Options for Treating Acid Mine Drainage at Stockton Mine, New Zealand

Acid Mine Drainage (AMD) emanating from Stockton Coal Mine and associated historic mines on Stockton plateau, New Zealand is unique in terms of volume with a mean of approximately 3500 litres a second potentially requiring treatment and peak flows of up to 10,0000 litres a second; and quality with Al:Fe ratios of 3:1. Dissolved Aluminium concentrations in streams at the coal mine licence boundary can be 100 mg/L and pH as low as 2.7. The site has over 6 metres of rainfall a year and the runoff coefficients are 0.6 to 0.9. The current open cast mine site operator Solid Energy New Zealand Ltd is reviewing the cost, permit issues and practicality of various options to manage AMD including: 1) Piping AMD to ocean via outfall 2) Piping AMD to ocean via a hydroscheme and outfall 3) Active treatment- dosing streams with ultrafine limestone, combined with capping acidic waste rock to minimize acid production 4) Passive Treatment (e.g., engineered wetlands) The positives and negatives of the four main treatments are discussed in this paper. The results of water quality and stream ecology improvements in the current active treatment with ultrafine limestone dosing direct to stream are detailed. Key words: acid drainage, ocean piping, passive treatment, active dosing, treatment costs, permitting, saturated covers