Numerical simulations of degassing scenarios for CO2-rich Lake Nyos, Cameroon

Abstract

A catastrophic outgassing of carbon dioxide (CO2) from Lake Nyos, Cameroon, occurred in 1986. Concentrations of dissolved CO2 have been increasing in the bottom waters since 1986, prompting an international effort to engineer controlled degassing that would maintain lake stability by reducing the gas concentration. A new model, based on the one-dimensional Dynamic Reservoir Simulation Model has been used to simulate the evolution and distribution of CO2 in Lake Nyos. Initial conditions for the simulations are taken from published observations and integrated forward with synthesized, daily meteorological data for 5 years. After model results were found to be in agreement with observed trends, a hypolimnetic piped withdrawal system for degassing the lake was then modeled to simulate its effect on the CO2 concentration profile over time. In the standard degassing operation, gas-rich bottom water continuously flows up through pipes, discharging as a fountain at the lake surface. Simulation results indicate that the lake density structure will most likely remain stable and CO2 concentrations will decrease at all depths above the pipe intake. Seven alternative operational strategies for the degassing system were also investigated, including a submerged outlet, an intake raised above the lake bottom, higher flow rate, a disrupted operation schedule, employment of a stabilization pond, multiple intake depths, and release of the withdrawn water from the lake. Multiple intake depths best maintain lake stability. A tentative conclusion is that degassing may need to be carried out indefinitely, albeit intermittently or at a reduced flow rate.