Water Quality Benefits of Grass Swales in Managing Highway Runoff

Grass swales are a vegetated stormwater management technology that can remove surface runoff contamination through sedimentation, filtration by the grass blades, infiltration to the soil, and likely some biological processes. Two full-scale grass swales in the median of a four-lane highway were monitored during 18 storm events to characterize the overall performance of grass swales as a stormwater management technology and to evaluate the effect of the shallow-sloped grass pre-treatment area adjacent to the swale in most designs. The study was designed as an input/output comparison between the water quantity and quality captured directly from the roadway and the effluent from the swales. Both swales exhibited significant removal of suspended solids (65–71% based on EMCs) and zinc (30–60% based on EMCs). No significant difference in treatment efficiency is noted between the swale with a pretreatment area and the grass swale alone.