Attenuation of copper in runoff from copper roofing materials by two stormwater control measures

Abstract

Concerns have been raised over diffuse and non-point sources of metals including releases from copper (Cu) roofs during storm events. A picnic shelter with a partitioned Cu roof was constructed with two types of stormwater control measures (SCMs), bioretention planter boxes and biofiltration swales, to evaluate the ability of the SCMs to attenuate Cu in stormwater runoff from the roof. Cu was measured as it entered the SCMs from the roof as influent as well as after it left the SCMs as effluent. Samples from twenty-six storms were collected with flow-weighted composite sampling. Samples from seven storms were collected with discrete sampling. Total Cu in composite samples of the influent waters ranged from 306 to 2863 μg L−1 and had a median concentration of 1087 μg L−1. Total Cu in the effluent from the planter boxes ranged from 28 to 141 μg L−1, with a median of 66 μg L−1. Total Cu in effluent from the swales ranged from 7 to 51 μg L−1 with a median of 28 μg L−1. Attenuation in the planter boxes ranged from 85 to 99% with a median of 94% by concentration and in the swales ranged from 93 to 99% with a median of 99%. As the roof aged, discrete storm events showed a pronounced first-flush effect of Cu in SCM influent but this was less pronounced in the planter outlets. Stormwater retention time in the media varied with antecedent conditions, stormwater intensity and volume with median values from 6.6 to 73.5 min. Based on local conditions, a previously-published Cu weathering model gave a predicted Cu runoff rate of 2.02 g m−2 yr−1. The measured rate based on stormwater sampling was 2.16 g m−2 yr−1. Overall, both SCMs were highly successful at retaining and preventing offsite transport of Cu from Cu roof runoff.