Workers recently completed construction of a project to reduce the discharge of pollutants into Alewife Brook in Arlington.
A collaboration between the town and Mystic River Watershed Association, the project involved diverting storm-water runoff into two new retension basins at the intersection of Egerton and Herbert roads.
Storm-water runoff is rainfall that pools on roads, parking lots, and other paved surfaces where it cannot soak into the ground. As the rainwater moves, it carries trash, oil, fertilizers, and anything else on these surfaces directly into streams and rivers, or indirectly into those water bodies through storm drains.
Untreated storm-water runoff is the leading cause of pollution in urban streams such as Alewife Brook, according to Erica Wood, outreach and communications manager for the watershed association.
The retension basins, situated at the edge of the roads, are filled with layers of soil, gravel, and native plants. Stormwater that flows into the basins is filtered by the plants and absorbed in the soil. The basins are expected to remove 20 percent to 70 percent of pollutants, depending on the type of pollutant.
Officials said they hope the project, paid for with US Environmental Protection Agency funds awarded by the state, can serve as a model for other green infrastructure improvements.