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.

Millions of contaminated sites worldwide need to be remediated to protect the environment and human health. Although numerous remediation technologies have been developed, selecting optimal technologies is challenging. Several multiple criteria decision-making methods for screening the optimal remediation technology have been proposed, but they mostly focus on a specific area rather than the whole contaminated site. In recent years, the “contamination source control—process blocking—in situ remediation” technology mix model has gradually gained high appreciation. Nevertheless, the screening of technologies within each chain of this model relies heavily on arbitrary personal experience. To avoid such arbitrariness, a petroleum-contaminated site containing light non-aqueous phase liquids (LNAPLs) was used as an example, and a scientific screening and combination procedure was developed in this study by considering the distribution characteristics of contaminants. Through the procedure, a technology mix, which includes institutional control, risk monitoring, emergency response, multiphase extraction, interception ditch, monitoring of natural attenuation, hydrodynamic control, as well as some alternative technologies, was found, aiming at different locations and strata. The clear spatial relationship concept promises to enhance the effectiveness of contaminated site remediation. The proposed method only gave us a technical framework and should be tested and enriched in future studies.