Geochemical identification of fresh water sources in brackish groundwater mixtures; the example of Lake Kinneret (Sea of Galilee), Israel

Fresh waters that dilute brines are considered to have a negligible effect on the ion ratios of the resultant mixture. We show that the major element composition of the fresh end-member can be deduced from the chemical composition of the mixed waters. That composition, then, can be used to differentiate between different neighboring carbonate aquifers, which supply the water. This is demonstrated for the Fuliya and Tabgha saline springs, located on the northwestern shore of Lake Kinneret (Sea of Galilee), Israel. At these springs, shallow fresh groundwater mixes with brines from deep aquifers. Seven saline springs and wells located at the Fuliya and Tabgha blocks were sampled over a year, and 32 eastern Galilee fresh springs and wells were sampled as representatives of the fresh water end-member. All samples were analyzed for major and minor ions. The saline spring data were used to construct mixing lines, followed by their extrapolation to low concentrations in order to derive the ion/chloride ratio characterizing the fresh component. We constructed ion/Cl vs. Cl curves; projection of the composition of fresh water on the calculated curve was used to identify a certain fresh water source as a possible end-member. Results indicate that the composition of the water feeding the Fuliya springs is different from that at Tabgha, reflecting interactions with different rocks in each basin. The major fresh water end-member diluting the Fuliya brines is characterized by high Mg/Cl and low Sr/Cl ratios, and is consistent with the composition of fresh groundwater in the dolomitic Cenomanian and Turonian aquifers widely exposed in the Fuliya drainage basin. The major fresh water end-member diluting the Tabgha brines, on the other hand, is characterized by low Mg/Cl and high Sr/Cl ratios, and is consistent with the composition of fresh groundwater in the chalky Eocene Timrat Fm. and Senonian outcrops. Although the chalky formations in the Tabgha drainage basin are exposed over only 20% of the area they contribute most of the solutes to the fresh water end-member. Rain flows over the chalky formations and then infiltrates into the Bar-Kokhba Eocene outcrops.