Sediment and nutrients (in the form of nitrogen and phosphorus) are top sources of pollution to the Potomac River. Encouragingly, long-term trends confirm that nutrient and sediment levels in the Potomac River have declined.
Sediment and nutrients naturally exist in streams and rivers, however, high concentration levels can severely degrade water quality and pollute stream ecosystems. Contributed by various sources including agricultural runoff and urban stormwater, excess nutrients and sediment pollute streams with toxins, create turbid waters that block out light, reduce underwater vegetation, and endanger people and animals alike. Equally troubling, excess nutrients can foster dangerous, bacteria-ridden algal blooms, threatening recreation spots and sources of drinking water.
Based on the latest* pollution data, the Potomac River region is currently on track to meet its 2025 federal pollution reduction goals assessed by the US EPA Chesapeake Bay Program. Reductions in nutrient pollution across the region can be attributed to a decades-long effort by public, private, and community interests to rein in industrial and point-source pollution dating back to the US Clean Water Act of 1972.
More recently, the US EPA Chesapeake Bay Program, in partnership with state and regional leaders, has made great strides to reduce pollution through establishing pollution benchmarks or Total Maximum Daily Loads (TMDL). Targeting non-point source pollution, or pollution flowing off the land from many sources, has been key to making progress toward these goals. Common sense mitigation measures including streamside forests, forest conservation, green infrastructure, and changes in farming practices are effectively reducing polluted runoff and restoring water quality.
Meeting short-term pollution reduction goals is a sign of progress that conservation efforts in the region are, indeed, working. If met, the 2025 goals will be one milestone in the journey to return safe, swimmable, and fishable waters to our community. Restoring the Potomac River to full health will require a continued decline in pollution in the years and decades to come.
Disconcertingly, rapid deforestation and the climate crisis present emerging threats that could derail progress to restore the Potomac River to full health. Rapid deforestation in the region is weakening the land’s ability to naturally absorb and filter excess stormwater, sending more runoff into local streams and tributaries. A 2022 comprehensive study of Maryland's forests confirmed that all but one region lost forest from 2013 to 2018, with Montgomery and Prince George's County accounting for nearly half of the statewide total. The climate crisis is throwing new wrenches into the mix. In our Rising to the Challenge local climate series, we unpack how more extreme storms and lengthening droughts are contributing to rising runoff levels. It is vital that Potomac River residents and leaders continue to support regional conservation efforts that protect water quality, forests, and public health.
*The results we report represent a snapshot in time. Given the nature of polluted runoff, there is a strong relationship between concentration levels of nutrients and sediment and annual and seasonal rainfall trends. While long-term trends are encouraging, year-after-year concentrations are in flux.
Since 1985, nitrogen levels in the Potomac watershed have been slowly improving to the point where in 2020, the Potomac River met 91 percent of its reduction goals. The United States EPA set nitrogen reduction goals for Chesapeake Bay subwatersheds to meet by the year 2025, with the Potomac watershed’s goal set at 48.01 million pounds per year by 2025. In 2020 total annual CAST-modeled nitrogen levels reached 52.6 million pounds, missing reduction goals by 4.6 million pounds.
Most nitrogen pollution in the watershed is contributed by rural sources with agriculture responsible for 47 percent of nitrogen loads registered on the Potomac. Nitrogen contributions from populated city centers have remained mostly consistent between 17 and 18 percent of total loads, with wastewater contributions falling dramatically since the 1985 baseline year to just 12 percent of total nitrogen contributions. Overall, nitrogen levels in the Potomac are trending in a direction we would hope to see due to improved water protection laws and policies, increased best management practices (BMPs), and more accurate tracking of annual levels. To continue progress, it’s essential that our communities champion nature-based solutions and continue to advocate for practices that further reduce nitrogen levels in both rural and urban places. In rural areas, we must continue to expand the adoption of sound BMPs while advocating for long-term planning that supports common sense growth and protects our streams and natural areas.
Pounds Delivered to Bay Per Year (Millions)
Source: Chesapeake Bay Program
Phosphorus totals across the Potomac have decreased in the long term since the baseline year for this report (1985). A total of 3.9 million pounds of phosphorus were measured annually in 2020, a level that met the EPA reduction goal established for the Potomac of 3.9 million pounds annually by 2025. The primary contributors to phosphorus totals in the Potomac were agricultural sources and urban sources. This trend has remained consistent for the past few years and mirrors trends seen in nitrogen totals with agriculture being the primary contributor. Farming has provided 35 percent of Potomac phosphorus levels, and urban centers contributed 22 percent.
Unlike nitrogen, phosphorus levels in the Potomac largely originate from urban sources, indicating that polluted runoff from cities and populated centers across the watershed is still a major concern. In highly developed areas with little ground cover, rooftops and streets divert excess rainwater into streams, polluting local waters with road chemicals, litter, and other debris. Making matters worse, city stormwater systems can be easily overwhelmed during heavy storms, sending diluted sewage straight into streams.
Phosphorus can originate from a variety of sources like lawn fertilizers, detergents, effluents from wastewater treatment plants, and many others. Efforts to reduce polluted runoff and combined sewage overflow in cities across the watershed have shown a positive impact in reducing overall nutrient pollution to local waters, however, phosphorus still appears to be a significant issue in developed areas. To combat this, municipalities must put more emphasis on increasing implementation of best management practices, like rain gardens and permeable pavement, that work in tandem with other infrastructure, including stormwater retention tunnels, to decrease urban polluted runoff so that phosphorus levels in the Potomac can start decreasing to baseline levels.
Pounds Delivered to Bay Per Year (Millions)
Source: Chesapeake Bay Program
Sediment pollution (excess loose dirt, clay, and soil) throughout the Chesapeake Bay has been a serious issue for decades, with past water quality and habitat health suffering because of it. Since the baseline reporting year (1985) for this report, annual Potomac River sediment levels have been slowly decreasing, with 2020 levels totaling 4.82 billion pounds of sediment. Like nitrogen and phosphorus, the US EPA and Chesapeake Bay Program have worked to set sediment reduction goals across the region. Our 2023 Report Card is the first since the Bay Program revised its Potomac sediment reduction goal raising the threshold of acceptable sediment from 2.27 billion pounds to 4.79 billion pounds annually by 2025. This target was adjusted to account for an updated model (the Phase 6 CAST) as well as loads contributed by shoreline erosion, which were excluded from previous models. Using this new reduction goal, 2020 marked a successful year in the Potomac with the watershed meeting 99% of the reduction goal for the year.
Sediment pollution is so intrinsically tied with other water quality parameters that if we can continue to reduce sediment pollution in the Potomac River then overall watershed health has a chance to continue improving. With less sediment comes the potential for an increase in underwater grasses, healthier environments for fish and other wildlife, as well as overall reductions in things like nutrients and other pollutants. Phosphorus most commonly flows into rivers and streams in the form of fertilizers, human-generated detergents, and sediments from farms.
In 2020, most sediment pollution was originating from natural causes, but unlike phosphorus and nitrogen, populated city centers were a greater contributor to local sediment pollution than agriculture. This is likely because of increased deforestation due to development and building as well as poor sediment control measures put into place during new construction projects. Local climate research also confirms that the Potomac region is experiencing stronger storms that produce extreme rainfall. Streams already stripped of natural defenses, like forests and vegetation, are more susceptible to erosion. Further, an emerging pattern of lengthening droughts combined with extreme storms puts strain on our ecosystem; dry landscapes are unable to absorb rainwater efficiently, resulting in harmful flooding and erosion.
Knowing this, it’s vital that the DC metro area and other city centers in the watershed continue to promote the installation of BMPs that are focused on reducing both stormwater and sediment runoff. Streamside forest buffers, increased urban tree canopy, rain barrels, and much more are just a few types of projects that have been used recently to help combat polluted runoff throughout the Potomac.
Pounds Delivered to Bay Per Year (Billions)
Source: Chesapeake Bay Program
As one of the largest sub-watersheds in the Chesapeake Bay, the Potomac River should set a precedent for pollution reduction and remediation. As part of the comprehensive plan to remediate the Chesapeake Bay the US EPA and the Chesapeake Bay Program set reduction goals for various pollutants across its different jurisdictions and watersheds. Reduction goals for nitrogen, phosphorus, and sediment have been firmly established for years as nutrient pollution was one of the leading contributors to degraded Bay health over the past few decades.
In order to evaluate cumulative annual nitrogen, phosphorus, and sediment loads in the Potomac River this report uses modeled data from the Chesapeake Bay Program’s Chesapeake Assessment and Scenario Tool (CAST), a modeling application that allows for the estimation of nitrogen, phosphorus, and sediment load source locations based on a variety of factors such as geography or watershed. This method differs from past reports, which used flow-normalized load estimates from USGS river input monitoring (RIM), and was selected based on its more accurate representation of watershed-wide load estimates. Values from 2020 were used as the assessment year in this report.
Nutrient and sediment load source data also come from the Chesapeake Bay Program’s Chesapeake Assessment Scenario Tool (CAST). CAST is the preferred method for load estimations as it considers both tidal and non-tidal portions of the watershed and gives a fuller, watershed-wide view of load dynamics.
Nitrogen, phosphorus, and sediment reduction benchmarks were taken from US EPA and Chesapeake Bay Program 2025 Planning Target load goals for 2025. This 2023 report marks the first time that the newly finalized sediment reduction goal is being used for our analysis. All grades and scores for this section were drawn from 2020 data.