Heavy metal contamination of settling particles in a retention pond along the A-71 motorway in Sologne, France.

A retention pond is a part of a drainage system designed to control water flow during rainstorms and to trap contaminated solid particles washed off by runoff water from a motorway. A series of studies have been carried out concerning the physico-chemical characteristics of the particles which settle down in such a pond in order to evaluate the effectiveness of the pond as a trap for heavy metals such as Pb, Zn and Cd. The highly contaminated roadside soil and the uncontaminated background soil were also studied for comparison. The settling particles had heavy metal concentrations 2-8 times higher than the background Sologne soil, depending on the metals. Heavy metal concentrations in the roadside soil were 7-26 times higher than those in the former. Sequential extractions, using the procedure of Tessier et al. (1979) illustrate that the highly contaminated roadside soil consisted mainly of the readily soluble fractions for all three heavy metals, with a limited proportion of residual metals. In the settling particles, the proportion of the latter is significantly increased, up to one-third of the total. The high concentration differences between the roadside soil and the settling particles indicates that most of the heavy metals are lost to the surroundings even before reaching the retention pond. Cadmium exhibited a specific behavior in that the most soluble fraction (exchangeable), which is negligible for Pb and Zn, occupied as much as one-fourth of the total in the roadside soil. Based on the 'enrichment factor' normalized to Fe introduced by Helz in 1976, the degree of contamination by heavy metals for the roadside soil and the settling particles was evaluated. The level of contamination was very severe in the roadside soil, while it was not so great in the settling particles. Suggestions are made to improve their removal efficiency.