Total Maximum Daily Load (TMDL) Determinations

When water quality criteria are exceeded in a waterbody, they may be placed on an EPA-required list of impaired waters, called the 303(d) list (based on sections of the Clean Water Act). In order to reduce pollutant loads to meet criteria, a total maximum daily load (TMDL) study is initiated. The TMDL is the maximum load (concentration x flow) for a pollutant that will still result in a waterbody meeting criterion. As part of a TMDL, sources for the pollutant are identified and their contribution to the total load are determined. Then, calculations are made to determine the recommended reductions (as a percent) in load necessary to assure a water quality criterion can be met.