Hydrogeological investigations and groundwater vulnerability assessment and mapping for groundwater resource protection and management: State of the art and a case study

Accepted Manuscript Hydrogeological investigations and groundwater vulnerability assessment and mapping for groundwater resource protection and management: state of the art and a case study Ismail chenini, Adel Zguibi, Lamia Kouzana PII: DOI: Reference:

S1464-343X(15)00118-1 http://dx.doi.org/10.1016/j.jafrearsci.2015.05.008 AES 2275

To appear in:

African Earth Sciences

Received Date: Revised Date: Accepted Date:

30 December 2014 12 May 2015 13 May 2015

Please cite this article as: chenini, I., Zguibi, A., Kouzana, L., Hydrogeological investigations and groundwater vulnerability assessment and mapping for groundwater resource protection and management: state of the art and a case study, African Earth Sciences (2015), doi: http://dx.doi.org/10.1016/j.jafrearsci.2015.05.008

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Hydrogeological investigations and groundwater vulnerability assessment and mapping

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for groundwater resource protection and management: state of the art and a case study 1*

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Ismail chenini, 1Adel Zguibi and 2Lamia Kouzana

UR13ES26, Department of Geology, Faculty of Sciences, Mathematics, Physics and Naturals of Tunis,University of Tunis El Manar, El Manar, 2092 Tunisia 2

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ISSTE Institute, University of Carthage, Tunisia

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*Corresponding author:

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Ismail Chenini

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Adress: UR13ES26, Department of Geology, Faculty of Sciences, Mathematics, Physics and Naturals of Tunis,University of Tunis El Manar, El Manar, 2092 Tunisia

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E-mail: [email protected]

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Tel: 00216 52952335

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Fax: 00216 71885408

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Abstract

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The available litterature was used in this work to review the methodologies for groundwater

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vulnerability mapping. The objective of the litterature review is to define the vulnerability

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concept and to discuss the best way to establish aquifer vulnerability maps and the utilities of

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these maps for groundwater protection. In this study, we explore the hydrodynamic properties

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of the Grombalia aquifer system in north Tunisia to evaluate the vulnerability of groundwater.

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The established vulnerability maps are used for groundwater managing and protection. In

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Grombalia basin, the groundwater resource is used for agriculture and drinking purposes. The

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intrinsic vulnerability of the phreatic aquifer of Grombalia is mapped using the standard

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DRASTIC, a modified DRASTIC and a DRIST models. The adopted methodology for the

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intrinsic vulnerability mapping is based on the hydrogeological system properties. The

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vulnerability index calculation was used to establish a map with areas of vulnerability degree.

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This method is based on the combination of various topographical, lithological and

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hydrogeological data using Geobased Information System software. These methods consider

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the attribution of a numeric index to each considered parameter. In the established map, 26%

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of the aquifer extension is vulnerable according to standard DRASTIC model. The modified

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DRASTIC method, which considers the vadose zone heterogeneity and the aquifer geometry,

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showed that 17% of the studied area is occupied by a high vulnerability. The application of

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the DRIST model showed a high vulnerability in area covering 66% of the extension of the

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shallow aquifer of Grombalia. This important vulnerability is due mainly to vertical

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parameters implicated in the infiltration of the pollutant. The established vulnerability maps

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provide recommendations for groundwater resource protection in the aquifer system of

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Grombalia. We conclude that the three used models for vulnerability assessment and mapping

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reveal the susceptibility of the aquifer system to the special effects of pollutants.

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Keywords: Hydrogeology, Groundwater vulnerability, Groundwater management, GIS,

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Tunisia.

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1. Introduction

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Because of the auto epuration function of the reservoir, groundwater is protected to the

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contamination. The surface water is the most sensitive to the pollution. Nevertheless, if the

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water resource is contaminated, it is not easy to modify its quality. Moreover, the groundwater

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quality is closely related to the lithology and the thickness of the vadose zone and the

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geometry of the reservoir. All the hydrogeological aspect of the aquifer system such as

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recharge zone, groundwater flow and land use must be involved in the evaluation of the water

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resource quality.

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Assessing the vulnerability of groundwater resource is a preventive tool for controlling

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groundwater contamination. (Farjad et al., 2012). Aquifer system protection is necessary for a

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sustainable use and protection of the groundwater resources (Gogu et al., 2003; Liggett and

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Talwar, 2009; Demiroğlu and Dowd, 2014). The aquifer protection issues are discussed using

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the groundwater vulnerability concept. Groundwater vulnerability to the pollution is a

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dimensionless parameter which is not directly measurable. The vulnerability is also identified

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as the hazard of the groundwater linked to the vadose zone lithology and the properties of the

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contaminant (Babiker et al., 2005; Musekiwa and Majola, 2013; Demiroğlu and Dowd, 2014).

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The vulnerability of an aquifer to the pollution is related to many parameters such as:

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lithology of the aquifer, geometry of the reservoir and hydrogeology (Varol and Davraz,

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2010; Moratalla et al., 2011). The available models for the assessment of the groundwater

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vulnerability are based on the combination of several hydrogeological parameters involved in

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the contamination process of groundwater.

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The first use of the vulnerability concept in hydrogeology was from 1970 (Albinet and

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Margat, 1970). The vulnerability concept was described based on the effect of the vadose

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zone to protect the groundwater quality. In fact, the vadose zone can play a key role to

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eliminate some pollutants infiltrated from surface water. From 1980's, various models and

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approaches for the vulnerability assessment and mapping have been developped and tested all

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over the world (Haertle, 1983; Aller et al., 1987; Foster, 1987; Foster and Hirata, 1988). The

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process of groundwater vulnerability mapping combines hydrogeological parameters of the

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aquifer to establish a map with a zoning related to the susceptibility of groundwater

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contamination by pollutant (Foster et al., 2002).

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The objectives of the manuscript are the establishment of the groundwater vulnerability map

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of the shallow aquifer of Grombalia and the elaboration of the groundwater contamination

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risk map of the aquifer system. The adopted approach is summarized in the following steps:

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(1) geological identification of the aquifer system; (2) geometry of the aquifer; (3)

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hydrodynamic characterization of the aquifer system; (4) intrinsic Vulnerability assessment

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and mapping (using DRASTIC model, Modified DRASTIC model and DRIST model); and

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(5) Comparison of the 3 generated vulnerability maps and elaboration of map showing the

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contamination risk map of the shallow aquifer of Grombalia. The overall objective of this

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study is to generate and compare groundwater maps of intrinsic vulnerability and risk using 3

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vulnerability assessing models, Geographical Information Systems (GIS) and available

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geological and hydrogeological data.

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2. Review of methodologies for aquifer vulnerability mapping

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The vulnerability to contamination of an aquifer system is a concept directly related to its

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sensitivity to pollutant (Vrba and Zaporozec, 1994). The vadose zone has an important

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function in the pollution of groundwater. It is the part of aquifer where the infiltaration and

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the transport of the contaminant takes place. Thus, the integration of the vadose zone in the

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vulnerability evaluation become with higher importance to have idea about the groundwater

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contamination susceptibility. As presented and defined in previous sections, the groundwater vulnerability can be assessed and mapped in two manners:

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- The intrinsic vulnerability which is assessed and mapped based on the hydrogeological properties of the aquifer system (Civita, 1994),

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- The specific vulnerability is related to some specific pollutants. It characterizes the

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sensitivity of groundwater to be contaminated by a specific contaminant (Schnebelen et al.,

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2002).

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The evaluation of the intrinsic vulnerability is the result of the superimposition of many maps

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reflecting hydrogeological parameters. The resulting map of the vulnerability mapping

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process is a zoning of the aquifer extension with specific degree of vulnerability (Farjad et al.,

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2012). A variety of models are available to assess and map the groundwater vulnerability. We

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can distinguish the following methods for the vulnerability assessment: (1) an approach using

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the vulnerability index; (2) a computer aided vulnerabilty mapping approach (Marcolongo and

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Perottov, 1987; Schnebelen et al., 2002); and (3) methods based on the statistical treatement

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(Antonakos and Lambrakis, 2007).

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The mapping method based on the intrinsic vulnerability index is applied using Geobased

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Information System software to assess and map all hydrogeological parameters considered in

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the vulnerability evaluation. This method consists on the attribution of an index to each

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hydrogeological parameter. The simulation methods take into account all the physical and

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dynamic properties of the aquifer. It is based on the resolution of the equations of pollutant

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transfer process. This method involves several hydrogeological data that are not usually

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available.

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The intrinsic vulnerability assessment and mapping considers the physical properties of the

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reservoir and the hydrodynamism of the groundwater in the aquifer system. The most

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common models for vulnerability mapping are: (1) DRASTIC, (2) SINTACS, (3) GOD, (4)

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AVI, (5) SYNTACS, (6) SI, and (7) EPIK. Some specific methods of intrinsic vulnerability

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mapping are applied to coastal aquifers such as GALDIT model (Chachadi and Lobo-Ferreira,

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2005). For vulnerability mapping in karst aquifer, EPIK model is applied (Doerfliger et al.,

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1999). The vulnerability map in fractured aquifer can be established using the DRASTIC-Fm

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model (Denny et al., 2005).

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The intrinsic vulnerability of groundwater and the susceptibility of the aquifer system to the

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pollution are presented in a map. The zoning map of the aquifer extension according to the

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vulnerability index is used by planners and decision makers to establish the policy of water

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resource protection and management. These vulnerability maps are used to establish a

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scenario to avoid groundwater pollution process and to protect and manage the available

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water resource.

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It is important to evaluate the reliability of the established maps by the application of more

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then two models and by the analysis and the validation of the quality of used data for

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hydrogeological parameter's mapping (Stigter et al., 2006). We give below some examples

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from the litterature to demonstrate the process of the vulnerability map's validation:

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Banton and Villeneuve (1998) used the DRASTIC rating system and the PRZM model

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to simulate the vulnerability. The established maps of the vulnerability were analysed

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to explore the relationship between the DRASTIC index and the PRZM approach,

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

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aquifer to the nitrates using modified DRASTIC approaches. 

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Antonakos and Lambrakis (2007) applied 3 models to establish the vulnerability of the

Ravbar and Goldscheider (2009) used 4 methods to elaborate the groundwater vulnerability map in a Slovene karstic aquifer catchment.

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Jose et al., (2012) attempt to assess the groundwater vulnerability in the Oaxaca Central Valleys by the application of the SINTACS model. The second method used is

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the geographic weighted regression method.

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approaches is then presented and discussed.

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A comparison between the two

Abbasi et al., (2013) propose the application of DRASTIC model base on the Analytic

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Element Method. The statistical modeling of the hydrogeological data by the

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application of the Weights of Evidence method is used to elaborate the groundwater

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vulnerability.

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Yu et al., (2014) propose the vulnerability assessment using transport modeling and groundwater age modeling with as an application to the Beijing Plain in China.

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3. Utilities of vulnerability maps for groundwater protection and management

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The management of water resources is a policy promoting the sustainable use by the economy

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of water resources to promote a good quality and significant quantity of water to the future

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generations. The main strategy of the water resource management is to support universal

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access to the water. The interpretation of the vulnerability map helps in identifying the basic

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of the strategy that will be adopted by planner to avoid the groundwater resource pollution. At

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a regional scale, the vulnerability mapping is a basic tool for promoting and identifying all

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zones susceptible to the pollution from the surface. The interpretations and analysis of the

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vulnerability maps coupled to the aquifer hydrodynamics and geometry can be used by

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planners and decision makers to specify the areas having the greatest contamination

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susceptibility. These maps are also used to guide hydrogeologist and planners to avoid the

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pollutant's impact on the quality of the groundwater resources.

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The intrinsic vulnerability assessment and mapping can be described as a combination of

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hydrogeological data and topographic maps into a simple map to be easily used by planners.

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The vulnerability map is superimposed to the land use map to assess the groundwater

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contamination risk map. The risk map is useful to protect the groundwater resource from all

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potential risks. In general, the vulnerability maps is used for: (1) Planning: vulnerability maps

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are considered as a preliminary investigations for the planning of water protection projects or

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to establish a management scenario for water resources's quality; (2) Protection of

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groundwater resource by the evaluation of area the most susceptible to the contamination; (3)

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establishment of a map showing the priority of the protection zone; (4) offering a support to

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the planners and decisions makers; (5) monitoring of the groundwater from possible

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contaminations; and (6) educational use of these maps for hydrogeological learning.

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A particular concentration should be adressed to high vulnerability areas. In theses areas the

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self protection of the aquifer made up by the vadose zone is not enough to protect the water

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quality. Particular measures can be instaured to avoid all activities having a potential of water

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resource pollution in most vulnerable area.

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4. Methodology

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According to Civita (1994), there are 24 methods for assessing the vulnerability of aquifers.

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The used methods for assessing vulnerability of groundwater are subdivided into 3 main

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classes (Liggett et al., 2009) : (1) parametric methods with indices ; (2)physical modeling ;

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and (3) Statistical methods. The used approach of the intrinsic vulnerability mapping in this

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study is based on the parametric methods. The methodology adopted used the standard

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DRASTIC model, the Modified DRASTIC model and the DRIST model in a Geobased

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Information System (GIS) environment to establish the intrinsic vulnerability and the risk

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maps.

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The GIS is ranked among the new techniques in mapping and analysing complex geological

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structure. It allows : (1) the storage, management and quick access to a large volumes of

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complex geographic data ; (2) the easy manipulations and varied analyzes of hydrogeological

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map data ; (3) the combination of different data in one hydrogeological database ; and (4) the

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speed and effectiveness of planning, management and decision-making. The adopted

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methodology is summarized in figure 1.

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(Here position of Figure 1)

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4.1. DRASTIC model

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According to Civita (1994), the DRASTIC method is based on the following assumptions: (1)

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a watershed with a flat relief and an area larger than 0,4Km2 ; (2) potential sources of

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contamination are located in the surface ; (3) Potential contaminants propagate from the

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surface and reach the aquifer by infiltration process ; (4) the nature and type of potential

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contaminants is not involved in calculating the vulnerability index ; and (5) The aquifer

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system is made up by porous media and the climate in the study area is semi-arid to arid.

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The intrinsic groundwater vulnerability mapping is established based on the hydrodynamic

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properties and the geometry of the aquifer (Babiker et al., 2005). DRASTIC is the first inial

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letter

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parameters by the DRASTIC model are : (1) D depth to water table ; (2) R net recharge ; (3)

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A aquifer media ; (4) S soil media ; (5) T topography ; (6) I impact of vadose zone ; and (7) C

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hydraulic conductivity. The groundwater vulnerability assessment using the DRASTIC model

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is based on a numerical ranking that considers weights, ranges, and ratings. For each one of

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the seven factors, weights from 1 to 5 are attributed by the contribution of each parameter in

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the sensitivity to the pollution (Table 1).

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for the 7 parameters used by this model (Aller et al., 1987). The 7 considered

(Here position of table 1)

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The seven DRASTIC parameters have a specific limit of variabiliy. This variability was

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defined based on effect of each parameter in the pollution phenomenon. The importance of

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each parameter was evaluated on the base of its contribution to the process of pollutant

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transfert from the surface to groundwater. For each parameter is attributed a rating value that

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veries from 1 to 10 accordint to the sensibility to the pollutant infiltration and transfert. The

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rating provides a classification of the clesses of each parameter considered in the DRASTIC

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model (Table 2).

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(Here position of table 2)

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The following linear combination is applyed to evaluate the index calculated in the

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DRASTIC model:

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DRASTIC index = Dr*Dw + Rr*Rw + Ar*Aw + Sr*Sw + Tr*Tw + Ir*Iw + Cr*Cw

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D, R, A, S, T, I, and C are the parameters used by the DRASTIC model, r : rating, w : the

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weight.

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Spatial analyst extension of the Arc GIS9.1 software is a helpful tool for the compilation of

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the used database. It is used to calculate the vulnerability index, and also to establish the map

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showing the spatial variability of the groundwater vulnerability. For each parameter

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considered in the DRASTIC model, digital geospatial data sets was created. Then, a model

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grid over the boudries of the study area and DRASTIC ratings are assigned to the grid cells.

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Using the grid layers extension, the DRASTIC indices are computed. Finally all maps are

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superimposed to generate the aquifer vulnerability map.

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The DRASTIC indices is an indicator of the pollution susceptibility. The high vulnerability

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index is located in area with high sensitivity of the groundwater to be contaminated (Table 3).

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(Here position of table 3)

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4.2. Hydrogeological parameters considered in the DRASTIC Model

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Depth to water table (D) : it is equal to the depth of the static level of the aquifer measured

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from the soil surface. It corresponds to the thickness of the strata, through which the pollutant

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is infiltrated before reaching the aquifer. This parameter controls the pollutant transfert

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phenomenon and therefore the possibility of contamination. The vulnerability of an aquifer is

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considered high while the depth to water table i low.

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Net Recharge (R) : it is about the amount of infiltrated water that reached the aquifer

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expressed in millimiters/year. The groundwater recharge process is the main mecanism

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involved in the transfert of the contaminant from the soil surface to the underground. The

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groundwater recharge process can be divided into three groups, which are frequently

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implicated in the same process (Fan et al., 2014): (1) Direct recharge : Water percolate

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vertically through the unsaturated zone ; (2) Indirect Recharge: percolation of water into the

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saturated zone through riverbeds and other watlands ; and (3)Localized recharge: Intermediate

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form of groundwater recharging, located in some areas of surface water concentration.The

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recharge is an essential process for the pollutant transfert in the vadoze zone to the saturated

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zone .When net recharge of aquifer is high, the possibility of contaminating the water table is

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higher.

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Aquifer Media (A) : The aquifer is defined as a porous and permeable geological formation

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that serve as a hydrogeological reservoir. In unconsolidated aquifers, the estimation of

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porosity and permeability is based on the granulometry of material. The groundwater flow is

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related to the porosity, permeability and transmissivity of aquifer.

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Soil media (S): The type of soil is considered as an important parameter that control the

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infiltration process and the aquifer recharge. The soil media is considered as an important

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parameter in contaminant percolation from the surface. Clayly soils increase the probability of

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the contaminant transfert from the soil surface to the reservoir.

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Topography (T) : This parameter represents the topography located above the aquifer. The

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slope controls the probability that a pollutant can be diffused by runoff water in the surface. In

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a low slopetopography region, the surface flow is little and the susceptibility of the pollution

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infiltration is high. Otherwise, when the flow is important, the potential for pollutant

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contamination is lower. The Digital Elevation Model extension DEM of the ARCGIS is used

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to calculate the slope value in a GIS environment.

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Impact of the vadose zone (I): The vadose zone is defined as the part of the aquifer that is

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temporarely unsaturated. The lithology of the vadose zone media is the parameter that

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influence the facility of the transfert of the pollutant. In the DRASTIC model, the rating

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attributed to the aquifer media (A) and the vadose zone impact (I) are equal in a shallow

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aquifer. A low rating is attributed to this parameter when the aquifer media consists of a

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permeable horizons. The grid layer was obtained using the geological map and from the

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hydrogeological correlation of well cross sections.

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Hydraulic conductivity of the aquifer (C):

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The hydraulic conductivity is the capacity of an aquifer to to conduct water. It controls the

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infiltration and dispersion of the pollutant from the soil surface to the reservoir. The

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vulnerability of the aquifer is considered high if the hydraulic conductivity is high.

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4.3. Modified DRASTIC model

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The lithology of the aquifer and the vadose zone media are the main parts of the qualitative

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parameters of the DRASTIC method. The lithological data available in the hydrogeological

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well cross section doesn’t respect the dominance of each lithological layer of the studied

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aquifer and therefore we can not use the weight and rating according to the classification of

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Aller et al., (1987). According to Saidi et al., (2010) a modified DRASTIC model has been

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implemented to estimate these parameters based on vertical and horizontal permeability data

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calculated from the reference permeability coefficients awarded for each lithological level of

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aquifer system (Castany, 1982. Banton and Bangoy, 1997).

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4.4. DRIST model

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The DRASTIC model involves the characterization of the saturated zone of the aquifer. In this

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model, the approach considers only the parameters related to the unsaturated zone of the

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aquifer. Indeed, only DRIST parameters are involved in the transfert of contaminant vertically

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from the soil surface. In this model, the parameters hydraulic conductivity (C) and nature of

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the aquifer(A) are not involved in the transfer. The vulnerability index calculation for the

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DRIST model is done by the same way as for the DRASTIC method. As the DRASTIC

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method, the assessment of vulnerability in the DRIST model refers to the classification of

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Aller et al. (1987).

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4.5. Study area

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Located in the northeastern part of Tunisia (Figure 2), the Grombalia basin has an area

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of about 660 km2. The major pollutants in this watershed are mainly caused by agricultural

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activities in the Grombalia plain and by industrial rejections. Thus, the assessment and

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mapping of the vulnerability of the Grombalia aquifer system is considered as a significant

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tool for decision makers to establish a sustainable strategy for groundwater resource

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protection.

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(Here position of Figure 2) 4.5.1. Hydrogeological identifications

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The Grombalia basin is a graben structure that was firstly identified by Schoeler

310

(1939) and Castany (1948). It is described as a sedimentary basin limited by two important

311

normal faults. The first fault with a NNW–SSE direction is called the fault of Borj Cedria.

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The sceond one, with a post miocene acitvity has a NE-SW direction and is called the fault of

313

Hammamet (Castany, 1948). The structure of the Grombali basin resulted from the activity of

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normal faults in the Middle Miocene (Chihi, 1995). The two normal faults are the natural

315

limits of the hydrogeological system of Grombalia.

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The Eocene deposits are made up by sand with glauconite and define the Souar

317

Formation located in the northweastern part of the basin. The Oligocene unit is composed of

318

sandstone (Burrolet, 1956 ; Blondel, 1991).

319

In the Grombalia Graben, the Quaternary deposits are made up by sands and clay (Figure 3).

320

(Here position of Figure 3)

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This filling gives rise to an entire aquifer which is the main water resource in the Grombalia

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basin. There are three superimposed levels constituting three types of aquifers: Phreatic

323

aquifer, semi-deep and deep aquifer (Figure 4).

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(Here position of Figure 4)

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To better define the structure and the geometry of the grombalia aquifer system, a

326

hydro stratigraphic correlation has been established in the northern part of the sutdied basin.

327

The correlation presented below has shown the multilayered aspect of the aquifer system of

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Grombalia (Figure 4).

329

The present study and hydrogeological investigation interested phreatic aquifer. It is

330

the first aquifer level formed mainly by sandstone and sandy. The substratum is clayey with a

331

thickness that increases from the SW to the NW (Figure 4).

332

The phreatic aquifer extension of Grombalia with a surface of 334 km² is limited to

333

Quaternary outcrops. It has been for a long period the subject of intensive exploitation to meet

334

the needs of irrigation. This operation is done primarily by shallow wells whose number has

335

increased from 5689 to 8814 wells between 1980 and 2012. These wells fail to meet the

336

irrigation water requirements. The renewable resources of the phreatic aquifer of Grombalia

337

are about 51*106m3 and the exploitation is about 104*106m3. The water balance deficit is

338

estimated at about 53*106m3.

339

4.5.2. Groundwater flow

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The piezometric map interpretation allows to know the groundwater flow direction,

341

with a variable uncertainty related to the density of measurement point used in the

342

establishment of the map. It also determines the aquifer recharge zone, the accumulation zone

343

and the outlet of the aquifer. Piezometric maps of the Grombalia phreatic aquifer were

344

established based on records of static levels and altitudes of wells. Two piezometric maps was

345

established (Figure 5) and interpreted in order to evoluate the hydrodynamic of the studied

346

aquifer.

347

(Here position of Figure 5)

348

The direction of the groundwater flow is the study area from the southeastern side to the

349

northwestearn part of the aquifer. The groundwater flow direction is the same for 2005 and

350

2013. The surexploitation is confirmed by the isopieze migration to the upstream part of the

351

basin and the groundwater level decline for 2013. The recharge zone is situated in the eastern

352

side of the aquifer extension; while the drainage zones are located in the northern part of the

353

plain.

354

5. Results and discussions

355

5.1. Application of the DRASTIC model

356

This part of the study is devoted to the development of the different maps related to the

357

spatial variability of the

358

parameter is assigned a definite weight according to the classification of Aller et al., (1987).

359

The establishment of a map showing the distribution of the DRASTIC index is done by the

360

superposing of layers related to each parameter.

361

5.1.1. Map of the « depth to water table (D) »

362

The study of this parameter is based on the use of static levels recorded by the CRDA of

363

Nabeul in 2013 from 17 wells located at Grombalia basin. The available values was processed

364

with the Arc Gis 9.3 software and was used to establish the map of the spatial variation of the

365

parameter « depth to water table ». This map is subdivided into 4 classes based on the used

366

classification. The corresponding sizes for these classes range from 3 to 9 (Figure 6).The

367

DRASTIC weight assigned to this map is 5.

368 369

DRASTIC parameters using GIS as mapping tool. For each

(Here position of Figure 6) 5.1.2. Map of the « Net Recharge (R) »

370

The groundwater recharge map of Grombalia basin was developed using the fluctuation

371

method of groundwater level (Water Table Fluctuation: WTF) (Fan et al., 2014) between

372

1972 and 2013. This method is based on the hypothesis that the increase of the piezometric

373

level is due to the aquifer recharge process. To apply this method, it is necessary to estimate

374

the porosity of the reservoir where the water level fluctuation is identified. According to

375

Castany (1982), the volume of free water that may contain a porous media in saturation

376

divided by its total volume defines the effective porosity. It is equivalent to the storage

377

coefficient of unconfined aquifer. Net recharge map of the study area (Figure 6) shows two

378

classes, the most important, covering almost all of the basin and has more than 250 mm

379

values. The DRASTIC ratings assigned range from 8 to 9.

380

5.1.3. Map of the « Aquifer media (A) »

381

This map is established by the integrationn of the analysis og the geological maps and the

382

correlation of the lithological information from 24 boreholes. There are three types of

383

lithological horizons: sand, sandstone and clay. The sands are the dominant lithological

384

horizon which occupies the center and the southern part of the Grombalia basin. For each

385

lithological horizon is assigned a DRASTIC rating according to the classification of Aller et

386

al. (1987) (Figure 6).

387

5.1.4. Map of the « Soil media (S) »

388

In the study area, data related to the soil media were extracted using the soil map of Tunisia at

389

the scale 1 / 500,000. The soil map shows that the Grombalia basin has a lithological

390

heterogeneity marked by 7 soil classes (Figure 6). A DRASTIC rating is assigned for each

391

class of soil.

392

5.1.5. Map of the « Topography (T) »

393

The digital terrain model in the Triangulated Irregular Network format (TIN) was the basic

394

information to calculate the variability of the slope in Grombalia basin. This model was

395

generated by using the 3D ArcMap extension. The slope map shows five classes (Figure 6).

396

The lowest slope class is 0-2% and occupy almost all of the studied basin.

397

5.1.6. Map of the « Impact of the vadose zone media (I)»

398

This map was established in the same way as the map of the lithology of the aquifer. It shows

399

the presence of three lithological horizons (sandstone, sand and clay) having different

400

permeabilities which explains the change in the DRASTIC rating assigned which varies

401

between 3 and 7 (Figure 6).

402

5.1.7. Map of the « Hydraulic conductivity of the aquifer (C)»

403

This map was established using the hydraudynamic data available from well and pumping

404

tests data. Then we assigned to each lithological level a permeability based on the

405

classification of Rodriguez et al., (2001).

406

The resulting map shows that hydraulic conductivity values varies between 4.6 10-7 m /s in

407

the northern part of the basin, and 9.4 10-4 m / s in the southern part of studied area.

408

According to the classification of Aller et al., (1987), these values are within six intervals

409

whose DRASTIC ratings range from 1 to 9 (Figure 6). For the hydraulic conductivity

410

parameter, the weight used is 3.

411

5.1.8. DRASTIC index map establishment

412

The groundwater vulnerability map of Grombalia at scale 1 / 50,000 using DRASTIC model

413

is obtained by multiplying the cote on each parameter with the value of the corresponding

414

weight. The DRASTIC index varies between 99 and 181 and represent three classes that make

415

up this map. The DRASTIC index map according to the classification of Aller et al., 1987

416

(figure 7 A) shows three zones : (1) low vulnerability (99-120), (2) medium vulnerability

417

(121-160), and (3) high vulnerability (161-181). High vulnerability occupies 26% of the total

418

area of the basin. It mainly covers the eastern side of the phreati aquifer extension where the

419

lithology of the vadose zone is permeable. At this part of the aquifer extension the depth to

420

water table has a high cote that varies between 7 and 9.

421

(Here position of Figure 7)

422

The DRASTIC index map according to the classification of Angel et al. 1986 (Figure 7 B)

423

shows two zones namely medium vulnerability (90-140) and high vulnerability (141-191).

424

5.2. Modified DRASTIC model Application

425

Compared to the DRASTIC method, this model has a difference in considering the following

426

parameters : (1) Aquifer media (A) and (2) Impact of the vadose zone (I). In the proposed

427

model, we have to substitute lithology by the permeability in the estimation of the parameters

428

(Saidi et al., 2010). All other maps are developed in the same way as for the standard

429

DRASTIC method. The results of the vertical and horizontal permeabilities was evaluated

430

using the hydraudynamic data available from 18 wells. A reference permeability coefficient

431

was assigned for each lithology according to Castany (1982) and Banton and Bangoy (1997).

432

5.2.1. Vadose zone impact

433

The permeability map of the vadose zone is established by applying the formula of the

434

vertical permeability by Castany (1982) : K = H / Σ (hi / ki)

435 436

With:

437

K : vertical permeability average (m / s);

438

H: Total thickness of the unsaturated zone (m);

439

hi: i layer thickness (m);

440

ki: permeability of layer i (m / s)

441

The permeability map for the vadose zone in the phreatic aquifer of Grombalia basin (Figure

442

8) shows values that range between 2.8 10-4 m / s and 1.1 10-2 m / s.

443

The rating values attributed to the permeability of the vadose zone are subdivided in two

444

intervals (figure 8).

445

(Here position of Figure 8)

446

5.2.2. Saturated zone impact

447

The permeability of the saturated zone map is established in the same way as that of the

448

vadose zone, based on the calculation of the equivalent horizontal permeability by applying

449

the formula of Castany (1982): K = Σ (hi ki) / Σ (hi)

450 451

With:

452

K : horizontal permeability average (m / s);

453

Σ (hi) : Total thickness of the saturated zone (m);

454

hi = layer thickness (m);

455

Ki = layer permeability (m / s).

456

The horizontal permeability average thus calculated allowed to draw the equivalent

457

permeability map of the saturated zone of the aquifer (Figure 9) and shows that values range

458

from 10-3 m / s and 1.4 10-2 m / s. Permeability of the saturated zone is subdivided in two

459

intervals. A higher rating (8) is assigned to the part of the phreatic aquifer extension with a

460

specific lithology dominated by coarse grain size and a high permeability located in the east

461

boundary and the center of Grombalia basin.

462

(Here position of Figure 9)

463

The lowest rating (6) is attributed to the low permeability because of the existence of clayey

464

horizons that inhibits the transport of contaminants.

465

The modified DRASTIC map established (Figure 10) shows that the vulnerability indices

466

range between 85 and 179. Grombalia basin is subdivided into 3 classes namely low

467

groundwater vulnerability (85 -120), medium groundwater vulnerability (121-160), and high

468

vulnerability (161-179). High vulnerability, covers the eastern part of the Grombalia basin,

469

occupies 17% of the phreatic aquifer extension and is related to the high permeability

470

recorded.

471

(Here position of Figure 10)

472

5.3. Application of DRIST model

473

The DRIST method requires the superposition of the five maps that are already established

474

previously in the DRASTIC model. This method is an improvement of the universally used

475

DRASTIC method (Saidi et al., 2010). In the DRIST method, the vulnerability indices range

476

between 75 and 140 and represent two classes that make up the map below (Figure 11). The

477

high vulnerability, with a DRIST index between 110 and 140, occupies 66% of the total

478

extension of the phreatic aquifer. In this method, the lithology of the aquifer and the hydraulic

479

conductivity are not considered. In the DRIST vulnerability map, the portion of the aquifer

480

formed by a vadose zone witha permeable lithology has a high vulnerability. While the

481

vadose zone is made up by clays, the vulnerability is medium.

482

(Here position of Figure 11)

483

5.4. Groundwater risk map

484

The risk of contamination of an aquifer is related to the suceptibility of a contaminant to be

485

infiltrated from the soil surface. According to Ferreira and Oliveira (1997), the contamination

486

risk of aquifer depends on the hydrogeological conditions and the pollutants. The

487

contamination risk of an aquifer is evaluated using the aquifer intrinsic specificities and the

488

land use map that reflected the anthropogenic impact.

489

The established risk map was based on the integration of the groundwater vulnerability map

490

of the phreatic aquifer of Grombalia basin and the land use map obtained from the Google

491

earth satellite imagery (Figure 12).

492

(Here position of Figure 12)

493

The land use map is converted to raster mode and a note for each category (Lr) is allocated

494

and will be affected by their relative weight (Lw = 5). This map will be superimposed to the

495

DRASTIC index map and create a risk map using

496

Gr(risk) = Vl(Intrinsic) + Lr * Lw

497

where Gr(risk) is the groundwater risk map, Vi is the groundwater vulnerability, Lr is a rating

498

assigned to each category of land use, Lw is the weight of the parameter. Gr (risk) obtained

499

range between 124 and 221 (Figure 13).

500

According to the groundwater risk map (Figure 13), the phreatic aquifer of Grombalia basin is

501

divides in four risk classes: (1) a high, (2) very high risk covering 59% of the total area, (3)

502

low risk covering a small portion (3%) located in the West boundary of the Grombalia basin,

503

and (4) a medium risk in the western part of the studied aquifer.

504

(Here position of Figure 13)

505

The comparison of the groundwater risk map with standard DRASTIC map shows a strong

506

resemblance at the eastern side of the phreatic aquifer extension. The established map of the

507

groundwater risk demonstrates a very high class, which is absent in the DRASTIC

508

vulnerability map. This is explained by the presence of irrigated perimeters that require

509

fertilizer and may contaminate the aquifer.

510

5.5. Discussion

511

The comparison of the standard and modified DRASTIC vulnerability maps (Figure 7 and

512

Figure 10) shows a slight difference in the extension of the high vulnerability zone. The high

513

vulnerability area occupies 26% of the phreatic aquifer extension using the standard

514

DRASTIC vulnerability model against only 17% of the total surface using the modified

515

DRASTIC method. This high vulnerability area is located in the eastern part of the phreatic

516

aquifer extension. This difference are explained by the limits of the DRASTIC model related

517

to the quality, the spatial distribution and the number of the hydrogeological data considered

518

in the model.

519

The elaboration of the vulnerability maps by the application of the three methods require the

520

use of parameters according to an interpolation process whose reliability depends on the data

521

used. The used interpolation can cause errors when establishing

522

hydrogeological parameter and when defining intervals. Rosen (1994) notes that the high

523

number of parameter often statistically decrease potential errors in the vulnerability index

524

calculation.

525

The standard class limits defined by vulnerabilty model does not correspond to the reality of

526

the study area. According to Labo-Ferreira et al., (2004), class and intervals can gather

527

various entities and vary from one region to another. The definition of class and intervals in

528

each parameter are relative to the specificity of each case study and are not absolute. We also

529

note that the vulnerability of an aquifer varies over time. It is related to the rainfall which is

530

involved in developping the depth of groundwater level and the net aquifer recharge rate. This

531

issues is very clear when comparing the results obtained in this study and that of Hamza et al.,

532

(2010) reflecting the vulnerability of the grombalia phreatic aquifer in 2005.

533

The classification of Aller et al., (1987) is considered for the DRASTIC and DRIST methods.

534

The comparison of the two maps (Figure 7 and fogure 11) shows the following points: (1) For

535

the two models, the area with a high vulnerability is concentrated in the eastern border of the

536

studied aquifer ; (2) In the DRASTIC vulnerability map, area with medium vulnerability has a

537

greater extension because it take in accounts the lithology and the hydraulic conductivity

538

which contributes to minimizing the effect of unsaturated zone. Weights assigned to these two

539

parameters are also importants ; and (3) The DRIST vulnerability map shows the effect of

540

factors that are related to the unsaturated zone and are essential to the transfer of pollutants to

541

the saturated zone of the aquifer.

maps for each

542

The risk of contamination of the phreatic aquifer of Grombalia basin demonstrates that a

543

major part of the aquifer extension is with high risk .This risk

544

hydrogeological characteristics which increases the aquifer sensitivity. Then, the presence of

545

irrigated perimeters in the eastern part of the Grombalia basin, which require the use of

546

fertilizers deteriorate the groundwater quality.

547

In the studied basin, groundwater vulnerability assessment and mapping involve the water

548

resource management through the water resource evaluation and the risk assessment. The

549

established maps are helpful for the management of water resources by giving the priority to

550

the projects that will be implemented in a zone with low vulnerablity. The temporal variation

551

of the spatial distribution of the vulnerability helps decisions makers and planner to control

552

the sensibility to pollution of each part of the aquifer.

553

6. Conclusion

554

The establishment of the groundwater vulnerability maps of the phreatic aquifer of Grombalia

555

basin is performed using GIS in order to evaluate potential contamination risks. The

556

vulnerability mapping is revealed by the application of three different methods. Indeed the

557

standard DRASTIC method demonstrated that a high vulnerability characterized an area of

558

26% of the total phreatic aquifer extension, while a medium vulnerability is located in the

559

most important part of the studied aquifer. The modified DRASTIC method, which takes into

560

account the heterogeneity of the lithology of the aquifer media and the unsaturated zone

561

showed that 17% of the studied area is occupied by a high vulnerability. A slight difference

562

emerged between the two methods that emphasize the importance of the type of data in the

563

establishment of the DRASTIC vulnerability maps. The application of the DRIST model

564

showed that 66% of the total area is occupied by a high vulnerability due mainly to the

565

parameters involved in the transfer of pollution.

is due firstly to the

566

The results thus obtained allow us to propose some recommendation supporting the

567

management and protection of the groundwater aquifer system of Grombalia basin: (1) Avoid

568

any activity with a risk of pollution in areas with high vulnerability, (2) Create

569

hydrogeological parks with uncultivated land whose purpose is the preservation of

570

groundwater areas where water quality has been deteriorated, and (3) Reduce any source of

571

pollution in order to limit its effects on the groundwater quality.

572

Acknowledgements

573

Authors express their appreciations to the anonymous reviewers who reviewed the two first

574

versions of the article. Their comments have contributed to the improvement of the scientific

575

quality of the manuscript.

576

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577

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Figures captions

692

Figure 1. Flowchart showing the adopted methodology

693

Figure 2. Location of the study area

694

Figure 3. geological map of the Grombalia basin (1. Recent quaternary; 2. Recent dunes; 3.

695

Recent alluvium; 4. Alluvium; 5. Marine Tyrrhenian; 6. Tyrrhenian dune; 7. Serravallian-

696

Tortonian; 8. Plio-Quaternary)

697

Figure 4. Hydro-startigraphic correlation and aquifer system levels identification

698

Figure 5. piezometric maps of the phreatic aquifer of Grombalia for 2005 (1) and 2013 (2)

699

Figure.6 Map layers of hydrogeological factors and parameters used in DRASTIC model

700

Figure 7. Standard DRASTIC vulnerability map of the phreatic aquifer of Grombalia: (A)

701

according to the classification of Aller et al, (1987) and (B) according to the classification of

702

Angel et al. (1986).

703

Figure 8. Permeability map of the unsaturated zone

704

Figure 9. Permeability map of the saturated zone

705

Figure 10. Modified DRASTIC vulnerability map of the phreatic aquifer of Grombalia

706

according to the classification of Aller et al., (1987).

707

Figure 11. DRIST vulnerability map of the phreatic aquifer of Grombalia classified according

708

to Aller et al. (1987).

709

Figure 12. Land use map of the Grombalia basin

710

Figure 13. Groundwater contamination risk map.

711

Table captions

712

Table 1. Weights assigned to parameter’s DRASTIC model (Aller et al., 1987).

713

Table 2. Classes and rating values of the DRASTIC model parameters (Aller et al., 1987)

714

Table 3. Vulnerability degree evaluation

Table 1

Parameter D R A S T I C

Weight 5 4 3 2 1 5 3

Table 2

D (m) Class Rating 0-1.5 10 1.5-4.5 9 4.5-9 7 9-15 5 15-23 3 23-31 2 > 31 1

R (mm) Class Rating 0-50 1 50-100 3 100-180 6 180-250 8 > 250 9

A Class metamorphic rocks massive shale limestone and shale sandstone and clay sandstone Sand and gravel karstic limestone

Rating 3 2 6 6 7 8 10

S Class Rating thin soil 10 sand 9 peat soil 8 loam 5 clay loam 3 Topsoil 2 clay 1

T (%) Class Rating 0-2 10 2-6 9 6-12 5 12-18 3 > 18 1

I Class impermeable layer Silt / clay metamorphic rocks sandstone Sand and gravel basalt karstic limestone

rating 1 3 4 6 8 9 10

C (m/day) Class rating 0.04-4 1 4-12 2 12-29 4 29-41 6 41-82 8 > 82 10

Table 3

Vulnerability degree

Vulnerability index (Aller et al, 1987)

Vulnerability index (Angel et al, 1986)

1-120

1-100

Moderate

121-160

101-140

High

161-200

141-200

> 200

> 200

Low

Very High

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Highlights 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65



The vulnerability mapping is a powerful tool for groundwater protection



Vulnerability mapping utilities

for groundwater resources management is

reviewed 

DRASTIC, modified DRASTIC and DRIST are used to establish vulnerability map



Vulnerability maps provide recommendations for the management of the groundwater