The purpose of this rulemaking is to protect human health from adverse effects of elevated arsenic (As) and protect sensitive aquatic life from cadmium (Cd) toxicity in low hardness waters.  The Department of Environmental Quality (DEQ) proposes to lower Idaho's arsenic (As) human health criteria from 50 ug/L to 10 ug/L and lower the low-end hardness cap used in calculation of cadmium (Cd) aquatic life criteria from 25 mg/L to 10mg/L or less (Section 210). In addition, DEQ proposes to make nonsubstantive revisions to portions of Section 210 identified as needing correction or clarification.

DEQ is also initiating this rulemaking in an effort to forestall pending legal action against the U.S. Environmental Protection Agency (EPA) that would force EPA to disapprove Idaho's As standard and require EPA to promulgate a federal rule to lower the As human health criteria applicable to Idaho waters. EPA is also prepared to disapprove Idaho's Cd standard to remove the low end hardness cap for calculating criteria. DEQ proposes to reduce the low end hardness cap for Cd from 25 mg/L to 10 mg/L or less. Idaho's current As human health criteria are also several times higher than EPA's recommendation for protecting human health.

After consideration of public comments, the rule has been adopted as initially proposed.