A REVIEW OF UNITED STATES ROAD SEDIMENT STUDIES WITH A FOCUS ON METAL POLLUTION: INSIGHTS AND OPPORTUNITIES

Compared to other environmental media, road sediment is an understudied material, particularly in the U.S. Road sediment studies of the conterminous U.S. and Hawaii evaluated from the early 1970s to present indicate several important findings. Nomenclature has been inconsistent in the area of road sediment and suggestions are made for uniformity. Input, redistribution and modification, retention, and output processes are extensive and are summarized. A range of metals have been investigated with Pb, Zn, Cu, Cr being most common in studies. Pb concentrations noticeably decrease based on sampling time period in studies from pre-1990 to post-1990, illustrating the effectiveness of policy changes that reduced and eventually eliminated leaded gasoline usage in the U.S. A limited comparison of U.S. road sediment heavy metal concentrations to data from China shows general similarities, with Cd, Cu, Zn, and Pb all significantly enriched relative to upper continental crust (UCC). Electron microscopy techniques are useful for constraining both processes and sources of metal-bearing pollutant particles. Mercury is perhaps the most commonly omitted metal in U.S. road sediment studies and new studies are warranted. More detailed studies of road sediment in rural, agricultural, and underserved areas are needed. Road sediment may be an underutilized tool to assess community scale pollution. The review provides constraints for the numerous opportunities that exist in road sediment research including citizen science, fate and transport of metals, sourcing, and exposure risk.