Isotopic and geochemical methods for studying water ...

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/324990136

Isotopic and geochemical methods for studying water?rock interaction and recharge mode: application to the Cenomanian?Turonian and PlioQuaternary aquifers of Essaouira Basin, Moro... Article in Marine and Freshwater Research · January 2018 DOI: 10.1071/MF17370

CITATIONS

READ

0

1

3 authors, including: Bahir Mohammed

Salah Ouhamdouch

Cadi Ayyad University

Cadi Ayyad University

150 PUBLICATIONS 363 CITATIONS

25 PUBLICATIONS 4 CITATIONS

SEE PROFILE

SEE PROFILE

Some of the authors of this publication are also working on these related projects:

Climate change and groundwater in semi-arid zone View project

Assessment, management and protection of water resources as climate change in semi-arid areas View project

All content following this page was uploaded by Salah Ouhamdouch on 13 May 2018.

The user has requested enhancement of the downloaded file.

CSIRO PUBLISHING

Marine and Freshwater Research https://doi.org/10.1071/MF17370

Isotopic and geochemical methods for studying water–rock interaction and recharge mode: application to the Cenomanian–Turonian and Plio-Quaternary aquifers of Essaouira Basin, Morocco Mohammed Bahir A, Salah Ouhamdouch A,C and Paula` M. Carreira B A

Laboratoire de Geósciences et Environnement (LGE), De´partement de Geólogie, Ecole Normale Supe´rieure, Universite´ Cadi Ayyad, BP 2400, Marrakech, Morocco. B Instituto Tecnologico e Nuclear, Universidade Tećnica de Lisboa, Estrada Nacional 10, PT-2686-953 Sacave´m, Portugal. C Corresponding author. Email: [email protected]

Abstract. Study of the Cenomanian–Turonian and Plio–Quaternary aquifers of Essaouira basin (Western Morocco), based on the interpretation of geochemical (major elements) and isotopic (18O, 2H, 13C and 14C) data, has aided the understanding of the hydrodynamics of these aquifers, which is greatly affected by tectonics. Hydrochemical characteristics based on the bivariate diagrams of major ions (Cl, SO42, NO3, HCO3, Naþ, Mg2þ, Kþ and Ca2þ) and electrical conductivity and mineral saturation indices indicate that the origins of groundwater mineralisation are the result of: (1) evaporite dissolution; (2) cation exchange reactions; (3) and evaporation processes. Radiogenic isotopes (3H and 14C) have highlighted the presence of significant recent recharge in the eastern part of the basin, with groundwater moving according to the general flow path (south-east to north-west). Stable isotope data from the Essaouira basin plot along the Global Meteoric Water Line and below the Local Meteoric Water Line. This suggests that groundwater has been recharged under several different climate regimes. Additional keywords: geochemistry, semi-arid area, stable isotopes, water resources.

Received 16 May 2017, accepted 17 January 2018, published online 7 May 2018

Introduction In arid and semi-arid areas, salinisation is the main threat to groundwater resources. This salinity may have different origins. It can be caused by primary salinisation processes that add solutes to the aquifer system, such as seawater intrusion (Demirel 2004; de Montety et al. 2008; Ayadi et al. 2016; Trabelsi et al. 2016; Ouhamdouch et al. 2017), the ascent of saline waters of the underlying aquifers through hydraulic thresholds in areas affected by tectonics (Zammouri et al. 2007; Trabelsi et al. 2009; Abid et al. 2010; Farid et al. 2015) and the geogenic dissolution of salt deposits (Carreira et al. 2014; Farid et al. 2015). It can also result from secondary processes leading to salinisation by redistribution or concentration of the solutes already present in the system, such as the return of irrigation water (Kattan 2008; Bel Hadj Salem et al. 2012; Qin et al. 2013; Ben Moussa et al. 2014), overexploitation of aquifers (Zammouri et al. 2007; Farid et al. 2015), evaporation (Kattan 2008) and mixing of different types of water (Trabelsi et al. 2009; Farid et al. 2013). In Morocco, most studies on groundwater salinisation have been focused on coastal areas (Hsissou et al. 1999; Fakir et al. 2002; Ben Kabbour et al. 2005; Chkir et al. 2008; Snoussi et al. 2008; Bahir et al. 2013; Re et al. 2013; Warner et al. 2013; Journal compilation CSIRO 2018

Ouhamdouch et al. 2016, 2017). These regions are densely populated and experience high pressure from human activities, with increasing demand for water resources to supply intensive industrial and tourism activities (Showers 2002; Chamchati et al. 2012). The central part of Morocco is similarly affected by high water demands for intensive agriculture activities. However, hydogeochemical evolution and groundwater salinisation issues in central Morocco remain underinvestigated. Sustainable management of water resources in semi-arid zones is almost impossible without adequate knowledge of the spatial distribution of these waters and an understanding of the processes that govern mineralisation. As a result, a multidisciplinary study using hydrogeochemical and isotopic methods was established to characterise these waters and to determine the mineralisation processes for the sustainable management of these resources in Morocco. Materials and methods Study area The study area is a set of synclinal structures located between Zelten wadi in the south and Hadid anticline in the north (Fig. 1). www.publish.csiro.au/journals/mfr

Marine and Freshwater Research

M. Bahir et al.

80 000

100 000

120 000

80 000

100 000

120 000

100 000 80 000 0

60 000

60 000

80 000

100 000

120 000

B

10 km

Sample

140 000

Study area

Geology Quaternary Alluvium (A)

Conglomerates (q1D)

Barkhanes (D)

Conglomerates and dunes (p-q)

Consolidated dunes (q2)

Colluvions (pM)

Consolidated dunes (q2M) Lakeside or paludal facies (q1)

Dune facies (M)

Paleogene Phosphated marls (eiPh)

Cretaceous

Jurassic

Dolomitic limestones (cs)

Dolomitic limestones (j-c)

Phosphated dolomitic limestones (cs-Ph)

Limestones, red clays, dolomites (jm)

Limestones and marls (ci)

Dolomites (ji)

Triassic

Dolomitic limestones (cm)

Saliferous red clay (tB)

Fig. 1. Water sampling locations and geological map of the study area.

The region has a population of ,400 000 inhabitants and is predominantly rural. The basin area is ,6000 km2 and is characterised by limited and discontinuous water resources. The Ouazzi and Meskala sub-basins are a part of the Essaouira coastal region, where the climate is a semi-arid type, with a mean annual rainfall of approximately 300 mm year1 and temperature of ,208C.

For the climate framework, the study area is characterised by a ‘cool’ dry season, followed by a ‘hot’ dry season and ultimately a ‘moderate’ rainy season. In general, the significant diurnal temperature fluctuations that are often observed during these seasons restrict the growth of plant species. Evaporation measured using a Colorado pan is ,2390.6 mm year1, whereas that measured using a Piche evaporimeter is ,2157.10 mL year1

Mineralisation and recharge mode of aquifers of the Essaouira basin


Section a

Section b

SSW

NNE

Igrounzar Wadi Zelten Bouabout Meskala bowl bowl

Kourimat bowl Kechoula anticline

SSW Igrounzar Marls, dolomitic limestones Limestones, dolomitic limestones

Wadi Bouabout bowl

Gray marls, limestone shells

4000 m

Section c

Kourimat bowl

W

Jehair anticline

200 m

Meskala bowl

4000 m

Sidi Ammar fault

200 m

Dolomitic limestones, dolomites Alternation of limestones and marls Dolomites, dolomitic limestones, sandstones Fault

NNE

Ouled Moumen bowl

Section d

E

N Alluvions, alluviums, Hadid sandstones, regs Marls, dolomitic limestones anticline Ksob wadi Limestones, dolomitic limestones Gray marls, limestone shells Dolomitic limestones, dolomites Fault

10 km

Marls, dolomitic limestones Limestones, dolomitic limestones Gray marls, limestone shells Dolomitic limestones, dolomites Alternation of limestones and marls Dolomites, dolomitic limestones, sandstones Fault

4000 m

1 km

200 m

C

S

Tidzi Amsitène Diapir anticline

Alluvions, alluviums, sandstones, regs Marls, dolomitic limestones Limestones, dolomitic limestones Alternation of limestones and marls Marls, limestones, sandstones Sandy marls, reef limestones, sandstones Dolomite, dolomitic limestones Salt clays, salt massif, dolomites Fault

Fig. 2. Schematic geological cross-sections of Sections a–d (see Fig. 1 for location details). SSW, south-south-west; NNE, north-north-east; W, west; E, east; N, north; S, south.

(results obtained from Tensift Basin Hydraulic Agency (ABHT), Marrakech, Morocco). On the geological plan, the upstream part of the study area (east) is dominated by the middle and upper Cretaceous outcrop (Albian, Vraconian, Cenomanian and Turonian). These formations consist of banks of limestone and dolomitic intercalated with marls and sandstones. The Albian and Vraconian are represented by green marls (thickness 150 m) and dolomitic limestones (thickness 140 m). The Cenomanian is characterised by an alternation of grey marls with anhydrite, lumachellic limestones and sandstones in small proportions. The Turonian consists of limestones where silica is very abundant. The downstream part of the study area (west) is characterised by the outcrop of the Plio-Quaternary formations. These are represented by conglomerates, alluviums, colluvium and sandstones (Figs 1, 2) Sampling and methods Thirty water samples were collected from boreholes (depth 9.50– 95 m), wells (depth 5.20–88 m) and springs within the Essaouira basin (Fig. 1). The depth of the water table is measured by a piezometric probe (SEBA Hydrometrie, Kaufbeuren, Germany; 200-m range). Temperature, pH and electrical conductivity (EC) were measured in situ using a pH-ORP-TEMP portable meter (AD111; ADWA Hungary Kft, Szeged, Hungary) and a portable conductivity meter (Hanna Instruments, Romania). HCO3 concentrations were analysed by titration using 0.1 M HCl. Analyses of major ions (Ca2þ, Mg2þ, Naþ, Kþ, Cl, SO42 and NO3) were performed on samples filtered through 0.45-mm filter paper using ionic chromatography with conductivity detection by The Laboratory at the University Cadi Ayyad of Marrakech (Morocco; Table 1). The precision of the Laboratory is better than 5%. Ionic balances were calculated and examined for each groundwater sample as a quality assurance check of the chemical analyses. Most groundwater samples had ion balances within the 5% range. The remaining samples were still included

and considered acceptable in the present study because the ionic balances were still relatively low and unmeasured constituents, such as organic anions, nutrients and trace metals, may have contributed to the higher ionic balances (Hem 1985). Stable isotope analyses of d2H (%VSMOW, per mille Vienna Standard Mean Ocean Water), d18O (%VSMOW) and d13C (%VPDB, per mille Vienna Pee Dee Belemnite), were performed by mass spectrometry in the International Atomic Energy Agency Isotope Hydrology Laboratory (AIEA–IHLS; Vienna, Austria; Table 2). The results are expressed as relative deviations d (%) from the VSMOW. The analytical precision (1s) was in the order of 0.1% for d18O and 1% for d2H. The 14C determinations were made on the total dissolved inorganic carbon (TDIC) of groundwater, precipitated in the field as BaCO3 at a pH .9.0. The counting rates of the 14C (benzene) were measured using a liquid scintillation counter in the AIEA– IHLS. The 14C content is expressed as the percentage of modern carbon (pMC). The errors associated with this method vary with the amount of carbon available in each sample, and increase where 14C content is low. The 13C values measured by the AIEA–IHLS (%VPDB) have an accuracy of 0.1% (Table 2). Evaluation of the groundwater data was performed using statistical methods and geochemical and isotopic methods were then used to establish or identify the processes controlling the groundwater chemistry (i.e. identifying and characterising the water–rock interaction mechanisms present within the aquifer system). The concentrations of major ions in the groundwater may be correlated depending on the underlying physical and chemical processes (Farnham et al. 2000; Gu¨ler and Thyne 2004; Thyne et al. 2004; Almasri and Kaluarachchi 2007). The evolution of geochemical processes in groundwater depends on the equilibrium between mineral phases and water and is commonly investigated using saturation indices. A positive value for saturation indices computed with regard to the solid phase, following Garrels and Christ (1965), indicates an oversaturated state. In the present study, the geochemical software

Code

BT1 BT2 WT3 ST4 WT5 WT6 ST7 WT8 BT9 BT10 WT11 ST12 WT13 BT14 WT15 WP16 WP17 SP18 SP19 WP20 BP1 ST2 BT3 WT4 ST5 ST6 ST7b WT8 BT9b ST10

Location

Aghbalou Aı¨t Si Mohamed Kourimat town Taouzart Aı¨t Laassaba Douar Jicht Sidi Aı¨ssa Chef lieu Aı¨t Kaddour Aı¨t Hmida Bouktaib Tfailia Hmida cherfa Od Mrah Boujlakh Laghouazi Boujlida Aı¨t Mahjar Tigmijou Nguiaa Rgaib Neknafa Firate Aı¨t Chaı¨b Aı¨n Tighssine El Khenga Timghazdane Chef lieu Bibaddazen Tahloucht

6.9 7.28 7.24 7.5 7.39 7.69 7.64 7.6 7.11 7.8 7.1 7.65 7.4 7 7.35 6.9 6.9 6.75 6.65 7.35 7.15 6.85 7.1 7.35 7.1 7.15 7.05 6.9 7.15 6.75

pH

2092 2600 1500 998 1065 994 3880 1600 2750 900 1900 3740 1220 2300 2540 2500 2800 3100 2700 2970 966 1810 1400 1500 1500 1008 1867 1450 1232 2430

EC (mS cm1) 20.8 22 20 17.4 21.9 21.3 17.4 18.3 21.5 23.2 22.1 26.4 25 26.2 24.4 26.6 22.8 22.2 24.8 22.2 24.4 29.5 25.8 23.5 22.8 22.9 20 24.4 24.6 20

Temperature (8C) 5 6.2 5.6 5.25 5.5 5 4.4 6.25 5.75 n.m. 5 n.m. 5.5 6.5 n.m. 6 6.6 5.5 7.5 3 6.75 6.7 6.4 n.m. 7.5 4.65 n.m. n.m. n.m. 8.04

HCO3 (mEq L1) 6.91 17.9 4.35 2.82 3.07 1.93 4.98 4.67 3.23 4.05 33.9 8.84 3.61 12.12 14.03 16.08 15.83 23.73 17.22 18.83 2.03 11.97 10.21 13.88 6.61 5.35 8.52 n.m. n.m. 10.88

Cl (mEq L1) 0.62 1.38 0.8 0.75 0.64 0.69 0 0.36 n.m. n.m. n.m. 0.39 n.m. 0.21 1.05 1.23 1.23 1.05 1.01 3.45 0.31 0.35 0.73 0.15 0.63 0.64 0.58 n.m. n.m. 0.27

NO3 (mEq L1) 10.77 1.79 4.06 0.49 1.57 1.42 49.81 4.59 27.62 3.68 0.16 49.94 4.47 7.4 8.71 8 7.61 8.65 7.77 4.46 0.47 7.8 1.87 1.86 6.41 4.07 6.55 n.m. n.m. 13.23

SO42 (mEq L1) 7.21 7.49 4.77 3.92 3.14 3.64 34.03 3.59 17.91 3.88 10.97 28.88 2.46 6.51 11.64 13.36 10.61 12.75 9.4 15.02 2.92 10.53 8.18 4.87 6.29 6.52 6.84 n.m. n.m. 11.01

Ca2þ (mEq L1) 10.17 8.52 6.85 2.9 4.56 4.22 27.81 6.21 19.95 4.37 10.44 23.26 4.33 11.5 14.48 13.09 14.72 16.49 14.66 7.41 4.63 9.49 6.39 4.25 9.95 5.06 8.15 n.m. n.m. 16.59

Mg2þ (mEq L1) 4.68 7.27 3.13 2.96 2.17 1.89 2.44 2.84 2.27 5.18 8.4 4.43 5.38 4.74 8.43 9.93 10.06 13.45 11.11 11.11 2.06 4.84 4.49 6.98 4.21 1.26 5.29 n.m. n.m. 7.17

Naþ (mEq L1)

Table 1. Physicochemical parameters of groundwater from Essaouira basin BT, borehole Turonian; WT, well Turonian; ST, spring Turonian; SP, Spring Plio–Quaternary; WP, well Plio–Quaternary; n.m., not measured; EC, electrical conductivity

0.19 0.14 0.06 0.08 0.05 0.06 0.32 0.11 0.21 0.36 0.12 0.28 0.36 0.14 0.18 0.18 0.14 0.12 0.12 1.7 0.06 0.16 0.11 0.09 0.11 0.11 0.35 n.m. n.m. 0.17

Kþ (mEq L1)

D Marine and Freshwater Research M. Bahir et al.



Table 2. Isotopic parameters of groundwater from the study area BT, borehole Turonian; WT, well Turonian; ST, spring Turonian; SP, Spring Plio–Quaternary; WP, well Plio–Quaternary; n.m., not measured; TU, tritium units; %MC, percentage modern carbon; PDB, Pee Dee Belemnite Sample

18

d O (%)

2

dH (%)

BT1 BT2 WT3 ST4 WT5 WT6 ST7 WT8 BT9 BT10 WT11 ST12 WT13 BT14 WT15 WP16 WP17 SP18 SP19 WP20 BP1 ST2 BT3 WT4 ST5 ST6 ST7b WT8 BT9b ST10

5.39 4.71 5.43 5.69 5.68 5.32 5.43 5.59 5.88 5.29 5.63 5.48 5.25 4.9 3.82 6.21 4.7 4.81 4.9 4.2 5.23 5.38 4.52 4.31 5.1 5.74 4.96 5.18 5.54 4.43

31.39 26.57 31.42 32.67 33.32 31.57 34.32 34.23 35.71 29.13 35.14 34.26 29.76 28.87 20.69 38.37 24.54 27.57 27.89 23.69 28.59 31.81 22.32 27.39 30.91 33.67 31 31.13 30.59 25.23

3

H (TU)

14

d C (%MC)

d C (%PDB)

1.25 0.42 1.86 3.43 1.69 2.02 0.43 n.m. n.m. 0.18 1 n.m. n.m. n.m. n.m. n.m. n.m. 0.76 n.m. n.m. 0.9 1.55 n.m. n.m. 1.5 n.m. 1.09 3.2 0.7 0.93

67.4 n.m. n.m. n.m. 60.5 n.m. n.m. n.m. 86 n.m. n.m. n.m. n.m. n.m. n.m. 60 n.m. 74.4 67.7 n.m. n.m. 48.26 27.03 n.m. 60.97 50.9 n.m. n.m. 60.7 n.m.

10.44 n.m. n.m. n.m. 10.82 n.m. n.m. n.m. 5.12 n.m. n.m. n.m. n.m. n.m. n.m. 7.41 n.m. 8.45 9.18 n.m. n.m. 8.57 8.87 n.m. 10.29 8.2 n.m. n.m. 9.47 n.m.

ð1Þ

where KIAP is the ionic activity product of the ions and Ksp is the solubility product of the mineral. Results and discussion Physicochemical data The hydrodynamic study allowed characterisation of the groundwater flow and provided an initial idea as to aquifer behaviour. This relationship was further clarified by the hydrochemical study. Fig. 3 shows that the main groundwater flow is towards the Atlantic Ocean (south-east to north-west).

Groundwater flow Piezometric level (m ASL) Tidzi diapir Hadid anticlinel Amsittene anticlinel Study area

13

DIAGRAMMES (Roland SIMLER, Laboratoire d’Hydrogeólogie d’Avignon, see http://www.lha.univ-avignon.fr/LHALogiciels.htm, accessed 15 December 2016) based on PHREEQC program (Parkhurst and Appelo 1999) was used to calculate the saturation indices (SI) with regard to calcite, dolomite and gypsum using the following formula: KIAP SI ¼ log Ksp

80

E

0

10 km

Fig. 3. Piezometric map of the study area.

Because the groundwaters of this aquifer are intended for use as drinking water and for irrigation, water must meet several quality standards depending on its use. Thus, the evolution of the groundwater hydrogeochemical content along its flow path was investigated not only to identify the chemical characteristics of the water and their relationship with the water–rock interaction processes, but also to define the potability of groundwater and its suitability for irrigation. The physicochemical parameters of the studied groundwater showed large variations, with high EC in the groundwater samples ranging from 900 to 4000 mS cm1 (Fig. 4) with a mean of 1900 mS cm1. It is of note that higher mineralisation was not found near the coastline, but near the inland limit of the Meskala sub-basin, pointing to an alternative source of salt to the system. The lowest EC was found in the centre of the basin near the village of Kourimat, with values increasing steadily to the north, west and south-east, with a maximum towards the coastline. For the Meskala sub-basin, the lowest EC was observed to the east of Meskala village (north part of the subbasin) and in the south part of the sub-basin. The areas of low EC represent recharge zones for the aquifers of the two basins, which was confirmed by the isotopic approach. The chemical composition of groundwater is primarily dependent on the geology, as well as on the geochemical processes that take place within the groundwater system. The ionic composition of groundwater samples collected from the different wells in the study area are plotted on the Piper diagram in Fig. 5, enabling classification and comparison of water types based on the ionic composition of different water samples (Hem 1985). Water types are often used in the characterisation of waters as a diagnostic tool (Pitkanen et al. 2002). Furthermore, water type is used to assess the hydrogeochemical facies. From the Piper diagram and radar chart (Fig. 5), the facies of the groundwater analysed were found to be of three types: Ca-Cl (25%), Mg-Cl (25%) and Mg-HCO3 (21%). This shows signs of

F


M. Bahir et al.

high chloride and magnesium content (.50%) compared with other ions. To determine the major elements that contribute to water mineralisation, a correlation between major element content and EC was established (Fig. 6). For the groundwater from Ouazzi basin, Ca2þ, Mg2þ, SO42 and Cl were found to be the main contributors to groundwater mineralisation. For the groundwater from Meskala basin, the main elements responsible for groundwater mineralisation were Mg2þ, SO42, Naþ and HCO3. In order to highlight the different mechanisms that contribute to groundwater mineralisation, the relationships between major elements were investigated (Fig. 7).

The binary diagram for Naþ and Cl (Fig. 7a) shows a positive correlation between these two ions, with R2 ¼ 0.658 and R2 ¼ 0.701 for the Ouazzi basin and Meskala respectively. This positive correlation indicates the same source of the two ions. Some samples are close to the halite dissolution line (1 : 1 line). The calculated SIs towards the halite dissolution line are very low, confirming that the dissolution of halite could contribute to the groundwater mineralisation (Fig. 8a). Other samples are placed below 1 : 1 line, indicating a deficit of Naþ v. Cl. This Naþ deficit is balanced by an excess of Ca2þ and Mg2þ, indicating a cation-exchange process. This hypothesis could be supported by Fig. 8f, which shows that most groundwater

Town Village 100

Iso-EC line Wadi Tidzi diapir Hadid anticlinel Amsittene anticlinel Study area

0

20 km

Fig. 4. Spatial distribution of the electrical conductivity in the Essaouira Basin. Iso-EC, iso-electrical conductivity.



samples were displaced towards a position that represent an increase in Ca2þ and Mg2þ content and a decrease in Naþ and Kþ content. The plot of Ca2þ v. SO42 (Fig. 7b) shows a positive correlation between these two ions in the samples taken from the Ouazzi and Meskala sub-basins. This positive correlation indicates that these two ions have the same source. In addition, Fig. 7b shows that only few samples are close to the dissolution line of gypsum, indicating that the dissolution of gypsum and anhydrite is limited. The dissolution hypothesis of gypsum and anhydrite is supported by the negative values of the calculated

SIs, reflecting an undersaturation with regard to gypsum and anhydrite (Fig. 8b, c). The excess of Ca2þ is supported by the cation-exchange process (Fig. 8f). The correlation diagram for Ca2þ and Mg2þ (Fig. 7c) shows a positive correlation between Ca2þ and Mg2þ reflecting the same source of the two cations. A few samples are plotted around the dissolution line of dolomite, indicating a limited contribution of this mineral to groundwater mineralisation. The rest of the samples are placed below the 1 : 1 line. Fig. 8e shows that the SIs with regard to the dolomite are positive and vary between 0 and 1.04. This corresponds to super-saturation towards the dolomite mineral. The relationship between Ca2þ and HCO3 (Fig. 7d) is low (R2 ¼ 0.108 and 0.093 for the Ouazzi and Meskala basin respectively). This weak correlation supports the absence of a relationship between these two ions. From Fig. 7d, it can be seen that the Ca2þ concentration increases while the HCO3 concentration remains relatively constant. With the exception of a few points, the calculated SIs are positive, thus reflecting the low contribution of the dissolution of calcite in the mineralisation of groundwater.

SO

4

g M Ca

Cl

NO

3

100

30 Ca-SO4 20 10 Mg-SO4 0 Ca-HCO3

0

0

Mg-Cl Ca-Cl

Mg-HCO3 Na-HCO3

Isotope data The stable isotopes of oxygen and hydrogen are generally considered to be transported conservatively in shallow aquifers (Kim et al. 2003). The use of 18O and deuterium isotopes in hydrogeology provides information on the origin and movement of groundwater. The close proximity to the meteoric water lines suggests that the water was of meteoric origin. The samples also plotted in the more negative area (i.e. left side) of the graph, which is appropriate for waters recharged at colder temperatures or in a colder climate, at different altitudes or by different systems. The points plotted below the meteoric water lines suggest some isotopic fractionation, possibly by evaporation. Measured stable isotope ratios were used as conservative tracers of water origin. The d18O and d2H values listed in Table 2

3

4

CO

K

Mg

SO

Na

HC

O

3

0

10

0

10

Na-Cl

0

0

0

0

10

0

0

10

100

Ca

100

0

Cl NO3

0

100

Fig. 5. Piper diagram and radar chart of the analysed water samples. Circles indicate the Ouazzi Basin; triangles indicate the Meskala Basin.

EC (μS cm1)

(a) 4000

(b)

3500 3000 2500 2000 1500 1000 500

R 2 0.1693

R 2 0.6762

0

5

10

(c)

4000 3500 3000 2500 2000 1500 1000 500

R 0.159 2

(e) 4000

R 0.3847

3500 3000 2500 2000 1500 1000 500

50

100

SO42 (mEq L1)

R 2 0.59

1000 0 10

20

30

40

0

Cl (mEq L1)

10

20

30

R 2 0.0669 R 2 0.318

0

1

K(mEq L1)

4000 3500 3000 2500 2000 1500 1000 500

R 2 0.8008

R 2 0.7554

0

40

Ca2 (mEq L1)

2

20

40

Mg2 (mEq L1)

(g) 4000

3500 3000 2500 2000 1500 1000 500

R 2 0.5111

0

2000

(f) 4000

R 2 0.7639

R 2 0.7488

3000

0

15

(d)

5000 4000

2

Na (mEq L1)

G

(h) 4000

3500 3000 2500 R 2 0.0038 2000 1500 1000 2 R 0.5081 500 0 2 4

6

8

HCO3(mEq L1)

10

3500 3000 2500 2000 1500 1000 500

R 2 0.0185

R 2 0.0796

0

2

4

NO3 (mEq L1)

Fig. 6. Bivariate plots of electrical conductivity (EC) plotted against chemical constituents of groundwater. Circles indicate the Ouazzi Basin; triangles indicate the Meskala Basin.

H


M. Bahir et al.

(a) 100

(b) 100

10

Ca2 (mEq L1)

R 2 0.6579

1: 1

10

R 2 0.6432

R 2 0.7011

1

1: 1

Na (mEq L1)

R 2 0.8291

1 1

10

Cl (mEq

100

0

1

L1)

10 2 (mEq

SO4 (d)

1:

1

(c) 100

100

L1)

R 2 0.5872

10

R 2 0.1086

Ca2 (mEq L1)

Ca2 (mEq L1)

20

R 2 0.0932

R 2 0.8318

1

2 1

10

100

Mg2 (mEq L1)

2

20

HCO3 (mEq L1)

Fig. 7. Correlation diagram of (a) Naþ v. Cl, (b) Ca2þ v. SO42, (c) Ca2þ v. Mg2þ and (d) Ca2þ v. HCO3. Note the log scale for all axes. Circles indicate the Ouazzi Basin; triangles indicate the Meskala basin.

are plotted in Fig. 9 in relation to the Global Meteoric Water Line (GMWL; Craig 1961) and the Local Meteoric Water Line (LMWL; Bahir et al. 2000) of Essaouira basin. The stable isotope composition of the analysed groundwater samples ranged from 6.21 to 3.82% d18O and from 35.71.2 to 20.69% d2H for the Ouazzi Sub-basin, compared with a range from 5.74 to 4.31% d18O and from 33.67 to 22.32% d2H for the Meskala Sub-basin. From Fig. 9, most of the points taken from the two Ouazzi and Meskala sub-basins are situated between the LMWL of the Essaouira basin and the GMWL or slightly below it, which excludes any evaporation effect of these waters. This enrichment in stable isotopes was recorded in the Kourimat region for the Ouazzi Sub-basin (Sample WP16) and in the eastern part for the Maskala Sub-basin (ST6). These areas have low EC values, confirming the current recharge of the Cenomanian– Turonian and Plio–Quaternary aquifer by infiltration of rainwater. However, given the low amounts of rainfall and its irregularity, aquifer inputs are low. In general, water with a 3H content .1 tritium unit (TU) is regarded as having a pre-1952 age, a date that represents the peak of the artificial release of tritium through nuclear tests, and such waters are said to have been affected by little or no secondary processes, such as evaporation before infiltration or isotopic exchange with the aquifer materials (Mazor 1991).

However, 3H concentrations above the detection limit (1 TU) indicate recent water infiltration. Tritium activities were measured in samples to establish the groundwater residence time. In fact, tritium values in the Turonian aquifer ranged from 0.18 to 3.43 TU (Table 1). The presence of measurable tritium concentrations (.1 TU) was observed upstream of the study area, confirming the presence of waters of modern infiltration (ST2, ST4, WT6, BT10, BP1 and WT3). This recent recharge is confirmed by low EC values (Fig. 2a). Determination of the 14C content of dissolved inorganic carbon (DIC) is useful for groundwater dating. However, in addition to radioactive decay, the 14C content (in DIC) can be affected by many geochemical and physical processes, such as dilution of the C content in soil CO2 caused by dissolution of carbonate minerals (which is the most important process) and isotopic exchanges that may take place between the different carbon-bearing species (gas, water and mineral) due to reversibility of the reactions. Changes in the d13C content of DIC are often used to deduce the processes that affect the carbon isotopic composition of DIC and the 14C value during the chemical evolution of groundwater. The relationship between 14C DIC and d13C DIC for groundwater systems will reflect: (1) the change in d13C values in DIC caused by isotopic exchanges between DIC and solid carbonate; (2) progression of the reaction


Na Cl (mEq L1)

(a) 5.0

0

20

40


60

0

20

40

60

80

100

0 0.5

SIGYPSUM

SIHalite

Ca2 SO42 (mEq L1)

(b)

5.5 6.0 6.5 7.0

1.0 1.5 2.0 2.5 3.0

7.5

(c)

I

(d )

0.5

0.6

Supersaturation

0 0.4

SICALCITE

SIAnhydrite

0.5 1.0 1.5 2.0

0.2 0 0.2

2.5

Undersaturation

3.0

0.4 0

50 2

Ca

100

SO4

2

(mEq

0

L1)

Ca2

1.2 1.0

SIDolomite

0.8 0.6 0.4 0.2 0 0.2 0.4 0

20

40

Ca2 HCO3 Mg2 (mEq L1)

[(Ca2 Mg2) (HCO3 SO42)] (mEq L1)

(f ) (e)

5

10

15

20

HCO3 (mEq

25

L1

)

20

(l)

10

Dissolution effect

0 40

20

0

10

[(Na

K)

20

(ll)

(CI)] (mEq L1)

Fig. 8. (a) Halite saturation index (SI) v. Naþ þ Cl, (b) gypsum SI v. Ca2þþSO42, (c) anhydrite SI v. Ca2þ þ SO42, (d) calcite SI v. Ca2þ þ HCO3, (e) dolomite SI v. Ca2þþMg2þ and ( f ) [(Ca2þ þ Mg2þ) – (HCO3 þ SO42)] v. [(Naþ þ Kþ) – (Cl)]. I, adsorption of Naþ and release of Ca2þ; II, adsorption of Ca2þ and release of Naþ. Circles indicate the Ouazzi Basin; triangles indicate the Meskala Basin.

or process with the ageing of the groundwater (i.e. with decay of 14C in DIC); and (3) the magnitude of the rate of change in d13C of DIC. The d13C data for water samples from the Meskala aquifers are relatively consistent, with values ranging from 8.2 to 10.29%. A wider range of d13C values (from 5.12 to 10.82%) was observed in samples from the Ouazzi Subbasin. Variations in d13C of DIC in the Ouazzi and Meskala sub-basins are plotted as a function of the 14C activities in Fig. 10. However, there do not appear to be appreciable trends in d13C of DIC within individual hydrochemical zones, which may indicate that source waters recharged to the aquifers are

similarly affected by the isotopic composition of carbonate minerals and by the relative abundance of different plants in the recharge areas. Groundwater ages have been estimated for all sites based on pMC data using the models of Fontes and Garnier (1979). The age values for water samples taken from the Ouazzi Sub-basin vary ,2300 bp. Water samples from the Meskala aquifers range from ‘post-bomb’ or modern to ,8500 bp (BT3 sample). Even if radiocarbon is an indispensable tool in the age tracing of groundwater, results must always be considered with caution. The reliability of the results depends strongly on the complexity of the geochemistry of the aquifer.

J


15

LMWL δ2H 7.73 δ18O 10.5

20

δ2H (‰VSMOW)

M. Bahir et al.

GMWL δ2H 8 δ18O 10

25 30

H2S exchange

35 Silicates

40

Evaporation

CO2 exchange

45 7

High Temperature

Condensation

6

5

4

GMWL

3

2

1

0

δ18O (‰VSMOW) Fig. 9. Stable isotope composition of the water samples analysed. Circles indicate the Ouazzi Basin; triangles indicate the Meskala Basin. GMWL; Global Meteoric Water Line; LMWL, Local Meteoric Water Line; VSMOW, Vienna Standard Mean Ocean Water.

groundwater samples, the most important process is a salinisation related to the presence of evaporites in the gypsiferous– marl substratum. The limited range of variation of most hydrochemical and isotopic contents indicates that waters in the aquifers are rather intermixed or subject to similar recharge. Groundwater isotopic composition clearly indicates a recent recharge that excludes any evaporation effect even if a borehole of the Meskala Sub-basin, situated in the central part of the area, produced water dating to ,8500 bp. Thus, despite of the arid dry climate conditions, the present study shows that some groundwaters are ancient groundwater, indicating that the areas had been subjected to wetter paleoclimate periods. Therefore, special care should be taken to protect this essential resource from being polluted by agricultural fertilisers and other human activities. Conflicts of interest The authors declare that they have no conflicts of interest. Acknowledgements

0

The authors thank the staff of International Atomic Energy Agency Isotope Hydrology Laboratory (AIEA–IHLS), Vienna, Austria.

δ13C (‰PBD)

2 4

References

6 8 10 12 0

20

40

δ14C

60

80

100

activities (%MC)

Fig. 10. Relationship between d13C and d14C in the water samples analysed. Circles indicate the Ouazzi Basin; triangles indicate the Meskala Basin. %MC, percentage of modern carbon; PDB, Pee Dee Belemnite.

Conclusion In most applications, complementary environmental isotope analyses of hydrogen and oxygen together with general hydrochemical measurements are recommended to facilitate a comprehensive and thorough interpretation of water resource characteristics. The goal of the present study was to use such methodology to analyse groundwater collected in the Meskala and Ouazzi sub-basins, east of Essaouira city (Morocco). This would allow delineation and characterisation of the major geological structures to describe the Turonian limestone aquifer in these basins where the waters are severely stressed by both natural conditions and new challenges. Most previous studies in the region considered only the hydrodynamic aspect, but were not able to reach an operative level. The comparison with other approaches enabled substantial progress in the surface hydrology of the upstream catchment and in the groundwater dynamics of the Ouazzi and Meskala sub-basins. The hydrochemical results of the present study lend support to the presence of both natural and anthropogenic processes that contribute to the groundwater mineralisation. Except for a few

Abid, K., Zouari, K., and Abidi, B. (2010). Identification and characterisation of hydrogeological relays of continental intercalaire aquifer of southern Tunisia. Carbonates and Evaporites 25, 65–75. doi:10.1007/ S13146-010-0008-3 Almasri, M. N., and Kaluarachchi, J. J. (2007). Modelling nitrate contamination of groundwater in agricultural watersheds. Journal of Hydrology 343, 211–229. doi:10.1016/J.JHYDROL.2007.06.016 Ayadi, R., Zouari, K., Saibi, H., Trabelsi, R., Khanfir, H., and Itoi, R. (2016). Determination of the origins and recharge rates of the Sfax aquifer system (southeastern Tunisia) using isotope tracers. Environmental Earth Sciences 75(636), 1–21. Bahir, M., Mennani, A., Jalal, M., and Youbi, N. (2000). Ressources hydriques du bassin synclinal d’Essaouira (Maroc). Estudios Geologicos. 56, 185–195. doi:10.3989/EGEOL.00563-4150 Bahir, M., El Moukhayar, R., Chkir, N., Chamchati, H., Galego Fernandes, P., and Carreira, P. (2013). Groundwater chemical evolution in the Essaouira Aquifer Basin – NW Morocco. Open Journal of Modern Hydrology 3, 130–137. doi:10.4236/OJMH.2013.33017 Bel Hadj Salem, S., Chkir, N., Zouari, K., Cognard-Plancq, A. L., and Valles, V. (2012). Hydrochemical and isotope evidence of groundwater contamination of cultivated fields of semi-arid environments in Tunisia. Arid Land Research and Management 26, 181–199. doi:10.1080/ 15324982.2012.680652 Ben Kabbour, B., Lahcen Zouhri, L., and Mania, J. (2005). Overexploitation and continuous drought effects on groundwater yield and marine intrusion: considerations arising from the modelling of Mamora coastal aquifer, Morocco. Hydrological Processes 19, 3765–3782. doi:10.1002/ HYP.5631 Ben Moussa, A., Mzali, H., Zouari, K., and Hezzi, H. (2014). Hydrochemical and isotopic assessment of groundwater quality in the Quaternary shallow aquifer, Tazoghrane region, north-eastern Tunisia. Quaternary International 338, 51–58. doi:10.1016/J.QUAINT.2014.01.023 Carreira, P. M., Marques, J. M., and Nunes, D. (2014). Source of groundwater salinity in coastline aquifers based on environmental isotopes (Portugal): natural vs. human interference. A review and reinterpretation. Applied Geochemistry 41, 163–175. doi:10.1016/J.APGEOCHEM. 2013.12.012


Chamchati, H., Bahir, M., Chkir, N., and Carreira, P. (2012). Vulne´rabilite´ de la ressource en eau et de´fis du de´veloppement durable du Bassin d’Essaouira. In ‘Proceedings of Second International Conference: Integrated Water Resources Management and Challenges of the Sustainable Development’, 24–26 March 2010, Agadir, Morocco. (Eds H. Treidel and M. Rubio.) pp. 383–395. (UNESCO: Paris, France.) Chkir, N., Trabelsi, R., Bahir, M., Hadji Ammar, F., Zouari, K., Chamchati, H., and Monteiro, J. P. (2008). Vulne´rabilite´ des ressources en eau des aquife`res coˆtie`res en zones semi-arides e´tude comparative entre les bassins d’Essaouira (Maroc) et la Jeffara (Tunisie). Comunicaço˜es Geolo´gicas 95, 107–121. Craig, H. (1961). Isotopic variation in meteoric waters. Science 133, 1702–1703. doi:10.1126/SCIENCE.133.3465.1702 de Montety, V., Radakovitch, O., Vallet-Coulomb, C., Blavoux, B., Hermitte, D., and Valles, V. (2008). Origin of groundwater salinity and hydrogeochemical processes in a confined coastal aquifer. Case of the Rhoˆne delta (Southern France). Applied Geochemistry 23, 2337–2349. doi:10.1016/J.APGEOCHEM.2008.03.011 Demirel, Z. (2004). The history and evaluation of saltwater intrusion into a coastal aquifer in Mersin. Turkey. Journal of Environmental Management 70, 275–282. doi:10.1016/J.JENVMAN.2003.12.007 Fakir, Y., El Mernissi, M., Kreuser, T., and Berjami, B. (2002). Natural tracer approach to characterize groundwater in the coastal Sahel of Oualidia (Morocco). Environmental Geology 43, 197–202. doi:10.1007/ S00254-002-0644-6 Farid, I., Trabelsi, R., Zouari, K., Abid, K., and Ayachi, M. (2013). Hydrogeochemical processes affecting groundwater in an irrigated land in Central Tunisia. Environmental Earth Sciences 68, 1215–1231. doi:10.1007/S12665-012-1788-7 Farid, I., Zouari, K., Rigane, A., and Beji, R. (2015). Origin of the groundwater salinity and geochemical processes in detrital and carbonate aquifers: case of Chougafiya basin (Central Tunisia). Journal of Hydrology 530, 508–532. doi:10.1016/J.JHYDROL. 2015.10.009 Farnham, I. M., Stetzenbach, K. L., Singh, A. K., and Johanneson, K. H. (2000). Deciphering groundwater flow systems in Oasis Valley Nevada, using trace element chemistry, multivariate statistics, and Geographical Information System. Mathematical Geology 32, 943–968. doi:10.1023/ A:1007522519268 Fontes, J. C., and Garnier, J. M. (1979). Determination of the initial 14C activity of the total dissolved carbon; a review of the existing models and a new approach. Water Resources Research 15, 399–413. doi:10.1029/ WR015I002P00399 Garrels, R. M., and Christ, C. L. (1965). ‘Solutions Minerals and Equilibrium.’ (Harper and Row: New York, NY, USA.) Gu¨ler, C., and Thyne, G. D. (2004). Hydrologic and geologic factors controlling surface and ground water chemistry in Indian Wells–Owens Valley area and surrounding ranges, California, USA. Journal of Hydrology 285, 177–198. doi:10.1016/J.JHYDROL.2003. 08.019 Hem, J. D. (1985). Study and interpretation of the chemical characteristics of natural water (3rd edn). Water-Supply Paper 2254, US Geological Survey, Alexandria, VA, USA. Hsissou, Y., Mudry, J., Mania, J., Bouchaou, L., and Chauve, P. (1999). Utilisation du rapport Br/Cl pour de´terminer l’origine de la salinite´ des eaux souterraines: exemple de la plaine du Souss (Maroc). Comptes Rendus de l’Acade´mie des Sciences – Series IIA. Earth and Planetary Science 328, 381–386. Kattan, Z. (2008). Estimation of evaporation and irrigation return flow in arid zones using stable isotope ratios and chloride mass-balance analysis:


case of the Euphrates River, Syria. Journal of Arid Environments 72, 730–747. doi:10.1016/J.JARIDENV.2007.10.011 Kim, Y., Lee, K. S., Koh, D. C., Lee, D. H., Lee, S. G., Park, W. B., Koh, G. W., and Woo, N. C. (2003). Hydrogeochemical and isotopic evidence of groundwater salinization in a coastal aquifer: a case study in Jeju volcanic island, Korea. Journal of Hydrology 270, 282–294. doi:10.1016/S0022-1694(02)00307-4 Mazor, E. (1991). ‘Applied Chemical and Isotopic Groundwater Hydrology.’ (Open University Press: Buckingham, UK.) Ouhamdouch, S., Bahir, M., Chkir, N., Carreira, P., and Goumih, A. (2016). Comportement hydrogeochimique d’un aquifere cotier des zones semiarides: cas de l’aquifere barremien-aptien du bassin d’Essaouira (Maroc occidental). Larhyss Journal 25, 163–182. Ouhamdouch, S., Bahira, M., and Carreira, P. M. (2017). Geochemical and isotopic tools to deciphering the origin of mineralization of the coastal aquifer of Essaouira basin, Morocco. Procedia Earth and Planetary Science. 17, 73–76. doi:10.1016/J.PROEPS.2016.12.038 Parkhurst, D. L., and Appelo, C. A. J. (1999). User’s guide to PHREEQC (version 2): a computer program for speciation, batch reaction, onedimensional transport and inverse calculation. Water-Resources Investigations Report 99-4259, US Geological Survey, Reston, VA, USA. Pitkanen, P., Kaija, J., Blomqvist, R., Smellie, J. A. T., Frape, S. K., Laaksoharju, M., Negral, P. H., Casanova, J., and Karhu, J. (2002). Hydrogeochemical interpretation of groundwater at Palmottu. Paper EUR 19118 EN, European Commission, Brussels, Belgium. Qin, R., Wua, Y., Xu, Z., Xie, D., and Zhang, C. (2013). Assessing the impact of natural and anthropogenic activities on groundwater quality in coastal alluvial aquifers of the lower Liaohe River Plain, NE China. Applied Geochemistry 31, 142–158. doi:10.1016/J.APGEOCHEM.2013.01.001 Re, V., Sacchi, E., Martin-Bordes, J. L., Aureli, A., El Hamouti, N., Bouchnan, R., and Zuppi, G. M. (2013). Processes affecting groundwater quality in arid zones: the case of the Bou-Areg coastal aquifer (North Morocco). Applied Geochemistry 34, 181–198. doi:10.1016/J.APGEO CHEM.2013.03.011 Showers, K. B. (2002). Water scarcity and urban Africa: an overview of urban–rural water linkages. World Development 30, 621–648. doi:10.1016/S0305-750X(01)00132-2 Snoussi, M., Ouchani, T., and Niazi, S. (2008). Vulnerability assessment of the impact of sea-level rise and flooding on the Moroccan coast: the case of the Mediterranean eastern zone. Estuarine, Coastal and Shelf Science 77, 206–213. doi:10.1016/J.ECSS.2007.09.024 Thyne, G., Gu¨ler, C., and Poeter, E. (2004). Sequential analysis of hydrochemical data for watershed characterization. Ground Water 42, 711–723. doi:10.1111/J.1745-6584.2004.TB02725.X Trabelsi, R., Kacem, A., Zouari, K., and Rozanski, K. (2009). Quantifying regional groundwater flow between continental intercalaire and Djeffara aquifers in southern Tunisia using isotope methods. Environmental Geology 58, 171–183. doi:10.1007/S00254-008-1503-X Trabelsi, N., Triki, I., Hentati, I., and Zairi, M. (2016). Aquifer vulnerability and seawater intrusion risk using GALDIT, GQISWI and GIS: case of a coastal aquifer in Tunisia. Environmental Earth Sciences 75(669), 1–19. Warner, N., Lgourna, Z., Bouchaou, L., Boutaleb, S., Tagma, T., Hsaissoune, M., and Vengosh, A. (2013). Integration of geochemical and isotopic tracers for elucidating water sources and salinization of shallow aquifers in the sub-Saharan Draâ Basin, Morocco. Applied Geochemistry 34, 140–151. doi:10.1016/J.APGEOCHEM.2013.03.005 Zammouri, M., Siegfried, T., El-Fahem, T., Samiha, K., and Kinzelbach, W. (2007). Salinization of groundwater in the Nefzawa oases region, Tunisia: results of a regional scale hydrogeologic approach. Hydrogeology Journal 15, 1357–1375. doi:10.1007/S10040-007-0185-X

www.publish.csiro.au/journals/mfr

View publication stats

K