Groundwater data were collected in the spring and fall of 2008 from three sites representing different geological settings and biogeochemical conditions within the surficial glacial aquifer of Long Island, NY. Investigations were designed to examine the extent to which average vadose zone thickness in contributing watersheds controlled biogeochemical conditions and processes, including dissolved oxygen concentration (DO), oxidation-reduction potential (Eh), dissolved organic carbon concentration (DOC), and microbial dinitrogen (N2) production. Greatest N2 production was observed at the south shore of Long Island, which is characterized by a thin vadose zone, low DO and Eh, and relatively high DOC. Limited N2 production occurred at the north shore of Long Island, which is characterized by a thick vadose zone, higher DO, higher Eh, and lower DOC. Our results show that vadose zone thickness exerts an important control on the extent of microbial N2 production in aquifers that lack a significant supply of sediment-bound reducing potential. We interpret heterotrophic denitrification to be the primary driver of N2 production in the present study, while acknowledging that anaerobic ammonium oxidation (anammox) likely plays an unquantified role as well.