Thermo-haline circulations in subsea confined aquifers produce saline, steady-state deep submarine groundwater discharge

Abstract

Deep Submarine Groundwater Discharge (DSGD) is a ubiquitous and highly significant phenomenon, yet it remains poorly understood. This work proposes a simple theoretical basis for steady-state DSGD from a confined aquifer. The simple theory is tested against numerical modeling (FEFLOW) of a case study of DSGD offshore northern Israel (eastern Mediterranean Sea). Modeling further investigates the hydrogeological setting that enables DSGD and the parameters that affect it – conductivity of the confining unit, head on land, seawater salinity and seawater temperature. The main findings are thus: steady-state DSGD eminates far offshore from a confined aquifer exposed by a submarine canyon, even under moderate heads. It is driven by a thermohaline circulation cell that forms around an intrinsic transition zone between salty, cold seawater and fresh, warm terrestrial groundwater. The circulation is driven by tilted isopycnals and consists of seawater entering the confined aquifer at the exposed section offshore, mixing with terrestrial groundwater within the aquifer, and seeping back out to the sea as saline warm water. The circulation mode and the character of the DSGD depend on the conductivity of the confining unit and on the hydraulic head. The seepage velocity is linearly related to the temperature and salinity gradients that develop in the confined aquifer during the thermohaline circulation. These new insights have potentially important implications for coastal hydrology, seawater chemistry, biogeochemistry, and submarine slope instability.