Reactive Solute Transport Through Two Contrasting Subterranean Estuary Exit Sites in the Ría de Vigo (NW Iberian Peninsula)

Subterranean estuaries (STEs), where continental groundwaters and saltwaters meet, are zones of intense biogeochemical reactivity. As such, STEs significantly modify groundwater-borne nutrient fluxes to the coastal zone. Thus, evaluating their reactive role is crucial to anticipate impacts of submarine groundwater discharge (SGD) over coastal ecosystems. Here, we studied the nitrogen biogeochemistry of two STEs with contrasting wave-exposure and redox conditions in Panxón and Ladeira beaches (Ría de Vigo, NW Iberian Peninsula). Seasonal surveys were performed at the permanently saturated zone of both beaches during low tide in February, May, July, and October 2019. Sediment was sampled and porewater samples collected using push-pull piezometers. Salinity, 222Rn and 226Ra activities were used to trace water circulation inside each beach. Porewater nitrate, ammonium, nitrite and dissolved oxygen were used to evaluate the role of these STEs as reactive sinks or sources of inorganic nitrogen. Our results showed a marked seasonal variability of water circulation inside both beaches, with strong salinity gradients in February and May and weakened circulation in July and October. The presence of a gravel layer in Panxón beach completely altered the typical structure of STEs by increasing porewater transport and mixing through the beach interior. As a result, Panxón beach profiles were highly enriched in nitrate and oxygen. Conversely, suboxic, and anoxic conditions were prevalent in Ladeira beach during the study period, with ammonium being the prevailing inorganic nitrogen form. High nitrate concentrations occurred associated to the tidal circulation cell during February and May, being the only effective mechanism of sediment oxygenation in Ladeira beach. Although nitrate reduction and production were observed in both STEs, comparison with averaged conservative mixing porewater profiles showed that Ladeira beach acted as a net nitrogen sink whereas Panxón beach acted as a net nitrogen source. The presence of a gravel layer oxygenates the interior of Panxón beach, thus limiting nitrate reduction and promoting the amplification of groundwater-borne nitrogen fluxes to the coast.

Nitrogen attenuation, dilution and recycling at the groundwater – surface water interface of a subtropical estuary inferred from the stable isotope composition of nitrate and water

Estuarine environments have a dynamic groundwater – surface water interface driven by terrestrial groundwater discharge, tidal cycles, waves and other processes. This interface also corresponds to an active biogeochemical environment. An assessment of discharging groundwater with elevated (>300 mg N L−1) NH4+ and NO3− concentrations at such an interface located in a subtropical estuary indicated that 80 % of the N was attenuated, 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 two or more by mixing before being discharged to the estuary. Most of the mixing occurred in a hyporheic zone in the upper 50 cm of the riverbed. However, groundwater entering this zone was already partially mixed (12–60 %) with surface water via a tidal circulation cell. Below the hyporheic zone (50–125 cm below the riverbed), 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). The discrepancy between δ15NNO3 (23–35 ‰) and δ18ONO3 (1.2–8.2 ‰) in all porewater samples indicated that the original synthetic nitrate pool (δ15N ~ 0 ‰; δ18O ~ 18–20 ‰) had turned-over during transport in the aquifer before reaching the riverbed. 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.

Hydrodynamic modelling of Port Foster, Deception Island, Antarctica

Over the last decade, the Antarctic continent has been the object of intensive scientific programmes. However, the emphasis of these studies rarely focuses on the Antarctic as a source of potential elements such as mercury. The release of mercury to the environment is known to occur at Deception Island, associated with volcanic activity. In this study, a 3D hydrodynamic model was used to assess the role of water circulation on the dispersion of released mercury. Sea level variation and tidal circulation data were obtained. Residence time was calculated using two different approaches. Internal tide generation in summer and winter were recognized and the barotropic tidal components obtained. Lagrangian tracers were used to depict particle circulation (simulating particulate mercury) in a three month summer simulation for barotropic and baroclinic conditions. The results show that particles accumulate in the northern and western parts of the bay. It is acknowledged that the results of the 3D model are associated with a non-negligible uncertainty, which can only be reduced with an ongoing commitment to monitoring. The findings of this study indicate that mercury accumulation is occurring in Port Foster (Deception Island), which is a potential threat to the local ecosystem.

Numerical modeling of the tide in the coast ar-ea Casablanca-Mohammedia (Morocco)

The objective of this study is to simulate the tidal circulation in the coastal area Casablanca-Mohammedia located on the Moroccan Atlantic. Simulations of the tidal currents of this zone use the 2D version of the MECCA (Model for Estuarine and Coastal Circulation Assessment). These simulations are based on the depth-integrated dynamical equations of turbulent motion. Equations are solved by using the implicit finite-differences techniques. The modelincorporates the actual bottom topography and the effects of the Earth rotation. As forcing mechanism, the model uses the tidal heights prescribed along the open boundaries.As first results, numerical experiments show that the model provides good results compared to those of the altymetric model TPXO.