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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.