CO2 flux and water geochemistry in Ribeira Quente river: A case study from a river draining an active central volcano (Furnas Volcano – São Miguel, Azores)

Building on previous studies that suggest that river water chemistry in Ribeira Quente watershed (Furnas Volcano) is influenced by hydrothermal discharges, measurements of the CO₂ fluxes along the major river and tributaries were made. The annual runoff in the river is estimated as 9.75 × 10⁶ m³ yr⁻¹, presenting a well-marked seasonal variation.

Waters are mainly of the Na-HCO₃ type, presenting a mean temperature of 19.53 ± 5.16 °C (median = 18.5 °C), reaching values as high as 38.5 °C, a mean EC equal to 325.24 ± 177.77 μS cm⁻¹ (median = 304.5 μS cm⁻¹), and are slightly acid to basic (mean pH = 7.07 ± 0.57; median = 7.1). Physico-chemical parameters and major-ion composition depict noticeable variations whenever hydrothermal discharges occur in the river channel. Besides mixing with hydrothermal discharges, water chemistry is also influenced by silicate weathering, processes also shown by the ¹³C/¹²C and ⁸⁷Sr/⁸⁶Sr isotopic ratios.

The total amount of CO₂ emitted along selected paths range between 20.5 t d⁻¹ and 23.8 t d⁻¹, values higher than the ones calculated dividing the river channel in separate sections (13 t d⁻¹–15.5 t d⁻¹), as the former values overestimates the area, thus leading to a higher overall flux calculation. CO₂ flux for all the watershed, estimated considering the contribution of all tributaries, is in the range of 14.6 t d⁻¹ to 17.7 t d⁻¹. Even if the CO₂ flux associated to surface water bodies in the study area is much lower than soil diffuse emissions, results point out to the need to characterize C-based GHG emissions from rivers draining active volcanic areas.