Deep Submarine Groundwater Discharge-Evidence From Achziv Submarine Canyon at the Exposure of the Judea Group Confined Aquifer, Eastern Mediterranean

摘要

Deep submarine groundwater discharge (DSGD) of fresh-brackish water was suggested to occur globally, yet its driving mechanism is not well constrained. Specifically, it is unclear whether the phenomenon may represent a steady-state condition of a hydrological system, in which the terrestrial recharge area is hydraulically connected with the submarine discharge area (by a confined aquifer, for example). Recently, our hydrogeological modeling suggested that such a system exists in northern Israel. The model predicted the location of a brackish water seepage within the Achziv Submarine Canyon, similar to 10 km offshore, where the Cretaceous Judea Group confined aquifer is carved by the canyon. The present work is a field study that attempts to identify these brackish seeps in the water column of the canyon at the aquifer exposure. The field observations include salinity and temperature profiling of the water column along the canyon during three cruises and Ra-224 measurements of water samples from the canyon. The results point to a significant near-bottom low-salinity and low-temperature anomaly right where the aquifer submarine outcrop is mapped that coincide with a high Ra-224 activity anomaly. Such near-bottom anomalies are absent in an adjacent submarine canyon, in which the confined aquifer is not exposed. The anomalies in the water column of the Achziv Submarine Canyon corroborate the results of the hydrogeological modeling that predicted brackish DSGD from the sea bottom at the aquifer outcrop. The possibility of DSGD in specific geo-hydrological settings is relevant for various biological, geochemical, hydrological, and geotechnical studies of the marginal ocean.