Coal mine drainage chemistry on the West Coast of the South Island is highly variable; pH ranges from about 2Á8, and chemical concentrations vary by several orders of magnitude. Factors that influence mine drainage chemistry on the West Coast include regional geology, mine type, hydrogeology, and local geology. At a regional scale, mine drainage chemistry is bimodal and relates to geological formations. Mines within the Paparoa Coal Measures have neutral mine drainage, whereas those within the Brunner Coal Measures have acid mine drainage. This is related to the availability of SO 4 during coal measures deposition and, in combination with Fe and organic material, the subsequent formation of pyrite during burial. Paparoa Coal Measures were deposited in alluvial to lucustrine environments where SO 4 was relatively unavailable, whereas Brunner Coal Measures were deposited in alluvial to estuarine to marginal marine settings where marine SO 4 was abundant. Brunner Coal Measures acid mine drainage chemistry is influenced by mine type; open cast mines have Al-and trace element-rich acid mine drainage compared to underground mines. Acid mine drainage forming reactions that release trace elements and Al proceed more rapidly and completely at open cast mines where mine waste has a higher reactive surface area compared to waste rock at underground mines. Brunner coal mine drainage chemistry is also influenced by hydrogeology where flooded underground mines release less acid than free-draining mines because there is less oxygen available to react with pyrite. In addition, local geology overprints mine drainage chemistry where differences in acid mine drainage chemistry arise from changes in contributing lithologies either within a single mine or between different coalfields. Identification of factors that control mine drainage chemistry enables prediction of mine drainage chemistry. These predictions have application to the mining industry for managing, mitigating, monitoring, and remediation of mine drainages that would otherwise cause negative environmental impact.
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.
Elevated dissolved antimony (Sb) and arsenic (As) are common environmental issues around orogenic gold mines. The metalloids occur principally in stibnite (Sb 2 S 3 ) and arsenopyrite (FeAsS), and are typically accompanied by pyrite (FeS 2 ). Samples of arsenopyrite-rich (&2.5 wt% As) and stibnite-rich (&10 wt% Sb) ore were collected from the Globe-Progress mine in the Reefton goldfield, New Zealand. Crushed samples (\5 mm particles) were placed in kinetic leach columns and monitored for a year. Leachate was collected and analysed monthly. After 12 months, only a small portion (\1 %) of the total As and Sb had leached from the ore samples. Over the course of each month, the pH of all leachates decreased from &7 to &3 due to pyrite oxidation. Dissolution of CaFe-Mg carbonates in the host rock was insufficient to neutralize the leachate. Dissolved As concentrations from the arsenopyrite-rich ore sample were initially 16 mg/L, but decreased to &2 mg/L over 12 months. Dissolved Sb concentrations from the stibnite-rich ore were [7 mg/L throughout the experimental period, with maxima of 12 mg/L. SEM analysis after 6 months showed secondary arsenolite (As 2 O 3 ) and Ca-Al-Mg sulphates on arsenopyrite-rich ore surfaces, but an absence of secondary minerals, including Sb oxides, on stibnite-rich ore surfaces.These experiments document geochemical and mineralogical processes associated with short-term (days to weeks) water-rock interactions that yield relatively high concentrations of dissolved metalloids ([5 mg/L) where localised (1-10 m) acidification and limited oxidation occur in sulphide-rich rocks.
Thirteen acid mine drainage seeps emanating from waste rock dumps and associated sediment ponds were monitored at Stockton Coal Mine near Westport, New Zealand to identify and quantify metal loads and delineate their spatial and temporal variability. Dissolved metal concentrations ranged from 0.05Á1430 mg/L Fe, 0.200Á627 mg/L Al, 0.0024Á0.594 mg/L Cu, 0.0052Á4.21 mg/L Ni, 0.019Á18.8 mg/L Zn, B0.00005Á0.0232 mg/L Cd, 0.0007Á0.0028 mg/L Pb, B0.001Á0.154 mg/L As and 0.103Á29.3 mg/L Mn and the pH ranged from 2.04Á4.31. Currently this acid mine drainage is treated further downstream by a number of water treatment plants employing a combination of ultra fine limestone and Ca(OH) 2 . However, in the interest of assessing more cost-effective technologies, biogeochemical reactors were assessed in the laboratory as potential cost-effective passive treatment options. Results of mesocosm-scale treatability tests showed that biogeochemical reactors incorporating mussel shells, pine bark, wood fragments (post peel) and compost increased pH to 6.7 and sequestered ]98.2% of the metal load from the Manchester Seep located within the Mangatini Catchment. Laboratory results demonstrated that the maximum loading rate was 0.8 mol total metals/m 3 substrate, and an average of 20.0 kg/day (7.30 tonnes/year) of metals could be removed by appropriately sized biogeochemical reactors.
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