The Diversity of Geochemical and Ecotoxicological Indices of Alluvial Deposits Reflects the Pattern of Landforms: The Case of the Vistula River Valley in the Małopolski Gorge (Poland)

Abstract

This study aimed to (1) determine the environmental risk resulting from the contamination of river valley sediments with trace elements of anthropogenic origin, (2) assess the relationship between this environmental risk and the geomorphology of the valley, and (3) identify areas that may become a source of contamination. This research was conducted in the Vistula River Valley between Sulejów and Kazimierz Dolny (Poland). Geochemical and ecotoxicological indices (for fraction < 1 mm) were analyzed (EF, I_geo, PI, CF, C_d, PI_Sum, PI_Avg, PI_Nemerow, PLI, ER, RI). Geomorphological mapping, supported by DEM and remote sensing analysis, was performed. High concentrations of trace elements in sediments, as determined by the ICP-OES and ICP-MS methods throughout the study area, indicate generally high environmental degradation and a moderate-to-considerable ecological risk. Contamination differs in the sediments of individual landforms: the highest levels are found in the sediments of the contemporary floodplain and oxbow lakes, while the lowest are observed in the Pleistocene terrace sediments. Only high concentrations of As, Pb, Zn, and Cd are of anthropogenic origin. Their source is probably the mining area of Upper Silesia (As, Pb, Zn) and agricultural activity (Cd). The differences in the values of geochemical indices in individual landforms confirm the influence of fluvial processes on the distribution of trace elements.

1. Introduction

Today, heavy metal contamination of sediments and waters represents a major environmental problem. Its significance is justified by its long-term impact on, among others, water and soil quality, aquatic ecosystems, and human health [1,2]. These metals originate from both natural (rock weathering, rock and soil erosion) and anthropogenic sources [3]. The high content of trace elements in river sediments can have multiple causes. Most often, it is a result of mining activities in the catchment area [2,4], industrial concentration [5,6], intensive agriculture [7,8], urban agglomerations [7], or the remobilization of older, heavily contaminated sediments [1,9]. Soil and water treatment technologies have become a prominent focus of contemporary scientific research [10,11].

In aquatic environments, sediments are widely regarded as a reliable indicator of contamination by trace elements [12]. It is assumed that 99% of trace elements are accumulated during the hydrological cycle just in sediments, which serve as their carrier at the time of transport and as the main sink [13,14,15]. The ability of sediment to bind heavy metals depends on its composition, particularly on the contents of clay fraction, organic matter, carbonates, iron oxides and hydroxides [16,17]. The content of these components, in turn, is highly dependent on the sedimentary conditions within the river channel or the floodplain, which are determined by the geomorphological conditions of the flow, the nature of the catchment area, including its geology, and the hydrological regime of the river [9,18,19].

The accumulation of heavy metals in channel and flood sediments poses a threat to water and other environmental elements. Fluctuations in the groundwater table and the associated changes in the physico-chemical conditions, as well as erosional processes, can trigger the mobilization of contaminated sediment [20,21]. The sediment is subsequently deposited elsewhere in the channel or in the floodplain area during the flood [8,22]. In environmental analyses, it is therefore important to consider the places where heavy metals are most likely to be remobilized and where there is the greatest ecological risk. Among the methods of assessing the degree of sediment contamination by trace elements is the determination of geochemical indices relating to the geochemical background or the Clarke of the elements under study. They make it possible to determine the origin of the contamination and its harmfulness [14,23]. The indices calculated for individual elements allow the precise identification of the pollution outbreak and type, while the integrated indices provide a general indication of the environmental hazard level [13,15,24]. Another method of assessing the contamination level is to calculate ecotoxicological indices determining the overall contamination of sediments and the risk it poses to living organisms [14,25]. Heavy metals and trace elements in river valleys pose significant ecological risks due to their toxicity, persistence, and bioaccumulation in sediments and aquatic systems [26,27,28]. To obtain a reliable environmental risk assessment, many authors advocate the simultaneous use of multiple indicators to analyze the contamination degree in conjunction with its biological effects [29]. However, for the ecological risk assessment of heavy metals, it is recommended to conduct empirical studies of their biological impact in the studied case [14,30].

To the best of our knowledge, studies using geochemical indices in river valleys are usually limited to channel deposits and the immediate vicinity of the channel. Single studies in which geomorphological landforms are considered [31,32] indicate a variation in the values of the indices between delineated geomorphological landforms. This supports the validity of the approach involving both geochemical and geomorphological aspects.

This study aimed to assess the environmental risk posed by the contamination of deposits forming various landforms in the Vistula River Valley at the Małopolska Gorge with heavy metals of anthropogenic origin (Cu, Pb, Zn, Ni, Co, As, Sr, Cd, Cr, Ba). Selected geochemical and ecotoxicological indices were used in this assessment. This study also aimed to identify areas that, due to their origin, may become secondary outbreaks of pollution by these elements.

2. Study Area

The analyzed section of the Vistula Valley is located between Sulejów and Zakrzów, in central Poland. Most of the section represents the Małopolska Gorge of the Vistula River (Figure 1), incised into Upper Cretaceous rocks—Campanian limestones, bedrocks (pillars), gaizes, marls, Maastrichtian chalks, glauconitic marls, sandy marls, and bedrocks (pillars) [33,34,35]. The differentiation of susceptibility to erosion resulting from the differentiation of lithological (facies) bedrock of alluvia influences the course and morphology of the Vistula River Valley in the studied cross-section. In the zones of occurrence of soft marl and chalk, the Vistula Valley is wider (northern part of the area). In the southern part of the studied area, the valley is narrower, because the substrate contains harder limestones and rocks with greater resistance to erosion [36,37,38].

Throughout the studied section, the Vistula River exhibits a draining character [39]. The hydrological regime of the Vistula can be described as typical for a braided river. It is characterized by the occurrence of mainly low and medium flow rates and sporadic high floods during the hydrological year, with a flow several times greater than the average flow [40].

The analyzed section can also be considered “free mature” [41]. This means that in the late Pleistocene and Holocene, the river responded freely to changes in the hydrological regime, adapting the dynamics of its erosional and depositional processes. This resulted in the formation of Pleistocene terraces and a bipartite floodplain. The terraces developed at the end of the Pleistocene, after the retreat of ice sheets, under cool climate conditions and the presence of poor plant communities in the drainage basin, as a result of the activity of a sediment-overloaded braided river [41]. In the Holocene, due to climate warming and the resulting development of a dense vegetation cover, the infiltration rate increased, while the surface runoff and the amount of material supplied to the river channel decreased. This caused a change in the developmental type of the Vistula riverbed from braided to meandering, resulting in the formation of a meandering river floodplain. In the Neo-Holocene, changes in land use in the catchment area took place. They gave rise to a decrease in the retention in the catchment area, an increase in surface runoff, and an increase in sediment supply to the river. This resulted in a change in the hydrological regime, manifested by an increase in the flow rate during floods and a lowering of the low water levels throughout most of the year. In consequence, a process referred to as “re-wilding of river” took place, resulting in a change of channel development type from meandering to braided [41,42]. Under the conditions of a braided river, the floodplain formed by the meandering Vistula was partially transformed during floods [40]. A meander plain, reworked by flows of braided river, was formed. Currently, the braided river, overloaded with material, has formed the most recent morphological landform—the contemporary floodplain stretching alongside the riverbed (Figure 2).

The study area—the Vistula River Valley in the investigated section—was under strong anthropogenic pressure. Contaminants associated with mining activities in Upper Silesia were reaching this region [4]. Moreover, these have been areas of concentrated settlement and intensive farming for many centuries.

3. Methods

This study involved the analysis of the geomorphological setting of the Vistula River Valley. Geological mapping, supported by the analysis of the digital terrain model and aerial photographs, was performed to determine the lithological composition of the alluvial sediments and to identify the landforms. During the field investigations, 192 sediment samples were collected for laboratory analysis. The samples were taken from a depth of 0.4–0.6 m. The lithological analysis included the determination of granulometric composition (areometric method), content of the organic matter (loss-of-ignition method at 450 °C), content of carbonates (Scheibler method), and iron content (iodometric method) [43]. For the determination of heavy metals in the sediments, the samples were washed through a plastic sieve with a mesh diameter of 1.0 mm [19,44]. Then air-dried samples were dissolved in a solution of H₂O-HF-HClO₄-HNO₃ (ratio 2:2:1:1) [19]. Concentrations of trace elements (Cu, Pb, Zn, Ni, Co, As, Sr, Cd, Cr, Ba, Fe, Al) were determined by the ICP-OES and ICP-MS method at Acme Labs (Bureau Veritas Commodities Canada Ltd., Vancouver, BC, Canada). The determination error was less than 5%.

The results have been archived in a GIS database (using ArcGis 10.6).

To determine the origin of trace elements, the degree of contamination of the studied section of the Vistula River Valley, and the environmental hazards in this area, several geochemical indices were calculated (EF, I_geo, PI, CF, C_d, PI_Sum, PI_Avg, PI_Nemerow, PLI, ER, RI) based on the results of the trace elements (for the fraction < 1 mm). All the data necessary for calculating the indices have been placed in the repository [45]. To assess the level of differentiation in the sediments of the individual landforms, an ANOVA analysis was used. The analyses were performed in Statistica 13.

The enrichment Factor (EF) is used for assessing the influence of anthropogenic contamination on the concentrations of individual heavy metals [46,47]. It was calculated in two ways, using Fe and Al as the reference elements. The following formula was used to calculate its value:

EF = (Ae × Bc)/(Ac × Be)

when

Ae—the content of the element in the sample, Ac—the Clarke of the element to be determined;

Be—the content of the reference element in the sample;

Bc—the Clarke of the reference element.

In this paper, the following assumptions are made based on Ergin et al. [48]:

EF < 2—depletion of mineral enrichment;

EF = 2–5—moderate enrichment;

EF = 5–20—significant enrichment;

EF = 20–40—very high enrichment;

EF > 40—extremely high enrichment.

The Geoaccumulation Index (I_geo) allows the assessment of contamination by comparing heavy metal contents in soils with the so-called “pre-industrial” concentrations. It is calculated using the formula [49,50]

$${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}={\log }_2{\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{1.5\cdot \mathrm{G}\mathrm{B}}}$$

when

C_e—the content of the element in the sample.

GB—geochemical background of the element. It was determined for the study area by Lis et al. [51].

1.5—natural variations in the content of a given element in the environment, with little anthropogenic impact.

For the assessment of this index, a 7-class classification of the Geoaccumulation Index was used [49,50] (

Table 1. Classification of the Geoaccumulation Index.

Class	Index Value	Sediment Quality
0	<0	Unpolluted
1	0–1	From unpolluted to moderately polluted
2	1–2	Moderately polluted
3	2–3	From moderately polluted to strongly polluted
4	3–4	Strongly polluted
5	4–5	From strongly polluted to extremely polluted
6	>5	Extremely polluted

).

The single pollution index (PI) is a single contamination index that relates to each of the elements analyzed. It was used to calculate cumulative indices, such as the Nemerow pollution index (PI_Nemerow) and the pollution load index (PLI). It is calculated according to the formula [24]

PI = C/GB

when

C—the content of the contaminant in the sample.

GB—geochemical background of the element. It was determined for the study area by Lis et al. [51].

It is assumed that

PI < 1—non polluted;

PI = 1–2—low level of pollution;

PI = 2–3—moderate level of pollution;

PI = 3–5—strong level of pollution;

PI > 5—very strong level of pollution.

The Contamination Factor (CF) was calculated to quantify the degree of contamination by the analyzed elements in the study area in relation to the geochemical background values for this area. It allows the area to be classified into the appropriate group depending on the multiplicity of exceedance of the geochemical background value. It is calculated using the formula [25]

CF = C_i/C_n

when

C_i—average content of toxic substance in samples taken from at least 5 sites;

C_n—pre-industrial reference level for the heavy metal (geochemical background or Clarke).

It is assumed that [25]

CF = 1—Low Contamination Factor;

CF = 1–3—Moderate Contamination Factor;

CF = 3–6—Considerable Contamination Factor;

CF > 6—Very High Contamination Factor.

For the study area, the Contamination Factor values were calculated by relating the measured trace element values to the geochemical background values determined by Lis et al. [51].

The degree of contamination (C_d) was calculated for a comprehensive assessment of the degree of contamination, as it is the sum of all contamination factors determined for the study area:

$${\mathrm{C}}_{\mathrm{d}}={\sum }_{\mathrm{i}=1}^{\mathrm{m}}{\mathrm{C}\mathrm{F}}^{\mathrm{i}}$$

when

CFⁱ—Contamination Factor for the particular substance;

m—the number of analyzed substances.

It is assumed that [25]

C_d < m—low degree of contamination;

C_d = m–2m—moderate degree of contamination;

C_d = 2m–4m—considerable degree of contamination;

C_d > 4m—very high degree of contamination.

Sum of pollution index (PI_Sum) was determined to find out the sum of all contaminants in the soil. It is calculated using the following formula [24]:

$${\mathrm{P}\mathrm{I}}_{\mathrm{s}\mathrm{u}\mathrm{m}}={\sum }_{\mathrm{i}=1}^{\mathrm{m}}{\mathrm{P}\mathrm{I}}_{\mathrm{i}}$$

when

PI_i—single pollution index for a particular substance;

m—the number of analyzed substances.

It is assumed that

PI_Sum < 1m—unpolluted, low level of pollution;

PI_Sum = 1m–3m—moderately polluted;

PI_Sum > 3m—heavily polluted.

The average pollution index (PI_Avg) is calculated using the following formula [24]:

$${\mathrm{P}\mathrm{I}}_{\mathrm{A}\mathrm{v}\mathrm{g}}={\displaystyle \frac{1}{\mathrm{m}}}{\sum }_{\mathrm{i}=1}^{\mathrm{m}}\left({\displaystyle \genfrac{}{}{0pt}{}{\mathrm{n}}{\mathrm{k}}}\right){\mathrm{P}\mathrm{I}}_{\mathrm{i}}$$

when:

PI_i—single pollution index for the particular substance;

m—the number of analyzed substances.

PI_Avg > 1 indicates low quality soil because of contamination.

Nemerow pollution index (PI_Nemerow) is calculated from the formula [24]:

$${\mathrm{R}\mathrm{I}}_{\mathrm{N}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{w}}\sqrt{{\displaystyle \frac{{\left(\frac{1}{\mathrm{m}}{\sum }_{\mathrm{i}=1}^{\mathrm{m}}{\mathrm{P}\mathrm{I}}_{\mathrm{i}}\right)}^2+{\mathrm{P}\mathrm{I}}_{\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{x}}^2}{2}}}$$

when:

PI_i—single pollution index for the substance ‘i’

PI_{i max—}maximum value of a single pollution index from all analyzed substances

m—the number of analyzed substances.

A classification based on Nemerow index [24] was used to assess the degree of risk from trace element contamination:

PI_Nemerow < 0.7—safety domain;

PI_Nemerow = 0.7–1.0—precaution domain;

PI_Nemerow = 1.0–2.0—slightly polluted domain;

PI_Nemerow = 2.0–3.0—moderately polluted domain;

PI_Nemerow > 3.0—seriously polluted domain.

To assess the overall toxic status of the area, the pollution load index (PLI) was calculated using the following formula [52]:

PLI = (PI₁ × PI₂ × PI₃ × … × PI_n)^1/n

when

PI—single pollution index for the particular substance;

n—the number of analyzed substances.

Values of PLI < 1 are assumed to indicate the absence of contamination, while a PLI > 1 indicates the presence of contamination [53]. However, a multi-level scale is also used to determine the level of contamination in the study area [52], also applied in this paper. According to it,

PLI < 1—unpolluted;

PLI = 1–2—unpolluted to moderately polluted;

PLI = 2–3—moderately polluted;

PLI = 3–4—moderately to highly polluted;

PLI = 4–5—highly polluted;

PLI > 5—very highly polluted.

The single index of ecological risk factor (E_r) developed by Hakanson [25] allowed a quantitative assessment of the potential ecological risk of the heavy metals studied:

E_r = T_{r i} × CF_i

when

T_{r i—}toxic-response factor for the particular substance;

CF_i—Contamination Factor.

To assess the ecological risk, a classification was applied [25]. According to it,

E_r < 40—low potential ecological risk;

E_r = 40–80—moderate potential ecological risk;

E_r = 80–160—considerable potential ecological risk;

E_r = 160–320—high potential ecological risk;

E_r > 320—very high potential ecological risk.

The potential ecological risk index (RI, or PERI according to other authors) was determined for the overall assessment of contamination by heavy metals under study. It is the sum of the individual ecological risk indices, calculated according to the following formula [25]:

$$\mathrm{R}\mathrm{I}={\sum }_{\mathrm{i}=1}^{\mathrm{m}}{\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$$

when

${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$—single index of ecological risk factor for the particular substance;

m—the number of analyzed substances.

It is assumed that [25]

RI < 150—low ecological risk;

RI = 150–300—moderate ecological risk;

RI = 300–600—considerate ecological risk;

RI > 600—very high ecological risk.

4. Results

4.1. Lithology

The following landforms are represented in the studied section of the Vistula River Valley: contemporary floodplain (cp), meander plain reworked by flows of the contemporary braided river (mpr), oxbow lakes (ol), and Upper Pleistocene terrace (pt) [19,41,54] (Figure 2).

The meander plain before embankment construction, and currently during their failures, is transformed by the flows of the contemporary braided Vistula River. This form occupies the largest part of the valley and is characterized by the greatest variability in the sediment composition (Figure 2). This part of floodplain (mpr) is composed of layers of silty and loamy deposits interbedded with sands.

Oxbow lakes are found within the transformed surface of the meander plain (mpr). These forms often show signs of changes due to contemporary flood flows. As a result, these forms are now filled with both organic material and mineral sediment.

The higher terrace formed during the Pleistocene that has been identified occurs as isolated small elevations and narrow sandy strips adjacent to the edge of the upland slope in the southern part of the studied area. In the northern part of the valley, however, the higher terrace forms an extensive sandy level with a width exceeding 2 km. The deposits of the Pleistocene river in other parts of the valley are covered by Holocene sediments that form the floodplain. The thickness of the sandy and sandy–gravel deposits of this level averages 12 m, according to Falkowska et al. [19].

The sediments in this area are characterized by a very high lithological diversity. The silt (0.002–0.05 mm content) varies from 0 to 80%, the clay (diameter of particles < 0.002 mm) content from 0 to 48%, and the organic matter content from 0.14 to almost 37%. The highest average contents of silt, clay, and organic matter were found in the sediments of oxbow lakes, while the lowest ones were in the Pleistocene terrace sediments. Carbonates occur locally. The highest contents were most often found in the contemporary floodplain sediments. The Fe/Al ratio in the study area ranges from 0.14 to 0.82. Its highest average values were found in the sediments of oxbow lakes, while the lowest ones were in the Pleistocene terrace sediments (

Table 2. The contents (%) of clay (<0.002 mm), silt (0.002–0.05 mm), sand (0.05–2 mm), organic matter, and carbonates, as well as the Fe/Al ratio in the study area.

	Sand (%)	Silt (%)	Clay (%)	Org (%)	CaCO3 (%)	Fe/Al (-)
	the entire study area
Maximum	100	80	48	36.90	5.90	0.82
Minimum	0	0	0	0.14	0.00	0.14
Mean	37	43	20	3.26	0.60	0.44
Median	34	48	18	2.47	0.00	0.44
Standard deviation	27	18	12	4.40	1.18	0.10
	contemporary floodplain (28 samples)
Maximum	100	56	31	7.77	5.90	0.63
Minimum	13	0	0	0.28	0.00	0.19
Mean	54	31	15	3.00	1.73	0.44
Median	52	31	17	2.92	1.20	0.45
Standard deviation	23	16	8	1.83	1.79	0.11
	meander plain reworked by flows of the braided river (130 samples)
Maximum	100	75	48	7.45	2.68	0.70
Minimum	0	0	0	0.14	0.00	0.22
Mean	36	44	20	2.56	0.22	0.44
Median	31	48	18	2.16	0.00	0.43
Standard deviation	26	18	11	1.42	0.48	0.08
	oxbow lakes (22 samples)
Maximum	40	80	48	36.90	3.75	0.82
Minimum	1	33	15	1.43	0.00	0.32
Mean	10	57	33	9.43	0.98	0.51
Median	7	56	34	5.70	0.00	0.49
Standard deviation	11	11	9	11.16	1.42	0.12
	Pleistocene terrace (12 samples)
Maximum	95	53	20	1.82	1.19	0.47
Minimum	33	5	0	0.61	0.00	0.14
Mean	68	23	10	1.19	0.40	0.29
Median	72	18	9	1.17	0.00	0.27
Standard deviation	23	17	7	0.51	0.56	0.09

).

4.2. Trace Elements Contents

The average concentrations of trace elements in the fraction <1 mm of the sediments in the study area are as follows: Ba > Sr > Zn > Cr > Ni > Pb > Cu > Co > As > Cd. The highest and lowest concentrations, the mean, the median, and the standard deviation are shown in

Table 3. The concentrations of trace elements analyzed [mg/kg].

Element	Cu	Pb	Zn	Ni	Co	As	Sr	Cd	Cr	Ba
The entire study area
Maximum	75.1	78.76	862.8	68.2	17.7	14.0	399	13.17	125	499
Minimum	1.5	4.20	4.1	1.2	0.5	0.1	19	0.01	1	56
Mean	17.7	17.94	77.8	29.4	8.5	6.2	85	0.53	54	325
Median	16.2	16.30	56.9	28.7	8.3	6.0	84	0.20	52	333
Standard deviation	10.7	10.09	101.9	14.8	3.8	2.9	37	1.57	29	86
Contemporary floodplain (28 samples)
Maximum	75.1	78.76	862.8	54.6	16.1	13.2	143	13.17	119	441
Minimum	2.0	4.60	9.4	2.5	1.0	1.0	24	0.08	4	126
Mean	23.7	28.12	199.9	28.5	8.6	6.5	85	2.38	54	315
Median	19.9	24.82	126.5	29.35	9.0	6.3	85	1.02	52	320
Standard deviation	18.3	19.50	219.1	15.1	4.0	3.3	28	3.56	32	86
meander plain reworked by flows of the braided river (130 samples)
Maximum	40.5	39.80	240.6	68.2	16.4	14.0	114	2.05	125	499
Minimum	3.0	5.20	7.7	3.5	1.5	0.1	32	0.01	5	149
Mean	16.3	16.29	57.5	30.0	8.7	6.1	82	0.22	56	335
Median	15.0	15.45	52.2	28.1	8.4	5.9	84	0.17	52	338
Standard deviation	7.6	5.32	29.9	12.9	3.2	2.5	16	0.22	25	72
oxbow lakes (22 samples)
Maximum	34	25.05	108.1	57.7	17.7	14	399	0.51	106	478
Minimum	8.3	8.80	17.1	7.8	1.8	2.7	55	0.08	5	56
Mean	24.7	20.47	76.7	40.2	10.3	8.1	127	0.27	72	348
Median	26.4	22.08	80.6	43.3	11.8	8.8	98	0.27	78	392
Standard deviation	6.3	4.29	25.6	13.9	4.7	2.9	82	0.10	27	114
Pleistocene terrace (12 samples)
Maximum	10.6	12.27	27.1	12.3	6.2	4.9	84	0.09	26	302
Minimum	1.5	4.20	4.1	1.2	0.5	1.1	19	0.01	1	114
Mean	5.5	7.35	14.1	5.6	2.6	2.6	42	0.04	12	195
Median	4.5	6.80	11.7	4.6	2.1	2.4	36	0.04	9	179
Standard deviation	3.1	2.56	7.8	3.8	1.7	1. 3	19	0.03	8	52

.

The highest average concentrations of Cu, Ni, Co, As, Sr, Cr, and Ba were found in the sediments of the oxbow lakes, while the lowest ones were observed in the Pleistocene terrace sediments. The highest average contents of Pb and Zn were found in the sediments of the contemporary floodplain and oxbow lakes, while the lowest ones were observed in the Pleistocene terrace sediments. The highest average concentration of Cd was measured in the contemporary floodplain sediments. In the sediments of the other landforms, the average concentrations of this metal are close to zero.

4.3. Geochemical Indices

The Enrichment Factor (EF) is used to assess the influence of anthropogenic contamination on the concentrations of individual heavy metals [46,47,48]. The analysis of the EF, using Fe as the reference element, indicates depletion of minerals for Sr, Cu, Co, Ni, and Cr and moderate enrichment for Ba, Zn, Pb, and Cd. The only element whose index value indicates significant enrichment is As. The analysis of the Cd distribution in the individual landforms shows that the EF values in the Pleistocene terrace sediments are lower compared to the other landforms, while in the contemporary floodplain sediments, they are higher. The contemporary floodplain sediments also display higher values for Pb and Zn compared to those of the other landforms.

When using Al as the reference element, the EF values are lower and indicate a depletion of minerals for most elements. Moderate enrichment was recorded for Pb and significant enrichment for As. In addition, in this case, an increase in the index value for Cd and Zn was observed in the contemporary floodplain sediments. Cd also had a higher enrichment value in the sediments of oxbow lakes.

The Geoaccumulation Index (I_geo) allows the assessment of contamination [49,50]. The analysis of the I_geo values shows that the majority of sediment samples were classified as unpolluted by Pb and Cd, from unpolluted to moderately polluted by Zn, As, and Cu, and from moderately polluted to strongly polluted by Co, Sr, Ni, Ba, and Cr.

When the relief landforms are considered, the analysis indicates that the I_geo values for Cu, Zn, Ni, Co, As, Sr, Cr, Ba were found in the Pleistocene terrace sediments. The values for Cu, Pb, Ni, and Cr are higher in the sediments filling oxbow lakes, while those for Pb, Zn, and Cd are higher in the sediments of the contemporary floodplain. On the contemporary floodplain, three points (P1, P2, P3) located near Piotrawin, Kazimierz Dolny, Janowiec, and downstream of the Zwolenka River mouth (Figure 1) are particularly outstanding. They indicate much higher contamination than in the rest of the contemporary floodplain area; in the case of Cd, the difference is three classes.

The single pollution index (PI) values [24] indicate nonpolluted (by Cd) to very strongly polluted (by Ni, Co, Sr, Cr, Ba) sediments. Most of the elements in the Pleistocene terrace sediments show low PI values. Distinctly different results were obtained for the Pb, Zn, and Cd in the contemporary floodplain sediments, as well as for the As in the oxbow lake sediments, in which the PI values were higher. On the contemporary floodplain, three points, P1, P2, and P3, are outstanding again (Figure 1). The PI value is much higher there compared to the rest of this landform area.

The Contamination Factor (CF) values [25] show very high contamination by Co, Sr, Ni, Ba, and Cr, moderate contamination by Pb, Zn, Cd, and As, and considerable contamination by Cu in the sediments of the entire study area. The Pleistocene terrace sediments obtained lower CF values for Cu, Pb, Zn, Ni, Co, Sr, and Cr. Higher values were found in the contemporary floodplain sediments for Cu and As and in the oxbow lake sediments for Cu, Zn, and Cd.

4.4. Integrated Geochemical Indices

The integrated geochemical indices, which combine the concentrations of all analyzed elements, were also determined during these studies.

The degree of contamination (C_d) values [25] indicate very high contamination (considerable in the Pleistocene terrace sediments) (Figure 3). Similar results were obtained in the case of the sum of pollution index (PI_sum). These values allow the classification of the study area sediments as heavily polluted, except for those in the Pleistocene terrace sediments, which were moderately polluted. The average pollution index (PI_avg) allows the categorization of these sediments as “low quality due to contamination”. The Nemerow pollution index (PI_Nemerow) indicates that these sediments represent a severely polluted domain.

The PLI—pollution load index [52] values vary between sediments of different landforms in the study area. The Pleistocene terrace sediments are classified as unpolluted to moderately polluted, the contemporary floodplain and oxbow lakes sediments—as very highly polluted. In the case of reworked meander plain sediments, this index is within the range of highly polluted sediments (Figure 4).

4.5. Geochemical Indices of Ecological Risk

The single index of the ecological risk factor (ER) [25] was calculated for eight trace elements (Cu, Pb, Zn, Ni, Co, As, Cd, Cr) for which the toxic-response factors were available in the literature. Based on the ER values, the study area sediments can be considered as having a low potential ecological risk for Zn, Pb, As, and Cu and a moderate potential ecological risk regarding the other elements. Taking landforms into account, the ER values indicate that the Pleistocene terrace sediments can also be classified as a low potential ecological risk regarding Ni, Co, Cd, and Cr. Different values were obtained for the contemporary floodplain sediments in the case of Cd; these indicate a high potential ecological risk.

The potential ecological risk index (RI), which involves all eight elements mentioned above [25], shows different values for the sediments of different landforms. In the sediments of the meander plain reworked by flows of the braided river, the RI takes values that enable to classify them as a moderate ecological risk (Figure 5). In the sediments of the contemporary floodplain and oxbow lakes, the values indicate a considerable ecological risk, while in the Pleistocene terrace sediments, they indicate a low ecological risk.

The existence of significant differences between landforms in the values of all calculated indices (p < 0.01) was confirmed using a one-way analysis of variance (ANOVA).

5. Discussion

The geochemical indices allowed the assessment of the accumulation level of trace elements in the Vistula Valley sediments and the risks to the biotic environment resulting from their presence. They also allowed the assessment of the level of their anthropogenic origin [55,56,57]. This is important for the ecosystem, as metals from anthropogenic sources have greater mobility and bioavailability compared to metals from geological sources [58].

Because there is a correlation between the distribution of trace elements and the processes that shaped the floodplain [19,59], analyses of the values of geochemical indices were performed concerning the landforms. This made it possible to identify the valley zones that, due to the nature of the process and the period of their formation, are most contaminated or are at risk of contamination [31,32,50]. This also allowed the tracing of mechanisms governing the distribution of harmful substances in fluvial deposits of the studied section of the valley. The analysis of geochemical indices showed that their values differ between the sediments of the Pleistocene terrace, meander plain reworked by flows of the contemporary braided river, contemporary floodplain, and oxbow lakes. Thus, it indicated variability in the level of anthropogenic contamination of the sediments in these landforms.

5.1. General Contamination by Trace Elements

Integrated indices, calculated for the fraction < 1 mm, indicate a high contamination level of the entire area and thus a generally high level of environmental degradation. The contemporary floodplain sediments and those filling the oxbow lakes show the highest contamination level. C_d is considerable only in the Pleistocene terrace sediments. The PI_Sum indicates moderate contamination, and the PLI classifies these sediments as unpolluted to moderately polluted.

The low values of the integrated indices measured in the sediments of the Pleistocene terrace, the oldest and morphologically the most elevated zone of the study area, are because it developed before the humans’ appearance in the river catchment area. The values are similar to those obtained for poorly developed upland areas [60]. Moreover, this landform was created by a braided river [41], whose sediments contain less organic matter and clay than those of the other landforms.

The younger, Holocene landforms (contemporary floodplain, the meander plain reworked by flows of the contemporary braided river), were created during the period of industrial development in the drainage basin [61,62,63]. However, the concentration and distribution of elements in the sediments differ between these landforms, which was also caused by different modes of river channel development at the time of their formation. Floodwaters encroaching on the meander floodplain carried debris and a load of pollutants, mainly from Upper Silesia [4]. These waters also washed out previously deposited sediments, leading to the mixing of washed-out and accumulated deposits. This phenomenon resulted in the “dilution” of the contaminant content relative to that originally transported by the river. The opposite phenomenon occurred on the contemporary floodplain that formed after the construction of levees limiting the accumulation area. Within the contemporary floodplain, floods are currently more frequent, they last longer, and the water level during floods is higher than before the construction of levees [64]. This results in the lithological variability of these sediments. Locally, they are enriched in calcium carbonate derived from the eroded bedrock. Additionally, the greatest pollutant load supply to the river occurred during the contemporary floodplain formation [65,66]. The single extreme excursions in the values of geochemical indices observed within this landform can be associated with local contemporary pollution outbreaks, e.g., industrial plants and roads [15,67,68].

5.2. Nature and Origin of Trace Elements in the Studied Section of the Vistula Valley Sediments

A more accurate distribution pattern of contaminants in the Vistula River Valley sediments, allowing for the assessment of their nature and origin [55], has been obtained from the geochemical indices analyses that relate to individual elements.

The I_geo, CF, and PI indicate that only some analyzed elements show elevated concentrations in the study area. High values of these indices for Sr, Cr, and Ba were found in the sediments of all landforms, while for Cu, Ni, and Co, high values are not recorded only in the case of Pleistocene terrace sediments. The highest contamination levels for Zn, Pb, and Cd are recorded in the contemporary floodplain sediments. The highest contamination with As is observed in the sediments filling oxbow lakes.

Due to the different chemical affinities of trace elements for different sediment components, the distribution of elements can be related to the lithological variability of landforms. The correlation analysis between trace element concentrations and environmental variables shows that Ni, Cr, and Ba are bound predominantly by the clay, and the Sr by silt. In contrast, the Cu concentration is related mostly to the presence of organic matter, the concentration of Pb, Co, and As is related to the Fe/Al ratio, while the concentrations of Zn and Cd are related to the carbonate content in the sediments (

Table 4. Correlation analysis between the contents of silt (0.002–0.05 mm), clay (<0.002 mm), organic matter, carbonates, and the Fe/Al ratio and the concentrations of trace elements in the sediments. The red color indicates statistically significant values (p < 0.05).

	Cu	Pb	Zn	Ni	Co	As	Sr	Cd	Cr	Ba
silt	0.5898	0.3856	0.1839	0.7529	0.7585	0.6617	0.7408	0.0487	0.7338	0.8129
clay	0.7358	0.5075	0.2797	0.8977	0.8851	0.7125	0.6680	0.0960	0.9000	0.8972
Org	0.8451	0.6961	0.5161	0.8652	0.8470	0.7255	0.7020	0.3488	0.8695	0.8304
CaCO3	0.4279	0.5379	0.5847	0.1945	0.2316	0.2678	0.5279	0.5265	0.2295	0.0960
Fe/Al.	0.8382	0.7091	0.5176	0.8687	0.8908	0.8559	0.7391	0.3558	0.8919	0.8455

). Thus, the values of the geochemical indices calculated for the individual elements reflect a tendency towards the accumulation of the individual compounds in deposits representing landforms of different origins and thus different lithological compositions.

The Enrichment Factor (EF) analysis is used for determining the origin of trace elements found in the sediments [13,55,56,57]. The analysis of this indicator shows that not all of the studied elements are anthropogenic in origin. Throughout the study area, the As and Pb originate from anthropogenic sources. In contrast, the lack of enrichment of the study area in Cu, Ni, Co, Sr, and Cr indicates a geogenic origin of these elements. This means that despite the high values of the I_geo, CF, and PI indices, these elements cannot be considered contaminants, as they do not originate from anthropogenic sources, despite their elevated concentrations. Bojanowski et al. [69] noted the enrichment in St of Campanian chalk near Mielnik, in eastern Poland.

The contemporary floodplain sediments exhibit elevated Pb, Zn, and Cd concentrations, expressed by the values of the I_geo, CF, and PI indices. The EF index supports the anthropogenic origin of these elements. The values of geochemical indices are similar to those obtained for the sediments of the contaminated sections of the Warta River [13] and Bystrzyca River [50] valleys, as well as of the Seine River downstream of Paris [57]. This may suggest that the origin of these elements is related not only to mining activities in the Silesian area but also to the presence of urbanized areas in the Vistula River basin. The particularly high values of geochemical indices for all analyzed elements, found at three points (P1, P2, P3) on the contemporary floodplain (Figure 1), indicate the presence of local contamination outbreaks [13,67]. Two of these points are located near the towns of Piotrawin, Kazimierz Dolny, and Janowiec and close to heavy traffic roads, affecting the sediment’s chemistry [68]. Due to the location of these towns in the upland area and on the Pleistocene terrace, surface runoff is probably another process that is additionally responsible for the accumulation of the elements [15]. The third of these points is located downstream of the Zwolenka River mouth. The high values of geochemical indices can be related to the supply of contaminants by the Zwolenka River, as its waters are characterized by poor quality [70].

The values of geochemical indices calculated for the sediments of the contemporary floodplain of the analyzed section of the Vistula River Valley have been compared with those determined for rivers elsewhere in the world. The comparison shows that although the values are lower than those from some heavily polluted rivers [71,72], they are higher than the values from many rivers flowing through mining [73,74], industrialized [75], and highly urbanized areas around the world [12,76]. Although no exceedances of permissible standards for heavy metals have been found in contemporary floodplain sediments, the high levels of these elements may pose a threat to the environment. This is because the contaminated sediments may be remobilized during flood flows to contaminate the Vistula River water and redeposit the elements in the lower sections of the river valley [19].

High values of geochemical indices were also found for the sediments of oxbow lakes, in which deposition of organic and, episodically, fine-grained mineral sediments takes place. Within the oxbow lakes, particularly high values were obtained for the I_geo, PI, and CF indices that relate to Cu, Ni, Co, Sr Cr, Ba, and As—elements that are strongly correlated with organic matter (Table 2). However, the EF analysis showed that only the As comes from anthropogenic sources. Cu, Ni, Co, Sr, Cr, and Ba, despite elevated concentrations, are derived from geogenic sources and cannot be considered contaminants. A similar phenomenon was observed in oxbow lakes of the Middle Oder River valley [77].

The EF values determined for the sediment of the meander plain reworked by flows of the contemporary braided river indicate a lower average enrichment in Pb and Cd compared to the contemporary floodplain sediments. Similarly, the I_geo, CF, and PI indices also show lower values, indicating a lower contamination level in the area. This is due to the aforementioned phenomenon of “dilution” of the braided river flood sediments by uncontaminated sediments of the reworked meander plain, leading to a reduction in trace element concentrations.

The I_geo, CF, and PI indices for the Pleistocene terrace sediments have lower values than in the other formations of the study area. The EF index showed the absence of contamination of anthropogenic origin for all elements, except for As, Pb, and Ba. The Pleistocene terrace is the only landform where the EF index indicates enrichment in Ba. The origin of this element is related to farm activities. It is possible to associate the presence of this typical domestic contaminant with the longest presence of human settlements on the Pleistocene terrace [78]. The large-scale development of the floodplain began only in the first, drier sub-period of warm Medieval Times [79].

Cd is a typical agricultural contaminant associated with the use of fertilizers [80,81]. Although the I_geo, CF, and PI indices pointed to an elevated content of this element only in the contemporary floodplain sediments, the EF index suggests its anthropogenic source in all landforms of the study area. This may be due to intensive agricultural activities carried out in the area. Although the Cd contamination is present only in the contemporary floodplain sediments, as shown by the geochemical indices, caution should also be exercised in the rest of the study area because of the toxicity of this element.

5.3. Ecological Risks

The RI takes on different values in the studied section of the Vistula River Valley, depending on the landform type. It indicates a considerable risk for the sediments of the contemporary floodplain and oxbow lakes, a moderate risk for the sediments of the meander plain reworked by the flow of the contemporary braided river, and a low risk for the Pleistocene terrace sediments. The ER calculated for the individual elements indicated a high environmental risk only for Cd contamination in the contemporary floodplain sediments. In the case of Ni, Co, and Cr, the environmental risk is moderate. As regards the other landforms and elements, the environmental risk is low. It therefore confirms the correlation observed during the analysis of the other indices; the highest contamination level, and thus the greatest threat to humans and the environment, is recorded on the contemporary floodplain and in the oxbow lakes, while the lowest one is found on the Pleistocene terrace. In addition, the comparison of the RI values of the contemporary floodplain with those of other rivers yields unfavorable results. Although the environmental risk is lower or similar to that of some of the polluted rivers [71,82], it reaches a higher level than that of many rivers flowing through heavily polluted mining areas [22], densely populated agricultural regions [3], or heavily urbanized and industrialized areas [83].

The geochemical indices calculated for the study area reveal the differences between the floodplain landforms of different origins. They also show that, despite low-to-moderate enrichment in heavy metals, the sediments of the studied section of the Vistula River Valley can be classified as low-quality soil because of contamination, making the area a severely polluted domain with a moderate-to-considerable ecological risk.

6. Conclusions

The differentiation of the geochemical and ecotoxicological indicators in alluvial deposits reflects the geomorphological diversity of the river valley. Hence, there is a different diversity of environmental risk across the individual landforms of the river valley, i.e., the contemporary floodplain, the meander plain reworked by flows of the contemporary braided river, the oxbow lakes, and the Pleistocene terrace.

The values of the geochemical indices indicate that the highest level of pollution in the study area is observed in the sediments of the contemporary floodplain and oxbow lakes. The lowest level of pollution is recorded in the sediments of the Pleistocene terrace.

The sediments of the contemporary floodplain and oxbow lakes show a significant ecological risk, the Pleistocene terrace shows a low risk, while the terrace of the meandering river shows a moderate risk.

The values of the geochemical indices indicate that despite the increased concentrations of Cu, Ni, Co, Sr, and Cr in the sediments, these elements cannot be considered as pollutants because they do not originate from anthropogenic sources. In turn, the increased contents of As, Pb, Zn, and Cd in the analyzed area are of anthropogenic origin. The high values of geochemical indices found in specific locations indicate the local nature of these pollution sources. These are human settlements and road infrastructure.

The presented studies revealed differences in the values of geochemical indices in the sediments of identified landforms, which indicates that the distribution of trace elements in river valley sediments depends on fluvial processes. For this reason, to obtain a reliable picture of the distribution of trace elements in alluvial sediments, the values of geochemical indicators should be analyzed taking into account the structure and pattern of the landforms. Only such an approach allows for the rapid identification of contaminated areas, which can also become secondary sources of trace element contamination. Moreover, to obtain a reliable assessment of the environmental risk, the analysis of the degree of contamination of sediments of individual landforms should be comprehensive and based on the simultaneous analysis of many geochemical indicators. These indicators are complementary in nature.