Mitigating AMD impacts in New Zealand using engineered wetlands

Abstract

Coal mining in New Zealand has caused perturbation of water resources and biodiversity. Contaminants impairing local waterbodies include acidity, iron, aluminum, arsenic, manganese, nickel, zinc, copper, sulfate and suspended solids. Exposure of sulfur containing rocks, such as pyrite, to atmospheric oxygen during mining operations produces acid mine drainage (AMD). Sulfuric acid and metal acidity are generated and can accentuate metal mobilization and bioavailability. Metals favor the dissolved state in acidic environments but form less toxic precipitates when exposed to adequate alkalinity. Metal toxicity effects are synergistic dependent on metals speciation and their concentrations. New Zealand is in the initial stages of acid mine drainage mitigation and has yet to develop proven treatment technologies. Implementation of passive treatment methods, such as engineered wetlands, have successfully reduced acid mine drainage impacts worldwide. Design criteria for these systems are improving while their limitations are well documented. We are currently collecting water quality and flow data from selected AMD-impacted sites. We are in the process of designing pilot-scale engineered wetland systems to ameliorate acid mine drainage in New Zealand. Sequential-treatment trains will be constructed and their performance evaluated in order to optimize design effectiveness. New Zealand acid mine drainage characteristics and complex topography offer unique challenges for implementing treatment systems. The AMD typically contains very high aluminum concentrations (commonly exceeding 50 g/m3) and has an aluminum to iron concentration ratio of three to one. Abundant steep topography can be exploited to create adequate driving head for implementing systems such as SCOOFI reactors while reducing and alkalinity producing systems can also be employed. Precipitation of up to six meters per year contributes to dynamic hydraulic characteristics and will offer unique design and treatment challenges.