The reclamation of acidic colliery spoil. I. Acid production potential.
Chemical analyses of spoil from twenty sites showed a considerable range of variation in pyrite content and acid neutralizing capacity (calcium carbonate equivalent). The acid-producing behaviour of nine spoils was related to their iron pyrites contents and acid neutralizing capacities. Most spoils were compacted with a high clay content and pyrite oxidation occurred in a shallow surface zone resulting in a material of low pH and low pyrite content. The underlying unoxidized zone exhibited high pH and high pyrite levels. It is recommended that spoils be analysed for pyrite content and acid neutralizing capacity immediately after regrading. The lime-requirement for permanent pH control can then be estimated. Assuming complete oxidation, 1% pyrite in spoil can give rise to a lime requirement of 40 t ha-1 for pH control to 15 cm depth. 1% of acid-neutralizing capacity is equivalent to 23 t ha-1 of limestone applied, for 15 cm of spoil. Limestone should be incorporated to 45-50 cm depth and the application rate increased accordingly. High pyrite spoils may have lime-requirements in the range 100-400 t ha-1 for pH control to 45 cm depth.