A geochemical weathering profile of a fine ash tailings dam and its impact on the underlying aquifers.

Affiliation

Institute for Groundwater Studies, University of the Free State, 205 Nelson Mandela Drive, Park West, 9301, Bloemfontein, Free State, South Africa. Electronic address: [Email]

Abstract

A weathering profile of an unlined fine ash tailings dam was established with the aid of static and kinetic tests to determine the leachate composition across a tailings dam profile and to evaluate its impact on the underlying aquifers. Kinetic leaching results indicated that SO42-, Ca2+, Na+, Cl-, Mg2+ and K+ ions were highly soluble in both, fresh and weathered fine ash. Leaching rates of elements including Ca, B, Ba and Al decreased with depth and age of the fine ash while V, Mo, Cu, Mn, Si, Li, Cr and SO4 had an increasing leaching rate. A constant leaching rate was measured for Se, Fe, Na, Mg, K, NO3 and Zn. At shallow depth, a weathering zone was identified by a decrease in leaching rates of most elements. Although the hydraulic conductivity of the fine ash dam was low (0.0067 m/d and 0.0367 m/d), long-term hydrochemical monitoring around the tailings dam showed alterations in groundwater types of the underlying aquifers due to seepage. Downstream of the tailings dam, the groundwater type of the shallow weathered aquifer changed from Ca/MgSO4/Cl to NaSO4/Cl and from Na-bicarbonate to Ca/MgSO4/Cl upstream. A reverse trend was observed in the deeper fractured aquifer indicating that the fine ash leachate plume migrated laterally and vertically from the shallower to the deeper aquifer. Hence, the natural clay layer below the tailings dam was insufficient in retaining soluble elements leached from the fine ash over two decades. Therefore, an impermeable liner is recommended prior to the tailings dam construction.

Keywords

Aquifer,Fine ash weathering,Hydrochemistry,Leaching rate,Soluble ions,

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