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Posted at 11:00 AM ET, 09/11/2008

Stormwater and Sewage: The Fate of D.C. Rainfall

By Ann Posegate

Wx and the City

Pop Quiz: What is the minimum amount of rainfall required for sewage and stormwater to overflow into the District's waterways?

Where D.C. rain drains: a Combined Sewer Overflow outfall. By Max Nepstad.

a) 3.0 inches
b) 1.5 inches
c) 0.7 inches
d) 0.1 inches

Maybe I've seen one too many basements flood with backed-up wastewater after heavy rains (including one last weekend due to Hanna), but the path of precipitation in the District really gets to me. In the oldest, pre-1900 part of the city -- you can view a map here -- stormwater and sewage flow through a primitive drainage system called a Combined Sewer System (CSS), where there is potential for them to mix and overflow into local waterways.

How much rain does it take for this to happen?

Keep reading for the answer, and more on the District's drainage system. Also, see our full forecast through the weekend, and SkinsCast for Sunday's regular season home opener.

In certain areas of the city such as Georgetown, the answer is d) 0.1 inches.

During dry weather, wastewater (aka sewage and greywater) flows to the Blue Plains sewage treatment plant. However, during wet weather, stormwater (rain or snowmelt) flows over impervious surfaces, picking up pollutants like litter, motor oil, lawn fertilizer, and the occasional dog waste along the way. Once it flows into storm drains, this stormwater can fill the CSS and mix with wastewater. Some of this concoction still makes it to Blue Plains to be treated. But a majority of it overflows into Rock Creek and the Anacostia and Potomac Rivers through a network of over 54 Combined Sewer Overflow (CSO) outfalls throughout the city. Hello contaminated clams and cancer-laden catfish.

In the mid-1800s, an underground CSS probably seemed like a tremendous improvement from the open canals such as that on Constitution Avenue that drained rain and sewage into the rivers. It's hard to imagine that we now drive and walk above an entire system of pipes that clear the city of stormwater and wastewater, sweeping it away into the waterways never to be seen again -- unless, of course, we see it backed up into a basement or we're out on a boat the next day. A storm can be so exciting until you think of its human and environmental consequences.

There are 722 communities in the U.S. that still have CSSs, including Baltimore, Alexandria and Richmond -- the problem is bigger than just the District. The more modern parts of D.C. and other cities have separate wastewater and stormwater systems. But, considering that almost 25 billion gallons of stormwater runoff enters the District's waterways each year, what is the city doing to reduce stormwater runoff and improve the very outdated CSS in the older areas? D.C.'s Long-Term Control Plan includes Low-Impact-Development techniques in various neighborhoods across the city, such as 20 million square feet of "green roofs" by 2020 to aid in absorbing rainwater before it hits the ground.

What can you do to help absorb rainwater on your property? Installing green roofs, rain gardens, rain barrels, "BayScaping" your lawn, and "scooping your pet's poop" are just some of the tips the District Department of the Environment recommends. What will you do to catch stormwater runoff before it starts?

By Ann Posegate  | September 11, 2008; 11:00 AM ET
Categories:  Environment, Posegate, Wx and the City  
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And on that note of what District Department of the Environment (DDOE) recommends, check out a newly launched stormwater management program for homeowners, by googling: "RiverSmart Homes."

Homeowners can receive up to $1,200 to adopt one or more of the following to help control stormwater:
- Shade Trees
- Above Ground Cisterns/Rain Barrels
- Permeable/Porous Pavers
- Rain Gardens
- BayScaping

Posted by: shelby | September 11, 2008 11:35 AM | Report abuse

Wonder what happens to Arlington County's stormwater. Do we have a combined CSS?

Posted by: El Bombo | September 11, 2008 12:32 PM | Report abuse

Thats all pretty shocking information, and nasty.

Posted by: CSH | September 11, 2008 12:40 PM | Report abuse

Posted by: . | September 11, 2008 12:57 PM | Report abuse

Don't rain barrels pose a mosquito problem though?

Posted by: Fred | September 11, 2008 1:15 PM | Report abuse

Right, thanks for that info ".". Arlington County operates on a Municipal Separate Storm Sewer System (MS4) similar to the newer parts of D.C. In an MS4, stormwater always drains into local streams and the Potomac without being treated, while wastewater (including sewage) always drains to the sewage treatment plant. They never mix.

With that said, rainfall is still an issue in any urban area, including Arlington, when it runs off over roadways into local waterways. Not only does it pick up pollutants, but the temperature of this runoff increases as it flows over concrete and dark surfaces that hold heat on hot summer days. This affects local water quality.

Posted by: Ann, Capital Weather Gang | September 11, 2008 1:19 PM | Report abuse

Rain barrels, if properly installed and maintained, do not pose mosquito problems. They are usually outfitted with very fine mesh screens that mosquitoes (even the tiny tiger mosquitoes) can't penetrate. And responsible rain barrel owners keep mosquito dunks in them to kill any larvae that might somehow find their way in. Bigger mosquito harbors include English ivy and holly (my holly shrubs swarm with mosquitoes--ick). Not to mention corrugated drain pipe extensions, or anywhere a few drops of water can collect. I am thrilled to hear of DDOE's homeowner stormwater management incentives and wish all our communities offered the same!

Posted by: ArlVA | September 11, 2008 1:51 PM | Report abuse

Fred - Most newer models of rain barrels have very efficient mesh screens on the top, so mosquitoes are not a problem. You can find out where to buy one here:

There's actually a great "rainscaping" and rain barrel display at the U.S. Botanic Garden through early October. The Chesapeake Conservation Landscaping Council set it up and can provide more info. Here's a fact sheet:

Posted by: Ann, Capital Weather Gang | September 11, 2008 2:02 PM | Report abuse

The local rain barrel manufacturer and distributor of many rain containment products is You can find some of their products at Amicus Green Building Center in Kensington MD. Not ALL rain barrels are made to function like the Aquabarrel products do. Before you buy or make a rain barrel consider the 'overflow port'. If it is not a balanced system - equal inlet=equal outlet - like the Aquabarrel designs - the overflow out the top of the unit may damage your foundation:-( DDOE may be purchasing some of Aquabarrel's 80 gallon units in the near future to offer to the public at reduced prices. (Aquabarrel was written up in the Post on Wednesday, April 23, 2008 page C-14 - during Earth Week)

Posted by: Barry | September 12, 2008 5:40 PM | Report abuse

The DC chapter of the Surfrider Foundation is hosting the 2008 Clean Water Paddle on the Potomac this Saturday, Sept. 13. This is a great way to learn more about DC's water quality issues, help to clean up our waterways, and get outdoors. Find out more here:

In addition, Leesburg and the Loudoun Soil and Water Conservation District are holding rain barrel workshops this month. See

Posted by: Ann, Capital Weather Gang | September 12, 2008 6:45 PM | Report abuse

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