An understanding of the ground water flow systems and the chemical reactions producing acid mine drainage is necessary for the successful design and selection of treatment procedures. The Bunker Hill Mine, a large underground lead-zinc mine developed in fractured quartzites, discharges an average of 3.4 Cfs of poor quality water. The zinc concentration averages 116 mg/l and the average ph is 2.8. Acid is produced from the oxidation of pyrite. Flow and water quality at 22 underground sites were monitored for 18 months. The chemical reactions controlling the oxidation of pyrite are presented. The spatial and temporal distribution of water quality and metal loading are presented and discussed. Of the zinc load discharging from the mine, 64% is carried by 2.9% Of the flow that originates in the upper levels of the mine. Hypothetical reclamation scenarios are presented and discussed. Effective targeting of reduction of inflow to acid producing areas of the mine would improve significantly the quality of water discharging from the mine.