Groundwater contamination through cyanide and metal migration from tailings dam operation

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April, 2016
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In the gold industry, only a few hundredths of an ounce of gold may be produced for every tonne of tailings generated. Tailings need to be properly managed because they constitute a major source of release of many metals and trace elements into the environment which is a potential source of groundwater contamination. The impact of Gold Fields Ghana Limited (GFGL) tailings dams were evaluated by chemical characterisation of metals and trace element concentrations in the decant pond, tailings slurry and underdrainage sump. The attenuation properties of the substrata were also evaluated with emphasis on hydraulic conductivity, permeability characteristics and geochemical properties (pH, Organic Matter Content (OMC), Cation Exchange Capacity (CEC), iron oxide (Fe2O3) and Anion Exchange Capacity (AEC). Finally, four years groundwater monitoring data was used to evaluate the impact of possible seepage from the dam. Results of the studies indicate that concentrations of Weak Acid Dissociable (WAD) cyanide and Total cyanide increased significantly with depth in the decant pond. pH of the pond is alkaline with mean values ranging from 9.3 to 10.6 corresponding to mean pH values in the tailings slurry at spigotting points and underdrainage sump. Total cyanide concentrations ranged from 0.36 mg/l to 1.11 mg/l over five meters depth and 0.10 mg/l to 0.29 mg/l over ten meters depth in the decant ponds of TSF 1 and TSF 2 . WAD cyanide concentrations ranged from 0.26 mg/l to 0.93 mg/l and from 0.01 mg/l to 0.16 mg/l over five and ten meters of decant depth respectively. Significant metals identified in the decant pond, underdrainage sump and tailings slurry were Sodium (Na) and Magnesium (Mg) with trace elements being Iron (Fe), Copper (Cu), Manganese (Mn), Arsenic (As), Nickel (Ni), Zinc (Zn) and Chromium (Cr), whiles alkali metals identified were Calcium (Ca) and Potassium (K). These metals and trace element were consistently present in the tailings and underdrainage. Low hydraulic conductivity and a combination of low acidic pH, relatively high Anion Exchange Capacity (AEC) and low Cation Exchange Capacity (CEC) were identified to be the major driving attenuation properties that favour cyanide attenuation in the Tailings Storage Facility (TSF) basin. The average concentration of Total cyanide detected within a radius of 3 m at the field leaching stations is 0.02 mg/l, constituting 99.9% attenuation of cyanide introduced. The hydraulic conductivity and permeability values computed from grain size analyses of the substrata material are 4.3 x 10 -6 to 9.7 x 10 -6 m/min and 6.1 x 10 -12 to 13.9 x 10 -11 m/min respectively. The TSF sub-strata material has a pH range of 4.3 to 4.8, a CEC range of 1.92 to 2.89 meq/100g and an AEC range of 881.25 to 1278.25 meq/l. The attenuation rate of cyanide at the five (5) stations remained constant at 0.021 mg/l/hr as time increased. Based on significant increase in levels of EC and pH levels as well as concentrations of Na, Ca, and Cl, it could be established that monitoring boreholes along the toe of TSF1 and TSF2 have received seepage from the tailings facilities. Similar concentrations were observed in boreholes at the South heap leach facility, specifically near the pregnant pond, monitoring boreholes located west of the North heap leach Facility and specifically around the South and North containment ponds
A thesis submitted to the Department of Civil Engineering, Kwame Nkrumah University of Science and Technology in fulfillment of the requirements for the degree of Doctor of Philosophy in Water Resources Engineering and Management,