Fluvial geochemistry in São Miguel Island (Azores, Portugal): Source and fluxes of inorganic solutes in an active volcanic environment Original Research Article

River water chemistry in two of the major rivers in São Miguel Island (Azores archipelago, Portugal) has been characterized monthly between June 2010 and October 2011 in order to estimate the main geochemical processes that explain water composition and to estimate solute fluxes and the thermal water input to rivers. Both rivers (Ribeira Grande — RRG, and Ribeira Quente — RRQ) drain active trachytic central volcanoes. The number of sampling stations is seven in RRG and six in RRQ. Rivers are mainly slightly acid to basic in nature (pH in the range 5.41–8.70 in RRG and 5.90–8.10 in RRQ) and from the Na–HCO3 and Na–HCO3–Cl water types. In both cases water temperature increases towards the river mouth and median values are slightly higher in Ribeira Quente (15.5–22.4 °C) comparing to Ribeira Grande river (15.2–19.7 °C). Electrical conductivity suggests poorly mineralized waters (139–456 μS/cm in RRG and 209–402 μS/cm in RRQ, and values increase downstream. Higher solute fluxes are associated to HCO3 and Na, and the total CO2 flux is equal to 3647 t/yr in RRQ and 7546 t/yr in RRG. Mixture with thermal water discharges also influences river water chemistry and in RRQ the contribution to the annual average discharge rate was estimated in 2.96 × 106 m3/yr (12.8% of overall discharge rate in the watershed). In RRG thermal water discharges were estimated in 2.4 × 106 m3/yr (14.9% of the discharge rate). The minimum total CO2-consumption associated with low-temperature weathering is equal to 0.58 × 106 mol/km2/yr in Ribeira Quente river and equal to 0.78 × 106 mol/km2/yr in Ribeira Grande river.