Incorporating the Soil Gas Gradient Method and Functional Genes to Assess the Natural Source Zone Depletion at a...

An increasing number of studies have demonstrated that natural source zone depletion (NSZD) in the vadose zone accounts for the majority (90%~99%) of the natural attenuation of light non-aqueous phase liquid (LNAPL). Until now, 0.05 to 12 kg/a.m² NSZD rates at tens of petroleum LNAPL source zones have been determined in the middle or late evolution stage of LNAPL release, in which limited volatile organic compounds (VOCs) and methane (CH₄) were detected. NSZD rates are normally estimated by the gradient method, yet the associated functional microbial activity remains poorly investigated. Herein, the NSZD at an LNAPL-releasing site was studied using both soil gas gradient methods quantifying the O₂, CO₂, CH₄, and VOCs concentrations and molecular biology methods quantifying the abundance of the pmoA and mcrA genes. The results showed that the methanogenesis rates were around 4 to 40 kg/a.m². The values were greater than the rates calculated by the sum of CH₄ escaping (0.3~1.2 kg/a.m²) and O₂ consuming (3~13 kg/a.m²) or CO₂ generating rates (2~4 kg/a.m²), suggesting that the generated CH₄ was oxidized but not thoroughly to CO₂. The functional gene quantification also supported the indication of this process. Therefore, the NSZD rates at the site roughly equaled the methanogenesis rates (4~40 kg/a.m²), which were greater than most of the previously studied sites with a 90th percentile value of 4 kg/a.m². The study extended the current knowledge of the NSZD and has significant implications for LNAPL remediation management.