Total Maximum Daily Load (TMDL) Determination

According to federal law, a TMDL must be developed for all impaired waters that are listed on State 303(d) lists. By EPA definition, a TMDL is “a calculation of the maximum amount of a pollutant that a waterbody can receive or assimilate and still meet water quality standards, and an allocation of that amount to the pollutant’s sources.” Technically, a TMDL is the sum of the wasteload allocations (WLA) for point sources, the load allocations (LA) for nonpoint sources and includes a margin of safety (MOS) or, a TMDL = WLA + LA + MOS. TMDL analyses are currently being used by regulatory agencies to address water quality problems around the country and more specifically, to bring waterbodies that are listed on State 303(d) lists into compliance with applicable water quality standards. The completion of a TMDL requires environmental data collection, mathematical modeling of present and future conditions, potentially involve development of viable management solutions and alternatives, and acceptance from environmental regulators and stakeholder groups. More recently, the results of TMDL studies are being used to develop watershed management plans that may include various forms of point and nonpoint source trading strategies.

HydroQual can provide support in the development of TMDLs through:

  • Identification and compilation of data resources and needs
  • Development of field monitoring programs
  • Selection and application of appropriate watershed/sewershed and receiving water models
  • Perform uncertainty analyses to select appropriate Margin of Safety
  • Identification and evaluation of management alternatives (including treatment processes and best management practices) for implementation of TMDLs
  • Visualization of field data and model computations
  • Presentation of study results to environmental regulators, public and other stakeholders

HydroQual has successfully completed many TMDL and water quality modeling studies for a wide variety of pollutants around the country for municipal, industrial and regulatory clients. Our wide range of clients (regulated and regulators) indicates that the studies we complete are scientifically objective, technically defensible and can be used in the TMDL process.


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