Hydrogeology of the South Fork of Long Island, New York CHARLES W. FETTER JR. Department of Geology, University of Wisconsin-Oshkosh, Oshkosh, Wisconsin 54901 ABSTRACT to be 9.15 x 104 m3/day. Key words: The overlying Magothy Formation con- geohydrology, ground water, water budget, tains fine gray sand interbedded with clay The South Fork of Long Island, New saline water interface, coastal aquifers, safe layers. The unit may contain either fresh or York, is underlain by unconsolidated Pleis- yield. saline ground water, depending upon the tocene and Cretaceous sediments resting on location. The contact between the Magothy crystalline bedrock. A two-layered aquifer INTRODUCTION Formation and the Raritan Formation was system contains fresh ground water with not discerned in test-well drilling. There saline ground water in the deeper strata. Long Island is a part of the Atlantic appears to be considerable relief on the sur- The average horizontal hydraulic conduc- Coastal Plain (Fig. 1). Unconsolidated strata face of this formation, because it ranged tivity of the upper aquifer is 49 m/day and rest on a crystalline basement complex the from 37 to 77 m below sea level at different of the lower aquifer is 25 m/day. surface of which slopes at 15 m/km2 to the locations. The average annual precipitation of 1.14 southeast (Fig. 2). Pleistocene sand and gravel form the up- m is the only natural source of fresh water. The lowermost sedimentary unit is the permost units. There are some zones of After consumptive losses the precipitation Raritan Formation of Cretaceous age. Its gravelly till and an occasional clay bed near provides about 1.85 x 10 s m^yr to re- base ranges from about 300 m below sea the coast. These are glacial deposits and charge the water table. Discharge of fresh level in the north to as much as 490 m tend to be heterogeneous. ground water occurs primarily as undersea below sea level in the south. This unit is Along the south shore are occasional de- outflow to the ocean at the perimeter of the clay to sandy clay and contains saline posits of clay below the Pleistocene sand area. The safe yield of the area is estimated ground water. and gravel. It is either the Gardners Clay of Geological Society of America Bulletin, v. 87, p. 401-406, 8 figs., March 1976, Doc. no. 60309. 401 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/3/401/3433702/i0016-7606-87-3-401.pdf by guest on 27 September 2021 402 C. W. FETTER, JR. AQUIFERS Glacio-fluvial deposits on Long Island Figure 2. Geologic cross tend to act as a single geohydrologic unit section of the South Fork. known as the glacial aquifer. This is under- lain by the Magothy aquifer. A number of pumping tests were made (Table 1). In ad- dition, specific capacity data were used to estimate the transmissivity of the glacial aquifer wells (Table 2). The average con- ductivity of the glacial aquifer is 48.5 m/day, whereas that of the Magothy aquifer is 25 m/day. The specific yield of the glacial aquifer was 0.2, which was similar to val- ues determined elsewhere on Long Island (Crandell, 1963; Warren and others, 1968). The storativity value for the Magothy aquifer indicates that it is confined. This is no doubt due to a number of thin, horizon- tal clay layers, because no single layer is continuous for any distance, except along the south shore where the Gardners Clay and (or) Monmouth Greensand may be present. CLIMATE An annual average of 116.8 cm of pre- cipitation was recorded in the area from 1951 through 1969. Evapotranspiration was computed using the Thornthwaite method (Thornthwaite and Mather, 1955, 1957), and it averaged 57.4 cm or almost half of the precipitation (Table 3). A soil a-600 moisture-balance diagram for 1962, a typi- cal year, shows no excess precipitation from May to September (Fig. 3). 0 5000 10000 15000 20000 25000 Meters Pleistocene age or the Monmouth Green- Pleistocene sand is in direct contact with the RECHARGE TO THE AQUIFERS sand of Cretaceous age. Both units are Magothy aquifer. marine deposits of clay and silt and act as The surface deposits are generally dune The water-balance equation (recharge = confining layers for the underlying sand, outwash sand, or very stony ice-con- precipitation — évapotranspiration — run- Magothy aquifer. The units are not con- tact drift. A glacial moraine runs lengthwise off) can be used to compute the annual re- tinuous. Where they are missing the upper along the island. charge to the aquifer system. There are few surface streams, and the total amount of TABLE 1. RESULTS OF PUMPING TESTS OF TEST WELLS overland runoff on Long Island is less than 5 percent of precipitation (Pluhowski and Well no. Aquifer Screen Drilling Pumping Conduc- Transmis- Storativity Kantrowitz, 1964; Warren and others, setting method rate tivity sivity or specific 1968). Annual recharge averaged 57 cm be- 2 (m below msl*) (1/min) (m/day) (m /day) yield tween 1951 and 1969 (Table 4). In addition to natural recharge, there is S-7570 Glacial -19.8 to -27.7 driven 1,646.5 104.3 0.2 from 11,000 to 15,000 m3 annually of S-31037-T-I Magothy -95.7 to-101.8 rotary 378.5 21.6 124.2 artificial recharge through septic tanks. Be- rotary 302.8 9.4 56.4 S-31037-T-II Magothy -69.8 to -75.9 cause this water was pumped from the S-31037-T-III Glacial -42.4 to -48.5 rotary 397.4 13.2 80.1 aquifer, it does not represent an addition to S-31037 Magothy -64.0 to -76.2 rotary 2,679.8 40.7 2,235.6 the water supply. S-31735 Glacial -17.7 to -23.8 rotary 492.1 31.8 190.6 S-33922-T-I Magothy -149.3 to-155.5 rotary 757.0 10.6 64.6 GROUND-WATER LEVELS S-33922-T-II Magothy -89.9 to -96.0 rotary 454.2 27.1 165.2 S-33922 Magothy -88.4 to-100.6 reverse 2,649.5 30.6 1,676.7 6.36 X IO"6 The water table does not extend more rotary than 6 m above sea level. Hydrographs of six observation wells are shown on Figure msl = mean sea level. 4. The elevation of the water table on Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/3/401/3433702/i0016-7606-87-3-401.pdf by guest on 27 September 2021 HYDROGEOLOGY OF THE SOUTH FORK OF LONG ISLAND, NEW YORK 403 » • March of précipitation • —1* Potential évapotranspiration * * Actual évapotranspiration il- / I \ \| Soil water recharge / M J J A MONTH IN 1962 Figure 3. Soil moisture balance diagram for 1962. March 15,1970, was close to the average in the observation wells, and a water-table TABLE 2. TRANSMISSIVITY AND CONDUCTIVITY ESTIMATED FROM map of that date shows the average water- SPECIFIC CAPACITY DATA FOR GLACIAL AQUIFER WELLS table condition (Fig. 5). A significant characteristic of the South Well no. Specific Screen Aquifer Transmissivity Conductivity capacity length thickness (m^day) Fork aquifer is the rapidity with which the (m/day) (1/min/m) (m) (m) water-table elevation responds to changes in the amount of recharge. The response of S-8980 496.7 6.7 16.8 745 44 the water table to changes in the amount of S-30778 1,080.4 24.1 31.7 1,739 54 recharge can be quantified. Jacob (1945) S-30227 1,080.4 24.4 32.0 1,739 54 correlated water levels on western Long Is- S-9470 1,055.5 4.6 13.1 1,739 132 land with cumulative departure from pro- gressive average values of precipitation, al- S-3615 372.5 7.9 11.0 522 47 though he recognized that the use of re- S-3658 198.7 6.1 6.1 248 41 charge rather than precipitation would be S-30207 409.8 6.4 15.2 596 39 more accurate. S-30208 471.9 6.4 14.9 720 48 Cumulative departures from progressive S-1340 372.5 6.4 17.4 522 30 average recharge on the South Fork for 1-, S-1341 447.0 6.7 15.2 671 44 2-, and 3-yr progressive averages were cor- S-17471 347.7 7.6 12.2 484 40 related with the annual average water level S-28928 360.1 7.6 19.5 497 26 in a nearby well (Fig. 6). Scales were S-16668 223.5 9.6 14.3 335 23 selected so the plotted magnitude of the two S-24323 379.4 9.1 14.9 571 38 curves would be approximately equal. The S-14921 931.3 9.6 23^8 1,490 63 departure curves were superimposed on the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/3/401/3433702/i0016-7606-87-3-401.pdf by guest on 27 September 2021 404 C. W. FETTER, JR. TABLE 3. EVAPOTRANSPIRATION AT BRIDGEHAMPTON, 1951-1969 upflow into the bottom of the ocean or tidal estuaries. Year Precipitation Potential Actual Actual (cm) évapotran- évapotran- évapotran- Streamflow spiration spiration spiration (cm) (cm) as a percentage of precipitation Ground-water discharge via springs and small streams is limited to coastal areas. 1951 119.1 67.1 54.4 45.6 There are few streams that drain from the 1952 124.2 68.3 58.1 46.9 interior of the South Fork.