Many total maximum daily loads (TMDLs) lump multiple wasteload allocations (WLA) for MS4 permittees into a single WLA. This single WLA is called a categorical WLA. This page provides guidance to MS4 permittees for interpreting categorical WLAs.
The primary advantage of a categorical WLA is that it provides a single value that all permitted MS4s can work toward achieving. This provides greater opportunity to implement pollution reduction practices efficiently. Examples include the following.
Categorical WLAs have disadvantages.
Prior to the 2020 MS4 General Permit issuance, it was not necessary to derive an individual waste load allocation (WLA) for a total maximum daily load (TMDL) that used a categorical MS4 (Municipal Separate Storm Sewer System) wasteload allocation (WLA), unless a permittee was claiming to meet a categorical WLA. This permit cycle, in order to make a determination as to whether permittees may or may not be meeting a WLA, they may need to derive an individual target load.
Permittees can work together to cumulatively meet required pollutant loads or pollutant load reductions. However, if a permittee chooses to derive its own pollutant target, there are several options. This section provides guidance for cumulatively meeting a categorical WLA or deriving an individual target.
Permittees can work together to meet a categorical WLA. This is a favored approach if the following conditions exist.
In Figure 1, two cities work cooperatively to implement two BMPs close to the impaired water, where they are most effective. Both cities can apply the reductions toward their permit requirements provided the permittees mutually agree to do this.
Option 1: Use the percent reduction found in Column I of the Applicable WLAs determination tab as the metric to determine if a WLA is being met or not. (The Simple Estimator provides percent reductions when BMPs are entered). If Column I shows “Insufficient data” or “Not Available” proceed to Option 2, 3 or 4. (If there are multiple flow zones, target the mid-range. See Interpreting wasteload allocations based on flow/load duration curves for more information).
Option 2: Using an aggregator - All permittees sharing a categorical WLA agree to use an aggregator (watershed district or another permittee who agrees to track BMPS and reductions). Use all BMPS since the baseline year and entire MS4 regulated area as outlined in the TMDL to determine whether they are meeting the WLA as a whole or not.
If a group of permittees agrees to address a categorical WLA together, they do not need to determine individual target loads. However, they are all equally responsible for items in the compliance schedule. Each permittee stills need to submit their TMDL Application Form. Categorical WLAs that are being addressed as a group should be noted in question 174 of the PDF portion of the application, and the partnerships should be noted in questions 6 and 7 of the PDF portion of the Application. Permittees are encouraged to submit additional spreadsheets or documents if that assists in tracking BMP ownership or partnerships.
Option 3: Use loading rates found in this spreadsheet. (To be completed and linked ASAP) MPCA staff have determined loading rates (lbs/ac/yr) that can be used for the following TMDL projects which did not have percent reductions listed in the MS4 WLA list.
Burandt Lake Excess Nutrients TMDL
Elk River Watershed TMDL
Golden Lake TMDL
Hardwood Creek Impaired Biota and Dissolved Oxygen TMDL
Lower Cannon River Turbidity TMDL
Miss. River - Saint Cloud WRAPS 2009
South Metro Mississippi TSS TMDL
Twin (Upper, Middle, and Lower) and Ryan Lakes TMDLs
Zumbro River Watershed Turbidity TMDL
Option 4: Permittees individually determine whether they are meeting their portion of the categorical WLA after identifying their target load. (See more information on how to determine this below).
If a permittee is part of a categorical WLA but wants an individual target load, there are several methods to calculate the target load. Permittee's should reference the wasteload allocation methodology section of the associated TMDL report in order to figure out how to derive their target load. Note that target loads are not approved WLAs. Permittees should consult the MPCA when developing target loads. The MPCA has geographic information systems (GIS) TMDL project area shape files that could be useful in estimating an MS4's portion of the categorical WLA. Available GIS files can be found here. The following approaches explain different methodologies that might be referenced in a TMDL report.
|Waterbody||MS4 Name||MS4 ID Number||MS4 Area within the contributing watershed (acres)|
|Bennett Lake||City of Roseville||MS400047||632|
|MnDOT Metro District||MS400170||55|
Figure 3 shows that the total WLA minus the WLA for construction and industrial stormwater is 21.7 lbs/yr. According to the table above, Roseville is 86% of the total MS4 area and Ramsey County is 6%, Roseville could assume their MS4 is 86% of the total WLA, so their individual allocation would be estimated to be 18.7 lbs/yr and Ramsey County's estimated WLA would be 1.3 lbs/yr. MnDOT already has an individual allocation of 1.6 lbs/yr.
If this information is not readily available, MPCA Stormwater staff can assist with determining the areas for each MS4 in the watershed area of the TMDL.
Every TMDL has a section called ‘wasteload allocation methodology’. That section should describe how the wasteload allocation for point sources such as wastewater treatment facilities, construction and industrial stormwater and MS4s was determined. There should be a subheading specifically discussing the MS4 WLA and which of the above approaches were used to calculate the WLA. This section should also state which permittees were included in the categorical WLA. All of this is necessary information in determining target loads.
Before calculating target loads, be sure to check your TMDL’s project page for an Implementation Plan. Many of the early TMDL reports had an association Implementation Plan that provided target loads as well as suggested BMPs. To find your TMDL Project page: Enter the name of the TMDL in the search box on this page The WRAPS report may also include helpful implementation suggestions. Check your Watershed webpage to see if a WRAPS report has been completed.