Stress-tolerant bioremediation strategy for cold-climate sites: laboratory, modelling, and field studies for extending petroleum hydrocarbon biodegradation to seasonally freezing and frozen contaminated soil phases

Bioremediation of petroleum hydrocarbon-contaminated soils in cold climates is challenging due to low temperatures, limited field accessibility, and low microbial activity. Conventional bioremediation practices have focused on short summers in cold climates, but the feasibility of extending hydrocarbon biodegradation to sub-zero temperatures has not been extensively explored. How biodegradation can be enhanced in freezing and frozen soils is unknown. Through laboratory, modelling, and field studies, this research investigated the enhancement of hydrocarbon biodegradation during phase changes in cold soils subjected to seasonal freeze-thaw conditions. In a pilot-scale biopile (3.5 tonnes) of nutrient-amended hydrocarbon-contaminated fine-grained soils operated over the winter at a cold-climate site, hydrocarbon biodegradation was enhanced during seasonal freezing and thawing in the field. The retention of unfrozen water was greater in the treated vs. untreated biopile. The removals of F2 (C10–C16) and F3 (C16–C34) hydrocarbons in the treated biopile were 57 and 58%, respectively, of which 26 and 39% were achieved between November and early March. To delineate the biodegradation activity below 0 °C, the generalized respiration model (GRESP) was modified to cover the soil phase change in hydrocarbon-contaminated northern soils during bioremediation. The Michaelis-Menten equation was modified and incorporated into GRESP to produce a new URESP model. Using soil temperature and unfrozen water content in frozen contaminated soil, URESP accurately estimated soil respiration related to hydrocarbon utilization. How soil amendments manipulate unfrozen water retention was also assessed and linked to hydrocarbon biodegradation during the freezing (4 to -5 °C) and frozen (-5 to -10 °C) soil phases. Nutrients significantly affected the freezing phase by inducing a freezing-point depression, shifting microbial communities, and stimulating biodegradation activity. Unfrozen water content had a significant effect during the frozen phase, which correlated with enhanced biodegradation. F3 hydrocarbons were significantly removed during the freezing and subsequent frozen phases (22–37%), and F3 biodegradation below 0 °C was positively correlated to the α-value (soil freezing index) of conventional soil freezing characteristic curves indexed for various soil types. This research informs the effective management and remediation of seasonally freezing and thawing petroleum hydrocarbon-contaminated sites in northern environments.