Using geochemical and isotopic tracers to characterize groundwater dynamics and salinity sources in the Wadi Guenniche coastal plain in northern Tunisia

Journal of Arid Environments 178 (2020) 104150

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Using geochemical and isotopic tracers to characterize groundwater dynamics and salinity sources in the Wadi Guenniche coastal plain in northern Tunisia

T

Safouan Ben Ammara,d,∗, Jean-Denis Taupinb, Mohsen Ben Alayac, Kamel Zouarid, Nicolas Patrisb, Mohamed Khouatmiae a

ISTEUB, La Charguia II, Univ. Carthage, Tunis, Tunisia Hydrosciences Montpellier, UMR 5569, IRD, CNRS, Univ. Montpellier, Montpellier, France LMU, INRAP Pôle Technologique Sidi Thabet, Tunisia d LRAE, ENI Sfax, Univ. Sfax route de Soukra, Sfax, Tunisia e CNSTN, Sidi Thabet, Tunis, Tunisia b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Geochemical tracers Isotopic tracers Salinity sources Coastal unconfined aquifer Tunisia

Northeastern Tunisia is a sub-humid region with annual rainfall ranging from 600 to 650 mm. Unconfined groundwater in this area is easily accessible and represents an important perennial water source for agricultural activities. The present study deals with a multi-parameter investigation of one of the most important basins in northeastern Tunisia: the Wadi Guenniche plain (130 km2). Hydrochemical (major elements and Br−) and isotopic (18O, 2H and 3H) investigations were carried out on 32 shallow wells to gain insights into groundwater recharge and salinity. The dominant water types were Ca–Mg/Cl–SO4 and Na/Cl with electric conductivity values varying between 0.86 and 6.6 mS/cm. Based on ion ratios and saturation indexes, groundwater salinity originated from weathering of carbonate and evaporite minerals and cation-exchange with clays. Additional influence of anthropogenic activities shown by high NO3− concentrations exceeding 100 mg/L was identified in different parts of the plain resulting from irrigation return flow. A six-year record of 18O and 2H signatures of daily rainwater (n = 293) enabled to define a Local Meteoric Water Line for the Bizerte area (δ2H = 7.02 δ18O + 8.27). The isotopic signature of groundwater was very similar to that of the mean annual rainfall in Bizerte suggesting direct recharge of the aquifer. Tritium levels in sampled wells were relatively high and indicated recent recharge and short residence time. Annual recharge of 100 mm was estimated using a mixing model based on 3H contents in precipitation and groundwater.

1. Introduction In many semi-arid and sub-humid regions, groundwater resources are scarce and subject to qualitative and quantitative deterioration because of high demand, anthropogenic activities and climate change (e.g. Leduc et al., 2007; Ben Ammar et al., 2016; Boufekane and Saighi, 2018; Voltz et al., 2018). In northeastern Tunisia, coastal groundwater resources are key to social and economic development and play a crucial role in the agricultural sector. More than 80% of groundwater resources are used in agricultural activities. Unlike in southern and central Tunisia, where large homogeneous basins and extended aquifer formations are developed, the northern hydrological basins are

comparatively smaller and hold limited groundwater resources because of a complex geological and structural context. Thus, northern coastal aquifers are characterized by their limited resources, which highly depend on recharge conditions, i.e. on the amount of annual rainfall, on the nature and extent of aquifer outcropping, and on connections with the natural hydrographic network. Moreover, intensive groundwater use in northeastern coastal plains leads to critical drawdown in piezometric levels, hence accentuating the marine intrusion risk. Unconfined groundwater reservoirs often represent vital freshwater resources and because of the low depth of water table and easy accessibility, they are usually over exploited to meet increased demand for irrigation, especially in the dry periods characterized by rising needs

∗

Corresponding author. Institut Supérieur des Technologies de l'Environnement de l'Urbanisme et du Bâtiment (ISTEUB), 2 Rue de l'Artisanat La Charguia 2, 2035, Tunis, Tunisia. E-mail addresses: [email protected] (S. Ben Ammar), [email protected] (J.-D. Taupin), [email protected] (M. Ben Alaya), [email protected] (K. Zouari), [email protected] (N. Patris), [email protected] (M. Khouatmia). https://doi.org/10.1016/j.jaridenv.2020.104150 Received 29 April 2018; Received in revised form 13 September 2019; Accepted 27 February 2020 0140-1963/ © 2020 Elsevier Ltd. All rights reserved.

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Fig. 1. Location map of Tunisia (a), geographic situation of the study area in Bizerte district (b), geological map of the study area (c) (after Burrolet and Dumon, 1952; Melki et al., 2002, 2011), simplified hydrogeological cross-sections along line A (d), simplified hydrogeological cross-sections along line B (e). Explanation for the legend: 1- Recent Quaternary (alluvial deposits); 2- Holocene (old dune); 3- Villafranchian (clay); 4- Late Pliocene (sand and sandstone); 5- Early Pliocene (clay); 6- Miocene (clay and gypsum); 7- Eocene (limestone); 8- Cretaceous (limestone); 9- Triassic (gypsum, sandstone and dolomite); 10- Anticline axis; 11- Fault; 12Borehole; 13- Cross-section; 14- Location of Bizerte meteorological station.

groundwater recharge using only conventional hydrogeological methods due to aquifers heterogeneity and anisotropy, uncertainty related to hydraulic parameters such as hydraulic conductivity, and human effects such as long-term exploitation and diffuse pollution (Li

and low recharge rates. In recent decades, understanding the hydrodynamic processes taking place in the aquifers has become a crucial issue to safeguard the sustainable use of groundwater resources. However, it is difficult to accurately identify sources and dynamics of 2

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Fig. 2. Seasonal fluctuations (October/May) of the static levels (meters below ground surface) in Pz209 and Pz226; geographical situation of the two piezometers is shown in Fig. 3.

hydrochemical data and to the complexity of the aquifer system. This study completes previous investigations of the Guenniche basin. The main objectives are to understand the evolution of the unconfined groundwater in relation to recharge mechanisms and salinization processes. To achieve this goal, both geochemical and isotopic tracers were used for the first time in the study area. A six-year long survey of the isotopic composition of rainwater at the Bizerte meteorological station (37° 16′ 33.20″ N and 09° 52′ 09.20″ E), situated 6 km to the NW of Guenniche basin, was undertaken as a basis for investigating the origin and subsurface history of unconfined groundwater in the area. The results of the study will be helpful for the effective management of groundwater resources, committed to preventing deterioration of groundwater quality due to strong anthropization.

et al., 2008; Zuber et al., 2011; Villeneuve et al., 2015; Wang et al., 2017). The hydrogeochemical facies concept, the application of equilibrium theory, the investigation of redox processes, and groundwater dating, can be used to interpret the type and origin of groundwater recharge, to decipher reactive processes, and to calibrate hydrological models (Glynn and Plummer, 2005; Yin et al., 2011; Clark, 2015; Tziritis et al., 2017). To identify the origin of groundwater salinity and to discriminate in particular sources of anthropogenic contaminants in groundwater, different ion ratios can be used. For example, the Cl/Br ratio, has been widely used in many hydrogeological studies to trace these processes (e.g. Cartwright et al., 2006; Alcalá and Custodio, 2008; Katz et al., 2011). The Cl/Br molar ratio of seawater is about 655. In most temperate coastal areas, the shallow groundwater ratio will range between 500 and 710. In coastal areas with arid climate, the Cl/Br ratio will increase and will range between 700 and 1300. Groundwater contaminated with waste water or fertilizers is characterized by Cl/Br ratios varying between 200 and 500. In groundwater affected by the dissolution of halite, the ratio will be greater than 1000 (Davis et al., 1998; Alcalá and Custodio, 2008; Katz et al., 2011). Stable isotopes and radioisotopes in water are valuable tracers of the hydrological cycle. In groundwater studies, stable isotopes (18O and 2H) may be used to depict recharge conditions, to determine groundwater provenance and to evaluate groundwater mixing processes. The weighted average of isotopic composition in long-term rainfall time series is usually used as the input signature to hydrological systems (Yurtsever and Gat, 1981). With a half-life of 12.32 years (Lucas and Unterweger, 2000), tritium (3H) provide insight into recent groundwater recharge processes. In a hydrogeological survey, the comparison of tritium values measured in groundwater with those found in precipitation provides a semi-quantitative estimate of groundwater age (Cartwright and Morgenstern, 2012; Clark, 2015). The unconfined Wadi Guenniche aquifer in northern Tunisia was selected as an example of a coastal aquifer in semi-arid conditions subjected to important exploitation. Few investigations have been conducted in this area in the past, focusing in particular on the understanding of the hydrogeological framework and hydrological conditions of the aquifer system. Ennabli (1980) estimated the groundwater resource volume and developed a groundwater-flow model. Hydrogeochemistry of the groundwater and its suitability for domestic and agricultural uses was recently discussed by Hammami et al. (2016) and Ben Ammar et al. (2018). However, groundwater management in this area often faces with the difficult task of assessing the origin, evolution and renewability of groundwater prone to excessive exploitation. The reasons are relate to insufficient hydrological and

2. The study area 2.1. Location and climate The plain of Wadi Guenniche covers about 130 km2. It is located in northeastern Tunisia, 6 km to the south-east of the city of Bizerte. It corresponds to a subsident coastal area characterized by low altitudes ranging between 0 and 50 m above sea level (masl). The plain is bordered by the lagoon of Bizerte in the west and surrounded by hills, locally called Jebel, with altitudes ranging between 150 and 300 masl: Er Rmel, Ain Saada and Sidi Ali Chebab in the north, Reyane in the east and Nacherine and Kechabta in the south (Fig. 1c). In Bizerte district, the climate is considered to be sub-humid and mild with an average annual rainfall of about 630 ± 153 mm/yr recorded at the Bizerte meteorological station (37° 16′ 33.20″ N and 09° 52′ 09.20″ E) (NIM, 2018). Approximately 95% of the total annual rainfall occurs during the wet season between September and April. The highest monthly rainfall occurs during December with an average of 100 mm, whereas the lowest monthly rainfall occurs in July with an average of only 2 mm. Potential evapotranspiration considerably exceeds the annual precipitation and reaches approximately 1350 mm/yr. Mean monthly temperatures range from 11.4 °C in February to 27.6 °C in August, the mean annual temperature being 19.0 °C. 2.2. Groundwater and land uses The Guenniche basin was one of the first aquifer systems in northeastern Tunisia exploited for agricultural activities, and represents the most important one in the Bizerte district. Use of groundwater from the unconfined aquifer expanded rapidly in the last two decades to support 3

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Static levels of the unconfined aquifer vary seasonally and locally as a result of the aquifer storage capacity and the dynamic interaction between the groundwater inputs (recharge) and outputs (exploitation). During the dry season, water levels decline as a consequence of intensive pumping to satisfy agricultural needs and lack of natural recharge (Fig. 2). Water levels rise during the rainy season following efficient recharge events and as a response to limited groundwater abstraction. Long-term records from 1972 to 2013 obtained from biannual monitoring (at the end of dry and wet seasons) of water levels in two piezometers situated in the north part of the study area showed seasonal fluctuations with an average amplitude of about 1–2 m, without marking a continuous or a general decline in groundwater levels (Fig. 2). To cope with seasonal and local decrease in piezometric levels especially around El Alia, artificial recharge using water from nearby dams has been tested. All selected sites used to test recharging the unconfined aquifer were small drilled basins situated in the north part of the plain: 2 sites in El Alia and one in El Kheraib. The results of the artificial recharge did not live up to expectations for a number of reasons: 1) low impact of the recharge tests compared to natural recharge. Only local and temporary increase of static levels attributed to natural recharge was observed, 2) at the recharge sites, low infiltration rates were observed. That’s why few wells were drilled to inject water directly to the aquifer, 3) low injected volumes compared to exploitation volumes and to natural recharge of the aquifer, and 4) to test artificial recharge, excess water in dams was needed, this condition correspond to a rainy year; therefore it was difficult to differentiate between the increase in piezometric levels caused by the artificial recharge from that linked to natural recharge.

the increasing irrigated production of vegetables. In the 1980s, groundwater abstraction was about 5.5 million m3/yr from 709 wells (Ennabli, 1980). According to recent estimates from the local authorities, a total of about 1550 hand-dug wells were drilled in the plain, with a density of approximately 12 wells/km2. As consequence, groundwater abstraction from the shallow aquifer increased twofold and reached 11 million m3/yr used exclusively for irrigation. Agricultural lands in the Guenniche plain are formed by poorly evolved soils and calcareous brown soils. The plain is limited to the north by a dense forest spreading over 20 km2 that is mainly composed of pines, eucalyptus and mimosas. Almost 80% of the plain is mainly used for annual crops, notably cereals and vegetables that consume large amount of water, especially during the dry season. This type of agricultural activity accounts with orchards for 65% of agricultural products in the Bizerte district. The remaining 20% of the plain is devoted to olive groves and vineyards that are cultivated in the northern part of the plain between El Alia and Menzel Jemil. 2.3. Geological setting The plain of Wadi Guenniche is part of the large subsident region known as collapse molasse basin situated between Bizerte and Ichkeul lakes. It represents a synclinal structure (Fig. 1d and e) related to the post-Villafranchian compressive phase affecting the north-eastern part of Tunisia (Miskovsky, 1983; Melki et al., 2011). The oldest outcrops in the study area are situated in Jebel Sidi Ali Chebab and consist of Triassic evaporite deposits formed by gypsum with rare marl levels, sandstone and dolomite overlain by Cretaceous and Eocene limestones and marls. The geological section continues with a thick clay sequence attributed to Miocene. The Pliocene marine sequence is divided into two formations: argillaceous Early Pliocene at the base locally known as Raf-Raf formation, with a thickness of about 60 m, and a sandstone formation attributed to Late Pliocene at the top, known as Porto Farina formation. The Pliocene formations are separated from Quaternary deposits by a Villafranchian sequence composed of clay and silt. The thickness of the Villafranchian varies between 50 and 100 m with a maximum of about 200 m observed in Jebel Kechabta. Aeolian and marine sand dunes attributed to Holocene outcrop in the northern part in Jebel Er Rmel (Fig. 1c). Recent alluvial Quaternary sediments composed of unconsolidated sands, gravels and silts with clay intercalations outcrop in large areas in the plain.

3. Methods The sampling campaign was conducted in March and April 2013. It involved 32 samples taken from irrigation wells. Their locations are shown in Fig. 3. Total well depths ranged from 4 m to 36 m with an average of 15 m. All sampled wells were pumped before sampling until constant pH, temperature and electrical conductivity (EC) were recorded. Field parameters were measured in situ using a WTW multiparameter instrument. Chemical analyzes of the major elements and bromide (Br−) were conducted in the laboratory of the CNSTN (National Center for Nuclear Science and Technology). An ion chromatograph (Dionex DX120) equipped with an AS14 column and a SRS300 suppressor was used for chloride, sulfate, nitrate and bromide analyzes with accuracy better than 3%. Bicarbonate contents were measured by titration with 0.1 N–HCl acid. Calcium and magnesium concentrations were determined by atomic absorption spectroscopy using an AAS Vario 6 instrument, with a precision of 3–5%. Sodium and potassium concentrations were measured by flame photometry. All reported values have an ionic balance (IB) within ± 5%, except for samples number 17, 28 and 32 with 5.2 ≤ IB ≤ 5.6%. Oxygen (18O/16O) and Hydrogen (2H/1H) ratios of groundwater were measured using a laser spectroscopy LGR LT-100 (Los Gatos Research) Liquid-Water isotope analyzer in line with analytical scheme recommended by the IAEA (Penna et al., 2010) at the Laboratory of Radio-Analysis and Environment (LRAE) of the National School of Engineers of Sfax (ENIS). The results were expressed as δ (‰) relative to the international standard V-SMOW (Vienna Standard Mean Ocean Water) as defined by Gonfiantini (1978). Analytical errors were ± 0.2‰ and ± 1.0‰ for δ18O and δ2H, respectively. Stable isotopes in precipitation were carried out on an Isoprime–GV Instruments mass spectrometer at the HydroSciences LAMA laboratory (Montpellier, France). Hydrogen isotopic composition was measured in continuous flow mode by water reduction over metallic chromium, while δ18O was analyzed by water-CO2 equilibration in dual inlet mode; both results are expressed in δ ‰ V-SMOW with an overall precision of ± 0.05‰ for δ18O and ± 0.8‰ for δ2H. Tritium contents in water were measured on

2.4. Hydrogeological functioning Unconfined groundwater is lodged in heterogeneous Quaternary detrital formations with a thickness varying between 50 and 100 m, while the deepest Pliocene (sand and sandstone) confined aquifer is situated between 75 and 300 m deep. The two aquifers are separated by the clayey and silty sequence (aquitard) attributed to Villafranchian (Fig. 1d and e). Static levels of the unconfined aquifer are generally between 5 and 10 m from the ground surface. The shallowest levels are observed in the central part of the plain (Hariza), where measured static levels range between 1 and 0.5 m. As shown in Fig. 3, piezometric levels of the unconfined aquifer vary from 45 to 5 masl with the highest values near the foothills of the surrounding Jebels, and the lowest ones around the Bizerte lagoon. Recharge to the unconfined aquifer results from rainfall infiltration over the plain and lateral flow from Wadi Guenniche and its tributaries during floods. Groundwater flow is topographically controlled from foot hill zones in the east towards the Bizerte lagoon in the west, which is the natural discharge area of the aquifer. The few pumping tests made in the study area showed that the transmissivity and hydraulic gradient vary from piedmont of the surrounding Jebels to the downstream parts of the plain. Transmissivity values ranged from 780 to 170 m2/day, and hydraulic gradient ranged from 0.015 to 0.003, from the piedmont to the plain, respectively (Ennabli, 1980). 4

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Fig. 3. Piezometric contour map of the unconfined aquifer, location of sampled wells in the study area and distribution of electric conductivity (mS/cm).

basic (pH = 8.4) with an average of 7.4, and from 16.6 to 21.0 °C (average of 18.9 °C), respectively. EC values measured in-situ varied from 0.86 to 6.6 mS/cm (Table 1). These values were similar to those measured by Hammami et al. (2016) in 2007 ranging between 0.6 and 5.5 mS/cm. The lowest EC values (≤2.5 mS/cm) were observed in the southern and eastern parts of the plain. The highest EC values (≥3.5 mS/cm) were observed in the lower central part of the plain (Hariza) and in the downstream part of the basin near the Bizerte lagoon where aquifer lithology is richer in clay and contains evaporites. Hydrogeochemical facies were consistent with the results presented by Ennabli (1980): all samples were dominantly Ca–Mg/Cl–SO4 and Na/Cl types. Cl− was the dominant anion ranging between 118 and 1883 mg/L, followed by SO42− and HCO3− with ranges of 118–1777 mg/L and 205–400 mg/L, respectively. As for cation concentrations, Na+ ranged from 71 to 987 mg/L, followed by Ca2+ and Mg2+ with respective concentrations varying from 65 to 509 mg/L, and from 31 to 293 mg/L, respectively. K+ was less than 41 mg/L. As a minor component of natural water, Br− varied from 0.02 to 7.32 mg/L. All groundwater samples were saturated with respect to calcite, aragonite and dolomite (−0.23 ≤ SIcalcite ≤ 0.99, −0.38 ≤ SIaragonite ≤ 0.84 and −0.13 ≤ SIdolomite ≤ 1.89), and undersaturated in lower mineralization to saturated in higher mineralization with respect to gypsum and anhydride with respective SI varying from −1.21 to 0.21 and from −1.46 to 0.03 (Table 1). These SI values indicated gypsum and anhydride dissolution as a probable origin of a part of Ca2+ and SO42− concentrations in groundwater. Chloride and sodium were the dominant chemical elements in groundwater and they were closely positively correlated with a linear relationship of Na+ = 0.86 Cl- and R2 = 0.91 (Fig. 4). The Na/Cl ratio of 0.86 is typical of the seawater value (Vengosh et al., 1999; Ben

ten samples by liquid scintillation counting following electrolytic enrichment at the LRAE. The results are expressed in tritium units (TU) with an accuracy of ± 0.3 TU. As shallow groundwater in the Guenniche plain is only used for irrigation, the Wilcox classification diagram (Evangelou, 1998) was used to assess the suitability of waters pumped in the investigated area for irrigation. This diagram is based on the EC (μS/cm) and on the sodium adsorption ratio (SAR) defined as:

S. A. R. =

Na+ 1 (Ca2 + 2

+ Mg 2 +)

The saturation indexes (SI) of a number of selected minerals were calculated with the PHREEQC program (Parkhurst and Appelo, 1999), and used to evaluate the equilibrium degree between water and these minerals. The SI were determined as: SImineral = log10 (IAPmineral/Kmineral), where IAP is the ion activity product of the dissociated chemical species in solution and K is the equilibrium constant. Equilibrium is indicated by SI = 0. IAP < Kmineral (SI < 0) corresponds to the undersaturation of the solution with respect to the mineral, and IAP > Kmineral (SI > 0) indicates the solution as being supersaturated with the mineral. 4. Results The chemical and isotopic compositions of samples collected in the Guenniche plain are displayed in Table 1. Values of pH and temperature for the shallow groundwater ranged from neutral (pH = 7.0) to slightly 5

6

10 m 8m 7m 13 m 15 m 7m 14 m 17 m 4m – 26 m 7m 6m – – 8m 12 m 14 m 36 m 13 m 32 m 22 m 12 m – 36 m – 10 m 4m 22 m 7m 6m 4m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

16.6 19.8 19.7 19.2 19 19.1 18.9 19.1 19.4 19.1 21 16.8 17.2 16.7 18.6 18.6 19.2 19.8 18.2 16.6 20.9 19.5 19.6 18.6 18.5 18.7 18 18.4 19.5 20.3 20.5 18.1

T° (°C)

7.3 7.5 7.3 7.5 7.3 7.9 7.2 7.4 7.4 7.7 7.3 7.3 7.5 7.6 7.4 7.3 7.1 7.1 7.7 8.4 7.3 7.1 7.4 7.5 7.5 7.6 7.7 7.2 7.4 7.3 7.7 7.0

pH

mags: meters above ground surface.

Well Depth (mags)

Sample ID

1.6 1.58 2.44 2.18 3.06 4.97 4.37 3.2 2.64 2.20 2.29 2.8 2.68 2.2 6.10 5.53 3.23 2.68 3.68 0.86 2.65 2.23 3.37 3.08 3.91 1.83 3.94 6.6 2.99 2.21 1.33 6.60

EC (mS/ cm)

172.80 125.60 178.80 164.40 214.00 170.80 158.80 205.20 175.60 126.40 109.60 152.00 93.60 138.40 337.60 247.60 141.20 107.20 206.80 65.20 126.00 152.00 328.40 194.00 508.80 138.00 202.20 256.80 232.00 114.20 83.00 120.00

Ca (mg/L)

49.29 38.06 65.15 79.79 107.12 55.88 82.96 35.87 73.93 82.72 46.60 100.53 91.01 51.97 241.80 178.36 100.53 82.23 74.91 30.74 103.94 93.94 116.88 177.63 226.43 64.66 228.75 293.04 193.49 81.13 57.22 271.08

Mg (mg/L)

115.00 108.10 195.50 190.90 285.20 809.60 708.40 480.70 349.60 271.40 292.10 181.70 262.20 271.40 535.90 563.50 255.30 301.30 561.20 209.30 733.70 423.20 791.20 821.10 931.50 211.60 443.90 966.00 92.00 236.90 71.30 986.70

Na (mg/L)

11.73 14.08 28.15 29.33 9.78 30.11 26.59 11.34 30.89 11.34 3.52 11.73 12.90 41.84 8.60 17.99 10.56 11.73 17.99 28.15 13.69 17.99 34.02 3.13 25.81 9.38 9.78 7.43 10.17 8.21 31.67 18.38

K (mg/L)

255.89 177.95 258.32 233.64 346.10 345.41 388.78 350.66 373.68 350.52 220.27 356.64 401.69 381.68 931.85 979.20 701.76 450.64 706.69 244.40 372.56 354.24 1285.52 1012.91 1570.79 393.29 1420.80 1776.96 1085.76 477.77 180.88 1476.00

SO4 (mg/L)

313.11 229.33 455.11 406.83 778.87 1480.35 1113.99 744.79 600.31 419.61 512.62 406.12 433.10 295.36 1420.71 1134.58 397.60 486.35 903.12 227.20 1300.72 913.06 1188.54 1384.50 1882.92 293.23 633.32 1597.50 275.48 355.00 118.57 1535.73

Cl (mg/L)

Table 1 Chemical and isotopic results from sampled wells in the Quaternary unconfined aquifer of Wadi Guenniche.

337.94 285.48 312.32 332.45 345.87 253.15 383.69 204.96 339.16 399.55 207.40 274.50 221.43 356.24 306.22 315.37 233.63 248.27 319.64 273.28 327.57 249.49 314.76 342.21 358.07 386.74 372.10 327.57 285.48 295.85 288.53 349.53

HCO3 (mg/L)

8.78 2.01 143.08 106.28 1.20 10.07 8.06 31.78 3.26 0.35 4.08 154.73 16.76 245.53 0.35 0.35 0.35 0.69 0.47 34.92 21.77 40.30 0.69 8.41 0.94 25.96 0.35 20.19 0.94 0.35 2.15 21.00

NO3 (mg/L)

1.83 0.12 0.1 0.05 0.27 0.15 0.07 6.66 0.68 0.1 2.05 0.05 0.1 0.05 0.07 0.07 0.07 0.1 0.07 7.32 4.56 0.15 0.12 2 0.17 5.42 0.07 0.02 0.2 0.07 0.05 0.07

Br (mg/ L)

IS

0.37 0.45 0.38 0.57 0.45 0.78 0.23 0.32 0.48 0.72 0.03 0.2 0.11 0.57 0.59 0.39 −0.09 −0.15 0.75 0.99 0.19 −0.03 0.61 0.51 0.88 0.65 0.76 0.25 0.48 0.17 0.5 −0.23

−29.31 −30.15 −30.56 −30.19 −29.24 −22.10 −26.00 −28.89 −29.44 −30.25 −30.40 −28.50 −26.26 −27.95 −29.34 −33.39 −31.10 −29.67 −30.38 −23.46 −31.75 −29.58 −27.94 −28.23 −26.88 −27.40 −26.70 −26.69 −27.30 −30.02 −29.37 −27.64 3.0 – 3.1 – 1.0 2.9 2.8 – 3.7 – – – 3.2 – – – – 2.4 – – – – – – – – – – – 4.9 – 3.1

H (TU)

3

−4.92 −5.27 −5.18 −5.25 −5.42 −2.88 −4.77 −5.74 −5.17 −5.26 −5.29 −5.58 −4.54 −4.76 −4.41 −4.93 −5.81 −5.45 −4.67 −3.68 −5.45 −5.02 −5.38 −4.81 −4.86 −4.78 −5.01 −3.83 −4.76 −5.34 −4.79 −4.51

H

2

Calcite

O

(‰ V-SMOW)

18

0.22 0.3 0.23 0.42 0.3 0.63 0.08 0.17 0.33 0.57 −0.11 0.05 −0.04 0.42 0.44 0.24 −0.24 −0.3 0.6 0.84 0.05 −0.18 0.46 0.36 0.73 0.5 0.61 0.1 0.33 0.02 0.35 −0.38

Aragonite

0.44 0.66 0.61 1.1 0.88 1.35 0.46 0.15 0.86 1.54 0 0.46 0.45 0.97 1.31 0.91 −0.04 −0.13 1.33 1.89 0.61 0.01 1.06 1.25 1.69 1.24 1.84 0.84 1.16 0.49 1.13 0.16

Dolomite

−0.8 −1.06 −0.81 −0.89 −0.66 −0.8 −0.77 −0.66 −0.69 −0.84 −1.07 −0.76 −0.91 −0.76 −0.14 −0.23 −0.52 −0.8 −0.4 −1.21 −0.89 −0.8 −0.01 −0.33 0.21 −0.74 −0.16 −0.03 −0.16 −0.74 −1.21 −0.42

Gypsum

−1.05 −1.29 −1.05 −1.13 −0.9 −1.04 −1.01 −0.9 −0.93 −1.08 −1.3 −1 −1.15 −1.01 −0.38 −0.47 −0.76 −1.04 −0.64 −1.46 −1.13 −1.04 −0.25 −0.58 −0.03 −0.98 −0.4 −0.27 −0.4 −0.98 −1.45 −0.66

Anhydrite

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Fig. 6. Plot of δ2H vs δ18O for the phreatic groundwater. The green square represents weighted mean values of precipitation in Bizerte.

Fig. 4. Chloride and sodium (meq/L) correlation in groundwater.

Ammar et al., 2016) but could not be considered in the case of Wadi Guenniche basin as an indicator of large seawater intrusion because actual hydrodynamic data do not support such phenomena. Cl− vs Na+ plot (Fig. 4) indicates that 75% of samples are scattered close to or slightly below the dissolution line of halite (Na/Cl = 1). As indicated in Fig. 5a, for low mineralization waters, Ca2+ and SO42− may derive from the dissolution of gypsum. In some samples with high mineralization, Ca2+ and SO42− values deviate from the equiline (1/1) and SO42− excess can be explained by the precipitation of calcite as indicated by its positive saturation index. In this case other cations (Na+ and K+) dominate groundwater chemistry and groundwater changes to Na–Cl type with high EC values. Ca2+ vs HCO3− and Mg2+ vs HCO3− plots show that for samples with lower mineralization the Ca2+/HCO3− and Mg2+/HCO3− ratios approximate 1.0 and the dots representing groundwater fall along the equiline (Fig. 5b and c). This suggests that Ca2+, Mg2+ and HCO3− derive from carbonate minerals dissolution. At higher mineralization, the Ca2+ and Mg2+ exceed the stoichiometric ratio and dots representing groundwater fall below the equiline. This Ca2+ and Mg2+ excess versus HCO3− can be explained by the cation exchange process. Significant variations were recorded for NO3− concentration in the Wadi Guenniche plain (Table 1), varying between 0.35 and 245.5 mg/ L. NO3− concentrations were found to exceed in some wells the permissible limits of 45 mg/L for drinking purposes. Nitrate levels ranged between 21 and 40 mg/L in the eastern part of the Guenniche basin between El Khetmine and El Alia. In the downstream part of the basin, between Jouaouda and Hariza, NO3− values ranged between 20 and 245.5 mg/L. High levels of NO3− can be attributed to excessive use of mineral and animal (manure) fertilizers in crop cultivation. Atmospheric deposition, which might provide high concentrations of nitrate under certain conditions of intense evaporation in very arid

environments, cannot be considered as a significant source in this area (Stadler et al., 2008; Heaton et al., 2012). Stable isotope (2H and 18O) signatures of rainwater in the Bizerte meteorological station were used to provide the rainfall end member of the system. Based on 293 daily precipitation samples collected between September 2008 and December 2014, δ18O and δ2H values ranged between −11.09 and −1.13‰ for δ18O (σ = 1.98), and between −74.6 and + 0.9‰ for δ2H (σ = 14.81) respectively (Fig. 6). The weighted averages of δ18O and δ2H were of −4.77‰ and −26.1‰ vs V-SMOW respectively, with a relationship of δ2H = 7.02 δ18O + 8.27 (R2 = 0.9). This equation defines the first the local meteoric water line (LMWL) established for the Bizerte region. The average isotopic composition of the Guenniche unconfined groundwater was quite similar to that of the mean rain water. Groundwater samples had δ18O composition between −5.81 and −2.88‰, and δ2H between −33.39 and −22.10‰ (Table 1), with mean values of −4.92‰ and −28.6‰, respectively. Tritium concentration in the Guenniche groundwater ranged between 1 TU (well 5) and 4.9 TU (well 30) which corresponds to “modern waters”. The most important values (> 3 TU) were measured in wells situated at the border and the center of the basin, except for well N°18 situated near El Alia with 2.4 TU. The lowest values, below 3 TU, were recorded in the downstream part of the basin near the Bizerte lagoon. 5. Discussion 5.1. Hydrogeochemical evolution and mineralization process The spatial distribution of EC (Fig. 3) shows that low salinity (< 3.5 mS/cm) groundwater zones are mainly located in the piedmont of the surrounding Jebels, whereas high salinity groundwater zones

Fig. 5. Correlation plots of Ca2+ vs SO42− (a), Ca2+ vs HCO3− (b) and Mg2+ vs HCO3− (c). 7

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converge in the central and downstream parts of the plain. This is in agreement with the nature of the aquifer formed essentially by coarsegrained alluvial deposits allowing for rapid groundwater circulation from recharge to discharge zones. The increase in groundwater salinity along the flow path from upstream to downstream parts of the basin would mainly result from the increasing clay and silt fractions of lithology and the larger water-rock interaction. From the major ion data, SI values and the various ion ratios, it is clear that the chemical proprieties of groundwater are controlled by natural geochemical processes: rock weathering and ion-exchange. Furthermore, in some areas groundwater quality is affected by anthropogenic practices such as excessive use of fertilizers and irrigation. Based on the Na+ versus Cl− plot (Fig. 4) dissolution of halite deposits, which are relatively abundant particularly in the Quaternary alluvial sediments, is assumed to be the origin of Cl− and Na+ in the groundwater. The depletion in Na+ observed in a number of points may be due to the ionic exchange process with Ca2+ between groundwater and clay minerals in the aquifer. Na+ can also originate from the dissolution of mirabilite (Na2SO4.10H2O) that is usually associated to evaporite minerals such as gypsum. The dissolution of mirabilite would explain excess of SO42− with respect to Ca2+ concentration shown in Fig. 5a. Similarly, the (Ca2+ + Mg2+) versus (SO42− + HCO3−) plot (Fig. 7) shows that Ca2+ and Mg2+ evolution can be explained by carbonate (calcite, dolomite and aragonite) and evaporite (gypsum and anhydride) weathering as indicated also by their saturation indexes. In addition to weathering of minerals, the cation exchange process can add solutes to groundwater. During the cation exchange process, clay particles of the aquifer exchange Ca2+ and Mg2+ against Na+ which is adsorbed onto the sediment (Huang et al., 2013). This process implies a decrease of Na+ in groundwater against an increase in Ca2+ and Mg2+. This phenomenon can also explain Cl− excess with respect to Na+ shown in Fig. 4 for samples with high mineralization. Active cation exchange taking place in the aquifer can be substantiated using a plot of corrected bivalent cations (Ca2+ and Mg2+) versus corrected Na+ (Fig. 8). During this process, concentrations of Ca2+ and Mg2+ involved in exchange reactions can be subtracted with the equivalent ones of associated anions (HCO3− and SO42 −) which would be derived from other processes such as carbonate dissolution or/and silicate weathering. Similarly, Na+ issued from the interaction between groundwater and the aquifer matrix can be accounted for by assuming that Na+ contribution of meteoric origin would be balanced by equivalent concentrations of Cl−. As a result, the slope of the bivariate plot of ((Na+ + K+) – Cl−) vs ((Ca2+ + Mg2+) – (SO42− + HCO3−)) should ideally be roughly equal to −1 (Jankowski et al., 1998). Fig. 8 shows that waters of the Guenniche plain have a slope of −0.75, indicating that cation exchange affects the water chemistry of the study area to some extent. From border areas

Fig. 8. Evidence of a cation exchange process in the study area.

characterized by lower values of EC to the central part of the plain where high values of mineralization were observed, the cation exchange process may be enhanced by low groundwater flow as indicated by low hydraulic gradient values and a more clayey aquifer formation. As a consequence, an increase in calcium released by clay particles in the aquifer against sodium concentration was observed in the lower part of the Guenniche plain. In the study area, Cl/Br ratios below seawater value and concomitant high NO3− concentrations characterized wells 1, 8, 20 and 26, indicating possible localized groundwater pollution caused by the excessive use of industrial and animal fertilizers in agriculture (Fig. 9). In the absence of measured values in the study and neighboring areas, we report from the literature a value for the Cl/Br ratio in a commercial KCl fertilizer of 510. For animal waste, a wide range of ratios was measured: from 86 to 329 for cattle and goats waste and from 2810 to 3730 for hog and horse waste (Panno et al., 2006). Recharge by direct rainfall infiltration revealed by the Cl/Br ratio values typical of coastal areas characterized only wells 11, 21 and 24 with respective Cl/Br ratios of 564, 643 and 1560. Cl/Br ratios for wells 11 and 21 are close to the seawater ratio but do not imply seawater intrusion, considering their location far from the coast. Indeed, in coastal regions under natural conditions, groundwater salinity comes mostly from marine aerosol, implying a recharge water Cl/Br ratio close to the seawater value of 655 (Alcalá and Custodio, 2008). Cl/Br ratios up to 2000 may indicate that the dissolution of halite and/or gypsum containing a variable fraction of halite are the main source of Cl− in groundwater. Halite and gypsum are frequently found in Quaternary deposits. High Cl/Br ratios up to 10,000 characterized wells situated in the lower and downstream parts of the study area where groundwater is of Na–Cl type. Enrichment of the soil zone with evaporites by evapotranspiration and the leaching of those minerals may cause an increase in Cl− concentration in the shallow groundwater. Sodium absorbed on clay surfaces as it substitutes for calcium and magnesium during the cation exchange process may alter the soil structure making it compact and impervious. The Wilcox classification diagram (Fig. 10) based on the SAR ratio was used as a common index of the suitability of groundwater for irrigation. The SAR ratio is of a particular importance, because high Na+ contents in irrigation waters may affect soil stability, increase soil hardness and reduce its permeability (Suarez et al., 2006). About 28% of sampled wells with low EC values and Ca–Mg/Cl–SO4 facies (except for well N°20), and mostly situated in the borders of the Guenniche basin, belong to the S1–C3 and S1–C4 classes which allow good use of the groundwater for irrigation. The water that belongs to the S2–C4 class allows only limited use of groundwater for agricultural purposes, while the water samples of the

Fig. 7. Plot of (Ca2+ + Mg2+) vs (SO42− + HCO3−) in meq/L. 8

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Fig. 9. Plot of Cl/Br molar ratio. Red squares: groundwater polluted by industrial and animal fertilizers; blue dots: groundwater recharged by direct infiltration of rain water; black dots: dissolution of halite and/or gypsum containing halite.

5.2. Isotopic data

during the air mass transport, rainfall intensity, origin of the vapor mass, and possible re-evaporation during rainfall. For all comparison between isotopic signature of groundwater, local precipitation and the Global Meteoric Water Line (GMWL), the slope of the Local Meteoric Water Line (LMWL) is usually set to 8 (Craig, 1961; Rozanski et al., 1993). In the Bizerte meteorological station, deuterium excess calculated with the theoretical slope of 8 was +13.9‰. This value is consistent with the Mediterranean MWL defined by Celle-Jeanton et al. (2001) for the Western part of the Mediterranean basin with d-excess of +14‰. The slope of the LMWL in Bizerte (7.02) is slightly lower than the slope of the Global Meteoric Water Line (GMWL) of 8. This advocates the effect of secondary evaporation during rainfall. This effect usually typifies arid and semi-arid environments with small rainfall amounts, low atmospheric relative humidity and relatively high air temperatures. The secondary evaporation effect during rainfall is expected to be the greatest for light rains; as a consequence rainwater in these events will be more enriched in heavier isotopes. Therefore, the slope of the meteoric water lines for light rainfall will depart from that of the GMWL, while the meteoric line for heavy rainfall will remain close to that of the GMWL. In the Bizerte station, for rainfall events ranging between 2 and 10 mm, weighted averages for δ18O and δ2H were −4.5 and −23.8‰, respectively. For rainfall ranging between 10 and 20 mm, the weighted averages were −5.6 and −30.1‰ respectively and for rainfalls greater than 20 mm the weighted averages were −5.9 and −33.2‰ respectively. Similar relationships between air temperature and isotopic composition of precipitation were observed. For air temperature > 20 °C, the isotopic composition of rainwater was −3.8 and −19.5‰ for δ18O and δ2H respectively, while the isotopic composition became more depleted for temperatures between 5 and 10 °C with respective weighted averages of −6.0 and −31.6‰.

5.2.1. Stable isotopes in precipitation Due to the large variability characterizing the isotopic content of daily precipitations (Fig. 11), the long-term weighted mean values of δ18O and δ2H calculated as recommended by the IAEA (2011), were used as input function into the hydrogeological system. Isotopic content of rain water is usually controlled by local climatic parameters including air temperature and relative humidity, amount of rainfall

5.2.2. Stable isotopes in groundwater The δ18O and δ2H values of groundwater samples fall on or close to the GMWL and the Bizerte LMWL (Fig. 6), except for a few points that fall below the LMWL. This indicates that most of groundwater is of meteoric origin and did not undergo significant evaporation before or during recharge. Because of this similarity, the isotopic signature of groundwater can be interpreted as an indicator of aquifer recharge

Fig. 10. Wilcox classification diagram of phreatic groundwater in the Guenniche basin.

S3–C4 and S4–C4 classes (generally with Na/Cl type) are not suitable for irrigation use. Samples with high EC (6, 15, 16, 28 and 32) falling within S3–C5 and S4–C5 are also not suitable for irrigation.

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Fig. 11. Daily rainfall amount (mm), δ18O (‰) and d-excess in precipitation from the Bizerte meteorological station for September 2008–December 2014 period.

required 3H input function from rainfall was reconstituted from the available data from IAEA Mediterranean network stations. First, we used available 3H records in Tunisia: Tunis (36.83°N10.23°E) between 1968 and 1997 and Sfax (34.72°N–10.68°E) between 1993 and 2015. The meteorological station of Tunis is the nearest one to Bizerte with the same climate conditions. Sfax station is situated about 300 km to the south with a more arid climate (212 mm/y). Even though the two chronicles overlap over 5 years (1993–1997), comparison is only possible for one year (1993). Data for the other years do not allow for calculating an annual value for Tunis (see supplementary material), and even for Sfax the data had to be reconstructed. The available data do not allow reconstructing the Tunis chronicle between 1993 and 2013. For this reason Tunis data covered the 1969 to 1992 period and Sfax data covered the 1993 to 2013 period. The earlier missing 3H data prior to 1968, containing the worldwide nuclear test era characterized globally by the peak of 1963, were reconstituted from data from the three Mediterranean stations, with latitude close to that of Tunis: the stations of Gibraltar (36.15°N-5.25°E) from 1961 to 1997, Heraklion (35.33°N-25.18°E) from 1963 to 1987 and Alexandria (31.2°N-29.95°E) from 1961 to 1989, with respective tritium peaks for the year 1963 of 482.4 TU, 1140.7 TU, and 682.7 TU (Fig. 12). An exponential function was used to complete data between 1950 and the first value for these three stations with assumed natural tritium content in 1950 equal to 4 TU. There is a bias in the dataset caused by distance from the study area, different annual rainfall amounts and the peak of 1963 related to anthropogenic activities, but the reconstructed dataset constitutes a realistic and complete temporal chronicle in our study zone. Some monthly data are missing in the Sfax and Tunis chronicles, only monthly tritium values representing more than 65% of total rainfall was calculated to avoid a bias on the weighted annual value, and if not, annual data was reconstructed (see supplementary material). The annual natural 3H content for the study area was defined from the Sfax chronicle, which showed a stabilization of the annual 3H signal over the period from 1992 to 2013 with an average of 3.87 ± 0.65 TU. For groundwater dating and renewal rates estimates, two models were tested: the piston flow model and the mixing model (see supplementary material for details on the use of the model). The residence time of groundwater in Wadi Guenniche basin was calculated using a piston flow model. In this model, 3H concentration in groundwater at the time of measurements in 2013 (A2013) is related to the 3H input of precipitation during the preceding year via

typical of present climate conditions by direct infiltration of rain water and from runoff in the Wadi Guenniche and its tributaries. The high infiltration capacity of the Quaternary sandstones allows a rapid infiltration and, in most of the area, the water table depth prevents evaporation after recharge and guarantees the preservation of the groundwater isotopic composition during the recharge process. Four samples taken from the lower part of the basin (Hariza) characterized by the shallowest levels of the water table (Wells number 13, 15, 28 and 32) are clearly situated below the LMWL, showing more evaporated isotopic compositions (Fig. 6). Based on the location of these samples and the lower thickness of the unsaturated zone (measured water table levels ranging between 1 and 0.5 m), this water is likely to be subject to the evaporative effect that could result from nearsurface evaporation or from irrigation return flow. The water used for irrigation is exposed longer to the atmosphere and therefore undergoes significant evaporation, which makes it more enriched in heavy isotopes. Part of this water infiltrates back into the aquifer as an evaporated return flow. Groundwater from well number 6, situated in the extreme downstream part of the basin near the Bizerte lagoon was found to have more enriched δ18O and δ2H values. Compared to other groundwater samples and precipitation, those values may indicate the possible onset of a seawater intrusion as a consequence of local intensive exploitation of the unconfined aquifer. This hypothesis can be supported by a Na/Cl ratio (0.84) close to that of the regional seawater (Ben Ammar et al., 2016). Considering the limited number of sampled wells near the Bizerte lagoon and in the absence of a systematic piezometric monitoring, the probable seawater intrusion in the extreme downstream part must be assumed as a locally limited phenomenon. A systematic hydrodynamic monitoring in this area is required. 5.2.3. Groundwater dating and renewability In recent years, groundwater dating became an important tool to assess groundwater renewability, understand groundwater circulation and achieve sustainable management of natural resources. As a constituent of the water molecule with relatively short half-life, 3H has been widely used to estimate recent groundwater residence times. In order to estimate the age of the unconfined groundwater in the Guenniche basin at least in relative terms, and because the current limitations of the tritium techniques due to the return of its natural atmospheric content since the end of the 1990's, we need to use a regionally significant 3H concentration time series in precipitation during the last 50 years. Because of the lack of regular 3H records in Tunisia and the absence tritium measurement in the Bizerte region, the

A2013 = Pi e-kt 10

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Fig. 12. Reconstructed rainfall 3H input in Tunisia from three Mediterranean stations Gibraltar (G), Alexandria (A) and Heraklion (H). Data from the Global Network of Isotopes in Precipitations (GNIP).

Where Pi is the annual mean 3H concentration in precipitation for year i; t is the transit time between year i and 2013 and k is the decay constant of 3H (0.0566/yr). Groundwater age can be ascertained from the simulated output curves (Fig. 13). Estimated age for samples 1, 3, 6, 7, 13, 18 and 32 with tritium values ranging between 2.4 and 3.3 TU indicates very recent recharge, inferior to 20 years. Except for well number 1, all these wells are situated around Wadi Guenniche and its tributaries. This does attest to an efficient recharge from Wadi floods. For samples 9 and 30 with respective 3H values of 3.7 and 4.9 TU, estimated age varies between 23 and 40 years. The lowest 3H value measured in the downstream part of the basin in well number 5 with only 1 TU indicates former recharge, prior to the nuclear test era. In this case groundwater is estimated to have infiltrated between 1956 and 1957. Reconstituted groundwater 3H contents were used to estimate renewal rates according to annual rainfall. The mixing model used (Leduc et al., 1996) assumes homogeneous tritium deposition over the entire aquifer at any given time, and a constant water reserve each year. The model simulates the aquifer tritium content depending on the renewal rate, which varies according to annual rainfall:

Ai = (1-ai) Ai-1 e-k + ai Pi with Ai the tritium content in the aquifer during year i, ai the renewal rate for the year i, Pi the rain tritium content during year i and k the decay constant of 3H. The simulated 3H output curves (Fig. 14) for 2013 depending on renewal rates ranging from 0 to 20% indicate that tritium contents between 2.4 and 3.0 TU may correspond to low renewal rates about 1–2%. Tritium contents above 3.0 TU appear to be consistent with renewal rates ranging between 2 and 20%. According to the aquifer lithology, renewal values about 2% would seem to be more plausible and realistic. For samples with respective 3H values of 3.7 and 4.9 TU, the renewal rate would be around 4–5%, representing to the maximum signal, although the uncertainties linked to the reconstruction of rainwater tritium values do not allow to match the peak value calculated. The lowest 3H value of 1 TU measured in well number 5 may imply a weak renewal rate around 0.3–0.5%. This well is situated in the downstream part of the basin where groundwater circulation is slower and the aquifer is more impermeable.

Fig. 13. Piston flow 3H output results in groundwater for sampling year 2013. Prior to 1968 data are reconstituted from the Gibraltar (G), Alexandria (A) and Heraklion (H) stations. 11

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Fig. 14. 3H content in the aquifer in 2013 depending on the mean renewal rates, calculated with the mixing model.

At the scale of the catchment, measured 3H in the Wadi Guennich basin provide a median value of 3 TU. This value corresponds to renewal rate of 2% (Fig. 14). Given a porosity of 11% (Ennabli, 1980), and a saturated thickness of 50 m, the corresponding mean annual recharge will be about 100 mm. This value represents 16% of the mean annual rainfall. This recharge rate is slightly higher than the one calculated by Ennabli in 1980 (11%) and can explain the stabilization of the piezometric levels of the unconfined aquifer in the face of a continuous increase of groundwater use.

information on the mechanisms of recharge and the residence time of shallow groundwater. A long-term survey of stable isotopes in daily rainfall in the Bizerte meteorological station allowed us to define a new LMWL and input function to regional aquifers. Groundwater stable isotopic composition indicated that groundwater is of meteoric origin resulting from recent local recharge by direct infiltration from atmospheric precipitation and by runoff in Wadi Guenniche and its tributaries. Tritium measurements on 10 samples were also useful to support the conclusions from the geochemical and stable isotopes data. Estimated 3H ages deduced from the piston flow model were within 40 years, indicating recent groundwater and fast groundwater circulation in line with aquifer lithology. Simulated renewal rates from groundwater 3H contents in 2013 using a mixing model agreed with residence times and estimated ratios that varied from about 2% in the upstream part of the plain to less than 0.5% near the Bizerte lagoon. Median values of groundwater 3H imply an annual recharge rate representing 16% of rainfall. This study displayed features which are common to other alluvial coastal aquifers in the Mediterranean basin where groundwater represents a valuable resource and plays a fundamental socio-economic role (e.g. Caschetto et al., 2016; Argamasilla et al., 2017). Fast recharge and high renewability of coastal alluvial aquifers make them more vulnerable to climate changes and to potential pollution from agricultural and other anthropogenic activities. Moreover, overexploitation of coastal groundwater resources can lead to seawater intrusion. In terms of groundwater resource management, our study of the Wadi Guenniche aquifer as a case study of the coastal Mediterranean basin highlights the urgent need for monitoring, protecting and mending if possible the coastal alluvial aquifers suffering from both human and natural constraints, in order to guarantee the sustainability of water resources.

5.3. Conceptual model A simplified conceptual model summarizing groundwater flow, recharge and geochemical and isotopic evolution in the unconfined aquifer of the Guenniche basin is presented in Fig. 15. The aquifer is recharged from direct infiltration of rainwater and from runoff in Wadi Guenniche and from return flow of irrigation water. Groundwater salinity is mostly controlled by water-rock interaction and cation exchange processes. From the piedmont of the surrounding Jebels to the lower downstream part of the plain, groundwater geochemical facies change from Ca–Mg/Cl–SO4 to Na–Cl type, and contamination from fertilizers used in agricultural activities becomes more important. High 3 H contents and stable isotopic signatures characterizing groundwater in the upstream part of the plain indicate recent recharge. High EC and Cl/Br ratios with enriched stable isotopic composition of groundwater in the lower part of the plain argue for an evaporative effect of groundwater caused by near-surface evaporation of the water table and the contribution of a return flow from agricultural water. 6. Conclusion A combination of hydrogeological, hydrochemical and isotopic methods allowed us to understand the geochemical evolution and the processes affecting groundwater salinization in the coastal plain of Wadi Guenniche. Useful information regarding the mechanism of recharge and residence time in the shallow unconfined aquifer was provided. The Hydrogeochemical study based on major elements and Cl/Br ratios highlighted that the principal natural hydrochemical processes are evaporites dissolution, cationic exchange, and evaporation of groundwater in the central part of the plain. Irrigation return flows have resulted in increasing NO3− values exceeding 100 mg/L in many parts of the plain. Isotopic contents of the precipitation and groundwater provided key

Acknowledgements This research is a part of Tunisian-French scientific program supported by collaboration agreement between ISTEUB (Univ. Carthage) and HSM (IRD). The authors appreciate the help of the staff of the Bizerte Water Resources Division for data collection and rain water sampling. We are thankful to Professor D.A. Ravetta editor-in-chief of Journal of Arid Environments and Dr. Canton, associate editor. We also like to thank the two anonymous reviewers for their fruitful comments and suggestions that greatly improved the manuscript. 12

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Fig. 15. Conceptual model (out of scale) of groundwater flow paths, dynamics of salinity, and isotopic signatures from recharge to discharge zones in the Wadi Guenniche plain.

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