Magnetic, geochemical and mineralogical properties of sediments from karstic and flysch rivers of Croatia and Slovenia

Abstract

The aim of the present work was to investigate links between the low-field magnetic susceptibility (MS) and chemical and mineral composition of sediments from several Croatian and Slovenian rivers, as well as to determine possible anthropogenic influence on these sediments. MS measurements are a fast and simple method, which serves as a proxy for the estimation of pollution in different environmental systems. The investigated rivers are predominantly unpolluted rivers from Croatian and Slovenian karstic and flysch areas, which belong to the Adriatic or the Black Sea watersheds: the Dragonja, the Mirna, the Raša, the Rižana, the Reka, the Rak, the Cerknišnica, the Unec and the Ljubljanica rivers. It was assumed that, due to their mostly unpolluted status, they could serve as a database for natural MS background values for this region. For comparison, several rivers and a lake from the Celje old metallurgic industrial area (Slovenia) were also investigated: the Savinja, the Hudinja, the Voglajna rivers and Slivniško Lake. They form a sub-basin of the Sava River drainage basin. Sediments of the clean karstic and flysch rivers showed extremely low MS values, with mass susceptibility values ranging from 0.58 × 10−7 to 5.11 × 10−7 m3/kg, and isothermal remanent magnetism (IRM) values ranging from 0.71 to 7.88 A/m. In the Celje industrial area, river sediments showed much higher MS values, with mass susceptibility values ranging from 1.31 × 10−7 to 38.3 × 10−7 m3/kg, and IRM values ranging from 0.91 to 100.42 A/m. The highest MS value was found in the Voglajna River at Teharje-Štore, a point which showed a significant number of anomalies of toxic metals in earlier investigations. Semiquantitative determination of relations between grain size and concentration of magnetite was performed using the Thompson-Oldfield method. X-ray diffraction (XRD) mineralogical analysis showed that sediments of the Celje area have mostly quartz as major mineral, with relatively small amount of carbonate minerals, while in sediments of karstic rivers carbonate minerals prevail. Statistically significant correlations were obtained between MS and Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd and Ba in the Slovenian karstic region and between MS and Cr, Fe, Co, Ni and Zn in flysch drainage basins. On the contrary, no correlation of MS and Hg content was obtained.

Introduction

Magnetic susceptibility represents the ease with which material can be magnetized in an external magnetic field. Determination of volume susceptibility is a cheap and fast method, and it is possible to use it as an indicator of potential anthropogenic contamination with certain metals.

The development of magnetic measurement application in environmental research begun in the ninth decade of the twentieth century. The application of this method to sediments was initiated by Thompson and Oldfield (1986), and a whole series of authors have been using it for different investigations in geosciences. Shortly after that, the application of magnetic measurements as proxies and indicators of anthropogenic contamination, which reduced the need for expensive and complicated chemical analyses, became one of the main research topics in contamination research (Oldfield et al. 1985; Strzyszcz 1993; Strzyszcz et al. 1996; Hay et al. 1997; Heller et al. 1998; Scholger 1998; Plater et al. 1998; Kapička et al. 1999; Petrovsky et al. 2000; Hanesch and Scholger 2002; Hanesch et al. 2003; Boyko et al. 2004; Fialová et al. 2006; Shoumkova 2006; Botsou et al. 2011; Novakova et al. 2012). Investigations performed in industrial areas have also shown that the distribution of magnetic susceptibility in soils is closely connected with deposition of industrial dust and that magnetic measurements could be used as a method for detection of heavy metals in soils. A detailed overview of magnetic monitoring methods in pollutant research was given by Petrovský and Ellwood (1999).

Numerous authors are in accordance with the conclusion that this method is promising and confident for the discrimination of polluted areas. Because this method is fast and cheap, it is possible to handle a dense network of sampling sites and construct magnetic susceptibility maps, which enable an efficient selection of the most important points on which chemical analyses will also be performed. This procedure cuts significantly the costs of heavy metal screening in the environment and contributes significantly to the quality of environmental research.

Until recently, magnetic susceptibility measurements of stream sediments have not been used with the purpose of environmental quality assessment in Croatia. First such measurements in Croatia have been performed by Frančišković-Bilinski (2008a) on samples from the Kupa River watershed. The area with the highest values of magnetic susceptibility was found to be the lower flow of the Mrežnica River, and it was shown that this anomaly originates from careless coal burning products disposal. Increased values of magnetic susceptibility were observed in stream sediments of the upper flow of the Dobra River, which are suggested not to be of anthropogenic influence (Frančišković-Bilinski et al. 2014).

To our knowledge, in Slovenia, there have been no previous investigations of magnetic susceptibility in surface sediments to assess the quality of rivers.

The aim of the present work was to perform measurements of the low-field magnetic susceptibility (MS) of stream sediments from several Croatian and Slovenian rivers and to find links with their chemical and mineral composition, as well as with possible anthropogenic influence. The results could serve as a database for natural MS background values for two presumably relatively unpolluted regions (Slovenian karstic rivers; flysch and alogene rivers of Croatia and Slovenia) in comparison with those from the Celje industrial region with intensive metal manufacturing processes.

Study area

The study area is composed of three regions (a, b and c) in Slovenia and Croatia (Fig. 1; Table 1). Region a, with 16 sampling points, is the Celje industrial region (Slovenia) drained by the Voglajna, Hudinja and Savinja rivers (Fig. 2). Region b (Fig. 3) is a karstic region in Slovenia with karstic rivers (the Rak, Cerkniščica, Unec and Ljubljanica) belonging to the Sava River drainage basin, represented by 8 sampling stations. Region c comprises drainage basins of rivers that flow through flysch with total 31 sampling stations (the Reka from Brkini Hills; the Rižana, Dragonja, Mirna and Raša from Istria) (Fig. 4).

Fig. 1
figure1

Location of three study areas within Slovenia and Croatia: a Celje industrial region (Slovenia); b karstic region in Slovenia; c flysch areas of Croatia and Slovenia

Table 1 Sample no., river and locality name, geographic coordinates (datum WGS 84), altitude and results of laboratory magnetic measurements (mass magnetic susceptibility–χ mass, volume magnetic susceptibility–χ v and isothermal remanent magnetism—IRM) of sediments (fraction <63 µm) from Croatian and Slovenian rivers
Fig. 2
figure2

Celje industrial region (Slovenia) drained by the Voglajna, Hudinja and Savinja rivers: geological map (simplified after FGI 1970) with position of sampling locations. C,P Carboniferous and Permian shale, sandstone and conglomerate; T triassic dolomite and limestone; η triassic andesite and tuff; Ol oligocene sand, sandstone, sandy marl; M Miocene sand, sandstone, marly limestone and lithothamnium limestone; Pl,Q Pliocene and Quaternary clay, quartz gravel and sand

Fig. 3
figure3

Karstic region in Slovenia drained by the Rak, Cerkniščica, Unec and Ljubljanica rivers, belonging to the Sava River drainage basin: geological map (simplified after FGI 1970) with position of sampling locations. T Triassic, mainly dolomite; J Jurassic, mainly limestone; C Cretaceous, mainly limestone; E 1,2 Eocene, flysch; Q Quaternary sediments

Fig. 4
figure4

Flysch areas of Croatia and Slovenia drained by the Reka from Brkini Hills and the Rižana, Dragonja, Mirna and Raša from Istria: geological map (simplified after FGI 1970) with position of sampling locations. J Jurassic, mainly limestone; C Cretaceous, mainly limestone; Pc,E Paleocene, Eocene limestone; E 2,3 Eocene, flysch; Q Quaternary sediments

  1. (a)

    Details about the study area of the Savinja, Voglajna and Hudinja drainage basins are given in Frančišković-Bilinski et al. (2006). This area is located in the eastern part of Slovenia. The studied area represents a young tectonic basin, which is filled with Quaternary deposits of the Savinja, Voglajna and Hudinja rivers. In the northern part, there are Pleistocene clays, quartz gravel and sand. In the SE part, Miocene sand, sandstone, marly limestone and lithothamnium limestone are also exposed as part of the Celje syncline. The oldest rocks (Triassic dolomite and limestone, as well as andesite and their tuff and Carboniferous and Permian shale, sandstone and conglomerate) crop out in the southern part. These rivers were poorly studied in terms of geochemistry; even though they are significantly affected by metallurgical industry around Celje. According to Štern and Förstner (1976), Voglajna was one of the most polluted water flows in Slovenia. They found that in the sediments of Voglajna River, chromium and zinc were enriched by a factor of more than 100, copper, lead and cadmium by a factor of 25–50, compared with the geochemical background. It was assumed that effluents from the old waste dumps of zinc works, from iron works near Štore, from a TiO2 factory and from an enamel pottery factory are the contributors to the unusually high metal contamination in Voglajna. Also, the consequence of more than 100 years of smelting of the sphalerite ore in Cinkarna Celje factory is heavy contamination of the Celje region with zinc and cadmium. In spite of the fact that industrial pollution was reduced in meantime, Frančišković-Bilinski et al. (2006) in samples taken during 2001–2002 still registered elevated Zn and Cd concentrations in river sediments (Table 2).

    Table 2 Concentration (in µg g−1, except when stated otherwise) of selected elements in <63 µm fraction of sediment samples from Regions a, b and c obtained by ICP-MS analysis
  2. (b)

    The investigated system of Slovenian karstic rivers (rivers which flow mostly through karstic areas) from Notranjsko podolje region stretches from the Cerkniško Lake in the south to the Ljubljanica River in the north. The most distant source branch of this system is the Trbuhovica, which springs on Prezid Polje in Croatia. Two main branches which are feeding the periodic Cerkniško Lake are the Cerkniščica and Stržen. The Cerkniško Lake has an area of 38 km2 when it is full, the maximal depth is 10 m, and it is 10.4 km long and 4.7 km broad. The Rak River drains underground the lake area, and it flows through the famous Rakov Škocjan valley and canyon. Rakov Škocjan is a protected area, known to its natural beauty, and the valley was basically a huge cave, whose ceiling has collapsed long ago as the karst surface slowly but steadily dissolved and cut it. There are two “natural bridges,” which are remnants of the cave ceiling. Then the river flows underground until the Planinska Jama cave. Within this cave, it unifies with the Pivka River and reaches the surface as Unica. This river sinks and, when it again reaches the surface near Vrhnika at Močilnik spring, it has the name Ljubljanica. It flows to Ljubljana, where it inflows the Sava River. The hydrology of this karstic system is explained by distribution of areas built of the Triassic semipermeable dolomites and permeable Jurassic and Cretaceous limestones (Gams 2004). Ljubljanica River was described earlier by Frančišković-Bilinski (2008b). The whole investigated area of Slovenian karstic rivers is very sparsely inhabited, without any bigger cities. Postojna, the largest settlement in the area with 8,500 inhabitants but without bigger industry, can influence quality of waters in the Pivka River. In the upper part of the drainage basin (in the area of the periodic Cerkniško Lake), the biggest settlement is Cerknica with about 3,500 inhabitants and the biggest settlement in the lower part of this drainage basin (near upper flow of the Ljubljanica River) is Vrhnika with about 7,500 inhabitants. There are no larger pollution sources in this region, there are no mining activities, neither heavy industry. Higher karstic areas (hills and mountains) are covered almost completely by forest and almost uninhabited, while smaller, mostly rural settlements are situated in karst poljes which are partly agriculturally cultivated. Smaller estates and mixed farming prevail, so it would not be expected that agriculture is larger source of pollution. The fact that the water from the karst spring, which receives among others waters from the Cerkniško Lake and the Rak River, is used for water supply (Poje 2008) indicates that water from this hydrological system can be considered as almost unpolluted.

  3. (c)

    Flysch (rivers which flow mostly through flysch) and alogene (rivers which have spring and part of its course in a non-karstic area and then enter karst) rivers of Croatia and Slovenia which were studied within this paper are located in the Istria peninsula, the Raša and Mirna in its Croatian part, the Dragonja at or close to the Croatian–Slovenian border and the Rižana in Slovenian part. Another studied river (the Reka) is located in nearby Brkini hills flysch area in SW Slovenia. The flysch of the Istrian peninsula is one of the three main flysch areas in the outer Dinarides, the other two occurring in the northern Dalmatian and Split areas. The flysch units in Istria have been considered as Middle to Late Eocene in age (Babić et al. 2007). From these rivers, Raša was studied earlier by Sondi et al. (1994, 1995), with respect to trace metal transport and sedimentation processes. Some results of contamination status of the Rižana, Dragonja, Mirna and Raša Rivers were earlier reported in two brief conference papers by Frančišković-Bilinski et al. (2003, 2007). The Dragonja River flows through the sparsely inhabited hilly area at the border between Slovenia and Croatia, with several small rural settlements and some Mediterranean agriculture. Small and short, the Rižana River has its spring in similar area, but its lower flow is surrounded by industrial areas of Koper (50,000 of inhabitants), which is center of the whole coastal region of Slovenia and one of the biggest Slovenian cities. The Mirna River flows through a sparsely inhabited valley, through a rural area with some Mediterranean agriculture. Only bigger settlement in the Mirna valley is town Buzet in its upper flow, with about 6,000 inhabitants, with a brewery, but without other heavier industry. The Raša River flows through sparsely populated hilly area and valley. It is a rural area with some Mediterranean agriculture. Also, coal-operated power plant in Plomin at eastern Istrian coast is located close to Raša valley, so its influence could be expected, especially due to careless disposal of slag, which is known to be radioactive and rich with heavy metals (Barišić et al. 2001). The Reka River valley in Slovenia is also a rather sparsely populated rural area, with some agriculture (mostly fruit growing), except in the middle flow of the Reka River, where Ilirska Bistrica, town with some 5,000 inhabitants, and small industry is located. Factory of organic acids, which operated in the town from 1955 till 1990, was known as a contamination source influencing the Reka River and turning it from source of drinking water to waste water channel. Since the closure of the factory, the situation has significantly improved (Kogovšek 2001).

Materials and methods

Sampling and sample preparation

Sampling of 55 stream sediments sites from Croatian and Slovenian rivers was performed during 3 years (2001–2003), in the frame of a Croatian–Slovenian bilateral project, in which geochemistry and mineralogy of river sediments was studied. Sediments were air-dried and dry sieved using standard sieves. Sieved samples were pulverized using a mortar grinder Pulverisette 2 (Fritsch, Germany).

Measurements of magnetic susceptibility and isothermal remanent magnetization

For measurements of mass susceptibility (χ in m3/kg), samples were placed in a standard cylindrical sample container (10 cm3) and weighed. A multifunction kappa-bridge MFK1-FA (AGICO, Brno, Czech Republic, Field 200 A/m, Frequency 976 Hz) was used. The procedure was earlier described in Hanesch et al. (2003).

A direct magnetic field of 1T was introduced, using a Pulse Magnetizer Model 660 (2G Enterprises, Ca, USA). After the magnetization step, IRM was measured on a DC-Squid Magnetometer (2G Enterprises, California, USA).

Mineralogical analysis using XRD

The mineral composition of the sediments (fraction <63 μm) was determined by X-ray powder diffraction using a Philips X-Pert MPD diffractometer (Cu tube, graphite monochromator, generator settings: 40 kV, 40 mA, range scanned 4–63º2θ). Crystalline phases were identified by comparison with Powder Diffraction File (1997) using the computer program X’Pert High score (Philips Analytical 2001).

ICP-MS analysis of sediment fraction <63 μm

Chemical analysis was performed by ACTLABS commercial laboratories, Ontario, Canada, in the fraction <63 μm, using ICP-MS (Inductively Coupled Plasma Mass Spectroscopy) by "Ultratrace 2" procedure. The procedure was as follows: 0.5 g of sample was dissolved in aqua regia at 90 °C in a microprocessor controlled microwave digestion unit for 2 h. The solution was diluted and analyzed using a PerkinElmer SCIEX ELAN 6100 ICP-MS instrument. The reference materials (USGS GXR-1, GXR-2, GXR-4 and GXR-6) were analyzed at the beginning and after analysis of each series of samples.

Although this digestion is not total, its use is justified because the international standard methods for determining action limits are based on aqua regia leach (Salminen and Tarvainen 1997). The aqua regia (3:1, v/v, HCl to HNO3) digestion procedure (ISO standard 11466) is considered adequate for analyzing total-recoverable heavy metals in soils of certain regions and is used to estimate the maximum element availability to plants. Residual elements that are not released by aqua regia digestion are mostly bound to silicate minerals and are considered unimportant for estimating the mobility and behavior of the elements (Chen and Ma 2001).

Statistical procedure for determination of anomalous values

Basic statistical parameters and correlation analysis were performed using STATISTICA 8.0. (StatSoft, Inc. 2007). Anomalous values of magnetic susceptibility and IRM were obtained from the experimental data in Table 1 using the two-dimensional scatter box diagrams (Tukey 1977; Reimann et al. 2005). The same procedure was used for assessing anomalous concentrations of toxic elements from the data in Table 2.

Results

Measurements of magnetic susceptibility and isothermal remanent magnetization (IRM)

Results of the measurements of mass susceptibility and isothermal remanent magnetization (IRM) are divided into three regions/datasets and, for each of them, mean values for every measured parameter are presented (Table 1). As expected, the highest values of all parameters are observed in the Celje industrial region, with mass susceptibility values ranging from 1.31 × 10−7 to 38.3 × 10−7 m3/kg and IRM values ranging 0.91–100.42 A/m. Sediments of the clean karstic and of flysch and alogene rivers showed extremely low MS values, with mass susceptibility values ranging from 0.5 × 10−7 to 5.11 × 10−7 m3/kg and IRM values ranging 0.7–7.88 A/m. Rivers from this clean region were divided into two groups; those from Slovenian karst showed a bit higher values than flysch and alogene rivers of Croatia and Slovenia.

Semiquantitative determination of relations between grain size and concentration of magnetite was performed using the Thompson-Oldfield method (1986) (Fig. 5). In this diagram, magnetic volume susceptibility (abbreviation: MS, dimensionless in SI units) is plotted versus IRM-intensity (A/m). The diagram shows that the concentration of magnetic phases varies largely between rivers. The values of magnetite concentrations spread approximately from 0.003 to 0.3 %. In Region c, the values are the lowest and in Region a are the highest. “Magnetic grain” sizes range between 1 and 16 µm in most samples of regions a and b. Most samples from Region c have grain sizes above 16 µm. The use of magnetic susceptibility versus IRM as magnetic grain size indicator rests on many assumptions; therefore, the results can be considered only as tentative ones.

Fig. 5
figure5

Diagram according to Thompson-Oldfield (1986), presenting semiquantitative determination of relations between grain size and concentration of magnetite: magnetic volume susceptibility (abbreviation: MS, dimensionless in SI units) is plotted versus SIRM-intensity (A/m)

Mineral composition of stream sediments

The mineral composition of river sediments (fraction <63 μm) of the Celje industrial region was earlier presented in details (Frančišković-Bilinski et al. 2006). Quartz was found as a major mineral in all sediment samples. Among other major minerals, albite was found in locations 1 and 3; muscovite in locations 2, 79, and 82; calcite in locations 4, 5, 6 and 82; dolomite in locations 6 and 81. Depending on locations, there are several minor minerals and also several trace minerals, some of which contain Fe-ion in the structure.

In stream sediments from the Slovenian karstic region quartz, calcite and dolomite are present in different proportions. Quartz predominates in samples 8, 9, 38, 39 and 17; calcite predominates in samples 58 and 10; while dolomite predominates in sample 11. There are also some traces of feldspars, muscovite and of chlorite.

In all samples from the Reka River, quartz is a dominant constituent. Feldspar (presumably albite) is present as the second abundant mineral in samples 53 and 54 and as a minor constituent in samples 12, 15, 16 and 56. Calcite is present in all samples as minor component, except in sample 12, where it is not present.

In sediments of the Mirna, Raša, Rižana and Dragonja rivers, quartz and calcite are the major minerals with slightly different proportions. Feldspars and muscovite are present as trace minerals.

Magnetite was not detected by XRD in any sample, which can be attributed to the generally low concentrations indicated by MS measurements.

Elemental composition of sediments

Concentrations of 38 elements were determined in sediments of regions b and c in the present work. From them only concentrations of 12 potentially toxic elements (Fe, Cr, Mn, Co, Ni, Cu, Zn, As, Cd, Cd, Ba, Pb and Hg), which are included in the list of existing sediment criteria issued by SMSP and FALCONBRIDGE NC SAS (2005), are shown in Table 2, in addition to macrocomponents (Mg, Al, Ca, S and K). In the Table 2 for comparison, concentrations of elements for Celje region from Frančišković-Bilinski et al. (2006) are also shown. Frančišković-Bilinski et al. (2006) showed that stream sediments of the Celje region are still contaminated, although industrial pollution decreased during the last decades. The most significant contaminants in the Celje region are Zn and Cd. In two locations within the town of Celje, several elements have concentrations above the threshold value that might cause serious toxic effects. In other locations, elements are present in concentrations that might cause only minor toxic effects. Descriptive statistics was used to obtain basic statistical parameters shown in Table 3 for all three studied regions. For each element, the values are given for the arithmetic mean, range and standard deviation. The value for the arithmetic mean ± standard deviation can be considered as baseline value for each element studied here. Comparison of mean concentrations of potentially toxic elements with sediment quality criteria (SMSP and FALCONBRIDGE NC SAS 2005) is presented in Table 4. In Region b, mean concentrations of Cu and Zn are below the level for the lowest toxic effects. Mean concentrations of Cr, Fe, Ni, As, Cd, Pb and of Hg are above the values that could cause the lowest toxic effects. Mean concentrations of Mn and of Ba are above the value capable for causing significant toxic effects. In Region c, mean concentrations of Cd and Pb are below the level for the lowest toxic effects. Mean concentrations of Cr, Mn, Fe, Cu, Zn and of As are above the values causing the lowest toxic effects. Mean concentrations of Ni, Ba and of Hg are above the values causing significant toxic effects. The highest values of Ni and of Hg are observed in the Reka River. Increased concentrations of Cu in samples 53, 54 and 56 are similar to, or even lower than, values recorded in the soils of nearby Coastal region of Croatia (Halamić and Miko 2009) with viticulture history. The Reka River area is not a wine growing area, but copper-based chemicals are also used in prophylaxis of apples which are cultivated in Brkini area. There is also a potential influence of chicken farming in the vicinity of Pivka because copper sulfate could be used in the diet of broiler chickens for stimulating appetite and producing increased growth rate. Cu content in two marl samples taken at outcrops upstream of Ilirska Bistrica is also in this range, 40 and 65 µg g−1, respectively. Therefore, natural origin of Cu cannot also be excluded without additional sampling in spite of the fact that stream sediments taken closer to analyzed marls contain lower Cu concentrations.

Table 3 Basic statistical parameters (mean, range and standard deviation) for 38 elements analyzed in stream sediments from regions a, b and c
Table 4 Comparison of mean concentrations of potentially toxic elements in regions a, b and c with sediment quality criteria (SMSP and FALCONBRIDGE NC SAS 2005); all values except Fe are in µg g−1

Concentrations of total Hg are elevated in several samples: In samples 38, 39 (Region b) and in 13, 15, 53 and 54 of the Reka River, the values are higher than 2 µg g−1, the value reported as capable of causing significant toxic effects. The values of Hg concentrations are the highest in sample 15 (Region c) and in sample 38 (Region b). These concentrations of total Hg are almost comparable with Hg concentrations of lower stretch of the Idrijca and Soča Rivers (Frančišković-Bilinski et al. 2005).

Anomalies in concentrations of selected elements and of magnetic susceptibility in sediments of regions a–c were determined by the boxplot method (Table 5). In other sediments, which are not included in Table 5, anomalies were not found.

Table 5 Anomalies in sediments of regions a–c determined by boxplot method

Comparison of the measured element concentrations with the available quality criteria for fresh water sediments (SMSP and FALCONBRIDGE NC SAS 2005) indicates that some of the investigated sediments contain several elements in concentrations that could have the lowest (e.g., Cr, Fe, Ni) or even significant toxic effects (Ba, Hg), i.e., such comparison indicates possible pollution of investigated sediments. But careful analysis of such elevated concentrations for several elements shows that they are most probably the result of natural processes and that they do not indicate any pollution. A typical example is Ba in sediments from the flysch region rivers. Sandstones from this region contain from 50 to 3,600 µg g−1 of Ba, and the highest values have been observed in the samples from the Pazin flysch basin (Mikes et al. 2006). These values are in accordance with the fact that authigenic barite is one of the most abundant heavy minerals found in marls of the central Istria flysch (Magdalenić 1972). The fact that sediment samples with the highest recorded Ba concentrations (99, 100, 107) were taken precisely in the streams which drain areas in which marls and sandstones contain authigenic barite supports the conclusion that Ba is naturally present in investigated sediments. Similar conclusions could be reached for Cr whose concentrations are lower in stream sediments than in flysch sandstones. The highest measured Cr concentrations are in the Reka river sediments that drain the Brkini area in which sandstones generally have higher Cr values. Cr is concentrated in chromium spinel, a key mineral in ophiolitic detritus which was more abundant in Brkini flysch sandstones than in sandstones from other NW External Dinarides flysch basins (Mikes et al. 2006). Elevated concentrations of Ni, another element that indicates ultramafic–mafic detritus origin, in the Reka river sediments are also in accordance with that.

The only element which has elevated concentrations in some sediment samples, which for the moment could not be explained by natural processes, is mercury. This can be said in spite of the fact that Idrija, where one of the biggest Hg mines in the world was located, is roughly 50 km away from the investigated area. Even closer, about 30 km, is Tršće where Hg ore occurrences are known. But both of these occurrences are within rocks formations which are not present in the analyzed area and drain to other watersheds. Analyses of two marls collected upstream of Ilirska Bistrica showed that their Hg content is very low: 87 and 25 ng g−1, respectively. Apart from that, there is no available data on Hg concentration in source rocks, but in the soils from the surroundings of nearby town Rijeka values higher than 170 ng g−1 were only rarely observed, and the median value for Croatian soils is 60 ng g−1 (Halamić and Miko 2009). Therefore, it can be presumed that some of the highest measured values (i.e., 57,800 ng g−1 in sample 15) indicate some pollution especially due the fact that sediments taken closer to the river sources (i.e., sample 12) have significantly lower Hg concentrations. Possibly, the elevated concentrations are the result of agriculture, in which mercury is used among other ways as a foliar spray against plant diseases. Other possible explanation is that Hg originates from inadequate waste dump of organic acids factory in Ilirska Bistrica. The factory was closed in 1990 but deposited waste influenced the environment long after the factory closure (Šoltes 2010). For more reliable conclusions about the origin of Hg in investigated sediments, more detailed sampling of stream sediments and source rocks is needed.

Such conclusions raise a question about usage of the available quality criteria for freshwater sediments (SMSP and FALCONBRIDGE NC SAS 2005). In our case, obviously unpolluted river sediments and even rocks from which they have been derived have some element concentrations that are supposed to have possibility to cause even significant toxic effect. Use of such criteria which are based merely on concentrations and not taking in account the bioavailability of elements should be done with judiciousness.

Discussion of magnetic properties of sediments in context of their geochemical and mineral composition

In the current discussion, we refer to our earlier papers (Frančišković-Bilinski et al. 2003, 2006, 2007), in which geochemistry and mineralogy of here studied sediments was described.

Earlier investigations in the Celje area (Frančišković-Bilinski et al. 2006) described in details the contamination of river sediments with following toxic metals for which are given their maximal measured values: Zn (1,040 µg g−1), Cd (7 µg g−1), Cu (138 µg g−1), Ni (82 µg g−1), Pb (133 µg g−1), Ag (3 µg g−1), Hg (1,086 ng g−1) and As (30 µg g−1). An extreme MS value is observed in Voglajna River at Teharje-Štore, the point which showed significant number of anomalies of toxic metals (Mo, Sb, P, Cr, Cu, Re, Pb, Bi, W). Another outlier MS value was measured in Voglajna River at Celje at the point which showed numerous anomalies (B, Na, Co, Zn, Sc, Zr, Cd, In, Pb, Tl).

XRD mineralogical analysis showed that sediments of the Celje area have mostly quartz as major mineral, with relatively small amount of carbonate minerals, while in sediments of karstic rivers carbonate minerals prevail.

According to Frančišković-Bilinski et al. (2007), high concentration of toxic elements Cd, Sb, Pb and Hg in sample 18 in upper flow of the Rižana River could be of concern; therefore, more sampling upstream can be suggested. Magnetic susceptibility of these samples shows values only slightly above the mean values for the investigated flysch and alogene rivers. Therefore, low value of MS does not always mean that there is no contamination with some elements. Raša, Mirna and Dragonja represent clean environments according to most of MS results. But sample 112 in the lower flow of the Raša River showed increased value of IRM (6.40 A/m), what could be due to the influence of nearby Plomin coal power plant. Coal combustion products which enter river systems are known to cause increased magnetic susceptibility of river sediments, what was investigated in details on the case of Mrežnica River, Croatia (Frančišković-Bilinski 2008a).

The results of correlation analysis between element concentrations and magnetic parameters χ v (SI) and IRM (A/m) for all three studied regions are presented in Table 6. In the polluted Celje industrial region (Region a), significant correlations of χ v (SI) and IRM (A/m) were obtained with S, Cr, Cu and Pb. Significant correlations were obtained between χ v (SI) and Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Ba and Pb in Region b and between χ v (SI) and Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn and Pb in Region c. It is interesting to notice that in regions b and c, there is no significant correlation between IRM (A/m) and any element. Also, correlation of χ v (SI) and Hg was not obtained in any of studied regions; therefore, the magnetic method is not applicable to detect all pollution sources, particularly of mercury, unless there was mercury ore processing, like in case of sample taken under smelter in town Idrija, Slovenia (Frančišković-Bilinski et al. 2005), which shows increased MS.

Table 6 Correlations between concentrations of selected analyzed elements and magnetic parameters of stream sediments from studied regions a, b and c

Zhang et al. (2011) proved the correlation of elevated MS and IRM values with heavy metal (Fe, V, Cr, Mo, Zn, Pb, Cd, Cu) concentrations from industrial activities (mainly the Fe-smelting plant) in Loudi City, Hunan Province (China). They demonstrated that magnetic parameters can be used as proxies for the anthropogenic contribution of pollutants to the river sediments. But, in contrast, they observed very weak correlations with Be, Cs, Rb, Nd, Co, Ni and Ba, what indicate that these metals stem from soil in the catchment region.

Dlouha et al. (2013) investigated polluted alluvial soils by magnetic susceptibility methods on the example of the Litavka River. Their research demonstrates a statistically significant correlation between magnetic susceptibility and soil concentration of Cu, Pb and Zn.

Research from the current paper is in accordance with the research of other authors.

Conclusions

Magnetic susceptibility measurements as fast and simple method for estimation of pollution with numerous toxic elements in different environmental systems were for the first time applied on stream sediments from several karstic and flysch rivers of Croatia and Slovenia. The presented results led to following conclusions:

  • The highest values of all measured parameters are present in the Celje industrial region with mass susceptibility values ranging from 1.31 × 10−7 to 38.3 × 10−7 m3/kg and IRM values ranging 0.91–100.42 A/m;

  • Sediments of the clean karstic and flysch rivers showed extremely low MS values, ranging from 0.5 × 10−7 to 5.11 × 10−7 m3/kg and IRM values ranging 0.7–7.88 A/m. The studied rivers were divided into two groups, the Slovenian karstic rivers (Region b) showed slightly higher MS values than flysch and alogene rivers of Croatia and Slovenia (Region c);

  • The results could serve as a database for the pollution state of sediments with respect to toxic elements, although not with Hg. No correlation of MS and Hg was obtained; therefore, magnetic method is not applicable to detect all pollution sources in the area, particularly of mercury.

  • Significant correlations were obtained between MS and Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd and Ba in Region b; between MS and Cr, Fe, Co, Ni and Zn in Region c.

In addition performed investigation gave information on chemical composition of stream sediments for unpolluted karst and flysch rivers in Notranjska and Istria regions which will be available for further environmental studies. In some sediments, elevated concentrations, in comparison with available quality criteria for freshwater sediments (SMSP and FALCONBRIDGE NC SAS 2005) of several elements, were recorded. Careful analysis of elevated concentrations for several elements (e.g., Ba, Cr and Ni) shows that they are most probably the result of natural processes and that they do not indicate any pollution.

Additional sampling of stream sediments and their source rocks is necessary for obtaining answers about the origin of elevated concentrations of other elements (e.g., Cu and Hg).

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Acknowledgments

Field work was organized within the Croatian–Slovenian bilateral project (2001–2003), principal investigators H. Bilinski and D. Hanžel. Magnetic measurements were performed at the Montanuniversität Leoben and at its Paleomagnetic Laboratory in Gams. A short stay of S. F–B. in Austria was covered from the Croatian–Austrian bilateral project (2010-2011), principal investigators S. Frančišković-Bilinski and T. Hofmann. Additional expenses were covered from projects of Croatian Ministry of Science Education and Sport No. 098-0982934-2720 (Principal investigator I. Pižeta) and No. 119-1191155-1156 (Principal investigator D. Balen). Authors are grateful to R. Košćal for preparing graphics. We also thank three anonymous reviewers, whose suggestions helped a lot to improve the overall quality of manuscript.

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Frančišković-Bilinski, S., Scholger, R., Bilinski, H. et al. Magnetic, geochemical and mineralogical properties of sediments from karstic and flysch rivers of Croatia and Slovenia. Environ Earth Sci 72, 3939–3953 (2014). https://doi.org/10.1007/s12665-014-3282-x

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Keywords

  • Karstic and flysch rivers
  • Croatia
  • Slovenia
  • Magnetic susceptibility
  • Trace elements
  • Heavy metals
  • Sediments
  • Mineralogy
  • Anthropogenic influence