With the implementation of the EPA's change in the maximum contaminant level (MCL) for arsenic in drinking water beginning in January 2006, water system compliance has also changed. EPA lowered the MCL, the legally allowable concentration of a chemical, physical, or biological constituent in drinking water based on health effects research, for arsenic from 0.05 (50 parts per billion [ppb]) to 0.010 mg/L (10 ppb) to provide the greater protectiveness indicated by the research. The agency has also identified technologies to treat arsenic to this level. However, many systems have not been able to achieve this standard.
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