Waterborne pollution from mining in cold climates : Potential catchment-scale immobilization of substances by microbial sulfate reduction (MSR)

Abstract: Waterborne pollution from mining is impacting groundwater and surface water resources in many regions of the world. Main problems include acidification and high levels of dissolved toxic metals that adversely affect humans and ecosystems. Over the past millennium, mineral extraction has left behind vast amounts of waste rock, tailings and exposed rocks across landscapes that, in contact with air and water, risk generating acid mine drainage (AMD). In comparison with large-scale mining sites, the impacts of the numerous abandoned small-scale mines have received limited attention in the scientific literature, in particular in the Arctic region. Furthermore, whereas the immobilization and retardation of toxic substances through sorption and (chemical) precipitation have been relatively well investigated, less is known about the potential impact of microbial processes on the large-scale transport and retardation of AMD. Main objectives of this thesis are to improve the understanding of contributions from abandoned small mines to the waterborne mining pollution, and to determine how the spreading of AMD via ground- and surface water may be mitigated on catchment scales by microbial sulfate reduction (MSR), which is a process that transforms sulfate into sulfide and facilitates metal precipitation from the aqueous solution. Multiple field measurement campaigns were conducted in Arctic Fennoscandia to evaluate the water quality downstream of mining sites, and a data-driven sulfur isotopic fractionation and mixing scheme was developed to quantify field-scale MSR. Results showed that small abandoned mines could contribute disproportionately to downstream water pollution, as compared with larger mines. Copper mass flows in a stream passing the abandoned Nautanen mines (northern Sweden) was for instance found to be 450 kg/year one century after mine closure. Furthermore, across five study areas (both mining-impacted and reference catchments) spanning geographically from southern Sweden to the Kola Peninsula (Russia), MSR was quantified as the percent reduction in sulfate concentration, showing within-catchment MSR magnitudes of 0 to 79%, between-catchment magnitudes of 2 to 28%, and a catchment-average of 13%. The overall magnitude of catchment-scale MSR was found to correspond relatively well with the presence of landscape elements that provided favorable conditions for sulfate-reducing microorganisms (SRM), such as forest providing organic material and wetland/lakes providing anoxic conditions which are both needed for the SRM. MSR has previously been neglected in freshwater systems due to assumed unsuitable conditions, however the results from this thesis have shown, for the first time, that MSR can in fact be wide-spread across landscapes. This opens the possibility of utilizing MSR as a nature-based solution for AMD by further enhancing favorable conditions for SRM. Moreover, MSR has not been accounted for in quantifications of large-scale pyrite weathering, which in presence of wide-spread MSR may be underestimated. This can have consequences for the global sulfur cycle as well as the carbon cycle, e.g., since pyrite weathering contributes with CO2-releases to the ocean-atmosphere system.