Influence of upwelling on the shallow water chemistry in a small wetland riparian zone (Basque Country)

Applied Geochemistry 27 (2012) 854–865

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Influence of upwelling on the shallow water chemistry in a small wetland riparian zone (Basque Country) M. Martínez-Santos a,⇑, E. Ruíz-Romera b, M. Martínez-López a, I. Antigüedad a a b

Department of Geodynamics, University of the Basque Country, Sarriena Auzoa z/g, Basque Country, Leioa 48940, Spain Department of Chemical and Environmental Engineering, University of the Basque Country, Alameda Urquijo z/g, Basque Country, Bilbao 48013, Spain

a r t i c l e

i n f o

Article history: Received 22 March 2011 Accepted 11 January 2012 Available online 25 January 2012 Editorial handling by W.M. Edmunds

a b s t r a c t Many hydrologic conceptual models in riparian areas assume that the alluvial deposits zone is hydraulically more active than the fractured bedrock below. Therefore, these models undervalue the possible contribution of deeper groundwater from the fractured bedrock system. A hydrochemical study, under various hydrological conditions, has been carried out in a small riparian zone of the Salburua wetland (Basque Country) in order to highlight the conceptual model. This wetland is included in a wide Quaternary aquifer, which has been declared a Nitrate Vulnerable Zone. The results of this study suggest that the fractured bedrock is at least as dynamic as the upper clayey deposits. The presence of more fractured zones, which act as hydraulic ‘‘windows’’, allow the upwelling of deeper groundwater and, consequently, make the upper alluvial deposits and the fractured bedrock water systems to be cross-connected. Nevertheless, this upwelling is limited to some small areas in the riparian zone. As a result of this local interaction, several chemical reactions have been observed and the hydrochemical characteristics of shallow groundwater undergo seasonal variations. The study shows that a hydrologic conceptual model, which does not consider the hydraulic activity of the fractured bedrock, can be too simplistic. The presence of hydraulic windows could be considered throughout the entire Quaternary aquifer, knowledge of which could help the managers of the Vulnerable Zone and the wetland to take more effective measures for regulation and conservation. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Riparian zones form an ecotone between terrestrial and aquatic ecosystems (Gregory et al., 1991), in which deep and shallow groundwater could converge and upwelling or downwelling processes could occur (Cey et al., 1999; Nwankwor and Anyaogu, 2000). Therefore, riparian zones are complex environments that are spatially heterogeneous in both horizontal and vertical dimensions in terms of hydrogeology, soil characteristics and biochemical processes (Maître et al., 2003, 2005). Inadequate understanding of the hydrogeology in riparian areas generally limits the quantitative interpretation of study results (Correll, 1996), while a good understanding is essential for effective resource management and for protecting sensitive ecosystems (Allen et al., 2010). Many hydrologic conceptual models of groundwater interaction in an alluvial deposits-fractured bedrock geological setting, as is the case in some riparian zones, firstly assume that the alluvial deposits zone is hydraulically more active than the fractured bedrock below. Although the bedrock can be often considered as impervious, several studies have reported that the fractured ⇑ Corresponding author. Fax: +34 946 012470. E-mail address: [email protected] (M. Martínez-Santos). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2012.01.007

bedrock could be highly dynamic (Haria and Shand, 2006; Banks et al., 2009; Abid et al., 2011). At least two types of conceptual model are used to explain the interaction between deep and shallow groundwater in a geological setting involving a complex superficial deposits-fractured bedrock system (Banks et al., 2009). In the first, there is no groundwater flow in the fractured bedrock, only in the superficial deposits. In the second model, groundwater flows through the fractured bedrock as well as through the superficial deposits. In this case, the mixing of deep and shallow groundwater could be sufficiently strong to cause visible changes in hydrochemistry of waters (Mallén et al., 2005). A critical challenge is that one rarely knows which is the most appropriate conceptual model (Banks et al., 2009). Regarding this challenge, the purpose of this study was to determine the hydrogeological behavior of marl bedrock underlying a riparian area, and its contribution to the shallow water system, including a wetland. The study area is the Salburua wetland (Basque Country) where Quaternary deposits overlie marl bedrock. For a long time the deposits-bedrock interface has been considered as a no-flow boundary, but new data enable this simplistic concept to be reconsidered and provide a basis for developing a reliable conceptual hydrologic model of the deep-shallow groundwater system in this area. As the piezometric information is insufficient to show

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

the complex pattern of groundwater flow hydrochemical data is used, collected during different hydrological conditions, for establishing the conceptual model. Consequently, the main questions that will be addressed in this study include: (1) is the fractured bedrock hydraulically active? (2) How to highlight this activity. (3) What is the effect of the deep groundwater on the hydrochemistry of the shallow ones? (4) How to consider this interaction for management purposes.

2. Study area This study was carried out in the riparian area of the Salburua wetland which is located near the city of Vitoria-Gasteiz (Basque Country), within the catchment of the Zadorra river (Fig. 1). In this catchment there is an aquifer associated with Quaternary materials, which forms the Hydrogeological Unit of Vitoria-Gasteiz (EVE, 1996). This aquifer covers an area of about 90 km2 and it has been divided into three sectors (East, West and Dulantzi) reflecting the geometry of the Quaternary deposits (Arrate et al., 1997). The East and Dulantzi Sectors were declared a Nitrate Vulnerable Zone (NVZ) for agricultural NO 3 contamination, according to EU Directive 91/676, by the Basque Government in 1998 (Fig. 1) and 2008, respectively. The NVZ includes fluvial and alluvial deposits of the Quaternary aquifer (clays, silt, sands and gravels), with average thicknesses of 5 m, and bordering marls. The Salburua wetland is located in the western edge of the East Sector. The predominance of the clay layers within the deposits in the area of the wetland act as an aquitard. The Quaternary aquifer is recharged both by rainwater infiltration and at certain times of the year by the streams, while water discharge occurs through evapotranspiration and through the wide network of drainage ditches. However, in the riparian zone of the study site lateral recharge seems to be the most important. The aquifer overlies a marl formation (lower-middle Campanian), of about 1000 m thick, where some fracturing has been observed in wells (30–40 m depth). The sulfur spring of Urgazi, which is particularly relevant to this study, rises through a fractured zone of the marls about 4.5 km from the wetland (Fig. 1). The river Alegria, which crosses the NVZ from east to west, receives the inflows of streams and drainage ditches and is the only drainage axis of groundwater in the area. The Salburua wetland, that is a part of the Green Belt of VitoriaGasteiz and has been included in the Ramsar Convention, is a point of local discharge from the Quaternary materials and is essentially formed by three ponds (Fig. 1): Betoño, Arkaute and Larregana. When the water table is at its highest, the three ponds extend over a total of 66 h, which is about one third of the total wetland surface. The wetland was drained in the 1950s for agricultural use. In 1998 it was restored with drainage ditches being closed and some open water layers (ponds) developed. This research has been carried out in the eastern riparian zone of the Arkaute pond, where a wide network of piezometers has been established (Fig. 1). The most common soil in this riparian zone is the Typic Ustorthent, with a silty clay loam texture in the upper horizons and, in some places, sandy loam texture in the deepest horizons. Soil moisture content is higher than 35% in the top meter of the soil between December and May, and the organic matter content is more than 5% in the surface horizons, which darkens the upper clay layer that is 1–2 m thick (Martínez, 2008). The thickness of the Quaternary deposit ranges from 2 (east) to 4 m (west) around the Arkaute pond. Piezometric levels in low water periods reach a depth of 2 m in the riparian zone while during very high water periods levels can reach the surface. Climate data have been taken from the Arkaute Meteorological Station (AME, Fig. 1). Rain is unevenly distributed throughout the

855

year. The period of highest rainfall is from November to January (800 mm/a), although precipitation is abundant until May. The driest and hottest (30 °C) months are from June to September, with rainfall below 30 mm/month. For the 2001–2009 period, average precipitation and temperature was lower than the long term average 614 mm and 11.0 °C. 3. Materials and methods 3.1. Monitoring network Until April 2007 there was only one piezometer (P5) in the study area. For the present study 14 piezometers (SM in Fig. 1) were installed to establish a monitoring network covering the southern and eastern areas of the Arkaute pond. This piezometer set-up was chosen in order to detect the flow direction of the groundwater and to provide adequate spatial coverage for hydrochemical sampling. The piezometers were built with 8 cm diameter PVC pipes that were sealed at the bottom. Piezometer P5 (0–2 m deposits, 2–12 m marls) was fitted with slotted casing (screen interval) between the depth of 1.5 and 12 m and accordingly could not be used for sampling different waters from both the Quaternary deposits and the marls. However, the other deeper piezometer (SM14; 0–1.8 m deposits, 1.8–14 m marls), sited within 3 m of P5, is screened between 3 and 14 m, which only allows the water coming from the marls to be sampled. The other piezometers (SM) are shallow (<4 m) and do not penetrate into the marls, so they enable the waters of the Quaternary deposits to be sampled. Sediment cores were extracted during drilling in all the piezometers (Fig. 2). Although the global study included the hydrochemistry of waters in all the piezometers of the network to differentiate between various different water types, in this paper only data from the two deep piezometers (P5 and SM14) are treated in detail to assess groundwater upwelling and its influence in the shallow groundwater. The Urgazi spring, meaning brackish water in the Basque language, is located further away, about 4.5 km SE of the wetland (Fig. 1), just in the marls. This small, low discharge spring (<0.1 L/s), has been sampled occasionally in the past with reports of anomalous characteristics, including high Na, important S concentrations and low temporal variability. It is now a subject of interest for the data it may provide to help understand the hydrogeological processes of the riparian area. Finally, waters of the Errekabarri stream (Fig. 1) have been also sampled. 3.2. Sampling and analysis Monthly water samples were collected from the piezometer network. In the case of the deep piezometers (SM14 and P5) a 12 VDC peristaltic pump (Eijkelkamp) was used. Three replicates were collected from each sampling point, at depths of 6 m (P5 and SM14) and 10 m (SM14), and stored in polyethylene bottles. Two of these replicate samples were filtered through 0.45 lm filters in the field after collection. One of them was then acidified to pH < 2 with HNO3 (65%) for cation analysis (Ca2+, Mg2+, Na+, K+, dissolved Fe), while anions were measured in the other sample    ðCl ; SO2 4 ; NO3 and F Þ. The unfiltered replicate was used to determine alkalinity and total Fe. Water samples were stored refrigerated in the dark and analyzed the same day in the laboratory. Cations were measured using ICP-OES (Perkin Elmer Optima 2000), while anions were determined using ion chromatography (DIONEX ICS 3000). Alkalinity was determined by titration with HCl (APHA-AWWA-WPCF, 1998) and total Fe using ICP-OES after the sample had been digested with HNO3 in a microwave oven

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Fig. 1. The study area localities. On the top, the catchment of Zadorra river, the limit of the Nitrate Vulnerable Zone (NVZ) and the position of Urgazi spring. On the bottom, Salbura wetland and the water sampling network. AME: Arkaute Meteorological Station.

(ETHOS 1, Millestone). The analysis of the acid volatile sulfide (AVS) in the samples of the Urgazi spring was conducted using the iodometric method, after adding 0.2 mL of zinc acetate 2 N per 100 mL of sample for conservation and storage in glass bottles. Temperature, pH, dissolved O2 (DO) and Eh were measured onsite with a YSI (6820) multiparameter probe, which was also used to carry out vertical profiles. Electrical conductivity (EC) was only measured in the samples collected at 6 and 10 m using a Hach HQ14d meter. The position of the piezometric level was recorded

hourly with data loggers (DiverÒ, Van Essen Instruments) and additionally, using a water level probe, when the samples were collected. Two pumping tests were performed with a Grundfos pump (P1). Before pumping, vertical profiles of some parameters were recorded in the piezometers and water samples were collected as explained above. In the first test, at the end of the drier period (15 October 2008), both piezometers (SM14 and P5) were pumped. Two days later (17 October) the vertical profile tests and sampling

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

857

Fig. 2. Stratigraphy of soil cores of piezometer sites. The dashed line to the right of the cores shows the screened interval of the piezometers, where the samples were taken.

were repeated. In the second test, at the end of the high water period (13 May 2009), only the SM14 piezometer was pumped. In this case, vertical profiles were recorded and water samples were taken again on the same day as soon as the piezometric level had reached the initial position. The aim of these tests was to observe the hydrochemical response to pumping during both hydrological conditions and, by comparing with previous waters, to identify the flows affected. Data has been analyzed with the hydrogeochemical program PHREEQC (Parkhurst and Appelo, 1999) to evaluate the chemical equilibrium of the water samples, by obtaining saturation indices and establishing their typology. SPSS software 17.0 was used for statistical analysis.

4. Results 4.1. Core statigraphy The stratigraphy of the cores from the SM1–SM11, SM14 and P5 piezometers is described in Fig. 2. The top layer of all cores consists of at least 1 m of dark clay. In all cores located in the riparian zone the clay is underlain by a silty layer of variable thickness (40– 130 cm) lying on the marl substrate. These layers have reducing patches of variable size, which implies at least temporary waterlogging of these strata, and also an abundance of calcareous nodules. The greatest differences are found in the southern cores (SM9–SM10) and in SM8, close to the Errekabarri stream, due to the presence of coarser textures (sands and gravels). The piezometer screen intervals have been represented as a dashed line in

Fig. 2. At SM14 only waters from the marls can be sampled; at P5, however, water from marls and Quaternary deposits can be sampled, and at the rest of piezometers only waters from Quaternary deposits. During drilling work, marl samples were collected for mineralogical analysis by X-ray diffraction (XRD). The analysis shows that 97% of its composition is comprised of calcite, phyllosilicates and quartz, accompanied by dolomite–ankerite. The S phases corresponds to pyrite and celestite. The pyrite has an alteration halo of gypsum. 4.2. Hydrogeology Fig. 1 shows the flow net near the Arkaute pond. Flow direction in the south area of the wetland (SM9, SM10) is northwards, towards the pond, but groundwater in the eastern riparian zone (SM1–SM8, SM14 and P5) is discharging mainly towards the Errekabarri stream. The lowest hydraulic head is systematically in the SM17 piezometer, where the head is below the water table in the pond. According to the flow net this stream acts as both an upstream source to the Quaternary deposits and a downstream sink from them. Nevertheless, this flow path does not directly affect the area of the piezometers (P5, SM14) under study. Groundwater levels were measured monthly, but measurements of a medium water period (20 May 2008) are used for constructuring piezometric contours. All other dates show a similar pattern to this. The general hydraulic gradient in the zone is low, less than 0.001 m/m (Antigüedad et al., 2009). The depth of the piezometric level varies between 0.30 m when the water table is high (in some places it reaches the surface in

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M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865 0

0

20 1

30

mm/day

40 1.5

16/01 02/12

10/04

50

22/05 02/04 29/04 13/05

2 03/07 29/07

2.5

Piezometric level

Manual measurement

Sampling day

14-07-09

16-06-09

19-05-09

21-04-09

24-03-09

24-02-09

27-01-09

30-12-08

02-12-08

04-11-08

12-08-08

15-07-08

17-06-08

20-05-08

22-04-08

25-03-08

26-02-08

29-01-08

01-01-08

3

07-10-08

15/10 17/10 18/09

09-09-08

Depth of piezometric level (m)

10 0.5

Rainfall

Fig. 3. The evolution of piezometric level in SM14. The depth of 0 m is the top of piezometer cap. The rainfall has been registered in the AME. The arrows indicate the sampling days.

periods of very high water levels) and 2.2 m in the drier season (Fig. 3). Groundwater levels show a clear seasonal fluctuation. These changes are smaller in the piezometers located near the pond (Antigüedad et al., 2009). The pumping tests performed at the piezometers have confirmed that the permeability of the Quaternary deposits is very low, generally K is below 1 m/d. Accordingly, this is an aquitard, although more permeable layers of sands at the bottom of the deposits are observed in some places. The marl substrate is located at a depth of between 1.8 and 2.5 m in the riparian zone. The information obtained during drilling reveals the existence of some fractured levels within the marls, which previously were considered as impervious. Nevertheless these fractures could allow upwelling of groundwater into the overlaying Quaternary deposits, increasing the complexity of flow patterns in the area. Water level measurements in piezometers which are close to one to another does not allow observation of a clear vertical hydraulic gradient. Therefore, an attempt is made to use hydrochemical data to identify the possibility of upwelling in this zone. 4.3. Water chemistry Table 1 shows physical and chemical data from piezometers P5 (at a depth of 6 m) and SM14 (at depths of 6 and 10 m) from April 2008 to May 2009 (Fig. 3). Data collected from the Urgazi spring in three samplings have been also included. It is noted that due to calibration problems of the YSI probe dissolved O2, temperature and Eh were not measured for some samples. 4.3.1. Physico-chemical parameters Vertical profiles of the physical variables measured in piezometers P5 and SM14 are shown in Fig. 4. Despite their proximity (about 3 m) these profiles vary from one piezometer to another. The pH and the Eh were constant along the P5 water column on every sampling date. In contrast, the profiles in SM14 show a pH increasing towards the bottom of the piezometer while Eh values become more negative. Regarding the changes over time, the Eh was found to have the greatest variability, especially at P5 (350 to 160 mV), while at SM14 it ranges from 320 to 20 mV, without taking into account the post-pumping tests (Table 1). The most reducing conditions were measured in both piezometers between the end of May and the middle of September 2008 at low water table (Fig. 3), while the most oxidizing conditions were measured between December 2008 and April 2009 when the water table was high.

Dissolved O2 (DO) concentration and temperature display similar patterns in both piezometers. The highest DO values (but normally below 2 mg/L) appear in the upper part of the water column (3–4 m). At deeper levels, the O2 concentration diminishes, falling to values below 0.05 mg/L at a depth of 10 m. The temperature shows a seasonal pattern with minimum values (below 10 °C) in winter and spring and maximum values (up to 14–15 °C) in summer and autumn. The temperature remains fairly constant (12 °C) at levels deeper than 8 m. The electrical conductivity (EC) measurements taken at a depth of 6 m in both piezometers are quite similar (Table 1). The average EC in the deepest sampling point (10 m in SM14) was 1940 lS/cm, compared to 1478 lS/cm (SM14) and 1248 lS/cm (P5) at a depth of 6 m. EC was more variable over time in P5, although there was no clear seasonality. In contrast, EC was fairly constant over time in SM14, especially at a depth of 10 m. Alkalinity increased with depth as did EC, the average being 580 mg/L in P5 and 605 mg/L in SM14 at 6 m; however, it reached 792 mg/L at 10 m in the last piezometer. Alkalinity did not follow a clear temporal pattern in either of the piezometers. Only a limited temporal variability was found in the hydrochemistry of the Urgazi spring, although it should be noted that only a small number of samples were taken. The average values of EC (2541 lS/cm) and alkalinity (1036 mg/L) were much higher than those observed in the piezometers located in the wetland. The pH values were around 8 and the redox potential was always negative (235 mV). Average temperature was 12.8 °C, but it varied over time. The significant linear correlation coefficients among physicochemical parameters show that the most significant positive correlation at 6 m in SM14 is between EC and pH, 0.62 (p < 0.01), while EC shows a negative correlation with DO (0.95, p < 0.01). At a depth of 10 m, alkalinity and DO are negatively correlated, 0.736 (p < 0.05). In P5 there is a good positive correlation between DO and Eh (0.75, p < 0.05) and a negative correlation between alkalinity and groundwater level depth (0.83, p < 0.01).

4.3.2. Ions and their relationships with other physico-chemical parameters All the monthly collected samples in all the points of the monitoring network are represented on the mixing diagrams shown in Fig. 5, from which a general view of the different water types that are present in the wetland area can be obtained.

Table 1 Physico-chemical parameters, depth of piezometer level (m), saturation index and type of water of SM14 (at 6 and 10 m), P5 and Urgazi spring. Electrical conductivity (EC, lS/cm), temperature (T, °C), dissolved oxygen (mg O2/L), redox potential (Eh, mV), pH, total dissolved solids (TDS), total alkalinity, HS-, H2S and all ions in mg/L, b.d.l, below detection limits (<0.1 mg/L). (1⁄) Pre-pumping sample and (2⁄) post-pumping sample. The statistical data (mean, standard deviation and standard error) do not include the post-pumping values. The empty spaces are missed or not measured data. SM14–6 m

Level

EC

T

O2

0.85

1427

22/05/2008

0.88

1530

11.1

0.16

03/07/2008

1.40

1281

11.4

29/07/2008

1.79

1517

18/09/2008

2.18

15/10/2008 (⁄1) 17/10/2008 (⁄2) 02/12/2008

pH

Ca2+

K+

Mg2+

Na+

Cl

NO 3

SO2 4

F

Dissol. Fe

Prec. Fe

Alkalinity

0.03

522

TDS

Saturation index

Type of water

Dolomite

Calcite

Siderite

662

0.48

0.32

1.39

512

666

1.09

0.46

1.19

7.4

81

6.1

15.8

156

76

b.d.l

194

0.02

197

7.7

59

3.4

24.0

105

72

b.d.l

200

4.01

0.37

194

7.5

62

6.0

34.0

270

76

b.d.l

210

2.72

4.06

0.59

641

740

0.95

0.33

1.04

11.5

0.09

302

7.6

22.1

281

76

b.d.l

210

3.41

5.47

0.95

702

769

1.23

0.64

1.34

1586

11.9

0.03

231

7.7

47

5.3

27.7

248

74

b.d.l

198

3.43

5.86

1.64

615

664

1.20

0.45

1.44

2.12

1527

12.1

192

7.5

62

6.5

18.3

247

58

b.d.l

161

2.35

6.19

4.21

606

610

0.68

0.33

1.23

2.09

1311

12.1

30

6.7

176

2.2

29.3

129

35

b.d.l

69

0.13

1.45

1.99

542

663

0.39

0.07

0.26

0.82

1389

12.2

165

7.2

113

5.5

30.4

81

74

b.d.l

147

1.08

9.01

595

621

0.65

0.34

1.14

16/01/2009 02/04/2009

0.88 1.26

1402 1503

11.6 10.8

0.30 0.13

38 178

7.3 7.3

76

5.7

29.6

196

82

b.d.l

227

1.73

4.52

3.71

588 619

721

0.68

0.27

0.94

29/04/2009

1.22

1542

10.8

0.03

194

7.7

69

4.6

25.5

217

106

b.d.l

268

3.05

7.77

2.60

617

732

1.27

0.61

1.49

13/05/2009 (⁄1) 13/05/2009 (⁄2) Mean STD SE

1.42

1551

10.7

0.06

202

7.8

71

5.8

25.9

248

84

b.d.l

244

1.63

8.50

2.19

634

752

1.63

0.76

1.73

1.50

1281

10.4

0.86

4.2

7.5

203

2.0

24.0

53

67

0.5

218

0.18

0.18

1.51

573

704

1.28

0.82

0.34

1478 91.3 27.5

11.4 0.5 0.2

0.15 0.13 0.04

189 65.2 20.6

7.5 0.2 0.1

71 18.6 6.2

5.4 0.9 0.3

25.3 5.6 1.8

205 69.3 21.9

78 12.3 3.9

206 35.7 11.3

2.4 0.9 0.3

5.5 2.6 0.8

2.0 1.5 0.5

605 53 16

694 56 18

0.99 0.36 0.11

0.45 0.17 0.05

1.01 0.88 0.28

SM14–10 m

Level

EC

T

O2

Eh

pH

Ca2+

K+

Mg2+

Na+

Cl

SO2 4

F

Dissol. Fe

Prec. Fe

Alkalinity

TDS

2.52

10/04/2008

0.85

1751

22/05/2008

0.88

1973

11.9

1.60

03/07/2008

1.40

1885

11.9

29/07/2008

1.79

2004

18/09/2008

2.18

15/10/2008 (⁄1) 17/10/2008 (⁄2) 02/12/2008 16/01/2009 02/04/2009 29/04/2009

NO 3

Saturation index

Na–Ca– HCO3–SO4 Na–HCO3– SO4 Na–HCO3– SO4 Na–Ca– HCO3–SO4 Na–HCO3– SO4 Na–Ca– HCO3–SO4 Ca–Na– HCO3–SO4 Na–Ca– HCO3–SO4 Na–Ca– HCO3–SO4 Na–Ca– HCO3–SO4 Na–Ca– HCO3–SO4 Ca–HCO3– SO4

Type of water

Dolomite

Calcite

Siderite

7.7

30.0

5.9

27.8

206

82.0

b.d.l

213

0.02

0.02

532

644

0.91

0.19

1.97

249

8.4

14.2

2.4

5.2

282

87.2

b.d.l

249

0.26

5.35

417

655

1.59

0.74

0.91

0.90

272

8.2

20.4

5.7

2.2

484

91.5

b.d.l

260

4.61

0.81

6.07

805

878

0.95

0.68

1.19

11.8

0.50

299

8.2

6.5

440

96.2

b.d.l

279

5.45

0.40

2.70

860

982

2.16

1.38

0.93

1955

11.8

0.10

248

8.3

7.6

5.2

9.2

448

95.8

b.d.l

279

5.38

0.16

3.96

747

861

1.30

0.34

0.57

2.12

2013

11.8

218

8.1

11.7

6.4

8.2

455

76.0

b.d.l

229

4.00

0.09

1.78

1054

805

1.37

0.49

0.28

2.09

1330

11.9

2.82

13

6.7

162

2.4

27.0

167

36.8

b.d.l

73

0.15

1.01

1.97

551

568

0.36

0.06

0.38

Na–HCO3– SO4 Na–HCO3– SO4 Na–HCO3– SO4 Na–HCO3– SO4 Na–HCO3– SO4 Na–HCO3– SO4 Ca–Na–HCO3

0.82 0.88 1.26

1845 2025 2005

11.9 12.0 11.9

188 248 229

7.8 7.9 7.6

21.5

6.7

12.7

237

81.1

b.d.l

173

1.76

0.47

1.02

1.02

0.35

0.59

Na–HCO3

8.3

6.5

8.9

434

108.7

b.d.l

261

4.07

0.50

1.86

820 866 919

677

0.70 0.60

844

0.08

0.25

0.44

1.22

2002

11.8

0.20

235

7.9

15.2

4.7

10.8

423

81.4

b.d.l

233

1.67

1.71

2.51

887

792

0.92

0.26

1.22

Na–HCO3– SO4 Na–HCO3– SO4 859

(continued on next page)

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

10/04/2008

Eh

860

Table 1 (continued) SM14–10 m

Level

EC

T

O2

Eh

pH

Ca2+

K+

Mg2+

Na+

Cl

NO 3

SO2 4

F

Dissol. Fe

Prec. Fe

Alkalinity

TDS

Saturation index

Type of water

Dolomite

Calcite

Siderite

13/05/2009 (⁄1) 13/05/2009 (⁄2) Mean STD SE

1.42

1881

11.8

0.40

244

8.1

26.6

5.9

16.0

388

96.8

b.d.l

260

2.45

3.92

0.89

803

830

1.61

0.65

1.72

1.50

1279

11.1

1.17

7.4

7.5

191

2.1

24.7

59

66.5

b.d.l

216

0.18

0.19

3.03

568

691

1.25

0.80

0.31

1940 87.9 26.5

11.9 0.1 0.0

0.63 0.47 0.17

243 29.7 9.4

8.0 0.3 0.1

17.3 7.9 2.6

5.5 1.3 0.4

10.7 7.14 2.26

380 99.8 31.6

90 9.9 3.1

244 32.5 10.3

3.7 1.5 0.5

0.83 1.2 0.4

2.61 2.0 0.6

792 178 53.5

797 109 34.4

1.2 0.6 0.2

0.5 0.4 0.1

0.6 1.0 0.3

P5

Level

EC

T

O2

Eh

pH

Ca2+

K+

Mg2+

Na+

Cl

SO2 4

F

Dissol. Fe

Prec. Fe

Alkalinity

TDS

0.04

0.01

646

0.90

1366

22/05/2008 03/07/2008

0.91 1.43

1408 1305

11.4 11.5

0.24 0.18

29/07/2008 18/09/2008

1.81 2.20

1150 1191

11.7 12.2

0.08 0.06

15/10/2008 (⁄1) 17/10/2008 (⁄2) 02/12/2008 16/01/2009 02/04/2009

2.14

1188

12.4

2.10

1185

12.5

0.84 0.91 1.28

1164 1072 1090

29/04/2009

1.22

13/05/2009 Mean STD SE

1.49

Urgazi

EC

13/12/1982 02/07/2009 16/12/2009 Mean STD SE

2552 2460 2610 2541 76 44

Saturation index

Type of water

Dolomite

Calcite

Siderite

623

0.68

0.56

1.32

7.1

235

18.6

23.4

24.4

18.0

b.d.l

150

179 201

7.0 6.7

213 219

10.3 9.6

26.1 36.5

13.2 50.0

16.2 44.3

b.d.l b.d.l

114 152

0.22

0.40 1.54

0.47 1.88

589 671

539 641

0.44 0.03

0.40 0.10

0.45 0.11

342 217

6.9 7.0

153

4.3

23.6 30.6

50.7 59.9

83.3 97.4

b.d.l b.d.l

121 104

0.35 0.64

2.33 1.85

2.29 0.45

549 460

658 571

0.01 0.12

0.18 0.14

0.14 0.11

17

6.8

161

4.3

19.8

53.7

76.2

b.d.l

79

0.24

1.81

0.70

469

505

0.38

0.00

0.07

2.63

61

6.7

158

3.2

29.3

55.4

40.3

b.d.l

47

0.13

0.34

0.50

469

456

0.30

0.05

0.84

11.9 11.6 11.0

7.0 7.0 7.1

251

19.6

20.5

7.6

18.0

0.7

110

0.24

0.00

0.12

0.42

0.48

2.02

182

8.4

22.4

49.0

27.5

b.d.l

136

0.29

0.56

0.47

630 612 597

584

0.12

158 51 16

550

0.45

0.40

0.23

1542

11.0

0.04

194

7.7

186

8.3

24.0

40.0

40.1

b.d.l

161

0.27

1.78

1.50

583

589

1.61

0.97

0.84

1248 152 48

11.0 11.6 0.5 0.2

0.03 0.99 2.5 0.9

82 101 152 48

7.5 7.0 0.3 0.1

200 35 12

10.4 5.8 2.0

25.2 5.3 1.8

38.7 19.0 6.3

46.8 31.2 10.4

125 26.6 8.9

0.32 0.15 0.06

1.15 0.9 0.3

0.88 0.8 0.3

580 70 22

584 50 17

0.4 0.6 0.2

0.4 0.3 0.1

0.3 0.8 0.3

T

Eh

pH

Ca2+

K+

Mg2+

Na+

Cl

NO 3

SO2 4

F

Alkalinity

Dissol. Fe

HS

H2S

TDS

Saturation index

225 245 235 14 10

8.0 8.8 8.0 8.3 0.5 0.3

5.0 5.0 5.1 5.0 0.0 0.0

3.6 5.3 4.6 4.5 0.9 0.5

5.5 6.9 5.7 6.0 0.8 0.4

475 536 594 535 59 34

280 320 309 303 20.6 11.9

18.0 0.04 0.15 6.1 10.3 6.0

14.0 13.9 14.0 0.1 0.0

1020 1020 1068 1036 27 16

13.9 11.7 12.8 1.6 1.1

1.5 2.5 2.0 0.7 0.5

b.l.d b.l.d

9.2

0.8

782 888 932 867 77 44

Ca–HCO3– SO4 Ca–HCO3 Ca–HCO3– SO4 Ca–HCO3 Ca–Na– HCO3–Cl Ca–HCO3 Ca–Na–Mg– HCO3 Ca–SO4 Ca–HCO3– SO4 Ca–HCO3– SO4

Type of water

Dolomite

Calcite

Magnesite

Fluorite

0.649 2.405 0.7 1.25 1.00 0.58

0.036 0.87 0.05 0.32 0.48 0.28

0.298 0.649 0.294 0.02 0.55 0.32

0.18 0.206 0.19 0.02 0.01

Na–HCO3–Cl Na–HCO3–Cl Na–HCO3–Cl

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

10/04/2008

NO 3

Na–HCO3– SO4 Ca–HCO3– SO4

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

861

(A)

(B)

Fig. 4. Vertical profiles of the physical parameters measured at piezometers P5(A) and SM14(B). Potential redox (Eh, mV), dissolved O2 (OD, mg/L), temperature (T, °C) and pH. Lithology column, constructive scheme and water sampling depth. The depth of 0 m is the top of piezometer cap.

 In Fig. 5A, five groups are shown as a function of SO2 4 and Cl concentrations (mmol/L). The Urgazi spring has not been included in this diagram, as the dissolved S is mostly in the form of HS (Table 1). The waters with the highest concentration of these two components came from the deep piezometers, SM14, at 6 and 10 m, and P5. However, in P5 two water types can be distinguished: one is found in the transition zone between the water in SM14 and that of the shallowest piezometers (Quaternary deposits), while the other has a lower Cl concentration and larger variability in SO2 4 . The two types appear at different times. Between November and May, when the piezometric level was less than 1–1.5 m deep, the Cl concentrations in P5 diminished significantly, while between June and October, with levels deeper than

1.5 m, the water chemical composition in P5 was similar to that observed in SM14 at a depth of 6 m. Therefore, the waters around the wetland can be classified into shallow waters of the riparian area associated with the Quaternary deposits (aquitard) and with low mineralization, and deep waters associated with the marl substrate (P5, SM14), with a high content  of Cl ; SO24 ; Naþ and F (Table 1). There are exceptions, such as waters in the shallow piezometer SM3, which were found to have Cl and SO2 4 concentrations similar to those in P5 during high water levels. The Urgazi spring was found to have high salt concentrations (Table 1), especially Na+ (535 mg/L) and Cl (303 mg/L), so it appears at the edge of the diagram in Fig. 5B. It is also important to highlight its high F concentration (14 mg/L), this being the fourth most impor-

862

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

(A)

(B)

Fig. 5. All sampling point analytical data are included in the water mixing diagram.

tant component after alkalinity (1036 mg/L), Na+ and Cl. As observed between Cl and SO2 4 , there is a gradient in the concentration of Na+ and Cl between the water with high mineralization, specifically the Urgazi spring, and the water in shallower piezometers around the wetland. Between these groups, the waters from SM14, P5 and SM3 lie in an intermediate position. Further, in P5 the concentration of Na+ was quite stable, without clear seasonal changes, while Cl was observed to have greater temporal variability. 2þ In P5, HCO > SO2 3 was the dominant ion, followed by Ca 4 >   þ 2þ þ 2þ  Cl > Na > Mg > K > Fe > prec. Fe > F > NO3 (Table 1), so this is a Ca–HCO3–(SO4) water type, except during the pumping test (Ca–Na–Mg–HCO3). There is a strong positive correlation be2þ 2 tween HCO 3 and Ca ; SO4 (>0.72, p < 0.01). During the low water season (GW level > 2 m deep), the dominant water type is Ca–Na– HCO3–Cl, where a good positive correlation between GW level depth and Cl, Na+, dissolved Fe (>0.87, p < 0.01) suggests that some ions are more abundant in the drier season. In SM14, HCO 3 is also the most abundant component; at 6 m the water is mostly of Na–Ca–HCO3–SO4 type and it becomes Na–HCO3–SO4 type 10 m depth. A positive correlation is found between HCO 3 and Na+ (>0.67, p < 0.05) and between Cl and SO2 (>0.69, p < 0.05) 4 at both depths. At 10 m, there is a higher content of F and precipitated Fe, but the concentrations of Ca2+, Mg2+ and Fe2+ are lower. 4.3.3. Water chemistry during the pumping tests There were differences in the chemical characteristics of the water before and after the pumping tests in piezometers P5 and SM14, especially in the latter. The water that filled the two

piezometers after the pumping tests of 13 May 2009 (SM14) and 15 October 2008 (SM14, P5) had remarkably similar physical and chemical characteristics. In P5, the type of water before and after the pumping test varies from Ca–HCO3 to Ca–Na–Mg–HCO3. The concentration of various   components (SO2 4 ; F , dissolved Fe and Cl ) decreased while the concentrations of Mg2+ and Na+ increased after the pumping test in October (Table 1). Nevertheless, despite these variations, the two sets of samples were fairly similar. In SM14, there is a change from Na–(Ca)–HCO3–SO4 to Ca–(Na)–HCO3–SO4 water type. The water filling the piezometer after the tests had a lower concentra  + tion of SO2 4 (especially in October), F , soluble Fe (at 6 m), Cl , Na , + K and alkalinity, and also lower pH, but higher concentrations of Ca2+ and Mg2+, and is similar to the water found in P5 (Table 1). In the Piper diagram (Fig. 6), the samples collected in both piezometers after the pumping tests occupy the same position as the samples in P5. A fairly good linear progression appeared in the Piper diagram from SM14 at 10 m depth, with the highest SO2 and 4 Cl concentrations, to P5 and the pumping test samples. Urgazi spring sample is also located together with the samples collected at 10 m, due to its high Cl content. The concentration of TDS diminishes clearly after the pumping tests, especially in SM14 at 10 m. The physical variables measured in both piezometers after the pumping tests have also changed. The DO concentration was higher and Eh became more oxidizing, particularly after the May pumping test (Fig. 4). On the contrary, the lowest pH values were measured after the pumping test in October.

863

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

Fig. 6. Piper diagram showing plot of P5, SM14 (at 6 and 10 m) and Urgazi spring analytical data. Pumping test data are included (October 2008 and May 2009). Total dissolved solids of each pumping test are represented in diagram, while Urgazi, P5 and SM14 mean values are represented outside.

5. Discussion Fig. 7 shows the hydrogeological conceptual model deduced from available information. Deep waters are characterized by a  þ  high salinity, being enriched in SO2 4 Na ; Cl and F . This enrichment is assigned to the dissolution of evaporitic levels (Iribar and Ábalos, 2011): halite for Na+ and Cl, fluorite and CaSO4 phases for F and SO2 4 . Fluoride is typically considered a chemical indicator of deep flows (Carrillo-Rivera et al., 1996). The Urgazi spring typically displays such characteristics with very abundant F- content (13 mg/L), elevated Na+, Cl and acid volatile sulfide (AVS), which can be related, as developed further on, to SO2 4 reduction. According to Iribar and Ábalos (2011), the evaporites are undoubtedly the source of the salinity of saline springs in the Basque Cantabrian basin. The fact that similar anomalous water is found in the deeper piezometers (SM14, P5) of the riparian area suggests the existence of a groundwater flow through the marls. In the study site, high concentrations of F- are only found in the waters of these piezometers (usually 2–5 mg/L), where there is always a concomitant increase of Na+, Cl and SO2 4 . However, although the waters found in the deeper piezometers (SM14 and P5) display deep water characteristics similar to the Urgazi spring, they display a lower salinity. Taking into account that there are almost 5 km between Urgazi and the wetland, it is most probable that water mixing occurs along this distance. This is in agreement with the fact that several ion concentrations, such as Cl, Na+ and F, are lower in deep piezometers than in Urgazi, due to a dilution process. However, the most important chemical difference between deep piezometers and the Urgazi spring is the SO2 4 content. The concentration of this ion is very high in deep water, while S is found as H2S and HS- in the Urgazi spring. In the case of this spring, bacterial SO2 4 reduction is feasible according to the following reaction (Appelo and Postma, 2005):  SO2 4 þ 2CH2 O ! H2 S þ 2HCO3

HCO 3

ð1Þ

The produced by bacterial reduction can also explain the higher alkalinity (1036 mg/L) and pH in Urgazi spring. Nevertheless,

2+ part of this HCO 3 precipitates as dolomite and calcite, so that Ca 2+ and Mg concentrations diminish. Sulfate reduction would simultaneously favor the formation of fluorite. The fractures in the marl substrate allow deeper water to rise toward the riparian area. At the same time, shallow water, associated with the Quaternary deposits, can flow into the aquifer below. This water has notably different physical and chemical characteristics to the shallow waters, which are characterized by higher concentrations of Ca2þ ; Mg2þ ; HCO 3 ; DO and oxidizing Eh, and lower of  þ Cl ; F ; Fe;SO2 4 ; Na and EC. The mixture of waters of different chemical composition and sources causes different chemical reactions during wet and drier seasons. Accordingly, a temporal variability has been observed in the piezometers of the riparian zone. During wet the season (December–April), when the piezometric levels are close to the surface, the main water inputs into the piezometers, especially in P5, are shallow waters from Quaternary deposits. Nevertheless, the very low hydraulic conductivity of alluvial deposits and the fact that they are not very thick make deep and shallow water interaction appreciable not only in the deepest piezometers but also in some shallow ones, as is the case in SM3  þ (Fig. 5), with a significantly higher SO2 concentra4 ; Cl and Na tion than in the other shallow piezometers. This suggests the upwelling flow through the marls is not generalized over the riparian zone but restricted to some places which act as hydraulic ‘‘windows’’ (Mallén et al., 2005; Stotler et al., 2011) of the fractured bedrock. The SO2 4 shows peculiar behavior in P5, and SM3. High concentrations are observed throughout the year but are highest in the wet season (Fig. 5A) when a decrease of Cl indicates an effect of dilution. Sulfate could stem from the oxidation of the acid volatile sulfides (AVS), which are transported by the deeper flow. Given the positive redox potential (16–158 mV, Table 1) and pH conditions (7.0–7.1) in P5 during the wet season (from December to April), all the S that is present in this piezometer should be in the form of SO2 4 , so the dominant reaction would be (Fig. 7):

þ  1=8H2 S þ 1=2H2 O () 1=8SO2 4 þ 5=4H þ e

pe0 ¼ 3:5

ð2Þ

864

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

Fig. 7. Conceptual hydrogeological and hydrochemical model of groundwater flow (through fractured marls of the substratum and alluvial Quaternary aquifer). The main chemical reactions are represented. Not to vertical scale.

The strongly reducing conditions (<200 mV) and the pH values observed during the drier season in P5 indicate that formation of AVS predominates over its oxidation (Eq. (2)). It has been suggested that in several wetlands and riparian areas that display reducing conditions for long time periods, and with a significant presence of clays and organic C, the dissimilatory reduction of SO2 4 by bacteria plays a major role (Mayer et al., 2010). As a consequence, the SO2 4 concentration diminishes, the formation of H2S increases and it can even precipitate forming secondary minerals. On the other hand, the analysis of rock cores from the bedrock using X-ray diffraction has corroborated the presence of pyrite, gypsum and celestite in the marl substrate. Pyrite oxidation seems to be the main source of Fe in these piezometers (Eq. (3), Fig. 7). Each molecule of FeS2 that is oxidized produces two protons. However, the decrease of pH is buffered in calcareous systems by the dissolution of CaCO3. Evidently the intensity of decalcification varies as a function of the available S (Van den Berg and Loch, 2000). The pH measured in both piezometers at a depth of 6 m was close to neutral, while in SM14 at a depth of 10 m the pH takes on a more basic character (Fig. 4, Table 1), indicating that proton formation is buffered. þ FeS2 þ 7=2O2 þ H2 O () Fe2þ þ 2SO2 4 þ 2H

ð3Þ

The pH/Eh values in Table 1 indicate that nearly all the Fe is in the form of Fe (II). Given that the oxidation from Fe (II) to Fe (III) is a slow reaction, it is expected that the concentrations of Fe (III) and the precipitation of Fe (III) phases are low (Kohfahl et al., 2008). Postma et al. (1991) observed that incomplete pyrite oxidation results in the simultaneous appearance of Fe2+ and SO2 4 in groundwater. Incomplete oxidation is particularly to be expected in flow systems where at least part of the Fe2+, produced during the pyrite oxidation, may transported away from the redoxcline before it is oxidized. This is in agreement with the presence of Fe2+ and the high concentration of SO2 4 in SM14. The pH values observed in this piezometer at a depth of 6 m, around 7.5, favor the predominance of dissolved Fe, while the higher values (up to 8.4) and the lower Eh at a depth of 10 m favor Fe precipitation as siderite. On the

other hand, the high alkalinity and pH values allow the precipitation of calcite and dolomite at a depth of 10 m. This fact explains the low Ca2+ and Mg2+ content at this depth. In the case of SM14, it does not receive direct water inflows from Quaternary deposits and does not show a marked response during rainy periods. Despite this, thanks to the pumping tests it has been observed that both formations, marls and deposits, are hydraulically connected. It involves a greater hydrochemical similarity between P5 and SM14 at a depth of 6 m, whilst the lesser variability in SM14 at 10 m reflects a more continuous inflow from marls. The Piper diagram (Fig. 6) provides a good representation of this behavior. The use of the PHREEQC program made it possible to establish the thermodynamic state of certain minerals by calculating their saturation indices (SI, Table 1). Therefore, in the deepest waters relatively weakly influenced by the shallow waters, the formation of dolomite, calcite and siderite is always taking place during any hydrological situation. However, this is not the case in P5, which shows a clear influence of the less mineralized waters from the Quaternary deposits, where the saturation indices change as a function of recharging with rainwater, leading to under-saturation, preventing siderite precipitation. In Urgazi spring, the F- concentration is high enough to reach fluorite saturation (Table 1). In fact, fluorite is very soluble in waters with high NaCl concentrations (Tropper and Manning, 2007), as is the case in this spring. 6. Conclusions Hydrologic conceptual models of groundwater interaction in an alluvial deposits-fractured bedrock geological setting often assume that the alluvial deposits are hydraulically more active than the fractured bedrock, and consequently the contribution of groundwater from below the bedrock interface is often ignored. Banks et al. (2009) suggested that models which treat this interface as a no-flow boundary may be overly simplistic and emphasized the need to understand the importance of the bedrock as a basis for

M. Martínez-Santos et al. / Applied Geochemistry 27 (2012) 854–865

reliable conceptual models. Accordingly, the purpose of this investigation was to determine the relative importance of both the alluvial deposits (aquitard) and fractured bedrock water systems and their interaction in a small riparian zone near the protected wetland of Salburua in the Basque Country. Mainly hydrochemical data of some of the piezometers to develop a conceptual model (Fig. 7) describing the significance of the bedrock. Hydrochemical data, for different hydrological situations, including pre- and post-pumping water chemistry, have allowed two different sources of water, shallow and deep water to be identified. Waters in the upper deposits (aquitard) are typically Ca– HCO3–(SO4) type as a result of oxidizing conditions during the wet season. In this period, a less-mineralized water, influenced by the relatively rapid arrival of rainwater and by the high piezometric level, prevailed in these alluvial deposits. Consequently, the dilution processes and the oxidation of the AVS and pyrite are the main cause of low concentration of Cl and high of SO2 4 and Fe. In the drier season, the influence of deep bedrock waters increases. It determines the highest Na+, F and Cl concentrations as well as the lack of dissolved O2, which favors the dissimilatory reduction of SO2 4 . Waters from the fractured bedrock, hydrochemically stabler over time, are mostly of the Na–HCO3–SO4 type. In the bottom of the deepest piezometer (SM14), the high SO2 4 concentration seems not to be affected by significant reduction processes. The fact that a similar anomalous water is found in the Urgazi spring, where all the S is in the form of HS- and H2S, reflects the existence of groundwater flow through the fractured marls. This upwelling flow through the fractured marls is not extended to the entire riparian zone, but it is limited to some small areas where even the shallow piezometers, as is the case of SM3, show water chemistry influenced by deep flow (Fig. 5). These areas act as hydraulic ‘‘windows’’ (Mallén et al., 2005; Stotler et al., 2011) in the fractured bedrock allowing different water-bearing systems to be cross-connected. Therefore, the hydrochemical investigation is important to help to locate the areas of superficial deposits influenced by these hydraulic windows. This fact can then be considered in hydraulic flow and transport models. Although in size the study area is local in scale the presence of hydraulic windows could be considered of more regional importance affecting other discharge areas along the entire Quaternary aquifer of Vitoria-Gasteiz. The presence of windows has to be taken into account when trying to explain the spatial hydrochemical variability observed along the Quaternary aquifer far away from the wetland area. The understanding of the flow characteristics using the hydrochemistry will help the managers to take more effective measures in this aquifer, which has been declared to be vulnerable with regard to NO 3 pollution. Acknowledgments The authors wish to thank the Basque Government (Group IT 516-10), the University of the Basque Country and the Spanish Ministry of Science and Technology (Project CGL2006-06485) for supporting this research. We thank Dr. Michel Loubet and an anonymous reviewer for their constructive comments and Dr. Mike Edmunds for corrections. References Abid, K., Zouari, K., Dulinski, M., Chkir, N., Abidi, B., 2011. Hydrologic and geologic factors controlling groundwater geochemistry in the Turonian aquifer (southern Tunisia). Hydrogeol. J. 19, 415–427.

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