EPA Arsenic Limit in Drinking Water

Under EPA's National Primary Drinking Water Regulations, the Maximum Contaminant Level (MCL) for arsenic in drinking water is 0.010 mg/L, which is 10 µg/L. It has applied since 23 January 2006 ^[1].

• MCL (the enforceable limit): 0.010 mg/L. EPA defines the MCL as "the highest level of a contaminant that is allowed in drinking water." Its table gives values in mg/L, and EPA notes that mg/L is the same as parts per million, so 0.010 mg/L is 0.010 ppm (EPA NPDWR page).
• MCLG (the health goal): 0. This is the level "below which there is no known or expected risk to health." It is a goal, not an enforceable limit ^[1].
• Long-term health effects EPA lists: skin damage, problems with the circulatory system, and possibly a higher risk of cancer ^[1].
• Sources EPA lists: natural deposits wearing away, runoff from orchards, and runoff from glass and electronics production wastes ^[1].

Approach: I looked up arsenic in EPA's National Primary Drinking Water Regulations contaminant table. I checked the units and the MCL and MCLG definitions on EPA's official NPDWR web page.