Analysis of Changes in Ground-Water Levels in a Sewered and an Unsewered Area of Nassau County, Long Island, New York


  • Dennis J. Sulam

    1. Hydrologist, U.S. Geological Survey, Water Resources Division, 5 Aerial Way, Syosset, New York 11791. Discussion open until March 1, 1980.
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  • Dennis J. Sulam is a Hydrologist with the Water Resources Division, U.S. Geological Survey. He has a B.S. degree in Geology and an M.S. degree in Environmental Sciences. He has 12 years of experience in urban hydrology with the U.S. Geological Survey. He has authored papers on artificial recharge and on the impact on water resources caused by urbanization.


From the 195O's to the early 1970's expansion of sanitary sewerage in southwest Nassau County contributed to progressive declines in ground-water levels. Since the early 197O's, however, 10 years after the area was fully sewered, water levels have not declined significantly, which suggests that the water table may have reached a new equilibrium position. Double-mass-curve analyses show that during 1953-76 the average weighted ground-water levels in a 32-square-mile (83-square-kilometer) part of the sewered area declined 12.2 feet (3.73 meters) more than those in the unsewered area to the east. However, by 1973 this decline was 13.5 feet (4.1 meters). Finite-difference digital-model results indicate that 3.6 feet (1.1 meters) of the relative 1953-76 decline was due to pumping in adjacent Queens County and that most of the remaining decline was a result of sewerage. Streamflow within the sewered area decreased in response to the lowered ground-water levels, and ground-water levels in the adjacent unsewered area were also lowered because of the sewerage.