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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.

Successful passive treatment of acid mine drainage can be improved through the use of small-scale pilot treatment systems to confirm appropriate system selection. Small-scale reducing and alkalinity producing systems were tested at two acid mine drainage sites in the West Coast Region, South Island, New Zealand: the Sullivan Mine and the Pike River Adit. A laboratory trial consisting of a limestone leaching column was conducted on the Blackball Mine acid mine drainage, West Coast Region. All three sites contain low pH (Sullivan Mine, 2.9; Pike River Adit, 3.2; Blackball Mine, 3.1), elevated Fe (Sullivan Mine, 47 mg/ L; Pike River Adit, 34 mg/L; Blackball Mine, 10.6 mg/L), elevated Al (Sullivan Mine, 14 mg/L; Pike River Adit, 1.6 mg/L; Blackball Mine, 14.1 mg/L) and minor concentrations of Mn (0.35Á0.51 mg/L), Ni (0.005Á0.13 mg/L) and Zn (0.14Á1.1 mg/L). The percentages of metals removed by the Sullivan Mine reducing and alkalinity producing system were: Fe (97%), Al (100%) and Ni (66%). The percentage metal removals at the Pike River Adit reducing and alkalinity producing system were: Fe (99%), Al (96%), Ni (95%) and Zn (99%). Percent metal removals for the Blackball Mine acid mine drainage were: Fe (87%), Al (91%), Mn (21%) and Zn (68%). Interpretation of data from these small-scale systems suggests that a reducing strategy may be successful at the Sullivan Mine and Pike River Adit and that an oxidising strategy may be appropriate for the Blackball Mine.

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