Metal extraction from road sediment using different strength reagents: impact on anthropogenic contaminant signals

Washoff of road deposited sediment into storm drain systems is one of the major contributors to nonpoint source pollution in urban fluvial networks. These sediments contain a variety of potentially toxic organic and inorganic contaminants. Road sediment from 13 locations in an urban (non-industrialized) drainage basin, and soil from 10 background (control) locations were collected to assess total and labile fractions of Al, Co, Cu, Fe, Mn, Ni, Pb and Zn. Four digestions, of varying strength, were used to assess contaminant levels, these included: a total four-acid digestion, a microwave-assisted digestion with concentrated nitric acid (USEPA Method 3051), a 0.5 M 'cold' HCl, and a 0.05 M EDTA (pH 7). Road sediment data indicate that Al, Co, Fe, Mn and Ni were primarily lithogenic in origin, while Cu, Pb and Zn showed very significant anthropogenic signals, most probably from vehicle-related sources. Median Pb concentration enrichment ratios for the EDTA extraction were about 42, indicating an extreme anthropogenic signal. The weak extractants (HCI and EDTA) are considered in this study to be superior in their ability to characterize the degree of anthropogenic contamination and should be utilized more widely in environmental contaminant studies.