Theme: 06. Petroleum and Heavy Hydrocarbon Site Strategies

Topic Area: 6d. Natural Source Zone Depletion

Formatted Title:

Continuous Soil Redox Sensing and Microbiome Characterization for Monitoring NSZD and Enhanced NSZD

Background/Objectives:

Although bioremediation commonly has been applied to hydrocarbon plumes, application of bioremediation to light non-aqueous phase liquids (LNAPL)-containing zones historically has been considered questionable, in part because we have lacked tools to monitor processes in situ. Both n atural source zone depletion (NSZD) and enhanced NSZD (ENSZD) are attractive low-resource remedies for LNAPL-impacted source zones. Acceptance of NSZD or ENSZD remedies relies on a multiple-lines-of-evidence approach and requires effective monitoring. Emerging use of in situ ORP sensing shows promise to resolve spatial and temporal redox dynamics during NSZD or more active remedies. Further, next generation gene sequencing of total and active microbial communities provides insight redox micro-niches and potential for biodegradation with varying electron acceptor regimes. Thus, redox monitoring combined with next generation sequencing can provide insight into ongoing biogeochemical LNAPL degradation pathways and provides information regarding NSZD and ENSZD progress. Further understanding of soil redox dynamics and how these conditions relate to subsurface microbial activities and LNAPL degradation is needed.

Approach/Activities:

Columns containing LNAPL-impacted soils from a former petroleum refinery were used to evaluate the use of soil redox sensors to monitor performance of NSZD along with two known NSZD enhancements, biostimulation with periodic sulfate addition, or periodic air sparging. Columns containing varying levels of LNAPL were included in the study. Standard geochemical analyses were conducted. Soil microbiomes were analyzed at the final time point via cryogenic preservation and 16S rRNA gene amplicon sequencing of both DNA and RNA using Illumina technology. DNA analysis provided a view of the total microbial communities present, while RNA analysis provided a view of the microbes that were active due to the enhancements.

Results/Lessons Learned:

Expected relationships between continuous soil redox measurements and active microbial communities were observed. Further, continuous ORP data provided insights into spatial and temporal ORP dynamics within columns that supported improved understanding of the hydrocarbon biodegradation data. NSZD columns were at methanogenic redox and showed lesser biodegradation, while columns enhanced via addition of sulfate or air showed enhanced biodegradation. Periodic sulfate doses of 1.3+/- 0.6g to 0.7L of the soil column system resulted in transient redox increases in sulfate enhanced columns. Continuous ORP data revealed that columns sparged with periodic pulses of 25L of air to 0.7L of the soil column system failed to reach aerobic redox values. Likely improved biodegradation due to air addition resulted from alternative anaerobic processes (e.g., iron reduction due to air oxidizing reduced iron). Additionally, hydrocarbon data combined with microbiome analysis indicated that differences in LNAPL composition affect naphthalene degradation mechanisms when sulfate is the electron acceptor. Microbiome analysis revealed insights into how hydrocarbon concentration and composition impacted microbiome structure and activity within investigated redox ranges. Overall, the study suggested that combining continuous redox sensing with microbiome analysis can provide insights beyond those possible with either monitoring tool alone.

Menu

Jorge Rico