Microbial sulfate reduction (MSR), which transforms sulfate into sulfide through the consumption of organic matter, is an integral part of sulfur and carbon cycling. Yet, the knowledge on MSR magnitudes is limited and mostly restricted to snap-shot conditions in specific surface water bodies. Potential impacts of MSR have consequently been unaccounted for, e.g., in regional or global weathering budgets. Here, we synthesize results from previous studies on sulfur isotope dynamics in stream water samples and apply a sulfur isotopic fractionation and mixing scheme combined with Monte Carlo simulations to derive MSR in entire hydrological catchments. This allowed comparison of magnitudes both within and between five study areas located between southern Sweden and the Kola Peninsula, Russia. Our results showed that the freshwater MSR ranged from 0 to 79 % (interquartile range of 19 percentage units) locally within the catchments, with average values from 2 to 28 % between the catchments, displaying a non-negligible catchment-average value of 13 %. The combined abundance or deficiency of several landscape elements (e.g., the areal percentage of forest and lakes/wetlands) were found to indicate relatively well whether or not catchment-scale MSR would be high. A regression analysis showed specifically that average slope was the individual element that best reflected the MSR magnitude, both at sub-catchment scale and between the different study areas. However, the regression results of individual parameters were generally weak. The MSR-values additionally showed differences between seasons, in particular in wetland/lake dominated catchments. Here MSR was high during the spring flood, which is consistent with the mobilization of water that under low-flow winter periods have developed the needed anoxic conditions for sulfate-reducing microorganisms. This study presents for the first time compelling evidence from multiple catchments of wide-spread MSR at levels slightly above 10 %, implying that the terrestrial pyrite oxidation may be underestimated in global weathering budgets.

Laboratory experiments and point observations, for instance in wetlands, have shown evidence that microbial sulfate reduction (MSR) can lower sulfate and toxic metal concentrations in acid mine drainage (AMD). We here hypothesize that MSR can impact the fate of AMD in entire catchments. To test this, we developed a sulfur isotope fractionation and mass-balance method, and applied it at multiple locations in the catchment of an abandoned copper mine (Nautanen, northern Sweden). Results showed that MSR caused considerable, catchment-scale immobilization of sulfur corresponding to a retention of 27 ± 15% under unfrozen conditions in the summer season, with local values ranging between 13 ± 10% and 53 ± 18%. Present evidence of extensive MSR in Nautanen, together with previous evidence of local MSR occurring under many different conditions, suggest that field-scale MSR is most likely important also at other AMD sites, where retention of AMD may be enhanced through nature-based solutions. More generally, the developed isotope fractionation analysis scheme provides a relatively simple tool for quantification of spatio-temporal trends in MSR, answering to the emerging need of pollution control from cumulative anthropogenic pressures in the landscape, where strategies taking advantage of MSR can provide viable options.

An emerging solution in mine waste remediation is the use of biological processes, such as microbial sulfate reduction (MSR), to immobilize metals, reducing their bioavailability and buffering the pH of acid mine drainage. Apart from laboratory tests and local observations of natural MSR in, for example, single wetlands, little is known about spatiotemporal characteristics of freshwater MSR from multiple locations within entire hydrological catchments. We here applied an isotopic fractionation (δ34S values in SO42−) and a Monte Carlo-based mixing analysis scheme to detect MSR and its variation across two major mining regions (Imetjoki, Sweden and Khibiny, Russia) in the Arctic part of Europe under different seasonal conditions. Results indicate a range of catchment-scale MSR values in the Arctic of ∼5%–20% where the low end of the range was associated with the non-vegetated, mountainous terrain of the Khibiny catchment, having low levels of dissolved organic carbon (DOC). The high end of the range was related to vegetated conditions provided by the Imetjoki catchment that also contains wetlands, lakes, and local aquifers. These prolong hydrological residence times and support MSR hot spots reaching values of ∼40%. The present results additionally show evidence of MSR persistence over different seasons, indicating large potential, even under relatively cold conditions, of using MSR as part of nature-based solutions to mitigate adverse impacts of (acid) mine drainage. The results call for more detailed investigations regarding potential field-scale correlations between MSR and individual landscape and hydroclimatic characteristics, which, for example, can be supported by the isotopic fractionation and mixing scheme utilized here.

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.

Pollution from small historical mining sites is usually overlooked, in contrast to larger ones. Especially in the Arctic, knowledge gaps remain regarding the long-term mine waste impacts, such as metal leakage, on water quality. We study the small copper (Cu) mines of Nautanen, northern Sweden, which had been in operation for only six years when abandoned approximately 110 years ago in 1908. Measurements from field campaigns in 2017 are compared to synthesized historical measurement data from 1993 to 2014, and our results show that concentrations of Cu, Zn, and Cd on-site as well as downstream from the mining site are order(s) of magnitude higher than the local background values. This is despite the small scale of the Nautanen mining site, the short duration of operation, and the long time since closure. Considering the small amount of waste produced at Nautanen, the metal loads from Nautanen are still surprisingly high compared to the metal loads from larger mines. We argue that disproportionately large amounts of metals may be added to surface water systems from the numerous small abandoned mining sites. Such pollution loads need to be accounted for in sustainable assessments of total pollutant pressures in the relatively vulnerable Arctic environment.

The Brahmaputra River in South Asia carries one of the world's highest sediment loads, and the sediment transport dynamics strongly affect the region's ecology and agriculture. However, present understanding of sediment conditions and dynamics is hindered by limited access to hydrological and geomorphological data, which impacts predictive models needed in management. We here synthesize reported peer-reviewed data relevant to sediment transport and perform a sensitivity analysis to identify sensitive and uncertain parameters, using the one-dimensional model HEC-RAS, considering both present and future climatic conditions. Results showed that there is considerable uncertainty in openly available estimates (260-720 Mt yr(-1)) of the annual sediment load for the Brahmaputra River at its downstream Bahadurabad gauging station (Bangladesh). This may aggravate scientific impact studies of planned power plant and reservoir construction in the region, as well as more general effects of ongoing land use change and climate change. We found that data scarcity on sediment grain size distribution, water discharge, and Manning's roughness coefficient had the strongest controls on the modelled sediment load. However, despite uncertainty in absolute loads, we showed that predicted relative changes, including a future increase in sediment load by about 40 % at Bahadurabad by 2075-2100, were consistent across multiple model simulations. Nevertheless, for the future scenarios we found that parameter uncertainty almost doubled for water discharge and river geometry, highlighting that improved information on these parameters could greatly advance the abilities to predict and manage current and future sediment dynamics in the Brahmaputra river basin.

Microbial sulfate reduction (MSR), which transforms sulfate into sulfide through the consumption of organic matter, plays an integral part in sulfur and carbon cycling. Yet, its magnitude beyond snap-shot conditions in specific surface water systems such as individual wetlands or lakes is with few exceptions unknown. We use stream water samples and a sulfur isotopic fractionation and mixing scheme to derive MSR at relatively large scales, both within and between five study areas located between southern Sweden and the Kola Peninsula, Russia. Results showed that the freshwater MSR ranged from 0 to 79% within the catchments and from 2 to 28% between the catchments, displaying a catchment-average value of 13%. The MSR-values showed differences between seasons, in particular in wetland/lake catchments where they were high during the spring flood, which is consistent with the mobilization of water that under low-flow winter periods can have developed anoxic conditions needed by the sulfate-reducing microorganisms (SRM). The combined abundance or deficiency of several landscape elements were found to indicate relatively well whether or not catchment-scale MSR would be high, such as the percentage of forest cover that can support MSR by the provision of organic matter to the SRM, and the areal percentage of lakes/wetlands providing longer hydrological residence times and the anoxic conditions needed by the SRM. This study, for the first time, shows compelling evidence in multiple catchments of MSR at levels around 10%, which is unaccounted for in global weathering budgets, implying for instance that the terrestrial pyrite oxidation may be underestimated.