Prevent Leakage and Mixture of Karst Groundwater

Abstract

Choosing optimal dam sites is a very complicated task due to the nature of karst and the insecurity of water storage often resulting in leakage from reservoirs. An appropriate project concept prior to exploration can significantly reduce the risks of water losses or at least minimize them to acceptable levels, while the absence or reduction of exploratory works can increase them. Many analyses show that once the reservoir is filled up, groundwater flow currently oriented toward the future reservoir would saturate the upper part of the karstified rocks, reactivate currently unsaturated (fossilized conduits) pathways, and form a reverse discharge outside of the reservoir area. The geological, hydrogeological, speleological, and other special investigation procedures should be permanent activities during the design stage, during the construction of the dam site and filling of the reservoir, as well as during exploitation. Having a good map, database, models, and geological, hydrogeological, and other 2D and 3D layers increases the chances of choosing a successful dam site and minimizes the possibilities of further leakage from reservoirs below the dam site and through the reservoir and dam site embankment. This chapter summarizes the necessary procedures for the acquisition of some of the basic information for choosing an optimal dam site and preventing leakage from reservoirs in karst formations through chosen characteristic examples. Several case studies involving mineral ore extraction and mine drainage in a karst aquifer environment are presented in this section. Mining operations often entail extremely high rates of groundwater inflow, which is a threat to safe mining. Insufficient knowledge about the hydrogeological setting and a lack of preventative drainage often lead to sudden inrushes. In the past, this has caused rapid mine flooding, material losses, and even human casualties. In the case of evaporite karst, ground subsidence resulting from rather fast dissolution of evaporite rocks is a special problem. The practical experience discussed in the section shows that various measures are undertaken to drain mining operations (including drainage wells on the ground surface, underground dewatering boreholes, drainage galleries, drainage shafts, and the like), as well as that grouting of karst conduits and caverns has not always been effective. The quality of karst groundwater, before it enters the zone of mining operations, is generally good. However, after the groundwater comes into contact with ore deposits, this quality frequently deteriorates. Numerous examples show that karst groundwater, when abstracted before it reaches mining operations, can be used for drinking water supply, irrigation water supply, and other similar purposes. The majority of karst terrains are characterized by a high degree of heterogeneity. The results obtained by applying methods for the assessment of local karstification (e.g., borehole tests) often cannot be reliable to extrapolate to a wider area. The use of remote sensing provides the opportunity to assess the spatial distribution of karstification in the subregional scale. Analysis of satellite and aerial images allows the identification of geomorphological and tectonic forms that may indicate the highly karstified zones. From the factors that indicate the karstification, and which can be mapped by remote sensing, two factors are selected: surface karstification (K _sf) and density of faults (T _f). By overlapping maps of these two factors using geographical information systems (GIS) techniques, the final map expressed through a KARST (karstification assessed by remote sensing techniques) index is obtained. For the first time, the mapping approach has been applied to the catchment area of Karuč springs (Montenegro). By surveying the catchment area after the preparation of the map of the KARST index, it was noted that the assessed degree of karstification by using remote sensing mainly matches to the field assessment of shallow karstification. The application of this approach provides an image of the spatial distribution of karstification, even for areas that are inaccessible for direct field research. The obtained map can be used as a basis for solving some of engineering problems in karst that are related to the regulation of water, extraction of groundwater, and protection of karst aquifers from contamination. The mixture of fresh groundwater and surface water is a frequent problem in karst, and most problematic for the sustainable use of fresh groundwater. This is mostly a result of a high permeability and low attenuation capacity of karst aquifers, particularly those formed in open (unconfined) structures. The problem becomes more complicated when karst aquifer is in contact with seawater and tapping coastal aquifers and distinguishing fresh from seawaters is regularly a very difficult task. For this purpose, the Phoenicians constructed special intake structures and still today, many attempts to address this problem are made and similar devices constructed. The regions in which a large number of submarine springs exist are the Mediterranean basin, Florida, the Caribbean basin, the Black Sea, the Persian Gulf, and the Pacific islands. The section includes an explanation of the classical Ghyben-Herzberg formula, which defines the relationship and interface between fresh and salty water, but also states that its application, as in the case of Darcy law, should be used with caution in the case of karst aquifers. Several chosen case studies from different locations (Yucatan Mexico, Libya, France, and Montenegro) provide an overview of problematic and very difficult management of littoral karstic aquifers. It is often the case that even implementation of sophisticated engineering works and controlled pumping of fresh groundwater cannot completely diminish salt water intrusion.