British Columbia Mine Reclamation Symposium

Heavy metal levels in grasses and legumes grown on Highland Valley Copper Mine tailings and the effect of artificial weathering on tailings growth production capability Hackinen, Coleen Marie, 1957-

Abstract

Samples of agronomic grasses and legumes grown on copper mine tailings under two surficial amendment regimes and two fertilizer treatments were analyzed for Ca, Cu, Fe, K, Mg, Mn, Mo, N, Ni, P, and Zn. These values were compared to National Research Council and Agriculture Canada diet recommendations for beef cattle. Foliar Ca, Fe, Mn, Ni and P concentrations were satisfactory. Copper and molybdenum concentrations were well above normal levels. Copper concentrations in the legumes and grasses averaged 63 and 44 mg/kg, respectively. Mean molybdenum concentrations were found to be 52 mg/kg in the grasses and 237 mg/kg in the legumes. Cu:Mo ratios in all species violated recommended dietary guidelines for beef cattle. Fluctuations in Cu:Mo ratios were attributable mainly to variations in foliar Cu. Most species were subjected to severe grazing by rodents, producing abnormal growth habits. As a result, assessment of relative species success was difficult. Grazing stress may also have affected foliar elemental levels. Tailings material was artificially weathered by leaching with 0.3 N acetic acid for seven weeks in a Soxhlet extraction chamber. Leached and unleached samples were analyzed for pH, 0.1 N HCl available Cu, Fe, Mn, Ni, and Zn, acid ammonium oxalate extractable Mo, total elemental levels and mineralogical composition. As a result of leaching, the pH of the tailings was reduced from 6.6 to 3.5. Declines in available Cu, Fe, Mn, Zn, and Mo were noted after leaching. No qualitative changes in tailings mineralogy were detected after artificial weathering. During leaching, three general elemental release patterns were observed. These were attributed to the sequential dissolution of readily soluble salts and carbonates, followed by the degradation of micas, pyroxenes, amphiboles and host ore minerals. The relatively inert minerals, such as quartz, some aluminosilicates and oxyhydroxides of Fe and Al represented the third group. At present, the tailings are unsuitable as a forage production area for beef cattle as a result of toxic Cu:Mo ratios in the foliage. Based on chemical changes induced by artificial weathering, it is probable that forage grown on the tailings will become less toxic over time. Various management practices may be employed to accelerate improvement in growth medium parameters.

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