Understanding arsenic behavior in carbonate aquifers: Implications for aquifer storage and recovery (ASR)

• Mobilization of geogenic arsenic under a range of redox conditions was investigated.
• Geochemical reactive transport modeling was coupled to leaching experiments.
• As behavior along flow path was controlled by a series of interconnected reactions.
• Oxidative dissolution of pyrite and simultaneous sorption of As onto neo-formed HFO.
• Reductive dissolution of HFO followed by secondary release of adsorbed As.

Geochemical reactive transport modeling was coupled to bench-scale leaching experiments to investigate and verify the mobilization of geogenic arsenic (As) under a range of redox conditions from an arsenic-rich pyrite bearing limestone aquifer. Modeling and experimental observations showed similar results and confirmed the following: (1) native groundwater and aquifer matrix, including pyrite, were in chemical equilibrium, thus preventing the release of As due to pyrite dissolution under ambient conditions; (2) mixing of oxygen- and nitrate-rich surface water with oxygen-depleted native groundwater changed the redox conditions and promoted the dissolution of pyrite, and (3) the behavior of As along a flow path was controlled by a complex series of interconnected reactions. This included the oxidative dissolution of pyrite and simultaneous sorption of As onto neo-formed hydrous ferric oxides (HFO), followed by the reductive dissolution of HFO and secondary release of adsorbed As under reducing conditions. Arsenic contamination of drinking water in these systems is thus controlled by the re-equilibration of the system to more reducing conditions rather than a purely oxidative process.