In Situ Arsenic Remediation Complicated by Carbonate

Remediation of dissolved arsenic (As) in groundwater at levels as high as 38 mg/L with pH values close to 1 and Fe and Al concentration of thousands of mg/L was attempted at a former phosphate fertilizer plant. Adsorption of As onto naturally occurring ferrihydrite was considered to be the dominant process that would reduce dissolved arsenic concentrations. Remediation was not completely effective because bicarbonate released from CaCO3 used as the neutralizing agent competed with As for adsorption sites on ferrihydrite. Dissolved carbonate concentration was higher than normal because CO2 gas was trapped in the groundwater. Field tests of the effectiveness of groundwater aeration showed that removing excess CO2 from the aquifer and reducing bicarbonate concentration has the potential to reduce dissolved As to the required level.