II. Karst Hydrogeology
Summary Sierra Blanca and Sierra Mijas make up a Hydrogeologic Unit of 170 km2 (in surface) situated in the South of Spain, 100 km in distance from the East of the Strait of Gibraltar and an average interior distance of 5 km from the Mediterranean Sea.
From the geological standpoint, it forms part of the Alpujarride Complex of the Betic Cordillera and as such, its stratigraphic serie is formed by an inferior metapelitic formation migmatite and gneisses of the Paleozoico age and other carbonated superior (white dolomitic marbles in the low part and blue limestone in the high part) and the estimated age is middle and superior Trias. The geological structure is very complex and permits differentiating three sectors: the western sector of Sierra Blanca made up of N-S and E-W folds which give rise to an interference in an egg carton , eastern sector of Sierra Blanca with an almost tabular structure and Sierra Mijas sector formed by ESEWNW folds, the vergence is always towards the interior of the Sierra. All this folded structure is found in turn truncated by fractures with preferencial NNE-SSW and NNW-SSE directions which have conditioned the superficial drainage net as well as groundwater. The two masses are separated by an outcrop of peridotites in the Puerto Pescadores.
The average annual rainfall on the Unit is approximately 700 mm although this varies from more than 800 mm in the western sector of Sierra Blanca to less than 600 mm in Sierra Mijas.
The recharge is produced by the infiltration of rain water and the discharge takes place in Sierra Blanca, through spring borders and in Sierra Mijas by means of pump in numerous catchments that exist. Between both Sierras there exists a hydrogeological connection, through the weathering zone of the peridotites of the Puerto Pescadores, but the transmissivity of this material is low, for this reason the transference of resources between the two Sierras is discarded. Taking the geological and hydrogeological information (hydrodynamic, hydrochemical, isotopic and hydrothermic) into account eight different hydrogeological systems have been distinguished, three in the western sector of Sierra Blanca and one in the eastern sector of the mentioned Sierra and four in Sierra Mijas.
The Alpuiarride carbonated aquifers have traditionally been considered to be little karstificated. However, in Sierra Blanca and Mijas two different basic types on aquifers can be distinguished, depending of fracturation and karstification grades as well as the lithology: karstic systems and fissured systems.
Borivoje F. MIJATOVIC
University of Novi Sad, Agricultural faculty,
It is well known that groundwaters in karstic terrains are very vulnerable to external pollution, not only because of low autopurification properties of karstic aquifers, but also of fact that underground collector (fracture) networks, although of limited extension, are able to drain very large catchment areas with disseminated and concentrated polluters. In such circumstances strict application of rules concerning the protection zones of groundwater caption objects is without any significance for dynamic processes of karstic waters, following natural hydrodynamic zoning and hierarchy of privileged collectors within drainage network (superposition of hydrodynamic zones, variable velocity of groundwater flow in space and time). Because of that, numerous tracer experiments that were performed are virtually unusable for quantitative analyses on pollutant propagation and its prediction through karstic aquifers.
Knowing the mentioned problem, the officials of water-management and environmental public services in European Union, long ago have understood and accepted the necessity of making hydrogeological complex studies (including monitoring control systems) because of rigorous conditions for water supply in their countries; these studies can give correct answers on relevant parameters regarding the protection of karstic aquifers. These are, first of all, parameters defining function of transport processes for groundwater mass flux (water flow + tracer) and prediction of pollutant propagation depending of transmissivity and storage capacity of karstic aquifers. Implication of tracer techniques method with artificial and natural tracers within hydrogeological research, significantly contribute to merritory defining of protection zones in the karstic terrains. In that sense, environmental impact of polluters into the karstic aquifers should be based on studying geological structures and their hydrogeological conditions and dynamic processes, caused by groundwater circulation beginning from precipitation and surface flows within catchment areas to different discharges of spring zones.
Hydrodynamic transport models. Karst Hydrogeological System
Theoretical approach of hydrodynamic transport models in karstic medium is based on a convective fluid moving through porous medium which by analogy of Fick's law is expressed as a Q= - D grad C, where D is coefficient of dispersion and C is fluid concentration. In a fractured medium consisting of some matrix-blocks, which are linked by the network of channels and cavities (collectors), mathematical concept of convective fluid flow could be expressed by means of differential equation dispersive flux (within axis direction) as:
where: Ci (x,t) fluid or tracer concentration [ML-3 ], vi mean velocity [LT- 1], xi flow distance [L], DI=a i·vi for Pe>10, ai defined as dispersivity [L], Pe=xi/a i Peclet's number, t transit time [T], i index relating to the number of individual flowpath in drainage network (i=1,2,3 .... N).
Figure 1 represents the conceptual flow model of the phenomenon described previously. It is clear that in nature some flow collectors may originate and somewhere in system.
In mathematical analyses of groundwater mass flux at hydrogeological systems often are used linear models of "black box" type. The basic principle is that two conditions are to be satisfied: (1) stationarity of the system, and (2) linearity of the system (Poitrinal and De Marsily, 1973).
Nevertheless, this model is convenient for every hydrogeological system with known input and output points. It is necessary ,therefore, to manage on response impulse (R.I.) that could be any of the outflow (amount of certain tracer) adequately injected at sinkpoint. For calculation of this R.I. of system it is necessary to manage a restitution curve at the point of spring. Analyzing the structure of the former curve, however, several parameters for the system under investigation could be obtained.
The noted water mass flux, defined that way, presents actually both input and output (inflow-outflow) of water in karst hydrogeological system (KHS). The KHS of this kind undergoes analysis of water input / output values, as a temporal series, which make reliable delimitation possible between transmissivity and storage capacity properties for karstic aquifers (Mijatovic, 1981).
Due to vertical superposition of unsaturated and saturated zones and general geometric heterogeneity of the KHS, experimentally obtained velocities for underground water flow differ considerably. Analyzing numerous tracer experiments in the Dinaric karst, it was proved that apparent velocities of groundwater flows in unsaturated zone during high water range between 15 to 60 cm/s, while in the saturated zone, during low flows, they are within the narrow range of 0.5 to few cm/s only. In karstic terrains of the Carpathian-Balkan mountain the apparent velocities of groundwater flows range from 22.20 to 0.45 cm/s, with an average of 6.31 cm/s. It is evident, therefore, that attempts in finding out certain mean velocity values have no sense for KHS of different hydrological conditions.
To be explained reasons for considerable differences of the foregoing data concerning to apparent velocity of groundwater flows in karst, it is of importance to mention that they are generally obtained by tracing water flows at zones with sink-holes of karst polje, due to which at the same time, it concerns to two kind of ground water circulations: (1) the predominantly vertical flows through unsaturated karst zone end (2) predominantly horizontal flows in karst saturated zone. It is clear that these two obtained velocity values could not be compared among themselves. Namely, while circulations of ground water through unsaturated zone are of predominantly flooding and periodical characteristics, the movement of water in saturated zone is strictly controlled by hydraulic gradient and conductivity in the karst drainage system (Figure 2).
In both cases the real velocity could be approximately equal for the same period of time after sinkhole injection; water at karst polje, sinks immediately to a great depths and further on it flows depending on hydraulic gradient of saturated zone, where actual value is h/d , rather than H/d .
However, that kind of repatriation of velocities in space and time points to the great vulnerability on pollution of groundwater, which otherwise is known for good quality.
The main postulates of groundwater protection
Karst has some specific physical and hydrodynamical properties which are consequences of geometric heterogeneity and anisotropy effect on velocity and hydraulic potential fields distribution. Taking all this in account the first task in hydrogeological research for protection of groundwater is the making of specific vulnerability maps for assessment of pollution risk in the catchment area and the groundwater discharge zones. The hydrogeological maps are to be made in two operational scales: the one, small regional--to represent the completeness of lithologic carbonate complex in the catchment area of KHS and the second, large--to fix all principal inlets and outlets points of the system (sinkholes and springs). Karst hydrogeological system is defined between this two operational scales by geological boundaries and boundary conditions whose determinations and delimitations frequently are the subject of long-term studies and terrain research, especially in the domain of groundwater hydrodynamic. It means that research of environmental impacts should be based on investigation of geological structures taking into account their hydrogeological conditions and dynamic processes caused by circulation of groundwater, beginning with precipitation and surface flows in catchment area (input function) to the different discharges in spring zones (output function). KHS defined in this way makes a point of transit karstic networks as on inflow-outflow systems with privileged and accessory zones of water flows inside the system (Figure 3).
In order to obtain full informations about groundwater between surface of karstic terrain and collector networks in subsystems of epikarst and endokarst as well as about transmissivity and storage capacity properties in main and secondary transit (drainage) zones we have to apply techniques with natural and artificial markers. From a practical point of view, it is impossible to make quantitative analyses of transport and prediction of the polluter propagation inside karstic aquifers, if these information are not obtain.
The protection measures for karstic aquifers and theirs catchment areas generally may be classified on: (1) preventive protection and (2) remedial measures or consequent protection.
The preventive protection (Hanzel and others, 1989) of water resources most of all considered the main aspect of environmental and groundwater protection in all phases of hydrological cycle and hydrodynamic processes into karst hydrogeological system under investigation. The measures concerning preventive protection are to preserve to natural conditions of groundwaters and to prevent their disturbance and its negative effects on their quality.
The consequent groundwater protection is necessary in case of already polluted waters owing to areal or point contamination of a latent (long-lasting) or accidental character. Remedial measures are usually very complex and costly, often with the uncertain outcome. Preventive protection, therefore, is the only reliable way if karstic water resources are to be rationally utilized in the future. These should include, first of all, the ban on the building of industrial and other objects with "dirty" technology, the obligated installation of the
equipment for waste water treatment, the construction of sewage systems in inhabited areas, the ban on the use of pesticides and artificial fertilizers, etc. All these measures to be accompanied by continuous monitoring and control system of quantity and quality characteristics of karstic waters (Stevanovic and Filipovic, 1994).
The choice of the sanitary protection zones is one of the most important problems, in view of geological and hydrological boundary conditions. Their delimitation in the terrain request a very competent knowledge of structural relationships and the hydrodynamic behavior of KHS in different hydrological conditions.
In some complex and specific conditions, after Stevanovic and Filipovic (1994), the number of protection zones can even be enlarged. As an example, the following schedule can be given: 1st protection zone (with most rigorous regime), 2nd protection zone (with rigorous regime), 3rd protection zone (with sanitary regime) and 4th protection zone (with mild regime).
Some specific problems of environmental impact on karstic groundwater
The pollution in the karstic terrains depends on the presence of potential pollutants. If they do exist, the possibility that the karstic aquifers gets polluted is considerable due to the presence of network of channel and cavities, where groundwater flows are not subjected to any filtration. Therefore the autopurificatory capabilities of aquifer are consequently reduced. The tracing experiments and correlative analysis between precipitations and spring discharges, confirms "quickly response" (reaction) of groundwater flows and faster propagation of the new waters produced from heavy precipitations in the remotest parts of the catchment area. This is especially characteristic of high water periods, with maximal spring discharge usually resulting in the lowest quality of groundwaters. Otherwise a favorable natural circumstances for the protection is the predominantly mountainous relief and uninhabited drainage areas. In such conditions the only sources of contamination are acid rains and local organic pollutants in the infrequent small villages.
An characteristic example of the pollution transport from a remote part of the catchment area has been recorded in the waters of Nemanja spring near Cuprija, in Eastern Serbia (Stevanovic, 1988). In these waters, a practically permanent bacteriological contamination has been recorded, most often of fecal character. Surface waters in the upper part of catchment area even more contaminated (coliform germs numbered 2,400,000, living bacteria 102,000, with E. coli and Citrobacter being is plated as well). Surface streams flow through several villages without any sewage system being installed. In the limestone section of the profile beyond the spring, the greatest part of these waters gravitating towards the zone of discharge, disappear. The Nemanja seepage spring has several levels of discharge. After Stevanovic (1988), the gravity springs on higher peaks, are characterized by the rapid transport of pollution of a higher rate, while the hypsometrically lower rising springs have more favorable characteristics and a much lower rate of bacteriological impurity (figure 4).
Similar forms of pollutants transport through surface streams have been recorded near Mirovsko and the spring Malo vrelo, in Eastern Serbia. In the former case, enormous bacteriological contamination resulted in hydric epidemic in 1980 (ECHO 1 1 viruses) in more than 1, 000 people living in the settlement of Boljevac (Stevanovic and Filipovic, 1994).
For more dangerous is radioactive contamination. Following the Chernobyl catastrophy, numerous examples of increased contents of radionucleides in some samples have been recorded. However, it is important to depict the examples of confined karstic aquifers in neogeneous basin of Kolubara near Lazarevac, where increase contents of radionuclides did not take place after the event in Chernobyl, which on the other hand was not the case for many shallow alluvial acquirers in this vast area.
In the other cases, the current mining can also result in undesirable occurrences. Thus, as the most probable cause of increased radioactivity of groundwaters in the area of Golubacka Mts., Serbia, on the riverbank of the Danube, a waste dump from the mine field on the Romanian side of the river in the zone of Moldava Veche has been designated (Vuasinovic and others, 1989).
The framework for vulnerability to pollution and control system for the remedial action of karstic aquifers
Regional hydrogeological system analysis is a tool to examine and describe complex dynamic units of water resources and flow media. In the case of KHS the hierarchical delimitation of groundwater flows between matrix-blocks and privileged networks within this system allows a reliable evaluation of the vulnerability risks and the control system for the remedial action under different lithological facies and for impacts at various places within such system. A systematic analysis on regional scale allows tracing of various categories of pollutants and their pathways trough primary and secondary karstic drainage networks.
Figure 5 presents a framework for the systematic analysis and description of complex regional hydrogeological system, connected with three types of input-sources to the left and three types of output-outflows to the right. KHS is hierarchically subdivided.
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