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

Abstract

The geochemistry and isotopic composition (H, O, S, Osulfate, 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 ∼16‰, which suggests local recharge via unconfined areas of the aquifer in the Negev anticline systems. The δ18OH2O and δ2H values (-6.5‰ and -35.4‰) of this groundwater are higher than those of groundwater in the Sinai Peninsula and southern Arava valley (-7.5‰ and -48.3‰) that likewise have lower deuterium excess values of ∼10‰. 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 (δ34SSO4 ∼-13‰; δ18OSO4 ∼+7.7‰) and dissolution of CaCO3 (87Sr/86Sr ∼0.70787; δ13CDIC = -3.7‰) occur. In the confined zone of the aquifer, bacterial SO4 reduction removes a significant part of dissolved SO42 -, thereby modifying its isotopic composition (δ34SSO4 ∼-2‰; δ18OSO4 ∼+8.5‰) and liberating dissolved inorganic C that contains little or no radiocarbon (14C-free) with low δ13CDIC values (