Long Island drinking water is provided by a sole source aquifer with nitrate levels exceeding the federal limit of 10ppm in some communities. Previous studies undertaken in the community of Northport identified a 50% loss of nitrogen in the system (MUNSTER, 2004; MUNSTER, 2008). A study of 15N- NO3- and 18O-NO3 identified sewage and fertilizer applications as the primary nitrogen contaminators in Northport (BLEIFUSS et al., 2000) but did not conclusively find evidence of denitrification or ammonia volatilization in the isotope signature. No study to date has been able to identify the mechanism for 50% nitrogen loss within the system.Groundwater denitrification was investigated as the mechanism for calculated 50% nitrogen deficit in a Long Island's coastal plain aquifer. Samples were taken from municipal supply wells and monitoring wells during May and October of 2008. Groundwater analysis included major ions, dissolved oxygen, dissolved organic carbon and dissolved N2/Ar ratios. Anion concentrations were used to estimate nitrogen inputs from three primary sources; on site wastewater disposal systems, fertilizer applications and atmospheric deposition. Field measured dissolved oxygen concentrations were used to identify hypoxic conditions that favor denitrification. Dissolved organic carbon was measured to categorize if sufficient electron donors exist to drive the denitrification reaction. Dissolved N2/Ar gases were measured using membrane inlet mass spectrometry (MIMS) to quantify excess N2 produced via denitrification.Dissolved oxygen concentrations exceed 4mg/l in all municipal supply samples and four of five monitoring well samples. Dissolved organic carbon concentrations are less than 0.5mg-C/l in municipal supply well samples. Nitrate (N-NO3-) denitrified is less than 2ppm for all samples. Samples with high dissolved oxygen concentration contain nitrate ranging from 2.0. 11.0mg/l N-NO3ª. We hypothesize thick vadose zone keeps dissolved oxygen content close to saturation during groundwater infiltration. Travel time from groundwater infiltration to well production is short, less than 50 years for municipal supply wells, which prevents typical age related oxygen depletion within the aquifer. A combination of thick vadose zone and short groundwater travel time oxidizes residual cesspool ammonium and dissolved organic carbon, eliminating electron donors that fuel denitrification. High dissolved oxygen concentrations in groundwater prevent saturated zone denitrification. We conclude that groundwater denitrification cannot account for observed 50% nitrogen loss in Northport, Long Island.