Effects of Bioretention Soil Mixtures on Metal Speciation and Toxicity to Aquatic Communities

Heavy metal concentrations in stormwater runoff are regulated and monitored because these pollutants can be toxic to aquatic communities. When evaluating stormwater toxicity to aquatic organisms, a heavy metal’s bioavailability and speciation should be considered in addition to the total metal concentration. The influent and effluent of four bioretention soil mixtures (BSMs) from 10 storms were evaluated for speciation shifts of typical heavy metals and changes to stormwater toxicity to find out if any of the BSMs were more adept at decreasing pollutant bioavailability. Visual MINTEQ 3.1 was used for metal speciation, and the Windward Environmental Biotic Ligand Model (BLM) (v 3.41.2.45) was used to predict toxic concentrations of heavy metal species. No noticeable speciation shifts were noted within bioretention cell (BRC) effluent for cadmium, copper, lead, and zinc. A multiple-factor analysis (MFA) indicated that the four BSMs do not differ from one another in reducing the BLM-identified toxic limit (relative toxicity) of stormwater effluent for the BLM-selected aquatic organisms. BRCs are most effective at reducing the toxicity of stormwater when the stormwater contains high pollutant concentrations. BRCs may increase the effluent stormwater’s toxicity at low pollutant concentrations through Cu export from compost.