Boron Isotopes as a Tracer for Nitrate in Coastal Settings

Water samples were analyzed for boron concentrations and isotope ratios and nitrate concentrations from the Setauket Mill Pond located near Stony Brook University on Long Island, New York. This work expands on a multi-year investigation into boron isotopes in Long Island surface and groundwaters, particularly in its use as a tracer of nitrates. Some areas of Long Island have groundwater nitrate concentrations above drinking water standards, which can pose significant health concerns. Across the island eutrophication of surface waters is also a major ecological issue. Measures are in place to reduce nitrate pollution, but non-point source additions are difficult to track. Previous studies have shown that boron isotopes can act as a tracer for nitrate sources, but early work on Long Island shows that seawater influence may make using boron isotopes to trace nitrate sources more challenging (Tamborski et al.,2020). This study aims to further evaluate boron isotopes as a tracer of natural and anthropogenic influences contributing to increased nitrate levels in systems influenced by seawater. The Setauket Pond is divided into two main parts, the north pond, which is closer to the Setauket Harbor of the Long Island Sound, and the south pond which is closer to the spring source that emerges near a local farm. They are connected below a narrow bridge. The north pond has elevated temperatures with low nitrate, and higher δ11B. This is consistent with rainwater inputs which generally have elevated δ11B due to the proximity to the ocean. The south pond has cooler temperatures, elevated nitrate levels, and lower δ11B. This was interpreted to reflect spring input into the pond, demonstrating that nitrate is being introduced through groundwaters. The south pond was also observed to have dramatically more and different types of algae, including hair algae which is damaging to ecosystems. Future work will continue to characterize possible nitrate sources within the pond, with the goal of informing new mitigation practices.