This issue paper identifies and summarizes experiences with proven aboveground treatment alternatives for arsenic in groundwater, and provides information on their relative effectiveness and cost. The information contained in this paper can also be found in the report Arsenic Treatment Technologies for Soil, Waste, and Water , EPA542-R-02-004 (Ref. 1.12), which provides cost and performance data for additional technologies that can treat arsenic in soil, waste, and water. This paper has been developed jointly by EPA s Engineering Forum and Technology Innovation Office. EPA's Engineering Forum is a group of professionals, representing EPA Regional Offices, who are committed to identifying and resolving the engineering issues related to remediation of Superfund and hazardous waste sites. The Forum is sponsored by the Technical Support Project. In January 2001, EPA published a revised maximum contaminant level (MCL) for arsenic in drinking water that requires public water suppliers to maintain arsenic concentrations at or below 0.010 milligrams per liter (mg/L) by 2006 (Ref. 1.1, 1.9). The revised standard may affect arsenic cleanup goals for groundwater. The information contained in this issue paper can help managers at sites with arsenic-contaminated groundwater to: ' Identify proven and effective treatment technologies ' Screen those technologies based on effectiveness, treatment goals, site characteristics, and cost ' Apply technology and experience from sites with similar remediation challenges ' Find more detailed arsenic treatment information using this issue paper as a reference Arsenic is a component of many industrial raw materials, products, and wastes, and is a contaminant of concern in groundwater at many remediation sites.