Introductory editorial: advances in karst hydrogeology

Abstract

Karst aquifers constitute 12 % of the global land surface and store water on which approximately 20–25 % of the world’s population depends, as it is stated in classical text books on karst. One-third of the surface area of Europe contains this type of aquifer, which supplies water to cities like London, Bristol, Paris, Montpellier, Vienna and Rome, to name just a few significant examples. In some countries, such as Slovenia and Austria, karst water contributes more than half of the drinking water supply and in many regions it is the only available source of fresh water. Moreover, karst aquifers are mostly upstream systems, acting as a water tower of fragile ecosystems that may be coastal and/ or correspond to urban areas subject to development. Karst areas also support unique ecosystems that are very rich in biodiversity, but their functioning, development and level of fragility remain largely unknown. Several karst systems are included in the UNESCO World Heritage list, and are characterised as strategic resources at local, national and international levels. The potential of karst areas in terms of hydro-geo-biodiversity, for instance, is widely recognised. These considerations illustrate the importance of the water stored in this type of medium and, therefore, the need for research into karst hydrogeology. Unlike porous or fissured formations, karst systems have a particular heterogeneous structure and behaviour, characterised by the concentration of groundwater flow through a network of karst conduits, enclosed within a less permeable matrix and connected to a local discharge area, the karst spring. This combination of areas of high hydraulic conductivity/low storage capacity (karst conduits) and medium to low hydraulic conductivity/medium to high storage capacity (fractures and matrix systems) means that contaminants may readily penetrate the subsurface, reaching groundwater and then being rapidly transported over large distances. Various research techniques have been developed, mainly based on the analysis of the natural response of karst aquifers, and include spring discharge (hydrographs), chemical composition (chemographs) and temperature and isotopes (both stable and radioactive). There has also been progress in the implementation of techniques such as dye tracers, with tracers (especially fluorescent ones) that are environmentally innocuous and provide a higher sensitivity. Recent experiments have been done with methods based on natural tracers, such as total organic carbon, natural fluorescence, turbidity and bacteria from the soil. The combined use of several of these methods allows the comparison of the results of various research programs and the ability to reach well-grounded conclusions. The specificity of the karst medium is fully accepted at all levels of hydrogeological knowledge, as shown by the fact that most of the international associations concerned with water, geological and environmental sciences have specific karst committees. One case is the Karst Commission of the International Association of Hydrogeologists (http://www.iah.org/karst), which organises many activities (conferences, meetings, training courses, publications, handbooks, etc.) on karst hydrogeology; in fact, this special issue is considered a contribution by the IAH Karst Commission. Moreover, there are specific international journals on karst (Acta Carsologica, Carbonates and Evaporites, International Journal of Speleology, Journal of Cave and Karst Studies), and contributions on this subject are published in many scientific journals, as occurs with this Environmental Earth Sciences special issue. In addition, B. Andreo (&) Department of Geology and Centre of Hydrogeology, University of Malaga, 28091 Malaga, Spain e-mail: andreo@uma.es