Controls on Nitrous Oxide Production in, and Fluxes from a Coastal Aquifer in Long Island, NY, USA

Nitrous oxide (N2O) has 265 times greater greenhouse potential than carbon dioxide and its atmospheric concentration has increased by about 20% since industrialization; however, N2O production and emissions from aquatic systems are poorly constrained. To evaluate N2O fluxes associated with meteoric groundwater discharge to coastal zones, we measured N2O concentrations in May and October 2011 from two discharge points of the Upper Glacial aquifer on Long Island, NY, USA. One coastal zone contains only fresh water and the other contains an upper saline zone. N2O concentrations decreased by around 40% for the fresh water and a factor of two for the salt water from May to October, 2011. Fluxes were around 100 to 200 times greater from the freshwater (246 to 448 mu mol/m shoreline/day) than saltwater aquifer (26.1 to 26.5 mu mol/m shoreline/day). N2O concentrations correlate positively with NO3- and dissolved oxygen concentrations and negatively with salinity, dissolved organic carbon (DOC) and N-2 denitrification concentrations. The smaller saltwater N2O export resulted from DOC enrichment in the upper saline zone, which appears to have driven denitrification to completion, removed N2O, and increased N-2 denitrification. DOC concentrations should be considered in global N2O flux estimates for coastal aquifers.