Hydrological characteristics and particle size distribution (PSD)
The increase of the SPM concentrations in the Sava River showed a good agreement with the higher water discharge (Fig. 2). The PSD of TIMS collected samples was not significantly influenced by either the discharge or the SPM concentration (Table 5). The SPM of samples 1 to 3 (August 2014, October 2014, and February 2015) predominantly contained silt-sized particles (> 75%) (Fig. 3, Table 2). In the same periods, the SPM concentrations and water discharge differed significantly (Fig. 2). The SPM collected in May 2015 (sample 4) contained the coarsest material (Fig. 3, Table 2), which was at first attributed to the positioning of TIMS. The low discharge coincided with low SPM concentration (Fig. 2), i.e., with low input of fine-grained material. In addition, low discharge brought TIMS closer to the riverbed where coarser material could have entered the sampler. However, the PSD results after H2O2 treatment disproved that conclusion (Fig. 3). The decrease of mean size and median after the treatment was most prominent in sample 4. Therefore, the cause for the observed PSD of sample 4 was believed to be due to flocculation processes inside TIMS promoted by high organic matter content (Table 2).
The dominance of composite particles in the riverine SPM, resulting from flocculation processes, was previously established in various catchments (Droppo and Ongley 1994; Droppo et al. 1997, 2005; Woodward and Walling 2007). According to the results presented here (Figs. 3 and 4; Table 2), the same processes can be confirmed in the Sava River. In addition, the flocculation was probably enhanced in TIMS due to SPM accumulation in a confined space with limited or no water movement.
Mineral composition and watershed lithology
The source of mineral components determined in the SPM samples can be traced back to the bedrock composition of the Sava River watershed and its tributaries. Quartz, muscovite/illite, feldspar, and kaolinite, found in all samples, probably derived from weathering of siliciclastic rocks along the Sava River watershed. These rocks were dominantly present in the catchments of the Savinja River and its tributaries Voglajna and Hudinja in Slovenia (Frančišković-Bilinski 2008; Szramek et al. 2011). Dolinar and Vrecl-Kojc (2010) specified muscovite/illite as a major mineral of the riverine suspended load upstream of our sampling location. A positive correlation established between muscovite/illite and the clay fraction indicates its presence as the main constituent of the fine-grained portion of the collected SPM. The source of kaolinite can be traced to the soils deriving from Pleistocene fluvioglacial terraces, which comprise 15% of siliciclastic components (Vidic et al. 1991). Indeed, Štern and Förstner (1976) observed that kaolinite content in sediments increased downstream of Sava–Savinja confluence.
Carbonates, more specifically calcite and dolomite, also comprised a significant portion of the SPM samples (Table 3). The dolomite bedrock in the spring area of the Sava River is identified as the most probable source of dolomite. Intense rainfall periods, in summer and autumn 2014, were accompanied by high dolomite content in the suspended sediment load confirming the detrital character of this mineral.
The origin of calcite in the Sava River SPM was harder to determine. Taking into account that limestone bedrock occurs in many areas of the Sava River watershed, portion of calcite found in riverine suspended load is undeniably also detrital in origin. However, high content of calcite (53.2%) found in sample 4 (May 2015) arose some questions.
Calcite positively correlated with sand fraction content in native samples which suggests that high content of calcite found in the SPM collected in May 2015 could be due to the detrital input of coarser material. This hypothesis was unfortunately proven null. The H2O2 treatment of sample 4 significantly lowered the sand content in favor of clay and silt fractions indicating flocculation processes as the primary reason for the apparent coarser grain size (Table 2). It is possible that particularly low discharge in May 2015 (Fig. 2) influenced the mineral composition of sample 4. The reduced delivery of alumosilicates and quartz, commonly originating from the Slovenian part of the Sava River watershed, could account for the atypically high amount of calcite in sample 4. Low discharge and low SPM concentrations were also observed in February 2015 (Fig. 2), but there was a surge in water discharge and suspended sediment load at the beginning of this sampling period. If most of the material was collected during this short event the similarities in mineralogical composition of the sample 3 with samples 1 and 2 are understandable.
The high content of calcite could also be attributed to the authigenic mineral precipitation processes induced by biological activity in the river. The evidence of algal control of calcite nucleation in the freshwater environment was found by Stabel (1986), the occurrence of biogenic calcite precipitation in lake sediments was also observed by Ivanić et al. (2017). Ollivier et al. (2011) assumed biogenic calcite precipitation processes in the River Rhône. The high TOC content (Table 3) and abundant algal remains (Fig. 4) in the riverine suspended load collected in May 2015 favor the possibility that biological activity induced the precipitation of calcite in the Sava River, but no definite conclusion can yet be reached. The similar problem concerning the detrital or biogenic origin of calcite in the SPM was also described in the Seine River by Roy et al. (1999). Additional sampling campaigns, targeting low river discharge and high primary production, are planned in the effort to determine the cause for higher calcite content in the Sava River suspended load.
Occasional occurrence of other minerals, i.e., vermiculite and chlorite, can be attributed to physical and chemical weathering processes (Setti et al. 2014). Chlorite was found in sample 2 (October 2014), in the period with the highest discharge, which suggests that the occurrence of this mineral is a consequence of the increased material input from the Sava River upper watershed (Milačič et al. 2017).
Vermiculite was present in traces only in sample 3 (February 2015). Its origin could be due to the chemical weathering of muscovite/illite during the pedogenesis along the Sava River banks (Vidic et al. 1991).
Trace element geochemistry
The major and trace elements geochemistry in the Sava River was examined based on their concentrations in the time-integrated SPM samples. The interrelations among elements were determined and used to assess their mode of transport in the Sava River.
A positive correlation between major elements Al, Fe, K, and to some extent Na (Table 4 and Table S2 in the Electronic supplementary material) indicates their presence in the Sava River SPM as main constituents of the suspended load mineral component. These elements also predominantly correlate with clay and silt content (Table 5) indicating their presence in detrital aluminosilicate particles. Good correlation between mineral-forming and trace elements denoted mineral surfaces as preferred means of transport for trace elements.
Concentrations of Al and associated trace elements in different sampling periods did not correspond to the SPM concentrations or river discharge, but rather to the PSD and mineral composition of samples (Table 5). Such behavior emphasizes the input of the different type of material during high/low discharge periods. Milačič et al. (2017) have recently shown that during the high river discharge, as occurred in September 2014, contaminated river bank sediments and soils are washed into the Sava River, resulting in elevated levels of potentially toxic trace elements (PTEs) in the riverbed sediments. Ogrinc and Ščančar (2013) investigated the SPM in the Sava River in the spring and autumn 2006 at 33 locations along the river and its tributaries. The differences they found between two sampling points were at times greater than the differences between seasons, indicating that the metal concentrations could not be entirely attributed to the discharge regime and the river SPM concentrations.
The relationship between metal concentrations and water discharge is rather complex, but different materials are indeed brought into the river under different flow regimes. The significantly different mineral composition of sample 4 (May 2015) is a good example. Low river discharge in May 2015 is also reflected in the low SPM concentrations (Fig. 2) and the elemental composition of the collected sample (Table S1, Electronic supplementary material). Whereas the origin of the high content of calcite in the sample 4 is somewhat undetermined, its influence on the elemental composition is quite clear. Diluting effect of calcite on the elements of detrital origin and trace elements associated with them is revealed through the negative correlation between calcite and the majority of analyzed elements (Table 5).
In the study conducted by Vidmar et al. (2016), the concentrations of metals in the SPM were lower than in the Sava River sediments and predominantly lower than in any of the TIMS collected samples described in this study (Fig. 5). The discrepancy probably arises from different SPM sampling methods. The SPM investigated in this work was collected during an extended period, and possible variances in daily transported SPM composition may have been evened out during settling time inside TIMS. The SPM sampled by Vidmar et al. (2016) was a 1-day sample taken during the Sava River high discharge period (approximately 690 m3 s−1). The SPM content is related to the water discharge, but inversely related to metal concentrations in particulate fraction, i.e., the increase of the water discharge is accompanied by an increment of detrital, metal-poor fraction of the SPM (Cobelo-Garcia and Prego 2004; Ollivier et al. 2011). Thus, the low metal concentrations in the Sava River SPM found by Vidmar et al. (2016) may be explained by the timing of sampling.
On the other hand, the metal concentrations in the riverbed sediment, found by Vidmar et al. (2016), corresponded rather well to samples 1–3 (Fig. 5). Horowitz and Stephens (2008) and Smith and Owens (2014) pointed out that the bed sediment fraction < 63 μm is a suitable representative of suspended sediment load which was confirmed by the results of this study.
Enrichment factor analysis
The enrichment factor (EF) analysis is often used in studies concerning the geochemical behavior of elements in rivers, either to determine anomalous metal concentrations of natural origin or to assess the anthropogenic influence on metals in suspended solids (Gaillardet et al. 1999; Meybeck et al. 2007; Viers et al. 2009; Chen et al. 2014). The EF is calculated by double normalization of the element concentration, with the average composition of the upper continental crust (UCC) used as the reference background value (Taylor and McLennan 1985; Hu and Gao 2008):
$$ \mathrm{EF}\left(\mathrm{X}\right)={\left(\mathrm{X}/\mathrm{Al}\right)}_{\mathrm{sample}}/{\left(\mathrm{X}/\mathrm{Al}\right)}_{\mathrm{UCC}}, $$
where EF(X) = enrichment factor of selected element, and X/Al = selected element concentration normalized to Al concentration. The basis for normalization to the UCC chemical composition is the premise that large river catchments incorporate different lithologies and as such can be approximated by the upper continental crust constitution (Gaillardet et al. 1999). Hence, this approach was chosen to analyze the geochemical data obtained in this study, more precisely to determine the origin and processes affecting trace elements concentrations in the Sava River SPM.
The EFs for 27 elements are presented in Fig. 6, arranged along the x-axis as a function of the increasing EF value. In literature, several criteria were suggested to assess the sources of trace elements in the SPM and sediments based on their EF values. The consensus is that calculated ratios close to 1 indicate that studied element concentration can be explained by its terrigenous origin. In the riverine SPM, the EF values between 0.5 and 1.5 suggest the crustal origin of studied elements, and only EF > 1.5 indicates that a portion of trace element load is probably derived from alternate sources (Zhang and Liu 2002).
In the Sava River SPM, the EF values of Na, Sr, K, Ba, Be, Rb, Tl, U, and Fe indicate their solely terrigenous origin. Na, Sr, K, and Ba showed EF < 1, with Na even slightly depleted due to its soluble character. The enrichment of these elements is commonly reported in the dissolved phase (Viers et al. 2009; Ollivier et al. 2011; Chen et al. 2014). Be, Rb, Tl, U, and Fe showed EF close to 1 with little variations.
In the Sava River suspended load, even Ti, Mn, Sn, Co, Mo, V, Cs, Mg, and Li can be considered predominantly geogenic in origin. Specifically, Sutherland (2000) suggested five contamination categories based on the EF values:
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EF < 2 minimal enrichment suggesting no or minimal pollution,
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EF = 2–5 moderate enrichment and pollution,
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EF = 5–20 significant enrichment and pollution,
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EF = 20–40 very high enrichment and pollution,
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EF > 40 enrichment indicating an extreme pollution signal.
In the analyzed SPM samples, Ti, Mo, Sn, and Co all showed EF < 2, except the EF values for Mo, Sn, and Co in sample 4 (May 2015) whose notably different mineralogical and geochemical composition was already discussed in previous sections.
The EFs between 2 and 3 were obtained for Mn, V, Cs, Mg, and Li, but their enrichment in the SPM could be of natural origin. Indeed, the abundance of Li in clay minerals was observed in the Sava River alluvial sediments, while V originates from Würm terraces (Šajn and Gosar 2014). It is probable that concentrations of other elements from this group also reflect regional lithology. Cu also displayed the EF mostly below 3 indicating only moderate enrichment of the SPM with this element despite its predominately anthropogenic origin.
The highest EF values (between 3 and 14) were obtained for Ni, As, Cr, Pb, Bi, Cd, Zn, and Sb, indicating pollution of the Sava River SPM with these elements during all sampling periods. Several authors (Štern and Förstner 1976; Kotnik et al. 2003; Frančišković-Bilinski 2008) detected the increased content of Cd, Cr, Cu, Ni, Pb, and Zn in sediment from the Slovenian part of the Sava River and associated the observed contamination with the metallurgical and mining activities in Slovenia. The anthropogenic input of Bi, Pb, Sb, and Zn into the environment could be the result of mining and smelting of galena and sphalerite, primary carriers of the mentioned trace metals. Pollution with Ni and Cr may also be related to the metal industry in Jesenice (Vidmar et al. 2016). Oreščanin et al. (2004) deduced that anthropogenic input of Cu, Zn, and Pb arises from fertilizing activities occurring in watersheds of the Sava River tributaries, the Krka and the Krapina Rivers.
It was recently shown (Milačič et al. 2017) that under high flood conditions of the Sava River, sediment pollution with ecotoxic elements may even pose an ecological risk. Therefore, the EF analysis of geochemical data collected in this study revealed that current environmental conditions of the Sava River are in concordance with previously reported moderate anthropogenic pressures on this aquatic ecosystem.