Infiltration Opportunities in Parking Lot Designs Reduce Runoff and Pollution

A low impact (dispersed) design demonstrated how small alterations to parking lots can reduce runoff and pollutant loads. A whole basin approach utilized the entire watershed for stormwater management. Storm runoff was treated as soon as rain hit the ground by routing it through a network of swales, strands and finally into a small wet detention pond. When the volume of water from all the different elements of the treatment train (the swales, the strand and the pond) were compared, almost all the storm runoff was retained on site. Further, the size of the wet detention pond used for final treatment could be greatly reduced because of more pervious areas. Individual basins in the parking lot, the various elements in the treatment train, and rainfall usually had significantly different water quality concentrations. Most of the nitrate and ammonia entered the system directly in rainfall and concentrations in runoff were usually reduced as it traveled through the system. Ammonia-nitrogen was highest in the runoff from the basin without a swale and organic nitrogen and phosphorus highest in the strand and pond; metal concentrations were highest in basins paved in asphalt. Polycyclic aromatic hydrocarbons (PAHs) were detected in the soils at the site and some approached the significantly toxic levels. Chlordane was the pesticide most often detected in measurable quantities in soils. Dichlorodiphenyltrichloroethante (DDT) and its daughter products were detected in almost all soils tested and DDE was found in measurable quantities.