Magnesium enrichment of desalinated seawater via managed aquifer recharge: insights from eight years of field observations and reactive transport modelling

Lack of magnesium (Mg 2+ ) in desalinated water used for drinking and irrigation is a well-known concern. We study Mg 2+ levels in the Menashe Streams managed aquifer recharge (MAR) site, where, in addition to traditional surface-runoff recharge, which has been ongoing for more than five decades, surplus desalinated seawater (DSW) recharge was initiated in 2015. We conduct an eight-year well-sampling campaign at the site and find that: (a) Mg 2+ levels at production wells downgradient from the DSW recharge pond are in decline. (b) The recharged DSW undergoes enrichment with Mg 2+ in the subsurface. The latter is attributed mainly to cation exchange and presents an opportunity to apply soil-aquifer-treatment (SAT) as a supplementary, relatively cheap, natural and sustainable approach to enrich DSW with Mg 2+ . To further evaluate this new approach, a 3D flow and reactive transport model of the site was constructed, allowing to quantify and predict the fate of DSW on its path to the production wells. Stochastic simulations show that long term MAR with an annually recurring volume of DSW, can contribute around 5 mg/l of magnesium to the DSW, out of which, 60% is attributed to the unsaturated zone and 40% to deeper aquifer processes. While recommended Mg 2+ levels in drinking water vary across regulatory bodies (generally on the order of ≥10 mg/L), this contribution represents a substantial fraction of the desired enrichment.