New isotopic evidence for the origin of groundwater from the Nubian Sandstone Aquifer in the Negev, Israel

The geochemistry and isotopic composition (H, O, S, O-sulfate, C, Sr) of groundwater from the Nubian Sandstone (Kurnub Group) aquifer in the Negev, Israel, were investigated in an attempt to reconstruct the origin of the water and solutes, evaluate modes of water-rock interactions, and determine mean residence times of the water. The results indicate multiple recharge events into the Nubian sandstone aquifer characterized by distinctive isotope signatures and deuterium excess values. In the northeastern Negev, groundwater was identified with deuterium excess values of similar to 16 parts per thousand, which suggests local recharge via unconfined areas of the aquifer in the Negev anticline systems. The delta O-18(H2O) and delta H-2 values (-6.5 parts per thousand and -35.4 parts per thousand) of this groundwater are higher than those of groundwater in the Sinai Peninsula and southern Arava valley (-7.5 parts per thousand and -48.3 parts per thousand) that likewise have lower deuterium excess values of -10 parts per thousand. Based on the geochemical differences between groundwater in the unconfined and confined zones of the aquifer, a conceptual geochemical model for the evolution of the groundwater in the Nubian sandstone aquifer has been reconstructed. The isotopic composition of shallow groundwater from the unconfined zone indicates that during recharge oxidation of pyrite to SO4 (delta S-34(SO4) similar to-13 parts per thousand; delta O-18(SO4) similar to+7.7 parts per thousand) and dissolution of CaCO3 (Sr-87/Sr-86 similar to 0.70787; delta C-13(DIC) = -3.7 parts per thousand) occur. In the confined zone of the aquifer, bacterial SO4 reduction removes a significant part of dissolved SO42-, thereby modifying its isotopic composition (delta S-34(SO4) similar to-2 parts per thousand; delta O-18(SO4) similar to+8.5 parts per thousand) and liberating dissolved inorganic C that contains little or no radiocarbon (C-14-free) with low delta C-13(DIC) values (<- 12 parts per thousand). In addition to local recharge, the Sr and S isotopic data revealed contribution of external groundwater sources to the Nubian Sandstone aquifer, resulting in further modifications of the groundwater chemical and isotopic signatures. In the northeastern Negev, it is shown that SO4-rich groundwater from the underlying Jurassic aquifer contributes significantly to the salt budget of the Nubian Sandstone aquifer. The unique chemical and isotopic composition of the Jurassic groundwater (delta S-34(SO4) similar to +14 parts per thousand; delta O-18(SO4) similar to 14 parts per thousand; Sr-87/Sr-86 similar to 0.70764) is interpreted as reflecting dissolution of Late Triassic marine gypsum deposits. In the southern Arava Valley the authors postulate that SO4-rich groundwater with distinctively high Br/Cl (3 x 10(-3)) low Sr-87/Sr-86 (0.70734), and high delta S-34(SO4) values (+ 15 parts per thousand) is derived from mixing with underlying brines from the Paleozoic units. The radiocarbon measurements reveal low C-14 activities (0.2-5.8 pmc) in both the northeastern Negev and southern Arava Valley. Taking into account dissolution of carbonate rocks and bacterial SO4 reduction in the unconfined area, estimated mean residence times of groundwater in the confined zone in the northeastern Negev are on the order of 21-38 ka, which sggests recharge predominantly during the last glacial period. The C-14 signal in groundwater from the southern Arava Valley is equally low but due to evidence for mixing with external water sources the residence time estimates are questionable. (c) 2007 Elsevier Ltd. All rights reserved.