Documenting stormwater quality on Texas highways and adjacent vegetated roadsides

The primary objective of this study is the documentation of stormwater quality of vegetated roadsides of two Texas highways (State Highway 6 in College Station and Loop 360 in Austin), both of which had high average daily traffic. Three sites each in Austin and College Station were monitored using passive first flush stormwater samplers for 16 months. Results from this study indicate that significant removal of sediment and heavy metals occurred over the width of vegetated roadsides, but no apparent nutrient (nitrogen and phosphorus) removal was observed. The results also show that vegetation density has a direct effect on the performance of vegetated roadsides. When roadsides are densely covered with grasses above 90%, significant sediment removal is expected, often within the first 4 m of the edge of pavement. A stepwise regression analysis identifies the antecedent dry period (ADP) as the most significant predictor to pollutant concentration. The pollutant event mean concentration was found to decrease with increasing ADP for all pollutants at the College Station sites, but not the Austin ones.