In response to the Jan. 19, 2026 collapse of a section of the 54-mile Potomac Interceptor and subsequent discharge of sewage into the Potomac River, DEQ initiated a special water quality monitoring initiative to measure bacteria levels along Virginia’s Potomac River shoreline, within Virginia embayments and at a handful of Maryland shoreline locations. DEQ’s sampling efforts will supplement the sampling efforts of DC Water, the DC Department of Energy & Environment, the Maryland Department of Environment and others.

On March 12, bacteria analytical results from DEQ's March 9 sampling event were posted to DEQ's interactive data map (below). Results from the three transect sampling locations were all below 100 CFU/100 mL for E coli. Sampling results for Virginia's southern Potomac embayments (below the Harry W. Nice Memorial/ US 301 Bridge) were all below the E. coli and Enterococci method detection limits of 10 CFU/100 mL.

On March 9, a DEQ sampling team collected water samples from three Virginia embayments located between the Harry W. Nice Memorial Bridge (US Route 301) and Coles Point. A separate DEQ sampling team collected water samples from three river transects: Key Bridge, 14th Street Bridge and Daingerfield Island. Each transect consisted of three samples: mid river, river right, and river left. Analytical results will be posted once DEQ has received the results and reviewed them.

The DC Department of Energy & Environment has launched a water monitoring dashboard that presents combined E. coli data from DC Water, DC Department of Energy & Environment (DOEE), Maryland Department of the Environment (MDE), Virginia Department of Environmental Quality (DEQ), and the Potomac Riverkeeper Network with University of Maryland (UMD)

Presented below is an interactive map of DEQ, VDH, and DEQ partner sampling locations and data:

View the DEQ Potomac Interceptor Collapse Sampling Dashboard full screen.

There are two thresholds that may be considered when evaluating bacteria results for recreational waters. These are generally distinguished between freshwater and estuarine waters (where freshwater meets saltwater). In the Potomac River, transitional and saltwater areas are generally downstream near Quantico, Virginia.

The first threshold is Virginia’s statistical threshold value for recreational primary contact — 410 counts/100 mL E. coli in freshwater and 130 counts/100 ml Enterococci in transitional and saltwater. The second is a beach action value VDH uses for coastal public beach advisories which is 104 /100 ml Enterococci. Fecal coliform is run as part of the suite of indicator bacteria, but Virginia does not have recreational thresholds for fecal coliform.

On March 5, the Virginia Department of Health (VDH) partially lifted their recreation advisory - issued Feb. 13. The lifted portion of the advisory includes the portion of the Potomac River from the Route 120 Chain Bridge to the Harry W. Nice Memorial Bridge (US Route 301). A recreation advisory remains in place from the American Legion Bridge (I-495) to the Route 120 Chain Bridge (image below).

VDH Revised Recreation Advisory Upper Potomac March 2026

On March 3, due to weather, DEQ made the decision to postpone water sampling that was scheduled for Wednesday, March 4. DEQ's next round of sampling is now scheduled for Monday, March 9.

On Feb. 27, DEQ's contractor finished collecting additional surface water grab samples. Analytical results will be posted as soon as they are available. Weather permitting, DEQ's next round of surface water sampling is scheduled for Wednesday, March 4.

On Feb. 26, DEQ's contractor completed transect sampling. They will return tomorrow to collect several grab samples.

On Feb. 25, DEQ collected water samples from nine embayments along the Virginia shoreline of the Potomac River. Simultaneously, a DEQ contractor began transect water sampling at several locations where the transect sampling involves collecting a water sample near each shoreline of the river, a sample from the middle of the river, and a sample half way between the middle sample and each shoreline sample. Data from these sampling efforts will be posted as soon as available.

On Feb. 17, DEQ conducted a special round of bacteria sampling consisting of 25 sampling locations along approximately 49 miles of river stretching from the Chain Bridge down to the mouth of Potomac Creek. Over the next several weeks, DEQ will be conducting additional sampling.

Also on Feb. 17, VDH staff conducted routine seawater sampling for shellfish growing areas from Colonial Beach to the Governor Harry W. Nice Memorial Bridge (Route 301) in King George County. They collected 36 water samples in total. Laboratory analyses indicate that there were no elevated fecal coliform bacteria concentrations, with the vast majority of the samples at or below the analytical method detection limit. See below for results of VDH's testing.

Potomac River sampling map

Please note, the areas shown in red are shellfish harvesting condemnations that existed prior to the Potomac River spill. More information on those closures is available at the links below:

Upper Machodoc Creek Shellfish Closure
Rosier Creek Shellfish Closure
Monroe Bay Shellfish Closure

On Feb. 13, the Virginia Department of Health (VDH) issued a recreational advisory for Virginia residents.

Will there be impacts to the health of the Potomac River and Chesapeake Bay?

The discharge of raw sewage into the Potomac River introduced not only bacteria into the river but also sediment and nutrients (phosphorus and nitrogen). To consider the potential for impacts to the health of the Potomac River and the Chesapeake Bay, DEQ looked at estimated sediment and nutrient loading from the Potomac Interceptor sewage discharge.

The amount of nutrient and sediment discharged from the collapse of the Interceptor were initially estimated based on typical pollutant concentrations found in suburban wastewater¹. These concentrations were then refined using data from AlexRenew, a wastewater treatment plant in Alexandria, VA that has data on sewage that is similar in composition to that which is normally conveyed via the Potomac Interceptor. Total loads (below) were calculated based on the total sewage overflow amount estimated by DC Water at 243,500,000 gallons.

Parameter	Typical Raw Sewage Concentrations from AlexRenew (mg/L)	Total Pounds²
Total Nitrogen	42.61	90,000
Total Phosphorus	5.53	10,000
Sediment	254.23	500,000

^₁_{Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal, and Reuse. New York: McGraw-Hill, 1991.}²_{Example calculation: contaminant concentration in mg/L x 1L/0.26 gal x 1 lb/453,592.37 mg x 243,500,000 gal}

These estimated pollutant loads were then compared to pollutant loads that were discharged into the Potomac River and the Chesapeake Bay in 2025.

Presented below in tabular and graphical formats are the estimated sediment and nutrient loads in terms of calculated loads for the Potomac River and the Chesapeake Bay.

Based on DEQ’s evaluation, the additional sediment and nutrient loading from the Potomac Interceptor sewage spill equates to less than ½ percent of the 2025 annual loading to the Potomac River.

Parameter	Estimated Loads From Interceptor Spill (lbs)	2025 Potomac Loads (lbs)	Chesapeake Bay Loads (lbs)	Potomac: % of 2025 Loads	Chesapeake: % of 2025 Loads
Total Nitrogen	87,000	51,000,000	237,800,000	0.2%	0.04%
Total Phosphorus	11,000	3,300,000	13,200,000	0.3%	0.09%
Sediment	516,000	4,495,000,000	17,693,000,000	0.01%	0.003%

Potomac nutrient graphs 02.26.2026

Potomac nutrient graphs 02.26.20262

updated potomac nutrient graphs

Concerning bacterial loads, modeling results generated by the Interstate Commission on the Potomac River Basin (ICPRB) predict that elevated fecal bacteria concentrations stemming from the Interceptor break will decline to near background levels due to die-off and flushing by river flow over approximately 10 days from the end of the discharge on Feb. 9. Results of DEQ’s Feb. 17 sampling supports ICPRB’s model predictions.

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