Investigation of intra - event variations of total, dissolved and truly dissolved metal concentrations in highway runoff and a gross pollutant trap - bioretention stormwater treatment train.

Metals in stormwater can be toxic to organisms, particularly when occurring in truly dissolved form (fraction <3 kDa). Here, using 153 samples collected during six rains, we investigated intra-events variations of total, dissolved and truly dissolved metal concentrations in highway runoff, and how they were affected by a stormwater treatment train, comprising a gross pollutant trap (GPT) and a bioretention system. Although intra-event variations in metal concentrations were observed in the highway stormwater, the effluent of the GPT and the effluent from the bioretention system, generally no "first flush" was observed. While total Pb, Cu and Zn concentrations were, on average, significantly reduced by the bioretention system (>76%, 79% and 94%, respectively), removal of dissolved metals was lower and truly dissolved Cd and Cu were generally not effectively removed (average removal Cd -11%, Cu 7.7%). Only for truly dissolved Zn was removal 65%. However, as with Cu, effluent Zn concentrations exceeded environmental threshold values. While Cu, Pb and Zn occurred in the highway runoff in particulate form, with average percentages of 86%, >99% and 72%, respectively, the speciation of Cu and Zn in the bioretention effluent shifted towards greater fractions of colloidal and truly dissolved metals. The GPT had no significant effect on metal concentrations and speciation.