Simulation of CO2 concentrations, temperature, and stratification in Lake Nyos for different degassing scenarios

[ 1] A large gas cloud erupted unexpectedly in 1986 from Lake Nyos, the larger of the two Cameroonian "Killer Lakes,'' with devastating consequences. Regular monitoring subsequently revealed that the deep water of the lake was gradually recharged with CO2. To preclude a similar event in the future, a degassing pipe was installed in the lake in 2001. In the present study a one-dimensional model is used to predict the effects of this pipe and other degassing options on the CO2 concentrations and the stratification within the lake for the next 50 years. The results of the simulations show that without degassing, total CO2 content would reach the preeruption value within a few decades. The presently installed pipe is sufficient to reduce CO2 pressures in the entire water column above the pipe inlet to < 5 bar within 10 years, and a steady state is reached within 50 years. Depending on the assessment of the risk due to the gas currently remaining in the lake and the costs involved, the installation of additional pipes could be considered ( 1) to remove the gas more quickly and ( 2) as a backup for long-term failures and maintenance. Once the steady state is reached, degassing with one pipe is a practicable long-term solution which can also be used for monitoring the approximate deep water CO2 concentrations by nonprofessionals. Assuming a doubled deep water input in the future as an upper limit of the expected source strength, the pipe is still able to prevent a CO2 accumulation. As a side effect, the degassing operation strongly changes the stratification in the lake. It transforms the lake from a meromictic to an oligomictic system and gradually removes the dissolved salts from the lake.