Investigating Boron Isotopes for Identifying Nitrogen Sources Supplied by Submarine Groundwater Discharge to Coastal Waters

Stable isotopes of oxygen, nitrogen, and boron were used to identify the sources of nitrate (NO3-) in submarine groundwater discharge (SGD) into a large tidal estuary (Long Island Sound, NY, United States). Potential contaminants such as manure, septic waste and fertilizer overlap in delta N-15 and delta O-18 but have been shown to have distinctive delta B-11 in non-coastal settings. Two distinct subterranean estuaries were studied with different land-use up gradient, representative of (1) mixed medium-density residential housing and (2) agriculture. These sites have overlapping delta N-15 and delta O-18 measurements in NO(3)(-)and are unable to discriminate between different N sources. Boron isotopes and concentrations are measurably different between the two sites, with little overlap. The subterranean estuary impacted by mixed medium-density residential housing shows little correlation between delta B-11 and [B] or between delta B-11 and salinity, demonstrating that direct mixing relationships between fresh groundwater and seawater were unlikely to account for the variability. No two sources could adequately characterize the delta B-11 of this subterranean estuary. Groundwater N at this location should be derived from individual homeowner cesspools, although measured septic waste has much lower delta B-11 compared to the coastal groundwaters. This observation, with no trend in delta B-11 with [B] indicates multiple sources supply B to the coastal groundwaters. The agricultural subterranean estuary displayed a positive correlation between delta B-11 and [B] without any relationship with salinity. Binary mixing between sea spray and fertilizer can reasonably explain the distribution of B in the agricultural subterranean estuary. Results from this study demonstrate that delta B-11 can be used in combination with delta N-15 to trace sources of NO(3)(-)to the subterranean estuary if source endmember isotopic signatures are well-constrained, and if the influence of seawater on delta B-11 signatures can be minimized or easily quantified.