Cation exchange and CaCO3 dissolution during artificial recharge of effluent to a calcareous sandstone aquifer

This research describes a field study and laboratory simulations of the geochemical evolution of groundwater following a recharge of effluent into aquifers. The study was conducted in the soil aquifer treatment (SAT) system of the Shafdan sewage reclamation plant, Israel. The SAT system recharges secondary effluent into the calcareous sandstone sediments of the Israeli Coastal Aquifer as a tertiary treatment. The reclaimed effluent is recovered ca. 500 m off the recharge basin and is used for unlimited irrigation. The laboratory simulations in which effluent was pumped through experimental columns packed with pristine Shafdan sediment showed that the chemical composition of the outflowing water was controlled mainly by cation exchange and CaCO3 dissolution. Na+, K+ and Mg2+ were adsorbed and Ca2+ was desorbed during the initial stage of recharge. The equilibrium distribution of the adsorbed cations was: Ca2+ ∼ 60%, Mg2+ ∼ 20%, and Na+ and K+ ∼ 10% each. The Ca2+ in the Shafdan production wells and in the experimental columns outflow (∼5 meq L−1) was always higher than the Ca2+ in the recharged effluent (∼3.5 meq L−1), indicating continuous CaCO3 dissolution. This study demonstrates that besides mixing, a suite of geochemical processes should be considered when assessing groundwater quality following artificial recharge of aquifers.