Chemistry of the Ferraria thermal water, S. Miguel Island, Azores: mixing and precipitation processes

Abstract

The Ferraria thermal water emerges at the sea level in the Ferraria lava delta (western edge of S. Miguel Island, Azores) with temperature of ca. 60°C and pH varying between 5.4 and 6.2. It is of sodium chloride type, resulting from ca. 50% seawater mixing with an acid brackish, at &100°C, denoting the presence of significant CO 2 (g) and the progress of water-rock interactions in open system conditions. The thermal Na-Cl water is strongly enriched with Sr and Mn and, comparatively, has low concentrations in Al, Fe and As. These elements are removed from the solution as critical conditions for the formation of several neo-formed mineral phases are gradually attained. Thermodynamic equilibrium calculations are consistent with this interpretation, showing that the thermal fluid can precipitate Fe 3? -(hydr-)oxides, kaolinite and noncrystalline silica. Wells logging show fracture planes and pores fully/partly filled up with polyphase botryoidal aggregates mostly composed of goethite ? ferrihydrite and displaying variable adsorbed contents of Si, P and As. These neo-formed phases result from the pristine fluid oxidation due to seawater mixing; its precipitation is easily affected by pH and redox variations of the brackish, due to volcanic gases pressure alterations, and fluid pressure or flowvelocity oscillation in the fractured aquifer.

Intertidal rocky shore seaweed communities subject to the influence of shallow water hydrothermal activity in São Miguel (Azores, Portugal)

“…The increase in temperature and acidity and the decrease in salinity are indicative of an input of hot and acidic freshwater, as evidenced by Carvalho et al (2011). Furthermore, at both sites, there is evidence that the effect of hydrothermal water is subject to dissipation with distance from its source, tidal height and wave exposure.…”

“…Seaweeds collected under such different scenarios exhibit similar increased metal accumulation (Woolsey and Wilkinson 2007;Wallenstein et al 2009;Wallenstein 2011). In both studies, intertidal communities are affected by acidic, metal-enriched, freshwater inputs (Carvalho et al 2011;Woolsey and Wilkinson 2007). In both cases, the input sources are localized and their effects in sea water dissipate due to dilution caused by the increasing influence of clean sea water and consequently the magnitude of their impacts on seaweed communities-lower species richness and a large proportion of early successional species in both studies-decreases with increasing distance from the input source.…”

“…Such habitats are naturally enriched in heavy metals (see Von Damm 1990;Carvalho et al 2011) and thus provide examples of naturally polluted sites to compare with those that are affected by human pollution. They also provide unusual ecological situations of increased temperature and acidity due to the input of hot hydrothermal water springs and the diffusion of acidic volcanic gases (mainly CO 2 ; Cruz and França 2006;Carvalho et al 2011).…”

“…Such habitats are naturally enriched in heavy metals (see Von Damm 1990;Carvalho et al 2011) and thus provide examples of naturally polluted sites to compare with those that are affected by human pollution. They also provide unusual ecological situations of increased temperature and acidity due to the input of hot hydrothermal water springs and the diffusion of acidic volcanic gases (mainly CO 2 ; Cruz and França 2006;Carvalho et al 2011). Organisms that live in such environments are therefore chronically exposed to thermal pollution and chemical pollution as well as to acidification that can affect biotic communities at various levels (e.g., productivity, species diversity, abundance and composition, and the bioaccumulation of contaminants), consequently affecting overall ecosystem health (Elliott and Cutts 2004).…”

Influence of wall-rock alteration and fluid mixing mechanisms in the chemistry of thermal fluids and mud-pool sediments at Caldeiras da Ribeira Grande (S. Miguel Island, Azores)

“…These manifestations are largely controlled by active fault zones and express emergences of water-dominated systems variably mixed with deeper constituents derived from moderate to high-temperature fluid fluxes related to the Sete Cidades, Furnas and Fogo active stratovolcanoes (Fig. 1a;Forjaz 1994;Carvalho 1999;Cruz et al 1999;Ferreira et al 2005;Carvalho et al 2011). Considerable efforts were carried out Miguel Island (Azores Archipelago) (I Nordeste, II Povoação, III Furnas, IV Fogo, V Picos, VI Sete Cidades), adapted from Nunes (2004).…”