Submarine groundwater discharge (SGD) along the Mediterranean coast of Israel

In the Mediterranean coast of Israel, two aquifers are draining to the sea - the Pleistocene coastal granular aquifer and the Cretaceous carbonate one. The Pleistocene aquifer consists of interlayered Nilotic sandstone, calcareous sandstone, siltstone, loam, and marine clays, which form several sub-aquifers, with total thickness of 100-200 meters. The upper sub-aquifer of about 50 m thickness is phreatic, while the lower ones are confined and there is a possibility that they drain to the sea in their submarine exposure along the continental slope, some 20-30 kms offshore. The carbonate aquifer is mainly confined, but drains to the sea at several locations along the northern coast. The northern coast is also characterized by many young, recently active, transversal faults, some of them are already known for being major pathways for seawater intrusion into the aquifers. Recently, we started a research of SGD along the Israeli coast, focusing on the role of transverse faults in SGD and on the proportions of recycled seawater in this discharge. The project involves geochemical tracing and continuous in-situ measurements of flow rates and solute fluxes, as well as a detailed ecological study of the coastal water. The geochemical tracing includes the study of Rn and the Ra quartet, as well as salinity and nutrients. Flow rates and solute fluxes and their temporal patterns will be determined using newly developed methodology, which apply the osmotic pumping principle to the in-situ continuous sampling of seawater and pore water. Tracing the profiles of a dye tracer previously injected into the sediment, flow rates as low as few cm/yr can be determined. Together with chemistry of pore fluids, this will allow the determination of fluxes of solutes, and especially of nutrients, into the coastal seawater. In the ecological part of the project, we will focus on the variability in microbial autototrophic and heterotrophic communities in relation with SGD-induced differences in the input of nutrients (nitrate, nitrite, phosphate, ammonium, silica) and other dissolved anthropogenic constituents (e.g. radium, polonium and heavy metals). Preliminary analyses of surface water along the Israeli coastline at 13-40 meter bottom depth show relatively high concentrations of 222Rn (40-50 dpm/100kg, excess argon - 30-40 dpm/100kg) with a peak at the Haifa bay (82 dpm/100kg). A transect taken perpendicular to the coastline, from Haifa to the northwest, shows a peak above the continental slope at bottom depth of 600 meter (62 dpm/100kg) with very low 226Ra content (3 dpm/100kg), implying the possible existence of a local plume. The source for this anomaly should be further studied