Volcanic CO2-dominated gases can slowly accumulate at the bottom of volcanic lakes by dissolution processes. The best studied cases are Lake Nyos and Lake Monoun (Cameroon), which in 1984 and 1986, respectively, violently released a massive amount of previously accumulated CO2, killing approximately 1700 people. These two disasters increased the general awareness of the potential danger this type of volcanic lake may represent. The only way to monitor the accumulation of CO2, or other gas species, in a volcanic lake is to lower Conductivity-Temperature-Depth/Pressure (CTD) probes, sample lake water and measure dissolved gases along vertical profiles. In most studies, these vertical profiles are performed only where the anomalous deep accumulation is known to occur, whereas the lateral variability and extension are often unknown.
We present a 3D mapping of the dissolved CO2 at Furnas Lake (São Miguel Island, Azores) obtained with a lightweight probe that uses infrared detection to measure the partial pressure of CO₂ gas dissolved in liquids. The equilibration time of the dissolved CO2 measurements is rather slow, approximately 15’. To compensate for the slow operational times of the CO2 probe, we executed measurements with a multi-parametric probe (pH, ORP, conductivity, dissolved O2, and temperature) at additional depths, which allowed to correlate pH with the dissolved CO2, and interpolate for the extra depths for which the dissolved CO2 was not directly measured. Interpolating both horizontally and vertically the total amount of dissolved CO2 content led to the estimate of a total CO2 mass of ~ 100 tons dissolved as CO2 (aq). If we also consider the carbon dissolved as bicarbonate (HCO3 concentrations obtained from Cruz et al., 2006 and Andrade et al., 2016), we obtain a total CO2 mass (CO2 + HCO3) ranging from 600-800 tons for Furnas Lake. Different plumes of dissolved CO2 are recognized in 3D; these spatially correspond to previously detected shallow anomalies of the diffusive CO2 degassing at the water-air interface (fumarolic inputs, structural alignments).
Our result represents the first direct quantification of the total dissolved CO2 in a volcanic lake. This novel methodology opens new perspectives for volcanic lake monitoring and gas hazard assessment.