Water Use, Ground-Water Recharge and 

Availability, and Quality of Water in the 

Greenwich Area, Fairfield County, 

Connecticut and Westchester County, 

New York, 2000–2002

In cooperation with the town of Greenwich, Connecticut

Water-Resources Investigations Report 03-4300

U.S. Department of the Interior

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

U.S. Geological Survey

Water Use, Ground-Water Recharge and 

Availability, and Quality of Water in the 

Greenwich Area, Fairfield County, 

Connecticut and Westchester County, 

New York, 2000–2002

In cooperation with the town of Greenwich, Connecticut

By John R. Mullaney

East Hartford, Connecticut

2004

Water-Resources Investigations Report 03-4300
District Chief

U.S. Geological Survey

101 Pitkin Street

East Hartford, CT 06108

http://ct.water.usgs.gov

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

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

only and does not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased 

from:

U.S. Geological Survey

Branch of Information Services

Box 25286, Federal Center

Denver, CO 80225
Contents iii

CONTENTS

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

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

Purpose and Scope ............................................................................................................................................ 3

Previous Investigations ..................................................................................................................................... 3

Acknowledgments ............................................................................................................................................. 3

Description of the Study Area ................................................................................................................................... 3

Geohydrology .................................................................................................................................................... 5

Surficial Deposits ..................................................................................................................................... 5

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

Precipitation and Runoff ................................................................................................................................... 9

Methods of Data Collection and Analysis ................................................................................................................. 13

Streamflow Measurements ................................................................................................................................ 13

Water-Level Measurements .............................................................................................................................. 15

Water-Quality Samples ..................................................................................................................................... 15

Water-Use Data ................................................................................................................................................. 15

Ground-Water Recharge in the Greenwich Area ....................................................................................................... 17

Factors Affecting Ground-Water Recharge ...................................................................................................... 17

Ground-Water Recharge and Discharge, 2001–02 ........................................................................................... 18

Water Use in the Greenwich Area ............................................................................................................................. 20

Water Use at Residences with Public Water Supply ......................................................................................... 20

Log-Linear Regression Models of Residential Water Use ................................................................................ 21

Variable Selection ..................................................................................................................................... 21

Prediction of Residential Water Use in Areas with Domestic Wells ....................................................... 22

Estimation of Consumptive Water Use .................................................................................................... 27

Water Use at Nonresidential Properties .................................................................................................... 27

Simulation of Ground-Water Flow in the Greenwich Area ....................................................................................... 28

Description of Flow Model and Model Assumptions ....................................................................................... 28

Boundary Conditions ......................................................................................................................................... 33

Ground-Water Recharge ................................................................................................................................... 33

Aquifer Properties ............................................................................................................................................. 37

Internal Sources and Sinks of Water ................................................................................................................. 38

Streamflow ................................................................................................................................................ 38

Reservoirs ................................................................................................................................................. 38

Ground-Water Withdrawals and Return Flow from Septic Systems ....................................................... 38

Model Calibration ............................................................................................................................................. 39

Hydrologic Budget ............................................................................................................................................ 41

Ground-Water Availability in the Greenwich Area ................................................................................................... 47

Water Quality in the Greenwich Area ....................................................................................................................... 50

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

References Cited ........................................................................................................................................................ 54

Appendixes 1-4 ........................................................................................................................................................... 57
iv Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

FIGURES

1. Map showing location of public water and sewer service areas, Greenwich area, Connecticut................. 2

2. Map showing location of subregional basins and data-collection sites for streamflow and 

ground-water-level measurements, Greenwich area, Connecticut .............................................................. 4

3. Map showing generalized surficial geology of the Greenwich area, Connecticut ...................................... 6

4. Map showing generalized bedrock geology of the Greenwich area, Connecticut ...................................... 8

5. Graph showing precipitation at Putnam Lake, Greenwich, Connecticut, 1967–2001 ................................ 10

6. Map showing additional data-collection sites in Fairfield County, Connecticut ........................................ 11

7. Graph showing annual mean runoff at two long-term streamflow-gaging stations near Greenwich, 

Connecticut, 1967–01.................................................................................................................................. 12

8. Graph showing precipitation at Bridgeport, Connecticut, and runoff at Sasco Brook near Southport, 

Connecticut (USGS station number 01208950), calendar years 1966–96 .................................................. 12

9. Cross section through wells installed for water-level measurements.......................................................... 16

10. Graph showing water levels in bedrock wells in the East Branch of the Byram River Basin and 

precipitation at Putnam Lake, Greenwich area, Connecticut, September 2001-October 2002................... 19

11. Graph showing quarterly base flow at U.S. Geological Survey streamflow-gaging station 

number 01211699, East Branch Byram River below Lake Mead at Round Hill, Connecticut................... 20

12. Graph showing frequency distribution for residuals and the “best-fit” normal distribution 

approximation for (A) average daily water use, (B) average daily summer water use, and 

(C) average daily winter water use, Greenwich area, Connecticut ............................................................. 23

13. Graph showing estimates of average daily residential water use per square mile for small basins, 

Greenwich area, Connecticut....................................................................................................................... 25

14. Graph showing estimates of average daily return flow per square mile from residential septic systems 

supplied by public water supply for small basins, Greenwich area, Connecticut ....................................... 26

15. Map showing extent of finite-difference ground-water-flow model grid, Greenwich area, Connecticut... 29

16. Cross section through row 14 finite-difference ground-water-flow model, Greenwich area, 

Connecticut.................................................................................................................................................. 30

17. Map showing location of observations for ground-water levels and streamflow, Greenwich area, 

Connecticut.................................................................................................................................................. 31

18. Graph showing ground-water levels and streamflow, March to May 2002, Greenwich area, 

Connecticut.................................................................................................................................................. 32

19. Map showing zone array based on geology used for (A) application of recharge to the model and 

(B) application of hydraulic properties to layers 1and 2, steady-state ground-water simulation, 

Greenwich area, Connecticut....................................................................................................................... 34

20. Graph showing instantaneous streamflows per square mile at two continuous streamflow-gaging stations, 

Greenwich area, Connecticut, March to April 2001.................................................................................... 36

21. Graph showing measured and predicted impervious areas for a random sample of model cells, 

Greenwich area, Connecticut....................................................................................................................... 37

22. Graph showing weighted simulated equivalent plotted against weighted observations of hydraulic head 

and streamflow for model simulation of the Greenwich area, Connecticut, April 16-24, 2002 ................. 41

23. Map showing simulated long-term, average annual ground-water recharge from precipitation 

aggregated by zones, Greenwich area, Connecticut.................................................................................... 44

24. Map showing simulated long-term average net residential consumptive water use for zones, 

Greenwich area, Connecticut....................................................................................................................... 46

25. Graph showing simulated ground-water outflow to streams, calculated base flow, and low flow for 

basins in the Greenwich area, Connecticut.................................................................................................. 48

26. Graph showing percentage of urban land use and concentrations of selected water-quality constituents, 

Greenwich area, Connecticut....................................................................................................................... 51
Contents v

TABLES

1. Streamflow measurements, Greenwich area, Connecticut, 2000–02 ........................................................... 13

2. Wells in the East Branch Byram River Basin, Greenwich, Connecticut ...................................................... 15

3. Median daily water use, and median daily seasonal water use by property size, in calendar year 2000 

at selected residences with public water supply.......................................................................................... 21

4. Parameter estimates, standard errors, t-statistics, and p-values for multiple linear regression model of 

average daily water use at residences with public water supply, calendar year 2000, Greenwich, area, 

Connecticut ................................................................................................................................................. 24

5. Parameter estimates, standard errors, t-statistics and p-values for multiple linear regression model of 

average daily summer water use at residences with public water supply, calendar year 2000, 

Greenwich area, Connecticut ...................................................................................................................... 24

6. Parameter estimates, standard errors, t-statistics and p-values for multiple linear regression model of 

average daily winter water use at residences with public water supply, calendar year 2000, 

Greenwich area, Connecticut ...................................................................................................................... 24

7. Water levels in April 2002 and average water levels at selected U.S. Geological Survey 

network wells in Fairfield County, Connecticut.......................................................................................... 33

8. Recharge estimates from historical values and nonlinear regression for April 2002 calibration................. 35

9. Parameter estimates, standard errors, t-statistics and p-values for multiple linear regression model of 

percentage of impervious area for randomly selected cells of the finite-difference ground-water-flow 

model, Greenwich area, Connecticut .......................................................................................................... 35

10. Simulated values of hydraulic conductivity in finite-difference ground-water-flow model, 

Greenwich area, Connecticut ...................................................................................................................... 37

11. Parameter estimates of hydraulic conductivity and recharge from nonlinear regression, finitedifference ground-water flow model, Greenwich area Connecticut ........................................................... 39

12. Summary of error statistics and comparison of observed and simulated ground-water levels, 

Greenwich area, Connecticut, April 16-20, 2002 ....................................................................................... 40

13. Observed and simulated streamflows, Greenwich area, Connecticut, April 24-25, 2002............................ 40

14. Annual ground-water budget for modeled zones, based on calibration data from April 18-25, 2002, 

Greenwich area, Connecticut ...................................................................................................................... 42

15. Annual ground-water budget for modeled zones, based on calibration data from April 18-25, 2002 and 

adjusted to average residential water withdrawals, Greenwich area, Connecticut ..................................... 43

16. Estimated net annual residential consumptive water use and the difference between estimated long-term 

average recharge and the 30-day 2-year low flow....................................................................................... 49

17. Percentage of urban land use and concentration of selected water-quality constituents in surface-water 

base-flow samples collected October 2000, Greenwich area, Connecticut ................................................ 51

18. Wastewater compounds detected on October 25, 2000 at station number 01211110, Greenwich area, 

Connecticut ................................................................................................................................................. 52

APPENDIXES

1. Records from two streamflow-gaging stations, March 2001-September 2002 ............................................ 58

2. Estimated average daily ground-water use, Greenwich area, Connecticut .................................................. 62

3. Estimated average daily summer ground-water use, Greenwich area, Connecticut..................................... 63

4. Estimated average daily winter ground-water use, Greenwich area, Connecticut ....................................... 64
vi Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

CONVERSION FACTORS AND VERTICAL DATUM 

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

as follows:

°C = (°F - 32) / 1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above or below sea level.

Concentrations of chemical constituents in water are given in milligrams per liter 

(mg/L) and micrograms per liter (μg/L). 

Concentrations of indicator bacteria are given in colonies per 100 milliliters (mL) 

of sample.

Multiply By To obtain

inch (in.) 25.4 millimeter

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

foot (ft) 0.3048 meter

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

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer 

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

gallon (gal) 3.785 liter

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

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

million gallons per day per square mile

 [(Mgal/d)/mi2] 1,461 cubic meter per day per square kilometer 
Abstract 1

Water Use, Ground-Water Recharge and Availability, 

and Quality of Water in the Greenwich Area, 

Fairfield County, Connecticut and Westchester County, 

New York, 2000–2002

by John R. Mullaney

ABSTRACT

Ground-water budgets were developed for 32 

small basin-based zones in the Greenwich area of 

southwestern Connecticut, where crystalline-bedrock 

aquifers supply private wells, to determine the status of 

residential ground-water consumption relative to rates 

of ground-water recharge and discharge. Estimated 

residential ground-water withdrawals for small basins 

(averaging 1.7 square miles (mi2)) ranged from 0 to 

0.16 million gallons per day per square mile 

(Mgal/d/mi2

). To develop these budgets, residential 

ground-water withdrawals were estimated using 

multiple-linear regression models that relate water use 

from public water supply to data on residential property 

characteristics. Average daily water use of households 

with public water supply ranged from 219 to 1,082 

gallons per day (gal/d). 

A steady-state finite-difference ground-waterflow model was developed to track water budgets, and 

to estimate optimal values for hydraulic conductivity of 

the bedrock (0.05 feet per day) and recharge to the 

overlying till deposits (6.9 inches) using nonlinear 

regression. Estimated recharge rates to the small basins 

ranged from 3.6 to 7.5 inches per year (in/yr) and relate 

to the percentage of the basin underlain by coarsegrained glacial stratified deposits. Recharge was not 

applied to impervious areas to account for the effects of 

urbanization. Net residential ground-water consumption was estimated as ground-water withdrawals 

increased during the growing season, and ranged from 

0 to 0.9 in/yr. 

Long-term average stream base flows simulated 

by the ground-water-flow model were compared to 

calculated values of average base flow and low flow to 

determine if base flow was substantially reduced in any 

of the basins studied. Three of the 32 basins studied had 

simulated base flows less than 3 in/yr, as a result of 

either ground-water withdrawals or reduced recharge 

due to urbanization. A water-availability criteria of the 

difference between the 30-day 2-year low flow and the 

recharge rate for each basin was explored as a method 

to rate the status of water consumption in each basin. 

Water consumption ranged from 0 to 14.3 percent of 

available water based on this criteria for the 32 basins 

studied.

Base-flow water quality was related to the 

amount of urbanized area in each basin sampled. 

Concentrations of total nitrogen and phosphorus, chloride, indicator bacteria, and the number of pesticide 

detections increased with basin urbanization, which 

ranged from 18 to 63 percent of basin area.

INTRODUCTION

New residential development in Connecticut is 

taking place in rural upland areas that are outside 

public water-supply areas. Residents in these areas rely 

on individual private wells drilled in the fractured crystalline-bedrock aquifer for their water supply. Very 

little information is available about recharge rates to 

this aquifer or about the water withdrawals and 

consumptive use from private domestic wells.

The majority of residents in Greenwich, a 

community in southwestern Connecticut (fig. 1), rely 

on public water supply from local and regional surfacewater reservoirs. Aquifers in the glacial stratified 

deposits in this part of Connecticut are limited in 

extent; therefore, any future development must rely on 

ground water from the crystalline-bedrock aquifer for 

water supply. Ground water is the primary source of 

water supply for approximately 12 percent of the 

town’s households that are outside the public watersupply area (fig. 1) (U.S. Department of Commerce, 

1991). Ground-water recharge is the ultimate source of
2 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

STAMFORD

GREENWICH

Westchester

County, New York

Westchester

County, New York

EXPLANATION

Area with private wells and septic 

systems

Greenwich

Connecticut

Massachusetts

New

York

Rhode

Island

Area with public water supply

and septic systems

Area with public water supply 

and sewers

Long Island Sound

Connecticut

New York

Putnam

Lake

Rockwood 

Lake

Converse

Lake

Bargh

Reservoir

Figure 1. Location of public water and sewer service areas, Greenwich area, Connecticut.
Description of the Study Area 3

water to the crystalline-bedrock aquifers, but information is sparse on recharge rates to these bedrock aquifers in New England. 

Increasing development and the lack of large 

glacial stratified aquifers for public water supply have 

led to a need to study ground-water availability and 

water use in basins in the Greenwich area. The U.S. 

Geological Survey (USGS) and the town of Greenwich 

began a cooperative study in 2000 on water use, 

ground-water recharge and availability, and quality of 

water in several coastal basins in southwestern 

Connecticut. The information presented in this report 

may be applicable to other parts of New England and 

New York where the issue of sustainable water use 

from bedrock aquifers is of increasing concern. Wateruse estimates are relevant especially to other parts of 

Fairfield County, Connecticut, and Westchester 

County, New York.

Purpose and Scope

The purpose of this report is to characterize residential ground-water use, ground-water recharge and 

availability, and quality of water in selected coastal 

basins in Fairfield County, Connecticut and 

Westchester County, New York. The report focuses 

primarily on the Greenwich, Connecticut area. It 

presents data collected during 2000 through 2002 on 

streamflow, ground-water levels, and water quality of 

base flow. Three multiple-linear regression models 

were used to estimate self-supplied water use from 

ground-water sources for average daily, winter, and 

summer conditions in each of 32 small basin-based 

zones. A steady-state ground-water-flow model was 

used as a tool to assist in estimating recharge rates, 

hydraulic conductivity of crystalline bedrock, and in 

the analysis of ground-water budgets and ground-water 

availability for each of the 32 zones. 

Previous Investigations

The ground-water resources of the study area 

were described previously in several reports. Gregory 

and Ellis (1916) described the hydrogeology, water 

supply, and water use in the Greenwich-Stamford area. 

Ryder and others (1970) conducted a water-resources 

inventory of the southwestern Connecticut coastal 

basins, including information on the hydrogeology, 

surface water, water quality, and water use of the 

region. Wolcott and Snow (1995) estimated groundwater recharge for northern Westchester County, 

including some areas in the Byram and Mianus River 

Basins that drain through Greenwich.

Acknowledgments

The following town of Greenwich officials and 

departments are thanked for their assistance with this 

investigation: Aleksandra Moch, Denise Savageau, and 

Steven Danzer of the Greenwich Conservation Department; the Greenwich Health Department; and the 

Greenwich Department of Public Works. The National 

Audubon Society and the Boy Scouts of America are 

thanked for allowing ground-water level and streamflow monitoring on their properties; the numerous residents of Greenwich are thanked who allowed access to 

their wells for water-level monitoring; and David Medd 

of the Aquarion Water Company of Connecticut is 

thanked for providing water-use and precipitation 

information. Thanks also are due to Anna Maria 

Marrone, assessor of the town of North Castle, New 

York, and Cindy Barber, GIS analyst for the city of 

Stamford, Connecticut. The following USGS 

employees are thanked for their contributions to this 

study: Gregory Schwarz, Bruce Davies 3rd, J. Jeffrey 

Starn, Remo Mondazzi, Staunton Williams Jr., 

Timothy Frick, Joseph Martin, and Barbara Korzendorfer. 

DESCRIPTION OF THE STUDY AREA

The study area (fig. 2), referred to in this report 

as the Greenwich area, contains 52.8 mi2 in Fairfield 

County in the southwestern coastal part of Connecticut 

and Westchester County, New York. The town of 

Greenwich, which makes up most of the study area, had 

a population of 61,101 in 2000 (data accessed on 

August 26, 2003, on the World Wide Web at URL 

http://www.census.gov). Development in the Greenwich area ranges from homes on small lots (less than 

0.5 acres) and commercial and business districts with 

public water supply and sewer in the southern part of 

the town, to large lots (more than 4 acres) and no 

commercial activity in the more rural northern areas. 

Residential water use in some parts of the town likely 

is higher than in other parts of Connecticut due to the 

large lot size and lawn area and the large number of 

swimming pools. In 1999, median annual household
4 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

01211699

01211110

01211010

01211040

CT-GW-21

CT-GW-23

CT-GW-22

01211450

01211600 01212100

01211210

01212550

01212600 Greenwich Creek

Byram River

Horseneck Brook

Putnam

Lake

Blind Brook

Mianus River 

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

STAMFORD

GREENWICH

Westchester

County, New York

Westchester

County, New York

EXPLANATION

A

A'

Long Island Sound

Rockwood 

Lake

Converse

Lake

Bargh

Reservoir

Kensico

Reservoir

A A' Line of section (fig. 8)

Subregional basin boundary

Study area boundary

01211600

Water-level observation well, 

with identifier

Continuous streamflow-gaging

station and water-quality site,

with identifier

Water-quality site with streamflow 

measurement(s) and identifier

Streamflow measurement site, 

with identifier

CT-GW-21

01211699

01211450

01211140

MILES

KILOMETERS

Figure 2, Location of subregional basins and data-collection sites for streamflow and ground-water-level measurements, 

Greenwich area, Connecticut.
Description of the Study Area 5

income in Greenwich was $99,086 as compared to the 

median annual household income of $53,935 in the 

State of Connecticut (data accessed on August 26, 

2003, on the World Wide Web at URL http://www.census.gov).

The town of Greenwich is in the Byram River 

and Mianus River drainage basins. Several smaller 

subregional basins that also drain to Long Island Sound 

are within the town (fig. 2). Altitude of the Greenwich 

area ranges from sea level on Long Island Sound to 

almost 600 ft near the northern border with Westchester 

County, New York. The water table is generally 

shallow and at a consistent depth below land surface at 

most places in the study area. Water levels in upland 

areas, such as Greenwich, typically are a subdued 

reflection of the topography.

Geohydrology

Two types of aquifers are present in the Greenwich area: (1) aquifers in surficial deposits including 

till, glacial stratified deposits, post-glacial alluvium, 

and swamp deposits; and (2) aquifers in the fractured 

crystalline bedrock. 

Surficial Deposits

The surficial geology of Connecticut has been 

most recently described by Stone and others (1992). 

The major surficial deposits in the Greenwich area are 

till and glacial stratified deposits. Till is an ice-laid 

deposit containing a nonsorted mixture of gravel, sand, 

silt, and clay. Till overlies the bedrock in most places in 

the Greenwich area and is the primary unconsolidated 

material. 

Tills of two separate glaciations are present in 

Connecticut. The characteristics of these two tills are 

summarized in Melvin and others (1992). The upper till 

(surface till) was deposited during the last glaciation 

from about 23,000 to 16,000 years before present. 

Surface till is generally sandy with many boulders and 

is thin in areas with numerous bedrock outcrops. 

Surface till has been noted up to 33 ft thick in Connecticut (Melvin and others, 1992). The average thickness 

of the upper till is 12 ft based on well-completion 

reports for 462 selected wells in Greenwich. The 

composition of the till is related to nearby bedrock 

types and to other surficial deposits that were present 

before the last glaciation. Drumlin till (lower till) is 

interpreted to have been deposited during an earlier 

glaciation (Melvin and others, 1992). Drumlin till is 

inferred to be present in the cores of drumlins and in 

thick till deposits (drumlin till possibly covered by 

surface till) in the Greenwich area (fig. 3), and may 

range in thickness from 30 to 100 ft. Based on the 

selected records of 121 wells in Greenwich, the average 

thickness of till deposits in areas with thick till is 35 ft. 

Thick till deposits containing drumlin till are present 

primarily in the northern half of Greenwich (fig. 3).

Glacial stratified deposits include sand and 

gravel, silt, and clay deposited in glacial meltwater 

streams or lakes by the retreating glacier (fig. 3). In 

other parts of Connecticut, these deposits are the most 

productive aquifers for public ground-water supplies. 

They are important areas for storing ground water and 

providing base flow to streams because ground-water 

recharge rates to these deposits are greater than 

recharge rates to deposits of till. Glacial stratified 

deposits are limited in the Greenwich area, as 

compared to other parts of Connecticut, and consist 

mostly of small and thinly saturated areas of sand and 

gravel. The largest area of glacial stratified deposits 

underlies the Tamarack swamp (fig. 3), in the northwestern part of Greenwich, where swamp deposits 

overlie fine-grained material and sand. According to 

Ryder and others (1970), this deposit may contain as 

much as 120 ft of saturated thickness with a transmissivity of up to 6,700 ft2/d. Several wells completed in 

another stratified deposit in the Banksville section (fig. 

3) of Greenwich (northeastern part of town) were 

reported to penetrate up to 53 ft of fine-grained sand. 

Some residents in Banksville obtain their water from 

shallow dug wells completed in this material 

(Aleksandra Moch, Greenwich Conservation Department, oral commun., 2001). 

Bedrock

The Greenwich area is underlain by metamorphic bedrock of two different geologic terranes 

(Rodgers, 1985). The northwestern part of Greenwich 

is underlain by Proto-North American terrane, which is 

thought to be the edge of the Proto-North American 

continent. The remainder of Greenwich is in the 

Iapetos terrane of the western uplands of Connecticut, 

which is thought to be oceanic sediments that were 

offshore from Proto-North America. Bedrock to the 

northwest of Cameron’s line (a fault, see fig. 4) 

includes granitic gneiss, gneiss, schistose marble, 

schist, and amphibolite (fig. 4). The oldest bedrock
6 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

STAMFORD

GREENWICH

Geology modified from Stone and others, 1992

Tamarack

Swamp

Banksville

Figure 3, Generalized surficial geology of the Greenwich area, Connecticut.
Description of the Study Area 7

FINE DEPOSITS--Composed of fine sand, silt, and clay particles generally

in well sorted, thin layers of alternating silt and clay and (or) very fine sand;

locally may contain lenses of coarser material. Fines, if present, overlie sand

and gravel described below

All sorted and stratified sediments composed of gravel, sand, silt and clay 

laid down by flowing meltwater during retreat of the last ice sheet; includes

minor lenses of flowtill and other diamict sediments

EXPLANATION

POSTGLACIAL DEPOSITS

WATER BODIES

SAND AND GRAVEL--Composed of mixtures of sand and gravel within individual 

layers and as alternating layers. Sand and gravel layers generally range from 

50-75 percent sand particles and from 25-50 percent gravel particles. Unit locally 

contains zones that are entirely sand

GLACIAL ICE-LAID DEPOSITS

ALLUVIUM OVERLYING SAND AND GRAVEL--Sand, gravel, silt, and

some organic material on the flood plains of modern streams; overlie

"sand and gravel" described below

SWAMP--Muck and peat that contain minor amounts of sand and silt

accumulated in poorly drained areas. Generally less than 10 feet thick

GLACIAL MELTWATER DEPOSITS

TILL--Poorly sorted, generally nonstratified mixture of grain sizes ranging

from clay to large boulders; the matrix of most tills is composed dominantly of

sand and silt. Darker green areas indicate till 15 feet or greater in thickness
8 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73o

40' 73o35'

Geology from Rodgers, 1985

Otf+Og

Otfc

Otfc

Otfs

Otfs Otfc Otfg

l

Otfg

Otfg

Cm

Oh

Cma

Ow

Owm

Yg

Oh

Yg

Cma

OCs

Cd

Yg

Oh

Ohn

Ogh?

Otfg

Ohn Otfg

Ohn

Ogh

Oh

Cd Cm

Cameron's Line

Figure 4. Generalized bedrock geology of the Greenwich area, Connecticut.
Description of the Study Area 9

Yg Gneiss of Highlands masifs

Cd Dalton Formation

OCs Stockbridge Marble

Ow Walloomsac Schist

Owm Basal marble member of Walloomsac Schist

Cm Manhattan Schist

Cma Amphibolite-bearing unit of Manhattan Schist

Ogh Golden Hill Schist

Oh Harrison Gneiss

Ohn Nodular member of Harrison Gneiss

Otfg Schist and granulite member of Trap Falls Formation

Otfs Shelton (white gneiss) Member of Trap Falls Formation

Otfc Carringtons Pond Member of Trap Falls Formation

EXPLANATION

Faults

Otf+Og Trap Falls Formation and granitic gneiss

units are the gneisses of Highland Massifs and are Proterozoic in age. The shelf-sequence rocks (marble and 

schistose marble) are Cambrian and Ordovician in age. 

Bedrock in the area southeast of Cameron’s line primarily includes schist, gneiss, and granitic gneiss. These 

rocks are lower to middle Ordovician in age.

Foliation of bedrock in the Greenwich area typically strikes N-NE and dips west at 34 to 80 degrees 

(Rodgers, 1985). The stream-drainage network apparently is strongly controlled by the underlying rock 

structure, and many streams are parallel to the strike of 

the foliation. Many high-angle fractures in the bedrock 

likely coincide with the direction of foliation. Another 

pattern in the stream-drainage network suggests structural features that strike N-NW; this could be another 

major direction of fractures in the underlying bedrock. 

Water moving through the crystalline bedrock, primarily through networks of interconnected fractures, 

supplies most private wells in Greenwich.

Precipitation and Runoff

The climate of the Greenwich area is humid, and 

the average annual precipitation during the 30-year 

period 1967–96 was 49.9 in/yr as measured at Putnam 

Lake (fig. 2) (David Medd, Aquarion Water Company
10 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

of Connecticut, written commun., 2002) (fig. 5). 

During this period, precipitation ranged from 30.5 in. 

(in 1995) to 68.9 in. (in 1983). Median annual precipitation in the Greenwich area during 1951–80 ranged 

from 46 to 50 in. (Hunter and Meade, 1983). Smaller 

amounts of precipitation occur near the coast; the larger 

amounts occur at higher elevations in northern Greenwich and are the results of orographic uplift.

Mean annual runoff from streams in the Greenwich area during 1930–60 ranged from about 21 to 23 

in. (Ryder and others, 1970). Annual mean runoff from 

two nearby USGS streamflow-gaging stations 

(01208950, 01208990, fig. 6) during 1967–01 ranged 

from 13 to 46 in. (fig. 7). The difference between 

precipitation and runoff represents water lost through 

evapotranspiration. The average difference between 

precipitation at Bridgeport, Connecticut, and annual 

mean runoff from Sasco Brook near Southport (station 

01208950, fig. 6), Connecticut, during 1966–96 was 

17.9 in. (fig. 8). This value is probably very similar to 

the evapotranspiration rate for the Greenwich area. 

Annual variations in the relation between precipitation 

and runoff can be attributed primarily to variations in 

ground-water storage and climatic factors such as 

temperature, humidity, wind speed, and cloud cover. 

Natural long-term differences in annual mean runoff 

are related primarily to long-term local differences in 

precipitation.

19671968196919701971197219731974197519761977197819791980198119821983198419851986198719881989199019911992199319941995199619971998199920002001

YEAR

20

30

40

50

60

PRECIPITATION, IN INCHE

S

70

Figure 5. Precipitation at Putnam Lake, Greenwich, Connecticut, 1967–2001.
Description of the Study Area 11

GREENWICH

BRIDGEPORT

FAIRFIELD

01208990

01208950

33

32

31

30 23

GREENWICH

STUDY AREA

21

23

22

BROOKFIELD

8

FAIRFIELD COUNTY,

CONNECTICUT

NEW YORK

NEW YORK

LONG ISLAND SOUND

01208990 Continuous streamflowgaging station and stationidentification number

Water-level observation well 

and town well number

21

EXPLANATION

Rain gage at Bridgeport 

climatological station

Figure 6. Additional data-collection sites in Fairfield County, Connecticut.
12 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

19671968196919701971197219731974197519761977197819791980198119821983198419851986198719881989199019911992199319941995199619971998199920002001

10

20

30

40

50

01208990, Saugatuck River near

Redding, Conn.

01208950, Sasco Brook near

Southport, Conn.

RUNOFF, IN INCHE

S PER YEAR

30 40 50 60 70

10

20

30

40

Least squares

fitted line

PRECIPITATION, IN INCHES

ANNUAL MEAN RUNOFF, IN INCHE

S

50

80

Figure 7. Annual mean runoff at two long-term streamflow-gaging stations near Greenwich, Connecticut, 1967–01.

Figure 8. Precipitation at Bridgeport, Connecticut, and runoff at Sasco Brook near Southport, Connecticut (USGS station 

number 01208950), calendar years 1966–96. 
Description of the Study Area 13

METHODS OF DATA COLLECTION AND 

ANALYSIS

Data collected for this study include (1) streamflow measurements at two continuous record streamflow-gaging stations (2) streamflow measurements at 

miscellaneous stations, (3) ground-water-level 

measurements, (4) water-quality samples, and (5) 

water-use data. 

Streamflow Measurements

Continuous streamflow was measured at two 

locations (USGS stations 01211699 and 01211110) 

from February 2001 to September 2002 (fig. 2; table 1) 

to estimate ground-water recharge, to examine the 

difference in peak flows among primarily urbanized 

and primarily forested basins, and to assess the status of 

streamflow conditions during the study period (2001–

02). Streamflow measurements at selected miscellaneous stations (table 1) were made on October 24-25, 

2000 in conjunction with collection of water-quality 

samples and on April 24-25, 2002 to calibrate a finitedifference ground-water-flow model. Records of 

streamflow were computed using methods described 

by Rantz (1982a, b). Streamflow records from 

February 2001 to September 2002 for stations 

01211699 and 01211110 (fig. 2) are in appendix 1. 

Table 1. Streamflow measurements, Greenwich area, Connecticut, 2000–2002.

[USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second; mi, miles; ft, feet; lat., latitude; long, longitude; Type of station: C, continuous; M, miscellaneous. Purpose: QW, water-quality samples collected; C, measurement to calibrate ground-water-flow model; R, measurement to create/maintain rating curve at continuous measurement site]

USGS station 

identification 

number and 

stream

Tributary to Location

Drainage 

area

 (mi2)

Type of 

station Purpose

Date of 

measurement

Discharge 

(ft3/s)

01211010

Brothers 

Brook

Mianus 

River

Lat. 41°03’26”, long 73°35’58”, Fairfield County, at Montgomery Pinetum Park (known in Greenwich as 

Strickland Brook)

1.39 M QW

C

10/25/00

04/24/02

0.173

1.01

01211040

Greenwich 

Creek

Long Island 

Sound

Lat. 41°03’56”, long 73°36’26”, Fairfield County, at bridge on Hill 

Street near Cos Cob (known in 

Greenwich as Beaver Brook)

0.69 M C 04/24/02 .538

01211110

Unnamed 

tributary

Greenwich 

Creek

Lat. 41°02’34”, long 73°36’59”, Fairfield County, on Old Church Road, 

at Cos Cob (known in Greenwich 

as West Brothers Brook)

2.19 C QW

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

10/25/00

02/22/01

03/13/01

03/22/01

03/31/01

04/19/01

05/09/01

05/18/01

06/04/01

06/11/01

09/07/01

10/09/01

10/11/01

11/06/01

11/30/01

04/10/02

04/20/02

06/07/02

06/13/02

08/06/02

09/24/02

.389

3.68

37.0

14.0

23.1

3.84

.977

.472

5.27

1.07

.314

.729

.686

.495

1.15

2.98

6.21

36.6

1.85

.040

.842
14 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

01211140

Horseneck 

Brook

Long Island 

Sound

Lat. 41°06’50”, long 73°38’00”, Fairfield County, at Lower Cross Road 

near Stanwich

0.52 M QW

C

10/25/00

04/24/02

0.091

.220

01211210

Horseneck 

Brook

Long Island 

Sound

Lat. 41°05’41”, long 73°35’58”, Fairfield County, at Valley Drive near 

Rock Ridge

4.81 M QW 10/25/00 .877

01211450

Byram River

Long Island 

Sound

Lat. 41°03’26”, long 73°42’18”, Fairfield County, at bridge on Bedford 

Road near North Greenwich

9.10 M C 04/25/02 4.55

01211600

Byram River

Long Island 

Sound

Lat. 41°03’38”, long 73°40’41”, Fairfield County, at bridge on Sherwood Avenue, at Riversville11.7 M C 10/24/00 1.87

01211699

East Branch 

Byram River

Byram 

River

Lat. 41°05’58”, long 73°41’02”, Fairfield County, below Lake Mead, 

200 ft upstream from John Street, 

at Round Hill (known in Greenwich as Middle Branch Byram 

River)

1.65 C QW

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R, C

R

R

R

R

10/24/00

02/22/01

03/13/01

03/20/01

04/19/01

05/18/01

06/04/01

06/11/01

09/07/01

09/28/01

11/06/01

11/30/01

01/03/02

02/05/02

04/10/02

04/24/02

05/20/02

06/07/02

08/06/02

09/24/02

.393

3.83

13.8

4.64

3.50

.654

2.89

.839

.029

1.60

.19

.620

.289

.687

1.98

1.09

5.53

25.6

.16

.069

01212100

East Branch 

Byram River

Long Island 

Sound

Lat. 41°03’39”, long 73°40’31”, Fairfield County, at bridge on 

Riversville Road just downstream 

from Merritt Parkway, 0.2 mi 

upstream from mouth

1

7.4 M QW 10/24/00 1.86

01212550

Unnamed 

tributary

Byram 

River

Lat. 41°01’39”, long 73°39’41”, Fairfield County, upstream of bridge 

on Pemberwick Road (known in 

Greenwich as Pemberwick Brook)

1.4 M C 04/25/02 .839

01212600

Byram River

Long Island 

Sound

Lat. 41°01’38”, long 73°39’41”, Fairfield County, downstream of 

unnamed tributary at Pemberwick

123.6 M C 04/25/02 13.6

1

Drainage area does not include basins diverted to Putnam Lake (fig. 2).

Table 1. Streamflow measurements, Greenwich area, Connecticut, 2000–2002.—Continued

[USGS, U.S. Geological Survey; mi2, square miles; ft3/s, cubic feet per second; mi, miles; ft, feet; lat., latitude; long, longitude; Type of station: C, continuous; M, miscellaneous. Purpose: QW, water-quality samples collected; C, measurement to calibrate ground-water-flow model; R, measurement to create/maintain rating curve at continuous measurement site]

USGS station 

identification 

number and 

stream

Tributary to Location

Drainage 

area

 (mi2)

Type of 

station Purpose

Date of 

measurement

Discharge 

(ft3/s)
Description of the Study Area 15

Water-Level Measurements

Three wells (table 2) were installed in the East 

Branch of the Byram River Basin, using air-rotary 

drilling techniques, to monitor water levels and 

ground-water storage in the bedrock aquifer. The wells 

were installed at different topographic positions in the 

watershed to evaluate the magnitude of water-level 

fluctuations in different parts of the ground-water-flow 

system (GW-21 on the hilltop on the drainage divide, 

GW-23 on the hillside, and GW-22 in the bottom of the 

valley, figs. 2, 9). This approach of installing wells in 

different parts of the ground-water-flow system was 

discussed by Melvin (1986) and has been implemented 

in many basins in Connecticut with USGS streamflowgaging stations. Water-level fluctuations generally are 

greater on hilltops and hillsides, where ground-waterflow gradients are low. Water-level fluctuations in the 

valley bottom typically are small, because these tend to 

be areas of ground-water discharge, and ground-waterflow gradients are high. 

Water levels were measured sporadically beginning in September 2001; water levels were monitored 

continuously using submersible pressure transducers 

with data loggers from November 20, 2001, to October 

2002. Data on daily water levels were published by 

Morrison and others (2003).

Water levels also were measured in 36 domestic 

wells on April 16-20, 2002 to calibrate the groundwater-flow model. Wells were selected from a list of 

voluntary homeowners who were interested in participating in the study. Wells were located using a global 

positioning system (GPS), and the altitude of the land 

surface was determined from 2-ft contour maps 

provided by the town of Greenwich. In addition, water 

levels measured by well drillers were obtained from 

402 well-completion reports as part of calibrating the 

ground-water-flow model.

Water-Quality Samples

Water-quality samples were collected from 

seven sites (fig. 2; table 1) during base-flow conditions 

on October 24-25, 2000. Samples were collected 

during a base-flow period to ensure that streams 

contained primarily ground-water discharge. These 

samples represented an integrated sample of groundwater quality in basins with different land-use characteristics. Samples were collected using methods 

described by Wilde and Radtke (1998), Wilde and 

others (1998a-c), and Wilde and others (1999a, b) and 

were analyzed at the USGS laboratory in Denver, Colorado, for major ions, dissolved trace elements, nutrients, volatile organic compounds (VOCs), pesticides, 

and indicator bacteria. Water-quality analyses were 

reported in Morrison and others (2002). An additional 

sample was collected at site 01211110 (fig. 2) for 

experimental analysis of pharmaceuticals, hormones, 

and other organic wastewater contaminants, and 

analyzed using methods described in Kolpin and others 

(2002). This sample was collected to determine if 

compounds associated with septic-system waste were 

entering streams from ground water. 

Water-Use Data

Water-use data were compiled from public water 

suppliers for calendar year 2000 (David Medd, 

Aquarion Water Company of Connecticut, written 

commun., 2002). These data were used to create a 

statistical model of water use at residences with private 

wells and to estimate return flow from private septic 

systems.

Table 2. Wells in the East Branch Byram River Basin, Greenwich, Connecticut.

[USGS, U.S. Geological Survey]

USGS local 

well

identifier

USGS

station identification 

number

Latitude

(NAD83)

Longitude

(NAD83)

Altitude

(feet)

Depth of 

well 

(feet)

Depth to 

bedrock 

(feet)

Depth to 

bottom of 

casing

CT-GW 21 410628073413301 41o 06’ 28.4” 73o 41’ 32.6” 465 350 11 18.7

CT-GW 22 410443073414101 41o 04’ 43.3” 73o 41’ 40.7” 222 250 14 18.6

CT-GW 23 410515073415901 41o 05’ 15” 73o 41’ 59.5” 365 250 9 17.8
16 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

0

3200100

00

500400

EA LEVEL S ALTITUDE, IN FEET ABOVE 

John Street

North Porchuck Road

Porchuck Road

A A'

Thick Glacial Till 

Walloomsac Schist Manhattan Schist

Cameron's Line Carrington's Pond Member of the Trap Falls Formation

(interlayered schistand gneiss)

CT-GW-21

CT-GW-23

CT-GW-22

00 1 2 KILOMETER

S

1 2 MILE

S

444

341

216

EXPLANATION

Inferred contact between bedrock and thick glacial tillInferred contact between bedrock typesWater-level altitude (feet) in well on April 16, 2002 444

Bend in section

Figure 9. Cross section through wells installed for water-level measurements. (Geology based on Rodgers, 1985.)
Ground-Water Recharge 17

GROUND-WATER RECHARGE IN THE 

GREENWICH AREA

In the Greenwich area, natural ground-water 

recharge to bedrock aquifers is derived primarily from 

precipitation that infiltrates from the land surface to the 

water table. Other natural sources of recharge include 

loss of water from lakes, wetlands, and streams. 

Previous studies in Connecticut have shown that there 

are differences in ground-water recharge in areas 

covered with till compared to areas covered with 

glacial stratified deposits. These differences have been 

observed in the base-flow component of streamflow 

(ground-water runoff). Mazzaferro and others (1979) 

determined the following mathematical relation 

between geology and ground-water runoff based on 

work by Thomas (1966):

, (1)

where

Y= Ground-water runoff, as a ratio to 

total basin runoff, and 

X= Percentage of the basin underlain by 

coarse-grained stratified drift (glacial stratified deposits).

Recharge rates calculated with this equation 

represent the total recharge in the basin, but do not 

describe how much recharge actually infiltrates to the 

bedrock aquifer from the glacial material. For instance, 

a large percentage of recharge to till areas discharges 

directly from the till into small nearby streams and 

intermittent watercourses during wet periods. Under 

natural conditions, most recharge to glacial stratified 

deposits does not enter bedrock, because glacial stratified deposits are mostly in valley-bottom locations. 

These areas are associated with ground-water 

discharge; therefore, ground water is more likely to 

flow upward from the bedrock aquifer to the glacial 

stratified deposits than downward from the surficial 

deposits to the bedrock aquifer. Under stressed conditions, such as those that take place when wells in the 

bedrock aquifer are pumped, water in the till and 

glacial stratified deposits, which normally would have 

discharged to a local surface-water body, may be drawn 

into the bedrock aquifer and into wells. 

Factors Affecting Ground-Water Recharge

Factors other than geologic properties may affect 

recharge rates locally and temporally. Annual and longterm variability and timing of precipitation and subsequent runoff from a basin are very important in determining recharge rates. In Connecticut, recharge takes 

place primarily during the nongrowing season from 

October to May (Melvin, 1986) although precipitation 

is generally distributed evenly throughout the year. In a 

typical year, ground-water levels begin to rise in 

October, because there is more recharge to the aquifer 

than discharge to streams. During the growing season, 

water levels typically follow a downward trend 

because of evapotranspiration and depletion of soil 

moisture, despite temporary peaks caused by large 

rainfall events. A lack of precipitation during the 

nongrowing season can cause lower than normal water 

levels in the aquifer even in a year with above-normal 

precipitation during the growing season. In addition, 

water-level fluctuations are likely to be larger in upland 

areas than in valley bottoms; therefore, the effects of 

drought conditions may be more pronounced in the 

upland areas.

In areas where the water table is at land surface, 

recharge is rejected as surface runoff (Kontis, 2001). 

Channelized or unchanneled surface runoff from till 

deposits may be another important source of recharge 

to glacial stratified deposits (Kontis, 2001). The natural 

drainage patterns, the degree to which channelized 

flow takes place, and the position of the water table 

during the nongrowing season may affect local 

recharge rates in any basin.

Other factors that may affect natural recharge 

include slope, vegetation cover, and local variations in 

geology and soil moisture. For example, Bauer and 

Mastin (1997) showed that areas with coniferous forest 

may have less ground-water recharge than areas with 

deciduous forest, due to retention and evaporation of 

rain and snow from the evergreen canopy.

In a developed area such as Greenwich, human 

activities can affect recharge rates and can change the 

ground-water budget of a particular basin. Development adds impervious surfaces that can impede natural 

recharge. Ground-water withdrawals can reduce the 

base flow of streams or increase the recharge from 

surface-water bodies and wetlands. Reduction in 

ground-water levels in areas where the water table is at 

land surface also may increase recharge from precipitation. The discharge of wastewater through septic

Y = 0.6X + 35
18 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

systems can affect ground-water-flow patterns, and, in 

areas like Greenwich, can be an additional source of 

recharge if the water is imported from public watersupply reservoirs inside or outside the basin. 

Additional recharge may be obtained from 

leaking water mains, storm sewers, and sanitary 

sewers, and ground-water withdrawals may occur from 

infiltration into sanitary sewer lines. The amount of 

water entering and leaving the ground-water system 

from these sources is difficult to quantify, but due to the 

aging water and sewer infrastructure in Greenwich, it is 

likely that some amount of water is gained or lost from 

these sources. A study of these issues would lead to 

enhanced understanding of the ground-water budget in 

the Greenwich area.

Ground-Water Recharge and Discharge, 

2001–02

Ground-water recharge and discharge can be 

observed in the hydrographs of ground-water levels in 

wells GW-21, 22, and 23 (fig. 10), and in records of 

streamflow (appendix 1). Ground-water recharge 

during the winter of 2001–02 was delayed in wells GW 

21 and 23 by a lack of precipitation. Ground-water 

levels began to rise in well GW-23 during November 

2001, but did not rise in well GW-21 until February 

2002 (fig. 10). The delay probably was caused by 

below-normal precipitation. Precipitation from 

October 2001 to February 2002 at nearby Putnam Lake 

was 8.11 in., compared to the 20-year average of 18.58 

in. for these months during 1982–01 (David Medd, 

Aquarion Water Company of Connecticut, written 

commun., 2002). The highest water levels in the three 

wells installed for this study were during May 2002. 

Water levels generally declined in all three wells during 

the summer of 2002; however, only GW-23 responded 

to summer precipitation. It may be that some additional 

source of recharge in this part of the basin allows water 

into the aquifer. One hypothesis is that an intermittent 

stream about 50 ft from this well affects ground-water 

recharge—during the spring, when this stream is 

flowing, it may be a conduit for ground-water 

discharge; when this stream dries up, it may become a 

source for ground-water recharge from upland surfacewater runoff collected during storms. Water-level fluctuations in well GW-22 (in the valley bottom of the 

basin) were much smaller than fluctuations in GW-21 

or GW-23, as expected, due to ground-water inflow 

from upgradient areas.

Stream base flow can be equal to ground-water 

recharge during periods when there is no net change in 

ground-water storage. Ground-water discharge to a 

stream can be determined by the separation of base 

flow from a streamflow hydrograph. Streamflow was 

continuously monitored (appendix 1) at two locations 

(fig. 2) from March 2001 to September 2002. A baseflow separation was conducted at station 01211699 to 

determine the ground-water discharge from this basin 

during April 2001–September 2002. The computer 

hydrograph separation program “PART” developed by 

Rutledge (1997) was used to estimate the quarterly 

ground-water runoff. (fig. 11).

During April–December 2001, the total runoff 

calculated for station 01211699 was 8.48 in. This 

compares with 8.60 in. of runoff at nearby Sasco Brook 

(station 01208950; fig. 6) during the same period. The 

mean annual runoff for April-December at Sasco 

Brook from 1967-2001 was 16.2 in., indicating that 

most of 2001 was drier than average.

At station 01211699 in Greenwich, the results of 

hydrograph separation from “PART” (fig. 11) indicate 

that most ground-water discharge from this basin took 

place from April to June (4.5 in.) with a very small 

amount (1.5 in.) from July to December. Ground-water 

runoff (6.0 in.) represented 70 percent of the total 

runoff from April to December 2001.

Data from USGS streamflow-gaging stations 

commonly are reported by water year,1 which represents the annual water cycle. If the baseflow-separation data from 01211699 are calculated on the basis of 

water year 2002 (October 2001 to September 2002), 

the total base flow for this period was 7.81 in. and the 

total runoff was 12.21 in (fig 10); therefore, base flow 

was about 64 percent of the total annual runoff. In 

comparison, the mean annual runoff at Sasco Brook 

(station 01208950; fig. 6) was 25.7 in. from 1967-2001. 

There probably was more base flow than recharge for 

water year 2002 because ground water that was in 

storage before the water year began was discharged and 

because 2002 was drier than normal. 

1

A water year is defined as the 12-month period October 1 

through September 30. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. 

Thus, the year ending September 30, 2002 is called the 2002 water 

year.
Ground-Water Recharge 19

Sep Oct Nov Dec

2002

Jan Fe

b

M

ar Apr M

ay Jun Jul Aug

Sep Oct

05

101520253

0

3

5

404550

332.521.510.50

.5

65.554.54

GW21 hilltopGW22 v

alley 

bottom

GW2

3 hill

side

Precipit

ation 

at Putn

am L

ake, Greenwich, Connecticut

EXPLANATION

PRECIPITATION, IN INCHES

GROUND-WATER LEVEL,

URFACE S IN FEET BELOW LAND 

2001

manual

measurements

G

a

p

s indic

ate missing record or equipment f

ailure

continuousmeasurements

Figure 10. Water levels in bedrock wells in the East Branch of the Byram River Basin and precipitation at Putnam Lake, Greenwich area, Connecticut, 

September 2001-October 2002.
20 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Apr-Jun Jul-Sep Oct-Dec Jan-Mar Apr-Jun Jul-Sep

0

8

2

4

6

2001 2002

RUNOFF, IN INCHE

S

STATION 01211699

OVERLAND RUNOFF

BASEFLOW

Water Year 2002

WATER USE IN THE GREENWICH AREA

Residential water use was estimated for basins in 

Greenwich to (1) determine water-use patterns, (2) 

provide data for input into a finite-difference groundwater-flow model, as either water withdrawals from 

wells or as return flow in areas with septic systems and 

public water supply, and (3) estimate consumptive 

water use, with respect to average rates of ground-water 

recharge.

Ground-water withdrawals for residences with 

private wells in Greenwich were estimated by evaluating water-use data and geographic information 

system (GIS) characteristics of residences with public 

water supply for calendar year 2000 and creating loglinear ordinary least-squares (OLS) regression models 

to estimate average daily water use, average summer 

water use, and average daily winter water use. Predictions of water use and confidence intervals were aggregated by drainage basin (appendix 2).

Additional estimates of water use for golf 

courses, commercial buildings, schools, churches and 

other government or institutional buildings, which 

would be necessary to determine total water use for 

each basin, would be useful to understand the full effect 

of water use on downstream resources, but were beyond 

the scope of this study.

Water Use at Residences with Public 

Water Supply

The water-use data (meter readings) for individual domestic users obtained from Aquarion Water 

Company of Connecticut for calendar year 2000 were 

matched to addresses in the town of Greenwich property GIS database. Average daily water use was calculated for each property with public water supply by 

summing the quarterly (or more frequent) meter readings and dividing by 365 days. Average daily summer 

water use was calculated by summing meter readings 

from April to September, and values for average daily 

winter water use were calculated by summing readings 

from January to March and October to December. 

These values were divided by 182.5 days, or one-half of 

a year. A subset of these properties had data that could 

be extracted only seasonally, because of water billing 

cycles, or meter readings that were not recorded at least 

quarterly; therefore, a smaller number of observations 

were used to predict winter and summer water use than 

were used for average daily water use. Data were 

summarized initially by property size (table 3).

Figure 11. Quarterly base flow at U.S. Geological Survey streamflow-gaging station number 01211699, East Branch 

Byram River below Lake Mead at Round Hill, Connecticut.
Ground-Water Recharge 21

Table 3. Median daily water use, and median daily seasonal water use by property size, in calendar year 2000 at selected 

residences with public water supply.

[gal/d, gallons per day; <, less than; >, greater than]

Residential 

property size 

(acres)

Median daily water use

calendar year 2000

(based on average daily use at 

individual properties)

Median daily water use, 

April-September 2000

(based on average daily 

summer use at individual 

properties)

Median daily water use, 

January-March 2000 and

October-December 2000

(based on average daily winter 

use at individual properties)

(gal/d) Number of 

households (gal/d) Number of households (gal/d) Number of households

<0.5 219 309 230 240 204 240

>.5-1 295 342 324 251 225 251

>1-2 357 1,297 410 1,011 275 1011

>2-4 535 828 704 723 385 723

>4 1,082 112 1389 89 848 89

Typical water use in Connecticut is estimated to 

be about 76 gal/person/d (Solley and others, 1998). 

Based on data compiled for this report (table 3), it is 

apparent that water use in the Greenwich area may be 

larger than what is typical, depending on the type of 

development. In areas with less than 1-acre lot size, per 

capita use is about 113 gal/person/d or less. In areas 

with larger lots, per capita use could be as high as 416 

gal/person/d (based on table 3 values and assuming an 

average of 2.6 persons per household (U.S. Department 

of Commerce, 1991)). 

Log-Linear Regression Models of 

Residential Water Use

The regression models that were used to predict 

water use for residences with private wells were of the 

form

(2)

where 

WU is water use, in gallons per day, 

β0 is an intercept, 

β1, β2, β3 are coefficients, 

X is an independent variable, and 

E is a random error. 

Information from the Greenwich GIS database 

that was compiled for each property included the presence and size of an outdoor swimming pool, the lot 

size, the number and total footprint of buildings, and 

the area of the lot that was forested. The area of forest 

cover was subtracted from the total lot area to create a 

variable related to lawn size. These variables were 

chosen because of their availability and the hypothesis 

that properties with large acreage, swimming pools, 

large or numerous buildings, and large lawns use more 

water. A study done by the American Water Works 

Association Research Foundation in several metropolitan areas in the United States determined that these are 

important factors in determining residential water use 

(American Water Works Association Research Foundation, 1999). A similar approach was undertaken in 

this study.

Variable Selection

The GIS information that was compiled was 

tested in different combinations as variables in a 

multiple linear-regression model to predict the natural 

log of average daily water use for each property with 

public water-supply information. A log-transformation 

of the response variable (water-use data) was necessary 

because the data appear to be log-normally distributed. 

Variables (tables 4, 5, and 6) were selected on the basis 

of physical plausibility, the evaluation of R2 (coefficient of determination), the distribution of residuals, 

ln( ) WU β0 β1 X1 ()β2 X2 ()β3 X3 ( )...

βn Xn ( ) E

+++

+

=
22 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

and whether or not the variable was statistically significant at the 95-percent confidence interval. Regression 

models were created for average daily water use, 

average summer water use, and average winter water 

use.

Selected variables (tables 4, 5, and 6) were 

similar for all three regression models. The variables as 

a group have some predictive ability and are highly 

significant, but the coefficients of determination (R2) 

for each model are low and indicate that additional 

variables would be useful. For example, a variable for 

the number of residents per household would help 

explain additional error; however, many of the variables for predicting water use probably are socioeconomic and therefore are difficult to quantify or are 

unavailable. Model residuals are generally normally 

distributed (figs. 12A-C). This indicates that it is 

reasonable to apply confidence intervals to water-use 

predictions made with these models.

Prediction of Residential Water Use in Areas with 

Domestic Wells

A major assumption in using the regression 

method to predict water use for residences with private 

wells is that residences with wells use water at similar 

rates to similar residences with public water supply. It 

is possible that water use in homes with private wells 

may differ from those with public water supply. For 

instance, homeowners with wells may use less water 

indoors because of the lower water pressure that is 

sometimes associated with private domestic water 

systems, or may be more or less inclined to fill or top 

off swimming pools, to water lawns, or to use water for 

other landscaping purposes. 

To use the OLS regression model described 

above for making water-use predictions, an algorithm 

was applied to correct for retransformation bias in 

converting the predictions from natural log space. Data 

that are retransformed from log space will not be 

normally distributed; therefore, predictions and the 

associated confidence intervals will be biased (Helsel 

and Hirsch, 1992). The method used to account for 

retransformation bias is the Minimum Variance Unbiased Estimator (MVUE) (Cohn and others, 1989). 

A computer program was written (Gregory 

Schwarz, U.S. Geological Survey, written commun. 

2002) to run the OLS regression model and make 

predictions using the MVUE. The program sorts 

predictions by water-use type (private well and septic 

system, or public water supply and septic system), 

aggregates data by basin (fig. 13), and computes the 90-

percent confidence interval of the aggregated values (

2). The program uses actual water-use data in areas 

with public water supply and private septic systems, or 

predicts water use for properties in the public watersupply area that were not matched to the GIS data. 

Return flow from septic systems (fig. 14) was assumed 

to be equivalent to the winter rate of water use. Return 

flow from septic systems in areas with public water 

supply were separated from return flow in areas with 

private wells, because this is generally an outside 

source of additional recharge to the aquifer. 

Predictions for areas outside the Greenwich town 

boundaries were made using different methods because 

of the availability and comparability of GIS datasets. 

Water-use predictions were made for the Stamford part 

of the study area using the same water-use model as for 

Greenwich, because the City of Stamford has comparable GIS datasets to Greenwich, except for forest 

cover. The unforested part of each property in the study 

area in Stamford was digitized from high-resolution, 

digital aerial photographs. Water-use estimates for residential properties in the Westchester County part of the 

study area were made based on property size only, 

because GIS data layers were unavailable. The number 

of residential properties of each size in each basin were 

summarized from town of North Castle, New York 

assessor’s records. Values from table 3 were applied to 

these properties. These estimates are subject to an 

unknown amount of error. The error will be less in 

basins where the estimate of water use for the 

Westchester part of the basin is within the 90-percent 

confidence interval for water use in the rest of the basin 

(appendix 2).

Average annual residential ground-water withdrawals from the study area are estimated to total 2.1 

Mgal/year (appendix 2). Return flow from septic 

systems at residences served by public water supply 

was estimated using the winter water use model to total 

1.4 Mgal/year.
Ground-Water Recharge 23

-5 -4 -3 -2 -1

0.0

0.1

0.2

0.3

0.4

0.5

DEN

SITY

-4 -3 -2 -1 0.0

0.1

0.2

0.3

0.4

-3 -2 -1 0.0

0.1

0.2

0.3

0.4

0.5

RESIDUAL

A.

B.

C.

DEN

SITY DEN

SITY

0 1 2 3

0 1 2 3

0 1 2

Figure 12. Frequency distribution for residuals and the “best-fit” normal distribution approximation for (A) average daily 

water use, (B) average daily summer water use, and (C) average daily winter water use, Greenwich area, Connecticut. 
24 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Table 4. Parameter estimates, standard errors, t-statistics, and p-values for multiple linear regression model of average 

daily water use at residences with public water supply, calendar year 2000, Greenwich, area, Connecticut.

[Coefficient of determination (R2) = 0.25]

Variable Units Parameter 

estimate Standard error t-statistic p-value

Intercept dimensionless 8.75 0.167 52.3 <0.0001

Log (unforested area) acres .219 .041 5.38 <.0001

Outdoor swimming 

pool size

acres 19.6 1.62 12.1 <.0001

Log (total footprint of 

buildings)

acres 3.62 .211 17.1 <.0001

Log (total footprint of 

buildings) squared

acres squared .915 .060 15.4 <.0001

Table 5. Parameter estimates, standard errors, t-statistics and p-values for multiple linear regression model of average 

daily summer water use at residences with public water supply, calendar year 2000, Greenwich area, Connecticut.

[Coefficient of determination (R2) = 0.26]

Variable Units Parameter 

estimate Standard error t-statistic p-value

Intercept dimensionless 9.23 0.210 44.0 <0.0001

Log (unforested area) acres .258 .050 5.20 <.0001

Outdoor swimming 

pool size

acres 22.4 1.99 11.3 <.0001

Log (total footprint of 

buildings)

acres 4.06 .268 15.2 <.0001

Log (total footprint of 

buildings) squared

acres squared 1.05 .078 13.6 <.0001

Table 6. Parameter estimates, standard errors, t-statistics and p-values for multiple linear regression model of average 

daily winter water use at residences with public water supply, calendar year 2000, Greenwich area, Connecticut.

[Coefficient of determination (R2) = 0.21]

Variable Units Parameter 

estimate Standard error t-statistic p-value

Intercept dimensionless 7.73 0.200 38.7 <0.0001

Log (property size) acres .345 .069 5.00 <.0001

Log (unforested area) acres .107 .046 2.34 .019

Outdoor swimming 

pool size

acres 12.5 1.73 7.25 <.0001

Log (total footprint of 

buildings)

acres 2.80 .246 11.4 <.0001

Log (total footprint of 

buildings) squared

acres squared .757 .069 11.0 <.0001
Ground-Water Recharge 25

Estimated average daily withdrawals

from residential wells, in million

gallons per day per square mile of 

basin drainage area

0 - 0.02

>0.02 - 0.04

>0.04 - 0.07

>0.07 - 0.1

>0.1 - 0.16

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

GREENWICH

STAMFORD

Westchester

County, New York

Westchester

County, New York

Long Island Sound

Basins with potential of 50 percent

or more increase in average daily

water use due to commerical, 

public, or institutional properties

Basin

number

EXPLANATION

Figure 13. Estimates of average daily residential water use per square mile for small basins, Greenwich area, Connecticut.
26 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Estimate average daily return flow

from residential septic systems

served by public water supply, 

in million gallons per day 

per square mile of basin drainage area

0

>0 - 0.02

>0.02 - 0.04

>0.04 - 0.05

>0.05 - 0.08

GREENWICH

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

STAMFORD

Westchester

County, New York

Westchester

County, New York

Long Island Sound

Basin

number

EXPLANATION

Figure 14. Estimates of average daily return flow per square mile from residential septic systems supplied by 

public water supply for small basins, Greenwich area, Connecticut.
Simulation of Ground-Water Flow 27

Estimation of Consumptive Water Use

Consumptive water use is the part of water withdrawn from the aquifer that is evaporated, transpired, 

incorporated into products or crops, consumed by 

humans or livestock, or otherwise removed from the 

immediate water environment. Estimates of consumptive water use are necessary to determine the amount of 

water withdrawn from the bedrock-aquifer system that 

is not returned, for comparison with available groundwater recharge and stream base flow. Estimates of 

consumptive use also are necessary to determine the 

amount of water imported from public water supply in 

Greenwich (surface-water sources) that is exfiltrated 

from septic systems and becomes an additional source 

of recharge.

The majority of consumptive water use is 

seasonal outdoor water use, including lawn or landscape watering, filling swimming pools, and washing 

vehicles. These uses are generally consumptive 

because water evaporates directly from surfaces or is 

taken up and transpired by plants to the atmosphere 

(U.S. Geological Survey, 1995).

Consumptive water use in Connecticut averages 

about 20 percent of water delivered by public water 

suppliers (Solley and others, 1998). In this report, 

consumptive use is estimated to be equal to outdoor 

water use. Indoor water use is probably returned to the 

aquifer or added as additional recharge to areas with 

public water supply and septic systems. Indoor water 

use is estimated to equal the winter water use. 

Consumptive use in the study area was estimated by 

subtracting the winter water-use data from the average 

daily water use at properties that had seasonal use 

information. The estimated average consumptive use 

for 2,315 properties (with public water supply) in the 

Greenwich area with seasonal-use information for 

calendar year 2000 is 20 percent, the median is 19, and 

the interquartile range is from 3 to 39 percent. This is 

consistent with published values for percentages of 

consumptive use (U.S. Geological Survey, 1995). 

Consumptive use estimated by the water-use models 

(tables 4 and 6, appendix 2) and aggregated by basin is 

higher than published values; consumptive use is estimated to average 29 percent.

Water Use at Nonresidential Properties

In addition to water use by residences, water also 

is withdrawn from wells for use at golf courses, country 

clubs, schools, churches, nurseries, and properties with 

commercial or institutional buildings. The determination of use for these properties is beyond the scope of 

this project and would require additional monitoring 

and willing participation from owners of these properties; however, if data were collected and analyzed, estimates of water use could be updated for each basin area 

shown in figure 13. Five of the 32 basins might have a 

large amount (50 percent or more increase) of additional average daily water use due to nonresidential 

water uses (fig. 13); however, some of this water use (at 

least in the case of golf-course irrigation) may not be 

from ground-water sources. More information is 

needed to update water-use estimates for these basins.

Water use at golf courses probably represents the 

largest individual category of water use in Greenwich. 

Water generally is withdrawn from on-site ponds and 

wells at golf courses, and information is limited on the 

ratio of ground-water to surface-water supply. Golf 

courses can use water stored in ponds from earlier 

spring runoff, overland runoff of surface water from 

storms during the irrigation season, and by pumping of 

ground water. Because of these different sources, the 

total ground-water use and consumption would be 

difficult to estimate without metering wells and total 

system uses simultaneously.

Water use is variable among golf courses and 

from year to year, depending on annual or seasonal 

precipitation, soil type, or other localized conditions. 

Estimates of typical golf course irrigation rates were 

provided by several golf course superintendents in 

Greenwich (Jeffrey Scott, Tamarack Country Club; 

Gary Glazier, Burning Tree Country Club; Scott Niven, 

Stanwich Club, oral commun., 2002) and range from 

60,000 to 150,000 gal/d during the growing season 

(May to September). This is equivalent to about 25,000 

to 60,000 gal/d averaged for 1 year, assuming a 150-

day irrigation season from May to September. Data 

from the Stanwich Club indicated that some irrigation 

also takes place during April and October (Aleksandra 

Moch, Greenwich Conservation and Inland Wetlands 

Department, written commun., 2002). 

SIMULATION OF GROUND-WATER FLOW 

IN THE GREENWICH AREA

Ground-water flow in the Greenwich study area 

was simulated using MODFLOW-2000 (Harbaugh and 

others, 2000). MODFLOW-2000 is a three-dimensional, finite-difference ground-water-flow model that 
28 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

includes a nonlinear regression algorithm to find the 

best fit among model parameters and hydrologic observations. The purpose of the ground-water-flow model 

is to test hypotheses about hydrologic properties and 

provide a framework to estimate ground-water budgets 

and water availability in small basin-based zones in the 

Greenwich area. The calibrated model can be used to 

simulate hypothetical conditions involving variation in 

consumption of water, changes in recharge, or other 

changes to the hydrologic system. Two model simulations were run: (1) the first simulation was for calibration to conditions during late April 2002 using winter 

water-use rates, and (2) the second simulation used the 

calibrated model with ground-water withdrawal rates 

adjusted to average water-use conditions.

Description of Flow Model and Model 

Assumptions

The ground-water model uses a finite-difference 

grid with 57 rows and 46 columns (fig. 15). Individual 

cells in the model are 1,017 by 1,017 ft. The model has 

five layers of constant thickness (fig. 16). The uppermost two layers simulate the surficial aquifers that 

consist of thin till, thick till, glacial stratified deposits, 

and bedrock underlying areas with thin till (fig. 3). The 

top of layer 1 is the land surface. Layer 1 is 15 ft thick, 

and layer 2 is 30 ft thick. The lower three layers (each 

200 ft thick) simulate the bedrock aquifer to a depth of 

645 ft below land surface. The depth was selected 

because it is greater than the average depth of wells in 

Greenwich. The average depth of 400 wells compiled 

from well completion reports was about 450 ft. Model 

layers are assumed to be confined, including layer 1, 

due to numerical instability in the model with thin 

uppermost layers. The model is a steady-state groundwater-flow model that is calibrated to a stable period 

(April 2002) when recharge and discharge were in 

balance and no major changes were taking place in 

ground-water storage. Steady-state ground-water-flow 

models are used to simulate long-term average conditions, or stable periods of increased or reduced 

recharge. The ground-water-flow model of the Greenwich area was calibrated to 34 ground-water levels 

measured during April 16-20, 2002; water levels from 

402 well-completion reports (fig. 17); and streamflow 

measurements at 8 sites (table 2). Water levels that 

were measured during April 2002 were assumed to 

represent average conditions because water levels in 

wells from the USGS statewide-monitoring network 

with monthly measurements (table 7; fig. 6) were at 

average (period of record) conditions during this time 

period. Normally during April, water levels in wells 

would be at above-average conditions; however, the 

spring of 2002 was very dry, and water levels in wells 

in the USGS statewide-monitoring network in Fairfield 

County were close to average conditions. Groundwater levels and streamflow in Greenwich during April 

16-25 were generally stable, with little change (fig. 18). 

The model also was calibrated to streamflow 

measurements from eight locations (table 1) in the 

Greenwich area on April 24-25, 2002. Streamflow 

during this time period was stable and represents base 

flow (fig. 18). These observations included seven 

measurements of discharge, and one determination of 

discharge from a rating curve at a continuous streamflow-gaging station number 01211110 (fig. 2).

The bedrock aquifer is simulated in this model as 

homogeneous and isotropic. This model is designed 

primarily to understand the water budget and should 

not be used for studies that require detailed flow-path 

analysis. In reality, the frequency and spatial orientation of fractures in the bedrock are likely to affect local 

ground-water-flow directions and possibly groundwater budgets. A similar approach modeling the water 

budget and hydraulic properties of fractured crystalline 

bedrock aquifers as an equivalent porous medium was 

used by Tiedeman and others (1997) and Wolcott and 

Snow (1995).

The steady-state ground-water-flow model was 

calibrated to observations of ground-water levels and 

streamflow. The calibration process is a nonlinear 

regression to provide optimal estimates of parameters 

based on the best fit to measured (or observed) values 

of hydraulic head and streamflow. Model parameters 

were estimated using a nonlinear-regression method 

developed by Cooley and Naff (1990) and applied to 

MODFLOW 2000 by Hill and others (2000). 
Simulation of Ground-Water Flow 29

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

Putnam

Lake

Rockwood 

Lake

Converse

Lake

Bargh

Reservoir

Kensico

Reservoir

EXPLANATION

Constant-head cell

5

 |

10

 |

15

 |

20

 | 25

 | 30

 |

35

 |

40

 |

45

 |

1

 |

--1

5--

10--

15--

20--

25--

30--

35--

40--

45--

50--

55--

No-flow and active model

area boundary

Model cell with simulated 

stream

40-- Row number

15

 |

Column number

STAMFORD

GREENWICH

Westchester

County, New York

Westchester

County, New York

Long Island Sound

14--

Figure 15. Extent of finite-difference ground-water-flow model grid, Greenwich area, Connecticut.
30 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

-500

-300

-100

100

300

500

4567 8 9 10 11 12 13 14 15 16 17 18 19 20 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Column

number

0 1 2 MILES

0

1 2 KILOMETERS

Layer 5 (bedrock)

Layer 4 (bedrock)

Layer 3 (bedrock)

Layer 1 Layer 2

ALTITUDE, IN FEET

Till and thick till

Coarse-grained glacial stratified deposits

Fine-grained glacial stratified deposits

Model layer boundary

No-flow boundary

EXPLANATION

Figure 16. Cross section through row 14 (see fig. 15), in the finite-difference ground-water-flow model, Greenwich area, 

Connecticut.
Simulation of Ground-Water Flow 31

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

STAMFORD

GREENWICH Westchester

County, New York

Westchester

County, New York

EXPLANATION

Water-level observation from

well-completion report

Water level measured April 16-20, 2002

Streamflow observation

Figure 17. Location of observations for ground-water levels and streamflow, Greenwich area, Connecticut.
32 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002 March

4

8 12 16 20 24 2

8

April

4

8 12 16 20 24 2

8

May

4

8 12 16 20 24 2

8

2002

50

325201510

0

3

5

4540

010203

0

405060

GW21 hilltop

GW22 v

alley 

bottom

GW2

3 hill

side

01211110

01211699

Gro

und-w

ater level

s (me

an d

aily)

Stre

amflow

(me

an d

aily)

tion period ar bli a ter-flow-model c a nd-w u Gro

e flow bas m a tre s nd a s ter level a nd-w u for gro

GROUND-WATER LEVEL, 

IN FEET BELOW LAND SURFACE

ECOND S TREAMFLOW, IN CUBIC FEET PER S

GW21 hilltopGW22 v

alley 

bottom

GW2

3 hillside

EXPLANATION

0121169901211110

Ground-w

ater level

Stre

amflow

Figure 18. Ground-water levels and streamflow, March to May 2002, Greenwich area, Connecticut.
Simulation of Ground-Water Flow 33

Table 7. Water levels in April 2002 and average water levels at selected U.S.Geological Survey network wells in Fairfield 

County, Connecticut.

[Water levels in feet below land surface; USGS, U.S. Geological Survey]

USGS local well 

identifier and 

station number

(see fig. 6)

Town April 2002

water level 

Average 

water

level

Period of 

record used Aquifer

FF23

411256073153101

Fairfield 8.13 7.89 1966-2002 Glacial stratified deposits

FF30

411124073172201

Fairfield 4.87 4.94 1993-2002 Crystalline bedrock

FF31

411118073175801

Fairfield 7.04 8.47 1993-2002 Crystalline bedrock

FF32

411030073181301

Fairfield 6.31 7.71 1993-2002 Crystalline bedrock

FF33

411058073182001

Fairfield 5.18 5.30 1993-2002 Till

BD8

413007073250501

Brookfield 32.37 30.56 1966-2002 Glacial stratified deposits

Boundary Conditions

The lateral model boundaries are based mostly 

on physical boundaries of the Greenwich area, 

including basin boundaries and streams (fig. 15). Basin 

boundaries on the perimeter of the model are treated as 

no-flow boundaries, and streams near the model 

boundaries are treated as head-dependent boundaries 

simulated with the MODFLOW stream package 

(Prudic, 1989). Other model boundaries include Long 

Island Sound on the southern side of the model, simulated as a constant-head boundary with altitude zero; 

the Byram River (fig. 2) in the southwestern part of the 

model, simulated as a head-dependent boundary; and 

the Mianus River on the eastern side of the model 

(head-dependent boundary) (fig. 15). To limit the size 

of the active model, three areas of the model were 

terminated across basin boundaries at a narrow point. 

These areas include the northeastern corner of the 

model area, and the Blind Brook Basin (fig. 2) in 

Westchester County, where streamflow leaves the 

model. The model also is terminated across a narrow 

point in the Byram River Basin (fig. 2) where streamflow enters the model. The large reservoirs, including 

the Kensico Reservoir, part of the Bargh Reservoir, 

Putnam Lake, Rockwood Lake, and Converse Lake 

(figs. 2 and 14), were simulated as constant head 

boundaries, because of their large size and typically 

large quantities of water in storage at all times other 

than extreme drought periods.

The lower model boundary is a no-flow 

boundary that is 645 ft below land surface. The upper 

boundary of the model is the land surface. Recharge is 

applied to layer 1 in the model and is considered a specified-flux boundary. 

Ground-Water Recharge

Ground-water recharge to areas with glacial till 

was estimated using optimal values based on nonlinear 

regression modeling in MODFLOW. Ground-water 

recharge to glacial stratified deposits was held constant 

at previously estimated values (table 8). Recharge rates 

in the model are effective recharge rates and account 

for the effects of ground-water evapotranspiration. A 

recharge parameter was defined in the model using a 

zone array (fig. 19A) corresponding to surficial 

geology, and a multiplication array corresponding to 

the percent pervious area in the model cell. The 

recharge parameter multiplied by the percentage of 

pervious area in each model cell determined the 

recharge value for the model input. This was done 

because peak streamflows (normalized to drainage 

area) in urbanized basins are higher than in basins with 

less urban development (fig. 20). The increased overland runoff limits the amount of water available for 

recharge, ultimately reducing the base flow of streams. 

These recharge estimates are conservative, because not 

all impervious areas drain directly into watercourses. 

Water from rooftops and driveways may infiltrate onto 

lawns or other unpaved areas, and stormwater drainage 

from some areas may infiltrate to the water table if the 

stormwater is discharged to retention basins or is 

allowed to flow onto pervious areas.
34 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Base from Connecticut Department of Environmental Protection 1994

Digital Line GraphProjection State Plane Feet Zone 3526

41o

41o5'

73

o40' 73

o

35'

STAMFORD

GREENWICH

Westchester

County, New York

Westchester

County, New York

Base from Connecticut Department of Environmental Protection 1994Digital Line GraphProjection State Plane Feet Zone 3526

41o

41o5'

73

o40' 73

o

35'

STAMFORD

GREENWICH

Westchester

County, New York

Westchester

County, New York

EXPLANATION

Glacial stratified deposits

(coarse grained)

Active modelarea boundary

Glacial stratified deposits

(fine grained)

Model cells not highlighted inside

active model are thin till

EXPLANATION

Glacial stratified deposits

Active modelarea boundary

Areas of thick till 

Model cells not highlighted inside

active model area are thin till in layer 1 

A. B.

Recharge rate used in

ground-water-flow simulations

(inches per year)

20

7.96.9

Hydraulic conductivity used in 

ground-water-flow simulations

(feet per day)

0.5

0.10

100

Figure 19. Zone array based on geology used for (A) application of recharge to the model and (B) application of hydraulic properties to layers 1and 2, steadystate ground-water simulation, Greenwich area, Connecticut.
Simulation of Ground-Water Flow 35

The percentage of impervious area in each model 

cell was estimated by two methods. For model cells 

completely within the town of Greenwich, impervious 

areas were estimated from 1997 Greenwich GIS layers 

as the sum of paved roadway areas, building areas, and 

driveway and sidewalk areas. In Greenwich GIS layers, 

driveways and sidewalk areas are linear features and 

have no area associated with them. To estimate the 

areas of these features, the length was divided by 2 and 

multiplied by the estimated width (driveways-13 ft; 

sidewalks-5 ft).

For model cells in areas outside Greenwich, 

impervious areas were estimated from a grid of land 

use/land cover based on 1995 Landsat thematic mapper 

images interpreted by Civco and others (1998). A 

regression model (fig. 21) was used to relate landuse/land-cover characteristics from Civco and others 

(1998) (table 9) to the calculated impervious areas in 

each of 120 randomly selected model cells in the 

Greenwich part of the model area. Predictions based on 

the regression model were applied to the model area 

outside Greenwich. Significant variables in the regression at the 95-percent confidence level included the 

urban categories “Commercial/Industrial/Paved,” 

“Residential and Commercial,” and “Turf and Tree 

Complex.” Other urban variables, including “Turf and 

Grass” and “Rural Residential” were not found to be 

significant predictors of impervious area. Model residuals were determined to be reasonably normally 

distributed (fig. 21).

Table 8. Recharge estimates from historical values and 

nonlinear regression for April 2002 calibration.

[*, value presented in model calibration section of report]

Recharge parameter Method

Recharge

(inches per 

year)

Till Nonlinear 

regression

*

Glacial stratified deposits, 

coarse grained

Historical 

values

120

1

Value from Mazzaferro, 1986.

Glacial stratified deposits, 

fine grained

2Estimated

2The area of fine-grained glacial stratified deposits represents a very 

small area in the ground-water flow model; a recharge rate similar to those 

for glacial till was used because of the low permeability of these deposits.

7.9

Table 9. Parameter estimates, standard errors, t-statistics and p-values for multiple linear regression model of 

percentage of impervious area for randomly selected cells of the finite-difference ground-water-flow model, 

Greenwich area, Connecticut.

[Coefficient of determination (R2)=0.87]

Variable Units Parameter 

estimate

Standard 

error t-statistic p-value

Intercept Dimensionless 2.0814 0.5464 3.8094 0.0002

Commercial/

industrial/paved 

Percentage of model 

cell area

0.6631 0.0475 13.9696 <.0001

Residential and 

commercial

Percentage of model 

cell area

0.3331 0.0167 19.9993 <.0001

Turf and tree

complex

Percentage of model 

cell area

0.1232 0.0166 7.4342 <.0001
36 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

TREAMFLOW, S S TANTANEOU S IN

QUARE MILE S ECOND PER S IN CUBIC FEET PER 

March

3

6 9 12 15 1

8 21 24 27

3

0

April

3

6 9 12 15 1

8 21 24 27

3

0

2001

0

0

5

5

10 1015 1520 2025 253

0

3

0

3

5

3

5

01211699

~

4

% impervious

are

a1

01211110 

~10

% impervious

are

a1

EXPLANATION

1. Estim

ated using equation based on table 12. 

% Impervious

are

a = 2.0

814 + .66

31(C) + .3331(R) + .12

32(T), where 

C = Commerci

al/industri

al/p

avement, R = Residenti

al and commerci

al, and T = Turf 

and tree complex from Civco 

and others (1998).

Figure 20. Instantaneous streamflows per square mile at two continuous streamflow-gaging stations, Greenwich area, Connecticut, March to April 2001.
Simulation of Ground-Water Flow 37

0 10 20 30 40 50

0

10

20

30

40

50

 PREDICTED IMPERVIOU

S AREA,

 IN PERCENT OF MODEL CELL AREA

MEASURED IMPERVIOUS AREA, IN PERCENT OF MODEL CELL AREA

1:1 LINE

60

Aquifer Properties

Hydraulic conductivity of aquifer materials 

either was assigned based on information from 

previous investigations or was estimated using 

nonlinear regression. Hydraulic conductivity of the 

crystalline bedrock was estimated by nonlinear regression in MODFLOW 2000. Hydraulic conductivity of 

glacial stratified deposits, thin till, and thick till were

based on values summarized in table 10. Values from 

table 10 were applied using zone arrays (fig. 19) for 

layers 1 and 2. Layers 3 to 5 were completely in bedrock, and a zone array was not used.

Table 10. Simulated values of hydraulic conductivity in finite-difference ground-water-flow model, Greenwich area, 

Connecticut.

[*, value presented in model calibration section of report]

Aquifer material Method Hydraulic conductivity 

(feet per day)

Median values from 

historical summary by 

Melvin and others (1992)

(feet per day)

Thin till Historical values 0.5 0.6-2.7

Thick till (drumlin till) Historical values .10 .06

Glacial stratified deposits

(combined coarse and fine grained)

Historical values 100 170 

(coarse grained)

Crystalline bedrock Nonlinear regression * .6

Figure 21. Measured and predicted impervious areas for a random sample of model cells, Greenwich area, Connecticut.
38 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Internal Sources and Sinks of Water

Ground water is added (source) or removed 

(sink) from the modeled area through streams, reservoirs, wells, and septic systems. Simulation of these 

sources and sinks is described below.

Streamflow

Streams were simulated using the stream 

package created for MODFLOW by Prudic (1989). The 

stream package is an accounting program that tracks 

the flow in streams that interact with ground water. The 

package allows for diversions of water among basins 

and provides output for comparisons with streamflow 

measurements made for model calibration. 

The active model area (fig. 15) contains 427 

stream segments and 1,375 reaches. Streamflows are 

estimated for every cell in the ground-water-flow 

model that contains a stream reach and are accumulated 

in a downstream direction. Discharge between each 

stream reach and the adjacent aquifer is calculated 

using Darcy’s Law and the equation from Prudic 

(1989):

(3)

where:

Q is leakage to or from the aquifer 

through the streambed

Hs is head in the stream

Ha is head in the aquifer

CSTR is conductance of the streambed, 

defined as KLW/M, where K is the hydraulic 

conductivity of the streambed, L is the 

length of the reach, W is the width of the 

stream in the reach, and M is the streambed 

thickness. 

Streambed hydraulic conductivity was assumed 

to be 1 ft/d, and streambed thickness was assumed to be 

1 ft. Streambed altitude was determined for each reach 

from a grid of 3,652 points regularly spaced every 225 

ft along all streams. The initial altitude of each point 

was determined using a digital elevation model. The 

values were then adjusted by visually inspecting each 

in comparison to USGS 10-ft contour quadrangle 

maps. Stream stage was assumed to be 2 ft higher than 

streambed altitude.

Streamflows predicted by the stream package 

downstream from the large reservoirs (Putnam and 

Rockwood Lakes) may be over- or underestimated 

because of the lack of information on daily diversions, 

withdrawals, and releases. Simulated streamflows for 

each zone represent the amount of water discharged 

from the ground-water system to the stream. Withdrawals from and diversions to these reservoirs will, in 

reality, affect the actual streamflow. Streamflows estimated for streams that are boundaries (fig. 15) in the 

model (that is, the mainstems of Mianus and Byram 

Rivers, fig. 2) do not include ground-water discharge or 

tributaries that may flow into these rivers from outside 

the model area.

Reservoirs

Reservoirs (Kensico, Converse Lake, Rockwood 

Lake, Putnam Lake, Bargh (figs. 2 and 14) were simulated as constant heads. This allows for an unlimited 

amount of water to enter the aquifer if heads in the 

aquifer are below those of the constant head. Another 

approach to simulating the reservoirs would be to use a 

general head boundary. Constant head boundaries were 

chosen for this simulation for the following reasons: (1) 

the discharge from reservoirs to the ground-water 

system was small, indicating that reservoirs were not a 

major source of recharge; (2) the hydraulic conductivity of the till deposits is small, and of the same order 

of magnitude or lower than reservoir bottom sediments, 

therefore it is likely that the flow of water to or from 

reservoirs is primarily controlled by the properties of 

the underlying aquifers; and (3) large quantities of 

water are stored at all times of year in these reservoirs.

Ground-Water Withdrawals and Return Flow from 

Septic Systems

In the calibration model simulation, groundwater withdrawals from residential land-use areas were 

summed for each model cell based on the water-use 

multiple-linear regression models described previously. The ground-water model was calibrated using 

streamflow and water-level data collected during midto late April 2002; therefore, ground-water withdrawals were assumed to represent nongrowing season 

conditions. Ground-water was withdrawn from layer 3 

of the model, which represents the top of bedrock. 

Several recent studies have shown that a large 

percentage of the discharge from wells in fractured 

rock comes from shallow fractures near the surface of 

the bedrock (Mullaney and others, 1999; Wolcott and 

Snow, 1995).

Q CSTR HS Ha = ( ) –
Simulation of Ground-Water Flow 39

Return flow from septic systems was simulated 

as recharging wells summed for each model cell and 

was assumed to be equivalent to indoor water use; 

therefore, winter water-use rates were used. Return 

flow from septic systems includes water returned in 

areas with private wells and in areas with public water 

supply and septic systems. Water from septic systems 

was returned to layer 1 in the model. It should be noted 

that based on the assumptions described above, net 

ground-water consumption (in areas with wells and 

septic systems) is considered to be negligible during 

April 2002. 

In the second simulation, in which average 

pumping conditions were simulated, the regression 

model estimates for average daily water use were 

applied to model cells; however, return flow was again 

assumed to be similar to winter water-use rates.

Model Calibration

The calibration process is a nonlinear regression 

to provide optimal estimates of parameters based on the 

best fit to measured (or observed) values of hydraulic 

head and streamflow. Two parameters in the model 

were estimated in this way—horizontal hydraulic 

conductivity of the bedrock aquifer and recharge to till 

deposits (table 11). These two parameters are important 

because the majority of ground-water flow takes place 

in the bedrock aquifer, and the majority of recharge 

takes place through till deposits. Other parameters that 

were not estimated using the nonlinear-regression 

method were based on values from previous investigations in Connecticut.

The ground-water-flow model of the Greenwich 

area was calibrated to 34 ground-water levels measured 

during April 16-20, 2002, water levels from 402 wellcompletion reports for wells (fig. 17) and streamflow 

measurements at 8 sites. Water levels from wellcompletion reports represent a wide range of waterlevel conditions. It is assumed that this range of conditions represents long-term average annual conditions. 

Data from water-level measurements and well-completion reports are subject to errors, including inaccuracy 

in determining location and land-surface elevation. 

Data from wells measured during April 2002 were 

given a larger weighting in the calibration process.

The weighting process for water level and 

streamflow measurements was accomplished as 

follows. Data that have a higher degree of accuracy are 

given a larger weight. The weights reflect the possible 

measurement error in the data and are equal to the 

inverse of the variance of the measurement. The variance in the measurements for both streamflow and 

water-level measurements was determined subjectively, based on an assessment of, in the case of water 

level measurements, the accuracy of the location and 

the land-surface elevation. The measurement error was 

assumed to be half the contour interval of the topographic map provided by the town of Greenwich (2 ft). 

This translates to a variance in water level measurements for wells measured in April 2002 that was estimated to be 0.37 ft2 at the 90-percent confidence level. 

Measurements obtained from well-completion reports 

were assumed to be less accurate because the wells 

were not located in the field. The weight of these 

measurements was assumed to be one, which translates 

to a measurement error of 1.65 ft at the 90-percent 

confidence interval. The methods of estimating model 

observation weights are described by Hill (1998).

 The weighting of the streamflow measurements 

in the calibration process was dependent on the assignment of whether the measurement was determined to 

be good, fair, or poor based on width of the river and 

stream cross-section characteristics. This weighting 

process for observations of streamflow loss or gain is 

described by Hill (1998).

A plot of the weighted simulated equivalents and 

weighted observations is shown in figure 21. This plot 

contains information on the fit among the observations 

and values predicted by the calibrated ground-water 

model. The model residuals appear to be normally 

distributed (fig. 22), but there are some wells with 

water-level observations in the ground-water-flow 

model where predicted water levels are much higher or 

lower than observed. There does not appear to be a 

geographic bias in water-level residuals. The average 

weighted error in the water-level measurements was 

1.9 ft (table 12). There was generally good agreement 

among streamflow observations (weighted and 

unweighted) and predictions (table 13, fig. 22).

Table 11. Parameter estimates of hydraulic conductivity and 

recharge from nonlinear regression, finite-difference groundwater flow model, Greenwich area Connecticut.

Estimated parameter Value Unit

Horizontal hydraulic 

conductivity crystalline 

bedrock

0.05 Feet per day

Recharge to till deposits 6.9 Inches per year
40 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Table12. Summary of error statistics and comparison of observed and simulated ground-water levels, 

Greenwich area, Connecticut, April 16-20, 2002.

Statistic Value

Sum of squared weighted errors 4.98 X 105

Average weighted error 

(ground-water level measurements)

1.90 feet

Average unweighted error 

(ground-water level measurements)

1.82 feet

Table 13. Observed and simulated streamflows, Greenwich area, Connecticut, 

April 24-25, 2002.

[USGS, U.S. Geological Survey; ft3/s, cubic feet per second]

USGS station identification 

number

(table 1, fig. 2)

Observed 

streamflow 

(ft3/s)

Simulated 

streamflow 

(ft3/s)

Difference 

(ft3/s)

01211010 1.01 0.99 0.02

01211040 0.54 0.71 -.17

01211110 1.60 1.63 -.03

01211140 .22 .25 -.03

01211699 1.09 1.28 -.19

01212550 .84 .75 .08

01212600 excluding the contributions from 01211450, 01212550, 

and 01211699

7.12 5.79 1.33
Simulation of Ground-Water Flow 41

0 200 400 600 800

0

200

400

600

800

-100

-100

1,000

1,000

WEIGHTED SIMULATED EQUIVALENT

WEIGHTED OBSERVATION

Water-level measurements made April 16-20, 2002

Water-level measurements from selected well completion reports

Streamflow (baseflow) measurements made April 24-25, 2002 1:1 line

Hydrologic Budget

The ground-water budget was summarized for 

two simulations. The first simulation was the modelcalibration phase, based on measurements of water 

levels and streamflow collected during mid- to late 

April 2002. For the second simulation, the calibrated 

model was used with residential ground-water withdrawals adjusted to average daily residential withdrawal rates (tables 14 and 15). The ground-water 

budget for each basin-based zone (first shown in 

fig. 13) (tables 14 and 15) was summarized using 

ZONEBUDGET (Harbaugh, 1990), a computer 

program that summarizes ground-water budgets for 

specific zones from MODFLOW-2000 output. Groundwater budgets for this report only include residential 

withdrawals. Further information is necessary on 

nonresidential properties.

Components of the ground-water budget for each 

basin include inputs and outputs. Inputs of water to 

each basin include recharge from precipitation, 

discharge from streams and reservoirs to the aquifer, 

discharge from residential septic systems to the aquifer, 

and underflow from other basins. Outputs of water 

from each basin include underflow to other basins, 

discharge to reservoirs or to Long Island Sound, 

discharge to streams, and ground-water withdrawals. 

Consumptive use of ground-water withdrawn from the 

bedrock aquifer was considered to be negligible during 

April 2002; therefore, estimates of the residential net 

consumption of ground water are included only in the 

model adjusted to average daily ground-water withdrawals (table 15).

Recharge rates from precipitation ranged from 

3.9 to 7.5 in/yr for zones in the model (fig. 23) and are 

based on the surficial geology and percentage of 

pervious area of each zone. Leakage from reservoirs or 

streams to the aquifer was small, and added 0.1-

0.8 in/yr of recharge to the aquifer in some zones.

Figure 22. Weighted simulated equivalent plotted against weighted observations of hydraulic head and streamflow for model 

simulation of the Greenwich area, Connecticut, April 16-24, 2002.
42 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002 Table 14. Annual ground-water budget for modeled zones, based on calibration data from April 18-25, 2002, Greenwich area, Con

[Based on calibration data from April 16-25, 2002; all values in inches per year; mi2, square miles] Zone 

(fig. 23)

Area

(mi2)

Recharge 

from precipitation Inflow from reservoirs Inflow from streams

Inflow from 

residential 

septic systems

Inflow from 

other 

basins

Outflow to 

other 

basins

Outflow to 

reservoirs 

or Long 

Island 

Sound

Ou

st

1 1.6 5.3 0.8 0.0 0.4 2.9 2.9 0.6 2 1.2 6.5 0 .5 .8 2.0 2.7 0 3 .6 6.8 0 0 1.2 1.0 2.9 0 4 3.1 6.4 0 0 .9 2.1 .9 0 5 .9 5.9 0 0 .1 1.2 4.9 0 6 1.7 7.5 0 .5 .8 2.9 .3 0 17 .8 6.4 0 0 1.5 1.2 4.3 0 8 1.6 6.4 0 0 .9 1.9 1.0 0 9 .3 5.5 0 0 1.1 1.3 1.9 0 10 1.4 6.6 0 .2 1.4 1.5 2.2 0 11 1.1 6.9 0 .9 1.0 .7 3.8 0 12 1.4 6.6 0 0 1.4 1.8 1.1 0 13 2.3 6.3 0 0 .7 1.4 .5 .4 14 1.2 6.8 0 0 1.7 2.0 .5 0 15 3.5 6.7 0 .1 .9 1.2 .9 0 16 2 6.8 0 0 .9 2.0 1.6 .5 17 1.7 6.7 0 0 1.9 1.4 1.3 0 18 3 4.3 0 0 .5 .8 .9 .5 19 3 6.7 .1 0 1.2 .8 .8 0 20 1 6.1 0 0 2.1 1.5 1.3 0 21 2.3 6.7 0 0 1.8 1.1 1.5 0 22 1.8 6.2 0 0 1.5 1.1 1.0 0 23 1.2 4.1 0 0 1.0 .7 .4 .8 24 .4 7.1 0 0 2.6 2.0 2.2 0 25 1.3 6.7 .1 0 .6 3.0 2.6 .6 26 1.5 7.0 0 .2 .9 .9 1.6 0 27 1.1 5.7 0 .1 1.5 3.7 1.9 0 28 1.2 7.2 0 0 1.9 1.4 .9 0 29 1.6 5.5 0 0 1.0 1.0 1.0 0 30 2.7 4.9 0 0 1.4 .9 1.6 .4 31 2.7 4.6 0 0 .5 1.3 1.1 1.2 32 1.8 3.6 0 0 .5 .4 1.4 1.3 
Simulation of Ground-Water Flow 43

Table 15. Annual ground-water budget for modeled zones, based on calibration data from April 18-25, 2002 and adjusted to averag

Greenwich area, Connecticut.

[All values in inches per year; mi2, square miles]

Zone 

(fig. 23)

Area 

(mi2)

Recharge 

from precipitation Inflow from reservoirs Inflow from streams Inflow from residential septic sys- tems Inflow from other basins Outflow to other basins

Outflow to 

reservoirs or 

Long 

Island 

Sound

Outflow to 

streams

1 1.6 5.3 0.8 0 0.4 2.9 2.8 0.6 5.5 2 1.2 6.5 0 .5 .8 1.9 2.7 0 5.8 3 .6 6.8 0 0 1.2 .8 2.8 0 4.3 4 3.1 6.4 0 0 1.0 2.1 .9 0 7.7 5 .9 5.9 0 0 .1 1.1 4.9 0 2.1 6 1.7 7.5 0 .6 .8 2.8 .3 0 10.3 7 .8 6.4 0 0 1.5 1.1 4.2 0 2.4 8 1.6 6.4 0 0 .9 1.7 .9 0 6.8 9 .3 5.5 0 0 1.1 1.3 1.8 0 4.6 10 1.4 6.6 0 .1 1.4 1.4 2.0 0 5.5 11 1.1 6.9 0 .9 1.0 .7 3.8 0 5.0 12 1.4 6.6 0 0 1.4 1.8 1.1 0 6.6 13 2.3 6.3 0 0 .7 1.4 .5 .4 6.5 14 1.2 6.8 0 0 1.7 2.0 .5 0 8.4 15 3.5 6.7 0 .1 1.0 1.2 .9 0 8.0 16 2.0 6.8 0 0 .9 1.9 1.5 .4 6.4 17 1.7 6.7 0 0 1.9 1.3 1.2 0 8.3 18 3.0 4.3 0 0 .5 .8 .9 .5 4.3 19 3.0 6.7 .2 0 1.2 .8 .8 0 6.5 20 1.0 6.1 0 0 2.1 1.4 1.2 0 6.5 21 2.3 6.7 0 0 1.8 1.1 1.5 0 7.8 22 1.8 6.2 0 0 1.5 1.1 1.0 0 7.1 23 1.2 4.1 0 0 1.0 .7 .4 .8 4.5 24 .4 7.1 0 0 2.6 2.1 2.1 0 6.1 25 1.3 6.7 .1 0 .6 3.0 2.6 .6 6.2 26 1.5 7.0 0 .2 .9 1.0 1.5 0 6.3 27 1.1 5.7 0 .1 1.5 3.6 1.9 0 8.1 28 1.2 7.2 0 0 1.9 1.3 .8 0 7.5 29 1.6 5.5 0 0 1.0 .9 1.0 0 6.1 30 2.7 4.9 0 0 1.4 .9 1.5 .4 4.8 31 2.7 4.6 0 0 .5 1.3 1.1 1.2 3.9 32 1.8 3.6 0 0 .5 .4 1.4 1.3 1.8 
44 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Estimated long-term average

ground-water recharge rates,

in inches per year

3.6 - 4.9

>4.9 - 5.9

>5.9 - 6.8

>6.8 - 7.3

>7.3 - 7.9

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

GREENWICH

STAMFORD

Westchester

County, New York

Westchester

County, New York

Long Island Sound

Basin

number

EXPLANATION

Figure 23. Simulated long-term, average annual ground-water recharge from precipitation aggregated by zones, 

Greenwich area, Connecticut.
Simulation of Ground-Water Flow 45

Inflow from residential septic systems is derived 

from ground-water withdrawals in the northern part of 

the Greenwich area and is not a net additional source of 

recharge over long-term average conditions. The redistribution of this water may be hydrologically important, because it is withdrawn from the deeper aquifer 

and discharged near the land surface, and therefore may 

change ground-water flow patterns. For instance, the 

water may discharge more quickly to a nearby surfacewater body.

In parts of basins with public water supply and 

septic systems, the discharge from septic systems 

represents an additional source of recharge. The net 

amount of additional recharge can be obtained from 

table 14 or 15 by subtracting residential ground-water 

withdrawals from inflow from residential septic 

systems in the calibration model.

In many cases, the outflow to streams in the calibration model (table 14) does not balance with the 

input of recharge from precipitation. Differences in 

these two values can be attributed to (1) input of additional sources of recharge described above, (2) underflow among basins, and (3) discharge of water to 

reservoirs or to Long Island Sound. In the simulation 

that was adjusted to average residential ground-water 

withdrawals, differences also can be attributed to 

consumptive use of ground water. Water-use totals may 

differ slightly compared to those in the water-use 

section because of discrepancies in delineating basin 

boundaries in the model grid and assigning a 

zone/basin code to each model cell.

Net annual consumptive residential water use 

was determined by subtracting the simulated input 

from septic systems from the average annual groundwater withdrawals in each zone (table 15). Some basins 

that have withdrawals of ground water have greater 

input from septic systems supplied by public water 

supply; therefore, the net residential consumption of 

ground water is zero (fig. 24) or is shown as a negative 

number in table 15.
46 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Estimated long-term average

net residential ground-water

consumption

0

>0 - 0.2 >0 - 0.01

>0.2 - 0.4 >0.01 - 0.02

>0.4 - 0.6 >0.02 - 0.03

>0.6 - 0.9 >0.03 - 0.04

Base from Connecticut Department of Environmental Protection 1994

Digital Line Graph

Projection State Plane Feet Zone 3526

41o

41o5'

73

o

40' 73

o

35'

GREENWICH

STAMFORD

Westchester

County, New York

Westchester

County, New York

Long Island Sound

Inches/

year

Million gallons/day/

per square mile

Basin

number

Basins with potential of 50 percent

or more increase in average daily

water use due to commerical, 

or public, or institutional properties

(-1.6) - (-0.1)*

*Basin has greater return flow from residential septic systems

served by public water supply than consumptive ground-water withdrawals

EXPLANATION

Figure 24. Simulated long-term average net residential consumptive water use for zones, Greenwich area, Connecticut.
Ground-Water Availability 47

GROUND-WATER AVAILABILITY IN THE 

GREENWICH AREA

Ground water is an important part of the water 

budget in Greenwich providing, at minimum, an estimated 35 percent of the annual streamflow (see eq. 1). 

This base flow contributes to maintaining river habitat 

and aquatic life, providing inflow to public watersupply reservoirs, and providing water to ponds used 

for fire protection during periods of little overland 

runoff. 

Determination of ground-water availability 

should consider what changes to the water budget of a 

particular basin might be acceptable. Changes to the 

water budget due to withdrawals may include decrease 

in base flow of streams, increase in the recharge from 

surface-water bodies, or the capture of ground water 

that would have originally been lost through groundwater evapotranspiration in wetlands and other areas 

with shallow water tables. One method to determine the 

status of each basin is to examine various streamflow 

criteria in comparison to the current water budget. To 

show this current status, several equations that predict 

statistical low streamflows were compared to the 

conditions described in the above section on groundwater budgets. 

Weiss (1983) produced equations based on 

streamflow statistics in Connecticut to determine the 7-

day 10-year low flow (Q7,10), the 7-day 2-year low flow 

(Q7,2), and the 30-day 2-year low flow (Q30,2). The 

(Q7,10) is the lowest flow for 7 consecutive days with a 

recurrence interval of 10 years, the (Q7,2) is the lowest 

flow for 7 consecutive days with a recurrence interval 

of 2 years, and the (Q30,2) is the lowest flow for 30 

consecutive days with a recurrence interval of 2 years. 

The (Q7,10) in Connecticut is considered to be 

the minimum amount of water required to assimilate 

wastewater (Connecticut Department of Environmental 

Protection, 1997). The (Q7,2) and (Q30,2) are statistical 

determinations of the typical summer flows of streams 

in Connecticut and probably represent periods of 

typical annual low base flow.

Equations for these streamflows from Weiss 

(1983) are:

(Q7,10)/A = 0.0065(%Asd+1) – 0.001 (4)

(Q7,2)/A = 0.0104(%Asd+1) – 0.001 (5)

(Q30,2)/A = 0.0124(%Asd+1) – 0.001 (6)

where

Q is the specified discharge, in cubic 

feet per second,

A is basin drainage area, in square 

miles, and

%Asd is the percentage of the basin containing coarse-grained glacial stratified deposits 

(glacial stratified deposits).

The specified flows are larger for basins containing 

larger percentages of glacial stratified deposits because 

of the greater recharge rates and the ability to store and 

transmit water.

Long-term average base flows predicted by the 

ground-water-flow simulation based on residential 

ground-water withdrawals are, in most cases, substantially greater than the low streamflow statistical criteria 

(fig. 25). The water budget for many of the basins probably is different now compared to pre-development 

conditions because of ground-water consumption and 

increasing urbanization.

In general, basins with consumptive residential 

use of ground water have long-term estimated average 

base flows that are lower than basins with little or no 

consumptive use. Some of the basins with little or no 

consumptive use of ground water (for example, basins 

31 and 32; fig. 21), however, are estimated to have 

reduced recharge rates due to urbanization, and therefore the estimated base flows approach low-streamflow 

criteria.
48 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002 21 17

30 23

15

22

29

27

18

32

11

20

4

14

5

1

28

19

16

2

6

13 26

9

8

25

3

10

12

7

24

15 0 2 4 6 8 10 12 14

PERCENT OF BASIN UNDERLAIN BY COARSE-GRAINED GLACIAL STRATIFIED DEPOSITS

0

12

8642

10

 PER YEAR S TREAMFLOW IN INCHE S

EXPLANATION

Q30, 2

Calculated baseflow (equation 1)

Q

 >0 - 0.4 in/yr>0.4 - 1.0 in/yr

7,10

Q7, 2

Estimated ground-water discharge

to streams for basins with net residential

1

consumptive use of ground water

17 Basin number

31

Calculated

low flows(equations 4-6)

-1.6* - 0 in/yr 

1. The effects of non-residential water use are not included*Negative number indicates basin has greater return flow from residential septic systems served by public water supply than consumptive ground-water withdrawals

Figure 25. Simulated ground-water outflow to streams, calculated base flow, and low flow for basins in the Greenwich area, Connecticut. [Q7,10 is the 7-day 

10-year low flow, Q7,2 is the 7-day 2-year low flow, and Q30,2 is the 30-day 2-year low flow.] 
Ground-Water Availability 49

When current estimated net residential groundwater consumptive use is compared to recharge rates 

for each zone in the study area (table 16), it can be seen 

that ground-water consumption is, in most cases, small. 

One criteria that might be used to make comparisons 

among zones in the Greenwich area is to subtract the 

Q30,2 from the recharge (from precipitation) for each 

basin normalized to drainage area. This sample criteria 

is conservative in the respect that it does not include 

additional recharge from septic systems or underflow 

from other basins. The basins with the largest relative 

estimated net residential consumptive water use 

include zones 7, 10, 12, and 24. It is estimated that for 

these zones, about 10 percent or more of the difference 

between long-term average recharge and the Q30,2 is 

being used (table 16). 

This sample criteria, although useful on a 

comparative basis, does not take into account other 

issues including (1) the additional nonresidential water 

use; (2) the physical plausibility of being able to 

capture the difference between the recharge and the 

Q30,2 by pumping; (3) the localized effects of lowering 

the water table on nearby wells, or on the habitat 

contained within wetlands and watercourses; and (4) 

the effects on summer streamflows (for example, if the 

average base flow is lowered to the Q30,2, summer base 

flow also will be substantially lower).

Output data from these ground-water models can 

be used to determine the current status of ground-water 

use relative to recharge or other water-budget criteria. 

In order for this data to be used properly, several qualifications and assumptions should be discussed.

In the ground-water flow simulations conducted 

for this study, it was assumed that impervious areas do 

not receive any recharge. Estimates of recharge from 

the nonlinear parameter estimation algorithm in 

MODFLOW were matched to streamflow observations 

and water levels to obtain the best fit for recharge to till 

deposits assuming no recharge beneath impervious 

areas. The recharge to urbanized basins is therefore 

reduced. It is believed that these estimates are conservative and that more research is needed in this area to 

determine the effects of impervious cover and alternative stormwater management on recharge and base 

flow. 

Table 16. Estimated net annual residential consumptive water use and the difference between estimated long-term 

average recharge and the 30-day 2-year low flow.

[Zones not shown in this table are estimated to have no net consumptive residential use of water; Q30,2, 30-day 2-year low flow]

Zone 

(fig. 22)

Recharge 

(inches)

Water consumption (inches)Q30,2 

(inches) 

Difference between 

recharge and Q30,2

(inches)

Percentage of 

difference consumed

1 5.3 0.13 0.9 4.4 2.9

2 6.5 0.33 1.5 4.9 6.6

3 6.8 0.49 0.2 6.7 7.4

5 5.9 0.03 0.2 5.8 0.6

6 7.5 0.33 0.9 6.6 5.0

7 6.4 0.81 0.2 6.2 13.0

8 6.4 0.38 0.6 5.7 6.7

9 5.5 0.35 0.3 5.2 6.8

10 6.6 0.63 0.7 6.0 10.6

12 6.6 0.74 0.2 6.3 11.6

13 6.3 0.34 0.4 5.9 5.7

16 6.8 0.32 0.3 6.5 5.0

19 6.7 0.31 1.3 5.5 5.6

24 7.1 0.94 0.5 6.6 14.3

25 6.7 0.38 1.8 4.8 8.0

26 7.0 0.35 1.6 5.5 6.4

28 7.2 0.19 0.9 6.3 3.0
50 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Residential water use was documented for this 

study, but there may be additional substantial groundwater withdrawals for commercial, institutional, and 

public buildings and golf courses. As shown in 

figure 12, additional water is used in some basins that 

requires further research. 

Water that has been returned to the aquifer by 

septic systems is assumed to remain in the basin as part 

of the water budget. In reality, some of this water may 

never recharge the bedrock aquifer, and may discharge 

to a nearby surface-water body and not be available for 

reuse. In the model simulations, return flow from septic 

systems was greater than ground-water withdrawals in 

nearly half of the basins studied (because of input from 

private septic systems at properties served by public 

water supplies), indicating that this is an important 

component of the water budget in the Greenwich area.

Wells drilled into the bedrock underlying glacial 

stratified deposits may have more water available under 

pumping stresses, if there is a good hydraulic connection between these coarse-grained deposits and the 

underlying bedrock, due to the storage characteristics 

of this aquifer. 

Data and interpretation of the water budget are 

provided for 32 basin-based zones (tables 14, 15). 

Human activities, including new development and 

ground-water withdrawals, may have different effects 

depending on their location in these basins. For 

instance, concentrated development with ground-water 

withdrawals near the headwaters of a basin may have a 

more localized effect on the water budget (that is, 

streamflow reduction or reduction in ground-water 

levels) than the same development if farther downstream in the basin, even though the overall change in 

pumping might not be large relative to the water budget 

for the entire basin. The potential for large fluctuation 

in water levels is greater in the hilltop setting and is less 

apparent in the valley bottom, due to ground-water 

contributions from upgradient parts of the basin 

(fig. 10).

Recharge rates and water budgets presented in 

this report are not fixed numbers, and the ground-water 

budget for any basin is dynamic. Changes in development in a particular basin may have consequences in 

addition to changes caused by increased withdrawal 

and use of ground-water. These changes include 

changes to recharge rates caused by impervious 

surfaces, changes to the water budgets of other basins 

(in the case where underflow from one basin to another 

is important), and the possibility of increased recharge 

if new development takes place in areas with public 

water supply and septic systems. The addition of sanitary sewers to areas with private wells can export water 

from a basin, and therefore affect ground-water availability. Other changes to the water budget include longterm changes in annual precipitation, changes in the 

seasonal variability of precipitation, or prolonged 

drought.

WATER QUALITY IN THE GREENWICH 

AREA

Analyses of seven surface-water-quality samples 

collected during base flow showed that concentrations 

of some indicators of water-quality degradation were 

higher in more urbanized basins than in less urbanized 

basins (fig. 26; table 17). Concentrations of total 

nitrogen, total phosphorus, chloride, indicator bacteria, 

and the number of different pesticide detections were 

generally higher with increasing urbanization of the 

basin. Samples from one site (USGS station number 

01211210), collected below a point of water diversion 

and storage, may not be entirely representative of the 

base-flow conditions of the lower part of Horseneck 

Brook (fig. 2).

The U.S. Environmental Protection Agency 

(USEPA) has set regional nutrient-criteria guidelines 

for State governments for determination of impaired 

waters (U.S. Environmental Protection Agency, 2000; 

2001). The criteria for total nitrogen for Ecoregion XIV 

(which includes Connecticut) is 0.7 mg/L for rivers and 

streams and 0.32 mg/L for lakes and reservoirs. The 

criteria for total phosphorus is 0.031 mg/L for rivers 

and streams and 0.008 mg/L for lakes and reservoirs. 

These criteria are considered a starting point in determining problem areas, and State governments may set 

more stringent criteria.

Nitrogen and phosphorus concentrations above 

ambient concentrations may originate from atmospheric deposition, septic systems, lawn fertilizers, 

waterfowl, and from point-source discharges. Elevated 

nitrogen concentrations are of concern for eutrophication and hypoxia in Long Island Sound (Long Island 

Sound Study, 1998) because nitrogen is the limiting 

nutrient in saltwater bodies. Data interpreted from 

Mullaney and others (2002) indicate that concentrations of total nitrogen in mostly forested basins range 

on average from 0.35 to 0.5 mg/L. As basins become
Water Quality 51

0 20 40 60

0

1

2

0 20 40 60

0.00

0.05

0.10

TOTAL PHOSPHORUS

IN MILLIGRAMS PER LITER

0 20 40 60

0

20

40

60

80

TOTAL NITROGEN CHLORIDE

0 20 40 60

1

10

100

1,000

FECAL COLIFORM

ENTEROCOCCI

0 20 40 60

URBAN LAND USE IN PERCENT

 OF BASIN AREA

0

1

2

3

4

5

NUMBER OF PESTICIDES

 DETECTED

URBAN LAND USE IN PERCENT

 OF BASIN AREA

COLONIESPER 100 ML

CONCENTRATION,

 IN MILLIGRAMS PER LITER

Figure 26. Percentage of urban land use and concentrations of selected water-quality constituents, Greenwich area, 

Connecticut.

Table 17. Percentage of urban land use and concentration of selected water-quality constituents in surface-water base-flow 

samples collected October 2000, Greenwich area, Connecticut.

[Urban land use determined from Civco and others (1998) by summing total area of each basins from the following land use/land cover categories: “Commercial, industrial, pavement; residential and commercial; rural residential; turf and tree complex; and turf and grass; USGS, U.S. Geological Survey; 

mg/L, milligrams per liter; mL, milliliters]

USGS

station 

identification 

number

(fig. 2)

Urban land 

use

(percentage 

of 

basin area)

Total 

nitrogen

concentration 

(mg/L)

Total 

phosphorus

concentration 

(mg/L)

Chloride 

concentration

(mg/L)

Fecal 

coliform bacteria

(colonies per

100 mL)

Enterococci 

bacteria

(colonies per 

100 mL)

Number of 

different 

pesticides 

detected

01211600 32.0 2.00 0.064 73.2 100 920 2

01211140 21.1 .380 .022 20.4 40 42 1

01211210 40.3 .600 .105 27.4 110 112 3

01211010 27.7 .530 .026 25.7 16 11 1

01211699 17.9 .290 .012 17.0 3 26 1

01211110 63.1 .880 .059 29.0 110 132 4

01212100 30.0 .500 .020 26.4 110 68 3
52 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

more urbanized, concentrations of total nitrogen due to 

nonpoint sources of nitrogen may range from less than 

0.5 mg/L to 2.0 mg/L. Basins containing municipal 

wastewater-treatment facilities may have total nitrogen 

concentrations as high as 5 mg/L or more if the wastewater is a large part of the total streamflow. Grady and 

Mullaney (1998) and Grady (1994) determined that 

median nitrate (the most common form of nitrogen in 

ground water) plus nitrite nitrogen concentrations in 

shallow ground-water beneath urbanized areas ranged 

from 1.1 to 2.4 mg/L, as compared with 0.11 to 0.14 

mg/L in shallow ground water beneath forested areas. 

Heisig (2000) determined that nitrate nitrogen concentrations in stream base flow were positively correlated 

to unsewered housing density in the nearby Croton 

Basin in New York.

Elevated phosphorus concentrations are of 

concern for algal blooms and eutrophication in freshwater bodies, where phosphorus is the limiting 

nutrient. Concentrations of total phosphorus in excess 

of 0.1 mg/L in moving water in streams may cause 

algal blooms or excessive nuisance plant growth (Litke, 

1999). Concentrations lower than 0.1 mg/L may be 

able to cause algal blooms in lakes and ponds. 

A water sample from an unnamed tributary to 

Greenwich Creek (USGS station number 01211110, 

fig. 2) was analyzed using an experimental procedure 

for wastewater compounds (Kolpin and others, 2002) 

to determine if this location (the most urbanized basin 

sampled) showed signs of input from septic systems. 

The compounds that were detected were present at very 

low concentrations—at, near, or below the reporting 

limits. Concentrations of most of the compounds are 

estimated due to the experimental nature of the analysis. The compounds detected (table 18) indicate that 

wastewater from septic systems or leaking sanitary 

sewer lines has entered this stream. The same 

compounds were detected in a national survey of pharmaceuticals, hormones, and other organic wastewater 

compounds (Kolpin and others, 2002).

Table 18. Wastewater compounds detected on October 25, 2000 at station number 01211110, Greenwich 

Connecticut.

[E, estimated concentration]

Wastewater compound Concentration

(micrograms per liter) Use or indication

Para-nonylphenol-total E 0.223 Nonionic detergent metabolite

Nonylphenol monoethoxylate- (NPEO1) (total) E 0.531 Nonionic detergent metabolite

Nonylphenol diethoxylate- (NPEO2) (total) E 0.933 Nonionic detergent metabolite

Triclosan 0.051 Anti-bacterial/disinfectant

3-beta-coprostanol E 0.428 Fecal indicator
Water Quality 53

SUMMARY AND CONCLUSIONS

Water use, ground-water availability, and quality 

of water were studied by the U.S. Geological Survey 

(USGS) in cooperation with the town of Greenwich, 

Connecticut, during 2000–02. The study area is 

52.8 square miles and includes Greenwich, part of 

Stamford, Connecticut, and adjacent parts of 

Westchester County, New York. Increasing development and the lack of large glacial stratified aquifers for 

public water supply have led to a need to study groundwater availability and water use in basins in the Greenwich area. Self-supplied ground water from a fractured 

crystalline-bedrock aquifer is the source of water 

supply to about 12 percent of the households in the 

Greenwich area. 

Households with public water supply (from a 

surface-water source) in Greenwich used from 219 to 

1,082 gal/d of water during 2000. Water use at these 

properties was less during the winter season (204 to 

848 gal/d) and more during the summer season (230 to 

1,389 gal/d). Water use was strongly dependent on the 

type and size of the residential property. Three 

multiple-linear regression models were used to estimate self-supplied water use (from a ground-water 

source) for average daily, winter, and summer conditions in 32 basin-based zones. The winter water-use 

model also was used to estimate return flow in areas 

with public water supply and septic systems. The 

regression models were developed with GIS characteristics of properties that were matched to a database of 

public water-supply information. Statistically significant predictors of water use included the amount of 

unforested area, the area of outside swimming pools, 

the sum of areas covered with buildings, and lot size. 

Estimates of average residential daily ground-water 

withdrawals by basin ranged from 0 to 0.16 million 

gallons per day per square mile (Mgal/d/mi2

), and 

totaled about 2.1 million gallons per day (Mgal/d). The 

estimated return flow from residential septic systems 

on properties served by public water supply ranged 

from 0 to 0.08 Mgal/d/mi2

, and totaled 1.4 Mgal/d. 

Residential consumptive water use in Greenwich 

was estimated to be about 20 percent of average daily 

water use, based on comparison of data aggregated by 

basin for seasonal regression models. Additional water 

use at nonresidential properties, including golf courses, 

churches, schools, and commercial properties, may 

amount to a 50-percent or more increase in average 

daily ground-water use in 5 of the 32 basin areas 

studied.

A steady-state finite-difference ground-waterflow model was used to study the water budgets of the 

32 basins in the study area. The ground-water-flow 

model was calibrated to streamflow measurements, 

water levels measured in private wells during April 

2002, and additional water levels reported on wellcompletion reports. Optimal values for hydraulic 

conductivity of the bedrock aquifer and recharge to till 

were estimated in the model using nonlinear regression. Ground-water recharge was not applied to impervious areas in the model. Impervious areas were 

estimated for each model cell from GIS information 

provided by the town of Greenwich, or from extrapolation of this information using interpreted LANDSAT 

imagery. The ground-water recharge to till deposits that 

cover most of the study area was estimated to be 6.9 

inches per year. Average (bulk) hydraulic conductivity 

of the bedrock was estimated to be 0.05 feet per day.

The calibrated ground-water-flow model was 

used to simulate average daily residential ground-water 

withdrawals, and water budgets were summarized by 

basin. Simulated long-term ground-water recharge by 

basin ranged from 3.9 to 7.9 in/yr. Basins with the 

smallest amount of recharge were primarily along the 

coastal sections of Greenwich where the degree of 

urbanization is greatest. Net consumptive residential 

ground-water use ranged from 0 to 0.9 in/yr. The 

largest net consumptive residential ground-water use 

was from small basins in the upper reaches of the East 

Branch of the Byram River Basin and in part of the 

Mianus River Basin in the Banksville section of Greenwich. 

Water-budget components were compared to 

statistically based calculations of average base flow 

and several low flows, including the 30-day 2-year flow 

(Q30,2), the 7-day 2-year flow (Q7,2), and the 7-day 10-

year (Q7,10)so that relative comparisons could be made 

among basins. A criteria of subtracting the Q30,2 from 

the estimated recharge for each basin was used as one 

possible estimate of ground-water availability. Based 

on this criteria, net residential ground-water consumption ranged from 0 to 14.3 percent of available water. 

Simulated streamflows (long-term average base flows) 

for each basin also were compared to the statistical 

calculation of base flow and low flow to determine if 

any of the long-term average simulated base flows 

approached these criteria. Possible reductions in base 
54 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

flow were noted in many basins that had either relatively large ground-water withdrawals or in basins with 

a high degree of urbanization. Two coastal basins and 

one basin in north-central Greenwich had simulated 

streamflows of less than 3 in/yr and approached statistical low flows. 

Users of this information should note that a 

percentage of the recharge in each basin (including, for 

example, artificial recharge from septic systems) may 

never enter the bedrock aquifer and may be unavailable 

for use. Changes to the water budget caused by new 

development in any basin may be more pronounced in 

the headwaters of a basin than in the downstream valley 

bottom, in terms of the localized effect on streamflow 

and ground-water levels. 

The water budgets estimated in this report are not 

fixed because the aquifer is a dynamic system. Changes 

in development in one basin may have effects on the 

water budgets of other basins, or may not be apparent 

for many years. New and existing development may 

have effects, in addition to ground-water withdrawals, 

on the water budget, such as loss of recharge, change in 

recharge patterns, or re-routing ground water through 

septic systems.

Results of the limited base-flow water-quality 

sampling indicate that the water quality is related to the 

degree of urbanization. Concentrations of nitrogen, 

phosphorus, chloride, and indicator bacteria appear to 

be related to the percentage of urban land in each of the 

basins sampled. More urbanized basins had detections 

of a larger number of pesticides, but concentrations 

were very low. In three basins, phosphorus concentrations were higher than regional nutrient criteria for 

rivers and streams and may cause excess algal growth. 

Total nitrogen concentrations exceeded regional 

nutrient criteria for lakes and ponds in water samples 

from six of seven basins sampled, indicating concentrations above ambient levels.

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U.S. Environmental Protection Agency, Office of Water 

EPA 822-B00-022, 84 p.
56 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

———2001, Ambient water quality criteria recommendations—Information supporting the development of 

State and Tribal nutrient criteria; lakes and reservoirs in 

nutrient ecoregion XIV: U.S. Environmental Protection 

Agency, Office of Water EPA 822-B01-011, 123 p.

U.S. Geological Survey, 1995, National handbook of recommended methods for water-data acquisition. Chap. 11--

Water use: U.S. Geological Survey Office of Water 

Data Coordination, Available online at 

http://water.usgs.gov/pubs/chapter11/ accessed 

February 20, 2003.

Weiss, L.A., 1983, Evaluation and design of a streamflowdata network for Connecticut: Connecticut Water 

Resources Bulletin no. 36, 31 p.

Wilde, F.D., and Radtke, 1998, National field manual for the 

collection of water quality data--Field measurements: 

U.S. Geological Survey Techniques of WaterResources Investigations, book 9, chap. A6, variously 

paginated. 

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1998a, National field manual for the collection of water 

quality data--Preparation for water sampling: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 9, chap. A1, 47 p. 

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1998b, National field manual for the collection of water 

quality data--Selection of equipment for water 

sampling: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 9, chap. A2, 94 p. 

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1998c, National field manual for the collection of water 

quality data--Cleaning of equipment for water 

sampling: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 9, chap. A3, 75 p. 

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1999a, National field manual for the collection of water 

quality data--Collection of water samples: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A4, 156 p. 

Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 

1999b, National field manual for the collection of water 

quality data--Processing of water samples: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 9, chap. A5, 149 p. 

Wolcott, S.W., and Snow, R.V., 1995, Computation of 

bedrock-aquifer recharge in northern Westchester 

County, New York, and chemical quality of water from 

selected bedrock wells: U.S. Geological Survey WaterResources Investigations Report 92-4157, 57 p.
References Cited 57

Appendixes 1–4. 
58 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Appendix 1. Records from two U.S. Geological Survey streamflow-gaging stations, Greenwich, 

Connecticut, March 2001-September 2002.

Station 01211699 E. Br. Byram R. Below Lake Mead At Round Hill 

STATION NUMBER 01211699 E. BR. BYRAM R. BELOW LAKE MEAD AT ROUND HILL SOURCE AGENCY USGS STATE 09 COUNTY 001

 LATITUDE 410558 LONGITUDE 0734101.5 NAD27 DRAINAGE AREA 1.65* CONTRIBUTING DRAINAGE AREA 1.65 DATUM 330 NGVD29

DISCHARGE, in CFS, WATER YEAR OCTOBER 2000 TO SEPTEMBER 2001. DATA COLLECTION BEGAN FEBRUARY 2001.

 DAILY MEAN VALUES 

 DAY OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP

 1 --- --- --- --- --- 4.3 8.5 1.9 1.1 1.1 0.03 0.57

 2 --- --- --- --- --- 4.1 7.2 1.9 11 1.5 0.02 0.31

 3 --- --- --- --- --- 4.0 6.4 1.7 6.0 0.88 0.01 0.17

 4 --- --- --- --- --- 4.0 5.7 1.7 3.2 0.79 0.07 0.11

 5 --- --- --- --- --- 4.4 5.2 1.4 2.1 1.5 0.14 0.08

 6 --- --- --- --- --- 4.5 5.2 1.2 1.6 2.4 0.16 0.04

 7 --- --- --- --- --- 3.9 4.9 1.1 1.3 1.2 0.11 0.03

 8 --- --- --- --- --- 3.7 4.7 1.1 1.2 1.4 0.06 0.02

 9 --- --- --- --- --- 4.1 5.8 1.2 0.95 1.4 0.03 0.02

 10 --- --- --- --- --- 4.1 12 1.1 0.88 0.84 0.04 0.40

 11 --- --- --- --- --- 4.2 6.1 0.99 0.83 0.79 0.06 0.61

 12 --- --- --- --- --- 4.4 5.9 0.95 0.94 0.62 0.17 0.38

 13 --- --- --- --- --- 12 5.3 0.78 0.90 0.49 0.67 0.22

 14 --- --- --- --- --- 10 4.4 0.61 0.83 0.40 0.61 2.4 

 15 --- --- --- --- --- 7.8 4.0 0.58 0.74 0.31 0.32 1.7 

 16 --- --- --- --- --- 6.9 3.8 0.54 0.66 0.27 0.17 0.67

 17 --- --- --- --- --- 8.1 4.0 0.59 18 0.24 0.12 0.34

 18 --- --- --- --- --- 7.0 3.6 0.70 8.4 0.33 0.12 0.21

 19 --- --- --- --- --- 5.5 3.3 0.75 3.3 0.31 0.08 0.15

 20 --- --- --- --- --- 4.7 3.1 0.62 2.2 0.25 2.4 0.52

 21 --- --- --- --- --- 5.9 3.2 0.59 1.8 0.20 3.0 5.2 

 22 --- --- --- --- --- 15 3.9 3.6 1.7 0.14 0.86 2.2 

 23 --- --- --- --- --- 15 3.3 4.9 2.0 0.11 0.71 0.84

 24 --- --- --- --- 3.3 8.2 3.0 3.7 2.7 0.10 1.1 0.49

 25 --- --- --- --- 4.9 6.7 2.6 2.1 1.9 0.10 0.54 1.9 

 26 --- --- --- --- 8.2 5.7 2.3 2.1 1.3 0.15 0.25 1.5 

 27 --- --- --- --- 6.1 5.2 2.4 2.2 1.1 0.21 0.15 0.78

 28 --- --- --- --- 5.0 4.7 2.2 1.8 0.88 0.13 0.13 0.53

 29 --- --- --- --- --- 4.3 1.9 2.0 0.67 0.11 0.10 0.41

 30 --- --- --- --- --- 20 1.9 2.2 0.60 0.09 0.06 0.36

 31 --- --- --- --- --- 14 --- 1.5 --- 0.06 0.32 ---

2001

MAM J J A SOND

2002

J FM A M J J A S

0

5

10

15

20

25

30

35

40

45

MEAN DAILY DISCHARGE, IN CUBIC FEET PER SECOND

01211699
Appendixes 59

Appendix 1. Records from two U.S. Geological Survey streamflow-gaging stations, Greenwich, 

Connecticut, March 2001-September 2002.—Continued

Station 01211699 E. Br. Byram R. Below Lake Mead At Round Hill—Continued

STATION NUMBER 01211699 E. BR. BYRAM R. BELOW LAKE MEAD AT ROUND HILL SOURCE AGENCY USGS STATE 09 COUNTY 001

 LATITUDE 410558 LONGITUDE 0734101.5 NAD27 DRAINAGE AREA 1.65* CONTRIBUTING DRAINAGE AREA 1.65 DATUM 330 NGVD29

DISCHARGE, in CFS, WATER YEAR OCTOBER 2001 TO SEPTEMBER 2002 

 DAILY MEAN VALUES 

 DAY OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP

 1 0.78 0.33 0.55 0.36 1.2 0.39 2.2 3.3 3.3 1.2 0.02 0.20

 2 0.72 0.29 0.47 0.34 1.3 0.33 1.9 3.6 2.7 1.4 0.50 3.7 

 3 0.52 0.64 0.43 0.32 1.1 11 1.9 5.3 2.2 1.3 0.71 2.4 

 4 0.36 0.58 0.37 0.31 0.90 4.7 2.2 3.0 2.0 2.2 0.37 3.6 

 5 0.29 0.63 0.38 0.33 0.72 2.1 1.8 2.1 2.4 1.4 0.22 1.2 

 6 0.55 0.35 0.36 0.44 0.64 1.6 1.6 1.9 7.1 1.1 0.14 0.37

 7 0.44 0.18 0.40 1.1 0.62 1.5 1.5 1.9 22 0.52 0.05 0.18

 8 0.27 0.26 0.44 0.93 0.64 1.3 1.5 1.9 7.2 0.48 0.02 0.12

 9 0.22 0.27 0.98 0.83 0.62 1.2 1.5 1.8 3.8 0.64 0.01 0.10

 10 0.26 0.29 1.6 0.84 0.65 1.8 1.9 1.8 2.8 0.73 0.01 0.08

 11 0.28 0.31 1.6 1.2 1.5 1.3 1.8 1.6 2.1 0.48 0.01 0.12

 12 0.29 0.27 0.79 1.4 1.3 0.97 1.6 3.4 1.8 0.41 0.01 0.05

 13 0.30 0.26 0.59 1.3 0.94 0.91 1.8 14 1.9 0.32 0.01 0.02

 14 0.36 0.33 0.59 1.0 0.67 1.3 2.1 23 3.8 0.25 0.0 0.02

 15 1.0 0.43 0.74 0.97 0.60 1.5 2.2 7.3 6.3 0.22 0.0 0.46

 16 0.83 0.45 0.63 0.87 0.61 1.5 1.9 5.0 4.2 0.11 0.02 2.2 

 17 0.71 0.44 0.66 0.84 0.67 1.4 1.6 4.3 3.2 0.05 0.01 1.2 

 18 0.75 0.44 1.6 0.79 0.61 1.8 1.6 15 2.2 0.03 0.01 0.48

 19 0.55 0.51 1.6 0.77 0.54 2.1 1.6 9.5 1.7 0.04 0.01 0.25

 20 0.62 0.60 1.1 0.85 0.54 5.2 1.7 5.5 1.5 0.08 0.01 0.17

 21 0.85 0.54 0.60 0.81 0.83 7.4 1.5 4.8 1.3 0.05 0.0 0.15

 22 0.91 0.53 0.36 0.78 0.89 3.3 1.9 4.4 1.2 0.03 0.01 0.13

 23 0.84 0.58 0.31 0.84 0.70 2.2 1.9 4.1 1.1 0.52 0.01 0.12

 24 0.71 0.64 1.8 1.4 0.56 2.0 1.1 3.8 0.96 0.86 0.01 0.08

 25 0.73 0.96 2.3 1.6 0.50 1.8 0.75 3.5 0.85 0.43 0.01 0.06

 26 0.59 1.2 1.3 1.3 0.48 1.8 1.2 3.3 1.3 0.18 0.01 0.14

 27 0.49 0.86 0.81 1.0 0.52 4.4 0.70 3.4 2.2 0.14 0.01 2.1 

 28 0.41 0.80 0.64 0.87 0.50 3.2 4.1 3.9 2.5 0.13 0.01 2.1 

 29 0.34 0.74 0.56 0.80 --- 2.2 10 3.7 1.7 0.11 0.58 0.97

 30 0.33 0.67 0.48 0.81 --- 1.9 3.7 3.2 1.3 0.06 0.39 0.55

 31 0.30 --- 0.41 0.87 --- 1.9 --- 3.0 --- 0.04 0.31 ---
60 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Appendix 1. Records from two U.S. Geological Survey streamflow-gaging stations, Greenwich, 

Connecticut, March 2001-September 2002.—Continued

Station 01211110 Unnamed Tributary To Greenwich Creek Nr Cos Cob 

STATION NUMBER 01211110 UNNAMED TRIBUTARY TO GREENWICH CREEK NR COS COB STREAM SOURCE AGENCY USGS STATE 09 COUNTY 001

LATITUDE 410233.5 LONGITUDE 0733659.3 NAD27 DRAINAGE AREA 2.19* CONTRIBUTING DRAINAGE AREA 2.19 DATUM 65 NGVD29

DISCHARGE, in CFS, WATER YEAR OCTOBER 2000 TO SEPTEMBER 2001. DATA COLLECTION BEGAN MARCH 2001.

DAILY MEAN VALUES

 DAY OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP

 1 --- --- --- --- --- 4.7 13 2.2 3.2 e1.3 0.10 0.75

 2 --- --- --- --- --- 4.3 9.6 2.2 20 e1.7 0.07 0.56

 3 --- --- --- --- --- 3.9 7.8 1.9 12 e1.1 0.04 0.36

 4 --- --- --- --- --- 3.7 6.6 2.2 4.8 e1.0 e0.25 0.30

 5 --- --- --- --- --- 4.3 5.9 1.5 2.8 e1.8 e0.25 0.33

 6 --- --- --- --- --- 4.5 5.6 1.3 2.4 e2.6 e0.25 0.29

 7 --- --- --- --- --- 4.0 5.3 1.2 2.2 1.0 0.31 0.26

 8 --- --- --- --- --- 4.0 4.8 1.1 2.5 e1.4 e0.23 0.21

 9 --- --- --- --- --- 4.8 10 0.97 1.6 e1.5 0.18 0.18

 10 --- --- --- --- --- 5.1 17 0.99 1.3 e1.1 e0.17 0.42

 11 --- --- --- --- --- 5.2 8.1 0.82 1.0 e1.1 e0.19 0.68

 12 --- --- --- --- --- 5.3 8.4 0.76 1.2 e0.94 1.6 0.36

 13 --- --- --- --- --- 26 7.0 0.85 1.2 e0.73 5.9 0.17

 14 --- --- --- --- --- 15 5.9 0.79 1.2 e0.60 2.5 6.2 

 15 --- --- --- --- --- 9.5 5.3 0.53 1.2 e0.52 1.5 2.0 

 16 --- --- --- --- --- 7.5 5.0 0.50 1.2 e0.49 1.3 0.98

 17 --- --- --- --- --- 8.2 4.8 0.59 23 e0.44 1.3 0.66

 18 --- --- --- --- --- 7.1 4.4 0.55 10 e0.44 1.3 0.49

 19 --- --- --- --- --- 5.1 3.8 0.60 5.2 e0.47 1.0 0.39

 20 --- --- --- --- --- 4.1 3.7 0.58 e3.4 e0.41 25 1.3 

 21 --- --- --- --- --- 6.6 3.8 1.1 e2.7 e0.39 12 19 

 22 --- --- --- --- --- 28 5.2 6.1 e2.3 e0.39 2.8 5.0 

 23 --- --- --- --- --- 26 3.8 8.6 e2.6 e0.33 4.0 3.0 

 24 --- --- --- --- --- 11 3.3 5.7 e3.1 e0.36 4.8 2.0 

 25 --- --- --- --- --- 7.3 2.9 4.0 e2.2 e0.31 1.8 9.9 

 26 --- --- --- --- --- 6.2 2.8 5.1 1.5 e0.39 1.2 4.0 

 27 --- --- --- --- --- 5.2 2.7 4.8 e1.2 e0.39 1.8 2.5 

 28 --- --- --- --- --- 4.3 2.5 4.0 e1.0 0.38 1.6 2.0 

 29 --- --- --- --- --- 3.9 2.2 3.4 e0.84 0.18 1.1 1.6 

 30 --- --- --- --- --- 41 2.3 3.1 e0.81 0.18 0.77 1.5 

 31 --- --- --- --- --- 24 --- 3.0 --- 0.10 0.68 ---

e Estimated

2001

MAM J J A SOND

2002

J FM A M J J A S

0

5

10

15

20

25

30

35

40

45

MEAN DAILY DISCHARGE, IN CUBIC FEET PER SECOND

01211110
Appendixes 61

Appendix 1. Records from two U.S. Geological Survey streamflow-gaging stations, Greenwich, 

Connecticut, March 2001-September 2002.—Continued

Station 01211110 Unnamed Tributary To Greenwich Creek Nr Cos Cob—Continued

STATION NUMBER 01211110 UNNAMED TRIBUTARY TO GREENWICH CREEK NR COS COB STREAM SOURCE AGENCY USGS STATE 09 COUNTY 001

LATITUDE 410233.5 LONGITUDE 0733659.3 NAD27 DRAINAGE AREA 2.19* CONTRIBUTING DRAINAGE AREA 2.19 DATUM 65 NGVD29

DISCHARGE, in CFS, WATER YEAR OCTOBER 2001 TO SEPTEMBER 2002 

DAILY MEAN VALUES 

 DAY OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP

 1 3.7 0.88 0.92 0.88 1.8 0.70 4.3 6.8 e4.0 e2.6 e0.92 0.93

 2 2.1 1.1 0.68 0.82 1.8 0.82 3.1 8.5 e3.4 e2.1 e1.1 25 

 3 1.6 1.0 0.62 0.80 1.4 13 3.2 7.6 e2.9 e2.3 e1.5 7.6 

 4 1.2 1.3 0.66 0.85 1.4 4.9 3.4 4.8 e2.9 e2.8 e1.2 17 

 5 0.94 1.2 0.67 0.72 1.2 3.2 2.7 4.1 e3.5 e2.2 e0.70 5.5 

 6 0.93 1.1 0.72 1.7 1.1 2.6 2.4 3.4 e9.4 e1.7 e0.40 3.6 

 7 0.81 1.0 0.89 4.5 1.0 2.2 2.1 3.2 e24 e1.4 0.02 3.0 

 8 0.64 1.8 1.2 1.9 1.0 2.0 2.1 2.8 8.9 e1.1 0.02 2.2 

 9 0.63 1.6 2.9 1.6 0.91 1.9 2.3 2.4 4.9 e1.3 0.02 2.4 

 10 0.56 1.5 1.6 1.7 1.5 1.9 3.2 2.3 3.6 e1.3 0.02 2.1 

 11 0.55 1.3 1.0 2.2 4.1 1.5 2.2 1.9 2.9 e1.1 0.02 1.9 

 12 0.69 1.2 --- 2.0 2.1 1.5 2.1 5.0 2.7 e0.92 0.02 1.4 

 13 0.77 1.3 --- 1.8 1.8 1.7 2.3 17 2.3 e0.85 0.02 1.5 

 14 0.67 1.4 --- 1.5 1.4 1.7 3.1 24 5.6 e0.77 0.01 1.9 

 15 2.1 1.5 1.2 1.4 1.3 1.5 2.8 7.9 5.9 e0.77 0.01 3.2 

 16 1.1 1.5 0.87 1.3 1.4 1.7 2.1 5.1 6.2 e0.70 0.05 9.5 

 17 0.95 1.5 0.95 1.3 1.4 1.2 1.7 4.3 5.1 e0.63 0.27 4.5 

 18 0.75 1.2 3.9 1.2 1.3 2.5 1.2 23 3.5 e0.63 0.06 2.9 

 19 0.57 1.3 2.1 1.1 1.0 2.7 1.5 11 2.8 e0.63 0.04 2.3 

 20 0.59 1.3 1.5 1.3 1.1 8.1 1.4 e7.0 2.4 e0.55 0.27 2.7 

 21 0.56 1.3 1.2 1.3 1.6 7.8 1.2 e5.6 2.2 e0.48 0.08 1.8 

 22 0.71 1.2 1.0 1.3 1.2 4.2 2.2 e5.0 1.8 e0.55 0.10 1.3 

 23 0.78 1.0 0.94 1.4 1.0 3.2 1.9 e4.6 1.8 e1.4 0.27 1.1 

 24 0.78 1.4 5.6 2.9 1.1 2.9 1.6 e4.3 1.7 e1.6 0.43 0.84

 25 1.1 2.3 2.6 2.6 0.94 2.5 3.3 e4.0 1.7 e1.4 0.31 0.84

 26 0.98 3.4 1.8 1.9 0.95 3.0 3.1 e3.8 2.7 e0.77 0.11 0.95

 27 0.91 1.3 1.6 1.8 0.93 9.7 1.9 e4.0 5.0 e0.77 0.04 6.2 

 28 0.82 0.98 1.3 1.6 0.78 4.8 18 e4.4 4.6 e0.77 0.04 3.4 

 29 0.53 0.97 1.2 1.5 --- 3.8 20 e4.3 e3.8 e0.85 5.3 1.9 

 30 0.60 1.0 1.1 1.4 --- 3.4 8.1 e3.8 e3.0 e0.85 1.7 1.3 

 31 0.72 --- 0.96 1.2 --- 3.5 --- e3.8 --- e0.85 0.87 ---

e Estimated
62 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Appendix 2. Estimated average daily ground-water use for the Greenwich area, Connecticut.

[gal/d, gallon per day]

Zone

Average daily water 

use Greenwich and 

Stamford Conn.

(gal/d)

Lower boundary 

of the 90-percent 

confidence 

interval

(gal/d)

Upper boundary 

of the 90-percent 

confidence 

interval

(gal/d)

Average daily 

water use in 

Westchester 

County, N.Y.

(gal/d)

Estimated total

(gal/d)

1 41,000 33,000 50,000 6,500 47,000

2 67,000 50,000 88,000 0 67,000

3 51,000 37,000 67,000 0 51,000

4 110,000 95,000 130,000 0 110,000

5 5,000 2,800 8,000 0 5,000

6 40,000 30,000 52,000 49,000 88,000

7 78,000 58,000 100,000 0 78,000

8 110,000 93,000 140,000 0 110,000

9 19,000 12,000 26,000 9,500 28,000

10 140,000 110,000 170,000 0 140,000

11 35,000 26,000 45,000 0 35,000

12 120,000 97,000 150,000 14,000 140,000

13 99,000 78,000 120,000 17,000 120,000

14 76,000 63,000 91,000 0 76,000

15 9,900 6,000 15,000 0 9,900

16 100,000 86,000 130,000 0 100,000

17 37,000 27,000 49,000 0 37,000

18 1,700 600 3,600 0 1,700

19 240,000 210,000 280,000 0 240,000

20 96,000 84,000 110,000 0 96,000

21 16,000 11,000 23,000 0 16,000

22 64,000 53,000 77,000 0 64,000

23 0 0 0 0 0

24 41,000 32,000 51,000 19,000 60,000

25 65,000 28,000 120,000 0 65,000

26 67,000 56,000 79,000 0 67,000

27 59,000 49,000 69,000 0 59,000

28 120,000 100,000 130,000 0 120,000

29 25,000 19,000 32,000 0 25,000

30 48,000 41,000 56,000 0 48,000

31 23,000 18,000 30,000 0 23,000

32 2,900 1,000 6,100 0 2,900
Appendixes 63

Appendix 3. Estimated average daily summer ground-water use for the Greenwich area, 

Connecticut.

[gal/d, gallon per day]

Zone

Average daily 

summer water use 

Greenwich and 

Stamford Conn., in 

gal/d

Lower boundary 

of the 90-percent 

confidence 

interval

(gal/d)

Upper boundary 

of the 90-percent 

confidence 

interval

(gal/d)

Average daily 

summer water 

use in Westchester County, N.Y.

(gal/d)

Estimated total

(gal/d)

1 53,000 41,000 67,000 7,700 61,000

2 95,000 65,000 130,000 0 95,000

3 71,000 48,000 100,000 0 71,000

4 150,000 120,000 180,000 0 150,000

5 6,400 3,300 11,000 0 6,400

6 55,000 39,000 75,000 59,000 110,000

7 110,000 78,000 160,000 0 110,000

8 160,000 120,000 200,000 0 160,000

9 25,000 16,000 38,000 11,000 36,000

10 190,000 150,000 250,000 0 190,000

11 46,000 33,000 63,000 0 46,000

12 170,000 130,000 230,000 18,000 190,000

13 140,000 110,000 190,000 22,000 170,000

14 100,000 81,000 130,000 0 100,000

15 13,000 7,400 21,000 0 13,000

16 150,000 120,000 180,000 0 150,000

17 51,000 35,000 70,000 0 51,000

18 2,200 680 4,900 0 2,200

19 330,000 270,000 410,000 0 330,000

20 120,000 110,000 150,000 0 120,000

21 21,000 13,000 32,000 0 21,000

22 85,000 67,000 100,000 0 85,000

23 0 0 0 0 0

24 52,000 37,000 70,000 23,000 75,000

25 110,000 35,000 240,000 0 110,000

26 87,000 72,000 100,000 0 87,000

27 75,000 61,000 91,000 0 75,000

28 150,000 130,000 170,000 0 150,000

29 32,000 24,000 41,000 0 32,000

30 60,000 50,000 72,000 0 60,000

31 30,000 22,000 39,000 0 30,000

32 4,000 1,200 8,900 0 4,000
64 Water Use, Ground-Water Recharge and Availability, and Quality of Water in the Greenwich Area, 2000–2002

Appendix 4. Estimated average daily winter ground-water use for the Greenwich area, 

Connecticut.

[gal/d, gallon per day]

Zone

Average daily 

winter water use 

Greenwich and 

Stamford Conn. 

(gal/d)

Lower boundary 

of the 90-percent 

confidence 

interval

(gal/d)

Upper boundary 

of the 90-percent 

confidence

interval

(gal/d)

Average winter 

season water use 

in Westchester 

County, N.Y.

(gal/d)

Estimated total

(gal/d)

1 31,000 25,000 37,000 4,900 36,000

2 48,000 38,000 59,000 0 48,000

3 35,000 27,000 44,000 0 35,000

4 82,000 71,000 94,000 0 82,000

5 4,200 2,500 6,400 0 4,200

6 27,000 21,000 35,000 37,000 64,000

7 51,000 41,000 63,000 0 51,000

8 80,000 67,000 94,000 0 80,000

9 13,000 9,400 18,000 7,300 20,000

10 89,000 76,000 100,000 0 89,000

11 24,000 19,000 30,000 0 24,000

12 80,000 67,000 94,000 10,000 90,000

13 67,000 54,000 81,000 13,000 80,000

14 54,000 46,000 63,000 0 54,000

15 6,900 4,500 10,000 0 6,900

16 73,000 62,000 85,000 0 73,000

17 26,000 20,000 33,000 0 26,000

18 1,200 470 2,300 0 1,200

19 170,000 150,000 190,000 0 170,000

20 72,000 64,000 81,000 0 72,000

21 11,000 7,800 15,000 0 11,000

22 45,000 38,000 53,000 0 45,000

23 0 0 0 0 0

24 29,000 24,000 36,000 15,000 44,000

25 40,000 22,000 65,000 0 40,000

26 54,000 46,000 63,000 0 54,000

27 44,000 38,000 51,000 0 44,000

28 90,000 82,000 100,000 0 90,000

29 19,000 15,000 23,000 0 19,000

30 38,000 32,000 43,000 0 38,000

31 17,000 14,000 21,000 0 17,000

32 2,000 800 4,100 0 2,000