Linking TMDLs and NPDES-Permitted Stormwater: Minnesota's Experience

A TMDL Wasteload Allocation (WLA) must include discharges from NPDES-permitted stormwater (construction, municipal, and industrial). Federal law requires NPDES permits to be consistent with the assumptions and requirements of any TMDL WLA. Based on these statements, the process for linking stormwater and TMDLs seems straightforward. However, linking NPDES-permitted stormwater to the TMDL WLA has proven challenging.In Minnesota we have developed several approaches for addressing the myriad issues that have arisen during development of TMDLs that have a regulated stormwater component. We discuss the following issues and use case studies to illustrate approaches for linking TMDLs and stormwater permitting.Setting WLAs. The Minnesota Pollution Control Agency (MPCA), responsible for developing TMDLs and administering the NPDES Program, has developed policy stating that the preferred form of the WLA is as an individual allocation for each regulated Municipal Separate Storm Sewer System (MS4). The watersheds of six nutrientimpaired lakes in the Minnehaha Creek Watershed District (MCWD) include 14 regulated MS4s. Individual WLAs were established for each MS4, thus providing clear targets that link with permit requirements.Future Growth. A TMDL WLA can include pollutant loads from future NPDES-permitted sources, and MPCA policy requires incorporation of future growth into the WLA when possible. This can be done by using local Comprehensive Plans to project future development. In the MCWD Lakes TMDL, several cities will see significant growth in the next 20 years, including annexation of currently undeveloped land, eventually resulting in the watershed being fully built out. The TMDL assigns all watershed loading to individual WLAs for the future boundaries of these developing cities. Since an MS4 must comply with a WLA through the permit, this allows a city to “grow into” its WLA and creates an opportunity for innovative stormwater management.Construction Stormwater. Construction stormwater is transient in nature. Consequently, Minnesota has developed a policy that considers compliance with the Construction Stormwater General Permit to meet the TMDL WLA. To provide a safeguard, the General Permit includes language that requires permittees to comply with specific BMPs named in the TMDL for construction stormwater. Hardwood Creek, impaired for biota, is a public ditch that must be periodically maintained. To minimize impacts from maintenance activities, the Hardwood Creek TMDL specifies BMPs that must be implemented during maintenance, thus linking the TMDL to the construction stormwater permit.Implementation. In Minnesota, TMDL implementation plans must be completed within one year of U.S. EPA approval of a TMDL. Although the implementation plan is not enforceable through the permit, MPCA has developed guidance for writing implementation plans that contain specific information useful for MS4 permittees in writing their Stormwater Pollution Prevention Programs (SWPPPs). Permittees choosing to implement BMPs described in the implementation plan may be considered in compliance with the WLA. Peltier Lake and Centerville Lake are impaired for nutrients. The TMDL implementation plan provides specific actions that should achieve the water quality objectives for the lakes. Permittees can simply use the BMPs in the implementation plan to develop their SWPPPs, which in turn can be easily reviewed by the MPCA and which will be considered to bring permittees in compliance with the WLA.The case studies and approaches discussed in this paper are consistent with EPA recommendations and should provide useful information for others who are attempting to link NPDES-permitted stormwater with TMDL requirements.