An overview of the structure, hazards, and methods of investigation of Nyos-type lakes from the geochemical perspective

Limnic eruptions represent a natural hazard in meromictic lakes hosted in volcanoes releasing CO2-rich magmatic gases. Biogeochemical processes also contribute to dissolved gas reservoirs since they can produce significant amounts of gases, such as CH4 and N2. Dissolved gases may have a strong influence of the density gradient and the total dissolved gas pressure along the vertical profile of a volcanic lake. An external triggering event, possibly related to uncommon weather conditions, volcanic-seismic activity, or landslides, or spontaneous formation of gas bubbles related to the progressive attainment of saturation conditions at depth, may cause a lake rollover and the consequent release of dissolved gases. This phenomenon may have dramatic consequences due to i) the release of a toxic CO2-rich cloud able to flow long distances before being diluted in air, or ii) the contamination of the shallow water layer with poisonous deep waters. The experience carried out over the past twelve years at Lake Nyos, where a pumping system discharges CO2- rich deep water to the surface, has shown that controlled degassing of deep water layers is the best solution to mitigate such a hazard. However, the application of this type of intervention in other lakes must be carefully evaluated, since it may cause severe contamination of shallow lake water or create dangerous density instabilities. Monitoring of physical and chemical parameters controlling lake stability and the evolution in time of dissolved gas reservoirs can provide essential information for evaluating the risk associated with possible rollover phenomena. Conceptual models for the description of limnological, biogeochemical and volcanic processes regulating water lake stability have been constructed by interpreting compositional data of lake water and dissolved gas compositions obtained by applying different sampling and analytical techniques. This study provides a critical overview of the existing methodological approaches and discusses how future investigations of Nyos-type lakes, aimed at mitigating the hazard for limnic eruptions, can benefit from i) the development of new technical and theoretical approaches aimed to constrain the physical-chemical mechanisms controlling this natural phenomenon, and ii) information from different scientific disciplines, such as microbiology, fluid dynamics and sedimentology.