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. 2022 Dec 30;13(1):114.
doi: 10.3390/life13010114.

Incorporating the Soil Gas Gradient Method and Functional Genes to Assess the Natural Source Zone Depletion at a Petroleum-Hydrocarbon-Contaminated Site of a Purification Plant in Northwest China

Zhuo Ning  1   2 , Yizhi Sheng  3 , Caijuan Guo  1   2 , Shuaiwei Wang  1   2 , Shuai Yang  4 , Min Zhang  1   2
Affiliations

Incorporating the Soil Gas Gradient Method and Functional Genes to Assess the Natural Source Zone Depletion at a Petroleum-Hydrocarbon-Contaminated Site of a Purification Plant in Northwest China

Zhuo Ning et al. Life (Basel). .

Abstract

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.

Keywords: functional gene; gradient method; natural source zone depletion (NSZD); petroleum hydrocarbon contaminated site.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 2
Figure 2
Schematic plot of the soil gas measurement borehole.
Figure 1
Figure 1
The characteristics of the study area and LNAPL contamination. Plot (a) shows the sampling location, groundwater flow direction, and the TPH contaminant concentrations (as represented by the concentration contour plot). The red rectangle near the storage tanks (i.e., block A, B, and C) represents the area where NSZD was evaluated. The three red triangles in the non-contaminated area, north of the storage tanks, represent the background sampling points. Plot (b) shows the vertical distribution of the LNAPLs and contamination plume.
Figure 3
Figure 3
Vertical profile of soil gas O2, CO2, CH4, and VOCs calculated in the study area. Some missed data points at 70~90cm indicated that the concentrations were greater than the upper limits of detection. (a–c) Plots represent the A, B, and C blocks in the contamination source area, respectively.
Figure 4
Figure 4
Vertical profile of pmoA and mcrA gene abundance in the study area. The (a–c) plots represent the A, B, and C blocks in the contamination source area, respectively.
Figure 5
Figure 5
The relationships between pmoA gene abundance and CH4 concentration.
See this image and copyright information in PMC

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