Chapter 19 Hydrogeochemical characterization of mineral waters in São Miguel Island, Azores

J. V. Cruz, P. Freire, A. Costa, J. Fontiela, L. Cabral and R. M. Coutinho
Geological Society, London, Memoirs, 44, 257-269, 2015, https://doi.org/10.1144/M44.19

Abstract

A dataset on the chemical composition of mineral water discharges from São Miguel, 76 samples, depicts a large variability of chemical types and dissolved solids content. The distribution of the discharges shows an association with the three active Quaternary central volcanoes that dominate the geology of São Miguel, namely Furnas, Fogo and Sete Cidades, 74% of which are springs, 13% fumaroles, 12% wells and 1% hand-dug wells. Total dissolved solids (TDS) values are in the range of 159–20 957 mg l−1, discharge temperature varies from 15 to 99.5°C and waters are mainly strongly acid to slightly alkaline (pH ranging between 2.2 and 7.71). Springs discharge mainly from perched-water bodies, corresponding to Na-HCO3 and Na-HCO3-Cl-type waters, with a composition influenced by absorption of CO2 and mixing between meteoric water and boiling waters with a hydrothermal component. Instead the majority of the wells are distributed along the coast and depict chloride enrichment owing to mixture with seawater. The lower pH values are associated with boiling pools at Fogo Volcano with a SO4-dominated chemistry, resulting from steam-heating of shallow perched-water bodies.

In active volcanic regions the occurrence of mineral water discharges is a widespread phenomenon, both during dormancy periods and associated with eruptive episodes. Worldwide case studies describing the associated hydrogeochemical processes can be drawn from the literature in order to characterize water composition during volcano dormancy (Chiodini et al. 1996; Valentino et al. 1999; Valentino & Stanzione 2003; Brombach et al. 2000; Evans et al. 2006; Grassa et al. 2006; Minissale et al. 2007; D'Alessandro et al. 2008; Taran et al. 2008; Dotsika et al. 2009) or changes associated with the onset of eruptive processes (Shevenell & Goff 1993; Goff et al. 2000; Varekamp et al. 2001).

Mineral water discharges in the volcanic environment are also useful to human populations in several ways, as numerous springs have been developed as spas, used for domestic heat production, greenhouse heat supply and power production, as well as for geothermal resources investigation and as a tool for active volcano monitoring (Martini et al. 1991; Wohletz & Heiken 1992; Allard 1994; Oskarsson 1995; Bonfanti et al. 1996; Tassi et al. 2003; Federico et al. 2004; Valentino & Stanzione 2004; Inguaggiato et al. 2011).

Owing to the volcanic nature of the Azores archipelago, 101 mineral water discharges occur in seven of the nine islands, namely on São Miguel, Terceira, São Jorge, Pico, Faial, Graciosa and Flores. The Azores are located in the North Atlantic Ocean, about 1500 km west of mainland Portugal, between latitudes 36° 55′ 43″ and 39° 43′ 23″ N and longitudes 24° 46′ 15″ and 31° 16′ 24″ W; São Miguel is one of the easternmost islands in the archipelago (Fig. 19.1). The islands spread along a 600 km-long NW–SE-trending lineament and have a total area of 2333 km2 and 246 102 inhabitants.

Fig. 19.1.
Fig. 19.1.

Location of São Miguel Island in the Azores archipelago.

Mineral waters in the Azores present a large variability of chemical types and dissolved solids contents. The associated therapeutic properties of some discharges were described for the first time in popular accounts dating from 1614, and the first comprehensive chemical analysis was made public in 1791 (Cruz et al. 2010a). Since the last quarter of the nineteenth century, data on mineral waters composition have been published by several researchers, but as 75% of the discharges are located in São Miguel, most of the results that have been made public are associated with discharges on that island (Fouqué & Cabral 1873; Lepierre 1917; Acciaiuoli 1940; Carvalho 1955; Bencini et al. 1982; Cruz et al. 1999, 2010a; Cruz 2003; Cruz & França 2006; Morell et al. 2008).

The main objective of this chapter is to provide a hydrogeological setting of mineral discharges located in São Miguel, and present a review on the chemical composition of these waters. As mineral waters in São Miguel are by far the largest and chemically most diverse group in the Azores, conclusions about the major processes that control water chemistry may also be applicable to other islands in the archipelago.

Geological setting

All nine islands and the islets of the Azores are of volcanic origin, and the volcanic activity responsible for the existence of these islands varies from effusive eruptions, characterized by a steady extrusion of lava, to explosive events, with violent fragmentation. Therefore, where the dominant eruptive styles are of Hawaiian and Strombolian types, basaltic lava flows and scoria cones, often associated with fissural zones, are formed. When volcanism is associated with highly explosive eruptive styles, the geology is dominated by central volcanic edifices, truncated by calderas at their summit, and this explains the large areas covered by pumice fall and pyroclastic flow deposits that can be observed on certain islands. Only on Santa Maria do some sedimentary units occur that are interbedded in the volcanic succession. The oldest volcanic rocks are dated at 8.12±0.85 Ma and may be observed on Santa Maria (Abdel-Monen et al. 1975). Since the settlement of the archipelago, which took place in the fifteenth century, about 30 eruptions have occurred both inland and offshore. The last eruption was a submarine event offshore from Terceira Island, between 1998 and 2000 (Gaspar et al. 2003).

São Miguel's geology is dominated by three volcanic centres that correspond to the major active trachytic central volcanoes of Fogo, Sete Cidades and Furnas, linked by rift zones. Nevertheless, several volcanic complexes on São Miguel are well defined and, from oldest to youngest, the following units can be considered: Nordeste Volcanic Complex; Povoação Volcanic Complex; Furnas Volcanic Complex; Sete Cidades Volcanic Complex; Fogo Volcanic Complex; and the Picos and Congro fissural volcanic systems.

Eruptive activity on the three active central volcanoes is shown by 57 volcanic events that took place during the last 5 ka, with an output of 4.6 km3 of dense rock (Booth et al. 1978). For all the island of São Miguel it is possible to estimate an overall erupted volume of 400 km3 in the past 4 Ma, with an average volume of at least 0.01 km3 per century (Moore 1990).

The oldest volcano on São Miguel is the Nordeste shield volcano, a highly eroded edifice in the easternmost region of the island, where the oldest outcrop dated is about 4 Ma in age and the most recent is 0.95 Ma (Fernandez 1982). Thereafter volcanic activity has evolved towards the west, forming the remaining volcanic complexes, while Picos Fissural Volcanic System is the most recent, corresponding to an area dominated by basaltic volcanism, characterized by numerous scoria cones and frequently aligned along fractures trending WNW–ESE and NW–SE. The NW–SE trend corresponds to the orientation of the so-called Terceira Rift, a regional leaky transform fault defined by Machado (1959).

Hydrogeological setting

The climate of the Azores can be considered as marine temperate, with an average annual precipitation of 1930 mm, exceeding by far the average annual actual evapotranspiration (581 mm). From October to March about 75% of the annual precipitation occurs, which also influences recharge distribution over the year.

The average annual precipitation on São Miguel is 1722 mm, being generally higher in the east sector of the island, as well as in the north flank compared with the south flank, and increasing with altitude (Bettencourt 1979; DROTRH/INAG 2001).

Estimates for the average discharge for the Azores show a value of 322 hm3 a−1, with a range between 8 hm3 a−1 (Graciosa Island) and 1731 hm3 a−1 (São Miguel) (DROTRH/INAG 2001).

Recharge rates range from 8 to 62%, with the higher values being observed on Pico, Terceira, Faial, São Miguel and Graciosa (Cruz 2004). The volume of groundwater resources in the archipelago is about 1580 hm3 a−1, with values ranging between 8 hm3 a−1 (Corvo island) and 582 hm3 a−1 (Pico island). At São Miguel resources are 370 hm3 a−1.

Despite differences in the volcanic successions observed on the several islands, resulting from volcanic eruptions of various types, groundwater occurrence can be generally described in terms of two main aquifer systems: (1) the basal aquifer system, which corresponds to freshwater lenses floating on underlying salt water; and (2) perched-water bodies, which are usually drained by springs spread on the slopes of volcanic edifices (Cruz & Silva 2001). A recent survey made in the archipelago points to the existence of 1673 springs and 150 drilled wells throughout the archipelago and, in certain areas, there are numerous traditional hand-dug wells. On São Miguel about 1100 springs and 26 wells were identified.

The hydrogeology characteristics of the archipelago are heterogeneous, resulting from the influence of a large number of factors such as the depositional and post-depositional characteristics of the volcanic rocks, and secondary factors such as weathering and tectonics. Specific capacity is in the range of 1.4×10−2 to 266.7 l (s m)−1, with a median value of 22 l (s m)−1. Transmissivity values also present a large range, with values between 2.65×10−6 and 4.03×10−1 m2 s−1, and a median of 1.62×10−2 m2 s−1. At São Miguel, specific capacity ranges between 0.49 and 100l (s m)−1 (median=1.11 l (s m)−1) and transmissivity between 5.98×10−4 and 1.22×10−1 m2 s−1 (median=1.35×10−3 m2 s−1).

Groundwater chemistry in the basal aquifer system and in the perched-water bodies depicts compositional differences (Cruz & Amaral 2004). Groundwater on perched-water bodies usually has low mineralization, shown by electrical conductivity values (36–725 μS cm−1; median=158 μS cm−1), and the main water types are Na-Cl to Na-HCO3 waters, with numerous samples lying in the intermediate compositional fields that characterize Na-Cl-HCO3 and Na-HCO3-Cl waters. Groundwater composition in the basal aquifer system is usually of the Na-Cl type, presenting a higher mineralization (median value for electrical conductivity equal to 1044 μS cm−1), as a result of the mixture with a seawater component.

Groundwater pollution owing to agricultural activities has also been reported in the majority of the nine islands, especially in the perched-water bodies, as shown by microbiology parameters or by occasional high contents of nitrogen species (Cruz et al. 2010b). Salinization in wells drilled in the basal aquifer system is also a pressure over the groundwater quality and this process has led to well abandonment in several islands, causing severe economic losses (Cruz & Silva 2000; Cruz et al. 2011).

Methodology and results

The dataset on the chemical composition of mineral water discharges from São Miguel is composed of 76 samples. The literature survey was careful to observe sampling and analytical procedures to achieve homogeneity, selecting whenever possible one reference per island, with the major data sources being the most recent and comprehensive works by Freire (2006), Costa (2006) and Cruz et al. (2010a), which are based on extensive field surveys.

Cations were analysed by atomic absorption and anions by ionic chromatography, following filtration carried out in the field after sampling (0.45 µm cellulose membrane filter). Samples for atomic absorption analysis were acidified with suprapur® nitric acid (65%) immediately after sampling. Temperature, pH, alkalinity and free CO2 titrations and electrical conductivity measurements were conducted in the field with specific portable equipment. Silica was analysed by the silicomolybdic method, using a spectrophotometer. Trace elements were analysed by ICP-MS, following sampling filtration in the field (0.10 μm filters) and acidification with suprapur nitric acid. The ion-balance values (the difference between the sum of cations and the sum of anions) show that 62.67% of the samples were within a ±5% interval and 78.67% of the samples were within a ±10% interval.

Discharges are not equally spread on the island, which reflects the influence of local hydrogeological conditions. The distribution of the discharges shows an association with the three active Quaternary central volcanoes that dominate the geology of São Miguel, in particular Furnas and Fogo volcanoes (Fig. 19.2). In Furnas and Fogo regions there are 45 (references AM1 to AM45) and 24 (references AM47 to AM70) discharges, respectively, whereas only six discharges are located at Sete Cidades (AM71 to AM76). The discharge identified as AM46 is located in the Povoação Caldera. Figure 19.2 also shows that discharges in Furnas and Fogo volcanoes are mainly associated with perched-water bodies, while at Sete Cidades all discharges are from coastal aquifers.

Fig. 19.2.
Fig. 19.2.

Distribution of mineral water discharge in São Miguel.

From the 76 mineral water discharges sampled on São Miguel, 74% are springs, 13% fumaroles, 12% wells and 1% hand-dug wells. Springs are mainly discharging from perched-water bodies, in the altitudinal range of 100–300 m, as only seven discharges occur from coastal aquifers at altitudes lower than 10 m. The majority of the wells are distributed along the coast, and only two wells (20%) are located on perched-water bodies.

Major-ion composition is reported in Table 19.1, while minor and trace element composition is reported in Table 19.2. Mineral waters in the dataset present a variable dissolved solids content, as shown by the electrical conductivity measurements, which range from 139 to 28 000 μS cm−1 (median=587 μS cm−1) and the total dissolved solids (TDS) content (159–20 957 mg l−1; median=949 mg l−1). Higher values are, in general, observed at Sete Cidades from discharges that emerge from coastal aquifers (AM71–AM76). Discharge temperatures range from 15°C to 99.5°C (median=35.6°C) and waters are mainly strongly acid to slightly alkaline, with pH ranging between 2.2 and 7.71 (median=5.71). The lower pH values are attained at samples AM58 to AM60 collected at Fogo Volcano (2.2–3.53) and sample AM40 from Furnas volcano (pH 2.8). Free dissolved CO2 ranges from 8.2 to 1126.2 mg l−1 (median=423 mg l−1), and the high CO2 in some mineral waters in São Miguel, as well as in the Azores, explains the qualification of ‘bitter waters’ given by the general public.

View this table:
Table 19.1.

Major-ion composition of mineral water discharges in São Miguel Island

View this table:
Table 19.2.

Minor and trace element composition of mineral water discharges in São Miguel Island

Discussion

Water chemistry is represented by plotting the samples on a Piper-type diagram, where three main groups of waters are defined (Fig. 19.3): (1) Na-HCO3 and Na-HCO3-Cl-type waters, to which almost all the thermal and cold CO2-rich waters belong, presenting a major ion relative content similar to the majority of cold springs discharging from perched-water bodies in the Azores (Cruz & Amaral 2004); (2) Na-Cl-type waters, to which samples from Sete Cidades area belong (AM71–AM76); and (3) Na-SO4-type waters, to which the boiling waters of Caldeira Velha (AM63) and Caldeiras da Ribeira Grande (AM58) belong. Samples of the Na-SO4-Cl-HCO3 and Na-SO4-HCO3-Cl types present intermediate compositions between HCO3-Cl- and SO4-dominated chemistries (samples AM61 and AM62).

Fig. 19.3.
Fig. 19.3.

Major-ion relative composition of the mineral waters in São Miguel represented by means of a Piper diagram.

Bicarbonate content controls water mineralization and samples with low electrical conductivity show a close positive relationship with HCO3 content (Fig. 19.4). On the same plot, samples AM65, AM71, AM73, AM74, AM75 and AM76 depict another trend, suggesting an increase in water mineralization independent of HCO3 content. This secondary trend is clearly shown on Figure 19.5, where the relationship between electrical conductivity and Cl content in the studied discharges is plotted, suggesting that the higher dissolved solids content of these samples is associated with a marine contribution, which explains the close relation between both variables and a Na-Cl-dominated composition (Fig. 19.6). Nevertheless, as shown by Cruz & França (2006), these samples present a HCO3 content clearly above that expected considering a binary mixture process between seawater and freshwater, with a seawater fraction in the range of 10–70.6%, suggesting also the hypothesis of the contribution of other processes on water chemistry, such as water–rock interaction and input of magmatic volatiles.

Fig. 19.4.
Fig. 19.4.

Relationship between electrical conductivity and bicarbonate content in mineral waters.

Fig. 19.5.
Fig. 19.5.

Relationship between electrical conductivity and chloride content in mineral waters.

Fig. 19.6.
Fig. 19.6.

Relationship between sodium and chloride content in mineral waters.

This contribution also explains why the majority of thermal waters on São Miguel lie in the peripheral water field on the Cl-SO4-HCO3 ternary diagram (Fig. 19.7). Na-HCO3-type waters are frequently discharged at the periphery of geothermal fields, where HCO3 enrichment is due to absorption of CO2-rich volatiles in the shallow groundwater, as shown for Furnas Volcano springs by Cruz et al. (1999), where a mantle CO2 contribution is indicated by δ13C data and, to a certain degree, by water–rock interaction, and the observed HCO3 enrichment is a result of neutralizing fluid acidity (Cioni et al. 1992; Chiodini et al. 1996; Lewicki et al. 2000). Silicate hydrolysis reactions can be considered the most important type of reactions in volcanic regions, contributing with alkali metals or alkali-earth metals, if rocks are more or less chemically evolved, while neutralizing fluid acidity and releasing HCO3 (Evans et al. 2002; Cruz & Amaral 2004). In the Azores these springs are the most common surface manifestations of geothermal reservoirs at depth and represent early stages of interaction between meteoric water and rocks (Cruz & França 2006).

Fig. 19.7.
Fig. 19.7.

Cl-SO4-HCO3 ternary diagram (Giggenbach 1988) for thermal waters in São Miguel.

The contribution of water–rock interaction to mineral water composition is shown by the relationship between HCO3 and SiO2, which depicts a common enrichment in mineral waters (Fig. 19.8). This contribution is controlled by water temperature, as suggested by the close relationship between this variable and the SiO2 content of mineral waters (Fig. 19.9).

Fig. 19.8.
Fig. 19.8.

Relationship between bicarbonate and silica content in mineral waters.

Fig. 19.9.
Fig. 19.9.

Relationship between mineral water temperature and silica content.

The relationship between water conductivity and SO4 content shows a group of mineral waters (AM40, AM58, AM63 and AM71) that depict very high sulphate contents, not associated with a marine fraction, as for samples AM73 to AM76, suggesting that water composition is controlled by a steam-heating process (Fig. 19.10). This process is also shown in Figure 19.7, as the same samples lie close to or in the steam-heated water field. The oxidation of reduced sulphur species in the shallow steam-heated aquifer explains the SO4 content and the lower pH values observed in São Miguel waters, as observed in similar conditions worldwide (Nordstrom et al. 2009; Varekamp et al. 2009).

Fig. 19.10.
Fig. 19.10.

Relationship between electrical conductivity and sulphate content in mineral water.

Using the minor and trace element relative composition shown on a Cl/1000–B–Li ternary plot, it is possible to depict the major geochemical processes that modify mineral water chemistry (Fig. 19.11). From the plot, it is possible to identify samples that lie close to the B corner (AM3, AM9, AM10, AM35, AM36 and AM40), and these high boron contents may be associated with absorption of low-Cl and high-B steam owing to hydrothermal processes at depth or rock leaching. The values of δ11B determined in mineral waters from São Miguel range between −6.2 and +6.3‰, but the narrower interval observed at Furnas volcano (−2.9 to +3.3‰), as well as the lack of a clear relation with B content, make it impossible to distinguish between geochemical processes, at least with regards to boron origin (Morell et al. 2008). Some samples show a trend towards the opposite corner of Li, suggesting a variable contribution of water–rock interaction processes (such as, for example, AM56-AM57). The marine contribution, also variable, is reflected by the samples that define a trend towards the relative seawater composition (AM48, AM49, AM53, AM58, AM64 and AM73 to AM76). The position of these groups suggests mixture between the marine evolution trend and samples showing high-B contents, suggesting that the contribution of water–rock interaction is also variable.

Fig. 19.11.
Fig. 19.11.

Cl/1000–B–Li ternary plot for mineral waters in São Miguel.

Nevertheless, and besides the considered geochemical processes, δ18O and δ2H data suggest that, despite a common meteoric origin, mixture between cold minerals waters and boiling thermal waters occurs at Furnas Volcano (Cruz & França 2006).

The application of the Na/1000–K/100–Mg1/2 ternary plot, defined by Giggenbach (1988), in order to couple Na–K and K–Mg geothermometers, shows that the majority of the samples lie close to the Mg1/2 corner, indicating immature conditions (Fig. 19.12). A trend is depicted by samples AM9 and AM10, suggesting an equilibration temperature of about 160°C for the geothermal fraction associated with their origin, despite measurements conducted in deep geothermal wells on Fogo Volcano that indicate a higher temperature, in the range of 230–240°C (Carvalho et al. 2006).

Fig. 19.12.
Fig. 19.12.

Na/1000–K/100–Mg1/2 ternary plot for thermal waters in São Miguel.

Saturation indices with respect to some mineral species, calculated at the measured discharge temperature, also reflect immature conditions (Cruz et al. 2010a). The majority of the waters are undersaturated with carbonate minerals (calcite and dolomite), reflecting their low alkali-earth metals content, oversaturated with quartz and slightly oversaturated with amorphous silica, which is typical of waters in volcanic terrains (Figs 19.13a, b). The saturation status with plagioclase end-members shows that mineral waters are mainly oversaturated in albite and undersaturated in anorthite, depending on the relative enrichment in alkali metals over alkali-earth metals, as well as the different standard-state thermodynamic parameters of relevant minerals and aqueous species (Fig. 19.13c).

Fig. 19.13.
Fig. 19.13.

Saturation status for several solid phases: (a) carbonate minerals; (b) silica; and (c) plagioclase end-member.

Summary and conclusions

On São Miguel 76 discharges of mineral waters were identified, associated with volcanic active structures, showing variable compositions. Electrical conductivity ranges from 139 to 28 000 μS cm−1 (median=587 μS cm−1) and the TDS content from 159 to 20 956.5 mg l−1 (median=949 mg l−1). Discharge temperature ranges from 15 to 99.5°C (median=35.6°C) and waters are mainly strongly acid to slightly alkaline, with a pH ranging between 2.2 and 7.71 (median=5.71). TDS content for the cold waters range from 159.11 to 1455.52 mg l−1, with a pH range from 4.61 to 6.77. In contrast, thermal waters present a higher TDS content, in the range from 196 to 20 956.5 mg l−1, and also a wider pH range (2.2–7.71).

In summary, this work follows the conceptual model proposed by Cruz & França (2006) and Cruz et al. (2010a), which defines a classification of mineral waters in the Azores according to their chemical composition and hydrogeological setting. The majority of the mineral discharges occur from perched-water bodies within the flanks of the major active trachytic central volcanoes of Fogo, Sete Cidades and Furnas, corresponding to Na-HCO3 and Na-HCO3-Cl types. The chemistry of these waters is mainly influenced by the absorption of magmatic volatiles, which have a CO2-dominated composition, as shown by several studies made on gaseous emanations, and by water–rock interaction. However, despite the enhancing of rock leaching owing to acidic conditions in the presence of in-depth CO2 input, for the cold mineral waters, which present a low TDS content, water–rock interaction is considered to be limited, typically from hydrogeological media characterized by high groundwater flow velocities and short residence times.

The highest temperatures observed in waters from boiling pools occurring in fumarolic fields, which are mainly from the Na-HCO3 type, are suggested to be representative of a hydrothermal component and discharge also from perched-water bodies. Thermal water discharges with a large range of temperatures are also influenced by mixing between meteoric water and boiling waters. Nevertheless, some samples collected in boiling pools are from the Na-SO4 water type, resulting from steam-heating of shallow perched-water bodies, with oxidation of H2S associated with the lowest pH values observed in natural waters on São Miguel.

In coastal aquifers, mineral waters may show Na-Cl-dominated compositions owing to mixture with seawater, such as for example the discharges on Sete Cidades Volcano, where the seawater fraction in groundwater composition reaches values as high as 71%. Local hydrogeological conditions, such as highly permeable coastal aquifers characterized by thin freshwater lenses overlying salt water, favour both seawater intrusion and mixing with mineral water.

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