Mohamed Hafedh Hamza, Abdellatif Maâlej, Mohamed Ajmi and Ayed Added - PDF

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Validity of the methods DRASTIC and SI applied by GIS technique to the study of diffuse agricultural pollution in two phreatic aquifers of a semi-arid region (Northeast of Tunisia) Mohamed Hafedh Hamza,

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Validity of the methods DRASTIC and SI applied by GIS technique to the study of diffuse agricultural pollution in two phreatic aquifers of a semi-arid region (Northeast of Tunisia) Mohamed Hafedh Hamza, Abdellatif Maâlej, Mohamed Ajmi and Ayed Added Abstract: The phreatic aquifer of Oued Guéniche located in the prefecture of Bizerta (northeast of Tunisia) and of the phreatic aquifer of Grombalia located in the prefecture of Nabeul (northeast of Tunisia) have a great economical importance because they are used for irrigation and domestic consumption. They occupy respectively areas of 83 km 2 and 392 km 2. Both aquifers encompass an area comprised mostly of agricultural zones, characterised by an increasing use of chemical fertilizers. Those chemical fertilizers threaten the quality of the ground waters. The study of the to pollution of those aquifers was made by applying two methods: the generic DRASTIC which is an intrinsic method, and the Susceptibility Index (SI) which is a specific to agricultural pollution method. This study employed the Geographical Information System (GIS) technology as a system for the acquisition, storage, analysis and display of geographic data. The validity of the two methods to agricultural pollution by nitrates was verified by comparing the distribution of nitrates in the two aquifers with the distribution of the different classes. That comparison demonstrated that the SI method is the more valid method in the studied systems. Keywords: aquifer, nitrates, GIS, DRASTIC, SI Mohamed Hafedh HAMZA Abdellatif MAÂLEJ Mohamed AJMI Ayed ADDED Université de Tunis El Manar, Faculté des Sciences de Tunis, Département de Géologie, CP 2092, El Manar II, Tunis, Tunisie. Tel : (+216) Fax : (+216) Mohamed Hafedh HAMZA Abdellatif MAÂLEJ Mohamed AJMI Ayed ADDED Received: 28 april 2010 / Accepted: 09 june 2010 Published online: 30 june 2010 Scribo 2010 Riassunto: I due acquiferi studiati sono l acquifero freatico di Oued Guéniche (governatorato di Bizerta) e l acquifero freatico di Grombalia (governatorato di Nabeul) situati nel Nord est della Tunisia. Essi hanno una grande importanza dal punto di vista economico poiché sfruttati per l irrigazione ed il consumo domestico. Questi acquiferi sono localizzati in una pianura prevalentemente adibita all agricoltura con un utilizzo crescente di fertilizzanti chimici. Occupano rispettivamente superfici di 83 km 2 e 392 km 2 e appartengono a una regione semiarida. Questo studio ha la finalità di verificare la validità del metodo generico parametrico DRASTIC (Aller et al. 1987) e del SI (Riberio, 2000) per la valutazione della vulnerabilità, per il diffuso inquinamento agricolo, essenzialmente a nitrati nelle due aree studiate. L uso della tecnologia GIS nella valutazione della vulnerabilità dell acquifero è necessaria poiché il GIS è un sistema per l acquisizione, l immagazzinamento, l analisi e la visualizzazione dei dati geografici. Il Generic DRASTIC è un metodo parametrico che fu sviluppato dall Agenzia di Protezione Ambientale degli Stati Uniti (US EPA) per la valutazione della vulnerabilità intrinseca verticale del sistema di acque sotterranee a scala regionale. SI è un metodo per valutare la vulnerabilità specifica verticale all inquinamento dovuto all attività agricola prevalentemente a causa di nitrati. Esso tiene conto delle proprietà dei contaminanti e delle relazioni con i vari componenti della vulnerabilità intrinseca. Il metodo DRASTIC è stato usato con 4 dei parametri iniziali di base mantenuti costanti: la profondità dell acquifero, la ricarica efficace annuale, l acquifero medio e la topografia, e ne è stato introdotto uno nuovo: il tipo di terreno di copertura. La validità delle mappe di vulnerabilità derivate dal DRASTIC generico (Aller et al. 1987) e dal SI (Riberio, 2000)per studiare l inquinamento da nitrati viene verificata nei due acquiferi studiati attraverso la comparazione della distribuzione dei nitrati nell acquifero e la distribuzione delle classi di vulnerabilità. Tale confronto mostra, nei due acquiferi studiati, che la carta migliore per la valutazione della vulnerabilità a causa dell inquinamento da nitrati è la quella del metodo SI, con la percentuale di coincidenza, tra la concentrazione dei nitrati e le differenti classi di vulnerabilità, del 70% nel caso dell acquifero di Oued Guéniche e del 73% nel caso dell acquifero di Grombalia. La carta del DRASTIC generico mostra una percentuale di coincidenza del 37% nel caso dell acquifero di Oued Guéniche e del 55% nel caso dell acquifero di Grombalia. Il metodo DRASTIC è un metodo di vulnerabilità intrinseca che non prende in considerazione né la natura degli inquinanti né i fattori che controllano la vulnerabilità specifica come il fattore dell uso del suolo. Il comportamento conservativo dei nitrati non permette una corretta valutazione di metodi intrinseci come il DRASTIC, che dà un grande significato alla capacità di attenuazione dei parametri idrogeologici coinvolti. Questi casi studiati mostrano il vantaggio di usare il metodo SI che è stato messo a punto prendendo in considerazione le proprietà chimiche dei nitrati assieme alle esistenti relazioni tra questi inquinanti e i vari componenti considerati nella vulnerabilità intrinseca. In questo specifico metodo di analisi, il parametro uso del terreno permette l integrazione di specifici fattori per ciascun utilizzo, come l effetto del riciclo nelle zone irrigate, e quindi permette una migliore sensibilità alle reali condizioni locali. AQUAmundi (2010) - Am01009: DOI /Am 58 Introduction The underground waters are usually contaminated by pollutants of different natures: biological, chemical or physical. The prevention against the aquifer pollution constitutes an important stage to which scientists are providing an increasing effort specially in studying the aquifer. The studied aquifers are the phreatic aquifer of Oued Guéniche (prefecture of Bizerta) and the phreatic aquifer of Grombalia (prefecture of Nabeul) located in the northeast of Tunisia. These aquifers are located in plains used mostly for agriculture caracterized by an increasing consumption of chemical fertilizers. Thus, this study aimed at verifying the validity of the following parametric methods: the generic DRASTIC (Aller et al., 1987) and SI (Ribeiro, 2000) in the assessment of the to pollution to diffuse agricultural pollution mainly by nitrates. The first method is an intrinsic one; however, the last one is a specific to pollution by agricultural pollution. Those methods consist in systems of numerical quotation based on the consideration of different factors affecting the hydrogeological system. The use of GIS technology in the evaluation of aquifer is necessary because GIS is a system for the acquisition, storage, analysis and display of geographic data. The used methods DRASTIC method DRASTIC (Aller et al., 1987) is a parametric method that was developed by the US Environmental Protection Agency (US EPA) for evaluating the intrinsic vertical of groundwater systems on a regional scale. Intrinsic is the term used to define the of groundwater to contaminants generated by human activities. It takes account of the inherent geological, hydrological and hydrogeological characteristics of an area, but is independent of the nature of human activities. The acronym DRASTIC stands for the parameters included in the method: Depth to water, net Recharge, Aquifer media, Soil media, Topography, Impact of vadose zone, and hydraulic Conductivity of the aquifer. DRASTIC indexes calculated are roughly analogous to the likelihood that contaminants released in a region will reach ground water, higher scores implying higher likelihood of contamination. The DRASTIC method includes two versions: the generic (or normal) DRASTIC version applied in the case of inorganic pollutants (e.g. in the case of nitrates), and the pesticides DRASTIC version applied in the case of pesticides. The method yields a numerical index that is derived from ratings and weights assigned to the seven model parameters. The significant media types or classes of each parameter represent rating between 1 and 10 based on their relative effect on the aquifer. The seven parameters are then assigned weights ranging from 1 to 5 reflecting their relative importance (Tab. 1). Tab. 1: Generic DRASTIC parameter weights (Aller et al., 1987) Parameter Weight D: depth to water 5 R: efficient or net recharge 4 A: aquifer media 3 S: soil media 2 T: topography 1 I: impact of the vadose zone 5 C: hydraulic conductivity of the aquifer 3 The DRASTIC Index is then computed applying a linear combination of all factors according to the following equation : DRASTIC Index = Dr Dw + Rr Rw + Ar Aw + Sr Sw + Tr Tw + Ir Iw + Cr Cw where D, R, A, S, T, I, and C are the seven parameters and the subscripts r and w are the corresponding rating and weights, respectively. The DRASTIC index values vary from 23 to 226 in the case of the generic version and fall into 4 classes corresponding to four degrees (Tab. 2). Tab. 2: Criteria for the evaluation of in the DRASTIC method (Aller et al.., 1987) SI method Vulnerability degree Vulnerability index Low Moderate High Very high 200 SI (Susceptibility Index) method (Ribeiro, 2000) is a method for evaluating the specific vertical to pollution originated by agricultural activities mainly by nitrates. Specific is the term used to define the of groundwater to a particular contaminant or group of contaminants. It takes into account of the properties of the contaminants and their relationship with the various components of intrinsic. The DRASTIC method has been used as a base, on which four original parameters have been maintained: depth to water, annual efficient recharge, aquifer media and topography, and a new one has been introduced: the land cover type. The principal types of land use and their assigned ratings provided by a team of Portuguese scientists (Ribeiro, 2000) are shown in Table 3. Tab. 3: Main soil occupation classes and correspondant Land Use (LU) values (Ribeiro, 2000) Land use class LU rating Industrial discharge, landfill, mines 100 Irrigated perimeters, paddy fields, Irrigated perimeters, paddy fields, Irrigated and non irrigated annual culture 90 Quarry, shipyard 80 Artificial covered zones, green zones, continuous urban zones 75 Permanent cultures (vines, orchards, olive trees, etc.) 70 Discontinuous Urban zones 70 Pastures and agro-forest zones 50 Aquatic milieu (swamps, saline, etc.) 50 Forest and semi-natural zones 0 The weight string (Table 4) has been modified in relation to the DRASTIC method. The SI index values measuring the aquifer fall into four classes corresponding to four degrees (Table 5). The different modifications have been effected taking into consideration the characteristics of common agricultural contaminants, such as the nitrate ion. The following factors were not taken into DOI /Am AQUAmundi (2010) - Am01009: 59 Tab. 4: Weights attributed to SI parameters (varying from 0 to 1, from the less to the most important) (Ribeiro, 2000) Vulnerability degree Vulnerability index Low Moderate High Very high 200 Tab. 5: Criteria for the evaluation of in the SI method (Ribeiro, 2000) Parameter D R A T OS Weight consideration: vadose zone media, soil type and permeability of the aquifer media. This last parameter is very difficult to evaluate spatially and it has already taken into consideration in the A parameter (Aquifer media) by the fracturation and granulometry factors. The soil type is indirectly represented by the land use type : For the purpose of groundwater pollution risk assessment, the soil zone can, in practice, either be allowed for, indirectly when estimating the subsurface pollution load from diffuse sources, or directly in combination with aquifer, the resulting categories applying only to diffuse source pollution (Foster, 1987). Attenuation processes in the soil and vadose zones relative to persistent contaminants are considered by Vrba and Zoporezec (1994) to be of less importance. Foster (1987) also minimizes the vadose zone role: In the case of persistent mobile pollutants, the unsaturated zone merely introduces a large time-lag before arrival at the water-table, without any beneficial attenuation. In many other cases, the degree of attenuation will be highly dependent upon the flow regime and residence time. In actual fact, besides the cleansing processes, which implicate the degradation of the contaminants, dilution processes are primordial in the assessment. They determine the restoration capability of the aquifer, which can also be expressed in terms of residence time (volume of water contained in the aquifer divided by the rate of recharge). Study areas Phreatic aquifer of Oued Guéniche The phreatic aquifer of Oued Guéniche is located in the prefecture of Bizerta (northeast of Tunisia) (Fig. 1). It is situated in a plain between latitudes North and m and longitudes East and m (Lambert North Tunisia coordinates) with a total surface of 83 km 2. The average of thickness of that aquifer is about 30 meters. The main towns and villages in the study area are El Alia, Menzel Jemil, El Khetmine, El Azib, Ejjouaouda, and Daouar Maghraoua. The main rivers crossing the study area are Oued Guéniche, Oued El Azib, Oued El Galaâ, Oued El Hella, Oued Jeddara, Oued El Meleh, and Oued Nechrine. The annual average rainfall varies between 485 and 599 mm and the annual average temperature is about 18 C (INM, ). The water resources of this aquifer have a great economical importance for this agricultural region. In fact, about 1400 wells are currently exploited in the study area and an annual volume of 10.5 million m 3 from these wells is used for irrigation (the equivalent of mm). The aquifer s area comprises agricultural zones characterised by an increasing use of chemical fertilizers which threatens the groundwater s quality. The identification of the hydrogeological units and subunits as well as the assessment Fig. 1: The phreatic aquifer of Oued Guéniche of the DRASTIC and SI parameters requires a good knowledge of the geology, the hydrogeology, the soil media, the topography, the meteorology and land usage in the study area. The data used in this study area are taken from different studies: geological (Burollet, 1951 and 1952; El Ghali and Ben Ayed, 2000), hydrogeological (Ennabli, 1966; Haj Ltaief, 1995; DGRE, 2005), geophysical (Ennabli, 1966; Haj Ltaief, 1995), climatic (INM, ), pedologic (Le Floc h, 1959; Gilson, 1995), topographic (OTC, 1981) and land-use (CRDA de Bizerte, 2000) studies. Phreatic aquifer of Grombalia The phreatic aquifer of Grombalia covering 392 km 2 is located in the prefecture of Nabeul (northeast of Tunisia) (Fig. 2). Its is situated in a plain between latitudes North and m and longitudes East and m (Lambert North Tunisia coordinates). The average of thickness of that aquifer located in a Plio-Quaternary sedimentary fill is about 25 meters. The main rivers crossing the study area are Oued Defla, Oued El Jorf, Oued El Bey, Oued Sidi Saïd, Oued Belli, Oued Tahouna and Oued Bezirk. The main towns and villages are Soliman, Soliman plage, Fondouk Djedid, Beni Khalled, Menzel Bou Zelfa, Grombalia, Turki, Nianou and Bou Argoub. The annual average rainfall varies between 356 and 536 mm and the annual average temperature is about 22 C (INM, ). More than 8000 wells are currently exploited in the study area and an annual volume of 90 million m 3 is taken from these wells for irrigation (the equivalent of 230 mm). As previously stated, mostly agricultural zones characterised by an increasing use of chemical fertilizers, occupy the study area. The data used in this study area are taken from dif- AQUAmundi (2010) - Am01009: DOI /Am 60 vertices; real-time projection/datum transformation of digitizer and GPS input data to meet mapping reference system specifications; feature filtering and extraction to new spatial databases based on feature attributes (filter) or location (clip). While, in the case of the aquifer of Gromabalia, ArcGIS version 9.2 was used. ArcGIS produced by ESRI, is a complete integrated collection of GIS software products that provides a standards-based platform for spatial analysis, data management, and mapping. At the desktop GIS level, ArcGIS can include: ArcReader, which allows one to view and query maps created with the other Arc products; ArcView, which allows one to view spatial data, create layered maps, and perform basic spatial analysis; ArcEditor which, in addition to the functionality of ArcView, includes more advanced tools for manipulation of shapefiles and geodatabases; or ArcInfo which includes capabilities for data manipulation, editing, and analysis. Application of the DRASTIC and SI methods in the phreatic aquifer of Oued Guéniche Fig. 2: The phreatic aquifer of Grombalia ferent studies: geological (Schoeler, 1939; Castany, 1948; Ben Salem, 1992), hydrogeological (Ennabli, 1970; DGRE, 2005), climatic (INM, ), pedologic (CRDA, 1997, 1998, 1999 and 2000), topographic (OTC, 1981) and land-use (CRDA de Nabeul, 2002) studies. GIS, an efficient tool for the study of aquifer GIS (Geographic Information System) technology allows a correct and continuous evaluation of the aquifers potential concerning the exploitation capacity and towards the danger of contamination with different pollutants, creating an objective overview. GIS means: Maps (document that contains data and describes hydrogeological events), information that is structured into a Database (information that can be used in the surveillance of the correct exploitation of groundwater) and Correlations between the two types of information: graphical and non-graphical (they are at the base of interpretations, studies, water supply systems design, evaluation and impact on the environment). In the case of Oued Guéniche aquifer, the software IDRISI AN- DES and CartaLinx produced by the Graduate School at Clark University (USA) were used to establish the different thematic maps and to generate the to pollution maps. IDRISI ANDES is the industry leader in raster analytical functionality, covering the full spectrum of GIS and remote sensing needs from database query, to spatial modeling, to image enhancement and classification. Special facilities are included for environmental monitoring and natural resource management, including land change modeling and time series analysis, multicriteria and multi-objective decision support, uncertainty and risk analysis, simulation modeling, surface interpolation and statistical characterization. CartaLinx is a spatial data Builder, a digital map development tool that serves as a companion to IDRISI. CartaLinx uses a full topological editor/digitizing system with capabilities for automatically building vector topology; automated generation of polygons and assignment of ID s by means of polygon locators (label points); insertion, deletion, or movement of nodes, arcs or arc Application of the generic DRASTIC method The depth to water map was obtained by interpolating 122 depth to water values homogenously distributed in the study area, recorded in 2005 by the DGRE. The aquifer net recharge was calculated according to the Williams and Kissel method (1991), the most suitable method to the studied area (Hamza et al., 2006). The aquifer media was determined using lithostratigraphic correlations based on the data of electric prospection, of boring logs and of geological maps of the study area (Haj Ltaief, 1995). Six geoelectric sections passing by 13 boring logs distributed in the study area have been established. In addition to these data, the values of hydraulic conductivity of the aquifer (Ennabli, 1966) measured in 9 well distributed localities were used to estimate the aquifer lithology in these localities based on tables giving approximate values of hydraulic conductivity for differ
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