The proliferation of mansions, redevelopment in Tysons Corner and new toll lanes on the Beltway are expected to harm an already imperiled network of streams in northeast Fairfax County unless new storm-water controls and strategies are established, according to a county report released Monday.
Expected growth in the middle Potomac River watersheds — which span 26 square miles and include Tysons and McLean — will increase the number of “impervious” surfaces like roads, sidewalks and parking lots that speed greater volumes of runoff into streams, which dumps pollutants into water and hastens erosion.
Among the most pressing environmental questions facing an area of Fairfax County set to see a rash of new construction in coming years: Whether new, cleaner storm-water technology can counterbalance the sheer volume of new development.
Fairfax County storm-water manager Randy Bartlett sees the planned growth as an opportunity to put those features in place.
“The whole science of storm water has been evolving very rapidly in recent years,” he said.
The Dulles Rail Project, back on track for critical federal funding, is expected to bring triple or even quadruple the existing building density of Tysons Corner. The Beltway’s high occupancy toll lanes project will replace dozens of aging ramps and bridges and add two new lanes in each direction of the highway. Also a factor, the report said, is an increase of “mansionization.” As a result, the report projects that about 28 percent of the watershed will be covered in impervious surfaces, 4 percent more than right now.
Existing damage to waterways is largely caused by development practices in recent decades that paid little heed to the damage of unchecked storm water. Three of the smaller watersheds in the area — Pimmit Run, Scotts Run and Dead Run — already rank in “very poor” environmental condition, according to the report, prepared by county staff.
It’ll be virtually impossible to restore the streams to their conditions before the growth occurred, the report said. Storm-water controls “will, however, help halt the degradation of the streams and loss of habitat and, with the implementation of the recommendations, the conditions of the streams should improve,” the report said.
