Mehdi Ramezanzadeh, Stephanie Slowinski,Fereidoun Rezanezhad, Kathleen Murr, Christina Lam, Christina Smeaton, Clement Alibert, Marianne Vandergriendt and Philippe Van Cappellen (2022). Impacts of Freeze-Thaw Cycles on Methanogenic Toluene Biodegradation: Experiment and Numerical Simulation. Proceedings of the GWF Annual Open Science Meeting, May 16-18, 2022.

In hydrocarbon-contaminated soils in cold regions , freeze-thaw cycles (FTCs) modulate the biogeochemical and physical processes controlling petroleum hydrocarbons (PHCs) biodegradation and the generation of associated by-products methane (CH4) and carbon dioxide (CO2). Thus, understanding the effects of FTCs on the soil biodegradation of PHCs is critical for the environmental risk assessment and the design of remediation strategies for contaminated soils in cold regions. In this study, we developed a diffusion-reaction model that accounts for the effects of FTCs on methanogenic toluene biodegradation. The model is verified against data generated in a 200 day-long batch experiment with soil collected from a PHC contaminated site in Canada. The fully saturated soil was exposed to successive 4-week FTCs under anoxic conditions with temperatures fluctuating between -10°C and +15°C. We measured the headspace concentrations and 13C compositions of CH4 and CO2, and analyzed the porewater for acetate, sulfate, dissolved organic and inorganic carbon, and toluene concentrations. The numerical model represents solute diffusion, volatilization, sorption, as well as a reaction network of 14 biogeochemical processes. The model successfully simulates the soil porewater and headspace concentration time series data by representing the temperature dependencies of microbial reaction and gas diffusion rates during FTCs. According to the model results, the observed increases in the headspace concentrations of CH4 and CO2 by 87% and 136%, respectively, following toluene addition are explained by toluene fermentation and subsequent methanogenesis reactions. The experimental results and numerical simulations both confirm that methanogenic degradation in anoxic soil is the dominant toluene attenuation mechanism, representing 74% of the attenuation, with sorption contributing to 11%, and evaporation contributing to 15%. Also, the model-predicted contribution of acetate-based methanogenesis to total produced CH4 agrees with that derived from the 13C isotope data. The freezing-induced soil matrix organic carbon release is considered as an important process causing DOC increase following each freezing period according to the calculations of carbon balance and SUVA index. The simulation results of a no FTC scenario indicate that, in the absence of FTCs, CO2 and CH4 emissions decrease by 29% and 26%, respectively, and that toluene is biodegraded 23% faster than in FTC scenario. Given its ability to represent the dominant processes controlling CH4 and CO2 fluxes and porewater chemical changes, our modelling approach can be used to simulate the sensitivity of soil biodegradation processes to FTC frequency and duration driven by temperature fluctuations.

This research aims to investigate the effects of freeze-thaw cycles on the biodegradation processes, that is critical for the environmental risk assessment and the design of remediation strategies for contaminated environments in cold regions.

AOSM2022 First Author: Mehdi Ramezanzadeh Additional Authors: Stephanie Slowinski (1),Fereidoun Rezanezhad (1), Kathleen Murr (1), Christina Lam (1), Christina Smeaton (2), ClementAlibert (1), Marianne Vandergriendt (1) and Philippe Van Cappellen (1) (1) University of Waterloo,(2) Memorial University of Newfoundland (Grenfell Campus)