Push-pull experiments to evaluate in Situ arsenic remediation in the Ogallala aquifer

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2010

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Arsenic concentrations exceed the U.S. Environmental Protection Agency maximum contaminant level (MCL) for drinking water (10 μg/L) in 220 Public Water Systems in Texas. Conventional treatments for arsenic removal are ex-situ treatments, which are costly and produce high concentration arsenic-contaminated sludge which must be disposed of properly.

A pilot study to assess an in-situ arsenic remediation treatment was conducted in the city of Andrews wellfield, Andrews County, Texas. This procedure involved adding dissolved iron to the aquifer through water wells. Under oxidizing conditions, the dissolved iron precipitates as iron oxides near the well. Water pumped from the well following iron precipitation flows through the iron-enriched zone and arsenic is adsorbed onto the iron oxides.

Two experiments were conducted in which 24 g and 72 g of Fe were injected into an experimental well. Prior to the experiments, arsenic concentration in the well water was 43 μg/L. Following the first experiment, arsenic concentration in produced water stabilized at 25 μg/L, and after the second experiment arsenic values stabilized at 20 μg/L, representing an approximate 50% reduction in produced arsenic under the test conditions.

These preliminary results suggest that there is potential for treating groundwater arsenic contamination in small public water systems using in situ treatment based on addition of iron to the wells. The reconnaissance tests showed a 50% reduction in arsenic concentration; however, additional testing is required to determine if arsenic concentrations can be reduced to less than the MCL of 10 mg/L and the frequency and volume (mass) of treatment injections required to maintain these low arsenic concentrations needs to be determined for full demand production rates.

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