​Santiago and Congro are two maar crater lakes, located at São Miguel, the largest island of the Azores archipelago. Santiago Lake is located on the Sete Cidades Volcano, at an altitude of 364 m, presenting a surface area of 0.25 km² and a maximumdepth of 33 m. Congro Lake is located on the Congro Fissural Volcanic System, at an altitude of 420 m, and has a surface area of 0.04 km² and a maximum depth of 22 m. Both lakes are monomictic in character, with low mineralized waters of meteoric origin, being the temperature in the range between 12.4 °C (winter period) and 23.8 °C (summer period). The main water type is Na-HCO₃, and the relative major-ion composition is HCO₃^- > Cl^- > SO₄^2- > F^- for anions and Na⁺ > K⁺ > Mg²⁺ > Ca²⁺ for cations.
Four sampling surveys were carried out between 2013 and 2016 in each lake in order to estimate surface diffuse CO₂ degassing. A total of 1612 and 713 CO₂ flux measurements were made with an accumulation chamber at Santiago and Congro lakes, respectively.

The higher CO₂ fluxes were measured during the winter surveys (0.2 t d^-1 in Congro and 5.6 t d^-1 in Santiago), while the lowest values (0.1 t d^-1 and 0.2 t d^-1, respectively, in Congro and Santiago lakes) were recorded in the summer. These seasonal differences observed in both lakes are associated with the monomictic character of the lakes, as the CO₂ is not able to ascend to the surface when the water column is stratified during the warmer period. Due to the physical processes that occur in both lakes, the CO₂ emission is mostly associated to a biogenic origin, but the higher CO₂ emissions measured in Santiago Lake suggest that a volcanic influence cannot be excluded, as also shown through the heavier δ¹³C isotopic measured in this water body. The tectonic structures that cross Santiago Lake probably enable the observed degassing.