THERMO-HALINE CIRCULATION IN CONFINED COASTAL AQUIFERS AND RESULTING DEEP SUBMARINE GROUNDWATER DISCHARGE

Thermally and salinity-driven groundwater convection and seawater recirculation play a crucial role in coastal zone and continental shelf hydrology. In phreatic aquifers, thermally driven circulation takes place at the deeper continental shelf and slope region, with pure seawater recirculating across the shelf break. In the nearshore scale seawater mixes with fresh water due to salt dispersion (e.g. Wilson, 2005). Both types of circulation cells produce submarine groundwater discharge (SGD).

In this work, theoretical calculations and numerical modeling test the effects of adding confinement to a coastal aquifer that is exposed to the sea at the continental slope. We find that in confined aquifers geothermal convection and salt dispersion combine to a single thermo-haline circulation cell that drives saline SGD in deep areas. The deep SGD comprises recirculated seawater that mix with fresh, terrestrial groundwater in the confined aquifer. Theoretical analysis suggests that the intensity of circulation and SGD depends on horizontal gradients of salinity and temperature, as these are the engine for the density-driven groundwater circulation. The numerical model, which is based on a case study in northern Israel, confirms the results of the theoretical analysis. The model is used to analyze the characteristics of deep SGD, and to test the sensitivity of the model to (1) Basal heat flux; (2) Temperature of the terrestrial groundwater; (3) Seawater salinity; (4) Aquifer conductivity. The main results of the numerical modeling are thus: (1) Deep saline SGD is expected where the confined aquifer outcrops offshore. (2) Seepage is slightly warmer than the surrounding seawater - temperature depends on the temperature differences between seawater and terrestrial water. (3) Salinity is found to be the main control on the combined thermo-haline circulation. This work contributes to the understanding of the mechanism of thermo-haline convection in confined aquifers, and the associated saline deep SGD. Hence it may have implications for a wide spectrum of marital and hydrological research: biogeochemical studies of marine ecosystems, geomechanical studies of seafloor morphology, and hydrogeological research of coastal environments.