Reliable reference for the methane concentrations in Lake Kivu at the beginning of industrial exploitation

Abstract

Abstract. Dissolved methane in Lake Kivu (East Africa) represents a precious energy deposit for the neighbouring countries, but the high gas loads have also been perceived as a threat by the local population. This is especially the case with respect to potential changes to the lake's stratification during planned industrial exploitation. Both of the above-mentioned issues require accurate and reliable measurements of dissolved gases and temporal changes to take responsible action. Previous data were unable to satisfactorily fulfil these requirements. Prior to our measurements, there was considerable disagreement about prognosticated new formation of methane. We show how measurement accuracy could be significantly improved by implementing equipment that is especially designed and modified for the complex gas conditions in Lake Kivu. Samples were taken from depths of 150 to 430 m to more reliably and more accurately determine the amount of dissolved methane and dissolved carbon dioxide. Beyond the provision of gas concentration profiles at the beginning of exploitation, this investigation can also potentially provide methods to survey the further evolution of gases over time. The use of gas-tight sampling bags produced highly reliable and accurate measurements. Our measurements confirmed the huge amount of stored methane, but clearly did not support the current belief of a significant recharge beyond diffusive loss. Direct measurements with a custom-made gas pressure sensor indicated no imminent danger due to limnic eruptions. However, a further survey of gas pressures is mandatory to detect changing conditions. Using sampling bags and a gas pressure sensor, we introduced reliable and highly accurate measurement approaches for the survey of the further development of gas concentrations. This equipment requires little effort with respect to calibration, which makes it suitable for use in the remote areas of Africa.

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

“…Regarding the stability of the lake, in Schmid et al (2005) raised the possibility that dissolved CH 4 in the lake was increasing at a rate of ∼ 0.5 % per year, with consistent repercussions for the safety of the surrounding population (Schmid et al, 2005). However, from the work of Pasche et al (2011) as well as the results from this recent field campaign, the hypothesis of a fast increase is today excluded, and the temporal variability appears to be slower than previously expected (Bärenbold et al, 2019;Boehrer et al, 2019;Schmid et al, 2019). In the future, regular monitoring of the lake is required to estimate the CH 4 and CO 2 budgets as well as their temporal variability, using reliable, fast and easy-to-use techniques.…”

“…In the future, regular monitoring of the lake is required to estimate the CH 4 and CO 2 budgets as well as their temporal variability, using reliable, fast and easy-to-use techniques. For a more precise estimation of the dissolved gas concentration, intercomparison between different sensors and methods is required, as conducted and presented in this work and in the even more comprehensive results from the entire intercomparison campaign (Bärenbold et al, 2019;Boehrer et al, 2019;Schmid et al, 2019). A fast-response sensor like the one proposed here could also be highly useful for estimating methane fluxes from the water surface and their spatial and seasonal variabilities.…”

“…During the campaign other types of measurements of dissolved methane and carbon dioxide were performed. The research team from Eawag (Switzerland) analyzed pumped water on the platform using a field mass spectrometer instrument (Brennwald et al, 2016), while a second team from the Helmholtz Centre for Environmental Research (UFZ, Germany) sampled water from a boat and measured the samples by headspace equilibration and GC analysis at the Lake Kivu Monitoring Program (LKMP) laboratory in Rubavu (Boehrer et al, 2019). The Sub-Ocean sensor was deployed from a research boat during 3 d of the campaign -10, 12 and 13 March -with a total of eight continuous profiles.…”

“…In this work we report a successful deployment of the Sub-Ocean sensor in a very different setting, highlighting the reliability and adaptability of the technique to different aquatic environments. Advantages and drawbacks of the technique are highlighted in the discussion section in comparison with other methods deployed during the same campaign: water sampling followed by laboratory gas chromatography (GC) analysis (Boehrer et al, 2019) and online water pumping followed by on-site mass spectrometry analysis (Brennwald et al, 2016). These results are not reported here as they focused on the concentrations in the deep waters (Bärenbold et al, 2019;Schmid et al, 2019).…”

Gas Pressure Dynamics in Small and Mid-Size Lakes

“…The deviation of the exponential fit and the more sophisticated approach lies within about 3% for temperature 10 to 35 • C; at lower temperatures deviation are even larger (see Supplementary Materials and Figure S1). The effect of dissolved solids on the Henry coefficient is small for freshwater (<3 g/L of dissolved solids) and quantifications are not available for salt compositions in inland waters (e.g., [28,32]) and hence has not been included in our evaluation.…”