INFLUENCE OF TIDAL FLUCTUATIONS ON TRANSPORT OF SUBSURFACE CONTAMINANTS DISCHARGING THROUGH SANDY COASTAL AQUIFERS

The flow and reactive processes in subterranean estuaries control the exit conditions and subsequent flux of subsurface pollutants discharging to coastal waters. With much of the world's coastlines subject to tidal fluctuations, being able to predict contaminant loading from tidally-influenced subterranean estuaries is crucial to ensuring appropriate pollution mitigation strategies are adopted. Tidal forcing strongly influences near-shore groundwater behaviour with the combined effects of tides, groundwater discharge and variable-density flow resulting in complex and dynamic flow, transport and biogeochemistry in subterranean estuaries. Numerical modeling, supported by field data, reveals that tidal forcing drives seawater recirculation through intertidal beach zones at significant rates compared with fresh groundwater discharge, leading to the creation of active and dynamic surficial salt-freshwater mixing zones. Such zones are key elements in that fate of pollutant transport to coastal waters. Simulations of a conservative land-derived tracer discharging through tidally influenced aquifers demonstrate that tidal effects modify a contaminant's subsurface flow path and decrease both the exit concentrations at the beach face and the rate of contaminant transfer across the aquifer-ocean interface. The conservative transport behavior is fundamental in the understanding of more realistic transport scenarios. Reactive transport simulations investigating the fate of land-derived BTEX contaminants show that tidally driven mixing processes enhance the aerobic biodegradation of BTEX in the subsurface prior to discharge, thus reducing the contaminant load to coastal waters. The extent of biodegradation is markedly influenced by the magnitude of tidal forcing (i.e., tidal amplitude) relative to the fresh groundwater flow rate as these factors control the intensity of salt-freshwater mixing, period of exposure of the contaminant to the mixing zone and biogeochemical zonations in the subterranean estuary. As numerous studies have identified that submarine groundwater discharge is a significant contributor of nutrients to coastal seas, a general conceptualization of the impact of tidal fluctuations on the transport and transformation of nutrients as they discharge through the subterranean estuary will also be discussed.