What is in bioretention systems? Depth of metal contamination in bioretention media filtering urban stormwater

Abstract

Bioretention is a mix of soil and vegetation that is widely used to remove metals from contaminated urban stormwater. Research on bioretention has focused on determining the effectiveness of filter media at removing contaminats from water. There is little published data on the accumulation of metal contamination in the bioretention media itself. These clearly have important implications for bioretention maintenance. The aim of this research is to determine the depth of metal contamination in bioretention media filtering urban stormwater runoff. Soil samples were collected from different depths including the surface mulch and soils at 50 – 100 mm and 150 – 200 mm deep from 45 bioretention systems in Auckland during the period from 18th February, 2011 to 4th March, 2011. Three systems were examined in more detail on 20th April, 2011 by collecting soil samples from surface sediments, mulch and soils at 20 – 70 mm, 120 – 170 mm and 350 – 400 mm and 550 – 600 mm deep. Soil samples were analysed in the lab for total copper, lead and zinc using standard methods and then extracted using a Toxicity Characteristics Leaching Procedure to determine the leaching potential of the metals. Metals accumulate in bioretention media in the field at concentrations that are a concern and require special management. Zinc concentrations in 45 samples (39.3%) require Class A landfill disposal according to national guidelines (Ministry for the Environment, 2004b). This is clearly an issue for soil disposal. Zinc concentrations in 73 samples (62.4%) exceeded the Canadian Environmental Quality Guidelines (CCME, 2002). There were clear relationships between land use and copper or zinc concentration. Metals clearly accumulated in top layers of the bioretention media. There was a significant difference in metal concentrations between the top layers and the sub soils. The results suggest that removing surfaces soils would be an effective method to remove metal contamination but these soils would require special disposal even after a short time of implementation. An alternative is to collect all the soils in the bioretention and in that way reduce the average concentration of metals to require a lower standard of disposal.