Infiltration of Sulfate to Enhance Sulfate-Reducing Biodegradation of Petroleum Hydrocarbons

The lack of sufficient electron acceptors, particularly sulfate, can limit the rate of biodegradation of petroleum hydrocarbons (PHCs). Hence, there is a growing interest by remediation practitioners to deliver sulfate to a PHC impacted saturated zone to enhance biodegradation. When shallow contamination is present in a relatively permeable aquifer and site constraints allow, a cost-effective approach is to apply sulfate on the ground surface. In this investigation a pilot-scale experiment was conducted to increase our understanding of the delivery of sulfate using a surface-based method and the resulting impact on a shallow PHC contaminated aquifer. A surficial infiltration pond positioned on the ground surface above a well-characterized residual PHC source zone was used to control sulfate dosing. A high-resolution network near the infiltration pond and downgradient of the source zone was employed to monitor relevant geochemical indicators and PHC concentrations. Compound-specific isotope analysis (CSIA) was used to identify biodegradation patterns and to investigate the occurrence of microbial sulfate reduction. Selected metabolites and reverse-transcriptase quantitative polymerase chain reaction analyses of expressed biodegradation genes (as mRNA) were also used to characterize the response of indigenous microorganisms (especially sulfate-reducing bacteria) to the added sulfate. Three sulfate application episodes (5000 L each) at various Na2SO4 concentrations were allowed to infiltrate under a constant hydraulic head. Although the applied sulfate solution was impacted by density-driven advection, detailed monitoring data indicated that the sulfate-enriched water mixed with upgradient groundwater as it migrated downward through the residual PHC zone and formed a co-mingled downgradient plume with the dissolved PHC compounds. The enrichment of delta S-34 of sulfate in conjunction with a decrease in sulfate concentration showed the occurrence of sulfate reduction due to the applied sulfate. Increased dissolved inorganic carbon (DIC) concentrations associated with a shift toward more depleted values of delta C-13 of DIC was indicative of an input of isotopically depleted DIC from biodegradation of PHCs. Despite fluctuations in benzene, toluene, and o-xylene (BTX) concentrations, the CSIA data for BTX showed that these compounds were biodegraded. The biomarker data provided supporting evidence that toluene and o-xylene were undergoing anaerobic biodegradation due to sulfate reduction. This study provides insight into factors controlling surface-based delivery of sulfate to shallow PHC impacted groundwater systems, and the value of isotopic and molecular-biological procedures to augment conventional monitoring tools.