When the River Gets Sick: Ecological Effects of the Potomac Sewage Spill

by Laura Gilmore

In winter, the Potomac River can look deceptively quiet. The boats are gone. The lilies are dormant. Many fish are absent from the shallows. Ecologically, however, the winter months are not an off-season.

Beneath the surface, the tidal freshwater Potomac River, which includes the stretch that flows past Arlington and Alexandria, is supporting overwintering waterfowl, dormant aquatic plants, brumating turtles (when their metabolism slows, they stop eating, bury themselves, and may wake occasionally to drink), freshwater mussels buried in the sediment, and the earliest life stages of spring fish spawning. When a large sewage release enters the system, the disturbance does not simply wash downstream. It moves through an interconnected food web. A sewage spill is best understood not as pollution in the abstract, but as a sudden biological and chemical shock to an ecosystem.

The Incident and Restoration Efforts

On January 19, 2026, a section of the 54-mile Potomac Interceptor sewer line collapsed upstream in Montgomery County, Maryland and released a significant overflow of untreated sewage into the Potomac River at the C&O National Historical Park. Emergency bypass systems were constructed to stop additional overflows, with the final major sewage release of approximately 600 thousand gallons occurring on February 8. Over this 21-day time period, an estimated 243.5 million gallons of untreated sewage entered the Potomac River. 

Clean-up efforts in the areas directly impacted by the overflows began on March 2. Emergency repairs were completed on March 14, and DC Water restored sewage flow to the Potomac Interceptor pipe and turned off the bypass systems. During this period, numerous agencies and organizations published recreational advisories and conducted water quality testing for elevated bacteria levels. On April 29, DC Water submitted its Potomac Interceptor Emergency Repair and Rehabilitation Plan prepared by Wetland Studies and Solutions to the Maryland Department of the Environment. The Environmental Protection Agency (EPA) announced it had accomplished its remediation of the Potomac Interceptor collapse and fully demobilized the federal presence onsite as of May 6. 

DC Water continued monitoring water quality daily through July 5 at the ten existing sampling sites and then has transitioned to weekly testing through September 10 to ensure that river bacteria levels are back to normal outside of expected seasonal or rainfall-related spikes. In addition, the first phase of comprehensive environmental restoration efforts is expected to be substantially completed by late summer. This ongoing work is focused on removing the remaining affected soil in the C&O Canal. In coordination with the U.S. Army Corps of Engineers, the EPA, and the National Park Service, the initial clean-up and reseeding of vegetation have been completed in the areas directly impacted by the overflow, along the drainage channel, culvert, and tributary to the river.

The second phase of restoration is expected to be completed in the fall. This includes a comprehensive plan to replant native herbaceous species, shrubs, and trees, regrade temporarily impacted wetlands, and restore natural hydrology.

Sewage In an Ecological Context

Raw sewage is not just dirty water. It is a concentrated pulse of biological material, containing organic matter, nitrogen and phosphorus, suspended solids, bacteria and pathogens, household chemicals, and trace pharmaceuticals. In a river ecosystem, this combination primarily triggers one process: a microbial bloom. Triggered by this overabundance of nutrients, bacteria rapidly multiply while decomposing the organic material. During that decomposition they consume dissolved oxygen; the same oxygen that fish, mussels, and aquatic insects need to survive. Scientists refer to this as biological oxygen demand. While some blooms are not deadly, harmful algal blooms can produce toxins that pose serious risks to human health, wildlife, and local economies dependent on water recreation and create dead zones in water bodies.

In the weeks following the spill, media reporting focused on the bacterial contamination confirmed in water sampling by government agencies, the Potomac Riverkeeper Network, and the University of Maryland. The Interstate Commission on the Potomac River Basin (ICPRB) provided analysis to responding agencies after the incident based on its Emergency River Spill Model (ERSM) to investigate bacteria concentrations downstream of the Potomac Interceptor break. The ERSM fecal coliform analysis, which factored in the sewage release rate, the Potomac River flow itself, and the concentration of fecal bacteria in raw sewage, led to estimated coliform (naturally occurring bacteria that are generally harmless but act as an indicator that the environment is compromised) and E coli levels. The model also noted that predicted bacteria concentrations will fall quickly to near background levels due to die-off and flushing by river flow when sewage stops flowing into the river.

Dr. Claire Buchanan, Director Emerita of ICPRB Program Operations, shared that past studies have shown that sediment coming into the Potomac at the fall-line (where the Piedmont area and Atlantic coastal plain meet in the eastern United States) settles to the river bottom in the tidal fresh and slightly brackish reaches between D.C. and Maryland Point (a prominent geographic cape on the western edge of Charles County, Maryland, which juts into the lower tidal portion of the Potomac River). The fact that the spill occurred during the winter season may also have helped the immediate impacts for the river. Biochemical cycling of nitrogen and phosphorus in the sediments is slower and biological oxygen demand of bacteria and other organisms in the sediment is lower due to the cold temperatures.

As temperatures warm, any settled organic material decomposes more rapidly. In slower-moving coves and shoreline habitats, this process can reduce dissolved oxygen and release additional nutrients back into the water column, potentially fueling algae growth later in the season.

Researchers from the University of Maryland Center for Environmental Science (UMCES) partnered with state scientists for a two-day sampling venture in early March. They sampled water, mud, and air at 17 different sites from upstream of the spill site down to the river’s mouth at the Chesapeake Bay. Their results at the spill site showed exceptionally high values for many indicators of untreated human sewage, including saltiness, E. coli, caffeine, and acetaminophen. Although the sewage had been contained by that point, the results suggest that some continued leaking out. The UMCES researchers also found nitrogen and phosphorus concentrations consistent with some of the worst years in recent memory. While these levels are not without precedent, the nature of the contamination raised concern about the potential for localized water quality issues during the warmer months. The team observed an algae bloom downstream from the spill in early March, but repeat or persistent blooms have not been reported this summer to state Harmful Algal Bloom dashboards.

Mike Rolband, Director of the Virginia Department of Environmental Quality (DEQ), provided some reassuring context during a Town Hall event held in Alexandria on March 19. Based on 243.5 million gallons of sewage discharged, DEQ estimated pollutant loads using AlexRenew wastewater data. The additional sediment and nutrient loading from the Potomac Interceptor sewage spill equated to less than 0.5 percent of the 2025 annual loading to the Potomac River.

The graphical representation of this nutrient and sediment loading analysis restores confidence that the Potomac River is naturally recovering from this year’s significant sewage spill.

Why the Arlington–Alexandria Stretch Matters

From a regional water-quality standpoint, monitoring agencies indicate the river is recovering relatively quickly. However, there is still the necessity to read the river at two scales. It is possible for the river to recover broadly while localized ecological responses continue to unfold over months. Big rivers can buffer disturbances, while small habitats reveal them.

By the time the spill reached our area, it had traveled into a tidal freshwater estuary rather than a typical downstream-flowing river. While most people think of an estuary as brackish, the Potomac is a freshwater estuary starting at the City of Alexandria since it mixes and reverses direction twice daily with the tides. Instead of moving quickly past the shoreline, material carried in the river—including sediment, nutrients, and bacteria—can circulate repeatedly or remain in the same locations.

This especially affects shoreline habitats, marinas, and tributaries such as Four Mile Run and Hunting Creek where contaminants may settle into sediment or remain trapped longer than in the open channel. Testing along the Alexandria waterfront in February found most main-channel sampling locations below common recreational thresholds, while Four Mile Run showed somewhat elevated bacterial levels relative to open water sites. Ecological responses may appear gradually and become more noticeable during the warming seasons as aquatic plants regrow and wildlife species begin reproducing.

At this time, there do not appear to be widespread confirmed reports of fish kills or mass wildlife mortality directly linked to the spill, according to area wildlife experts. However, ecological disturbances do not always produce dramatic visible effects. Sewage solids can settle into sediments and smother habitat, while high organic loads reduce oxygen available to aquatic organisms. Sensitive species include aquatic insect larvae (e.g. mayflies, caddisflies), small forage fish, freshwater mussels, and the submerged aquatic vegetation (SAV). Subtle impacts on insects, mussels, and small fish can ripple upward through the ecosystem over time. The Arlington and Alexandria waterfront sits within a major “wintering corridor” of the Atlantic Flyway. Common winter species, such as canvasbacks, buffleheads, common mergansers, American coots, and the occasional tundra swans depend heavily on SAVand aquatic invertebrates for food. While no immediate crises were observed in the aftermath of the spill, observations over the subsequent seasons and water quality monitoring for benthic macroinvertebrates can speak to any underlying concerns for these species.

Along the Waterfront: What Residents and Naturalists Might Notice This Season

During a public meeting in Glen Echo, Maryland on May 18, residents seemed to disagree that recovery following the massive raw sewage spill into the Potomac River has been successfully achieved. Several residents reported lingering odors near portions of the river corridor, reinforcing the value of continued observation and monitoring, even as broader river conditions improve.

Most ecological changes following a disturbance are subtle. Many of these observations are also part of normal seasonal variation. What matters most is noticing patterns over time and changes from what is typically observed in a familiar location. Residents or naturalists may be best positioned to help scientists and other monitors recognize the first signs of possible concern due to their familiarity with what is typical for a site. For residents or naturalists in Northern Virginia along the Potomac, observations shared to iNaturalist can help paint a picture of normal weather variations, new pollution events, or speak to a river in trouble (independent of the February spill).  None of the observations below alone confirms ecological harm, but together they can help tell the story of how a tidal river responds and recovers over time.

Water & Shoreline

• Unusual odors along the shoreline
• Foam lines collecting in coves or marinas
• Murky water during otherwise calm conditions
• An early green tint to the water
• Stringy algae attached to docks, rocks, or vegetation
• Clusters of empty mussel shells along beaches or riprap
• Reduced water clarity or unusually shallow visibility depth

Birds & Wildlife

• Waterfowl concentrated in fewer locations than usual
• Birds feeding heavily at stormwater outfalls or creek mouths
• Increased gull or crow scavenging along shorelines
• Osprey repeatedly missing fish during hunting attempts
• Bald eagles scavenging more frequently instead of fishing
• Wildlife repeatedly avoiding specific coves or tributaries

Fish, Amphibians & Reptiles

• Fewer minnows or forage fish in shallow warm areas
• Dead or visibly stressed fish after warm days. After a recent heat wave, the Maryland Department of the Environment is investigating a large number of fish deaths in the Potomac River. Experts believe this fish kill event is owed to “summer turnover,” when warmer water from the surface and cooler water from lower down suddenly mix. Scientists hypothesize a combination of heat stress, drought conditions (river flow is at half the typical July median), and potential underlying bacterial infections or parasites are behind the event.
• Reduced frog calling activity near tidal marshes at dusk
• Turtles basking less frequently than expected

Aquatic Plants & Invertebrates

• Submerged aquatic vegetation (SAV) beds absent where previously observed (apart from seasonal weather impacts)
• Patchy underwater vegetation instead of continuous meadows
• Waterfowl or turtles absent from historically productive feeding areas
• Unusual reduction of dragonflies hunting over shoreline water
• Unusual reduction of aquatic insects around lights at night

Photographs taken repeatedly from the same location over time can be especially valuable. In tidal rivers like the Potomac, ecological recovery is often gradual and easiest to notice through seasonal comparison. For those interested in phenology monitoring (the tracking of recurring plant and animal life cycle events over time), Nature’s Notebook provides information on the value of becoming a consistent observer and shares frequently asked questions about how to submit observations.

Local hotspots for observation in the Arlington/ Alexandria area include:

• Four Mile Run mouth
• Gravelly Point shoreline
• Columbia Island Marina
• Daingerfield Island
• Hunting Creek boardwalks and shoreline
• Jones Point Park

The Resilience of Rivers

The size of the sewage spill, as well as the unusually low river flow when it occurred, were almost without precedent. Yet it is important to remember that rivers are dynamic systems. Tides mix water. Wetlands filter particles. Microbes break down organic matter. Plants regrow. Natalie Exum, an Assistant Professor of Environmental Health and Engineering at Johns Hopkins University, reminded concerned residents that “dilution is [often] the solution to pollution.”

The Potomac has recovered from major pollution events before, and current monitoring suggests the river is recovering again without widespread long-term impacts to wildlife populations. Public health advisories have gradually been lifted, bacterial levels across much of the river have declined (independent of summer storm events that stir up the sediment), and widespread wildlife mortality has not been observed.

At the same time, ecological recovery is rarely instantaneous or uniform. As noted above, tidal rivers, smaller habitats such as tributaries, coves, and shoreline sediments may respond differently than the main channel, and some effects may not have become apparent until this growing season when aquatic plants regrow and fish and insects return to shallow waters.

For residents and naturalists alike, this is an opportunity to observe not only disturbance, but resilience. Small changes will be most visible to those willing to slow down and notice: clear water over submerged vegetation, minnows gathering again in the shallows, diving ducks returning to familiar winter coves. The Potomac is still telling its story, and careful observation remains one of the most valuable ways we can understand it.

Local Stream Monitoring in ARMN Jurisdictions

Arlington, Alexandria, and Falls Church all participate in long-term watershed monitoring programs that track stream health to include bacterial and biological testing.

While monitoring suggests that many streams in the Potomac watershed are showing gradual improvement or stabilization rather than continued decline, stream recovery is rarely immediate; ecological resilience can become visible only with years of careful observation. To become a volunteer stream monitor in Arlington, contact Lily Whitesell; for those interested in Alexandria, contact Anthony Minnick.