The TMDL for each lake was apportioned between the waste load allocation (WLA) and the load allocation (LA). The WLA includes loads from sites currently covered by an NPDES permit: the City of Forest Lake MS4, two large sewage treatment systems, and construction and industrial stormwater sites. The WLA also includes sites expected to be covered by an NPDES permit in the future: City of Scandia MS4, City of Wyoming MS4, and City of Chisago City MS4. The LA includes loads from stormwater runoff that originate in unregulated MS4 communities (Chisago Lake Township), unregulated MS4 portions of permitted MS4 or future permitted MS4 communities (City of Scandia, City of Chisago City, City of Wyoming and City of Forest Lake) livestock loading, internal loading, and atmospheric deposition. The watershed load (including regulated MS4, future regulated MS4 and unregulated or non-MS4 areas) was divided between the WLA and LA according to the amount of upland area estimated in each category. The upland area was selected to represent the developable area in the watershed; it includes the total watershed area with the lake and wetland area subtracted out. Total area was not used due to the high amount of surface water in some of the watersheds. MS4 wasteload was allocated based on the portion of the lake's developable watershed area contained within the estimated regulated portions of the MS4. The boundaries of the regulated portion of the MS4s were estimated by excluding the portions of MS4 communities that are not technically covered under NPDES permits (i.e., areas that are either agricultural or otherwise not projected to be served by stormwater conveyances, such as open space, park and recreation, and rural residential). The portion of each municipality that is not estimated to be within the regulated boundaries of an MS4 is provided with a LA determined based on the developable watershed area. To calculate TMDL allocations, upstream impaired lakes were assumed to have outflow meeting the phosphorus standard because each of these lake impairments is also addressed through the TMDL. In the TMDL report, the WLAs and LAs were presented in terms of phosphorus loading per day. The percent reductions were presented only to provide further information.
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Minnesota Pollution Control Agency (St. Paul, Minnesota)
Minnesota Pollution Control Agency