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TMDL overview
Overview of TMDLs.
TMDL development
TMDLs in development.
TMDL approvals
List of TMDLs approved by USEPA.
TMDL implementation
TMDL implementation.
Wastewater, Storm Water, CAFOs
Point source issues in TMDLs.
Nonpoint source
Runoff and stormwater related to TMDLs.
TMDL map and projects
Map of TMDL Projects and New Project Pages.
Contact information
For information contact:
Andrew Craig,
Nonpoint Source Project Coordinator

Kari Fleming,
Environmental Toxicologist

TMDL overview

Hardies Creek

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 developed after consideration of all sources of pollution to an impaired waterbody and is stated as the amount of pollutant that the waterbody can assimilate and not exceed water quality standards. TMDL pollutant loads are determined in consideration of in-water targets that must be met for the waterbody to respond favorably. Targets may be based on promulgated numeric water quality criteria or may be based on narrative criteria developed in consideration of local data and/or nearby reference sites.

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). Thus, three components make up the TMDL: WLA + LA + 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 landspreading, 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 modeling and calculating WLAs and LAs.

Once the TMDL is developed and approved, federal and state regulations then require implementation of TMDLs to meet water quality standards where there are implementation mechanisms in place and supported by law. 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.

Last revised: Wednesday December 04 2019