Karst Hydrology (Proceedings of Workshop W2 held at Rabat, Morocco, April-May 1997). IAHSPubl. no. 247, 1998. 45 Vulnerability of karst aquifers CHRIS LEIBUNDGUT Institute of Hydrology, University of Freiburg, Werderring 4, D-79098 Freiburg, Germany Abstract The susceptibility of karst systems, especially to point source pollution, is discussed. A distinction is made between intrinsic and specific vulnerability. The spatial aspect in vulnerability assessment and the possibility of showing the results in vulnerability maps are discussed. Different conceptual approaches to assessing the vulnerability of karst groundwater resources are compared. One approach to the investigation of mountain karst regions with respect to the potential vulnerability of their water resources is presented in more detail. The approach is illustrated by two case studies from extended karst massifs in the Swiss Alps. The investigations are based on the monitoring of spring water. It is shown, how the point information at the springs can be disaggregated to give information on the whole karst system. Besides catchment size the location of karst springs, storage and transfer characteristics can also be obtained. Artificial tracer experiments play a crucial role in investigating connections and characterizing the dynamics of the fast conduit flow connections. The conduits connect the most vulnerable locations of the karst area, that are endangered by point source pollutions, with the karst springs. The results of the vulnerability investigations can be depicted on maps. Vulnérabilité des aquifères karstiques Résumé La susceptibilité des systèmes karstiques, tout particulièrement aux sources de pollution ponctuelles, est discutée. Une distinction est faite entre vulnérabilité intrinsèque et spécifique. L'aspect spatial dans l'estimation de la vulnérabilité et la possibilité de montrer des résultats sous forme de cartes de vulnérabilité sont discutés. Différentes approches conceptuelles pour estimer la vulnérabilité des eaux souterraines du karst sont comparées. Une approche appliquée dans des régions montagneuses karstiques et concernant la vulnérabilité potentielle des ressources en eau, est présentée avec plus de détails. L'approche est illustrée par deux cas d'études dans des massifs karsti- ques dans les Alpes suisses. Les études sont basées sur les mesures réalisées aux résurgences. Il est montré, comment des mesures ponctuelles aux ré- surgences peuvent être traitées pour fournir de l'information sur l'ensemble du système karstique. En plus de la taille du bassin et de la localisation des résurgences karstiques, les caractéristiques de stockage et de transfert peuvent être obtenues. Des tests avec des traceurs artificiels jouent un rôle crucial dans l'investigation des connexions et pour caractériser la dynamique des conduits à écoulement rapide. Les conduits mettent en connexion les zones les plus vulnérables de la zone karstique, qui sont mises en danger par les sources ponctuelles de pollution, avec les résurgences karstiques. Les résultats des investigations sur la vulnérabilité peuvent être décrits sur des cartes. INTRODUCTION Around 25% of the earth's population are largely or entirely dependent upon karst aquifers for their drinking water, and water for agricultural and industrial use. In
Transcript
Karst Hydrology (Proceedings of Workshop W2 held at Rabat, Morocco, April-May 1997). IAHSPubl. no. 247, 1998. 45
Vulnerability of karst aquifers
CHRIS LEIBUNDGUT Institute of Hydrology, University of Freiburg, Werderring 4, D-79098 Freiburg, Germany
Abstract The susceptibility of karst systems, especially to point source pollution, is discussed. A distinction is made between intrinsic and specific vulnerability. The spatial aspect in vulnerability assessment and the possibility of showing the results in vulnerability maps are discussed. Different conceptual approaches to assessing the vulnerability of karst groundwater resources are compared. One approach to the investigation of mountain karst regions with respect to the potential vulnerability of their water resources is presented in more detail. The approach is illustrated by two case studies from extended karst massifs in the Swiss Alps. The investigations are based on the monitoring of spring water. It is shown, how the point information at the springs can be disaggregated to give information on the whole karst system. Besides catchment size the location of karst springs, storage and transfer characteristics can also be obtained. Artificial tracer experiments play a crucial role in investigating connections and characterizing the dynamics of the fast conduit flow connections. The conduits connect the most vulnerable locations of the karst area, that are endangered by point source pollutions, with the karst springs. The results of the vulnerability investigations can be depicted on maps.
Vulnérabilité des aquifères karstiques
Résumé La susceptibilité des systèmes karstiques, tout particulièrement aux sources de pollution ponctuelles, est discutée. Une distinction est faite entre vulnérabilité intrinsèque et spécifique. L'aspect spatial dans l'estimation de la vulnérabilité et la possibilité de montrer des résultats sous forme de cartes de vulnérabilité sont discutés. Différentes approches conceptuelles pour estimer la vulnérabilité des eaux souterraines du karst sont comparées. Une approche appliquée dans des régions montagneuses karstiques et concernant la vulnérabilité potentielle des ressources en eau, est présentée avec plus de détails. L'approche est illustrée par deux cas d'études dans des massifs karstiques dans les Alpes suisses. Les études sont basées sur les mesures réalisées aux résurgences. Il est montré, comment des mesures ponctuelles aux résurgences peuvent être traitées pour fournir de l'information sur l'ensemble du système karstique. En plus de la taille du bassin et de la localisation des résurgences karstiques, les caractéristiques de stockage et de transfert peuvent être obtenues. Des tests avec des traceurs artificiels jouent un rôle crucial dans l'investigation des connexions et pour caractériser la dynamique des conduits à écoulement rapide. Les conduits mettent en connexion les zones les plus vulnérables de la zone karstique, qui sont mises en danger par les sources ponctuelles de pollution, avec les résurgences karstiques. Les résultats des investigations sur la vulnérabilité peuvent être décrits sur des cartes.
INTRODUCTION
Around 25% of the earth's population are largely or entirely dependent upon karst aquifers for their drinking water, and water for agricultural and industrial use. In
46 Chris Leibundgut
Europe there are around 3 000 000 km2 of carbonate rocks most of which contain karst aquifers. Many important cities such as Grenoble, Vienna and Innsbruck are supplied entirely by karst water. Karst water also plays an important role in water supply for human consumption and for irrigation in North Africa. At the regional scale karst water is often the only water resource. That leads to the absolute requirement for sustainable management of karst water resources.
Vulnerability has become a modern catchword and the vulnerability of ecosystems and other topics such as the vulnerability of karst systems are modern research fields. The special characteristics of karst aquifers result in their high potential vulnerability, in terms of both water quantity and water quality. These were summarized by Gunn (1986) in an instructive schematic model of a karst aquifer (Fig. 1).
Karst systems are highly vulnerable compared to other groundwater systems, since potential contaminants can easily reach the groundwater. This is due to the high permeability of karst aquifers in the solutionally enlarged fissures and channels and to the lack of effective attenuation mechanisms. In some karst regions the soil cover is thin or absent (bare karst). As a consequence, the breakdown of contaminants by microorganisms and by physical and chemical processes, that normally occurs effectively in the soil zone, is very weak in these karst areas. Where streams sink underground, the soil zone is completely bypassed. Furthermore, the recharged water can pass quickly through the unsaturated zone via shafts and well-developed integrated fissure systems and can be transported directly to springs or wells through large conduits and channels in the saturated zone. In consequence, the unsaturated zone looses its usual filtration function in which it effectively delays the arrival of contaminants and further attenuates them by physical and chemical processes. In the saturated zone mainly dilution and hydrodynamic dispersion take place. Very often the residence time of water, that reaches the springs and pumping wells by conduit type flow is far too short for pathogens to die. Thus, karst water might be bacteriologically contaminated if the recharge areas are not appropriately protected. These recharge areas are often located far away from the springs. The vulnerability of karst systems is obvious and the need to assess this vulnerability, to locate the most sensitive areas and to protect them accordingly is an important consequence. At the same time this is a very difficult task.
It is the aim of this keynote paper to give an introduction to the relevant concepts of how to assess the vulnerability of karst areas. A special focus is given on integrative concepts. Fundamental aspects in general aquifer vulnerability are given by Foster (1987) and Smith (1993).
DEFINING THE VULNERABILITY OF KARST AQUIFERS
The physical environment has a certain potential with respect to specific purpose of land use and owns some degree of protection of groundwater against natural and human impact. The term vulnerability is divided into intrinsic vulnerability and specific vulnerability. Intrinsic vulnerability is defined as the properties of the karst system itself, depending on the characteristics of the soil zone, the unsaturated zone and the saturated zone. These groundwater protecting properties are responsible for the degree of sensitivity of the karst system to potential threats from outside. Specific
Vulnerability ofkarst 47
vulnerability is defined as the degree of vulnerability caused by actual threats, mainly human impact such as land use, spills etc.
According to Quinlan et al. (1991), the degree of intrinsic vulnerability depends on four main characteristics:
the existence or absence of a soil and debris cover, which is crucial for the protection of the karst groundwater from contaminants;
- the infiltration characteristics. Diffuse, spatial infiltration on covered karst generally results in lower vulnerability. A linear or point infiltration, which is typical for karst, leads to a higher vulnerability;
- the intensity of epikarst and - the development of the karst system.
Diffuse autogenic recharge (may be concentrated in subcutaneous zone)
Interg rated vadose flows
Concentrated autogenic recharge from closed depressions
Diffuse allogenic recharge through permeable cap rock
As these characteristics vary from one karst system to another, the relevant parameters (flow velocity, residence time and purification capacity) also vary. The latter consists of filtration, chemical processes and biochemical processes and, together with the flow velocity, determines the degree of vulnerability of a karst system.
SPATIAL ASPECTS IN THE VULNERABILITY ASSESSMENT OF KARST AREAS
The mapping of groundwater vulnerability is a useful instrument for regulatory, managerial and decision-making purposes at all levels of government. With the spatial information on vulnerability, an environmentally sound planning of land use and groundwater protection measures can be facilitated. In non-karstified areas, the information, that is relevant for the natural purification capacity of the environment and thus for its vulnerability is mainly spatial. The vulnerability is dependent on the potential for attenuation of contaminants in the soil zone, the unsaturated zone and in the aquifer itself (Vrba & Zaporozec, 1994). However, due to the enormous inhomogeneity of karstified areas, the relevant information for a vulnerability assessment in karst areas is not only spatial, but mainly point or linear. The most vulnerable parts of karst regions are areas with a fast connection to the conduit system. These are dolines (point), which are often connected to vertical shafts and swallow holes (point), where streams (linear) sink underground. When lakes (spatial) have a connection to the karst system, the open water bodies are very vulnerable too. Contaminants, that are released in these locations are hardly attenuated, as they bypass the soil zone and are transported fast to the springs via conduits with a very low purification capacity. The short travel time is responsible for the bacteriological risk. When these locations lie within the catchment area of springs, used for public water supply, they have to get special protection.
The relevant results of investigations concerning the vulnerability of a karst area can be presented in a map. The most important element is the delimitation of the catchment areas of the springs as areas of potential threat. Within these areas especially threatened elements like lakes, sinking streams and dolines can be depicted. In areas that are used for agricultural activity it would be helpful to show the vulnerability spatially differentiated. This could be achieved by using information on soil parameters or the density of dolines. Ketelaere et al. (1997) show the practicability of using GIS techniques to estimate the vulnerability of karst catchments in Malta.
CONCEPTS IN THE ASSESSMENT OF THE VULNERABILITY OF KARST WATER RESOURCES
Concept 1: The hydroecological approach
This concept was developed in the framework of the UNESCO programme "Man and Biosphere" project no. 30 "Man's impact on mountain ecosystems". The evaluation of vulnerability is part of an integrated approach, in order to determine
Vulnerability ofkarst 49
the potential, the variability and the persistence of the karst water system. Karst water resources are often used by capturing karst springs. To assess the vulnerability of these water resources with regard to their suitability for public water supply, their potential, variability and their persistence must be known (Leibundgut, 1987). Karst springs might show a high discharge variability, that reduces their potential, when the aquifer storage is limited and discharges during dry periods get low. Another limiting factor for their potential is the threat of contamination. The persistence of a karst system represents its stability to changes from outside, namely from the impact of human activity. The persistence is a critical factor of karst water resources, because karst systems are very susceptible to pollution, that might be caused by human activity.
To assess this vulnerability to pollution it is a basic and at the same time a difficult task to determine the catchment area of any spring that is used for public water supply. Once the catchment area has been determined, the especially sensitive parts of it must be identified and appropriately protected. The storage characteristics of the aquifer and water transfer mechanisms must be further investigated, to assess, how fast and over which pathways contaminants might be transported, and to what extend they are diluted and adsorbed.
Karst springs are the most logical, efficient, reliable and economical places to monitor pollutants and natural and artificial tracers, and to obtain information about the karst system (Quinlan & Ewers, 1985). This is especially valid in well developed karst systems, where recharged water from extended areas converges via conduits to big karst springs. Accordingly, Leibundgut (1987) applied the principle of convergence. All information from the whole catchment area converges on the springs and must be disaggregated, in order to be able to draw conclusions concerning the karst system. Physical, chemical, biological and isotopic data can be
Fig. 2 Overlay of spatial information.
50 Chris Leibundgut
A I B I C I I
|< < « | colluvial deposits 1***1 s n o w a n d i c e
V//A aalenian schists recent infiltration
I | limestone waterlevel summer
|+ + +| crystalline waterlevel winter
Fig. 3 Model of the Klecki spring with its catchment and subcatchments A, B and C.
monitored at the springs and can be interpreted with the help of geofactors like geology and topography. The spatial information can be combined with the point information at the springs to produce a functional model of the karst system or even a quantitative model. This is an interdisciplinary task.
The spatially differentiated evaluation of vulnerability can be assessed by an overlay of spatial information. The information from the hydrological investigation of subcatchments may be overlaid with the information on geofactors and land use (Fig. 2). Each resulting subsystem shows a characteristic response to a given input (specific vulnerability). The result are subsystems with varying vulnerability where adapted measures of protection can be planned. The example of the model of the Klecki spring in the Swiss Alps shows the situation with three subcatchments (Fig. 3). A detailed description of the assessment of intrinsic vulnerability is given in Leibundgut (1984). The subcatchments are evaluated regarding their vulnerability. In order to give information to the planners the results are evaluated with a scenario of intensified land use (Table 1).
Concept 2: COST Action 65
Another concept was developed in the framework of the European Coordination Programme, where 16 countries took part in the action "Hydrological aspects of groundwater protection in karstic terrains". During the five-year programme, different investigations, concerning the threats to karst aquifers and necessary counter-measures, were carried out. The aim was to develop a uniform and
Vulnerability ofkarst 51
Table 1 Rating system to evaluate the vulnerability of the subcatchments of the Klecki spring.
Permeability Filter capacity Storage Allogenic recharge Influence on infiltration rate Influence on chemical water quality Influence on hygienic water quality
Limestone Karst/fissure water Unused Mountain climbing
transferable concept for the protection of karst groundwater resources. A summary of the results from Germany is presented by Hôtzl (1996), while the complete report can be found in COST-Action 65 (1995). The concept distinguishes between intrinsic and specific vulnerability. The different investigation methods for the evaluation of the intrinsic vulnerability in karst terrains are summarized. Tracer techniques in combination with more classical methods and modelling play an important role in the investigations that are to assess the properties of the karst system (Table 2).
In the same intensity the potential threats (specific vulnerability) for karst aquifers, mainly imposed by humans, have to be investigated. These are infrastructural development, industrial activity, land use, aquifer overstress and long term effects of air pollution. As an example, Table 3 shows the human activity with sewage (production, storage, etc.) and the processes that might occur (contamination, migration to the groundwater, infiltration of sewage, etc.). The negative consequences (microorganisms, no filtration, etc.) have to be assessed with hydrological techniques.
Table 2 Investigation methods in karst for the evaluation of intrinsic vulnerability.
Geology, geomorphology Information on the karst medium:
Geophysics Structural and tectonic conditions, Hydraulically effective disturbance, Degree of karstification, Draining system
Spatial heterogeneities, Preferential flow paths, Draining system
Hydrodynamics Water balance Information on the flow system and transport processes:
Hydrochemistry Tracer techniques Modelling Hydrodynamic parameters, Flow behaviour of the system
Groundwater recharge, Available groundwater potential
Origin, Interactions with surrounding matrix, Mixing processes, Water quality
Flow direction, Flow velocity, Residence times, Dispersion, Retardation, Determination of the catchment
Flow and transport parameters, Calibration, Prediction
52 Chris Leibundgut
Table 3 Examples of potential threats for karst aquifers.
Human activity
Sewage Production Storage Transport Disposal
Traffic Roads Railway Accidents
Building activities Construction of tunnels and dams Other building activities Waste disposal Domestic waste Sewage sludge
Processes
Contaminant migration to the groundwater: - infiltration of untreated sewage - leaks of septic systems - leaks of sewage pipes
Infiltration of mineral oil and other contaminants: - contaminant containing runoff
from roads - spill of contaminants from
accidents
Destruction of protection cover (soil etc.) Artificial drainage
Migration of contaminants, Leaking from sites
Emission of fluid contaminants Storage of domestic chemicals Use of domestic chemicals Air pollution Traffic Heating
Leaks Spills
Air pollution
Negative consequences
Microorganisms (bacteria, viruses) Ammonia, nitrate No filtration Only self purification
Salts, hydrocarbons, heavy metals, pesticides, microorganisms, ammonia ...
Increase in vulnerability Qualitative and quantitative deterioration
Organic compounds, heavy metals, ammonia, sulphate, chloride, pesticides, hydrocarbons
Salts, hydrocarbons
Nitrate and sulphate oxides, organic microcontaminants, heavy metals, indirect effects like acidic rain and deterioration of forests
Based on the determined degree of vulnerability, the karst systems have to be protected. The proposed protection programme differentiates groundwater resources as such from those which are used for the abstraction of drinking water. It follows the concept of zonal protection and risk management. The resource protection is based on the importance of the aquifer for a potential use for water supply. The protection of aquifers that are used for water supply comprises the special protection of the area around the water tapping and overall protection of the catchment. The catchment has to be divided into protection zones according to the occurrence of features that make the aquifer especially vulnerable (lack of soil cover, intensity of epikarst, open cracks, shafts, dolines and swallow holes). The spatial registration of these features should result in a vulnerability map as a basis for protection zonation. Examples for a differentiated distribution of protection zones based on the intrinsic vulnerability were developed in one contribution to the COST project (Doerflinger & Zwahlen, 1995).
Concept 3: Tracer hydrological approach
This approach is illustrated by studies in extended alpine karst massifs in eastern Switzerland, where hydrological conditions were investigated in order to assess the
Vulnerability ofkarst 53
potential of the karst water resources and their vulnerability. The karst massif of the "Alpstein" and the karst massif of "Churfirsten" form the Helvetian Sântis Nappe, which is overthrust over the Subalpine Molasses in the northwest. The two karst regions, which are hydrologically interconnected, are bordered by the "Walensee" lake and the Seez Valley in the south and by the Rhine Valley in the east. The Alpstein massif is characterized by several southwest-northeast striking folds and plates, whereas the Churfirsten-Alvier massif forms a uniform arch. The rocks in the area are marls and limestones, with different degrees of karstification. The massifs with highest peaks over 2200 m a.s.l. (area approximately 400 km2) drain in all directions. They are open hydrological systems and show mainly subsurface drainage to karst springs or to the adjacent porous aquifers of the river valleys (Rieg et al., 1993). Parts of the massifs where marly rocks can be found are drained by rivers that might be connected to the subsurface drainage via sinkholes. Special hydrological features of the areas are several mountain lakes. The karst water resources are in conflict with livestock and tourism, which are the main potential threats. The results of the investigations are the basis for karst water protection measures.
The data collection includes a discharge measuring network at surface streams and springs, where samples are also taken for hydrochemical analysis. From the resulting understanding of the system, multi-tracer or single tracer experiments can be planned in an efficient way and prove different connections. If necessary, detailed investigations can complete the data collection. The interpretation of the data results in a synthesis on catchment area size and location and the storage characteristics and transfer mechanisms of single spring systems. On this basis the vulnerability of the water resources can be evaluated (Leibundgut, 1995).
Different methods exists to calculate the catchment size of karst springs. All analysis should take into account that the size may vary over time. One method is to use the water balance equation (Bonacci, 1988). However, it is difficult to determine all water balance elements in mountainous regions, especially when the systems are not closed. A different method has been applied for the Alpstein system. Catchment sizes were calculated from the mean annual discharge of the spring (m3 s"1) and the estimated specific discharge (m3 s"1 km"2). The mean annual discharge was calculated from a long discharge series. The specific discharge is estimated from a regression function using climatic and physiographic data. The regression function was developed by Aschwanden (1985), who investigated many alpine catchments with statistical methods. The catchment size of the spring can then be obtained by forming the quotient of the mean annual discharge and the specific discharge of the spring. This is an iterative process, since the specific discharge changes with changing catchment size, as its mean altitude changes as well. The catchment sizes have an accuracy of ±10% (Leibundgut, 1995).
The most likely location of the karst springs catchment areas can be determined with the help of the information that is obtained from the spring water chemistry together with geological information. Among the different measured and analysed parameters, the electrical conductivity (total mineralization), the Ca share of total hardness (calcite dissolution) and the sulphate equivalent percentage (dissolution of non-carbonates) proved most suitable to give information on the origin of the spring water. With these geochemical tracers and the knowledge of the geology it was possible to draw conclusions on the origins of the spring water. The parameters
54 Chris Leibundgut
could be used to see the influence of marls in the catchment areas, but could hardly be used to distinguish between different limestones. The parameters water temperature and isotope content further help to locate the catchment area, as they give information on the mean altitude of the catchment. In mountainous regions, use can be made of the fact that their values in the springs depend on the mean altitude of the catchment. In the Churfirsten-Alvier area, a regression line for the mean spring water temperature spring altitude relationship was calculated. The gradient was 0.49°C per 100 m. The temperature residuals of single springs from the regression line are indications of the altitude of their catchment. The interpretation has to take into consideration though, that water from moraine and slope debris springs is influenced by the air temperature, as their water flows close to the surface. For karst springs uninfluenced by slope debris or moraine material the temperature is a valuable parameter. The information from the water temperature could be confirmed by 180 and 2H contents of the spring waters (Leibundgut & Rieg, 1994). A quantification of the mean spring catchment altitude is possible when the isotope altitude relationship of rain and snow input water of the area is established.
In addition to the size and the location of the karst springs catchment, the transfer mechanisms of the karst aquifers are important for the evaluation of their potential and vulnerability. Monitoring of discharge gives valuable information on the storage characteristics (recession curve analysis) and the transfer mechanisms (single event analysis) of the aquifers considered as a whole. The recession coefficient of the slowest storage and its volume give information on the usable volume of water in blocks (small fissures and porosity). The recession coefficient and the volume of the fast storage gives information on the conduit system, that is responsible for the fast water transfer and thus important for fast contaminant transport. Recession curve analysis gives information on the degree of karstification. In the Churfirsten-Alvier area it was possible to distinguish between well karstified limestone aquifers (Schrattenkalk) and aquifers with a recession characteristic that indicates a fissure drainage system (Kieselkalk).
Analyses of single events showed that the response of discharge from well developed conduit karst systems to precipitation is fast and strong. Chemographs, together with the electrical conductivity and the temperature during single events, can give information on the origin of the discharged water. These parameters show qualitatively the contribution of event water that is transported fast to the springs and might contain potential pathogens. In the study areas the decrease of the electrical conductivity and geogene tracers like sulphate and hardness in the karst springs during events showed the influence of event water (dilution effect). The simultaneous increase of agrochemicals like nitrate and chloride indicate the same mechanisms. Springs that are influenced by soft rocks showed an increase in electrical conductivity, that is due to water pressed out from the soft rocks (piston flow effect). The quantification of event water contribution can be achieved by isotopes (Lakey & Krothe, 1996). It has to be decided from case to case which additional information is needed for a vulnerability assessment.
The seasonal fluctuation of parameters like isotopes and temperature gives qualitative information on the mean residence time and the volume of the total karst water, which can be much bigger than the dynamic storage calculated by the recession curve analysis. For example, an extensive study of an Alpine karst system,
Vulnerability ofkarst 55
the Lurbach system in the Austrian Alps, provided detailed results on different karst storages. The total karst water volume (calculated from tracer data) was 93 times bigger than the dynamic storage (calculated from the recession curve analysis), which was 23 times bigger than the volume of the conduit drainage channels calculated from artificial tracer experiments (Behrens et al, 1992). The total karst water storage and its mean water residence time is a measure for the time it takes to replenish the whole water. The higher it is, the longer it takes for non point source pollutions to reach the springs, but at the same time the longer they stay in the karst aquifer. From case to case it has to be decided whether quantitative determination of the volume and water residence time of the total water storage is necessary for a vulnerability assessment of the karst system. It becomes important when dealing with nonpoint source pollution from agricultural practice. For point source pollution it is much more important to characterize the fast flow component through the conduits, since they present the most vulnerable part of the karst system.
Artificial tracer experiments for flow path definition and characterization
Artificial tracer experiments play a crucial role in karst hydrology because they can prove fast underground connections and at the same time characterize the fast flow component. The results about the location of catchment areas from other methods can be confirmed or corrected. In many cases catchment areas of different springs overlap and this can be seen from tracers injected in one location appearing in different springs. In the case studies multi-tracer experiments with fluorescent dyes were carried out. The conclusions about catchment sizes and locations helped to develop an efficient plan for injection and sampling points. For an assessment of karst water vulnerability it is favourable to conduct tracer experiments when karst groundwater levels are high, as additional connections might be activated and the highest flow velocities occur (worst case scenario in vulnerability assessment). Tracing campaigns were conducted in different summer times, when karst water levels have been relatively high. The tracers were directly applied to the conduit system in sinking streams, cave streams or lake swallow holes. Automatic samplers and fluocapteurs (activated charcoal) were installed to survey the tracer reappearance. Flow connections and catchment locations could be proved. The recovery rates gave information on the importance of the connection. Where tracer breakthrough curves existed, transport parameters could be determined. The mean tracer transport velocity of the different tracer experiments in the Alpstein massif ranged from 11 to 260 mhf1, while the dispersivity ranged from 9 to 232 m. The residence times are too short for efficient degradation of contaminants. They might be used as a criterion for the available time to intervene when contaminants are accidentally released on the surface. Intervention would mean to stop extraction of water or to bypass it from endangered springs (Janez, 1995).
Analytical solutions of 1-D mathematical transport models were fitted to the tracer breakthrough curves to determine the parameters of the models which describe the aquifer. For more details of the models refer to Maloszewski & Zuber (1992). Two mathematical models proved suitable to describe the tracer breakthrough curves that generally shows a long tailing. These were the MDM (Multi Dispersion Model)
56 Chris Leibundgut
and the GMDM (Good Mixing Dispersion Model). The SFDM (Single Fissure Dispersion Model) also gave good fitting results, but was not physically justified as fast water transport in the studied areas takes place in conduits and not in fissures (Seifert, 1996). The MDM would physically correspond to different flow paths in conduits, while the GMDM would correspond to a series of a reservoir (siphon or lake) and a conduit. It was not possible to decide which model represented the conditions more realistic. This is not a big drawback for a vulnerability assessment, as the mean travel time and parameters that describe dispersion can be calculated with both models. For a vulnerability assessment it is more important to know how the tracer breakthrough curve changes with changing hydrological conditions.
In the Churfirsten study area, tracer experiments in one of the main conduits to the biggest spring (Rinquelle) in the area were carried out under varying hydrological conditions. The Rinquelle (MQ = 3.9 m3 s"1) has an estimated catchment of 52 km2
and is the overflow of an extended cave system (Rieg & Leibundgut, 1992). The injection point was at a distance of 6 km in a stream of the cave system (Q about 7 1 s4). The water flows with a mean gradient of 17% through all geological units of the area. The first experiment took place in June, under low water conditions (2 m3 s"1), the second in the following year during snow melt in May (4.5 m3 s"1) and the third in July during heavy convective precipitation (7.2 m3 s"1). The higher the spring discharge during the experiment was, the higher were the peaks of the normalized tracer breakthrough fluxes and the shorter were the residence times of the tracer (Fig. 4). An attempt was made to interpolate the results for different hydrological conditions. In order to do this, Mull & Smoot (1986) proposed a method to get a mean response function from tracer breakthrough curves under
g(t) [1/h] Churfirsten - Kernels
0,1
0,08
0,06
0,04
0,02
0
A A \r IT
\ £-\ \
i-iAi--. f/\: K: ../, ~^~ ,'
\ \
-\ ~~——~—-_
100 120 20 40 60 80
hours after injection Fig. 4 Tracer fluxes, normalized with the mass of recovered tracer for the tracer experiments under varying hydrological conditions: June 1992 2 m3 s'1, May 1993 4.4 m3 s~', July 1993 7.2 m3 s"1 (Seifert, 1996).
three
Vulnerability ofkarst 57
different hydrological conditions which is independent of discharge (hydrological condition) and the amount of injected tracer. This response function describes the aquifer between the injection point and the sampling point. The following parameters have to be calculated from the tracer breakthrough curves of the single experiments and plotted versus the spring discharge, during the experiment: - peak concentrations Cp per kg injected tracer, - times of tracer peaks tin„ - standard deviation of mean residence times a,. A linear relationship for Cp and discharge and tinl and discharge and a logarithmic one for a, and discharge have to be determined. For each tracer breakthrough, a dimensionless transfer function can be determined with the above three parameters. The concentration, normalized to the peak concentration (C/Cp), has to be plotted versus the dimensionless time((/ - tint)/(Jt). From several transfer functions the mean transfer function can be determined by forming arithmetic means (Fig. 5). With the help of the mean transfer function and the information of a hypothetical spring discharge and tracer injection amount, a synthetic tracer breakthrough can be simulated. Consequently, the breakthrough of a potential point source pollution (e.g. a spill of chemicals in a location with access to the conduit system) can be simulated for a given discharge. That way it is possible to interpolate the results of tracer experiments for unmeasured hydrological conditions. An extrapolation to extremely high or extremely low discharges is problematic, when the function is not based on experiments under such conditions.
In the presented case studies, tracer experiments were essential in proving underground connections and characterizing the inner structure of the fast flow connections. As an overall result of the studies a vulnerability map of the karst massifs was developed. It shows the catchment areas of the different springs, which are at the same time areas of latent vulnerability. Within the catchments, areas with direct access to the conduit system were separated as being of acute vulnerability (Fig. 6). An advanced technique to delineate protection zones in karst areas using a modelling approach is given by Dassargues & Derouane (1997).
Churfirsten - mean transfer function
-June 92
- May 93
- July 93
C / C 1
0,9
0,8
0,7
0,6
0,5
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Vulnerability ofkarst 59
When assessing the specific vulnerability in mountainous regions, often livestock and tourism are the main threats to groundwater quality. To avoid bacteriological risk, livestock exclusion from the most sensitive karst areas is effective. The animals have to be fenced away from sinkholes, cave entrances and from open water bodies (sinking streams and lakes) (Berryhill, 1989). The problem of sewage disposal from tourist locations has to be solved in an appropriate way. The sewage has to be treated mechanically and biologically before it infiltrates in the underground. The infiltration should take place in locations, where diffuse flow dominates (distant from dolines). The remainders from mechanical filtration should be taken out of the karst area or used for limited land application. The appropriate rate and time of application to areas with a considerable soil cover has to be well planned. Waste disposal has to be managed in a sensible way and has to be realized far enough away from the most vulnerable locations like sinkholes and dolines. The systematic use of Sanitary Bacterial Dynamics as tracer offer further perspectives in investigation of intrinsic vulnerability and relate it to the specific vulnerability of an area (Gunn et al., 1997).
CONCLUSIONS
Karst aquifers are extremely vulnerable as there are fast water connections from the surface to the groundwater with only little protection against contaminants. A spatial assessment of vulnerability is difficult as the most sensitive parts of karst areas, namely dolines, swallow holes, bare karst and sinking streams, are not only spatial, but also point and linear. In the sense of water resource protection, karst areas should be protected as a whole. The catchments of springs which are used for water supply are areas of latent vulnerability and have to be protected as a whole. Within these areas, especially sensitive areas must be distinguished and accordingly protected, as they are areas of acute vulnerability. For the investigation ofkarst areas and the assessment of their vulnerability only integral approaches are efficient. The methods used are numerous. The observation of spring discharge and analyses of natural tracers occurring in water can efficiently be combined with geological information to obtain an understanding of the karst system. Other advanced techniques like GIS techniques and remote sensing are suitable in providing spatial information and vulnerability maps. Tracer experiments are essential to prove flow connections and can be modelled to characterize fast flow connections.
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