Introduction

The content of main rock-forming and trace elements in recent fluvial sediments is an important indicator of the intensity of weathered underlying rocks and transporting conditions. Evaluating a potential risk and a degree of environmental pollution is incomplete without taking natural geological conditions and associated processes into account (Grba et al. 2015; Sindern et al. 2016; Singh et al. 2003). Recent reports that studied distribution of trace elements in stream sediments have revealed that distinguishing the origin of these phases is crucial for assessing the proportion of natural and anthropogenic sources (Bábek et al. 2015; Famera et al. 2013; Nováková et al. 2015; Sakan et al. 2014).

Elements enter fluvial systems through natural processes, such as physical and chemical weathering, as well as through human activities that include waste and sewage treatment, road runoff, or, atmospheric deposition coming from burning, mine tailings (Langedal 1997), traffic, and open-cast mining. These inputs present a significant portion of elements in the global geochemical cycle; as a result, detailed surveys at the regional level provide important information about element cycling (Doležalová Weismannová et al. 2015; Nováková et al. 2015; Rieuwerts and Farago 1996). Elements tend to adsorb onto organic matter, clays, hydrated oxides of iron and manganese, and, in particular, fine-grained particles; then, they enter the river system as complexes or hydrated ions (Vuković et al. 2014). Desorption of elements in stream sediments pose a significant threat to aquatic life due to potential bioaccumulation in tissues (Schäfer et al. 2015).

The River Bílina belongs to the most polluted tributaries of the River Elbe. Covering a total of 1071 km2 approximately, the River Bílina catchment area is divided into a zone of mountain streams draining the Krušné Hory Mts., a flatland located in the Most Basin, and a territory lining the river stretch passing through the České Středohoří Mts. In this catchment area, the hydrological regime is influenced by anthropogenic activity; it is assumed that the area upstream of the place where the river enters the Jirkov Reservoir is the only portion with a natural hydrological regime (Kaiglová et al. 2015a, b).

The region has been known from the Middle Ages for metallic ore and brown-coal mining. The subsequent development of various industries was closely linked with brown coal mines which saw their biggest expansion in the 1960s to 1980s. Surface mining caused changes to deep circulation of groundwater with pressurized water entering from the underground into the area of open-cast mines directly. For these reasons, a system was developed to drain the catchment area; it replaced the small watercourses and altered, in terms of quantity and quality, the materials entering the River Bílina. Numerous studies have documented the degree of contamination of River Bílina sediments, water, and biota (Blahová et al. 2012; Jurajda et al. 2010; Kohušová et al. 2011; Kružíková and Svobodová 2012; Orendt et al. 2012; Pribylova et al. 2006). High levels of heavy metals, such as Hg, Cd, Cr, Cu, and Zn, were also found in River Elbe sediments downstream of the river’s confluence with the River Bílina (Brügman 1995).

While the area has been the subject of numerous studies from the aspect of presence of toxic contaminants, the importance of geological background for the overall environmental burden of the river systems has still not been assessed. As a result, any data on the content of elements in geological units are only available in archives and databases. There are detailed studies that cover distribution of trace elements in the coal extracted in the Most Basin (Bouška and Pešek 1999; Pešek and Sivek 2016; Voroš et al. 2018; Fojtík et al. 2018). These results show that for brown coal and any other coal, Clarke values (Ketris and Yudowich 2009) regarding As, Cu, Cr, Ni, Pb, V, and Zn are significantly lower compared to the median of content of elements given for the upper and lower benches of the main coal seam in the Most Basin. In addition, an increased amount of all the trace elements mentioned above was found in the coal claystone (Fojtík et al. 2018). According to the published studies, enrichment factor (EF) can be used to assess any anthropogenic input into the environmental system and presents a good tool for evaluating heavy metal additions; hence, EF was calculated for selected elements in order to eliminate provenance effects in downstream trends. The main aim of this study was to define background levels of As, Zn, Pb, Cr, Ni, V, and Cu in sediments using known elemental composition of the surrounding geological units, as well as describe enrichment of the sediments from the River Bílina by calculating enrichment factors.

Material and methods

Description of samples

Twenty samples (Fig. 1) were collected in the River Bílina within its catchment area. Each sample was collected at 20 cm of the bottom layer. The total content was determined for each of the identified elements, i.e., As, Cu, Cr, Ni, Zn, Pb, and V. A total of 116 samples of rocks were used to characterize the potential entry of natural elements into the river system; they represent the main geological units—Proterozoic crystalline, Neogene volcanics, sediments and coal mass, and Quaternary loess. This information was compared with the present content of trace elements identified in stream sediments of the River Bílina (Table 1).

Fig. 1
figure 1

The course of the River Bílina with indicated sampling sites and main geological units in the adjacent area. The watercourse is segmented based on the nature of its channel; the stretches are indicated in the distribution charts for each element

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Table 1 The median content of the evaluated elements (μg g−1) in the geological units. n.d. = under the limit of determination
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In upper reaches of the River Bílina, the geological basement is made up of the Krušné Hory Mts. crystalline complex (samples R001 and R002). This part of the catchment area lacks any broad floodplains, thus is characterized by a limited space for overbank sediment deposition. It is located in a remote area with low density of settlement and no direct industrial factors. Downstream of this territory, the greater surroundings of the town of Jirkov present a strongly anthropogenically transformed area. At sampling site R003, the stream was channelled; at sampling site R004, the riverbed is lined with concrete. Some extent of this reach was also enclosed in pipes. This section is known for an active open-cast mine located in the surroundings. Downstream of the piped reach, the channel cuts deep into the ground and its surroundings present a strongly anthropogenically affected area (sampling site R005). The river passes by a refinery (sampling sites R006–R008) to flow through the Most Corridor where it was turned into a medium- to deep-cut, stabilized, straight channel (sampling sites R009–R013). The area of the river passing through the České Středohoří Mts. (English: Central Bohemian Uplands) is characterized by the lowest amount of anthropogenic interventions except that the river channel was straightened and adjusted (R014–R018). Just upstream of the confluence with the River Elbe, there is a highly urbanized area with heavy anthropogenic modifications. From this part onwards, the channel of the River Bílina was completely straightened, made considerably deep-cut, and modified through engineering measures (R019–R020).

Sample preparation; trace element analysis

All samples were air-dried and, subsequently, sieved to achieve a grain size below 0.063 mm. The content of As, Cu, Cr, Ni, Zn, Pb, V, Al, and Ti was analyzed in the laboratory of Czech Geological Survey, Prague, Czech Republic (see http://geology.cz for details). The laboratory was accredited by the Czech Accreditation Institute under ISO 17025. The method of determining elements was as follows: 5 g was taken from each sample, placed in a high-pressure digestion vessel made of polytetrafluoroethylene and digested using 1 ml HF (40%) and 3 ml HNO3 (65%). For Al and Ti, the determination was carried out using a flame atomic absorption spectroscope (Perkin-Elmer Analyst 200). For As, Cr, Ni, Zn, Cu, Pb, and V, the content was determined using ICP-MS Thermo Scientific X, Series 2. Arsenic was determined using hydride generation atomic absorption spectroscopy. The analytical procedure used solely analytical-grade reagents. Deionized water was used throughout the study. Analytical blanks were run in the same way as the samples.

The methodology was validated by analysis of the following certified reference materials: NIST SRM 2702 marine sediment; NIST SRM 2781 domestic sludge (either of them National Institute of Standards and Technology, USA) and METRANAL 32 sandy soil (Analytika Czech Republic). Limits of detection were established as follows:

$$ \mathrm{Al}=0.01\%;\mathrm{As}=0.1\ \upmu \mathrm{g}\ {\mathrm{g}}^{-1};\mathrm{Cr}=2\ \upmu \mathrm{g}\ {\mathrm{g}}^{-1};\mathrm{Cu}=1\ \upmu \mathrm{g}\ {\mathrm{g}}^{-1};\mathrm{Ni}=5\ \upmu \mathrm{g}\ {\mathrm{g}}^{-1};\mathrm{Pb}=1\ \upmu \mathrm{g}\ {\mathrm{g}}^{-1};\mathrm{Ti}=7\ \upmu \mathrm{g}\ {\mathrm{g}}^{-1};\mathrm{V}=2\ \upmu \mathrm{g}\ {\mathrm{g}}^{-1};\mathrm{and}\ \mathrm{Zn}=1\ \upmu \mathrm{g}\ {\mathrm{g}}^{-1}. $$

Normalization of the data was applied to the assessment of anomalous metal contribution using Al and Ti as proxies. The following formula was used for enrichment factor calculation:

$$ \mathrm{EF}=\left(\mathrm{A}/\mathrm{An}\right)/\left(\mathrm{B}/\mathrm{Bn}\right) $$
A:

The content of the element in the river sample

An:

The content of the normalized element in the river sample

B:

The content of the element in the background samples

Bn:

The content of the normalized element in the background samples

Local background values were calculated for each element as a median content for the dominant rocks within the catchment area. The background values from the Proterozoic rocks and the Neogene sediments were used as background levels to calculate EF for samples R001–R003 and R004–R020.

Results and discussion

Lithogenic origin

The River Bílina flows down from the Krušné Hory Mts. to territories known as the Most Basin and the České Středohoří Mts. The geological background (Fig. 1) consists of the Proterozoic Krušné Hory Crystalline Complex (Vilímek and Raška 2016), Variscan volcanic rocks and granitoids (Cajz et al. 1999), Cretaceous marine sediments (Caracciolo et al. 2011), Neogene sediments (Pešek and Sivek 2016), and Quaternary loess and loess loams.

The reciprocal ratio of Al2O3, SiO2, and TiO2 allows distinguishing the mutual relations between the various rocks in the area (Fig. 2a, b). The Proterozoic crystalline and Neogene volcanic rocks present a primary source of material for Neogene and Quaternary sequences. Thus, association of studied elements should be comparable with them and a genetic relationship is expected (Bouška and Pešek 1999; Rieder et al. 2007). This is visible for values related to Proterozoic crystalline, Quaternary loess, and some Neogene clays (Fig. 2a).

Fig. 2
figure 2

a SiO2-Al2O3 ratio (wt.%) in different types of rocky outcrops in the region. b Al2O3-TiO2 ratio (wt.%) in main types of rocky outcrops in the region

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Arsenic

The arsenic (As) content ranged from 21 to 969 μg g−1 in stream sediments (Fig. 3, Table 1). We found a lower As content in fluvial samples R001–R003 originating from the upper reach. Proterozoic crystalline is outcropping in the upper reach of the River Bílina catchment area. The amount of As significantly increased after the river entered into the area of open-cast coal mining, i.e., from sample R004 onwards. A continuous rapid decrease in As levels was observed down the stream (sample R004 to sample R019, Fig. 3). Significant drops of As values—displayed for samples R006, R011, and R014 in Fig. 3, could be interpreted by the arsenic content becoming diluted due to the tributaries (Fig. 1) that flow from areas not influenced by mining. Samples R018 and R019 were collected in the lowermost reach of the catchment area—one built up by Neogene volcanics, which explains the lowest As content for these samples.

Fig. 3
figure 3

The content of arsenic (μg g−1) in the geological matrices related to the geological background and samples from River Bílina sediments (R001–R020). The individual river stretches indicated in Fig. 1 are indicated by the line. Captions: Ptz, Proterozoic; Ngv, Neogene volcanics; Ngs, Neogene sediments; Ngc, Neogene coal; Ql, Quaternary loess

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Basic volcanic rocks and Neogene sediments contain low arsenic amounts while a highly variable content of the element was repeatedly documented for coal and Proterozoic rocks (Bouška and Pešek 1999; Pešek et al. 2005). This suggests that increased arsenic amounts probably originate from Neogene coal. It was found that no parent rock contains so much arsenic as the most polluted River Bílina sediments (Fig. 3). This can be caused by the high mobility of the element and its presence on surfaces exposed to rain. Considerable amounts of sorption can be assumed to be underway on hydrated Fe and Al oxides.

Copper

A low copper content was detected in Quaternary loess (median 14 μg g−1) and Proterozoic rocks (Table 2). A higher copper content was detected in samples from Neogene volcanics and coal (Fig. 4). Contrary to the above, a lower Cu content was found in samples R001–R008 that represent the area of the upper and the middle reach of the watercourse. The Cu values started increasing from sample R009 to fluctuate downstream. The highest values were detected in samples R009, R010, and R013 that were collected from the wider surroundings of the towns of Most and Bílina. This part of the region is surrounded by open-cast mines and defunct tailings (Fig. 4).

Table 2 The content of study elements (μg g−1) in stream sediments of the Bílina River
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Fig. 4
figure 4

The content of copper (μg g−1) in the geological matrices related to the geological background and samples from River Bílina sediments (R001–R020). The individual river stretches indicated in Fig. 1 are indicated by the line. Captions: Ptz, Proterozoic; Ngv, Neogene volcanics; Ngs, Neogene sediments; Ngc, Neogene coal; Ql, Quaternary loess

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The copper content ranged from 18 to 203 μg g−1 in the studied stream sediments and exceeded median values in parent rocks. The remarkable decrease in the Cu content as measured for samples R011, R012, R014, and R018 was influenced by the geological background consisting of Cretaceous sediments and Neogene volcanics. In contrast to the natural content of Cu (1–30 μg g−1) in the rocks located in the Krušné Hory Mts., we assume that the element accompanies fine-grained Neogene deposits and coal in increased amounts (Fig. 4).

Zinc

The results show that the Zn content in coal is lower than in stream sediments neighboring open-cast mines (Fig. 5). The median of the zinc content was 265 μg g−1, 63 μg g−1, and 1006 μg g−1 for Quaternary loess, Neogene coal, and Neogene volcanic rocks, respectively. In the investigated fluvial material, a highly variable content of Zn was observed (Fig. 5). The lowest values were found in the samples from upper reaches (R001–R004). The zinc content significantly varied in the samples from the middle and lower reaches. The highest values were noticed in sediments in the wider surrounding of the towns of Bílina and Teplice.

Fig 5
figure 5

The zinc content (μg g−1) in the geological matrices related to the geological background and samples from River Bílina sediments (R001–R020). The individual river stretches indicated in Fig. 1 are indicated by the line. Captions: Ptz, Proterozoic; Ngv, Neogene volcanics; Ngs, Neogene sediments; Ngc, Neogene coal; Ql, Quaternary loess

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To conclude, we assume weathering of volcanic rocks and Neogene sandstones to be the main process affecting the zinc content in the sediments (Fig. 5). Since the EF Zn/Al Zn/Ti (Table 3) is rising from point R007, the input of zinc into the river system can be expected to be one of anthropogenic origin. Here, dissolved zinc is usually the issue; it comes from frequently used objects such as railings, barriers, and parts of bridges that undergo treatment using galvanizing or zinc-coating technology.

Table 3 Enrichment factor (EF) values for the content of As, V, Cr, Ni, Cu, Zn, and Pb in River Bílina stream sediments relative to Al and Ti as standardization elements
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Lead

Lead (Pb) fluctuated in stream sediments throughout the flow of the River Bílina with concentrations (27–103 μg g−1) being consistent with those found in the natural background of surrounding units (Fig. 6), i.e., Quaternary loess, Neogene volcanics and sediments and coal mass. The rare instances of extreme values in samples R017 (430 μg g−1) and R020 (368 μg g−1) revealed an unknown anthropogenic input. The EF Pb/Al and Pb/Ti (Table 3) identified in R004 increases downstream. It is possible to deduce that the extensive industrial activity in the region is responsible for the systematic anthropogenic input of lead into the river system; emission from industrial activity—except coal mining and processing—is typically considered an anthropogenic source.

Fig 6
figure 6

The lead content (μg g−1) in the geological matrices related to the geological background and samples from River Bílina sediments(R001–R020). The individual river stretches indicated in Fig. 1 are indicated by the line. Captions: Ptz, Proterozoic; Ngv, Neogene volcanics; Ngs, Neogene sediments; Ngc, Neogene coal; Ql, Quaternary loess

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Chromium

The content of chromium (Cr) in stream sediments (Fig. 7) highly increased in the upper reach (R002, 650 μg g−1). As the river enters an artificial channel, amounts of chromium gradually fall to 116 μg g−1 (R007). In the subsequent river stretch, high Cr levels alternating low levels was an apparent feature with no direct link to the surrounding environment. After the river channel enters industrial premises, the content increased up to the maximum at R018 and R019 where the values reached concentrations 623 and 602 μg g−1, respectively. Extreme Cr content was found in Neogene volcanics and sediments (Fig. 7, Table 2).

Fig 7
figure 7

The chromium content (μg g−1) in the geological matrices related to the geological background and samples from River Bílina sediments (R001–R020). The individual river stretches indicated in Fig. 1. are indicated by the line. Captions: Ptz, Proterozoic; Ngv, Neogene volcanics; Ngs, Neogene sediments; Ngc, Neogene coal; Ql, Quaternary loess

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Nickel

The nickel (Ni) content was found to be highest in the upper reach (Fig. 8, R002, 358 μg g−1). As the river enters the artificial channel, nickel decreases gradually to as little as 57 μg g−1 at point R006. In the following reach downstream, high and low levels take turn without any apparent relation to the environment. The overall pattern for Ni is the same as for Cr, indicating the same source. The median of Ni in the geological underlay ranged from 5 to 156 μg g−1 (Table 2). Any extreme amounts were established in Neogene sediments when they significantly exceeded the concentrations measured in stream sediments (Fig. 8).

Fig 8
figure 8

The nickel content (μg g−1) in the geological matrices related to the geological background and samples from River Bílina sediments (R001–R020). The individual river stretches indicated in Fig. 1 are indicated by the line. Captions: Ptz, Proterozoic; Ngv, Neogene volcanics; Ngs, Neogene sediments; Ngc, Neogene coal; Ql, Quaternary loess

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Vanadium

The vanadium content was very low in sediments of the upstream part of the river (Fig. 9) between points R001 and R007 (61–182 μg g−1). Subsequently, there is a sharp increase in the content (982 μg g−1) at point R009, followed by high values alternating low values without any obvious trend. The highest vanadium content was found in coal with a median of 311 μg g−1. In Neogene volcanic rocks, the vanadium content was below the limit of determination and in the Quaternary loess and Neogene sediments, it does not exceed 227 μg g−1 (Table 2).

Fig 9
figure 9

The vanadium content (μg g−1) in the geological matrices related to the geological background and samples from River Bílina sediments (R001–R020). The individual river stretches indicated in Fig. 1 are indicated by the line. Captions: Ptz, Proterozoic; Ngv, Neogene volcanics; Ngs, Neogene sediments; Ngc, Neogene coal; Ql, Quaternary loess

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Natural background levels

Ti and Al were applied as normalization elements for River Bílina sediments due to the following reasons: (1) the natural content of both elements tends to be stable; (2) Al and Ti oxides reflect the genetic ratio in the lithogenic basement of the study area; (3) Al and Ti are associated with fine solid materials and are resistant to weathering.

EF values were interpreted as suggested by Sakan et al. (2014), where EF < 1 indicates no enrichment, < 3 is minor enrichment;, 3–5 is moderate enrichment, 5–10 is moderately severe enrichment, 10–25 is severe enrichment, 25–50 is very severe enrichment, and > 50 is extreme enrichment. The contamination degrees are indicated by the line in Fig. 10.

Fig. 10
figure 10

Enrichment factor (EF) values calculated for the content of As, Pb, V, Cu, Ni, Cr, and Zn in River Bílina stream sediments relative to Al and Ti as standardization elements

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Enrichment factor values of the studied elements in stream sediments using the average values of EFAl varied from extreme severe to minor enrichment and decreased in the following order: As > Pb > V > Cu > Ni > Cr > Zn (Fig. 10). The same order of elements was obtained when Ti was used as a normalization factor for EFTi, i.e., As > Pb > V > Cu > Ni = Cr > Zn. Since the river reach between the towns of Most and Bílina passes in the close vicinity of mine tailings and coal-storing premises and is therefore directly affected by mining (Kaiglová et al. 2015a), it can be assumed that coal and coal-bearing sediments present an important input into the River Bílina system and coal can be expected to be a source of element enrichment. The increased amounts of Pb and As seemed not to be consistent with the natural background conditions. Therefore, it is possible to assume their anthropogenic origin, which supports the conclusions reached by Kohušová et al. (2011).

Conclusion

The present study of chemical composition of River Bílina sediments and the surrounding rocks from geological units demonstrated the importance of a comprehensive approach in areas heavily affected by anthropogenic activity. Open-cast brown-coal mines and the related activities have changed the overall character of the landscape not only in terms of morphology but also from the aspect of the quantity of the material entering into the river setting. It involves defunct tailings that contain Neogene sediments, i.e., gangue and a small proportion of coal mass, and are subject to intense weathering on contact with precipitation. As a result, the Neogene sediments are supposed to be the main natural source for assessing environmental pollution in the area. For these sediments, median values of evaluated elements are 2.5 μg g−1 for As, 25.5 μg g−1 for Cu, 199.0 μg g−1 for Zn, 14.0 μg g−1 for Pb, 94.5 μg g−1 for Cr, 44.0 μg g−1 for Ni, and 53.5 μg g−1 for V. Due to high percentage of organic matter and reduction conditions, there is enhanced content of the elements in the fluvial environment of the River Bílina.

Since in industrial processes other than coal processing activity As is not present at any larger scale, Ng sediments can be considered a primary source of increased levels of this element in river sediments. Industrial sources of the study elements unrelated to coal mining and processing can be assumed only for Cu, Zn, and Pb.