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Dissolved Hydrogen Dynamics Associated with Emulsified Vegetable Oil Bioremediation of Chloroethenes

Emulsified vegetable oil (EVO) is widely used as a fermentable substrate to enhance the reductive dechlorination of chlorinated ethenes (CEs) in groundwater systems. The fermentation of EVO by naturally occurring bacteria produces molecular hydrogen (H2) which acts as an electron donor driving microbially mediated reductive dechlorination. While dissolved H2 drives much of the dechlorination associated with CE bioremediation, the dynamics of H2 production and consumption associated with EVO addition to groundwater systems is seldom documented. The present study shows how H2 concentrations changed over a 4‐year period following EVO addition to a sandy coastal plain aquifer underlying Naval Air Station Pensacola, Florida, USA. Prior to EVO addition, H2 concentrations at the site were in the range characteristic of Fe(III)‐reducing conditions (0.2–0.6 nM). Following EVO addition, H2 concentrations increased exponentially, peaking at approximately 25,000 nM. Hydrogen concentrations then began decreasing exponentially, and by 4 years after EVO addition had stabilized at 4.0 nM. That pattern suggests symbiotic cross‐feeding between fermentative and respirative microbial populations resulting in a Gaussian rise and fall of H2 concentrations. That, in turn, suggests while EVO biostimulation can temporarily increase H2 concentrations to very high levels, those higher concentrations are unlikely to be sustained indefinitely.