Abstract
Stable isotopes of oxygen, nitrogen and boron were used to identify the sources of NO3- in submarine groundwater discharge into a large estuary (Long Island Sound, NY). Potential contaminants such as manure, septic waste and fertilizer overlap in 15N and18O but have been shown to have distinctive 11B in non-coastal settings. Two sites on the north shore of Long Island (NY) were studied with different, up-gradient land use, representative of mixed medium-density residential housing and agriculture. These sites have overlapping 15N and18O measurements in nitrate. Boron isotopes and concentrations are measurably different between the two sites, with little overlap. There is little correlation between 11B and [B] or salinity, demonstrating that direct mixing relationships between the fresh groundwater and seawater are unlikely to account for the variability, although the groundwater mixtures appear to have a seawater origin. Well water and rain water samples show a range of 11B that can explain the high values of the submarine groundwater samples. Potential nitrate endmembers, including septic system samples, fertilizers and a manure sample were also analyzed for 11B to compare to two sites with fresh submarine groundwater discharge up to 75 cm d-1that delivers significant NO3- to this coastal area. Several interesting conclusions emerge from this survey. One is that seawater provides boron but not salinity to fresh groundwaters collected within the subterranean estuary. This is likely through sea spray and boric acid volatilization. Another is that the large range of 11B with no trend in [B] 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.
Highlights
Submarine groundwater discharge (SGD) is an important vector for the delivery of nutrients to the coastal ocean (e.g., Taniguchi et al, 2019)
Isotopes of N and O in nitrate (NO3−) have a proven utility in sorting out the sources of nitrogen delivered to groundwaters (Aravena et al, 1993; Bateman and Kelly, 2007; Bannon and Roman, 2008; Kendall et al, 2008)
This site has been well studied (Tamborski et al, 2015, 2017a,b) and was chosen to represent a shoreline impacted by mixed anthropogenic N sources; there was no evidence of groundwater denitrification or nitrogen attenuation in the subterranean estuary (STE) (Tamborski et al, 2017a)
Summary
Submarine groundwater discharge (SGD) is an important vector for the delivery of nutrients to the coastal ocean (e.g., Taniguchi et al, 2019). Anthropogenic contaminant sources including cesspools, septic systems and fertilizers are implicated in FIGURE 1 | Long Island, NY, United States (A) and the subterranean estuary (STE) study sites of Callahans Beach (B) and Iron Pier Beach (C), indicated by yellow stars. Land cover within the watershed of Callahans Beach is composed of medium-density residential housing (2770 people km−2; town of Fort Salonga); the beach itself is directly down-gradient from a golf course (Figure 1) This site has been well studied (Tamborski et al, 2015, 2017a,b) and was chosen to represent a shoreline impacted by mixed anthropogenic N sources (fertilizer, manure and/or septic); there was no evidence of groundwater denitrification or nitrogen attenuation in the STE (Tamborski et al, 2017a). Identifying the sources of nitrogen in groundwater is of particular importance to managers here because an effort to reduce nitrogen in Suffolk County, NY, United States, called “Reclaim our Water Initiative,” is predicated on nitrogen pollution from septic systems (Suffolk County Government, 2019)
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