| dc.description.abstract | In subsurface environments contaminated by petroleum hydrocarbons (PHCs), the steep geochemical and redox gradients near the water table, the oxygen availability, moisture content, salinity, pH, nutrient concentrations and temperature, modulate microbial pathways and process rates that affect the fate of hydrocarbons. The reactive transport of PHCs is strongly controlled by hydrological and climatic forcings, including water table fluctuations (WTFs) and freeze-thaw cycles (FTCs), which cause large temporal variations in the local geochemical conditions and the distributions of temperature and soil water content which are key determinants of natural source zone depletion (NSZD) process rates. FTCs and WTFs modify the biogeochemical and physical processes controlling the biodegradation of PHCs and the associated generation of methane (CH4) and carbon dioxide (CO2). Therefore, understanding the impacts of FTCs and WTFs in PHC-contaminated soils and groundwater is critical for environmental risk assessment and natural attenuation of PHCs.
A diffusion-reaction model that accounts for the effects of FTCs on methanogenic toluene biodegradation was developed. The model is verified against data generated from a 215 day-long batch experiment with soil collected from a PHC-contaminated site in Ontario, Canada. The fully saturated soil was exposed to successive 4-week FTCs under anoxic conditions with temperatures fluctuating between -10°C and +15°C. The headspace gas for the concentrations and 13C isotope compositions of CH4 and CO2, and the porewater for pH, acetate, dissolved organic and inorganic carbon, and toluene were analyzed. The model represents solute diffusion, volatilization, and sorption, as well as a reaction network of 13 biogeochemical processes. The model successfully simulated the soil porewater and headspace concentration time series 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 due to toluene fermentation and subsequent methanogenesis reactions. The experimental results and model simulations both confirm that methanogenic degradation under anoxic soil conditions is the dominant reaction for toluene attenuation, representing 74% of the attenuation, with sorption contributing 11%, and volatilization contributing 15%. Also, the model-predicted contribution of acetate-based methanogenesis to the total produced CH4 agrees with that derived from the 13C isotope data. The freezing-induced soil matrix organic carbon release is considered an important process causing an increase in dissolved organic carbon (DOC) following each freezing period according to the calculations of carbon balance and specific UV absorbance (SUVA) index. The simulation results of a no FTC scenario indicate that, in the absence of FTCs, CO2 and CH4 generation would decrease by 29% and 26%, respectively, and that toluene would be biodegraded 23% faster than in the FTC scenario. Given its ability to represent the dominant processes controlling CH4 and CO2 fluxes and porewater chemical changes, this modeling approach can be used to simulate the sensitivity of soil biodegradation processes to FTC frequency and duration driven by temperature fluctuations in anoxic soil conditions.
A ten-month soil column experiment to simulate the effects of water table fluctuations on methanogenic PHCs biodegradation rates and pathways was conducted. Eight columns were filled with 45 cm of undisturbed soil core samples collected from a PHC-contaminated site in London, Ontario. Four columns simulating fluctuating water table conditions were subjected to three successive 3-week cycles of drainage and imbibition. In the remaining four columns, the soils remained saturated over the period of the experiment, simulating a static water table. The responses to the imposed water table fluctuations and ethanol/naphthalene injections were monitored by measuring soil surface CO2 and CH4 effluxes, dissolved CO2 and CH4 concentrations, depth-dependent moisture content, δ13C isotope composition of CO2 and CH4, DOC, dissolved inorganic carbon (DIC), and major anions at the end of each drainage-imbibition cycle. The results show that maximum CO2 and CH4 effluxes were up to 10 times higher during the drainage periods than during the imbibition periods due to the release of accumulated CO2 and CH4 and aerobic degradation. Also, the average dissolved CH4 concentration decreased by 29% during the drainage periods because of the release of CO2 and CH4, aerobic CH4 oxidation, and inhibition of methanogenesis in the presence of O2, while the average dissolved CO2 increased by 105% due to the oxidation of DOC and CH4. The results of δ13C for CO2 and CH4 show that the prevailing methanogenic pathway shifted from hydrogen-based methanogenesis to acetate-based methanogenesis in the ethanol/naphthalene spiked soils due to the increase in acetate concentrations. In the fluctuating columns, CH4 oxidation became the prevailing pathway controlling CH4 flux dynamics after the first drainage period. Moreover, naphthalene was consumed 29% faster in the fluctuating columns compared to the static soil columns. Both experiment and model demonstrate that there is a trade-off associated with water table fluctuations: lowering the water level can exacerbate global warming via more CO2 and CH4 effluxes, while this is effective for PHC attenuation. The results of this study shed new light on the role of soil drying and rewetting effects on methanogenic hydrocarbon degradation and CO2 and CH4 effluxes. | en |
