Magnetic, geochemical and mineralogical properties of sediments fromclean karstic and flysch rivers of Croatia and Slovenia
Stanislav Frančišković-Bilinski1*, Robert Scholger2, Halka Bilinski1 and Darko Tibljaš3
1Institute «Ruđer Bošković», Division for marine and environmental research, POB 180, 10002 Zagreb, Croatia
2Montanuniversität Leoben, Department of applied geosciences and geophysics, Peter-Tunner-Straße 25, 8700 Leoben, Austria
3University of Zagreb, Faculty of Science, Department of Geology, Institute of Mineralogy and Petrology, Horvatovac 95, 10000 Zagreb, Croatia
*Corresponding author, E-mail: ; Phone: +385 1 4561081; Fax: +385 1 4680242
Abstract
The aim of the present work was toinvestigate links between the low-field magnetic susceptibility (MS) of sediments from several Croatian and Slovenian rivers and their chemical and mineral composition, as well as possible anthropogenic influence. MS measurements are a fast and simple method, which serves as a proxy for the estimation of pollution in different environmental systems. As far as we know, it is the first time applied on stream sediments in the studied region.
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 andthe 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 and the 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 0.5*10-7 to 5.11*10-7 m3/kg, and isothermal remanent magnetism (IRM) values ranging from 0.7 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.
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.
Keywords
karstic and flysch rivers; Croatia; Slovenia; magnetic susceptibility; trace elements; heavy metals; sediments; mineralogy; anthropogenic influence
1. Introduction
Magnetic susceptibility is 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 anthropogenic contamination with certain metals.
The development of magnetic measurements application in environmental research begun in the eighthdecade of the twentiethcentury. The application of this method to sediments was initiated by Thompson and Oldfield (1986) and awhole series of authors have beenusing it for different investigations in geosciences. Shortly after that,the application of magnetic measurements as proxies and indicators 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; Fialova et al.,2006;Shoumkova et al., 2006; Botsou et al., 2011Novakova et al. 2012).Investigations performed in industrial areas have also shown, that the distribution of magnetic susceptibility in soils is closely connected with depositionof 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).
The majority of the authors are in accordance with the conclusion, that this method is promising and confident for the discriminationof polluted areas. Because this method is fast and cheap, it is possible to handle a dense network of sampling sitesand construct magnetic susceptibility maps, which enable an efficient selection of the most important points on which chemical analyses will also be performed. This procedurecuts 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 assessmentin Croatia. First such measurements in Croatia have been performed by Frančišković-Bilinski(2008a) on the samples from the Kupa River watershed. The area with thehighest values of magnetic susceptibility was found to be the lower flow of the Mrežnica River. It was shown that this anomaly originates from uncareful 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 et al., 2014).
To our knowledge, in Slovenia there were no 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 clean 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.
2. Study area
The study area is composed of three regions in Slovenia and Croatia (Figure 1). The first, with 16 sampling points, is the Celje industrial region (Slovenia) drained by the Voglajna, Hudinja and Savinja rivers (Figure 2). The second (Figure 3) is a karstic region inSlovenia with karsticrivers (the Rak, Cerknišnica, Unec and Ljubljanica) belonging to the Sava River drainage basin, represented by 8 sampling stations. The third region comprises drainage basins of rivers that flow through flysch (the Reka from Brkini Hills with 8 sampling stations; the Rižana, Dragonja, Mirna and Raša from Istria, with 23 sampling points) (Figure4).
a)Details about the study area of the Savinja, Voglajna and Hudinja drainage basins are given inFrančišković-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 lithothamnian 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 aresignificantly affected by metallurgical industry around Celje.
b)The investigated system of Slovenian karstic rivers 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 Obrh 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 the lake area and it flows through the famous Rakov Škocjan valley and canyon and then 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 inflowsthe Sava River. The hydrology of this karstic system is explained by distribution of areas built of the Triassic semi-permeable dolomites and permeable Jurassic and Cretaceous limestones (Gams, 2004). Ljubljanica River was described earlier by Frančišković-Bilinski(2008b).
c)Flysch and alogene 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).
3. Materials and methods
3.1. Sampling and sample preparation
Sampling of 57 stream sedimentssites from Croatian and Slovenian rivers was performed during three years (2001-2003), in the frame of a Croatian-Slovenian bilateral project,in which geochemistry and mineralogy of river sedimentswas studied. Sediments were air dried and dry sieved (standard sieves, Fritsch, Germany). Sieved samples were pulverized using a mortar grinder Pulverisette 2 (Fritsch, Germany). Samples were used for different purposes within the above mentioned project and the rest material was stored for possible further research in the future. In the present paper, their magnetic susceptibility and isothermal remanent magnetism (IRM) were measured for the first time.
3.2. Measurements of magnetic susceptibility and isothermal remanent magnetisation
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 magnetisation step, IRM was measured on aDC-Squid Magnetometer (2G Enterprises, Ca, USA).
3.3. 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 4-63 º2θ). Crystalline phases were identified by comparison with Powder Diffraction File (1997) using the computer program X’Pert High score (Philips, 2001).
3.4. 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) and “Ultratrace 2” software. The procedure was as follows: 0.5 g of sample was dissolved in aqua regia at 90 ºC in a microwave digestion unit. The solution was diluted and analyzed using a Perkin Elmer SCIEX ELAN 6100 ICP-MS instrument. Thereference 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).
3.5 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 concentrationsof toxic elements in Region 2 and Region 3 from the data in Table 2 while anomalies for the elements in Region 1, determined using same statistical approach, are taken from Frančišković-Bilinski et al. (2006).
4. Results
4.1. Measurements of magnetic susceptibility and isothermal remanent magnetisation (IRM)
Results of the measurements of mass susceptibility and isothermal remanent magnetisation (IRM) are divided in three regions/datasets and, for each of them, mean valuesfor 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*10-7m3/kg and IRM values ranging from 0.91 to 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 from 0.7 to 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) (Figure 5). In this diagram,magnetic volume susceptibility(abbreviation: MS, dimensionless inSI units) is plotted vs. 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 3 the values are the lowest and in region1 are the highest. “Magnetic grain” sizes range between 1 and 16 µm in most samples of regions 1 and 2. Most samples from region 3 have grain sizes above 16 µm. The use of magnetic susceptibility vs. IRM as magnetic grain size indicator rests on many assumptions, therefore the results can be considered only as tentative ones.
4.2. Mineral composition of stream sediments
The mineral composition of river sediments (fraction <63 μm) of the Celje industrial region was earlier presented in details (see Table 1, 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 fromthe 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 arealso some traces of feldspars, muscovite and of chlorite.
In all samples from the Reka River, quartz is a dominant constituent. Feldspar (presumably albite ordered) 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, whereit is not present.
In sediments of the Mirna, Raša, Rižana and Dragonja Rivers(Figure 4) 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.
4.3. Elemental composition of sediments
Concentrations of 38 elements were determined in sediments of regions 2 and 3 in the present work. From them onlyconcentrations 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). For concentrations of elements and their anomalies in the Celje Region, we refer to our earlier paper(Frančišković-Bilinski et al.2006), which 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, serious toxic effects are possible due to several elements. In other locations, only minor toxic effects are possible.Descriptive statistics was used to obtain basic statistical parameters shown in Tables 3a and 3b. For each element the values are given for the arithmetic mean, geometric mean, median, minimal and maximal concentrations, variance, standard deviation, skewness and kurtosis. 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 Tables4a and 4b, for regions 2 and 3, respectively. In Region 2 mean concentrations of Cu and Znare 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 3 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 could be due to intensive fruit growing in this region (field observation of D.T., one of authors of the current paper).
Concentrations of total Hg are elevated in several samples:in samples 38, 39 (Region 2) and in 13, 15, 53 and 54 of the Reka River, the values are higher than 2 ppm, the value reported as capable of causing significant toxic effects. The values of Hg concentrationsare the highest in sample 15 (Region 3) and in sample 38 (Region 2). These concentrations of total Hg are almost comparable with Hg concentrations of lower stretch ofthe Idrijca, and Soča Rivers(see Table 2 and Table 4, in Frančišković-Bilinski et al., 2005).
Anomalies in concentrations of selected elements and of magnetic susceptibility in sediments of regions 1-3 were determined by the boxplot method(Table5). (In other sediments, which are not included in Table 5, anomalies werenot found.