Enhanced Parking Lot Design for Stormwater Treatment

A low impact (dispersed) design demonstrates how small alterations to parking lots can reduce runoff and pollutant loads. Storm runoff was treated as soon as rain hit the ground by encorporating a network of swales, strands and a small wet detention pond into the overall design (Figure 1). When the volume of water discharged from all the different elements to the treatment train (the swales, the strand, and the pond) are compared, calculations showed that almost all the runoff was retained on site. The most effective method for reducing pollutant loads is to keep runoff on site and allow time for infiltration as well as for chemical, biological and hydrological processes to take place. Basins paved with porous pavement had the best percent removal of pollution loads with many removal rates for metals greater than 75 percent in the basin with a smaller garden area and greater than 90 percent with larger gardens. More phosphorus loads were discharged from basins with vegetated swales than from basins with no swales. It should be emphasized here that even with some poor removal rates by swales in the parking lot for phosphorus, when the entire system is evaluated, efficiencies are good since the site retained over 99 percent of the storm runoff during the year that it was evaluated. Sediment sampling identified polycyclic aromatic hydrocarbons, chlordane and DDT products as problems. Phosphorus and nitrogen in the sediments increased from year one to year two. Metal and nutrient pollutants in the sediments were not found to be migrating to the deeper strata.