The volcanic lakes located on active volcanic edifices are systems that possess specific dynamics, constituting natural reservoirs where natural condensations of fluids of volcanic origin occur.
The main lakes of São Miguel Island were selected taking in consideration their area, depth, origin and location, in order to investigate their CO2 emissions into the atmosphere. The majority of the studied lakes are located on the three active volcanoes of São Miguel Island: Sete Cidades Volcano (Blue Lake and Green Lake, and Santiago Lake), Fogo Volcano (Fogo Lake) and Furnas Volcano (Furnas Lake), and also Congro Lake, which is located on the Congro Fissural Volcanic System.
The application of the GSA method for statistical analysis allowed the identification of different populations, sometimes associated with hydrologic processes occurring in the lakes (e.g. thermal stratification). In lakes with an identified volcanic fluid contribution these processes are indicated by various populations. Also, the study of stable isotopic content confirmed the meteoric origin of the lakes and the deep origin of a fraction of the CO2 emitted at Furnas and Santiago. The δ13C content in the other lakes proves that the emitted CO2 has a biogenic origin, even when more than one set of CO2 flux data was identified.
Estimated values of emitted CO2 range from 0.15 t d-1 (Congro Lake; summer survey) to 600 t d-1 (Furnas; winter survey). Relating the output values and the surface area of the water bodies gives values ranging from 1.5 t km-2 d-1 (Santiago Lake) to ~321 t km-2 d-1 of CO2 (Furnas Lake). This latter value suggests that Furnas Lake is among the eight volcanic lakes with the highest CO2 emissions rate per unit area. The total annual CO2 emission flux from all the studied lakes is estimated to be ~168 × 103 t yr-1, of which approximately 47% is of volcanic origin.