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Geoscientific Instrumentation, Methods and Data Systems An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 20 Nov 2019

Submitted as: research article | 20 Nov 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Instrumentation, Methods and Data Systems (GI).

Continuous In Situ Measurement of Dissolved Methane in Lake Kivu Using a Membrane Inlet Laser Spectrometer

Roberto Grilli1, François Darchambeau2, Jérôme Chappellaz1, Ange Mugisha3, Jack Triest4, and Augusta Umutoni3 Roberto Grilli et al.
  • 1CNRS, Univ. Grenoble Alpes, IRD, Grenoble INP, IGE, F-38000 Grenoble, France
  • 2KivuWatt Ltd., Kigali, Rwanda and Chemical Oceanography Unit, Université de Liège, Belgium
  • 3Lake Kivu Management Program LKMP, Gisenyi, Rwanda
  • 4KM Contros, Kongsberg Maritime, Kiel, Germany

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 six 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 % (2 s) from the standard deviation of eight profiles at 80 m of depth, corresponding to ±112 μBar of CH4 in water or ±160 nmol L−1 at 25 °C and 1 atm. The instrument was able to continuously measure the top 150 m of water depth within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 % at 150 m depth, which at this depth and thermal condition of the lake corresponds to 3.5 mmol L−1. At deeper depth, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results were in good agreement with discrete in situ measurements conducted with the commercial HydroC sensor. The fast profiling feature will be highly profitable for future monitoring of the lake, while the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and which would allow to measure at higher concentrations of CH4.

Roberto Grilli et al.
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Roberto Grilli et al.
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Publications Copernicus
Short summary
We report the results from the deployment of a newly developed in situ sensor for dissolved gases measurements. Its adaptation to high gas concentrations and dissolved gas pressures was proven. The campaign leads to a first continuous profile of methane on the first 150 m, and allowed to compare the data with previous measurements. The fast response of the instrument makes this technique a good candidate for regular monitoring of those type of lakes, for anticipating disastrous gas eruptions.
We report the results from the deployment of a newly developed in situ sensor for dissolved...