The maximum amount of a pollutant that can enter a river, stream, lake, or estuary and still allow it to meet the water quality standards set for it by the state and the federal government is known as the Total Maximum Daily Load (TMDL). TMDLs are necessary when a water body does not meet standards even after standard pollution control technology has been installed on all regulated dischargers like wastewater treatment plants.

Identifying Pollutants

To determine a TMDL, all current sources of a pollutant must be identified, including non-point sources like stormwater or agricultural runoff. Usually, a computer simulation model is used to explain the link between current pollutant loads and observed water quality, and to predict by how much the loads need to be reduced in order for the water body to meet water quality standards. Learn more about TMDLs from the EPA.

TMDL Databases

Each jurisdiction in the basin maintains a database of their TMDL reports:

Maryland │ Virginia │ Washington, D.C.  │ Pennsylvania │ West Virginia

Current ICPRB Projects

Potomac tributaries toxic TMDL (D.C.)

ICPRB assisted U.S. EPA and the District of Columbia Department of Energy and Environment (DOEE) in the redevelopment of toxics TMDLs in the Potomac and Rock Creek watersheds for impairments identified in DOEE’s 2014 Integrated Report. The update previously established TMDLs by incorporating additional monitoring data and expressing the TMDLs as daily loads. The document was approved by EPA Region 3 in 2016. We will post links to the TMDLs when they are available.

Accotink Creek TMDL (Va.)

The overall goal of the Accotink Creek project was to assist the Virginia Department of Environmental Quality (Va. DEQ) in developing TMDLs to address the biological impairments in Accotink Creek. These TMDLs replaced the flow TMDL vacated by the U. S. District Court, who asserted that the Clean Water Act did not give the EPA the authority to regulate flow. In the first stage of the project, ICPRB assisted Va. DEQ by performing a stressor identification analysis to determine the most likely stressors of the benthic communities in Accotink Creek. Sediment, chlorides, habitat modification, and hydromodification were identified as the most likely stressors, but under the Clean Water Act, only sediment and chloride can be addressed through TMDLs. In the second stage of the project, ICPRB assisted Va. DEQ in developing the TMDLs for sediment and chloride in the Accotink Creek watershed. The document is currently undergoing the approval process.

Completed TMDL and Water Quality Analyses Reports

Listed below are previous TMDL reports or Water Quality Analyses (WQA) in which ICPRB was involved. ICPRB’s work in each project varied but may have included data analyses, modeling, and/or writing reports.