QUANTIFYING GREENHOUSE GAS CONCENTRATIONS IN COASTAL GROUNDWATER DISCHARGING TO GREAT SOUTH BAY AND LONG ISLAND SOUND, NY

Fluxes of the greenhouse gases N₂O and CH₄ from coastal ecosystems are potentially significant but critically under-examined. Increasing nutrient input to estuaries via groundwater may enhance production of N₂O and CH₄. Further, previous studies in Florida and Massachussetts have indicated that submarine groundwater discharge (SGD) can be a dominant source for CH₄ and N₂O in estuaries and that much of the dissolved gas likely fluxes to the atmosphere. However, greenhouse gas flux due to SGD has been studied only in a couple of locations, and controlling processes are unknown. Concentrations of dissolved N₂O and CH₄ were measured in intertidal and subtidal groundwater samples collected along the northern and southern shores of Long Island, NY. While both areas are densely populated and heavily impacted by nutrient loading from wastewater, they differ geologically and hydrodynamically. The northern shore has high tidal ranges (~3 m) and steep onshore topography and hydraulic gradients. Piezometer sampling at the northern shore sites showed a thick (up to 7 m) surficial zone of recirculating brackish groundwater. The southern embayment of Great South Bay is a back-barrier lagoon underlain by Holocene muds, peat, and sands, with lower hydraulic gradients and lower tidal ranges than the northern shore. Observed patterns of chemical parameters in the groundwater reflect the contrasting physical and chemical environments of the two sites, and provide evidence about the mechanisms that control the production of CH₄ and N₂O. The aquifer along the south shore is oxygen-poor, providing a reducing environment which is conducive to CH₄ production, while the surficial aquifer of the north shore is better oxygenated. CH₄ values were higher in south shore groundwater samples (avg: ~22 µM) than in north shore samples (avg: ~5 µM), while N₂O concentrations were lower on the south shore (≤ 87 nM vs. ≤ 700 nM). In north shore samples, N₂O concentrations increased with NO₃ and with decreasing salinity. These results suggest that NO₃ contamination in these sites leads to enhanced N₂O production in the groundwater. Fluxes of N₂O and CH₄ to the estuaries in the study locations will be estimated and compared to known fluxes from other estuaries.