Metal speciation and bioavailability in acid mine drainage from a high Arctic coal mine waste rock pile: Temporal variations assessed through high-resolution water sampling, geochemical modelling and DGT

This study investigates acid mine drainage from an abandoned coal mine waste rock pile in Svalbard (78° N). A trench was dug to collect all water from the pile and water sampled daily to determine temporal variations in runoff chemistry and to calculate the release of dissolved metals (filtered to < 0.45 μm in size) from the pile during the entire period with running water from June to September 2005. Dissolved and ‘labile’ fractions of ‘total’ Al, Mn and Cu were quantified in July and August 2006 by using diffusive gradients in thin films (DGT). Further, metal speciation was calculated for both 2005 and 2006 using the chemical equilibrium model WHAM VI. Predictions of the dissolved inorganic metal fraction using WHAM VI agreed fairly well with in situ measurements of ‘labile’ Al, Mn and Cu using DGT. However, for Al, the model overestimated the observed degree of organic complexation at the highest pH. Observations and model results for 2005 indicate that the highest dissolved metal concentrations, lowest pH, and highest inorganic (readily ‘bioavailable’) metal fractions occurred during an initial two-week ‘spring flush’. In fact, more than 96% of all dissolved metals were predicted to be ‘bioavailable’ during that period, which is considered affected by a release of pollutants slowly generated and accumulated within the pile during winter, but released upon thawing. Later in the summer period, both dissolved metal concentrations and the fractions considered bioavailable decreased markedly. This study shows that collection/treatment of spring runoff is essential to reduce the environmental impact at the study site. That conclusion is thought to have wider applications and may include waste depositions containing other types of material deposited in cold regions.