Source vulnerability mapping in carbonate (karst) aquifers by extension of the COP method: application to pilot sites B. Andreo & N. Ravbar & J. M. Vías
Abstract A step from resource to source vulnerability mapping is presented, based on the European COST Action 620 approach for karst groundwater protection. Guidelines on vulnerability assessment of the horizontal groundwater flow path within the karst saturated zone (K factor) are proposed. By integrating this into the previously existing COP method for intrinsic resource vulnerability mapping, adequate source protection can be assessed. The proposed “Karst saturated zone (K) factor” assessment considers groundwater travel time (t subfactor), connection and contribution to the source (r subfactor) and active conduit or fissured network (n subfactor). The extended COP method was applied in two carbonate aquifers in southern Spain with different geological, hydrogeological and climate settings. The results are coherent with previous research results of the studied areas. Moreover, they are consistent with the occasional groundwater contamination detected in one of the springs. On the other hand, an absence of contamination, despite high risk, justifies the lower degrees of vulnerability assigned to the sources surveyed. The source vulnerability maps obtained can thus be used as a basis for the delineation of protection zones.
Received: 15 May 2008 / Accepted: 15 October 2008 * Springer-Verlag 2008 B. Andreo ()) Department of Geology, Faculty of Science, University of Málaga, Campus Universitario de Teatinos, Málaga, 29071, Spain e-mail:
[email protected] Tel.: +34-952-132004 Fax: +34-952-132000 N. Ravbar Karst Research Institute SRC SASA, Titov trg 2, 6230, Postojna, Slovenia B. Andreo Department of Geography, University of Málaga, Avda, Cervantes 2, Málaga, 29071, Spain J. M. Vías Department of Geography, University of Málaga, Campus Universitario de Teatinos, Málaga, 29071, Spain Hydrogeology Journal
Keywords Carbonate aquifer . Spain . Groundwater protection . Vulnerability mapping . COP method
Introduction Groundwater from karst aquifers is an important drinkingwater resource worldwide (Ford and Williams 2007). The functioning and behaviour of karst aquifers have some peculiarities due to the concentrated recharge of water and its rapid infiltration, the high permeability of aquifer systems and fast transport in karst conduits over large distances. Consequently, such aquifers are particularly vulnerable to contamination. Therefore, COST Action 620 proposed a European approach on Vulnerability and Risk Mapping for the Protection of Carbonate (Karst) Aquifers (Zwahlen 2004). However, several other methodologies for groundwater vulnerability assessment exist. Overviews of some of the most commonly used ones have been prepared by Vrba and Zaporozec (1994), Gogu and Dassargues (2000) and others. The COST Action 620 proposal of the European approach (Daly et al. 2002) for groundwater vulnerability mapping is founded on the assessment of basic factors that control the infiltration of water and contaminants from the land surface towards the groundwater (Fig. 1). Two types of intrinsic vulnerability have been distinguished, depending on the target or receptor of the potential contamination. For resource vulnerability, the target is the groundwater surface or water table within the aquifer and thus only the vertical pathway within the aquifer is considered. The vulnerability assessment includes the overlying layers factor (O), the concentration of flow factor (C) and the precipitation regime factor (P). For source vulnerability, water in wells or springs is the target. Consequently, the pathway additionally includes horizontal flow in the saturated zone, depicted as the karst network development factor (K). The European approach does not prescribe detailed guidelines as to how the component factors should be measured and categorised, but the proposed conceptual framework could be adopted into methods appropriate for use in individual karst aquifer systems. The derived COP method (Vías et al. 2006) represents an integral interpreDOI 10.1007/s10040-008-0391-1
Fig. 1 Conceptual sketch based on the European approach (Daly et al. 2002). The parameters influencing water flow within the karst aquifer system that are considered within the extended COP method for karst source vulnerability mapping are presented
tation of the COST Action 620 proposal for intrinsic vulnerability assessment (Fig. 1). Detailed guidelines, tables and formulae for resource protection have been developed and specific characteristics of karst aquifers have been considered. To obtain source vulnerability, an additional factor describing the groundwater pathway within the saturated zone (K factor) needs to be included. The main aim of this work was to extend the COP method (Vías et al. 2006) for source vulnerability mapping by proposing a new comprehensive possibility for K factor mapping. To date, some approaches have been made to include the pathway in the saturated zone of the karst aquifer in vulnerability mapping. The first method evaluating specific transport processes in the karst saturated zone was the EPIK method (Doerfliger and Zwahlen 1998). Other methodologies such as VURAAS (Cichocki et al. 2001) and VULK (Jeannin et al. 2001) present a view of how the K factor could be assessed. The Slovene approach (Ravbar 2007; Ravbar and Goldscheider 2007) was proposed for the application in Slovene karst areas, although its source vulnerability assessment was developed simultaneously to present work, as cooperation between the authors. It has been tested and validated at a small test site, the catchment of Podstenjšek springs extending over an area of less than 10 km2. However, the detailed conceptual basis and application of the COP+K method for source vulnerability to large aquifers (several tens of km2), with different climatic and hydrogeological characteristics, constitutes the object of the present manuscript. In fact, the pilot sites Hydrogeology Journal
considered in the present work are the same as those used to develop the COP method (Vías et al. 2004, 2005, 2006; Andreo et al. 2006) and their hydrogeological behaviour is well known, through hydrodynamic, hydrochemical and isotopic research, as well as tracer testing in Sierra de Libar, which permit us to contrast and validate the present proposal on a more representative and realistic scale.
Proposed methodology Conceptual basis for the K factor evaluation The early EPIK method (Doerfliger and Zwahlen 1998), which significantly influenced later ones, remains one of the few methods developed for karst source vulnerability assessment. To denote the horizontal flow path in the saturated zone, the term K factor–karstic network (development) factor was used, and has later been adopted by some later methodologies. Although this factor should not only represent the degree of karst network development within the saturated zone, but should also consider phreatic drainage characteristics and the behaviour of the aquifer itself, the term “karst saturated zone” (K factor) is proposed for use. The newly proposed K factor assessment involves the degree of karstification or drainage system development and the identification of the underground water flow paths. It also includes information on groundwater flow velocities within the saturated zone and the variability of water flow and drainage divides arising from different hydrogeological settings or hydrological conditions. DOI 10.1007/s10040-008-0391-1
Very often, scant data are available on water flow paths within the saturated zone and hydraulic connections of different parts of an aquifer. To obtain the spatial distribution of the main differences in groundwater flow characteristics within the saturated zone, several sources of direct and indirect information should be combined. Speleological exploration is the only direct observation of the karst network, but unfortunately the active cave passages are usually developed at a local scale and they are often not accessible. This criterion also has drawbacks and may not be especially relevant, as cave registering can reflect the degree of research work in a certain area. The conduit size aspect may not be an acceptable parameter either, because small karst conduits, too, permit fast flow and can result in rapid contaminant transport without significant attenuation. Indirect information can be obtained by observing the hydrological behaviour of the aquifer (Doerfliger et al. 1999; Zwahlen 2004). Thus, hydrodynamic, physico– chemical characteristics, both from springs (hydrograph analyses) and from boreholes (water table variations, pumping test), and laboratory analyses can be used. Tracer experiments using natural and artificial tracers can provide information on groundwater flow velocities, retardation and response to rainfall infiltration. Crucial for source vulnerability assessment is the size of the catchment area. However, watersheds are sometimes difficult to determine due to their variability in time and the strong dependence on the respective hydrological conditions. Thus, catchment boundaries may vary by several tens of kilometres (Ravbar and Goldscheider 2006; Ravbar 2007). Furthermore, the catchment areas of different springs can overlap and the flow paths cross each other.
Guidelines for the K factor assessment To protect a source from contamination, three main questions should be considered (Daly et al. 2002; Brouyère 2004): – Within what time period will a contaminant arrive at the source (days, weeks, months...)? – What proportion of the contaminant will arrive (only traces, 1%, 10% or all)? – How long will a contamination episode last? Therefore the K factor assessment should be based mainly on transit time or groundwater flow velocities and on connection and degree of contribution to the source, which are among the most important contamination aspects. In contrast, the duration of a contamination event could be an optional aspect. Additionally, reliable information on active conduit networks should also be considered. Hence, the proposed assessment of the K factor should mainly be based on the hydraulic properties of the aquifer, together with its geological, geomorphological, speleological and hydrological characteristics. Besides conventional survey techniques such as speleological investigation, geological mapping, borehole analyses, hydrograph analHydrogeology Journal
yses, chemical and isotopic analyses, tracing experiments, remote sensing, geophysical measurements and the quantitative characterisation of karst hydrological systems are important. The transit time and recovery rate information is thus the fundamental concept for the newly proposed K factor assessment. However, this information cannot be mapped, so additional criteria that can be mapped in the field should be identified. Hence, an assessment scheme is proposed that considers the following subfactors (Figs. 1 and 2): travel time (t subfactor), connection and contribution degree of different parts of aquifer to the spring (r subfactor) and information of karst conduits with active drainage (n subfactor). The t subfactor is based on the groundwater flow velocities information. Only information on travel time explicitly within the saturated zone should be considered. These data identify only the groundwater-flow characteristics and are independent of the drainage system within the unsaturated zone. However, due to the general inaccessibility of these data, a very high heterogeneity of karst aquifers and their strong dependence on various hydrogeological conditions may present several difficulties in making an assessment of the t subfactor. A certain degree of estimation according to the hydrogeological knowledge of the aquifer’s behaviour or its individual parts is consequently indispensable. The t subfactor assessment should be done for high water conditions, when the flow is faster and the contaminant can more rapidly arrive at the spring without considerable attenuation. Shorter travel times imply extensively karstified drainage system, while longer times indicate poorly developed and weakly connected conduit systems, which are not very efficient in draining the aquifer. In diffuse flow systems or poorly karstified aquifers which do not show high groundwater-flow velocities, the distance to the source parameter would significantly contribute to the final source vulnerability mapping much more than in conduit flow systems or highly karstified aquifers. Three classes for transit time are proposed in Fig. 2 (10 d). However, the limits of the classes can also be adapted to the national legislation of each country.
Fig. 2 Karst saturated zone (K factor) assessment scheme. The data range expressed as, for example, (1–10 days] means that value of 1 d is not included in this class whilst 10 days is included DOI 10.1007/s10040-008-0391-1
The r subfactor considers the contribution and connection rates of different parts of the aquifer to the spring. Both concepts are considered in a complementary way. Thus, in response to hydrogeological and hydrological settings, parts of an aquifer system can either permanently or temporarily (e.g. during high water conditions) contribute to the source. Some parts can contribute in different proportions to the source (e.g. if catchments from several springs overlap). Furthermore, parts of the aquifer system can be either directly or indirectly connected to, and drained by, the source (e.g. parts of aquifer can be separated by an aquiclude, as can be seen in Fig. 3). Thus, an inner, intermediate and outer zone of contribution and connection are distinguished. The inner zone comprises parts of the system that always contribute (normally in a proportion >10%) and the connection is sure and direct to the spring (Fig. 4). The groundwater velocities flowing towards the spring are very high. Therefore, these areas should be classified as more vulnerable. The outer zone comprises parts of the system that contribute only a small portion of the total amount (3 to ≤30 indicate medium vulnerability and values from >30 to 125 indicate a high degree of protection and thus a very low degree of vulnerability. The spatial distribution of the K factor should be shown on the K map. The Slovene approach (Ravbar and Goldscheider 2007) considers that vulnerability is high only when the K value is 1; in areas where transit time is 1 d, clear evidence of an active karst network exist and contribution rate is >10%. However, for example, in many carbonate aquifers with high karstification development, a high degree of connection exists between karst features and karst network. Well permeable karst conduits are thus often present, but an active cave network can either not be explored or is not else evident. Thus, the area under consideration corresponds to the n subfactor class “intermediate, unknown”, but could still permit rapid transit times and/or high contribution rates. For this reason, COP+K method considers such areas as highly vulnerable and classifies K score ≤3, whereas Slovene approach considers them as moderately vulnerable.
Adaptation of the COP method for source vulnerability mapping Based on the European approach (Daly et al. 2002) the parameters taken into account for the source vulnerability mapping, as explained above, are shown in Fig. 1. To integrate the K factor into the existing COP method and to make it suitable for source vulnerability assessment, the two maps should be superimposed. To enable this combination, primarily the K scores and COP index values (resource scores) have to be transformed in the relevant indexes, as shown in the assessment scheme in Fig. 5. The summing of the COP (resource) and K indexes offers a source vulnerability score assessment ranging between 1 and 7. The values are distributed in three classes of vulnerability: values 1 and 2 signify high source vulnerability, 3 and 4 medium vulnerability and more than 4 low source vulnerability.
Fig. 3 Different hydrogeological settings, which can influence the extent of the catchment area of a spring. Arrows indicate the groundwater flow path towards the karst spring depending on the hydrological conditions (high and low water) and the position of the groundwater divide Hydrogeology Journal
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Fig. 4
Illustration of the A source catchment division into inner, intermediate and outer zone
Consequently, the resulting source vulnerability equals the resource vulnerability when the K factor value indicates high vulnerability. When it indicates medium or low vulnerability, the source vulnerability values are low in comparison to the resource ones. The source vulnerability map obtained can be used as a basis for the delineation of source protection zones by transformation of the vulnerability classes into the
protection zones. The transformation should take into account the requirements for source protection established in each country. Additionally, insets of the separate factor maps should be added to the final presentation, affording decision makers an immediate insight into the situation and showing which factor controls the final vulnerability values in each particular area.
Fig. 5 Source vulnerability assessment scheme. The data range expressed as, for example, (1–2] means that a value of 1 is not included in this class whilst 2 is included Hydrogeology Journal
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Source vulnerability mapping in pilot sites The extended COP method for source-vulnerability mapping has been applied in two aquifers in the South of Spain, the aquifers of Sierra de Líbar and Sierra de Mijas, both in the province of Malaga (Figs. 6 and 7). The first is a karst aquifer with conduit flow behaviour and with high rainfall quantities throughout the year, whilst the second is a fissured carbonate aquifer with diffuse flow behaviour and moderate-to-low rainfall conditions. Intrinsic vulnerability mapping (using the COP and other methods) and validation by tracer test and hydrogeological tools (hydrodynamic, hydrochemical and isotopic analyses) have previously been applied in both aquifers (Vías et al. 2005, 2006; Andreo et al. 2006). Detailed descriptions of the pilot sites, including the C, O and P factors, are provided by Vías et al. (2006). For this reason, only characteristics related to the K factor are described below.
Sierra de Líbar Sierra de Líbar has an abundance of karstic landforms (Delannoy 1987), including karren, sinkholes and poljes with swallow holes in which sinking streams infiltrate. Most of the discharge from the aquifer is via springs (Fig. 6): Cueva del Gato (S-1), Benaoján (S-2), Jimera de Líbar (S-3) and Charco del Moro (S-4). These springs show typical karst behaviour of quick response to precipitation and rapid changes in flow and hydrochemical and isotopic compositions (Benavente and Mangin 1984; Jimenez et al. 2004). For the source vulnerability mapping, the catchment areas of each spring have been delineated according to current geological and hydrogeological knowledge, and the results of a multi-tracer test (Andreo et al. 2004, 2006), which also provided information on groundwater flow velocities for the t subfactor assessment. Groundwater flow velocities vary between 40 and 90 m/h, although travel times higher than 100 m/h have also been detected. These data are typical of a karst conduit system and are coherent with source hydrograph analyses as well as with the high variability of hydrochemical and isotopic values in response to rainfall events (Andreo et al. 2006). The individual catchment areas of springs often overlap. The degree of contribution from a given injection point is between 3 and 74%, depending on the spring and injection point (Andreo et al. 2004). The sinking stream of the Pozuelo polje (H-1) is connected to the S-1 and S-2 sources, and the Llanos de Libar polje (H-2) proved to be within the catchment area of the S-2 and S-3 springs. The recovery rates strongly depend on hydrological circumstances, although further information is not available. The swallow hole H-3 is only connected to the spring S-4. To assess the r subfactor values, tracer test data were used. Furthermore, the information on the active karst network (n subfactor) can be deduced from the speleological mapping of the Hundidero-Gato cave system, to which the highest degree of vulnerability has been assigned. Hydrogeology Journal
Despite hydrogeological data demonstrating conduit flow behaviour of the aquifer, in the rest of the area water conduits are unknown and the area was classed in the intermediate class of the n subfactor. The resource vulnerability map shows vulnerability to be high in the more karstified areas and moderate in the less karstified outcrops of limestone, whilst the Cretaceous marls present low vulnerability for groundwater (Fig. 6a). The final K map shows the highly vulnerable zones near the springs and over the large Hundidero-Gato cave system (Fig. 6b). Most of the area is considered as moderately vulnerable due to high travel times and the unknown location of karst conduits. The source vulnerability map (Fig. 6c) was obtained by combining the previously assessed resource vulnerability (Fig. 6a) and the K factor map (Fig. 6b). The resulting map shows that most of the area is moderatelyto-highly vulnerable. There is only a slight change between the resource and source vulnerability maps. Thus, the areas close to the springs are highly vulnerable; but this is also the case in distant zones, where the Jurassic limestones are highly karstified. The high vulnerability of the Sierra de Líbar aquifer, as assessed using the extended COP method, is consistent with actual known episodes of groundwater contamination. These episodes are normally short lasting and are detected when water is abstracted from the S-2 spring to supply the village of Benaoján.
Sierra de Mijas Karst morphological features are very scarce in Sierra de Mijas and, hydrogeologically, several systems can be distinguished (Andreo et al. 1997). Hydrograph analysis of the springs and chemical monitoring show diffuse flow behaviour of the aquifer (Andreo et al. 1997; Andreo and Carrasco 1999). In the immediate vicinity of the springs, many boreholes have been drilled for drinking-water supply and, consequently the spring dried up. Therefore, source protection in Sierra de Mijas involves both springs and pumping wells. No data on tracer tests are available for Sierra de Mijas. However, several pumping tests have been made. A storage capacity (effective porosity) of 0.02, and transmissivity values of 103 m2/d have been estimated. The saturated thickness of the aquifer has an order of magnitude of 102 m. Consequently, the permeability is estimated to be about 10 m/d. Considering this latter value and the effective porosity, the average groundwater velocity of 20 m/d has been taken into account for assessing the t subfactor. This assumption is congruent with the historical discharge data from the springs, as well as chemical and isotopic records from wells, showing an absence of sharp peaks in the graphs and attenuated variations occurring several months after the periods of high rainfall. Tritium data from the springs indicate that some of the groundwater abstracted in the eastern part of Sierra de Mijas in 1994 originated from recharge in 1986 (Andreo and Carrasco 1999). DOI 10.1007/s10040-008-0391-1
Fig. 6 Source vulnerability mapping of Sierra de Líbar. Although the aquifer is drained by several springs, the entire aquifer system is presented to enable a comprehensive insight of the results; where catchment areas overlap the higher degree of vulnerability is taken into account. a Intrinsic-resource-vulnerability map, b map of the K index and c source vulnerability map
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Fig. 7
Source vulnerability mapping of Sierra de Mijas. Although the aquifer is drained by several springs, the entire aquifer system is presented to enable a comprehensive insight of the results; where catchment areas overlap the higher degree of vulnerability is taken into account. a Intrinsic-resource-vulnerability map, b map of the K index and c source vulnerability map
The aquifer system of Sierra de Mijas is thus highly fractured and poorly karstified. It shows diffuse flow behaviour and there is no evidence of a developed karst network. In assessing the K factor, the fissured nature of the aquifer was evaluated as the highest value of the n subfactor and considered for the entire area. However, the precise catchment areas of each well and spring are difficult to determine, due to overlapping. Particular areas can contribute to several springs and boreholes. Although the proportions of each contribution are not known, nevertheless, from the physico–chemical properties of the springs and wells, a very scarce contribution from any given area is inferred. Therefore, the whole area has been categorised as the outer zone and classified with the highest value of the r subfactor. The resource vulnerability map of Sierra de Mijas shows high vulnerability at the aquifer edges and moderate vulnerability in the uplifted area (Fig. 7a), depending mainly on the thickness of the unsaturated zone. The K factor map shows a low degree of vulnerability for the entire Sierra de Mijas (Fig. 7b). Only very close to the springs and wells are moderate and high degrees of vulnerability assigned. The source vulnerability map was obtained (Fig. 7c) by combining the resource vulnerability map (Fig. 7a) and the K factor map (Fig. 7b). The resource vulnerability is, in general, lower. Moderate vulnerability is assigned to the marginal areas of the aquifer, very close to the springs and wells, mainly due to longer travel times. Low vulnerability has been assigned to the uplifted parts of the aquifer due to the thick unsaturated zone. This result is in agreement with the absence of groundwater contamination in Sierra de Mijas, despite extensive hazards over the aquifer (e.g. landfill, waste pipe lines in urban areas and petrol stations), and it is also coherent with previous research on this area (Vías et al. 2005, 2006). This source vulnerability is a consequence of a thick unsaturated zone, relatively low hydraulic conductivity, strong hydraulic inertia and, therefore, diffuse flow behaviour.
and recovery rates. In areas where the required data is not available a certain simplification assessing the K factor could be carried out, according to the hydrogeological settings and other knowledge that may be available. The extended COP method has been applied to two aquifers in southern Spain—Sierra de Líbar and Sierra de Mijas—in order to evaluate source vulnerability mapping in carbonate aquifers with different degrees of karstification and with different types of available data. Both aquifers discharge into several springs, some of which are used for drinking water supply. In Sierra de Mijas more than 150 boreholes have been drilled to pump water for supply purposes. Due to the large number of sources with overlapping catchment areas in the two test sites, the vulnerability mapping carried out was not just of one source, but of the entire aquifer system, enabling decision makers to obtain a comprehensive insight into the situation. Where catchment areas overlap, the higher degree of vulnerability has been presented on the final map. Thus, while planning particular land use that extends over a large scale (e.g. roads, industry, etc.) a compatible decision for each source can be taken. The study shows that the extended COP method can be used both in diffuse and conduit flow systems, and in aquifers under different climatic conditions. The results obtained are coherent with previous research results of the studied areas. The occasional groundwater contamination detected in one of the springs of Sierra de Líbar and the high water quality of the Sierra de Mijas sources, despite these being at high risk, justify the resulting vulnerability maps. The vulnerability maps obtained can thus be used as environmental management tools. Nevertheless, to evaluate the applicability of the extended COP method, it should be applied to other test sites and validated by means of natural or artificial tracers. Acknowledgements This work is a contribution to the projects CGL2005–05427 and CGL2008–06158 BTE of DGICYT, P06RNM 2161 of Junta de Andalucía and IGCP 513 of UNESCO and to the Research Groups RNM-308 and HUM-776 of the Junta de Andalucía. The research was also conducted within the framework of the Slovenian Ministry of Higher Education, Science and Technology. The authors thank Nico Goldscheider (University of Neuchatel) for his comments and suggestions, and the anonymous reviewers and the Managing Editor (M.T. Schafmeister) for their constructive criticism.
Conclusions
References
A proposed method for source vulnerability mapping in carbonate aquifers has been developed. The COP method for intrinsic vulnerability assessment has been adapted by adding a new factor indicating the karst groundwater flow within the saturated zone (K factor). Thus a complete interpretation of the European approach conceptual framework has been made. The K-factor assessment requires information on the location of the underground water flow paths, travel times
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