Variation in total and extractable elements with distance from roads in an urban watershed, Honolulu, Hawaii

Roads play a major role in transporting sediment associated nonpoint source pollutants to urban stream networks via storm drains. In urban areas the relationship of erodible soil to roads may be of critical importance in controlling metal contributions to roads. Two 50-m transects (Park and School) were investigated perpendicular to roads in Manoa basin, Oahu, Hawaii. Concentrations of nine elements were compared to background control soil locations and to five supplemental samples from nearby recreational parks. Sediment from curbside areas of roads (road deposited sediment) was collected as the starting point of each transect, and subsequently soil was sampled from two depths (0-2.5 cm and 7.5-10.0 cm) along the transects. Total and 0.5 M HCl extractable concentrations were determined for aluminum (Al), calcium (Ca), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb) and zinc (Zn) using either inductively coupled plasma-atomic emission spectroscopy (ICP-AES) or flame atomic emission spectroscopy (FAAS). Ca, Cu, Pb and Zn exhibited anthropogenic enhancement, with Pb and Zn having the greatest enrichment in road sediment followed by locations nearest the road. Copper displayed a narrower band of contamination than either Pb or Zn, and this may reflect larger aerosol associations and more rapid fall velocities. Lead and Zn exhibited substantial decay in concentration at 50 m compared to the road sediment, but enrichment was still apparent. The positioning of a band of soil between the road-curb area and the sidewalk for the Park transect facilitated deposition and storage of trace metals, and with subsequent erosion by splash or concentrated flow this area can account for continued transport of contaminated sediment to adjacent road surfaces. On the other hand the School transect had no soil directly beside the road, and the nearest sample from the road (5 m) displayed enrichment but substantially lower than the Park transect. These preliminary data suggest that remobilization of soil stored metals in close proximity to roads can significantly prolong the environmental contamination of urban road systems and eventually stream sediments.