Abstract
Emissions of the greenhouse gas CH4, which is often produced in contaminated aquifers, are reduced or eliminated by microbial CH4 oxidation in the overlying vadose zone. The aim of this field study was to estimate kinetic parameters and isotope fractionation factors for CH4 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 CH4 injection concentrations ranging from 17 to 80 mL L-1 were performed. An apparent V-max of 0.70 +/- 0.15 mmol CH4 (L soil air)(-1) h(-1) and an apparent K-m of 0.28 +/- 0.09 mmol CH4 (L soil air)(-1) was estimated for CH4 oxidation at 2.7 m depth, close to the groundwater table. At 1.1 m depth, K-m (0.13 +/- 0.02 mmol CH4 (L soil air)(-1)) was in a similar range, but Vmax (0.076 +/- 0.006 mmol CH4 (L soil air)(-1) h(-1)) was an order of magnitude lower. At 2.7 m, apparent first-order rate constants determined from a CH4 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 Vmax much higher than the in situ CH4 oxidation rate prior to GPPTs indicated a high buffer capacity for CH4. At both depths, known methanotrophic species affiliated with Methylosarcina and Methylocystis were detected by cloning and sequencing. Apparent stable carbon isotope fractionation factors a for CH4 oxidation determined during GPPTs ranged from 1.006 to 1.032. Variability was likely due to differences in methanotrophic activity and CH4 availability leading to different degrees of mass transfer limitation. This complicates the use of stable isotopes as an independent quantification method.