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Similarities and differences in hydraulic, hydrologic and
hydrogeologic approaches to karst groundwater investigations
OGNJEN BONACCI
Civil Engineering Faculty, University of Split
21000 Split, Matice hrvatske str.15
Phone: 385 21 303 340; Fax: 385 21 524 162; e-mail: obonacci@gradst.hr
The objective of this paper is to demonstrate the necessity of close co-operation of hydraulic, hydrologic and hydrogeologic scientific approach to karst groundwater investigations. The particularities of the karst aquifer are presented. The karst aquifer contains significant water resources, but development of these reservoirs is difficult due to the complexity of the aquifer. The key to understanding and predicting groundwater flow through them is an ability to accurately define the features and location of the heterogeneities using different scientific and engineering methods and approaches. Time and space scale issues are of a special importance for understanding and modelling of karst water circulation. Hydrogeology and geology are strong frameworks for water circulation in and over the karst. The importance of karstification index, e, and effective porosity, ne, is explained. The main hydrologic tasks are water budget equation, definition of the catchment area and existence of strong inflow-outflow relationship. Measurement of hydrogeologic, hydrologic and hydraulic parameters in piezometer boreholes is stressed as particularly useful for the karst groundwater investigations. The hydraulic conductivity , K, is a key parameter for understanding and modelling of groundwater circulation in karst. The example of definition of hydraulic conductivity , K, for the Ombla karst spring is given.
Keywords: karst aquifer, diffuse flow, conduit flow, hydraulic conductivity, inflow-outflow relationship.
Karst represents a specific area consisting of surface relief and a surface-underground hydrographic network resulting from the water circulation and its aggressive chemical and physical action in cracks, joints and fractures along the layers of soluble rocks, such as limestone, chalk and dolomite as well as gypsum and salt. Karst is characterised by soluble rocks located near or at the surface. The karstification process results from the physical and chemical water action on the solution and transportation of elements from the rocks. Over time, the permeability of the rock mass is greatly enhanced, and rainwater, instead of being mostly diverted over the surface and into open streamflows may be infiltrated. Thus, karst terrain is characterised by a high proportion of underground drainage. A soluble rock formation may contain voids with a spectrum of sizes from submicroscopic cracks (10-3 mm) to caverns tens of meters across (Atkinson, 1986).
Karst aquifers contain significant
water resources in many parts of the world, but the development and management
of these resources is difficult due to the complexity of the aquifer. The key
to understanding and predicting groundwater flow through these complex aquifers
is an ability to accurately define the feature and location of the
heterogeneities or an ability to reproduce the behaviour of surface and
groundwater circulation (Bonacci, 1987).
Karst water circulation has many particular features that often clearly distinguish it from water circulation of other geologic formation. The essential principles of the hydrologic, hydrogeologic and hydraulic process are identical in karst and non-karst terrain, but the variations are more specific and numerous in the conditions of flow in karst. Occasional, quite frequent "surprises" make some laymen, and often some experts, believe that certain "mysterious" phenomena, unexpected and inexplicable, occur in karst. A careful analysis of these phenomena can yield precise answers to the position, composition and dimensions of underground and surface karst forms. In the modelling procedure, the non-homogeneous and anistropic features of the karst medium often present a great problem, which sometimes cannot be solved.
Another important problem is the strong interaction between surface water and groundwater in karst. The basic hydraulic and/or hydrologic principles of physics, which underlie this interaction, are not particularly complex and have been described in detail in the literature. The problems related to their interpretation appear only due to the fact that it is not easy to observe these processes in wide and non-homogeneous karst terrain. Numerous measurements and an accumulated stock of experience in organising them are necessary to obtain accurate and reliable data indispensable for defining parameters reliable for hydrological analyses, especially regionalization and modelling.
Scale issues are of a special importance for understanding and modelling of karst water circulation. Conditions in karstified medium are very different in their space and time scale.
The karst aquifers are two-component systems in which the major part of storage is in the form of true groundwater in narrow fissures, where laminar flow prevails. On the other hand the majority of water is transmitted through the karst underground by turbulent flows in solutinally enlarged conduits. Slow or so-called diffuse flow occurs through karst fissures of small dimensions generally in the laminar regime. Turbulent fast flow, or conduit flow, occurs in large fissures through irregular karst conduits, with dimensions varying from 1 cm to few meters. It should be noted that it is never only one type of flow that occurs in karst. There is a significant and permanently present interaction between the above-mentioned two types of flow. The flow type and consequently the hydrograph shape are significantly influenced by the presence and continuous interaction between essentially different flow types. Due to the dualistic flow system, the integration of measured parameters into the model calibration is not straight forward (Teutsch and Sauter, 1997).
Additional troubles are due to the fact that a space of karst system is characterised by an epikarst, a vadose zone and a phreatic zone. The vadose zone is an area of continuous and quick oscillations of groundwater level (GWL). The maximum velocities of the rising and lowering GWL are very high which proves of low capacity of karst.
The result of the all above-mentioned is the extreme heterogeneity and variability of hydrogeologic, hydrologic and hydraulic parameters in space and time. It should be stressed that such a complex space needs multy- and inter-disciplinary scientific approach. In order to understand and model it, at least the hydrogeology, hydrology and hydraulics should be taken into consideration. However this co-operation should be deep and close based on physical principles of water circulation in heterogeneous and anistropic media. This process is in the beginning. As its definite result a new scientific discipline called karstology may be expected.
Geology as well as hydrogeology is the first and the strongest frameworks for water circulation in and over the karst terrain. Therefore they govern the main characteristics of the karst aquifer. Karstrification is primarily a geologic characteristic important for the water flow exploration, and can be defined as the density, frequency and number of all types of karst voids (intergranular voids, fractures, fissures, conduits and caves). It is the greatest at the surface and decreases with the depth of a karst massif. According to experimental investigations the following law for a karstification index, e, which decreases with depth, H, measured from the surface is defined by the following equation:
e = a ×
exp(-bH) (1)
where, a, and, b, are empirical constants which have to be determined for a given region.
The capacity of karst for water retention is not great. Numerous analyses carried out in many regions of the world (Bonacci, 1987) showed that the effective porosity, ne, amounts from 0.05 to 1 % on average. Effective porosity, ne, is the relationship between the volume of the pores saturated by the gravitational water, V0, and the volume of all pores, V. It is expressed by the percentage of the total volume, V, by the following equation:
ne
= (Vo/V)*100 (2)
The occurrence of large caves and transportation conduits changes this characteristic locally; thus, apparently, a karst massif has a high storage capacity. A clear proof of the low storage capacity of a karst massif, however, is the sudden rising and lowering of GWL. In the Cetina River catchment an average rate of increase in GWL is greater than 2 m*h-1, and an average decrease in GWL is greater than 0.25 m*h-1. Those data illustrate another specific feature of the karst massif, i.e. its great transportation capacity, Physical and chemical processes in limestone, dolomites and other soluble rocks which form karst, accompanied by tectonic movements, have formed well linked fissure systems in karst massifs with dimensions varying from micrometers to several meters. Water circulates rapidly through such systems and thus makes their water storage capacity seemingly great. The storage in superficial deposits overlying limestone and in the subcutaneous zone can play an important role in sustaining baseflow recession.
Until now the role of bedding-planes
in karst circulation has not been understood enough. Knez (1996) gives some
promising explanations dealing with the relationship between positions and
dimensions of bedding-planes, karst passage development and groundwater
circulation in ?kocjanska jama (Slovenia).
While considering groundwater in karst one should bear in mind that the karstification process, i.e. the solution of soluble rocks, is a continual process which cannot be stopped, so that even the most detailed and reliable modelling is only temporary. The time unit is relatively long when compared with the human lifetime, but it is very short when compared with the geological time scale. A good linear correlation between magnitude of rain and dissolution-rate has been observed. The dissolution rates evaluated for karst region vary between 3 and 1000 m3*km-2 per year.
Hydrology is basically an interpretative science, which encompasses the occurrence, distribution, movement and properties of water on and over the surface of the earth. The system of hydrologic conclusions is based on water budget equation, which is generally very simple. The problem of pragmatic use of this equation is often extremely complex due to interpretation of details and especially in karst regions.
The determination of the catchment boundaries and the catchment area is the starting point in all hydrological analyses. This is essential data, which serve as a basis for using of the water budget equation. The definition of the catchment areas or groundwater recharge areas for karst springs and rivers is crucial for estimation of the groundwater supplies and for identification of the possible source, directions and velocities of the contaminant movement. Only a few karst terrains have been studied well enough as to make it possible to define the catchment precisely. Surface and sub-surface catchment boundaries in karst can differ extremely. The position of the watershed divide line strongly depends upon the GWL, which changes in time.
Water circulation in karst terrain is characterised by the existence of strong inflow-outflow relationship. The swallow-holes generally serve as the inflow patterns. The ground water outflows through different types of karst springs.
The karst spring hydrograph refers in various ways to complex problems of the hydrological transformation of rainfall into runoff in conditions of karst terrain. The reaction of the karst spring hydrograph to the catchment rainfall is strongly influenced by surface and underground karst morphological forms (Bonacci, 1993). The term hydrograph in karst refers not only to hydrographs of water levels and discharges, but also to the representation in time of all other phenomena and natural tracers. By analysing the hydrographs of karst springs and rivers it is possible to identify aquifer characteristics and, accordingly, the main features of a karst rock-fissure massif.
Measurements of hydrogeologic, hydrologic and hydraulic parameters in piezometer boreholes are extremely important for explanation of water circulation in and over the karst terrain. One should bear in mind that the karst massive is frequently intersected by impervious layers of different thickness. This fact influences the hydraulic, hydrogeologic and hydrologic characteristics, governs the groundwater circulation, and creates double or even multiple GWL.
Hydraulics represents the branch of applied mechanics dealing with the behaviour of fluids at rest and in motion. In this paper water in motion is considered. For karst groundwater investigations the most important segments of hydraulics are groundwater recovery and flow in pipes. Karst aquifers may be conceptually classified according to whether the diffuse, conduit or fissure component predominates (Atkinson, 1986).
Gale (1984) states that the movement of groundwater through fissured media particularly in which non-Darcian flows prevail, is less well comprehended than the flow through porous media, which is governed by well-understood hydraulic principles.
For many applications it is important to estimate the hydraulic conductivity, K, of a karst massif which is defined by Darcy's equation:
q = K × grad h (3)
where, q, is seepage discharge per unit cross-sectional area and, h, is hydraulic head. Fig. 1 presents the results of the computation of, K, for the Ombla karst spring (Bonacci, 1995), The value of, K, decreases as the spring discharge increases. This can be explained by the position of the main karst conduits in the Ombla catchment. The figure also shows that various pairs of piezometers give various, K, values. The highest value is obtained for piezometers 9 and 8, whereas the lower value is recorded for piezometers 8 and 18. The reason for this is due to the fact that the karst massif nearer to the Ombla spring shows more intensive fracturing than the part further from the spring, hence, K, decreases in the part of the karst massif where there are less evident karstification processes. This points to a significant influence of the space scale effect upon the results obtained by investigations and measurements in karst.
Fig. 1. Relationship between hydraulic conductivity, K, and Ombla Spring discharge, Q
Under the assumption that water flow through karst conduits is similar to the flow in pipes, Bernoulli's equation can be applied. Many investigations confirmed that flow regime in karst conduits is completely roughly turbulent. It is well known that karst conduits have an irregular cross section, differing more or less from a circular shape. It is also necessary to take into account irregularities of cross section in karst conduits resulting from rock slides and existence of siphons. In addition, occasional large caves and significant narrow sections occur along the conduit. Hydraulic consequences of these underground features can be very important for water flow and thus for its mathematical modelling.
The velocity of water flow through karst conduits is controlled by the slope of the passage in free-surface stream flow and pressure in the phreatic zone. The velocity and type of the water flow are controlled by the friction and by the loss of energy. The water frequently transports sediments thus influencing hydraulic characteristics of flow.
Gale (1984) states that in many karst aquifers the bulk of the water is transmitted by turbulent flow in soultionally-enlarged conduits. By using the soultionally-developed bedformes (Slabe, 1995), which are found on the walls of these conduits and the hydraulically transported sediments, which are found within them, it is possible to obtain some indication of the hydraulic conduits under which conduit flows occur.
Karst terrains are characterised particularly by special landforms and subsurface drainage. The various actions of water result in numerous variations of surface and subsurface karst forms. The scientific disciplines such as hydrogeology, hydrology and hydraulics cannot easily be applied to karst regions with a very complex underground drainage system. Therefore a special and common approach is necessary in order to understand and predict water circulation in karst. A careful and extensive study of the water circulation in karst involves numerous, well-organised measurements and detailed scientific analyses. The hydraulic, hydrologic and hydrogeologic approaches have many similarities and many differences, which should be efficiently utilised in order to solve complex karst water related problems. Evidently, until now it has not been usual practice. This paper can be understood as a call for new, close multy- and inter-disciplinary co-operation in karstology.
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