Effect of Freeze–Thaw on Microbial Transformation of Humic Substances during Groundwater‒River Water Interaction

A study of the role of environmental factors (abiotic and microbiological) in the formation of groundwater (GW) quality in the river bank filtration zone under cyclic freeze–thaw (CFT) is presented. GW samples from the Tunguska field (the Amur region) taken in the fall period from different aquifer depths from wells located at various distances from the river bank filtration zone are used. We perform CFT in vitro in the presence of different carbon sources at two temperatures: from –18 to +4°C for slow thawing and from ‒18 to +23°С for rapid thawing. The activity of microbial communities (MCs) from GW and river water after CFT is confirmed by their cultivation on a readily available carbon source (peptone) and hard-to-mineralize sodium humate (HNa), as well as under conditions of their co-metabolism. The activity after CFT upon a thawing temperature of 4°С in the presence of HNa is maximal for MCs from a depth of 41 m from boreholes 1500 m away from the bank. Microorganisms from river water and GW from wells close to the bank are highly active at a thawing temperature of 23°С, regardless of the composition of carbon sources during the CFT period. The growth activity of MCs from wells distant from the bank at a thawing temperature of 23°С depends on the depth of water sampling and the presence of a readily available co-substrate. It is shown for the first time that the thawing temperature during CFT affects changes in the spectral characteristics of humic substances (HSs) as a result of their microbial transformation.