Submarine Groundwater Discharge (SGD)

​

We study fluxes of both fresh and recirculated seawater to the sea, both in the local and the regional scales.

1. It all started in the Sea of Galilee (lake, NE Israel), where we used the MOSQUITO flux meter  to both continuously monitor the composition of brine-related pore water composition, as well as to measure fluxes to the lake (10’s of cm/yr).

2. A survey of the Israeli coast from north to south showed that the main fresh SGD occurs at the northern part, with especially large fluxes in areas, where the Cretaceous carbonate rocks are in direct contact with the sea.

3. Our main SGD research focused on Dor Bay (see figure), located at the southern Carmel coastal plane, where 40-50 m of Quaternary sands and calcareous sands cover Cretaceous rocks. First, we characterized SGD flow patterns and hydrogeology and determined SGD fluxes. Using 222-Rn time series and short-lived Ra isotopes, as well as coastal boreholes and seabed seepage meters, we showed that at this site, fresh water discharge is 3.5 cm/d or >1x106 m3/yr per 1 km of coastline. We also showed that fresh groundwater mainly derives from a hydrologically-confined unit, while seawater circulation is limited to the local phreatic unit.

4. The next step was to characterize and to assess the nutrient loads conveyed to the sea by the SGD. We showed that nitrates and silica are mainly conveyed by fresh water discharge, while ammonium and phosphate are mainly about seawater circulation. We also showed that while in the subsurface (the so called ‘subterranean estuary’) some of the nitrate (~20%) is going through denitrification, probably also DNRA (= dissimilatory nitrate reduction to ammonium).

5. Then, together with a group from Barcelona (J. Garcia-Orellana and V. Rodellas), we took the challenge to estimate the total SGD and nutrient loads to the whole Mediterranean. This was accomplished by 228-Ra balance of the Mediterranean. Water was mainly sampled during a one-month cruise (April 2011) with the German R/V Meteor, which went from Istanbul to Vigo, with complementary samples from other cruises and coastal groundwater from all around the Mediterranean. Results showed that the total SGD (fresh + saline) to the Mediterranean is 0.2-4.3x1012 m3, which means 4-93x106 m3 per 1 km of coastline. This also translates into large loads of nutrients, which are similar to the inputs of dust and rivers.