Real Time Measurement of Concentration and δ13C-CH4 in Water

A new, high temporal and spatial resolution, in-situ method for the fast and precise determination of methane (CH 4 ) concentration and its δ 13 C isotope value (δ 13 C-CH 4 ) in water was developed. Methane is extracted from water by the application of a vacuum to a membrane gas/liquid exchange module and analysed by a portable cavity ring-down spectroscopy (CRDS) analyser (Picarro® G2201-i). The system was calibrated for concentration and δ 13 C of CH 4 using synthetic water standards. Reference samples were analysed with gas chromatography (GC) and mass spectroscopy (GC-C-IRMS) via the headspace method. All results show good correlations for CH 4 concentration (R 2 = 0.994) and a measuring accuracy of 0.002 ppm (CH 4 concentration) and 0.6‰ (δ 13 C-CH 4 ) . Calculated response times τ were averaged, resulting in 13.5 s. This allows much faster analyses compared to previously published methods. This method was successfully tested in a field campaign at Lake Stechlin (Germany), which has a pronounced CH 4 peak at the thermocline in summer. Hypolimnic CH 4 concentrations up to 0.6 μmol/l indicate microbial methanogenesis in the well-oxygenated upper 10 m of the water column. Simultaneous measurements by the CRDS-membrane system (M-CRDS), a membrane combined with off-axis integrated cavity output spectrometer system (M-ICOS) and gas chromatography (GC) show similar results. 24-hour measurements were performed at the depth of maximum CH 4 concentration and indicate short-term variations of 0.2 μmol/l (CH 4 concentration) and 7‰ (δ 13 C-CH 4 ) within 24 hours. The M-CRDS provides a high temporal and spatial data resolution for accurate detection, mapping and quantification of CH 4 distribution in aquatic systems.