An investigation of the spatial and temporal variability of the saline interface in a sandy aquifer subject to storm wave runup and rainfall recharge

Investigations at Hat Head in northern New South Wales, Australia, have shown that the depth and location of the saline interface changes significantly in response to storm-induced wave runup, rainfall recharge and regional groundwater discharge. The interplay between these three factors creates moving zones of fresh and salty water that displace each other over time, leading to the development of complex geochemical patterns. The results of an extensive hydrogeological investigation incorporating surface and borehole geophysics, drilling, monitoring and hydrochemical sampling on multiple occasions has demonstrated that the mixing zone between fresh groundwater and seawater occurs as zones of variable chemical composition which extend further inland and to greater depths than anticipated. The location and magnitude of these mixing zones varies over time scales of weeks. The chemical processes within these mixing zones are dominated by redox reactions that may never reach an equilibrium, with the system being episodically disturbed by new storm events. Diel changes from tides do not have an observable impact on the interface. An improved understanding of these processes will require regular monitoring and sampling from a range of vertical sampling points in the coastal zone, combined with routine monitoring using borehole induction logs.