Water-rock interactions during recharge of effluents into a calcareous sandstone aquifer

The water-rock interactions following an artificial recharge of effluents into a calcareous sandstone aquifer were studied in the Soil Aquifer Treatment (SAT) system of the Shafdan sewage reclamation plant, Israel, and in laboratory column experiments. The Shafdan SATsystem is based on rapid infiltration of secondary effluents into the Israeli Coastal Aquifer and their recovery, for reuse in irrigation, after a residence time of a few months within the aquifer. Aset of laboratory column experiments was designed to simulate the geochemical evolution of the effluents during their flow in the aquifer. Both, experimental and field observations indicate that the main water-rock interactions altering the chemical composition of cations in the effluents are cation exchange and CaCO ₃ dissolution. The initial cation exchange includes massive adsorption of Na+, K+ and Mg ²⁺ and desorption of Ca ²⁺. CaCO ₃ dissolution caused constant increase of the Ca ²⁺ concentrations in the groundwater by 1-2 meq L-1.