Intercalibration campaign for gas concentration measurements in Lake Kivu
1.The 2018 intercalibration campaign aimed at quantifying the methane (CH4) and carbon dioxide (CO2) gas content and the recharge rate of CH4 in Lake Kivu using a range of different measurement methods. Measurements were performed by research teams from the Helmholtz Centre for Environmental Research (UFZ) in Magdeburg (Germany), the Swiss Federal Institute for Aquatic Science and Technology (Eawag, Switzerland), the French National Centre for Scientific Research (CNRS) in Grenoble (France), and from KivuWatt Ltd (Kigali, Rwanda).
2. The following measurement methods were applied: two sensors for the in-situ observation of total dissolved gas pressure; two sensors for the in-situ observation of the partial pressure of dissolved CH4; and two methods for quantifying the concentrations of dissolved CH4 and CO2 in samples retrieved from the lake either using sampling bags or a tubing system. These methods were specifically customized for the application under the special conditions in Lake Kivu. Since some of the methods quantify partial pressures of CH4 and/or CO2, and other methods quantify their concentrations, a procedure for converting between partial pressures and concentrations was developed and implemented.
3. The observations yielded a consistent picture of the vertical profiles of dissolved concentrations of CH4 and CO2 as well as the total gas pressures in Lake Kivu. The observed variability between the datasets is related to the limited accuracy of the different measurement methods.
4. The observed CH4 concentrations were within the range of previous observations. However, in the resource zone (below 260 m depth), they were approximately 5-20 % below the concentrations measured by M. Halbwachs and J.-C. Tochon in 2003, which had previously been used as the standard for estimating the CH4 content in the lake.
5. The CH4 content in the resource zone (between 260 and 480 m depth) was estimated to ~40 km3 STP (volume of gas at a temperature of 0°C and a pressure of 1 atm) (range, 36.4 - 42.2 km3). These numbers are somewhat lower than the previous estimate of 44.7 km3 by Wüest and Schmid (2012) based on the measurements of M. Halbwachs and J.-C. Tochon in 2003. The CH4 content in the potential resource zone (200-260 m) was estimated to range between 8.2 and 8.6 km3 for all methods, which agrees with the previous estimate of 8.5 km3. The whole-lake CO2 content was estimated to 285 km3 STP.
6. The differences in CH4 concentrations and content compared to previous estimates are due to the limited accuracy of the different measurement methods and were not caused by the comparably small amount of CH4 removed by the past and ongoing gas extraction operations.
7. The 2018 measurements do not confirm the previous hypothesis that the CH4 concentrations were increasing during the last decades in Lake Kivu. They rather indicate approximately constant concentrations since the first observations in the 1950's within the uncertainty range of the present and previous measurements.
8. Recommendations for monitoring the evolution of gas concentrations in the lake cover two important aspects. Required monitoring equipment and frequency are defined based on different monitoring purposes. In addition, a need for building up sampling routines and skills is identified.