The First Flush Blues
October 17, 2011 | UC Berkeley
It rained last week. In most places this wouldn’t be a remarkable event, but it hasn’t rained in Berkeley for over half a year. When I heard the sound of rain running across the roof, I knew that summer was really over and that we are moving from brown hillsides to fragrant mud on our way to the oddly premature spring that starts in February. As required by the local building code, the rain that runs across my roof passes through a PVC pipe and into the street. The runoff from my roof, along with that of the roofs of my neighbors and all of the water from the roads, driveways, and parking lots all flow through the streets on their way to the storm sewers that drain the city into San Francisco Bay.
The first rain of the year is special not only because it marks the start of the rainy season. It is an important time for people concerned with the health of the Bay because the runoff carries with it all of the pollutants that have accumulated on the paved surfaces during the dry season. This phenomenon, which urban hydrologists refer to as the first flush, delivers a lot more than rainwater into the Bay. In fact, the first flush often contains some of the highest concentrations of metals, pesticides and trace organic compounds of the entire year. And if you are one of those hardy souls who insists on swimming in the Bay’s icy waters you probably want to take a few days off after the first flush.
In Singapore, where rain that falls within the city is routed to local reservoirs to augment the drinking water supply, they have engineered the system to bypass the first flush of every rainstorm. As a result, the more polluted water flows out to sea thereby minimizing the need to remove the contaminants during the drinking water treatment process. Of course, bypassing the first flush does nothing to protect the aquatic ecosystem or prevent beach closures. But then again, that has never been a high priority in Singapore.
Can we capture and treat storm urban runoff in the Bay Area? In many cities, rain gardens, permeable pavement and other forms of green infrastructure are being used to reduce the volume of the first flush. But in a city with steep topography and real risks of flooding it might be difficult to implement many of these low-impact development methods. It might just be more effective to keep the pollutants off the streets. For example, a fleet of modern, vacuum assisted street-sweeping trucks might be able to capture the dust from the streets before it rains. If modern street sweepers are too expensive or ineffective, simple approaches like cutting back on the use of toxic materials might do the trick. For example, by reducing the amount of copper in brake pads, Californians have achieved major reductions in the amount of copper pollution entering the bay.
To cure the first flush blues, an ounce of prevention might trump a million gallons of treatment.