Biogeochemical assessment of the impact of Zn mining activity in the area of the Jebal Trozza mine, Central Tunisia

Soil pollution associated with potentially toxic elements (PTEs) from mining residues is a significant problem worldwide. The decommissioned Jebal Trozza mine, located in central Tunisia, may pose a serious problem because of the possible high concentrations of PTEs present in its wastes. This mine is a potential source of contamination for agriculture in this area due to both direct causes (pollution of agricultural soils) and indirect causes (pollution of sediments that accumulate in a dam used for irrigation). The aim of the study reported here was to assess the effects of local mining activity in two respects: (1) in terms of soil quality, as determined by soil edaphological parameters and PTEs contents in the mining wastes and local soils; and (2) in terms of biological quality, as evaluated by quantification of enzymatic activity as an indicator of bacterial activity in soils and wastes. The mine tailings contained high levels of Pb (1.83-5.95%), Zn (7.59-12.48%) and Cd (85.95-123.25 mg kg^-1). The adjacent soils were also highly contaminated with these elements, with average concentrations of Pb, Zn and Cd that exceeded the European standard values for agricultural soils (3, 300 and 300 mg kg^-1 for Cd, Pb and Zn, respectively). Enzymatic dehydrogenase showed zero activity in waste piles and very low activity in PTE-contaminated soils, but this activity returned to normal values as the pollution decreased, thus demonstrating the effect that the contamination load had on the health of the studied soils. A Statistical Factor Analysis clearly distinguished three groups of samples, and these are related to the influence that mining on the soils and sediments had on the PTE concentrations and their effects on the biological quality of the soil. An environmental assessment based on the enrichment factor criteria indicated risk levels that varied from strongly to severely polluted. The risk appears to be greater close to the mine, where the highest PTE levels were determined.