Controls on Nitrogen Biogeochemistry in a Subterranean Estuary, Stony Brook Harbor, NY

Current subterranean estuary models predict a well-defined submarine groundwater discharge (SGD) 'freshwater tube' discharging at or slightly below the low tide point. Nutrient modeling of the subterranean estuary relies on redox defined end -member mixing of fresh and marine waters to produce either nutrient attenuation or remineralization within the SGD 'freshwater tube'. This study, based in Stony Brook Harbor (Long Island, NY), employs a combination of electrical resistivity surveys, pore water sampling and seepage meter measurements. In a glacial sands aquifer setting we find the 'freshwater tube' actually occurs as diffuse freshwater zone that can discharge for tens of meters offshore and is capped by a thin harbor floor 'muck' layer. This 'muck' layer acts as a zone of nutrient attenuation, despite near saturation dissolved oxygen levels in both discharging groundwater and overlying sea water. Stony Brook Harbor is rated highest in ecosystem diversity among north shore Long Island Sound embayments and represents embayments currently experiencing minimal anthropogenic impact. Electrical resistivity surveys were used to determine the placement of piezometers for porewater sampling. Porewater sampling was done using retract-a-tip piezometers (AMS) to a maximum 10m depth, at intervals of 0.5- 1.0m. Porewater was sampled in May and October of 2011; an additional intertidal cluster well was sampled for 30 days to confirm stabile salinity distribution. A combination of N₂/Ar and δ¹⁵N-NO₃^- are used to determine the extent of denitrification in the subterranean estuary. Samples were also analyzed for dissolved organic carbon (DOC), dissolved oxygen (DO), iron and manganese to determine the geochemical framework of nitrogen transformations. Month long intertidal cluster well measurements show stable porewater salinity and DO to a depth of 10m in the intertidal zone with a 2m tide range. Off shore sampling and electrical resistivity work indicate the freshwater zone extends for tens of meters beyond the low tide line. Fresh zone porewater contains DO concentrations greater than 70% saturation and DOC concentrations ranging 0.3- 2.5 mgL^-1. Nitrate concentrations are elevated in the freshwater zone, ranging 8.1-12.5mgL^-1 NO₃^--N at a depth of 3.5-10m below the sediment water interface in samples taken off shore, beneath the observed muck layer. Despite high nitrate concentrations in the underlying fresh zone, significant denitrification occurs in the brackish muck layer, up to 4mg-N L^-1, fueled by DOC concentrations reaching a maximum of 15.1mgL^-1. These results provide new insight into the processes controlling nitrogen attenuation in oxic freshwater-oxic seawater mixing during SGD.