Hazards in Karst and Managing Water Resources Quality

Abstract

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 (t_tot) is calculated to obtain the travel time from any point in the catchment area to the tapping structure. For the ponor catchment area, t_tot is the sum of the surface water travel time to the ponor (t_s) 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.