Evolution of brackish groundater in a typical arid region: Northern Arava Rift Valley, southern Israel

Abstract

Brackish groundwater in the northern Arava Valley south of the Dead Sea is characterized by salinities up to 4000 mg/l. This groundwater originates from two main aquifers. The first is a relatively shallow aquifer composed of clastic and carbonates of the Neogene-Recent Hazeva and Arava fill formations. The second aquifer occurs usually within a depth of 700–1500 m and is composed of Lower Cretaceous sandstones of the Kurnub Group. No significant differences could be discerned between the concentrations and ionic ratios of these two main groundwater bodies. The chemical evolution of the groundwater is the result of two major processes: (1) dissolution—Groundwater flowing through the Kurnub Group (beyond the boundaries of the Arava Rift Valley) and through part of the Hazeva and Arava fill aquifers, is characterized by typical ratios Na/Cl≈ 1 and by Ca/(HCO 3+SO 4) <0.75. These ratios from dissolution of evaporites (60–70% of all dissolved minerals) and of carbonates (10–20%) in an arid environment resembling contemporary regional conditions; (2) mixing—In certain cases, groundwater flowing through the shallow Hazeva and Arava fill aquifers and through the Kurnub Group (within the boundaries of the Rift Valley) are characterized by Na/Cl < 1and by Ca/(HCO 3+SO 4) > 0.75. The chemical composition of this groundwater indicates a contribution of salts derived from ancient Ca Cl brines. Such brines are known to have evolved since the Neogene in the Arava Rift Valley. They are encountered throughout the whole sedimentary succession in the Dead Sea and northern Arava Rift Valley area, intermixing to various degrees with brackish or fresh groundwater. The stratigraphic relation between the brackish and saline groundwater bodies is not consistent. Usually, the less concentrated groundwater overlies the more saline. There are, however, cases of transposition, possibly indicating the occurrence of unflushed pockets of Ca Cl brines. Brackish groundwater derived from the two main aquifers is characterized by an excess of Mg and SO 4. Such an excess may have evolved from residual salts precipitated as the result of recent or subrecent fractional evaporation of rain and/or floodwater under arid conditions. Water which precipitated crusts of calcite, gypsum and halite became enriched in Mg and SO 4, and to a lesser degree in K and Na. Another possible explanation for the excess Mg and SO 4 is the complete evaporation of surface water resulting in the precipitation of K Mg sulphates. Subsequent dissolution by freshflood water would then preferentially dissolve and mobilize these highly soluble minerals.