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.

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.

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Microbial sulfate reduction (MSR) has increasingly been investigated for its potential to immobilize metals and reduce their bioavailability while also increasing the pH of acid mine drainage (AMD; e.g., Nielsen et al., 2018). The process involves microbes (bacteria and archaea) converting sulfate into sulfide that together with toxic dissolved metals precipitate into less mobile forms. Laboratory bioreactor experiments on MSR show a metal retention of 70% or more under favorable conditions (e.g., Sinharoy et al., 2020;Zhang & Wang, 2016). The activity depends on several factors, such as (bioavailable) carbon and sulfate supply, oxygen level, pH, and temperature (Xu & Chen, 2020). MSR has also been observed in the field at certain locations and time periods, for example, in individual wetlands or near tailing deposits at particular points of time (Mandernack et al., 2000;Praharaj & Fortin, 2004). Recently, Fischer, Jarsjö, et al. (2022) additionally showed evidence of ongoing and considerable MSR in multiple locations (so-called "hot spots") within an AMD-impacted catchment (Imetjoki, Northern Sweden), which is essential if MSR is to be used as an effective mitigation solution for spatially extensive mining sites and their downstream regions. However, large knowledge gaps remain regarding catchment-scale MSR in freshwater systems, where specific catchment and seasonal conditions could differ substantially from site to site. It is therefore not known to what extent MSR more generally could provide a basis for viable bioremediation, for instance, as part of nature-based solutions for sites impacted by (acid) mine drainage across the world. This includes the Arctic, which counts as one of the world's larger mining regions with numerous examples of largescale mine drainage development, and where cold conditions and pronounced seasonality may hamper the activity of freshwater sulfur-reducing microbes (SRM).Current evidence shows that point locations which are relatively favorable for MSR contain soil and sediments with sufficiently high organic matter content to support the metabolism of SRM, and that they are associated with