Testing a Bioswale to Treat and Reduce Parking Lot Runoff

Davis, CA (March 1, 2009)- Dr. Qingfu Xiao has just released a report on Davis Soil used in a bioswale to mitigate parking lot stormwater runoff. The results show its effectiveness in trapping pollutants, increasing stormwater retention, and making water available to trees. Davis Soil is made of natural materials readily and inexpensively available in California, and Dr. Xiao’s earlier report gave it high marks when compared with other engineered soils from Cornell and Carolina State.


A bioswale integrating Davis Soil (a mixture of Lava Rock and regular soils) and trees was installed in a parking lot on the University of California Davis’s campus to evaluate the system’s effectiveness on reducing storm runoff and pollutant loading from the parking lot and supporting tree growth. The control and treatment (best management practices or BMP) sites each had 8 parking spaces. These two sites were adjacent to one another and identical with the exception being that there was no Davis soil bioswale retrofit for the control site. A tree was planted at both sites. Storm runoff, pollutant loading, and tree growth were measured and monitored during February 2007 thru October 2008. There were 50 total storm events with a total precipitation of 563.8 mm (22.2 in) during this period.
Compared with the runoff from the control site, the BMP site reduced amount of surface runoff by 88.8 percent. The loading reduction for nutrients, metals, organic carbon, and solids were 95.3%, 86.7%, 95.5%, and 95.5%, respectively. The total loading reduction was 95.4 percent. Petroleum hydrocarbons (i.e., gas, diesel, and motor oil) from both sites were below the laboratory detectable limit. The nature of Davis soil proved a better aeration and drainage system for tree roots during high moisture season. The performance of this BMP demonstrated its potential use for reducing runoff from parking lot and supporting tree growth.
Urbanization converts largely pervious landscapes into buildings, roads, parking lots, and other impervious surfaces that increase storm runoff volume and contaminant loads. Urban storm runoff causes property damage, adds pollutants to receiving water bodies, increases the cost of infrastructure maintenance, and reduces groundwater recharge because of reduced infiltration.
Engineered soils are a type of soil that integrates soil and stones to support runoff storage, increase infiltration, and promote deep rooting that reduces the heaving of sidewalks, curbs and gutters by tree roots. They are highly porous, and have been used to expand the soil volume for trees in small tree wells in plazas and parking lots. In this study, pollutant removal rates of contaminated storm runoff and runoff storage capacities were tested for three different types of engineered soils. Surface runoff was collected from parking lots and streets in different types of land uses for a variety of storm sizes and seasons. The laboratory test results indicated that 29 to 84 percent of the nutrients in the storm runoff were removed by these engineered soils. The heavy metal removal rate ranged from 75 to 92 percent. Pollutant removal rates were strongly related to the type and size of rainfall event, runoff pollutants concentration, as well as the pollutants constituents and engineered soil types.
Related Resources:
Testing a Bioswale to Treat and Reduce Parking Lot Runoff
Urban Runoff Pollutants Removal of Three Engineered Soils