Assessment of microbial methane oxidation above a petroleum‐contaminated aquifer using a combination of in situ techniques

Emissions of the greenhouse gas CH 4 , which is often produced in contaminated aquifers, are reduced or eliminated by microbial CH 4 oxidation in the overlying vadose zone. The aim of this field study was to estimate kinetic parameters and isotope fractionation factors for CH 4 oxidation in situ in the vadose zone above a methanogenic aquifer in Studen, Switzerland, and to characterize the involved methanotrophic communities. To quantify kinetic parameters, several field tests, so‐called gas push‐pull tests (GPPTs), with CH 4 injection concentrations ranging from 17 to 80 mL L −1 were performed. An apparent V max of 0.70 ± 0.15 mmol CH 4 (L soil air) −1 h −1 and an apparent K m of 0.28 ± 0.09 mmol CH 4 (L soil air) −1 was estimated for CH 4 oxidation at 2.7 m depth, close to the groundwater table. At 1.1 m depth, K m (0.13 ± 0.02 mmol CH 4 (L soil air) −1 ) was in a similar range, but V max (0.076 ± 0.006 mmol CH 4 (L soil air) −1 h −1 ) was an order of magnitude lower. At 2.7 m, apparent first‐order rate constants determined from a CH 4 gas profile (1.9 h −1 ) and from a single GPPT (2.0 ± 0.03 h −1 ) were in good agreement. Above the groundwater table, a V max much higher than the in situ CH 4 oxidation rate prior to GPPTs indicated a high buffer capacity for CH 4 . At both depths, known methanotrophic species affiliated with Methylosarcina and Methylocystis were detected by cloning and sequencing. Apparent stable carbon isotope fractionation factors α for CH 4 oxidation determined during GPPTs ranged from 1.006 to 1.032. Variability was likely due to differences in methanotrophic activity and CH 4 availability leading to different degrees of mass transfer limitation. This complicates the use of stable isotopes as an independent quantification method.