Heavy metal contamination in soils of urban highways: Comparison between runoff and soil concentrations at Cincinnati, Ohio

Rainfall runoff from urban roadways often contains elevated amounts of heavy metals in both particulate and dissolved forms (Sansalone and Buchberger, 1997). Because metals do not degrade naturally, high concentrations of them in runoff can result in accumulation in the roadside soil at levels that are toxic to organisms in surrounding environments. This study investigated the accumulation of metals in roadside soils at a site for which extensive runoff data were also available. For this study, 58 soil samples, collected from I-75 near Cincinnati, Ohio, were examined using X-ray fluorescence, C-S analyzer, inductively coupled plasma spectroscopy, atomic absorption spectrometry and X-ray diffraction. The results demonstrated that heavy metal contamination in the top 15 cm of the soil samples is very high compared to local background levels. The maximum measured amount for Pb is 1980 ppm (at 10-15 cm depth) and for Zn is 1430 ppm (at 0-1 em depth). Metal content in the soil falls off rapidly with depth, and metal content decreases as organic C decreases. The correlation to organic C is stronger than the correlation to depth. The results of sequential soil extraction, however, showed lower amounts of Pb and Zn associated with organic matter than was expected based on the correlation of metals to % organic C in the whole soil. Measurement of organic C in the residues of the sequential extraction steps revealed that much of the carbon was not removed and hence is of a more refractory nature than is usual in uncontaminated soils. Cluster analysis of the heavy metal data showed that Pb, Zn and Cu are closely associated to one another, but that Ni and Cr do not chow an association with each other or with either organic C or depth. ICP spectroscopy of exchanged canons showed that only 4.5% of Pb, 8.3% of Zn, 6.9% of Cu and 3.7% of Cr in the soil is exchangeable. Combined with the small amounts of metals bound to soluble organic matter, this result shows that it is unlikely that those contaminants can be remobilized into water. At this site, clays are not an important agent in holding the metals in place because of low amounts of swelling clays. Instead, insoluble organic matter is more important. Mass balance calculations for Pb in soil showed that most of the Pb came from exhausts of vehicles when leaded gasoline was in use, and that about 40% of this Pb is retained in the soil. This study shows that, highway environments being a relatively constant source of anthropogenic organic matter as well as heavy metals, heavy metals will continue to remain bound to organic matter in-situ unless they are re-mobilized mechanically. Removal of these heavy metals as wind-blown dust is the most likely mechanism. Another possibility is surface run-off carrying the metals into surface drainages, bypassing the soil. This study also shows that for those countries still using leaded gasoline, important reductions in Pb contamination of soils can be achieved by restricting the use of Pb additives.