Total Maximum Daily Loads (TMDLs)

Interstate Commission on the Potomac River Basin

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:

MarylandVirginiaWashington, D.C.PennsylvaniaWest Virginia

Current ICPRB Projects

Fine-scale Modeling of Suspended Sediment and Phosphorus

ICPRB, in conjunction with the Maryland Department of the Environment (MDE), is currently developing a modeling framework for finer-scale modeling of suspended sediment and phosphorus that could be implemented in urban watersheds across the State of Maryland. This work is ongoing and consists of the following tasks:

  1. (a) developing a Soil and Water Assessment tool (SWAT) for the Jones Falls watershed thus providing a finer resolution model with improved stream bed and bank simulation and source tracking, and (b) investigating the impact of urban best management practices using the SWAT model.
  2. Developing a Spatial Statistical model to forecast water quality endpoints and help provide recommendations on the development of finer scale regional models. The outcome of this study can provide new lines of evidence that would allow the Chesapeake Bay Program Partnership improving its urban land-use simulation. This will also provide an avenue for MDE to utilize their short-term continuous water quality monitoring (e.g., temperature and sediment), and satellite derived products such as NLDAS climate data.

Completed TMDL and Water Quality Analyses Reports

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 Model Report for the Revised Polychlorinated Biphenyl (PCB) TMDLs for Small Tributaries in the Rock Creek Watershed in the District of Columbia was approved by U.S. EPA Region 3 in 2016.

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.

Published 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.

Body of Water Impairment Year Completed
Accotink Creek, Va. Chlorides 2017
Accotink Creek, Va. Sediment 2017
Accotink Creek, Va. Benthic Macroinvertebrates 2017
Rock Creek and Potomac River, D.C. Polychlorinated Biphenyl (PCB) 2016
Rock Creek and Potomac River, D.C. Pesticides 2016
Accotink Creek, Va. Sediment/Chlorides 2015
Tripps Run and Holmes Run, Va.  Hydromodification/Riparian Habitat 2014
Fairview Beach, Va. Bacteria 2014
Antietam Creek, Md. Phosphorus 2012
Catoctin Creek, Md. Phosphorus 2012
Double Pipe Creek, Md. Phosphorus 2012
Liberty Reservoir, Md. Phosphorus/Sediment 2012
Lower Monocacy River, Md. Phosphorus 2012
Rock Creek, Md. Phosphorus 2012
Upper Monocacy River, Md. Phosphorus 2012
Conowingo Dam (WQA) Phosphorus 2011
Conowingo Dam (WQA) Sediment 2011
Lower North Branch Potomac, Md. (WQA) Phosphorus 2011
Lower North Branch Potomac, Md. (WQA) Sediment 2011
Potomac River, Washington Co., Md. Sediment 2011
Potomac River, Montgomery Co., Md. Sediment 2011
Potomac River, Montgomery Co., Md. (WQA) Eutrophication 2011
Deep Creek Lake, Md. (WQA) Eutrophication 2010
Four Mile Run, Va. Bacteria 2010
Hunting Crk/Cameron Run/Holmes Run, Va. Bacteria 2010
Potomac River, Wash. Co., Md. Eutrophication 2010
Anacostia River Trash 2009
Anacostia, Md. and D.C. Nutrients/BOD 2008
Triadelphia Reservoir and Rocky Gorge Reservoir, Md. Phosphorus/Sediment 2008
Anacostia, Md. and D.C. Sediment 2007
Potomac River Polychlorinated Biphenyls (PCBs) 2007
Prettyboy Reservoir and Loch Raven Reservoir, Md.  Nutrients, Sediment 2007
Potomac/Anacostia, D.C./Md./Va. Polychlorinated Biphenyls (PCBs) 2007
Goose Creek, Va. Sediment 2004
Goose Creek, Va. Bacteria 2003
Small Tributaries, D.C. Multiple 2003