TMDL & ARAPS

Total Maximum Daily Loads (TMDLs) & Assumptions & Requirement Attainment Plans (ARAPs)

MSD, St. Louis County, and 59 other municipal co-permittees work together to protect and improve water quality in our region’s waters through the strategic development and implementation of best management practices that prevent, reduce, and/or eliminate pollutants that can contaminate stormwater runoff from urbanized areas.  This partnership operates in compliance with permit requirements for the St. Louis Metropolitan Small Municipal Separate Storm Sewer System (MS4).  Among those MS4 permit requirements, there are special conditions for TMDLs.

TMDLs are a calculation that sets the amount of pollutant a water body can assimilate without exceeding the water quality criterion for that particular pollutant.  In other words, a TMDL is pollutant specific diet for a waterbody.

The TMDL allocates the pollutant load among three components:

  • Wasteload allocation (WLA) for all point sources and MS4 Operators;
  • Load allocation (LA) for all nonpoint sources and natural background sources; and,
  • Margin of Safety (MOS) to account for any uncertainty in data or model assumptions.

The TMDL is equal to the sum of the wasteload allocation, load allocation, and margin of safety as expressed in the following equation:

TMDL = ΣWLA + ΣLA + MOS

TMDLs in the St. Louis MS4 Area

Within the MS4 area, TMDLs have been developed for waters placed on Missouri’s Section 303d list of impaired waters for elevated concentrations of Escherichia coli (E. coli) bacteria as a result of polluted stormwater runoff.  Through 2019, eight bacteria TMDLs have been approved by the United States Environmental Protection Agency.

EPA Approved Bacteria TMDLs

Stream EPA Approval Date Stream EPA Approval Date
Coldwater Creek 7/13/2016 Gravois Creek 1/16/2018
Creve Coeur Creek 7/13/2016 Maline Creek 4/25/2018
Fishpot Creek 7/13/2016 Deer Creek 6/26/2019
Watkins Creek 7/13/2016 Black Creek 6/26/2019

Please visit the Missouri Department of Natural Resource’s Impaired Waters and TMDL website to view EPA approved TMDLs and learn more about this requirement.

TMDL ARAPs

MS4 permittees must submit a TMDL ARAP to the Missouri Department of Natural Resources as soon as practicable but no later than 30 months after the date EPA approves or establishes the TMDL or the effective date of their operating permit, whichever is later.  The purpose of bacteria TMDL ARAPs developed in the St. Louis area is to address the TMDL wasteload allocation assigned to the MS4 area in the watershed of interest.  Each ARAP includes a description of procedures and implementation activities used to target known sources of bacteria in the watershed.   BMPs targeting bacteria are scheduled in a manner which aligns with activities carried out under the St. Louis County Phase II Stormwater Management Plan (as updated) and Project Clear.  BMP effectiveness is evaluated at scheduled intervals, and those that are found to be ineffective are modified or replaced.

In compliance with MS4 permit requirements, TMDL ARAPs have been developed for bacteria impairments in Coldwater Creek, Creve Coeur Creek, Fishpot Creek and Watkins Creeks.  These ARAPs were approved by the Missouri Department of Natural Resources on August 14, 2019.  TMDL ARAPs for bacteria in Gravois, Maline, Deer and Black creeks are under development.

If you would like to learn more about bacteria TMDL ARAP implementation in the St. Louis MS4, you may review the approved TMDL ARAPs provided below.

Coldwater Creek

Creve Coeur Creek

Fishpot Creek

Watkins Creek