Assessment of seawater intrusion and nitrate contamination on the groundwater quality in the Korba coastal plain of Cap-Bon (North-east of Tunisia)

Accepted Manuscript Assessment of seawater intrusion and nitrate contamination on the groundwater quality in the Korba coastal plain of Cap-Bon (North-east of Tunisia) Adel Zghibi, Jamila Tarhouni, Lahcen Zouhri PII: DOI: Reference:

S1464-343X(13)00132-5 http://dx.doi.org/10.1016/j.jafrearsci.2013.07.009 AES 1884

To appear in:

African Earth Sciences

Received Date: Revised Date: Accepted Date:

10 July 2012 3 July 2013 12 July 2013

Please cite this article as: Zghibi, A., Tarhouni, J., Zouhri, L., Assessment of seawater intrusion and nitrate contamination on the groundwater quality in the Korba coastal plain of Cap-Bon (North-east of Tunisia), African Earth Sciences (2013), doi: http://dx.doi.org/10.1016/j.jafrearsci.2013.07.009

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Assessment of seawater intrusion and nitrate contamination on the

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groundwater quality in the Korba coastal plain of Cap-Bon (North-east of

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Tunisia)

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Adel Zghibi a,b,*, Jamila Tarhouni a, Lahcen Zouhri b

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a

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Avenue Charles Nicolle, Mahrajène, 1082 Tunis, Tunisia

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b

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France

Water Sciences and Techniques Laboratory, National Agronomic Institute of Tunisia. 43

HydrISE, LaSalle Beauvais Polytechnic Institute, 19 Rue Pierre Waguet, 60026 Beauvais,

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* Corresponding author. Tel.: +216 20 81 81 46; Fax: +216 71 79 93 91

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E-mail address: [email protected]

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Abstract

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In recent years, seawater intrusion and nitrate contamination of groundwater have become a

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growing concern for people in rural areas in Tunisia where groundwater is always used as

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drinking water. The coastal plain of Korba (north-east of Tunisia) is a typical area where the

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contamination of the aquifer in the form of saltwater intrusion and high nitrate concentrations

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is very developed and represents the major consequence of human activities.

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The objective of this study is to evaluate groundwater resource level, to determine

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groundwater quality and to assess the risk of NO3- pollution in groundwater using

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hydrogeochemical tools. Groundwater were sampled and analysed for physic-chemical

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parameters : Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3-, NO3-, Total Dissolved Solid and of the

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physical parameters (pH, electrical conductivity and the temperature). The interpretation of

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the analytical results is shown numerically and graphically through the ionic deviations, Piper

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Diagram, seawater fractions and binary diagrams. Moreover, electrical conductivity

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investigations have been used to identify the location of the major intrusion plumes in this

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coastal area and to obtain new information on the spatial scales and dynamics of the fresh

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water–seawater interface. Those processes can be used as indicators of seawater intrusion

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progression.

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First, the hydrogeochemical investigation of this aquifer reveals the major sources of

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contamination, represented by seawater intrusion. Thus, the intensive extraction of

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groundwater from aquifer reduces freshwater outflow to the sea, creates several drawdown

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cones and lowering of the water table to as much as 12 m below mean sea level in the center 1

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part of the study area especially between Diarr El Hojjej and Tafelloun villages, causing

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seawater migration inland and rising toward the wells.

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Moreover, the results of this study revealed the presence of direct cation exchange linked to

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seawater intrusion and dissolution processes associated with cations exchange.

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Second, a common contaminant identified in groundwater is dissolved nitrogen in the form of

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nitrate. The average nitrate concentration of the aquifer is about 30.44 mg/l, but contents as

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great as about 50 mg/l occur in the central region where seawater has been identified. Nitrate

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survey reveals that nitrate concentration above the drinking water standard (50 mg/l) covered

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an area of 122.64 km², which accounts for 28% of the whole area. Irrigation with the nitrogen

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fertilizers, domestic sewage, industrial wastewater and movement of contaminants in areas of

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high hydraulic gradients within the drawdown cones probably are responsible for localized

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peaks of the nitrate concentration.

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It is suggested that risk assessment of nitrate pollution is useful for a better management of

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groundwater resources, preventing soil salinisation and minimizing nitrate pollution in

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groundwater.

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Keywords: Seawater intrusion; Over-exploitation; Cap-Bon; Nitrate contamination;

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Salinisation processes; Groundwater resources

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1. Introduction

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Groundwater is a significant or sole source of water in the Mediterranean African coastal

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country (Morocco, Algeria, Tunisia, Libya and Egypt) where rapid population growth and

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intensive economic activity has increased the need for fresh-water supplies. This need is

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mainly being satisfied by pumping groundwater from coastal aquifer systems.

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However, groundwater quality patterns are complex because of the input from many different

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water sources (Trabelsi et al., 2007; Zouhri et al., 2008). These include seawater intrusion,

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ascending deep groundwater, and anthropogenic sources such as wastewater or irrigation

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return flow (Steinich et al., 1998; Trabelsi et al., 2007).

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Consequences such as lowering of groundwater levels, land subsidence, and the related

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damage to buildings and other infrastructure harm the economy of the irrigation districts and

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endanger their future development (Steinich et al., 1998).

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Understanding the spatial variations in chemical composition of groundwater that result from

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different types of input is extremely difficult, especially if concentrations have varied over

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time or if boundary conditions have changed (Hussein, 2004) and, at the same time, has

2

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important practical implications for water resource evaluation and management (Bouchaou et

66

al., 2008).

67

In Tunisia, numerous irrigation regions are located near the coast, principally in northern-

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eastern part of Cap-Bon peninsula, where extensive areas have been established in the late

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1970s and have evolved into an advanced agricultural production zone in Tunisia which have

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gained considerable importance in the economy of the country.

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Because climatic conditions in those regions are not favorable for intensive agricultural

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practices, these activities are primarily dependent on groundwater extraction.

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This situation has led to two significant problems linked to human activity: (1) salinization

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due to the formation of large piezometric drawdown cones, which in turn have accelerated

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seawater intrusion by reversing the hydraulic gradients into aquifers and (2) direct input of

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nitrate mainly from fertilizers.

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Agriculture is based on intensive irrigation and fertilization to improve the soils. Most of the

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fields have been covered with greenhouses and several hectares of citrus trees. The crops

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include mainly seasonal crops such as vegetables mainly potatoes, tomatoes, tobacco and

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strawberries but also citrus plantations, vineyards and chilly peppers.

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Sufficient-to-excess amounts of fertilizer and water have been applied annually. It was

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reported that N-fertilizer was applied at 80 - 120 kg N/ha during the cropping practice in 2004

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(Grava, 2005; Aouissi et al., 2012). Furthermore, the mainly wadis (wadi: dry stream except

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during periods of rainfall) have been severely polluted by wastewater discharge from industry

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and domestic use which resulted in high NO3 concentrations in the groundwater.

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The present study presents the results of a hydro-geochemical study of the Korba aquifer, in

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order to control and manage groundwater quality by an understanding of groundwater

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contamination and identification of the factors affecting seawater intrusion process and nitrate

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concentration distribution.

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The objectives are (1) to determine what part of the aquifer is currently affected by seawater

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intrusion due to the over-exploitation of the groundwater reserve, and (2) to describe the

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anthropogenic contamination of the aquifer by nitrate.

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2. Study area

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The Korba aquifer is located in the northern-eastern part of the Tunisia. It extends from

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Nabeul in the south to the city of Kélibia, and is bounded by the Mediterranean sea in the east

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and the Djebel Sidi Abderrahmen anticline in the west (Fig. 1a,b). The study area is

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characterised by a semi-arid climate with an average annual precipitation of 420 mm and an

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3

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annual mean temperature of 17°C. The relative humidity varies between 71 and 81% (I.N.M.,

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2005). According to the Korba station data, 60% of the annual rainfall is concentrated

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between November and March, and the climatic deficit (Rainfall – Evapotranspiration) covers

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a period of about 10 months, reaching its maximum (150 mm) in July and August.

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The topography of the study area is a flat wide valley with altitudes from a few meters above

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mean sea level near the Mediterranean coast and increasing steadily toward the Djebel Sidi

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Abderrahmen anticline, which is 150–250 m above sea level (Fig. 1c).

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3. Geological and hydrogeological Settings

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The Korba aquifer is composed from the Quaternary deposits, Pliocene, and Upper Miocene

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(Sable of the Somâa Formation) laying unconformably on middle Miocene (sandstones and

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marls formations) that constitutes the aquifer basement (Fig. 2a).

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Three main geological formations constitute the aquifer system. The first is the Quaternary

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deposits. They are usually composed of two units: the lower unit of marine facies corresponds

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to sandy limestone with molluscs indicating the maximum flooding surface (MFS) of the

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Tyrrhenian transgression. The upper unit is mainly composed of a continental facies (Ozer et

113

al., 1980) with the occurrence of oolitic limestone and coprolites or pelloids. The Tyrrhenian

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age forming approximately a 1.2 km width band parallel to the coast all along the domain. It

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is constituted mainly by arenitic limestone underlined by a conglomeratic layer (Oueslati,

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1994). Its thickness varies between 10 and 50 m. The second formation is of Pliocene age and

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corresponds to marine sediments deposited in the Dakhla syncline in the North of the city of

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Korba (Kerrou, 2008). The Dakhla syncline was formed during the Atlasic folding phase. The

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trangressional marine Pliocene sediments were unconformably deposited on these folded and

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eroded formations. These deposits are mainly composed of sandstone-sand-marl alternations

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topped by sandstones and sand. The dominant lithologies in that formation are yellow sands

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with alternating clay and sandstone levels. The thickness of the Pliocene formation is about 80

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m in the central part of the area, reaches 250 m offshore, decreasing towards the west (Kerrou

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et al., 2010). The third formation, called “the sands of Somâa”, is of late Miocene age and is

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localized only in the South of the study area. It is composed mainly of thick fine sand layers

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of continental origin including conglomeratic level sand clay lenses (Kerrou et al., 2010). The

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lower part of the Middle Miocene corresponds to detrital deposits known as the Beglia

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Formation (Kouzana et al., 2010). The upper part is composed of lenticular sandstones and

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marls with lignite levels called the Saouaf Formation. The Upper Miocene is absent in the

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study area. This is believed to have been caused by the widespread erosional activity due to 4

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the Miocene orogeny (Ennabli, 1980). This unit is mainly composed by thick fine sand layers

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of continental origin including conglomeratic levels and clay lenses. To the south of the area,

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the thickness of the Somâa formation might exceed 400 m (Kerrou et al., 2004; Kerrou,

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2008).

135

The Korba aquifer is alluvial-phreatic and it covers about 438 km². It was formed during the

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Pliocene and Quaternary periods by sedimentation processes of eroded products from Djebel

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Sidi Abderrahmen anticline. Two principal geological units form this aquifer (Fig. 2b)

138

(Tarhouni et al., 1996; Paniconi et al., 2001; Ennabli, 1980; Kerrou et al., 2010). The first unit

139

was formed during the Pliocene and Quaternary ages by deposition of eroded products from

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the Djebel Sidi AbedErrahmen anticline and the Dakhla syncline. The Pliocene formation is

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sandstone with alternating marl units and having a mean thickness of 85 m. The Quaternary

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alluvium is composed of detrital sediments (sand, gravel, and silt) with thin clay lenses and

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has a thickness that varies between 20 m and 25 m. The second unit is constituted of a

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Miocene marl formation underlying the first unit (Paniconi et al., 2001). Transmissivities in

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the Korba aquifer range from 5 × 10-5 to 10-2 m2/s (Ennabli, 1980). Generally, the aquifer has

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good hydrodynamic characteristics (e.g., Zghibi et al., 2011; Ennabli, 1980; Kerrou et al.,

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2010; Paniconi et al., 2001). The highest permeability values, reaching 7.4 × 10-3 m/s, are

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found in the north of Somâa and in the center zone. The higher value related to the Plio-

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Quaternary units is related to the most productive and highest transmissivity about 10-2 m2/s

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(Kerrou, 2008). These obtained values of hydraulic conductivity are in tight agreement with

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the lithological and geological settings of aquifer. The hydraulic conductivity estimated by

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Ennabli (1980) is about 10-4 m/s for the Quaternary calcarenitic limestone and about 5 × 10-5

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m/s for the Pliocene deposits. The minimum value was displayed in proximity to Menzel

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Temime City (10-4 m/s) in the north. The smaller values were attributed randomly to the less

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permeable lithofacies, the intermediate values were attributed to the sandstone and the most

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permeable values were attributed to the sand (Zghibi et al., 2011).

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4. Agricultural activity

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The cultivation of the fields in the Governorate of Nabeul began in the late 1970s (Ennabli,

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1980), and the area has evolved into an advanced agricultural production zone in Tunisia.

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The soils of the study area are mainly fluvisols and have predominantly silt loam texture. The

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major land use in the region is intensive agriculture. Vegetables (potatoes, tomatoes,

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strawberries,…), vineyards plantations and citrus are the main crops, as can be seen from Fig.

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3a. Other minor crops, namely: olives, wheats and tobacco are also cultivated in the area. 5

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The irrigated area located mainly between Korba, Diarr El Hojjej, El Mida and Menzel Horr,

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covers over 5000 ha, representing 7.7% of the entire irrigated public soil surface in Tunisia

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(Khlaifi, 1998).

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The high nitrate levels are largely caused by man, particularly related to the use of nitrogen

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fertilizers and manure, which constitutes a diffuse source (EEA, 2000).

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The fertilizers comprise three major nutrients: nitrogen, phosphorus, and potassium.

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addition, large quantities of commercial fertilizers such as urea [CO(NH2)2], N–P–K

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fertilizers, ammonium sulfate, magnesium sulfate and potassium chloride are applied during

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the growing season, but some fertilizers are also applied during winter to enhance the growth

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of fruits and vegetables cultivated in greenhouses.

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It was very hard to know the applied quantities but Grava (2005) estimated that N-fertilizer

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was applied at 80 - 120 kg N/ha during the cropping practice in 2004.

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Nitrate in groundwater can also result from different sources, including septic tank, sewage

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discharge and the oxidation of organically bound nitrogen in soils (Appelo and Postma, 2005).

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The local water management authorities inventoried more than 9000 wells pumping altogether

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around 54 Mm3/year (CRDA, 2005).

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Most of these wells are located in very small private farms and are traditionally dug and

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equipped with oil motor pumps. In 2004, the total amount of irrigation water is approximately

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ranging between 37.5 – 40 Mm3/year (Fig. 3b). Regional groundwater is heavily exploited

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despite the efforts of the government to mobilize most of the regional surface water and to

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transport water from the north of the country via the Medjerda Cap-Bon canal. Moreover, in

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the beginning of the 90’s, the government encouraged the farmers to adopt drip irrigation

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systems by contributing to 60% of the cost of the installation in order to reduce water

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consumption. The result was that irrigated surfaces increased at the coast of the same water

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consumption (Kerrou, 2008).

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5. Materials and methods

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In June 2005, 35 water samples were collected in the study area from piezometers and

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irrigation wells. Wells were chosen in order to achieve a relatively uniform distribution of the

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samples within the study area. The location of the well was precisely recorded using a Global

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Positioning System (GPS) receiver (Fig. 4).

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Also, at the time of sampling, basic information such as well location and depth, contact

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information of the cropping system and land use type surrounding the well were all recorded.

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In

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Furthermore, the groundwater quality can be described directly in observation wells by

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electrical conductivity measurement. Therefore, the electrical conductivity was measured at

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the wellhead in five wells (A3, A10, A11, A12 and A13) located between Diarr El Hojjaj and

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Menzel Horr (Fig. 5b).

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The groundwater chemistry of Korba aquifer of Cap-Bon has been studied in terms of the

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major ionic constituents Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3-, NO3 , Total Dissolved Solid

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(TDS) and of the physical parameters (pH, electrical conductivity and the temperature).

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Parameters measured in the field were temperature, pH, and conductivity, using a multi-

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parameter WTW Universal Conductivity Meter.

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Water samples were collected from pumping wells after minimum of several hours of

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pumping prior to sampling. Samples were collected into 250 cm3 polyethylene bottles without

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preservation. All the samples were stored in an ice chest at a temperature lower to 4 °C and

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later transferred to the laboratory of Water Sciences and Techniques at the National

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Agronomic Institute of Tunisia.

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The samples used for anions and cations were analyzed using the DX100 and DX120 Ion

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Chromatograph. The analyses of SO4 were undertaken by a gravimetric method, those of

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HCO3, Cl, and Ca by the titrimetry method and Mg and Na were analyzed by spectrometry of

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atomic adsorption (SAA).

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Samples were analyzed using a spectrophotometric method to measure NO3 –N concentration

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(Rodier, 1984). Nitrate concentrations were obtained after multiplying the measured NO3 –N

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concentration by 4.43 (Aouissi et al., 2012).

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The charge balance errors for the analyses were generally within ∓5%.

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The statistical summaries of the major hydrochemical parameters in the Korba aquifer from

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year 2005 are presented in Table 1.

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The seawater fraction in the groundwater is often estimated using chloride concentration since

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this ion has been considered as a conservative tracer, not affected by ion exchange (Custodio,

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1987) and it is calculated as follows (Appelo and Postma, 2005):

_

_

_

f sea 

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CCl ,sample  CCl , fresh ClCl ,sea  ClCl , fresh

(1)

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Where CCl, sample is the Cl- concentration of the sample, CCl, sea is the Cl- concentration of the

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Mediterranean sea and CCl, fresh represents the Cl- concentration of the fresh water.

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The fresh water sample will be chosen considering the lowest measured value of the electrical

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conductivity (Slama, 2010). In effect, Cl- is not usually removed from the system due to its

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high solubility (Appelo and Postma, 1993). The only inputs are either from the aquifer matrix

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salts or from a salinisation source like seawater intrusion etc (Kouzana et al., 2009).

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Once calculated, the seawater fraction is used to calculate the theoretical concentration of

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each ion i resulting from the conservative mixing of seawater and the fresh water:

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Ci ,mix  f sea × Ci ,sea  (1  f sea ) × Ci , fresh

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where Ci,sea and Ci,fresh are the concentration of the ion i of respectively seawater, and fresh

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water.

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For each ion i, the difference between the concentration of the conservative mixing Ci,mix and

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the measured one Ci,sample simply represents the ionic deltas (∆) (Fidelibus et al., 1993)

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resulting from any chemical reaction occurring with mixing:

(2)

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Ci  Ci ,sample  Ci ,mix

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In addition, the calculation of these ionic deltas is important for determining and quantifying

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the hydrogeochemical processes and potential chemical reactions that take place in the aquifer

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(Slama et al., 2010; Pulidio-Leboeuf, 2004; Grassi and Cortecci, 2004). So, when ∆Ci is

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positive, groundwater is getting enriched for ion i whereas a negative value of ∆Ci indicates a

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depletion of the ion i compared to the theoretical mixing (Andersen et al., 2005; Slama, 2010).

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6. Results and Discussions

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6.1. Seawater Intrusion

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Several factors probably led to the seawater intrusion into the aquifer system: (1) groundwater

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extraction for industrial and agricultural use, (2) increased pumping times and (3) damming

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wadis Chiba in 1963, M’Laabi in 1964 and Lebna in 1986.

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Thus, building these dams caused a decrease of the groundwater table downstream due to a

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reduction of recharge by its effluents (Kerrou, 2008). In addition, this situation caused

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groundwater level to decline steadily leading to the creation of a wide depression between

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Diarr El Hojjej and Tafelloun villages where the measured hydraulic head reached 12 m

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below the mean sea level in 2005 (Fig. 5a). So, the potentiometric maps of 2005 allow to

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highlight a powering of the hydraulic heads below sea level and an estimation of the annual

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drawdown of 2 meters.

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(3)

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In order to obtain new information on the spatial scales and dynamics of the fresh water-

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seawater interface of coastal groundwater exchange, electrical conductivity (EC) methods was

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conducted with a multielectrode profiler at the center part of Korba aquifer (Fig. 5b),

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precisely at the concentric piezometric depression between Diarr El Hojjej and Tafelloun, at

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1.5 km from the coastline correspond to Piezometers (PZ) A-10, A-11 and A-13 and near to

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Mediterranean sea represented by Piezometers A-3 and A-12, measured at 1 m intervals from

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the water surface. Rapid increase of EC is seen at depths of 5 m below sea-level in

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Piezometers A-3 and A-12 at 1 km far from sea coastline (Fig. 5c). This is the result of the

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increase in bulk conductivity with increasing pore water conductivity. These depths represent

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the interface between freshwater and saltwater. Depth profiles of conductivity showed a sharp

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gradient between fresh groundwater above and saline groundwater below. Also at PZ: A-11,

268

A-13 and A-10 located on the piezometric depression, far to the coast, there is a gradual

269

increase in EC from a depth of 10 m to about 40 m below sea level. Their EC values are

270

respectively 13, 18 and 35 mS/cm. Obviously, the concentric depression of 12 m below sea

271

level led to further inland propagation of the salt-water wedge and accelerated seawater

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intrusion by reversing the hydraulic gradients and saltwater up-coning. EC in the lower

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aquifer was around 30 mS/cm. However, in the upper aquifer, brackish water (3–6 mS/cm)

274

has been found in some sectors. Thus, the land uses around investigated Piezometers include

275

industrial greenhouses, citrus fields, and private farm fields where the uses of fertilizers

276

contribute to the high EC values. This finding has been interpreted as being due to the

277

recycling of irrigation water (Zghibi et al., 2012).

278

From Table 1, the averages of the major ions are within acceptable ranges for domestic and

279

irrigation uses (Yidana et al., 2010). However, there are some cases of extreme values which

280

suggest some local pollution cases.

281

Thus, the average of EC values is 5.93 mS/cm, corresponding to total dissolved solids (TDS)

282

of 4074.08 mg/l exceeding the WHO (2008) TDS limits for potable water.

283

The main contributors to the groundwater salinity are Cl-, Na+, Mg2+, Ca2+, K+ and SO42- that

284

shows a great value of concentration (Table 1) respectively 47.7, 37.53, 12.04, 12.08, 0.6 and

285

7.13 meq/l. These higher scores of concentration are the result of seawater intrusion into the

286

aquifer system from the eastern boundary of the study area.

287

From Fig. 6a, the well sampling analysis shows a very high electrical conductivity (EC) with

288

80% of the samples have an EC greater than 3 mS/cm.

289

The zone where the fresh water could be found is located in the south and the north (Grava,

290

2005) but the center part of aquifer represents the most contaminated area where the presence 9

291

of shallow depth piezometric level within the coastal area favourable to seawater intrusion by

292

reversing the hydraulic gradients.

293

Furthermore, the seawater phenomena is confirmed from the seawater percentage distribution

294

map showing in Fig. 6b, where 62% of the whole area presents values more than 25% of

295

seawater. The seawater intrusion has advanced far inland (approximately a distance of 3 km)

296

and indicates a relatively narrow transition zone between seawater and freshwater.

297

The extent of seawater intrusion is indicated by the distribution of chloride as shown in Fig.

298

6c. The chloride concentration was measured up to 120 meq/l near to Menzel Horr and Lebna

299

villages. From the coast to the inland, especially at a distance of 1.5 km from the coast

300

between Diarr El Hojjej and Tafelloun, chloride concentration attaint 350 meq/l, showing a

301

change that could be attributed to saltwater upconing that occurs in coastal aquifers due to

302

over-pumping of the groundwater reserve.

303

Based on these distributions, Tarhouni et al. (2002) estimated that the rate of seawater

304

intrusion in the study area was approximately 0.8 km/yr which resulted from intensive

305

pumping.

306

The total dissolved solids (TDS) values in the Korba aquifer range between 690 mg/l and

307

26850 mg/l, with an average value of 4074 mg/l (Table 1). From Fig. 6d, the lower TDS

308

values (<2000 mg/l) occur at the southwestern part of the region, between the villages of

309

Diarr Ben Salem and Tazerka as well as in the north part around Menzel Temime village.

310

However, in the center area and along the coastal zone, the TDS values area dramatically

311

increased, ranging in values between 4000 and 30000 mg/l.

312

Towards the piezometric depression of Tafelloun – Diarr El Hojjej villages, a progressive

313

increase of TDS is observed, which is attributed to: (1) seawater intrusion from Mediterranean

314

sea and (2) various human activities such as the construction of septic tanks, agricultural

315

activities and the use of saline water for irrigation. Thus, there are many wells which receive

316

pollution from domestic and animal effluents.

317

The water quality of each sample was plotted on a classical Piper diagram, referring to the

318

concentrations of hydrochemical components (Piper, 1944) (Fig. 7).

319

The Piper diagram was preferred to other conventional methods because it allows a more

320

precise identification of the water samples and some dominant geochemical processes in the

321

water chemistry (Cusimano et al., 2006).The groundwater is sodium–calcium-chloride type

322

and this composition is controlled by the interaction of the water with the sediments of

323

alluvial deposits. Thus, the diagram shows that a number of wells plot on the Theoretical

324

Mixing Line (TML) indicating that mixing processes between seawater and freshwater are 10

325

taking place. If simple mixing between seawater and fresh groundwater takes place, there will

326

be a perfectly linear relationship between the electrical conductivity and the chloride

327

concentration (Milnes et al., 2006).

328

Therefore, Fig. 8a shows the relationship between electrical conductivity (mS/cm) and the

329

chloride concentration (meq/l) in the Korba aquifer and reveals a rather good correlation for

330

most samples.

331

Nevertheless, we can see some samples which are located in the higher domain of the simple

332

mixing line can either result from pure mixing between chloride enriched freshwater with

333

seawater overprinted by an additional chloride source (Milnes et al., 2006) and suggest the

334

presence of several possible salinisation processes.

335

The bivariate diagram between sodium and chloride shows an increase of sodium linearly

336

with chloride (Fig. 8b). Most ground samples have a Na+ versus Cl- mostly in agreement with

337

the mixing line between seawater and freshwater, the latest defined by the sample chosen

338

considering the lowest measured value of the electrical conductivity (Slama, 2010). This

339

enrichment in Na+ relative to Cl-, and the corresponding depletion in Ca2+ (Fig. 7c) and Mg2+

340

(Fig. 8d) over Na+ at some points, suggests a strong water aquifer interaction related to

341

dilution of halite and direct cation-exchange reactions between groundwater and the clay

342

fraction of the aquifer material. This reaction takes place during the refreshening stage when

343

freshwater flushes seawater in a coastal aquifer, displacing the freshwater/saltwater interface

344

towards the Mediterranean sea (El Yaouti et al., 2009). In addition, the concurrence of the

345

sodium and chloride values with the mixing line is due to the high solubility product of halite

346

deposits relatively abundant in this part of the basin (Ben Hammouda et al., 2010), which is

347

considered as the main source of both elements. However, some samples show specific

348

deviations from the mixing line which could be attributed to water–soil interaction, suggesting

349

additional processes of chloride and sodium enrichment (Carol et al., 2009). In particular, a

350

sample from the coastal plain and located on the piezometric depression shows sulphate

351

enrichment versus chloride (Fig. 8e) associated with calcium enrichment and/or depletion

352

(Fig. 8c). These samples present a large derivation from the theoretical line of mixing as a

353

result of their higher value of SO42- and Ca2+ content. The enrichment of these points with

354

SO42- suggests other sources, including the dissolution of gypsum and agricultural

355

contamination (where some of these water samples recorded high NO3 content).

356

The most likely source of this sulphate is from dissolution of the small amounts of gypsum

357

scattered through the aquifer, present in the catchment area or the evaporation of the irrigation

_

11

358

water excess (Kouzana et al., 2009). Therefore, SO42− level in groundwater can be explained

359

partially by gypsum dissolution and partially by surface sources. Gypsum is widely

360

distributed in subsurface layer of the aquifer and in general, its dissolution produces mineral

361

SO42− as (Suthar et al., 2009): (4)

362

CaSO4 .2H 2O  Ca 2  SO42  2H 2 0

363

Calcium is another important product of this reaction reflecting the cation exchange and it

364

directly contributes to hardness situation of groundwater. However, calcite precipitation or

365

dissolution are suggested as the factors that remove the Ca2+ from groundwater and decrease

366

its content (Pulido-Leboeuf, 2004; Pulido-Leboeuf et al., 2003).

367

In order to more thoroughly identify the processes that modify the theoretical content and to

368

determine the behavior of these cations, the ionic delta was plotted versus chloride content for

369

each of the cations in question (Fig. 8f). The majority of samples are depleted in Na+ and

370

enriched in Ca2+ (and Mg2+). The loss of Na+ and gain of Mg2+ and clearly Ca2+ suggests a

371

direct cation exchange usually observed in similar situations when the seawater is replacing

372

fresh water (e.g., Zghibi et al., 2012; Slama et al., 2010; Yaouti et al., 2009; Kouzana et al.,

373

2009; Appelo and Postma, 2005). However, some samples are always or seasonally depleted

374

in Mg2+ and enriched in Ca2+ indicating a possible exchange between these two cations. There

375

is absorption of Mg2+ and a release of Ca2+ from the clay minerals in the case of salinisation,

376

while the reverse occurs in the case of dilution (desalinization) in some samples. However,

377

the excess of Ca2+ suggests the existence of other sources contributing to the enrichment of

378

groundwater. In this case, gypsum dissolution due to dedolomitization as well as fertilizers

379

can be cited as potential sources of Ca2+ (e.g., Custodio, 1987; Slama et al., 2010; Milnes and

380

Renard, 2004; El Yaouti et al., 2009).

381

6.2. Nitrate Contamination

382

The Korba unconfined aquifer supports extensive irrigation activity and is therefore subject to

383

contamination by nitrate (NO3 ).

384

Fig. 9 shows the spatial distribution of nitrate concentrations, characterized by the highest

385

nitrate level. They vary in a wide range, between 20 and 150 mg/l.

386

The same Figure reveals that nitrate concentration above the drinking water standard (50

387

mg/l) covered an area of 122.64 km², which accounted for 28% of the whole area.

388

The most contaminated wells are located mainly in the central part of the study area,

389

especially between Diarr El Hojjej, Menzel Horr, Tafelloun and Lebna villages.

_

12

390

In this area, a considerable quantity of fertilizers, domestic sewage and industrial wastewater

391

with high level of NO3 was drained into the surrounding lands, which resulted in high NO3

392

concentrations in the groundwater.

393

However, in the north and the south, the nitrate concentrations in the groundwater was low

394

due to the presence of fast-growing trees with deep roots (citrus, Olives, Vineyards,…), that

395

are able to assimilate nitrate from deeper soil, lessening NO3 contamination (Cheng et al.,

396

2010). Also, the presence of few wadis and a small population might be the reason for that.

397

In addition to the perturbation of surface water regime, the growth of groundwater abstraction

398

for the 21st century (54 Mm3 in 2005) especially near the coast, lead to the appearance of a

399

concentric depression of 12 m below sea level with a diameter of approximately 5 km in the

400

region of Diarr El Hojjej and Tafelloun villages (Fig. 5a). This means that the hydraulic

401

gradients generally homogeneous (2–5‰) were reversed mainly toward the central part of the

402

aquifer leading to an acceleration of seawater intrusion.

403

The quite hydraulic gradients associated with the depressions promote the transportation of

404

contaminants toward the center of the cone and may be the reason that locally high

405

concentrations of nitrates occur in the groundwater (Cheng et al., 2010).

406

Thus, nitrate concentrations distribution in Korba aquifer is similar to electric conductivity

407

and chloride concentration maps. The EC (Fig. 6a), Cl- (Fig.6c) and NO3 (Fig. 9) contour

408

maps show that the lowest values were found in the north and the south sector of the study

409

area (EC < 4 mS/cm, Cl < 40 meq/l and NO3 < 30 mg/l). However, the center part of Korba

410

aquifer is the most contaminated area. It can clearly be seen that the area that was identified as

411

seawater intrusion dominated has the biggest nitrate concentrations values.

412

In addition, it is interesting to investigate the relationship between nitrate concentration and

413

seawater intrusion. Fig. 10a revealed that, in general, some samples mixed with seawater, at a

414

ratio above 5%, are less concentrated with regards to nitrate. On the others hand, it clearly

415

indicates that groundwater of some rural areas of Korba aquifer like Diarr El Hojjej,

416

Tafelloun, Lebna and Menzel Horr was severely polluted in terms of NO3 level with regards

417

to seawater mixing.

418

We concluded that groundwater NO3 concentrations were higher in areas with fast-growing

419

agriculture and industry, large populations, abundant wadis, and over-exploitation than other

420

areas.

_

_

_

_

_

_

_

_

13

_

_

_

421

In the case of contamination by nitrate, the ratio NO3 /Cl versus Cl (Fig. 10b) can be used as

422

an indicator to identify seawater mixing and anthropogenic pollution (Park et al., 2005).

423

Saline water typically presents a low ratio, because the majority of sample show a small value

424

of concentration in NO3 with respect to chloride and so the relative ratio is close to 0

425

confirming seawater intrusion process into freshwater. A second group of sample shows an

426

increase in the NO3 /Cl ratio indicating a strong anthropogenic contamination. This

427

relationship is particularly important as it gives a measure of the contamination caused by

428

products rich in NO3 used in agriculture. Thus, salinisation is not only the result of typical

429

seawater ions addition but also of those elements used in agriculture as NO3 , K , SO4 and

430

Ca that increase the value of the TDS (Grava, 2005).

431

Therefore, the nitrate contamination is a result of the local hydrogeological setup coupled

432

with the traditionally applied flood irrigation and the complete lack of environmental

433

awareness regarding the over-fertilisation (Ben Moussa et al., 2010). The plot of SO4 versus

434

NO3 (Fig. 10c) exhibits a well defined relationship which indicates an origin of SO4 and

435

NO3 related to either the gypsum dissolution and partially by the contribution of SO4-

436

fertilizers (Gi-Tak et al., 2004), commonly used in Korba agricultural areas. Such good

437

correlation indicates the contamination of groundwater with nitrate and sulphate form

438

different sources, i.e. fertilizers, sewage and animal wastes (Suthar et al., 2009). Furthermore,

439

the positive relationship between NO3 and Ca

440

are utilized in the highly polluted regions of Tafelloun and Diarr El Hojjej as Ca(NO3)2−

441

fertilizers (Stigter et al., 2006; Ben Moussa et al., 2008).

442

7. Conclusions

443

The hydrogeochemical investigation carried out in June 2005 in Korba aquifer allowed

444

distinction between two main salinisation processes: (a) seawater intrusion within the center

445

area where the presence of shallow depth piezometric level is favourable by reversing the

446

hydraulic gradients and (b) contamination by nitrate caused mainly by extensive irrigation

447

activity.

448

Thus, growth of groundwater abstraction, increasing of pumping times combined with

449

damming wadis (Chiba, M’Laabi and Lebna) contributes to the reduction of the recharge by

450

its effluents and accelerate seawater intrusion into aquifers from Mediterranean sea.

_

_

_ _

+

2-

2+

2_

2-

_

_

14

2+

(Fig. 10d) also suggests that both elements

451

The mixing of seawater with fresh-brachich water was analyzed and confirmed by using ionic

452

deviations, Piper Diagram, seawater fractions and binary diagrams.

453

Nevertheless, mixing freshwater-seawater was not conservative and accompanied by other

454

geochemical processes. This investigation finds that the major water type is the Na‐Ca‐Cl

455

water type and the mineralization results from the dissolution of abundant halite and gypsum

456

in the studied area. In the second step, these methods combined the spatial distribution of

457

variables with nitrate concentrations in groundwater throughout the classified range and

458

helped us understand the relationships between the effects of agricultural activities and

459

seawater on groundwater contamination.

460

The average of NO3

461

concentration of nitrate reaches values as high as 150 mg/l.

462

The highest NO3 concentration was mainly located within the central region and surrounding

463

Menzel Horr and Tafelloun, which may be attributed to the expanding of irrigated agriculture

464

area.

465

Irrigation by fertilizers as SO4-fertilizers and Ca (NO3)2-fertilizers, sewage and animal wastes

466

enormously increased groundwater NO3 , whereas vegetables citrus plantations, vineyards,

467

chilly peppers, and Tobacco cropping systems promoted an increase in groundwater NO3

468

concentrations accompanied with well irrigation. Nevertheless, rainfall from the upper soil

469

layer, soil texture and quality, other agricultural residual substances and the accumulative

470

amount of nitrate in soil profile all influence groundwater NO3 concentrations, which merit

471

further investigation. The results from this study can be used to propose better groundwater

472

management policies in order to minimize the negative anthropogenic impact on the aquifer in

473

the irrigation area in the Cap-Bon peninsula.

_

concentration in 2005 was 30.44 mg/l. In several zones, the

_

_ _

_

474 475

Acknowledgements

476

We wish to express our gratitude to the Editor and the referee for their careful readings of the

477

first version of this manuscript. Their comments, suggestions, and remarks proved

478

indispensable in helping us improve the style and presentation of this paper.

479 480

15

481 482 483 484 485

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Table Table 1 Summary of the concentrations of the major hydrochemical parameters in the Korba aquifer (June 2005) Parameters

Mean

Minimum

Maximum

EC (mS/cm)

5.93

0.99

39.1

TDS (mg/l)

4074.08

690

26850

Temp. (°C)

19.23

16.2

22

pH

39.07

6.4

520.8

Ca2+ (meq/l)

12.08

2.04

67.71

Mg2+(meq/l)

12.04

1.8

88.65

Na+(meq/l)

37.53

2

K+(meq/l)

0.6

0.05

3.786

Cl-(meq/l)

47.7

3.77

361.23

SO42-(meq/l)

7.13

0.1

59.7

HCO3-(meq/l)

3.03

0.02

7.3

NO3 –N (meq/l)

4.8

0.09

28.83

% seawater

6.66

0.06

54.67

∆Na+

-0.507

-21.75

22.82

∆Ca2+

5.96

-2.27

31.2

∆ Mg2+

-0.155

-18.41

16.26

_

309.87

Figure

Fig. 1. (a) Location of Tunisia, (b) location of Cap-Bon peninsula and (c) topographic framework of the study area (square size is 10 x 10 km).

Fig. 2. (a) Geologic map of the Korba aquifer system of Cap-Bon: 1. Eocene sandstone; 2. Oligocene marl; 3. Oligocene sand; 4. Miocene marl; 5. Miocene sandstone; 6. Pliocene sand; 7. Tyrrhenian sandstone; 8. Quaternary alluvial deposits; 9. Marine Quaternary and (b) simplified geological cross-section (A-A’) through the aquifer system.

Fig. 3. (a) Land use map of the Korba aquifer of Cap-Bon (1. Dam; 2. Wheats; 3. Olives; 4. Vineyards and citrus; 5. Vegetables; 6. Pastures) and (b) evolution of amount of irrigation water (Mm3).

Fig. 4. Sampling map of shallow groundwater (1. Dam; 2. Wadis; 3. water samples) of the Korba aquifer of Cap-Bon.

Fig. 5. (a) Potentiometric map of Korba aquifer of Cap-Bon and flow direction (June 2005); (b) the location of Piezometers investigated by electrical conductivity methods and (c) electrical conductivity logs carried out at five Piezometers showing the freshwater-saltwater relationships.

Fig. 6. Spatial distributions of (a): electric conductivity (mS/cm); (b) seawater percentage; (c) chloride concentration (meq/l) and (d) TDS (mg/l) of the Korba aquifer.

Fig. 7. Water sampling analytical results plotted in the Piper Diagram of the Korba aquifer.

Fig. 8. (a) Electrical conductivity (mS/cm)/Cl- (meq/l) relationship; (b) Na+/Cl- relationship; (c) Ca2+/Cl- relationships; (d) Mg2+/Cl- relationships; (e) SO42-/Cl- relationships and (f) ∆Na+, ∆Ca2+ and ∆Mg2+ versus Chloride (meq/l) of groundwater samples of the Korba aquifer of Cap-Bon.

Fig. 9. Distribution of Nitrate concentrations (mg/l) in the Korba aquifer, showing high concentration in the piezometric depression area.

_

_

Fig. 10. (a) Seawater fraction versus NO3 (meq/l); (b) Chloride versus NO3 /Cl- contents; (c) _

_

SO42-/NO3 relationship and (d) Ca2+/NO3 relationship for all measured samples in Korba aquifer of Cap-Bon.

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Abstract In recent years, seawater intrusion and nitrate contamination of groundwater have become a growing concern for people in rural areas in Tunisia where groundwater is always used as drinking water. The coastal plain of Korba (north-east of Tunisia) is a typical area where the contamination of the aquifer in the form of saltwater intrusion and high nitrate concentrations is very developed and represents the major consequence of human activities. The objective of this study is to evaluate groundwater resource level, to determine groundwater quality and to assess the risk of NO3- pollution in groundwater using hydrogeochemical tools. Groundwater were sampled and analysed for physic-chemical parameters : Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3-, NO3-, Total Dissolved Solid and of the physical parameters (pH, electrical conductivity and the temperature). The interpretation of the analytical results is shown numerically and graphically through the ionic deviations, Piper Diagram, seawater fractions and binary diagrams. Moreover, electrical conductivity investigations have been used to identify the location of the major intrusion plumes in this coastal area and to obtain new information on the spatial scales and dynamics of the fresh water–seawater interface. Those processes can be used as indicators of seawater intrusion progression. First, the hydrogeochemical investigation of this aquifer reveals the major sources of contamination, represented by seawater intrusion. Thus, the intensive extraction of groundwater from aquifer reduces freshwater outflow to the sea, creates several drawdown cones and lowering of the water table to as much as 12 m below mean sea level in the center part of the study area especially between Diarr El Hojjej and Tafelloun villages, causing seawater migration inland and rising toward the wells. Moreover, the results of this study revealed the presence of direct cation exchange linked to seawater intrusion and dissolution processes associated with cations exchange. Second, a common contaminant identified in groundwater is dissolved nitrogen in the form of nitrate. The average nitrate concentration of the aquifer is about 30.44 mg/l, but contents as great as about 50 mg/l occur in the central region where seawater has been identified. Nitrate survey reveals that nitrate concentration above the drinking water standard (50 mg/l) covered an area of 122.64 km², which accounts for 28% of the whole area. Irrigation with the nitrogen fertilizers, domestic sewage, industrial wastewater and movement of contaminants in areas of high hydraulic gradients within the drawdown cones probably are responsible for localized peaks of the nitrate concentration. It is suggested that risk assessment of nitrate pollution is useful for a better management of groundwater resources, preventing soil salinisation and minimizing nitrate pollution in groundwater. Keywords: Seawater intrusion; Over-exploitation; Cap-Bon; Nitrate contamination; Salinisation processes; Groundwater resources

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Highlights 

The groundwater resources of Korba are very important for public water supply

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Intensive agriculture and seawater intrusion are the main source of pollution

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We evaluate the groundwater vulnerability to pollution using hydrogeochemical tools

 There are a relationship between seawater intrusion and agricultural contamination 

Mixing freshwater-seawater accompanied by other geochemical processes