Geological controls and tidal forcing of submarine groundwater discharge from a confined aquifer in a coastal sand dune system

Submarine groundwater discharge (SGD) into the ocean is increasingly recognized as an important component of land‐ocean interaction. Numerous geological settings may be the source of terrestrially derived SGD, including spatially extensive sand dune systems, which are a common coastal setting in tropical, subtropical, and temperate regions throughout the world. Within the catchment of the Great Barrier Reef lagoon, numerous large‐scale sand dune systems exist. Creeks and rivers draining these systems are largely absent; however, terrestrially derived groundwater discharges from discrete springs in the intertidal zone. The geological and hydrological controls of the discharge were investigated, using a combination of geological, geophysical, and hydrological methods. From sedimentary and geoelectric data it is inferred that a unit of sandy clay provides a confinement of the shallow aquifer system in the dune complex, with groundwater discharging where this layer is breached. Groundwater discharge flux was inversely correlated with tidal water level, but remained positive throughout the tidal cycle. A simple hydrological model indicates that the continuous discharge is the result of a change in hydraulic pressure of the confined aquifer in response to tidal forcing at the seaward limit of the aquitard, which is a previously undescribed effect of tidal modulation of SGD. The observed groundwater discharge is likely to have a significant effect on the biogeochemistry of the coastal environment, particularly in the absence of surface water drainage.