Depth of the vadose zone controls aquifer biogeochemical conditions and extent of anthropogenic nitrogen removal

Abstract

We investigated biogeochemical conditions and watershed features controlling the extent of nitrate removal through microbial dinitrogen (N) production within the surficial glacial aquifer located on the north and south shores of Long Island, NY, USA. The extent of N production differs within portions of the aquifer, with greatest N production observed at the south shore of Long Island where the vadose zone is thinnest, while limited N production occurred under the thick vadose zones on the north shore. In areas with a shallow water table and thin vadose zone, low oxygen concentrations and sufficient DOC concentrations are conducive to N production. Results support the hypothesis that in aquifers without a significant supply of sediment-bound reducing potential, vadose zone thickness exerts an important control of the extent of N production. Since quantification of excess N relies on knowledge of equilibrium N concentration at recharge, calculated based on temperature at recharge, we further identify several features, such as land use and cover, seasonality of recharge, and climate change that should be considered to refine estimation of recharge temperature, its deviation from mean annual air temperature, and resulting deviation from expected equilibrium gas concentrations.

Carbon Budget of Tidal Wetlands, Estuaries, and Shelf Waters of Eastern North America

“…Rates of fresh groundwater discharge show large spatial variability and are difficult to quantify, but global estimates suggest that the rate of discharge to estuaries and coasts (submarine groundwater discharge or SGD) is in the range of 6 to 10% of riverine discharge (Burnett et al, 2003). Concentrations of DIC and DOC in groundwater are also highly variable (e.g., Cai et al, 2003;Kroeger & Charette, 2008;Pabich et al, 2001;Santos et al, 2008;Sawyer et al, 2014;Szymczycha et al, 2017). Based on typical carbon concentrations and a global estimate of SGD rate, Cole et al (2007) estimated that SGD of carbon is 27% of the riverine flux of carbon.…”

Oxygen‐controlled recirculating seepage meter reveals extent of nitrogen transformation in discharging coastal groundwater at the aquifer–estuary interface

“…The NO 3 − loss mechanisms may include removal via denitrification (e.g., Kroeger and Charette 2008; Colman et al 2018) or anammox, possibly within reducing microsites (e.g., Sawyer 2015) or assimilation into microbial biomass (e.g., Marchant et al 2014). Denitrification could have been enabled by available electron donor DOC (e.g., Szymczycha et al 2017), which was ~ 35–85 μ M at lowest salinity and increased to ~ 310–390 μ M at high salinity; or by dissolved Fe (Böhlke and Denver 1995), or dissolved Mn (Labbé et al 2003), both of which followed a similar trend as DOC with respect to salinity (Brooks et al 2021). Previous studies have shown that within coastal sediments, anammox is generally more important where the ratio of NO 3 − : DOC is large (Babbin and Ward 2013), and can remain active even when NH 4 + concentrations are low, as was the case in the fresh or near‐fresh groundwater, but generally decreases in proportion to denitrification in the presence of labile forms of DOC (Smith et al 2015).…”

“…include removal via denitrification (e.g., Kroeger and Charette 2008;Colman et al 2018) or anammox, possibly within reducing microsites (e.g., Sawyer 2015) or assimilation into microbial biomass (e.g., Marchant et al 2014). Denitrification could have been enabled by available electron donor DOC (e.g., Szymczycha et al 2017), which was~35-85 μM at lowest salinity and increased to~310-390 μM at high salinity; or by dissolved Fe (Böhlke and Denver 1995), or dissolved Mn (Labbé et al 2003), both of which followed a similar trend as DOC with respect to salinity (Brooks et al 2021). Previous Table 1.…”

Noise-Resilience Horizon of Groundwater Potential Maps Generated by Frequency-Ratio Data Integration

“…The stochastic approach thus assumes an inherent random uncertainty from the generated input values. Szymczycha et al (2017) used the recharge temperature as a proxy for the GWP interpreted as the aquifer recharge rate. The calculation of the recharge temperature hinged on several assumptions, the uncertainties of which could significantly alter the estimates within and among sites.…”