15. Karst Groundwater Availability and Sustainable Development

The hydraulic behavior of karst aquifers under several hydrologic conditions is described. Different parts of a spring hydrograph have been associated with hydrodynamics of karst aquifers, starting from a strong recharge event up to a long-term flow recession condition. As karst aquifers are characterized by a complex conduit network developing in a fractured carbonate rocks, an exchange of water between conduit and matrix/fractures generally occurs. Under strong recharge conditions, concentrated infiltration favors an increase in the hydraulic head in the conduit network, and a rapid pressure pulse is forced through the phreatic conduits giving a hydrographic peak at the spring. Later, after the end of recharge processes, the karst conduit network drains the saturated zone of the aquifer, and water table decreases. After prolonged rainfall deficit, a meteorological drought can induce a groundwater drought, and the water table falls to a minimum height; this hydrologic condition highlights the hydraulic behavior of karst aquifers under droughts. Statistical analyses could forecast a groundwater drought using rainfall data series, and provide a useful tool for water management. An example for karst aquifers of Picentini Mountains (Southern Italy) has been described, where spring discharge time series are available for several decades. Besides, the exceptional spring discharge increase after 1980 Irpinia earthquake is also described. The underground flow through karstic aquifer depends on the size, shape, nature, filling, and interconnection of cavities and voids. The transformation of rainfall into karst spring discharge can be very rapid, or the process can be rather slow and long lasting, due to the storage potential and retardation capacity of the underground voids. Autocorrelation and cross-correlation analyses can be very useful tools for evaluation of discharge regime and forecast of karstic aquifer behavior under different circumstances. In addition to these analysis, this section provides a theoretical base of a linear multiple regression model used for karst spring discharge simulations based on daily values, as well as examples based on autoregressive models (AR), cross-regressive models (CR), and most often used hybrid models (ARCR). The theoretical basis of transformation functions which is used for simulation of daily values of karst spring discharges was explained in detail. For the purposes of assessing partially (insufficiently) gauged karst springs, developed model presented in this section can be used for extending recorded discharge time series, for computing the real evapotranspiration, catchment size, and dynamic storage volume of the karst aquifer. The estimation of the groundwater potential for exploitation is of utmost importance, particularly during the planning or in the initial stages of the regulatory system development. This section examines two different concepts of the possible implementation of regulatory systems: first one, based on water “borrowing” from natural storage—static reserves—and second one is based on the increase in the dynamic groundwater storage by the construction of underground reservoir. By simulating different scenarios of groundwater exploitation, along with knowledge of hydrogeological behavior, a realistic basis for future optimal control of karst outflow regime can be created. This implies the analyses of storage changes in karst water reservoirs under natural conditions, and calculation of the future potential exploitation. The applied concept is based on the karst groundwater budget, which provides valuable information on storage changes in the karst aquifer, enables further predictions of the optimal exploitation rate, and facilitates karst groundwater management. Several examples are use to illustrate model performances and outputs. In hydrogeological and engineering geological surveys of karst interior, the only research methods that allow direct observation, investigation, and exact geological mapping of channels and caverns are speleology and cave diving. Data collected during speleological and cave diving exploration significantly contribute to the reconstruction of the evolution of karst processes, which is very important in assessing the depth of karstification which is in turn important for tapping of deep siphonal springs for water supply, as well as for other hydrotechnical structures in karst. Section 15.4 is an overview of four phases of speleological and cave diving explorations, from planning through exploration to the final data analysis. The examples of research given in this chapter are only a part of the experiences that point to significant development of karstic channels in karst areas. Speleodiving and speleology research in defining the position and functioning of karstic aquifer have contributed significantly to solving water supply problems, and to the implementation of hydrotechnical and other structures in karst. Although karst aquifers are one of the main water sources for drinking water supply worldwide, it is well known that they are at the same time one of the most problematic resources. Due to the unstable flow and great variation of the discharge of the karstic springs, during recession (lean) periods, the local population often suffers from water shortage. If aquifer is well karstified and has adequate storage in its deeper parts, it is often possible, just as it is in the case of open water reservoirs, to regulate and manage minimal flow by various engineering interventions. Such an option provides opportunities to satisfy water demands not only of direct consumers but of ecosystems as well by ensuring ecological flow downstream. In this section, possible engineering solutions to regulate groundwater flow in karst and physical, ecological, and economical implications of such interventions are discussed. The three prerequisites for implementation of an engineering regulation project which should be fulfilled are as follows: Regulation is physically possible; regulation is environmentally sound and friendly; and regulation is economically feasible and sustainable. Each of them is explained, and guidance for a realistic evaluation is provided. Several case studies facilitate understanding of the regulation opportunities and threats as well as identify some helpful surveying methods used in water practice. The two main groups of engineering regulation for controlling karstic groundwater are discussed in detail: (1) regulation of discharge zone and (2) regulations, i.e., interventions in the wider catchment area. The former can be achieved by spring overpumping, drilling the wells or other supplementary intakes, constructing a subsurface (underground) dam, or by artificial recharge. The riverbed regulation, directing groundwater to other more promising catchment areas, closing or regulating ponors (swallow holes), and building impermeable barriers are some of the measures which can be applied in the wider catchment to maintain seasonal flow. A few successfully implemented projects of aquifer discharge control and lessons learned in management of groundwater reserves are also presented in this section.