Investigating geochemical aspects of managed aquifer recharge by column experiments with alternating desalinated water and groundwater.

Managed aquifer recharge (MAR) events are occasionally carried out with surplus desalinated seawater that has been post-treated with CaCO3 in infiltration ponds overlying the northern part of the Israeli Coastal Aquifer. This water's chemical characteristics differ from those of any other water recharged to the aquifer and of the natural groundwater. As the MAR events are short (hours to weeks), the sediment under the infiltration ponds will intermittently host desalinated and natural groundwater. As part of comprehensive research on the influence of those events, column experiments were designed to simulate the alternation of the two water types: post-treated desalinated seawater (PTDES) and natural groundwater (GW). Each experiment included three stages: (i) saturation with GW; (ii) inflow of PTDES; (iii) inflow of GW. Three runs were conducted, each with different sediments extracted from the field and representing a different layer below the infiltration pond: (i) sand (<1% CaCO3), (ii) sand containing 7% CaCO3, and (iii) crushed calcareous sandstone (35% CaCO3). The results from all columns showed enrichment of K+ and Mg2+ (up to 0.4meq/L for 20 pore volumes) when PTDES replaced GW, whereas an opposite trend of Ca2+ depletion (up to 0.5meq/L) was observed only in the columns that contained a high percentage of CaCO3. When GW replaced PTDES, depletion of Mg2+ and K+ was noted. The results indicated that adsorption/desorption of cations are the main processes causing the observed enrichment/depletion. It was concluded that the high concentration of Ca2+ (relative to the total concentration of cations) and the low concentration of Mg2+ in the PTDES relative to natural GW are the factors controlling the main sediment-water interaction. The enrichment of PTDES with Mg2+ may be viewed as an additional post-treatment.

Saline Groundwater from Coastal Aquifers As a Source for Desalination.

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.

Hydrogeochemical tool to identify salinization or freshening of coastal aquifers determined from combined field work, experiments, and modeling.

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.