Continuous in situ measurement of dissolved methane in Lake Kivu using a membrane inlet laser spectrometer

Abstract

Abstract. We report the first high-resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented membrane-based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to 6 orders of magnitude with respect to the original prototype (from nmol L−1 to mmol L−1 detection) to fit the range of concentrations at Lake Kivu. The accuracy of the instrument was estimated to ±22 % (2σ) from the standard deviation of eight profiles at 80 m depth, corresponding to ±0.112 mbar of CH4 in water or ±160 nmol L−1 at 25 ∘C and 1 atm. The instrument was able to continuously profile the top 150 m of the water column within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 %, which at 150 m depth and under thermal conditions of the lake corresponds to 3.5 mmol L−1. Deeper down, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results are in good agreement with discrete in situ measurements conducted with the commercial HydroC® sensor. This fast-profiling feature is highly useful for studying the transport, production and consumption of CH4 and other dissolved gases in aquatic systems. While the sensor is well adapted for investigating most environments with a concentration of CH4 up to a few millimoles per liter, in the future the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and total dissolved gas pressure, for exploring settings with very high concentrations of CH4 such as the bottom waters of Lake Kivu.

No increasing risk of a limnic eruption at Lake Kivu: Intercomparison study reveals gas concentrations close to steady state

“…The results of the dissolved gas measurements (CH 4 and CO 2 ) of Eawag, UFZ [ 13 ] and CNRS [ 29 ] are shown in Fig 2 . For both CH 4 and CO 2 , the measurements agree well within the uncertainties of the different approaches.…”

“…The results of the dissolved gas measurements (CH 4 and CO 2 ) of Eawag, UFZ [13] and CNRS [29] are shown in Fig 2. For both CH 4 and CO 2 , the measurements agree well within the uncertainties of the different approaches. The profile from Eawag shows higher CH 4 concentrations (up to 10%) than UFZ between 250 and 350 m depth, whereas UFZ measured higher CH 4 and CO 2 concentrations (up to 5%) below 400 m. In particular, the UFZ profile indicates further increasing CH 4 and CO 2 concentrations with depth below 400 m, while the Eawag profile levels off or even decreases.…”

Inter-Comparison of the Spatial Distribution of Methane in the Water Column From Seafloor Emissions at Two Sites in the Western Black Sea Using a Multi-Technique Approach

“…It is described in detail in a previously published paper (Grilli et al, 2018), where a laboratory comparison with measurements of discrete water samples at different water temperatures and salinities was conducted. It was deployed successfully during two campaigns, over a methane seepage area in western Svalbard in 2015 (Jansson et al, 2019b), and at Lake Kivu in Rwanda in March 2018 (Grilli et al, 2020). The instrument allows a dynamic range from a few nmol L −1 up to a few μmol L −1 .…”

A novel high-resolution in situ tool for studying carbon biogeochemical processes in aquatic systems: The Lake Aiguebelette case study

“…Because of the manual nature of these measurements, only a few or even just one sample is often taken in systems, particularly during multi-lake surveys (e.g., Rasilo et al, 2015). Recently, however, equilibrator systems have been used to extract dissolved gas from water (either in situ or on site) which is then directed either to a laser-based optical spectrometer (Gerardo-Nieto et al, 2019;Gonzalez-valencia et al, 2014;Grilli et al, 2020;Wankel et al, 2013;Yuan et al, 2020) or to a compact mass spectrometer (Bärenbold et al, 2020;Brennwald et al, 2016;Short et al, 2006) for highly resolved measurements. Note that this is a not extensive list of studies.…”

“…In this work, we present a first deployment of a novel membrane inlet laser spectrometer (MILS) instrument that is an upgraded version of the SubOcean probe (Grilli et al, 2018(Grilli et al, , 2020Triest et al, 2017). A newly developed mid-infrared spectrometer for simultaneous detection of CH4, C2H6 and  13 CH4 (Lechevallier et al, 2019) was implemented on the in situ instrument.…”