A total maximum daily load (TMDL) is the amount of pollutant that a body of water can receive and still meet water quality standards. After a TMDL has been set for a body of water, NPDES permittees that discharge to that water (including municipal stormwater systems) are assigned a wasteload allocation (WLA). WLAs specify the amount of each pollutant (a portion of the TMDL) that the permittee can discharge.
WLAs approved by the U.S. EPA prior to the effective date of the MS4 General Permit must be addressed by permittees in their stormwater pollution prevention program (SWPPP) documents. Applications for general permit coverage must include information on applicable WLAs and how the permittee will move toward complying with them over the five-year permit term.
See the Minnesota Stormwater Manual for TMDL-related permit requirements and technical guidance.
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