Water Quality PDF Print E-mail

While the water flowing through the upper stretches of Rock Creek is relatively clean, the creek’s water quality decreases quickly as it flows downstream.  By the time the creek empties into the Potomac near Thompson’s Boathouse, it is contaminated with trash, chemicals, and even sewage.  Because of this contamination, the National Park Service prohibits swimming and wading in the lower portions of the creek.

Where does the pollution come from?  Two major sources are stormwater runoff and combined sewer overflows (CSOs).


Stormwater Runoff

During a storm, any rain that does not percolate into the ground turns into “runoff”—water flowing downhill.  As it flows, runoff can pick up pollutants and carry them to rivers and streams like Rock Creek.  Those pollutants include excess fertilizers, spilled chemicals, eroded sediments, and trash, oil and salt from streets. Stormwater from any area can be polluted, but the problem is especially bad in urban areas, where much of the ground is covered with roads, rooftops, and parking lots. Because rain doesn’t percolate into these impermeable surfaces, they generate larger amounts of runoff than rural and undeveloped areas.

Above Route 28 in Maryland, much of Rock Creek’s watershed remains in good condition.  To protect the uppermost areas of that watershed, Montgomery County established an Upper Rock Creek Special Protection Area. In the area, new development must not increase the amount of impermeable surface beyond specified amounts, and developers must use various stormwater management techniques to limit runoff.

trash from beth mullinBelow Route 28, the Rock Creek watershed becomes increasingly urban and populated.  In addition, much of the development in lower portions of the watershed happened in the 1960s, before engineers and planners learned to use stormwater management techniques.  Montgomery County and Washington D.C. are taking steps to address stormwater runoff problems in these areas by, for instance, installing permeable pavement, green roofs, and vegetated “buffer zones” to absorb rainwater. Area residents can do their part to reduce stormwater pollution by reducing fertilizer use, avoiding spills, and diverting downspouts onto permeable ground and away from storm drains. Montgomery County provides more tips for preventing stormwater pollution on its website.

cso sign from dc wasa site

Combined Sewer Overflows

Combined Sewer Overflows (CSOs) are another major reason for low water quality in Rock Creek.  A “combined” sewer uses the same underground piping network to carry both rainwater from storm drains and wastewater from businesses and homes (i.e. sewage).  A “separate” sewer, by contrast, carries rainwater and wastewater in two separate piping networks.  During dry weather, a combined system carries sewage to a wastewater treatment plant where it is treated before being discharged.  But when it rains, the combined flow from rainwater and sewage can exceed the capacity of the piping network and the treatment plant.  When that happens, a combined sewer system discharges the excess flow from a CSO.  While CSOs ensure that storm drains continue to function and help prevent flooding, they also release untreated sewage into the environment.

rock creek cso map from dc wasaApproximately one-third of Washington D.C. is served by a combined system, some parts of which are over 100 years old. The system includes a total of 52 CSOs, 28 of which drain directly into Rock Creek. (DC Water’s website includes a detailed map of its CSOs.)  Those 28 CSOs discharge an estimated 52 million gallons of untreated water into the creek every year, the majority of which comes from the large CSO outfall at Piney Branch.  These discharges are a major source of damage to Rock Creek.

As a result of a consent decree with citizen groups and the federal government, Washington D.C. and DC Water have developed a Long Term Control Plan (now known as the Clean Rivers Project) for reducing the impacts of CSOs on Rock Creek. Current projects include CSO regulator adjustments, and the Rock Creek Sewer Separation Project.

For more information

• The U.S. Geological Survey’s website allows users to access detailed water quality data for Rock Creek.  There is a station near Joyce Road in Washington, DC. that provides continuous water quality monitoring data.  Information on stream height, flow, and water chemistry can be used to track changes in runoff patterns and pollution over time.  This information will be crucial in evaluating the success of upstream pollution control efforts.

• The Montgomery County Department of Environmental Protection’s website includes additional data, detailed watershed information, and a watershed mapping tool.

• The New York Times maintains a comprehensive database of water pollution violations.

• DC Water submits quarterly reports on its Clean Rivers Project to EPA.