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

Abstract

Stable isotopes of oxygen, nitrogen, and boron were used to identify the sources of nitrate (NO 3 −) 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 δ 15 N and δ 18 O but have been shown to have distinctive δ 11 B 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 δ 15 N and δ 18 O 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 δ 11 B and [B] or between δ 11 B 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 δ 11 B of this subterranean estuary. Groundwater N at this location should be derived from individual homeowner cesspools, although measured septic waste has much lower δ 11 B compared to the coastal groundwaters. This observation, with no trend in δ 11 B with [B] indicates multiple sources supply B to the coastal groundwaters. The agricultural subterranean estuary displayed a positive correlation between δ 11 B 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 δ 11 B can be used in combination with δ 15 N to trace sources of NO 3 − to the subterranean estuary if source endmember isotopic signatures are wellconstrained, and if the influence of seawater on δ 11 B signatures can be minimized or easily quantified.

Boron Isotopes in Fresh Surface Waters in a Temperate Coastal Setting

“…An earlier study of boron that focused on sources of nitrate into the Long Island Sound via a subterranean groundwater discharge (SGD) found that δ 11 B combined with δ 15 N and δ 18 O of nitrates in water samples do not provide values that fit cleanly within the range of published values for its use as a tracer of nitrates [15]. However, while this study could not pinpoint specific sources, it did identify differences between the two study areas.…”

“…However, while this study could not pinpoint specific sources, it did identify differences between the two study areas. These two areas, which were selected to represent agricultural (fertilizers and manure) vs. urban sources (septic and fertilizer), both showed a large range in δ 11 B [15]. The agricultural site had higher boron concentrations and lighter δ 11 B, while the urban source area had lower boron concentrations that trend toward the seawater δ 11 B value [15].…”

Response of Nitrate Processing to Bio-labile Dissolved Organic Matter Supply Under Variable Oxygen Conditions in a Sandy Beach Seepage Face

“…This interaction between the NO 3 − production and DOC supply reveals that the retention of NO 3 − in coastal systems might be higher than previously thought, since the SOM dependent NO 3 − removal capability tends to be stable during the high loading of bio-labile DOM. Within many coastal ecosystems, a large amount of terrestrial NO 3 − , derived from overuse of chemical fertilizer or leakage from sewage pipes, reaches the coastal seawater via the benthic system (Tamborski et al, 2020). Compared with seawater concentration, submarine groundwater discharge-derived NO 3 − concentration was several orders of magnitude higher at our site (e.g., more than 10 times higher in Sanggou Bay, China, Wang et al, 2014).…”

Application of using a boron tracer to evaluate groundwater contamination in the Quaternary limestone aquifer in the southern region of Okinawa Island, Japan

“…Both dams effectively block saltwater intrusion into the watershed (Nawa and Miyazaki 2009). In the coastal areas, sea spray blown inland by lapping waves results in boron characteristics that are similar those of seawater (Tamborski et al 2020). We assume that the island region exhibits similar phenomena, wherein the endmember of groundwater includes the precipitation content of some seawater components.…”

“…Dissolved nitrogen has been utilized as a tracer of anthropogenic activities involving nitrogenous compounds (Yoshimoto et al 2013;Nakano et al 2013). However, using nitrogen as a single tracer has limitations, as the oxygen and nitrogen isotopic composition in nitrate ions cannot identify pollution sources (Tamborski et al 2020). To address this issue, combining nitrate with boron has emerged as a new way to achieve higher accuracy in pollutant identification (Widory et al 2005).…”