We study geochemical processes along the variably‐saturated zone during managed aquifer recharge (MAR) with reverse‐osmosis desalinated seawater (DSW). The DSW, post‐treated at the desalination plant by calcite dissolution (remineralization) to meet the Israeli water quality standards, is recharged into the Israeli Coastal Aquifer through an infiltration pond. Water quality monitoring during two MAR events using suction cups and wells inside the pond indicates that cation exchange is the dominant subsurface reaction, driven by the high Ca2+ concentration in the post‐treated DSW. Stable isotope analysis shows that the shallow groundwater composition is similar to the recharged DSW, except for enrichment of Mg2+, Na+, Ca2+, and HCO3–. A calibrated variably‐saturated reactive transport model is used to predict the geochemical evolution during 50 years of MAR for two water quality scenarios: (i) post‐treated DSW (current practice) and (ii) soft DSW (lacking the remineralization post‐treatment process). The latter scenario was aimed to test soil‐aquifer‐treatment (SAT) as an alternative post‐treatment technique. Both scenarios provide an enrichment of ∼2.5 mg L−1 in Mg2+ due to cation exchange, compared to practically zero Mg2+ currently found in the Israeli DSW. Simulations of the alternative SAT scenario provide Ca2+ and HCO3– remineralization due to calcite dissolution at levels that meet the Israeli standard for DSW. The simulated calcite content reduction in the sediments below the infiltration pond after 50 years of MAR was low (<1%). Our findings suggest that remineralization using SAT for DSW is a potentially sustainable practice at MAR sites overlying calcareous sandy aquifers.

This study proposes a hydrogeochemical tool to distinguish between salinization and freshening events of a coastal aquifer and quantifies their effect on groundwater characteristics. This is based on the chemical composition of the fresh-saline water interface (FSI) determined from combined field work, column experiments with the same sediments, and modeling. The experimental results were modeled using the PHREEQC code and were compared to field data from the coastal aquifer of Israel. The decrease in the isotopic composition of the dissolved inorganic carbon (delta(13)C(DIC)) of the saline water indicates that, during seawater intrusion and coastal salinization, oxidation of organic carbon occurs. However, the main process operating during salinization or freshening events in coastal aquifers is cation exchange. The relative changes in Ca(2+), Sr(2+), and K(+) concentrations during salinization and freshening events are used as a reliable tool for characterizing the status of a coastal aquifer. The field data suggest that coastal aquifers may switch from freshening to salinization on a seasonal time scale.

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Reverse osmosis (RO) seawater desalination is currently a widespread means of closing the gap between supply and demand for potable water in arid regions. Currently, one of the main setbacks of RO operation is fouling, which hinders membrane performance and induces pressure loss, thereby reducing system efficiency. An alternative water source is saline groundwater with salinity close to seawater, pumped from beach wells in coastal aquifers which penetrate beneath the freshwater-seawater interface. In this research, we studied the potential use of saline groundwater of the coastal aquifer as feedwater for desalination in comparison to seawater using fieldwork and laboratory approaches. The chemistry, microbiology and physical properties of saline groundwater were characterized and compared with seawater. Additionally, reverse osmosis desalination experiments in a cross-flow system were performed, evaluating the permeate flux, salt rejection and fouling propensities of the different water types. Our results indicated that saline groundwater was significantly favored over seawater as a feed source in terms of chemical composition, microorganism content, silt density, and fouling potential, and exhibited better desalination performance with less flux decline. Saline groundwater may be a better water source for desalination by RO due to lower fouling potential, and reduced pretreatment costs.

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