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Dec 3, 2004 - carbonate aquifer, in the NE part of Sierra de Mijas. (Andalusia, S Spain), to the west of the town of Torre- molinos, in the Costa del Sol area ...

Environ Geol (2005) 47: 586–595 DOI 10.1007/s00254-004-1185-y

J. M. Vı´ as B. Andreo M. J. Perles F. Carrasco

Received: 18 May 2004 Accepted: 27 September 2004 Published online: 3 December 2004 Ó Springer-Verlag 2004

J. M. Vı´ as Æ B. Andreo (&) M. J. Perles Æ F. Carrasco Department of Geography, University of Ma´laga, Ma´laga, 29071, Spain E-mail: [email protected] Tel.: +34-95-2132004 Fax: +34-95-2132000

ORIGINAL ARTICLE

A comparative study of four schemes for groundwater vulnerability mapping in a diffuse flow carbonate aquifer under Mediterranean climatic conditions

Abstract This paper shows the results of a comparative study involving application of the vulnerability mapping methods known as AVI, GOD, DRASTIC and EPIK to a pilot carbonate massif in southern Spain, namely the Torremolinos aquifer. The main objectives of the study were to determine which methods are most suitable for diffuse flow carbonate aquifers such as in southern Spain, and to evaluate variations in the degree of vulnerability associated to the rainfall variations that normally occur in a Mediterranean climate. According to three of the above methods, the aquifer is moderately vulnerable, but the AVI method evaluated it as highly vulnerable—this, however, is improbable. The vulnerability maps reflect the great importance of geol-

Introduction Groundwater is a natural resource often subjected to severe human impact. In Mediterranean regions, pressure on groundwater resources involves two factors: water has traditionally been scarce due to climatic conditions, i.e., low rainfall and high evaporation, and there has been an increase in the tourist population along the whole Mediterranean coast and particularly in the Costa del Sol area (Andalusia, southern Spain), where the population doubles during the summer months. In this respect, a new problem has arisen because many potentially contaminating human activities have been developed above the aquifers. Therefore, strategies such as vulnerability mapping are required to preserve

ogy-related parameters (mainly those concerned with lithology) and, to a lesser degree, that of the depth of the groundwater table which is related to the rainfall. After this latter parameter, it is possible to distinguish between humid and dry climatic situations; thus, vulnerability increases in a humid year, especially according to the GOD and AVI methods. In conclusion, the GOD method seems the most adequate of the methods applied in this work for vulnerability mapping of diffuse flow carbonate aquifers in the Mediterranean domains. Keywords Vulnerability mapping Æ DRASTIC Æ GOD Æ AVI and EPIK methods Æ Diffuse flow carbonate aquifer Æ Mediterranean climate Æ Southern Spain

optimum groundwater quantity and quality. Therefore, management of this vital natural resource has become a worldwide priority. Vulnerability maps have become an ever more essential tool for groundwater protection and environmental management. Between 1997 and 2003, the European Union supported COST Action 620 ‘‘Vulnerability mapping for the protection of carbonate (karst) aquifers’’ to compare assessment methods for vulnerability mapping and to develop a general procedure for application in European carbonate aquifers (Daly et al. 2002; Zwahlen 2004). Over large areas of Europe, groundwater from carbonate aquifers constitutes an important natural resource for drinking water supply and, for this reason, protection schemes have

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been adopted. The project was given additional impetus by the European Water Framework Directive (2000), which is intended to provide a common framework for water resource policy and management. In the last 30 years, the international scientific community has shown great interest in groundwater quality and, thus, many publications focused on environmental management for groundwater protection (Foster 1987; Adams and Foster 1992; Robins et al. 1994; Vrba and Zaporozec 1994; Ho¨tzl 1996; Daly and Drew 1999). Since the concept of vulnerability to the contamination was introduced by Albinet and Margat (1970), many methods have been proposed for vulnerability mapping of aquifers, including DRASTIC (Aller et al. 1987), GOD (Foster 1987), AVI (Van Stempvoort et al. 1993), SINTACTS (Civita 1994), EPIK (Doerfliger et al. 1999); and PI (Goldscheider et al. 2000). The above acronyms normally stand for the factors that are considered for vulnerability assessment. These are explained in the ‘‘Methodology’’ section. Gogu and Dassargues (2000) have done a complete overview of several existing methods on groundwater vulnerability assessment, especially on the way these methods have been developed and applied and the future challenges on vulnerability mapping. The present manuscript completes the former work because it is not a theoretical discussion on the different methods of vulnerability mapping, but a practical application of several methods in a pilot site with climatic, geological and hydrogeological characteristics representative of Mediterranean carbonate aquifers poorly karstified. Thus, the results obtained in this manuscript could be potentially useful for vulnerability mapping in this type of aquifer. This work contains new contributions to the paper written by Gogu and Dassargues (2000) in connection with two main aims: (1) to analyze the results obtained by different methods of vulnerability mapping to determine which is the most suitable for diffuse flow carbonate aquifers in southern Spain and (2) to evaluate how vulnerability depends on the quantity of precipitation. The work was carried out in the Torremolinos carbonate aquifer, in the NE part of Sierra de Mijas (Andalusia, S Spain), to the west of the town of Torremolinos, in the Costa del Sol area (Fig. 1). This aquifer was selected as a Spanish experimental area as part of the COST 620 Action (Zwahlen 2004), because the two conditions mentioned at the beginning of this chapter are present (Andreo et al. 2000, 2002): water resources are scarce and there is a potential risk of contamination due to human impact.

Characteristics of the pilot site The Torremolinos aquifer extends over 56 km2 and its topography, like that of most carbonate aquifers in

southern Spain, is very abrupt except at the northern and eastern edges. The average annual temperature is 18.3°C and the rainfall is 630 mm per year, although it is highly irregular in time. Two rainwater patterns can be distinguished: humid years (precipitation >700 mm) and dry years (100 0.4

Results After using each method to evaluate the vulnerability indexes, these are expressed within a range of five intervals, from ‘‘very high’’ to ‘‘very low’’, to standardize the legends of the figures and to obtain comparable vulnerability maps for different rainfall conditions. The map obtained by the DRASTIC method (Fig. 2) shows a ‘‘low’’ degree of vulnerability for the marl rocks and a ‘‘moderate’’ vulnerability for the marbles. The areas of ‘‘very low’’ vulnerability are due to the existence of clay soils (mainly calcisol according FAO classification) with low hydraulic conductivity. Only small differences in vulnerability were found between a humid year (Fig. 2a) and a dry one (Fig. 2b) but, in any case, these are due to variations in the piezometric level and, especially, to the variations in the recharge. Thus, the vulnerability is lesser, although not greatly so, in a dry year than in a humid year. Moreover, the recharge is highly dependent on the hydraulic conductivity of the unsaturated zone and, therefore, on the lithology. Using the GOD method, the Torremolinos aquifer has a ‘‘moderate’’ degree of vulnerability in the carbonate rocks and a ‘‘low’’ degree in the Pliocene-Quaternary materials (Fig. 3a, b). The variations in vulnerability between the maps corresponding to humid and dry years are also due to differences in the depth of the water table. Thus, in several zones near the borders of the aquifer, where human activities are more evident (Andreo et al. 2002, Vı´ as 2003), the vulnerability increases from a ‘‘moderate’’ to a ‘‘high’’ degree if a dry year is compared with a humid year, because the water table rises closer to the surface. Using the AVI method, ‘‘high’’ and ‘‘very high’’ degrees of vulnerability are found in the marbles and ‘‘low’’ and ‘‘very low’’ degrees in the Pliocene-Quaternary marls (Fig. 4a, b). The hydraulic conductivity of the unsaturated zone (especially the soil) is the parameter that most determines vulnerability, but in the Torremolinos aquifer, soil is practically absent and this fact greatly increases the vulnerability. Within the marbles, the vulnerability varies, depending especially on the thickness of the unsaturated zone and, thus, on the

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Table 4 Rating values of the vulnerability parameters for EPIK method E (epikarst)

Range Rating

Highly fractured in quarries and roads E1=1

Rest of catchment area E3=4

P (protection cover)

Range

Leptosols and soils on quarries

Regosols, anthrosols, calcisols

Rating Range (out of catchment area)

P1=1 Areas collecting runoff water (buffer 50 and 100 m) and slopes feeding those areas (slope higher than 10% for cultivated sectors and 25% for meadows and pastures) I3=3 Triassic marbles K3=3

P2=2 Rest of area

Soils on layers that have very low hydraulic conductivity and thickness >400 m P4=4

I (infiltration)

K (karst network)

Rating Range Rating

depth of the water table. The vulnerability increases by one degree where the water table is less than 100 m deep (Vı´ as 2003). The vulnerability distribution in the map for a dry year is similar to that obtained by the GOD method for a humid year, although the degrees of vulnerability are higher in the map deduced by the AVI method.

Fig. 2 Vulnerability maps for 1996–1997 humid hydrological year (a) and 1994–1995 dry hydrological year (b) obtained by the DRASTIC method

I4=4

Finally, the EPIK method permits us to obtain only one vulnerability map (Fig. 5) because it only considers the intrinsic parameters of the aquifer; thus the vulnerability map is the same for humid and dry climatic conditions. In general, this method gives a similar vulnerability distribution to that obtained with the DRASTIC and GOD methods, that is, a ‘‘low’’ degree

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Fig. 3 Vulnerability maps for 1996–1997 humid hydrological year (a) and 1994–1995 dry hydrological year (b) obtained by the GOD method

of vulnerability in the Pliocene-Quaternary materials and a ‘‘moderate’’ degree of vulnerability in the Triassic marbles (Table 5). However, there are exceptions in Fig. 4 Vulnerability maps for 1996–1997 humid hydrological year (a) and 1994–1995 dry hydrological year (b) obtained by the AVI method

zones where an important degree of fracturing occurs, such as in quarries or in the slopes of the roads, which raises the vulnerability from ‘‘moderate’’ to ‘‘high’’;

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Fig. 5 Vulnerability map obtained by the EPIK method

where infiltration conditions are supposed favorables, vulnerability becomes ‘‘very high’’.

Discussion Each method of vulnerability evaluation results in a different map, although they all show the same distribution of spatial variability, i.e., a smaller degree of vulnerability of the Pliocene-Quaternary marls with respect to the Triassic marbles. To compare and discuss the results of the four methods, the authors calculated the percentage of the surface of the study area assigned to each degree of the vulnerability mapping method (Table 2). The vulnerability classed as ‘‘moderate’’ by the DRASTIC, GOD and EPIK methods (Figs. 2, 3 and 5) basically concurs with the presence of marbles (Fig. 1), approximately 85% of the study area. The AVI method, however, evaluates near 75% of the aquifer marbles as presenting a ‘‘High’’ degree of vulnerability and assigns a ‘‘very high’’ degree to a further 10% (Fig. 4 and Table 2). Therefore, the AVI method reports a higher degree of vulnerability of the aquifer than do the other methods; this seems improbable because no evidence of contamination in the groundwater has been detected in several decades, despite the existence of various human activities Table 5 Percentage of surface area according to the degree of vulnerability calculated by the different methods for humid and dry climatic conditions

Vulnerability

Very high High Moderate Low Very low

potentially contaminant developed over the marbles (landfill, waste pipe lines in urbanisations, petrol station). Thus, a ‘‘Moderate’’ vulnerability for the marbles is coherent with the characteristics of the aquifer (Andreo 1997; Andreo et al. 1997, 2000; Andreo and Carrasco 1999): high thickness of the unsaturated zone, relatively low hydraulic conductivity, strong inertia and, therefore, diffuse flow behavior. The highest degrees of vulnerability, ‘‘high’’ and ‘‘very high’’, obtained with the GOD and AVI methods respectively, coincide with zones where anthropic pressure is highest because of the presence of built-up areas, landfill, roads and crops, all of which increase the risk of contamination. This agrees with the results obtained previously by the SINTACS method (Longo et al. 2001). In any case, these aspects underline the importance of contamination vulnerability mapping in carbonate massifs in southern Spain. Vulnerability mapping constitutes an important tool for environmental management to preserve the quality of groundwater. In fact, after this pilot experience in the Torremolinos aquifer, landfills and cemeteries have been closed. The highest vulnerability classes in the map obtained by the EPIK method correspond to the quarries, which are considered as ‘‘artificial dolines’’, and the slopes of the roads where the absence of protective cover and the high fracturation provoke an increase in infiltration (after Doerfliger et al. 1999). However, in these ‘‘artifi-

DRASTIC

GOD

AVI

EPIK

Humid

Dry

Humid

Dry

Humid

Dry

0 0.1 86 13 1

0 0 83 16 1

0.1 2 84 14 0

0 0.1 86 14 0

10 76 0 14 0

2 84 0 14 0

1 2 83 14 –

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cial dolines’’ swallow holes, which permit rapid infiltration into the aquifer, do not exist. The Torremolinos system has a diffuse flow behavior (Andreo et al. 1997, 2000), and infiltration normally occurs slowly because the marbles are highly fractured but poorly karstified. The influence of rainfall variations and therefore of the water table is clear in the maps obtained by the AVI and GOD methods, in contrast to the DRASTIC vulnerability maps, which are less influenced by the groundwater table variations arising from differences between years of higher or lower rainfall. This could be because the rating range of parameters D and R in the DRASTIC method are not well suited to the Torremolinos aquifer. In any case, it is clear that the interannual variations of rainfall that normally occur in the Mediterranean climate affect the aquifer’s vulnerability to contamination. Thus, in a humid year the vulnerability is higher than in a dry year because the groundwater table rises closer to the surface. The similarity in the distribution of the vulnerability maps obtained by the different methods is due to the importance of the lithology, and to the influence of the final classification of the intervals used in each method. These aspects are particularly relevant in the DRASTIC method, which evaluates several parameters by means of the lithology; moreover, it does not establish a universal classification in intervals and consequently is exposed to subjective interpretations (Vı´ as 2003). The assignation of ratings in the methods involves a certain degree of subjectivity that is difficult to eliminate. In this sense, the DRASTIC method is the most subjective, because of the wide range of the rating of some parameters. The EPIK and GOD methods involve a more selective choice of variables and a reduction of the ratings; as a result, the risk of subjectivity, with respect to the ratings, is smaller. The AVI method does not have any subjective element, as it does not rate the parameters. Finally, concerning the scale, the EPIK method needs a more detailed scale to obtain the E and I parameters, and so it is advisable to use a larger scale (1/25,000) to delimitate the karst landforms (mainly karren field in Sierra de Mijas) with influence in the vulnerability map. The DRASTIC, GOD and AVI methods do not need such a detailed scale to evaluate vulnerability, and a moderate scale (1/50,000) could be very effective, because the differences in vulnerability depend on the geology and the groundwater table depth, both of which can be determined at a scale of 1/50,000.

Conclusions For the DRASTIC, GOD and AVI methods of vulnerability mapping, the parameters related to geology, and

especially to the lithology, are most relevant, while the depth of the groundwater table has less influence. The latter does, however, determine variations in vulnerability between humid and dry years, especially when applying the AVI and GOD methods. In the EPIK method, the vulnerability is defined by the characteristics of the protection cover and, to a lesser degree, by the presence of highly fractured zones. The higher or lower number of parameters used for each method does not establish significant differences in the final vulnerability map. Thus, with the DRASTIC method, which uses seven parameters, it is possible to obtain a vulnerability mapping similar or even with less class than the one obtained by the GOD method, which only uses three variables. The kind of assessment model used (parametric or analogical relations) influences the different degrees of vulnerability. Thus, the AVI method reports a higher vulnerability than that found with the parametric methods (GOD, EPIK and DRASTIC) but this is in disagreement with the hydrogeological knowledge available. The high vulnerability zones deduced by EPIK for very fractured areas (quarries and the slopes of roads) are in contradiction with the very low karstification in such areas and, consequently, the slow infiltration into the marbles because of the diffuse flow behavior of the aquifer. Vulnerability mapping with the DRASTIC, GOD and AVI methods seems very useful for land use management, using moderate and small scales that provide an overall view. However, there is a need for methods with a larger scale to establish protection zones for the aquifers. For carbonate aquifers, the EPIK method offers better performance for the establishment of protection zones. From the results of this work, the authors conclude that the GOD method could be adequate for vulnerability mapping in poorly karstified carbonate aquifers in southern Spain, at least at small–moderate scales. If a large scale is used, or if the study is performed in areas where karstification is well developed, it could be useful to compare the results of the GOD method with those obtained by the EPIK method, or other methods created specifically for karst aquifer in the framework of COST Action 620, before adopting a vulnerability map for groundwater protection. Acknowledgements This paper is a contribution to European COST Action 620, to the projects IGCP 448 of UNESCO, REN2002-01797/HID and REN2003-01580/HID of the DGES and to the Research Groups RNM 308 and HUM 776 of the Junta de Andalucı´ a. The comments of Prof. Alain Dassargues (University of Lie`ge, Belgium), Prof. Jacques Mudry (University of Franche-Comte´) and an anonymous reviewer are much appreciated.

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