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.