Rapid Seasonal Transition from Neutral to Acidic Drainage in a Waste Rock Test Pile at the Antamina Mine, Peru

摘要

Distinct seasonal changes were observed in the chemical composition of water draining from an experimental waste rock pile composed of intrusive material at the Antamina Mine in Peru. Effluent water pH was predominantly circumneutral (i.e. pH 7-8) and was characterized by low dissolved metal concentrations for the first two years following pile construction. In the middle of the third wet season, the pH dropped rapidly to pH 6.4, coinciding with significant increases in dissolved Cu and Zn concentrations from 3 mg/L to 65 mg/L and 20 mg/ L to 60 mg/ L, respectively. As expected, the decrease in pH also induced a dramatic decrease in dissolved Mo and As concentrations. Towards the end of the wet season pH rebounded to 6.7 and Cu and Zn concentrations again dropped to 4 mg/L and 41 mg/L, respectively, while Mo and As concentrations increased. Data from four basal collection lysimeters and smaller-scale field cells suggests that lower-pH water from isolated regions of highly reactive material strongly influences the overall chemical composition of the drainage from the test pile, even after dilution with higher-pH water infiltrating through much larger volumes of waste rock. Localized pH decreases are likely caused by carbonate depletion and exaggerated by longer residence times in fine-grained, highly reactive materials.