Element 3 -TMDL Development

The TMDL process quantitatively assesses the impairment factors so that states can establish water quality based controls to reduce pollution and restore and protect the quality of their water resources. The purpose of a TMDL is to determine the pollutant loading a water body can assimilate without exceeding state water quality standards. Missouri's Water Quality Standards consist of three components: designated uses, water quality criteria to protect those uses, and an antidegradation policy.

The TMDL establishes the pollutant loading capacity necessary to meet the water quality standards established to protect the designated uses for each water body. The TMDL identifies the relationship between the pollutant, its sources and instream water quality conditions. A TMDL consists of a wasteload allocation, a load allocation, and a margin of safety. The wasteload allocation is the fraction of the total pollutant load apportioned to point sources and/or permitted entities. The load allocation is the fraction of the total pollutant load apportioned to nonpoint sources and/or non-permitted sources. The margin of safety is a percentage of the TMDL that accounts for any uncertainty associated with modeling and data inadequacies.

TMDLs are typically established to bridge the gap between the assimilative capacity of the water body, the required effluent limitations within the wasteload allocation, any relevant pollution controls, and water quality standards to address water quality problems.

In accordance with EPA guidance, TMDLs are typically scheduled for development within 8 to 13 years from the date they appear on the 303(d) Impaired Waters List. Scheduling may also be determined by the level of threat to human health and aquatic life, data availability, department initiatives, recovery potential and stakeholder involvement. The department's current TMDLs under development activity can be found online.