Water Quality: Docket No. 58-0102-1803

This rulemaking has been initiated to allow de minimis additions of heat when waters exceed applicable temperature criteria due to man-made causes. Currently, Idaho’s point source treatment requirements allow point sources of heat to raise receiving water temperatures up to 0.3°C only when the receiving water is naturally warmer than numeric criteria. There is not an allowance for any increase, however small, when it cannot be shown receiving water temperatures are naturally warmer than criteria.

Idaho has many very small point sources. All add some heat to the waters to which they discharge. And, in most cases, the water bodies to which they discharge are warmer than Idaho’s numeric temperature criteria set to protect aquatic life for a portion of each year. Heat is a non-conservative pollutant, and the sources of heat can be relatively small. This rulemaking proposes allowing NPDES/IPDES regulated human sources of heat loading to cause no more than a de minimis 0.3°C increase in receiving water temperatures. This would allow a 0.3°C increase to waters that are exceeding the numeric temperature criteria upstream for the designated aquatic life use even in cases where the exceedance of numeric criteria is not due to natural conditions.