In situ Arsenic Remediation in a fractured, alkaline aquifer

Abstract

In situ removal of arsenic from ground water used for water supply has been accomplished in circum-neutral ground water containing high dissolved iron concentrations. In contrast, the ground water at our study site is alkaline, contains measurable dissolved oxygen and little dissolvediron. Because the dissolved iron concentration is low in the basalt aquifer, the iron oxide content of the aquifer would not increase with successive pumping cycles unless iron is added to the injected water. Additionally, the high pH limits adsorption onto iron oxide present in the aquifer. Having the ability to lower arsenic concentrations in high-pH, oxic ground water could have wide application because similar high arsenic ground water is present in many parts of the world.

Laboratory and field results show that the basalt has limited capacity for adsorption of As(V), presumably by naturally occurring hydrous ferric oxide (HFO). However, addition of HFO can significantly increase As(V) adsorption. Lowering the pH combined with increasing the iron oxide content in the basalt aquifer reduces arsenic concentrations in produced ground water. Arsenic removal was very effective in laboratory experiments and during early part of the initial push-pull experiment. Moderate arsenic removal after increasing the iron oxide content of the basalt aquifer and lowering the pH during the cross-flow experiments was limited to about 50% at best. This relatively low removal may be due to a variety or combination of factors, including local hydraulics of the aquifer, chemical reaction kinetics, and changing aqueous chemistry caused by the lowered pH.