The geochemical environment during rehabilitation of the Wangaloa opencast coal mine, Southeast Otago, New Zealand
Baker, Michelle Amy
This study investigates the geochemistry of the Wangaloa opencast mine during the early stages of its rehabilitation. New Zealand's coal industry is currently growing, and with increasing pressure from an environmentally conscious society, it is likely that the number of rehabilitation projects will increase. Understanding the geochemical environment of coal mine sites leads to a better awareness of the challenges facing rehabilitation workers, and encourages a targeted and direct approach to remediation efforts. The Wangaloa opencast coal mine closed in 1989 after a 43 year mining history, and since 2002 an extensive rehabilitation project has been in operation. An earlier attempt to grow Pinus radiata on quartz-rich overburden dumps was not successful, and the site is currently being rehabilitated with re-profiling and native plant establishment. The Barclay coal seam at Wangaloa contains elevated levels of boron (370 mgkg⁻¹) and reduced sulfur (>4.1 wt% S), mostly in the form of pyrite, FeS₂. Waters interacting with this coal therefore display elevated boron and sulfate, as well as lowered pH from the oxidation of pyrite. Quality of discharge waters from the site via the main pit lake is one of the principal environmental issues. The rehabilitation project aims to increase mine discharge pH and reduce dissolved metal and metalloid loads. Water analysis completed as part of this project includes pH, conductivity, alkalinity, sulfate, dissolved oxygen, major cations, chloride, total reactive phosphorus, NH₄⁺, NO₂⁻ +NO₃⁻, IC, OC, TOC and Al, As, B, Ba, Cu, Fe, Mn, Sr and Zn. The main pit lake on the site had low pH (ca. 4.5) and elevated trace element levels prior to dosing with slaked lime in November 2002. After this addition, the lake pH gradually rose to ca. 6, and began rapid, large scale fluctuations (4.3<pH<6.4) after one lake water residence period. This project uses monthly sample collections from several surface water sampling sites and five ground water collection sites to characterise site water quality over the period of one year, and to create an overview of the fluxes and budgets contributing to the main pit lake chemistry. This study concludes that the main pit lake is dominantly controlled by ground water fluxes, and that the slaked lime pH remediation was only effective until the residence time of lake waters was reached, upon which time the lake water pH began to move toward ground water pH. Aside from pH, differences between ground and surface waters are minor due to the proximity of the unconfined aquifer to the land surface, relatively high hydraulic conductivities and short residence time of ground water. Native plants are being established in a range of substrates that consist of variably disaggregated Tertiary coal measure sediments derived from the mine sequence. This material has low organic matter content (typically <10%), originating from coal fines or carbonaceous sediment. Maps of substrate type and age since last disturbance have been compiled, and substrate samples were collected during March, June and December 2003 from a range of sites varying in age, slope, and parent material. Major nutrient analysis includes nitrate, ammonia, total N, total P, available P and readily exchangeable base cations. Other analysis includes pH, total C, total S, and total recoverable Fe, Mn, Al, Ba, Cu, Ni, Pb, Sr and Zn. Total nitrogen and phosphorus are low on areas of recently disturbed, quartz-rich overburden, with total N ≤1200 mgkg⁻¹, and total P ≤200 mgkg⁻¹. Background forested areas have total N ranging between 2800-3300mgkg⁻¹ and total P of 420-500mgkg⁻¹. Quartz-rich overburden sites contain available nitrogen levels of less than 6 mgkg⁻¹ mostly as ammonium, while background forested areas are generally >20mgkg⁻¹, as approximately even proportions of nitrate and ammonium. The substrates are generally acid, with pH down to 3. Links have been made between total carbon and boron in overburden materials, indicating the presence of coal in overburden. This study concludes by suggesting that the site is recovering in terms of substrate and water pH, but that boron levels in pit lake waters may remain elevated due to an internal supply of coal on the lake bed. This study also recommends targeted substrate fertilisation to assist native plant establishment.
Advisor: Craw, Dave; Martin, Candace; Smith, Carol
Degree Name: Master of Science
Degree Discipline: Geology
Publisher: University of Otago
Research Type: Thesis
x, 176 leaves :ill. (some col. and folded), maps (some col., 1 folded) ; 30 cm. Includes bibliographical references (leaves 132-137) University of Otago department: Geology.