- Portada
- volume 3 (2000)
- number 3-4 - Volume dédié à Jos BOUCKAERT Herdenki...
- Soil geochemistry in and around the town of Marche-en-Famenne (Wallonia, S-Belgium)
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Soil geochemistry in and around the town of Marche-en-Famenne (Wallonia, S-Belgium)
Abstract
The geological substrate of the town of Marche-en-Famenne, and its surrounding area, consists mainly of Givetian limestone and Frasnian to Emsian siliciclastics. The soil geochemistry within and around the town was studied by the collection of 214 samples from the urban area, and from a section of 3.5 by 20 km to the SW of the town. These samples were analysed for Ca, Cd, Cu, Fe, K, Mg, Mn, Ni, Pb and Zn by AAS. One of the most remarkable feature is that the contrast between limestone and sandstone is only weakly reflected in the major and trace element distribution patterns. This may be explained by the presence of loamy soils, which develop on limestone after weathering, which are very comparable to loamy soils developed on top of sandy shale and shale. A clear relation between geological substrate, soil-type and Fe-K-Ni content is apparent, and is also reflected in the bivariate correlation matrix. Further, red coloured lithologies of the so-called Burnotian age (i.e., Emsian) display an increase in Fe-content. Some particular dispersion patterns are clearly geogenetic in origin (e.g., in relation with mineralised faults, presence of overbank sediments, etc.), while in and around industrial sites, elevated heavy metal concentrations have been recorded and reflect an anthropogenic influence on the natural environment. Highest recorded Zn, Pb and Cd concentrations are respectively about 1300 mg/kg, 500 mg/kg and 4 mg/kg. However, values do not exceed trigger values, commonly used in environmental studies (e.g., Flemish Soil Sanitation Act). A speciation study, based on selective extraction, showed that about 20% of Zn and Pb are relatively easily dissolved; Cd can also potentially be leached, since more than 40% relates to the operationally defined "carbonate fraction". However, the Cd concentration is generally low. More important here is that overall heavy metals are dominantly bound to Mn- and/or Fe oxyhydroxides and to a fraction, which is only dissolved by strong acids (HClconc-HNO3 conc-HFconc). This aspect, together with the absence of real anomalous concentrations in heavy metals, suggest that the eco- and human toxicological risk in the studied area is low.