Prepared in cooperation with the New York City Department of Environmental Protection

Geophysical and Hydrologic Analysis of an Earthen 

Dam Site in Southern Westchester County, New York

Scientific Investigations Report 2012–5247

U.S. Department of the Interior

U.S. Geological Survey
Cover. 1. A panoramic view of Hillview Reservoir

2. U.S. Geological Survey scientists deploying an accelerated weight drop for a seismic survey in Westchester County, New York

3. Discharge from stonewall seep along Hillview Avenue, Westchester County, New York

4. U.S. Geological Survey scientist deploying a slug for a single-well aquifer test in Westchester County, New York

5. Discharge from instrumented hillside seep in Westchester County, New York

6. U.S. Geological Survey scientist evacuating water from a well in Westchester County, New York

7. U.S. Geological Survey scientist measuring groundwater levels in Westchester County, New York

1

2 3 4

5 6 7
Geophysical and Hydrologic Analysis of an 

Earthen Dam Site in Southern Westchester 

County, New York

By Anthony Chu, Frederick Stumm, Peter K. Joesten, and Michael L. Noll

Prepared in cooperation with the 

New York City Department of Environmental Protection 

Scientific Investigations Report 2012–5247

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Chu, Anthony, Stumm, Frederick, Joesten, P.K., and Noll, M.L., 2013, Geophysical and hydrologic analysis of an 

earthen dam site in southern Westchester County, New York: U.S. Geological Survey Scientific Investigations Report 

2012–5247, 64 p. (Also available at http://pubs.usgs.gov/sir/2012/5247/.)
iii

Acknowledgements

The authors thank Matthew Osit, Richard Zunino, George Schmitt, and Masud Ahmed of the 

New York City Department of Environmental Protection for their technical assistance and 

logistical support. The authors also thank William Capurso, Michael Como, Irene Fisher, Robert 

Pearsall, and Agnes Cwalina of the U.S. Geological Survey for their field support.
This page has been left blank intentionally.
v

Contents

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

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

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

Study Area..............................................................................................................................................4

Hydrogeologic Setting .........................................................................................................................4

Methods of Investigation..............................................................................................................................5

Water-Quality Sample Collection.......................................................................................................5

Single Well Slug Test Methods...........................................................................................................5

Surface Geophysics .............................................................................................................................5

Hydrologic Measurements................................................................................................................10

Water Quality................................................................................................................................................10

Hydrogeology................................................................................................................................................10

Reservoir Tidal Effects .......................................................................................................................11

Precipitation and Groundwater Levels ...........................................................................................11

Slug Test Analysis...............................................................................................................................11

Shear Wave Velocity and Resistivity Survey Observations.........................................................13

Hydrologic Conditions........................................................................................................................20

Groundwater Levels ..................................................................................................................20

Shallow Saturated Zone ..................................................................................................20

Deep Saturated Zone .......................................................................................................23

Temperature as a Tracer ..........................................................................................................23

Embankment Seepage .......................................................................................................................26

Seep A ................................................................................................................................26

Seep B ................................................................................................................................27

Seep C ................................................................................................................................30

Seep D ................................................................................................................................30

Seep E ................................................................................................................................30

Summary........................................................................................................................................................30

References Cited..........................................................................................................................................32

Appendix 1.—Water Elevation and Precipitation at Selected Wells at the Hillview Reservoir,

City of Yonkers, Westchester County, New York.......................................................................55

Figures

1. Map showing the location of the Hillview Reservoir study area and selected

observation wells and boreholes, City of Yonkers, Westchester County, New York.........2

2. Map showing the location of wells and seeps on the southern embankment at the

Hillview Reservoir, City of Yonkers, Westchester County, New York ...................................3

3. Graphs showing response of water levels in nearby wells A, TB–2S; B, TB–17S; 

C, TB–2D; and D, TB–17D to East Basin water levels in the Hillview Reservoir, City of

Yonkers, Westchester County, New York, in 2007 .................................................................12
vi

4. Graph showing annual precipitation as recorded at Central Park, New York County,

New York, during 1870–2007 .....................................................................................................13

5. Map showing the distribution of hydraulic conductivity estimated from slug test

results of wells on the southern embankment at Hillview Reservoir, City of Yonkers,

Westchester County, New York................................................................................................14

6. Map showing locations of geophysical surveys on the southern embankment at the

Hillview Reservoir, City of Yonkers, Westchester County, New York .................................15

7. Cross-sections showing two-dimensional resistivity along A, A–A′, B, B–B′, C, C–C′, 

D, D–D′, and E, E–E′ at the Hillview Reservoir, City of Yonkers, Westchester County, 

New York ......................................................................................................................................16

8. Cross-section showing two-dimensional shear wave velocity along F–F′ at 

the Hillview Reservoir, City of Yonkers, Westchester County, New York ..........................18

9. Cross-section showing two-dimensional shear wave velocity along G–G′ at 

the Hillview Reservoir, City of Yonkers, Westchester County, New York ..........................19

10. Map showing interpreted elevation of a subsurface clay-rich unit delineated from

two-dimensional resistivity and shear wave seismic surveys at the Hillview

Reservoir, City of Yonkers, Westchester County, New York ................................................21

11. Graphs showing elevation of water levels in wells A, TB–17S, B, MR–100P, 

C, TB–17D, and D, MR–100PA in relation to daily precipitation at the Hillview 

Reservoir, City of Yonkers, Westchester County, New York, between April 1, 2005 

and March 1, 2008.......................................................................................................................22

12. Map showing water-level elevation in the shallow water-bearing unit at the

southern embankment of the Hillview Reservoir, City of Yonkers, Westchester

County, New York, in September 2007.....................................................................................24

13. Map showing water-level elevation in the deep water-bearing unit at the southern

embankment of the Hillview Reservoir, City of Yonkers, Westchester County, New

York, in September 2007.............................................................................................................25

14 Graph showing water temperature in the East Basin and wells TB–1S, TB–1D, 

TB–2D, and MR–100PA at the Hillview Reservoir, City of Yonkers, Westchester 

County, New York, between April 1, 2005 and March 1, 2008..............................................27

15. Chart showing discharge rates of seeps measured on the southern embankment

at the Hillview Reservoir, City of Yonkers, Westchester County, New York, and daily

precipitation between April 1, 2005, and March 1, 2008, recorded at Central Park,

New York County, New York .....................................................................................................28

16. Chart showing groundwater levels in well TB–17S and discharge rates at seep B at

the Hillview Reservoir, City of Yonkers, Westchester County, New York, between

June 1, 2007 and March 1, 2008................................................................................................29

Tables

1. Site information for A, groundwater wells and B, reservoir basins and seeps at the

Hillview Reservoir, City of Yonkers, Westchester County, New York ...................................6

2. Concentrations of detected constituents in water samples collected from the 

Hillview Reservoir, City of Yonkers, Westchester County, New York, in 2006...................34 

3. Hydrologic properties of selected wells at the Hillview Reservoir in the City of

Yonkers, Westchester County, New York..................................................................................9

4. Suspended sediment in surface-water samples from seep B at the Hillview

Reservoir, City of Yonkers, Westchester County, New York ................................................29
vii

Conversion Factors and Datum

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

Area

acre 0.004047 square kilometer (km2

)

Volume

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

)

Flow rate

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

Hydraulic conductivity

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

Transmissivity*

foot squared per day (ft2

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

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

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

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

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

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

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

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

or micrograms per liter (μg/L).

Abbreviations

2D two dimensional

BLS below land surface

CCA chromated copper arsenate

MASW multichannel analysis of surface-wave

MCL maximum contaminant level

NYC New York City

NYSDEC New York State Department of Environmental Conservation

NYCDEP New York City Department of Environmental Protection

USGS U.S. Geological Survey
This page has been left blank intentionally.
Geophysical and Hydrologic Analysis of an Earthen Dam 

Site in Southern Westchester County, New York

By Anthony Chu, Frederick Stumm, Peter K. Joesten, and Michael L. Noll

Abstract

Ninety percent of the drinking water for New York City 

passes through the Hillview Reservoir facility in the City of 

Yonkers, Westchester County, New York. In the past, several 

seeps located downslope from the reservoir have flowed 

out from the side of the steepest slope at the southern end 

of the earthen embankment. One seep that has been flowing 

continuously was discovered during an inspection of the 

embankment in 1999. Efforts were made in 2001 to locate 

the potential sources of the continuous flowing seep. In 2005, 

the U.S. Geological Survey, in cooperation with the New 

York City Department of Environmental Protection, began a 

cooperative study to investigate the relevant hydrogeologic 

framework to characterize the local groundwater-flow system 

and to determine possible sources of the seeps. The two 

agencies used hydrologic and surface geophysical techniques 

to assess the earthen embankment of the Hillview Reservoir. 

Between April 1, 2005 and March 1, 2008, water levels were 

measured manually each month at 46 wells surrounding the 

reservoir, and flow was measured monthly at three of the 

five seeps on the embankment. Water levels were measured 

hourly in the East Basin of the reservoir, at 24 of 46 wells, and 

discharge was measured hourly at two of the five seeps. Slug 

tests were performed at 16 wells to determine the hydraulic 

conductivity of the geologic material surrounding the screened 

zone. Estimated hydraulic conductivities for 25 wells on the 

southern embankment ranged from 0.0063 to 1.2 feet per 

day and averaged 0.17 foot per day. The two-dimensional 

resistivity surveys indicate a subsurface mound of electrically 

conductive material (low-resistivity zone) beneath the terrace 

area (top of dam) surrounding the reservoir with a distinct 

elevation increase closer to the crest. Two-dimensional shear 

wave velocity surveys indicate a similar structure of the high 

shear wave velocity materials (high-velocity zone), increasing 

in elevation toward the crest and decreasing toward the 

reservoir and toward the northern part of the study area. Waterquality samples collected from 12 wells, downtake chamber 

1 of the reservoir, and two seeps detected the presence of 

arsenic, toluene, and two trihalomethanes. Water-quality 

samples collected at the two seeps detected fluoride, indicating 

a connection with reservoir water.

Shallow wells on the southern embankment exhibited 

the largest seasonal water-level fluctuations ranging between 

6 feet and 12 feet. The embankment is constructed from 

reworked low-permeability glacial deposits at the site. Waterlevel responses in observation wells within the embankment 

indicate that there is a shallow (approximately the upper 

45 feet of the embankment) and a deep water-bearing unit 

within the embankment with a large downward vertical 

gradient between the shallow and deep water-bearing units. 

Precipitation strongly affected water levels in shallow wells, 

whereas the basin appears to be the main control on water 

levels in the deep wells. Seeps on the embankment slope 

appear to be caused by above-average precipitation that 

increases water levels in the shallow water-bearing unit, but 

does not easily recharge the deep water-bearing unit. Based 

on the data that have been analyzed, source water to the seeps 

appears to be primarily groundwater and, to a lesser extent, 

water from the East Basin of the reservoir.

Introduction

The Hillview Reservoir in Yonkers, southern Westchester 

County, New York, (fig. 1) was constructed between 1913 

and 1916, contains more than 900 million gallons of water, 

and maintains a hydrostatic head of about 293 feet (ft) on the 

New York City (NYC) water-supply distribution system to the 

south. Ninety percent of NYC’s drinking water passes through 

the Hillview Reservoir facility from the Kensico Reservoir, 

which is fed by the Delaware and Catskill aqueducts in 

upstate New York. Water is chlorinated at the reservoir and is 

piped from the southern end of the reservoir for distribution 

to users in NYC. The concrete lined reservoir, which has 

an area of about 90 acres, is about equally divided into the 

East Basin and West Basin by a concrete dividing wall, and 

has operated continuously since the first water tunnel was 

completed in 1917. A 14-ft-diameter concrete reinforced 

conduit (connecting conduit) hydraulically connects downtake 

chamber 1 to downtake chamber 2 (fig. 2; Malcolm Pirnie, 

Inc. and TAMS Consultants, Inc., 2002).
2 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

EAST

 BASIN 

YONKERS 

 RACEWAY

73°52'26" 73°51'52"

40°54'30"

40°54'55"

Base from New York City Department of Environmental Protection, digital data

Projection New York State Plane fipszone 3101; Datum NAD 1983

NEW YORK STATE THRUWAY

HILLVIEW AVENUE

KIMBALL AVENUE

HILLVIEW AVENUE

CENTRAL PARK AVENUE

MB–4W

TB–10

X

PD

MR–131

DT–2

Z–PD

Y–PD

110–P

109–P

106–P

105–P

104–P

HESF–8D

PA

Z–PA

Y–PA

106–PA

104–PA

HESF–8S

WE–5079

WE–5078

WESTCHESTER

 COUNTY

 NEW

 JERSEY

Central Hudson River

Park

City of 

Study area Yonkers

BRONX

COUNTY

KINGS

COUNTY

QUEENS

COUNTY

NEW YORK

COUNTY

LONG ISLAND SOUND

WEST

 BASIN Basin

dividing

wall

Edge of roadway

0 200 FEET

0 100 200 METERS

WE–1256

Buildings

Observation well or 

borehole and identifier

Shaded region indicates 

extent of earthen dam

TB–10

EXPLANATION

Roads

Path

 EXTENT OF FIGURE 2

Figure 1. Location of the Hillview Reservoir study area and selected observation wells and boreholes, City of Yonkers, 

Westchester County, New York.
Introduction 3

Base from New York City Department of Environmental Protection

Projection New York State Plane fipszone; Datum NAD 1983

0 50 FEET

0 25 50 METERS

73°52'12" 73°52'01"

40°54'25"

40°54'35"

HILLVIEW AVE

KIMBALL AVE

HILLVIEW AVE

Downtake 

chamber

1

Basin

dividing

wall

Control

chamber

Downtake

chamber

2

Connecting

conduit

WEST

 BASIN 

EAST

 BASIN 

Seep B

Seep C

Seep E

Seep A

Seep D B–4

B–3P

MB–3

MB–2

MR–121

B–5A

MB–5

MB–4W

MB–1W

TB–9

TB–8

TB–16

TB–15

TB–13

TB–12

TB–10

TB–5D

TB–4D

TB–3D

TB–2D

TB–1D

TB–18D

TB–17D

TB–14D

TB–11B

MR–123P MR–100P

TB–5S

TB–4S

TB–3S

TB–2S

TB–1S

TB–18S

TB–17S

TB–14S

MR–123PA

MR–100PA

CMB–1

CMB–2W

EXPLANATION

Seep E

TB–10

Seep A

Observation well and identifier

Seep area and identifier

Seep area with continuous data recorder

Basin gage with continuous data recorder

Roads

Path

Parking area

Figure 2. Location of wells and seeps on the southern embankment at the Hillview Reservoir, City of Yonkers, Westchester 

County, New York.
4 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

The earthen embankment comprises low-permeability 

glacial clays that were excavated from the site and rest 

on a veneer of low-permeability glacial deposits that 

overlie crystalline bedrock. The earthen embankment was 

subsequently modified by other construction and maintenance 

projects near downtake, uptake, and control chambers; 

connecting shafts; connecting conduits; the reservoir dividing 

wall; and the bypass tunnel.

Groundwater generally flows outward from the reservoir 

toward the surrounding glacial drift. In the past, several seeps 

located downslope from the reservoir have flowed out from 

the side of the steepest slope at the southern end of the earthen 

embankment. One seep that flowed continuously during the 

study was discovered during an inspection of the embankment 

in 1999 (George Schmitt, New York City Department of 

Environmental Protection, written commun., 2007). In 2001, 

the New York City Department of Environmental Protection 

(NYCDEP) drilled 25 wells at the southern end of the 

reservoir, adding to the 32 wells previously installed around 

the reservoir (for a total of 57 wells around the reservoir) in an 

effort to locate the potential sources of the continuous flowing 

seep. The NYCDEP approach included taking periodic depthto-water measurements and sampling reservoir and spring 

water for major ions. The results were inconclusive (Malcolm 

Pirnie, Inc. and TAMS Consultants, Inc., 2002).

In 2005, the U.S. Geological Survey (USGS) began 

a cooperative study with the NYCDEP to investigate the 

relevant hydrogeologic framework to characterize the local 

groundwater-flow system and to determine possible sources of 

the seeps. The USGS was able to access 46 of the 57 wells for 

monitoring water levels, temperature, and water quality. The 

principal objectives of this study were to (1) characterize the 

distribution of groundwater levels near the reservoir to obtain 

a better understanding of the embankment groundwater-flow 

system; (2) characterize possible flowpaths of water to the 

seeps; and (3) monitor the long- and short-term changes in 

groundwater levels and seep discharge within the study area.

Purpose and Scope

This report presents a summary of the hydrologic 

conditions in the southern part of the Hillview Reservoir, 

and hydrologic, surface geophysical, and basic water-quality 

data collected by the USGS to investigate the hydrogeologic 

framework of the southern embankment to determine 

the possible sources of seeps. Selected hydrographs of 

seasonal water-level elevations and distribution of hydraulic 

conductivities are shown in illustrations.

Study Area

The Hillview Reservoir in southern Westchester County 

in the City of Yonkers was put into service in 1917 when 

the first water tunnel was completed. The reservoir has a 

surface area of more than 90 acres and contains more than 

900 million gallons of water (fig. 1). The reservoir is bounded 

to the north and west by the New York State Thruway, to the 

north and east by the Yonkers Raceway and residences, to the 

south and east by residences along Kimball Avenue, and to 

the south and west by residences along Hillview Avenue and a 

business district.

Hydrogeologic Setting

The Hillview Reservoir study area is underlain by 

unconsolidated Holocene deposits, artificial fill (modified 

glacial clays), and glacial-drift deposits of Pleistocene age. 

These sediments consist of boulders, gravel, sand, silt, and 

clay, which are underlain by crystalline bedrock. The bedrock 

is fractured and permeable where transmissive fractures 

or faults exist. In general, the bedrock forms a relatively 

impermeable base of the groundwater-flow system at the site.

The Hillview Reservoir earthen dam consists of an 

assemblage of reworked Pleistocene materials, manmade or 

artificial fill, and an underlying layer of Pleistocene glacial 

till and drift deposits. These unconsolidated deposits rest 

upon bedrock. The groundwater levels within the earthen 

embankment at the Hillview Reservoir are strongly affected 

by recharge from precipitation and from the reservoir to the 

unconsolidated embankment. The reservoir water levels 

fluctuate as a result of increased or decreased water demand 

during daily cycles. This cyclic demand produces an artificial 

“tidal” load on the surrounding embankment materials and 

local groundwater-flow system. References in this report to 

tidal effects on groundwater levels refer to this artificial tidal 

loading created by the rising and falling water levels within 

the reservoir basins.

Southern Westchester County is underlain by a highgrade metamorphic bedrock sequence consisting of gneiss, 

schistose-gneiss interlayered with granite, and marble 

(Asselstine and Grossman, 1955; Baskerville, 1982, 1992). 

The bedrock in southern Westchester County consists of a 

series of northeast-trending ridges and valleys. The ridges 

generally are underlain by gneiss and granite (Asselstine and 

Grossman, 1955; Baskerville, 1982). Hillview Reservoir is 

located on a ridge that is underlain by gneiss that is likely the 

Yonkers Gneiss or Fordham Gneiss. The bedrock contains 

many fractures, some of which are transmissive. The gneiss is 

considered a poor-to-moderate groundwater producer, whereas 

the marble is the most productive bedrock in Westchester 

County (Asselstine and Grossman, 1955). Depth to bedrock 

ranges from less than 1 ft below land surface (BLS) to 

125 ft BLS within the southern part of Westchester County. 

The range in thickness of the natural till at the reservoir 

was estimated to be between 45 and 70 ft (Malcolm Pirnie, 

Inc., and TAMS Consultants, Inc., 2002); however, records 

of wells installed along the lower lying and northern areas 

of the reservoir indicate depths to bedrock are about 20 ft. 

Records for one well, WE–1256, to the north of the reservoir 
Methods of Investigation 5

at Yonkers Raceway indicated a depth to bedrock of 24 ft 

(Asselstine and Grossman, 1955).

At least two groundwater-flow zones—one shallow and 

the other deep—appear to exist at the study area. Wells in 

the shallow flow zone have the highest water levels, are only 

slightly affected by reservoir tidal influences, and seem to 

respond to substantial precipitation recharge. In contrast, wells 

in the deep flow zone have lower water-level elevations, are 

highly affected by reservoir tidal influences, and only slightly 

respond to precipitation-induced recharge.

Methods of Investigation

The NYCDEP installed 57 wells in the earthen 

embankment surrounding Hillview Reservoir before this 

investigation. Only 46 of the original 57 wells were available 

for use in this study; the remaining 13 had been damaged 

during maintenance. At least five separate seeps located 

downslope from the reservoir have been documented at the 

Hillview Reservoir. The five seeps are identified as seeps A, B, 

C, D, and E. Seeps A, B, and E have been flowing the longest 

at the site (fig. 2).

Hydrologic data were collected from the 46 groundwater 

observation wells, a surface-water-level gage programmed 

to measure water-level and temperature fluctuations in the 

East Basin of the reservoir, and the four seeps that flowed 

during the study (table 1); seep D was not flowing during the 

time of the study. Surface-geophysical data were collected 

along the roadways and embankment slope in the vicinity of 

the seeps. Twelve wells, seeps A and E (which were flowing 

during the study), and the East Basin were sampled for 

water-quality analysis. Twenty-five wells were hydraulically 

tested to estimate the hydraulic conductivity of the earthen 

embankment in the southernmost part of the site.

Water-Quality Sample Collection

In winter and spring 2006, the USGS sampled 12 

groundwater observation wells (TB–1S, TB–1D, TB–2D, 

TB–5S, TB–5D, TB–12, TB–15, TB–17S, TB–17D, TB–18S, 

TB–18D, and B–3P) to determine water-quality conditions in 

the westernmost part of the study area in relation to the seeps 

on the embankment. Samples were also collected from the 

East Basin (at downtake chamber 1) and at seeps A (from the 

flume) and E (along the stone wall) (fig. 2). Water samples 

were collected in accordance with standard USGS methods 

as described in the USGS National Field Manual for the 

Collection of Water Quality Data (U.S. Geological Survey, 

variously dated) and were analyzed at the USGS National 

Water Quality Laboratory (NWQL) in Denver, Colorado, for 

nutrients, major ions, metals, pesticides, pesticide degradates, 

and volatile organic compounds (table 2). Temperature, pH, 

and specific conductance were recorded in the field for each 

water sample at the time of collection and are listed in table 2.

Single Well Slug Test Methods

Water displacement tests, commonly known as “slug 

tests,” were conducted in 16 wells at the Hillview Reservoir 

in August 2007 to estimate the hydraulic conductivity of the 

water-bearing zones in which the screened intervals of the 

wells were completed (table 3). The water in the well was 

displaced by a solid object, called a slug, and the water-level 

recovery was measured as a function of time. Based on the 

rate of recovery in the well, the hydraulic conductivity of the 

screened interval was calculated.

The slugs consisted of 1.07-inch-diameter (outside 

dimension) polyvinyl chloride pipes with caps on both ends. 

The slugs were suspended in the wells by a polypropylene 

rope attached to an eye bolt at the top of each slug, and the 

rope was secured to a section of pipe at the top of the well.

Water levels in the wells were measured during the slug 

tests using digital pressure transducers that were suspended 

in the water column by a cable that was secured at the top 

of the well. The pressure-transducer data loggers recorded 

water-level data based on the pressure of the water column 

in the well; water levels are accurate to 0.01 ft. The data 

loggers were programmed to record data on a logarithmic 

time scale. Water-level data were collected every 0.3 second 

at the beginning of the test, and the sampling interval between 

measurements was increased logarithmically to 5 minutes 

after 1 hour. This sampling interval provided detailed data 

coverage during the early portion of the slug test and less 

detailed coverage during the latter portion of the test, where 

less change was expected. The data loggers were started a 

few seconds before the insertion of the slug to record the 

background water level and to ensure that the exact time the 

slug was inserted was recorded. After the end of the test, the 

water-level data were downloaded to a computer and analyzed 

using slug test analysis software.

Care was taken to avoid dropping the slug in the water to 

minimize splashing because this could make early data from 

the test difficult to interpret. After the slug was inserted, the 

well was allowed to recover without any outside influence.

The slug tests ran for a minimum of 2 hours, and many of 

the tests ran for 5 hours or more. The duration of the tests was 

based on well construction, hydraulic characteristics, observed 

water-level changes under ambient conditions, and in some 

cases, previous hydrologic testing in the wells. The slug tests 

were allowed to run until the rate of recovery was less than 

0.01 ft in a 5-minute interval.

Surface Geophysics

Surface-geophysical data were collected in the vicinity of 

seep A. The two surface-geophysical methods used were twodimensional (2D) resistivity and 2D multichannel analysis of 

surface-wave (MASW) surveys.

The 2D resistivity surveys measure the subsurface 

electrical resistivity distribution by injecting electrical current 
Table 1. Site information for A, groundwater wells and B, reservoir basin and seeps at the Hillview Reservoir, City of Yonkers, Westchester County, N[All depths are in feet below land surface. Latitude and longitude are referenced to the North American Datum of 1983 (NAD 83). All elevations are in feet above National Geod(NGVD 29). NYSDEC, New York State Department of Environmental Conservation; ID, identifier; USGS, United States Geological Survey; —, no data] A. Groundwater wells Local well name NYSDEC well ID Longitude Latitude USGS site ID Measuring point elevation Land surface elevation Well depth Sounded depth Screen depth Satu

zo

scre

TB–1S WE5051 -735210 405428 405428073520901 302.01 300.69 40 41.8 30–40 Sha

TB–1D WE5062 -735210 405428 405428073520902 302.37 300.49 122 72.05 60–70 Dee

TB–2S WE5058 -735206 405427 405428073520302 300.23 299.73 41 38.45 30–40 Sha

TB–2D WE5072 -735209 405427 405427073520802 300.23 300.16 71 67.45 60–70 Dee

TB–3S WE5032 -735207 405427 405426073520701 300 300.28 41 39.5 30–40 Sha

TB–3D WE5057 -735207 405427 405426073520702 300.2 300.29 71 62.2 60–70 Dee

TB–4S WE5039 -735205 405427 405427073520401 302.79 300.29 — 43.3 — Sha

TB–4D WE5045 -735205 405427 405427073520402 302.46 300.05 103 73.1 60–70 Dee

TB–5S WE5024 -735210 405426 405426073521002 300.92 299.17 51 50.9 30–40 Sha

TB–5D WE5071 -735210 405426 405426073521004 301.13 299.03 77 76.15 66–76 Dee

TB–8 WE5040 -735204 405425 405425073524001 276.87 275.14 42 42.1 29–39 Sha

TB–9 WE5043 -735204 405425 405424073520501 268.63 269.03 41 42.2 30–40 Toe 

TB–10 WE5050 -735204 405423 405423073520401 245.44 243.39 41 42.35 20–40 Toe 

TB–11B WE5048 -735203 405425 405425073520201 255.85 253.46 31 31.7 20–30 Sha

TB–12 WE5035 -735203 405427 405424073520301 249.27 247.11 52 32.1 20–30 Toe 

TB–13 WE5033 -735210 405424 405423073521001 220.66 217.97 31 32.4 10–30 Toe 

TB–14S WE5028 -735205 405422 405422073520501 241.45 238.9 48 — 20–30 Sha

TB–14D WE5041 -735205 405422 405422073520502 242.97 240.54 50 50.9 29–49 Toe 

TB–15 WE5046 -735212 405426 405426073521201 229.77 227.95 33 30.17 12–32 Toe 

TB–16 WE5027 -735210 405427 405427073521001 299.33 299.75 49 48.75 — — 

TB–17S WE5022 -735211 405427 405426073521001 299 297.32 40 39.9 30–40 Sha

TB–17D WE5063 -735211 405427 405426073521003 299.2 297.37 80 79.75 70–80 DeeTB–18S WE5056 -735212 405428 405427073521102 277.99 275.32 27 26.8 — Sha

TB–18D WE5049 -735212 405428 405427073521101 278.02 275.43 60 63.55 50–60 Dee

MB–1W WE5065 -735212 405430 405430073521101 300.38 300.55 107 77 60–80 — 

MB–4W WE5069 -735213 405430 405429073521201 288.31 286.82 103 61.5 50–60 Dee

MB–5 WE5070 -735211 405428 405427073521103 299.31 299.67 105 75.25 60–80 Dee

MR–100P WE5042 -735210 405428 405427073520901 300.71 299.02 — 39.45 — Sha

MR–100PA WE5066 -735208 405428 405427073520902 300.88 298.96 — 61.35 — Dee
Table 1. Site information for A, groundwater wells and B, reservoir basin and seeps at the Hillview Reservoir, City of Yonkers, Westchester County, N[All depths are in feet below land surface. Latitude and longitude are referenced to the North American Datum of 1983 (NAD 83). All elevations are in feet above National Geod(NGVD 29). ID, identifier; NYSDEC, New York State Department of Environmental Conservation; USGS, United States Geological Survey; —, no data]

A. Groundwater wells

Local well 

name

NYSDEC 

well ID Longitude Latitude USGS site ID

Measuring 

point 

elevation

Land surface 

elevation

Well 

depth

Sounded 

depth

Screen 

depth

Satu

zo

scre

MR–121 WE5067 -735202 405427 405427073520102 265.19 262.33 — 20.5 — Sha

MR–123P WE5068 -735204 405428 405427073520801 302.49 299.73 — — — — 

MR–123PA WE5031 -735204 405428 405428073520301 302.06 299.35 — — — — 

MR–131 WE5074 -735222 405440 405439073522101 273.2 270.6 33 32.55 — — 

B–3P WE5026 -735206 405427 405426073520501 299.79 300.53 — 22.55 — Sha

B–4 WE5060 -735206 405426 405425073520502 283.81 282.15 — 35.67 — Sha

B–5A WE5055 -735206 405425 405425073520501 279.45 277.35 — 35.45 — Sha

HESF–8S WE5037 -735228 405443 405442073522801 243.88 241.78 10 10.7 5–10 Sha

HESF–8D WE5029 -735228 405443 405442073522701 244.15 241.81 19 19.3 14–19 Dee

CMB–2W WE5064 -735205 405427 405426073520502 301.03 299 — 35.4 — Sha

104–P WE5059 -735217 405436 405436073521702 301.72 300 — 41.3 — — 

104–PA WE5047 -735217 405436 405436073521701 302.98 300 — 20.7 — — 

105–P WE5053 -735220 405441 405440073522001 302.65 300.7 — 42.65 — — 

106–P WE5061 -735224 405444 405444073522301 301.2 298.9 — 42.4 — — 

106–PA WE5054 -735224 405444 405444073522401 301.33 296.6 — 42.62 — — 

109–P WE5073 -735223 405454 405454073522201 303.1 300.1 — 34.75 — — 

110–P WE5034 -735220 405458 405458073521901 303.4 302.1 — 32.65 — — 

111 — -735215 405502 — — — — — — — 

X WE5021 -735209 405459 405458073520901 302.1 299.9 — 38.35 — — 

Y–PA WE5030 -735207 405454 405454073520701 302.63 300 — 21.14 — — 

Y–PD WE5044 -735207 405454 405454073520702 302.45 300 — 42.4 — — 

Z–PA WE5036 -735203 405445 405445073520201 302.75 300.4 — 19.86 — — 

Z–PD WE5052 -735203 405445 405445073520202 302.06 299.4 — 46.12 — — 

PA WE5038 -735159 405432 405432073515801 303.17 300.3 — 22.05 — — 

PD WE5023 -735159 405437 405436073515801 302.76 300.2 — 42.3 — — 

DT–2 WE5025 -735212 405503 405502073521101 300.9 298.92 — 44.8 — — 

WE–1256 WE1256 -735158 405508 405508073515801 — 250 500 — — — 

WE–5078 WE5078 -735157 405441 405441073515701 290.6 287.6 124 124.2 — — 

WE–5079 WE5079 -735201 405449 405449073520001 277.58 274.6 91 91.15 — — 
Table 1. Site information for A, groundwater wells and B, reservoir basin and seeps at the Hillview Reservoir, City of Yonkers, Westchester County, N[All depths are in feet below land surface. Latitude and longitude are referenced to the North American Datum of 1983 (NAD 83). All elevations are in feet above National Geod(NGVD 29). ID, identifier; NYSDEC, New York State Department of Environmental Conservation; USGS, United States Geological Survey; —, no data] B. Reservoir basin and seeps Local site name Longitude Latitude USGS site ID Downstream order number Land surface elevation 

East Basin -735210 405433 405432073521001 1302006 — Seep A -735210 405427 405426073521101 1302007 255.34 

Seep B -735210 405426 405425073520901 1302009 234.14 

Seep C -735210 405426 405426073521005 — 259.84 

Seep D -735210 405426 405426073521006 — 234.61 

Seep E -735210 405426 405425073521101 1302008 221.61 

1The terms shallow and deep are used to identify wells screened in the shallow and the deep saturated zones. The term toe refers to wells located near the edge of the embank

a single water bearing unit where the shallow and deep saturated zones converge.
Methods of Investigation 9

Table 3. Hydrologic properties of selected wells at the Hillview Reservoir in the City of Yonkers, 

Westchester County, New York.

[Well locations are shown in figure 2. —, no data]

Local 

well name

Delay in watertemperature 

response,1

in months

Distance from 

reservoir,2

in feet

Apparent linear 

groundwater 

velocity, 

in feet per month

Hydraulic 

conductivity, 

in feet per day

TB–1S 6 25 4.2 0.0097

TB–1D 5 25 5 0.0063

TB–2S 6 31 5.2 0.014

TB–2D 5 31 6.2 0.007

TB–3D 5 38 7.6 3

0.351

TB–4S 6 31 5.2 3

0.373

TB–4D — 31 — 0.074

TB–5S — 200 — 3

0.076

TB–5D — 200 — 0.0078

TB–8 — 169 — 0.023

TB–9 — 288 — 3

0.02

TB–10 — 513 — 3

0.07

TB–11B — 300 — 3

0.048

TB–12 — 400 — 0.01

TB–13 — 431 — 3

0.096

TB–15 — 325 — 0.44

TB–17S — 188 — —

TB–17D — 188 — 0.23

TB–18S — 213 — —

TB–18D — 213 — 0.79

MB–1W — 63 — 30.017

MB–4W 18.5 150 8.1 1.2

MB–5 18 138 7.7 0.051

MR–100P 18.5 75 4 —

MR–100PA 19 75 4 —

B–3P — 69 — 0.014

B–4 — 175 — 3

0.294

B–5A — 206 — 0.01

HESF–8D — 396 — —

CMB–2W 16 69 4 0.036

106–P 6 53 8.8 —

Z–PA — 26 13 —

DT–2 17.5 132 7.5 —

1Delay between temperature change as recorded at the East Basin of the Hillview reservoir and corresponding 

temperature change in well.

2Linear distance from the subject well to the perimeter of the reservoir.

3Slug test data (unpublished) from TAMS Consultants, Inc. and analyzed by the U.S. Geological Survey.
10 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

into the ground through a pair of electrodes, then measuring 

the potential difference between a second pair of electrodes. 

The resistance of the subsurface material will be directly 

proportional to the measured potential difference and inversely 

proportional to the current injected (Zohdy and others, 

1974). After a geometric correction is applied, the resultant 

resistivities are defined as apparent because they are based on 

a homogeneous subsurface.

Surface-wave surveys measure the shear, or surface 

wave, propagated from a seismic source. MASW, a recently 

developed seismic method, uses a seismic source in the 

form of a hammer or accelerated weight drop and an array 

of geophones that measures the ground roll (Rayleigh wave) 

generated from the nearby seismic source (Park and others, 

1999). The multichannel recording method is effective in 

identifying and isolating noise from the data. The resulting 

one-dimensional shear wave data are inverted, and a 2D 

section is produced.

Hydrologic Measurements

Water-level elevations in all 46 wells were calculated 

from depth-to-water measurements collected with an electric 

water-level tape or a chalked steel tape. All the wells were 

monitored monthly throughout the course of the study. 

Of the wells in the southern embankment study area, 24 

were instrumented with continuous-record digital pressure 

transducers programmed to measure water-level elevation 

and temperature hourly in each of the 24 wells. Volumetric 

measurements were taken monthly at seeps A, B, C, and 

E using a calibrated container and stopwatch. Continuous 

measurements were taken at seeps A and B with a digital 

pressure transducer programmed to measure stage and water 

temperature on an hourly schedule.

Water Quality

Water-quality samples were collected for this study from 

12 groundwater observation wells, the East Basin, and at 

seeps A and E (fig. 2) to provide a basic understanding of the 

connections of the wells and the seeps to the basin. Pesticides 

and pesticide degradates were not detected in any of the 

samples collected. However, byproducts from disinfectants 

used in drinking water as well as metals and arsenic were 

detected in the samples collected. Arsenic concentrations 

greater than 1 microgram per liter (μg/L) were detected at 

8 of the 15 sites sampled. The highest arsenic concentration 

(21.8 μg/L) was detected at well B–3P. This elevated 

concentration may have been affected by the proximity of a 

pressure-treated wood deck constructed at a nearby trailer. 

Chromated copper arsenate (CCA) is a chemical preservative 

used to pressure-treat wood to protect it from rotting due 

to insects and microbes. Studies indicate that residue from 

CCA can leach into soil (U.S. Consumer Product Safety 

Commission, 2011). The sample collected from well B–3P 

also had the highest concentrations of metals and chloride of 

the 15 sites that were sampled. Elevated chromium and copper 

concentrations at well B–3P may also be attributed to the 

proximity of the pressure-treated wood deck.

Toluene was detected at concentrations greater than the 

detection limit at 5 of the 15 sites sampled. The concentrations 

at these five sites ranged from 4.78 μg/L at TB–2D, to 

0.022 μg/L (estimated) at TB–17D (table 2).

Trihalomethanes are disinfection byproducts that are 

formed when chlorine is added to water for disinfection 

and reacts with naturally occurring organic matter or 

bromides. Two trihalomethanes—bromodichloromethane and 

trichloromethane—were detected in water samples collected 

from the study area. Trace amounts of bromodichloromethane 

were detected at downtake chamber 1, well TB–17S, 

and seeps A and E, with the concentration highest at the 

downtake chamber, then dropping off with distance from the 

downtake chamber where disinfection occurs. This drop off 

in concentration indicates a connection between the downtake 

chamber, well TB–17S, and seeps A and E, with the downtake 

chamber as the source of the tracer compound (tables 1 

and 3). Trichloromethane was detected at 13 of the 15 sites 

sampled. With the exception of the TB–17 paired wells (pair 

of shallow and deep wells), the deeper wells had the highest 

trichloromethane concentrations. Trichloromethane, an 

immiscible dense nonaqueous phase liquid that sinks when 

mixed in water, was not detected in samples collected from 

shallow wells TB–1S, B–3P, and TB–18S. Concentrations 

of trichloromethane in samples collected at well TB–15, 

downtake chamber 1 (East Basin), and seeps A and E were 

in the middle of the range of concentrations for the deep and 

shallow well samples, which suggests that seeps A and E may 

have a connection with the basin. 

Municipalities add flouride to their drinking water to 

support dental health. Flouride, which is added to the New 

York City water supply, was detected in samples collected 

from both seeps at concentrations of 0.82 milligrams per 

liter (mg/L) at seep A, and 0.77 mg/L at seep E, indicating a 

connection with water from the reservoir (table 2).

Hydrogeology

All the wells monitored in this study were registered 

with the New York State Department of Environmental 

Conservation, and New York State well identification numbers 

were obtained for all the wells (table 1). Digital pressure 

transducers were installed in the East Basin (as a basin gage), 

at selected observation wells, and at two seeps to monitor 

hourly, daily, and seasonal changes in groundwater levels 

and temperature in relation to East Basin water levels and 

precipitation. Groundwater levels in all observation wells, 

discharge amounts at flowing seeps, and water levels at the 

East and West Basins were measured manually at monthly 

and quarterly intervals. Precipitation amounts were recorded 
Hydrogeology 11

by the National Weather Service at Central Park, New 

York, and were compared with the 138-year record. Surface 

geophysical methods were used to delineate variations in the 

subsurface geology.

Reservoir Tidal Effects

The Hillview Reservoir consists of two separate basins 

known as the East and West Basins. Both basins store water 

from several aqueducts and are used to maintain a constant 

hydraulic head within NYC’s water supply system. A digital 

pressure transducer was installed in the East Basin. Water 

levels and temperature were recorded at 1-hour intervals. The 

data indicate that the water level in the basin ranges by 3 to 

4 ft several times a day, creating an artificial basin tidal cycle 

(fig. 3). The effect of the basin tide on the water levels in the 

wells on the southern embankment appears to be related to the 

distance of the wells from the East Basin and the depth of the 

wells. Analysis of the hydrographs (fig. 3) indicates that water 

levels in the wells that are closest to the East Basin (fig. 2) and 

screened in the deep saturated zone are most strongly affected 

by the basin tidal cycle. This basin tidal cycle is referred to in 

this report as tidal or the basin tide. The mean water elevation 

within the East Basin is 293 ft (fig. 3).

Precipitation and Groundwater Levels

The National Weather Service defines (meteorological) 

spring as March through May, summer as June through 

August, autumn as September through November, and winter 

as December through February. The National Weather Service 

has maintained a precipitation recording station in Central 

Park since 1869. The 138-year mean annual precipitation is 

44.98 inches. Based on the precipitation records, it appears 

that the region is in an extended wet period that began in 

2002. The 138-year annual average has been exceeded every 

year from 2002 to 2007, and 7 of the previous 8 years dating 

back to 2000 (fig. 4). In addition, since 2003, total annual 

precipitation has exceeded 50 inches. Only once since 1970 

has annual precipitation exceeded 50 inches for more than 

2 years in a row. In 2003, 2004, 2005, 2006, and 2007, the 

annual precipitation exceeded the 138-year average by 30, 16, 

24, 33, and 37 percent, respectively. According to the National 

Oceanic and Atmospheric Administration (2008) data from 

Central Park, 2006 and 2007 were the seventh and fourth 

wettest years recorded, respectively. October 2005, April 2007, 

September 2004, September 2005, June 2006, November 

2006, and February 2008 were the first, second, third, fourth, 

fifth, eighth, and ninth wettest months ever, respectively. 

Conversely, September 2005 was the fourth driest month 

recorded. Seasonally, autumn 2005 and summer 2006, spring 

2007 and summer 2007, and autumn 2006 were the third 

(tied), fourth (tied), and tenth wettest of those seasons ever, 

respectively. April 15, 2007, is the date of the second greatest 

daily precipitation ever recorded (7.57 inches) at Central Park.

All the wells showed a downward trend in water-level 

elevations during spring 2006. This trend corresponds to a 

relatively dry period that occurred that spring, which included 

the driest March in the Central Park precipitation record. 

Water levels appeared to rebound in response to increased 

precipitation in the months that followed the dry period. Most 

of the hydrographs presented in appendix 1 show an upward 

trend in water levels during that period. During April 2007, 

the hydrographs of all the instrumented wells documented a 

sharp upward shift in water-level elevation. This abrupt shift 

correlates with the largest single precipitation event during this 

study, where 7.57 inches of precipitation fell at Central Park 

within a 24-hour period.

Slug Test Analysis

The slug test data were analyzed using the Bouwer and 

Rice (1976) method with spreadsheets developed by Halford 

and Kuniansky (2002) using Microsoft Excel. The ratio of the 

change in water level to the initial change in water level after 

the insertion of the slug was plotted log-linearly as a function 

of time. A line was fit to the data points. Using the slope of 

this line and the well construction and screen length, hydraulic 

conductivity was calculated (Bouwer and Rice, 1976).

One common problem with slug test analyses is that the 

data often are not log-linear for the duration of the test. One 

reason these data are not log-linear is that well-screen intervals 

commonly are surrounded by sand packs, which often have 

a higher hydraulic conductivity than native aquifer materials. 

The groundwater stored in the sand pack quickly enters the 

well, whereas water from the aquifer enters the well at a much 

slower rate. Typically a dataset collected from a well with a 

substantial sand pack will have two log-linear sections of the 

dataset—a steep-dipping trend in early time from the water in 

the sand pack and a shallow-dipping trend in later time from 

the aquifer.

The analysis of the data collected at Hillview Reservoir 

was complicated by variations in well construction. Different 

screen lengths, drilled apertures, sand-pack properties, 

and missing or inaccurate records made comparisons of 

the interpretations difficult. At some of the wells in the 

southwestern area of the reservoir, the water level recovered 

too quickly to determine whether the measured hydraulic 

conductivity was from the sand pack or from the waterbearing unit. Consultants for the NYCDEP had previously 

completed slug tests on many wells (Malcolm Pirnie, Inc. and 

TAMS Consultants, Inc., 2002), but some of these wells were 

subsequently destroyed before the USGS study began. Data 

from these previous slug tests were reanalyzed by the USGS 

using the methods described above. Data from 9 consultant 

slug tests and 16 USGS slug tests (a total of 25 wells) were 

analyzed to estimate hydraulic conductivity (table 3).

Estimated hydraulic conductivities range from 0.0063 

to 1.2 feet per day (ft/d) and averaged 0.17 ft/d (table 3). 

The contoured data shown in figure 5 indicate that an area 
278.0 278.1 278.2 278.3 278.4 278.5 278.6 278.7 278.8 278.9 290 291 292 293 294 295 296 

290 291 292 293 294 295 296 255.8 255.9 256.0 256.1 256.2 256.3 256.4 256.5 256.6 256.7 290 291 292 293 294 295 296 

290 291 292 293 294 295 296 A. Shallow well TB–2S B. Shallow well TB–17S C. Deep well TB–2D D. Deep well TB–17D Water level in the East Basin

Elevation of water level, in feet above NGVD 29

21 22 23 24 25 26 27 28

January 2007

21 22 23 24 25 26 

January 2007

EXPLANATION

Water level in well

Figure 3. Response of water levels in nearby wells A, TB–2S; B, TB–17S; C, TB–2D; and D, TB–17D to East Basin water levels in the Hillview Reserv

Yonkers, Westchester County, New York, in 2007. (NGVD 29, National Geodetic Vertical Datum of 1929.)
Hydrogeology 13

0

10

20

30

40

50

60

70

80

90

1870

1875188018851890189519001905191019151920

1925

1930193519401945

1950

19551960196519701975198019851990199520002005

Year

Total annual precipitation, in inches

(44.98 inches)

Figure 4. Annual precipitation as recorded at Central Park, New York County, New York, during 1870–2007. Solid black line 

indicates annual mean precipitation during the 138-year record of observation. Data are from National Oceanic and Atmospheric 

Administration (2008); modified from Stumm and others (2012).

of lower than average hydraulic conductivity appears to be 

present between the TB–17 and TB–3 well pairs and south 

toward Hillview Avenue along the southwestern part of the 

earthen dam. The hydraulic conductivity appears to increase 

away from this area toward the northwestern part and to some 

degree the northeastern part of the study area. There appears 

to be an abrupt transition from low permeable units to higher 

permeable units between wells MB–1W and MB–4W in the 

northwestern part of the study area (fig. 5). Two isolated areas 

of higher than average permeable deposits near wells TB–3D, 

B–4, and TB–4S were also indicated (figs. 2 and 5).

The lowest hydraulic conductivities in the shallow 

water-bearing unit were measured in wells TB–1S, B–5A, 

B–3P, and TB–2S at 0.0097 ft/d, 0.01 ft/d, 0.014 ft/d, and 

0.014 ft/d, respectively (figs. 2 and 5; table 3), whereas the 

lowest hydraulic conductivities in the deep water-bearing unit 

were measured in TB–1D, TB–2D, TB–5D, and MB–5 at 

0.0063 ft/d, 0.007 ft/d, 0.0078 ft/d, and 0.051 ft/d, respectively 

(figs. 2 and 5; table 3). The highest hydraulic conductivities 

in the shallow water-bearing unit were measured in wells 

TB–4S, B–4, CMB–2W, and TB–8 at 0.373 ft/d, 0.294 ft/d, 

0.036 ft/d, and 0.023 ft/d, respectively (figs. 2 and 5; table 3), 

whereas the highest hydraulic conductivities in the deep 

water-bearing unit were measured in wells MB–4W, TB–18D, 

TB–3D, and TB–17D at 1.2 ft/d, 0.79 ft/d, 0.351 ft/d, 

and 0.23 ft/d, respectively (figs. 2 and 5; table 3). Due to 

the variation in well construction and scarcity of data, no 

inference with respect to hydraulic conductivity and depth 

could be made with the data available.

Shear Wave Velocity and Resistivity Survey 

Observations

The 2D resistivity surveys indicate a subsurface mound 

of electrically conductive material (low-resistivity zone) 

beneath the terrace area (top of dam) surrounding the reservoir 

with a distinct elevation increase closer to the crest. The 

conductive zone appears to decrease in elevation toward the 

reservoir beneath the terrace (A–A′ and C–C′) and downslope 

toward Hillview Avenue (B–B′, D–D′, and E–E′) (figs. 6 and 

7A–E).

Two-dimensional shear wave velocity surveys indicate 

a similar structure of the high shear wave velocity materials 

(high-velocity zone), increasing in elevation toward the 

crest and decreasing toward the reservoir and toward the 

northern part of the study area. The bedrock was imaged along 

cross-section F–F′ where depths ranged from 35 meters (m) 
14 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

HILLVIEW AVE

KIMBALL AVE

HILLVIEW AVE

?

?

? ? 20

20

120

100

60

40

20

?? ? ? ?

?

?

?

?

?

?

?

80

37

5

4

2

1

1

1

1

2

1

1

44

7

1

35

29

23

79

8

7

2

5

120

10 

WEST

 BASIN 

EAST

 BASIN 

2

Base from New York City Department of Environmental Protection, digital data

Projection New York State Plane fipszone 3101; Datum NAD 1983

EXPLANATION

Hydraulic Conductivity—Shows lines of equal hydraulic

conductivity. Dashed where inferred

Estimated hydraulic conductivity value of 

screened zone (x100) and well identifier—Values 

are in feet per day and rounded to the nearest integer

Seep with continuous data recorder, and identifier

Seep and identifier

40

120

Seep A

Seep E

MB–4W

73°52'12" 73°52'01"

40°54'25"

40°54'35"

0 50 FEET

0 25 50 METERS

Seep B

Seep A

Seep E

TB–4S

TB–3D B–3P TB–4D

CMB–2W

TB–8

TB–11B

TB–12

TB–10

TB–13

TB–15

MB–1W

MB–4W

TB–18D

B–4

B–5A

TB–1S

TB–1D

TB–2S

TB–2D

MB–5

TB–5D

TB–5S

TB–17D

TB–9

Seep C Seep D

Downtake 

chamber 1

Basin

dividing

wall

Control

chamber

Downtake

chamber 2

Connecting

conduit

Roads

Path

Parking area

Figure 5. Distribution of hydraulic conductivity estimated from slug test results of wells on the southern embankment at 

Hillview Reservoir, City of Yonkers, Westchester County, New York. Hydraulic conductivity values are listed in table 3 and are 

shown in this figure multiplied by 100 and rounded to the nearest integer.
Hydrogeology 15

Seismic survey line

EXPLANATION

2D resistivity survey lines

Well locations and identifier

Seep A

Seep E

Seep area with continuous data recorder

and identifier

Seep area and identifier

Base from New York City Department of Environmental Protection, digital data

Projection New York State Plane fipszone 3101; Datum NAD 1983

F'

F

A'

A

G

G'

C'

C

B

E

D

D’

E'

B’

MB–4W

HILLVIEW AVE

KIMBALL AVE

HILLVIEW AVE

Seep A

WEST

 BASIN 

EAST

 BASIN 

73°52'12" 73°52'01"

40°54'25"

40°54'35"

Downtake 

chamber 1

Control

chamber

Downtake

chamber 2

Connecting

conduit

F F’

E E’

0 50 FEET

0 25 50 METERS

MR–100P

Seep B

Seep E

Seep C

TB–13

MB–4W

B–3P

TB–18D

MB–1W

TB–2S

Seep D

CMB–2W

 EXTENT OF FIGURE 10

Basin

dividing

wall

TB–1D

MB–5

Roads

Path

Parking area

Figure 6. Locations of geophysical surveys on the southern embankment at the Hillview Reservoir, City of Yonkers, Westchester 

County, New York. 2D, two dimensional.
Elevation , in meters above NGVD 29

 

312295279262246230213

MItItItItItU  

Seep A

MR–100P

TB–17 Road

0 12 24 36 48 60 72 92

90888684828078767472

95908580757065

302295289282276269262256249243236

Distance, in meters along survey line

0 39 79 118 157 197 236

0 12 24 36

48

72

0 39 79 118 157 197 236

MR–100P A A'

B B'

Elevation , in feet above NGVD 29

MB–5 TB–5D

C C'

0 MB–1W 20 40 60 80 100

90858075706560

312295279262246230213197

95

0 66 131 197 262 328 394 

Distance, in feet along survey line

120

R

60

Figure 7. Cross-sections showing two-dimensional resistivity along A, A–A′, B, B–B′, C, C–C′, D, D–D′, and E, E–E′ at the Hillview Reservoir, City of Yo

County, New York. The locations of the cross-sections are shown in figure 6. Profiling was through a Schlumberger array. ft, foot; m, meter; RMS, roo
D D' Seep C Seep A TB–18D

0

79 157 236

24 48 0 72

Distance, in meters along survey line

269

262

256

249

243

236

230

223

217

210

203

8280787674727068666462

0

236 0 39 79 118 157 197

Seep D

Seep A

TB–1D

E E'

Road 95

90858075706560

Distance, in feet along survey line

60

12 24

36

48

72

312295279262246230213197

0

Elevation, in feet above NGVD 29

Elevation, in meters above NGVD 29 

Figure 7. Cross-sections showing two-dimensional resistivity along A, A–A′, B, B–B′, C, C–C′, D, D–D′, and E, E–E′ at the Hillview Reservoir, City of Yo

New York. The locations of the cross-sections are shown in figure 6. Profiling was through a Schlumberger array. ft, foot; m, meter; RMS, root mean s
TB–1D MR–100P TB–2D TB–3D 

Downtake chamber 1 89.9 87.9 85.9 83.9 81.9 79.9 77.9 75.9 73.9 71.9 69.9 67.9 65.9 63.9 61.9 59.9 57.9 55.9 53.9 51.9 

Elevation, in meters above NGVD 29

Location of 1D seismic stations Unit seismic station spacing = 5 meters

EXPLANATION

Shear wave velocity in meters per second

F

200 400 600 800 1,000 1,200

∆

Figure 8. Cross-section showing two-dimensional shear wave velocity along F–F′ at the Hillview Reservoir, City of Yonkers, Westchester County, Ne

location of the cross-section is shown in figure 6. 1D, one-dimensional; NGVD 29, National Geodetic Vertical Datum of 1929.
G 

MB–4W MB–5 MR–100

Downtake chamber 1

89.987.985.983.981.979.977.975.973.971.969.967.965.9

TB–18D

∆ ∆ ∆

Elevation, in meters above NGVD 29

Location of 1D seismic stations Unit seismic station spacing = 5 meters

EXPLANATION

Shear wave velocity in meters per second

200 400 600 800 1,000 1,200

∆

Figure 9. Cross-section showing two-dimensional shear wave velocity along G–G′ at the Hillview Reservoir, City of Yonkers, Westchester County, 

New York. The location of the cross-section is shown in figure 6. 1D, one-dimensional; NGVD 29, National Geodetic Vertical Datum of 1929.
20 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

(114.8 ft) BLS near B–3P to 36 m (118.1 ft) BLS at TB–1D 

(fig. 8). The depth of the imaged bedrock along cross-section 

G–G′ ranged from 20 m (65.6 ft) to greater than 24 m (78.7 ft). 

A high-velocity zone that decreases in elevation from south 

to north along G–G′ (fig. 9) correlates with the decrease in 

elevation of conductive material from south to north along the 

2D resistivity cross-section A–A′ (fig. 7).

Velocities in the high-velocity zone were on average 

almost 200 meters per second (656.2 feet per second) greater 

than the overlying material (figs. 8 and 9). This high-velocity 

zone ranges from only 6 m (19.7 ft) BLS near well MB–5 

to almost 28 m (91.9 ft) BLS north of the TB–1 paired wells 

(figs. 8 and 9). When the low-resistivity zone (fig. 7) and the 

high-velocity zone (figs. 8 and 9) are compared, they seem 

to correlate within a few meters. This finding indicates that 

a zone of high electrical conductivity and high shear wave 

velocity underlies the westernmost part of the study area. This 

zone may be a well consolidated unit with higher clay content 

than the surrounding material. The water table appears to have 

less of an effect on resistivity in the most conductive material.

A contour map of the top of the apparent clay-rich zone 

was constructed, which combined both the 2D resistivity 

(low-resistivity zone) and shear wave velocity survey 

elevations (high-velocity zone) (fig. 10). This unit of low 

resistivity (clay-rich) and high shear wave velocity (dense) 

appears to be at its highest elevation in the vicinity of wells 

TB–16, MR–100P, MR–100PA, and MB–5 (fig. 10). The unit 

ranges in elevation from a maximum of 86 m (282.2 ft) above 

NGVD 29 near well MR–100P to a minimum of 64 m (210 ft) 

above NGVD 29 north of well TB–1D (fig. 10). Three areas 

were less conductive than other areas along the slope below 

wells TB–17 and TB–5 and may indicate gaps in the dense 

clay-rich material (figs. 5 and 10).

The unsaturated zone shows moderate-resistivity 

zones that correlate with low-velocity horizons at 2 to 8 m 

(6.6–26.2 ft) BLS, which may correlate to a sandier horizon 

that has less artificial compaction. Conductive (low-resistivity) 

zones were present in the deep subsurface. The elevation and 

distribution of these low-resistivity zones correlate with a 

high-velocity zone detected in the deep subsurface along both 

seismic survey lines F–F′ and G–G′ (figs. 6, 8, and 9).

Hydrologic Conditions

Groundwater levels were used to define the shallow 

and deep groundwater zones at the Hillview Reservoir. In 

addition, groundwater levels were evaluated to determine the 

hydrologic conditions in the earthen embankment.

Groundwater Levels

Groundwater levels were measured manually at 

46 observation wells during synoptic surveys and recorded 

digitally at selected wells at hourly intervals. Evaluation of 

groundwater levels at the Hillview Reservoir site indicates the 

presence of at least two separate saturated zones—a shallow 

water-bearing unit and a deep water-bearing unit. The shallow 

saturated zone had the highest water levels and appears to 

be most affected by precipitation. Groundwater levels in the 

wells closest to the reservoir were slightly affected by basin 

tides; the shallow wells farther away were not affected by the 

artificial tides created by the fluctuations in the water levels 

of the reservoir’s East Basin. During the study, shallow wells 

exhibited the largest seasonal water-level fluctuations, ranging 

between 6 ft and 12 ft at wells MR–100P and TB–17S, 

respectively (fig. 11). Within the south embankment study 

area, water levels in the shallow saturated zone fluctuated 

an average of 7 ft for all wells during this investigation. The 

largest fluctuations of 19 ft, 12 ft, and 10 ft occurred at wells 

TB–18S, TB–17S, and TB–13, respectively (appendix 1).

During the study period from April 2005 through 

February 2008, variation of water levels within 100 ft of the 

reservoir ranged from 3 to 6 ft in the shallow saturated zone 

and from 4 to 5 ft in the deep saturated zone. During this same 

period, variation of water levels in wells beyond 100 ft of the 

reservoir in the shallow saturated zone ranged from 5 to 20 ft 

whereas water levels in the deep saturated zone ranged only 2 

to 5 ft. Observation wells TB–10, TB–12, and TB–14D, which 

are located southeast along the toe or edge of the embankment, 

had the largest range in water levels (12 to 20 ft) (appendix 1). 

In contrast, observation wells TB–13 and TB–15 had a 

relatively small variation with only a 2- to 4-ft range in water 

levels during the study, which indicates that the hydrologic 

characteristics of the embankment materials at wells TB–10, 

TB–12, TB–13, TB–14D, and TB–15 are highly variable.

Shallow Saturated Zone

The shallow water-bearing unit at the southern 

embankment of the Hillview Reservoir was defined as the 

uppermost 45 ft of the embankment materials below the crest 

of the reservoir. Most of the materials within the shallow 

water-bearing unit consist of modified glacial till and artificial 

fill. The material in the shallow water-bearing unit appears 

to be variable in composition with respect to the amount of 

fine-grained sediments. Hydrographs of observation wells 

TB–2S and TB–17S, screened in the shallow water-bearing 

unit, indicate water levels near the reservoir (well TB–2S) 

were only slightly tidally affected, and water levels about 

150 ft from the reservoir (well TB–17S) were nontidal (fig. 3). 

Average water levels observed in paired wells (fig. 11) were 

14 to 16 ft higher in the shallow observation wells near the 

reservoir than in the deep wells. Some shallow wells farther 

from the reservoir had water levels up to 20 ft higher than 

nearby deep wells. Most deep wells are screened about 20 

to 30 ft deeper than shallow wells. Thus, a large downward 

vertical gradient exists between the shallow and deep saturated 

zones underlying the area surrounding the reservoir. The toe 

of the embankment is a low-permeability transition zone that 

constrains horizontal flow for both saturated zones.
TB–18D

TB–18S

CMB–2W

B–3P

TB–3D 

TB–3S

TB–2D

TB–2S

TB–1D

TB–1S

TB–5D

TB–5S

Downtakechamber 2

Controlchamber

Connectingconduit

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40°54'25"40°54'30"

73°52'10" 73°52'05"

EXPLANATION

Well location

Seep areaElevation of clay-rich unit—Elevation in meters above NGVD 29

"

No data64–69 70–7475–79 80–86

Point of increased resistivity—Potential gap in clay-rich unit

70 Structure contour line of clay-rich unit—Elevation 

in meters above NGVD 29. Interval is 5 meters. Dashed where inferredZone of increased electrical resistivity

Base from New York City Department of Environmental Protection, digital dataProjection New York State Plane fipszone 3101; Datum NAD 1983

Seep B

Seep C

Seep E

Seep A

Seep D

?

?

?

?

?

?

?

?

?

?

?

?

70

75

80

85

65

75

70

HILLVIEW AVE

EAST BASIN 

Seep A

0 40 FEET

0 20 

MB–5

MR–100PMR–100PA TB–16

MB–4W

MB–4W

RoadsPath

Figure 10. Interpreted elevation of a subsurface clay-rich unit delineated from two-dimensional resistivity and shear wave seismic surveys at the Hi

Reservoir, City of Yonkers, Westchester County, New York. (NGVD 29, National Geodetic Vertical Datum of 1929.)
A. Shallow well TB–17S C. Deep well TB–17D D. Deep well MR–100PA B. Shallow well MR–100P 012345678 254 256 258 260 262 264 266 268 270 272 274 276 

254 256 258 260 262 264 266 268 270 272 274 276 012345678 228 230 232 234 236 238 240 242 244 246 248 250 

238 240 242 244 246 248 250 252 254 256 258 260

04/01/05

05/01/05

05/31/05

06/30/05

07/30/05

08/29/05

09/28/05

10/28/05

11/27/05

12/27/05

01/26/06

02/25/06

03/27/06

04/26/06

05/26/06

06/25/06

07/25/06

08/24/06

09/23/06

10/23/06

11/22/06

12/22/06

01/21/07

02/20/07

03/22/07

04/21/07

05/21/07

06/20/07

07/20/07

08/19/07

09/18/07

10/18/07

11/17/07

12/17/07

01/16/08

02/15/08

04/01/05

05/01/05

05/31/05

06/30/05

07/30/05

08/29/05

09/28/05

10/28/05

11/27/05

12/27/05

01/26/06

02/25/06

03/27/06

04/26/06

05/26/06

06/25/06

07/25/06

08/24/06

09/23/06

10/23/06

11/22/06

12/22/06

01/21/07

02/20/07

03/22/07

04/21/07

Continuous water level

EXPLANATION

Precipitation

Continuous water level

EXPLANATION

Precipitation

Manual water level—Dashed 

lines are included as a visual aid

Date Date

Water level elevation, in feet above NGVD 29

Figure 11. Elevation of water levels in wells A, TB–17S, B, MR–100P, C, TB–17D, and D, MR–100PA in relation to daily precipitation at the Hillview Res

Westchester County, New York, between April 1, 2005 and March 1, 2008. Blank where data are missing. Data are from National Oceanic and Atmosph

NGVD 29, National Geodetic Vertical Datum of 1929.
Hydrogeology 23

In general, shallow wells respond fairly quickly to 

precipitation variations. Exceptions to this observation 

occurred in wells B–3P, CMB–2W, TB–2S, and TB–17S 

(fig. 11; appendix 1). Three of these wells, B–3P, CMB–2W, 

and TB–2S, had among the lowest hydraulic conductivity 

values of any shallow wells tested (fig. 5; table 3). Since 2003, 

annual precipitation amounts have exceeded the 138-year 

average (fig. 4). Groundwater elevations in the shallow 

water-bearing unit have increased at the Hillview Reservoir 

accordingly. During the study period from April 2005 through 

February 2008, water-level elevations in the shallow waterbearing unit appear to have been above average. A water-level 

contour map of the shallow water-bearing unit for September 

2007 indicates that water levels ranged from 286 ft along 

the reservoir to 192 ft at the toe (fig. 12). Two groundwater 

mounds, one on the westernmost part and the other near the 

easternmost part of the study area, are indicated in the contour 

map. Groundwater appears to be mounding in the vicinities 

of wells B–3P, CMB–2W, and TB–4S, and wells TB–17S, 

TB–2S, and MR–100P. These groundwater mounds correlate 

with areas of low hydraulic conductivity (fig. 5). Water-level 

elevations appear to be at or above land-surface elevations 

in the vicinity of the known seeps in the westernmost part of 

the study area (fig. 12). The localized groundwater mounds 

indicate that the shallow water-bearing unit is underlain or 

surrounded by low-permeability materials in these areas. 

The two surface geophysical surveys delineated a dense, 

electrically conductive zone underlying the westernmost 

groundwater mound. This zone is interpreted as being a clayrich unit (fig. 10) and correlates with the location of an area of 

low hydraulic conductivity (fig. 5).

The westernmost groundwater mound is larger than the 

easternmost mound and appears to supply water to seeps A 

and B. These large groundwater mounds coupled with the 

steep slopes found at the Hillview Reservoir appear to be 

the main controlling source of water to the seeps in this part 

of the reservoir. The lack of a groundwater mound during 

drier months (lower precipitation) in the vicinity of TB–17S 

(fig. 2) indicates that the contribution of water from the 

connecting conduit is limited or has other controlling factors 

—hydrogeology, precipitation, and reservoir infrastructure. 

(fig. 11). A leaking connecting conduit in this area might 

create a consistently above-average water elevation 

independent of variations in precipitation throughout the study 

period. The correlation of the emergence of several seeps at 

the site with the period of above-average precipitation during 

the past 10 years strongly suggests a connection with the high 

precipitation and local geologic control. Further study of this 

area could verify this hypothesis. In general, groundwater 

tends to flow from the reservoir and the two groundwater 

mounds outward toward the surrounding glacial overburden. 

There is also a downward component of groundwater flow into 

the underlying deep saturated zone at the embankment.

Deep Saturated Zone

Water levels in the deep saturated zone appear to be 

fluctuating at regular daily intervals as a result of the artificial 

basin tide. The largest tidal range was observed in wells 

screened in the deep water-bearing unit closest to the reservoir. 

Hydrographs of observation wells TB–2D and TB–17D, 

screened in the deep water-bearing unit, indicate that water 

levels near the reservoir were strongly affected by the basin, 

and those about 150 ft from the reservoir were less influenced 

(fig. 3). Water-level elevations in the deep water-bearing unit 

were consistently lower than those in the overlying shallow 

water-bearing unit. Hydrographs of wells screened in the 

deep water-bearing unit had small seasonal water-level ranges 

(excluding short term peaks) in the westernmost part of the 

study area (fig. 11). For example, seasonal water-level ranges 

for observation wells TB–13 and TB–15 were less than 2 ft 

(appendix 1). In contrast, water-level fluctuations in the 

easternmost part of the study area at well TB-12 were greater 

than 10 ft (appendix 1), which indicates that the materials 

underlying the easternmost part of the study area may be 

more permeable than those underlying the westernmost part. 

In general, the farther away a well is from the reservoir, the 

greater the range in water levels over the period of study. A 

water-level contour map of the deep water-bearing unit for 

September 2007 indicates water levels ranged from 192 to 

269 ft (fig. 13). The area along the toe of the embankment is a 

low-permeability transition zone that constrains flow for both 

saturated zones. The rapid thinning of both the shallow and 

deep saturated zones along the toe of the embankment with 

a shallow depth to bedrock below, forces the groundwater 

from both zones to flow through this area. Recharge from the 

reservoir appears to be a major contributor to the deep waterbearing unit with lesser influence from precipitation-induced 

recharge or recharge from the overlying shallow water-bearing 

unit. In general, groundwater tends to flow from the reservoir 

outward toward the surrounding glacial drift.

Temperature as a Tracer

One of the earliest applications of using heat as a tracer 

in groundwater studies was by Slichter in 1905. Winslow 

(1962) measured a thermal plume of warm water from a 

nearby river that was induced to flow toward a pumping well 

field. Conductive transport of heat occurs only in moving 

groundwater (Freeze and Cherry, 1979). The seasonal 

variation in heat transported along with flowing groundwater 

can be measured and analyzed. The transfer of heat by 

the circulation of recharged or discharged groundwater 

is a common phenomenon (Anderson, 2005). In general, 

the amplitude of temperature fluctuations decreases with 

depth (Sillman and Booth, 1993). Below about 1.5 m (5 ft) 

groundwater temperatures are not substantially affected by 

diurnal temperature fluctuations at land surface.

Groundwater temperature curves were used as an 

indicator of groundwater-flow velocities at the site along the 
24 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

210

220

230

240

200

250

270

280

260

HILLVIEW AVE

KIMBALL AVE

HILLVIEW AVE

73°52'12" 73°52'01"

40°54'25"

40°54'35"

Seep A

Seep B

Seep E

B–4

255

B–3P

286

MR–121

252

B–5A

249

TB–8

255

TB–3S TB–5S

256

TB–4S

265

TB–2S

280

TB–1S

271

TB–18S

252

TB–11B

240

MR–100P

263

TB–9

239

TB–15

220

TB–13

192

TB–12

223

TB–10

212

CMB–2W

285

TB–17S

270

TB–14D

205

Base from New York City Department of Environmental Protection, digital data

Projection New York State Plane fipszone 3101; Datum NAD 1983

EXPLANATION

Observation well—Upper number represents well 

identifier. Lower number indicates elevation above 

NGVD 29 in feet

Water level elevation of shallow aquifer system—

Shows equipotential surface of hydraulic head in the 

shallow water-bearing unit. Dashed where inferred. 

Contour interval is 10 feet

TB–1S

271

Seep with continuous data recorder and identifier

Seep A

Seep C Seep D

Seep and identifier Seep C

WEST

 BASIN

EAST

 BASIN 

Downtake 

chamber 1

Control

chamber

Downtake

chamber 2

Connecting

conduit

270

0 50 FEET

0 25 50 METERS

Figure 12. Water-level elevation in the shallow water-bearing unit at the southern embankment of the Hillview Reservoir, City of 

Yonkers, Westchester County, New York, in September 2007. NGVD 29, National Geodetic Vertical Datum of 1929.
Hydrogeology 25

Figure 13. Water-level elevation in the deep water-bearing unit at the southern embankment of the Hillview Reservoir, 

City of Yonkers, Westchester County, New York, in September 2007. NGVD 29, National Geodetic Vertical Datum of 1929.

220

230

240

250

260

270

Base from New York City Department of Environmental Protection, digital data

Projection New York State Plane fipszone 3101; Datum NAD 1983

200

210

EXPLANATION

Observation well—Upper number represents the well 

identifier. Lower number indicates elevation above 

NGVD 29, in feet 

Water level elevation of the deep aquifer system—

Shows equipotential surface of hydraulic head in 

the deep water-bearing unit. Dashed where 

inferred. Contour interval is 10 feet

Seep area with continuous data recorder 

and identifier

Seep area and identifier

73°52'12" 73°52'01"

40°54'25"

40°54'35"

Control

chamber

260

Basin

dividing

wall

0 50 FEET

0 25 50 METERS

Connecting

conduit

Seep A

Seep C

Seep B

Seep C

Seep E

Seep A

Seep D

MB–5

 249

MB–4W

250

TB–4D

242

TB–3D

269

TB–2D

258

TB–18D

241

TB–17D

241

TB–14D

205

MR–100PA

247

TB–13

192

TB–12

223

TB–15

220

TB–10

212

MB–4W

250

TB–5D

235

WEST

 BASIN 

EAST

 BASIN 

TB–1D

257

Downtake

chamber 2

Downtake 

chamber 1

HILLVIEW AVE

KIMBALL AVE

HILLVIEW AVE
26 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

150-ft perimeter of the reservoir’s East Basin. The seasonal 

variation in temperature of the surface water within the 

reservoir basins provides a source of heat for use as a tracer 

of recharge and groundwater flow at the site. Groundwater 

contour maps of the site indicate water from the East Basin 

recharges the shallow and deep saturated zones. Using the 

temperature curves recorded from the East Basin’s digital 

pressure transducer as a guide, the time delays in the high- and 

low-temperature curves for wells instrumented with digital 

pressure transducers were evaluated for wells within 150 ft 

of the reservoir. Based on hydraulic-head gradients in the 

shallow and deep saturated zones, groundwater flows outward 

from the East Basin across the embankment. Recharge from 

precipitation also appears to be a factor in the shallow waterbearing unit. Limiting this evaluation to the 150-ft perimeter 

of the East Basin provided an estimate of the apparent linear 

groundwater-flow velocity in this zone.

The East Basin reached a high water temperature 

of 18.5 degrees Celsius (°C) in mid-August 2006 and a 

low temperature of 1.0°C in mid- to late February 2007. 

Groundwater in well TB–1S reached a high temperature in 

mid- to late February 2007 and a low in mid-August 2007, a 

6-month delay in the groundwater response, which equated 

to an estimated linear velocity of 4.2 feet per month (ft/mo) 

(0.14 foot per day (ft/d)) based on the 25-ft distance from the 

East Basin (fig. 14). Groundwater in well TB–1D reached 

a high temperature in mid-January 2007 and a low in midJuly 2007, a 5-month delay in the groundwater response or 

an estimated linear velocity of 5 ft/mo (0.17 ft/d). Similar 

velocities were estimated for wells TB–2S, TB–2D, TB–3D, 

TB–4S, CMB–2W, MR–100P, and MR–100PA (table 3). 

Within a given saturated zone, the temperature curve delays 

increased as the distance between wells and the East Basin 

increased. In addition, differences in temperature curve delays 

were observed between wells screened in the shallow saturated 

zone and those in the deep saturated zone (fig. 14). In general, 

the temperature amplitude decreased and the delay increased 

in the deep water-bearing unit compared with the amplitude 

and delay in the shallow water-bearing unit. Although wells 

TB–1S and TB–2D are about the same distance from the 

East Basin, their temperature curves are different (fig. 14). 

The temperature data indicate that the estimated linear 

groundwater velocities in the deep saturated zone at TB–2D 

are slightly higher than those in the shallow saturated zone at 

TB–1S. A lack of instrumented wells in this area limited the 

application of this technique. Based on the temperature data, 

the estimated groundwater-flow velocities ranged from about 4 

to 9 ft/mo (table 3). One exception was for well Z–PA, which 

had a delay of only 2 months for an apparent velocity of 13 

ft/mo. Wells beyond 150 ft from the reservoir had delays that 

were longer than 18 months and, thus, estimated groundwaterflow velocities could not be accurately determined. However, 

the wells within the southern part of the study area appear to 

be relatively similar in apparent linear velocities. There were 

some indications of an increase in estimated linear velocities 

in wells farther northwest of MR–100P and in the well pairs 

at TB–1 and TB–2. This finding seems to correlate with the 

slug test estimates of increased hydraulic conductivity in 

wells northwest of MR–100P, TB–1S, and TB–2S and the 

location of a dense clay-rich unit delineated by the surface 

geophysical surveys.

Embankment Seepage

At least five separate seeps located downslope from the 

reservoir have been documented at the Hillview Reservoir, and 

are defined as seeps A, B, C, D, and E. Based on the data that 

have been analyzed, source water to the seeps appears to be 

primarily groundwater and, to a lesser extent, water from the 

East Basin. A long-term monitoring network would provide 

valuable data for understanding hydrologic controls on the 

seeps at the reservoir. The seeps are described in detail below.

Seep A

Seep A, one of the longest running seeps at the site, is 

located 85 ft downslope from well TB–17S (fig. 2). The seep 

has an elevation of 255.3 ft. Manual discharge measurements 

of seep A by NYCDEP consultants were made sporadically 

from September through November 2001 and ranged from 0 

to 8.3 gallons per minute (gal/min). The seep appears to have 

continued to flow sporadically through February 2008. In May 

2005, the USGS installed a Parshall flume at this seep with a 

digital pressure transducer to record stage at 1-hour intervals. 

Stage was recorded digitally using a pressure transducer 

at hourly intervals in both the flume and weir to calculate 

discharge. A rating equation was used to calculate discharge 

from the stage measurements collected at the flume.

Manual measurements were collected to calibrate the 

digital data. In May 2005, discharge was measured to be about 

8 gal/min; thereafter, discharge rapidly declined (fig. 15). 

Precipitation from May through October 2005 was below 

average. Discharge at seep A dropped to less than 1 gal/min 

until November 2005. A sudden increase to about 4.8 gal/min 

of discharge was measured at seep A in November 2005 

through April 2006. This increase in discharge corresponds to 

the third wettest autumn (2005) and the wettest October ever 

recorded in Central Park. However, another rapid decrease in 

discharge occurred during May 2006 that appeared to coincide 

with the installation of a new french drain system behind the 

retaining wall downslope from this seep. It appears the new 

drain system, which does not have a discharge monitor, may 

have altered the source of water to this seep. Increases in 

precipitation in 2006 and 2007 no longer caused increases in 

discharge at this seep. In May 2006, a weir was installed (to 

replace the flume) with a digital pressure transducer at seep 

A to provide more accurate low-discharge measurements. A 

rating equation was used to calculate discharge from the stage 

measurements collected at the weir. From May 2006 through 

February 2008, discharge at seep A did not exceed 2 gal/min 

and averaged about 1 gal/min or less (fig. 15).
Hydrogeology 27

0

2

4

6

8

10

12

14

16

18

20

TB–1D

East Basin

MR–100PA

TB–1S

TB–2D

EXPLANATION

Water temperature, in degrees Celsius

Date

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

Figure 14. Water temperature in the East Basin and wells TB–1S, TB–1D, TB–2D, and MR–100PA at the Hillview Reservoir, City of 

Yonkers, Westchester County, New York, between April 1, 2005 and March 1, 2008. Blank where data are missing.

Seep B

Seep B is located 100 ft downslope from the control 

chamber and about 70 ft upslope from the retaining wall along 

Hillview Avenue. (fig. 2). During a field visit by the USGS in 

August 2004, a small stream (seep B) was observed flowing 

from the steep embankment. On subsequent field visits to 

the embankment in autumn 2004, the discharge had stopped 

flowing from seep B. The discharge was not quantified at that 

time but appeared to resemble discharge amounts observed 

during visits in 2007. During a field visit in September 

2007, when the USGS installed a Parshall flume at this seep, 

discharge from seep B was observed to be flowing again. 

This seep appears to flow only after periods of sustained 

above-average precipitation and high water-level elevations 

in the shallow saturated zone. The flume was instrumented 

with a digital pressure transducer to record stage at hourly 

intervals. The seep was observed to produce large quantities 

of discharge and formed a small stream during a field visit in 

August 2004. The seep appears to have stopped flowing near 

the end of fall 2004; it began flowing again in August 2007. 

Manual discharge measurements were as high as 21.4 gal/min. 

A rating equation was used to calculate discharge. Spring and 

summer 2007 were the fourth wettest seasons ever recorded 

at Central Park. April 2007 was the second wettest month on 

record at Central Park. Discharge from this seep appeared 

to erode sediment at the toe. Sediment grab samples were 

obtained near the source of this seep. Discharge from seep B 

steadily declined through February 2008. From November 

2007 to the end of this study on February 1, 2008, discharge 

averaged 6 gal/min (fig. 15). However, seasonal increases 

in the discharge of this seep have been observed since 

February 2008.

Based on observations of seep B, discharge may increase 

during periods of above-average precipitation and high waterlevel elevations in the shallow water-bearing unit. Because of 

the discontinuous nature of this seep, erosion of fine materials 

from within the embankment may occur during these periods 

of increased discharge. Results of analysis of sediment grab 

samples from seep B indicate that the suspended sediment 

concentration ranged from 1 to 9 mg/L. The suspended 

sediment size ranged from 31 to 89 percent less than the 
28 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York Discharge, in gallons per minute

Precipitation, in inches

Date

0

1

2

3

4

5

6

7

8

0

5

10

15

20

25

30

EXPLANATION

Seep A

Continuous record measurement

Volumetric measurement—Dashed lines are intended as a visual aid

Seep B

Continuous record measurement

Volumetric measurement—Dashed lines are intended as a visual aid

Seep E

Volumetric measurement—Dashed lines are intended as a visual aid

Precipitation

Unmeasured french drain 

system installed

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

Figure 15. Discharge rates of seeps measured on the southern embankment at the Hillview Reservoir, City of Yonkers, 

Westchester County, New York, and daily precipitation between April 1, 2005, and March 1, 2008, recorded at Central Park, 

New York County, New York. Blank where data are missing.

0.063-millimeter (mm)-sieve size (table 4). This finding 

indicates that most of the sediment load is a fine-grained silt 

or clay.

Hydrographs indicate an increase in groundwater 

elevations at two wells completed in the shallow water-bearing 

unit, TB–17S and TB–2S, near seep B (fig. 11; appendix 1). 

Increased precipitation in 2007 produced the fourth wettest 

spring and summer. Increases in water levels in the shallow 

water-bearing unit were recorded during this period at the 

two wells. At well TB–2S, water levels were at a historic 

high, and those at TB–17S, nearly so. A relatively dry autumn 

(2007) appears to have lowered groundwater levels in many 

shallow wells at the southern embankment of the study area. 

The steady decline in discharge measured at seep B appears to 

correlate closely with water-level elevations at well TB–17S 

(fig. 16).

It is unclear why there was no discharge flowing at the 

seep B site during similar high water levels in TB–17S in 

2006. One possible explanation may be that a low permeable 

zone is present in the vicinity of TB–17S and TB–2S, 

indicating an impediment to drainage in this area. This low 

permeable zone was delineated by the surface geophysical 
Hydrogeology 29

Table 4. Suspended sediment in surface-water samples from seep B at the Hillview Reservoir, City 

of Yonkers, Westchester County, New York.

[U.S. Geological Survey (USGS) groundwater spring designated as seep B with USGS identification number 

405425073520901. <, less than; mm, millimeter; mg/L, milligrams per liter]

Sample date 

and time Sample medium

Suspended 

sediment, 

<0.063-mm-sieve 

diameter, 

in percent

Suspended 

sediment 

concentration, 

in mg/L

Sampling 

method

9/6/2007 9:44 Surface water 31 8.0 Grab

9/6/2007 11:07 Surface water 54 9.0 Grab

3/11/2008 13:30 Surface water 89 1.0 Grab

3/11/2008 13:35 Surface water 88 1.0 Grab

0

5

10

15

20

25

30

258

260

262

264

266

268

270

272

Date

Water level elevation, in feet above NGVD 29

EXPLANATION

Seep B discharge

TB–17S water level

Discharge, in gallons per minute

06/01/0707/01/0707/31/0708/30/0709/29/0710/29/0711/28/0712/28/0701/27/08

Figure 16. Groundwater levels in well TB–17S and discharge rates at seep B at the Hillview Reservoir, City of Yonkers, 

Westchester County, New York, between June 1, 2007 and March 1, 2008. (NGVD 29, National Geodetic Vertical Datum of 

1929.) Blank where data are missing.
30 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

surveys and hydraulic testing. The low permeable zone 

correlates with low estimated linear groundwater velocities, 

calculated using groundwater temperature curves, and the 

location of a large groundwater mound. The low permeable 

zone appears to function much like a bowl that fills partially 

from basin recharge and mostly during periods of aboveaverage precipitation. To further explore the effect that the 

low permeable zone has on drainage in this area, water-level 

fluctuations in well TB–17S located in the low permeable zone 

were compared with water-level fluctuations in well TB–18S 

located outside the low permeable zone. Evaluation of water 

levels in TB–18S indicates rapid increases in groundwater 

elevations after prolonged above-average precipitation 

(appendix 1). Whereas water levels in TB–17S increase, the 

elevated water levels in TB–18S appear to decline, and the 

shallow saturated zone in that area returns to a lower level. 

The water level at TB–17S returns to a background level at a 

slow rate (fig. 11). This appears to correlate with geophysical 

delineation of a bowl-like ridge of dense, electrically 

conductive material (low-permeable clay) that underlies the 

westernmost part of the study area. Further study would be 

required to better understand this mechanism.

Seep C

Seep C, located 30 ft downslope from the control 

chamber, appears to have started flowing at the same time 

as seep B (fig. 2). Seep C, at an elevation of 259.8 ft, likely 

flowed for only a few weeks. Seep C was observed during 

a field visit in September 2007. Seep C does not have a 

specific point source and could be better characterized as a 

saturated soil area. Discharge was difficult to quantify and 

never exceeded 1 gal/min. Discharge at seep C appears to have 

stopped in November 2007.

Seep D

Seep D is located 50 ft downslope from seep A, was 

observed throughout this study and during field visits in 2003. 

Seep D is characterized by a small area of constantly saturated 

soil near the base of the embankment about 50 ft downslope 

from seep A. No discharge has been observed emanating from 

this seep. Seep D appears to have been a wet area since field 

visits by the USGS began in 2003.

Seep E

Seep E, encompassing the embankment toe or edge 

along Hillview Avenue, is the largest seep of a group of seeps 

at the base of a stone retaining wall that runs along a part of 

Hillview Avenue at the toe of the embankment downslope 

of seeps A, B, C, and D. Although most of these seeps can 

be characterized as small wet areas or minor discharges, 

one opening in the retaining wall produced large discharges. 

This seep is expressed as wet areas at the base of a stone 

embankment retaining wall. One particular area along the 

retaining wall has an opening from which discharge was 

measured manually and ranged from less than 1 to 30 gal/min 

(fig. 15). During July 2005, a 30-gal/min discharge was 

measured manually. Subsequent field visits to this seep 

indicated a rapid decrease in discharge during late spring 

and summer 2005. In February 2006, the discharge dropped 

to less than 2 gal/min, and by July 2006, the discharge was 

below measureable levels. After the installation of the french 

drainage system behind the retaining wall, no discharge was 

observed from this seep. However, many wet areas continued 

to be observed along the retaining wall, indicating a potential 

buildup of groundwater in the clay-rich sediments there.

Summary

The earthen embankment at the Hillview Reservoir 

comprises low-permeability clays and glacial till that were 

excavated from the site; the embankment rests on a thin 

veneer of glacial-drift deposits that overlie crystalline bedrock. 

The study area is underlain by two permeable water-bearing 

units—the shallow and deep saturated zones. In the past, 

several seeps located downslope from the reservoir have 

flowed from the steepest slope at the southern end of the 

earthen embankment including one seep (seep A) that has been 

flowing continuously since about 1999. In 2001, the New York 

City Department of Environmental Protection (NYCDEP) 

installed 25 wells at the southern end of the reservoir (for a 

total of 57 wells around the reservoir) in an effort to locate the 

source of the continuous flowing seep. The U.S. Geological 

Survey (USGS) was able to use 46 of the 57 wells to monitor 

water levels, temperature, and water quality.

Water-Quality Samples.—Water samples were collected 

from 12 groundwater observation wells, the East Basin of 

the reservoir, and two seeps for chemical analysis. Samples 

were analyzed for nutrients, major ions, metals, pesticides, 

pesticide degradates, and volatile organic compounds. Results 

of the water-quality analysis of groundwater samples collected 

in this study identified arsenic concentrations greater than 

1 microgram per liter (μg/L) at 8 of the 15 sites sampled. Well 

B–3P had the highest concentration of arsenic at 21.8 μg/L. 

The sample collected from B–3P also had the highest 

concentrations of metals and chloride of the 15 sites that were 

sampled. Toluene was detected at concentrations greater than 

the detection limit at 5 of the 15 sites sampled.

Trace amounts of bromodichloromethane, a 

trihalomethane, were detected at downtake chamber 1, well 

TB–17S, and seeps A and E. Using bromodichloromethane 

as a tracer, there appears to be a connection between the 

downtake chamber, well TB–17S, and seeps A and E. 

Trichloromethane was detected at 13 of the 15 sites sampled. 

With the exception of the TB–17 well pair, the deep wells 

had the highest trichloromethane concentrations. No 

trichloromethane was detected at shallow wells TB–1S, B–3P, 

and TB–18S. Concentrations of trichloromethane in samples 

collected at well TB–15, downtake chamber 1 (East Basin), 
Summary 31

and seeps A and E were in the middle of the concentration 

range of the deep and shallow well samples, which suggest 

a connection between the downtake chamber and the seeps. 

Fluoride detected in samples collected from both seeps 

indicate a connection with water from the reservoir.

Reservoir Tidal Effect.—Evaluation of the hydrograph 

data indicates that water levels in the wells closest to the East 

Basin are strongly affected by the artificial tides created by 

fluctuations in the elevation of the water level in the basin as a 

result of water use. This cyclic demand produces an artificial 

“tidal” load on the surrounding embankment materials and 

local groundwater-flow system. The effect of the basin tides 

on the water levels in the wells appears to be related to the 

distance of the wells from the basin and the depth of the wells. 

In general, the water-level elevation in the basin ranges from 

3 to 4 feet (ft) several times a day; the mean water elevation in 

the basin is 293 ft. 

Precipitation and Groundwater Levels.—In 2003, 2004, 

2005, 2006, and 2007, the annual precipitation exceeded 

the 138-year average by 30, 16, 24, 33, and 37 percent, 

respectively. Between January 2004 and March 2008, 

7 months had recorded monthly precipitation totals that were 

among the top 10 wettest on record for specified months. Five 

of the top 10 wettest seasons on record also occurred during 

2005–07. 

All the wells showed a downward trend in water-level 

elevations during spring 2006. This decline corresponds to a 

relatively dry period that occurred that spring, which included 

the driest March in the Central Park, New York, precipitation 

record. Water levels appeared to rebound in response to 

increased precipitation in the months that followed the dry 

period.

Hydraulic Conductivity.—Water-level data from 25 wells 

were used to estimate hydraulic conductivity in the study 

area. The results indicate a range of estimated hydraulic 

conductivities from 0.0063 to 1.2 feet per day (ft/d) and 

averaged 0.17 ft/d. An area of lower than average hydraulic 

conductivity was delineated between the TB–17 and TB–3 

well pairs and south toward Hillview Avenue along the 

southwestern part of the earthen embankment. There appears 

to be an abrupt transition from low permeability units to 

higher permeability units in the northwestern part of the study 

area. Two isolated areas of higher than average permeable 

deposits at wells TB–3D, B–4, and TB–4S also were indicated 

by the results of slug tests.

Surface Geophysics.—Results of the two-dimensional 

(2D) resistivity surveys indicate a mound of electrically 

conductive material beneath the terrace area (top of dam) 

surrounding the reservoir (crest) with a distinct increase in 

elevation closer to the crest. The conductive zone appears to 

decrease in elevation toward the reservoir beneath the terrace 

and downslope toward Hillview Avenue.

Results of the 2D shear wave velocity surveys indicate a 

similar structure with a zone of high velocity and increasing 

in elevation toward the crest and decreasing in elevation 

toward the reservoir and the northern part of the study area. 

The bedrock was imaged along a northwest-southeast trending 

line (G–G′) west of the reservoir; the bedrock ranged from 

35 meters (m) (114.8 feet (ft)) below land surface near well 

B–3P to 36 m (118.1 ft) below land surface at well TB–1D. 

The high-velocity zone averaged almost 200 meters per 

second (656.2 feet per second) greater than the overlying 

material. This high-velocity zone ranges from only 6 m 

(19.7 ft) below land surface (BLS) near MB–5 to almost 

28 m (91.9 ft) BLS north of the TB–1 well pair. When 

compared, the low-resistivity zone identified with the 2D 

resistivity survey and the high shear wave velocity zone 

appear to correlate within a few meters, indicating that a zone 

of high conductivity and high shear wave velocity underlies 

the westernmost part of the study area. This zone may be 

a well-consolidated unit with higher clay content than the 

surrounding material.

Hydrologic Conditions.—Groundwater elevations were 

measured manually at 46 observation wells during the study 

and digitally monitored at selected wells at hourly intervals. 

Water levels in the shallow zone (upper 45 ft of embankment) 

are the highest in the study area and appear to be affected 

by precipitation.

Water levels in shallow observation wells near the 

reservoir averaged 14 to 16 ft higher in elevation than those 

in the deep wells at well pairs. Some shallow wells farther 

from the reservoir had water-level elevations up to 20 ft higher 

than those in nearby deep wells. A large downward vertical 

gradient exists between the shallow and deep saturated zones 

in the area surrounding the reservoir. The toe (edge) of the 

embankment represents a constrained flow transition zone for 

both saturated zones.

Two groundwater mounds, one in the westernmost 

part and the other near the easternmost part of the study 

area, correlate with areas of low hydraulic conductivity. 

Groundwater elevations are at or above land surface in the 

vicinity of the known seeps in the westernmost part of the 

study area. A dense, electrically conductive zone (clay) that 

underlies the westernmost groundwater mound correlates 

with an area of low hydraulic conductivity. The westernmost 

groundwater mound is larger than the easternmost mound 

and appears to supply water to seeps A and B. These large 

groundwater mounds coupled with the steep slopes found at 

the southern end of the Hillview Reservoir appear to be the 

source of the seeps in this part of the reservoir. The lack of 

a groundwater mound near some wells during drier months 

indicates the contribution of water from the connecting 

conduit is either limited or has other controlling factors—

hydrogeology, precipitation, and reservoir infrastructure.

Temperature as a Tracer.—Delays in groundwatertemperature responses between the East Basin and the 

groundwater observation wells were used as an indicator of 

groundwater-flow velocity. Based on hydraulic-head gradients 

in both the shallow and deep saturated zones, flow appears 

to move from the reservoir outward across the embankment. 

A 6-month delay in groundwater flow (which equates to 

an estimated linear velocity of 4.2 feet per month (ft/mo) 
32 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

(0.14 foot per day (ft/d)) based on the 25-ft distance from the 

East Basin) was calculated between the East Basin and well 

TB–1S, the shallow well in the TB-1 well pair. Similarly, a 

5-month delay or a linear velocity of 5 ft/mo (0.17 ft/d) was 

calculated between the East Basin and well TB–1D, the deep 

well in the TB-1 well pair. Similar velocities were found at 

TB–2S, TB–2D, TB–3D, TB–4S, CMB–2W, MR–100P, and 

MR–100PA. As the distances between the East Basin and the 

wells increased, temperature curve delays increased.

Seeps.—At least five separate seeps have been 

documented at the Hillview Reservoir. The five seeps are 

defined as seeps A, B, C, D, and E.

Seep A is one of the longest continuous running seeps at 

the site and is located 85 ft downslope from well TB–17S at 

an elevation of 255.3 ft. Discharge from seep A was measured 

from September through November 2001. The seep appears 

to have continued to flow sporadically through February 

2008. Precipitation appeared to be a substantial contributing 

factor to flow from seep A until a french drain system was 

installed downslope from the seep in May 2006. It appears 

the new drain system may have affected the flow from seep 

A; discharge at the seep did not exceed 2 gallons per minute 

(gal/min) after the drain system was installed.

Seep B is located 100 ft downslope from the control 

chamber and appears to flow only after periods of sustained 

above-average precipitation and high water-level elevations 

in the shallow saturated zone. Because of the discontinuous 

nature of discharge from this seep, erosion of fine materials 

from within the embankment may occur during periods of 

increased discharge. The steady decline in discharge measured 

at seep B appears to correlate closely with the water-level 

elevations in well TB–17S. It is unclear why no discharge 

was flowing at seep B during similar high water levels in well 

TB–17S in 2006. The seep stopped flowing near the end of 

fall 2004 and began to flow again in August 2007. Manual 

discharge measurements were as high as 21.4 gal/min.

Seep C appears to have started at the same time as seep 

B and is located 30 ft downslope from the control chamber. 

Seep C was observed during a field visit in September 2007. 

Discharge from this seep averaged less than 1 gal/min. 

The seep appears to have stopped in November 2007. The 

elevation of seep C is 259.8 ft.

Seep D is characterized by a small area of constantly 

saturated soil near the base of the embankment about 50 ft 

downslope from seep A. No discharge has been observed 

emanating from this seep. Seep D appears to have been a wet 

area since field visits by the USGS began in 2003.

Seep E is the largest seep of a group of seeps coming 

from the base of a stone retaining wall that runs along a part 

of Hillview Avenue at the toe of the embankment downslope 

from seeps A, B, C, and D. Although most of these seeps 

can be characterized as small wet areas or minor discharges, 

one opening in the retaining wall produced large discharges. 

During July 2005, discharge was manually measured to be 

30 gal/min.

The hydrologic factor that appears to be controlling the 

seeps is above-average precipitation, which has a substantial 

effect on water levels in the shallow saturated zone. When 

the precipitation recharges the shallow saturated zone, the 

groundwater flows primarily through the higher hydraulic 

conductivity materials away from the reservoir. Based on 

the data that have been analyzed, source water to the seeps 

appears to be primarily groundwater and, to a lesser extent, 

water from the East Basin. A long-term monitoring network 

would provide valuable data for understanding hydrologic 

controls on the seeps at the reservoir.

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Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. BLS, below land surface; μS/cm, microsiemens per centimeter; °C, degrees Celsius; DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocarbamate; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; SiO2, silicon dioxide; μm, —, no data] Local well name Sampling date Depth to water level, in feet BLS (72019) pH, water, unfiltered, field, standard units (00400) Specific conductance, water, unfiltered, μS/cm (00095) Temperature, water, in °C (00010) Elevation of land surface, in feet (72000) Depth of well, in feet BLS (72008) Dissolved solids dried at 180 °C, water, filtered, in mg/L (70300) Calcium, water, unfiltered, recoverable, in mg/L (00916) 

East Basin 2/8/2006 — 7.3 103 4.1 — — 55 5.45 Seep A 2/6/2006 — 7.2 393 5.4 255 — 206 13.9 

Seep E 2/8/2006 — 6.6 356 7 220 — 199 18.4 

TB–17S 2/7/2006 37 7.4 73 — 297 40 398 42.9 TB–5S 5/1/2006 48 7.8 1,850 11.7 299 51 11,300 217 

B–3P 5/1/2006 13 8 4,070 12.2 301 23 3,060 185 

TB–12 5/1/2006 21 7.7 2,350 15.5 247 52 1,270 43.1 

TB–15 5/2/2006 10 7.1 440 13.1 228 33 240 34.4 

TB–18D 5/1/2006 36 6.8 2,430 13.2 275 60 988 174 

TB–1S 2/7/2006 30 7.1 1,140 — 301 40 655 127 

TB–18S 5/2/2006 17 6.8 1,520 10.2 275 27 965 202 

TB–1D 2/7/2006 44 6.7 95 12.1 300 122 49 6.46 

TB–5D 2/6/2006 65 7 155 9.9 299 77 790 82.9 

TB–2D 2/7/2006 43 7.6 103 11.1 300 71 54 5.92 

TB–17D 2/7/2006 60 7 195 10.2 297 80 869 138 
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than]

Local well 

name

Sampling 

date

Sodium, 

water, 

unfiltered, 

recoverable, 

in mg/L 

(00929)

Chloride, 

water, 

filtered, 

in mg/L 

(00940)

Fluoride, 

water, 

filtered, 

in mg/L 

(00950)

Silica, water, 

filtered, 

in mg/L as 

SiO2

(00955)

Sulfate, 

water, 

filtered, 

in mg/L 

(00945)

Ammonia, 

water, 

filtered, 

in mg/L as N 

(00608)

Nitrate plus 

nitrite, 

water, 

filtered, 

in mg/L as N 

(00631)

Nitrite, 

water, 

filtered, 

in mg/L as N 

(00613)

O

East Basin 2/8/2006 8.71 9.93 1 3.6 7.43 <0.04 0.27 <0.008 

Seep A 2/6/2006 56 86.5 0.82 4 14.3 <0.04 0.28 <0.008 

Seep E 2/8/2006 42 68.9 0.77 4.5 13.2 <0.04 0.23 <0.008 

TB–17S 2/7/2006 106 144 1.37 7.6 20.5 <0.04 0.51 <0.008 TB–5S 5/1/2006 3,790 <1.00 0.27 12.5 77.9 <0.04 2.16 <0.008 

B–3P 5/1/2006 2,710 1,630 0.32 e0.01 34.1 0.08 0.24 0.015 TB–12 5/1/2006 452 652 0.31 <0.2 23.7 e0.02 1.91 <0.008 TB–15 5/2/2006 31.8 77.7 0.15 8.3 13.2 0.09 0.25 <0.008 TB–18D 5/1/2006 190 398 0.16 <0.2 31.1 0.21 0.22 <0.008 TB–1S 2/7/2006 34 142 0.18 17.9 15.7 0.22 e0.05 e0.006 TB–18S 5/2/2006 39.1 258 0.14 14.4 50.8 <0.04 5.11 <0.008 TB–1D 2/7/2006 8.51 9.44 0.9 4.4 5.93 <0.04 0.31 <0.008 TB–5D 2/6/2006 208 367 0.31 8.5 19.8 0.05 0.26 e0.006 TB–2D 2/7/2006 7.64 10.4 0.84 3.7 6.72 e0.03 0.3 <0.008 TB–17D 2/7/2006 195 408 0.32 8.9 52.1 0.07 <0.06 <0.008 
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than] Local well name Sampling date Barium, water, unfiltered, recoverable, in μg/L (01007) Cadmium, water, unfiltered, in μg/L (01027) Chromium, water, unfiltered, recoverable, in μg/L (01034) Copper, water, unfiltered, recoverable, in μg/L (01042) Iron, water, unfiltered, recoverable, in μg/L (01045) Lead, water, unfiltered, recoverable, in μg/L (01051) Manganese, water, unfiltered, recoverable, in μg/L (01055) Mercury, water, unfiltered, recoverable, in μg/L (71900) 

East Basin 2/8/2006 15.4 <0.040 0.15 0.9 39.8 0.24 14.3 <0.010 

Seep A 2/6/2006 26.2 <0.040 0.61 2 251 0.42 6.2 e0.008 

Seep E 2/8/2006 20.7 e0.022 0.42 1.2 55.1 0.38 1.5 <0.010 

TB–17S 2/7/2006 283 0.271 71.8 71.2 50,100 26.5 1,710 0.633 

TB–5S 5/1/2006 495 0.233 61.2 30.9 13,900 9.28 248 0.13 

B–3P 5/1/2006 658 0.83 132 276 102,000 123 3,370 0.65 

TB–12 5/1/2006 158 1.53 13 18.1 7,370 7.58 159 0.032 

TB–15 5/2/2006 24.5 e0.022 3.4 2.4 3,760 0.79 579 0.021 

TB–18D 5/1/2006 310 0.275 163 89.5 41,900 18.8 5,840 0.085 

TB–1S 2/7/2006 305 0.391 31.8 46 25,100 22.2 4,190 0.047 

TB–18S 5/2/2006 134 0.226 16.7 11.4 5,130 2.85 97.1 0.015 

TB–1D 2/7/2006 16.1 <0.040 3.7 2.8 1,120 1.08 32.6 <0.010 

TB–5D 2/6/2006 83 e0.029 2.9 2.9 1,210 0.47 1,210 <0.010 

TB–2D 2/7/2006 10.7 <0.040 2.2 1.9 328 1.11 16.2 <0.010 

TB–17D 2/7/2006 96.9 0.105 6.8 10.4 3,520 13.7 3,550 0.011 
Local well 

name

Sampling 

date

Zinc, water, 

unfiltered, 

recoverable, 

in μg/L 

(01092)

1,2,3-

Trichloropropane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77443)

1,2-

Dibromo-3-

chloropropane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(82625)

1,2-

Dibromoethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77651)

1,2-

Dichloroethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(32103)

1,2-

Dichloropropane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34541)

1,3-

Dichloropropane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77173)

1,4-

Dichlorobenzene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34571)

 

(

r

East Basin 2/8/2006 <2 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

Seep A 2/6/2006 4 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

Seep E 2/8/2006 2 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–17S 2/7/2006 116 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–5S 5/1/2006 44 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

B–3P 5/1/2006 415 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–12 5/1/2006 40 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–15 5/2/2006 5 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–18D 5/1/2006 226 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–1S 2/7/2006 73 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–18S 5/2/2006 14 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–1D 2/7/2006 4 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–5D 2/6/2006 3 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–2D 2/7/2006 3 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

TB–17D 2/7/2006 22 <0.18 <0.51 <0.036 <0.13 <0.029 <0.06 <0.034 

Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than]
Local well name Sampling date 2-Chloro- 2’,6’-diethyl- acetanilide, water, filtered, recoverable, in μg/L (61618) 2-Chloro-4- isopropyl- amino-6- amino-s- triazine, water, filtered, recoverable, in μg/L (04040) 2-Ethyl- 6-methyl- aniline, water, filtered, recoverable, in μg/L (61620) 3,4-Dichloro- aniline, water, filtered, recoverable, in μg/L (61625) 3,5-Dichloro- aniline, water, filtered, recoverable, in μg/L ( 61627) 3-Chloro- propene, water, unfiltered, recoverable, in μg/L (78109) 4-Chloro-2- methyl- phenol, water, filtered, recoverable, in μg/L (61633) Acetochlor, water, filtered, recoverable, in μg/L (49260) A

r

East Basin 2/8/2006 <0.005 e0.005 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

Seep A 2/6/2006 <0.005 e0.004 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

Seep E 2/8/2006 <0.005 e0.004 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

TB–17S 2/7/2006 <0.005 e0.004 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

TB–5S 5/1/2006 <0.006 e0.006 <0.010 <0.0045 <0.012 <0.50 <0.0050 <0.006 

B–3P 5/1/2006 <0.006 e0.010 <0.010 <0.0045 <0.012 <0.50 <0.0050 <0.006 

TB–12 5/1/2006 <0.006 e0.007 <0.010 <0.0045 <0.012 <0.50 <0.0050 <0.006 

TB–15 5/2/2006 <0.006 e0.006 <0.010 <0.0045 <0.012 <0.50 <0.0050 <0.006 

TB–18D 5/1/2006 <0.006 e0.006 <0.010 <0.0045 <0.012 <0.50 <0.0050 <0.006 

TB–1S 2/7/2006 <0.005 <0.006 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

TB–18S 5/2/2006 <0.006 <0.014 <0.010 <0.0045 <0.012 <0.50 <0.0050 <0.006 

TB–1D 2/7/2006 <0.005 e0.005 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

TB–5D 2/6/2006 <0.005 e0.005 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

TB–2D 2/7/2006 <0.005 <0.006 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

TB–17D 2/7/2006 <0.005 <0.006 <0.004 <0.0045 — <0.50 <0.0056 <0.006 

Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than]
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than]

Local well 

name

Sampling 

date

alphaEndosulfan, 

water, 

filtered, 

recoverable, 

in μg/L 

(34362)

Atrazine, 

water, 

filtered, 

recoverable, 

in μg/L

(39632)

 Azinphosmethyl oxygen 

analog, 

water, 

filtered, 

recoverable, 

in μg/L 

(61635)

Azinphosmethyl, 

water, 

filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82686)

Benfluralin, 

water, filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82673)

Bromomethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34413)

Carbaryl, 

water, 

filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82680)

Carbofuran, 

water, 

filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82674)

East Basin 2/8/2006 — e0.005 <0.070 <0.050 <0.010 <0.26 <0.041 — 

Seep A 2/6/2006 — e0.005 <0.070 <0.050 <0.010 <0.26 <0.041 — 

Seep E 2/8/2006 — e0.005 <0.070 <0.050 <0.010 <0.26 <0.041 — 

TB–17S 2/7/2006 — e0.005 <0.070 <0.050 <0.010 <0.26 <0.041 — 

TB–5S 5/1/2006 <0.011 e0.004 <0.042 <0.050 <0.010 <0.33 <0.041 <0.020 

B–3P 5/1/2006 <0.011 e0.007 <0.042 <0.050 <0.010 <0.33 <0.041 <0.020 

TB–12 5/1/2006 <0.011 e0.005 <0.042 <0.050 <0.010 <0.33 <0.041 <0.020 

TB–15 5/2/2006 <0.011 e0.004 <0.042 <0.050 <0.010 <0.33 <0.041 <0.020 

TB–18D 5/1/2006 <0.011 e0.004 <0.042 <0.050 <0.010 <0.33 <0.041 <0.020 

TB–1S 2/7/2006 — <0.007 <0.070 <0.050 <0.010 <0.26 <0.041 — 

TB–18S 5/2/2006 <0.011 <0.007 <0.042 <0.050 <0.010 <0.33 <0.041 <0.020 

TB–1D 2/7/2006 — e0.005 <0.070 <0.050 <0.010 <0.26 <0.041 — 

TB–5D 2/6/2006 — e0.005 <0.070 <0.050 <0.010 <0.26 <0.041 — 

TB–2D 2/7/2006 — <0.007 <0.070 <0.050 <0.010 <0.26 <0.041 — 

TB–17D 2/7/2006 — <0.007 <0.070 <0.050 <0.010 <0.26 <0.041 — 
Local well name Sampling date Chlorpyrifos, water, filtered, recoverable, in μg/L (38933) cis-1,3- Dichloro propene, water, unfiltered, recoverable, in μg/L (34704) cis- Permethrin, water, filtered (0.7 μm glass fiber filter), recoverable, in μg/L (82687) cis-Propi- conazole, water, filtered, recoverable, in μg/L (79846) Cyanazine, water, filtered, recoverable, in μg/L (04041) Cyfluthrin, water, filtered, recoverable, in μg/L (61585) Cyper- methrin, water, filtered, recoverable, in μg/L (61586) DCPA, water, filtered (0.7 μm glass fiber filter), recoverable, in μg/L (82682) 

r

East Basin 2/8/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 Seep A 2/6/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 

Seep E 2/8/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 TB–17S 2/7/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 TB–5S 5/1/2006 <0.0050 <0.05 <0.006 <0.013 <0.018 <0.053 <0.046 <0.0030 B–3P 5/1/2006 <0.0050 <0.05 <0.006 <0.013 <0.018 <0.053 <0.046 <0.0030 TB–12 5/1/2006 <0.0050 <0.05 <0.006 <0.013 <0.018 <0.053 <0.046 <0.0030 TB–15 5/2/2006 <0.0050 <0.05 <0.006 <0.013 <0.018 <0.053 <0.046 <0.0030 TB–18D 5/1/2006 <0.0050 <0.05 <0.006 <0.013 <0.018 <0.053 <0.046 <0.0030 TB–1S 2/7/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 

TB–18S 5/2/2006 <0.0050 <0.05 <0.006 <0.013 <0.018 <0.053 <0.046 <0.0030 TB–1D 2/7/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 TB–5D 2/6/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 

TB–2D 2/7/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 TB–17D 2/7/2006 <0.0050 <0.05 <0.006 — — <0.027 <0.009 <0.0030 

Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than]
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than]

Local well 

name

Sampling 

date

Diazinon, 

water, 

filtered, 

recoverable, 

in μg/L 

(39572)

Dichlorvos, 

water, 

filtered, 

recoverable, 

in μg/L 

(38775)

Dicrotophos, 

water, 

filtered, 

recoverable, 

in μg/L 

(38454)

Dieldrin, 

water, 

filtered, 

recoverable, 

in μg/L 

(39381)

Dimethoate, 

water, 

filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82662)

Disulfoton 

sulfone, 

water, 

filtered, 

recoverable, 

in μg/L 

(61640)

Disulfoton, 

water, 

filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82677)

Endosulfan 

sulfate, 

water, 

filtered, 

recoverable, 

in μg/L 

(61590)

E

(

r

East Basin 2/8/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 

Seep A 2/6/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 

Seep E 2/8/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 

TB–17S 2/7/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 

TB–5S 5/1/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 <0.014 <0.021 <0.022 

B–3P 5/1/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 <0.014 <0.021 <0.022 

TB–12 5/1/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 <0.014 <0.021 <0.022 

TB–15 5/2/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 <0.014 <0.021 <0.022 

TB–18D 5/1/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 <0.014 <0.021 <0.022 

TB–1S 2/7/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 

TB–18S 5/2/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 <0.014 <0.021 <0.022 

TB–1D 2/7/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 

TB–5D 2/6/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 

TB–2D 2/7/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 

TB–17D 2/7/2006 <0.0050 <0.01 <0.08 <0.009 <0.0061 — — — 
Local well name Sampling date Ethion, water, filtered, recoverable, in μg/L (82346) Ethoprop, water, filtered (0.7 μm glass fiber filter), recoverable, in μg/L (82672) Fenamiphos sulfone, water, filtered, recoverable, in μg/L (61645) Fenamiphos sulfoxide, water, filtered, recoverable, in μg/L (61646) Fenamiphos, water, filtered, recoverable, in μg/L (61591) Fipronil sulfide, water, filtered, recoverable, in μg/L (62167) Fipronil sulfone, water, filtered, recoverable, in μg/L (62168) Fipronil, water, filtered, recoverable, in μg/L (62166) 

r

East Basin 2/8/2006 <0.004 — <0.049 <0.04 <0.029 <0.013 <0.024 <0.016 

Seep A 2/6/2006 <0.004 — <0.049 <0.04 <0.029 <0.013 <0.024 <0.016 

Seep E 2/8/2006 <0.004 — <0.049 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–17S 2/7/2006 <0.004 — <0.049 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–5S 5/1/2006 <0.016 <0.012 <0.053 <0.04 <0.029 <0.013 <0.024 <0.016 

B–3P 5/1/2006 <0.016 <0.012 <0.053 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–12 5/1/2006 <0.016 <0.012 <0.053 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–15 5/2/2006 <0.016 <0.012 <0.053 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–18D 5/1/2006 <0.016 <0.012 <0.053 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–1S 2/7/2006 <0.004 — <0.049 <0.04 <0.029 e0.006 <0.024 <0.016 

TB–18S 5/2/2006 <0.016 <0.012 <0.053 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–1D 2/7/2006 <0.004 — <0.049 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–5D 2/6/2006 <0.004 — <0.049 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–2D 2/7/2006 <0.004 — <0.049 <0.04 <0.029 <0.013 <0.024 <0.016 

TB–17D 2/7/2006 <0.004 — <0.049 <0.04 <0.029 <0.013 <0.024 <0.016 

Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than]
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO2, silicon 

<, less than]

Local well 

name

Sampling 

date

 Iodomethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77424)

 Iprodione, 

water, 

filtered, 

recoverable, 

in μg/L 

(61593)

 Isofenphos, 

water, 

filtered, 

recoverable, 

in μg/L 

(61594)

lambdaCyhalothrin, 

water, 

filtered, 

recoverable, 

in μg/L 

(61595)

 Malaoxon, 

water, 

filtered, 

recoverable, 

in μg/L 

(61652)

 Malathion, 

water, 

filtered, 

recoverable, 

in μg/L 

(39532)

Metalaxyl, 

water, 

filtered, 

recoverable, 

in μg/L 

(61596)

 Methidathion, 

water, 

filtered, 

recoverable, 

in μg/L 

(61598)

w

r

East Basin 2/8/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.005 <0.006 

Seep A 2/6/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.005 <0.006 

Seep E 2/8/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.005 <0.006 

TB–17S 2/7/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.005 <0.006 

TB–5S 5/1/2006 <0.50 <0.026 <0.011 <0.014 <0.039 <0.027 <0.012 <0.007 

B–3P 5/1/2006 <0.50 <0.026 <0.011 <0.014 <0.039 <0.027 <0.007 <0.009 

TB–12 5/1/2006 <0.50 <0.026 <0.011 <0.014 <0.039 <0.027 <0.008 <0.017 

TB–15 5/2/2006 <0.50 <0.026 <0.011 <0.014 <0.039 <0.027 <0.007 <0.009 

TB–18D 5/1/2006 <0.50 <0.026 <0.011 <0.014 <0.039 <0.027 <0.007 <0.009 

TB–1S 2/7/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.008 <0.006 

TB–18S 5/2/2006 <0.50 <0.026 <0.011 <0.014 <0.039 <0.027 <0.007 <0.009 

TB–1D 2/7/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.005 <0.006 

TB–5D 2/6/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.005 <0.006 

TB–2D 2/7/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.005 <0.006 

TB–17D 2/7/2006 <0.50 <0.538 <0.003 — <0.030 <0.027 <0.005 <0.006 
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2 <, less than] Local well name Sampling date Metolachlor, water, filtered, recoverable, in μg/L (39415) Metribuzin, water, filtered, recoverable, in μg/L (82630) Molinate, water, filtered (0.7 μm glass fiber filter), recoverable, in μg/L (82671) Myclobutanil, water, filtered, recoverable, in μg/L (61599) Oxyfluorfen, water, filtered, recoverable, in μg/L (61600) Pendimethalin, water, filtered (0.7 μm glass fiber filter), recoverable, in μg/L (82683) Phorate oxygen analog, water, filtered, recoverable, in μg/L (61666) Phorate, water, filtered (0.7 μm glass fiber filter), recoverable, in μg/L (82664) 

r

East Basin Seep A Seep E TB–17S TB–5S B–3P TB–12 TB–15 TB–18D TB–1S TB–18S TB–1D TB–5D TB–2D TB–17D 2/8/2006 2/6/2006 2/8/2006 2/7/2006 5/1/2006 5/1/2006 5/1/2006 5/2/2006 5/1/2006 2/7/2006 5/2/2006 2/7/2006 2/6/2006 2/7/2006 2/7/2006 e0.005 e0.005 e0.005 e0.005 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.006 <0.028 <0.028 <0.028 <0.028 <0.028 <0.006 <0.028 <0.006 <0.006 <0.006 <0.006 ————<0.0030 <0.0030 <0.0030 <0.0030 <0.0030 —<0.0030 ———— <0.008 <0.008 <0.008 <0.008 <0.033 <0.033 <0.033 <0.033 <0.033 <0.008 <0.033 <0.008 <0.008 <0.008 <0.008 ————<0.017 <0.017 <0.017 <0.017 <0.017 —<0.017 ———— <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.022 <0.105 <0.105 <0.105 <0.105 <0.027 <0.027 <0.027 <0.027 <0.027 <0.105 <0.027 <0.105 <0.105 <0.105 <0.105 <0.011 <0.011 <0.011 <0.011 <0.055 <0.055 <0.055 <0.055 <0.055 <0.011 <0.055 <0.011 <0.011 <0.011 <0.011 
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2

<, less than]

Local well 

name

Sampling 

date

 Prometon, 

water, 

filtered, 

recoverable, 

in μg/L 

(04037)

 Prometryn, 

water, 

filtered, 

recoverable, 

in μg/L 

(04036)

Propanil, 

water, 

filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82679)

 Propargite, 

water, 

filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82685)

Propyzamide, 

water, 

filtered 

(0.7 μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82676)

 Simazine, 

water, 

filtered, 

recoverable, 

in μg/L 

(04035)

Tebuthiuron, 

water, 

filtered (0.7 

μm glass 

fiber filter), 

recoverable, 

in μg/L 

(82670)

Tefluthrin, 

water, 

filtered, 

recoverable, 

in μg/L 

(61606)

r

East BasinSeep ASeep ETB–17STB–5S

B–3PTB–12TB–15TB–18DTB–1STB–18STB–1DTB–5DTB–2DTB–17D

2/8/20062/6/20062/8/20062/7/20065/1/2006

5/1/20065/1/20065/2/20065/1/20062/7/20065/2/20062/7/20062/6/20062/7/20062/7/2006

<0.010<0.010<0.010<0.010<0.010

<0.010<0.010<0.010<0.010<0.010<0.010<0.010<0.010<0.010<0.010

<0.005<0.005<0.005<0.005<0.006

<0.006<0.006<0.006<0.006<0.005<0.006<0.005<0.005<0.005<0.005

————

<0.011<0.011<0.011<0.011<0.011

—

<0.011

————

————

<0.023<0.023<0.023<0.023<0.023

—

<0.023

————

<0.0040<0.0040<0.0040<0.0040

<0.0040<0.0040<0.0040<0.0040<0.0040<0.0040<0.0040<0.0040<0.0040<0.0040<0.0040

e0.007e0.006e0.006

<0.005

<0.005<0.005<0.005<0.005<0.005<0.005<0.005

e0.007e0.006e0.006

<0.005

<0.016<0.016<0.016<0.016

<0.016<0.016<0.016<0.016<0.016<0.016<0.016<0.016<0.016<0.016<0.016

————

<0.003<0.003<0.003<0.003<0.003

—

<0.003

————
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2 <, less than] Local well name Sampling date Terbuthylazine, water, filtered, recoverable, in μg/L (04022) Thiobencarb, water, filtered (0.7 μm glass fiber filter), recoverable, in μg/L (82681) trans-1,3- Dichloro- propene, water, unfiltered, recoverable, in μg/L (34699) trans- Propiconazole, water, filtered, recoverable, in μg/L (79847) Tribuphos, water, filtered, recoverable, in μg/L (61610) Trifluralin, water, filtered (0.7 μm glass fiber filter), recoverable, in μg/L (82661) 1,1,1,2-Tetra- chloroethane, water, unfiltered, recoverable, n μg/L (77562) 1,1,1-Tri- chloroethane, water, unfiltered, recoverable, in μg/L (34506) 1

ch

re

East Basin Seep A Seep E TB–17S TB–5S B–3P TB–12 TB–15 TB–18D TB–1S TB–18S TB–1D TB–5D TB–2D TB–17D 2/8/2006 2/6/2006 2/8/2006 2/7/2006 5/1/2006 5/1/2006 5/1/2006 5/2/2006 5/1/2006 2/7/2006 5/2/2006 2/7/2006 2/6/2006 2/7/2006 2/7/2006 <0.010 <0.010 <0.010 <0.010 <0.008 <0.008 <0.008 <0.008 <0.008 <0.010 <0.008 <0.010 <0.010 <0.010 <0.010 ————<0.010 <0.010 <0.010 <0.010 <0.010 —<0.010 ———— <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 <0.09 ———— <0.034 <0.034 <0.034 <0.034 <0.034 — <0.034 ———— ————<0.035 <0.035 <0.035 <0.035 <0.035 —<0.035 ———— <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.009 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 <0.032 
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2

<, less than]

Local well 

name

Sampling 

date

 1,1,2-Trichloroethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34511)

 1,1-Dichloroethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34496)

1,1-Dichloroethene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34501)

1,1-Dichloropropene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77168)

1,2,3,4- 

Tetramethylbenzene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(49999)

 1,2,3,5- 

Tetramethylbenzene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(50000)

1,2,3-Trichlorobenzene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77613)

 1,2,3-Trimethylbenzene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77221)

1,2,

u

re

East Basin

Seep A

Seep E

TB–17S

TB–5S

B–3P

TB–12

TB–15

TB–18D

TB–1S

TB–18S

TB–1D

TB–5D

TB–2D

TB–17D

2/8/2006

2/6/2006

2/8/2006

2/7/2006

5/1/2006

5/1/2006

5/1/2006

5/2/2006

5/1/2006

2/7/2006

5/2/2006

2/7/2006

2/6/2006

2/7/2006

2/7/2006

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.040

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.035

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.026

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.14

<0.140

<0.140

<0.140

<0.140

<0.180

<0.180

<0.180

<0.180

<0.180

<0.140

<0.180

<0.140

<0.140

<0.140

<0.140

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.18

<0.060

<0.060

<0.060

<0.060

<0.090

<0.090

<0.090

<0.090

<0.090

<0.060

<0.090

<0.060

<0.060

<0.060

<0.060
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2 <, less than] Local well name Sampling date 1,2-Dichloro- benzene, water, unfiltered, recoverable, in μg/L (34536) 1,3,5-Trimethyl- benzene, water, unfiltered, recoverable, in μg/L (77226) 1,3-Dichloro- benzene, water, unfiltered, recoverable, in μg/L (34566) 2,2-Dichloro- propane, water, unfiltered, recoverable, in μg/L (77170) 2-Chloro- toluene, water, unfiltered, recoverable, in μg/L (77275) 2-Ethyl- toluene, water, unfiltered, recoverable, in μg/L (77220) 4-Chloro- toluene, water, unfiltered, recoverable, in μg/L (77277) Isopropyl- toluene, water, unfiltered, recoverable, in μg/L (77356) 

r

East Basin Seep A Seep E TB–17S TB–5S B–3P TB–12 TB–15 TB–18D TB–1S TB–18S TB–1D TB–5D TB–2D TB–17D 2/8/2006 2/6/2006 2/8/2006 2/7/2006 5/1/2006 5/1/2006 5/1/2006 5/2/2006 5/1/2006 2/7/2006 5/2/2006 2/7/2006 2/6/2006 2/7/2006 2/7/2006 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.048 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.044 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.040 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.050 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 <0.08 
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2

<, less than]

Local well 

name

Sampling 

date

Bromobenzene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(81555)

 Bromochloromethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77297)

Bromodichloromethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(32101)

 Bromoethene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(50002)

 Chlorobenzene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34301)

 Chloroethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34311)

 Chloromethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34418)

 cis-1,2- 

Dichloroethene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77093)

r

East Basin

Seep A

Seep E

TB–17S

TB–5S

B–3P

TB–12

TB–15

TB–18D

TB–1S

TB–18S

TB–1D

TB–5D

TB–2D

TB–17D

2/8/2006

2/6/2006

2/8/2006

2/7/2006

5/1/2006

5/1/2006

5/1/2006

5/2/2006

5/1/2006

2/7/2006

5/2/2006

2/7/2006

2/6/2006

2/7/2006

2/7/2006

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

3.65

0.666

0.543

0.932

e0.035

<0.028

e0.034

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.10

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.028

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.12

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.170

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024

<0.024
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2 <, less than] Local well name Sampling date Dichlorodi- fluoro- methane, water, unfiltered, recoverable, in μg/L (34668) Dichloro- methane, water, unfiltered, recoverable, in μg/L (34423) Diethyl ether, water, unfiltered, recoverable, in μg/L (81576) Diisopropyl ether, water, unfiltered, recoverable, in μg/L (81577) Ethyl methacrylate, water, unfiltered, recoverable, in μg/L (73570) Ethyl methyl ketone, water, unfiltered, recoverable, in μg/L (81595) Ethyl- benzene, water, unfiltered, recoverable, in μg/L (34371) Hexachloro- butadiene, water, unfiltered, recoverable, in μg/L (39702) H

r

East Basin Seep A Seep E TB–17S TB–5S B–3P TB–12 TB–15 TB–18D TB–1S TB–18S TB–1D TB–5D TB–2D TB–17D 2/8/2006 2/6/2006 2/8/2006 2/7/2006 5/1/2006 5/1/2006 5/1/2006 5/2/2006 5/1/2006 2/7/2006 5/2/2006 2/7/2006 2/6/2006 2/7/2006 2/7/2006 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.06 e0.04 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.10 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <0.18 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <2.0 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 <0.14 
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2

<, less than]

Local well 

name

Sampling 

date

 Isopropylbenzene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(77223)

Methyl 

acrylate, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(49991)

 Methyl 

acrylonitrile, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(81593)

 Methyl 

methacrylate, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(81597)

 Methyl 

tert-butyl 

ether, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(78032)

Methyl 

tertpentyl 

ether, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(50005)

Methylene 

blue active 

substances, 

water, 

unfiltered, 

recoverable, 

milligrams 

per liter 

(38260)

 m-Xylene 

plus pxylene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(85795)

 N

East BasinSeep ASeep ETB–17STB–5SB–3PTB–12TB–15TB–18DTB–1STB–18S

TB–1DTB–5DTB–2DTB–17D

2/8/20062/6/20062/8/20062/7/20065/1/20065/1/20065/1/20065/2/20065/1/20062/7/20065/2/2006

2/7/20062/6/20062/7/20062/7/2006

<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038

<0.038<0.038<0.038<0.038<0.038

<1.0<1.0<1.0<1.0<1.0<1.0<1.0<1.0<1.0<1.0

<1.0<1.0<1.0<1.0<1.0

<0.40<0.40<0.40<0.40<0.40<0.40<0.40<0.40<0.40<0.40

<0.40<0.40<0.40<0.40<0.40

<0.20<0.20<0.20<0.20<0.20<0.20<0.20<0.20<0.20<0.20

<0.20<0.20<0.20<0.20<0.20

<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10

<0.10<0.10<0.10<0.10<0.10

<0.04<0.04<0.04<0.04<0.04<0.04<0.04<0.04<0.04<0.04

<0.04<0.04<0.04<0.04<0.04

<0.100<0.100<0.100<0.100

0.1210.1230.194

<0.100<0.100

e0.085

<0.100

—

<0.100<0.100

e0.072

<0.06<0.06<0.06

e0.03

<0.06<0.06<0.06<0.06<0.06<0.06

<0.06<0.06<0.06<0.06<0.06
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New [Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocasurface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon 

2 <, less than] Local well name Sampling date n-Butyl- benzene, water, unfiltered, recoverable, in μg/L (77342) n-Propyl- benzene, water, unfiltered, recoverable, in μg/L (77224) o-Xylene, water, unfiltered, recoverable, in μg/L (77135) sec-Butyl- benzene, water, unfiltered, recoverable, in μg/L (77350) Styrene, water, unfiltered, recoverable, in μg/L (77128) tert-Butyl ethyl ether, water, unfiltered, recoverable, in μg/L (50004) tert-Butyl- benzene, water, unfiltered, recoverable, in μg/L (77353) Tetrachloro- ethene, water, unfiltered, recoverable, in μg/L (34475) T

r

East Basin Seep A Seep E TB–17S TB–5S B–3P TB–12 TB–15 TB–18D TB–1S TB–18S TB–1D TB–5D TB–2D TB–17D 2/8/2006 2/6/2006 2/8/2006 2/7/2006 5/1/2006 5/1/2006 5/1/2006 5/2/2006 5/1/2006 2/7/2006 5/2/2006 2/7/2006 2/6/2006 2/7/2006 2/7/2006 <0.12 <0.12 <0.12 <0.12 <0.14 <0.14 <0.14 <0.14 <0.14 <0.12 <0.14 <0.12 <0.12 <0.12 <0.12 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.038 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.060 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.042 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.060 <0.060 <0.060 <0.060 <0.080 <0.080 <0.080 <0.080 <0.080 <0.060 <0.080 <0.060 <0.060 <0.060 <0.060 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 <0.030 
Table 2. Concentrations of detected constituents in water samples collected from the Hillview Reservoir, City of Yonkers, Westchester County, New York, in 2006.—Continued[Numbers in parentheses are National Water Information System (NWIS) parameter codes. DCPA, dimethyl tetrachloroterephthalate; e, estimated; EPTC, S-ethyl dipropylthiocarbamate; BLS, below land surface datum; mg/L, milligrams per liter; μg/L, micrograms per liter; μS/cm, microsiemens per centimeter at 25 degrees Celsius (°C); N, nitrogen; P, phosphorus; SiO , silicon dioxide; μm, micrometer; <, less than] 2

Local well 

name

Sampling 

date

Toluene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34010)

 trans-1,2- 

Dichloroethene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34546)

trans-1,4-

Dichloro-2-

butene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(73547)

Tribromomethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(32104)

 Trichloroethene, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(39180)

Trichlorofluoromethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(34488)

Trichloromethane, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(32106)

 Vinyl 

chloride, 

water, 

unfiltered, 

recoverable, 

in μg/L 

(39175)

East BasinSeep ASeep ETB–17STB–5SB–3PTB–12TB–15TB–18DTB–1STB–18STB–1DTB–5DTB–2DTB–17D

2/8/20062/6/20062/8/20062/7/20065/1/20065/1/20065/1/20065/2/20065/1/20062/7/20065/2/20062/7/20062/6/20062/7/20062/7/2006

e0.018

<0.020<0.020

0.107

e0.060e0.023e0.012

<0.020<0.020<0.020<0.020<0.020<0.020

4.78

e0.022

<0.032<0.032<0.032<0.032<0.032<0.032<0.032<0.032<0.032<0.032<0.032<0.032<0.032<0.032<0.032

<0.7<0.7<0.7<0.7<0.7<0.7<0.7<0.7<0.7<0.7<0.7<0.7<0.7<0.7<0.7

<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10<0.10

<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038<0.038

<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08

14.5

7.577.49

12

1.75

<0.024

1.54

14.320.9

<0.024

e0.042

22.922.424.1

0.625

<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08<0.08
This page has been left blank intentionally.
Appendix 1.—Water Elevation and Precipitation at 

Selected Wells at the Hillview Reservoir, City of Yonkers, 

Westchester County, New York

Hydrographs in the following figures show elevation of water levels in relation to daily 

precipitation at the Hillview Reservoir, City of Yonkers, Westchester County, New York, 

April 1, 2005–March 1, 2008.
0

1

2

3

4

5

6

7

8

274

276

278

280

282

284

286

288

290

292

294

296

Manual water level—Dashed lines 

are intended as a visual aid

EXPLANATION

Daily precipitation, in inches

Water level elevation, in feet above NGVD 29

Precipitation

Date

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

Figure 1–1. Elevation of water levels at well B–3P in relation to daily precipitation at the Hillview Reservoir, City of 

Yonkers, Westchester County, New York, April 1, 2005–March 1, 2008, as recorded at Central Park (National Oceanic 

and Atmospheric Administration, 2008). The location of the well is shown in figure 1.
Appendix 1 57

0

1

2

3

4

5

6

7

8

275

277

279

281

283

285

287

289

291

293

295

297

Daily precipitation, in inches

Water level elevation, in feet above NGVD 29

Date

Manual water level—Dashed lines 

are intended as a visual aid

EXPLANATION

Precipitation

Continuous record water level

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

Figure 1–2. Elevation of water levels at well CMB–2W in relation to daily precipitation at the Hillview Reservoir, City 

of Yonkers, Westchester County, New York, April 1, 2005–March 1, 2008, as recorded at Central Park (National Oceanic 

and Atmospheric Administration, 2008). The location of the well is shown in figure 1.
58 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

0

1

2

3

4

5

6

7

8

267

269

271

273

275

277

279

281

283

285

287

289

Daily precipitation, in inches

Date

Water level elevation, in feet above NGVD 29

Manual water level—Dashed lines 

are intended as a visual aid

EXPLANATION

Precipitation

Continuous record water level

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

Figure 1–3. Elevation of water levels at well TB–2S in relation to daily precipitation at the Hillview Reservoir, City of 

Yonkers, Westchester County, New York, April1, 2005–March 1, 2008, as recorded at Central Park (National Oceanic 

and Atmospheric Administration, 2008). The location of the well is shown in figure 1.
Appendix 1 59

0

1

2

3

4

5

6

7

8

206

208

210

212

214

216

218

220

222

224

226

228

Daily precipitation, in inches

Date

Water level elevation, in feet above NGVD 29

Manual water level—Dashed lines 

are intended as a visual aid

EXPLANATION

Precipitation

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

Figure 1–4. Elevation of water levels at well TB–10 in relation to daily precipitation at the Hillview Reservoir, City of 

Yonkers, Westchester County, New York, April 1, 2005–March 1, 2008, as recorded at Central Park (National Oceanic 

and Atmospheric Administration, 2008). The location of the well is shown in figure 1.
60 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York 04/01/05 05/01/05 05/31/05 06/30/05 07/30/05 08/29/05 09/28/05 10/28/05 11/27/05 12/27/05 01/26/06 02/25/06 03/27/06 04/26/06 05/26/06 06/25/06 07/25/06 08/24/06 09/23/06 10/23/06 11/22/06 12/22/06 01/21/07 02/20/07 03/22/07 04/21/07 05/21/07 06/20/07 07/20/07 08/19/07 09/18/07 10/18/07 11/17/07 12/17/07

01/16/0802/15/08

0

1

2

3

4

5

6

7

8

220

222

224

226

228

230

232

234

236

238

240

242

Daily precipitation, in inches

Date

Water level elevation, in feet above NGVD 29

Manual water level—Dashed lines 

are intended as a visual aid

EXPLANATION

Precipitation

Continuous record water level

Figure 1–5. Elevation of water levels at well TB–12 in relation to daily precipitation at the Hillview Reservoir, City of Yonkers, 

Westchester County, New York, April 1, 2005–March 1, 2008, as recorded at Central Park (National Oceanic and Atmospheric 

Administration, 2008). The location of the well is shown in figure 1.
Appendix 1 61

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

0

1

2

3

4

5

6

7

8

182

184

186

188

190

192

194

196

198

200

202

204

Daily precipitation, in inches

Water level elevation, in feet above NGVD 29

Date

Manual water level—Dashed lines 

are intended as a visual aid

EXPLANATION

Precipitation

Continuous record water level

Figure 1–6. Elevation of water levels at well TB–13 in relation to daily precipitation at the Hillview Reservoir, City 

of Yonkers, Westchester County, New York, April 1, 2005–March 1, 2008, as recorded at Central Park (National 

Oceanic and Atmospheric Administration, 2008). The location of the well is shown in figure 1.
62 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York

0

1

2

3

4

5

6

7

8

196

198

200

202

204

206

208

210

212

214

216

218

Daily precipitation, in inches

Date

Water level elevation, in feet above NGVD 29

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

Manual water level—Dashed lines 

are intended as a visual aid

EXPLANATION

Precipitation

Figure 1–7. Elevation of water levels at well TB–14D in relation to daily precipitation at the Hillview Reservoir, City of 

Yonkers, Westchester County, New York, April 1, 2005–March 1, 2008, as recorded at Central Park (National Oceanic and 

Atmospheric Administration, 2008). The location of the well is shown in figure 1.
Appendix 1 63

04/01/0505/01/0505/31/0506/30/0507/30/0508/29/0509/28/0510/28/0511/27/0512/27/0501/26/0602/25/0603/27/0604/26/0605/26/0606/25/0607/25/0608/24/0609/23/0610/23/0611/22/0612/22/0601/21/0702/20/0703/22/0704/21/0705/21/0706/20/0707/20/0708/19/0709/18/0710/18/0711/17/0712/17/0701/16/0802/15/08

Manual water level—Dashed lines 

are intended as a visual aid

EXPLANATION

Precipitation

Continuous record water level

0

1

2

3

4

5

6

7

8

209

211

213

215

217

219

221

223

225

227

229

231

Daily precipitation, in inches

Date

Water level elevation, in feet above NGVD 29

Figure 1–8. Elevation of water levels at well TB–15 in relation to daily precipitation at the Hillview Reservoir, City of 

Yonkers, Westchester County, New York, April 1, 2005–March 1, 2008, as recorded at Central Park (National Oceanic and 

Atmospheric Administration, 2008). The location of the well is shown in figure 1.
64 Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York 04/01/05 05/01/05 05/31/05 06/30/05 07/30/05 08/29/05 09/28/05 10/28/05 11/27/05 12/27/05 01/26/06 02/25/06 03/27/06 04/26/06 05/26/06 06/25/06 07/25/06 08/24/06 09/23/06 10/23/06 11/22/06 12/22/06 01/21/07 02/20/07 03/22/07 04/21/07 05/21/07 06/20/07 07/20/07 08/19/07 09/18/07 10/18/07 11/17/07

12/17/0701/16/0802/15/08

EXPLANATION

Precipitation

Continuous record water level

0

1

2

3

4

5

6

7

8

250

252

254

256

258

260

262

264

266

268

270

272

Daily precipitation, in inches

Date

Water level elevation, in feet above NGVD 29

Figure 1–9. Elevation of water levels at well TB–18S in relation to daily precipitation at the Hillview Reservoir, City of Yonkers, 

Westchester County, New York, April 1, 2005–March 1, 2008, as recorded at Central Park (National Oceanic and Atmospheric 

Administration, 2008). Blank where data are missing. The location of the well is shown in figure 1.
Prepared by the Pembroke Publishing Service Center 

For additional information write to:

New York Water Science Center

U.S. Geological Survey

2045 Route 112, Bldg. 4

Coram, NY 11727

Information requests:

(518) 285-5602

or visit our Web site at:

http://ny.water.usgs.gov
Chu and others—Geophysical and Hydrologic Analysis of an Earthen Dam Site in Southern Westchester County, New York—Scientific Investigations Report 2012–5247