Operationally defined metal fractions in road deposited sediment, Honolulu, Hawaii

Road deposited sediments (RDSs) in urban watersheds represent key storage areas for potentially contaminated materials and they are also the primary source of nonpoint pollutants entering aquatic systems. In addition to their association with degradation of stormwater quality, exposure to contaminated RDS has also been implicated in a variety of health issues, particularly among children. Limited data are available to characterize the geochemical properties of RDSs, specifically the association of metals with operationally defined solid fractions. To address this a new optimized four-step (acid extractable [AEx], reducible, oxidizable, and residual) sequential extraction procedure was applied to 13 road sediments from an urban watershed in Oahu, Hawaii. Eight elements were examined (Al, Co, Cu, Fe, Mn, Ni, Pb, and Zn) in the <2-mm fraction. Data indicate that more than 50% of Al, Co, Fe, Mn, and Ni was associated with the residual fraction and reflects lithogenic-pedogenic control. Of the three remaining trace metals that are considered to be anthropogenically enhanced, Cu was dominantly associated with the residual fraction (45%), followed by the oxidizable fraction (25%). Lead was primarily associated with the reducible fraction with a mean of 71%, a value among the highest so far reported in the literature. Zinc had only one-fifth of its total concentration in the residual fraction, but one-third in the most mobile and/or bioavailable fraction (AEx), which is susceptible to release into solution with decreases in pH.