Retention ponds reduce pollutants but make mosquito habitats

WASHINGTON -- Public officials nationwide are inadvertently creating vast breeding grounds for mosquitoes -- including those that carry the West Nile virus -- by installing stormwater retention ponds near businesses and homes in an effort to reduce contaminants that collect in water.

As concern over the mosquito-borne virus heightens, the effort to create new ponds and clean up old ones has pit two environmental causes against each other.

The ponds help keep pollutants out of streams, bays and drinking water reservoirs, advocates say, while meeting federal and local water quality standards. Their purpose is to collect rainfall, urban runoff and chemicals from farms and lawns, and help control flooding.

But the stormwater basins, which drain over days, create an ideal habitat for mosquitoes, encouraging the spread of diseases such as West Nile virus and St. Louis encephalitis.

"It drives us crazy," said Cyrus Lesser, chief of the mosquito control section of the Maryland Department of Agriculture. The ponds "are everywhere, and they give us fits. The worst part is that they are being installed adjacent to business and residential areas, right next to where people live."

The basins, some as large as lakes, others just ditches, are a long-accepted environmental practice, designed when "West Nile wasn't even a blip on the radar screen," said Thomas Grizzard, head of the Occoquan Monitoring Laboratory at Virginia Tech. Now that it is, some state and local health officials want the stormwater basins redesigned.

Effort seems futile

Susan Erskine watched warily this week as construction crews created a standing-water pond next to her home in suburban Centreville in Fairfax County, Va. The basin borders about 10 houses, many occupied by children and adults over 50, two groups considered at risk from West Nile.

Erskine, 56, has been careful not to let standing water accumulate in her yard. But she said those efforts now seem futile considering that, after the next major rain, hundreds of gallons of water will sit just beyond her property.

"The county keeps telling us: Don't keep still water lying around, tip over birdbaths," the retired teacher said. "But they are the ones putting gallons and gallons of still water right next to us. Excuse me, but something's wrong with the message."

The ponds work by collecting stormwater and allowing pollutants to settle at the bottom, while the cleaner water drains into streams and larger bodies of water. Scott St.Clair, director of Fairfax's maintenance and stormwater management, said many of the ponds are designed to drain and dry within 48 hours, faster than mosquito larvae can develop.

But a national study by California researchers found that even the best basins harbored mosquitoes, mainly because their drains eventually clogged, said Vicki Kramer, a senior official with the state's Department of Health Services who oversaw the project.