Effect of time-varying wave conditions on the fate of nitrogen in a freshwater unconfined nearshore aquifer

Abstract

Coastal forcing including waves lead to complex and dynamic mixing between discharging groundwater and recirculating coastal water in permeable unconfined nearshore aquifers. This mixing can drive biogeochemical transformations that affect the fate of pollutants in nearshore aquifers and their ultimate discharge to coastal waters. The objective of this study was to examine the effect of time-varying wave conditions on the fate of nitrogen in a freshwater unconfined nearshore aquifer. This was achieved by conducting numerical groundwater flow and reactive transport simulations using MODFLOW-NWT 2005 with a new modified wave-PBC package together with PHT3D v2.10. The simulations demonstrate that time-varying wave conditions enhance mixing in the nearshore aquifer compared to simulations that only consider constant wave conditions, and in doing so impact nitrogen transformations. For the conditions and reaction network simulated, time-varying wave conditions resulted in up to 99% of nitrogen removal in the nearshore aquifer due to the intensified mixing and increased delivery of organic matter into the mixing zone. Sensitivity analysis shows that the abundance and reactivity of coastal-derived organic matter plays a key role in nitrogen removal in aquifers along wave-influenced shorelines. The simulations highlight the importance of considering time-varying wave conditions when designing field investigations and analysing field data. The findings have implications for prediction and estimation of chemical fluxes to the coastal waters and also suggest that increasingly frequent storm events and higher wave conditions as the climate changes may modify pollutant fluxes from nearshore aquifers to coastal waters.