Variation in Total and Extractable Elements with Distance from Roads in an Urban Watershed, Honolulu, Hawaii

Roads play a major role intransporting sediment associated nonpoint sourcepollutants to urban stream networks via storm drains. In urban areas the relationship of erodible soil toroads may be of critical importance in controllingmetal contributions to roads. Two 50-m transects(Park and School) were investigated perpendicular toroads in Manoa basin, Oahu, Hawaii. Concentrations ofnine elements were compared to background control soillocations and to five supplemental samples from nearbyrecreational parks. Sediment from curbside areas ofroads (road deposited sediment) was collected as thestarting point of each transect, and subsequently soilwas sampled from two depths (0–2.5 cm and 7.5–10.0 cm)along the transects. Total and 0.5 M HCl extractableconcentrations 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-atomicemission spectroscopy (ICP-AES) or flame atomicemission spectroscopy (FAAS). Ca, Cu, Pb and Znexhibited anthropogenic enhancement, with Pb and Znhaving the greatest enrichment in road sedimentfollowed by locations nearest the road. Copperdisplayed a narrower band of contamination than eitherPb or Zn, and this may reflect larger aerosolassociations and more rapid fall velocities. Lead andZn exhibited substantial decay in concentration at 50 m compared to the road sediment, but enrichment wasstill apparent. The positioning of a band of soilbetween the road-curb area and the sidewalk for thePark transect facilitated deposition and storage oftrace metals, and with subsequent erosion by splash orconcentrated flow this area can account for continuedtransport of contaminated sediment to adjacent roadsurfaces. On the other hand the School transect hadno soil directly beside the road, and the nearestsample from the road (5 m) displayed enrichment butsubstantially lower than the Park transect. Thesepreliminary data suggest that remobilization of soilstored metals in close proximity to roads cansignificantly prolong the environmental contaminationof urban road systems and eventually stream sediments.