Push-pull test for in situ determination of microbial metabolic activities: denitrification and methanogenesis

Abstract

Quantitative information on subsurface microbial processes is needed for many applications, but information obtained by existing methods can be difficult to interpret. The goal of this project was to develop a single well, 'push-pull' test for the determination of in situ microbial metabolic activities. The method consists of an injection of a test solution into an existing monitoring well followed by the extraction of the groundwater/test solution mixture from the same well. The test solution consists of a tracer and one or more reactive solutes selected to investigate the activity of a specific microbial system. During the extraction phase, the concentrations of all components of the test solution and potential reaction products are measured and used to develop breakthrough curves. A series of tests were used to investigate the processes of denitrification and methanogenesis by injecting solutions with nitrate, nitrite, or dissolved hydrogen gas in a petroleum contaminated, anaerobic zone of an unconfined, alluvial aquifer and in an uncontaminated, aerobic zone of the same aquifer located directly upgradient of the contaminated zone. Nitrate and nitrite injections yielded utilization rates that were approximately 2.5 times greater in the contaminated zone (than in the uncontaminated zone) where 80% of the injected nitrate mass and 90% of the injected nitrite mass were utilized. During the nitrate injection tests, trace amounts of produced nitrite were detected. The hydrogen injection test resulted in a 60% loss of hydrogen with no detectable methane production in the contaminated well. The push-pull field procedure developed in this project was shown to be capable of determining and quantifying microbial metabolic activity in situ.