Abstract

A new, high temporal and spatial resolution, in-situ method for the fast and precise determination of methane (CH4) concentration and its delta C-13 isotope value (delta C-13-CH4) 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 (R) G2201-i). The system was calibrated for concentration and delta C-13 of CH4 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 CH4 concentration (R-2 = 0.994) and a measuring accuracy of 0.002 ppm (CH4 concentration) and 0.6 % (delta C-13-CH4). Calculated response times t 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 CH4 peak at the thermocline in summer. Hypolimnic CH4 concentrations up to 0.6 mu mol/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 (MICOS) and gas chromatography (GC) show similar results. 24-hour measurements were performed at the depth of maximum CH4 concentration and indicate short-term variations of 0.2 mu mol/l (CH4 concentration) and 7% (delta C-13-CH4) within 24 hours. The MCRDS provides a high temporal and spatial data resolution for accurate detection, mapping and quantification of CH4 distribution in aquatic systems. (C) 2017 The Authors. Published by Elsevier B.V.