Investigating boron isotopes for identifying nitrogen sources supplied by submarine groundwater discharge to the Long Island Sound

Abstract:

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). Potential contaminants such as manure, septic waste and fertilizer overlap in δ¹⁵N and δ¹⁸O but have been shown to have distinctive δ¹¹B 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 δ¹⁵N and δ¹⁸O measurements in NO₃^-. Boron isotopes and concentrations are measurably different between the two sites, with little overlap. While one of the studied estuaries shows little correlation between or between δ¹¹B and salinity, demonstrating that direct mixing relationships between fresh groundwater and seawater were unlikely to account for the variability, the second studied estuary show a positive correlation between without a correlation to salinity. Well water and rainwater samples show a range of δ¹¹B that can explain the high values of the submarine groundwater samples as an atmospheric input for B, through boric acid volatilization. Potential anthropogenic NO₃^- endmembers, including septic system samples, fertilizers and manure samples were also analyzed for δ¹¹B to compare to the two sites, and these fall within the range of reported values. The large range of δ¹¹B and concentrations well above rainwater at both studied sites suggests multiple N (and B) sources, consistent with our working knowledge that submarine groundwater discharge brings diffuse, non-point sourced contaminants to Long Island Sound.