Effects of Water Table Fluctuations on Natural Source Zone Depletion of Petroleum Hydrocarbons in Contaminated Soils

In soils contaminated with petroleum hydrocarbons (PHCs), water table fluctuations (WTFs) affect the kinetics of PHC biodegradation and the generation and transport of carbon dioxide (CO₂) and methane (CH₄). Thus, understanding the impacts of WTFs on natural source zone depletion processes is critical for environmental risk assessment and the design of soil remediation strategies. In this study, a 300 day-long column experiment was conducted to simulate the effects of water table fluctuations on the aerobic and anaerobic PHC biodegradation pathways and rates. Eight columns were each filled with 45 cm of soil from undisturbed cores collected at a site formerly contaminated with PHCs. Four columns simulating fluctuating water table conditions were subjected to three successive 6-week cycles of drainage and imbibition. The remaining four columns remained fully saturated over the period of the experiment, simulating a static water table. Except for the controls, the columns received injections of ethanol or ethanol plus naphthalene after 111 days of pre-equilibration. Over the duration of the experiment, soil moisture, soil surface CO₂ and CH₄ effluxes, dissolved CO₂ and CH₄ concentrations, δ¹³C compositions of CO₂ and CH₄, dissolved naphthalene concentrations, and ancillary geochemical parameters were monitored. The remaining naphthalene depth distributions in the soil columns were also measured at the end of the experiment. A reactive transport model representing 13 biogeochemical reaction pathways was verified against the acquired data. The experimental and modeling results confirmed that the prevailing pathway generating CH₄ shifted from hydrogen-based to acetate-based methanogenesis in the ethanol and ethanol plus naphthalene spiked columns, while CH₄ oxidation played a key role in controlling the CH₄ efflux during the drainage periods. Compared to the static water table columns, the WTF columns exhibited significantly faster naphthalene attenuation while the cumulative CO₂ and CH₄ effluxes were about twice as high. These observations were attributed to the periodic incursion of air during the WTFs, which increased the porewater-air interface area for gas transfer while also accelerating the aerobic degradation of soil organic matter and naphthalene. Overall, our study advances the quantitative modeling of the biogeochemical reaction network in PHC contaminated soils under WTFs, including the role of methanogenic pathways.