Rehabilitation of mine sites normally requires restoration of or even improving land use and environmental factors in the affected region. The present studies have assessed environmental parameters that are the principal constraints for mine development in the Stockton Plateau, including in particular surface water and underflow leachate quality. Gently dipping quartzose sandstone and grits, localised siltstones, and carbonaceous mudstones of early Tertiary age comprise the Buller Coalfield. The Brunner Coal Measures (BCM) unconformably overlie rocks of early Cretaceous Berlins Porphyry and Greenland Group of Ordovician age. These sequences, in turn, are underlie the recently nominated Millerton sandstones and mudstones of the Kaiata Formation, which were almost entirely eroded from the area investigated. This geological setting in combination with the harsh climatic conditions of the area inhibits any significant ecosystem development. However, dominant medium size vegetation such .as manuka, hebe and rata develop mainly in shaded areas.

Chemical, physical and mechanical evaluation of the overburden waste rock materials has identified three principal lithotypes. The coarse to very coarse massive sandstone and grit and granule conglomerates were grouped into LIT I (A-B).The fine to coarsely laminated micaceous siltstone and fine to medium sandstones compose LIT II, and the carbonaceous mudstone and the fine muddy siltstones are grouped in LIT III. The carbonaceous mudstones, which are a subordinate unit in the Brunner Coal Measures, contains substantial disseminated pyrite concentrations; whilst the other assessed mudstone of the Kaiata Forn1ation, is the most reactive with framboidal pyrites present in various micromorphologies.

These sulphide bearing minerals in contact with water and oxygen trigger acid rock drainage (ARD). The liquid solutions are being released from the mined areas as underflows from overburden stockpiles, and contribute significantly to the surface water chemistry. Acid mine drainage (AMD) at Stockton coal mine thus is characterised by low pH, high sulphur concentration and subordinate concentrations of aluminium and iron (possible in its two phases Fe²⁺ and Fe³⁺). The acid mine drainage prediction techniques (AMDP) have confirmed that the carbonaceous mudstone, within the Brunner Coal Measures, is the principal lithotype with acid production potential, and that the Kaiata mudstone is the principal acid producer of all the assessed lithotypes.

Release of strongly acidic leachates (pH < 3.00), with high concentrations of sulphates up to 4000 mg/1, aluminium and iron was characteristic of both mudstones. Early low pH and high concentrations of sulphates in leachates originated by the laminated sandstone of LIT II indicated that these are net acid producers in the short term, but in the long term the quality of released leachates appears to steadily increase. The coarser graded sandstones (LIT IB) and siltstones produced weakly acidic leachates with low concentrations of heavy metals and sulphates.

Although a large percentage of overburden stockpiles are stable, it was observed that some of the stockpiles presented shallow settled failures and one (Mt. Frederick sidecast) presents a serious problem of instability. A water pressure build-up within the sidecast spoil appears to have affected the possible thin weak capping layer of finer material and triggered the initial failure of this spoil pile. Rehabilitation on this area thus requires first hand stabilisation programme for the spoil pile.

The findings of this study show that long term rehabilitation programme needs to be settled using the whole environmental baseline data provided in the present thesis and other already agreed measures. Adequate handling of spoils, which include encapsulation of the carbonaceous-pyritic mudstones, seems the most effective methodology of acid mine drainage remediation. Equally, remediation of spoil piles using chemicals and/or other nutrients release material, needs to be firstly evaluated in both field and laboratory conditions.