Assessment of the historical evolution of the total and labile Pb, Zn, and Cd fractions and their environmental risks of the Jbel Ressas tailings and agricultural soil (NE Tunisia)

The Jbel Ressas (NE Tunisia) is an ancient mining site, where the three tailings were since 1952 and until today exposed without any protection. It is a site of great interest, because it is typical of carbonate Pb–Zn deposits, where the environmental stakes are enormous. To consider the remediation or protection of these dumps and their surrounding soil, the evolution evaluation of their total and labile toxic trace elements (TTE) quantities and the pollutants spatial distribution and risks, since their initial deposition is a crucial step. The draw up of their complete balance sheet showed that the total Pb, Zn and Cd quantities in the initial waste were 46,640 t (tone), 134,780 t and 1150 t, respectively. In the initial waste, 98% of pollutants were in stable form (without environmental risk). From 1952 to 2021, around 75% of the initial TTE was mobilized and a large quantity unloaded from the tailings toward the surrounding areas. In 2021, the three dumps kept 10,300 t of Pb, 27,500 t of Zn and 291 t of Cd. The range values of the whole-soil removal (Uₜₛ) and relative binding intensity (IR) indexes (“equilibrium” parameters) of the TTE tailings from 1952 to 2021 were 0.24–13.2 and 0.4–6.2, respectively. These data confirmed the mining waste instability and the high environmental risk levels. It was also carried out that the danger levels increased with its exposition time. Among the strong TTE quantities released from the dikes, only 6.1%, 4.8%, and 4.1% of Pb, Zn, and Cd, respectively, were found in the agricultural soil. The highest percentages of the mobile TTE were in the agricultural soil surrounding the tailings. Then, the bioavailable TTE concentrations decreased with distance from the pollution sources. Based on the background values, the soil potential ecological risk (Eⁱ r), except 3%, was very high (14 < Eⁱ r < 3200). The soil equilibrium parameters of Jbel Ressas tailings and soil confirmed their high instability and risk. These results proved the interest of the historical study of a polluted site. Considering the soil and tailing pollution levels, the remained TTE quantities in the tailings and the acceleration speed of the pollutant’s remobilization, the management of this site is urgent.