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A GIS-based DRASTIC vulnerability and net recharge reassessment in an aquifer of a semi-arid region (Metline-Ras Jebel-Raf Raf aquifer, Northern Tunisia)
ARTICLE IN PRESS
Journal of Environmental Management 84 (2007) 12–19 www.elsevier.com/locate/jenvman
A GIS-based DRASTIC vulnerability and net recharge reassessment in an aquifer of a semi-arid region (Metline-Ras Jebel-Raf Raf aquifer, Northern Tunisia) M.H. Hamzaa, A. Addeda,, R. Rodrı´ guezb, S. Abdeljaoueda, A. Ben Mammoua a
Universite´ de Tunis El Manar, Faculte´ des Sciences de Tunis, De´partement de Ge´ologie, Laboratoire des Ressources Mine´rales et Environnement, 2092 Tunis, Tunisie b Universidad Nacional Autonoma de Me´xico, Instituto de Geofı´sica. Ciudad Universitaria., 04510 Me´xico D.F., Me´xico Received 28 November 2004; received in revised form 8 April 2006; accepted 9 April 2006 Available online 25 July 2006
Abstract This paper aims to elaborate new generic DRASTIC aquifer vulnerability maps of the coastal aquifer of Metline-Ras Jebel-Raf Raf (Northeast of Tunisia) using the GIS technique, making the data analyses easier to handle and providing better capabilities of dealing with large spatial data. A similar study was carried out in 1999 in the same aquifer using a method based on the soil water balance equation to determine the net recharge parameter. Unfortunately, the lack of data in the study area made the results unsatisfactory. By applying the Williams and Kissel equation and the Rao relationship, we intend to demonstrate that we could correctly evaluate the net recharge parameter. Moreover, new data related to the aquifer hydraulic conductivity, the soil cover and the vadose zone lithology have become available, and allowed us to elaborate suitable DRASTIC maps. r 2006 Elsevier Ltd. All rights reserved. Keywords: Reassessment; Net recharge; Aquifer vulnerability; DRASTIC; GIS
1. Introduction The use of chemical fertilizers for agricultural reasons and the settlement of factories in the study area located in the plain of Metline-Ras Jebel-Raf Raf (northern east of Tunisia) during the last decades, are the main causes of groundwater pollution. Thus, a map of aquifer vulnerability to pollution is highly needed to locate the vulnerable areas that can be affected. Vulnerability of the Metline-Ras Jebel-Raf Raf aquifer to pollution by inorganic pollutants was studied in 1999, by using the generic DRASTIC model (Hamza, 1999; Added and Hamza, 1999). In the present study, the DRASTIC parameters were improved by using new available data Corresponding author. Tel.: +216 71 872 600; fax: +216 885 408.
E-mail addresses: [email protected] (M.H. Hamza), [email protected] (A. Added), [email protected] (R. Rodrı´ guez). 0301-4797/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2006.04.004
related to the aquifer lithology, hydraulic conductivity, vadose zone lithology and soil cover. Furthermore, the parameter of aquifer net recharge was recalculated and used again in the elaboration of new generic DRASTIC maps. In our previous work, the net recharge was determined using the soil water balance equation. Indeed, some of the approximations applied affected the accuracy of the results, because some data are not available. The Williams and Kissel (1991) equation and the Rao (1970) relationship were used to recalculate the aquifer net recharge. These methods take into consideration fewer factors than the soil water balance equation, and consequently they allow an easier determination of the net recharge. The GIS technique used in this study was an effective tool. Making the data analyses easier to handle and providing high capabilities of dealing with large spatial data.
ARTICLE IN PRESS M.H. Hamza et al. / Journal of Environmental Management 84 (2007) 12–19
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Fig. 1. The study area.
2. Study area
3. Aquifer net recharge reassessment
The plain of Metline-Ras Jebel-Raf Raf (Fig. 1) is a coastal plain located in the northern east of Tunisia, between the latitudes North 514.411 and 526.215 and the longitudes East 431.657 and 440.351 (UTM Ref). The watershed has a surface of 50 km2. The alluvial aquifer occupies 35 km2 and is located in the Quaternary parts of the watershed. The main towns and villages in the watershed area are Metline, Ras Jebel, Raf Raf, Beni Ata and Sounine. The annual average rainfall varies between 510 and 638 mm. The water resources of this alluvial aquifer have a great economical importance for this agricultural region. In fact, more than 200 wells are currently exploited in the study area. A volume of 1 million m3/year from these wells (the equivalent of 43 mm) is used for irrigation. Moreover, a part of the study area surface belongs to an irrigated perimeter occupying 22 km2, created in the region of Ras Jebel by the Ministry of Agriculture in the 1990s. A volume of about 2.5 million m3 (the equivalent of 112 mm) is supplied every year from the river Oued Mejerda located outside the watershed, to this area.
In 1999, the soil water balance method (Thornthwaite, 1948) was used to calculate the annual aquifer net recharge. The following is the equation used in this method: R¼
12 X
ðPi þ Iri Þ R=Oi þ DW i ETRi ,
i¼1
where R ¼ annual net recharge, Pi ¼ the rainfall in the ith month (mm/month), Iri ¼ irrigation in the ith month (mm/ month), R/Oi ¼ runoff in the ith month (mm/month), DWi ¼ change in soil water storage in the ith month (mm/ month), and ETRi ¼ real evapotranspiration in the ith month (mm/month). Many approximations were made to calculate the aquifer net recharge (Hamza, 1999) because there is no a land use map with an appropriate scale to evaluate the runoff parameter and there are no data related to the change in soil water storage. Under these conditions, the calculated net recharge values varied between 46 and 178 mm, and were divided into three classes: 0–50 mm, 51–100 mm and
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M.H. Hamza et al. / Journal of Environmental Management 84 (2007) 12–19
Fig. 2. Metline-Ras Jebel-Raf Raf aquifer: net recharge map according to the soil water balance method (1999).
101–180 mm. The rating values are respectively 1, 3 and 6 (Fig. 2). The aim of this study was to compare results of the aquifer net recharge calculated with two simple methods (the Williams and Kissel equation and the Rao relationship), and then to reassess the vulnerability of the studied aquifer. The methods under consideration take into account the amount of precipitation and irrigation, and the Williams and Kissel equation includes one more factor: the hydrologic soil groups. The soil groups are defined according to the soil infiltration rates. The net recharge (R) was determined by applying the Williams and Kissel equation according to the following relations: R ¼ ðP 10:28Þ2 =ðP þ 15:43Þ for hydrologic soil group A; R ¼ ðP 15:05Þ2 =ðP þ 22:57Þ for hydrologic soil group B; R ¼ ðP 19:53Þ2 =ðP þ 29:29Þ for hydrologic soil group C; R ¼ ðP 22:67Þ2 =ðP þ 34:00Þ for hydrologic soil group D; where P is the annual average rainfall plus irrigation, expressed in inches. The irrigation includes the amount of water driven from wells (43 mm/year in all the study area) and that provided from the Mejerda river to the irrigated perimeter of Ras Jebel which covers 22 km2. The criteria used to classify the soil groups as A, B, C and D are the following: Soil group A: These soils have a high infiltration rate even when thoroughly wetted. They chiefly consist of deep, well drained to excessively drained sands or gravels. They have a high rate of water transmission.
Soil group B: These soils have a moderate infiltration rate when thoroughly wetted. They are chiefly moderately deep to deep, moderately well drained to well drained soils that have moderately fine to moderately coarse textures. They have a moderate rate of water transmission. Soil group C: These soils have a slow infiltration rate when thoroughly wetted. They chiefly have a layer that impedes downward movement of water or have moderately fine to fine texture. They have a slow rate of water transmission. Soil group D: These soils have a very slow infiltration rate when thoroughly wetted. They chiefly consist of clay soils that have a high swelling potential, soils that have a permanent high water table, soils that have a clay layer at or near the surface, and shallow soils over nearly impervious material. They have a very slow rate of water transmission. The hydrologic soil groups map of the area understudy was extracted from the soil map (with a scale of 1/12 500), and from some other specific studies where the soil map is not available. Therefore, three hydrologic groups were observed:
Group B: sandy soils, sandstone forest soils, and loamy sand soils, Group C: sandy loam soils, clayey sand soils, sandy clay soils and nodular crust soils, Group D: clayey hydromorph soils and urban perimeters soils.
The results showed that the net recharge varied between 38 and 112 mm. In limited climatologic homogenous areas (as it is the case in the present study), Rao (1970) has proposed
ARTICLE IN PRESS M.H. Hamza et al. / Journal of Environmental Management 84 (2007) 12–19
empirical relationships in order to determine the aquifer net recharge (R): R ¼ 0:20ðP 400Þ for areas with P between 400 and 600 mm; R ¼ 0:25ðP 400Þ for areas with P between 600 and 1000 mm; R ¼ 0:35ðP 600Þ for areas with P above 1000 mm; where R and P expressed in mm. The Rao relationship does not take into account the soil infiltration rates as the Williams and Kissel equation does. Since the P values in the study area belong to the second climatologic class (situated between 600 and 1000 mm), the calculated net recharge is between 61 and 108 mm. In fact, the use of the Williams and Kissel equation and the Rao relationship depend on the availability of data in the study area. The Williams and Kissel equation is recommended when the soil distribution in the area is available at an adequate scale, and the Rao relationship is preferred in territories where the soils are not precisely studied.
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Table 1 DRASTIC index ranges for vulnerability degrees Vulnerability degree
Drastic index
Low Medium High Very high
1–100 101–140 141–200 4200
4.2. The data The identification of the hydrogeological units and subunits as well as the assessment of the DRASTIC parameters, require a good knowledge of the geology, the hydrogeology, the soil media, the topography and the meteorology in the study area. The data used in this study are taken from geological studies (Burollet, 1951; El Ghali and Ben Ayed, 2000), a geophysical study (Essayeh, 1996), hydrogeological studies (Ennabli, 1969; Direction Ge´ne´rale des Ressources en Eau (DGRE), 1993–2003), a climatologic study (Institut de la Me´te´orologie Nationale (INM), 1993–2003), soil studies (Mansour, 1988; Fournet and Mouri, 1990; Arrondissement des Sols de Bizerte (ASB), 1997–2000) and a topographic study (Office de la Topographie et de la Cartographie (OTC), 1981).
4. Application of the generic DRASTIC method 4.3. Generic DRASTIC maps of the study area 4.1. The DRASTIC method DRASTIC is an empirical method that was developed by the US Environmental Protection Agency (US EPA) for evaluating the pollution potential of groundwater systems on a regional scale (Aller et al., 1987). This method is being used more and more in Europe and in Latin America (Lobo Ferreira and Oliveira, 1997; Ramos and Rodrı´ guez, 2003, etc.). 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. GIS is considered an adequate tool to use in the application of the DRASTIC methods. 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 pesticide DRASTIC version applied in the case of pesticides. The last version was already applied in the study area (Hamza et al., 2004). The DRASTIC index values measuring the aquifer vulnerability vary from 23 to 226 in the case of the generic version, and fall into four classes corresponding to four vulnerability degrees (Table 1.).
To elaborate the generic DRASTIC maps the GIS software ARC/Info and Idrisi were used: The depth to water map was obtained by interpolating the depth to water values recorded in 2003. These values were registered in about 60 wells homogenously distributed in the study area. The depth to water map obtained was classified according to the DRASTIC classes of depth to water. The values of aquifer net recharge calculated according to the Williams and Kissel equation and to the Rao relationship, were divided into classes according to the DRASTIC net recharge classification: – 0–50, 50–100 and 100–180 mm classes when the Williams and Kissel equation is used (Fig. 3). – 50–100 and 100–180 mm classes when the Rao relationship is used (Fig. 4). The aquifer media was determined in two ways. In the first way, we used the data related to the boring logs done by Ennabli in 1969, then we evaluated and updated the thickness of the aquifer zone according to the depth to water variation noted between 1969 and 2003. In the second way, we used the aquifer hydraulic conductivity data (see below), particularly in the areas where boring logs do not exist, to estimate the aquifer lithology (Ennabli, 1969). Finally, the map was established and classified according to the DRASTIC aquifer media classes.
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Fig. 3. Metline-Ras Jebel-Raf Raf aquifer: net recharge map according to the Williams and Kissel equation (2003).
Fig. 4. Metline-Ras Jebel-Raf Raf aquifer: net recharge map according to the Rao relationship (2003).
The soil data were extracted both from the 1/12 500 soil map covering a great part of the aquifer surface (Mansour, 1988; Fournet and Mouri, 1990), and from some other specific soil studies achieved by ASB (1997–2000). Nine soil classes were extracted and to each one a rating value was attributed. As regard to the soils of urban perimeters (not included in the DRASTIC soil classes), we attributed a rating of 1 because of their weak infiltration rate. The other soil classes of the study area are in conformity with the classes proposed by the DRASTIC method. The surface slope was calculated by using the three topographic maps (1/25 000 scale) covering the study area.
The slope map has been classified according to the surface slope classes of the DRASTIC method. In order to ascertain the lithology of the vadose zone, we established lithologic correlations between the available boring logs, and we used the geologic data extracted from the 1/50 000 maps of the study area (Burollet, 1951; El Ghali and Ben Ayed, 2000). The obtained map was classified according to the DRASTIC classes of the vadose zone. In some cases, the vadose zone lithology does not conform to the DRASTIC classification and in those cases, some considerations were made in order to give a logical value to the classes. For example, the DRASTIC rating value assigned to ‘‘sand and gravel’’ lithology is 8, and 6 to
ARTICLE IN PRESS M.H. Hamza et al. / Journal of Environmental Management 84 (2007) 12–19
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Fig. 5. Metline-Ras Jebel-Raf Raf aquifer: generic DRASTIC map determined by using the Williams and Kissel equation (2003) to calculate the net recharge parameter.
‘‘sandstone’’. Hence, we assigned a rating value equal to 7 for an intermediate lithology composed of ‘‘sand and little sandstone’’. Finally, the hydraulic conductivity of the aquifer was calculated according to the following equation: k ¼ T/b, where k is the hydraulic conductivity of the aquifer (m/s), T is the transmissivity (m2/s), and b the thickness of the aquifer expressed in m. As opposed to the study conducted in 1999, the transmissivity and the thickness of the aquifer have become known and this allows a better determination of the hydraulic conductivity of the aquifer. Additionally, taking into account the faults crossing the area, the hydraulic conductivity was reviewed and new values were assigned to faulted areas (Rodrı´ guez et al., 2001). Finally, by multiplying each mapped parameter by the value of its weight and also by adding the seven numerical maps, we were able to establish the generic DRASTIC vulnerability map.
5. Results and discussion The net recharge, calculated through the application of the Williams and Kissel equation and the Rao relationship, shows that the obtained results are close as they are between 38–112 and 61–108 mm, respectively. Three classes in the first method and two in the second were determined with respect to the DRASTIC net recharge classification (0–50, 50–100 and 100–180 mm). The corresponding rating values are 1, 3 and 6 in the first method and 3 and 6 in the second. Both generic DRASTIC maps were achieved at a better scale (1/25 000) than in 1999 (1/100 000), and show three classes of vulnerability: low, medium and high. When the
Williams and Kissel equation is used, the zones with medium vulnerability represent 59% of the total area, and those with low and high vulnerability occupy 35% and 6%, respectively (Fig. 5). However, using the Rao relationship, the zones of medium and high vulnerability (64% and 10%) are slightly more extended than in the previous case (Fig. 6). Differences between the two methods do not exceed 4–5% for the high and the medium and 9% for the low vulnerability. On one hand, it seems better to use the Williams and Kissel equation which takes into consideration the parameter of hydrologic soil group and indirectly the soil infiltration rates. On the other hand, as indicated previously, the Rao relationship can be recommended mainly in areas where soil data are not too accurate.
6. Conclusion The values obtained by the recalculation of net recharge in 2004 using Williams and Kissel’s equation (between 38 and 112 mm) and Rao’s relationship (between 61 and 108), are very different from the results obtained in 1999 using the soil water balance method. In fact, the latter method requires a lot of approximations because of lack of data in the study area, and that decreased the accuracy of the results. Compared to Rao’s relationship, Williams and Kissel’s equation takes into consideration the hydrologic soil group and consequently the soil infiltration rates. This method is preferentially used in cases in which the soil type was previously well studied in the study area. However, in a great part of Tunisia, Rao’s relationship could be used since the soil type has been not studied at a small scale.
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M.H. Hamza et al. / Journal of Environmental Management 84 (2007) 12–19
Fig. 6. Metline-Ras Jebel-Raf Raf aquifer: generic DRASTIC map determined by using the Rao relationship (2003) to calculate the net recharge parameter.
The generic DRASTIC vulnerability maps obtained in 2004 show that the greater part of the groundwater has a medium vulnerability to pollution (between 59% and 64% in accordance with the net recharge method used). The highly vulnerable zones occupy a surface varying between 6 and 10%. The new vulnerability maps are considered as operative maps (scale 1/25 000). They allow a better vision of the vulnerability to pollution and consequently a better aquifer management. Acknowledgments The authors would like to thank all those who provided us with the necessary data to accomplish our study. Appreciations go specially to Mr. Mohamed Habib Ben Said and Mr. Rachid Sta of the water resource institute (DGRE) of Bizerta and the reviewers for their critical comments. References Added, A., Hamza, M.H., 1999. Evaluation of the vulnerability in Metline aquifer (Northeast of Tunisia). ESRI User Conference, San Diego, USA. Aller, L., Bennet, T., Lehr, J.H., Petty, R.J., Hacket, G., 1987. DRASTIC: a standardised system for evaluating ground water pollution potential using hydrogeologic settings. US Environmental Protection Agency Report (EPA/600/2-87/035), Robert S. Kerr Environmental Research Laboratory, 455pp. Arrondissement des Sols de Bizerte (ASB), 1997–2000. Comptes rendus d’e´tudes pe´dologiques e´tablis a` Metline, Ras Jebel et Raf Raf. Burollet, P.F., 1951. Etude ge´ologique des bassins Mio-Plioce`nes du Nord Est de la Tunisie. Annales des Mines et Ge´ologie 7, 82 (+annexes+cartes 1/50.000). Direction Ge´ne´rale des Ressources en Eau (DGRE), 1993–2003. Annuaires pie´zome´triques de la Tunisie.
El Ghali, A., Ben Ayed, N., 2000. Carte ge´ologique au 1/50 000 de Metline. Publications du Service Ge´ologique de Tunisie. Ennabli, M., 1969. Etude hydroge´ologique de la plaine de Ras Jebel. Rapport interne DGRE, re´f 7/57, 134pp. +annexes. Essayeh, F., 1996. Apport de la me´thode de prospection e´lectrique a` l’e´tude des proble`mes d’intrusion marine dans la plaine de Metline Ras Jebel Raf Raf. DEA, Univ. Tunis II, FST,, 100pp. +annexes. Fournet, A., Mouri, A., 1990. Reconnaissance pe´dologique des pe´rime`tres d’irrigation de Beni Ata et de Chaab Eddoud. Rapport interne, Direction des Sols. Hamza, M.H., 1999. Etude de la vulne´rabilite´ a` la pollution potentielle de la nappe de Ras Jebel par les syste`mes d’information ge´ographique. DEA, University of Tunis II,, 102pp. Hamza, M.H., Added, A., Ben Mammou, A., Abdeljaoued, S., Rodrı´ guez, R., 2004. E´valuation de la vulne´rabilite´ a` la pollution potentielle par les pesticides, de la nappe coˆtie`re alluvionnaire de la plaine de Metline-Ras Jebel-Raf Raf, Nord-Est tunisien, selon la me´thode DRASTIC applique´e par les syste`mes d’information ge´ographique. La Houille Blanche Revue Internationale de l’Eau 2004/ 2005, 86–94. Institut de la Me´te´orologie Nationale (INM), 1993–2003. Tableaux climatologiques mensuels, Station Bizerte-Sidi Ahmed. Lobo Ferreira, J.P., Oliveira, M.M., 1997. DRASTIC groundwater vulnerability mapping of Portugal. In: Groundwater: An Endangered Resource. Proceedings of Theme C of the 27th Congress of the International Association for Hydraulic Research, San Francisco, USA. Mansour, H., 1988. Carte pe´dologique de Beni Ata, Ras Jebel et Raf Raf au 1/12.500. Publ. Direction des sols de Tunisie. Feuilles de Metline S.O., Metline S.E., Ghar El Melh N.E. 1981. Office de la Topographie et de la Cartographie (OTC). Carte topographique de la Tunisie au 1/25.000. Ramos, J.A., Rodrı´ guez, R., 2003. Aquifer vulnerability mapping in the Turbio River valley, Mexico: a validation study. Geofisica International 42 (1), 141–156. Rao, K., 1970. Hydrometeorological aspects of estimating ground water potential. In: Seminar on Ground Water Potential, Bangalore, Geological Society of India, pp. 1–18. Rodrı´ guez, R., Reyes, R., Rosales, J., Berlin, J., Mejia, J.A., Ramos, A., 2001. Estructuracion de mapas tematicos de indices de vulnerabilidad
ARTICLE IN PRESS M.H. Hamza et al. / Journal of Environmental Management 84 (2007) 12–19 acuifera de la mancha urbana de Salamanca Gto. Technical Report, Municipio de Salamanca, CEAG, IGF-UNAM, 120pp. Thornthwaite, C.W., 1948. An approach toward a rational classification of climate. Geographical Review, New York 38 (1), 55–94.
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Williams, J.R., Kissel, D.E., 1991. In: R.F. Follett, D.R. Keeney, R.M. Cruse (Eds.), Water Percolation: An Indicator of Nitrogen-leaching Potential in Managing Nitrogen for Groundwater Quality and Farm Profitability, pp. 59–83.
Journal of Environmental Management 84 (2007) 12–19 www.elsevier.com/locate/jenvman
A GIS-based DRASTIC vulnerability and net recharge reassessment in an aquifer of a semi-arid region (Metline-Ras Jebel-Raf Raf aquifer, Northern Tunisia) M.H. Hamzaa, A. Addeda,, R. Rodrı´ guezb, S. Abdeljaoueda, A. Ben Mammoua a
Universite´ de Tunis El Manar, Faculte´ des Sciences de Tunis, De´partement de Ge´ologie, Laboratoire des Ressources Mine´rales et Environnement, 2092 Tunis, Tunisie b Universidad Nacional Autonoma de Me´xico, Instituto de Geofı´sica. Ciudad Universitaria., 04510 Me´xico D.F., Me´xico Received 28 November 2004; received in revised form 8 April 2006; accepted 9 April 2006 Available online 25 July 2006
Abstract This paper aims to elaborate new generic DRASTIC aquifer vulnerability maps of the coastal aquifer of Metline-Ras Jebel-Raf Raf (Northeast of Tunisia) using the GIS technique, making the data analyses easier to handle and providing better capabilities of dealing with large spatial data. A similar study was carried out in 1999 in the same aquifer using a method based on the soil water balance equation to determine the net recharge parameter. Unfortunately, the lack of data in the study area made the results unsatisfactory. By applying the Williams and Kissel equation and the Rao relationship, we intend to demonstrate that we could correctly evaluate the net recharge parameter. Moreover, new data related to the aquifer hydraulic conductivity, the soil cover and the vadose zone lithology have become available, and allowed us to elaborate suitable DRASTIC maps. r 2006 Elsevier Ltd. All rights reserved. Keywords: Reassessment; Net recharge; Aquifer vulnerability; DRASTIC; GIS
1. Introduction The use of chemical fertilizers for agricultural reasons and the settlement of factories in the study area located in the plain of Metline-Ras Jebel-Raf Raf (northern east of Tunisia) during the last decades, are the main causes of groundwater pollution. Thus, a map of aquifer vulnerability to pollution is highly needed to locate the vulnerable areas that can be affected. Vulnerability of the Metline-Ras Jebel-Raf Raf aquifer to pollution by inorganic pollutants was studied in 1999, by using the generic DRASTIC model (Hamza, 1999; Added and Hamza, 1999). In the present study, the DRASTIC parameters were improved by using new available data Corresponding author. Tel.: +216 71 872 600; fax: +216 885 408.
E-mail addresses: [email protected] (M.H. Hamza), [email protected] (A. Added), [email protected] (R. Rodrı´ guez). 0301-4797/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2006.04.004
related to the aquifer lithology, hydraulic conductivity, vadose zone lithology and soil cover. Furthermore, the parameter of aquifer net recharge was recalculated and used again in the elaboration of new generic DRASTIC maps. In our previous work, the net recharge was determined using the soil water balance equation. Indeed, some of the approximations applied affected the accuracy of the results, because some data are not available. The Williams and Kissel (1991) equation and the Rao (1970) relationship were used to recalculate the aquifer net recharge. These methods take into consideration fewer factors than the soil water balance equation, and consequently they allow an easier determination of the net recharge. The GIS technique used in this study was an effective tool. Making the data analyses easier to handle and providing high capabilities of dealing with large spatial data.
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13
Fig. 1. The study area.
2. Study area
3. Aquifer net recharge reassessment
The plain of Metline-Ras Jebel-Raf Raf (Fig. 1) is a coastal plain located in the northern east of Tunisia, between the latitudes North 514.411 and 526.215 and the longitudes East 431.657 and 440.351 (UTM Ref). The watershed has a surface of 50 km2. The alluvial aquifer occupies 35 km2 and is located in the Quaternary parts of the watershed. The main towns and villages in the watershed area are Metline, Ras Jebel, Raf Raf, Beni Ata and Sounine. The annual average rainfall varies between 510 and 638 mm. The water resources of this alluvial aquifer have a great economical importance for this agricultural region. In fact, more than 200 wells are currently exploited in the study area. A volume of 1 million m3/year from these wells (the equivalent of 43 mm) is used for irrigation. Moreover, a part of the study area surface belongs to an irrigated perimeter occupying 22 km2, created in the region of Ras Jebel by the Ministry of Agriculture in the 1990s. A volume of about 2.5 million m3 (the equivalent of 112 mm) is supplied every year from the river Oued Mejerda located outside the watershed, to this area.
In 1999, the soil water balance method (Thornthwaite, 1948) was used to calculate the annual aquifer net recharge. The following is the equation used in this method: R¼
12 X
ðPi þ Iri Þ R=Oi þ DW i ETRi ,
i¼1
where R ¼ annual net recharge, Pi ¼ the rainfall in the ith month (mm/month), Iri ¼ irrigation in the ith month (mm/ month), R/Oi ¼ runoff in the ith month (mm/month), DWi ¼ change in soil water storage in the ith month (mm/ month), and ETRi ¼ real evapotranspiration in the ith month (mm/month). Many approximations were made to calculate the aquifer net recharge (Hamza, 1999) because there is no a land use map with an appropriate scale to evaluate the runoff parameter and there are no data related to the change in soil water storage. Under these conditions, the calculated net recharge values varied between 46 and 178 mm, and were divided into three classes: 0–50 mm, 51–100 mm and
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M.H. Hamza et al. / Journal of Environmental Management 84 (2007) 12–19
Fig. 2. Metline-Ras Jebel-Raf Raf aquifer: net recharge map according to the soil water balance method (1999).
101–180 mm. The rating values are respectively 1, 3 and 6 (Fig. 2). The aim of this study was to compare results of the aquifer net recharge calculated with two simple methods (the Williams and Kissel equation and the Rao relationship), and then to reassess the vulnerability of the studied aquifer. The methods under consideration take into account the amount of precipitation and irrigation, and the Williams and Kissel equation includes one more factor: the hydrologic soil groups. The soil groups are defined according to the soil infiltration rates. The net recharge (R) was determined by applying the Williams and Kissel equation according to the following relations: R ¼ ðP 10:28Þ2 =ðP þ 15:43Þ for hydrologic soil group A; R ¼ ðP 15:05Þ2 =ðP þ 22:57Þ for hydrologic soil group B; R ¼ ðP 19:53Þ2 =ðP þ 29:29Þ for hydrologic soil group C; R ¼ ðP 22:67Þ2 =ðP þ 34:00Þ for hydrologic soil group D; where P is the annual average rainfall plus irrigation, expressed in inches. The irrigation includes the amount of water driven from wells (43 mm/year in all the study area) and that provided from the Mejerda river to the irrigated perimeter of Ras Jebel which covers 22 km2. The criteria used to classify the soil groups as A, B, C and D are the following: Soil group A: These soils have a high infiltration rate even when thoroughly wetted. They chiefly consist of deep, well drained to excessively drained sands or gravels. They have a high rate of water transmission.
Soil group B: These soils have a moderate infiltration rate when thoroughly wetted. They are chiefly moderately deep to deep, moderately well drained to well drained soils that have moderately fine to moderately coarse textures. They have a moderate rate of water transmission. Soil group C: These soils have a slow infiltration rate when thoroughly wetted. They chiefly have a layer that impedes downward movement of water or have moderately fine to fine texture. They have a slow rate of water transmission. Soil group D: These soils have a very slow infiltration rate when thoroughly wetted. They chiefly consist of clay soils that have a high swelling potential, soils that have a permanent high water table, soils that have a clay layer at or near the surface, and shallow soils over nearly impervious material. They have a very slow rate of water transmission. The hydrologic soil groups map of the area understudy was extracted from the soil map (with a scale of 1/12 500), and from some other specific studies where the soil map is not available. Therefore, three hydrologic groups were observed:
Group B: sandy soils, sandstone forest soils, and loamy sand soils, Group C: sandy loam soils, clayey sand soils, sandy clay soils and nodular crust soils, Group D: clayey hydromorph soils and urban perimeters soils.
The results showed that the net recharge varied between 38 and 112 mm. In limited climatologic homogenous areas (as it is the case in the present study), Rao (1970) has proposed
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empirical relationships in order to determine the aquifer net recharge (R): R ¼ 0:20ðP 400Þ for areas with P between 400 and 600 mm; R ¼ 0:25ðP 400Þ for areas with P between 600 and 1000 mm; R ¼ 0:35ðP 600Þ for areas with P above 1000 mm; where R and P expressed in mm. The Rao relationship does not take into account the soil infiltration rates as the Williams and Kissel equation does. Since the P values in the study area belong to the second climatologic class (situated between 600 and 1000 mm), the calculated net recharge is between 61 and 108 mm. In fact, the use of the Williams and Kissel equation and the Rao relationship depend on the availability of data in the study area. The Williams and Kissel equation is recommended when the soil distribution in the area is available at an adequate scale, and the Rao relationship is preferred in territories where the soils are not precisely studied.
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Table 1 DRASTIC index ranges for vulnerability degrees Vulnerability degree
Drastic index
Low Medium High Very high
1–100 101–140 141–200 4200
4.2. The data The identification of the hydrogeological units and subunits as well as the assessment of the DRASTIC parameters, require a good knowledge of the geology, the hydrogeology, the soil media, the topography and the meteorology in the study area. The data used in this study are taken from geological studies (Burollet, 1951; El Ghali and Ben Ayed, 2000), a geophysical study (Essayeh, 1996), hydrogeological studies (Ennabli, 1969; Direction Ge´ne´rale des Ressources en Eau (DGRE), 1993–2003), a climatologic study (Institut de la Me´te´orologie Nationale (INM), 1993–2003), soil studies (Mansour, 1988; Fournet and Mouri, 1990; Arrondissement des Sols de Bizerte (ASB), 1997–2000) and a topographic study (Office de la Topographie et de la Cartographie (OTC), 1981).
4. Application of the generic DRASTIC method 4.3. Generic DRASTIC maps of the study area 4.1. The DRASTIC method DRASTIC is an empirical method that was developed by the US Environmental Protection Agency (US EPA) for evaluating the pollution potential of groundwater systems on a regional scale (Aller et al., 1987). This method is being used more and more in Europe and in Latin America (Lobo Ferreira and Oliveira, 1997; Ramos and Rodrı´ guez, 2003, etc.). 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. GIS is considered an adequate tool to use in the application of the DRASTIC methods. 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 pesticide DRASTIC version applied in the case of pesticides. The last version was already applied in the study area (Hamza et al., 2004). The DRASTIC index values measuring the aquifer vulnerability vary from 23 to 226 in the case of the generic version, and fall into four classes corresponding to four vulnerability degrees (Table 1.).
To elaborate the generic DRASTIC maps the GIS software ARC/Info and Idrisi were used: The depth to water map was obtained by interpolating the depth to water values recorded in 2003. These values were registered in about 60 wells homogenously distributed in the study area. The depth to water map obtained was classified according to the DRASTIC classes of depth to water. The values of aquifer net recharge calculated according to the Williams and Kissel equation and to the Rao relationship, were divided into classes according to the DRASTIC net recharge classification: – 0–50, 50–100 and 100–180 mm classes when the Williams and Kissel equation is used (Fig. 3). – 50–100 and 100–180 mm classes when the Rao relationship is used (Fig. 4). The aquifer media was determined in two ways. In the first way, we used the data related to the boring logs done by Ennabli in 1969, then we evaluated and updated the thickness of the aquifer zone according to the depth to water variation noted between 1969 and 2003. In the second way, we used the aquifer hydraulic conductivity data (see below), particularly in the areas where boring logs do not exist, to estimate the aquifer lithology (Ennabli, 1969). Finally, the map was established and classified according to the DRASTIC aquifer media classes.
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Fig. 3. Metline-Ras Jebel-Raf Raf aquifer: net recharge map according to the Williams and Kissel equation (2003).
Fig. 4. Metline-Ras Jebel-Raf Raf aquifer: net recharge map according to the Rao relationship (2003).
The soil data were extracted both from the 1/12 500 soil map covering a great part of the aquifer surface (Mansour, 1988; Fournet and Mouri, 1990), and from some other specific soil studies achieved by ASB (1997–2000). Nine soil classes were extracted and to each one a rating value was attributed. As regard to the soils of urban perimeters (not included in the DRASTIC soil classes), we attributed a rating of 1 because of their weak infiltration rate. The other soil classes of the study area are in conformity with the classes proposed by the DRASTIC method. The surface slope was calculated by using the three topographic maps (1/25 000 scale) covering the study area.
The slope map has been classified according to the surface slope classes of the DRASTIC method. In order to ascertain the lithology of the vadose zone, we established lithologic correlations between the available boring logs, and we used the geologic data extracted from the 1/50 000 maps of the study area (Burollet, 1951; El Ghali and Ben Ayed, 2000). The obtained map was classified according to the DRASTIC classes of the vadose zone. In some cases, the vadose zone lithology does not conform to the DRASTIC classification and in those cases, some considerations were made in order to give a logical value to the classes. For example, the DRASTIC rating value assigned to ‘‘sand and gravel’’ lithology is 8, and 6 to
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Fig. 5. Metline-Ras Jebel-Raf Raf aquifer: generic DRASTIC map determined by using the Williams and Kissel equation (2003) to calculate the net recharge parameter.
‘‘sandstone’’. Hence, we assigned a rating value equal to 7 for an intermediate lithology composed of ‘‘sand and little sandstone’’. Finally, the hydraulic conductivity of the aquifer was calculated according to the following equation: k ¼ T/b, where k is the hydraulic conductivity of the aquifer (m/s), T is the transmissivity (m2/s), and b the thickness of the aquifer expressed in m. As opposed to the study conducted in 1999, the transmissivity and the thickness of the aquifer have become known and this allows a better determination of the hydraulic conductivity of the aquifer. Additionally, taking into account the faults crossing the area, the hydraulic conductivity was reviewed and new values were assigned to faulted areas (Rodrı´ guez et al., 2001). Finally, by multiplying each mapped parameter by the value of its weight and also by adding the seven numerical maps, we were able to establish the generic DRASTIC vulnerability map.
5. Results and discussion The net recharge, calculated through the application of the Williams and Kissel equation and the Rao relationship, shows that the obtained results are close as they are between 38–112 and 61–108 mm, respectively. Three classes in the first method and two in the second were determined with respect to the DRASTIC net recharge classification (0–50, 50–100 and 100–180 mm). The corresponding rating values are 1, 3 and 6 in the first method and 3 and 6 in the second. Both generic DRASTIC maps were achieved at a better scale (1/25 000) than in 1999 (1/100 000), and show three classes of vulnerability: low, medium and high. When the
Williams and Kissel equation is used, the zones with medium vulnerability represent 59% of the total area, and those with low and high vulnerability occupy 35% and 6%, respectively (Fig. 5). However, using the Rao relationship, the zones of medium and high vulnerability (64% and 10%) are slightly more extended than in the previous case (Fig. 6). Differences between the two methods do not exceed 4–5% for the high and the medium and 9% for the low vulnerability. On one hand, it seems better to use the Williams and Kissel equation which takes into consideration the parameter of hydrologic soil group and indirectly the soil infiltration rates. On the other hand, as indicated previously, the Rao relationship can be recommended mainly in areas where soil data are not too accurate.
6. Conclusion The values obtained by the recalculation of net recharge in 2004 using Williams and Kissel’s equation (between 38 and 112 mm) and Rao’s relationship (between 61 and 108), are very different from the results obtained in 1999 using the soil water balance method. In fact, the latter method requires a lot of approximations because of lack of data in the study area, and that decreased the accuracy of the results. Compared to Rao’s relationship, Williams and Kissel’s equation takes into consideration the hydrologic soil group and consequently the soil infiltration rates. This method is preferentially used in cases in which the soil type was previously well studied in the study area. However, in a great part of Tunisia, Rao’s relationship could be used since the soil type has been not studied at a small scale.
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Fig. 6. Metline-Ras Jebel-Raf Raf aquifer: generic DRASTIC map determined by using the Rao relationship (2003) to calculate the net recharge parameter.
The generic DRASTIC vulnerability maps obtained in 2004 show that the greater part of the groundwater has a medium vulnerability to pollution (between 59% and 64% in accordance with the net recharge method used). The highly vulnerable zones occupy a surface varying between 6 and 10%. The new vulnerability maps are considered as operative maps (scale 1/25 000). They allow a better vision of the vulnerability to pollution and consequently a better aquifer management. Acknowledgments The authors would like to thank all those who provided us with the necessary data to accomplish our study. Appreciations go specially to Mr. Mohamed Habib Ben Said and Mr. Rachid Sta of the water resource institute (DGRE) of Bizerta and the reviewers for their critical comments. References Added, A., Hamza, M.H., 1999. Evaluation of the vulnerability in Metline aquifer (Northeast of Tunisia). ESRI User Conference, San Diego, USA. Aller, L., Bennet, T., Lehr, J.H., Petty, R.J., Hacket, G., 1987. DRASTIC: a standardised system for evaluating ground water pollution potential using hydrogeologic settings. US Environmental Protection Agency Report (EPA/600/2-87/035), Robert S. Kerr Environmental Research Laboratory, 455pp. Arrondissement des Sols de Bizerte (ASB), 1997–2000. Comptes rendus d’e´tudes pe´dologiques e´tablis a` Metline, Ras Jebel et Raf Raf. Burollet, P.F., 1951. Etude ge´ologique des bassins Mio-Plioce`nes du Nord Est de la Tunisie. Annales des Mines et Ge´ologie 7, 82 (+annexes+cartes 1/50.000). Direction Ge´ne´rale des Ressources en Eau (DGRE), 1993–2003. Annuaires pie´zome´triques de la Tunisie.
El Ghali, A., Ben Ayed, N., 2000. Carte ge´ologique au 1/50 000 de Metline. Publications du Service Ge´ologique de Tunisie. Ennabli, M., 1969. Etude hydroge´ologique de la plaine de Ras Jebel. Rapport interne DGRE, re´f 7/57, 134pp. +annexes. Essayeh, F., 1996. Apport de la me´thode de prospection e´lectrique a` l’e´tude des proble`mes d’intrusion marine dans la plaine de Metline Ras Jebel Raf Raf. DEA, Univ. Tunis II, FST,, 100pp. +annexes. Fournet, A., Mouri, A., 1990. Reconnaissance pe´dologique des pe´rime`tres d’irrigation de Beni Ata et de Chaab Eddoud. Rapport interne, Direction des Sols. Hamza, M.H., 1999. Etude de la vulne´rabilite´ a` la pollution potentielle de la nappe de Ras Jebel par les syste`mes d’information ge´ographique. DEA, University of Tunis II,, 102pp. Hamza, M.H., Added, A., Ben Mammou, A., Abdeljaoued, S., Rodrı´ guez, R., 2004. E´valuation de la vulne´rabilite´ a` la pollution potentielle par les pesticides, de la nappe coˆtie`re alluvionnaire de la plaine de Metline-Ras Jebel-Raf Raf, Nord-Est tunisien, selon la me´thode DRASTIC applique´e par les syste`mes d’information ge´ographique. La Houille Blanche Revue Internationale de l’Eau 2004/ 2005, 86–94. Institut de la Me´te´orologie Nationale (INM), 1993–2003. Tableaux climatologiques mensuels, Station Bizerte-Sidi Ahmed. Lobo Ferreira, J.P., Oliveira, M.M., 1997. DRASTIC groundwater vulnerability mapping of Portugal. In: Groundwater: An Endangered Resource. Proceedings of Theme C of the 27th Congress of the International Association for Hydraulic Research, San Francisco, USA. Mansour, H., 1988. Carte pe´dologique de Beni Ata, Ras Jebel et Raf Raf au 1/12.500. Publ. Direction des sols de Tunisie. Feuilles de Metline S.O., Metline S.E., Ghar El Melh N.E. 1981. Office de la Topographie et de la Cartographie (OTC). Carte topographique de la Tunisie au 1/25.000. Ramos, J.A., Rodrı´ guez, R., 2003. Aquifer vulnerability mapping in the Turbio River valley, Mexico: a validation study. Geofisica International 42 (1), 141–156. Rao, K., 1970. Hydrometeorological aspects of estimating ground water potential. In: Seminar on Ground Water Potential, Bangalore, Geological Society of India, pp. 1–18. Rodrı´ guez, R., Reyes, R., Rosales, J., Berlin, J., Mejia, J.A., Ramos, A., 2001. Estructuracion de mapas tematicos de indices de vulnerabilidad
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Williams, J.R., Kissel, D.E., 1991. In: R.F. Follett, D.R. Keeney, R.M. Cruse (Eds.), Water Percolation: An Indicator of Nitrogen-leaching Potential in Managing Nitrogen for Groundwater Quality and Farm Profitability, pp. 59–83.