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.