A numerical model has been developed to simulate the effectiveness and costs of a number of conceptual remedial measures aimed at reducing acidity and metal concentrations in the Ngakawau River and its tributaries near the Stockton Coal Mine, situated on the West Coast of New Zealand._x000D_
The Stockton Coal Mine has been in operation over the past 100 years. During that time, seepage from waste rock dumps has contributed to elevated acidity and metal concentrations in the Ngakawau River and some of its tributaries. The yearly mean rainfall at the mine site is 6000 mm._x000D_
The model, developed using GoldSim software, simulates rainfall, run-off and stream discharge for 26 tributaries. It then calculates acidity and metal concentrations in the mine impacted tributaries and the dilution downstream from the mine. The model has been calibrated to measured flow and acidity and metal concentrations in the Ngakawau River and some of its tributaries._x000D_
Acidity and metal loads are removed from the system in accordance with specified efficiencies and capacities of the respective conceptual remedial measures. The user can compare specific remedial measures, or a combination of conceptual remedial measures._x000D_
The model predicts the acidity and metal concentrations at a number of monitoring stations for any remedial strategy selected by the user. It also calculates the capital and operational costs for that particular remedial strategy._x000D_
This paper presents the development of the numerical model and describes how the end-user can use it to compare the efficiency and costs of various remedial strategies by running different what if' scenarios._x000D_
FORMAL CITATION:Vermaak, J and Lindsay, P, 2006. A numerical model to simulate the effectiveness of remedial measures aimed at reducing acidity and metal concentrations in the Ngakawau River and its tributaries near Stockton Coal Mine, West Coast, New Zealand, in Proceedings Water in Mining 2006, pp 379-386 (The Australasian Institute of Mining and Metallurgy: Melbourne).