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HomeMy WebLinkAboutNassau Point Ground Water Qualityv INVESTIGATION OF GROUND WATER QUALITY OF THE NASSAU POINT PENINSULA TOWN OF SOUTHOLD, SUFFOLK COUNTY SUFFOLK COUNTY DEPARTMENT OF HEALTH Herbert W. Davids, Director Division of Environmental Health Services · · · Nassau Potnt ......'..:..;.'.'..i...'. ... '-.'..'~, X '. '," · ~ '.'~;'¢-'./ ~'~'. '.':.'~ ..... :.."4-" ~.~ X '.. ~.';.~" . / ~.~'.- ~ ~:' ~ '.:~:.- ...... v' ,. :.:...~' ~...,.,.4.~ r,.-~.~..,~ '~~[... ,. .. -. ~ .... ~:'.~.~"' X , ~....,,..-. .,,-s, .. ~r~, -..~ .. :...; .... . -. ;:-.~.M'. _ 4-.~..* .%. '- · .'...'. '.' '.... ~'% ocEAN Fig. 1 Nassau Point - Little Nog Neck - Town of Southold Location: Nassau Point, also known as Little Hog Neck, ts.a peninsula located on 'the south shore of the North Fork In the Town of Southold, Suffolk County (Fig. 1). The peninsula has an approximate land area of 0.75 sq. mi. and ts surrounded by salt water bodies. Great Peconic Bay borders the peninsula on the west and the Little Pecontc Bay on the east. Haywaters and Broadwaters separate the peninsula from the mainland on the north. A causeway, approximately 1500 feet tn length by 150 feet in width, connects the peninsula to the mainland. Geology: Underlying soil conditJons of Nassau Point consist of primarily glacial outwash deposits. These deposits are chiefly sands and gravels of Upper Pliestocene age. An area, near Wunneweta Pond, consists of ground moraine deposits which are a mixture of clay, sand and pebbles to boulder sized gravel. Shoreline deposits of.recent geologic age comprise the east coast of the Point. Thts formatfon ts well sorted sand and gravel deposited by currect and wave action. The topography elevations range .from sea level to over 50 feet. ~limate: The predominate climate for the area is temperate marine. Temperatures are moderate and precipitation is abundant during the fall, winter and spring. Summer usually is dry. The mean annual precipitation is 44.S inc'hes. The April-September mean precipitation is 20.5 inches. Fig. 2 - Cross Section from L.I. Sound to Little Pecontc Bay -2- Ground Water: A small body of fresh water underlies Nassau Point (Fig. 2). Precipitation provides the only source of replenishment of the ground water reservior. A part of the precipitation flows overland to the surrounding salt water bodies and fresh water ponds. A part is returned to the atmosphere by evaporation and by trans- piration of plants. The remainder of the precipitation seeps into the ground. Some of the seepage into the ground eventually reaches the ground water reservtor and becomes available for withdrawal by wells (Fig. 3). At the present time, all the homes Information supplied by individual depths vary l0 to 100 feet. on the peninsula use private wells. homeowners indicate that well It is estimated that approximately 5 inches of the annual precipitation enters the surface waters as direct runoff and 17 inches is lost throug~ the combined effects of evaporation and transpiration {evapotranspir- ation). Recharge to the ground water reservior is the difference between the amount of precipitation and the sum of direct runoff and evapotrans- piration. The annual recharge for the Nassau Point area is 160 million gallons. A lO~ reduction is included to allow for local clay soil conditions, paving, and residential homes. All the fresh ground water available for use on the contai ned Since the peninsula is under water table conditions in upper Pliestocene deposits. Point is virtually surrounded by salt water, the area is \ \ ,,, Ffg. 4 Ghyben-Herzberg Principle Fig. 5 - Vertfcal Hovemenb of Salty Ground Water Toward a Pumping ~el1 -3- treated as an island of ground water or The specific gravity of the fresh water underlying salt water, therefore, the fresh the salt water within the boundaries of the to the Ghyben-Herzberg principle {Fig. 4). a discrete fresh water lens. is less than that of the water tends to float on island genera'lly according This principle states that equilibriun, the depth of fresh water below sea level at any point on the island, is proportional to the fresh water head above sea level and dependent on the relation of the specific gravities of fresh and salt water. Generally, the relationship shows that fresh water would extend 40 feet below sea level for each foot it extends above sea level. This theory does not take into account the zone of mixed water around the fresh water lens. Little Hog Neck, based on 1963 report, contains a fresh water mound with a crest altitude of slightly more than 1 foot. A report by the Geological Survey of water tabJe elevations as of March lg20 was not of sufficient detail to include Nassau Point since the contour map developed had elevations shown at 5' intervals. The movement of the ground water moves toward the surrounding salt water bodies along flow lines whose direction is normal or perpendicular to the water table contopr. It is estimated by the USGS that there is 600 million cubic feet of deposits satuated with fresh water. This water is not totally availabl( The specific yield of the aquifer is assumed to be about 22% based on studies performed in the Town. This would leave the peninsula with an estimated volume of fresh water in storage as 1,O00 million gallons. -4- In an ideal hydrologic system, discharge equals recharge plus or min~ changes in ground water storage. Pumpage could be allowed to approach \ the daily recharge from precipitation. However, withdrawal must be kept lower than this amount to minimize demand on storage. On the basis of conditions, withdrawal should not exceed 30% of annual recharge Annual recharge = 160 mg - 0.44 mgZday 30% Annual recharge - 0.132 mg/D = Allowable Withdrawal Assuming Use of lO0 gal/capita/day 132000 MGD 100 GPCD = 1320 People Assuming 3.7 Persons Per Home '1320 ~ = 356 Homes. Theroetically there ts sufficient water available based on 1963 data to support 350 homes, however, several factors must be incorporated into any allowable density in this area. These include salt water intrusion and water quality. Salt Water Intrusion: Fresh water generally has a chloride concen~ tration of less than 40 mg/1. The concentration within the zone of diffusion varies from 40 to 1800 mg/1. The underlying body of salty ground water has a chloride concentration of 1600 to 18,000 ppm. Intrusion occurs when pumpage neat the interface draws the salty water upward (Fig. 5). High tides and wave action can inundate the fresh water lens by downward movement. Oredging also causes the inward move- ment of the interface. In a USGS report (1963) four shoreline wells had chloride concentration ranging from 103 to 1600 ppm. Drinking Water Standards states a recommended limit of 250 mg/1. These wells were within 500' of the shoreline. Water Quality: On August 26, 1970, this Unit sampled sixteen wells for partial chemical analysis. The analysis included ammonia, nitrates, -5- detergents, chlorides, tron and pH. Additional samples March 2, lg71. One home was sampled on both occasions seasonal variations. No variation was found. were taken on to determine Nitrates find there way into water supplies from the decomposition of organic nitrogen and ammonia, both of which are present in household sewage.. Lawn and garden fertilizers are also a source of nitrates. Maximum permissible levels of 10 mg/1 due to serious and occasionally fatal waters containing nitrates. nitrate nitrogen has been set poisioning of infants ingesting Detergents have disposal system and therefore a good entering an individual water supply. gents is 0.5 mg or MBAS. their orgin specifically from the household waste indicator that sewage wastes are The recommended limit for deter- Chlorides are a dual indicator. High concentrations can be used to indicate salt water intrusion while lower values show etther domestic sewage pollution and/or salt water intrusion. The Drinking Water Standard for ch]orides is 250 mg/1. Iron concentrations in excess of the recommended Drinking Water Standards is an esthetic problem causing discoloration, taste, and clothes and fixture staining. pH is 14 point range indicating the acidity or alkalinity of the water with a value of 7 being neutral. 0-7 indicates a acidic condition. .Samples - 20 Determination Chiori~es 20 Detergent 13 ~onst!tutent NBAS Chloride Nitrate Nitrate 13 Iron 13 Ammonia 13 pN 20 Range % Of Samples Within Range * 0.1 mg/1 g5 0.1-0.5 5 0-20 20.0 20-40 30.0 40-250 50.0 250 or greater 0 0-7 95 7-10 0 10 or greater 5 Iron 0-0.3 77 0.3 or greater 23 * indicates less than appeared to be receiving detrect cesspool leach- Only one shallow well tngs. There was no evidence of detergent in the aquifer except in the aforementioned case. were not found in a concentrations were except for the east balance in the area Fifty percent of the wells sampled showed varying degrees of salt water intrusion. The chloride concentrations indicating possible intrusion specific geographical area although the higher found in the areas closer to the salt water bodies bluff. It is obvious that there is a tenuous between the salt and fresh water. Certainly any large withdrawals will have drastic effects. The one case of nitrate concentration exceeding standards was caused by cesspool leaching. The generally low concentrations can be attributed to seasonal population -7- fluctuation and the sam11 percentage of land receiving fertilizers. The 1ton concentrations tn excess of standards were probably caused by well inactivity, plumbing or the well casings. · Conclusions 1. Nassau Point h~s a limited quantity of fresh water tn under- ground storage due to the salt water of the Bay's existing below the peninsula. 2. The only source of replenishment of the fresh ground water is by natural precipitation of which only about one half finds it way into the water table by the tnflitratton prOcess. 3. Chemical analyses (chlorides) indicated that salt water intrusion is evident t.n 50~ of the wells surveyed while an additional 30~ had chloride levels in which differentiation between salt intrusion and cesspool pollution could not be actually determined. Eighty percent of the homes are thus experiencing chloride levels in their water supply greater than normal accepted ground water concentrations although none exceeded drinking water standards. 4. The rate of salt water intrusion cannot be accurately es- tablished but the rate wi1'1 be a function of development, population growth, ground water withdrawal and the amount of natural precipitation. Another drought with the resultant lowering of the ground water fresh salt water balance and deterioration. table will further aggrevate the increase the rate of ground water -8- R~Rommendations 1. Establish low density zontng tn the area. Prevent any dredging of the shoreline 3. If the condition of the ground water shows detertoa'tto~, a central water supply for the peninsula should be established to provide management of the resource. Controlled move- ment of the salt water interface could thus be achieved. The feasibility of connecting to a central mainland public supply shoulc be studied. Prepared by: ~ames Hetl, P.E. ~ater Quality Section