figure 24

Basin-scale model

Fairfield Hills well-field-scale model

United Water well-field-scale model

Well 7

Well 3

Well 2

Pootatuck River

Pootatuck River

EXPLANATION

Layer 1

Layer 2

Layer 3

B B'

C C'

ALTITIUDE, IN FEET ABOVE NGVD29 

100

150

700

200

300

250

350

400

500

550

450

600

100

150

200

300

250

350

400

500

550

450

600

650

100

150

200

300

250

350

400

500

550

450

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000 22,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 13,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 13,000

14,000

LINEAR DISTANCE, IN FEET Pootatuck River Pootatuck 

River

Pootatuck River

See figure 24 for map

Figure 25. Cross sections showing model layers, Pootatuck River Basin, Newtown, Connecticut. Locations 

shown in figure 24.
Development of the Groundwater-Flow Models 37

hydraulic properties to which hydraulic conductivity values 

could be assigned. This resulted in three principal hydrologic 

units—coarse-grained glacial deposits, which consist of 

sands and gravels; fine-grained deposits, which consist of 

till and fine-grained stratified deposits (silts and clays); and 

bedrock. The unconsolidated materials were assumed to be 

consistent from land surface to bedrock, as evidenced by the 

stacked units mapped by Stone and others (1992). Trial and 

error adjustments to hydraulic conductivity in these larger 

areas were made until simulated water levels more closely 

matched observed water levels. The resulting values of 

hydraulic conductivity were consistent with values calculated 

from the aquifer-test data and also with accepted ranges of 

hydraulic conductivity associated with the type of geologic 

material present. Although the calibrated groundwater-flow 

model uses reasonable parameters, it is important to note that 

groundwater-flow models produce nonunique solutions, such 

that the same model-calculated response in water levels or 

base flows might be obtained by using different combinations 

of hydraulic parameters.

Well-Field-Scale Models

The purpose of the well-field-scale models was twofold: 

(1) to determine hydraulic conductivity values for the aquifer 

materials in the vicinity of the production wells, and (2) to 

determine areas contributing recharge to simulated wells. The 

Fairfield Hills and the United Water well-field-scale models 

(fig. 24) were run in transient mode to simulate the aquifer 

tests and in steady-state mode to determine recharge areas 

to the simulated production wells. Features common to both 

well-field-scale models are summarized first. 

Each model grid had a variable cell size that was centered 

on the production and observation well locations. The grid 

spacings of the cells were 10, 50, 100, and 400 ft, increasing in 

size from the vicinity of the production and observation wells 

to the model boundary. The well-field-scale models mostly 

covered the stratified glacial deposits. 

In each well-field-scale model, three layers were used 

to represent the primary hydrogeologic units, based on the 

surficial materials map (fig. 4) and the bedrock surface map 

(fig. 3). The stratified glacial deposits were divided into model 

layers 1 and 2. The minimum thickness for combined layers 

1 and 2 was 15 ft. Horizontal hydraulic conductivity values 

varied spatially and reflected the type of material present; 

vertical hydraulic conductivity was specified at one tenth 

of the horizontal hydraulic conductivity. Initial hydraulic 

conductivity values were based on those determined during 

the aquifer tests, those used in a previous groundwater model 

of the Pootatuck River Valley (Haeni, 1978), and the surficial 

materials map (fig. 4). The spatial extent of the active cells in 

layers 1 and 2 were identical and coincided with the extent 

of the sand and gravel in the stratified glacial deposits. The 

active extent of layer 3 was greater than that of layers 1 and 2 

and represented the bedrock underlying the valley-fill deposits 

and the till-covered upland areas (fig. 4) that occur within the 

Pootatuck River Basin. Layers 1 and 2 were specified as convertible between confined and unconfined groundwater-flow 

conditions and layer 3 was specified as a confined layer. 

The well-field-scale models covered only part of the 

Pootatuck River Basin. Where possible, the model boundaries were chosen to coincide with the basin boundary and, 

therefore, were no-flow boundaries. Other boundaries were 

set far enough away from the production wells so they would 

not affect the aquifer response to the wells and so the modeled 

area would include the full extent of the areas contributing 

recharge to those wells (fig. 24). Water entered the modeled area across these arbitrarily set lateral boundaries and 

was simulated using the Constant-Head Boundary feature of 

MODFLOW-2000 (Harbaugh and others, 2000). Initial water 

levels specified along these boundaries were determined from 

the preliminary basin-scale model, and final water levels were 

determined from the basin-scale model as part of the iterative 

process described above. Recharge, specified as one-half the 

precipitation, was added at the times it rained during the test.

Values of base flow in the Pootatuck River and its 

tributaries where they entered the modeled area were estimated using equations developed by Ahearn (2010). These 

equations calculate a specified duration flow at an ungaged 

site in separate “bioperiods” that represent the different flow 

needs of the aquatic ecosystem at certain times of the year. 

The flow duration for the Rearing and Growth bioperiod (July 

through October) at the nearby Pomperaug River streamgage 

was determined for the dates of each of the aquifer tests, and 

that duration flow was estimated at the points where streams 

entered the well-field-scale models. Basin characteristics used 

in the regression equation were total drainage area and the 

percent of area underlain by coarse stratified deposits.

Initial calibration of the well-field-scale models was 

accomplished by comparing observed drawdown and baseflow values, measured during the aquifer tests, and the 

simulated values. Simulated drawdown values were calculated 

as the difference between the simulated water levels during the 

aquifer tests and the simulated water levels that represented 

steady-state conditions just prior to the start of the aquifer test.

Fairfield Hills Well-Field-Scale Model

The transient well-field-scale model of the Fairfield Hills 

area is centered on production Wells 3 and 7 in the northern 

part of Newtown. The total active modeled area is about 

6.2 mi2 and consists of 222 rows and 234 columns (fig. 26). 

Thickness of layer 1 ranged from 7.5 to 23 ft, thickness of 

layer 2 ranged from 7.5 to 138 ft, and thickness of layer 3 was 

80 ft (fig. 25, cross section B–B′ shown on fig. 27). Stream 

reaches in the Fairfield Hills model contained 1,679 model 

cells grouped into 39 stream segments. In the stratified glacial 

valley-fill deposits, stream cells were assigned to layer 1, 

and in the upland till and bedrock areas, stream cells were 

assigned to layer 3, the uppermost layer in that area (fig. 27). 

The area of the streambed in each modeled stream cell (used 
38 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

ll 3 (NT-111 )

73°16' 73°14'

41°25'

41°24'

41°23'

Well 3

Well 7

Well 8

EXPLANATION

Constant-head cell

Stream cell

Inactive area

Active cell

Active cell, fine mesh area

Production wells and identifier

Model cells

0 1/2 1 MILE

0 0.5 1 KILOMETER

Figure 26. Fairfield Hills well-field-scale model, Pootatuck River Basin, Newtown, Connecticut.
Development of the Groundwater-Flow Models 39

73°16' 73°14'

41°25'

41°24'

41°23'

EXPLANATION

Stream cell

Inactive area

Feet per day, layer 1 and layer 2 Stream cells, inactive cells

18, 16 (layer 2)

3

55

60

220

Line of section B-B' (See fig. 25)

Production well 

Well 3

Well 7

Well 8

B B'

0 1/2 1 MILE

0 0.5 1 KILOMETER

Simulated hydraulic conductivity

Feet per day, layer 3

Figure 27. Hydraulic conductivity in the Fairfield Hills well-field-scale model, Pootatuck River Basin, Newtown, Connecticut.
40 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

pumping and observation wells matched the observed water 

levels as closely as possible. The changes were made to groups 

of cells that corresponded to different geologic materials, as 

denoted on the surficial materials map (fig. 4). There were four 

hydraulic conductivity zones in layers 1 and 2 (fig. 27, table 9) 

and one in layer 3. Calibrated values of specific yield in layers 

1 and 2 were 0.15 where hydraulic conductivity was less than 

30 ft/d and 0.25 where hydraulic conductivity was greater than 

or equal to 30 ft/d. Specific storage was 0.00001 in all layers.

Simulated and observed water levels from the calibrated 

model at the 10 observation wells and 2 production wells in 

the Fairfield Hills well field are shown in figures 28 and 29. 

Visual comparison of simulated and observed water levels 

shows the initial water-level decline at the start of pumping 

and the transition period into aquifer-test pumping conditions 

match reasonably well as do the transition period to nonpumping (recovery) conditions toward the end of the aquifer test. 

Water levels in Well 7 showed the best match. 

Simulated and observed water levels were compared in 

four ways: (A) the difference at midnight on July 31, 2007 

(about half way through the aquifer test); (B) the mean absolute error (the mean of the absolute value of the residuals); 

(C) the median error of synchronous pairs (simulated and 

observed water levels with the same date and time) during the 

period July 26 through August 6, 2007 (table 10); and (D) as 

plots of simulated residuals (the difference between observed 

and simulated water levels) compared to simulated water levels and observed compared to simulated water levels (fig. 30). 

Simulated and observed water levels at Wells 3 and 7 

matched reasonably well with mean absolute errors of 1.3 

and 0.4 ft for Wells 3 and 7, respectively (table 10B). Based 

on median error, the best overall matches between simulated 

and observed water levels were at Well 3 and its associated 

observation wells with median errors that ranged from -1.2 to 

-0.2 ft; median errors for Well 7 and associated observation 

wells ranged from -1.5 to +3.4 ft (table 10C). Two observation 

wells (NT-85 and NT-94) associated with Well 7 had the worst 

median errors, -1.5 and 3.4 ft, respectively. Observation well 

NT-94 was located near a mapped geologic contact and this 

relatively large median error could be because of differences 

in aquifer properties between the units; however, data were 

insufficient to specify more precisely different properties for 

the various mapped geologic units. 

Model calibration included a determination of the mean 

of the residuals and the mean absolute error. Ideally, the 

mean of the residuals should be randomly distributed and 

close to zero, indicating no bias in the results and the mean 

absolute error should be less than 5 percent of the total range 

in the water-level measurements used for model calibration 

(Anderson and Woessner, 1992). Figure 30 shows the residuals 

of simulated and observed water levels of synchronous pairs at 

the Fairfield Hills and United Water well fields. The residuals 

are all less than 5 ft and are distributed about the zero line 

(fig. 30A) indicating a good fit between the observed and 

simulated transient water levels. The data all lie about the line 

of 1:1 slope (fig. 30B), also indicating a good fit between the 

to calculate streambed conductance) was determined using the 

length of stream reach in each modeled cell as the cell length 

in the downstream direction and the width of the stream as a 

function of the stream order. The streambed was assumed to be 

1 ft thick. 

Simulation of Aquifer Test

The transient model of the Fairfield Hills well field used 

data from the aquifer test conducted from July 23 to August 

6, 2007, at production Wells 3 and 7. To simulate the various stages of the aquifer test—shut off, pumping, and recovery—the groundwater model had a total of 145 stress periods 

and covered the period July 25 through August 6, 2007. The 

simulation began with a steady-state stress period equilibrating the initial water levels to a steady-state solution, followed 

by 144 transient stress periods representing the different 

stresses—pumping rates and recharge—during the aquifer 

test. Hourly stress periods, each with a single time step, were 

used to simulate the rapid water-level changes at the beginning 

and end of the pumping part of the aquifer test. At other times 

when the water level was more stable, stress periods were 4 or 

8 hours in length with an hourly time step. 

Simulated production wells were assigned to layer 2, and 

the pumping rates used in the simulation were those measured 

at specific times during the aquifer test; each pumping rate 

in the model was held constant until the time of the next 

pumping-rate measurement. The pumping rate at Well 3 

varied from 100 to 140 gal/min and at Well 7 from 438 to 

465 gal/min. Flow in the Pootatuck River where it crossed the 

upgradient (southern) boundary of the model was estimated 

at 4.36 ft3/s, assuming a 50-percent flow duration in the 

Rearing and Growth Bioperiod (Ahearn, 2010). Flows in Deep 

Brook, Tom Brook, and an unnamed tributary that enters the 

Pootatuck River from the east were estimated at 1.20, 

0.27, and 0.11 ft3/s, respectively; each of these tributaries 

enters the Pootatuck River downstream from the well field 

and downstream from the Berkshire streamgage (012035055 

on fig. 24).

Model Calibration and Hydraulic Properties

Water levels measured in the production well and in the 

observations wells during the aquifer test were used for model 

calibration of the transient model. Figure 5 shows locations 

of the pumping and observation wells. Because of the limited 

duration of data available for this one transient event, only 

streambed conductance, hydraulic conductivity, and storage 

values were varied during the model calibration process. 

The water levels specified at the boundaries of the model 

were not changed during the initial calibration process but 

later were updated with values from the basin-scale model. 

The final calibrated value for vertical hydraulic conductivity 

of the streambed was 2 ft/d, which was used to calculate 

streambed conductance.

Hydraulic conductivity values were adjusted through 

trial and error until the simulated water levels at the individual 
Development of the Groundwater-Flow Models 41

observed and simulated transient water levels. All errors were 

within the requirements established by CTDEP for Level A 

mapping (State of Connecticut, 1991).

Simulated base flow was compared to the observed base 

flow at each of the staff gages upstream and downstream 

from Wells 3 and 7 and at the streamgages at Berkshire 

(012035055) and Sandy Hook (01203510) (fig. 5) at two times 

during the aquifer test: on July 28, 2007, when measured 

flows were at their lowest before a rain event and on August 3, 

2007, immediately after the production wells were turned off. 

The best comparison was near Well 3 on July 28, 2007, when 

simulated and observed flows were identical. On August 3, 

2007, the simulated flows were from 13 to 52 percent greater 

than the observed flows (table 11).

Steady-State Model

The well-field-scale model was run in steady state mode 

to determine areas contributing recharge to the production 

wells. A steady-state model represents long-term average base 

flows, which are equivalent to median annual streamflow 

(State of Connecticut, 1991). Although the two streamgages 

on the Pootatuck River, at Berkshire and Sandy Hook, were 

operating during the aquifer tests, data from these streamgages 

were not sufficient for calibration of a steady-state model. 

Therefore, median flow in the Pootatuck River at the Berkshire 

and Sandy Hook streamgages was estimated from data at the 

streamgage on nearby Pomperaug River (fig. 1, insert), which 

has operated continuously since 1932. A relation between the 

two rivers was developed using concurrent flow data, and the 

median flow on the Pootatuck River was estimated using the 

long-term data at the Pomperaug River. Because the estimated 

flow represented natural conditions, the model was run with no 

wells pumping. Calibration of the steady-state Fairfield Hills 

model for nonpumping conditions included a comparison of 

estimated and simulated flows. The estimated base flow 

of the Pootatuck River was 20.4 ft3

/s at Berkshire and 

28.7 ft3

/s at Sandy Hook. The simulated steady-state, nonpumping streamflows of 19.2 and 28.9 ft3

/s at Berkshire and 

Sandy Hook differed by 5.9 and 0.7 percent, respectively.

The only natural flow of water to the Pootatuck River 

Basin aquifer enters the groundwater system as recharge from 

precipitation. Many groundwater-flow models use a constant 

rate of recharge over the entire modeled area. However, the 

amount of recharge at a given location is a function of the 

physical and land-use conditions in that area. An important 

feature of this study was to develop a map of areallydistributed recharge representing steady-state conditions 

based on the surface and land-cover conditions in the basin. 

The distribution of recharge was based on a statistical relation 

between various basin characteristics and recharge that was 

developed for the nearby Pomperaug River Basin (Bjerklie 

and others, in press). The relation was developed using a 

precipitation runoff modeling system (PRMS) basin model, 

developed by the USGS (Leavesley and others, 1996; Mastin 

and Vaccaro, 2002). A description of the method is provided 

in the appendix. Using areally-distributed recharge to a 

groundwater-flow model allows for analysis of changes to the 

groundwater system caused by changing land use or climate 

change. The approach used in this study could be used in other 

basins in western Connecticut and a similar approach could be 

developed for any basin.

United Water Well-Field-Scale Model

The transient well-field-scale model of the United 

Water area is centered on production Wells 1 and 2 in the 

southern part of Newtown. The total active modeled area 

is about 7.3 mi2

 (fig. 31) and consists of 222 rows and 224 

columns. Thickness of layer 1 ranged from 7.5 to 23 ft and 

layer 2 ranged from 7.5 to 142 ft. Thickness of layer 3 was 

80 ft (fig. 25, cross section C–C′ shown on fig. 32). Stream 

reaches in the United Water well-field-scale model contained 

1,725 model cells grouped into 34 stream segments. The 

layering approach for active cells, stream-cell assignment, and 

determination of streambed area and thickness was the same 

as that used in the Fairfield Hills model.

Table 9. Calibrated hydraulic parameters of the Fairfield Hills well-field-scale model, Pootatuck River Basin, Newtown, Connecticut.

[ft/d, feet per day]

Zone

Hydraulic conductivity (ft/d)

Layer 1 Layer 2 Layer 3

Horizontal Vertical Horizontal Vertical Horizontal Vertical

Valley-fill sediments near Well 3 18 1.8 16 1.6

3 3 Valley-fill sediments near Well 7 55 5.5 55 5.5

Alluvium along river near Well 7 60 6 60 6

Area near river channel in southern part of model area 220 22 220 22
42 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

NT-107 observed

NT-107 simulated

NT-109 observed

NT-109 simulated

NT-108 observed

NT-108 simulated

NT-110 observed

NT-110 simulated

Well 3 observed

Well 3 simulated

NT-106 observed

NT-106 simulated

250

251

252

253

254

255

256

257

258

259

260

245

246

247

248

249

250

251

252

253

254

255

245

246

247

248

249

250

251

252

253

254

255

250

251

252

253

254

255

256

257

258

259

260

220

225

230

235

240

245

250

255

260

248

249

250

251

252

253

254

255

256

257

258

WATER-LEVEL ALTITUDE, IN FEET ABOVE NGVD29

DATE

Figure 28. Simulated and observed water levels during Fairfield Hills aquifer test at Well 3, Pootatuck River Basin, Newtown, 

Connecticut.
Development of the Groundwater-Flow Models 43

240

245

250

255

260

265

270

275

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

7/23 7/24 7/25 7/26 7/27 7/28 7/29

2007

7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8

Well 7 observed

Well 7 simulated

NT-85 observed

NT-85 simulated

NT-86 observed

NT-86 simulated

NT91 observed

NT-91 simulated

NT-90 observed

NT-90 simulated

NT94 observed

NT-94 simulated

260

261

262

263

264

265

266

267

268

269

270

260

261

262

263

264

265

266

267

268

269

270

260

261

262

263

264

265

266

267

268

269

270

260

261

262

263

264

265

266

267

268

269

270

260

261

262

263

264

265

266

267

268

269

270

WATER-LEVEL ALTITUDE, IN FEET ABOVE NGVD29

DATE

Figure 29. Simulated and observed water levels during Fairfield Hills aquifer test at Well 7, Pootatuck River Basin, Newtown, 

Connecticut.
44 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut Table 10. Simulated and observed water levels in wells at the Fairfield Hills well field, Pootatuck River Basin, Newtown, Connecticut. [All units in feet] Water-level altitude Wells associated with Well 3 Wells associated with Well 7 Date and time Well 3 NT-106 NT-107 NT-108 NT109 NT-110 Well 7 NT-85 NT-86 NT-90 NT-91 NT-94 Simulated 7/31/07 12:00 a.m. 224.4 250.7 252.0 249.8 249.6 252.1 246.4 263.4 265.7 265.9 265.0 264.8 Observed 7/31/07 12:00 a.m. 223.8 252.4 252.3 250.7 250.6 252.6 246.5 265.2 265.8 264.9 265.5 261.4 A. Residual 0.6 -1.7 -0.3 -0.9 -1.0 -0.5 -0.1 -1.8 -0.1 1.0 -0.5 3.4 Number of synchronous pairs1 67 105 113 109 113 117 18 118 107 118 83 111 B. Mean absolute error 1.3 1.3 0.2 0.7 0.6 0.2 0.4 0.6 0.1 0.5 0.4 1.6 C. Median error -1.2 -1.1 -0.2 -0.7 -0.5 -0.2 0.2 -1.5 0.1 0.9 -0.3 3.4

1Synchronous pairs: simulated and observed water levels with the same date and time used to calculate mean absolute error and median error for period July 26 through August 6, 2007.
Development of the Groundwater-Flow Models 45

 SIMULATED RESIDUALS

Well-field-scale models

Well-field-scale models

220

230

220 230

SIMULATED WATER LEVELS, IN FEET IN FEET ABOVE NGVD

 OBSERVED WATER LEVELS, IN FEET ABOVE NGVD29

Well 3

Well 3 observation wells

Well 7

Well 7 observation wells

TW-2, 4, and 6

250

260

270

250 260 270

320

330

320 330

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

220 230 250 260 270 320 330

A

B

Figure 30. Comparison of simulated and observed 

water levels at Fairfield Hills and United Water 

well fields, Pootatuck River Basin, Newtown, 

Connecticut; A, simulated residuals compared to 

simulated water levels, and B, observed water 

levels compared to simulated water levels. Different 

colors represent data from different wells.
46 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Model Calibration and Hydraulic Properties

Water levels recorded throughout the 11-day period in the 

three observation wells (Wells TW-2, TW-4, and TW-6, fig. 6) 

were used for calibration. It was not possible to record water 

levels in Well 2. Hydraulic conductivity of the aquifer and of 

the streambed and specific yield values were varied during 

the model calibration process. The initial water levels specified at the boundaries of the model, based on the topographic 

map, were not changed during the initial calibration, but were 

updated with values from the basin-scale model as part of the 

iterative model development described earlier in this report. 

The final calibrated value for vertical hydraulic conductivity of 

the streambed was 2 ft/d, which was used to calculate streambed conductance.

Hydraulic conductivity values were adjusted until the 

simulated water levels at the individual observation wells 

matched the observed water levels as closely as possible. The 

changes were made to groups of cells that corresponded to different geologic materials as denoted on the surficial materials 

map (fig. 4). There were two hydraulic conductivity zones in 

layers 1 and 2 and one in layer 3 (fig. 32, table 12). Calibrated 

value of specific yield was 0.25 in layers 1 and 2, and specific 

storage was 0.00001 in all layers.

Simulated and observed water levels from the calibrated 

model are shown in figure 33. Visual comparison of simulated and observed water levels shows the initial water-level 

increase when the pump was first turned off and during the 

transition period into the aquifer-test nonpumping conditions 

match reasonably well, as do those in the transition period to 

pumping conditions at the end of the aquifer test. Observed 

and simulated water levels were compared in six ways: 

(A) the difference at 2 a.m. on September 5, 2007 (about 

halfway through the shut off portion of the aquifer test); 

(B) the mean absolute error and (C) median error of synchronous simulated and observed pairs of water levels during the 

period September 3 through September 6, 2007; (D) the mean 

absolute error and (E) median error of synchronous simulated 

and observed pairs of water levels during the period September 4, at 10 p.m. through September 5, 2007 at 8 a.m.—the 

part of the aquifer test when the well was shut off (table 13); 

and (F) as plots of simulated residuals compared to simulated 

water levels and observed compared to simulated water levels 

(fig. 30). For the overall time period of September 3 through 

September 6, 2007, the mean absolute error ranged from +0.4 

to +0.5 ft (table 13B), and the median error for the observation wells ranged from -0.2 to -0.1 ft (table 13C). For the 

period September 4, at 10 p.m. through September 5, 2007 at 

8 a.m., during shut off, the mean absolute error ranged from 

+0.9 to +2.2 ft (table 13D) and the median error ranged from 

-2.6 to -0.8 ft (table 13E). Figure 30 shows a good fit between 

simulated and observed water-level values and their residuals 

(see discussion earlier in this report). These differences are 

within the CTDEP requirements for Level A mapping (State of 

Connecticut, 1991). 

Simulation of Reverse Aquifer Test

The transient model of the United Water well field 

used data from the reverse aquifer test conducted from 

September 4 to 5, 2007, at Well 2. To simulate the various 

stages of the aquifer test—increased pumping of Well 2, shut 

off, and return to regular pumping—the groundwater model 

had a total of 162 stress periods and covered the period 

August 25 through September 5, 2007. The initial steadystate stress period was followed by transient stress periods 

starting about 10 days before the aquifer test so that effects 

of any initial conditions of starting water level and continued 

pumping would be minimal by the time of the aquifer test. 

Stress periods that covered August 25 through 31 were 4 hours 

long, with hourly time steps; stress periods after September 1 

were hourly, each with a single time step.

The simulated production well was assigned to layer 2 

and the pumping rates used in the simulation were those measured before and after the 9.5-hour reverse aquifer test when 

the pump was turned off. The pumping rate for the initial 

steady-state stress period was set at 650 gal/min, the average 

of the pumping rates recorded from August 25 to September 

4, 2007. Flow in the main stem and in the North Branch of the 

Pootatuck River was estimated at 0.70 and 0.30 ft3/s, respectively, assuming a 75-percent flow duration in the Rearing and 

Growth bioperiod (Ahearn, 2010).

Table 11. Simulated and observed base flow, Fairfield Hills well 

field, Pootatuck River Basin, Newtown, Connecticut.

[ft3/s, cubic feet per second; USGS, U.S. Geological Survey]

Site

Base flow, in ft3/s

Simulated Observed Difference

July 28, 2007

Well 3 upstream 5.69 5.71 0.02

Well 3 downstream 5.66 5.66 -0.00

Well 7 upstream 5.31 4.65 -0.66

Well 7 downstream 5.20 4.70 -0.50

Berkshire streamgage1 5.66 5.60 -0.06

Sandy Hook streamgage2 8.78 9.20 0.42

August 3, 2007

Well 3 upstream 5.45 4.66 -0.79

Well 3 downstream 5.40 4.79 -0.61

Well 7 upstream 5.20 3.43 -1.77

Well 7 downstream 5.05 3.79 -1.26

Berkshire streamgage1 5.40 4.70 -0.70

Sandy Hook streamgage2 8.50 6.90 -1.60

1

USGS streamgage number 012035055.

2

USGS streamgage number 01203510.
Development of the Groundwater-Flow Models 47

Well 2

73°18' 73°16'

41°23'

41°22'

41°21'

Well 1

EXPLANATION

Constant-head cell

Stream cell

Inactive area

Active cell

Active cell, fine mesh area

Production wells

Model cells

0 1/2 1 MILE

0 0.5 1 KILOMETER

Figure 31. United Water well-field-scale model, Pootatuck River Basin, Newtown, Connecticut.
48 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Figure 32. Hydraulic conductivity in the United Water well-field-scale model, Pootatuck River Basin, Newtown, Connecticut.

Well 2

73°18' 73°16'

41°23'

41°22'

41°21'

Well 1

C C'

EXPLANATION

Stream cell

Inactive area

Feet per day, layer 1 and layer 2

Feet per day, layer 3

Stream cells, inactive cells

60

3

220

Line of section C-C' (fig. 25)

Production well

Simulated hydraulic conductivity

0 1/2 1 MILE

0 0.5 1 KILOMETER
Development of the Groundwater-Flow Models 49

Table 12. Calibrated hydraulic parameters of the United Water well-field-scale model, Pootatuck River Basin, Newtown, 

Connecticut.

[ft/d, feet per day]

Zone

Hydraulic conductivity (ft/d)

Layer 1 Layer 2 Layer 3

Horizontal Vertical Horizontal Vertical Horizontal Vertical

Valley-fill sediments near Well 2 60 6 60 6

3 3

Alluvium along river near Well 2 220 22 220 22

316

318

320

322

324

326

328

330

9/3 9/4 9/5 9/6

2007

316

318

320

322

324

326

328

330

9/3 9/4 9/5 9/6

2007

316

318

320

322

324

326

328

330

9/3 9/4 9/5 9/6

2007

TW-2 TW-4

TW-6

EXPLANATION

WATER-LEVEL ALTITUDE, IN FEET ABOVE NGVD29

Observed

Simulated

Figure 33. Simulated and observed water levels during United Water aquifer test, Pootatuck River Basin, Newtown, Connecticut.

Location of wells shown in figure 6.
50 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

No direct model calibration to streamflow was possible 

in the United Water model because no streamgages were 

located within the United Water modeled area, and there were 

no streamflow measurements made during the reverse aquifer 

test. The simulated flows appear reasonable; simulated flows 

were 1.51 and 0.62 ft3/s at the upstream and downstream 

measuring sites, respectively. The piezometer data, discussed 

earlier, showed a substantial downward gradient between 

the surface water and the groundwater (fig. 19), which 

corroborates the simulated loss of streamflow near the 

production well.

Limitations of Well-Field-Scale Models

The Fairfield Hills and United Water well-field-scale 

models were calibrated to a limited set of field data and to a 

short time period. Field conditions precluded accurate streamflow measurements in the vicinity of the production wells and 

streamflow into each model area was estimated. In addition, 

there was a small number of data points to use for calibration 

of water levels, especially for the United Water model. For the 

transient models, the initial steady-state stress period was calibrated to water levels measured before the aquifer tests that 

did not represent long-term average conditions, both because 

the field conditions were much drier than average and, in the 

United Water model, the water levels in the aquifer had not 

recovered to a static level. In the steady-state models, water 

levels at the boundaries were based on those simulated in the 

basin-scale model and not on actual field conditions. 

The transient and steady-state models simulated all 

water withdrawn by the production wells as removed from 

the system. However, during the aquifer tests, some, if 

not all from the water was returned to the Pootatuck River 

downstream from the well fields. In addition, some of the 

water delivered to residents in Newtown is returned to the 

groundwater through septic systems.

Basin-Scale Model

The total active modeled area of the Pootatuck River 

basin-scale model was about 26 mi2 (fig. 24). The finitedifference grid representing this area consisted of 111 rows 

and 81 columns of uniformly spaced model cells that were 

400 ft on a side. This cell size represented the hydrogeologic 

features of the Pootatuck River Basin as accurately as possible 

while balancing the size of the numerical model in terms of 

the complexity of data storage and computer solution time. 

The Pootatuck River Basin was subdivided vertically into 

three layers of variable thickness that extended from the water 

table into the bedrock using the same procedure as for the 

well-field-scale models.

Layer Distribution

The three model layers represented the primary hydrogeologic units based on the surficial materials map (fig. 4) 

and bedrock surface map (fig. 3). Cross-section A-A′ 

Table 13. Simulated and observed water levels in wells at the United Water well field, Pootatuck River Basin, 

Newtown, Connecticut.

[All units in feet]

Date and time

Water-level altitude

TW-2 TW-4 TW-6

Simulated 9/05/2007 2:00 a.m. 322.0 322.1 323.8

Observed 9/05/2007 2:00 a.m. 324.5 324.6 324.5

A. Residual -2.5 -2.5 -0.7

September 3 through September 6, 2007

Number of synchronous pairs1 68 53 68

B. Mean absolute error 0.5 0.5 0.4

C. Median error -0.1 -0.1 -0.2

September 4, 10 p.m. through September 5, 8 a.m.

Number of synchronous pairs1 11 11 11

D. Mean absolute error 2.2 2.2 0.9

E. Median error -2.5 -2.6 -0.8

1

Synchronous pairs: simulated and observed water levels with the same date and time used to calculate mean absolute error and 

median error for a specified period.
Development of the Groundwater-Flow Models 51

Recharge

A map of areally-distributed recharge, similar to that used 

in the well-field-scale models, was developed for the basinscale model based on the surface and land-cover conditions 

in the basin. A description of the method is provided in the 

appendix. Using areally-distributed recharge was important 

because the basin-scale model was used to estimate changes in 

future recharge due to potential land-use changes. 

The regression equation was used to compute a recharge 

estimate for each cell in the basin-scale model (fig. 35). The 

estimated recharge averaged 16 in/yr for the entire basin with 

a maximum of 28 in/yr for grid cells composed entirely of 

coarse stratified glacial deposits. These values are reasonable 

because average annual precipitation is about 48 in/yr (Miller 

and others, 2002). The recharge estimates were constrained 

by setting the minimum possible value to 1 in/yr to prevent 

unrealistic negative values. Initially, the high end value was 

constrained to 24 in/yr based on previous estimates of recharge 

in coarse stratified glacial deposits in western Connecticut 

(Mazzaferro, 1986). However, recent work done in the nearby 

Pomperaug Basin (Bjerklie and others, in press) suggests the 

recharge used by Mazzaferro (1986) underestimated base-flow 

percentages. Thus, the high-end constraint was removed to 

provide the best simulation results in the basin-scale model. 

Pumping

Production wells were assigned to layer 2 and the 

simulated pumping rates were those registered for each well. 

Although only Wells 2, 3, and 7 were pumped during the 

aquifer tests conducted for this study, in order to simulate 

the maximum possible effect of pumping, all the wells were 

included in the basin-scale model. Therefore, Well 1 in the 

United Water well field, which is used in periods of high 

demand, and Well 8 in the Fairfield Hills well field, which is 

a backup well (and never pumped at the same time as Well 7), 

also were simulated in the basin-scale steady-state model. The 

total pumping simulated in the model was about 3.41 Mgal/d, 

which is the sum of the registered rates for the production 

wells shown in table 14. 

Hydraulic Properties

The values and distribution of horizontal and vertical 

hydraulic conductivity for different geologic deposits were 

determined from the calibrated well-field-scale models. These 

values were applied to areas of similar deposits generalized 

from the surficial materials map (fig. 4). The values of hydraulic conductivity for most of the area ranged from 3 to 220 ft/d 

(fig. 34); the value for all of layer 3—both beneath the stratified glacial deposits and in the upland till and bedrock areas—

was 3 ft/d. It was assumed that wetlands in the upland areas 

functioned as open water bodies with less resistance to flow 

than the surrounding aquifer; therefore, they were assigned a 

horizontal and vertical hydraulic conductivity of 5,000 ft/d. 

(fig. 25) shows the modeled layers (cross-section location shown on fig. 24). As with the well-field-scale models, 

the spatial extent of the active cells in layers 1 and 2 were 

identical and coincide with the extent of the stratified glacial 

deposits that consist of areas of differing proportions of sand 

and gravel in the center of the valley (fig. 4). Two layers were 

used in order to locate the production wells in the lower part 

of the aquifer and to differentiate minor differences in hydraulic conductivity (fig. 34). Layer 3 represents (1) bedrock 

underlying the stratified glacial deposits and (2) the combined 

till and bedrock areas of the uplands; the active area of layer 

3 was larger than the active area of layers 1 and 2 and is the 

entire watershed area shown in figure 34. Layers 1 and 2 were 

specified as convertible between confined and unconfined 

groundwater-flow conditions and layer 3 was specified as a 

confined layer.

The top of the model in the area of stratified glacial 

deposits was specified as land surface, derived from digital 

elevation data of 10-ft contour lines (Scott Sharlow, Newtown, 

written commun., 2007). Total thickness of the stratified 

glacial deposits in layers 1 and 2 ranged from 15 to 133 ft with 

a median value of 30 ft. The thickness of layer 1 ranged from 

7.5 to 23 ft and the thickness of layer 2 ranged from 7.5 to 

110 ft. The bottom of layer 2 represented the bedrock surface. 

During model calibration, a thickness of 80 ft for layer 3 was 

found to be a reasonable value for most model cells. However, 

in the upland areas where layer 3 was the uppermost active 

layer, the thickness of layer 3 was increased in approximately 

40 percent of the active model cells to smooth out the lower 

surface and avoid numerical instabilities in the model. The 

thickness of layer 3 in areas representing till and bedrock 

ranged from 80 to 413 ft. 

Hydrologic Boundaries

The bedrock was modeled explicitly because it was 

assumed that water flows across the boundary between the 

upper part of bedrock and the surficial materials, especially 

beneath the upland areas where thin till deposits overlie 

bedrock. The lateral boundaries of the model, set at the basin 

boundary of the Pootatuck River Basin, were assumed to be 

no-flow boundaries. The stream reaches in the model consisted 

of 681 model cells grouped into 117 stream segments (fig. 24). 

In the area of stratified glacial deposits, the stream cells were 

assigned to layer 1, and in the upland bedrock and till areas the 

stream cells were assigned to layer 3. 

Hydrologic Stresses

There were two hydrologic stresses simulated in the 

basin-scale model. Precipitation recharged the groundwater 

at the water table and pumping from the production wells 

removed groundwater from the model. 
52 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Figure 34. Simulated hydraulic conductivity of the basin-scale model, Pootatuck River Basin, Newtown, Connecticut.

Newtown

Monroe

Bethel

Redding

Southbury

Easton

Brookfield

73°20' 73°16'

41°24'

41°20'

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Simulated hydraulic conductivity

Feet per day, layer 1 and layer 2

18, 16 (layer 2)

55

60

220

Inactive model area

Production wells

Feet per day, layer 3

3
Development of the Groundwater-Flow Models 53

Figure 35. Areal distribution of recharge used in the basin-scale groundwater-flow model, Pootatuck River Basin, Newtown, 

Connecticut.

73°20' 73°16'

41°24'

41°20'

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Simulated groundwater recharge

Inches per year

0 to 5

greater than 5 to 10

greater than 10 to 15

greater than 15 to 20

greater than 20 to 25

greater than 25 to 30

(highest value was 28)

Inactive model area
54 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

land surface; therefore, the simulation represents a steady-state 

condition that is drier than what may be expected in the area.

All the wells with reliable data are located in the valleyfill sediments and, because the model cells in the basin-scale 

model are 400 ft on a side, some cells contain multiple observation wells; the five observation wells near each of the production wells in the Fairfield Hills well field were in only two 

model cells at each production well location. These cells were 

the calibration locations. The distances from observation wells 

to production wells ranged from 5 to 280 ft. The three observation wells in the United Water well field were located in two 

model cells, which were the calibration locations. These six 

calibration locations and Well NT-15 produced a total of seven 

well calibration points for the basin-scale model.

Single groundwater-level measurements were available at 

many domestic wells in other parts of the study area, usually 

measured at the time the well was drilled (fig. 1). The data 

from these single measurements do not necessarily represent 

long-term average conditions in the aquifer. Furthermore, 

errors in measurement-point altitudes of these wells may be 

more than 5 ft in areas of high topographic relief, such as in 

the upland tills. Water levels in the wells in these areas were 

not used for model calibration but were used to assess qualitatively if the simulated water levels were reasonable. 

Water levels measured in Wells 3 and 7 and in the 10 

observation wells (fig. 5) immediately prior to the start of 

pumping at the Fairfield Hills well field were assumed to 

represent steady-state conditions and were used to calibrate 

the basin-scale model. In addition, because it was not possible 

to measure nonpumping water levels in the area of Well 2, the 

water levels measured in Wells TW-2, TW-4, and TW-6 

(fig. 6) most likely do not represent steady-state conditions and 

may not be appropriate for comparison to simulated steadystate water levels. Using all seven calibration points, the mean 

of the residuals was +1.1 ft and the mean absolute error was 

+2.0 ft, less than 2.5 percent of the total range (78 ft) in water 

levels. Excluding the two observation points associated with 

Well 2 because they may not represent steady-state conditions, 

the mean of the residuals was -0.5 ft, and the mean absolute 

error was +0.7 ft; however, because the range was less (14 ft), 

this represented approximately 4.7 percent of the total range.

Figure 36 compares the simulated and observed water 

levels in all the wells for which data exist. The data at the 

seven calibration points are shown with black dots; all the 

residuals plot close to zero (fig. 36A), and the observed and 

simulated pairs plot near the 1:1 line (fig. 36B) indicating a 

good fit between the observed and simulated values. The many 

water levels from wells of uncertain altitude and location in 

other parts of the study area are shown with gray dots, indicating less emphasis was placed on these data. These data are 

more randomly distributed—some show a good fit between 

observed and simulated values and many others show simulated water levels below the observed value (this may indicate 

that some geologic feature is not represented in the model). 

Basin-Scale, Steady-State Model Calibration

Ideally, data that represent average conditions would be 

used to calibrate a steady-state model. In this investigation, 

available data included water levels measured as part of 

the aquifer tests, water levels of unknown quality from 

various other periods, and streamflow data recorded at two 

streamgages that include the periods of the aquifer tests. 

Calibration of this steady-state simulation was based mainly 

on estimated streamflow data at the two streamgages, and 

water levels measured in observation wells before the aquifer 

tests. Because the estimates of streamflow assumed natural, 

nonpumping conditions, none of the simulated wells were 

pumped during calibration. Hydraulic-conductivity values of 

the aquifer were not changed during calibration of the basinscale model; however, streambed conductance was varied.

Water-Level Data

In addition to water-level data measured in observation 

wells for the aquifer tests, water-level measurements made 

since 1967 by the USGS at a long-term observation well 

(NT-15) were used for model calibration. Well NT-15 is 

located about 2,100 ft west-northwest of Well 3 (fig. 4). The 

water level in Well NT-15 was 8.26 ft below land surface on 

July 28, 2007, during the Fairfield Hills aquifer test and 

9.38 ft below land surface on August 28, 2007, during the 

United Water aquifer test. These water levels were substantially below the long-term median water level of 5.72 ft below 

Table 14. Registered pumping rates and distance from stream 

for all production wells in the basin-scale model, Pootatuck River 

Basin, Newtown, Connecticut.

[Registered pumping rate, maximum permissible pumping rate registered 

with the Connecticut Department of Environmental Protection; ft3/d, cubic 

feet per day; Mgal/d, million gallons per day; gal/min, gallons per minute; 

<, less than]

Production 

well

Registered pumping rate Distance 

from stream, 

in feet ft3

/d Mgal/d gal/min

Well 1 66,841 0.50 350 60

Well 2 144,376 1.08 750 < 50

Well 3 51,868 0.39 270 140

Well 7 89,967 0.67 467 130

Well 8 102,600 0.77 533 < 50
Development of the Groundwater-Flow Models 55

225

275

325

375

425

475

525

575

625

225 275 325 375 425 475 525 575 625

-200

-175

-150

-125

-100

-75

-50

-25

0

25

50

75

100

125

150

175

200

225 275 325 375 425 475 525 575 625

A

B

Basin-scale model

Basin-scale model

SIMULATED WATER LEVELS, IN FEET ABOVE NGVD29

OBSERVED WATER LEVELS, IN FEET ABOVE NGVD29 SIMULATED RESIDUALS

Observation points

Seven points in valley-fill sediments near production wells

Upland wells of uncertain altitude and location

Figure 36. Comparison of simulated 

and observed water levels in the 

basin-scale model A, simulated 

residuals compared to simulated 

water levels and B, observed 

compared to simulated water levels, 

Pootatuck River Basin, Newtown, 

Connecticut.
56 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Streamflow Data

The basin-scale groundwater-flow model was run under 

steady-state conditions that represent long-term averages. As 

mentioned earlier in this report, natural (nonpumping) base 

flow was estimated from long-term data from the streamgage 

on the nearby Pomperaug River. Estimated flow data at the 

two streamgages on the Pootatuck River were compared to 

simulated flow data from the calibrated model with no wells 

pumping. The estimated base flow was 20.4 ft3/s at Berkshire 

(012035055) and 28.7 ft3/s at Sandy Hook (01203510). The 

simulated steady-state, nonpumping streamflow was 19.1 

and 28.4 ft3/s at Berkshire and Sandy Hook, respectively, and 

differed from the estimated values by 6.4 and 1.0 percent, 

respectively. 

Simulated streamflows where four upland streams 

crossed onto the stratified glacial deposits were similar to 

estimates of mean base flow at those locations made using a 

technique developed by Mazzaferro and others (1979). The 

calibrated value for vertical hydraulic conductivity of the 

streambed was 2 ft/d. This value falls in the range of laboratory determined vertical hydraulic conductivities of streambed 

samples of 1.3 to 3.9 ft/d reported by Haeni (1978). 

Model Limitations

The steady-state, basin-scale groundwater-flow model of 

the Pootatuck River Basin has a large modeled area compared 

to the number and location of available calibration points. 

Ideally, data from calibration locations scattered throughout 

the modeled area, in the valley-fill sediments and till uplands, 

would be used for model calibration. In addition, very few 

data were available that represented steady-state conditions. 

As previously mentioned, median streamflow had to be 

estimated from the data in the nearby Pomperaug River Basin, 

and water levels measured just before pumping began in the 

aquifer tests were assumed to represent steady-state even 

though they were substantially lower than long-term average 

conditions. The model simulated all groundwater withdrawn 

by the production wells as removed from the system; 

however, as with the well-field-scale models, some of the 

water delivered to the residents of Newtown is returned to the 

groundwater through septic systems.

The basin-scale groundwater-flow model of the Pootatuck 

River Basin was calibrated to only seven well locations and 

two streamgage locations and estimates of base flow. Model 

calibration comparisons show a good match between simulated and observed steady-state water levels in the vicinity of 

the production wells during the periods of the aquifer tests, but 

the simulated water levels in the till uplands cannot be verified. Because of the size of each model cell and the distances 

between wells and the Pootatuck River, each simulated production well occurs within the same cell location as a stream 

reach, although the stream is simulated in layer 1 and the well 

in layer 2. This may cause the simulated contribution of stream 

water to the well to be greater than it actually is. 

Sensitivity Analysis of Groundwater-Flow 

Models

A sensitivity analysis was conducted to determine how 

the results of the basin-scale and well-field-scale models may 

change if the parameters of streambed conductance, aquifer 

hydraulic conductivity, and recharge to the water table are 

altered. The values of streambed conductance were increased 

and decreased by a factor of 5, and most of the other parameters were changed by 50 percent; however, hydraulic conductivity was decreased by 30 percent for the well-field-scale 

models because those models would not solve with a larger 

decrease in hydraulic conductivity. The results of each sensitivity analysis were compared to those of the calibrated model.

Streambed Conductance

The values for streambed conductance in the calibrated 

steady-state basin-scale and well-field-scale models were 

based on a vertical hydraulic conductivity of the streambed of 

2 ft/d. Streambed conductance in the basin-scale model ranged 

from 4,000 to 32,000 ft2/d, depending on stream order and in 

the well-field-scale models ranged from 100 to 32,000 ft2/d, 

depending on stream order and cell size. 

To check the sensitivity of models to changes in 

streambed conductance, the values were increased and 

decreased by a factor of 5. The large range of values affected 

the performance of the numerical model; the closure criteria 

specified in the MODFLOW solver had to be increased for 

the simulation to solve, and the budget error ranged from 0 

to -0.64 percent. In the Fairfield Hills well-field-scale model, 

a different MODFLOW solver was used. Also, the simulated 

water level in the cell containing Well 1 was below the bottom 

of the cell; the simulated well “went dry” in the basin-scale 

model, most likely because the recharge area for Well 2, with 

a pumping rate much larger than Well 1, now included the 

location of Well 1.

Hydraulic Conductivity

To determine the sensitivity of the models to changes 

in aquifer properties, all horizontal and vertical hydraulic 

conductivity values were increased by 50 percent from the 

calibrated values. In the basin-scale model, the horizontal 

and vertical hydraulic conductivity values were decreased by 

50 percent and, as mentioned above, in the well-field-scale 

models, horizontal and vertical hydraulic conductivity values 

were decreased by 30 percent. When horizontal and vertical 

hydraulic conductivity values were decreased by 50 percent 

in the basin-scale model, the closure criteria specified in 

the MODFLOW solver had to be increased in order for the 

simulation to solve (budget error was 0.06 percent instead 

of -0.01 percent in the calibrated solution), thus increasing 

the model error and decreasing the reliability of the model 

results. In the well-field-scale models when hydraulic 
Delineation of Areas Contributing Recharge to Production Wells 57

to define the nonpumping water table. In each modeled cell, 

the drawdown caused by the simulated production wells was 

calculated as the difference between the nonpumping and 

the pumping water levels from the steady-state simulations 

(fig. 37). The drawdown occurred only near the simulated 

production wells. The greatest drawdown was about 9 ft for 

the Fairfield Hills model and about 5 ft for the United Water 

model. As can be observed in figure 37, the largest areas of 

drawdown were near Wells 7 and 8. Pumping affects the water 

levels near the wells, but the groundwater does not necessarily 

flow to the production well (Reilly and Pollock, 1993; and 

Franke and others, 1998). Thus, the areas of drawdown cannot 

be assumed to be the same as the recharge areas discussed in 

the following section.

Delineation of Groundwater-Recharge Areas

Land-use decisions can affect the water quantity and 

quality of an aquifer and thus affect of the water pumped 

for public supply. Local planners need information on the 

source of the groundwater to wells in order to make wise 

land-use decisions. In addition, protection of water quality 

in production wells is a major concern of the CTDEP, and 

the Level A mapping regulations require delineation of 

areas contributing recharge to wells under the maximum 

stress possible (State of Connecticut, 1991). To simulate 

the maximum extent of recharge areas to production wells, 

the well-field-scale models were run with all potential wells 

pumping at their registered (maximum allowable) rates, even 

though this is not the normal operation of the Fairfield Hills or 

United Water systems.

In the well-field-scale models, the USGS particletracking program MODPATH (Pollock, 1994) was used in 

conjunction with MODFLOW (Harbaugh and others, 2000) to 

determine the areas contributing recharge to production wells 

in the Pootatuck River Basin. In the particle-tracking analysis, 

the top face of each active model cell at the water table was 

“seeded” with 16 particles in a 4 × 4 array. These particles 

were tracked forward through the simulated flow system until 

they reached the production wells. The recharge area to a well 

was the starting location of the particles captured by that well. 

The Fairfield Hills well-field-scale model simulated 

pumping from Wells 3, 7, and 8. However, when simulated 

pumping in Well 3 was greater than 25,000 ft3/d (0.19 Mgal/d), 

the simulated water level in the well cell dropped below the 

bottom of the cell—it went dry; therefore, only Wells 7 and 8 

were pumped at their registered rates, 0.67 and 0.77 Mgal/d, 

respectively. Even though simulated pumping in Well 3 was 

less than the registered rate (0.39 Mgal/d), it was higher than 

the maximum rate that could be maintained during the aquifer 

test (0.16 Mgal/d). However, the inability of the model to 

simulate the maximum pumping rate of Well 3 is an additional 

limitation of the model and could indicate that an unknown 

geologic feature is not included in the groundwater model. 

The United Water well-field-scale model was run with 

conductivity was decreased by 30 percent Well 3 went dry. 

The MODLFOW solver used the same values as in the 

calibrated simulation and the budget error was -0.01 percent 

for the Fairfield Hills model and -0.3 percent for the United 

Water model. 

Recharge

Steady-state recharge rates to the aquifer were increased 

and decreased by 50 percent from the calibrated rates. 

Groundwater-recharge values, estimated from 1960 to 2008 

as part of a regional model, ranged from an increase of 

40 percent to a decrease of 40 percent from the long-term 

average (David Bjerklie, U.S. Geological Survey, written 

commun., 2010); therefore, this range of values assumes a 

somewhat more extreme range is possible. When recharge 

rates were increased by 50 percent in the basin-scale model, 

the closure criteria specified in the MODFLOW solver had to 

be increased in order for the simulation to solve—increasing 

the model error; however, the budget error was -0.01 percent, 

the same as in the calibrated solution. In the well-field-scale 

models, the MODFLOW solver values were unchanged from 

those in the calibrated solution. 

Delineation of Areas Contributing 

Recharge to Production Wells

Groundwater-model simulations of the Pootatuck River 

Basin show that the source of water pumped from each 

production well is a combination of water that recharged the 

groundwater in the contributing area and water from stream 

inflow. The calibrated well-field-scale groundwater-flow 

models were used to determine the drawdown in water levels 

caused by the five simulated production wells in Newtown 

under steady-state conditions and to define the areas contributing recharge to those wells. The size and shape of the 

drawdown areas and the simulated areas contributing recharge 

are controlled by the hydraulic properties of the aquifer, the 

boundary conditions of the flow system, and the changes in 

aquifer stresses such as pumping and recharge rates. Analytical methods can be used to delineate the areas contributing 

recharge to production wells for simple flow conditions; 

however, accurate delineation of areas contributing recharge 

to production wells in systems affected by nearby streams 

and spatially variable recharge rates, such as in the Pootatuck 

River Basin, is facilitated by the use of a numerical model 

with particle tracking.

Simulated Groundwater Levels and Drawdown

The calibrated well-field-scale models simulated the 

steady-state groundwater system with all five production wells 

pumping. In addition, the well-field-scale models were used 
58 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

73°18' 73°16' 73°14'

41°24'

41°22'

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

0.5 to 1

Greater than 1

Areas with glacial-stratified deposits

Pootatuck River Basin

Production well, and identifier

Drawdown, in feet

Well 2

Well 1

Well 3

Well 7 Well 8

Newtown

Monroe

Figure 37. Drawdown caused by simulated production wells in the Pootatuck River Basin, Newtown, Connecticut.
Delineation of Areas Contributing Recharge to Production Wells 59

Wells 1 and 2 pumping at their registered rates, 0.50 and 

1.08 Mgal/d, respectively.

Areas contributing recharge for conditions represented by 

the calibrated well-field-scale, steady-state groundwater-flow 

models of the Pootatuck River Basin are shown in figure 38. 

The area contributing recharge in the Fairfield Hills model 

(northern box on figure 38) included narrow parts of the 

aquifer that extended beyond the immediate upgradient areas, 

probably because of deeper groundwater-flow paths. The ends 

of these “tails” are close to the boundary at the southwestern 

corner of the model. However, this is a very small part of the 

recharge area with little flow from the boundary.

As mentioned previously, the groundwater-flow models 

described in this report include the upland till areas; therefore, parts of the recharge areas shown in figure 38 are in the 

uplands. Because data from this study will be used to fulfill 

the Level A mapping regulations, a second figure is provided 

to be consistent with the guidelines for that program (State of 

Connecticut, 1991). Figure 39 shows the areas contributing 

recharge in the stratified glacial deposits and the upland area 

from which surface water would flow to those areas. 

Recharge to each model cell varied depending on the 

physical and land-use conditions at that location. Water 

coming from the recharge area to each well in the well-fieldscale models was determined using a Geographic Information 

System processing technique to assign the cell-specific 

recharge rate to each cell or portion of a cell in the total 

recharge area. (Each of the 16 particles in each cell occupied 

a portion of the cell area, and in some cells not all of the 

particles tracked to the production well.) That recharge rate 

was then multiplied by the corresponding area to determine 

the volume of water originating in each cell; the volumes 

were then summed for all the cells that comprise the area 

contributing recharge for each production well. Because of 

the numerical problem with simulation of Well 3, results from 

that well are shown in the table but are not included in the 

following discussion.

The volume of water pumped from each well derived 

from only the area contributing recharge ranged from 20 

to 62 percent (table 15); because areal recharge and stream 

inflow are the only sources of water to the wells in this model, 

the volume from stream inflow ranged from 38 to 80 percent. 

These results show that stream inflow to the groundwater 

system can be a substantial part of the water from a production 

well in the Pootatuck River Basin. Many production wells near 

streams induce stream inflow and production, wells are often 

sited near streams to capture this source of water. 

Sensitivity Analysis of Groundwater Recharge 

Areas

The recharge areas resulting from each sensitivity analysis were compared to those of the calibrated model in each 

of the two well-field-scale models. Table 16, Parts A through 

C, shows the size of the contributing area and the amount 

of recharge from the contributing area as well as the percent 

change from the calibrated results. Part D shows the volume 

of water from the contributing area for the well-field-scale 

models and the percent change from the calibrated results. 

Streambed Conductance

When streambed conductance was decreased by a factor 

of 5, less stream inflow was available to the production well 

and the size of the recharge areas and the volumes of water 

derived from the recharge areas increased. The recharge areas 

increased by 40 to 273 percent and the recharge volumes by 

34 to 250 percent (table 16A). The results also show that the 

percentage of the total volume of pumped water derived 

from only the recharge areas increased to 69 to 83 percent 

(table 16D). Well 3, pumping at 25,000 ft3/d was less sensitive 

to a change in streambed conductance than the other wells and 

Well 1 was the most sensitive.

When streambed conductance was increased by a factor 

of 5, more water was available from stream inflow and the 

recharge areas and the volumes of water derived from these 

recharge areas decreased by 0 to 20 percent. The results show 

that the total volume of water to each production well derived 

from only the recharge area ranged from 16 to 63 percent; 

the remainder of the volume was derived from stream inflow 

(tables 16A and D). 

Table 15. Source of water to production wells, Pootatuck River 

Basin, Newtown, Connecticut.

[ns, not simulated; italics, results for Well 3 presented in table but are not 

discussed]

Source

Percent of volume pumped from each well 

derived from stream inflow and from the 

contributing area

Well 1 Well 2 Well 3 1 Well 7 Well 8

Calibrated result for well-field-scale models

Stream inflow 80 38 ns 49 46

Recharge area 20 62 100 51 54

Total 100 100 100 100 100

1

Pumping rate at Well 3 set to 25,000 cubic feet per day. Well 3 went 

dry above that rate, perhaps because of an unknown geologic feature not 

simulated in the groundwater model.
60 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Figure 38. Simulated areas contributing recharge to production wells in the well-field-scale models, Pootatuck River Basin, 

Newtown, Connecticut.

Newtown

Monroe

73°18' 73°16' 73°14'

41°24'

41°22'

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Well 1

Well 2

Well 3

Well 7

Well 8

Areas with glacial-stratified deposits

Pootatuck River Basin

Simulated areas contributing 

recharge to wells

Well 2

Well 1

Well 3

Well 7 Well 8

Production well, and identifier

Well-field-scale model area

Fairfield Hills well-field-scale model area

United Water well-field-scale model area
Delineation of Areas Contributing Recharge to Production Wells 61

Newtown

Monroe

73°18' 73°16' 73°14'

41°24'

41°22'

2

0 1 2 MILES

0 1 KILOMETERS

EXPLANATION

Wells 1 and 2 combined

Wells 7 and 8 combined

Well 3

Simulated areas contributing 

recharge to wells

Areas with glacial-stratified deposits

Pootatuck River Basin

Well 2

Well 1

Well 3

Well 7 Well 8

Production well, and identifier United Water well-field-scale model area Fairfield Hills well-field-scale model area

Figure 39. Simulated areas contributing recharge to production wells using Level A mapping regulations in the well-field-scale 

models, Pootatuck River Basin, Newtown, Connecticut.
62 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut Table 16. Results of the sensitivity analyses on recharge areas for the well-field-scale models, Pootatuck River Basin, Newtown, Connecticut. [Increases and decreases are based on the calibrated values; -, indicates a decrease; nd, no data because well went dry in simulation; ns, no simulated streamflow component, italics, results for Well 3 presented in table but not discussed] Well 1 Well 2 Well 3 1 Well 7 Well 8 Well 1 Well 2 Well 31 Well 7 Well 8 Volume pumped from each well, in cubic feet per day 66,800 144,000 25,000 90,000 103,000 A. Streambed conductance Recharge area, in square feet Change in percent from the calibrated value Calibrated result 2,520,000 17,100,000 5,820,000 10,000,000 13,000,000 Streambed conductance decreased by a factor of 5 9,410,000 24,200,000 6,350,000 14,800,000 18,200,000 273 42 9 48 40 Streambed conductance increased by a factor of 5 2,030,000 17,100,000 5,680,000 9,120,000 11,500,000 -19 0 -2 -9 -12 Volume of water from recharge area, in cubic feet Change in percent from the calibrated value Calibrated result 13,200 89,300 27,600 46,300 54,900 Streambed conductance decreased by a factor of 5 46,200 120,000 29,700 65,100 78,200 250 34 8 41 42 Streambed conductance increased by a factor of 5 10,600 90,800 27,000 42,200 48,800 -20 2 -2 -9 -11 B. Hydraulic conductivity Recharge area, in square feet Change in percent from the calibrated value Calibrated result 2,520,000 17,100,000 5,820,000 10,000,000 13,000,000 Hydraulic conductivity decreased by 30 percent 2,030,000 15,200,000 nd 8,950,000 11,400,000 -19 -11 nd -11 -12 Hydraulic conductivity increased by 50 percent 2,970,000 18,400,000 6,265,600 10,900,000 14,300,000 18 8 8 9 10 Volume of water from recharge area, in cubic feet Change in percent from the calibrated value Calibrated result 13,200 89,300 27,600 46,298 54,900 Hydraulic conductivity decreased by 30 percent 11,000 77,800 nd 43,000 50,000 -17 -13 nd -7 -9 Hydraulic conductivity increased by 50 percent 15,000 93,600 28,300 47,300 57,000 14 5 3 2 4 C. Recharge Recharge area, in square feet Change in percent from the calibrated value Calibrated result 2,520,000 17,100,000 5,820,000 10,000,000 13,000,000 Areal recharge decreased by 50 percent 2,630,000 23,000,000 8,670,000 13,000,000 18,500,000 4 35 49 29 42 Areal recharge increased by 50 percent 2,140,000 13,400,000 4,450,000 8,200,000 10,100,000 -15 -22 -24 -18 -23 Volume of water from recharge area, in cubic feet Change in percent from the calibrated value Calibrated result 13,200 89,300 27,600 46,300 54,900 Areal recharge decreased by 50 percent 12,000 118,000 38,700 55,600 72,600 -9 33 40 20 32 Areal recharge increased by 50 percent 11,600 68,700 21,600 39,700 43,600 -12 -23 -22 -14 -21
Delineation of Areas Contributing Recharge to Production Wells 63

Table 16. Results of the sensitivity analyses on recharge areas for the well-field-scale models, 

Pootatuck River Basin, Newtown, Connecticut.—Continued

[Increases and decreases are based on the calibrated values; -, indicates a decrease; nd, no data because well went dry 

in simulation; ns, no simulated streamflow component, italics, results for Well 3 presented in table but not discussed]

D. Percent of volume pumped from each well derived from stream inflow and from the contributing area

Well 1 Well 2 Well 3 1 Well 7 Well 8

Source Calibrated result2

Stream inflow 80 38 ns 49 46

Recharge area 20 62 100 51 54

Total: 100 100 100 100 100

Streambed conductance decreased by a factor of 5

Stream inflow 31 17 19 28 24

Recharge area 69 83 81 72 76

Total: 100 100 100 100 100

Streambed conductance increased by a factor of 5

Stream inflow 84 37 8 53 52

Recharge area 16 63 92 47 48

Total: 100 100 100 100 100

Hydraulic conductivity decreased by 30 percent

Stream inflow 83 46 nd 52 51

Recharge area 17 54 nd 48 49

Total: 100 100 nd 100 100

Hydraulic conductivity increased by 50 percent

Stream inflow 78 35 ns 47 44

Recharge area 22 65 100 53 56

Total: 100 100 100 100 100

Areal recharge decreased by 50 percent

Stream inflow 82 18 ns 38 29

Recharge area 18 82 100 62 71

Total: 100 100 100 100 100

Areal recharge increased by 50 percent

Stream inflow 83 52 14 56 58

Recharge area 17 48 86 44 42

Total: 100 100 100 100 100

1

Pumping rate at Well 3 set to 25,000 cubic feet per day. Well 3 went dry above that rate, perhaps because of an 

unknown geologic feature not simulated in the groundwater model.

2

Data presented in table 15.
64 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

ensures that if one of the instances described in a sensitivity 

simulation were to be true in the Pootatuck River Basin, 

the resulting recharge area has been identified—these areas 

would be analogous to maximum “error bars” for the recharge 

areas and are shown in figure 40. The maximum extent of 

the simulated recharge areas intersects the boundaries of the 

models, particularly the contributing area to Well 3. Therefore, 

the arbitrary constant-head boundary is supplying water to 

the pumped wells. If the model were extended, the simulated 

recharge area in the till and bedrock uplands would be larger 

than that shown on the map.

The results of the sensitivity simulations varied by 

production well and simulation; there was no apparent overall 

pattern that would indicate the calibrated values were incorrect. In terms of overall change from the calibrated model, a 

few results stand out: 

1. The sensitivity analyses where values of the parameters 

were decreased showed larger changes in recharge areas 

than those simulations in which parameter values 

were increased; 

2. A decrease by a factor of 5 in streambed conductivity had 

the greatest effect of all of the sensitivity analyses and 

increased the areal extent of the recharge areas, thereby 

decreasing the percent of pumped water derived from 

stream inflow and increasing the percent of water derived 

from the areal recharge; and

3. A decrease of 50 percent in areal recharge had the second 

greatest effect, also increasing the areal extent of the 

recharge areas. 

Decreases in both streambed conductance and areal recharge 

decrease the amount of water available to satisfy the demand 

for water at the production wells. 

Simulation of the Interaction between 

Groundwater and Surface Water

The basin-scale model was used to assess the effects of 

the production wells on the groundwater levels and base flow 

in the Pootatuck River. In addition, it was used to determine 

the water budget—the relative amounts of water coming from 

the recharge area and from stream inflow. 

Sensitivity Analysis of Simulated Groundwater 

Levels

The basin-scale groundwater-flow model was used to 

define the steady-state water table with all the wells pumping. 

The discussion focuses on the center of the valley because all 

the calibration points in the model are in the stratified 

glacial deposits.

Hydraulic Conductivity

When the horizontal and vertical hydraulic conductivity 

values were decreased by 30 percent, recharge areas decreased 

by 11 to 19 percent and the volume of water derived from 

these recharge areas decreased by 7 to 17 percent (table 16B). 

The results show that the percentage of the total volume of 

water to each production well derived only from the recharge 

area ranged from 17 to 54 percent for the well-field-scale 

models; the remainder of the volume was derived from stream 

inflow (table 16D).

When horizontal and vertical hydraulic conductivity 

values were increased by 50 percent from the calibrated 

values, the recharge areas for the production wells and the 

volumes derived from those recharge areas increased by 8 

to 18 percent and 2 to 14 percent, respectively (table 16B). 

The results show that the percent of the total volume of water 

to each production well derived only from the recharge area 

ranged from 22 to 65 percent; the remainder of the volume 

was derived from stream inflow (table 16D). The differences 

in these percentages were attributed to the areally-variable 

recharge rate as well as the large influence of the stream on the 

hydrologic system as described above. The areas contributing 

recharge to Wells 1, 3, and 8 were sensitive to a decrease in 

hydraulic conductivity (Well 3 went dry in the well-field-scale 

model) and the area contributing recharge to Well 1 was most 

sensitive to an increase in hydraulic conductivity (table 16B). 

Recharge

When recharge rates were decreased by 50 percent, the 

recharge areas that resulted were 4 to 42 percent larger and the 

volumes of water derived from these recharge areas ranged 

from 9 percent smaller to 33 percent larger (table 16C). The 

results show that the percent of the total volume of water to 

each production well derived only from the recharge area was 

18 to 82 percent in the well-field-scale models; the remainder 

of the volume was derived from stream inflow (table 16D). 

When the recharge rates were increased by 50 percent, 

the recharge areas that resulted decreased by 15 to 23 percent and the change in volumes of water derived from these 

recharge areas decreased from 12 to 23 percent (table 16C). 

The results show that the percentage of the total volume of 

water to each production well derived only from the recharge 

area was 17 to 48 percent in the well-field-scale models 

(table 16D); the remainder of the volume was derived from 

stream inflow. The differences in these percentages were 

attributed to the areally-variable recharge rate and to the influence of the stream on the hydrologic system. Wells 2 and 8 

were the most sensitive to an increase and decrease 

in recharge.

The spatial extents of the recharge areas determined 

during the sensitivity analyses were stacked on top of each 

other to show the maximum possible extent or possible change 

in geographic position of simulated recharge areas. This 
Simulation of the Interaction between Groundwater and Surface Water 65

Newtown

Monroe

73°18' 73°16' 73°14'

41°24'

41°22'

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Well 1

Well 2

Well 3

Well 7

Well 8

Areas with glacial stratified deposits

Pootatuck River Basin

Production well, and identifier

Well 2

Well 1

Well 3

Well 7 Well 8

Simulated areas contributing 

recharge to wells

Fairfield Hills well-field-scale model area

United Water well-field-scale model area

Figure 40. Combination of simulated recharge areas resulting from the sensitivity simulations, Pootatuck River Basin, 

Newtown, Connecticut.
66 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

The sensitivity simulations done on the basin-scale 

model were the same as those done on the well-field-scale 

models except the hydraulic conductivity was reduced by 

50 percent. As discussed in the “Model Limitations” section of 

this report, there were only seven calibration points (cells) for 

groundwater levels because of the coarse discretization (cells 

were 400 × 400 ft). Observation wells NT-85, NT-86, NT-90, 

and NT-91 near Well 3 were in a single cell; observation wells 

NT-106, NT-107, NT-108, and NT-109 near Well 7 also were 

in a single cell. Therefore, the average of the observed water 

levels in these wells was compared to the simulated water 

level in the observation cell. In addition, because it was not 

possible to measure water levels near Wells 1 and 2 in nonpumping conditions, the model may not represent steady-state 

conditions in this area.

The mean of the residuals for each of the sensitivity 

simulations at each of the calibration points is shown in 

table 17. The sensitivity model with lowered recharge had the 

lowest mean residual, 0.05 ft, less than that in the calibrated 

model (1.10 ft), but there was a greater range of values—the 

positive residuals balanced out the negative ones; also, one of 

the observation wells went dry, as might be expected because 

there was less water available to the groundwater system. The 

calibrated model had the lowest mean absolute error (1.95 ft); 

the lowered recharge model was 2.33 ft. If the two calibration points near Wells 1 and 2 are excluded from the analysis, 

the mean of the residuals for the calibrated model was -0.49. 

The sensitivity model with lowered streambed conductance 

had the greatest residual. The connection between the aquifer 

and the stream is an important component of the hydrologic 

system; a reduction of stream inflow near the production wells 

would cause water levels in the aquifer to be lower, and lower 

streambed conductance in other areas could prevent aquifer 

discharge to the stream, thereby causing higher water levels. 

Figure 41 shows a comparison of the groundwater levels in 

the calibrated model and in each of the sensitivity models. 

The simulated water levels near the United Water well field 

are consistently higher than the observed water levels; this 

supports the assumption that the observed water levels were 

affected by the production wells and did not represent steadystate conditions. Although the points on figure 41 representing 

calibrated model of the United Water well field do not plot 

near the 1:1 line, they are in the middle of the data points from 

the sensitivity simulations.

In several of the sensitivity simulations, some of the 

calibration cells went dry. One or more wells went dry in all 

but the calibrated model and the simulation with increased 

recharge; Well NT-94, the background well, went dry in four 

of the sensitivity simulations. The model was most sensitive to 

reduction in streambed conductance—the mean of the residuals was -5.35 ft, and the mean absolute error was +4.51 ft.

Sensitivity Analysis of Stream Base Flow in the 

Pootatuck River

The calibrated basin-scale model was used to simulate 

base flow along the main stem Pootatuck River (fig. 42). 

Because figure 42 shows cumulative flow, the sharp increases 

on the graph represent inflow from tributary streams, and 

the decreases represent reduction in base flow downstream 

from the production wells. The sensitivity of the base flow 

to changes in model parameters also is shown in figure 42. 

Changing the streambed and conductivity parameters had little 

effect on the base flow; the simulated base flow is not sensitive 

to these parameters. Changes in the recharge, however, have 

a substantial effect on the streamflow. Future development in 

the basin will increase the impervious surface area, thereby 

changing the amount of recharge that reaches the groundwater 

system and affecting the base flow in the Pootatuck River. 

Simulation of future scenarios is described later in this report. 

Simulated Water Budgets

Simulated average annual water budgets for the nonpumping and pumping scenarios in the basin-scale-model 

were summarized for 26 subbasins of the Pootatuck River 

Basin ranging in size from less than 0.1 to 3.9 mi2 (fig. 43). 

Geographic and hydrologic position, as well as the rate of 

recharge to each subbasin, determined what component of 

the water budget was the greatest source of water to that 

subbasin. For subbasins located in upland, headwater areas 

(subbasins 1–15 and 26), recharge from precipitation was the 

greatest source of water, whereas in subbasins located further 

downstream (subbasins 16–25), stream inflow was the greatest 

source of water. When comparing pumping and nonpumping 

conditions in the calibrated model, the steady-state simulations 

show that streamflows downstream from the production well 

are generally decreased by the amount of pumping; streamflow 

in subbasin 16, near the outlet of the Pootatuck River Basin, 

was decreased by about 5.3 ft3/s, the total amount of pumping 

in the basin.
Simulation of the Interaction between Groundwater and Surface Water 67

Table 17. Results of sensitivity analysis on water levels in the basin-scale model, Pootatuck River 

Basin, Newtown, Connecticut. 

[Residual, difference between simulated and observed water levels]

Simulation Range of residuals,

in feet

Mean 

residual,

in feet

Mean 

absolute error, 

in feet

Number of dry 

well cells

Calibrated model -1.72 to 5.12 1.10 1.95 0

Streambed divided by 5 -13.82 to 1.62 -5.35 4.51 3

Streambed times 5 -1.88 to 6.00 1.32 2.19 1

Conductivity minus 50 percent -4.30 to 4.10 1.09 2.01 2

Conductivity plus 50 percent -2.27 to 4.94 0.87 2.09 1

Recharge minus 50 percent -3.31 to 4.53 0.05 2.33 1

Recharge plus 50 percent -2.35 to 4.84 1.08 2.34 0

245

255

265

275

285

295

305

315

325

335

245 255 265 275 285 295 305 315 325 335

 SIMULATED WATER LEVELS, IN FEET ABOVE NGVD29

OBSERVED WATER LEVEL IN FEET ABOVE NGVD29

Calibrated model

Streambed divided by 5

Streambed times 5

Conductivity minus 50 percent

Conductivity plus 50 percent

Recharge minus 50 percent

Recharge plus 50 percent

Fairfield Hills well field

United Water well field

Figure 41. Comparison of simulated and observed water levels in the basin-scale model sensitivity analysis, Pootatuck River 

Basin, Newtown, Connecticut.
68 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

50

4540353025201510

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000

SIMULATED BASE FLOW, IN CUBIC FEET PER SECOND

SIMULATED DISTANCE FROM HEADWATERS, IN FEET

Calibrated model

Streambed divided by 5Streambed times 5Conductivity minus 50 percentConductivity plus 50 percentRecharge minus 50 percentRecharge plus 50 percent

Cold Springs Rd.

Hattertown Rd.

Turkey Hill Rd.

Wasserman Way

Sandy Hook

1, 2 Wells

8 Wells 7,

Well 3

Housatonic River

Figure 42. Cumulative downstream base flow in the Pootatuck River in basin-scale model sensitivity analysis, Newtown, Connecticut.
Simulation of the Interaction between Groundwater and Surface Water 69

Newtown

Monroe

Redding

Bethel

Southbury

Easton

Brookfield

73°20' 73°16'

41°24'

41°20'

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Subbasins used in future water-budget scenarios, and identifier

Pootatuck basin boundary

Continuous-record streamgage, and identifier

Production well

5

16

23

6

11

7

19

15

18

10 20

12

22

17

25

21

24

3

9

1

13

2

26

8

14

4

26

01203510

012035055

Base from Connecticut Department of Environmental Protection 

Digital Line Graphs, 2005

Figure 43. Model subbasins in the basin-scale model, Pootatuck River Basin, Newtown, Connecticut.
70 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

2, the furthest upstream, and increase near Wells 7 and 8, and 

again at Well 3.

Changes in the cumulative downstream base flow under 

potential future recharge scenarios also are shown in figure 45. 

The base flow is reduced in each scenario because there is less 

recharge; the commercial land-use scenario, which represents 

the greatest increase in impervious area, shows the greatest 

change. Changes in base flow become apparent very near the 

headwaters of the Pootatuck River and the hydrographs continue to diverge downstream, although the general shape of the 

cumulative base flow shown in the figure remains the same. 

Water Budgets in Subbasins of Pootatuck 

River—Future Scenarios

The simulations of potential future recharge showed 

variation in the cumulative downstream base flow, flow 

from individual subbasins, and water budgets in each of 26 

subbasins in the Pootatuck River Basin. Base flow decreased 

in all subbasins because of decreased areal recharge. Most 

changes because of pumping occurred in subbasins with, and 

downstream from, production wells (table 18). Subbasin 6, 

in the headwaters, was the only subbasin with no outflow in 

any simulation; the stream draining this subbasin crosses from 

the till uplands onto the more transmissive stratified glacial 

deposits and loses water to the underlying aquifer.

Change in base flow was the greatest in the commercial 

land-use scenario because it had the greatest reduction in 

recharge. Cumulative downstream flow and flow from individual subbasins decreased by more than 20 percent in several 

subbasins (table 19). Most of these subbasins were located 

in upland headwater areas and the simulated flow was less 

than 1 ft3/s. These small changes may be within the expected 

error of the model. In other, downstream subbasins (16–25), 

cumulative downstream flow decreased from 10 to 20 percent 

(table 19) and flow from individual subbasins decreased from 

0 to 30 percent (table 20). In the mixed-use and residential 

land-use scenarios, cumulative downstream flow and flow 

from individual subbasins decreased by less than 1 ft3/s (most 

less than 0.1 ft3/s) for most subbasins, with the exception of 

cumulative base flow in subbasins 16 through 18 in the mixeduse scenario where flows decreased by as much as 1.46 ft3

/s 

(table 19). Wells 1 and 2 were located in subbasin 21—one 

of the smallest subbasins with an area of 0.1 mi2. Stream base 

flow from subbasin 21 showed a loss and a substantial percent 

change in flow as compared to the nonpumping simulation 

(table 20) because of the relatively large amount of pumping at 

Wells 1 and 2 and the small amount of areal recharge.

Simulated Effect of Future Recharge 

Conditions

Future development in the Pootatuck River Basin will 

affect the amount of recharge to the groundwater system and 

the areal distribution of that recharge because the percentage 

of effective impervious cover will change when agricultural 

and forested land is converted to residential or commercial 

land uses. The basin-scale, steady-state, groundwater-flow 

model was used to simulate how these potential changes may 

affect base flows in the main stem of the Pootatuck River and 

base flows and water budgets in each of the 26 subbasins of 

the Pootatuck River Basin. 

Three development scenarios were simulated to evaluate 

the potential effect of future development on recharge, and 

therefore, on base flow in the Pootatuck River. In each of these 

scenarios, only the spatially-variable rates of recharge were 

altered in those areas that could be developed, based on the 

potential change in effective impervious area. An areally-distributed recharge map was developed for each scenario using 

the regression equation discussed in an earlier section and in 

the appendix of this report. No attempt was made to account 

for potential increasing water needs in the future scenarios 

because the simulated pumping rates were the registered rates 

(except for Well 3, which was pumped at 25,000 ft3/d, still 

higher than what it was able to pump during the aquifer test).

The town of Newtown provided maps, developed by 

H.C. Planning Consultants, Inc. (2008), that identified areas 

that could be developed. It was assumed that future commercial development would have 85 percent effective impervious 

area (EIA), residential development would have 35 percent 

EIA, and mixed use development would have 50 percent 

EIA. Therefore, each of the three development scenarios—

commercial, residential, and mixed land-use scenarios—

altered recharge in the areas to be developed to account for the 

increase in impervious area. The commercial land-use scenario 

simulates the greatest change in recharge (fig. 44). In the 

discussion that follows, results of each scenario are compared 

to the calibrated basin-scale, steady-state, groundwater-flow 

model and changes are noted. 

Base Flows in the Main Stem Pootatuck River—

Future Scenarios

The basin-scale model was used to determine changes in 

the cumulative downstream base flow in the main stem of the 

Pootatuck River. Simulated base flow under nonpumping and 

pumping conditions is shown in figure 45. Changes in base 

flow caused by pumping become apparent near Wells 1 and 
Simulated Effect of Future Recharge Conditions 71

73°20' 73°16'

41°24'

41°20'

EXPLANATION

Change (reduction) in recharge, in inches per year

0 to 1

greater than 1 to 3.5

greater than 3.5 to 7

greater than 7 to 11

greater than 11

Inactive model area

Town boundary

0 1 2 MILES

0 1 2 KILOMETERS

Figure 44. Difference in recharge in commercial land-use scenario as compared to the calibrated model, Pootatuck 

River Basin, Newtown, Connecticut.
72 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Figure 45. Cumulative downstream base flow in the Pootatuck River for the basin-scale model future-recharge analysis, Newtown, 

Connecticut.

0

5

10

15

20

25

30

35

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000

DISTANCE FROM HEADWATERS, IN FEET

SIMULATED BASE FLOW, IN CUBIC FEET PER SECOND

Well 2

Well 1 

Well 8

Well 7

Well 3

Hattertown, Road

Sandy Hoook 

Housatonic River

Deep

Brook

Cold Spring

Brook

N. Branch 

Pootatuck River

Cold Springs RoadTurkey Hill Road

Wasserman Way

40,000 45,000

No pumping

With pumping

Residential land use

Mixed land use

Commercial land use 

Basecase

Future recharge scenario with pumping

EXPLANATION
Simulated Effect of Future Recharge Conditions 73

Table 18. Annual water budget for modeled subbasins under pumping and nonpumping conditions, Pootatuck River Basin, Connecticut.—Continued[mi2, square miles; pumping at registered rates; values in cubic feet per second; subbasins 1–15 and 26 are in headwaters; subbasins 16–25 are downstream; subbasins are shown on figure 43]

A. Current calibrated basin-scale model

Pumping conditions1 Nonpumping conditions

Subbasin Area (mi2) Recharge from precipitation Inflow from streams Inflow from other subbasins Outflow to other subbasins Outflow to streams

Groundwater 

withdrawals 

for public 

supply

Difference 

(inflow 

minus 

outflow)

Percent 

discrepancy

Inflow 

from 

streams

Inflow 

from other 

subbasins

Outflow to 

other 

subbasins

Outflow 

to 

streams

Difference 

(inflow 

minus 

outflow)

Percent 

discrepancy

1 1.9 1.85 0 0.52 1.43 0.91 0 0.04 1.8 0 0.48 1.43 0.91 -0 -02 0.6 0.68 0 0.23 0.85 0.05 0 -0 -0 0 0.23 0.85 0.05 -0 -03 3.9 4.21 0.55 2.97 1.71 6.13 0 -0.10 -1.3 0.55 2.97 1.71 6.13 -0.10 -1.34 0.4 0.39 0 0.19 0.29 0.32 0 -0.02 -3.9 0 0.19 0.29 0.32 -0.02 -3.95 1.5 1.78 0 1.22 2.43 0.55 0 0.02 0.5 0 1.21 2.43 0.56 0.01 0.36 0.6 0.68 0 0.15 0.84 0.00 0 -0.02 -2.5 0 0.15 0.84 0.00 -0.02 -1.87 0.6 0.66 0 0.44 1.05 0.05 0 -0 -0 0 0.44 1.05 0.05 -0 -0.08 0.5 0.49 0 0.52 0.80 0.18 0 0.02 2.3 0 0.52 0.80 0.18 0.02 2.39 3.7 4.16 7.32 1.13 1.93 10.7 0 0 0 7.33 1.13 1.93 10.7 -0 -0

10 0.4 0.42 0 0.23 0.59 0.05 0 0 0 0 0.23 0.59 0.05 0 0

11 1.1 1.78 0 0.70 1.36 1.12 0 -0 -0 0 0.70 0.97 1.51 -0 -0

12 0.4 0.49 0 0.06 0.55 0.05 0 -0.05 -8.2 0 0.06 0.55 0.05 -0.05 -8.213 1.5 1.64 0 0.52 1.50 0.49 0 0.17 8.0 0 0.52 1.50 0.49 0.17 8.014 0.8 0.84 0 0.95 0.78 1.02 0 0 0 0 0.95 0.78 1.02 0 015 0.8 0.92 0 0.13 0.59 0.47 0 -0 -0 0 0.13 0.59 0.47 -0 -016 1.9 2.62 24.2 1.30 0.11 27.9 0 0.12 0.4 27.5 1.30 0.11 31.2 0.12 0.417 0.1 0.29 23.9 0.25 0.35 24.2 0 -0.12 -0.5 27.2 0.25 0.35 27.5 -0.12 -0.4

218 0.8 1.15 15.8 1.15 0.50 16.9 0.60 0.10 0.6 18.5 1.16 0.50 20.2 0.10 0.5319 0.9 1.41 12.7 2.87 0.72 15.2 1.04 -0 -0 14.4 2.87 0.73 17.9 -0 -0

20 0.6 0.75 11.6 1.48 0.57 12.7 0 0.59 4.3 12.3 1.61 0.52 14.4 -0.22 -1.5

421 0.1 0.13 10.7 0.98 0.26 10.5 1.67 -0.59 -5.0 10.7 0.55 0.39 10.7 0.22 1.9

22 0.3 0.61 6.62 0.51 0.46 7.28 0 0 0 6.62 0.51 0.46 7.28 -0 -023 1.4 1.72 4.17 1.29 0.52 6.62 0 0.05 0.6 4.17 1.29 0.52 6.62 0.05 0.624 0.1 0.08 2.75 0.32 0.21 3.11 0 -0.17 -5.5 2.75 0.32 0.21 3.11 -0.17 -5.525 0.1 0.07 2.04 0.39 0.25 2.25 0 -0 -0 2.04 0.39 0.25 2.25 -0 -026 1.2 1.37 0 0.52 0.32 1.57 0 -0 -0 0 0.52 0.32 1.57 -0 -0
74 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut Table 18. Annual water budget for modeled subbasins under pumping and nonpumping conditions, Pootatuck River Basin, Connecticut.—Continued [mi2, square miles; pumping at registered rates; values in cubic feet per second; subbasins 1–15 and 26 are in headwaters; subbasins 16–25 are downstream; subbasins are shown on figure 43] B. Future recharge scenario with mixed land use Pumping1 Nonpumping Sub- basin Area (mi2

)

Recharge 

from 

precipitation

Inflow 

from 

streams

Inflow 

from other 

subbasins

Outflow 

to other 

subbasins

Outflow 

to 

streams

Groundwater 

withdrawals 

for public 

supply

Differnece 

(inflow 

minus 

outflow)

Percent 

discrepancy

Inflow 

from 

streams

Inflow from 

other subbasins

Outflow to 

other 

subbasins

Outflow 

to 

streams

Difference 

(inflow 

minus 

outflow)

Percent 

discrepancy

1 1.9 1.73 0 0.45 1.42 0.76 0 0 0 0 0.45 1.42 0.76 0 02 0.6 0.63 0 0.18 0.78 0.03 0 0 0 0 0.18 0.78 0.03 0 03 3.9 3.64 0.41 2.82 1.64 5.32 0 -0.09 -1.3 0.41 2.82 1.63 5.32 -0.09 -1.34 0.4 0.27 0 0.16 0.22 0.22 0 -0.02 -4.2 0 0.16 0.22 0.22 -0.02 -4.25 1.5 1.54 0 1.22 2.14 0.46 0 0.15 5.5 0 1.22 2.13 0.47 0.15 5.56 0.6 0.58 0 0.15 0.88 0.00 0 -0.15 -20.6 0 0.15 0.88 0.00 -0.15 -20.77 0.6 0.54 0 0.42 0.96 0.01 0 -0 -0.1 0 0.42 0.96 0.01 -0 -0.28 0.5 0.44 0 0.52 0.78 0.17 0 0.02 1.6 0 0.52 0.78 0.17 0.02 1.69 3.7 3.52 6.16 1.06 1.95 8.79 0 -0 -0 6.16 1.06 1.95 8.79 -0 -0

10 0.4 0.31 0 0.26 0.54 0.03 0 0 0 0 0.26 0.54 0.03 0 0

11 1.1 1.60 0 0.63 1.31 0.92 0 -0 -0 0 0.63 0.92 1.31 -0 -0

12 0.4 0.43 0 0.06 0.49 0.04 0 -0.04 -8.1 0 0.06 0.49 0.04 -0.04 -8.113 1.5 1.36 0 0.47 1.52 0.15 0 0.16 8.7 0 0.47 1.52 0.15 0.16 8.714 0.8 0.67 0 0.94 0.75 0.86 0 -0 -0 0 0.94 0.75 0.86 -0 -015 0.8 0.77 0 0.11 0.54 0.35 0 0 0 0 0.11 0.54 0.35 0 016 1.9 2.49 20.3 1.32 0.11 23.9 0 0.12 0.5 23.6 1.32 0.11 27.2 0.12 0.417 0.1 0.29 20.0 0.24 0.35 20.3 0 -0.12 -0.6 23.3 0.25 0.35 23.6 -0.12 -0.5

218 0.8 1.00 13.0 1.11 0.48 13.9 0.60 0.09 0.6 15.7 1.12 0.48 17.2 0.09 0.5319 0.9 1.28 10.3 2.58 0.64 12.5 1.04 -0 -0 12.0 2.57 0.66 15.2 -0 -0

20 0.6 0.63 9.49 1.35 0.54 10.3 0 0.60 5.3 10.2 1.48 0.48 12.0 -0.21 -1.7

421 0.1 0.12 8.79 0.97 0.25 8.56 1.67 -0.60 -6.1 8.79 0.53 0.39 8.85 0.21 2.2

22 0.3 0.55 5.53 0.49 0.43 6.13 0 0 0 5.53 0.49 0.43 6.13 -0 -023 1.4 1.56 3.27 1.23 0.49 5.53 0 0.04 0.7 3.27 1.23 0.49 5.53 0.04 0.724 0.1 0.06 2.03 0.30 0.19 2.36 0 -0.16 -6.7 2.03 0.30 0.19 2.36 -0.16 -6.725 0.1 0.07 1.69 0.36 0.23 1.88 0 -0 -0 1.69 0.36 0.23 1.88 -0 -026 1.2 1.08 0 0.54 0.28 1.34 0 -0 -0 0 0.54 0.28 1.34 -0 -0
Simulated Effect of Future Recharge Conditions 75

Table 18. Annual water budget for modeled subbasins under pumping and nonpumping conditions, Pootatuck River Basin, Connecticut.—Continued[mi2, square miles; pumping at registered rates; values in cubic feet per second; subbasins 1–15 and 26 are in headwaters; subbasins 16–25 are downstream; subbasins are shown on figure 43]

C. Future recharge scenario with commercial land use

Pumping1 Nonpumping

Subbasin Area (mi2) Recharge from precipitation Inflow from streams Inflow from other subbasins Outflow to other subbasins Outflow to streams

Groundwater 

withdrawals 

for public 

supply

Difference 

(inflow 

minus 

outflow)

Percent 

discrepancy

Inflow 

from 

streams

Inflow 

from other 

subbasins

Outflow 

to other 

subbasins

Outflow 

to 

streams

Difference 

(inflow 

minus 

outflow)

Percent 

discrepancy

1 1.9 1.83 0 0.48 1.43 0.89 0 0 0 0 0.48 1.43 0.89 0 02 0.6 0.68 0 0.21 0.85 0.05 0 -0 -0 0 0.21 0.85 0.05 -0 -03 3.9 4.13 0.53 2.95 1.70 6.01 0 -0.10 -1.3 0.53 2.95 1.69 6.01 -0.10 -1.34 0.4 0.37 0 0.19 0.28 0.30 0 -0.02 -4.0 0 0.19 0.28 0.30 -0.02 -4.05 1.5 1.73 0 1.24 2.37 0.53 0 0.07 2.3 0 1.23 2.36 0.55 0.06 2.06 0.6 0.65 0 0.15 0.87 0 0 -0.07 -8.7 0 0.15 0.86 0 -0.06 -8.07 0.6 0.64 0 0.44 1.03 0.05 0 -0.01 -0.6 0 0.44 1.03 0.05 -0.01 -0.68 0.5 0.48 0 0.52 0.80 0.18 0 0.02 2.2 0 0.52 0.80 0.18 0.02 2.29 3.7 4.01 7.01 1.11 1.94 10.2 0 0 0 7.01 1.11 1.93 10.2 0 0

10 0.4 0.39 0 0.23 0.58 0.04 0 0 0 0 0.23 0.58 0.04 0 0

11 1.1 1.75 0 0.68 1.35 1.08 0 -0 -0 0 0.68 0.96 1.47 -0 -0

12 0.4 0.47 0 0.06 0.53 0.04 0 -0.04 -8.2 0 0.06 0.53 0.04 -0.04 -8.213 1.5 1.56 0 0.51 1.50 0.40 0 0.17 8.2 0 0.51 1.50 0.40 0.17 8.214 0.8 0.79 0 0.95 0.77 0.97 0 -0 -0 0 0.95 0.77 0.97 -0 -015 0.8 0.89 0 0.12 0.58 0.44 0 -0 -0 0 0.12 0.58 0.44 -0 -016 1.9 2.58 23.4 1.30 0.11 27.1 0 0.12 0.4 26.7 1.30 0.11 30.4 0.12 0.417 0.1 0.29 23.1 0.25 0.35 23.4 0 -0.12 -0.5 26.4 0.25 0.35 26.7 -0.12 -0.4

218 0.8 1.14 15.1 1.14 0.50 16.2 0.60 0.10 0.6 17.8 1.15 0.50 19.5 0.10 0.5319 0.9 1.38 12.1 2.81 0.70 14.6 1.04 -0 -0.0 13.8 2.80 0.71 17.3 -0 -0

20 0.6 0.74 11.1 1.45 0.57 12.1 0 0.59 4.5 11.8 1.58 0.51 13.8 -0.22 -1.5

421 0.1 0.13 10.2 0.98 0.26 9.98 1.67 -0.59 -5.3 10.2 0.54 0.40 10.3 0.22 2.0

22 0.3 0.60 6.32 0.50 0.46 6.97 0 0 0 6.32 0.50 0.46 6.97 0 023 1.4 1.67 3.93 1.28 0.51 6.32 0 0.04 0.6 3.93 1.28 0.51 6.32 0.04 0.624 0.1 0.07 2.57 0.32 0.21 2.92 0 -0.17 -5.8 2.57 0.32 0.21 2.92 -0.17 -5.825 0.1 0.07 1.96 0.39 0.25 2.16 0 0 0 1.96 0.39 0.25 2.16 0 026 1.2 1.30 0 0.53 0.31 1.52 0 -0 -0 0 0.53 0.31 1.52 -0 -0
76 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut Table 18. Annual water budget for modeled subbasins under pumping and nonpumping conditions, Pootatuck River Basin, Connecticut.—Continued [mi2, square miles; pumping at registered rates; values in cubic feet per second; subbasins 1–15 and 26 are in headwaters; subbasins 16–25 are downstream; subbasins are shown on figure 43] D. Future recharge scenario with residential land use Pumping1 Nonpumping Sub- basin Area (mi2

)

Recharge 

from 

precipitation

Inflow 

from 

streams

Inflow 

from other 

subbasins

Outflow 

to other 

subbasins

Outflow 

to 

streams

Groundwater 

withdrawals 

for public 

supply

Difference 

(inflow 

minus 

outflow)

Percent 

discrepancy

Inflow 

from 

streams

Inflow 

from other 

subbasins

Outflow 

to other 

subbasins

Outflow 

to 

streams

Difference 

(inflow 

minus 

outflow)

Percent 

discrepancy

1 1.9 1.81 0 0.48 1.42 0.86 0 0 0 0 0.48 1.42 0.86 0 02 0.6 0.67 0 0.21 0.83 0.05 0 0 0 0 0.21 0.83 0.05 0 03 3.9 4.04 0.51 2.92 1.68 5.88 0 -0.10 -1.3 0.51 2.92 1.68 5.88 -0.10 -1.34 0.4 0.35 0 0.18 0.27 0.28 0 -0.02 -4.0 0 0.18 0.27 0.28 -0.02 -4.05 1.5 1.70 0 1.24 2.33 0.52 0 0.10 3.3 0 1.24 2.32 0.53 0.09 3.16 0.6 0.64 0 0.15 0.88 0 0 -0.10 -12.4 0 0.15 0.88 0 -0.09 -11.97 0.6 0.62 0 0.43 1.02 0.04 0 -0.00 -0 0 0.43 1.02 0.04 -0 -08 0.5 0.47 0 0.52 0.79 0.18 0 0.02 2.1 0 0.52 0.79 0.18 0.02 2.19 3.7 3.92 6.85 1.10 1.94 9.94 0 -0 -0 6.85 1.10 1.94 9.94 -0 -0

10 0.4 0.38 0 0.24 0.57 0.04 0 0 0 0 0.24 0.57 0.04 0 0

11 1.1 1.73 0 0.67 1.34 1.06 0 -0 -0 0 0.67 0.95 1.44 -0 -0

12 0.4 0.46 0 0.06 0.52 0.04 0 -0.04 -8.2 0 0.06 0.52 0.04 -0.04 -8.213 1.5 1.53 0 0.50 1.51 0.35 0 0.17 8.3 0 0.50 1.51 0.35 0.17 8.314 0.8 0.77 0 0.95 0.77 0.95 0 -0 -0 0 0.95 0.77 0.95 -0 -015 0.8 0.87 0 0.12 0.57 0.42 0 0 0 0 0.12 0.57 0.42 0 016 1.9 2.56 22.8 1.30 0.11 26.5 0 0.12 0.4 26.1 1.31 0.11 29.8 0.12 0.417 0.1 0.29 22.5 0.25 0.35 22.8 0 -0.12 -0.5 25.8 0.25 0.35 26.1 -0.12 -0.4

218 0.8 1.11 14.7 1.13 0.49 15.8 0.60 0.10 0.6 17.4 1.14 0.50 19.1 0.10 0.5319 0.9 1.36 11.8 2.77 0.69 14.2 1.04 -0 -0 13.5 2.76 0.70 16.9 -0 -0

20 0.6 0.72 10.8 1.43 0.56 11.8 0.00 0.60 4.6 11.5 1.56 0.51 13.5 -0.21 -1.6

421 0.1 0.13 9.94 0.98 0.26 9.71 1.67 -0.60 -5.4 9.94 0.54 0.39 10.0 0.21 2.0

22 0.3 0.59 6.17 0.50 0.45 6.81 0 0 0 6.17 0.50 0.45 6.81 0 0.023 1.4 1.65 3.81 1.27 0.51 6.17 0 0.04 0.6 3.81 1.27 0.51 6.17 0.04 0.624 0.1 0.07 2.47 0.31 0.21 2.82 0 -0.17 -5.9 2.47 0.31 0.21 2.82 -0.17 -5.925 0.1 0.07 1.91 0.38 0.25 2.11 0 -0 -0 1.91 0.38 0.25 2.11 -0 -026 1.2 1.26 0 0.53 0.31 1.49 0 -0 -0 0 0.53 0.31 1.49 -0 -0

1Wells pumping at maximum permissible pumping rate registered with the Connecticut Department of Environmental Protection.2 Well 3 located in subbasin 18.3 Wells 7 and 8 located in subbasin 19.4 Wells 1 and 2 located in subbasin 21.
Simulated Effect of Future Recharge Conditions 77

Table 19. Cumulative base flow in future pumping scenarios, Pootatuck River Basin, Newtown, Connecticut.[ft3/s, cubic feet per second; -, decrease; <, less than; subbasins 1–15 and 26 are in headwaters; subbasins 16–25 are downstream; subbasins are shown on figure 43]

Subbasin

Nonpumping Current conditions1 Future recharge scenarios Mixed use Commercial Residential

Simulated 

streamflow

(ft3/s)

Simulated 

streamflow 

(ft3/s)

Difference from 

simulated 

nonpumping 

streamflow 

Simulated 

streamflow 

(ft3/s)

Difference from 

simulated current 

streamflow

Simulated 

streamflow

(ft3/s)

Difference from 

simulated current 

streamflow

Simulated 

streamflow

(ft3/s)

Difference from 

simulated current 

streamflow

ft3/s percent ft3/s percent ft3/s percent ft3/s percent

1 0.90 0.90 0 0 0.84 <-0.10 -7 0.73 -0.17 -19 0.86 <-0.10 -42 <0.10 <0.10 <0.10 <0.1 <0.10 <0.10 <0.1 <0.10 <-0.10 -25 <0.10 <0.10 <0.13 6.10 6.10 0 0 5.76 -0.34 -6 5.11 -0.99 -16 5.9 -0.2 -34 0.30 0.30 0 0 0.25 <-0.10 -17 0.15 -0.15 -50 0.27 <-0.10 -105 0.53 0.52 <-0.10 -2 0.47 <-0.10 -10 0.39 -0.13 -25 0.49 <-0.10 -66 0 0 0 0 0 0 0 0 0 0 0 0 07 <0.10 <0.10 <0.10 <0.1 <0.10 <-0.10 -25 <0.10 <-0.10 -50 <0.10 <-0.10 -258 0.15 0.15 0 0 0.14 <-0.10 -7 0.12 <-0.10 -20 0.14 <-0.10 -79 10.7 10.7 0 0 9.97 -0.73 -7 8.69 -2.01 -19 10.3 -0.44 -4

10 <0.10 <0.10 <0.10 <0.1 <0.10 <0.10 <0.1 <0.10 <-0.10 -20 <0.10 <0.10 <0.1

11 1.41 1.13 -0.28 -20 1.09 <-0.10 -4 1.03 <-0.10 -9 1.11 <-0.10 -2

12 <0.10 <0.10 <0.10 <0.1 <0.10 <0.10 <0.1 <0.10 <-0.10 -25 <0.10 <0.10 <0.113 0.51 0.51 0 0 0.38 -0.13 -25 0.17 -0.34 -67 0.43 <-0.10 -1614 1.00 1.00 0 0 0.94 <-0.10 -6 0.81 -0.19 -19 0.96 <-0.10 -415 0.47 0.47 0 0 0.46 <-0.10 -2 0.45 <-0.10 -4 0.47 0.00 016 31.2 25.9 -5.27 -17 24.5 -1.46 -6 21.8 -4.11 -16 25.1 -0.86 -317 27.5 22.3 -5.27 -19 20.8 -1.42 -6 18.3 -4.01 -18 21.4 -0.84 -4

218 20.2 14.9 -5.27 -26 13.9 -1.01 -7 12.1 -2.85 -19 14.3 -0.6 -4319 18.0 13.3 -4.64 -26 12.4 -0.95 -7 10.7 -2.68 -20 12.8 -0.57 -4

20 14.4 11.9 -2.45 -17 11.1 -0.81 -7 9.68 -2.26 -19 11.5 -0.48 -4

421 10.8 9.87 -0.91 -8 9.15 -0.72 -7 7.86 -2.01 -20 9.44 -0.43 -4

22 7.27 7.27 0 0 6.85 -0.42 -6 6.11 -1.16 -16 7.02 -0.25 -323 6.61 6.61 0 0 6.22 -0.39 -6 5.56 -1.05 -16 6.37 -0.24 -424 3.11 3.11 0 0 2.90 -0.21 -7 2.53 -0.58 -19 2.98 -0.13 -425 2.24 2.24 0 0 2.16 <-0.10 -4 2.02 -0.22 -10 2.19 <-0.10 -226 1.55 1.55 0 0 1.49 <-0.10 -4 1.37 -0.18 -12 1.51 <-0.10 -3

1Wells pumping at maximum permissible pumping rate registered with the Connecticut Department of Environmental Protection.2Well 3 located in subbasin 18.3Wells 7 and 8 located in subbasin 19.4Wells 1 and 2 located in subbasin 21.
78 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut Table 20. Base flow from individual subbasins in future pumping scenarios, Pootatuck River Basin, Newtown, Connecticut. [ft3/s, cubic feet per second; -, decrease; <, less than; subbasins 1–15 and 26 are in headwaters; subbasins 16–25 are downstream; subbasins are shown on figure 43] Subbasin Nonpumping Current conditions1 Future recharge scenarios Mixed use Commercial Residential Simulated streamflow (ft3/s) Simulated streamflow (ft3/s) Difference from simulated nonpumping streamflow Simulated streamflow (ft3/s) Difference from simulated current streamflow Simulated streamflow (ft3/s) Difference from simulated current streamflow Simulated streamflow (ft3/s) Difference from simulated current streamflow ft3/s percent ft3/s percent ft3/s percent ft3/s percent

1 0.90 0.90 0 0 0.84 -0.06 -7 0.73 0.17 -19 0.86 -0.04 -42 <0.10 <0.10 <-0.10 <-0.1 <0.10 <0.10 <0.1 <0.10 <0.10 -25 <0.10 <0.10 <0.13 5.62 5.62 0 0 5.34 -0.28 -5 4.81 -0.81 -14 5.45 -0.17 -34 0.30 0.30 0 0 0.25 <-0.10 -17 0.15 -0.15 -50 0.27 <-0.10 -105 0.53 0.52 <-0.10 -2 0.47 <-0.10 -10 0.39 -0.13 -25 0.49 <-0.10 -66 0 0 0 0 0 0 0 0 0 0 0 0 07 <0.10 <0.10 <-0.10 <-0.1 <0.10 <-0.10 -25 <0.10 <-0.10 -50 <0.10 <-0.10 -258 0.15 0.15 0 0 0.14 <-0.10 -7 0.12 <-0.10 -20 0.14 <-0.10 -79 3.38 3.37 <-0.10 <-0.1 3.08 -0.29 -9 2.55 -0.82 -24 3.20 -0.17 -5

10 <0.10 <0.10 <0.10 <0.1 <0.10 <0.10 <0.1 <0.10 <-0.10 -20 <0.10 <0.10 <0.1

11 1.41 1.13 -0.28 -20 1.09 <-0.10 -4 1.03 -0.10 -9 1.11 <-0.10 -2

12 <0.10 <0.10 <0.10 <0.1 <0.10 <0.10 <0.1 <0.10 <-0.10 -25 <0.10 <0.10 <0.113 0.51 0.51 0 0 0.38 -0.13 -25 0.17 -0.34 -67 0.43 <-0.10 -1614 1.00 1.00 0 0 0.94 <-0.10 -6 0.81 -0.19 -19 0.96 <-0.10 -415 0.47 0.47 0 0 0.46 <-0.10 -2 0.45 <-0.10 -4 0.47 0 016 3.68 3.68 0 0 3.64 <-0.10 -1 3.59 <-0.10 -2 3.65 <-0.10 -117 0.33 0.33 0 0 0.33 0 0 0.33 0 0 0.33 0 0

218 1.68 1.07 -0.61 -36 1.06 <-0.10 -1 1.03 <-0.10 -4 1.06 <-0.10 -1319 3.60 1.40 -2.2 -61 1.26 -0.14 -10 0.98 -0.42 -30 1.32 <-0.10 -6

20 2.16 0.90 -1.26 -58 0.86 <-0.10 -4 0.77 -0.13 -14 0.87 <-0.10 -3

421 <0.10 -0.82 -0.89 -1,271 -0.83 <0.10 1 -0.83 <0.10 1 -0.83 <0.10 1

22 0.67 0.66 <-0.10 -1 0.63 <-0.10 -5 0.55 -0.11 -17 0.64 <-0.10 -323 2.45 2.45 0 0 2.35 -0.10 -4 2.18 -0.27 -11 2.39 <-0.10 -224 0.36 0.36 0 0 0.35 <-0.10 -3 0.34 <-0.10 -6 0.36 0 025 0.21 0.21 0 0 0.21 0.00 0 0.20 <-0.10 -5 0.21 0 026 1.55 1.55 0 0 1.49 <-0.10 -4 1.37 -0.18 -12 1.51 <-0.10 -3

1Wells pumping at maximum permissible pumping rate registered with the Connecticut Department of Environmental Protection.2Well 3 located in subbasin 18.3Wells 7 and 8 located in subbasin 19.4Wells 1 and 2 located in subbasin 21..
Summary 79

Summary

The aquifer system that includes stratified glacial, 

valley-fill sediments and till and bedrock uplands in the 

Pootatuck River Basin is the sole source of water to Newtown, 

Connecticut. Continued land development and population 

growth have caused concerns regarding future water supplies 

and increasing competition between water for human use 

and protecting aquatic habitat. To address these concerns, 

the U.S. Geological Survey and Newtown did a cooperative 

hydrogeologic study of the Pootatuck River Basin. Three 

aquifer tests were conducted—two at the Fairfield Hills well 

field and one at the United Water well field—to determine 

aquifer properties and the effect of well pumping on the 

interaction between surface water and groundwater near the 

wells. Transmissivities calculated using data from the aquifer 

ranged from 3,200 to 20,000 ft2/d. These data were used in 

groundwater-flow models of the well fields and the basin.

Several methods were used to analyze the interaction 

between the stream and the aquifer. During the aquifer 

test at the Fairfield Hills well field, streamflow increased 

between the upstream and downstream measuring sites even 

when the wells were pumping, perhaps because of bank or 

wetland storage of surface water from recent rain events and 

groundwater inflow from the opposite side of the stream; 

however, there was a reduction in the streamflow gain. A 

similar result was observed in the water-level measurements 

and stream stage made at each of the streambed piezometers 

near Well 3; the head gradient indicated groundwater was 

discharging to the stream, but the gradient was smaller 

during the pumping phase of the test. The head gradients in 

the piezometers near Well 7 indicated that during pumping, 

water moves from the stream into the groundwater system. 

Groundwater temperatures at various depths in piezometers 

near Well 3 became more similar to the temperature of the 

surface water when the well was pumping. This also implied 

movement of surface water into the aquifer. A similar pattern 

was observed in a piezometer near Well 7.

Normalized stable-isotope ratios of hydrogen and oxygen, δ2

H and δ18O, were used to determine possible mixing of 

surface water and groundwater at the Fairfield Hills production 

wells. Samples collected near Well 7 near the end of pumping 

showed from 32 to 69 percent of the pumped water came from 

the river and a sample collected near Well 3 near the end of the 

pumping showed from 68 to 97 percent of the pumped water 

came from the river.

Lack of water storage precluded running a typical aquifer 

test on Well 2 at the United Water well field, so data from 

a previous test were used to calculate transmissivity, and a 

9.5-hour production-well recovery test was run to collect data 

to analyze the effect of the pumping on the surface watergroundwater connection. Stage measurements made at the 

piezometers indicated there was stream inflow even during the 

short period when the well was not pumping. When 

Well 2 was pumping, the temperatures of the groundwater 

below the streambed were relatively constant and similar to 

each other with some fluctuation of temperature at a depth of 

2 ft, implying that surface water flowed to the groundwater 

system during pumping. When Well 2 was shut down, the 

temperature at 2 ft below the streambed became less similar to 

the temperature at the other depths in the piezometers on the 

near bank of the stream. 

Two well-field-scale and one basin-scale threedimensional, numerical groundwater-flow models were 

developed for the Pootatuck River Basin to (1) synthesize the 

available hydrogeologic data including aquifer tests (wellfield scale); (2) determine areas contributing recharge to five 

simulated production wells for steady-state conditions (wellfield scale); and (3) assess the effect of three potential future 

recharge scenarios on base flow and water budgets in the 

Pootatuck River (basin scale). 

Groundwater-flow models were developed for each well 

field and calibrated to the data collected at each location. 

These models were used to determine parameters to be used in 

a basin-scale model as well as recharge areas at the well-field 

scale. Calibrated values of hydraulic conductivity for areas 

representing the stratified glacial deposits ranged from 16 to 

60 ft/d at the Fairfield Hills well field and from 60 to 220 ft/d 

at the United Water well field. The calibrated value for the 

bedrock and till underlying the stratified glacial deposits and 

in the uplands surrounding the primary aquifer was 3 ft/d. 

The simulated and observed water levels from the aquifer 

test at the Fairfield Hills well field matched reasonably well 

with mean absolute errors of 1.3 and 0.4 ft for Wells 3 and 7, 

respectively. Overall, Well 3 and associated observation wells 

had the best match with median errors that ranged from -1.2 

to -0.2 ft; median errors for Well 7 and associated observation wells ranged from -1.5 to +3.4 ft. Two observation wells 

associated with Well 7 had the worst median errors of -1.5 and 

+3.4 ft, respectively.

Comparison of simulated and observed water levels from 

the short-duration, reverse aquifer test at the United Water well 

field show that the initial water-level increase at the start of the 

aquifer test and during the transition period into aquifer-test 

nonpumping conditions match reasonably well, as do those in 

the transition period to pumping conditions at the end of the 

aquifer test. For the overall time period of the test, the mean 

absolute error ranged from +0.4 to +0.5 ft and the median 

error for the observation wells ranged from -0.2 to -0.1 ft. During the shutdown, the mean absolute error ranged from +0.9 to 

+2.2 ft and the median error ranged from -2.6 to -0.8 ft.

Steady-state models were developed for both well fields 

and for the basin. Because the amount of recharge to the 

groundwater system depends on the physical and land-use 

conditions, an areally-distributed map of recharge was 

developed for the basin. The estimated recharge averaged 

16 in/yr for the entire basin and ranged from a set value of 

1 to 28 in/yr. All five production wells in the Newtown area 

that are available for use were simulated simultaneously even 

though it is acknowledged that this is not a normal operating condition. Simulating all five wells provides the most 
80 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

conservative estimate of the effect of pumping on the hydrologic system. The total pumping simulated was about 3.41 

Mgal/d, which is the sum of the registered rates.

The calibrated well-field-scale groundwater-flow models 

were used to define the areas contributing recharge to five 

simulated production wells in Newtown under steady-state 

conditions. The results of the modeling showed that water 

pumped from each well is a combination of groundwater 

from the recharge area and stream inflow. The percent of the 

pumped water derived from the recharge area ranged from 20 

to 62 percent; therefore, 38 to 80 percent of the pumped water 

came from the stream. This result is similar to that determined 

from the stable isotopes described above and potentially has 

large implications for protection of the quality of the water 

pumped from the wells. 

The basin-scale model was run under steady-state conditions that represent long-term averages and was calibrated 

to average water levels and streamflow estimated from the 

nearby Pomperaug River. Because of the scale of the model 

and the lack of available groundwater measurement points, 

only seven groundwater calibration points were available. The 

mean of the residuals at these points was +1.1 ft and the mean 

absolute error was +2.0 ft, less than 2.5 percent of the total 

range (78 ft) in water levels. 

The calibrated, basin-scale groundwater-flow model was 

used to estimate the effect on base flows in the main stem of 

the Pootatuck River, and base flows and water budgets in each 

of 26 subbasins under three potential development scenarios. 

These scenarios assumed impervious area in all newly 

developed areas would increase to 35, 50, or 85 percent of the 

total area. The simulations showed that base flow in the main 

stem of the Pootatuck River is reduced in each future recharge 

scenario and base flow decreased in all subbasins, with some 

subbasins showing a decrease of more than 20 percent. For 

subbasins located in upland headwater areas, recharge from 

precipitation was the greatest source of water, whereas in 

subbasins located further downstream and near the Pootatuck 

River, stream inflow was the greatest source of water. When 

comparing pumping to nonpumping conditions, the steadystate simulations show that streamflows downstream from a 

production well are generally decreased by the amount 

of pumping. 

The areas contributing recharge delineated in this investigation and the potential effects of future development on base 

flow are valid only for the specific pumping, recharge, and 

aquifer property conditions detailed in this report. If pumping 

and recharge conditions are modified in the future, or if additional long-term water-level data are collected for different 

calibration locations in the modeled area, the areas contributing recharge to production wells in the Pootatuck River Basin 

could be evaluated on the basis of the new conditions and data. 

The models described in this report can serve as tools to evaluate the recharge areas to production wells in response to future 

changes in pumping and recharge conditions.

Acknowledgments

The authors wish to thank the town of Newtown, 

Newtown Health District, United Water Connecticut 

Company, Aquarion Operating Services, and the Potatuck 

Sportsman Club, especially the late John (Jack) Stokes, for 

the high level of cooperation received in preparation for and 

conducting the aquifer tests using the Fairfield Hills and 

United Water production wells. The authors also wish to 

thank Jeffrey Starn, Joseph DePalma, Karen Beaulieu, Laura 

Colangelo, and Daniel Wright of the U.S. Geological Survey 

for their assistance with field work prior to and during the 

aquifer tests and Virginia de Lima and John Mullaney for help 

in preparing the manuscript. 

References Cited

Ahearn, E.A., 2010, Regional regression equations to estimate 

flow-duration statistics at ungaged stream sites in 

Connecticut: U.S. Geological Survey Scientific Investigations Report 2010–5052, 45 p.

Anderson, M.P., and Woessner, W.W., 1992, Applied groundwater modeling: San Diego, Calif., Academic Press, Inc., 

381 p.

Barlow, P.M., and Dickerman, D.C., 2001, Numericalsimulation and conjunctive-management models of the 

Hunt-Annaquatucket-Pettaquamscutt stream-aquifer system, 

Rhode Island: U.S. Geological Survey Professional 

Paper 1636, 88 p.

Bjerklie, D.M., Starn, J.J., and Tamayo, Claudia, in press, 

Estimation of the effects of land use and groundwater 

withdrawals on streamflow for the Pomperaug River, 

Connecticut: U.S. Geological Survey Scientific Investigations Report 2010–5114.

Brown, C.J., Starn, J.J, Stollenwerk, K.G., Mondazzi, R.A., 

and Trombley, T.J., 2009, Aquifer chemistry and transport 

processes in the zone of contribution to a public-supply 

well in Woodbury, Connecticut, 2002–06: U.S. Geological 

Survey Scientific Investigations Report 2009–5051, 158 p. 

Clark, I.D., and Fritz, Peter, 1997, Environmental isotopes in 

hydrogeology: New York, CRC Press LLC, 328 p.

Cooper, H.H., and Jacob, C.E., 1946, A generalized graphical 

method for evaluating formation constants and summarizing 

well field history: American Geophysical Union Transactions, v. 27, p. 526–534.

Coplen, T.B., 1994, Reporting of stable hydrogen, carbon, and 

oxygen isotopic abundances: Pure and Applied Chemistry, 

v. 66, p. 273–276.
References Cited 81

Coplen, T.B., Wildman, J.D., and Chen, Julie, 1991, Improvements in the gaseous hydrogen-water equilibration technique for hydrogen isotope ratio analysis: Analytical 

Chemistry, v. 63, p. 910–912.

Craig, Harmon, 1957, Deuterium–oxygen 18–tritium relationships in natural waters, in New Research Methods in 

Hydrology, Committee on Research in Water Resources, 

University of California.

Craig, Harmon, 1961, Isotopic variations in meteoric waters: 

Science, v. 133, p. 1702–1703.

Darling, W.G., and Bath, A.H., 1988, A stable isotope study of 

recharge processes in the English chalk: Journal of Hydrology, v. 101, p. 31–46.

DeSimone, L.A., Walter, D.A., Eggleston, J.R., and Nimiroski, 

M.T., 2002, Simulation of ground-water flow and evaluation of water-management alternatives in the Upper Charles 

River Basin, eastern Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 02–4234, 94 p.

Dickerman, D.C., Kliever, J.D., and Stone, J.R., 1997, Hydrogeology, water quality, and simulation of ground-waterdevelopment alternatives in the Usequepaug-Queen groundwater reservoir, southern Rhode Island: U.S. Geological 

Survey Water-Resources Investigations Report, 97–4126, 

48 p.

Epstein, Samuel, and Mayeda, Toshiko, 1953, Variation of 18O content of waters from natural sources: Geochimica et 

Cosmochimica Acta, v. 4, no. 5, p. 213–224.

Fishman, M.J., and Friedman, L.C., 1989, Methods for 

determination of inorganic substances in water and fluvial 

sediments: U.S. Geological Survey Techniques of WaterResources Investigations, book 5, chap. A1, 545 p.

Förstel, H., 1982, 18O/16O ratio of water in plants and their 

environment, in Schmidt, H.L., Förstel, H., and Heinzinger, 

K., eds., Stable isotopes: Amsterdam, The Netherlands, 

Elsevier, p. 503–516.

Franke, O.L., Reilly, T.E., Pollock, D.W., and LaBaugh, J.W., 

1998, Estimating areas contributing recharge to wells, 

lessons from previous studies: U.S. Geological Survey 

Circular 1174, 14 p.

Friesz, P.J., 2004, Delineation of areas contributing recharge 

to selected public-supply wells in glacial valley-fill and 

wetland settings, Rhode Island: U.S. Geological Survey 

Scientific Investigations Report 2004–5070, 64 p.

Gat, J.R., 2008, The isotopic composition of evaporating 

waters—review of the historical evolution leading up to 

the Craig-Gordon model: Isotopes in Environmental and 

Health Studies, v. 44, p. 5–9.

Grady, S.J., and Weaver, M.F., 1988, Preliminary appraisal 

of the effects of land use on water quality in stratified-drift 

aquifers in Connecticut—U.S. Geological Survey Toxic 

Waste—Ground-Water Contamination Program: 

U.S. Geological Survey Water-Resources Investigations 

Report 87–4005, 41 p.

Granato, G.E., Barlow, P.M., and Dickerman, D.C., 2003, 

Hydrogeology and simulated effects of ground-water withdrawals in the Big River area, Rhode Island: U.S. Geological Survey Water-Resources Investigations Report 03–4222, 

76 p.

Haeni, F.P., 1978, Computer modeling of groundwater 

availability in the Pootatuck River Valley, Newtown, 

Connecticut: U.S. Geological Survey Water-Resources 

Investigations Report 78–77, 64 p.

H.C. Planning Consultants, Inc., 2008, Newtown, buildout 

analysis & population projections, 21 p.

Halford, K.J., and Kuniansky, E.L., 2002, Documentation of 

spreadsheets for the analysis of aquifer-test and slug-test 

data: U.S. Geological Survey Open-File Report 02–197, 

51 p.

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, 

M.G., 2000, MODFLOW-2000, the U.S. Geological Survey 

modular groundwater model—User guide to modularization 

concepts and the ground-water flow process: U.S. Geological Survey Open-File Report 00–92, 121 p.

Konikow, L.F., and Reilly, T.E., 1999, Groundwater modeling, 

in Delleur, J.W., ed., The handbook of groundwater engineering: Boca Raton, Fla., CRC Press, 40 p.

Kruseman, G.P., and DeRidder, N.A., 1990, Analysis and 

evaluation of pumping test data (2d ed.), Publication 47, 

International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands, 370 p.

Leavesley, G.H., Restrepo, P.J., Markstrom, S.L, Dixon, M., 

and Stannard, L.G., 1996, The modular modeling system 

(MMS)—Users manual: U.S. Geological Survey Open-File 

Report 96–151.

Mastin, M.C., and Vaccaro, J.J., 2002, Documentation of precipitation runoff modeling system modules for the modular 

modeling system modified for the watershed and river 

systems management program: U.S. Geological Survey 

Open-File Report 02–362.

Mazzaferro, D.L., 1986, Ground-water availability and water 

quality in Southbury and Woodbury, Connecticut: 

U.S. Geological Survey Water-Resources Investigations 

Report 84–4221, 105 p.
82 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

Mazzaferro, D.L., Handman, E.H., and Thomas, M.P., 1979, 

Water resources inventory of Connecticut, part 8, 

Quinnipiac River Basin: Connecticut Water Resources 

Bulletin 27, 88 p.

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

three-dimensional finite-difference ground-water flow 

model: U.S. Geological Survey Techniques of WaterResources Investigations, book 6, chap. A1, 586 p.

McKeegan, D.K., 1986, Depth to bedrock and classification of 

unconsolidated materials above the water table in stratifieddrift areas of part of the Pootatuck River Valley, Newtown, 

Connecticut: U.S. Geological Survey Miscellaneous Investigations Series, Map I–1702.

Miller, D.R., Warner, G.S., Ogden, F.L., and DeGaetano, A.T., 

2002, Precipitation in Connecticut: Storrs, Conn., Connecticut Institute of Water Resources, University of Connecticut, 

66 p. 

Pollock, D.W., 1994, User’s guide to MODPATH/

MODPATH_PLOT, version 3—A particle tracking postprocessing package for MODFLOW, the U.S. Geological 

Survey modular three-dimensional finite-difference groundwater-flow model: U.S. Geological Survey Open-File 

Report 94–464, 234 p.

Pool, D.R., and Dickinson, J.E., 2007, Ground-water flow 

model of the Sierra Vista subwatershed and Sonoran 

portions of the Upper San Pedro Basin, Southeastern 

Arizona, United States, and Northern Sonora, Mexico: 

U.S. Geological Survey Scientific Investigations Report 

2006–5228, 60 p.

Prudic, D.E., 1989, Documentation of a computer program to 

simulate stream-aquifer relations using a modular, finitedifference, ground-water flow model: U.S. Geological 

Survey Open-File Report 88–729, 113 p.

Reilly, T.E., 2001, System and boundary conceptualization 

in ground-water flow simulation: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. B8, 38 p.

Reilly, T.E., and Harbaugh, A.W., 2004, Guidelines for evaluating ground-water flow models: U.S. Geological Survey 

Scientific Investigations Report 2004–5038, 30 p.

Reilly, T.E., and Pollock, D.W., 1993, Factors affecting areas 

contributing recharge to wells in shallow aquifers: 

U.S. Geological Survey Water-Supply Paper 2412, 21 p.

Rodgers, John (compiler), 1985, Bedrock geological map of 

Connecticut: Connecticut Geological and Natural History 

Survey, Natural Resources Atlas Series Map, 2 sheets, 

scale 1:125,000.

Starn, J.J., Stone, J.R., and Mullaney, J.R., 2000, Delineation 

and analysis of uncertainty of contributing areas to wells 

at the Southbury Training School, Southbury, Connecticut: 

U.S. Geological Survey Water-Resources Investigations 

Report 00–4158, 52 p.

State of Connecticut Department of Environmental Protection, 1991, Regulations for mapping wells in stratified drift 

to level A mapping standards, General Statutes Section 

22a–354b–1, 27 p.

Stone, J.R., Schafer, J.P., London, E.H., DiGiacomo-Cohen, 

M., Lewis, R.S., and Thompson, W.B., 1998, The Quaternary geologic map of Connecticut and Long Island Sound 

Basin: U.S. Geological Survey Open-File Report 98–371, 

72 p.

Stone, J.R., Schafer, J.P., London, E.H., and Thompson, W.B., 

1992, Surficial materials map of Connecticut: U.S. Geological Special Map, 2 sheets, scale 1:125,000. 

Stonestrom, D.A., and Constantz, Jim, eds., 2003, Heat as 

a tool for studying the movement of ground water near 

streams: U.S. Geological Survey Circular 1260, 96 p. 

Wilson, W.E., Burke, E.L., and Thomas, C.E., Jr., 1974, 

Water resources inventory of Connecticut, part 5, lower 

Housatonic River Basin: Connecticut Water Resources 

Bulletin 19, 79 p. 

Zimmermann, U., Ehhalt, D., and Münnich, K.O., 1967, Soil 

water movement and evapotranspiration—Changes in the 

isotope composition of water, in Isotopes in hydrology: 

Vienna, International Atomic Energy Agency, p. 567–84.4.
Appendix. Spatially-Distributed Recharge 83

The amount of recharge at a given location is a function 

of the physical and land-use conditions in the basin. A 

distributed recharge map was created for the Pootatuck 

River Basin based on a statistical relation between various 

basin characteristics and recharge that was developed for 

the adjacent Pomperaug River Basin (Bjerklie and others, in 

press). The relation was developed using a precipitation runoff 

modeling system basin model (PRMS), developed by the U.S. 

Geological Survey (Leavesley and others, 1996; Mastin and 

Vaccaro, 2002).

In the PRMS model, the key physical attributes of the 

land surface that control runoff quantity and source are the 

(1) surficial geology; (2) soil type; (3) land cover—in particular the amount of effective impervious surface; and (4) drainage density, which is a measure of the length of streams within 

a specified subbasin. These attributes are described below.

• Surficial Geology—the presence of coarse stratified 

glacial deposits (sands and gravels) as a percent of the 

total area of each subbasin. These deposits form the 

principal water-bearing units in the basin and transmit 

the greatest amount of water to production wells. 

• Soil Type—The important soil type is the class D soils 

(also known as Hydrologic Soil Group D), which are 

clayey soils with low permeability. These soils hold 

water but do not readily transmit water vertically (as 

recharge to groundwater) and thus tend to cause higher 

surface runoff and a higher permanent water table.

• Percent Effective Impervious Surface—Impervious 

surfaces prevent vertical recharge of precipitation 

and result in high surface runoff, especially where the 

runoff from these surfaces is collected in storm drains 

and routed directly into streams. Effective impervious surface is a part of the total impervious surface, 

and acknowledges that some runoff from impervious 

areas does infiltrate the ground. Effective impervious 

cover was estimated using a method developed for 

Connecticut (Bjerklie and others, in press). This was 

based on estimation of total impervious area using the 

Impervious Surface Analysis Tool (ISAT) described by 

Chabaeva and others (2004) and coefficients developed for Connecticut land-use and population-density 

classes (Prisloe and others, 2003). 

• Drainage Density—The drainage density (length of 

stream per unit area) is an indicator of the perennial 

drainage characteristics of the subbasin. Where the 

density is higher and the drainage network is well 

established, a more stable discharge regime is indicated, which also indicates groundwater dominance.

The effect of the key physical factors on groundwater 

recharge was assessed in the model by multiple linear regression analysis. The simulated recharge was regressed against 

the physical attributes listed above for each subarea in the 

PRMS, and a simplified predictive equation derived. The 

drainage density provided only marginal improvement in the 

regression and was not included as a predictor variable. The 

resulting equation was,

R = c E1 2 lA+ c CSD+c C3 4 lassD+ c (1)

where

R is the recharge, in inches per year,

c1–c4 are regression coefficients 

EIA is the effective impervious cover, in fractional 

percent,

CSD is the fraction of coarse stratified glacial 

deposits, in fractional percent, and

ClassD is the Class D soils, in fractional percent.

The coefficients of the derived regression equation and 

their statistical significance are shown in table 21. The negative values for the coefficient of effective impervious cover 

and Class D soils indicate that these variables reduce recharge; 

the positive value for the coefficient of coarse stratified glacial 

deposits indicates this variable increases recharge.

Appendix. Spatially-Distributed Recharge

Table 21. Coefficients and statistics for regression equation 

to determine groundwater recharge, Pootatuck River Basin, 

Newtown, Connecticut.

[EIA, effective impervious area; CSD, coarse stratified glacial deposits; 

ClassD, Class D soils; Pr (>|t|), probability value is not significant at 95-percent probability level]

Variable Value Standard error Pr(>|t|)

EIA -0.1049 0.0280 0.0004

CSD 0.0386 0.0043 0.0000

ClassD -0.0419 0.0108 0.0003

Intercept 0.0392 0.0019 0.0000

The regression statistics for the equation are:

Residual standard error: 0.009473 on 60 degrees of freedom,

Multiple R-Squared: 0.6395, 

Adjusted R-squared: 0.6215, 

F-statistic: 35.48 on 3 and 60 degrees of freedom, and 

p-value: 2.57e-013. 
84 Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut

References

Bjerklie, D.M., Starn, J.J., and Tamayo, Claudia, in press, 

Estimation of the effects of land use and groundwater 

withdrawals on streamflow for the Pomperaug River, 

Connecticut: U.S. Geological Survey Scientific Investigations Report 2010–5114.

Chabaeva, A.A., Civco, D.L., and Prisloe, S., 2004, Development of a population and land use based regression model 

to calculate the amount of imperviousness, in Proceedings 

of the annual ASPRS convention, Denver, Colorado, 

2004, 9 p., accessed on September 10, 2009, at 

http://nemo.uconn.edu/tools/impervious_surfaces/pdfs/

Chabaeva_etal_2004.pdf.

Leavesley, G.H., Restrepo, P.J., Markstrom, S.L., Dixon, M., 

and Stannard, L.G., 1996. The modular modeling system 

(MMS)—Users manual: U.S. Geological Survey Open-File 

Report 96–151.

Mastin, M.C., and Vaccaro, J.J., 2002, Documentation of precipitation runoff modeling system modules for the modular 

modeling system modified for the watershed and river 

systems management program: U.S. Geological Survey 

Open-File Report 02–362.

Prisloe, M., Wilson, E.H., and Arnold, C., 2003, final report—

Refinement of population-calibrated land-cover-specific 

impervious surface coefficients for Connecticut: Nemo 

FY02 Workplan, DEP project 01-08, University of 

Connecticut, Middlesex County Extension Center, 20 p.
Prepared by the Pembroke and Lafayette Publishing 

Service Centers.

For more information concerning this report, contact:

Director

U.S. Geological Survey

Connecticut Water Science Center

101 Pitkin Street

East Hartford, CT 06108

dc_ct@usgs.gov

or visit our Web site at:

http://ct.water.usgs.gov
Carlson and others—Hydrogeology and Simulation of the Unconsolidated Glacial Aquifer in the Pootatuck River Basin, Newtown, Connecticut—SIR 2010–5142