TMDLs_Content Page_TMDL overview

TMDL Overview

Waldo Tributary
Waldo Tributary

Section 303(d) of the Clean Water Act (CWA) requires delegated states to determine on a biennial basis whether waterbodies are impaired (not meeting designated uses or water quality criteria). One of the underlying goals of the CWA is to restore all impaired waters so they meet applicable water quality standards. One of the key tools to meet this goal is the development of a total maximum daily load (TMDL).  A TMDL is the amount of a pollutant a water can receive and still meet water quality standards.

To establish a TMDL, Wisconsin’s numeric water quality standards or applicable water quality targets based on narrative water quality standards are used. Water quality monitoring and measured flow in the watershed quantify current pollutant loads to the impaired water. To calculate the pollutant loads, computer models are used. Specific information for the model is needed such as: weather, topography, soil types, and land use. With these and other data inputs, the model simulates physical processes associated with the flow of water, sediment movement, nutrient cycling, crop growth, etc. Models can also be used to predict impacts of changes in land use, climate, and management practices on water quality. Once targets are set for the waterbody, the TMDL is established by allocating the allowable load between the point sources (WLA) and the nonpoint sources (LA) with some amount of the total load set aside as a margin of safety (MOS). The analysis can be expressed as a formula:

TMDL = Wasteload Allocation (WLA) + Load Allocation (LA) + Margin of Safety (MOS)

  • The wasteload allocation (WLA) is the total allowable pollutant load from all point sources (e.g. municipal, industrial, CAFOs, MS4 stormwater). Reserve capacity may either be built into the WLA or be a separate component of the total loading capacity to allow for future growth in the watershed.
  • The load allocation (LA) is the allowable pollutant load from nonpoint sources (agricultural, CAFO off-site land spreading, residential runoff, etc.). Natural sources (e.g., runoff from non-disturbed areas) are typically covered under the load allocation, and whenever possible nonpoint source loads and natural background loads should be distinguished.
  • The margin of safety (MOS) accounts for uncertainty in the analysis.

Public input is very important during the development of TMDLs. All TMDLs require a minimum 30–day public comment period and public informational meeting or hearing. Once comments are addressed, the TMDL is approved by the State of Wisconsin and the US Environmental Protection Agency.  Once approved, TMDLs are automatically amended to the Areawide Water Quality Management Plan.

Federal and state regulations require implementation of TMDLs through programs such as the WPDES program and the NPS program. For point source discharges, WLAs delineated in the TMDL need to be expressed in Wisconsin Pollutant Discharge Elimination System (WPDES) permits. Nonpoint source implementation is an adaptive process, requiring the collaboration of diverse stakeholders and the prioritization and targeting of available programmatic, regulatory, financial, and technical resources.