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 underlain by the Magothy aquifer. A number of pumping tests were made (Table 1). In addition, specific capacity data were used to estimate the transmissivity of the glacial aquifer wells (Table 2). The average conductivity 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 values 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, horizontal 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 precipitation 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 contact 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 progressive 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. Many stream 1953 161.8 70.9 63.2 39.0 courses are little more than low-lying swampy areas with little significant flow. A 1954 134.6 67.3 60.2 44.6 number of tidal estuaries are called streams, 1955 110.5 69.6 59.2 53.5 although fresh-water discharge to them is 1956 116.8 64.5 61.5 52.6 limited to springs. 1957 98.6 68.8 50.0 50.9 1958 157.5 64.8 61.5 39.1 Direct Evapotranspiration 1959 119.1 70.0 66.3 55.7 1960 121.7 65.5 59.9 49.2 Evaporation from lakes and ponds on the 1961 133.6 66.3 60.7 45.5 South Fork represents a direct discharge 1962 118.4 63.5 59.7 50.4 from the water table. Based on evap- 1963 97.0 66.3 55.4 56.9 oration-pan measurements in Mineola on 1964 97.0 66.6 48.0 49.4 western Long Island, evaporation from surface water bodies was estimated to be about 1965 78.0 65.0 53.8 69.3 86 cm/yr (Pluhowski and Kantrowitz, 95.0 65.3 50.3 53.1 1966 1964). The surface of fresh-water bodies 1967 122.7 61.5 60.2 49.0 is a fraction of one percent of the total 1968 100.1 67.1 50.8 50.6 land area, and the total volume evaporated 1969 114.8 67.6 59.4 51.6 is insignificant. Average 116.8 66.5 57.4 49.1 Some direct évapotranspiration of ground water probably occurs wherever the water table lies within 1.5 m of the ground water-level curves and adjusted vertically This correlation demonstrates the rapid surface (Pluhowski and Kantrowitz, 1964). until the best possible fit was obtained. The response of the water table to changes in The total area where the water table is departure from a 2-yr progressive average recharge. The significance of this from a within 1.5 m of the land surface is small has the best correlation with water levels. water resources standpoint cannot be over- because of the prevailing low elevation of Excluding years 1954 and 1956, this corre- emphasized. The response of the water the water table, and the amount of direct lation is almost perfect. table to periods of decreased accretion is évapotranspiration is very small. both rapid and severe. On the other hand, TABLE 4. ANNUAL RECHARGE OF PRECIPITATION the recovery of water levels after a drought Undersea Outflow can be equally rapid. Year Recharge The magnitude of extremes of the water The major portion of ground water is (cm) table is a function of the seasonal and an- discharged by diffusion along the salt-water nual variability in recharge. Extremes in interface and upflow into the ocean and 1951 62.2 water levels were plotted against average tidal estuaries. Recharge over the 340.6 1952 66.0 water levels for six observation wells (Fig. km2 land area of the South Fork is esti- 1953 94.5 7). The extremes are shown either as a per- mated to be about 1.85 X 108 m3/yr. 1954 71.4 cent of average water level or as the actual Gauged streamflow, consumptive with- 1955 49.3 range in feet during the period 1951 to drawals, and direct évapotranspiration ac- 1956 53.1 1969. Since years with approximately the count for about 10 percent of the total. The X 8 1957 46.5 highest and lowest recorded precipitation remaining 90 percent, or 1.67 10 miyr, are included, the range of extremes is close is discharged as undersea outflow. 1958 91.9 to the maximum probable. This graph can 50.8 1959 be used to predict fluctuations in the water 1960 59.2 table. SAFE YIELD 1961 69.9 1962 56.4 DISCHARGE FROM THE The safe yield of an area such as the 1963 39.9 GROUND-WATER RESERVOIR South Fork can be difficult to evaluate. It is 1964 47.0 a function of both hydrologie and ecologic 1965 31.2 Water is discharged from the ground- factors (Fetter, 1972a). Excessive with- 1966 30.1 water reservoir by several natural mech- drawals may not only cause saline-water encroachment but may also lower ground- 1967 59.9 anisms and well withdrawals. Natural dis- water levels and drain streams and ponds. 1968 47.2 charge occurs from small springs and streams along the coast, direct évapotrans- The author developed a method for de- 1969 53.1 piration from the ground-water table, dif- termining the position of the saline-water Average 56.9 fusion along the salt-water interface, and interface beneath oceanic islands (Fetter,

I- It GO

> L_

Figure 4. Hydrographs of water-table observation wells.

! s 3 § 1 O 1 S 3 a 1 S ; s i S 4 o ? S ? S ? » PE C ; s * S 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 I960 1961 1962 1963 1964 1965 1966 1967 1968 1969

CUMULATIVE DEPARTURE OF ANNUAL RECHARGE 1972b). A digital-computer model for the it will not be harmful from a water-supply FROM N-YEAR PROGRESSIVE AVERAGE RECHARGE, South Fork was developed, which com- standpoint. IN INCHES WATER LEVEL RECHARGE puted the water-table elevation and the If one makes the conservative estimate depth to saline ground water under varying that 20 percent of the 1.67 X 108 m3 of conditions of recharge and withdrawal. annual ground-water discharge lost as sub- Any additional ground-water withdraw- sea outflow were recoverable, a pumping als will result in an upward movement of rate of 9.15 x 104 m3/day would result. the saline-water interface, which is nor- This value was tested on the digital model mally from 30 to 180 m below sea level. If by means of a series of 91.5 m3/day wells this movement stabilizes at a position evenly spaced, with none located within where it is below the depth of well screens, 914 m of a coastline.

Figure 6. Correlation of ground-water levels Figure 5. Position of the water table in March 1970 when it was near the long-term average level. with recharge from precipitation.

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 406 C. W. FETTER, JR.

10 A cross section of the resulting salt-water 9 interface is shown on Figure 8. The amount 8 of movement of the salt-water interface 7 would not interfere with public supply 6 wells of moderate depth. The value could 5 probably be increased by careful location of wells and by increasing the withdrawal 4 rates of interior wells. The effects of ground-water withdrawals on the coastal ecology, however, are not known. Environmental impacts should be established and then the safe yield taking them into account should be determined. Because it will be many years before consumptive withdrawals of 9.15 X 104mMay are realized, there is sufficient time to make this determination. 5 6 7 8 9 10 20 40 60 80 200 ACKNOWLEDGMENTS

\ This work was done as part of a doctoral \ \ dissertation at Indiana University. Jas- 100 minko Karanjac was of great assistance "V 90 with suggestions regarding various aspects 80 of the hydrogeology. Robert V. Ruhe and ; 70 J. D. Winslow critically reviewed the manu- j, 60 script. Robert Holzmacher, president of ; 50 Holzmacher, McLendon and Murrell, has « • I 40 permitted use of data from the files of his company. 1 REFERENCES CITED ! 30 Crandell, H. C., Jr., 1963, Geology and ground- water resources of the Town of Southold, ' 20 Suffolk County, New York: U.S. Geol. Sur- vey Water-Supply Paper 1619-GG, 36 p. Fetter, C. W., Jr., 1972a, The concept of safe ground water yield in coastal aquifers: 10 1 2 3 4 5 6 7 8 9 10 20 40 60 80 Water Resources Bull., v. 8, no. 6, p. 1173-1176. AVERAGE WATER LEVEL IN OBSERVATION WELLS IN FEET ABOVE MEAN SEA LEVEL 1972b, Saline water interface beneath Figure 7. Extremes of water level as a function of average water levels in observation wells. oceanic islands: Water Resources Research, v. 8, no. 5, p. 1307-1315. Jacob, C. E., 1945, Correlation of ground-water levels and precipitation on Long Island, New York: New York State Water Power and Control Comm. Bull. GW-14, 20 p. Pluhowski, E. J., and Kantrowitz, I. H., 1964, Hydrology of the Babylon-Islip area, Suf- folk County, Long Island, New York: U.S. Geol. Survey Water-Supply Paper 1768, 119 p. Thornthwaite, C. W., and Mather, J. R., 1955, ra The water balance: Drexel Inst. Technol- •Ji ogy, Pubs, in Climatology, v. 8, no. 1, p. ? 1-86. o 1957, Instruction and tables for computing potential évapotranspiration and the water balance: Drexel Inst. Technology, Pubs, in Climatology, v. 10, no. 3, p. 185-311. Warren, M. A., de Laguna, Wallace, and Lusc- zynski, N. J., 1968, Hydrology of Brook- haven National Laboratory and vicinity, Suffolk County, Long Island, New York: U.S. Geol. Survey Bull. 1156-C, 125 p.

MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 4, 1975 Figure 8. Salt-water interface with pumping REVISED MANUSCRIPT RECEIVED JULY 11, 1975 withdrawals of 9.15 m May. MANUSCRIPT ACCEPTED JULY 17, 1975 Printed in U.S.A.

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