While strolling around our new park, you might have noticed this landscaped depression near the mural.

This is a rain garden.  The storm drains within the park empty into this garden so the ground has an opportunity to absorb rainwater.  Obviously there’s a limit to what the ground can absorb in a downpour, so the grate at the bottom of the photo carries the overflow into the sewers.

Under each street in areas of the city built before 1900 is a single pipe that carries both sewage and storm water. The problem with this combined system is that heavy rain storms force the combined system to overflow at 53 discharge points into Rock Creek, the Potomac, and the Anacostia.

Building these rain gardens helps alleviate the pressure on the sewer system during storms and thus helps protect the water quality of our rivers.

The Japanese Zelkova (Zelkova serrata) is one of the many species of trees regularly planted in DC

The District’s Urban Forestry Administration is responsible for planting street trees throughout the city. Just before Christmas we published a map (below) of the UFA’s 2010 tree planting list for LeDroit Park and Bloomingdale.

What a great joy it was to walk home Thursday night to spot a new Japanese zelkova (pictured above) taking root between the sidewalk and street beside our house.  A quick walk around the neighborhood revealed that the other trees had been planted, too.

View LeDroit Park-Bloomingdale Tree Planting Schedule in a larger map

In older parts of the city built before 1900 (see the map), our sewers and our storm drains are the same system, meaning that the water leaving your sink joins up with the same water running into the street grates.  Only older cities have this combined system; the rest of the the Washington area, including newer parts of the District, have separate pipes for sewage and for storm water.

This combined system is usually considered an environmental problem, since occasional heavy rainfalls inundate the combined system, forcing it to eject both storm water and raw sewage into the rivers and Rock Creek for a few hours through 53 outfall points.

However, with melting snow, the system is reversed into an environmental virtue, since the melting slush—salt, sand, grease, and all— are filtered with household waste at the Blue Plains treatment plant at the southern end of the District.

Let’s call this the combined system paradox: an environmental threat to the city’s waterways in the spring and summer becomes an environmental steward when winter snows melt.