Below is a summary of Minnesota Pollution Control Agency (MPCA) guidance for setting Wasteload Allocations (WLAs) for regulated stormwater. This includes construction, Municipal Separate Storm Sewer Systems (MS4), industrial, and individually-permitted stormwater. Additional information can be found here under the guidance documents heading .
During development of a Total Maximum Daily Load (TMDL), data are generally not available to support identification of specific Best Management Practices (BMPs) for construction stormwater. It is therefore MPCA’s preference to utilize the Construction Stormwater General Permit to manage compliance with a TMDL. National Pollutant Discharge Elimination System (NPDES) permitted construction stormwater must be given a WLA for TMDLs where the pollutant/stressor is total suspended solids (TSS) (turbidity), phosphorus (excess nutrients or eutrophication), dissolved oxygen, or biota (plant, fish, or macroinvertebrate). Construction stormwater should be given a single categorical WLA (i.e. one WLA for construction stormwater). However, it is acceptable to include construction stormwater as part of an overall categorical WLA for stormwater when the entire watershed load is from regulated stormwater discharges. The WLA for construction stormwater will typically range from 0.05 to 0.15 percent of the overall TMDL, minus the Margin of Safety. TMDLs that do not prescribe BMPs for construction stormwater should contain the following language in the implementation section of the TMDL report: The WLA for stormwater discharges from sites where there are construction activities reflects the number of construction sites one or more acres expected to be active in the watershed at any one time, and the best management practices (BMPs) and other stormwater control measures that should be implemented at the sites to limit the discharge of pollutants of concern. The BMPs and other stormwater control measures that should be implemented at construction sites are defined in the State's NPDES/SDS General Stormwater Permit for Construction Activity (MNR100001). If a construction site owner/operator obtains coverage under the NPDES/State Disposal System (SDS) [Permit] General Stormwater Permit and properly selects, installs and maintains all BMPs required under the permit, including those related to impaired waters discharges and any applicable additional requirements found in Appendix A of the Construction General Permit, the stormwater discharges would be expected to be consistent with the WLA in this TMDL. It should be noted that all local construction stormwater requirements must also be met.
During development of a TMDL, data are generally not available to support identification of specific BMPs for industrial stormwater. It is therefore MPCA’s preference to utilize the Industrial Stormwater Multi-sector General Permit to manage compliance with a TMDL. Industrial stormwater must receive a WLA if the pollutant of impairment is part of benchmark monitoring for any industry in the impaired watershed. Data resulting from benchmark monitoring of industrial stormwater discharges cannot be used to establish a separate WLA for industrial stormwater. When effluent limits are established for an industrial sector, that sector may be given a separate WLA consistent with those effluent limits. Industrial stormwater should be given a single categorical WLA (i.e. one WLA for industrial stormwater). However industrial stormwater may be part of an overall categorical WLA for stormwater when the entire watershed load is from regulated stormwater discharges. TMDLs that do not prescribe additional BMPs for industrial stormwater should contain the following language in the implementation section of the TMDL report: The WLA for stormwater discharges from sites where there is industrial activity reflects the number of sites in the watershed for which NPDES industrial stormwater permit coverage is required, and the BMPs and other stormwater control measures that should be implemented at the sites to limit the discharge of pollutants of concern. The BMPs and other stormwater control measures that should be implemented at the industrial sites are defined in the State's NPDES/SDS Industrial Stormwater Multi-Sector General Permit (MNR050000) or facility specific Individual Wastewater Permit (MN00XXXXX) or NPDES/SDS General Permit for Construction Sand & Gravel, Rock Quarrying and Hot Mix Asphalt Production facilities (MNG490000). If a facility owner/operator obtains stormwater coverage under the appropriate NPDES/SDS Permit and properly selects, installs and maintains all BMPs required under the permit, the stormwater discharges would be expected to be consistent with the WLA in this TMDL. It should be noted that all local stormwater management requirements must also be met.
Each regulated (permitted) MS4 within a TMDL study area must be given a WLA. In addition to standard expressions of WLA (e.g. pounds/day), the allocation section of the TMDL should include a required reduction (e.g. percent, mass) from a clearly defined baseline (e.g. a year, a specific BMP condition). When data allow, expressing the WLA as a loading rate (e.g. lbs/acre) is encouraged. The TMDL should list the name, permit number and ID for each MS4. This WLA will be categorical or individual. Individual WLAs should be given to each permitted MS4, although categorical WLAs may be appropriate in certain situations. If a categorical WLA is included in a TMDL report, the report should provide an explanation of the rationale for using a categorical WLA.
WLAs for existing regulated MS4 stormwater should consider future loads. Methods for addressing future loads should be consistent with File:Guidance on What Discharges Should be Included in the TMDL Wasteload Allocation for MS4 Stormwater.docx. The following language should be included in all TMDL/WRAPS reports to allow for transfer of load without re-opening the TMDL:
Future transfer of watershed runoff loads in this TMDL may be necessary if any of the following scenarios occur within the project watershed boundaries:
Load transfers will be based on methods consistent with those used in setting the allocations in this TMDL [Specify method, if needed, e.g., “Loads will be transferred on a simple land-area basis.”]. In cases where WLA is transferred from or to a regulated MS4, the permittees will be notified of the transfer and have an opportunity to comment.
A TMDL report can address allocations for non-regulated MS4 stormwater in one of three ways. First, the report may assign a categorical LA to non-regulated MS4 stormwater and describe a clear procedure for transferring load from the LA to the WLA. A second and preferred approach is to assign individual LAs to all non-regulated MS4s in the watershed. In this situation, the TMDL report should include the following statement: Individual LAs for a MS4 will be transferred to individual WLAs for MS4s that come under NPDES permit coverage. A third approach utilizes a combination of approaches 1 and 2. With this approach, individual LAs are provided to some MS4s and a categorical LA is assigned to a group of MS4s. At a minimum, individual LAs should be given to MS4 stormwater that is likely to be covered under a NPDES permit in the future.