Performance of commercially available soil amendments for enhanced Cu attenuation in bioretention media

Bioretention structures such as planter boxes, swales and rain gardens are being increasingly utilized in built landscapes as a strategy to attenuate both stormwater flows and contaminant loads. Copper (Cu) roofing materials contribute significantly higher mass loads of dissolved Cu per unit area than other surfaces such as parking lots and roadways. While a recent study demonstrated that conventional bioretention media can remove greater than 90% of Cu from copper roof runoff, the median Cu concentrations at the point of discharge from bioretention structures (66 μg L^-1) still did not achieve Cu concentrations in stormwater discharges sought in some jurisdictions (for example, < 14 μg L^-1). Consequently, commercially available soil amendments were assessed to improve bioretention Cu removal. The ability of biochar, greensand, and zeolite to improve Cu removal was evaluated in laboratory column studies. Additionally, the performance of zeolite as an underlayer amendment was evaluated in bioretention planter boxes treating stormwater from a picnic shelter with a partitioned copper roof. Cu was measured in the planter box influent and effluent. The field setup included 2 control planter boxes containing only standard bioretention media and 2 amended with the zeolite underlayer. Samples from ten storms were collected with flow-weighted composite sampling. Total Cu in composite samples of the influent waters ranged from 445 to 1683 μg L^-1 and had a median concentration of 934 μg L^-1. Total Cu in the effluent from the control planter boxes ranged from 10 to 64 μg L^-1, with a mean of 29 μg L^-1. Total Cu in effluent from the zeolite amended planter boxes ranged from 4 to 44 μg L^-1 with a mean of 18 μg L^-1. Attenuation in the control planter boxes ranged from 90 to 99% with a median of 93.4% by concentration and ranged from 95 to 99% with a median of 97.5% in the zeolite amended planter boxes.