Total Maximum Daily Load (TMDL); What’s It All About?

People interested in stormwater management and water resource issues have likely heard the term Total Maximum Daily Load and its acronym TMDL.  A TMDL is the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards.  TMDL regulations were authorized under Section 303 of the 1992 Clean Water Act.   Section 303 requires that water quality standards be established (e.g. for recreational use, water supply, and protection of aquatic life), and that waterways be monitored to determine if the standards are being met.  Many rivers and streams have been found to be “impaired”, meaning that pollutants are being measured at concentrations above water quality standards.  When this occurs, Section 303 requires that a TMDL for the water body be established.

In addition to identifying the total amount of pollution that the waterbody can receive and still meet water quality standards, the TMDL also evaluates how much pollution is reaching the creek from existing sources of pollution.  Using this information, the total amount of pollution reductions that are needed to meet water quality standards can then be estimated.  These pollutant reductions are assigned to specific entities like treatment plants and municipalities, who must then develop a TMDL plan.  The TMDL plan must identify activities and a timeline for reducing pollution to levels that met the water quality standards.

The TMDL calculations consider pollutants that are caused by both point sources (e.g. a pipe discharge to a stream) and non point sources (e.g. fertilizer carried by runoff into a stream).  Point sources of pollution receive wasteload allocations (WLA) in the TMDL calculations.  These point sources include all sources subject to regulation under the National Pollutant Discharge Elimination System (NPDES) program.  Municipalities with municipal separate storm sewer system (MS4) permits are treated as point sources in TMDL assessments.  Nonpoint sources of pollution receive load allocations (LA) in the TMDL calculations.  Nonpoint source are typically managed through a variety of state, local, and federal programs which may be regulatory, non-regulatory, or incentive-based, depending on the program.

A Wissahickon Watershed sediment TMDL was established in 2003, under which each municipality was assigned a sediment reduction requirement in pounds of sediment per year.   The municipalities were required to submit sediment TMDL plans under their most recent MS4 permit (submitted in September 2012), specifying how they would achieve the required pound per year reductions in sediment pollution reaching the Creek.  The Pennsylvania Department of Environmental Protection (PA DEP) is responsible for reviewing and approving these TMDL plans.

The nonprofit organization PennFuture successfully challenged the Upper Gwynedd Township TMDL plan approved by PA DEP; the outcomes of this lawsuit apply to the state-wide TMDL approval process and will improve Pennsylvania’s program for preventing stormwater runoff from polluting streams and rivers.  Click here for more information on this settlement; the next issue of Upstream/Downstream will also provide more details.

The U.S. Environmental Protection Agency is also in the process of updating a second Wissahickon Watershed TMDL that will address nutrient and dissolved oxygen impairments.  The draft updated nutrient and dissolved oxygen TMDL will be published for public comment in the near future.

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