Shallow aquifers as an element of methane biogeochemical cycle in West Siberia

Methane seeps – cm-sized holes and craters with an active release of gas bubbles and water – were recently found within boreal floodplains of Western Siberia. Seep-abundant areas are hot spots of methane emission; fluxes reach hundreds of mgCH₄ m^-2 h^-1 from individual seeps. Methane in seeps is of modern primary microbial origin. We suggest that it is produced in raised bogs covering 30-40% of the region area. Lateral methane transport from raised bogs to seeps occurs through shallow aquifers where dissolved methane concentration reaches tens of percent saturation. The objectives of this study were to assess: i) a spatial distribution and key drivers of CH₄ accumulation in groundwater on a regional scale, ii) methane origin, iii) groundwater discharge to rivers, and iv) oxidation of methane in groundwater.

The study sites in West Siberia were located within the latitudinal and longitudinal transects representing tundra, taiga and steppe biomes: 1000 km in north-south (65.5°-56.4° N) and 700 km in east-west (63.3°-76.5° E) directions. Ground water samples (N = 25) were collected in active wells of municipal water supply stations with depths varying from 25 to 300 meters. Dissolved hydrocarbons and carbon dioxide in groundwater samples were obtained by headspace technique. Along with determination of C-containing gas concentrations in samples we measured their isotopic composition (δ¹³С in СО₂, δ¹³С and δD in СН₄ and ¹⁴C in СН₄). In parallel, groundwaters were measured for pH, electrical conductivity, inorganic and organic carbon concentrations, water stable isotope ratios (δD and δ¹⁸О), ³H content and concentrations of major anions and cations. To estimate an input of groundwaters to a river network of the study region we analyzed the dissolved methane concentrations and water stable isotope ratios (δD and δ¹⁸О) in different order rivers (N = 60).

Groundwaters of shallow aquifers in forest-tundra, middle and southern taiga of Western Siberia are highly saturated with methane (average value 38.8% of the full saturation or 827 μmol l^-1), while in the northern taiga and forest-steppe the dissolved methane concentration did not exceed 0.7% of the full saturation (15 μmol l^-1). The key driver was the site location in relation to bogs: groundwater CH₄ concentration was higher in aquifers, that are located downstream of wetland-dominated areas (along the groundwater flow direction). Values of δ¹³С, δDand ¹⁴С in СН₄ and δ¹³С in СО₂ indicated the modern primary microbial origin of methane and a negligible methane oxidation in aquifers. Dissolved organic carbon concentration positively correlated with dissolved methane concentration (R² = 0.55, p < 0.0001 for a power fit). Higher concentrations of dissolved organic carbon and methane corresponded to waters that are more depleted in δD and δ¹⁸О. Groundwater contributed to at least 20% of river discharge throughout the region during the summer low-flow period. Thus, shallow aquifers of West Siberia provide an effective pathway for methane migration from raised bogs to rivers and, further, to the atmosphere.

This study was supported by a grant of the Russian Science Foundation (No. 19-77-10074).