Transmissivity and permeability distribution in hard rock environment: a regional approach

Transcription

1 Hard Rock Hydrosyslems (Proceedings of Rabat Symposium S2, May 1997). n, IAHSPubi.no. 241, 1997 ol Transmissivity and permeability distribution in hard rock environment: a regional approach JIRI KRÂSNY Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Faculty of Science, Charles University, Albertov 6, Praha 2, Czech Republic Abstract A simple standardized approach has been applied during regional hydrogeological studies in hard rock environment (igneous, metamorphic and sedimentary highly cemented and/or folded non-carbonate rocks) of the Bohemian Massif (Czech Republic). Statistical samples of transmissivity and permeability data, selected mainly according to different pétrographie rock types and géomorphologie and hydrogeological position of water wells, were treated in different areas. This approach have led to conclusions regarding permeability and transmissivity regional distribution. Transmissivity magnitude and variability usually change in dependence on type of test or procedure used within areas of different extension. Four levels of inhomogeneity elements of distinct size causing the phenomenon are defined. INTRODUCTION Depending on hydrogeological and climatic conditions either the magnitude of natural groundwater resources or hydraulic parameters of rocks represent the limits of groundwater development. In a hard rock environment of the Earth's temperate climatic zone, due to often high natural groundwater resources, the magnitude of transmissivity (permeability) is a decisive factor for groundwater abstraction possibilities as a rule. During regional hydrogeological studies performed in recent years in the Bohemian Massif (Czech Republic) a quantitative and standardized approach was applied in hydrogeological data processing which have led to general conclusions regarding permeability and transmissivity data regional distribution. This approach is suitable especially for hard rock environment as there, in contrast to hydrogeological basins, generally accepted methodology for regional hydrogeological data processing and representation had not existed. HYDROGEOLOGICAL ENVIRONMENT OF HARD ROCK In contrast to geological bodies usually determined by their stratigraphie pertinence and/or lithologie composition, spatial arrangement of hydrogeological bodies (aquifers and aquitards) is often partly or even entirely independent of these geological properties and features. This is particularly the case of the "hard rock" environment. It is well known that no general agreement has been reached among hydrogeologists which types of rocks should be considered as "hard rocks". Even though there is no exact definition what does hard rock mean, it is generally understood by geologists that hard rocks are crystalline, i.e. igneous and

2 82 Jifi Krâsny metamorphic rocks. This concept was taken over e.g. by Larsson et al. (1987) who defined "hard rocks" as igneous and metamorphic, non-volcanic and non-carbonate rocks. Yet, hydrogeologists may feel insufficiency of such definition and its limited content due to the fact that also well cemented sedimentary rocks may be characterized by the same hydrogeological environment as crystalline rocks. Recently Gustafsson (1993) proposed that the term "hard rock" might, from a groundwater exploration point of view, include all rocks without sufficient primary porosity and conductivity for feasible groundwater extraction. Consequently, it is obvious that the content of the term "hard rock" from hydrogeological point of view should have wider content as that of crystalline rock. To the specific hydrogeological "hard rock" environment should belong at least crystalline (igneous and metamorphic) rocks and sedimentary highly cemented and/or folded rocks. Following this consideration hydrogeological hard rock environment (sometimes designated as "hydrogeological massif") can be characterized by three decisive features (Krâsny 1996a): - no rocks with interstitial porosity (with the exception of the uppermost part of the vertical sequence where regolith etc. and/or Quaternary deposits form so called weathered zone) occur there; - no stratiform aquifer (feature characteristic of hydrogeological basins) occurs there except for relatively non extensive and usually folded layers of crystalline limestones (marbles) and/or some other lithologically specific intercalations, forming joint aquifer systems with surrounding crystalline rocks; - vertical sequence of three zones, characteristic by distinct hydrogeological conditions, may be defined there from the ground surface downwards as follows: - upper or weathered zone [formed by regolith, talus and/or Quaternary deposits where intergranular (interstitial) porosity prevails], - middle or fissured zone [formed by more or less regularly (from the regional point of view) fissured bedrock with prevailing fracture porosity] and, - lower or massive zone [formed mainly by a massive bedrock with usually isolated deep-seated faults or fault zones]. This vertical zoning of the "hard rock" environment and the defining and characterization of each of the zones might be of use for the implementation of both conceptual and mathematical models. Extended occurrences of carbonate and neo-volcanic rocks cannot be considered as hard rocks even though they might sometimes comply with the above-mentioned conditions. Carbonate and neo-volcanic rocks represent as a rule completely distinct hydrogeological (and also hydrogeochemical) environment compared with hard rocks (hydrogeological massif) as defined above. The above-mentioned upper and middle zones form together the principal regionally extended "near-surface" aquifer occurring more of less conformably to the land surface. The thickness and character of this composite and heterogeneous aquifer, however, changes from place to place particularly in relation to petrography of respective rocks and their tectonic deformations (faulting and Assuring), character of their weathering and, morphological and climatic conditions. Its usual thickness reaches up to a few or more tens of metres. Permeability decreases in general downwards. It is just this aquifer which is decisive for the magnitude of regional groundwater runoff and, consequently, natural groundwater resources formation.

3 Transmissivity and permeability distribution in hard rock environment 83 The near-surface aquifer usually enables better groundwater abstraction possibilities as well. GEOLOGICAL AND HYDROGEOLOGICAL BACKGROUND The prevailing part (about 84%) of the Czech Republic is built by the Bohemian Massif, an old, geologically heterogeneous platform block consolidated by the Variscan orogeny and extended outside the Czech territory to Austria, Germany and Poland. Igneous, metamorphic and Precambrian and Lower Palaeozoic non-carbonate rocks ("hard rocks") form approximately 68% of the Bohemian Massif within the Czech Republic being covered by younger deposits of sedimentary basins in the remaining part (Fig. 1). >o OSTRAVA ALPINE-Cf^ 1 ^ SK km 8 Fig. 1 Geological position of the Bohemian Massif in central Europe (after Krâsny, 1996b). 1: superficial boundaries of the Bohemian Massif; 2: post-variscan deposits outside the superficial boundaries of the Bohemian Massif; 3-4: basement of the Bohemian Massif intensively folded by Variscan tectogenesis: 3: crystalline, Precambrian and Palaeozoic mostly non-carbonate ("hard") rocks; 4: Silurian and/or Devonian karstified limestones; 5-8: post-variscan cover of the Bohemian Massif: 5: Permocarboniferous basins; 6: Bohemian Cretaceous basin; 7: Tertiary basins (mostly Neogene, in southern Bohemia also Cretaceous deposits); 8: Tertiary volcanic rocks; 9: frontiers between states.

4 84 Jifi Krâsny Hydrogeological studies have evidenced the importance of "hard rock" environment for natural groundwater resources formation (Krâsny et al., 1981, 1982). In the summit parts of the highest mountains of the Bohemian Massif specific groundwater runoff (=natural groundwater resources) exceeds 10 1 s" 1 km" 2 and in some drainage basins even reaches round 15 1 s" 1 km" 2. Recharge may be estimated even more than 20% of the average annual precipitation there which corresponds to a mean value of more than 250 mm/year of recharge. With decreasing elevation groundwater runoff generally diminishes up to values s" 1 km" 2. Mechanism of groundwater runoff formation in hard rock regions was described by Knëzek & Krâsny (1990). Because of high natural groundwater resources knowledge of the permeability and transmissivity distribution is the decisive viewpoint for any groundwater development considerations in hard rock areas. Therefore, during recent hydrogeological studies and hydrogeological maps compilation in the Czech Republic, attention has been paid to the regional distribution of transmissivity (and permeability) values (Krâsny, 1996b). Methodology of these studies is described hereafter. PROCEDURE OF DATA ANALYSIS Thousands of pumping tests from drilled and dug wells have been performed in hard rocks of the Czech part of the Bohemian Massif in recent decades. During many hydrogeological studies, especially when compiling the hydrogeological map of the Czech Republic at 1: (Krâsny, 1993b; Krâsny et al, 1997), a quantitative and standardized approach was applied. Transmissivity has been accepted as the best hydraulic parameter to express groundwater abstraction possibilities and to be represented in hydrogeological maps. Statistical treatment of available transmissivity data and the objective classification of respective statistical samples were the principal applied procedures (Krâsny, 1993a). Following the combined classification of transmissivity magnitude and variation six classes are defined after orders of transmissivity magnitude from very high (/ class more than 1000 m 2 day _1 ) to imperceptible transmissivity (VI class less than 0.1 m 2 day"'). As the decisive factor by which to determine a class of transmissivity magnitude of a statistical sample, the interval x ± s (3c = arithmetic mean, s = standard deviation) considered as "transmissivity background" was chosen and the percentage of this interval belonging to particular classes determined - cf. Fig. 2. Likewise transmissivity magnitude, variation in transmissivity is also classified into six classes, a to /, on the basis of a standard deviation of a statistical sample. Transmissivity may be expressed by the index of transmissivity Y (Jetel & Krâsny, 1968), by the specific capacity q or by the coefficient of transmissivity T. The last two parameters, however, have to be expressed as a logarithm. All six classes of transmissivity variation have the same extent, always 0.2 of the respective standard deviation: class a (insignificant variation) has a standard deviation less than 0.2, class b (small variation) etc. and the class / (extremely large variation) more than 1.0 (cf. Fig. 3). More about classification procedure can be found in Krâsny (1993a).

5 Transmissivity and permeability distribution in hard rock environment 85 Y*2%- --S -A - x-2s- I HIM [ i i < 11 HI OXXE aoos offi 0.1 i r ri n i i 11 i I 20 index Y I I I'll 1 r q(l/sm) i i '"i i i i i i n i i i mu so ,000 2poo 5floo 10,000 T (m 2 /d) x*s Fig. 2 Cumulative relative frequencies of transmissivity samples of different rock types in the Jizerské hory Mts. and Krkonose Mts. (after Krâsny, 1993c, modified). q: specific capacity in 1 s" 1 m' 1, T: coefficient of transmissivity in m 2 day" 1, Y: index of transmissivity; x : arithmetic mean; s: standard deviation; x ± s: interval of prevailing transmissivity values (transmissivity background); +A, -A: fields of positive and negative anomalies (+4-/4, A: extreme anomalies) outside the interval x ± s; KJ: field of phyllites, mica schists and gneisses; L: Lusatian granodiorite pluton; K: Krkonose-Jizera granite pluton; +Q: samples where possible, probable of evident inflow into well comes from Quaternary deposits covering the bedrock. All transmissivity samples are based on results of pumping tests from water wells. Thus prevailing transmissivity values and their ranges ("background") can be assessed under different natural conditions; in addition to these "transmissivity background" values, ranges of positive and negative anomalies can be estimated (cf. Fig. 2). This procedure enables realistic, not exaggerated assessment of groundwater abstraction possibilities in different areas and considerations and discussion on influences of natural features causing differences in transmissivity values. MAIN RESULTS AND DISCUSSION Hydrogeological studies following above-mentioned procedures have brought important information on hard rock hydrogeology: statistical samples were treated in different areas, according to different pétrographie rock types and géomorphologie and hydrogeological position of water wells. Differences in well depths were also taken into account.

6 86 Jifi Krâsny Fig. 3 Prevailing transmissivity magnitude and variation of particular statistical samples of "hard rocks" of the Bohemian Massif (after Krâsny, 1990). Transmissivity magnitude and variation classified after Krâsny (1993a): the limits between the classes of transmissivity magnitude (0.1; 1; 10; 100; 1000) are expressed in m day, the limits between the classes of transmissivity variation (0.2; 0.4 etc.) represent the standard deviation of a statistical sample of a transmissivity parameter expressed as logarithm. On a local scale, irregular changes in permeability and transmissivity spatial distribution within the "near-surface aquifer" are common. They are evidenced by yields and hydraulic parameters differences in near-by wells drilled in the same rocks. These differences may reach several (usually up to three but sometimes even up to four) orders of transmissivity (permeability) magnitude (e.g. Krâsny, 1993c, Fig. 2). Therefore the positive and negative anomalies (i.e. extreme values of transmissivity) within the same environment (represented by a statistical sample) might differ considerably. These differences are mainly attributed to changes in character and thickness of the weathered zone and to distinct character and abundance of faults and joints in the middle zone. Nevertheless, on a regional scale, in spite of the above-mentioned locally significant variability, transmissivity tends to attain considerably closer values in prevailing ranges, arithmetic means and anomalies. The regionally prevailing transmissivity values (hydrogeological background) as determined from results of drilled (and partly also dug) wells in many hard rock areas of the Czech part of the

7 Transmissivity and permeability distribution in hard rock environment 87 Bohemian Massif can be usually characterized by units m 2 day" 1 up to slightly more than 10 m 2 day" 1 thus belonging to classes IV(-III) c,d of transmissivity magnitude and variation after the classification of Krâsny (1993a) [low transmissivity (IV class, i.e. prevailing transmissivity ranging from 1 to 10 m 2 day" 1 and partly III class, i.e. intermediate transmissivity) with moderate to large transmissivity variation (classes c,d)] Fig. 3. "Background" or mean values determined for particular rock type in an area may be considered as representing also the respective Representative Elementary Volume (REV). Distinct petrologic types of rocks usually do not display any significant difference in their prevailing transmissivity. Only areas built up completely or partially by crystalline limestones (marbles) were characterized by higher transmissivity, usually half to one order of magnitude higher compared with other crystalline rock types (e.g. Krâsny, 1997). There are some indications that relatively higher permeability may be expected in areas formed by basic igneous rocks, too. Unfortunately, few data are available to prove this assumption in the Bohemian Massif. In some regions, however, differences between transmissivity of other rock types (granites, phyllites) have been found (e.g. Krâsny, 1993c). The influence of weathering and generally the presence of better permeable Quaternary deposits (especially in valleys) results in higher transmissivity of wells where hard rocks are covered by these loose deposits. Transmissivity variation of samples with the occurrence of Quaternary deposits is usually lower than that of samples of hard rocks without Quaternary cover. This indicates an equalizing effect of hydraulically more homogeneous Quaternary deposits (cf. Fig. 2). Except for the mentioned differences, however, influence of petrography upon permeability and transmissivity cannot be considered significant and usually is masked by other factors. Superimposed upon the regional transmissivity background caused by a regional more or less regular Assuring, significant regional differences in transmissivity magnitude were proved due to the presence of inhomogeneity elements of a higher scale level. They belong usually to tectonically strongly affected zones and/or to belts of regional higher permeability along the river valleys. First of them are in areas where considerably higher transmissivity was determined in more water wells thus indicating more extensive "anomalous" areas. Two examples from petrographically distinct hard rock environment in southern Bohemia, apparently belonging to tectonically strongly affected zones, with prevailing transmissivity between 40 and 90 m 2 day"' are mentioned by Krâsny (1996c); these zones, represented by prevailing transmissivity almost one order of magnitude higher compared with surrounding areas (considered hydrogeological background), might be of importance for groundwater abstraction in hard rock environment. Similar deep-seated anomalous tectonic zones are in some regions obviously indicated by occurrences of thermal and mineral waters. Other significant regional differences in transmissivity magnitude were found when comparing samples of water wells sited in distinct géomorphologie (and hydrogeological) position. Since the first attempt was made to quantify these differences (Krâsny, 1974) other results have been achieved. In different areas built up by hard rocks prevailing transmissivity was estimated 2 to 4.5 times higher in zones of discharge (valleys) than in recharge zones (above all slopes and elevations). Irrespective of which are the main causes of these differences in permeability

8 Jin Krâsny Fig. 4 Example of regional differences in transmissivity values due to distinct géomorphologie (hydrogeological) conditions (without scale - after Krâsny, 1996c). 1: river courses and zones of regionally higher transmissivity (valleys); 2: zones of regionally lower transmissivity; IV-IlIc: class of transmissivity magnitude and variation (after classification of Krâsny, 1993a), transmissivity characteristics of a sample expressed by coefficient of transmissivity in m 2 day"'; : probable interval of 68% of transmissivity values (prevailing transmissivity = hydrogeological background); (9.0): arithmetic mean. (transmissivity) regional distribution this fact should be taken into account: hard rock environment should not be considered as regionally homogeneous but as a complex system where belts of regionally higher prevailing permeability follow the valleys (Fig. 4, Krâsny, 1996c). In the Bohemian Massif regional tendencies in prevailing transmissivity values were determined in some extended areas, probably due to different neo-tectonic activities: lower transmissivity belongs to areas with smaller neo-tectonic activity, higher to zones where neo-tectonic deformations have been more intensive (generally mountains). As a result, these differences between prevailing transmissivity values may somewhere reach more than one order of transmissivity magnitude: in Fig. 5 regional differences are evident between the Sumava Mts. in the southwest and the environs of the south Bohemian basins. This may represent an important "shift" of the regional transmissivity background which should be considered and upon which local changes of transmissivity are superimposed (Krâsny, 1996c). CONCLUSIONS Many geological features can be classified according to their order of magnitude (e.g. stratigraphie units, structural elements as folds, fissures and faults Rats, 1967). The same principles were applied also to the classification of inhomogeneity elements for hydrogeological purposes (Rats & Chernyshov, 1967). Hydrogeological properties strongly depend on inhomogeneity elements of different size. The quantitative expression of these properties, however, changes in dependence on the size of a study area: the greater the study area the smaller the variability of results caused by inhomogeneity elements of a certain size. Consequently, relation of the size of an inhomogeneity element to the extension of the study area (or used test, procedure) causes so called scale effect. The size of inhomogeneity elements determines also the extension of the REV.

9 Transmissivity and permeability distribution in hard rock environment 89 Fie. 5 Tendencies in regional transmissivity values in southern Bohemia (after Krasny, 1996c). 1: Cretaceous, Tertiary and Quaternary deposits of the south Bohemian basins, Quaternary deposits of the main river valleys; 2: varied group of crystalline rocks with frequent limestone intercalations (with higher prevailing transmissivity cf. the text above); 3: monotonous group of different types or crystalline rocks; V-III: classes of transmissivity magnitude of crystalline rocks (after classification of Krâsny, 1993a). In a hard rock environment hierarchy of decisive hydrogeological properties (inhomogeneity elements) in respect of the size of a considered study area, thus influencing permeability and transmissivity spatial distribution, might be determined as follows: - The smallest inhomogeneity elements belong to the upper (weathered) zone with the occurrence of loose deposits with intergranular porosity. Results of permeability laboratory tests of these deposits may differ considerably even though they belong to the same geological unit; aquifer tests, however, result in smaller variation of hydraulic parameters. - Fissures in the middle (fissured) zone may cause local differences of three or even four orders of magnitude in transmissivity (and mean permeability) values based on individual pumping tests (Fig. 2); regionally averaged values (and prevailing values, i.e. regional hydrogeological background), however, usually do not differ very much in distinct types of rocks and in distinct areas (Fig. 3). Superimposed upon this regional transmissivity background caused by a regionally more or less regular fissuring, significant regional differences in transmissivity magnitude may be found due to the presence of inhomogeneity elements of a higher scale level; these can be represented by fault zones of regional importance forming more extensive and tectonically strongly affected zones and/or by belts of regionally higher permeability along the river valleys (Fig. 4); these inhomogeneity elements may often be of a linear form up to more kilometres long.

10 90 Jin Krâsny - Within the highest level we can obtain even closer results equalizing influences of inhomogeneity elements of all the preceding levels; yet changes (tendencies) in prevailing regional transmissivity values were proved in extended areas of hundreds of square kilometres, probably due to differences in neo-tectonic activities (Fig. 5). REFERENCES Gustafsson, P. (1993) SPOT satellite data for exploration of fractured aquifers in southeastern Botswana. Mem. 24th Congress IAH1, As. Jetel, J. & Krâsny, J. (1968) Approximative aquifer characteristics in regional hydrogeological study. Vest. Ûstf. Ûst. Geol. 43(5), Praha. Knëzek, M. & Krâsny, J. (1990) Natural groundwater resources mapping in mountainous areas of the Bohemian Massif. Mem. 22nd Congress IAH 2, Lausanne. Krâsny, J. (1974) Les différences de la transmissivité, statistiquement significatives, dans les zones de l'infiltration et du drainage. Mém. de l'aih 10, 1. Communication, Montpellier. Krâsny, J. (1990) Regionalization of transmissivity data: hard rocks of the Bohemian Massif. Mem. 22nd Congress IAH 1, Lausanne. Krâsny, J. (1993a) Classification of transmissivity magnitude and variation. Groundwater 31(2), Krâsny, J. (1993b) Hydrogeological map of the Czech Republic: a quantitative and standardized approach to representation of groundwater in hard rocks. Mem. 24th Congress IAH 2, As. Krâsny, J. (1993c) Prevailing transmissivity of hard rocks in the Czech part of the Krkonose and Jizerské hory Mts. Mem. Symp. "Wspôlczesneproblemy hydrogeologii" (Polanica Zdroj), Wroclaw. Krâsny, J. (1996a) Hydrogeological environment in hard rocks: an attempt at its schematizing and terminological considerations. First Workshop on "Hardrock hydrogeology of the Bohemian Massif" Acta Universitatis Carolinae Geologica 40, 2, Krâsny, J. (1996b) State-of-the-art of hydrogeological investigations in hard rocks: the Czech Republic. First Workshop on "Hardrock hydrogeology of the Bohemian Massif" Acta Universitatis Carolinae Geologica 40, 2, Krâsny, J. (1996c) Scale effect in transmissivity data distribution. First Workshop on "Hardrock hydrogeology of the Bohemian Massif" Acta Universitatis Carolinae Geologica 40, 2, Krâsny, J. (1997) Crystalline limestones: a specific hydrogeological environment in hard rock areas. 2nd Workshop on "Hardrock hydrogeology of the Bohemian Massif Acta Universitatis Wratislaviensis (in press). Krâsny, J. (éd.), Dankovâ, H., Hanzel, V., Knëzek, M., Matuska, M. & Suba, J. (1981) Map of Groundwater Runoff in Czechoslovakia 1: Cesky hydrometeor. ûst. Praha. Krâsny, J., Knëzek, M., Subovâ, A., Dankovâ, H., Matuska, M. & Hanzel, V. (1982) Odtok Podzemni Vody na Ûzemî Ceskoslovenska (Groundwater runoff in the territory of Czechoslovakia). Cesky hydrometeor. ûst. Praha. Krâsny, J. (éd.), Curda, J., Hazdrovâ, M., Hercfk, F., Hrkal, Z., Kacura, G., Kessl, J. & Michlicek, E. (1997) Hydrogeological Map of the Czech Republic 1: Czech Geological Survey Praha (in press). Larsson, I. et al. (1987) Les Eaux Souterraines des Roches Dures du Socle. Etudes et Rapports d'hydrologie no. 33, UNESCO, Paris. Rats, M. V. (1967) Neodnorodnost Gornykh Porod i Ikh Fizicheskikh Svojstv. Nauka Moskva. Rats, M. V. & Chernyshov, S. N. (1967) Statistical aspect of the problem on the permeability of the jointy rocks. In: Hydrology of Fractured Rocks (Proc. Dubrovnik Symp., October 1965), vol. I, IAHS Publ. no. 73.