Hydro-chemical tracers and hydro-chemical modeling assessing groundwater leakage for a karst aquifer in Yarkon-Taninim basin, Israel

Study region: Yarkon-Taninim groundwater basin, Israel. Study focus: A Paleocene-Eocene thermal episode caused an anomaly of barium and silica concentrations in the Eocene aquitard that were used as natural tracers for identifying and quantifying leakage into the Turonian - Upper Cenomanian aquifer through the Senonian-Paleocene (Mount-Scopus Group) strata. Increased concentrations of both Ba2+ and SiO2 in Turonian - Upper Cenomanian aquifer groundwater were supposed to be a sign of downward leakage from the Eocene aquitard. Within the Menashe syncline where these tracers pointed to the leakage, it was estimated by the balance method. Using this estimate, the area-averaged hydraulic conductivity of the intermedium layer was assessed from the Darcy's law by using this layer thickness and a hydraulic gradient between the Avdat-Group aquitard and the Turonian - Upper Cenomanian aquifer. The calculation and integration of its results over the balance area by a grid with 1x1 km2 cells was performed by using the GIS-based ARGUS ONE software. Cl-and SO42-con- centrations in the leakage have been estimated by using the mixing model considering the assessed leakage. PHREEQC software was used for solubility calculation of amorphous silica, quartz and barite at different temperatures, pH and salinities and for calculation of saturation indices of Ba-bearing minerals. New hydrological insights for the region: Previously the Senonian layers within synclines in the Yarkon-Taninim basin, which is the primary source of the groundwater supply in Israel, were supposed to be impermeable. This study has shown that the main leakage is concentrated in the Menashe syncline vicinity north of the Yarkon-Taninim basin. The two tracers first provided a possibility of the leakage assessment within the Menashe syncline: more than 2.3*106 - 11.4*106 m3 yr- 1 (leakage rate of 5.4*10-5 - 2.6*10-4 m day- 1 at the Eocene outcrops of the syncline). The average regional permeability of the Senonian aquiclude within the Menashe syncline was estimated in the range 7.7*10- 5 - 3.8*10-4 m day- 1 , essentially higher as compared to the values for the Bet-Govrin syncline. The salinity of the slow leakage was assessed to be low; therefore, it will not damage the high-quality groundwater in the Turonian - Upper Cenomanian aquifer that is exploited for the potable water supply.