RESUMO
Trace elements (TE) are indicative of industrial pollution in soils, but geochemical methods are difficult to implement in contaminated sites with large numbers of samples. Therefore, measurement of soil magnetic susceptibility (MS) has been used to map TE pollutions, albeit with contrasted results in some cases. Effective radium concentration (ECRa), product of radium concentration by the emanation factor, can be measured in a cost-effective manner in the laboratory, and could then provide a useful addition. We evaluate this possibility using 186 topsoils sampled over about 783km(2) around two former lead and zinc smelters in Northern France. The ECRa values, obtained from 319 measurements, range from 0.70±0.06 to 12.53±0.49Bq·kg(-1), and are remarkably organized spatially, away from the smelters, in domains corresponding to geographical units. Lead-contaminated soils, with lead concentrations above 100mg·kg(-1) <3km from the smelters, are characterized on average by larger peak ECRa values and larger dispersion. At large scales, away from the smelters, spatial variations of ECRa correlate well with spatial variations of MS, thus suggesting that, at distance larger than 5km, variability of MS contains a significant natural component. Larger ECRa values are correlated with larger fine fraction and, possibly, mercury concentration. While MS is enhanced in the vicinity of the smelters and is associated with the presence of soft ferrimagnetic minerals such as magnetite, it does not correlate systematically with metal concentrations. When multiple industrial and urban sources are present, ECRa mapping, thus, can help in identifying at least part of the natural spatial variability of MS. More generally, this study shows that ECRa mapping provides an independent and reliable assessment of the background spatial structure which underlies the structure of a given contamination. Furthermore, ECRa may provide a novel index to identify soils potentially able to fix leached components.
