LimnoTech provided an improved inventory of TMDLs and WLAs to guide and support development of an improved TMDL Implementation Plan.


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Success Story

MS4 TMDL Implementation Planning for Washington, DC

The District of Columbia, like all major cities, has an MS4 permit with obligations to manage urban stormwater. The current permit requires development of a Consolidated TMDL Implementation Plan for all of the TMDL wasteload allocations (WLAs) assigned to District MS4 discharges. This requirement may become more common in MS4 permits across the U.S. as permitting authorities use this instrument to achieve pollution reduction.


The MS4 area within the District contains over 600 stormwater outfalls that drain approximately 20,000 acres. Development of a consolidated implementation plan is challenging because more than 380 WLAs are assigned to MS4 discharges for 23 different pollutants across 45 different water bodies. The landscape is further complicated by the presence of a substantial amount of federal property, persistent legacy pollutants, and the fact that upstream sources outside the district represent a major source of pollutants that affect water quality.


Working with the District Department of the Environment, LimnoTech sought the most effective and affordable ways to prioritize pollutant removal and protect water quality. Key elements included:

  • Careful review of the adequacy of supporting data used to list the impairments that brought about the TMDLs and WLAs.
  • Linkage of pollutant removal to the 1.2-inch storm retention standard.
  • Assessment of forecast development and redevelopment trends.
  • Consideration of BMPs and source control methods capable of reducing multiple pollutants.
  • Development of a city-wide modeling tool to account for and track pollutants and pollutant reduction.

LimnoTech developed an improved inventory of TMDLs and WLAs to guide planning. Preliminary results suggest that some TMDLs and WLAs may not be necessary or required because of data quality issues. A methodology designed to model and track pollution reduction and interim benchmarks and milestones was established. Development of the Consolidated Implementation Plan with support from stakeholders is underway.