Monitoring and inverse modelling of hydrogeochemical processes during managed aquifer recharge in Southwestern Bangladesh

Managed Aquifer Recharge (MAR) has been applied as Storage, Transfer, and Recovery (ASTR) to provide fresh drinking water for local communities at 99 locations in southwest Bangladesh since 2009. Aerobic freshwater from ponds is filtered subsequently infiltrated into anaerobic shallow brackish aquifers. At approximately 45% of these sites, relatively higher levels Fe As were observed recovered water, which requires a better understanding the hydrogeochemical processes that govern Fe, Mn, MAR systems. Therefore, two representative sites with above (74 ± 11 μg/L site GMF11) below (19 6 JJS91), standard 50 weekly monitored on hydrochemical changes Dec 2017 2018. Hydrogeochemical occurring during storage quantified inverse forward geochemical mass balance models developed PHREEQC. The following explained quality: 1) mixing infiltration native groundwater (∼90%:∼10%); 2) consumption O2 by a) dissolved Fe2+ precipitated Fe(OH)3 GMF11 b) sedimentary organic matter (OM) JJS91; 3) reduction SO4 coupled oxidation OM both sites; 4) corrosion freshening induced cation-exchange (Ca sorption; Na desorption) triggering calcite siderite dissolution GMF11. Dissolution carbonate minerals occurred lesser extent JJS91, while cation exchange (Na Ca suggested was displaced because inadequate JJS91. Distinct pH values reflected dominance versus oxidation. Siderite led 4.3 3.1 1.0 0.5 mg/L respectively. Elevated Mn caused max. 20% min. 80% mobilization processes, mainly desorption Fe-oxides Mn-bearing siderite. Recommendations are provided improve quality.