Methane formation and future extraction in Lake Kivu
This chapter summarises the current knowledge on the vertical distribution of methane (CH4) and its formation in Lake Kivu. Additionally, we review the objectives and restrictions under consideration for sustainable extraction (safe, environmentally acceptable, and economically effective) of the enormous amount of CH4 from the lake. The harvested CH4 will be used to produce electricity which is desperately needed in both neighbouring countries: the Democratic Republic of the Congo and Rwanda. From a system-analysis point of view, the following processes need to be included as the minimum for adequately evaluating the vertical and temporal development of the lake CH4 during extraction: (1) in situ CH4 formation occurring in the permanently stratified, anoxic deep-water, (2) CH4 oxidation in the oxic surface water, (3) natural lake-water upwelling caused by subaquatic springs, (4) artificial lake-water up- and downwelling due to extraction- and reinjection-related flows, and (5) upward diffusion caused by double diffusive convection and weak turbulence. Water density is parameterised as a function of temperature, salinity, and the two gases carbon dioxide and CH4. For the sake of clarity of the presentation, we use a simplified 4-box analysis and are neglecting the diffusion process (5). This allows for the essence of the CH4 extraction challenge to be conveyed while avoiding excessive complexities. The system analysis for different CH4 extraction concepts clearly reveals that the depth of reinjection of the CH4-depleted deep-water is critical for the sustainability of the extraction and an optimal CH4 harvesting plan. Here, the suitability of different reinjection scenarios is compared by evaluating each of them in terms of the objectives "safety" (water column stability), "lake ecological integrity" (nutrient upward fluxes), and "economic viability" (amount of harvestable CH4). Comparison of model simulations, run over 50 years, revealed that (1) using lake surface (dilution) water for adjusting the density of the reinjection water and (2) reinjecting the nutrient-rich deep-water in the top 190 m are both unacceptable in terms of sustainability.