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Mobility of lead in the roadside soils of the 176th Street and Transcanada Highway Interchange, Surrey, BC Lee, Norman

Abstract

Tetra ethyl lead was used as a fuel additive to improve the octane rating of automotive gasoline until the early 1990s. Consequently soil lead concentrations in excess of the provincially determined standard are present at a number of highway construction sites. Presently, the Ministry of Transportation is obligated to mitigate risks by excavating and disposing of the contaminated soil present at each highway project at considerable cost. This is done without consideration of a significant risk-determining factor, the mobility of the contaminant. The contaminated soils of the 176th Street TransCanada Highway Interchange, Surrey BC were studied to determine whether it would be acceptable to leave lead-contaminated roadside soils in place. Significant lead accumulations of up to 1430 mg/kg were found in the roadside soils of the 176th Street TransCanada Highway Interchange Site. The distribution of lead contamination followed the characteristic distribution given in the literature. Soil lead concentrations decreased rapidly to background levels at 10 m away from the roadside and at the 60 cm depth. Leachant concentrations of all 24 samples (5 depths from 6 borehole sets) subjected to the batch desorption test (24 hours, pH 3.5) were below the 5 ppm method detection limit. Estimated adsorption capacities of 24 samples (from 6 borehole sets) subjected to the batch adsorption test ranged from 4000-17000 mg/kg. Estimated capacities were 4 to 17 times the highest accumulated soil lead concentration found on the site. Annual source indicators have dropped drastically with the phase out of leaded gasoline, therefore it is unlikely that adsorption capacity will be reached in the near future. The results suggest leaving the lead-contaminated soil in place may be acceptable. A multiple regression analysis with commonly measured soil properties (soil pH, total soil lead, silt and clay fraction, soil carbon) showed estimated adsorption capacity was strongly correlated with soil carbon content. Therefore, organic matter is recommended as a remedial measure in the event of a lead spill.

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