A new drainage system constructed on Capitol Hill is helping relieve pollution that flows into the Anacostia River. It has one unusual feature: flowers.

The botanical structure, known as a rain garden, absorbs almost 90 percent of the pollutants that rainwater carries into it, according to Doug Siglin, director of the Anacostia River Initiative. The garden replaced a conventional lawn that stood between the Senate parking lot on D Street Northeast and the sidewalk.

The rain garden has a threefold effect: It helps relieve the pollutants released into the Anacostia River, which flow to the Potomac River and ultimately into the Chesapeake Bay, Siglin explained.

Sen. Dick Durbin (D-Ill.) led the initiative to have the filtration system constructed on the Hill after he and Siglin met last year to address the effects of pollution on the Chesapeake Bay. Architect of the Capitol Alan Hantman also worked on the project.

In urban areas, once factory pollution has been regulated, the largest remaining source of pollution are the toxins found on parking lots, streets, sidewalks, rooftops and even grass, Siglin said.

Although invisible to the eye, heavy metals, carcinogenic chemicals, and bacteria from animals and pets lie on the ground. In parking lots, oil and gas residue along with rust and other pollutants sit on the surface, Siglin said. All of these wash off when the rain hits.

“When there are heavy storms, there are sewage overflows,” said Ed Hopkins, an environmental director at the Sierra Club. This excess water often bypasses the treatment plant and heads into the Anacostia River.

An EPA report on storm-water runoff states that residential, commercial and industrial areas are responsible for 21 percent of impaired lakes and 45 percent of impaired estuaries in the United States. In the mid-Atlantic states alone, storm water is responsible for more than 5,000 miles of impaired streams.

Once in the river system, these toxins affect wildlife. In May, The U.S. Fish and Wildlife Service reported that vehicle emissions and road runoff are linked with high tumor rates in brown bullhead catfish from the Anacostia River. Tumor prevalence in the river, which ranges from 50 percent to 70 percent, is the highest reported for U.S. rivers.

The bacteriological action in the garden breaks down pollutants, Siglin said. The garden thrives while its soil, bacteria and roots decompose the incoming toxins. Siglin described this 10-year-old technology as a “classic back-to-the-future” concept.

The rain garden contains rocks and a coarse mix of soil composed of 50 percent sand, 30 percent topsoil and 20 percent organic material.

“It is a recent approach based on what is an increasing recognition of how bad the pollution that’s washing off city parking lots and streets really is,” Siglin said. He estimated that 10 such rain gardens exist in the Washington metro area.

“Communities can do a lot to follow the example being set on the Capitol,” said Bonnie Smith, an EPA spokeswoman. “It’s the kind of activity that is great to see happening throughout the mid-Atlantic region.”

There are no visible features that distinguish a rain garden from any other garden. “If you’re not looking for it, it would be easy to miss,” Hopkins said.

One rain garden may not make an appreciable difference to the problem, Hopkins explained. But the benefit is these low-cost drainage structures are relatively small scale and many can be constructed around the city, Hopkins said.