Recent pH and CO2 profiles at Lakes Nyos and Monoun, Cameroon: implications for the degassing strategy and its numerical simulation

Abstract

In situ pH profiles are reported for the first time for Lakes Nyos and Monoun. The pH profiles were converted to CO2 profiles using HCO3− profiles calculated from conductivity data. Recent observations (1993–1996) at Lake Nyos indicates that CO2 still accumulates below 180m depth at a rate of 125Mmolyear−1. At Lake Monoun, the majority of CO2 is present below a depth of 60m, only 25m below the saturation depth. Consequently, a potential danger of gas explosion is high at both lakes, and artificial degassing of the lakes should be performed as soon as possible. A system for industrial degassing of the lakes is proposed. The system, based on the self-sustained gas lift principle, consists of multiple pipes (14cm in diameter) with different intake depths; 12 pipes for Lake Nyos (four each at 185, 195 and 205m) and three pipes for Lake Monoun (at 70, 80 and 90m). The stepped degassing at different depths is intended to keep the maximum stability of the lakes. The proposed degassing operation was simulated using the dyresm code for both lakes. In 5years, approximately 50% of currently dissolved CO2 in Lake Nyos and 90% in Lake Monoun will be removed. The expected changes in the thermal and chemical structures of the lakes as degassing proceeds will be most easily monitored with a carefully calibrated CTD equipped with a pH sensor. The simulation indicates that the discharged degassed water will sink to a level of neutral buoyancy, i.e. to a maximum of 70m at Lake Nyos and 35m at Lake Monoun. There would be no possibility of triggering a gas explosion by this plunge of discharged water because the water present there would have already been replaced by water at lower CO2 concentration, during the degassing from shallower pipes.