A total maximum daily load (TMDL) is the amount of pollutant loading that can occur and have a water body meet water quality standards. A TMDL may be written as an equation which allocates pollutant loading to four separate categories:
TMDL = WLA + LA + MOS + RC
where WLA is wasteload allocation, LA is load allocation, MOS is margin of safety, and RC is reserve capacity. WLA includes pollutant loading from sources covered by a NPDES permit (often called point sources), LA includes sources not covered by a NPDES permit (often called nonpoint sources), MOS accounts for uncertainty in the estimates of WLA and LA, and RC allows for future growth.
When assessing which pollutant loads should be placed into the WLA, it is necessary to clearly define what is covered under an NPDES permit. Minnesota’s Phase 2 MS4 General Permit states that only the portions of the storm sewer system that are under a permittee's operational control are authorized by the permit. MPCA interprets this to mean an MS4 is responsible for ensuring its discharge is in compliance with water quality based effluent limits (WQBELs), since the discharge is within the publicly owned or operated conveyance system. The permittee must take appropriate measures to bring its MS4 into compliance. All discharges originating within a regulated MS4 or to a regulated MS4 from property owned or operated by the regulated MS4 are put into the WLA. Other discharges to an MS4 are also put into the WLA when the permittee has regulatory authority to control the discharges.
The Wasteload Allocation (WLA) is the portion of a receiving water's loading capacity that is allocated to one of its existing or future point sources of pollution. WLAs constitute a type of water quality-based effluent limitation (40 CFR 130.2). NPDES-permitted stormwater discharges must be included in the WLA. TMDL requirements in Stormwater permits (MS4, Construction, and Industrial) are triggered by the inclusion of a WLA in an EPA-approved TMDL prior to the issuance of the permit.