PIT LAKE PREDICTION IN NEW ZEALAND; A HUMID CLIMATE ANALOGUE FOR ACIDIC PIT LAKES IN THE APPALACHIANS

Pit lakes resulting from open pit mining often develop low pH surface water owing to acid runoff from sulfide-bearing wall rocks. To date, most pit lake research in the United States has focused on mining areas in the Western United States, which have arid climates characterized by evaporation rates that exceed precipitation rates. Because rainfall and evaporation have a significant influence on lake chemistry, studies in the Eastern United States may benefit from international pit lake research conducted in humid climates, such as New Zealand. This paper presents the methodology used to predict the water chemistry of a future pit lake at the Martha gold mine, North Island, New Zealand. Upon closure in 2007, the mine will be flooded with river water to create a 192-m-deep pit lake, which the mining company intends to rehabilitate into a recreational amenity. Owing to the high local precipitation rate (213 cm/yr), runoff from pit walls would constitute a significant component of the lake water balance, and was expected to influence lake chemistry. To explore the effects of runoff, the geochemical model PHREEQC was used to construct three, 55-year predictions of epilimnion and mixolimnion water quality in the proposed Martha pit lake, based on recent mineralogic and limnologic investigations. Model 1 considered the current closure strategy, whereas Model 2 covered wall rocks that produced highly-acidic runoff before mine closure, and Model 3 covered the same regions after mine closure. After 50 years of steady-state conditions, Model 1 predicted that surface water will have a pH < 5.0 and will not comply with New Zealand water quality guidelines for recreational use. By covering the wall rock regions that produced highly-acidic runoff before mine closure, Model 2 showed the epilimnion pH increased to 7.0. Moreover, Model 3 showed that covering the same areas after mine closure produced similar result to Model 2 after 50 years of steady-state conditions, increasing pH to 6.8. Because only 10% of the subaerially exposed pit area contains acid-generating rocks, this is a feasible option for the mining company that may significantly improve future pit lake water quality. A similar approach may be applied to minimize AMD remediation efforts and expenses at abandoned mine sites in the Appalachians.