Heavy Metal Contamination in Soils of Urban Highways Comparison Between Runoff and Soil Concentrations at Cincinnati,Ohio

Rainfall runoff from urban roadways oftencontains elevated amounts of heavy metals in both particulate anddissolved forms (Sansalone and Buchberger, 1997). Because metalsdo not degrade naturally, high concentrations of them in runoffcan result in accumulation in the roadside soil at levels thatare toxic to organisms in surrounding environments. This studyinvestigated the accumulation of metals in roadside soils at asite for which extensive runoff data were also available.For this study, 58 soil samples, collected from I-75 nearCincinnati, Ohio, were examined using X-ray fluorescence, C-Sanalyzer, inductively coupled plasma spectroscopy, atomicabsorption spectrometry and X-ray diffraction. The resultsdemonstrated that heavy metal contamination in the top 15 cm ofthe soil samples is very high compared to local backgroundlevels. The maximum measured amount for Pb is 1980 ppm (at 10–15 cm depth) and for Zn is 1430 ppm (at 0–1 cm depth). Metal content in the soil falls off rapidly with depth, and metalcontent decreases as organic C decreases. The correlation toorganic C is stronger than the correlation to depth. The resultsof sequential soil extraction, however, showed lower amounts ofPb and Zn associated with organic matter than was expected basedon the correlation of metals to % organic C in the whole soil.Measurement of organic C in the residues of the sequentialextraction steps revealed that much of the carbon was not removedand hence is of a more refractory nature than is usual inuncontaminated soils. Cluster analysis of the heavy metal datashowed that Pb, Zn and Cu are closely associated to one another,but that Ni and Cr do not show an association with each other orwith either organic C or depth. ICP spectroscopy of exchanged cations showed that only 4.5%of Pb, 8.3% of Zn, 6.9% of Cu and 3.7% of Cr in the soil isexchangeable. Combined with the small amounts of metals bound tosoluble organic matter, this result shows that it is unlikelythat these contaminants can be remobilized into water. At thissite, clays are not an important agent in holding the metals inplace because of low amounts of swelling clays. Instead, insoluble organic matter is more important. Mass balancecalculations for Pb in soil showed that most of the Pb came fromexhausts of vehicles when leaded gasoline was in use, and thatabout 40% of this Pb is retained in the soil.This study shows that, highway environments being a relativelyconstant source of anthropogenic organic matter as well as heavymetals, heavy metals will continue to remain bound to organicmatter in-situ unless they are re-mobilized mechanically. Removalof these heavy metals as wind-blown dust is the most likelymechanism. Another possibility is surface run-off carrying themetals into surface drainages, bypassing the soil. This studyalso shows that for those countries still using leaded gasoline,important reductions in Pb contamination of soils can be achievedby restricting the use of Pb additives.     

Effect of Non-Vehicular Sources on Heavy Metal Concentrations of Roadside Soils

Soils along highway environments typically contain high concentrations of heavy metals because of non-point contamination sources, most commonly vehicle exhaust and wear of vehicle parts. Pb, Zn, Cu, Cr, Sb, Ba and Ra concentrations of soils along roadways with very low traffic volumes in a rural portion of San Patricio County, Texas, have been studied in order to distinguish the effects of a point source (an industrial waste landfill) from the effects of traffic. The highest concentrations of Zn, Cr, Sb, Ba and Ra were detected in soils along the access road to the landfill. The association of high Ba values with high Ra values suggests that the most probable sources of this contamination were oilfield waste from storage tank bottoms, which were disposed of in landfill waste pits and were also spread over roads within the landfill to decrease dust. Outside the landfill along a different roadway, a second contamination area which was characterized by very high Ba but low Sb and Ra concentrations compared to background values, has been detected. Drilling mud spill is the most probable source of this contamination. The contaminations caused by these two point sources differ from typical vehicle-sourced contamination in two ways: their areal extent was limited and contaminants were not typical for highway environments (like Ba and Ra). Like vehicular contamination, however, these areas were confined to the immediate vicinity of the roadways. Another difference is that Pb in most vehicle-contaminated areas is strongly bound to the soil constituents, mostly organic matter, whereas the point source Pb was found to be readily exchangeable and to have no association with organic matter.