- Email: [email protected]
Major ion chemistry of groundwater from perched-water bodies of the Azores (Portugal) volcanic archipelago
Applied Geochemistry 19 (2004) 445–459 www.elsevier.com/locate/apgeochem
Major ion chemistry of groundwater from perched-water bodies of the Azores (Portugal) volcanic archipelago J. Virgı´lio Cruz*, Catarina S. Amaral Department of Geosciences, Azores University, Apartado 1422, 9501-801 Ponta Delgada, Portugal Received 3 February 2003; accepted 10 June 2003 Editorial handling by R.L Bassett
Abstract A dataset of major ion composition of 246 samples from cold-water springs discharging from perched-water bodies at volcanic islands (Azores archipelago, Portugal) reveal waters with low mineralization, which evolve due to two main geochemical processes: (1) seawater spraying and (2) dissolution of primary minerals of volcanic rocks. As a result, water facies range from Na–Cl to Na–HCO3 type waters. The relationship between alkali, alkali–earth metals and HCO3 shows differences between waters discharging from perched-water bodies in basaltic rocks comparing to more evolved rocks of trachytic nature. The use of principal component analysis shows that water-rock interaction is limited, which is compatible with the geochemical observations and with the hydrogeological environment. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Background Despite the fact that volcanic rocks correspond to a small fraction of the earth surface, hydrogeology of aquifers made of volcanic deposits is a vital research issue in numerous islands and continental areas where these materials are prominent (Peterson, 1972, 1993; Custo´dio, 1978; Hunt et al., 1988; Stieltjes et al., 1988; Ingebritsen and Scholl, 1993; Lau and Mink, 1995; Yamamoto, 1995; Cruz and Silva, 2001). Studies on the geochemistry of aqueous fluids in volcanic regions have been conducted in rivers worldwide in order to study weathering of basaltic rocks (Gı´slason et al., 1996; Louvat, 1997), giving an input to the study of geochemical cycles and atmospheric CO2 consumption. Data on groundwater composition in basaltic aquifers are also encountered in the literature (Gı´slason and Eugster, 1987; Silva, 1988; Wood and Fernandez, 1988; Gı´slason and Arno´rsson, 1993; Join et al., 1997; Allard * Corresponding author. Tel.: +351-296650143; fax: +351296650141. E-mail address: [email protected] (J.V. Cruz).
et al., 1997; Cruz and Silva, 2000; Aiuppa et al, 2000). There are fewer data related to groundwater composition for aquifers consisting of volcanic rocks of acid nature (Giggenbach, 1992; Rowe et al., 1995). Due to the volcanic nature of the Azores, groundwater geochemistry is partially influenced by the dissolution of primary minerals from volcanic rocks. The release of major ions depends on the saturation states of primary minerals, precipitation of secondary minerals, the aqueous chemistry of each component leading to formation of soluble and insoluble species, and the acidic character of the environment (Aiuppa et al., 2000). However, as in other areas, the chemical composition of the groundwater depends also on other factors, such as rainfall chemistry, climate, rock type, residence time of water, rock division, temperature and pressure (Custo´dio, 1989). In addition an anthropogenic input of variable magnitude is also to be considered and, in the case of islands or coastal regions the interaction with seawater can be relevant (Cruz and Silva, 2000; Cruz, 2001a; Kim et al., 2003). Rock contribution to groundwater composition in volcanic regions is also strongly dependent on in-depth volatile release (Brusca et al., 2001; Federico et al., 2002). In the Azores, numerous mineral and thermal springs show the influence of these processes (Cruz et
0883-2927/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0883-2927(03)00135-5
446
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
al., 1999; Cruz and Franc¸a, 2001a, in press; Cruz, in press). In certain areas of the archipelago, volcanic CO2 diffusive emanations were studied, sometimes corresponding to a hazardous situation for inhabitants, and the CO2-dominated composition of the gaseous phase in fumaroles was clearly established (Baxter et al., 1999; Ferreira and Oskarsson, 1999).
The contribution of volcanic rock dissolution to the groundwater relative anionic composition is considered to be limited, and in humid or semi-humid climates HCO3 +CO23 are dominant due to CO2 in soil and in rain water (Custo´dio, 1989). Therefore, and especially in islands like the Azores, atmospheric deposition is very important. Previous studies using trend surface analysis
Fig. 1. Geologic framework of the Azores archipelago. The number near the islands corresponds to the maximum age in Ma (data from Abdel-Monem et al., 1975; Fe´raud et al., 1980; Azevedo, 1998).
Fig. 2. Density of springs and drilled wells in the Azores archipelago (data from Cruz, 2001b).
447
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Table 1 Groundwater chemical data statistics: main variables (pH, temperature, electrical conductivity) and major ions composition. Island description: SM- Sa˜o Miguel; SA- Santa Maria; TE- Terceira; SJ- Sa˜o Jorge; GR- Graciosa; PI- Pico; FA- -Faial; FL- Flores; CO- Corvo Island
SM
SA
TE
SJ
GR
PI
FA
FL
CO
Azores
Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min.
ph
T ( C)
Cond (mS/cm)
HCO3 (mg/l)
Cl (mg/l)
SO4 (mg/l)
SiO2 (mg/l)
Na (mg/l)
K (mg/l)
Ca (mg/l)
Mg (mg/l)
7.11 0.69 7.26 8.20 4.70 6.11 0.67 6.05 8.09 4.80 7.11 0.46 7.20 8.60 6.00 6.90 0.32 6.90 7.50 6.40 7.33 0.30 7.30 8.10 6.80 7.45 0.18 7.45 7.70 7.20 7.34 0.36 7.30 7.80 6.50 7.19 0.18 7.20 7.50 6.80 6.97 0.47 7.01 7.55 6.02 7.01 0.67 7.20 8.60 4.70
14.25 1.60 14.20 19.00 8.80 17.41 1.50 17.50 20.70 14.20 14.00 1.42 13.50 17.00 11.00 14.55 0.84 14.50 16.00 12.50 15.31 0.50 15.50 16.00 14.50 14.05 0.68 14.05 15.00 13.10 14.59 2.20 13.65 19.80 12.40 16.00 0.83 16.00 18.00 14.00 15.90 0.74 15.90 16.80 14.30 14.97 1.83 15.00 20.70 8.80
174.12 78.33 152.00 440.00 36.00 304.71 119.78 270.00 725.00 125.00 154.52 100.49 138.00 607.00 40.00 172.00 73.71 148.00 336.00 100.00 277.15 64.24 258.00 444.00 195.00 110.38 26.18 104.25 151.00 82.00 132.37 48.81 122.50 237.00 79.00 146.79 53.19 140.00 270.00 70.00 222.11 71.82 221.00 367.00 148.00 188.89 97.63 157.95 725.00 36.00
49.63 24.26 45.75 152.50 10.37 64.21 41.48 54.29 207.40 4.88 37.71 20.52 33.10 112.00 16.30 27.00 7.59 25.00 39.00 17.00 44.45 14.51 45.00 69.60 25.00 32.53 5.47 33.55 39.00 24.00 45.33 18.93 43.30 84.20 24.40 42.86 14.36 42.15 77.00 18.30 31.52 9.92 31.72 51.24 17.69 47.08 25.83 41.65 207.40 4.88
29.94 15.27 25.56 96.92 12.07 65.68 23.52 58.93 134.19 32.66 28.97 14.94 24.90 78.10 13.60 28.10 11.16 24.90 46.20 14.20 55.76 18.15 44.10 90.60 40.10 16.70 1.71 16.00 19.50 15.30 16.36 9.37 13.10 46.80 8.20 32.64 10.45 28.40 60.40 21.30 50.10 19.24 40.12 94.79 34.79 35.58 21.11 28.40 134.19 8.20
9.44 7.03 7.40 32.50 1.32 7.97 4.72 7.00 19.30 1.20 5.63 6.73 4.10 36.20 2.20 6.32 5.68 3.80 21.60 1.60 7.35 1.37 7.10 10.20 4.90 5.78 3.52 4.20 11.80 2.90 4.86 1.77 4.10 8.40 3.30 5.10 2.27 5.15 9.00 1.20 8.30 1.48 8.70 10.66 5.96 7.75 5.93 6.30 36.20 1.20
47.93 19.10 46.50 121.10 4.00 25.18 12.59 24.59 53.93 2.98 35.21 12.44 33.90 68.50 3.80 18.79 6.66 18.40 31.10 11.70 34.28 5.47 34.50 47.20 27.40 20.15 5.42 17.45 29.50 16.20 40.73 9.10 40.30 56.50 19.00 29.72 8.64 29.20 50.10 17.50 29.12 8.11 27.80 42.60 17.80 38.17 17.86 36.05 121.10 2.98
28.23 12.67 26.00 72.00 9.00 44.10 22.98 39.42 131.20 17.81 29.90 14.50 24.20 82.10 17.50 21.23 8.66 18.20 35.40 11.60 32.74 7.48 30.30 52.50 24.20 11.05 2.18 10.35 14.60 8.90 18.29 5.54 18.20 30.40 11.20 26.51 4.92 26.10 36.10 16.30 29.68 11.07 28.20 54.20 19.20 29.24 14.84 26.30 131.20 8.90
6.94 3.58 6.50 22.10 1.20 2.18 1.01 2.06 4.38 0.12 5.88 13.53 2.00 68.90 1.60 3.71 7.09 1.00 25.70 0.30 3.59 1.30 3.10 6.10 1.80 1.98 0.46 1.95 2.60 1.40 3.93 1.80 3.45 8.40 1.30 2.68 1.66 1.85 7.10 0.80 3.03 2.46 2.50 9.40 0.83 5.01 5.48 3.80 68.90 0.12
5.01 2.79 4.56 14.03 0.40 10.92 4.30 9.60 24.00 4.00 3.54 2.13 3.80 6.30 0.10 3.29 2.45 2.40 9.50 1.00 11.90 6.21 7.50 23.70 6.20 4.35 1.25 5.00 5.20 2.20 4.85 3.04 4.35 14.20 1.40 6.52 3.00 6.35 12.50 1.70 6.55 2.54 6.41 12.42 3.41 6.11 4.10 5.54 24.00 0.10
3.27 2.36 2.70 13.22 0.20 10.20 4.79 9.74 25.79 2.19 4.03 4.29 3.60 23.30 1.20 4.67 1.58 5.20 7.20 2.50 7.88 2.16 6.60 12.50 6.00 3.58 0.35 3.70 3.90 3.00 1.58 1.23 1.05 4.90 0.50 3.95 1.91 3.95 10.20 0.30 4.39 1.55 4.02 6.87 2.55 4.53 3.75 3.54 25.79 0.20
448
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 3. Piper diagram showing major ion chemistry of cold springs discharging from perched-water bodies at Azores.
have shown that areas where springs discharging from perched-water bodies are particularly enriched in Cl correspond to regions exposed to dominant winds (Cruz et al., 1992). Based on the cationic content it is possible to establish a relationship between the groundwater composition and the rock type, and several relationships have been proposed in the literature: olivine-rich basalts can show values of r(Mg+Ca)/r(Na+K) higher than 10, while volcanic rocks of acid nature can show values lower than 0.1 (Custo´dio, 1989). Grunberger et al. (1988) have also proposed the use of the ratio between the activities of Mg2+ and H4SiO04 as a tool to discriminate between groundwater discharging from basaltic rocks and from more evolved rocks. Despite the progress in understanding groundwater composition made in the Azores by several authors, an effort was needed to compile all the data provided and characterize groundwater composition at the archipelago scale. This work compares results, looking to similarities that are explained by geological and hydrogeological features. The present paper, following a previous contribution by Cruz and Franc¸a (2001b), is directed to fill
that gap. In order to study relations between variables and samples principal component analysis is applied. 1.2. Study area The Azores is an archipelago of volcanic nature made of 9 islands and a few islets, about 1000 miles from the mainland of Portugal. Located between 37–40 N latitude and 25–31 W longitude, the archipelago has an area of 2,333 km2 and approximately 246,030 inhabitants, mainly living on Sa˜o Miguel (54.1%) and Terceira (23.3%) islands. The islands are spread along a NW trending strip, about 500 km long. The Azores climate can be considered as marine temperate, and is characterized by a sharp difference between a colder, wet season that occurs between October and March and when 75% of the annual precipitation falls, and a dry season. Average annual precipitation is equal to 1,930 mm, and exceeds by far the average annual actual evapotranspiration, which is equal to 581 mm (DROTRH-INAG, 2001). Groundwater is a vital resource in the Azores archipelago, playing important roles as a drinking water
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
449
Fig. 4. Cl (in meq/l) vs. Na+ (in meq/l) plot for the studied waters (best fit line: y=0.756x+0.042).
source, and as an ecosystem support. Despite the environmental, social, and economic value of groundwater, aquifers of the Azores support increased use, which is reflected in groundwater quality deterioration. In the Azores about 97% of the water supply is due to groundwater sources, corresponding to annual volume of 24.7106 m3. This exceeds by far the water withdrawal from surface sources (0.8106 m3; DROTRHINAG, 2001). Groundwater sources correspond mainly to springs discharging from perched-water bodies (444 springs). Therefore, the geochemical characterization of the fluids discharging in these springs is a key issue to groundwater management in the Azores and the present dataset was pivotal for regional water planning.
2. Geological and hydrogeological setting The archipelago of the Azores is made of 9 islands entirely of volcanic origin. The islands geologic history includes successive volcanic eruptions of various types, from Hawaiian and Strombolian eruptions that produced basaltic s.l. lava flows, to highly explosive events related to more acidic magmas accounting for the large areas on certain islands covered by pumice-fall deposits and pyroclastic flow deposits (ignimbrites). The landscape also shows the differences between the different eruptive styles, from prominent central volcanoes trun-
cated at the summit by large calderas due to highly explosive eruptions, and fissure zones and scoria cone fields typical of eruptions of magma of basic nature. In Fig. 1 the geological setting of the Azores archipelago is summarized. The oldest outcrop of the archipelago was dated from about 8.12 Ma at Santa Maria island (Abdel-Monem et al., 1975). Nevertheless, a large portion of the archipelago has potentially active volcanoes, and since settlement in the 15th century more than 20 eruptions have taken place. The last eruption was a submarine event 10 km NW of Terceira island that took place between 1998 and 2000. Hydrogeology characteristics in the archipelago are heterogeneous, depending on multiple factors, from the depositional and post-depositional characteristics of the volcanic rocks, to secondary factors such as weathering and tectonics. Specific capacity is on the range of 1.4010 2–266.67 Ls 1 m 1 (n=65), with a median of 32.29 Ls 1 m 1 (Cruz, 2001b). Transmissivity also shows a large range of values, between 1.6510 5 and 4.0310 1 m2 s 1, and a median of 3.6610 2 m2 s 1. The higher values correspond to wells drilled in fissured basaltic lava flows, while the lower estimates are from wells in volcanic deposits of similar origin but much older and weathered. Studied recharge rates range from 8.5 to 62.1% of the total precipitation; the higher values correspond to areas where soil cover is sparse over fractured basaltic
450
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 5. Relationship between electrical conductivity (in mS/cm) and: (a) Cl (in meq/l; best fit line: y=0.005x+0.025); (b) HCO3 (in meq/l; best fit line: y=0.003x+0.236).
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
451
Fig. 6. Cl (in meq/l) vs. SO24 (in meq/l) plot for the dataset. Seawater chemistry data from Cruz (1997) and thermal discharge composition from Cruz and Franc¸a (in press): (a) Caldeira Velha boiling pool; (b) Caldeira Velha spring.
lava flows (Cruz, 2001b; Cruz and Silva, 2001). Groundwater resource calculations indicate annual recharge of 1.6109 m3/a, asymmetrically distributed all over the archipelago (Cruz, 2001b). Groundwater recharge at Corvo, the smallest island, is estimated to be 8.3106 m3/a, while on Pico, the second largest island recharge is 5.8108 m3/a. Values higher than the median (1.0108 m3/a) are observed also at Sa˜o Miguel, Sa˜o Jorge, Terceira and Flores islands. Groundwater occurs in two main aquifer systems: (1) the basal aquifer system, corresponding to fresh-water lenses floating on underlying salt water, generally with a very small hydraulic gradient and with water quality affected by mixing with seawater, and (2) perched-water bodies which correspond to confined layers or to leaky aquifers, at elevation, where waters generally have a small residence time as shown by the low dissolved solids content.
3. Methodology The dataset on the major ion composition of cold spring discharges from perched-water bodies of the Azores is made of 246 samples. The literature survey
was made in order to evaluate uniformity of sampling and analytical procedures, selecting whenever possible only one data source per island. These springs are not equally distributed on the islands, reflecting the influence of local hydrogeological conditions that constrain the spring density (Fig. 2). This is shown by the Pico island example, which despite being the second largest island in the Azores, with an area equal to 450.7 km2, has only 1.3310 2 springs/km2 due to the occurrence of high permeability volcanics, often densely fissured (Cruz, 2001b; Cruz and Silva, 2001). Results from several islands are used in this study: Santa Maria (Cruz, 1992; Cruz et al., 1992), Sa˜o Miguel (Coutinho, 1990; Lobo, 1993; Cruz et al., 1999; Carvalho, 1999), Terceira (Lobo, 1993), Sa˜o Jorge (Lobo, 1993), Pico (Cruz, 1997; Cruz and Silva, 2000), Faial (Coutinho, 2000), Graciosa (Lobo, 1993), Flores (Lobo, 1993) and Corvo island (Cruz et al., 2002). All analysis with charge balance higher than 10% were rejected. Field measurements of temperature, pH and electrical conductivity, as well as alkalinity titration with 0.05M H2SO4 to a pH of 4.45, were made immediately after sampling. Cations were analysed by atomic absorption spectroscopy (AAS) and anions by ion chromatography (IC), following filtration carried out in the field after sampling (0.45 mm cellulose membrane filter). Samples
452
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 7. HCO3 (in meq/l) vs. Na++K+ (in meq/l) plot for the dataset (seawater chemistry data from Cruz, 1997). Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
for AAS were acidified by concentrated HNO3 following sample collection. SiO2 was analysed mainly by visible spectrophotometry.
4. Results Major-ion compositions including statistical measures (average, standard deviation, median, maximum, minimum) are reported in Table 1 for all the samples included from the Azores archipelago. Groundwaters in this dataset have low mineralization, as shown by the electrical conductivity measurements, which range from 36 to 725 mS/cm, with an average value equal to 189 mS/cm and a median of 158 mS/cm. Higher values are generally observed at Santa Maria, Graciosa and Corvo islands. As the dataset summarizes only cold-water discharges are studied. The temperature distribution reflects this feature, and the average value is equal to 15 C. Waters are mainly slightly acid to slightly alkaline, with a pH ranging between 4.70 and 8.60, but showing an average value equal to 7.01 and a median equal to 7.20. Lower values are attained at Sa˜o Miguel and Santa Maria. Only at Santa Maria island is the pH median above neutrality by almost one pH unit.
Groundwater composition is dominated by Na++K+ and Cl +HCO3 , which is reflected in the water types that vary between Na–Cl to Na–HCO3 waters. Numerous samples lie in the intermediate compositional fields that characterize Na–Cl–HCO3 and Na–HCO3–Cl waters in the Piper plot (Fig. 3). Chloride and Na+ can account for respectively 7.3– 40.8% and 18.7–50.4% of the relative anionic and cationic content of the groundwater (in meq/l). These major ionic species in solution are positively correlated (r=0.819; Fig. 4), and their contribution to the overall chemical composition of the groundwater is shown by the correlation between Cl and the electrical conductivity (r=0.848; Fig. 5a). These results suggests the influence of seawater spraying in the groundwater compositional evolution, which is essentially explained due to the seawater control of the rain water chemistry over islands and coastal regions worldwide (Berner and Berner, 1996). Other samples show HCO3 enrichment, which can account for 3.4–43.2% of the relative anionic composition. Fig. 5b shows the relationship with electrical conductivity and depicts the contribution of HCO3 to the groundwater chemistry. Despite the positive correlation between these variables, the correlation coefficient is lower than for Cl (r=0.653). The relationship between Cl and SO24 shows that the majority of the samples are plotted near the
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
453
Fig. 8. HCO3 (in meq/l) vs. Ca2++Mg2+ (in meq/l) plot for the dataset (seawater chemistry data from Cruz, 1997). Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
seawater line, suggesting a marine contribution to water composition (Fig. 6). The weak correlation (r=0.414) and the scattered pattern, with numerous samples plotted above the seawater line, suggest a volcanic contribution beyond sea salts input, which explain the SO24 enrichment by oxidation of reduced S species from volatile releases. To show this effect on the Cl /SO24 ratio, two SO4-rich thermal water discharges from Sa˜o Miguel island, representing waters that evolve by steam heating and mixing with meteoric cold waters, are also plotted in Fig. 6 (lines a and b). This plot shows that numerous samples fall in the field between the seawater line and the steam-heated thermal discharges line. These samples are mainly from Sa˜o Miguel and correspond to springs located in active volcanic centres of Quaternary age (Furnas and Fogo).
5. Discussion The alkali metals present are positively correlated with HCO3 (r=0.707) and the data depict an excess of Na++K+ in relation with HCO3 as shown by the fact that waters fall in the field above the Na++K+=HCO3 line (Fig. 7). Nevertheless, the trend defined by the data deviates from seawater composition and therefore suggests other mineralization processes
besides seawater spraying alone. If the HCO3 is derived from CO2 in the soil, then the enrichment in alkalis may be attributed to silicate weathering, which also contributes to increased HCO3 content. From incongruent dissolution of primary aluminosilicate minerals in volcanic rocks, an enrichment of alkali content over Cl is expected, as Cl is practically unavailable for dissolution during water percolation (Join et al., 1997). In Fig. 4 it is possible to show that the line of best fit is plotted in the field under the Cl =Na+ line, clearly pointing toward a Na+ enrichment in solution beyond the rain composition. The relationship between alkali–earths and HCO3 shows a more scattered plot with less correlation (r=0.522; Fig. 8), suggesting a different behaviour between islands. The majority of the samples from Sa˜o Miguel, Terceira and Faial, where perched-water aquifers are made of more alkaline rocks, plot in the field beneath the Ca2++Mg2+=HCO3 line. By contrast, samples from islands where perched-water bodies are made of basic rocks, including Santa Maria, Sa˜o Jorge, Graciosa, Corvo, Pico and Flores, plot mainly above the Ca2++Mg2+=HCO3 line and show stronger correlation (r=0.739). The relationship between alkali and alkali–earth metals depicts a scattered pattern, and the samples plot between the basic and acid rock compositions used for
454
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 9. Na++K+ (in meq/l) vs. Ca2++Mg2+ (in meq/l) plot for the dataset [seawater chemistry data from Cruz (1997); basaltic and trachytic rocks s.l. data from Schmincke and Weibel (1972) and White et al. (1979)]. Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
reference (Fig. 9). This is consistent with the slight water mineralization and shows that these springs discharge waters with very short residence times, in an environment characterized by high groundwater flow velocities. This implies a high water-rock ratio (Langmuir, 1997) and the effects of silicate hydrolysis on water composition are limited. Characterization of hydrographs from springs in Santa Maria and Corvo islands revealed a two months delay between precipitation events and spring discharge, which shows the short residence time of groundwater in these aquifers and is consistent with geochemical data interpretation (Cruz et al., 1997, in press). Groundwater from Sa˜o Miguel, Terceira and Faial mainly plot near the seawater line, with ratios similar to rain water, while samples from islands with perchedwater aquifers of basic nature fall mainly in the field between the Ca2++Mg2+=Na++K+ and seawater lines. These observations for the latter islands suggest that the enrichment of alkali–earth metals is caused by silicate hydrolysis as aquifers have a typical basaltic mineral assemblage of Ca-rich plagioclases, pyroxenes and olivines. In fact, silicate hydrolysis reactions can be consider the most important types of reactions in volcanic terranes, contributing alkali metals or alkali–earth metals if rocks are, respectively, more or less chemically
evolved (Evans et al., 2002). The effect of this lithologic control over water composition has already been shown in the Azores for the river waters of Sa˜o Miguel island, where the Ca/Na ratio is sensitive to trachytic, basaltic or intermediate rock composition (Louvat and Alle`gre, 1998) Lithologic influences on groundwater composition are also shown by the relationship between Ca2+ and the complex H4SiO04 which was determined using HIDSPEC speciation software (Fig. 10; Carvalho and Almeida, 1989). The use of the Ca2+/H4SiO04 ratio to separate lithologic influences was made in order to prevent seawater signature bias, which influences Na+, K+ and Mg2+ content to a larger extent, and assuming that lithologic influence can be viewed in simplest terms as a result of silicate rocks dissolution that neutralises fluid acidity and releases SiO2 and cations. In this plot a sharp difference between samples from Sa˜o Miguel, Terceira and Faial, which drain more evolved rocks, and waters from other islands is shown. The highest content of Ca2+ is observed at Santa Maria and Graciosa islands, while silica content is higher in Sa˜o Miguel waters. Despite the fact that Ca2+ is an alkali earth not influenced by seawater spraying the scattered pattern depends on the sea salts input magnitude, as this process dilutes the original relative content considering only
455
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 10. Relationship between H4SiO04 (in mmol/L) and Ca2+ (in mmol/L). Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
water-rock interaction. This is supported by the weak correlation between Na++K+ and H4SiO04 (r=0.233) showing that even in islands dominated by volcanic rocks of basic nature, such as Pico or Santa Maria islands, the increase in silica is followed by an increase of alkali content due to a seawater component of variable magnitude, resulting in a scattered pattern (Fig. 11).
6. Principal component analysis Principal component analysis (PCA) is a multivariate statistical technique that can reveal the underlying structure of a dataset by means of the distances between variables or samples in a multi-dimensional data space (factorial plans). The reader is referred to the work of Davis (1986) for an in-depth account of the theory. PCA is a useful tool in hydrogeochemical studies on perchedwater bodies at volcanic islands and there are a few examples in the literature (Join et., 1997; Cruz and Franc¸a, in press). PCA was performed on the dataset and the eigenvalues and the percentage of the variance explained by each eigenvector are listed (Table 2). For that purpose the software ANDAD 6.00 was used (CVRM, 2000). The scores of variables and samples onto the principal component axis are plotted, which shows the similarities
between variables on the dataset (Fig. 12a and b). On the plots it is possible to show that pH is opposed to the other variables and presents positive scores along axis 1. This can be explained by the CO2 input from soils, which contributes to the acidic character of waters and considering that dissolution of primary minerals is limited and not able to consume much CO2. As weathering of silicate minerals is expected to cause pH to increase, as well as HCO3 and alkali or alkali–earth metals, it is possible that in certain areas the solution of CO2 in soils Table 2 Results from the principal component analysis vectors, eigenvalues and cumulative variance (principal vectors in bold) No.
Eigenvalue
Cumulative variance
1 2 3 4 5 6 7 8 9 10 11
5.57279 1.65311 1.19228 0.94228 0.94254 0.56265 0.30485 0.18765 0.11884 0.02042 0.01563
50.66 15.03 10.84 8.57 5.12 3.90 2.77 1.71 1.08 0.19 0.14
456
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 11. H4SiO04 (in mmol/L) vs. and Na++K+ (in mmol/L) (best fit line: y=0.558x+1.047). Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
or in-depth will occur in an open system, to which the release of volcanic volatiles contributes. In fact, in large areas diffusive gaseous emanations in soils were identified, supporting this hypothesis (Baxter et al., 1999). Scores along axis 2 exhibit the effect from two groups of variables: electrical conductivity presents negative scores along axis 2 and is associated with variables such as Cl and Mg2+, while SiO2, K+ and HCO3 present positive scores (Fig. 12a). The association between electrical conductivity and Cl suggest seawater contribution to water compositions. Therefore, two main evolution trends are observed: (1) seawater salt influence due to spraying, and (2) dissolution of silicate minerals (Fig. 12a). The plot of scores along axis 1 vs. axis 3 also shows the same effects and discriminates Ca2+ and HCO3 , which can account for the weathering of basaltic minerals (Fig. 12b). The plot of the sample scores on the factorial plans shows that the majority of the samples present positive scores along axis 1, suggesting, as explained, that the studied waters have short residence times with limited contributions from dissolution of primary rock minerals.
7. Conclusions Results of the major-ion chemistry from 246 samples of volcanic perched-water bodies have proved to be useful in order to identify two major trends of geochemical evolution. Seawater spray influence explains the Na–Cl water type and the control that Cl , a conservative ion, exerts on water mineralization. The HCO3 enrichment depicts, besides the CO2 soil contribution, the influence of the dissolution of primary silicate minerals of volcanic rocks. Enrichment of alkali metals beyond the seawater ratio, which controls rainwater chemistry, shows the contribution of silicate mineral hydrolysis. The alkali– metal enrichment is characteristic of the majority of samples from Sa˜o Miguel, Faial, and Terceira, from perched-water aquifers mainly made of evolved volcanic rocks of trachytic nature. In islands where perchedwater aquifers contain basaltic rocks s.l. the waters have generally higher concentrations of alkali–earth metals, positively correlated with HCO3 , whereas HCO3 is less correlated with alkali metals because Na++K+ is influenced by sea-spray.
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
457
Fig. 12. Principal components analysis results for variables and samples: (a) axis 1 vs. axis 2, and (b) axis 1 vs. axis 3. Solid square— variables; open square—samples.
Principal component analysis identifies the two main trends, but also shows that dissolution of minerals is limited. This is consistent with the low dissolved solids content. The relationship between alkali and alkali– earth metals also shows that because of limited evolu-
tion, the waters plot closer to the seawater ratio line and not to the basic and acidic rock lines. This is consistent with the hydrogeologic setting, i.e. limited, incongruent water-rock interaction in highly permeable perchedwater bodies.
458
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Acknowledgements The authors are grateful for the constructive and useful reviews (by JML and an anonymous referee), as well as by the editorial handling provided by Dr. R. Basset.
References Abdel-Monem, A.A., Fernandez, L.A., Boone, G.M., 1975. K/ Ar ages from the eastern Azores group (Santa Maria, Sa˜o Miguel and the Formigas islands). Lithos 4, 247–254. Aiuppa, A., Allard, P., D’Alessandro, W., Michel, A., Parello, F., Treuil, M., Valenza, M., 2000. Mobility and fluxes of major, minor and trace metals during basalt weathering and groundwater transport at Mt. Etna volcano (Sicily). Geochim. Cosmochim. Acta 64, 1827–1841. Allard, P., Jean-Baptiste, P., D’Alessandro, W., Parello, F., Parisi, B., Flehoc, C., 1997. Mantle-derived helium and carbon in groundwaters and gases of Mount Etna, Italy. Earth Planet. Sci. Lett. 148, 501–516. Azevedo, J.M.M., 1998. Geology and Hydrogeology of the Flores Island (Azores-Portugal). PhD thesis, University of Coimbra Coimbra. (in Portuguese with English abstract). Baxter, P.J., Baubron, J.-C., Coutinho, R., 1999. Health hazards and disaster potential of ground gas emission at Furnas volcano, Sa˜o Miguel, Azores. J. Volcanol. Geotherm. Res. 92, 95–106. Berner, E.K., Berner, R.A., 1996. The Global Water Cycle, Geochemistry and Environment. Prentice-Hall, Englewood Cliffs. Brusca, L., Aiuppa, A., D’A´lessandro, W., Parello, F., Allard, P., Michel, A., 2001. Geochemical mapping of magmatic ga´s-water-rock interactions in the aquifer of Mount Etna volcano. J. Volcanol. Geotherm. Res. 108, 199–218. Carvalho, M.R., 1999. Estudo hidrogeolo´gico do macic¸o vulcaˆnico de A´gua de Pau/Fogo (Sa˜o Miguel-Ac¸ores). PhD thesis, University of Lisbon, Lisbon (in Portuguese, with English abstract). Carvalho, M.R., Almeida, C., 1989. HIDSPEC, um programa de especiac¸a˜o e ca´lculo de equilı´brio a´gua/rocha. Geocieˆncias 4, 1–22. (in Portuguese with English abstract).. Coutinho, R., 1990. Estudo Hidrogeolo´gico do Macic¸o das Sete Cidades. MSc thesis, University of Lisbon, Lisbon (in Portuguese, with English abstract). Coutinho, R., 2000. Elementos para a Monitorizac¸a˜o Sismovulcaˆnica da ilha do Faial (Ac¸ores): Caracterizac¸a˜o Hidrogeolo´gica e Avaliac¸a˜o de Anomalias de Rn Associadas a Feno´menos de Desgaseificac¸a˜o. PhD thesis, University of Azores, ponta Delgada (in Portuguese, with English abstract). Cruz, J.V., 1992. Hidrogeologia da ilha de Santa Maria. MSc thesis, University of Lisbon, Lisbon (in Portuguese, with English abstract). Cruz, J.V., 1997. Estudo hidrogeolo´gico da ilha do Pico. PhD thesis, University of Azores, Ponta Delgada (in Portuguese with English abstract). Cruz, J.V., 2001a. Salinization of the basal aquifer system at volcanic islands: Azores archipelago (Portugal) case study.
In: Proc. 3th Internat. Conf. Future Groundwater Resources at Risk, Lisbon. Cruz, J.V., 2001b. Recursos Subterraˆneos. Plano Regional da A´gua da Regia˜o Auto´noma dos Ac¸ores. Relato´rio Te´cnicoCientı´fico 03/DGUA/01, Centro de Geologia Ambiental, Ponta Delgada (in Portuguese). Cruz, J.V. Groundwater and volcanoes: examples from the Azores archipelago. Environ. Geol. (in press). Cruz, J.V., Franc¸a, Z., 2001a. Mineral and thermal waters in the Azores archipelago (Portugal): geological setting and hydrogeochemical outline. In: Seiler, K.-P., Wohnlich, S. (Eds.), Proc. XXXI IAH Congress—New Approaches to Characterising Groundwater Flow. Balkema Publishers, Lisse, pp. 477–481. Cruz, J.V., Franc¸a, Z., 2001b. Groundwater composition of perched-water bodies at Azores volcanic islands. In: Cidu, R. (Ed.), Proc. 10th Internat. Symp. Water Rock Interaction. Balkema Publishers, Lisse, pp. 481–484. Cruz, J.V., Franc¸a, Z. Thermal and mineral waters springs at Azores archipelago (Portugal): hydrogeochemical outline. J. Hydrol. (in press). Cruz, J.V., Silva, M.O., 2000. Groundwater salinisation in Pico island (Azores, Portugal): origin and mechanisms. Environ. Geol. 39, 1181–1189. Cruz, J.V., Silva, M.O., 2001. Hydrogeologic framework of the Pico island (Azores, Portugal). Hydrogeol. J. 9, 177–189. Cruz, J.V., Silva, M.O., Carvalho, M.R., 1992. Hidrogeoquı´mica das a´guas subterraˆneas da ilha de Santa Maria (Ac¸ores). Geolis 6, 121–135. (in Portuguese with English abstract).. Cruz, J.V., Silva, M.O., Carvalho, M.R., 1997. Sı´ntese hidrogeolo´gica da ilha de Santa Maria (Ac¸ores). Geolis 7, 31–44. (in Portuguese with English abstract). Cruz, J.V., Coutinho, R.M., Carvalho, M.R., Oska´rsson, N., Gislason, S.R., 1999. Chemistry of waters from Furnas volcano, Sa˜o Miguel, Azores: fluxes of volcanic carbon dioxide and leached material. J. Volcanol. Geotherm. Res. 92, 151– 167. Cruz, J.V., Franc¸a, Z., Nunes, J.C. e Duarte, J.F. Hidrogeologia da ilha do Corvo (Ac¸ores): resultados preliminares. In: Proc. 3a Assembleia Luso-Espanhola de Geodesia e Geofı´sica, Valeˆncia (in press). Custo´dio, E., 1978. Geohidrologia de Terrenos e Islas Volca´nicas. Inst. Hidrologia—Centro Estu´dios Hidrogra´ficos, Madrid (in Spanish). Custo´dio, E., 1989. Groundwater characteristics and problems in volcanic rock terrains. In: Isotopic Techniques in the Study of the Hydrology of Fractures and Fissured Rocks, IAEA, Vienna, pp. 87–137. CVRM., 2000. Programa ANDAD; Manual do utilizador. CVRM—Centro de Geosistemas, IST, Lisboa (in Portuguese). Davis, J.C., 1986. Statistics and Data Analysis in Geology. John Wiley & Sons, New York. DROTRH-INAG, 2001. Plano regional da a´gua; Relato´rio te´cnico; versa˜o para consulta pu´blica. DROTRH-INAG, Ponta Delgada (in Portuguese). Evans, W.C., Sorey, M.L., Cook, A.C., Kennedy, B.M., Shuster, D.L., Colvard, E.M., White, L.D., Huebner, M.A., 2002. Tracing and quantifying magmatic carbon discharge in cold groundwaters: lessons learned from Mammoth Mountain, USA. J. Volcanol. Geotherm. Res. 114, 291–312.
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459 Federico, C., Aiuppa, A., Allard, P., Bellomo, S., Jean-Baptiste, P., Parello, F., Valenza, M., 2002. Magma-derived gas influx and water-rock interaction in the volcanic aquifer of Mt. Vesuvius, Italy. Geochim. Cosmochim. Acta 66, 963–981. Fe´raud, G., Kaneoka, I., Allegre, J.C., 1980. K/Ar ages and stress pattern in the Azores: geodynamic implications. Earth Planet. Sci. Lett. 46, 275–286. Ferreira, T., Oskarsson, N., 1999. Chemistry and isotopic composition of fumarole discharges of Furnas caldera. J. Volcanol. Geotherm. Res. 92, 169–179. Giggenbach, W.F., 1992. Isotopic shift in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. Earth Planet. Sci. Lett. 113, 495–510. Gı´slason, S., Eugster, H.P., 1987. Meteoric water-basalt interactions. II: a field study in N.E. Iceland. Geochim. Cosmochim. Acta 51, 2841–2855. Gı´slason, S., Arno´rsson, S., 1993. Dissolution of primary basaltic minerals in natural waters: saturation state and kinetics. Chem. Geol. 105, 117–135. Gı´slason, S., Arno´rsson, S., A´rmannsson, A´, 1996. Chemical weathering of basalt in southwest Iceland: effects of runoff, age of rocks and vegetative/glacial cover. Am. J. Sci. 296, 837–907. Grunberger, O., Coudray, J., Olek, M., 1988. Essai de corre´lation entre mine´ralisation des eaux souterraines et ge´ochimie des roches dans le masif volcanique du Piton des Neiges (ıˆle de la Re´union). Hudroge´ologie 2, 183-189, (in French with English abstract). Hunt, C.D., Ewart, C.J., Voss, C.I., 1988. Region 27, Hawaiian islands. In: Back, W., Rosenshein, J.S., Seaber, P.R. (Eds.), Hydrogeology, The Geology of North America, vol. O-2. Geol. Soc. America, Boulder, pp. 255–262. (chapter 30). Ingebritsen, S.E., Sholl, M.A., 1993. The hydrogeology of Kilauea volcano. Geothermics 22, 255–270. Join, J.-L, Coudray, J., Longworth, 1997. Using principal components analysis and Na/Cl ratios to trace groundwater circulation in a volcanic island: the example of Reunion. J. Hydrology 190, 1–18. Kim, Y., Lee, K.-S., Koh, D.-C., Lee, D.-H., Lee, S.-G., Park, W.-B., Koh, G.-W., Woo, N.-C., 2003. Hydrogeochemical and isotopic evidence of groundwater salinization in a coastal aquifer: a case study in Jeju volcanic island, Korea. J. Hydrol. 270, 282–294.
459
Langmuir, D., 1997. Aqueous Environmental Geochemistry. Prentice-Hall, Englewood Cliffs. Lau, L.S., Mink, J.F., 1995. Groundwater modelling in Hawaii: a historical perspective. In: El-Kadi, A.I. (Ed.), Groundwater Models for Resources Analysis and Management. CRC Press, Boca Raton, pp. 253–274. Lobo, M.A., 1993. Contribuic¸a˜o Para o Estudo Fı´sico-quı´mico e Microbiolo´gico da A´gua para Consumo Humano do Arquipe´lago dos Ac¸ores. PhD thesis, University of Azores, Angra do Heroı´smo (in Portuguese). Louvat, P., 1997. E´tude Ge´ochimique de l’E´rosion Fluviale d’ıˆles Volcaniques a` l’Aide des Bilans d’E´le´ments Majeurs et Traces. PhD dissertation, IPGP, Paris (in French). Louvat, P., Alle`gre, C.J., 1998. Riverine erosion rates on Sa˜o Miguel volcanic island, Azores archipelago. Chem. Geol. 148, 177–200. Peterson, F.L., 1972. Water development on tropic volcanic islands type example: Hawaii. Ground Water 10, 18–23. Peterson, F.L., 1993. Hydrogeology of volcanic oceanic islands. In: Sakura, Y. (Ed.), Selected Papers on Environmental Geology, Vol. 4. Heise, pp. 163–171. Rowe, G., Brantley, S., Fernadez, J., Borgia, A., 1995. The chemical and hydrologic structure of Poa´s Volcano, Costa Rica. J. Volcanol. Geotherm. Res. 64, 233–267. Schminke, H.-U., Weibel, M., 1972. Chemical study of rocks from Madeira, Porto Santo and Sa˜o Miguel, Terceira (Azores). N. Jb. Miner. Abh. 117, 253–281. Silva, M.O., 1988. Hidrogeologia da ilha da Madeira. Geolis 2, 96–102. (in Portuguese with English abstract).. Stieltjes, L., Gourgand, B., Steenhoudt, M., 1988. Modes de circulation et de gisement de l’eau souterraine dans an volcan bouclier basaltique; example de l’ıˆle de la Reunion, milieu oce´anique tropical. Hydroge´ologie 2, 83–94. (in French with English abstract).. Yamamoto, S., 1995. Volcano Body Springs in Japan. KokonShoin, Tokyo. White, W.M., Tapia, M.D.M., Schilling, J.-G., 1979. The petrology and geochemistry of the Azores islands. Contrib. Mineral. Petrol. 69, 201–213. Wood, W.W., Fernandez, L.A., 1988. Volcanic rocks. In: Back, W., Rosenshein, J.S., Seaber, P.R. (Eds.), Hydrogeology, The Geology of North America, Vol. O-2. Geol. Soc. America, Boulder, pp. 353–365. (chapter 39)..
Major ion chemistry of groundwater from perched-water bodies of the Azores (Portugal) volcanic archipelago J. Virgı´lio Cruz*, Catarina S. Amaral Department of Geosciences, Azores University, Apartado 1422, 9501-801 Ponta Delgada, Portugal Received 3 February 2003; accepted 10 June 2003 Editorial handling by R.L Bassett
Abstract A dataset of major ion composition of 246 samples from cold-water springs discharging from perched-water bodies at volcanic islands (Azores archipelago, Portugal) reveal waters with low mineralization, which evolve due to two main geochemical processes: (1) seawater spraying and (2) dissolution of primary minerals of volcanic rocks. As a result, water facies range from Na–Cl to Na–HCO3 type waters. The relationship between alkali, alkali–earth metals and HCO3 shows differences between waters discharging from perched-water bodies in basaltic rocks comparing to more evolved rocks of trachytic nature. The use of principal component analysis shows that water-rock interaction is limited, which is compatible with the geochemical observations and with the hydrogeological environment. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Background Despite the fact that volcanic rocks correspond to a small fraction of the earth surface, hydrogeology of aquifers made of volcanic deposits is a vital research issue in numerous islands and continental areas where these materials are prominent (Peterson, 1972, 1993; Custo´dio, 1978; Hunt et al., 1988; Stieltjes et al., 1988; Ingebritsen and Scholl, 1993; Lau and Mink, 1995; Yamamoto, 1995; Cruz and Silva, 2001). Studies on the geochemistry of aqueous fluids in volcanic regions have been conducted in rivers worldwide in order to study weathering of basaltic rocks (Gı´slason et al., 1996; Louvat, 1997), giving an input to the study of geochemical cycles and atmospheric CO2 consumption. Data on groundwater composition in basaltic aquifers are also encountered in the literature (Gı´slason and Eugster, 1987; Silva, 1988; Wood and Fernandez, 1988; Gı´slason and Arno´rsson, 1993; Join et al., 1997; Allard * Corresponding author. Tel.: +351-296650143; fax: +351296650141. E-mail address: [email protected] (J.V. Cruz).
et al., 1997; Cruz and Silva, 2000; Aiuppa et al, 2000). There are fewer data related to groundwater composition for aquifers consisting of volcanic rocks of acid nature (Giggenbach, 1992; Rowe et al., 1995). Due to the volcanic nature of the Azores, groundwater geochemistry is partially influenced by the dissolution of primary minerals from volcanic rocks. The release of major ions depends on the saturation states of primary minerals, precipitation of secondary minerals, the aqueous chemistry of each component leading to formation of soluble and insoluble species, and the acidic character of the environment (Aiuppa et al., 2000). However, as in other areas, the chemical composition of the groundwater depends also on other factors, such as rainfall chemistry, climate, rock type, residence time of water, rock division, temperature and pressure (Custo´dio, 1989). In addition an anthropogenic input of variable magnitude is also to be considered and, in the case of islands or coastal regions the interaction with seawater can be relevant (Cruz and Silva, 2000; Cruz, 2001a; Kim et al., 2003). Rock contribution to groundwater composition in volcanic regions is also strongly dependent on in-depth volatile release (Brusca et al., 2001; Federico et al., 2002). In the Azores, numerous mineral and thermal springs show the influence of these processes (Cruz et
0883-2927/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0883-2927(03)00135-5
446
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
al., 1999; Cruz and Franc¸a, 2001a, in press; Cruz, in press). In certain areas of the archipelago, volcanic CO2 diffusive emanations were studied, sometimes corresponding to a hazardous situation for inhabitants, and the CO2-dominated composition of the gaseous phase in fumaroles was clearly established (Baxter et al., 1999; Ferreira and Oskarsson, 1999).
The contribution of volcanic rock dissolution to the groundwater relative anionic composition is considered to be limited, and in humid or semi-humid climates HCO3 +CO23 are dominant due to CO2 in soil and in rain water (Custo´dio, 1989). Therefore, and especially in islands like the Azores, atmospheric deposition is very important. Previous studies using trend surface analysis
Fig. 1. Geologic framework of the Azores archipelago. The number near the islands corresponds to the maximum age in Ma (data from Abdel-Monem et al., 1975; Fe´raud et al., 1980; Azevedo, 1998).
Fig. 2. Density of springs and drilled wells in the Azores archipelago (data from Cruz, 2001b).
447
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Table 1 Groundwater chemical data statistics: main variables (pH, temperature, electrical conductivity) and major ions composition. Island description: SM- Sa˜o Miguel; SA- Santa Maria; TE- Terceira; SJ- Sa˜o Jorge; GR- Graciosa; PI- Pico; FA- -Faial; FL- Flores; CO- Corvo Island
SM
SA
TE
SJ
GR
PI
FA
FL
CO
Azores
Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min. Average St. Dev. Median Max. Min.
ph
T ( C)
Cond (mS/cm)
HCO3 (mg/l)
Cl (mg/l)
SO4 (mg/l)
SiO2 (mg/l)
Na (mg/l)
K (mg/l)
Ca (mg/l)
Mg (mg/l)
7.11 0.69 7.26 8.20 4.70 6.11 0.67 6.05 8.09 4.80 7.11 0.46 7.20 8.60 6.00 6.90 0.32 6.90 7.50 6.40 7.33 0.30 7.30 8.10 6.80 7.45 0.18 7.45 7.70 7.20 7.34 0.36 7.30 7.80 6.50 7.19 0.18 7.20 7.50 6.80 6.97 0.47 7.01 7.55 6.02 7.01 0.67 7.20 8.60 4.70
14.25 1.60 14.20 19.00 8.80 17.41 1.50 17.50 20.70 14.20 14.00 1.42 13.50 17.00 11.00 14.55 0.84 14.50 16.00 12.50 15.31 0.50 15.50 16.00 14.50 14.05 0.68 14.05 15.00 13.10 14.59 2.20 13.65 19.80 12.40 16.00 0.83 16.00 18.00 14.00 15.90 0.74 15.90 16.80 14.30 14.97 1.83 15.00 20.70 8.80
174.12 78.33 152.00 440.00 36.00 304.71 119.78 270.00 725.00 125.00 154.52 100.49 138.00 607.00 40.00 172.00 73.71 148.00 336.00 100.00 277.15 64.24 258.00 444.00 195.00 110.38 26.18 104.25 151.00 82.00 132.37 48.81 122.50 237.00 79.00 146.79 53.19 140.00 270.00 70.00 222.11 71.82 221.00 367.00 148.00 188.89 97.63 157.95 725.00 36.00
49.63 24.26 45.75 152.50 10.37 64.21 41.48 54.29 207.40 4.88 37.71 20.52 33.10 112.00 16.30 27.00 7.59 25.00 39.00 17.00 44.45 14.51 45.00 69.60 25.00 32.53 5.47 33.55 39.00 24.00 45.33 18.93 43.30 84.20 24.40 42.86 14.36 42.15 77.00 18.30 31.52 9.92 31.72 51.24 17.69 47.08 25.83 41.65 207.40 4.88
29.94 15.27 25.56 96.92 12.07 65.68 23.52 58.93 134.19 32.66 28.97 14.94 24.90 78.10 13.60 28.10 11.16 24.90 46.20 14.20 55.76 18.15 44.10 90.60 40.10 16.70 1.71 16.00 19.50 15.30 16.36 9.37 13.10 46.80 8.20 32.64 10.45 28.40 60.40 21.30 50.10 19.24 40.12 94.79 34.79 35.58 21.11 28.40 134.19 8.20
9.44 7.03 7.40 32.50 1.32 7.97 4.72 7.00 19.30 1.20 5.63 6.73 4.10 36.20 2.20 6.32 5.68 3.80 21.60 1.60 7.35 1.37 7.10 10.20 4.90 5.78 3.52 4.20 11.80 2.90 4.86 1.77 4.10 8.40 3.30 5.10 2.27 5.15 9.00 1.20 8.30 1.48 8.70 10.66 5.96 7.75 5.93 6.30 36.20 1.20
47.93 19.10 46.50 121.10 4.00 25.18 12.59 24.59 53.93 2.98 35.21 12.44 33.90 68.50 3.80 18.79 6.66 18.40 31.10 11.70 34.28 5.47 34.50 47.20 27.40 20.15 5.42 17.45 29.50 16.20 40.73 9.10 40.30 56.50 19.00 29.72 8.64 29.20 50.10 17.50 29.12 8.11 27.80 42.60 17.80 38.17 17.86 36.05 121.10 2.98
28.23 12.67 26.00 72.00 9.00 44.10 22.98 39.42 131.20 17.81 29.90 14.50 24.20 82.10 17.50 21.23 8.66 18.20 35.40 11.60 32.74 7.48 30.30 52.50 24.20 11.05 2.18 10.35 14.60 8.90 18.29 5.54 18.20 30.40 11.20 26.51 4.92 26.10 36.10 16.30 29.68 11.07 28.20 54.20 19.20 29.24 14.84 26.30 131.20 8.90
6.94 3.58 6.50 22.10 1.20 2.18 1.01 2.06 4.38 0.12 5.88 13.53 2.00 68.90 1.60 3.71 7.09 1.00 25.70 0.30 3.59 1.30 3.10 6.10 1.80 1.98 0.46 1.95 2.60 1.40 3.93 1.80 3.45 8.40 1.30 2.68 1.66 1.85 7.10 0.80 3.03 2.46 2.50 9.40 0.83 5.01 5.48 3.80 68.90 0.12
5.01 2.79 4.56 14.03 0.40 10.92 4.30 9.60 24.00 4.00 3.54 2.13 3.80 6.30 0.10 3.29 2.45 2.40 9.50 1.00 11.90 6.21 7.50 23.70 6.20 4.35 1.25 5.00 5.20 2.20 4.85 3.04 4.35 14.20 1.40 6.52 3.00 6.35 12.50 1.70 6.55 2.54 6.41 12.42 3.41 6.11 4.10 5.54 24.00 0.10
3.27 2.36 2.70 13.22 0.20 10.20 4.79 9.74 25.79 2.19 4.03 4.29 3.60 23.30 1.20 4.67 1.58 5.20 7.20 2.50 7.88 2.16 6.60 12.50 6.00 3.58 0.35 3.70 3.90 3.00 1.58 1.23 1.05 4.90 0.50 3.95 1.91 3.95 10.20 0.30 4.39 1.55 4.02 6.87 2.55 4.53 3.75 3.54 25.79 0.20
448
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 3. Piper diagram showing major ion chemistry of cold springs discharging from perched-water bodies at Azores.
have shown that areas where springs discharging from perched-water bodies are particularly enriched in Cl correspond to regions exposed to dominant winds (Cruz et al., 1992). Based on the cationic content it is possible to establish a relationship between the groundwater composition and the rock type, and several relationships have been proposed in the literature: olivine-rich basalts can show values of r(Mg+Ca)/r(Na+K) higher than 10, while volcanic rocks of acid nature can show values lower than 0.1 (Custo´dio, 1989). Grunberger et al. (1988) have also proposed the use of the ratio between the activities of Mg2+ and H4SiO04 as a tool to discriminate between groundwater discharging from basaltic rocks and from more evolved rocks. Despite the progress in understanding groundwater composition made in the Azores by several authors, an effort was needed to compile all the data provided and characterize groundwater composition at the archipelago scale. This work compares results, looking to similarities that are explained by geological and hydrogeological features. The present paper, following a previous contribution by Cruz and Franc¸a (2001b), is directed to fill
that gap. In order to study relations between variables and samples principal component analysis is applied. 1.2. Study area The Azores is an archipelago of volcanic nature made of 9 islands and a few islets, about 1000 miles from the mainland of Portugal. Located between 37–40 N latitude and 25–31 W longitude, the archipelago has an area of 2,333 km2 and approximately 246,030 inhabitants, mainly living on Sa˜o Miguel (54.1%) and Terceira (23.3%) islands. The islands are spread along a NW trending strip, about 500 km long. The Azores climate can be considered as marine temperate, and is characterized by a sharp difference between a colder, wet season that occurs between October and March and when 75% of the annual precipitation falls, and a dry season. Average annual precipitation is equal to 1,930 mm, and exceeds by far the average annual actual evapotranspiration, which is equal to 581 mm (DROTRH-INAG, 2001). Groundwater is a vital resource in the Azores archipelago, playing important roles as a drinking water
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
449
Fig. 4. Cl (in meq/l) vs. Na+ (in meq/l) plot for the studied waters (best fit line: y=0.756x+0.042).
source, and as an ecosystem support. Despite the environmental, social, and economic value of groundwater, aquifers of the Azores support increased use, which is reflected in groundwater quality deterioration. In the Azores about 97% of the water supply is due to groundwater sources, corresponding to annual volume of 24.7106 m3. This exceeds by far the water withdrawal from surface sources (0.8106 m3; DROTRHINAG, 2001). Groundwater sources correspond mainly to springs discharging from perched-water bodies (444 springs). Therefore, the geochemical characterization of the fluids discharging in these springs is a key issue to groundwater management in the Azores and the present dataset was pivotal for regional water planning.
2. Geological and hydrogeological setting The archipelago of the Azores is made of 9 islands entirely of volcanic origin. The islands geologic history includes successive volcanic eruptions of various types, from Hawaiian and Strombolian eruptions that produced basaltic s.l. lava flows, to highly explosive events related to more acidic magmas accounting for the large areas on certain islands covered by pumice-fall deposits and pyroclastic flow deposits (ignimbrites). The landscape also shows the differences between the different eruptive styles, from prominent central volcanoes trun-
cated at the summit by large calderas due to highly explosive eruptions, and fissure zones and scoria cone fields typical of eruptions of magma of basic nature. In Fig. 1 the geological setting of the Azores archipelago is summarized. The oldest outcrop of the archipelago was dated from about 8.12 Ma at Santa Maria island (Abdel-Monem et al., 1975). Nevertheless, a large portion of the archipelago has potentially active volcanoes, and since settlement in the 15th century more than 20 eruptions have taken place. The last eruption was a submarine event 10 km NW of Terceira island that took place between 1998 and 2000. Hydrogeology characteristics in the archipelago are heterogeneous, depending on multiple factors, from the depositional and post-depositional characteristics of the volcanic rocks, to secondary factors such as weathering and tectonics. Specific capacity is on the range of 1.4010 2–266.67 Ls 1 m 1 (n=65), with a median of 32.29 Ls 1 m 1 (Cruz, 2001b). Transmissivity also shows a large range of values, between 1.6510 5 and 4.0310 1 m2 s 1, and a median of 3.6610 2 m2 s 1. The higher values correspond to wells drilled in fissured basaltic lava flows, while the lower estimates are from wells in volcanic deposits of similar origin but much older and weathered. Studied recharge rates range from 8.5 to 62.1% of the total precipitation; the higher values correspond to areas where soil cover is sparse over fractured basaltic
450
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 5. Relationship between electrical conductivity (in mS/cm) and: (a) Cl (in meq/l; best fit line: y=0.005x+0.025); (b) HCO3 (in meq/l; best fit line: y=0.003x+0.236).
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
451
Fig. 6. Cl (in meq/l) vs. SO24 (in meq/l) plot for the dataset. Seawater chemistry data from Cruz (1997) and thermal discharge composition from Cruz and Franc¸a (in press): (a) Caldeira Velha boiling pool; (b) Caldeira Velha spring.
lava flows (Cruz, 2001b; Cruz and Silva, 2001). Groundwater resource calculations indicate annual recharge of 1.6109 m3/a, asymmetrically distributed all over the archipelago (Cruz, 2001b). Groundwater recharge at Corvo, the smallest island, is estimated to be 8.3106 m3/a, while on Pico, the second largest island recharge is 5.8108 m3/a. Values higher than the median (1.0108 m3/a) are observed also at Sa˜o Miguel, Sa˜o Jorge, Terceira and Flores islands. Groundwater occurs in two main aquifer systems: (1) the basal aquifer system, corresponding to fresh-water lenses floating on underlying salt water, generally with a very small hydraulic gradient and with water quality affected by mixing with seawater, and (2) perched-water bodies which correspond to confined layers or to leaky aquifers, at elevation, where waters generally have a small residence time as shown by the low dissolved solids content.
3. Methodology The dataset on the major ion composition of cold spring discharges from perched-water bodies of the Azores is made of 246 samples. The literature survey
was made in order to evaluate uniformity of sampling and analytical procedures, selecting whenever possible only one data source per island. These springs are not equally distributed on the islands, reflecting the influence of local hydrogeological conditions that constrain the spring density (Fig. 2). This is shown by the Pico island example, which despite being the second largest island in the Azores, with an area equal to 450.7 km2, has only 1.3310 2 springs/km2 due to the occurrence of high permeability volcanics, often densely fissured (Cruz, 2001b; Cruz and Silva, 2001). Results from several islands are used in this study: Santa Maria (Cruz, 1992; Cruz et al., 1992), Sa˜o Miguel (Coutinho, 1990; Lobo, 1993; Cruz et al., 1999; Carvalho, 1999), Terceira (Lobo, 1993), Sa˜o Jorge (Lobo, 1993), Pico (Cruz, 1997; Cruz and Silva, 2000), Faial (Coutinho, 2000), Graciosa (Lobo, 1993), Flores (Lobo, 1993) and Corvo island (Cruz et al., 2002). All analysis with charge balance higher than 10% were rejected. Field measurements of temperature, pH and electrical conductivity, as well as alkalinity titration with 0.05M H2SO4 to a pH of 4.45, were made immediately after sampling. Cations were analysed by atomic absorption spectroscopy (AAS) and anions by ion chromatography (IC), following filtration carried out in the field after sampling (0.45 mm cellulose membrane filter). Samples
452
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 7. HCO3 (in meq/l) vs. Na++K+ (in meq/l) plot for the dataset (seawater chemistry data from Cruz, 1997). Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
for AAS were acidified by concentrated HNO3 following sample collection. SiO2 was analysed mainly by visible spectrophotometry.
4. Results Major-ion compositions including statistical measures (average, standard deviation, median, maximum, minimum) are reported in Table 1 for all the samples included from the Azores archipelago. Groundwaters in this dataset have low mineralization, as shown by the electrical conductivity measurements, which range from 36 to 725 mS/cm, with an average value equal to 189 mS/cm and a median of 158 mS/cm. Higher values are generally observed at Santa Maria, Graciosa and Corvo islands. As the dataset summarizes only cold-water discharges are studied. The temperature distribution reflects this feature, and the average value is equal to 15 C. Waters are mainly slightly acid to slightly alkaline, with a pH ranging between 4.70 and 8.60, but showing an average value equal to 7.01 and a median equal to 7.20. Lower values are attained at Sa˜o Miguel and Santa Maria. Only at Santa Maria island is the pH median above neutrality by almost one pH unit.
Groundwater composition is dominated by Na++K+ and Cl +HCO3 , which is reflected in the water types that vary between Na–Cl to Na–HCO3 waters. Numerous samples lie in the intermediate compositional fields that characterize Na–Cl–HCO3 and Na–HCO3–Cl waters in the Piper plot (Fig. 3). Chloride and Na+ can account for respectively 7.3– 40.8% and 18.7–50.4% of the relative anionic and cationic content of the groundwater (in meq/l). These major ionic species in solution are positively correlated (r=0.819; Fig. 4), and their contribution to the overall chemical composition of the groundwater is shown by the correlation between Cl and the electrical conductivity (r=0.848; Fig. 5a). These results suggests the influence of seawater spraying in the groundwater compositional evolution, which is essentially explained due to the seawater control of the rain water chemistry over islands and coastal regions worldwide (Berner and Berner, 1996). Other samples show HCO3 enrichment, which can account for 3.4–43.2% of the relative anionic composition. Fig. 5b shows the relationship with electrical conductivity and depicts the contribution of HCO3 to the groundwater chemistry. Despite the positive correlation between these variables, the correlation coefficient is lower than for Cl (r=0.653). The relationship between Cl and SO24 shows that the majority of the samples are plotted near the
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
453
Fig. 8. HCO3 (in meq/l) vs. Ca2++Mg2+ (in meq/l) plot for the dataset (seawater chemistry data from Cruz, 1997). Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
seawater line, suggesting a marine contribution to water composition (Fig. 6). The weak correlation (r=0.414) and the scattered pattern, with numerous samples plotted above the seawater line, suggest a volcanic contribution beyond sea salts input, which explain the SO24 enrichment by oxidation of reduced S species from volatile releases. To show this effect on the Cl /SO24 ratio, two SO4-rich thermal water discharges from Sa˜o Miguel island, representing waters that evolve by steam heating and mixing with meteoric cold waters, are also plotted in Fig. 6 (lines a and b). This plot shows that numerous samples fall in the field between the seawater line and the steam-heated thermal discharges line. These samples are mainly from Sa˜o Miguel and correspond to springs located in active volcanic centres of Quaternary age (Furnas and Fogo).
5. Discussion The alkali metals present are positively correlated with HCO3 (r=0.707) and the data depict an excess of Na++K+ in relation with HCO3 as shown by the fact that waters fall in the field above the Na++K+=HCO3 line (Fig. 7). Nevertheless, the trend defined by the data deviates from seawater composition and therefore suggests other mineralization processes
besides seawater spraying alone. If the HCO3 is derived from CO2 in the soil, then the enrichment in alkalis may be attributed to silicate weathering, which also contributes to increased HCO3 content. From incongruent dissolution of primary aluminosilicate minerals in volcanic rocks, an enrichment of alkali content over Cl is expected, as Cl is practically unavailable for dissolution during water percolation (Join et al., 1997). In Fig. 4 it is possible to show that the line of best fit is plotted in the field under the Cl =Na+ line, clearly pointing toward a Na+ enrichment in solution beyond the rain composition. The relationship between alkali–earths and HCO3 shows a more scattered plot with less correlation (r=0.522; Fig. 8), suggesting a different behaviour between islands. The majority of the samples from Sa˜o Miguel, Terceira and Faial, where perched-water aquifers are made of more alkaline rocks, plot in the field beneath the Ca2++Mg2+=HCO3 line. By contrast, samples from islands where perched-water bodies are made of basic rocks, including Santa Maria, Sa˜o Jorge, Graciosa, Corvo, Pico and Flores, plot mainly above the Ca2++Mg2+=HCO3 line and show stronger correlation (r=0.739). The relationship between alkali and alkali–earth metals depicts a scattered pattern, and the samples plot between the basic and acid rock compositions used for
454
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 9. Na++K+ (in meq/l) vs. Ca2++Mg2+ (in meq/l) plot for the dataset [seawater chemistry data from Cruz (1997); basaltic and trachytic rocks s.l. data from Schmincke and Weibel (1972) and White et al. (1979)]. Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
reference (Fig. 9). This is consistent with the slight water mineralization and shows that these springs discharge waters with very short residence times, in an environment characterized by high groundwater flow velocities. This implies a high water-rock ratio (Langmuir, 1997) and the effects of silicate hydrolysis on water composition are limited. Characterization of hydrographs from springs in Santa Maria and Corvo islands revealed a two months delay between precipitation events and spring discharge, which shows the short residence time of groundwater in these aquifers and is consistent with geochemical data interpretation (Cruz et al., 1997, in press). Groundwater from Sa˜o Miguel, Terceira and Faial mainly plot near the seawater line, with ratios similar to rain water, while samples from islands with perchedwater aquifers of basic nature fall mainly in the field between the Ca2++Mg2+=Na++K+ and seawater lines. These observations for the latter islands suggest that the enrichment of alkali–earth metals is caused by silicate hydrolysis as aquifers have a typical basaltic mineral assemblage of Ca-rich plagioclases, pyroxenes and olivines. In fact, silicate hydrolysis reactions can be consider the most important types of reactions in volcanic terranes, contributing alkali metals or alkali–earth metals if rocks are, respectively, more or less chemically
evolved (Evans et al., 2002). The effect of this lithologic control over water composition has already been shown in the Azores for the river waters of Sa˜o Miguel island, where the Ca/Na ratio is sensitive to trachytic, basaltic or intermediate rock composition (Louvat and Alle`gre, 1998) Lithologic influences on groundwater composition are also shown by the relationship between Ca2+ and the complex H4SiO04 which was determined using HIDSPEC speciation software (Fig. 10; Carvalho and Almeida, 1989). The use of the Ca2+/H4SiO04 ratio to separate lithologic influences was made in order to prevent seawater signature bias, which influences Na+, K+ and Mg2+ content to a larger extent, and assuming that lithologic influence can be viewed in simplest terms as a result of silicate rocks dissolution that neutralises fluid acidity and releases SiO2 and cations. In this plot a sharp difference between samples from Sa˜o Miguel, Terceira and Faial, which drain more evolved rocks, and waters from other islands is shown. The highest content of Ca2+ is observed at Santa Maria and Graciosa islands, while silica content is higher in Sa˜o Miguel waters. Despite the fact that Ca2+ is an alkali earth not influenced by seawater spraying the scattered pattern depends on the sea salts input magnitude, as this process dilutes the original relative content considering only
455
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 10. Relationship between H4SiO04 (in mmol/L) and Ca2+ (in mmol/L). Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
water-rock interaction. This is supported by the weak correlation between Na++K+ and H4SiO04 (r=0.233) showing that even in islands dominated by volcanic rocks of basic nature, such as Pico or Santa Maria islands, the increase in silica is followed by an increase of alkali content due to a seawater component of variable magnitude, resulting in a scattered pattern (Fig. 11).
6. Principal component analysis Principal component analysis (PCA) is a multivariate statistical technique that can reveal the underlying structure of a dataset by means of the distances between variables or samples in a multi-dimensional data space (factorial plans). The reader is referred to the work of Davis (1986) for an in-depth account of the theory. PCA is a useful tool in hydrogeochemical studies on perchedwater bodies at volcanic islands and there are a few examples in the literature (Join et., 1997; Cruz and Franc¸a, in press). PCA was performed on the dataset and the eigenvalues and the percentage of the variance explained by each eigenvector are listed (Table 2). For that purpose the software ANDAD 6.00 was used (CVRM, 2000). The scores of variables and samples onto the principal component axis are plotted, which shows the similarities
between variables on the dataset (Fig. 12a and b). On the plots it is possible to show that pH is opposed to the other variables and presents positive scores along axis 1. This can be explained by the CO2 input from soils, which contributes to the acidic character of waters and considering that dissolution of primary minerals is limited and not able to consume much CO2. As weathering of silicate minerals is expected to cause pH to increase, as well as HCO3 and alkali or alkali–earth metals, it is possible that in certain areas the solution of CO2 in soils Table 2 Results from the principal component analysis vectors, eigenvalues and cumulative variance (principal vectors in bold) No.
Eigenvalue
Cumulative variance
1 2 3 4 5 6 7 8 9 10 11
5.57279 1.65311 1.19228 0.94228 0.94254 0.56265 0.30485 0.18765 0.11884 0.02042 0.01563
50.66 15.03 10.84 8.57 5.12 3.90 2.77 1.71 1.08 0.19 0.14
456
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Fig. 11. H4SiO04 (in mmol/L) vs. and Na++K+ (in mmol/L) (best fit line: y=0.558x+1.047). Islands: SM-Sa˜o Miguel; SA-Santa Maria; TE-Terceira; SJ-Sa˜o Jorge; GR-Graciosa; PI-Pico; FA-Faial; FL-Flores; CO-Corvo.
or in-depth will occur in an open system, to which the release of volcanic volatiles contributes. In fact, in large areas diffusive gaseous emanations in soils were identified, supporting this hypothesis (Baxter et al., 1999). Scores along axis 2 exhibit the effect from two groups of variables: electrical conductivity presents negative scores along axis 2 and is associated with variables such as Cl and Mg2+, while SiO2, K+ and HCO3 present positive scores (Fig. 12a). The association between electrical conductivity and Cl suggest seawater contribution to water compositions. Therefore, two main evolution trends are observed: (1) seawater salt influence due to spraying, and (2) dissolution of silicate minerals (Fig. 12a). The plot of scores along axis 1 vs. axis 3 also shows the same effects and discriminates Ca2+ and HCO3 , which can account for the weathering of basaltic minerals (Fig. 12b). The plot of the sample scores on the factorial plans shows that the majority of the samples present positive scores along axis 1, suggesting, as explained, that the studied waters have short residence times with limited contributions from dissolution of primary rock minerals.
7. Conclusions Results of the major-ion chemistry from 246 samples of volcanic perched-water bodies have proved to be useful in order to identify two major trends of geochemical evolution. Seawater spray influence explains the Na–Cl water type and the control that Cl , a conservative ion, exerts on water mineralization. The HCO3 enrichment depicts, besides the CO2 soil contribution, the influence of the dissolution of primary silicate minerals of volcanic rocks. Enrichment of alkali metals beyond the seawater ratio, which controls rainwater chemistry, shows the contribution of silicate mineral hydrolysis. The alkali– metal enrichment is characteristic of the majority of samples from Sa˜o Miguel, Faial, and Terceira, from perched-water aquifers mainly made of evolved volcanic rocks of trachytic nature. In islands where perchedwater aquifers contain basaltic rocks s.l. the waters have generally higher concentrations of alkali–earth metals, positively correlated with HCO3 , whereas HCO3 is less correlated with alkali metals because Na++K+ is influenced by sea-spray.
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
457
Fig. 12. Principal components analysis results for variables and samples: (a) axis 1 vs. axis 2, and (b) axis 1 vs. axis 3. Solid square— variables; open square—samples.
Principal component analysis identifies the two main trends, but also shows that dissolution of minerals is limited. This is consistent with the low dissolved solids content. The relationship between alkali and alkali– earth metals also shows that because of limited evolu-
tion, the waters plot closer to the seawater ratio line and not to the basic and acidic rock lines. This is consistent with the hydrogeologic setting, i.e. limited, incongruent water-rock interaction in highly permeable perchedwater bodies.
458
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459
Acknowledgements The authors are grateful for the constructive and useful reviews (by JML and an anonymous referee), as well as by the editorial handling provided by Dr. R. Basset.
References Abdel-Monem, A.A., Fernandez, L.A., Boone, G.M., 1975. K/ Ar ages from the eastern Azores group (Santa Maria, Sa˜o Miguel and the Formigas islands). Lithos 4, 247–254. Aiuppa, A., Allard, P., D’Alessandro, W., Michel, A., Parello, F., Treuil, M., Valenza, M., 2000. Mobility and fluxes of major, minor and trace metals during basalt weathering and groundwater transport at Mt. Etna volcano (Sicily). Geochim. Cosmochim. Acta 64, 1827–1841. Allard, P., Jean-Baptiste, P., D’Alessandro, W., Parello, F., Parisi, B., Flehoc, C., 1997. Mantle-derived helium and carbon in groundwaters and gases of Mount Etna, Italy. Earth Planet. Sci. Lett. 148, 501–516. Azevedo, J.M.M., 1998. Geology and Hydrogeology of the Flores Island (Azores-Portugal). PhD thesis, University of Coimbra Coimbra. (in Portuguese with English abstract). Baxter, P.J., Baubron, J.-C., Coutinho, R., 1999. Health hazards and disaster potential of ground gas emission at Furnas volcano, Sa˜o Miguel, Azores. J. Volcanol. Geotherm. Res. 92, 95–106. Berner, E.K., Berner, R.A., 1996. The Global Water Cycle, Geochemistry and Environment. Prentice-Hall, Englewood Cliffs. Brusca, L., Aiuppa, A., D’A´lessandro, W., Parello, F., Allard, P., Michel, A., 2001. Geochemical mapping of magmatic ga´s-water-rock interactions in the aquifer of Mount Etna volcano. J. Volcanol. Geotherm. Res. 108, 199–218. Carvalho, M.R., 1999. Estudo hidrogeolo´gico do macic¸o vulcaˆnico de A´gua de Pau/Fogo (Sa˜o Miguel-Ac¸ores). PhD thesis, University of Lisbon, Lisbon (in Portuguese, with English abstract). Carvalho, M.R., Almeida, C., 1989. HIDSPEC, um programa de especiac¸a˜o e ca´lculo de equilı´brio a´gua/rocha. Geocieˆncias 4, 1–22. (in Portuguese with English abstract).. Coutinho, R., 1990. Estudo Hidrogeolo´gico do Macic¸o das Sete Cidades. MSc thesis, University of Lisbon, Lisbon (in Portuguese, with English abstract). Coutinho, R., 2000. Elementos para a Monitorizac¸a˜o Sismovulcaˆnica da ilha do Faial (Ac¸ores): Caracterizac¸a˜o Hidrogeolo´gica e Avaliac¸a˜o de Anomalias de Rn Associadas a Feno´menos de Desgaseificac¸a˜o. PhD thesis, University of Azores, ponta Delgada (in Portuguese, with English abstract). Cruz, J.V., 1992. Hidrogeologia da ilha de Santa Maria. MSc thesis, University of Lisbon, Lisbon (in Portuguese, with English abstract). Cruz, J.V., 1997. Estudo hidrogeolo´gico da ilha do Pico. PhD thesis, University of Azores, Ponta Delgada (in Portuguese with English abstract). Cruz, J.V., 2001a. Salinization of the basal aquifer system at volcanic islands: Azores archipelago (Portugal) case study.
In: Proc. 3th Internat. Conf. Future Groundwater Resources at Risk, Lisbon. Cruz, J.V., 2001b. Recursos Subterraˆneos. Plano Regional da A´gua da Regia˜o Auto´noma dos Ac¸ores. Relato´rio Te´cnicoCientı´fico 03/DGUA/01, Centro de Geologia Ambiental, Ponta Delgada (in Portuguese). Cruz, J.V. Groundwater and volcanoes: examples from the Azores archipelago. Environ. Geol. (in press). Cruz, J.V., Franc¸a, Z., 2001a. Mineral and thermal waters in the Azores archipelago (Portugal): geological setting and hydrogeochemical outline. In: Seiler, K.-P., Wohnlich, S. (Eds.), Proc. XXXI IAH Congress—New Approaches to Characterising Groundwater Flow. Balkema Publishers, Lisse, pp. 477–481. Cruz, J.V., Franc¸a, Z., 2001b. Groundwater composition of perched-water bodies at Azores volcanic islands. In: Cidu, R. (Ed.), Proc. 10th Internat. Symp. Water Rock Interaction. Balkema Publishers, Lisse, pp. 481–484. Cruz, J.V., Franc¸a, Z. Thermal and mineral waters springs at Azores archipelago (Portugal): hydrogeochemical outline. J. Hydrol. (in press). Cruz, J.V., Silva, M.O., 2000. Groundwater salinisation in Pico island (Azores, Portugal): origin and mechanisms. Environ. Geol. 39, 1181–1189. Cruz, J.V., Silva, M.O., 2001. Hydrogeologic framework of the Pico island (Azores, Portugal). Hydrogeol. J. 9, 177–189. Cruz, J.V., Silva, M.O., Carvalho, M.R., 1992. Hidrogeoquı´mica das a´guas subterraˆneas da ilha de Santa Maria (Ac¸ores). Geolis 6, 121–135. (in Portuguese with English abstract).. Cruz, J.V., Silva, M.O., Carvalho, M.R., 1997. Sı´ntese hidrogeolo´gica da ilha de Santa Maria (Ac¸ores). Geolis 7, 31–44. (in Portuguese with English abstract). Cruz, J.V., Coutinho, R.M., Carvalho, M.R., Oska´rsson, N., Gislason, S.R., 1999. Chemistry of waters from Furnas volcano, Sa˜o Miguel, Azores: fluxes of volcanic carbon dioxide and leached material. J. Volcanol. Geotherm. Res. 92, 151– 167. Cruz, J.V., Franc¸a, Z., Nunes, J.C. e Duarte, J.F. Hidrogeologia da ilha do Corvo (Ac¸ores): resultados preliminares. In: Proc. 3a Assembleia Luso-Espanhola de Geodesia e Geofı´sica, Valeˆncia (in press). Custo´dio, E., 1978. Geohidrologia de Terrenos e Islas Volca´nicas. Inst. Hidrologia—Centro Estu´dios Hidrogra´ficos, Madrid (in Spanish). Custo´dio, E., 1989. Groundwater characteristics and problems in volcanic rock terrains. In: Isotopic Techniques in the Study of the Hydrology of Fractures and Fissured Rocks, IAEA, Vienna, pp. 87–137. CVRM., 2000. Programa ANDAD; Manual do utilizador. CVRM—Centro de Geosistemas, IST, Lisboa (in Portuguese). Davis, J.C., 1986. Statistics and Data Analysis in Geology. John Wiley & Sons, New York. DROTRH-INAG, 2001. Plano regional da a´gua; Relato´rio te´cnico; versa˜o para consulta pu´blica. DROTRH-INAG, Ponta Delgada (in Portuguese). Evans, W.C., Sorey, M.L., Cook, A.C., Kennedy, B.M., Shuster, D.L., Colvard, E.M., White, L.D., Huebner, M.A., 2002. Tracing and quantifying magmatic carbon discharge in cold groundwaters: lessons learned from Mammoth Mountain, USA. J. Volcanol. Geotherm. Res. 114, 291–312.
J.V. Cruz, C.S. Amaral / Applied Geochemistry 19 (2004) 445–459 Federico, C., Aiuppa, A., Allard, P., Bellomo, S., Jean-Baptiste, P., Parello, F., Valenza, M., 2002. Magma-derived gas influx and water-rock interaction in the volcanic aquifer of Mt. Vesuvius, Italy. Geochim. Cosmochim. Acta 66, 963–981. Fe´raud, G., Kaneoka, I., Allegre, J.C., 1980. K/Ar ages and stress pattern in the Azores: geodynamic implications. Earth Planet. Sci. Lett. 46, 275–286. Ferreira, T., Oskarsson, N., 1999. Chemistry and isotopic composition of fumarole discharges of Furnas caldera. J. Volcanol. Geotherm. Res. 92, 169–179. Giggenbach, W.F., 1992. Isotopic shift in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. Earth Planet. Sci. Lett. 113, 495–510. Gı´slason, S., Eugster, H.P., 1987. Meteoric water-basalt interactions. II: a field study in N.E. Iceland. Geochim. Cosmochim. Acta 51, 2841–2855. Gı´slason, S., Arno´rsson, S., 1993. Dissolution of primary basaltic minerals in natural waters: saturation state and kinetics. Chem. Geol. 105, 117–135. Gı´slason, S., Arno´rsson, S., A´rmannsson, A´, 1996. Chemical weathering of basalt in southwest Iceland: effects of runoff, age of rocks and vegetative/glacial cover. Am. J. Sci. 296, 837–907. Grunberger, O., Coudray, J., Olek, M., 1988. Essai de corre´lation entre mine´ralisation des eaux souterraines et ge´ochimie des roches dans le masif volcanique du Piton des Neiges (ıˆle de la Re´union). Hudroge´ologie 2, 183-189, (in French with English abstract). Hunt, C.D., Ewart, C.J., Voss, C.I., 1988. Region 27, Hawaiian islands. In: Back, W., Rosenshein, J.S., Seaber, P.R. (Eds.), Hydrogeology, The Geology of North America, vol. O-2. Geol. Soc. America, Boulder, pp. 255–262. (chapter 30). Ingebritsen, S.E., Sholl, M.A., 1993. The hydrogeology of Kilauea volcano. Geothermics 22, 255–270. Join, J.-L, Coudray, J., Longworth, 1997. Using principal components analysis and Na/Cl ratios to trace groundwater circulation in a volcanic island: the example of Reunion. J. Hydrology 190, 1–18. Kim, Y., Lee, K.-S., Koh, D.-C., Lee, D.-H., Lee, S.-G., Park, W.-B., Koh, G.-W., Woo, N.-C., 2003. Hydrogeochemical and isotopic evidence of groundwater salinization in a coastal aquifer: a case study in Jeju volcanic island, Korea. J. Hydrol. 270, 282–294.
459
Langmuir, D., 1997. Aqueous Environmental Geochemistry. Prentice-Hall, Englewood Cliffs. Lau, L.S., Mink, J.F., 1995. Groundwater modelling in Hawaii: a historical perspective. In: El-Kadi, A.I. (Ed.), Groundwater Models for Resources Analysis and Management. CRC Press, Boca Raton, pp. 253–274. Lobo, M.A., 1993. Contribuic¸a˜o Para o Estudo Fı´sico-quı´mico e Microbiolo´gico da A´gua para Consumo Humano do Arquipe´lago dos Ac¸ores. PhD thesis, University of Azores, Angra do Heroı´smo (in Portuguese). Louvat, P., 1997. E´tude Ge´ochimique de l’E´rosion Fluviale d’ıˆles Volcaniques a` l’Aide des Bilans d’E´le´ments Majeurs et Traces. PhD dissertation, IPGP, Paris (in French). Louvat, P., Alle`gre, C.J., 1998. Riverine erosion rates on Sa˜o Miguel volcanic island, Azores archipelago. Chem. Geol. 148, 177–200. Peterson, F.L., 1972. Water development on tropic volcanic islands type example: Hawaii. Ground Water 10, 18–23. Peterson, F.L., 1993. Hydrogeology of volcanic oceanic islands. In: Sakura, Y. (Ed.), Selected Papers on Environmental Geology, Vol. 4. Heise, pp. 163–171. Rowe, G., Brantley, S., Fernadez, J., Borgia, A., 1995. The chemical and hydrologic structure of Poa´s Volcano, Costa Rica. J. Volcanol. Geotherm. Res. 64, 233–267. Schminke, H.-U., Weibel, M., 1972. Chemical study of rocks from Madeira, Porto Santo and Sa˜o Miguel, Terceira (Azores). N. Jb. Miner. Abh. 117, 253–281. Silva, M.O., 1988. Hidrogeologia da ilha da Madeira. Geolis 2, 96–102. (in Portuguese with English abstract).. Stieltjes, L., Gourgand, B., Steenhoudt, M., 1988. Modes de circulation et de gisement de l’eau souterraine dans an volcan bouclier basaltique; example de l’ıˆle de la Reunion, milieu oce´anique tropical. Hydroge´ologie 2, 83–94. (in French with English abstract).. Yamamoto, S., 1995. Volcano Body Springs in Japan. KokonShoin, Tokyo. White, W.M., Tapia, M.D.M., Schilling, J.-G., 1979. The petrology and geochemistry of the Azores islands. Contrib. Mineral. Petrol. 69, 201–213. Wood, W.W., Fernandez, L.A., 1988. Volcanic rocks. In: Back, W., Rosenshein, J.S., Seaber, P.R. (Eds.), Hydrogeology, The Geology of North America, Vol. O-2. Geol. Soc. America, Boulder, pp. 353–365. (chapter 39)..