Image of a heavy rainstorm in the streets of Washington, DC. — Urban stormwater runoff is one of the leading causes of river pollution, especially in the northeastern United States. © Greg Kahn

Stories in Maryland and D.C.

Curbing Stormwater Pollution

Working in our cities to help protect water in the Bay.

March 09, 2026

Stormwater runoff is the fastest growing source of freshwater pollution in the world, including here in the Chesapeake Bay watershed. Stormwater pollution is caused when rainwater falls on impervious surfaces—including sidewalks, parking lots and roads—where it mixes with oil, sediment, trash and other pollutants. Stormwater then flows into our cities’ sewer systems and rivers, eventually reaching the Bay.

Picture of a green roof stormwater garden in Washington, DC. — **Washington, D.C.** Kristof Grina, Co-founder and Director of Agriculture of Up Top Acres, harvests vegetables from a rooftop garden covering a third of an acre in southeast Washington, D.C. © Greg Kahn

In order to meet its Clean Water Act obligation, in 2013 Washington, D.C. implemented new regulations requiring major, new construction and renovation projects within D.C. to capture stormwater runoff from their property sites through green infrastructure. To maximize design flexibility for developers, the city has created the Stormwater Retention Credit (SRC) market, a first-of-its-kind voluntary trading program that allows developers to meet a portion of their retention requirements by purchasing credits generated on other properties.

A photograph features a rain garden within a community park. — **Capturing Runoff** A new rain garden at Knollwood Life Plan Community in DC captures runoff from 1.6 acres of impervious surface. The native plants provide food for birds and pollinators. © Matt Kane / TNC

Since the SRC market was established, TNC has been a key player in helping prove and shape market conditions by both implementing credit-generating green infrastructure projects and by working closely with elected officials and D.C. Department of Energy and Environment to make necessary adjustments the market’s regulatory framework. The green infrastructure projects that generate credits in this market not only help clean our waterways, but they also help reduce local flooding, cool the summer heat, and expand green space and native plants for urban wildlife and biodiversity. Because of all these benefits to both people and nature, TNC continues to stay invested in the success of the SRC market as a model for other cities around the world.

Given our deep experience and relationships in this space, TNC is now pivoting our strategy to lead a mentorship program that helps startup businesses enter the market as credit-generators that work with a variety of D.C. landowners to build green stormwater infrastructure projects.

With critical financial support of Forbright Bank, TNC launched this Green Stormwater Infrastructure Mentorship Program to help startup SRC aggregation companies learn how to better maneuver projects and the DC SRC marketplace. We currently have four companies under mentorship and look forward to scaling this critical work to reduce stormwater pollution to the Potomac River and Chesapeake Bay.

Learn more about our stormwater and cities work.