Assessment of environmental impact of tailing dumps in Chadak mining area, Uzbekistan

Resumen

Of all the diversity of man-made wastes, the problems associated with mining wastes have a great concern. The characterization is the first step to find a solution to the potential pollution related to these areas and to prevent environmental risk to living organisms. A set of processes that adversely affect the environment is associated with mine wastes that include pollution of adjacent soils, water and air with toxic elements. In this work, the assessment of potential trace elements contamination in tailing dumps and soils was characterized in the Chadak mining area (Uzbekistan). Hence, the main objective of this work is to assess the environmental impact of metals and metalloids coming from waste rock piles and tailing dumps generated during processing of the Chadak Au-Ag deposit, by characterizing the potentially contaminating elements and assessing their toxicity. The studies and results obtained have covered a knowledge gap in relation to the evaluation of the potential pollution of the environment in this area. Concentrations of trace elements (V, Cr, Co, Ni, Cu, Zn, As, Cd, Sb and Pb) were determined by X-ray fluorescence analysis and compared with background (BC) and intervention values (IV). The concentrations of As, Zn, Sb, and Pb exceeded the BC in most cases and were higher in the abandoned than in the active tailing dump, ranging from 42–1689 mg/kg for As, 73–332 mg/kg for Zn, 14–1507 mg/kg for Sb, and 27–386 mg/kg for Pb. Moreover, As and Sb showed mean values strongly exceeding the IV (up to 20- and 62-fold, respectively), indicating a high risk of pollution in the area for these two elements. Physico-chemical properties of soils and tailing samples showed a moderate critical load of pollutants related to the alkaline pH and calcium carbonate content, low EC, CEC and OC. The moderate presence of calcium carbonate in the tailings, along with the high acidification potential, results in the potential acid generation in the active tailing dump, while the abandoned tailing dump is not susceptible to generate acidification over time. Mineralogical studies of the tailings by XRD showed that the samples were dominated by quartz, feldspars, calcite, micas and chlorite. The SEM-EDS imaging made on the tailings and open pit waste materials identified primary and secondary minerals consisting of sulfides, silicates and secondary precipitations of Fe and Mn oxyhydroxides. The presence of yukonite-like precipitation of Ca–Fe–As phases were also detected and could be related to potentially available forms of As. Selective extractions were applied in order to assess the mobility and availability of metal(loid)s in samples. Oxyanion-forming elements such as As and Sb were immobilized by Fe oxides, although to some extent also extractable with acetic acid and soluble-in-water forms were detected, indicating potential bioavailability that can impose a potential toxicity risk for the environment. Selective extractions data also showed that Zn and Pb were relatively immobile, although in tailing dumps significant amounts of these elements were also extractable with acetic acid. In tailing materials Zn and Pb mobility were negatively correlated by the cation-exchange capacity (CEC) and clay content, indicating the importance of these factors in the reduction of the potential toxicity for these elements. Total concentration of As, Sb, and Pb were also negatively correlated with soil pH, indicating that the oxidation process of sulphide tailings and thus the generation of acidic conditions may lead to release of contaminants over time. However, due to the calcium carbonate content, the acid neutralization capacity of the tailings is not yet exhausted and contaminant concentrations in soil-pore water are still relatively low. Three toxicity tests (plant accumulation, toxicity bioassays and human health risk) were performed to evaluate the toxicity of metal(loid)s. The test of plants accumulation indicated that Artemisia absinthium can be suitable plant for phytoextraction of Cu, Zn and Sb in polluted areas, and Phragmites australis may be efficiently utilized for metal removal (mainly Cu) and, in any case, can also both be used for phytostabilization of contaminated sites. The toxicity bioassays made with the soluble-in-water phase using Lactuca sativa and Vibrio fischeri showed no risk of toxicity in the short-term, indicating that the metal(loid)s in tailings are mainly associated to secondary phases related to carbonates and iron/manganese forms that reduce the mobility and toxicity of these soluble phases. The results of human health risk assessment indicate a high potential of toxicity by accidental ingestion route for As, Sb and Pb. Special high risk was detected in the case of urban soils exposed to children (gardens or parks) and in agricultural soils. The results of our investigation suggest that environmental risk associated with these wastes in semi-arid climate is therefore not a short-term problem but rather requires constant monitoring. In addition, remediation actions are recommended in order to mitigate or prevent the risk of toxicity to the population close to the mining area.