Spring geochemistry in an active volcanic environment (Sao Miguel,Azores): Source and fluxes of inorganic solutes

摘要

Mineral waters were monthly sampled in selected springs from Furnas and Fogo trachytic central volcanoes (Sao Miguel, Azores, Portugal). Water temperatures between 15.1 ℃ and 90.2 ℃, characterize poorly mineralized waters of Na-HCO3 and Na-HCO_3-Cl types. According to the spring location, two watersheds were selected in each volcano to evaluate solute fluxes and chemical weathering rates (Fogo volcano: Ribeira Grande river - RRG; Furnas volcano: Ribeira Quente river - RRQ). Na, Mg, K and Ca fluxes in groundwater represented 43%, 60%, 46% and 57% of the total (subsurface plus surface fluxes) in RRG, and respectively 43%, 53%, 46% and 49% in RRQ. Average HCO_3 flux is ten times higher in RRQ (130.1 × 10~6 mol/yr) compared to RRG (13.8 × 10~6 mol/yr), reflecting the volcano degassing and the lower ratio between groundwater and river water fluxes. Based on these values, total CO_2-consumption by weathering ranged from 1.5 × 10~6 mol/km~2/yr (RRG) to 4 × 10~6 mol/km~2/yr (RRQ). TDS load varied between 3772 t/yr (RRG) and 15388 t/yr (RRQ), and the ratio between values in groundwater and in surface water is respectively equal to 0.72 and 2.04. The associated chemical weathering rates in groundwater were 206 t/km~2/yr (RRG) and 399 t/km~2/yr (RRQ). Coupled to river water, these values indicate that total chemical weathering rates are respectively equal to 493 t/km~2/yr and 594 t/km~2/yr. A similar approach developed for the entire archipelago showed that the chemical weathering rates due to groundwater are in the range from 33 to 321 t/km~2/yr, being partially controlled by the age of the islands. Results of the present study point out to the need to consider groundwater solute fluxes due to weathering when establishing geochemical budgets.