Nitrogen attenuation, dilution and recycling in the intertidal hyporheic zone of a subtropical estuary

Abstract

Abstract. Tidal estuarine channels have complex and dynamic interfaces controlled by upland groundwater discharge, waves, tides and channel velocities that also control biogeochemical processes within adjacent sediments. In an Australian subtropical estuary, discharging groundwater with elevated (> 300 mg N L−1) NH4+ and NO3- concentrations had 80 % of the N attenuated at this interface, one of the highest N removal rates (> 100 mmol m−2 day−1) measured for intertidal sediments. The remaining N was also diluted by a factor of 2 or more by mixing with surface water before being discharged to the estuary. Most of the mixing occurred in a hyporheic zone in the upper 50 cm of the channel bed. However, groundwater entering this zone was already partially mixed (12 %–60 %) with surface water via tide-induced circulation. Below the hyporheic zone (50–125 cm below the channel bed), NO3- concentrations declined slightly faster than NH4+ concentrations and δ15NNO3 and δ18ONO3 gradually increased, suggesting a co-occurrence of anammox and denitrification. In the hyporheic zone, δ15NNO3 continued to become enriched (consistent with either denitrification or anammox) but δ18ONO3 became more depleted (indicating some nitrification). A high δ15NNO3 (23 ‰–35 ‰) and a low δ18ONO3 (1.2 ‰–8.2 ‰) in all porewater samples indicated that the original synthetic nitrate pool (industrial NH4NO3; δ15N ∼ 0 ‰; δ18O ∼ 18 ‰–20 ‰) had turned over completely during transport in the aquifer before reaching the channel bed. Whilst porewater NO3- was more δ18O depleted than its synthetic source, porewater δ18OH2O (−3.2 ‰ to −1.8 ‰) was enriched by 1 ‰–4 ‰ relative to rainfall-derived groundwater mixed with seawater. Isotopic fractionation from H2O uptake during the N cycle and H2O production during synthetic NO3- reduction are the probable causes for this δ18OH2O enrichment. Whilst occurring at a smaller spatial scale than tide-induced circulation, hyporheic exchange can provide a similar magnitude of mixing and biogeochemical transformations for groundwater solutes discharging through intertidal zones.

Spatiotemporal Variation of Groundwater Nitrate Concentration Controlled by Groundwater Flow in a Large Basin: Evidence From Multi‐Isotopes (15N, 11B, 18O, and 2H)

“…In contrast, in zone A (Figure 1), $${{\text{NO}}_{3}}^{-}$$ concentration in confined water decreases significantly along the flow direction, accompanied by the increase of δ 15 N‐$${{\text{NO}}_{3}}^{-}$$ and δ 18 O‐$${{\text{NO}}_{3}}^{-}$$ and Fe 2+ concentration and the obvious decrease of ORP (Figure 7a). However, $${{\text{NH}}_{4}}^{+}$$ concentration in groundwater also increases, indicating that the decrease in $${{\text{NO}}_{3}}^{-}$$ concentration might result from DNRA (Lamontagne et al., 2018). As the lakeside floodplain, terrain in zone A is gentle, and a set of sub‐sand and sub‐clay of fluvial and lacustrine facies with a thickness of several meters are generally deposited (Guo et al., 1985).…”

“…Groundwater $${{\text{NO}}_{3}}^{-}$$ concentration is affected by human activities, biogeochemical processes (e.g., dissimilatory $${{\text{NO}}_{3}}^{-}$$ reduction to $${{\text{NH}}_{4}}^{+}$$ (DNRA), denitrification, and sorption), and groundwater flow (Archana et al., 2018; Böhlke & Denver, 1995; Hofmeister et al., 2022; Lamontagne et al., 2018; Rivett et al., 2008). Among them, groundwater flow is an important driving force for distribution, migration, and transformation of $${{\text{NO}}_{3}}^{-}$$ in groundwater (Umezawa et al., 2008; Yue et al., 2019).…”

Temporal variations in porewater fluxes to a coastal lagoon driven by wind waves and changes in lagoon water depths

“…The length of the porewater flowpath have a large influence on the biogeochemical processes occurring within sediments and on the chemical composition of porewaters discharging across the sediment-water interface (Heiss et al, 2017;Lamontagne et al, 2018;Weinstein et al, 2011). Consequently, the spatial scale of porewater fluxes needs to be considered to evaluate the overall magnitude of solute inputs driven by porewater fluxes.…”

Modeling Hydrologic Controls on Particulate Organic Carbon Contributions to Beach Aquifer Biogeochemical Reactivity

“…Seawater delivers oxygen and reactive organic carbon to intertidal sediments. Various reactions including aerobic respiration (Anschutz et al, 2009; Charbonnier et al, 2013; Seidel et al, 2015), denitrification (Jiao et al, 2018; Kroeger et al, 2006; Kroeger & Charette, 2008; Lamontagne et al, 2018), and metal transformations (Charette & Sholkovitz, 2002; McAllister et al, 2015; Roy et al, 2010) have been observed in intertidal aquifers, impacting concentrations of solutes discharging to the coastal ocean.…”