Effects of water table fluctuations on naphthalene degradation: Soil column experiment and modeling

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 (CO 2 ) and methane (CH 4 ). In this study, a 300‐day‐long column experiment was conducted to simulate the effects of WTFs on the aerobic and anaerobic PHC biodegradation pathways. Eight columns were each filled with 45 cm of PHC‐contaminated soil cores. Four columns simulating WTFs were subjected to cycles of drainage and imbibition. The remaining columns remained fully saturated, 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, surface CO 2 and CH 4 effluxes, dissolved concentrations and δ 13 C compositions of CO 2 and CH 4 , dissolved naphthalene concentrations, and geochemical parameters were monitored. 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 CH 4 shifted from hydrogen‐based to acetate‐based methanogenesis in the ethanol and ethanol‐plus‐naphthalene‐spiked columns, while CH 4 oxidation played a key role in controlling the CH 4 efflux during the drainage periods. The WTF columns exhibited significantly faster naphthalene attenuation, while the cumulative CO 2 and CH 4 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.