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It is not necessary or required for an MS4 to derive an individual waste load allocation (WLA) for a Total Maximum Daily Load (TMDL) that developed categorical MS4 WLAs. However, if you do want to determine your MS4’s portion of the categorical WLA, the following approaches can be used:
- Area approach. The WLA can be divided based on the relative area of each MS4. Only the areas within the study watershed should be considered. For example, if two MS4s have a WLA of 100 lbs/day, MS4 A covers 75% of the study area and MS4 B covers 25% of the study area, the target loads would be 75 lbs/day for MS4 A and 25 lbs/day for MS4 B. This is a desirable approach if the pollutant loading per unit area is considered similar across all MS4s.
The TMDL report may contain information that is useful for this approach. For example, the Ramsey Washington Metro Watershed District Watershed TMDL report (2017) contains the following table. This information, along with the TMDL table can be used to derive an individual WLA.
Potential sources of bacteria and actions to address
Link to this table
| General Category | Source | Action |
|---|---|---|
| Municipal Sanitary Infrastructure | Combined Sewer Overflows1 | Replace infrastructure, if possible |
| Sanitary Sewer Bypasses/Overflows | Line sanitary sewer pipes | |
| Inflow and Infiltration | Minimize and eliminate through repairing damaged sewers | |
| Illicit discharges from unsewered communities | Detect and eliminate illicit discharges | |
| Human Sanitary Sources | Leaky/failing septic systems | Update and enforce subsurface sewage treatment system (SSTS) ordinances |
| Educate homeowners on septic system maintenance and opportunities for replacement | ||
| Implement higher standards for setbacks for installing septic systems near receiving water bodies | ||
| Porta-potties | Ensure construction site temporary toilets have appropriate pollution prevention BMPs | |
| Domestic pets | Dogs, cats, dog parks, walking trails, residential yard and sidewalk runoff (spring runoff after winter accumulation) | Establish pet waste disposal stations along trails, in dog parks, etc. |
| Pass and enforce pet waste ordinances and educate pet owners about them | ||
| Add infiltration BMPs downstream of parks/residential areas and upstream of stormwater pipes (i.e., somewhere between the park/residential area and the stormwater outfall so as to intercept and infiltrate some or all of the flow from these areas) | ||
| Reduce transport from parks, residential, and other areas by the use of buffer zones and other disconnection of flow pathways (e.g. impervious surface, disconnection, downspout disconnection) | ||
| Urban wildlife | Rodents (raccoons, squirrels, etc.), deer, etc., birds (geese, ducks, swallows, etc.), areas where wildlife congregate such as fields, parks, and golf courses | Wildlife feeding bans and control of nuisance populations, including ducks and geese and other wildlife |
| Remove community facilities such as vending machines for feeding ducks and geese | ||
| Add buffers in riparian zones near waterbodies to deter waterfowl congregation | ||
| Consider wildlife barriers if wildlife (e.g. raccoons) are found to be living in storm sewers | ||
| When possible, use infiltration BMPs instead of detention ponds in residential developments and other areas where wildlife may congregate | ||
| Other Urban Sources (non-stormwater) | Excessive irrigation/overspray | Assess the extent to which irrigation over-spray occurs at city-owned facilities and eliminate overspray as appropriate |
| Education on water conservation | ||
| MS4 Infrastructure | Illicit sanitary connections to MS4 | Detect and eliminate illicit discharges |
| Leaky sewer pipes | Line sewer pipes | |
| Biofilms/regrowth | Reduce dry weather flows, which provide conditions that promote bacteria growth. Dry weather flows could be from night time irrigation of lawns/parks or leaky stormsewer pipes. | |
| Decaying plant matter, litter, and sediment in the storm drain system | Enhance street sweeping program. | |
| Prevent the discharge of grit chamber clean out effluent | ||
| Remove leaf litter and soil in street and gutters: implement an Adopt-a-storm drain program | ||
| Other Natural Sources | Stream Bank Erosion | Streambank restoration |
1Note only a few combined sewers remain in Minneapolis
*This table was created using information from the Upper Mississippi River Bacteria TMDL Study (2014) and Protection Plan and the Upper Mississippi River Bacteria TMDL Implementation Plan (2016), Pathogens in Urban Stormwater Systems (2014), and the Minnehaha Creek Bacterial Source Identification Study (2017).