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Groundwater contamination due to historical magnetite mine tailings deposited in a near-shore, marine environment

Paper from the Proceedings of Mine Closure 2015 conference held in Vancouver, Canada, June 1-3, 2015. (downloadable PDF)

Authors: R.C. Dickin and S.E. Dickin, AECOM, Canada; L. Groat, University of British Columbia, Canada; K. Jia, AECOM, Canada; U. Mayer, University of British Columbia, Canada; R. Mills and M. Sanborn, AECOM, Canada

Published by InfoMine Inc.
ISBN: 978-0-9917905-9-3
Copyright: 2015, InfoMine Inc.


Abstract

In the 1960s, mine tailings were produced from the processing of iron ore from a magnetite skarn deposit in a near-shore intertidal marine area on the west coast of British Columbia. The mine and processing facility were abandoned over 40 years ago. The tailings formed a beach that was used for recreational purposes for many decades. Recent investigations of the tailings identified concentrations of total arsenic (As) (26 µg/g to 19,200 µg/g) and cobalt (Co) (50 µg/g to 1,820 µg/g) that exceed the applicable BC Contaminated Sites Regulation (CSR) soil and sediment standards for parkland and marine aquatic life land use. Groundwater from monitoring wells screened in the near-shore and intertidal tailings had concentrations of dissolved As (125 µg/L to 1,860 µg/L) and dissolved Co (40 µg/L to 1,040 µg/L) that exceed applicable CSR standards for the protection of marine aquatic life. However, water quality in monitoring wells, located more than 30 m inland of the high tide level and screened in tailings, meet the applicable CSR groundwater quality standards. A clear understanding of the tailings mineralogy and the geochemical reactions that are causing the elevated As and Co concentrations in groundwater is required before remedial options can be developed and evaluated.

Concentrations of As and Co in soil, intertidal sediments and groundwater were measured along a profile, parallel to the groundwater flow path that extends from the uplands into the intertidal zone. Representative tailings samples from various depths and subject to different oxidation-reduction environments were collected, including the saturated and unsaturated zone in the uplands region, the beach, as well as the shallow and deep intertidal zones. These samples were analysed for mineralogy, total CSR metals, and shake flask extractable metals (using distilled water and seawater). Sequential extraction procedures were also undertaken to identify metal concentrations associated with the following fractions: F1 exchangeable, F2 carbonates, F3 bound to iron (Fe) and manganese (Mn) oxides, and F4 bound to organic matter.

The study concluded that trace (<0.01) sulphide mineralisation (arsenopyrite and cobaltite) associated with the skarn wall rock is the source of the As and Co. As and Co concentrations in the tailings are higher in the shallower tailings near the shoreline, which suggests higher concentrations of the sulphide mineralisation present in tailings deposited late in the mine life. Oxidation of these sulphide minerals in the shallow intertidal and near-shore zones is the likely cause of elevated As and Co concentrations in groundwater and intertidal porewater.

A conceptual site model was developed to assist in understanding the geochemical reactions governing the release of As and Co, and to allow assessment of the geochemical effects that could occur after implementation of various proposed remedial options. In the uplands freshwater zone, As and Co appear to be sufficiently adsorbed and sequestered by secondary Fe and Mn oxides or other secondary minerals that the groundwater quality standards are not exceeded. However, As and Co in the near-shore water table fluctuation zone are much higher, likely because the large daily water table fluctuations promote the oxidation and release of As and Co from the sulphide minerals present in the tailings and/or inhibit the attenuation potential for these contaminants.


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