Adamcova et al.indd Problems of Hydraulic Conductivity Estimation in Clayey Karst Soils Renata ADAMCOVA1, Franz OTTNER2, Goran DURN3, Sabine GREIFENEDER2, Ivan DANANAJ4, Maria DUBIKOVA5, Rastislav SKALSKY6, Slobodan Miko7 and Sanja KAPELJ7 1. INTRODUCTION Karren fields are a typical karst phenomenon in the ero- sion zones in karst areas, where the uncovered fissured carbonate rock enables very rapid infiltration of surface water, with all pollutants included. Groundwater vul- nerability is therefore extremely high. Elsewhere, there are places continuously covered with soil, which seem to function as natural pollution barriers. A considerable thickness of the soil cover can be found in depressions or in other accumulation zones. The degree of karst groundwater protection by soils depends not only on their thickness, but also on both their hydraulic conduc- tivity and retention properties. Between 2000–2003, soils from selected karst ter- rains in Slovakia, Croatia and Austria (Fig. 1) were stu- died within one Slovakian and three international inter- disciplinary research projects. This paper summarizes one part of the results, dealing only with the hydraulic conductivity of fine-grained karst soils. It can be con- cluded that: – wide intervals of hydraulic conductivity were meas- ured by different methods in selected soil types; – hydraulic conductivity assessment methods based sole- ly on the grain-size distribution in the fine-grained soils are unsuitable; – factors influencing the hydraulic conductivity are complex. 2. STUDY AREAS Selected karst areas were chosen for investigation in all three countries. The Brezovske Karpaty Mts. (the Small Geologia Croatica 58/2 195–203 10 Figs. 2 Tabs. ZAGREB 2005 Key words: Karst soils, Hydraulic conductivity, Tri- axial permeameter, Consolidometer, Guelp permea- meter, Grain size. 1 Comenius University in Bratislava, Faculty of Natural Sciences, Mlynska dolina, 84215 Bratislava 4, Slovak Republic; e-mail: adamcova@fns.uniba.sk 2 University of Natural Resources and Applied Life Sciences Vienna, Institute of Applied Geology, Peter-Jordan-Strasse 70, A-1190 Vienna, Austria; e-mail: ottner@mail.boku.ac.at 3 e-mail: gdurn@rgn.hr 4 State Geologic Survey of Slovak Republic, Mlynska dolina 1, 81704 Bratislava 1, Slovak Republic; e-mail: dananaj@fns.uniba.sk 5 University of New South Wales, School of Materials Science & Engineering, 2052 Sydney, Australia; e-mail: dubikova@materials.unsw.edu.au 6 Soil Science and Conservation Research Institute, Gagarinova 10, 82713 Bratislava 2, Slovak Republic; e-mail: skalsky@vupu.sk 7 Croatian Geological Survey, Sachsova 2, 10000 Zagreb, Croatia; e-mail: smiko@hgi-cgs.hr, sanja.kapelj@zg.t-com.hr Abstract Even in karst areas, considerably thick soils can be found in accumu- lation zones. Here, the degree of groundwater vulnerability depends not only on the thickness, but also on the hydraulic conductivity and retention properties of the soil cover. The hydraulic conductivity of fine-grained karst soils from Slovakia, Croatia and Austria was stud- ied within several international research projects, by the application of four different test methods. Results are discussed from different points of view. Triaxial tests yielded a very broad interval between the maximum and minimum hydraulic conductivity (from 5.83x10-7 m.s-1 to 3.50x10-11 m.s-1), therefore the mean value cannot be used in any calculations. The consolidometer method gave lower values in gen- eral, between 9.40x10-10 m.s-1 to 3.59x10-8 m.s-1. However, this meth- od overestimates the soil “impermeability”. Estimates based on grain size are unsuitable, as fine-grained soils did not fulfil the random con- ditions of known formula. Finally, the “in situ” hydraulic conductivity was measured using a Guelph permeameter. As expected, “in situ” tests showed 100 to 1000-times higher kf than the laboratory tests. This method best reflects the real conditions. Therefore, only this type of data should be considered in any environmental modelling. In a soil profile, hydraulic conductivity depends on the mineral com- position, depth, secondary compaction, etc. The degree and duration of saturation with water is very important for young soils containing smectite. Their hydraulic conductivity might be very low when satu- rated for long time, but also very high, when open desiccation cracks occur. A very slight trend was found, but only in Slovak soils, show- ing a decrease in the hydraulic conductivity with increasing content of the clay fraction <0.002 mm. These results should contribute to a better estimate of the protective role of soils in groundwater vulner- ability maps. 196 Geologia Croatica 58/2 Carpathians Mts.) and the Slovensky kras Mts. (the Slo- vak Karst Mts.) were studied in Slovakia, the Istrian Peninsula in Croatia and the Dobratsch Mts. (the Gail- taler Alps) in Austria (Fig. 1). Slovakia The karst area in the Brezovske Karpaty Mts. is built up by Triassic carbonates of the Jablonica Group (Car- nian Wetterstein limestones and dolomites, and Upper Carnian–Norian dolomites). They belong to the Ned- zov Nappe. Both limestone and dolomite complexes of the Jablonica group are highly permeable, but the karst processes are mainly developed in limestones, produc- ing wide karst joints, caverns and caves. The Quater- nary of the Brezovske Karpaty Mts. is mainly repre- sented by Pleistocene aeolian sediments covering the SE foot slopes of Plesiva Hora, as well as by Holocene fluvial sediments and slope sediments (BEGAN et al., 1984). The Slovensky kras Mts. is an area with the most typical karst development in Slovakia. These exempla- ry karst phenomena are on the UNESCO List of World Heritage sites. The research was undertaken on the Sil- icka plateau. In synclinal bedding, the MiddleTriassic limestone–dolomite complex overlies the less perme- able to impermeable Lower Triassic sediments that also occasionally appear on the surface. Both complexes belong to the Silica Nappe (MELLO et al., 1996). In the SW part of the Silicka plateau, the Mesozoic complexes are covered by Tertiary clays, gravels and sands of the Poltar formation. Small remnants of these sediments also occur in the eastern part of the area. Quaternary sediments are represented mostly by slope debris. Flu- vial sediments are reduced to brook alluvium (MELLO et al., 1996). Croatia Karst terrains cover approximately 50% of Croatian territory and predominantly consist of karstified Meso- zoic and Tertiary limestones and dolomites. The Istrian peninsula represents the NW part of the spacious Adri- atic Carbonate Platform and consists predominantly of carbonate rocks ranging in age from Late Middle Jurassic to Eocene, with subordinate Eocene silici- clastic rocks, flysch and calcareous breccia, and Qua- ternary terra rossa and loess. The Istrian upper Middle Jurassic to Eocene succession can be divided into four large-scale sequences (VELIĆ et al., 1995). The 1st, Fig. 1 Most important carbonate rock outcrops in Slovakia, Croatia and Austria and studied karst areas. 197Adamcova et al.: Problems of Hydraulic Conductivity Estimation in Clayey Karst Soils 2nd and 3rd of these are composed of carbonates, each terminated by important, lengthy periods of emersion, i.e. type 1 sequence boundaries (TIŠLJAR et al., 1998). The 4th large scale sequence consists of carbonate and clastic rocks and unconformably overlies the palaeore- lief developed on carbonate rocks. The most widespread sediments in this sequence are flysch deposits. Since the formation of flysch, the surface has been affected by tectonics, karst processes and weathering which has led to the development of both surficial and underground features. Different types of sediments, polygenetic pal- aeosols and soils have been formed. For the most part, they irregularly cover all the four aforementioned large- scale sequences of Istrian carbonates and flysch. Austria The Dobratsch Mt. belongs to the Oberostalpin tectonic unit and is very similar to the north part of the Kara- vanken Mts. COLINS & NACHTMANN (1978) gave a complete description of the geology, beginning from the crystalline basement, through sedimentary rocks of Car- boniferous and Permian ages, up to the Triassic com- plexes that prevail. The Dobratsch Mt. is mainly built up of the Wetterstein-type limestone of Ladinian age. It is up to 700 m thick, with deep open vertical fissures. These are mostly of tectonic origin, connected with the major N–S and E–W faults bordering the mountain against the valleys. The upper part of the karstic rock is typically dry, but huge and important karst springs occur at the contact with the “impermeable” Werfenian complex at the bottom, in the north, north-east and east. Due to the deep circulation along the faults, some ther- mal springs are also present. The summit of the moun- tain is built up by reef limestones (Ladinian to Carnian in age). Glacial gravelly moraines are the main Quater- nary sediments, covering mostly the northern parts of the mountain with a considerable thickness. Under the steep southern slopes, huge masses of a prehistoric rock- fall can be found, but the slopes are still not stable due to the active Periadriatic fault at the foot. 3. MATERIALS AND METHODS Fifteen soil profiles at eight different locations were studied. Table 1 shows the studied sites, together with the numbers of studied soil profiles at each. Data on the pedological characteristics of the studied areas, as well as mineral composition of the soil samples have already been described in detail (ADAMCOVA et al., 2001, 2002) and will not be presented here. Both undis- turbed and disturbed soil samples were taken from up to 3 horizons of every studied soil profile, depending on the soil thickness. After a macro-morphological descrip- tion “on site”, micro-morphological analyses of thin soil sections have been carried out. Both the qualitative and the semi-quantitative mineral compositions were studied by XRD. The soils were classified according to ISSS–ISRIC–FAO (SPAARGAREN, 1994) as Cam- bisols (10 profiles), Luvisols (4 profiles) and Leptosol (1 profile). The following physical properties of the soils were tested on 38 undisturbed samples in the laboratory: – grain size distribution: measured on the <2 mm fraction by combining wet sieving either with the hydrometer method (BS 1377 – PART 2, 1990) or with SediGraph; – Atterberg limits and plasticity index (necessary for the engineering-geological classification): plastic lim- it by rolling and liquid limit by the one-point Casa- grande method, both according to BS 1377 – PART 2 (1990); – hydraulic conductivity: determined by up to 3 differ- ent laboratory methods: a) permeameter with a triaxial pressure chamber – method also required by soil scientists (STN 72 1020, 1990 – Method G); Austrian samples have been tested in an other permeameter type, results are incomparable – GREIFENEDER (2000); b) consolidometer: the filtration coefficient kf was calculated from the consolidation curve (DAN- ANAJ et al., 2005); c) calculation of kf from the grain-size curve: apply- ing the most suitable empirical formulas selected by the PC software GeoFil; – in order to define the retention ability of the soils regarding heavy metals, other special laboratory tests have also been done that are not discussed here. To complete the data on hydraulic conductivity and to check the results of laboratory tests, field “in situ” tests were carried out using the Guelph permeameter, but only in Slovakia and Austria (GREIFENEDER, 2000; FIALA, 1999). Country Slovakia Croatia Austria Area the Small Carpathians Mts. the Slovak Karst Mt. Istria Peninsula the Gailtaler Alps W Slovakia E Slovakia NW Croatia S Austria Site Ardovo (1) Plomin (2) (number of Dobrá Voda (2) Silica (2) Medulin (2) Dobratsch (3) profiles) Silicka Brezova (1) Pekići (2) Table 1 Location of sampling points (soil profiles). 198 Geologia Croatica 58/2 4. RESULTS AND DISCUSSION Table 2 summarizes the hydraulic conductivity results, showing the mean, minimum and maximum values, specified by the applied method and studied soil type. The mean value of all triaxial tests is 9.47x10-8 m.s-1, but instead of applying this number in any modelling (e.g. ZENISOVA et al., 2002; MALIK & VOJTKOVA, 2004), the very broad interval between the maximum and minimum should be remembered. In general, lower values with smaller extremes have been measured by the consolidometer method that over- estimates the soil “impermeability”. The estimate from grain size produced poor results since no differences were determined between the soils, as the formulas do not include natural porosity or bulk density. Although these results are often similar to the results produced by the consolidometer, they are unacceptable, because none of the soils fulfilled the random conditions of the applied formulas (e.g. d10>0.05 mm for the most fre- quent formula of Carman–Kozeny) (MELIORIS et al., 1986). All of the studied soils could be classified as clays in the terms of engineering-geological classifi- cation (STN 73 1001, 1987). Therefore, this computer output is not factually valid, and the method should not be applied to fine-grained soils. As expected, “in situ” tests showed 100 to 1000- times higher hydraulic conductivity than the laboratory tests, reflecting the presence of big macro-pores. Such pores are usually not present in undisturbed laboratory samples. For this reason, the results of laboratory tests cannot be applied when evaluating the hydraulic con- ductivity of soils “in situ”, as the differences from real- ity are too great. The hydraulic conductivity of the studied soils also changes from one horizon to another within the same profile. Trends observed by one test method are often opposite to the trends seen by other methods (Fig. 2). This is because hydraulic conductivity reflects the simul- taneous effect of many different factors, both primary and secondary. Grain size is only one of them. In general, the studied Croatian soils are finer than similar soils in Slovakia which are again finer than the Austrian ones. No real correlation between grain size and hydraulic conductivity tested in the triaxial per- meameter could be found. Only a very weak descend- ing trend with the increasing content of the clay frac- tion (<0.002 mm) could be observed (Fig. 3), similar to the results of the consolidometer method (Fig. 4). There was no correlation between the contents of the whole fine fraction <0.063 mm and kf (Fig. 5). A very draft estimate of the filtration coefficient kf from the content of the clay fraction might be possible, but only for Slovak soils despite the soil type. Here, an exponen- tial trend was found, however, the reliability is very low (Fig. 6). The differences between the classified soil types are small. However, some differentiation can be seen look- ing at the results of field tests (Fig. 7). Unfortunately, there are not enough data on the Leptosols. Surprising- ly, the Leptosol from Silica yielded the lowest hydrau- lic conductivity in the triaxial test: 3.5x10-11 m.s-1 in the uppermost horizon (depth 0–10 cm). However, this was probably due to imperfections in the method (secondary Method kf (m.s-1) all soils Cambisol Luvisol Leptosol Permeameter with number of tests 30 19 9 2 triaxial pressure mean value 9.47x10-8 8.89x10-8 7.17x10-8 2.60x10-7 chamber minimum 3.50x10-11 1.45x10-10 1.97x10-10 3.50x10-11 maximum 5.83x10-7 3.97x10-7 5.83x10-7 5.22x10-7 number of tests 33 23 8 2 Consolidometer mean value 7.13x10-9 6.50x10-9 8.22x10-9 1.00x10-8 minimum 9.40x10-10 9.40x10-10 1.40x10-9 7.02x10-9 maximum 3.59x10-8 1.73x10-8 3.59x10-8 1.30x10-8 number of tests 40 26 12 2 GeoFil (grain size) mean value 2.98x10-9 2.17x10-9 2.78x10-9 1.56x10-9 minimum 1.01x10-9 1.08x10-9 1.01x10-9 1.21x10-9 maximum 9.96x10-9 9.96x10-9 4.11x10-9 1.92x10-9 number of tests 51 23 22 6 Guelph mean value 9.33x10-6 8.20x10-6 1.28x10-5 9.67x10-7 permeameter minimum 8.09x10-9 1.13x10-8 5.63x10-8 8.09x10-9 maximum 6.88x10-5 5.49x10-5 6.88x10-5 2.36x10-6 La bo ra to ry Fi el d Table 2 Hydraulic conductivity of studied soil types – results of four test methods. 199Adamcova et al.: Problems of Hydraulic Conductivity Estimation in Clayey Karst Soils factor). The chamber pressure might compact the very loose sample. Also 15 cm deeper, the volume reduction after the test reached 16%. The highest hydraulic con- ductivity measured by this method was 5.8x10-7 m.s-1 in the upper Luvisol sample from Dobra Voda 2, probably due to many roots and cracks (primary factor) and short saturation before the test (secondary factor). Fig. 2 Different test methods yielded partly opposing trends of hydraulic conductivity variability with depth. Legend: a – permeameter with triaxial chamber; b – consolidometer; c – GeoFil; d – Guelph field permeameter. Fig. 3 Hydraulic conductivity – results of tests using the permeameter with a triaxial pressure cham- ber. A – distribution according to soil type; B – dis- tribution according to country. A B 200 Geologia Croatica 58/2 Sometimes the changing kf could be easily explai- ned by the mineralogy, e.g. in Slovak samples contain- ing smectite (Dobra Voda, Silica). Here, permeability decreased considerably during 2 weeks in the triaxial chamber, as the pores were closed due to smectite swelling (Fig. 8). Both the degree and duration of satu- ration, as well as porosity are very important, whereby these factors are in very close relationships in soils con- taining swelling clay minerals. There, the results of “in situ” tests are very season-dependent. In the autumn, Fig. 4 Hydraulic conductivity – results of tests using the consolidometer. A – distribution according to soil type; B – distribution according to country. A B Fig. 5 Hydraulic conductivity – results of tests using the Guelph field permeameter, related to the content of the whole fine fraction (silt and clay). A – distribution according to soil type; B – distribu- tion according to country. 0.1 1 10 100 1000 70 80 90 100 content of the fraction <0.063 mm (%) Cambisol Luvisol Leptosol 0.1 1 10 100 1000 70 80 90 100 SK A content of the fraction <0.063 mm (%) 0.1 1 10 100 1000 70 80 90 100 content of the fraction <0.063 mm (%) Cambisol Luvisol Leptosol 0.1 1 10 100 1000 70 80 90 100 SK A content of the fraction <0.063 mm (%) A B 201Adamcova et al.: Problems of Hydraulic Conductivity Estimation in Clayey Karst Soils when the soil was highly saturated for long periods, considerably lower hydraulic conductivity (10-8 m.s-1) was measured in Dobra Voda than in the hot dry sum- mer (10-5–10-6 m.s-1). This also explains the fact that Slovak Luvisols (containing smectite) seem to be slight- ly less permeable than Cambisols (without smectite) in the lengthy triaxial tests, having enough time for the smectite expansion, but are more permeable during the short field tests by good weather. It can be concluded that if any accident with liquid pollutants occurs on the surface covered with expandable soils (here the luvi- sols), the chance of successful remediation and preven- tion of groundwater contamination is high in the wet seasons, when the soil is saturated for long periods, but the groundwater vulnerability is very high there in dry seasons, due to the occurrence of contraction cracks. In older soils, without smectite, secondary com- paction can lower the effective porosity and thus the hydraulic conductivity. This was observed in the upper- most horizons at the Dobratsch Mt. due to cattle graz- ing (Figs. 9 and 10), but the primary compacting effect of the geostatic pressure was also evident in the deeper horizons of thick soils in Slovak samples. Fig. 6 Increasing content of the clay fraction should reduce the hydraulic conductivity. However, this trend could be proved only within the field tests on Slovak soils. Unfortunately, no field tests could be done at the Croatian profiles yet. Fig. 7 Hydraulic conductivity – results of tests using the Guelph field permeameter, related to the con- tent of the clay fraction. A – distribution according to soil type; B – distribution according to country. A B 202 Geologia Croatica 58/2 5. CONCLUSIONS The detailed study illustrates the difficulties and the complexity of the problem of determining the hydrau- lic conductivity of soils. This should be taken into account when preparing a methodology for groundwa- ter vulnerability mapping (MALIK & SVASTA, 1999; MARSCHALKO & IDES, 2000). Some attempts have already been published, where the assessment of soil permeability was based solely on an field estimate of the content of the clay fraction. But, such results are far away from the hydraulic conductivity measured “in situ”. Therefore, the methodology needs further devel- opment, based on results collected from many field tests and maybe sorted according to the exact soil type in terms of soil science (the weather conditions during the test period should be also taken into account). This would allow soil maps to be used as an important input for groundwater vulnerability mapping. Acknowledgement We gratefully acknowledge the Research Support Scheme of the Open Society Foundation (project No. 1326/2000), Action Austria–Slovakia (projects No. 37s11 and 42s6), as well as the Slovak Granting Agency VEGA (project No. 1/9160/02) for financial support. 6. REFERENCES ADAMCOVA, R., DUBIKOVA, M., SKALSKY, R., DURN, G., MIKO, S., KAPELJ, S. & OTTNER, F. (2001): Vys- kum vybranych typov pod ako barier znecistenia podzem- nych vod anorganickymi latkami [Research of the barrier effect of selected soil types against inorganic groundwater pollutants – in Slovak].– Conf. 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