Nataša Ravbar

Hazards in Karst and Managing Water Resources Quality

Karst is an extremely fragile natural environment. The geological, morphological, hydrological, and hydrogeological features of karst determine an overall high vulnerability to a number of potentially dangerous events. The delicate equilibrium of karst ecosystems can be dramatically and irreversibly changed, as a consequence of both natural and anthropogenic impacts. This contribution examines the main peculiarity of karst and discusses the main natural and anthropogenic hazards affecting karst. Sinkholes, mass movements, floods, and loss of karst landscape are dealt with and discussed also by means of description of some case studies. Actions to mitigate the hazard in karst are also treated, highlighting the necessity to protect karst, an environment that needs specific regulations to be properly safeguarded. In particular, the Karst Disturbance Index, to evaluate the degree of disturbance done by man to the natural karst, is discussed. Groundwater contamination is by the World Health Organization listed among the world’s severest problems. Globally, water resources are limited and under pressure from urbanization and climate change. Among available drinking water resources, groundwater from karst aquifers is progressively becoming more valuable for potable, irrigation, and other agricultural and industrial use due to its abundance (high flow rate springs up to some tens of m3/s) and relatively high quality of water. However, its efficient use and protection poses a great challenge to urban karstology due to the very high susceptibility to contamination. The concept of groundwater vulnerability and contamination risk assessment is presented as an alternative approach for source protection zoning and land-use planning in karst. Specifically, vulnerability assessment has in some countries already been adopted by some national water-related policies as it confirmed to be a practical tool for protection zoning. It offers balance between groundwater protection and economic interests. The resulting maps are useful for planners and developers dealing with the protection and management of karst groundwater. However, caution needs to be taken when selecting the appropriate method for vulnerability assessment and when interpreting the results. Karst groundwater protection mostly relies on the implementation of sanitary protection zones where different restrictions apply. A review of the relevant legislation of several European countries showed that the groundwater travel time is the most frequent criterion for the delineation of sanitary protection zones, where the horizontal travel time to the groundwater source is generally considered. As a result, some countries increasingly use groundwater vulnerability maps to define sanitary protection zones and to implement more stringent measures where groundwater is vulnerable. A step further in the optimization of the sanitary protection zone delineation approach is to include the travel time through the vadose zone and to take into account surface water flow to the ponor. The total travel time (ttot) is calculated to obtain the travel time from any point in the catchment area to the tapping structure. For the ponor catchment area, ttot is the sum of the surface water travel time to the ponor (ts) and the travel time from the ponor to the tapping structure, based on dye-tracing tests. For any point outside the catchment area of the ponor, the total travel time is the sum of the vertical (t v) and horizontal (t h) groundwater travel times. Apart from test results obtained using natural and artificial dye tracers, the vertical travel time can be estimated based on vulnerability assessment, while the horizontal time can be assessed by analyzing spring hydrographs. The vulnerability map produced on the basis of total travel time calculations can easily be converted into a map of sanitary protection zones, depending on national legislation. The Remediation of Groundwater in Karst section describes aggressive technologies currently being applied to remediate karst aquifers, including in situ thermal treatment, in situ chemical oxidation, in situ bioremediation, and pump and treat. The fundamentals of each technology are discussed, including design principles, failure mechanisms, and amenable contaminants. The authors first provide an overview of trends in the groundwater remediation industry, which is followed by thought-provoking discussion on the politics of remediation in karst. Special attention is given to the technical challenges presented by karst, such as conduit flow and dissolution features, which may make remediation impracticable. On the technical side, this chapter includes a demonstration of modeling tools to assist with remedial evaluation and design. For example, the authors illustrate the use of VS2DTI for heat transport modeling in thermal remediation design, and the conduit flow process (CFP) for pump and treat design. Each example illustrates the need to incorporate conduit geometry and flow in the remedial analysis, as the use of equivalent porous media (EPM) techniques would lead to poor remedial performance. The hydrogeology of the thick karstified carbonate regions is challenging not only theoretically but also from a practical point of view. In these systems different types of groundwater flow are operating on distinct timescales associated with different types of permeability. Practical and scientific concerns related to karst hydrogeology are often on a regional scale such as sustainable water management, contamination of aquifers, and geothermal utilization. It is key issue to understand the regional and hydraulically connected nature of carbonate systems and to find appropriate solution for these particular problems. The importance of the gravity-driven flow concept is that it helps to understand the common genesis of thermal flow. The paper presents a deduced generalized flow pattern for deep carbonate regions which can provide a basis for finding similarities between thermal springs connected to continental carbonates. The understanding of the scale effect is highlighted to resolve practical problems. An important consequence of the hydraulic continuity and relatively higher hydraulic diffusivity of karst is that the effects of natural or artificial stresses on the groundwater level can propagate greater distances and depths than in siliciclastic sedimentary basins. The Transdanubian Range, Hungary can give an “in situ example” for the operation of hydraulic continuity based on a “long-term pumping test.” The fact of hydraulic continuity operating on a different scale can be used also during the planning of geothermal doublet systems and in the necessity of the use of heat content of effluent lukewarm and thermal springs and wastewater of spas in discharge zones of thermal water. Inadequate management of transboundary aquifers can lead to various groundwater quality (changes in groundwater flow, levels, volumes) and quantity (dissolved substances) problems. These problems are more difficult to prevent, mitigate, and solve in an international context than in the case of national aquifers. International cooperation is necessary to ensure an appropriate assessment, monitoring, and management of transboundary groundwater resources. International agreements are made to prevent potential conflicts and to improve the overall benefit from groundwater. In practice, agreements, to be made and respected, require a sufficient knowledge on the resource, its current state, and the trends. This is often a challenge for invisible groundwater and especially in a complex hydrogeological environment like karst. Aquifers in karst are very vulnerable as well, asking for an additional attention of national and international water authorities. This chapter describes DIKTAS, a case study of transboundary aquifers in the Dinaric karst region; it addresses motivation for international water cooperation, methodological approach, achieved results, and current efforts.

Analysis of human induced changes in a karst landscape — the filling of dolines in the Kras plateau, Slovenia

A comprehensive analysis of the increased pressure on karst landscapes due to expansive economic and urban development is presented with the aim of evaluating changes in land use and their deleterious effects on karst relief forms. The study focuses on two areas surrounding the relatively quickly growing settlements of Hrpelje-Kozina and Divača on the Kras plateau (Slovenia) that have been subjected to intensive urban and business development and traffic since the motorway was brought to their vicinity fifteen years ago. National legislation loopholes and technological improvement were the cause of the commonly unsupervised human encroachment which caused the widespread degradation of the landscape. By comparing different topographical and ortophotographical materials from the past four decades and by detailed field inspection of land use and environmental changes, as well as the morphometrical characterization of dolines, the following results have been found: due to the population growth in the past four decades (39% and 50%, respectively), an increase of settlement area by 18 and 11 percentage points took place. Consequently, between 25 and 27% of dolines have disappeared or have been extensively modified (filled up and leveled). According to the local spatial plans, an additional 18% to 28% dolines are endangered. Broad human induced changes in the karst landscape have resulted in a noticeable increase in landscape deterioration, which is consistent with similar phenomena observed in other regions. Due to the extreme susceptibility of the karst to human activities that may lead to the degradation of its exceptional esthetic and environmental value, the alteration of karst processes such as corrosion, endangering of unique habitats and the quality of non-renewable natural resources, it is necessary to promptly define measures for its protection at the national level.

Extreme hydrological events in karst areas of Slovenia, the case of the Unica River basin

The study presents natural hazards in Slovenias karst, focusing on flooding in karst poljes. A specific study was done on the flood dynamics of two typical and connected karst poljes (Cerknica and Planina) of the Classical Karst region. In the case of particularly extreme hydrological conditions in the autumn of 2008, detailed analyses of the recharge-discharge regime and the interrelationship of flooding on the two poljes were done. Daily precipitation, discharge, and water level values from several monitoring sites were analyzed and cross-correlated, and additional hydrological analyses were done using a digital elevation model in order to acquire water level increase and decrease intensity, flood water volumes, and the extent of flooding and to understand the conditions controlling karst flooding. The results reveal that the hydrological functioning of the studied karst poljes is influenced by the hydrogeological and temporary hydrological conditions in the catchment area. The response of the binary karst system (i.e., the influence of autogenic and allogenic recharge) is especially distinct. The study shows that during extremely intense recharge, the reactions of karst aquifer systems to precipitation are as rapid as the response of surface waters (the water level of Cerknica Lake increased with an intensity of 38-63 cm/day or 55 m3/s respectively) while retention capacities are negligible. In contrast to flash floods, floods in karst areas may last from several weeks to several months. For the observed period a three-dimensional simulation of the flooding was made. At the maximum recorded water level, the volume of water on the Cerknica polje was 51 million m3, and 26 million m3 on the Planina polje. The maximum extent of flooding on the Cerknica polje was 23 km2 and on the Planina polje 9.5 km2. On the basis of the study, information was provided regarding future hazard mitigation. However, the study demonstrated that a sufficiently dense monitoring network is necessary to predict the occurrence and duration of floods with greater certainty.

Application of Methods for Resource and Source Vulnerability Mapping in the Orehek Karst Aquifer, SW Slovenia

In a shallow karst aquifer in SW Slovenia assessment of groundwater and drinking water source vulnerability has been performed using different methods. The small, but well defined, Orehek karst is mainly drained by the no longer exploited Korentan spring. The recharge area of the spring is well karstified, densely wooded, and covered by thin soils. Besides precipitation, the spring is additionally recharged by temporally active sinking streams. To assess the vulnerability, two methods, COP+K and Slovene approach, have been applied. Both methods provide tools for assessing vulnerability in carbonate rocks. They both rank among very sophisticated methods that require vast amounts of data, time, and financial and technical resources. Both methods share the same methodological procedure and consider the same type of information, categorized by the same factors. On the other hand, the scoring, classification and weighting of individual parameters between the methods are different. Consequently, the resulting maps differ significantly. Major differences between the results are identified and discussed. In addition, sensitivity analysis of individual factors have been performed and compared to cross-correlation, autocorrelation and water budget calculations. The results show high dependence of the COP+K method to the parameters characterizing the infiltration conditions (the so-called C factor) and distance to water source. The Slovene approach vulnerability classes are mainly influenced by thickness of the soil cover, presence of morphological features and the temporal hydrological variability that is justified by the performed statistical analysis.

Validation of Vulnerability Assessment Using Time Series Analysis—the Case of the Korentan Spring, SW Slovenia

In the shallow Orehek karst aquifer in southwest Slovenia, the use of hydrological data analysis for the purpose of validating the assessment of water source vulnerability was tested. The appropriate criteria for the aquifer and groundwater flow characterization were identified and major drawbacks highlighted. Results of water budget calculations were used to determine the extent of the catchment of the Korentan spring, which is the main outflow from the aquifer. The vulnerability assessment was verified by autocorrelation and cross-correlation analyses of available daily hydrological time series data. The small variability of the Korentan spring water temperature and electric conductivity time series points to the dominance of autogenic recharge and that sinking streams in the catchment contribute to the spring to a minor degree. The analysis indicated relatively small storage capacity of the aquifer and its high degree of karstification and rapid groundwater flow. The results justify the small proportion of highly vulnerable areas and lower vulnerability for marginal parts of the aquifer. Time series analyses proved to be time- and cost-effective, but have limited applicability for vulnerability validation purposes, as they do not provide direct and clear spatially resolved information on the vulnerability of the catchment.

Vulnerability and protection aspects of some Dinaric karst aquifers: a synthesis

The present paper aims to address the sustainable management issues of karst water sources in Slovenia and Croatia. It focuses on the Mediterranean part of the Dinaric karst, which holds important groundwater reserves. The importance of selected karst aquifers for water supply is presented. In addition, national drinking water management strategies are examined to synthesise the issues relating to the exposure of groundwater to contamination and over-exploitation. Finally, the most significant problems, pressing needs and challenges are identified. These depend predominantly on revision of existing drinking water protection strategies and integration of karst-specific provisions, aspects of over-exploitation and proper water quality control. The need for harmonised multi-country policies regulating resource abstraction and contamination issues is stressed.

Abandoned water resources as potential sources of drinking water – the Korentan karst spring near Postojna

In the area of the Orehek karst intrinsic vulnerability maps of groundwater and the Korentan karst spring, hazard maps and risk to contamination map have been made using Slovene Approach. For this purpose cartographic and other published data on geological, geomorphological, pedological, hydrological and meteorological characteristics have been gathered and extensive field inventory of the missing geomorphological and pedological characteristics has been made. Publically available land use and census data have been examined and checked in the field, manure heaps and illegal waste dumps have been mapped. The final thematic maps are excellent tools for national and local authorities when planning water protection and land use. The presented study in the catchment of an actual water source is an example of good practice of the revival of the in the past abandoned water sources that can in the future serve as a substitution water source in case of main water source overexploitation or contamination.

Impact of Motorways on Karst Waters

Impervious surfaces of road networks accumulate contaminants and pollutants, which are washed off during rain and snowstorm events into nearby waters and lands. Especially in areas with a developed traffic network, runoff from motorways is a significant source of pollution. Various protection and remediation measures were developed and implemented for preventing or at least diminishing its negative influences. Three major sources of pollution are associated with the road network: vehicles (emissions of motor vehicles, spilled and released oil, tires’ particles, de-icing agents), road characteristics and paint markers, and atmospheric depositions influenced by the adjacent land use. Besides pollutants directly associated with traffic, the impermeable surfaces can collect and drain a negligible quantity of organic waste, nitrogen and phosphorous, herbicides, pesticides and faecal pathogens. Soil infiltration treatment, building of collection basins and constructed wetlands are used as protection and remediation measures. In karst areas, motorway runoff has an amplified impact on ground water, compared to other types of landscapes. The soil layer is commonly thin or non-existent and thus soil infiltration treatment is practically inexistent. Stormwater runoff may immediately flow into the aquifer through subsurface conduit networks, fractures, sinkholes and sinking streams, which makes karst groundwater especially vulnerable to pollution. Accidents that cause greater quantities of hazardous substances to flow into the karst, for whatever reason, endanger our environment, karst waters, and even in smaller quantities also the quality of the karst springs that have been captured for drinking water supply. Especially dangerous are spills of oil derivatives, since we know too little about the flow of such substances that do not dissolve in water and are lighter than it. Based on observations of Globocec spring following the accident near Ortnek and of the Rižana Spring following the spillage near Obrov and other similar cases, we anticipate a longer retention time and washing away of oil derivatives due to the possibility of adsorption on sediments and retention in siphons.

Unroofed Caves Provide Important Clues to the Karst Development

Unroofed caves are old caves that were revealed on account of the lowering of the karst surface. They are preserved by their infill—mostly alluvium and flowstone. It became clear during the motorway construction undertaking in Slovenia that unroofed caves constitute a relatively common karst landform. In fact, more common than karstologists had imagined before the karst surface was uncovered through earthworks. The various types of notches occurring on the surface have long been interpreted as types of dolines or as the result of the lithological properties of rock and its fracturing. 75 km long and, on average, 25 m wide stretch of the motorway across the classical Karst gave up 350 caves, of which 90 are unroofed caves. Some of them make up the same cave system. New findings prompted karstologists to become more aware of these unique surface karst forms. In the process, numerous unroofed caves filled with all types of alluvium were discovered. There were several attempts at typification of the characteristic shapes of unroofed caves and to design partial models to explain their typical formation processes. The surface and subcutaneous dissolution of carbonate rock and its disintegration from back in the Ice Age, brought about the lowering of the karst surface. Old caves, which were formed by erstwhile water flows and are partly intersected by shafts which drain water from the permeable karst surface, pop up as either empty or filled with alluvium. The caves were formed as a part of a system of cavities in a period when impermeable rocks had enclosed the aquifer higher up, causing the underground water in the aquifer to be at a higher level. The hypothesis was that the karst topography and its remarkable systems of valleys can be traced back to former surface water throughflow. However, revealing the surface did not provide the evidence to support this hypothesis, instead obvious signs of former water throughflow in carbonate rock were identified—manifested as open and cut through old caves.

Unroofed Caves Near Kozina and Their Identification

Unroofed caves are an important karst form that makes up a part of the karst surface and epikarst, and provides us with valuable evidence of the karst development. They are old caves that became exposed by the lowering of the karst surface. In fact they are preserved by their fill—mostly fine-grained alluvium. It is also often that they feature preserved flowstone and an intact rock rim. During earthworks preceding the motorway construction, this important karst feature, also characterizing the surface, attracted special attention. Earthworks revealed that the karst surface is scattered with several distinct types of unroofed caves, which in itself are not an uncommon phenomena. The typical shapes of unroofed caves found on karst terrain could be singled out, i.e. individual doline-like forms that occur in strings, and oblong notches. Because the surface of the Karst region has lowered so dramatically, there are old caves and shafts opening up all the time in the course of the construction of the motorways. Old caves are either void or filled with alluvium. The caves were formed as a part of a system of cavities in a period when impermeable rocks had enclosed the aquifer higher up, causing the ground water in the aquifer to be at a higher level. But karstification gave rise to a drop in the water table in the aquifer—today it is 200 m and more below ground, and the karst surface is still lowering. Unroofed caves are therefore regarded as distinct surface karst forms which were in part reshaped by surface processes that make up an important part of epikarst. In the course of the earthworks for the construction of the Kozina motorway, the typical shapes of unroofed caves, passages and large cave systems carved in horizontal or inclined karst surfaces were discovered.