HomeMy WebLinkAboutHydrogeology of Capt. Kidd area 1987.%
LEBGETTE, BI~ASHEARS ~c {~RAHAM, INr:.
r.E]NSLILTINr~ I-~RE]UND-WATER BE:OLmI~ISTS
December 18,1987
Mr. William Schickler
Suffolk County Water Authority
Sunrise Highway and Pond Road
Oakdale, NY 11769
RE:
Hydrogeology of the Captain
Kidd Area, Mattituck, New York
Dear Mr. Schickler:
As requested, Leggette, Brashears & Graham, Inc, (LBG)
has reviewed the hydrogeology of the Captain Kidd Water
Company, located in Mattituck, New York. The purpose of
this study is to determine the potential for supplying
adquate water to the area without inducing salt-water
intrusion.
INTRODUCTION
The Captain Kidd Water Company supplies potable water
to the Captain Kidd Estates in Mattituck, New YoYk. The
system is comprised of two sand and gravel production wells
located six feet apart near the intersections of East Drive
and Inlet Drive. These wells were drilled in 1949 and 1959
and have corresponding Suffolk County well numbers S-06513
and S-17835. Well S-06513 was completed to a depth of 110
feet with a 12-foot length of screen at its base~ The
second well, S-17835 was completed to 101 feet with 11 feet
of screen at its base. According to the Suffolk County
Department of Health. Services (SCDHS) the average daily
demand is 17,000 gpd (gallons per day) or 11.8 gpm (gallons
per minute). LBG understands that an independent assessment
of the water system by Stone and Webster states that the
average annual usage is 50,000 gallons per customer. This
Mr. William Schickler -2- December 18, 1987'
demand is only one third of the average demand for Suffolk
County, reflecting the seasonal nature of these residences.
Therefore, the maximum peak demand could exceed three times
the 17,000 gpd (or 51,000 gpd).
AREAL GEOLOGY
According to McClymonds's et al. (1972), this area is
dominated by the Harbor Hill Moraine. This geologic unit
exists throughout most of northern Long Island, and is
considered part of the Upper Glacial Aquifer. (Leggette,
Brashears & Graham, Inc., March 1985) discussed the areal
extent of a clay layer in the Upper Glacial on the North
Fork of the ISland. The clay layer has been observed in the
area between depths of 80 and 180 feet. Where it exists,
this clay helps prevent the intrusion of salt water into
potable water wells in the vicinity. Unfortunately, the
extent of the clay layer is not well documented, and it is
not known whether it exists in the Captain Kidd area.
Underlaying the Harbor Hill Moraine is the Magothy
Formation, a geologic unit used extensively for water supply
across Long Island. Krulikas et al. (1983) published a map
of the Magothy Formation indicating the surface to be
approximately 300 to 400 feet below sea level (see fig-
ure 1). Because of its depth, the Magothy Formation is
considered too saline in this area to be used for potable
water.
SALT WATER INTRUSION
Knowing the depth of the salt water interface is
important for the proper design and operation of production
wells. Lacking specific data for the location of the
interface in the Captain Kidd area, the Ghyben-Herzberg
principal can be used to estimate the depth to saline water.
The Ghyben-Herzberg principal predicts that for every foot
LEGG£TTE, BRASHEARS & r'~RAHAM, INC.
SUFFOLK COUNTY WATER AUTHORITY
CAPTAIN KID AREA
MATTITUCK. NEW YORK
STRUCTURAL CONTOUR MAP
OF THE MAGOTHY FORMATION
( FROM KROLIKAS ET. AC,. 1983)
-400
-300
·-24~
FIGURE I
·-356
SCALE IN MILES
2 4
·-135
LEGGETT~ BRASHEARS & GRAHAM~ INC.
Mr. William Schickler -3- December 18, 1987
of water above sea-level, there are forty feet of fresh
water below sea-level. The nearest well from which wa-
ter-table data could be obtained is the SCDHS observation
well 53325. The well is located on Mill Road in Mattituck
and has a static water-table elevation of 1.77 feet above
sea-level, or about 40-feet bg (below grade). A review of
the County water table data supplied by the SCDHS indicates
that the elevation of the water in the Captain Kidd area has
fluctuated about 1 to 2.5 feet above sea-level between 1980
and 1987. Therefore, the expected thickness of the fresh
water lens is about 40 to 100 feet. Although the
Ghyben-Herzberg principle is a good first estimation, the
actual thickness of the fresh water lens can vary depending
on the local geology.
The screened intervals of the two production wells are
27 to 39 feet, and 31 to 41 feet below sea-level. Since
saline water has not been reported in either of the wells
(see table 2 below), the fresh water/salt water interface is
probably substantially below 40 feet.
WATER QUALITY
LBG has reviewed the water chemistry data logged by the
SCDHS in order to determine the past and present quality of
the groundwater in the area. Table 1 shows the chemical
data with road or street designations, obtained in or near
the Captain Kidd subdivision. Unfortunately, the well data
from which these samples were taken was not obtainable,
therefore the depths which the data represent are unknown.
It is most likely that these data represent water skimmed
off the top of the saturated zone and not deeper regions of
the aquifer. The chloride values are consistently about
30 +1-_5 ppm (parts per million) except for two samples
taken on Sound Beach Road, the street closest to Long Island
Sound. The two anomolous values are 70 and 129 ppm, values
well below the New York State drinking water standard of
250 ppm. Nitrate levels, as indicated in Table 1, range
LI:'I~'r'£T?£, ~I~A'=HI='AR~ & I"~'I~AHAM, IN?~.
Mr. william Schickler -4- December 18, 1987
between 0.4 and 12.4 ppm, but average about 6.25 ppm. The
higher values are most likely elevated due to the close
proximity of some septic leach fields to the homeowner
wells, and are not representative of a regional elevated
nitrate problem.
Water-quality data of the Captain Kidd Water Company
was also reviewed (see table 2). These data indicate that
the salinity of the production wells is not excessive,
ranging from 27 to 44 ppm, averaging about 32.6 ppm.
Nitrates values average approximately 5 ppm, well below New
York State potable water standards. All other water quality
parameters are well within drinking water standards.
WATER-SUPPLY POTENTIAL
The lack of specific data regarding the geology at
depth, elevation of the water table, and/or the nature and
depth of the salt-water interface reduces our ability to
predict water supply potential. Conceptually, since there
is no present information indicating that the water supply
is inducing salt-water intrusion, the Authority could
continue pumping the existing system, or develop a new
supply capable of similar yields and not expect salinity
problems.
A limiting factor to developing additional supplies in
the area is the regional recharge. The Comprehensive Public
Water Supply Study for Suffolk County (CPWS-24) recommends a
maximum drought year recharge rate of approximately 0.35 mgd
(million gallons per day) per square mile. The Captain Kidd
area encompasses about one square mile and therefore should
be capable of supplying up to 350,000 gpd.
Calculations based on the methods reviewed by
(Leggette, Brashears & Graham, Inc., March 1985), using
estimated values for various parameters, such as permea-
bility or depth to interface, can be used as guidelines for
system design. Assuming a fresh water lens thickness of
/I:'G(~E:TT£, BI~ASHE:ARS & GRAHAM, INC.
Mr. William Schickler -5- December 18, 1987'
100 feet, penetrated 30 percent by the well, and a permea-
bility of 2t000 gpd/sq.ft. (gallons per day per square
foot), the maximum sustainable yield from a single well
would be on the order of about 250 gpm or 360,000 gpd. This
is a conservative estimate, assuming individual wells are
located such that there is no interference between cones of
influence, the aquifer in the area has no clay layer below
the screened interval, and that the aquifer is isotropic in
nature.
CONCLUSIONS
Based on the above information LBG feels that the
Captain Kidd Water Company has the potential to provide
roughly 20 times the current annual demand of 17,000 gpd.
This maximum capacity is based on both the areal recharge Of
350,000 gpd and the withdrawal rate at which upconing might
reasonably occur, about 360,000 gpd.
The authority should not try to withdraw water near Or
equal to the regional recharge on other than a peak demand
basis to protect the aquifer from lateral salt-water
intrusion. LBG recommends that because of the seasonal
nature of the area that a maximum of 250 gpm (350,000 gpd)
could be supplied during the summer months, but that the
average annual withdrawal should not exceed 200,000 gpd.
Any future water-supply development should entail the
installation of a pilot hole to locate the fresh-water/
salt-water interface and to define the geology. Monitor
wells should be utilized beneath production well screens,
and just above the interface to monitor potential upconing.
Thank you for the opportunity to review this site for
LE:GGE:TT£, BI~ASHE:ARS & GI~AHAI~I, INC.
SELECTED WATER ANALYSES FROM HOMEOWNER WELLS
LOCATED IN, OR NEAR THE CAPTAIN KIDD WATER DISTRICT
TABLE I
Year Street Chloride Nitrate
1972-1983
1984-1986
Sound Beach Road
Captain Kidd Drive
Summit Drive
Central Drive
Ruth Road
Stanley Road
Sound Beach Road
Captain Kidd Drive
Summit Drive
Central Drive
Ruth Road
Stanley Road
129 0.4
70 2.7
33.5 0.96
25 8.0
27 3.6
32 5.6
31 9.7
30 5 4
31 4 7
31 5 2
26 5 2
32 9 8
29 9 0
30 5 1
28 7.1
34 12.4
30 6.3
27 7.1
31 6.3
46 5.4
20 9.1
20 7.8
30 5.4
34 5.3
28 5.4
32 5.4
6 8.2
32 11.9
32 4.7
32 5
32 3.9
32 7.2
35 8.6
22 4.1
25 7.3
33 5.9
CEGGETTE, BRASHEARS & ~RAHA~,INC.
Mr. William Schickler -6- December 18, 1987'
the Authority. If you have any questions on any of the
above, please feel free to call.
LEGGETT~, BRASHEARS & GRAHAM,
Robert C. Luhrs
Hydrogeologist
Reviewed by:
Vice-President
RCL:gtk
87r17
INC.
LEG(SE'rTE, BRA:SHE:ARS & GRAHAM, INC.
BIBLIOGRAPHY
Holzmacher, McLendon & Murrell, "Comprehensive
Public Water Supply Study, Suffolk County, New
York, CPWS-24", Vol. 1-III, 1970.
Leggette, Brashears & Graham,
J.B. Lennox, "Factors Affecting
Development in the Southhold Area,
New York," March 1985.
Inc. (LBG),
Water-Supply
Long Island,
R.K. Krulikas, E.J. Koszalka and T.P. Doriski,
"Altitude of the Top of the Matawan Group-Magothy
Formation, Suffolk County, Long Island, New York,"
U.S.G.S. Open File Report 83-137.
LE:GGE:TTE:, BRASHI:'ARS & GRAHAM, INC.
SELECTED WATER'PARAMETERS FOR THE
CAPTAIN KIDD WATER COMPANY
FROM SUFFOLK COUNTY DEPARTMENT
OF HEALTH SERVICES DATABASE
TABLE II
Well Date pH NH3 NO3 C1
S-06513
08/07/79 6.2 5.00 32.0
09/01/81 6.2 *0.04 5.70 32.0
09/22/81 5.9 *0.05 5.30 27.0
11/09/81 6.2 *0.04 5.20 30.0
02/04/82 6.3 *0.04 5.20 32.0
12/22/82 6.2 2.70 5.10 44,0
06/14/84 5.4 *0.04 5.10 31.0
12/31/84 7.3 *0.02 4.60 27.0
11/13/85 6.3 *0.02 5.30 31.0
S-17835
07/28/79 6.1 6.50 28.0
12/02/79 6.3 0.09 4.90 33.0
05/02/80 6.4 *0.02 4.80 29.0
07/09/80 6.0 *0.04 5.50 31.5
03/30/81 6.4 *0.02 5.10 30.0
04/27/81 6.2 *0.04 4.80 34.0
07/09/81 0.26 5.10
09/01/81 6.4 *0.04 5.50 32.0
09/22/81 5.8 *0.05 5.40 27.0
12/01/83 6.3 *0.04 5.10 32.0
12/29/83 6.6 1.20 1.20 34.0
06/14/84 6.4 *0.04 4.50 33.0
12/31/84 7.2 0.03 4.80 28.0
05/24/85 6.5 *0.02 6.30 33.0
11/13/85 6.3 0.02 5.40 33.0
* Denotes less than.