Size fractionation of metals present in highway runoff: beyond the six commonly reported species

Highway runoff is an important non-point source of pollutants, especially metals. This study reports monitoring results from 17 storm events at three highway sites for six commonly sampled metals: cadmium, chromium, copper, lead, nickel, and zinc. The study also reports the results of eight additional metals: aluminum, arsenic, cobalt, iron, manganese, selenium, strontium, and vanadium. Soluble phase, defined as passing a 0.45 microm filter, and particulate phase concentrations on four size fractions from 0.45 to larger than 100 microm are reported. The greatest metal masses were typically observed in the 8 to 20 microm fraction. The size distribution shows that sedimentation as a treatment process typically found in stormwater management can remove only 65% of the mass of most particulate phase metal species. Additional processes, such as coagulation or filtration, are needed to obtain greater removal rates. The results indicate the need to study particle size distribution (PSD) in order to better select treatment alternatives or assess environmental impacts.