On the risk of a dissolved gas-triggered limnic eruption in Lake Kivu

Abstract

Lake Kivu is distinguished by several unique characteristics that set it apart from other lakes around the world. One of the notable features is a temperature increase with depth, accompanied by unusual staircase-like patterns in the thermodynamic and environmental parameters. The lake also experiences suppressed vertical mixing due to stable density stratification, with its deep water separated from the surface water by chemoclines. Additionally, Lake Kivu contains high concentrations of dissolved methane (CH4) and carbon dioxide (CO2), and there is no standard method for measuring their concentrations. The lake is also recognized as a renewable energy source due to its continuous supply of CH4, and it demonstrates a quadruple-diffusive convection transport mechanism. These factors contribute to the lake’s distinctiveness. The occurrence of catastrophic limnic eruptions at Lakes Nyos and Monoun, along with the structural similarities between these lakes and Lake Kivu, raises serious concerns about the likelihood of a similar disaster in Lake Kivu in the future. The scale of threats posed in Lake Kivu can be orders of magnitude greater than the other two lakes, given its 3,000 times larger size, two to four orders of magnitude higher content of dissolved CO2, containing substantial quantities of CH4 in addition to CO2 in solution, and holding a far denser population living in its much wider catchment area. The present study aims to assess the probability of a future gas outburst in this giant lake by numerical modeling of its hydrodynamics over the next half a millennium. The turbulent transport is calculated using the extended k-ϵ model. An implicit Euler method is applied to solve the governing partial differential equations on a vertically staggered grid system, discretized using a finite-volume approach. Since the previously calibrated model successfully reproduces the measured lake profiles, the same tuned parameter values are used in this study, assuming a stable, steady-state condition in the future. The results of our simulations effectively address common concerns regarding the risk of a gas burst in the lake due to buoyancy instability-triggered overturn and/or supersaturation of the water column.