Surface water occurrence in river deltas is governed by precipitation, evaporation, and the influx and outflux of water to and from the delta. Although studies of changes in water occurrence have been conducted at large scales, precise detection of changes in water occurrence is missing for most important river deltas. We take the case of the endorheic Selenga River Delta in Russia and train an accurate classification and quantification of water occurrence in its domain. We utilize remotely sensed observations of the Landsat satellite imagery during the last 33 years and implement supervised classification to map the surface water extent and its changes between periods of 1987-2002 and 2003-2019. We find that water occurrence has decreased in the Delta, with seasonally inundated areas presenting more pronounced decreases in water occurrence than permanent water bodies. We show that the change in the surface runoff is the main driver of changes in the spatial patterns of surface water with R2 = 0.58, while changes in water level in the recipient Lake Baikal do not influence water occurrence in the Delta. Our results show that the shrinkage and expansion of the water surface reflect the change in the freshwater supply of the Delta, and the management of the Selenga River needs to consider the impact of changes on the water occurrence.

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 e.g. single wetlands, little is known about spatio-temporal characteristics of freshwater MSR from multiple locations within entire hydrological catchments. We here applied an isotopic fractionation (δ34S-values in SO42-) and 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%. 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 hydro-climatic characteristics, which e.g. can be supported by the here utilized isotopic fractionation and mixing scheme.

Abstract Introduction: Nowadays it is nebulous how much matter is delivered by the largest rivers into the Arctic ocean and how much sediments are eroded by bank and watershed erosion. Thus, these investigations were made for the two biggest watersheds of the Arctic Ocean (the Ob’ and Yenisei) and their components of the sediment budget estimated. Methods and results: 1.The fieldwork campaign allowed us to obtain characteristic models of the distribution of suspended and bed sediments in the estuaries of the Yenisei and Ob’ river 2.The method for estimating the instantaneous sediment yield is developed based on the author’s program for extrapolating ADCP measurement data (language R), water velocity and backscatter intensity. The average annual sediment runoff for the Ob’ is 63.5 Mt/year, for the Yenisei – 32.5 MT/year; 3.The watershed component of sediment runoff was estimated by the RUSLE, taking into account the trapping of sediments by reservoirs and lakes. For the Ob’ potential watershed is 85 MT/year, and 53.6 MT/year for the Yenisei 4.The Bank erosion was calculated based on GSWE, Arctic-DEM, and HYDROATLAS and GRWL databases for the downstream of rivers, which was 35.0 MT/year for the Ob and 21.9 MT/year for the Yenisei. 5.The total deposition of sediments was calculated as a difference between total sediment yield and total erosion in catchments. For the Ob’ total deposition was 56.5 MT/year, for the Yenisei is 43 MT/year. Discussion and conclusion The sediments deposited in the catchment area or in the bedforms can be eroded again by snowmelt and rainfall erosion in the catchment area or directly due to the erosion of the banks. The products of erosion gradually move to the end of the river, with the exception of the part of sediment trapped by reservoirs. For instance, for the Ob’, it is only 10% of the watershed erosion, and for the Yenisei, it is 17% of eroded soil on the catchment. According to the calculations, bank erosion for both downstream is less than the watershed component of sediment runoff. The fact that these components are comparable and it gives some progress in solving the still unexplored question of the role of the bank and watershed components in the origin of sediment runoff of large rivers. Acknowledgments This work powered by the grant №18-05-60219 of the Russian Foundation for Basic Research(RFBR)