Using pilot trials to test geochemical treatment of acid mine drainage on stockton plateau

Abstract

Herbert Stream, a tributary of the Waimangaroa River on the Stockton Plateau, South Island, New Zealand, has elevated metal concentrations (Al 7.68 ppm, Fe 1.37 ppm, Mn 0.69 ppm, and Zn 0.12 ppm) and low pH (2.3–3.4) characteristic of acid mine drainage. Average flow rate is 5.3 L/s. To determine the effectiveness of different geochemical treatment strategies, small-scale trials consisting of a reducing and alkalinity producing system (RAPS), a limestone leaching bed (LLB), and an open limestone channel (OLC) were operated for 8 months. All three trial systems performed well, removing metals and raising pH. Maximum removal rates were: Al 99% (all three systems); Fe 97% (RAPS), 99% (LLB), and 95% (OLC); Mn 95% (RAPS), 92% (LLB), and 74% (OLC); and Zn 87% (RAPS) and 91% (LLB). The OLC was less effective than the other trial systems in raising pH, and the effectiveness of Al removal decreased with time, probably due to armouring of the limestone by hydroxide precipitates. Minimal armouring of the limestone in the RAPS and LLB occurred, and the RAPS was successful at reducing oxidised Fe to Fe monosulfides (most likely mackinawite). Based on monitoring of the trial AMD treatment systems, a full-scale LLB was designed to treat the entire flow of Herbert Stream. © 2008 Taylor & Francis Group, LLC.