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Total Maximum Daily Loads

Improving Waters – The Clean Water Act

When too much pollution flows to a lake, river, or coastline, it may no longer be safe for people to drink, boat on, or swim in, and it may no longer support a healthy wildlife population.

When lakes or rivers have more contaminants than the levels states have determined to be acceptable, the water body is considered impaired.  These levels, or criteria, are based on a water body’s designated use under state water quality standards.

The Clean Water Act requires states to create a list of these impaired water bodies and develop plans to improve water quality. Scientists analyze impaired water bodies to determine the daily amount, or load, of a pollutant they can absorb without significantly harming the water’s designated use. States then identify major sources of the contaminant within a watershed and specify where and when reductions must be made so the water body once again meets its water quality requirements. This proposed “Total Maximum Daily Load” (TMDL) plan is then sent to the EPA for approval.

The banks of a river

Pollution Budgets

A TMDL is a pollution budget and a reduction plan. It guides all stakeholders in a watershed, including the state, on how to improve water quality.

Developing and implementing a TMDL can be complex, difficult, and expensive. NEIWPCC has contributed to TMDL work on many water bodies in New England and New York State, including atmospheric mercury deposition across the Northeast, nitrogen flows to the Long Island Sound, and work on nonpoint source pollution reductions across the region.


Our TMDL workgroup connects people working on TMDLs across our member states and EPA regions. Workgroup meetings provide a forum for staff to share knowledge, develop solutions, and anticipate and plan for future challenges and potential conflicts.

NEIWPCC hosts national informational webinars to train state, federal, and tribal TMDL program staff, as well as other non-governmental stakeholders when appropriate.

NEIWPCC also monitors EPA’s TMDL guidance and writes comment letters, if necessary, on behalf of our member states.

NEIWPCC worked with the U.S. Geological Survey to develop the New England Regional SPARROW (Spatially Referenced Regressions on Watershed Attributes) model, helpful in developing nutrient TMDLs.

For more information on NEIWPCC’s work with TMDLs contact Courtney Botelho.