Applied Geochemistry 29 (2013) 162–173
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Groundwater composition and pollution due to agricultural practices at Sete Cidades volcano (Azores, Portugal) J. Virgílio Cruz a,⇑, M.O. Silva b, M. Isabel Dias c, M. Isabel Prudêncio c a
CVARG, Centre of Volcanological and Geological Hazards Assessment, Department of Geosciences, University of the Azores, Apartado 1422, 9501-801 Ponta Delgada, Portugal Geology Department and Geology Centre, Faculdade de Ciências da Universidade de Lisboa, Edifico C6, 1749-016 Lisboa, Portugal c Applied Geochemistry & Luminescence on Cultural Heritage Group, Unit of Chemical and Radiopharmaceutical Sciences, IST/ITN, Estrada Nacional 10, 2686-953 Sacavém, Portugal b
a r t i c l e
i n f o
Article history: Received 11 March 2012 Accepted 18 November 2012 Available online 10 December 2012 Editorial handling by Peter Birkle
a b s t r a c t Groundwater is a strategic resource of the Azores archipelago given that agriculture is a major economic activity. A field study was undertaken at Sete Cidades volcano (São Miguel, Portugal) to characterize the composition of soil water at several depths in two sites: one without anthropogenic pressure (village site) and the other pasture land (Pa I site), and groundwater in the saturated zone of both sites. The composition of groundwater from springs discharging on the volcano flanks and inside the summit caldera is similar, composed principally of poorly mineralized Na–HCO3 and Na–HCO3–Cl water types, as suggested by median electrical conductivity values. Samples of groundwater collected in seven piezometers spread inside the Sete Cidades caldera are characterized by a conductivity between 95 and 232 lS/cm and the dominant water type is Na–HCO3. Soil water at the Sete Cidades village site is of Na–Cl type and its compositional similiarity to rain water suggests control by evapotranspiration. The limited soil depth of the site, as well as the high precipitation and soil hydraulic conductivity, cause vertical homogenization of soil water composition. In contrast, soil water sampled at the pasture site (Pa1) shows greater mineralization when compared to the previous site, and waters are mainly HCO3 to HCO3–Cl types with a Mg–Ca trend for cations. A trend was indicated for the relative soil water composition at site Pa I, from the more superficial suction cup (G1) to the deeper G14–15. This is an evolution similar to perched-water bodies inside the Sete Cidades Caldera. The Mg–Ca-dominated composition at lower depth is explained by the application of fertilizers. Nevertheless, other processes are also influencing water chemistry evolution, and the observed relative decreases in Ca, Mg and K with depth may result from clay formation and the uptake by ion exchange of Ca2+ and Mg2+ for Na+. Although, the relative increase in Na could also result from silicate weathering. Besides contributing to the Na content, silicate dissolution may also explain the relative increase in HCO3, both being associated with silica in solution. The impact of the excessive application of fertilizers is reflected by the NO3 contents of soil water at Pa I and the village sites, as values are higher where agriculture is developed, with a similar trend being observed in the saturated zone. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The impact of agricultural activity on freshwater resources has been widely acknowledged, and its impact on surface water systems has been described in numerous studies (e.g. Moss, 2008; Kowalkowski et al., 2012). Similarly, N leaching to groundwater resulting from agricultural practices has been studied, revealing a generally high N content in groundwater in the 27 EU-members states (Velthof et al., 2009). However, studies have shown that the adoption of corrective measures can prevent these high concentrations, thus contributing to a reversal of NO3 pollution in aquifers (Hansen et al., 2011). ⇑ Corresponding author. Tel.: +351 296 650 147; fax: +351 296 650 142. E-mail address:
[email protected] (J. Virgílio Cruz). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.11.009
The relationship between agricultural practices and the dissolution of NO3 in groundwater, as well as other pollutants, and the subsequent hydrogeochemical processes, have been studied worldwide and described in large-scale overviews and in numerous case studies (Houzin et al., 1986; Keeney, 1989; Stamatis et al., 2011; Menció et al., 2011; Jayasingha et al., 2011; Heaton et al., 2012). The emphasis here is on the application of fertilisers. Nevertheless, the grazed pasturelands also exhibit high N leaching to groundwater due to animal wastes, naturally deposed or by manure dissemination (Ryden et al., 1984; Burden, 1986; Olson et al., 2009; Infascelli et al., 2009). The role of water flow and solute transport through the vadose zone is an important research area, given that that it may affect various geochemical processes and the overall composition of groundwater. The evolution of water chemistry will depend on
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the interaction of several factors, such as atmospheric deposition and evapotranspiration, organic-driven mechanisms such as organic matter decay and nutrient uptake, and inorganic processes as mineral weathering and cation exchange. The crucial role of these processes in the interface between water table and the atmosphere is well characterized in a series of recent publications (White et al., 1998, 2005, 2008, 2009; Maher et al., 2009). Groundwater is a strategic resource in the Azores archipelago as about 98% of the water supply is supported by aquifers. However, and despite the environmental and economic value of groundwater in the archipelago, aquifers are under increasing exploitation, which is reflected in the deterioration of groundwater quality. Surface water and groundwater pollution due to the impact of agricultural activities has been reported in the majority of the islands, as shown by high contents of N species. Nitrate, derived from the application of synthetic and organic fertilizers, and from animal waste leaching, and from microbiology processes, has resulted in some cases in failure to comply with EU and national water quality regulations (Santos et al., 2005; Ribeiro et al., 2008; Martins et al., 2008; Cruz et al., 2010a,b). The objective of the present paper is to characterize the impact of agriculture on groundwater quality of the Sete Cidades volcano, located in the westernmost sector of São Miguel, the largest of the nine volcanic islands that make the Azores archipelago (Fig. 1). The field methodology included the characterization of the soil water composition at several depths, as well as the study of groundwater composition in the saturated zone, excluding thermal waters. The present paper describes results obtained for both media. A coupled methodology was used, to link the pollutant sources and the NO3 leaching in the vadose zone and the aquifer. Related scientific contributions on surface and groundwater quality have already been published (Cruz et al., 2007a,b). A similar scheme was tested and considered more feasible, when compared to other approaches, in a study of a coastal aquifer heavily polluted with NO3 (Guimerà, 1998). Rekha et al. (2011) used piezometers to char-
163
acterize NO3 leaching to shallow aquifers, a similar approach to the one developed in the present study. São Miguel island is located between 37°550 N to 37°040 N and 25°520 W to 25°080 W, and has an area of about 747 km2 and 137,699 inhabitants (in 2011) (Fig. 1). It belongs to the Azores Eastern group, and, like for the remaining islands, agriculture is one of the main economic activities with 8.6% of the regional Gross Domestic Product (2010). Livestock is the main agricultural activity, as about 94% of the land is used for pasturage with more than 100,000 bovines in 2003.
2. Geological and hydrogeological setting 2.1. Geology The Azores archipelago is located near the triple junction between the American, African and Eurasian plates. The geodynamic setting is complex and several main tectonic structures are observed in the Azores region. The Middle Atlantic Ridge separates Flores and Corvo islands (western group) from the other islands, which are spread eastward. The maximum age observed in the archipelago is 8.12 Ma on Santa Maria island (Abdel-Monen et al., 1975). Since its settlement in the 15th century, about 30 eruptions have occurred, the last of which was a submarine event that occurred near Terceira island in 1998–2000 (Gaspar et al., 2004). São Miguel is dominated by three volcanic centers, which correspond to the major active trachytic central volcanoes of Fogo, Sete Cidades and Furnas, linked by rift zones. Several volcanic complexes at São Miguel can be distinguished (listed from older to younger): Nordeste, Povoação, Furnas, Sete Cidades, Fogo and Picos. In the last 5 ka the activity of these three active central volcanoes has been evidenced by 57 volcanic eruptions, with an output of 4.6 km3 of dense rock (Booth et al., 1978).
Fig. 1. Location of the Azores archipelago in the North-Atlantic Ocean. São Miguel island is shown on top.
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Sete Cidades volcano is the westernmost of the three active central volcanoes of São Miguel island. This volcano occupies an area of 110 km2 and has a subaerial volume of 45 km3 (Moore, 1990; Queiroz, 1998). This steep volcanic cone, with an average slope of 12°, has a maximum altitude of 845 m and an average basal diameter of approximately 12 km. The present field study was undertaken inside the Sete Cidades volcano caldera, a 5 km diameter circular shaped depression, with an average depth of 300 m. This summit caldera is the result of several distinct collapses. The three main phases of collapses are dated from 36 ka to 16 ka and are due to major eruptions that produced major fall and flux pyroclastic deposits. Steep walls from 30 m to 400 m surround the depression. Several volcanic features can be observed inside the caldera, namely pumice cones, maars (s.l.) and a few domes. Four lakes inside the caldera are related to volcanic center craters that resulted from post-caldera eruptions. The two principal lakes cover the caldera floor, and are are known as Lagoa Azul (3.58 km2) and the Lagoa Verde (0.82 km2). These lakes are connected and receive direct input from rainfall and of drainage from the Sete Cidades caldera drainage basin, with an area of approximately 18 km2. About 24% of the drainage basin area is pasture land and the local economy is strongly dependent on agriculture. 2.2. Hydrogeology Groundwater in the Azores occurs in two major aquifers systems (Cruz, 2003, 2004): (1) the basal aquifer system, which corresponds to fresh-water lenses floating on underlying salt water, and (2) perched-water bodies. The basal aquifer system located in the coastal area generally has a very low hydraulic gradient, and groundwater abstraction is from drilled wells. It is suggested that a hydraulic contact occurs with the perched-water bodies at altitude, which eventually releases water to the basal unit. The perched-water bodies correspond to pervious units, with impermeable to very low permeable layers at the bottom and, where topographic conditions are favorable, are drained out by numerous springs on the volcano slopes. Therefore, these aquifers at altitude correspond to confined layers or to leaky aquifers, which can lose water through aquitards bounding them from above. There is no evidence to support the hypothesis for the existence of dike-impounded aquifers (Peterson, 1972), as in the Hawaiian conceptual model for volcanic islands; however, this is not excluded with the present information (Cruz and Silva, 2001). At São Miguel island perched-water bodies are drained by 1100 cold water springs, mainly on the flanks of the central volcanoes, like Sete Cidades. Discharge is, in general, higher in springs from lava flow aquifers, when compared to aquifers made of pyroclastic deposits. The study has shown that, in the case of São Miguel island, the average recession discharge in lava flow aquifers is about 5.6 103 L/s, higher than the 3.82 104 L/s observed in springs from pyroclastic formations (Paradela, 1980). There is a sharp difference between discharge in winter and summer periods, reflecting a seasonal influence: in the Sete Cidades aquifer system the average discharge is 0.48 L/s in winter and 0.23 L/s in summer. Specific capacity at São Miguel ranges between 0.49 and 100 L/ s m, with a median value of 1.1 L/s m (Cruz, 2004; Cruz et al., 2011). Transmissivity range is 5.98 104–1.22 101 m2/s, with a median value of 1.35 103 m2/s, and about 45.4% of the estimates can be considered as intermediate values from 1.16 104 to 1.16 103 m2/s (Krásny´, 1993). Groundwater resources are estimated at 370 Mm3/a with recharge rates between 16% and 45%, with the highest values observed for the Ponta Delgada–Fenais da Luz aquifer system (Cruz, 2004).
3. Methodology The research programme includes the characterization of the soil water composition at several depths, as well as a study of the groundwater composition in the saturated zone. Water in the unsaturated zone was sampled by means of ceramic suction cups, which represents a common method with economic and operational advantages over other approaches, despite the small volume of sample collected (Stephens, 1995; Tindall and Kunkel, 1999). Suction cups of 21, 31 and 63 mm diameter were used at the end of PVC tubes which height was levelled with the land surface, enabling an easier sampling procedure in the field. The PVC tube acts as a reservoir and in the opposite end a stopper is placed with two holes, one to retrieve the sample from inside with a syringe and the other for creating a vacuum inside the system. Immediately after retrieving the sample, a vacuum of about 60 cbar (60 kPa) was created with a hand pump inside the sampling device, adjusted with the vacuum dial gauge, in order to establish a hydraulic gradient from the soil into the sampler through the ceramic cup. The disadvantages of using such soil–water sampling devices are essentially associated with the depth limitation of a maximum of 10 m, cup-water chemical interactions, the soil–water content and to the small capture area (Stephens, 1995; Tindall and Kunkel, 1999). Considering the objectives of the present work, these disadvantages can be considered as negligible. Nevertheless, all ceramic cups were submitted to several stages of washing prior to installation, according the methodology proposed by Perez and Evangelista (1993). The laboratory procedure consists of several stages of washing in supra pure water, each for 24 h, under vacuum at about 60 cbar, repeated until the aliquot conductivity stabilizes. In a few cases sample conductivity decreased rapidly, while in other suction cups values remained low, suggesting water good quality. During these washing stages, it was possible to conclude that the suction cup diameter was not influencing the conductivity of samples. Five pasture locations were selected and monitored each fortnight when logistical conditions were propitious, as well as a site with similar physical conditions, but without agricultural activity. The results from one pasture will be considered here, obtained in the so-called Pavão I (Pa I), which corresponds to the most extensive data set (Fig. 2A). On this site, six ceramic suction cups were installed, at several depths (G1:0.35 m; G4: 0.70 m; G3: 1 m, G7: 1.30 m; G11: 1.60 m; G14–15: 1.90 m). The site without agricultural activity is located near the Sete Cidades village, and two ceramic cups were installed there, respectively at 0.25 (S62) and 0.50 m (S61) depth. Despite the low density of sampling sites inside the Sete Cidades caldera (0.64 sites/km2), the homogeneous physical and chemical soil properties, the similar geological background and the land use, with two main classes (pasture and non-agriculture lands), allows considering results gathered in the present study as representative. Groundwater in the saturated zone was sampled and characterized with a coupled approach, namely by studying water composition in springs and in piezometers. Therefore, a regional groundwater quality survey was made in the whole of the Sete Cidades volcanic edifice, along which 28 springs spread on the flanks of the volcano. A set of four springs located inside Sete Cidades Caldera were monitored twice a week, and seven piezometers were installed in selected pasture lands and in one site, near Sete Cidades village, with no agricultural activity (Figs. 2B and 3). The piezometers were installed in pasture lands at depths ranging from 1.23 m to 3.23 m (Pz I: 2.19 m; Pz II: 2.45 m; Pz III: 1.52 m; Pz V: 3.23 m; Pz VII: 2.66 m), while the piezometers near the village were installed at a depth of 1.23 m (Pz IV) and 0.57 m (Pz VI).
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Fig. 2. Location of the sites where suction cups were installed for soil water collection (A) and location of the springs and piezometer sites inside Sete Cidades caldera (B).
Field measurements of temperature, pH and electrical conductivity, as well as an alkalinity titration with 0.05 M H2SO4 to a pH of 4.45, were performed immediately after sampling, preferably twice a month. Cations were determined by atomic absorption and anions by ion chromatography, following filtration carried out in the field after sampling (0.45 lm cellulose membrane fil-
ters). Samples for AAS were acidified with concentrated HNO3 following collection. In addition to the collection of soil water and groundwater samples, rain water aliquots were also collected over a year, using suprapur acid prewashed polyethylene bottles installed close to the village site, at the west shore of the lake.
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Fig. 3. Location of the sampled springs to characterize the regional groundwater chemistry.
Drilling for piezometer completion was used to collect soil samples for characterization. Chemical analyses were performed by ICP-MS (Activation Laboratories, Canada), cation exchange capacity (CEC) was determined using barium chloride–triethanolamine as extractants at high pH (8.1) and granulometric analysis followed a procedure which was begun by a wet separation with a 63 lm (0.0625 mm) sieve to get coarser and a finer fractions. Laser analysis was used for the finer fraction.
4. Results 4.1. Groundwater discharging in springs Groundwater discharging from springs on the central volcano flanks is mainly of Na–HCO3 and Na–HCO3–Cl types (Fig. 4A). These waters have an average temperature of 14.4 °C, are slightly acid (median pH = 6.4) and have low mineralization, as suggested by electrical conductivity values between 105 and 341 S/cm (median = 212 S/cm) (Table 1). Nitrate content ranges between 0.2 and 53.5 mg/L (median = 7.2 mg/L), and 21% of the sampled springs do not complying with the 25 mg/L-recommended value according to Portuguese drinking water standards (Law 236/98, from August 1, 1998). One spring exceeded the maximum standard value of 50 mg/L. Groundwater sampled in the four monitored springs located inside the caldera are of Na–HCO3 and Na–HCO3–Cl types, having a pH range between 5.7 and 8.6, and low mineralization, as suggested by the electrical conductivity values (176–686 S/cm) (Fig. 4B). Considering the NO3 content, a sharp contrast was detected between samples from the Pedras Brancas (0.8–4.8 mg/L) and Moinhos springs (0.4–3.6 mg/L), and the values observed in waters collected in Tanque (19.9–40.7 mg/L) and Seara springs (38.9–78.4 mg/L).
4.2. Piezometers Samples of groundwater collected in the seven piezometers spread along Sete Cidades caldera have a pH range between 5.3 and 8.1, with median values for each piezometer ranging between 6.4 and 7.0, and the dominant water type is Na–HCO3 (Fig. 4C) (Table 1). Median values for the water conductivity range between 95 and 232 S/cm, respectively in Pz IV and Pz V. The later is the deepest piezometer installed in the field, showing that groundwater is little mineralized. Alkalinity values determined for the piezometers Pz II, Pz III, Pz IV and Pz VII range between 16 and 50 mg CaCO3/L. The NO3 content ranges between 0.1 and 49.6 mg/L, with median values of 37.8 and 30.1 mg/L in piezometers Pz V and Pz VII, which are higher than values for Pz I (8.3 mg/L), Pz II (1.3 mg/L), Pz III (3.2 mg/L), Pz IV (1.1 mg/L) and Pz VI (3.1 mg/L). Soil composition was determined for 21 samples collected during piezometers completion and is dominated by SiO2 (52.2– 62.8%), Al2O3 (15.7–17.0%), Na2O (4.6–6.8%) and K2O (3.0–5.1%), reflecting the association with volcanic rocks of acidic character as pumice-fall deposits of trachytic composition. Only in the pasture, where piezometer V was installed, do the SiO2 and alkali contents of soils suggest a basaltic-trachyandesite rock composition, whereas the main sites are trachytic. CEC results are low, ranging between 6.9 and 7.2 Cmol/kg at the village site (piezometer IV), respectively, at 0.63 and 0.86 m depth, and between 4.8 and 11.2 Cmol/kg at the Pa I site (piezometers VII) with lower values observed at depth (2.16 m). These results are consistent with the sand to silty-sand character of the soils, revealed by the granulometric analysis: in Pa I, sand and coarser material corresponds to 64.3–87.0% of the sample, while material of silt and clay grain size ranges between 12.3% and 34.1% and 0.7–1.3%, respectively. Soils at the village site have a small coarse fraction, and material with a sand grain size ranges between
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Fig. 4. Major-ion relative composition represented by means of a Piper-type diagram for sampled water types (A – regional groundwater; B – monitored springs inside Sete Cidades caldera; C – piezometers; D – soil water).
52.9% and 68.2%, whereas silt and clay proportions are from 30.4– 45.1% to 1.4–1.9%, respectively. 4.3. Soil water suction cups Soil water at the Sete Cidades village site is of Na–Cl type and median values for all major species are similar in range. Chloride content is generally higher at 0.25 m depth (Fig. 4D) (Table 1). The pH ranges between 5.4 and 8.4, without significant differences with depth: pH at 0.25 m ranges between 5.6 and 8.4 (suction cup S62), and at 0.50 m varies between 5.4 and 7.9 (suction cup S61). Water conductivity ranges from 40 and 495 S/cm and the maximum value at 0.25 m depth (495 S/cm; median = 90.5 S/cm) is higher than that observed at 0.50 m depth (292 S/cm; median = 78 S/cm). The maximum NO3 contents observed in the suction cups S62 and S61 are equal to 2.8 mg/L (median = 1.2 mg/L) and 5.4 mg/L (median = 1.4 mg/L), respectively. These results are lower compared to values observed in sites with agriculture activity, which will be discussed further. Median values for NO2 content range from 0.6 (S62) to 0.7 mg/L (S62). Soil water sampled at the Pa I site has a higher mineralization compared to the previous site, as suggested by the electrical conductivity measurements with median values between 141 and 357 S/cm for suction cups G1, G4, G3, G7, G11 and G14–15 (Table 1).
Waters at this site are mainly of HCO3 to HCO3–Cl types and with a Mg–Ca trend for cations, which constitutes a major difference compared to the village site (Fig. 4D). The Cl and HCO3 contents are higher at suction cups near the surface, showing the effects of sea salt deposition and CO2 dissolution in the upper layers of the soil. The pH shows a decreasing trend with depth, from a median value of 8.2 at 0.35 m depth to values between 7.0 and 7.2 from 0.70 m depth to the water table; the latter with a value of 6.9 (Piez VII; Table 1). Nitrate contents range between 2.0 and 87.7 mg/L, with different intervals according to depth: 2.0–61.4 mg/L (G1), 7–50.7 mg/L (G4), 2.1–54.5 mg/L (G3), 4.5–31.9 mg/L (G7), 10.1– 23.9 mg/L (G11) and 5.3–87.7 mg/L (G14–15). In the suction cup installed at 1.90 m depth, the median value is 39.4 mg/L (G14– 15; Table 1). Therefore, the NO3 content of Pa I pasture land indicates the continuous input of inorganic fertilizers, usually applied eight times a year, as well as leaching of animal wastes. The NO2 content were usually below detection limit, with a range of 0.25– 0.4 mg/L.
5. Discussion The chemistry of rain water is dominated by Na–Cl due to the influence of the seawater, as evidenced by the close relationship between both variables (r = 0.887; Fig. 5A). The variation of the
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Table 1 Results for master variables (pH, temperature, electrical conductivity) and major-ion composition. Point
Statistics
pH
T (°C)
Conductivity (lS/cm)
Na (mg/L)
K (mg/L)
Moinhos
Max Min Median
8.2 6.4 7.4
20.5 13.6 15.0
686 97 188
57.5 29.3 34.1
Pedras Brancas
Max Min Median
8.6 6.4 7.3
21.6 11.7 15.4
378 100 186
Seara
Max Min Median
8.4 6.4 7.6
20.7 13.6 15.1
Tanque
Max Min Median
7.7 5.7 6.8
S61
Max Min Median
S62
Mg (mg/L)
Ca (mg/L)
12.4 2.4 3.4
2.9 0.4 0.6
6.3 3.4 4.6
29.5 16.5 22.4
6.4 2.9 4.5
85.4 53.7 61.0
3.6 0.4 2.7
65.1 18.8 41.9
29.8 23.0 26.5
11.9 3.7 6.4
5.3 0.8 1.1
8.3 4.9 6.6
38.3 24.2 28.9
7.3 3.4 5.1
117 40.3 44.8
4.8 0.8 3.7
78.8 25.0 54.8
387 120 285
50.0 19.0 39.2
27.7 9.4 13.1
3.8 1.6 2.2
8.6 4.4 5.6
53.0 28.0 33.7
14.6 5.2 8.1
56.1 36.0 44.5
78.4 38.9 48.1
96.5 30.0 65.7
20.5 15.3 16.1
417 122 308
75.0 30.0 46.0
22.9 5.3 9.3
2.9 1.2 2.1
17.6 7.0 8.9
29.8 14.9 22.7
23.0 4.3 16.2
97.0 46.4 87.8
40.7 19.9 32.4
109 32.7 71.7
7.9 5.4 6.7
25.1 12.9 16.9
292 41 78
24.0 5.7 10.0
50.8 1.8 5.4
7.8 0.2 0.5
11.9 0.8 2.2
53.1 3.7 12.8
5.0 0.2 0.9
26.8 4.9 17.1
5.4 0.0 1.4
114 15.8 43.0
Max Min Median
8.4 5.6 6.8
24.9 12.9 17.3
495 40 91
28.1 6.4 9.5
56.0 3.1 6.3
9.3 0.2 1.0
16.8 1.1 2.4
73.0 4.9 16.2
7.4 0.2 0.9
24.4 7.3 17.1
5.9 0.0 1.2
61.6 11.1 29.1
G1
Max Min Median
8.9 6.7 8.2
25.2 13.6 17.5
691 95 357
22.8 7.6 15.2
85.5 1.4 15.9
52.5 1.8 21.0
42.5 4.8 19.6
51.6 4.5 19.2
9.0 0.1 4.4
250 41.5 155
61.4 2.0 13.9
177 58.4 100
G4
Max Min Median
8.8 6.8 7.1
22.3 13.6 16.0
470 81 251
18.0 10.7 14.2
14.8 4.6 7.9
36.3 0.8 5.6
22.3 5.4 9.8
23.6 10.3 13.3
8.7 2.2 7.1
154 31.7 48.8
50.7 7.0 27.4
108 55.6 75.8
G3
Max Min Median
9.0 6.7 7.3
23.5 13.5 17.0
398 89 264
20.3 7.1 11.0
32.4 3.5 9.2
52.3 1.1 9.8
29.0 5.0 11.0
36.0 6.5 11.8
18.6 3.6 6.3
164 24.4 42.7
54.5 2.1 17.9
152 36.6 89.4
G7
Max Min Median
8.8 6.2 7.2
24.4 13.5 17.7
366 82 141
15.2 8.9 10.9
22.0 2.3 6.4
35.4 0.7 2.4
21.6 3.2 8.1
23.8 4.2 9.8
13.2 4.5 7.3
129.3 22.0 35.4
31.9 4.5 9.5
165 7.5 67.4
G11
Max Min Median
7.5 6.7 7.0
23.4 14.3 20.9
264 79 171
17.6 9.4 12.8
8.3 4.3 5.6
6.6 0.8 3.7
11.0 5.2 8.2
12.3 4.6 8.6
10.5 6.1 6.8
105 31.7 85.4
23.9 10.1 14.4
101 56.3 81.5
G14–15
Max Min Median
8.4 6.2 7.2
24.9 14.0 17.5
433 77 146
27.5 9.6 15.7
92.8 4.0 9.5
15.3 0.9 4.0
20.5 5.0 10.3
83.6 4.7 17.6
15.4 1.6 8.3
115 26.8 39.0
87.7 5.3 39.4
89.0 35.1 74.0
Piez I
Max Min Median
7.1 5.7 6.4
20.6 14.5 15.6
206 74 104
19.1 8.7 10.1
14.2 4.2 8.0
2.2 0.7 1.1
8.0 2.1 5.1
38.2 5.0 8.7
16.5 0.5 5.0
n.d. n.d. n.d.
22.0 1.1 8.3
64.6 34.4 54.6
Piez II
Max Min Median
7.6 6.2 7.0
22.3 13.8 16.5
349 77 100
21.8 9.4 14.0
12.5 2.9 4.3
4.0 0.1 1.1
11.7 3.0 4.8
47.5 5.3 14.2
14.0 0.5 2.6
61.0 19.5 32.9
5.7 0.3 1.3
80.7 14.3 42.0
Piez III
Max Min Median
8.1 5.6 6.9
23.1 12.4 16.2
159 47 99
35.0 4.7 9.8
18.4 2.6 5.8
3.8 0.3 1.0
30.8 1.5 5.9
61.0 0.4 9.8
13.7 0.1 1.4
58.6 22.0 40.3
16.8 0.3 3.2
122 13.3 49.5
Piez IV
Max Min Median
7.6 5.8 6.6
25.0 13.8 16.5
151 69 95
24.5 6.1 11.7
13.1 2.4 7.0
1.4 0.0 0.6
9.0 1.5 4.2
53.9 6.6 14.4
6.2 0.1 0.7
56.1 22.0 31.7
5.1 0.1 1.1
133 27.6 56.6
Piez V
Max Min Median
6.9 5.7 6.4
17.4 15.2 15.9
255 212 233
37.2 28.0 29.9
23.0 14.1 20.7
3.3 2.6 2.9
21.5 8.6 14.2
34.5 22.5 24.6
11.7 4.6 5.4
n.d. n.d. n.d.
40.8 24.5 37.8
76.6 64.4 74.5
Piez VI
Max Min Median
7.1 5.3 6.2
16.5 13.4 14.6
209 56 156
28.4 8.3 20.3
11.9 3.0 8.7
3.3 0.8 1.3
27.2 1.5 3.8
50.8 10.4 30.4
6.9 0.9 3.9
n.d. n.d. n.d.
42.4 0.4 3.1
57.6 6.6 41.9
Piez VII
Max Min Median
8.0 6.0 6.9
22.3 13.2 16.9
283 75 108
21.8 6.4 12.9
8.7 3.0 6.0
5.9 0.6 1.1
12.4 5.2 7.6
25.1 6.3 10.8
5.8 1.3 2.5
56.1 22.0 37.8
49.6 2.1 30.1
68.9 14.1 51.6
major-ion composition in rain water with time shows that the Cl content varies depending on the climatic conditions (Fig. 5B). The regional groundwater chemistry also reveals the influence of a seawater component as shown by the relationship between Cl content and water conductivity (r = 0.586; Fig. 6A) and the in-
Cl (mg/L)
SO4 (mg/L)
HCO3 (mg/L)
NO3 (mg/L)
SiO2 (mg/L)
verse relationship with altitude (Fig. 6B). Nevertheless, a close relationship is also shown by the conductivity – HCO3 (r = 0.699; Fig. 6C) and the Na–HCO3 plots (r = 0.604; Fig. 6D), reflecting the contribution of soil CO2 and silicate dissolution as observed in the majority of springs discharging from perched-water bodies in
J. Virgílio Cruz et al. / Applied Geochemistry 29 (2013) 162–173
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the Azores (Cruz and Amaral, 2004). However, the low conductivity values suggests that water–rock interaction is limited, which also explains the SiO2 content (30.82–94.59 mg/L). The groundwater conductivity time series for the monitored springs reveals that data is quite stable with time despite some minor fluctuations, and that the Seara and Tanque springs have similar but higher values when compared to Moinhos and Pedras Brancas discharges (Fig. 7). Despite being difficult to establish a clear relationship between groundwater conductivity and rainfall in the caldera area the time delay between both variables is very short, suggesting a limited residence time. From the NO3 content observed in samples from the Seara and Tanque springs, both discharging in the northern region of the Sete Cidades caldera, it can be inferred that the higher dissolved solids in these waters is caused by agricultural activities. The NW sector of the Sete Cidades
Fig. 5. Relationship between Na and Cl content of rain water (A) and variation of rain water chemistry (B).
Fig. 7. Variation of groundwater conductivity in monitored springs. Monthly precipitation in the caldera of Sete Cidades is also shown for the same time interval.
Fig. 6. Relationship between conductivity and Cl (A), altitude and Cl (B), conductivity and HCO3 (C) and Na and HCO3 in groundwater samples from Sete Cidades volcano.
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caldera is dominated by pasture lands, and the associated impacts on water quality have motivated water authorities to divert surface flow to prevent discharge to the lake to reduce eutrophication. The major-ion content time series in the caldera springs show that the majority of the elements have limited variability through time, the major fluctuations being associated with HCO3 and SiO2, which suggests that water–rock contributions to groundwater chemistry is more sensitive to seasonal control (Fig. 8). Groundwater sampled in piezometers has dissolved solid contents within a similar order of magnitude as springs, as revealed by water conductivity measurements. The variation of water conductivity in piezometers shows that values, despite some fluctuations with higher amplitudes, follow a similar trend (electronic Supplementary data – Fig. IA). Major element variations for each piezometer through time show that the dissolved content is relatively stable for most of the analyzed species, whereas major fluctuations are associated with HCO3 and SiO2 contents in piezometers I, II, III, IV, V, VI and VII, and also Cl for piezometer VI (electronic Supplementary data – Fig. IB–IH). The former trend can be explained by the dissolution of volcanic glass, releasing Si to groundwater, which is a process favoured by faster dissolution rates of basaltic glass comparing to crystalline basalt, plagioclase or olivine (Gislason and Eugster, 1987). The observed fluctuations are the result of a variable degree of mixture with lake water depending on lake level variations. The latter corresponds to a shallow-depth piezometer (0.57 m) located in the village site and the Cl content is influenced by rain water chemistry and evapotranspiration. In both suction cups installed close to Sete Cidades village (S61 and S62), conductivity values are somewhat variable and generally
higher during summer or drier periods, which suggests the effect of salt leaching following rain, while in winter leaching occurs during the entire period (Fig. 9). Soil water conductivity values are similar for both cups, as the limited research depth on this site, as well as the high precipitation and soil hydraulic conductivity, favors fluid homogenization. As soil water is dominated by Na–Cl, similar to rain water, it can be suggested that evapotranspiration is alone controlling its composition (Langmuir, 1997). The higher Na–Cl content shows the effect of evapotranspiration losses, as the low HCO3 content results in a limited chemical weathering contribu-
Fig. 9. Variation of soil water conductivity in suction cups S62 (0.25 m) and S61 (0.50 m). Monthly precipitation in the caldera of Sete Cidades for the same time interval is also shown.
Fig. 8. Variation of major ions and SiO2 in the monitored springs (A – Moinhos; B – Pedras Brancas; C – Seara; D – Tanque).
J. Virgílio Cruz et al. / Applied Geochemistry 29 (2013) 162–173
tion. The actual monthly evapotranspiration calculated by the Thornthwaite (1948) ranges between 77.3 mm to 83.1 mm from June to September, and from 46 mm and 46.5 mm from November to April, and exceeds monthly precipitation between June and August. The major element variation during the studied time-period is represented in Fig. 10. The variation pattern suggests that in both suction cups the higher variability corresponds to Cl and K, resulting from the flushing of the soil profile after higher rain episodes, a process which is more marked in summer, where evapotranspiration is higher and salt accumulation reaches a maximum. Species like Cl, carried by sea salts, and K, which is a fairly mobile element during weathering of volcanic glass in rocks of acid character (e.g. in Sete Cidades; Shikazono et al., 2005), and HCO3 from CO2 solubilization, are transported by soil water following infiltration episodes. The comparison of results from the Village and Pa I sites suggest that the dissolved solid content is higher in the second. At the Pa I site, median values of water conductivity range between 141 and 357 S/cm, and higher values are measured in the more superficial cups (G1, G3, G4) (Fig. 11). Maximum values occur in summer, which suggests salt flushing following rain episodes during this dryer period, when evapotranspiration reaches highest values. The time-series plot for major-ion content shows that major variability is associated with HCO3 and NO3, suggesting that NO3 loading to groundwater results from soil flushing following rainfall episodes (electronic Supplementary data – Fig. II). Considering the relative soil water composition of site Pa I, it is possible to depict a trend, from the more superficial suction cup (G1) to the deeper G14–15, which presents a relative water composition similar to the perched-water bodies inside the Sete Cidades
Fig. 10. Variation of major-ion composition in soil water sampled in suction cups S62 (A) and S61 (B).
171
Fig. 11. Variation of soil water conductivity in suction cups G1 (0.35 m), G4 (0.70 m), G3 (1 m), G7 (1.3 m), G11 (1.60 m) and G14–15 (1.9 m). Monthly precipitation in the caldera of Sete Cidades for the same time interval is also shown.
Caldera. This trend is marked by a shift from a Mg–Ca cation composition to Na-dominated waters (Fig. 4). From suction cup G1 (0.35 m) to G11 (1.60 m), median values for cations decrease 0.84 times for Na, 0.42 times for Ca, 0.18 times for Mg and 0.35 times for K (values were not calculated for G14–15 as this suction cup may be affected by water table fluctuation and is, therefore, not fully representative of soil water). The Cl content also shows an absolute decrease equal to 0.44 times between 0.35 m and 1.6 m depth. However, using this species as a conservative element, net changes relative to Cl are equal to 1.91 times for Na, 0.95 times for Ca, 0.41 times for Mg and 0.8 times for K. The absolute decrease of Cl shows that the contribution of rain water to soil water composition in deeper soils, at least compared to the Village site, is not relevant. The Mg–Ca-dominated composition in the more superficial levels may result from the application of fertilizers, as Pa I site is pasture land. In Sete Cidades caldera, studies to characterize nutrient export to the lake, have shown that there are six applications of the so-called ‘‘nitrolusal 26’’, a fertilizer made of NH4NO3 with Ca and Mg carbonate, as well as to the application of the so-called ‘‘foscamónio 111’’ in March and October (DROTRH, 2002). The latter is a fertilizer made of NH4NO3, P and K. Nevertheless, the cumulative enrichment in Mg and Ca from silicate weathering cannot be excluded in explaining the Mg–Ca-dominated composition. The impact of the excessive application of fertilizers can be traced by a comparison of NO3 contents in pasture and village sites, as values are higher where agriculture is developed (2.0–87.7 mg/ L), when compared to the maximum observed in the village site (5.4 mg/L). The observed relative decreases in Ca, Mg and K with depth may result from clay formation and uptake by ion exchange and, while the relative increase in Na can also result from ion exchange processes, silicate weathering is expected to contribute to the Na content, as well as to the relative increase in HCO3, both associated with silica in solution. Despite the NO3 biological uptake, the NO3 median values in Pa I suction cups range between 13.9 and 39.4 mg/L. The higher value is observed in cup G14–15, at a depth of 1.9 m, and is similar to the median value from samples collected in piezometer VII installed in the same pasture. It could be that this cup is affected by groundwater table fluctuation. Data from Pa I was used to design a conceptual model for the evolution of water composition with depth (Fig. 12). Conductivity and HCO3 concentrations depict a similar decreasing trend to the 130 cm-depth level, followed by a more stable variation in-depth, despite moisture conditions (Fig. 12A). The higher HCO3 content
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A
0 -20
0
100
200
300
400
B
Atmospheric deposition: Na-Cl dominated Fertilizers: N-P-Ca-Mg -K dominated
20
30
Depth (cm)
-80
G3
-120
50
Mg-Ca soil-water
-80 -100
Na soil-water
-120
G7 -140
-140 -160
40
Nitrification (pH decrease)
-60
G4 Depth (cm)
10
-40
-60
-100
0
-20
G1
-40
0
-160
G11
water table variation
-180
G14-15
-180 -200
-200 pH
Cond
HCO3
Na
K
Mg
Ca
Cl
SO4
NO3
Fig. 12. Variation of water composition with depth in Pa I (median values): A – pH, electrical conductivity (lS/cm), and HCO3 (mg/L); B (all values in mg/L) – Na, K, Mg, Ca, Cl, SO4, NO3.
in the upper level of the soil profile reflects both the contribution from organic-derived CO2 as well as acidity neutralization by weathering. Nevertheless, the magnitude of the weathering contribution should be lower compared to the former, as pH is relatively stable from 100 cm to 190 cm-depth levels, explaining the slight Na enrichment in-depth. The higher NO3 content at suction cup G4 is suggested to be the result of nitrification in the upper level of the soil, which could also explain the sharp pH decrease for cups G1–G4. The NO3 will travel to great depths without changes associated with anoxic conditions, as suggested by the NO 2 ion, being below detection. Water in G14–15 is enriched in several ions, which is thought to result from mixture with groundwater (Fig. 12B). The cycling fluctuation of the lake level will enhance groundwater table variations.
6. Conclusions The present field study is focused on the characterization of water from unsaturated and saturated zones in the Sete Cidades caldera, located in São Miguel, the westernmost island of the Azores archipelago. Regional groundwater generally has low mineralization with Na–HCO3 and Na–HCO3–Cl water types, resulting from the effect of several processes, such as addition of sea spray, CO2 dissolution in soils and water–rock interaction. Groundwater sampled in four monitored springs has a pH range between 5.7 and 8.6 with low mineralization. Water types from these discharges are similar to the composition found all over the volcano as part of a regional hydrogeochemical survey. Waters collected in seven piezometers have a pH range between 5.3 and 8.1, and HCO3–Na is the dominant water type. The concentration of major elements for the studied time period is relatively stable for the majority of the analyzed species with the exception of HCO3 and SiO2. These variations are explained by different degrees of mixing between groundwater, more enriched in those species, and lake water. With regard to soil water composition in the unsaturated zone, conductivity values for the village site are quite variable with a Na–Cl dominant water type. Vadose zone water from the Pa I site is more mineralized with HCO3 to HCO3–Cl types and a Mg–Ca trend for cations, which constitutes a major difference comparing to the village site.
Data from site Pa I shows a trend, from the more superficial suction cup (G1), having a Na–Cl composition similar to rain water, to the deeper G14–15 with a composition similar to perched-water bodies inside the Sete Cidades Caldera. This trend is marked by a shift from a Mg–Ca cation composition to Na-dominated waters, whereas the soil water composition in the more superficial levels results from the application of fertilizers associated with rock weathering. Nevertheless, the observed relative decreases in Ca, Mg and K with depth reflect the contribution of other processes, such as clay formation and ion exchange of Ca2+ and Mg2+ for Na+, while the relative increase in Na can also result from silicate weathering. The impact of excessive applications of fertilizers are reflected by NO3 contents in pasture and the village sites, as values are higher where agriculture is developed. A similar trend is also shown by the NO3 content in piezometers. Spatial variability of the NO3 content was also observed in the monitored springs that discharge from perched-water bodies inside the Sete Cidades caldera. Nitrate contents are substantiality lower in areas without agriculture (springs Pedra Branca and Seara), compared to values observed at springs located in areas where grazed pasture land are predominant, namely in the NW sector of the Sete Cidades caldera (springs Tanque and Seara; Fig. 2). Acknowledgements The authors are grateful to the Fundação para a Ciência e Tecnologia for the funding of the Project POCTI/CTA/36493/1999. The detailed and useful comments on early versions of the manuscript provided by two anonymous reviewers, as well as by the Editors (Dr. Ron Fuge and Dr. Peter Birkle), are acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apgeochem. 2012.11.009. References Abdel-Monen, A., Fernandez, L., Boone, G., 1975. K/Ar ages from the eastern Azores group (Santa Maria, São Miguel and the Formigas Islands. Lithos 4, 247–254.
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