Geogenic influence and impact of mining activities on water soil and plants in surrounding areas of Morila Mine, Mali

• Published in: Journal of geochemical exploration 2020-02, Vol.209, p.106429, Article 106429
• Main Authors: Bokar, H., Traoré, A.Z., Mariko, A., Diallo, T., Traoré, A., Sy, A., Soumaré, O., Dolo, A., Bamba, F., Sacko, M., Touré, O.
• Format: Article
• Language: English
• Subjects: Geochemical background; Geogenic contamination; Mali; Morila goldmine; Trace metal contamination
• ISSN: 0375-6742; 1879-1689
• DOI: 10.1016/j.gexplo.2019.106429

The Morila Gold mine, a joint venture between AngloGold Ashanti and Randgold Resources, is one of the 10 operating mines in Mali. The mine is located at 180 km southeast of Bamako, Mali, and has been operating since 2001. With the depletion of the orebody, a closure plan was established by the government and shareholders since 2012, and rehabilitation strategies are currently operating. The concentration of major, minor and traces elements were analyzed in 90 water samples (45 water supply boreholes, 13 shallow wells, 20 piezometers of the mine and 12 surface water), 46 stream sediment samples, 10 mine soil samples and 57 plant samples. All the samples were collected from June 2015 to December 2016 in three study areas: Morila mine, an intermittent stream at downstream of the mine and a controlled geochemical background area free of mining activities (located 15 to 20 km upstream of the mine). Chemical water analyses show that arsenic (As: 0.8 to 132 ppb), manganese (Mn: 2.52 to 5200 ppb) and total iron (Fe: 0.462 to 78 ppm) are present at high levels. The ANOVA tests demonstrate significant differences between the three sites for most major ions and trace metals in water, stream sediments and mine soils. The mean concentrations within the mine were significantly higher than those outside mine boundaries. Factor analysis and cluster analysis show a geogenic and pollution factors influencing the soils and the water quality in the study area. Arsenic values were mostly found in groundwater of the village of Sanso (the nearest village to the Morila mine), where 7 of the 10 sources of drinking water boreholes had arsenic contents higher than the standard value of 10 ppb. This arsenic contamination in groundwater and those located in background area could not be linked to mining activities, and attributed to geogenic influences. In stream sediments and soil samples, the values of the analyzed elements were ranged bellow the maximum allowed concentration except for As, Fe, Al and S. Mine soils have high Fe (~10 wt%) and high Al (~3 wt%) concentrations, likely derived from naturally sources. Sulfur contents in soil and stream sediments were high only in the mining environment, and derive from sulfide mineral oxidation. Arsenic contents were elevated in most plant species samples, and concentrations of samples located within the mine were higher than those of the background area. This study indicates that most of the metals and toxic contaminations were from geoge