Spatial distribution and ecological risk assessment of potentially toxic elements in agricultural soils, stream sediments, and plants around Lakhouat mine (northwestern Tunisia)

Abstract

Potentially toxic element (PTE) distribution in 21 soil samples, 07 stream sediment samples and wheat near an abandoned mining site in the region of Lakhouat (northwestern Tunisia), sampled during dry season was investigated. The main objective was to characterize the environmental impacts of the former Pb-Zn extractive activity in the region. Chemical analysis revealed the presence of high PTE concentrations up to 28,040 mg kg−1 (Pb) and 94,420 mg kg−1 (Zn). Regarding nearby agricultural soils, PTE concentrations are exceeding the Tunisian standards for non-contaminated soils. The same contamination was detected in the stream sediments for which the Pb and Zn levels were found to be 54 to 147 times higher compared to the geochemical tolerance. Moreover, important concentrations of PTEs up to 638 mg kg−1 (Pb) and 939 mg kg−1 (Zn) were also found even in deep horizons (beyond 40 cm). Pollution index (PI) and Nemerow integrated pollution index (NIPI) gave several statuses of sediment quality revealing a polymetallic contamination dominated by the two elements Pb and Zn. The contamination found in Lakhouat soils is the result of the chemical and mineralogical composition of the soil and the infiltration of water loaded with PTEs from the discharges nearby. Extremely high PTE concentrations were also found in the roots (Pb = 1086 mg kg−1; Zn = 16.9 mg kg−1) and aerial parts (Pb = 3231 mg kg−1; Zn = 150 mg kg−1) of wheat grown on the contaminated soils, exceeding the thresholds for plant toxicity. Our study helps to assess the ecological risk due to potentially toxic elements contamination in agricultural soils, stream sediments and plants in Tunisia and other similar regions.