TMDLs address specific waterbodies, segments of waterbodies, or even entire watersheds, and are pollutant specific. TMDLs must allow for seasonal variations and a margin of safety, which accounts for any lack of knowledge concerning the relationship between pollutant loads and water quality. The TMDL calculation can be represented by the following equation:
TMDL = The total maximum daily pollutant load of the receiving stream. This represents the allowable pollutant loading the stream can receive while maintaining applicable water quality standards. TMDLs can be expressed in terms of mass per time, toxicity, or other appropriate terms.
= The sum of waste load allocations for this segment of the receiving stream. This represents the portion of the receiving stream’s loading capacity that is allocated to one or more existing or future point sources dischargers.
= The sum of load allocations for this segment of the receiving stream. This represents the portion of the stream’s loading capacity that is allocated to one or more existing or future nonpoint sources of pollution or to natural background sources.
MOS = A margin of safety that accounts for the uncertainty about the relationship between the pollutant loads and the quality of the receiving stream.
Most Surface Water Discharge permits contain technology-based effluent limits, which are usually the best available technology that is economically achievable. In cases where technology-based limits are not sufficient to protect water quality standards, water quality-based effluent limits are incorporated into the permits. In many cases, the development and implementation of water quality-based limits includes the development of a TMDL for the receiving water.
TMDLs are developed for water bodies that do not meet the water quality standards due to nonpoint source pollution, or might not meet water quality standards with the application of technology-based controls for point sources. In these cases, point sources of pollution would need additional water quality-based controls. Nonpoint sources of pollution are considered on a watershed basis.
TMDLs developed and used as a basis for water quality-based effluent limits in a Surface Water Discharge permit are commonly called "Point Source TMDLs." For more information about point source TMDLs, contact
Kelli Buscher by calling (605) 394-2229.