| Soils along highways typically contain high amounts of heavy metals because of nonpoint contamination sources, most commonly vehicle exhaust and wear of vehicle parts. Since the heavy metals do not degrade naturally, accumulation of high concentrations in soils can be toxic to organisms living in surrounding environments. To determine the mechanisms that control both the retention and remobilization of metals, roadside soil samples collected from three different cities have been analyzed for their heavy metal contents: Cincinnati, OH, Corpus Christi, TX and Sinton, TX.; Analysis of the soils collected from urban highways of Cincinnati (I-75) and Corpus Christi (I-37) showed that heavy metal concentrations of the highway soils are very high in the top 15 cm of the soil. There is a negative relationship between metal content and depth, and a positive relationship between heavy metal concentration and the organic C content of the soil. As distance from the road increases, the amount of contamination decreases, except where an additional point source occurs close to the highway.; The similarity of heavy metal concentration profiles from Cincinnati and Corpus Christi suggests that road salting during winter in Cincinnati has little effect on remobilizing the heavy metals.; To distinguish the effects of a point source (an industrial waste landfill in this case) from effects of traffic, heavy metal, Sb, Ba and Ra concentrations of roadways with very low traffic volumes in Sinton, TX, have been studied. The results showed two different areas of contamination: one a high Ba, Ra, Sb plume and one a high Ba, low Sb plume. These two point sources differ from typical vehicle-sourced contamination in three ways: their areal extent is limited, the spectrum of contaminants is unusual and the correlation of the metals to organic C is not seen.; Insoluble organic C content of the soils is found to be a very important agent in holding the metals in place. Vehicles contribute organic matter as well as heavy metals to roadside soils. The heavy metals are strongly bound to this organic matter and remain in-situ unless they are re-mobilized mechanically. Pb in the low traffic environment of Sinton, however, was not associated with organic matter and was found to be very mobile because of the absence of vehicular-sourced insoluble organic C that would hold Pb in place. |
