A Total Maximum Daily Load (TMDL) is a pollution budget, or “diet”, for a body of water.  The TMDL represents the maximum amount of a pollutant that can occur in a waterbody while still meeting its water quality standards.  These water quality standards are determined by the Clean Water Act (CWA) with a goal that all waters of the United States be clean enough to be “fishable” and “swimmable”. A number of water bodies throughout Virginia have a TMDL, including streams in Alexandria.

The City of Alexandria has developed action plans to address both the Chesapeake Bay TMDL and local TMDLs. These action plans assess potential significant sources of pollutants of concern; evaluate the efficacy of measures to be implemented; and identify best management practices, measurable goals and milestones to demonstrate the effectiveness of the plan in reducing pollution. Requirements for developing these action plans are outlined in the City's municipal separate storm sewer system (MS4) permit. Please visit the City's MS4 program page for more information.

Most of the Chesapeake Bay and its tidal waters, such as the Potomac River, are impaired because of too much nitrogen, phosphorus, and sediment.  These pollutants cause algae blooms that consume oxygen and create “dead zones” where fish and shellfish cannot survive, block sunlight that is needed for underwater bay grasses, and smother aquatic life on the bottom. Nitrogen, phosphorus, and sediment enter the water from agriculture, urban and suburban stormwater runoff, wastewater facilities, air pollution, forested areas, and other sources, including septic systems. Despite some reductions in pollution during the past 25 years of restoration there has not been enough progress toward meeting the water quality goals for the Chesapeake Bay and its tidal waters.

On December 29, 2010, the United States Environmental Protection Agency (EPA) established the Chesapeake Bay Total Maximum Daily Load (TMDL) for nitrogen, phosphorous, and sediment. This TMDL – the largest and most complex ever developed by the EPA – sets pollution reductions for the six Chesapeake Bay watershed states (including Virginia) and the District of Columbia. The TMDL is required under the CWA and responds to consent decrees in Virginia and the District of Columbia from the late 1990s.

The TMDL is actually a combination of 276 smaller nitrogen, phosphorus, and sediment TMDLs for 92 individual Chesapeake Bay segments.  Pollutant limits are set to meet water quality standards for dissolved oxygen, water clarity, underwater Bay grasses, and chlorophyll-a, an indicator of algae levels.  The TMDL will be implemented using an accountability framework that includes Watershed Implementation Plans (WIPs), two year milestones, and tracking and assessment tools.  These tools, as well as other possible consequences, may be used if a jurisdiction does not meet their commitments or EPA's expectations toward achieving nutrient and sediment reductions.

The first two elements of the accountability framework are the WIPs and the two-year milestones. The Bay states had to complete their first set of two-year milestones by December 31, 2011 and second set by January 2014. The TMDL is designed to ensure that all pollution control measures from all source sectors that are needed to fully restore the Bay and its tidal rivers are in place by 2025, with at least 60 percent of the actions from all sectors to be in place by 2017. 

The City’s Environmental Management Ordinance (1992) provides for safeguarding of Chesapeake Bay Resource Protection Areas and protects water quality by requiring stormwater quality best management practices (BMPs) for new development and redevelopment.  The Erosion and Sediment Control Ordinance (1981) provides the basis of the City’s vigorous plan review and inspection program to address construction site controls.  The City's BMP inspection program continues to be aggressive in requiring proper functioning and maintenance of facilities.  The City also provides education and outreach to schools, interested organizations, and the general public about ways to protect the City's water resources.  

Please visit the Chesapeake Bay page for more information on what the City is doing in response to this TMDL. 

As with most cities and urban areas, too much bacteria is a problem in many of the streams in Alexandria.  The City has two bacteria TMDLs, one for Holmes Run, Cameron Run, and Hunting Creek and one for Four Mile Run.  These TMDLs were both finalized in 2010 and available on the DEQ's website.  There are many sources of bacteria in our local streams including pet waste, overflows from the City’s combined sewer system, and from wildlife droppings from geese, deer, etc.

Currently, the City addresses these TMDLs by providing education and outreach, opportunities for public involvement, proactive illicit discharge detection, and enforcement of City code that requires owners to pick up after their pets.  The City has also made tremendous progress in minimizing its combined sewer system discharges and implementing the National CSO Policy.  

The Bacteria TMDL Action Plan was finalized in 2022 in response to the requirements outlined in the 2018 MS4 Permit. Please visit the MS4 program page for more information about the City's MS4 permit and program.  

There is a Polychlorinated Biphenyls (PCBs) TMDL for Tidal Portions of the Potomac and Anacostia Rivers in the District of Columbia, Maryland, and Virginia, including the portion of the Potomac River in Alexandria.  This TMDL was established in 2007. 

PCBs are man-made compounds, first manufactured in 1929, and used for a variety of industrial applications, including coolants, lubricants in electrical equipment, dust control, pesticides, fire retardants, paints and coatings, printing inks, caulking, and wood treatment. There are no natural sources of PCBs.  PCBs do not break down easily, so even though production of them was banned in 1979, they continue to be an environmental pollutant. 

The City performs the following in support of implementation of the TMDL:

• Standard contaminated land condition for development Special Use Permit’s (SUPs) requiring screening for PCBs as part of the site characterization 
• Assess municipal properties for sources of PCBs and assign any “high risk” facilities that currently store, or have transferred, transported or disposed of PCBs in a manner that would expose it to precipitation  (none found)
• Characterize stormwater runoff from “high risk” properties (none found)
• Cleanup of the Hume-VEPCO Power Substation (complete)
• Compliance with combined sewer system Long Term Control Plan
• Developed brochure for developers, business and homeowners

The Tidal Potomac PCB TMDL Action Plan was updated in 2020 in response to the requirements outlined in the 2018 MS4 Permit. Please visit the MS4 program page for more information about the City's MS4 permit and program.