Water quality programs across the country, including Colorado’s, often address unacceptable mercury levels in local water bodies through the Total Maximum Daily Load (TMDL) process. The process is prescribed in the federal Clean Water Act to ensure all sources of pollutant loading are accounted for when devising strategies to meet water quality standards.
The Total Maximum Daily Load is an estimate of the greatest amount of a specific pollutant that a water body or stream segment can receive without violating water quality standards. The state compares recent information about the physical, chemical and biological condition of a stream segment with the associated water quality standards for that stream segment.
Where technology-based effluent limits in discharge permits alone aren’t stringent enough to ensure water quality standards are met, these stream segments are designated "Water Quality Limited" and added to a list. This list, called the 303(d) List, identifies the specific component (such as mercury, nitrate or copper) that’s the basis for the water quality problem in that segment of the water body.
Total Maximum Daily Loads are required for all components listed for each stream segment on the 303(d) List. This list is updated every two years.