Hydrogeochemical changes during managed aquifer recharge (MAR) in a salinised coastal aquifer

Highlights: • Artificial recharge freshens groundwater at a salinised detrital aquifer • Salinity decreases from 3000 to 350–550 μS/cm at 80 m • Recharge produces multiple hydrogeochemical changes and modifies reaction times • Main geochemical processes: cation exchange, sulfate and carbonate dissolution • Additionally, oxidation of organic matter and sulphide minerals The southern part of the Plana de Castellón aquifer (Spanish Mediterranean coast) has experienced persistent seawater intrusion over the last 50 years. The salinisation process is related to the lateral advance of the saline wedge and saline upconing as a result of intensive local groundwater pumping. In this context, managed aquifer recharge (MAR) was performed to aid in the recovery of this coastal aquifer. The artificial recharge water (ARW) was characterised by very low mineralisation, as evidenced by a mean electrical conductivity of 330 μS/cm, whereas native groundwater (NGW) was brackish, with EC values of approximately 3000–3500 μS/cm. The dynamics of the injected water and physicochemical changes to groundwater during the MAR event were evaluated using a combination of hydrogeochemical methods. EC, chloride ions, nitrate ions, and stable water isotopes (oxygen-18 and deuterium) were used as tracers to quantify the presence of injected water in groundwater (mixing ratios). Hydrochemical analysis (of major cations and anions) was used to determine changes to water characteristics and hydrogeochemical processes associated with the mixing of ARW and NGW and water-rock interactions. The injection significantly reduced groundwater salinity: EC and major ions rapidly and continuously declined after the first days of recharge. Percentage decreases in electrical conductivity of 80–90% and 30% were detected at 80 and 450 m from the recharge wells (values of 350–550 μs/cm), respectively. The principal process occurring in the aquifer during injection was dilution due to the displacement of NGW and mixing of ARW and NGW. Carbonate and sulfate mineral dissolution, cation exchange, and redox reactions were also identified, and their magnitudes were estimated. In quantitative terms, cation exchange was the most important geochemical process occurring during the MAR event.