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

PLoS One. 2020 Aug 25;15(8):e0237836. doi: 10.1371/journal.pone.0237836. eCollection 2020.

Abstract

Lake Kivu, East Africa, is well known for its huge reservoir of dissolved methane (CH4) and carbon dioxide (CO2) in the stratified deep waters (below 250 m). The methane concentrations of up to ~ 20 mmol/l are sufficiently high for commercial gas extraction and power production. In view of the projected extraction capacity of up to several hundred MW in the next decades, reliable and accurate gas measurement techniques are required to closely monitor the evolution of gas concentrations. For this purpose, an intercomparison campaign for dissolved gas measurements was planned and conducted in March 2018. The applied measurement techniques included on-site mass spectrometry of continuously pumped sample water, gas chromatography of in-situ filled gas bags, an in-situ membrane inlet laser spectrometer sensor and a prototype sensor for total dissolved gas pressure (TDGP). We present the results of three datasets for CH4, two for CO2 and one for TDGP. The resulting methane profiles show a good agreement within a range of around 5-10% in the deep water. We also observe that TDGP measurements in the deep waters are systematically around 5 to 10% lower than TDGP computed from gas concentrations. Part of this difference may be attributed to the non-trivial conversion of concentration to partial pressure in gas-rich Lake Kivu. When comparing our data to past measurements, we cannot verify the previously suggested increase in methane concentrations since 1974. We therefore conclude that the methane and carbon dioxide concentrations in Lake Kivu are currently close to a steady state.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Africa, Eastern
  • Carbon Dioxide / analysis
  • Gases / analysis*
  • Lakes / chemistry*
  • Methane / analysis
  • Pressure
  • Risk

Substances

  • Gases
  • Carbon Dioxide
  • Methane

Grants and funding

MS received grant 160114 from the Swiss National Science Foundation (www.snf.ch) for the Eawag measurements. RG received funding from the European Community’s Seventh Framework Programme ERC-2015-PoC under grant agreement no. 713619 (ERC OCEAN-IDs) and from the Agence Nationale de la Recherche (ANR) under grant agreement ANR-18-CE04-0003- 01. Authors AU and AM are staff of the unit LKMP (Lake Kivu Monitoring Programme) of EDCL (Energy Development Corporation Limited) which is a subsidiary of the Rwandan Energy Group Limited (REG). EDCL/LKMP supported the involved research teams with local transport and help with field work. In addition, they paid the flight to Rwanda and accommodation in Rwanda for the participants of UFZ (authors BB and WvT) and CNRS (author RG). The specific roles of these authors are articulated in the ‘author contributions’ section. The funders had no other role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.