A Comparison of Nitrogen in Shallow Ground Water from Sewered and Unsewered Areas, Nassau County, New York, from 1952 through 1976a

Abstract

ABSTRACTMedian nitrogen concentrations of ground water and water from streams in Nassau County were statistically analyzed on an areal and temporal basis and by individual well to compare concentrations of nitrogen in ground water beneath sewered and unsewered areas. The comparison is discussed in terms of hydrogeologic conditions and major point and nonpoint sources of nitrogen.Although no significant differences in median nitrate concentrations during the 25‐year period of record were observed between the sewered and unsewered areas, recent (1972–76) ammonium and nitrate data suggest a relative improvement in water quality in the sewered area. This finding is based on (1) significantly lower ammonium and nitrate concentrations in stream water of the sewered area during base flow, (2) significantly lower median ammonium concentrations in ground water in the sewered area, (3) significant decreasing long‐term nitrate trends in water from 8 of 13 wells in the sewered area where records were sufficient for analysis, and (4) significantly lower median nitrate concentrations near the water table beneath the sewered area.The lack of significant difference between median nitrate concentrations in the sewered and unsewered areas may be in part due to sampling bias and to the considerable length of time necessary for ground water in the sewered area, which was contaminated by cesspool and septic‐tank effluent before sewering, to reach a point of discharge in a stream or bay.In addition, ammonium leached from landfills and nitrate originating from nonpoint sources such as fertilizers and animal wastes, cannot be removed by sewers and may be obscuring water‐quality improvements brought about by sewering.