THE POTENTIAL FORMATION OF ACID-MINE DRAINAGE IN PYRITE-BEARING HARD-COAL TAILINGS UNDER WATER-SATURATED CONDITIONS - AN EXPERIMENTAL APPROACH

Annually, an amount of approximately 13 million cubic meters of hard-c oal tailings must be disposed of in the German Ruhr Valley. Besides th e waste of land in a densily populated region, the disposal of the pyr ite-bearing material under atmospheric conditions may lead to the form ation of acid mine drainage (AMD). Therefore, alternative disposal opp ortunities are of increasing importance, one of which being the use of tailings under water-saturated conditions, such as in backfilling of abandoned gravel pits or in the construction of waterways. In this cas e, the oxidation of pyrite, and hence the formation of AMD, is control led by the amount of oxygen dissolved in the pore water of tailings de posited under water. In case the advective percolation of water is sup pressed by sufficient compaction of the tailings, oxygen transport can be reduced to diffusive processes, which are limited by the diffusive flux of dissolved oxygen in equilibrium with the atmospheric pO(2). C alculations of the duration of pyrite oxidation based on laboratory ex periments have shown that the reduction of oxygen is mainly controlled by the content of organic substance rather than the pyrite content, a fact that is supported by results from oxidation experiments with nit rate. A ''worst case'' study has lead to the result that the complete oxidation of a 1.5-m layer of hard-coal tailings deposited under water -saturated conditions would take as much as several hundred thousand y ears.