6 YEARS OF CHANGE AT LAKE NYOS, CAMEROON, YIELD CLUES TO THE PAST ANDCAUTIONS FOR THE FUTURE

The catastrophic release of gas from Lake Nyos, Cameroon, in 1986 caus ed substantial but incomplete mixing of the stratified water column. T he post-release evolution of water-column structure has been monitored through April 1992. Changes began immediately after the event as rain fall and inflow brought dilute fluid into the surface layer. Inflow an d surface mixing have gradually deepened the chemocline. The Total Dis solved Solids (TDS) values in the upper 40 m of the water column have dropped from a few hundred mg/kg just after the release to <100 mg/kg. The chemocline is presently strongest at 50 m depth; 5 m below this, the TDS = 570 mg/kg. From 55 to 150 m depth is a gentle gradient in wh ich TDS reaches 920 mg/kg. Little change in water-column chemistry has occurred in this depth interval since the release. Between 150 m dept h and lake bottom at 210 m depth, a strong secondary chemocline has fo rmed. Temperature, CO2 concentration ([CO2]), and TDS have all increas ed in the deepest layer in response to recharge by warm, mineralized w ater, reaching values of 25.0-degrees-C, 320 mmol/kg, and 1800 mg/kg, respectively, 1 m above lake bottom. Considering all these changes in part as a ''recovery'' process, it is possible to construct a model of the pre-release water column. The data indicate that the pre-release chemocline was at least 50 m deep. Above the chemocline was a dilute l ayer containing a seasonal thermocline; below the chemocline was proba bly a gradient zone(s) with correlated increases in TDS and [CO2] and a secondary chemocline near lake bottom. Maximum values of TDS and [CO 2] calculated for pre-release bottom water are 2400 mg/kg and 430 mmol /kg, respectively, based on tritium data. From this pre-release struct ure, a model of the gas release is proposed that is consistent with av ailable chemical and observational data. An important feature of the m odel is that disruption of the pre-existing stratification was much mo re extensive than previously proposed, and even the deepest water laye rs were involved in the event. This model is not intended to limit pos sible gas release mechanisms, and thus complete re-establishment of pr e- 1986 water-column conditions is not a prerequisite for a future rel ease. Spontaneous instability could occur at lake bottom in <20 yr if dissolved gas pressures continue to increase in this zone by 0.5-1 bar /yr as they have for the last 6 yr.