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 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– 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 δ11B and [B] or between δ11B 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 δ11B of this subterranean estuary. Groundwater N at this location should be derived from individual homeowner cesspools, although measured septic waste has much lower δ11B compared to the coastal groundwaters. This observation, with no trend in δ11B with [B] indicates multiple sources supply B to the coastal groundwaters. The agricultural subterranean estuary displayed a positive correlation between δ11B 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 δ11B can be used in combination with δ15N to trace sources of NO3– to the subterranean estuary if source endmember isotopic signatures are well-constrained, and if the influence of seawater on δ11B signatures can be minimized or easily quantified.

中文翻译：

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研究硼同位素以确定由海底地下水排放到沿海水域的氮源

氧、氮和硼的稳定同位素被用于确定排放到大型潮汐河口（美国纽约长岛湾）的海底地下水 (SGD) 中硝酸盐 (NO3–) 的来源。粪便、化粪池废物和肥料等潜在污染物在 δ15N 和 δ18O 中重叠，但已显示在非沿海环境中具有独特的 δ11B。研究了具有不同土地利用梯度的两个不同的地下河口，代表 (1) 混合中密度住宅和 (2) 农业。这些站点在 NO3– 中具有重叠的 δ15N 和 δ18O 测量值，并且无法区分不同的 N 源。两个位点之间的硼同位素和浓度明显不同，几乎没有重叠。受混合中密度住宅影响的地下河口显示 δ11B 和 [B] 之间或 δ11B 与盐度之间几乎没有相关性，表明淡水和海水之间的直接混合关系不太可能解释这种变化。没有两个来源可以充分表征这个地下河口的 δ11B。该位置的地下水 N 应来自个人房主污水池，尽管与沿海地下水相比，测量的化粪池废物的 δ11B 低得多。这一观察结果表明，δ11B 没有趋势，[B] 表明沿海地下水有多个来源供应 B。农业地下河口δ11B与[B]呈正相关，与盐度无关。海喷雾和肥料的二元混合可以合理解释 B 在农业地下河口的分布。这项研究的结果表明，如果源端元同位素特征受到良好约束，并且海水对 δ11B 特征的影响可以最小化或易于量化，则 δ11B 可与 δ15N 结合使用，以将 NO3 的来源追踪到地下河口。