Variation of zinc concentrations in lake bottom sediments and implications for recreational use of water bodies

Abstract

Zinc content in bottom sediments was tested in 50 water bodies on the Silesian Upland (southern Poland). The metal was found in concentrations that ranged from 83 mg/kg to 38,400 mg/kg, that is, in amounts not previously found in any other water bodies worldwide (e.g., Hubertus I—38,400 mg/kg, Brzeziny—38,200 mg/kg, Morawa—35,300 mg/kg). It has been established that the bottom sediments of water bodies used for recreational purposes on the Silesian Upland are contaminated with zinc to varying degrees, as evidenced by geoecological indicator values, that is, the geoaccumulation index (−2.2 < I_geo < 8.7), the sediment contamination factor ($0.5\le C_f^i<219.4$), the anthropogenic enrichment factor (1.2 ≤ I_AP < 512.0), and the ratio of the concentrations found to the regional geochemical background (0.3 < I_RE < 148.3). Zinc contamination of bottom sediments has been caused by human activity, with only minimal influence of natural conditions. The water bodies surveyed overwhelmingly do not meet the geoecological conditions for safely engaging in recreation and leisure activities. A ratio of the concentrations found to the regional geochemical background of I_RE ≤ 5.0 has been proposed as the acceptable range for recreational use of water bodies involving direct contact of living organisms with zinc-containing bottom sediments. The need to revitalize, reclaim, and protect the water bodies in question has been pointed out.

Introduction

Zinc is included in the group of heavy metals with densities greater than 5 g/cm³. It is a relatively soft metal with a bluish-white color (Järup, 2003). Despite the fact that it is relatively common in the Earth’s crust, it is included in the group of rare elements. Under natural conditions, zinc usually combines with sulfur to form the mineral sphalerite (ZnS). Most commonly, it crystallizes from hydrothermal solutions in the form of veins in carbonate rocks—limestones and dolomites (Ali et al., 2018; Hussain et al., 2022). Under natural conditions, its concentrations in the Earth’s crust range from 10 ppm to 120 ppm in rocks (Kabata-Pendias and Pendias, 1993), averaging 78 ppm (Hussain et al., 2022).

Zinc is one of the most important trace elements found in the human body. Alongside playing the catalytic and structural roles, it is also included in many enzymes and proteins (Roohani et al., 2013). It is estimated that for proper functioning, a human body needs to absorb about 15 mg of zinc per day (King and Turnlund, 1989). Insufficient amounts of zinc negatively affect human health, but its deficiencies can be easily supplemented by oral administration (Prasad, 2013). Being one of the heavy metals, zinc also exhibits toxic properties, which was demonstrated many years ago (Fosmire, 1990). The essential or destructive physiological role of Zn (any element) depends on its forms and concentrations. Zinc poisoning may be of an acute nature and in this case, it is detected early and complete recovery can be achieved through appropriate treatment (Qu et al., 2012). Chronic effects are usually associated with exposure to low doses over an extended period of time (Nriagu, 2011).

Zinc has been widely used in the metallurgical industry. It is commonly used to coat steel and iron to prevent corrosion, and it is also alloyed with other metals to make brass and bronze (Craig et al., 2001). The primary source of zinc in the environment is metal mining and processing. Zinc most often enters the various components of the geographic environment (air, water, and soil) together with dust from smelters that produce zinc and other nonferrous metals, and also during the combustion of hard coal (Wang et al., 2017). The highest concentrations of this metal are recorded near nonferrous smelters. In water, zinc in general does not exhibit high toxicity, but its presence increases the toxic effects of copper, nickel, and cadmium (Świderska-Bróż, 1993).

Bottom sediments of anthropogenic water bodies serve as a kind of “archive,” documenting the various stages of environmental changes that occurred in their surroundings as well as in their catchment areas. The geo-indicative significance of bottom sediments becomes greater especially after considering the role of post-sedimentation diagenesis of sediments, their resuspension or biotic activity, including the activities of burrowing organisms, and so on. The results of such studies are especially useful for water bodies that function under varying degrees of human pressure. Bottom sediments are a very good indicator of the ecological changes that occur as a result of human activity (Rzetala, 2016; Machowski et al., 2019).

The quality of bottom sediments of lakes and artificial water bodies is important both for the environment and for human health and life. Various hazardous pollutants can accumulate in bottom sediments; under certain conditions, these may be a source of secondary pollution of limnic waters, thus negatively affecting plants, animals, and humans. In connection with growing concerns about the potential negative impact of zinc on the aquatic environment, the relevant agencies of, among others, the World Health Organization (Simon-Hettich et al., 2001) and the European Union (Aschberger et al., 2010) have turned their attention to these issues. Based on relevant regulations in force within the European Union, technical guidelines have been developed for conducting risk assessments of zinc’s impact on the natural environment and on human health (Van Sprang et al., 2009). In Japan, an Environmental Quality Standard was developed in 2003 that concerned the protection of aquatic organisms from the negative effects of zinc (Tsushima et al., 2010). After the establishment of zinc standards, it was stressed that information must be collected in the following areas: identifying the sources of zinc contamination, establishing the amount of zinc in the environment, and clarifying its ecological effects. In Japan, a comprehensive study was conducted to provide a scientific basis for the development of realistic zinc risk mitigation measures. The results of these assessments have great bearing on policies regarding risk mitigation measures for hazardous substances, which include zinc (Naito et al., 2010).

Due to the primarily anthropogenic origin of zinc in the geographic environment and its possible harmful effects, this element is used as a bioindicator. The level of zinc contamination in the aquatic environment is determined by, inter alia, studies of aquatic organisms (Sakowski and Simmons, 2010), such as fish (Sheriff et al., 2014) or crayfish (Mamdouh et al., 2022). Due to its potential toxic effects on aquatic organisms and human health, testing the bottom sediments of anthropogenic water bodies for zinc is a very common procedure (Varol, 2020; Panda et al., 2023; Sojka et al., 2023). Particular danger is associated with the recreational use of artificial water reservoirs located in urban areas (Rzetala et al., 2023). In cities with large populations, there is an increasing demand for areas that could be used for recreational purposes, including water bodies and their surroundings (Kuś et al., 2022). In addition, water bodies located in urban and industrial areas are used, for instance, as a source of water for domestic use, agriculture and industry (Rzetala and Jaguś, 2012).

The objectives of the present study were as follows: determining zinc concentrations in the bottom sediments of water bodies on the Silesian Upland and its periphery, assessing the factors that determine the spatial variation in the concentration of this element between water bodies in the region, identifying levels of zinc contamination of bottom sediments, assessing the zinc content of bottom sediments as an indicator of anthropogenic pollution, and assessing the possibility of using the presence of zinc in bottom sediments as a geoecological indicator conditioning the recreational use of inland water bodies.

Materials and methods

Research area

The study area includes the Silesian Upland together with the peripheries of adjacent areas (southern Poland) (Solon et al., 2018). An important phase in the evolution of this area’s landscape has been the cultural formation process (including without limitation urbanization and industrialization) that has lasted for more than 2 centuries. The anthropogenic transformation of environmental conditions took place in the wake of the industrial revolution, which was accompanied by, inter alia, the mining of mineral resources (e.g., iron ore, zinc and lead ores, coal, sands, gravels, clays), the development of the processing industry, an intensive influx of population, and progressing urbanization (Figure 1).

Figure 1.

The distribution of surface formations (A) and the geological structure of the substrate (B) on the Silesian Upland and major mineral resource deposits, their mining and processing (Rzetala, 2016; revised and supplemented); the changes in Figure 1 were made in the open access Quantum GIS software version 3.12. 1—sands, gravels, alluvial soils, peat, and silts (Holocene); 2—eolian sands, locally in dunes (Quaternary); 3—loesses, sandy loesses, and loess-like dusts (Quaternary); 4—clays, sands, and clays with gruss, solifluctive-deluvial (Pleistocene); 5—sands, gravels, and alluvial loams (Pleistocene); 6—marginal clays, loams, and sands (Pleistocene); 7—outwash sands and gravels (Pleistocene); 8—end moraine gravels, sands, boulders and clays, glacial tills, glacial till waste, and glacial sands and gravels (Pleistocene); 9—organodetritic limestones, sulfur-bearing limestones, gravels, clays, loams, sands, locally gypsum, and lignite (Neogene); 10—limestones, marls, dolomites, mudstones, sandstones, claystones, with flints and siderite insertions, gravels, conglomerates, sands, clays, and fire clays (Jurassic); 11—claystones, mudstones, sandstones, limestones, dolomites, marls, oolitic limestones, gypsum, anhydrite, epigenetic ore-bearing dolomites, iron ores (Triassic); 12—conglomerates, arkosic sandstones, mudstones, claystones (Perm); 13—sandstones, conglomerates, claystones, mudstones, coal (Carboniferous); 14—waterways and water bodies; 15—watershed; 16—actively mined coal deposits; 17—zinc and lead ore deposits; 18—power plants; 19—cement factories; 20—steelworks, smelters; 21—metallurgical processing plants or smelters (zinc and lead); 22—former smelters (zinc and lead); 23—water body labels (1—Dzierżno Duże, 2—Dzierżno Małe, 3—Pogoria I, 4—Pogoria II, 5—Pogoria III, 6—Chechło, 7—Stawiki, 8—Morawa, 9—Hubertus I, 10—Gliniak, 11—Hubertus II, 12—Borki, 13—Borki Małe, 14—Sosina, 15—Pławniowice, 16—Mały Zalew, 17—Rogoźnik III, 18—Rogoźnik II, 19—Rogoźnik I, 20—Balaton, 21—Czeladź Norwida, 22—Kazimierz, 23—Gliniok, 24—Koparki, 25—Amendy, 26—Kozłowa Góra, 27—Przeczyce, 28—Paprocany, 29—Wielikąt, 30—Łężczok, 31—Kradziejówka, 32—Harmęże, 33—Sławków, 34—Przeczyce—pond, 35—Ostrożnica, 36—Przetok, 37—Żabie Doły, 38—Bobrek—rozlewisko, 39—Pekin—basin E, 40—Pekin—basin S, 41—Pekin—basin W, 42—Przy Leśnej, 43—Makoszowy, 44—Milicyjny, 45—Mały, 46—Kajakowy, 47—Łąka, 48—Ozdobny, 49—Makoszowy Las, 50—Brzeziny).

On the Silesian Upland and its periphery, zinc ores were mined on an industrial scale and smelted on site for many years. Indeed, some of the largest deposits of zinc and lead ores in Europe were found in these areas (Heijlen et al., 2003; Cabała et al., 2020). Initially (as early as the 12th century) silver and lead were mined, and only later, as technology developed, zinc also began to be extracted from the same deposits (Warchulski et al., 2018). After many years of zinc and lead ore mining in the Silesia-Kraków region, mining activities were first discontinued in the Bytom and Chrzanów area; in 2020, they ceased in the Olkusz area, while deposits in the Zawiercie area have not been mined so far. Nowadays, there are also few zinc and lead smelters in the region—many nonferrous metal smelters have been closed down (e.g., in the vicinity of the Hubertus, Morawa, Stawiki, and Borki water bodies), and only the largest metallurgical plants remain in operation (e.g., the smelter in the vicinity of the Chechło water body). The Upper Silesian Industrial Region (Górnośląski Okręg Przemysłowy—GOP), which developed on the Silesian Upland and its periphery, was one of Europe’s largest industrial districts (and at the same time largest environmental disaster areas) for decades (Rzetala et al., 2023).

The presence of ca. 4,700 mostly small water bodies with a total area of 185.4 km² is associated with the past economic development of the Silesian Upland and its periphery (Rzetala and Jaguś, 2012). Natural bodies of water (lakes) are virtually absent in the study area (this is a consequence of, inter alia, the progressing karstification of the substrate in part of the study area, its old-glacial relief and broadly understood human impact) (Rzetala, 2016). Water-filled anthropogenic basins dominate to such an extent that the Silesian Upland, together with parts of adjacent regions, is dubbed the Upper Silesian Anthropogenic Lake District (Rzetala and Jaguś, 2012).

The changes related to Poland’s political transformation were accompanied by socioeconomic shifts, which resulted in the closure of many large industrial plants. Numbers of active mines, smelters, and associated plants in the region are steadily decreasing, restructuring processes are underway and the employment structure is changing (with a reduction in labor demand in industry boosting employment in the service sector, including tourism and recreation). The region’s industrial heritage and the presence of large forest areas and numerous bodies of water provide the basis for the present development of postindustrial tourism, alongside various forms of ecotourism and recreation for the conurbation’s approximately 2.2 million residents (Dwucet et al., 2016; Rzetala et al., 2023). For these reasons, water bodies have broadly understood recreational significance despite the fact that they function under varying degrees of human pressure (e.g., urban, industrial, agricultural).

Field research

The study included 50 water bodies that varied in terms of their ages, morphometric features, the functions performed, and catchment land use and land cover forms (Figure 2; Table 1). The catchments of the 25 water bodies studied are dominated by urbanized and industrialized areas; areas used for agriculture and wastelands prevail in the catchments of 15 water bodies, and the catchments of 10 water bodies are dominated by forests.

Figure 2.

Locations of recreationally used water bodies tested for zinc content in their bottom sediments on the Silesian Upland and its periphery (southern Poland): A—former mineral workings; B—reservoirs impounded by dams; C—levee ponds; D—water bodies in subsidence basins and hollows; E—water bodies with multiple origins; F—water body numbering (see Figure 1).

Table 1.

Morpho- and hydrometric parameters of the water bodies in the Silesian Upland^a

No. of Water Body	Geographical Coordinates		Total Capacity	Maximum	Electrolytic	Year of	Predominant Land Use and Catchment	Recreational Functions of
(See Figure 1)	Latitude	Longitude	(dam3)	Area (ha)	Conductivity (μS/cm)	Creation	Cover Typeb	the Water Bodyc
1	50°22′24.00″N	18°33′25.00″E	94,000	561.0	5,632.0	1964	a, b	F, N, O
2	50°23′16.30″N	18°33′51.10″E	12,600	160.0	679.0	1938	a, b	S, C, F, W, M, B, O
3	50°21′27.00″N	19°14′15.00″E	3,600	75.0	736.0	1943	c, b	S, C, F, M, B, N, O
4	51°27′28.22″N	19°13′13.50″E	500	26.0	628.5	1977	c, b	F, N, O, B
5	50°21′13.11″N	19°12′05.00″E	12,000	208.0	483.0	1974	c, b	S, C, F, D, M, B, O
6	50°28′04.00″N	18°54′49.10″E	1,300	90.0	183.0	1970	c, d	S, C, F, M, B, O
7	50°16′25.56″N	19°06′35.59″E	131	7.6	784.5	1955	b, d	C, F, W, M, B, N, O
8	50°16′24.56″N	19°07′19.57″E	693	34.7	380.0	1965	b, d	S, C, F, M, B, N, O
9	50°15′46.37″N	19°06′41.59″E	142	6.7	1,102.0	1928	b, d	F, O
10	50°15′53.55″N	19°07′00.54″E	824	38.7	512.1	1928	b, d	S, C, F, W, M, B, N, O
11	50°15′37.59″N	19°07′23.27″E	140	6,7	1,650.0	1928	b, d	F, O
12	50°16′30.01″N	19°06′06.99″E	202	12.0	311.0	1965	b, d	F, O, B
13	50°16′42.91″N	19°05′49.36″E	11	1.1	372.3	1965	b, d	F, B
14	50°14′27.00″N	19°19′50.05″E	1,000	50.0	547.4	1977	c, d	S, C, F, W, M, B, O
15	50°23′29.23″N	18°28′08.00″E	29,100	240.0	617.0	1970	a, c	S, C, F, W, M, B, O
16	50°23′20.45″N	18°29′55.77″E	143	6.5	488.0	1970	a, c	C, F, W, M, B
17	50°24′17.23″N	19°03′37.39″E	9	0.9	699.5	1975	a, c	F, B
18	50°24′13.40″N	19°02′40.03″E	340	25.0	651.0	1975	a, c	F, M, B, O
19	50°23′54.59″N	19°01′43.58″E	360	12.1	644.0	1975	a, c	S, C, F, M, B, O
20	50°16′31.21″N	19°15′11.16″E	71	9.0	535.5	1939	c, b	C, F, M, B, O
21	50°18′28.10″N	19°06′45.13″E	89	1.0	685.2	1965	a	F, N, B
22	50°17′17.26″N	19°14′40.44″E	82	4.1	571.0	1920	b, c	F, O, B
23	50°14′25.49″N	18°59′33.22″E	769	1.7	7,520.0	1950s	b	F
24	50°13′42.52″N	19°18′40.77″E	440	4.0	707.0	1999	b, c	D, B, O
25	50°19′36.20″N	18°55′35.09″E	21	1.3	341.0	1890s	b	F
26	50°25′46.87″N	18°58′22.20″E	15,300	587.0	344.7	1938	a, c	C, F, N, B
27	50°26′30.56″N	19°11′33.55″E	20,700	510.0	362.0	1963	a	S, C, F, W, M, B, O
28	50°05′05.59″N	18°59′02.22″E	1,600	110.0	315.0	1870	c, a	S, C, F, W, M, B, N, O
29	50°00′44.59″N	18°17′34.53″E	1,424	356.0	887.8	XIXd	a	F, N, O
30	50°08′26.15″N	18°16′37.57″E	948	237.0	447.3	XIXd	a	F, N, B, O
31	50°03′40.00″N	18°40′42.55″E	72	18.0	889.1	XIXd	a	F
32	50°00′59.00″N	19°09′17.13″E	123	30.7	482.0	XIXd	a	F
33	50°18′03.11″N	19°24′02.59″E	12	2.4	888.0	XXe	b	F, B
34	50°27′04.31″N	19°14′00.00″E	163	40.7	401.0	1960s	a	F, O
35	50°26′58.35″N	19°57′37.20″E	22	4.0	233.0	XIXd	c	F
36	50°19′18.05″N	19°03′08.35″E	20	1.0	683.7	1946	b	F, B
37	50°21′49.54″N	18°57′29.46″E	200	25.0	1,476.7	XXf	b	F, N, B, O
38	50°16′59.65″N	19°13′52.00″E	5	1.8	1,941.7	1950s	b	F
39	50°17′02.44″N	19°14′05.59″E	1	0.8	1,326.0	1960s	b	F
40	50°16′50.38″N	19°14′11.44″E	7	0.2	3,022.0	1950s	b	F
41	50°16′58.55″N	19°13′36.07″E	6	0.4	649.5	1960s	b	F
42	50°22′10.13″N	19°52′59.01″E	2	0.3	3,222.0	1970s	b	F
43	50°15′40.00″N	18°46′37.00″E	65	5.4	2,888.2	1970s	c	F
44	50°14′07.58″N	19°02′31.56″E	43	4.7	730.0	1920	b	F, B, O
45	50°14′25.57″N	19°02′42.59″E	9	1.0	689.0	1920	b	F, B, O
46	50°14′45.00″N	19°02′43.00″E	250	10.2	716.1	1920	b	C, F, M, B, O
47	50°15′00.58″N	19°02′42.00″E	290	12.7	774.5	1920	b	F, B
48	50°14′46.29″N	19°02′34.24″E	6	1.2	401.0	1920	b	B, O
49	50°15′30.09″N	18°46′35.59″E	29	1.9	700.1	1970s	c	F
50	50°20′24.58″N	18°58′31.57″E	10	1.0	3,092.4	1920s	b	F

^aAccording to Rzetala (2014); Rzetala et al. (2023); revised and supplemented.

^bPredominant land use and catchment cover type: a—agricultural land and wasteland, b—urban and industrial areas, c—forest, d—water.

^cRecreational functions of the water body: S—sailing, C—canoeing, F—fishing, D—diving, W—water sports, M—swimming, B—beach and waterfront recreation, N—nature conservation within the water body, O—others.

^dSecond half of the 19th century.

^eEarly 20th century.

^fFirst half of the 20th century.

A total of 134 bottom sediment samples were collected from water body basins. As in the case of numerous other similar surveys carried out within water bodies in the region (Rzetala, 2016; Rzetala et al., 2023), samples were collected in accordance with the principle of uniform sampling of sediment cover and morphometric variation of water bodies (i.e., their shapes, dimensions, and depth variation) (Machowski et al., 2019). During the field work period, the depths of the water bodies studied ranged from 0.7 m to 18 m, which was reflected by the locations of bottom sediment sampling sites. Samples were collected in the deepest parts of the water bodies as well as in zones corresponding to their average depth, and additionally in bays if these were present within the water bodies. These are the locations recognized in limnological studies as the most representative for reconstructing the occurrence and quantitative as well as qualitative variation of bottom sediments, designated for sampling based on bathymetric charts (maps showing depth distribution in lakes). Samples were collected using the Beeker sediment core sampler (version 04.20.S.A.) manufactured by Eijkelkamp, and, as auxiliary tools, Van Veen grab samplers with capacities of 1.25 dm³ or 2.50 dm³ (Rzetala, 2016; Rzetala et al., 2023). Scoops were used where low sediment thickness prevented sampling with a core sampler (for instance, in the Koparki water body, which is less than 20 years old, sediment thickness is negligible, making it impossible to effectively use a pneumatic core sampler). Using the material sampled from a given vertical profile, a mixed sample representative of the site in question was prepared. A decision was made to use mixed samples in the study for several reasons. Above all, the relatively short period during which water bodies operated under environmental conditions identified with strong human pressure is representative of the period during which this pressure occurred. The thickness of sediments in the anthropogenic water bodies studied is small (among other things, due to their young age), and these bottom sediments have the characteristics of sapropel that has been mixed both due to natural factors and through human activity. Not insignificant for changes in bottom sediment deposition conditions is the occurrence of resuspension, which intensifies especially in shallow bodies where naturally or anthropogenically conditioned mixing occurs (e.g., within the range of effective wave action). Therefore, sediment redeposition caused by natural factors as well as the use of water for economic purposes cannot be ruled out. Another important consideration is the varying occurrence of sedentation, or the partial (e.g., in the Dzierżno Duże and Paprocany water bodies) or complete (e.g., in the Stawiki water body) dredging of bottom sediments from water body basins.

The thickness of bottom sediments in the water bodies studied varied but was generally low compared to natural lakes that have often existed for many thousands of years. The thickness of bottom sediments measured at the sampling sites ranged from 0.2 cm (in the Koparki water body) to about 180 cm (in the Gliniok water body), with an average value of just over 20 cm and a median value of about 16 cm. The basic pattern governing the distribution of bottom sediments in water bodies is that these are found mainly at the mouths of surface tributaries and in deep spots, and much smaller amounts of these sediments are present within elevated areas of water body basins and in their remaining coastal parts that exhibit even a slight slope. The rate of formation of bottom sediments varies widely, given the multiple factors that determine the delivery and deposition of allochthonous and autochthonous matter (Jaguś and Rzetala, 2012; Rzetala et al., 2013).

The conditions for the occurrence of bottom sediments in young anthropogenic bodies of water listed in the article underpinned the collection of mixed samples using appropriate approaches and techniques applied to core and surface sampling (Förstner et al., 1974; Förstner, 1976; Salomons and Mook, 1980; Moore et al., 2009; Forghani et al., 2012). The prepared mixed samples were placed in polyethylene bags and transported to the laboratory.

Laboratory research

Laboratory work was carried out at the laboratory of the Institute of Earth Sciences at the University of Silesia in Sosnowiec and at Activation Laboratories Ltd. at Ancaster, Canada. At the former laboratory, bottom sediment samples were prepared for zinc level determination purposes. They were dried at 105°C and subsequently homogenized using a mortar. After the material had been ground, the <0.063 mm fraction was isolated using chemically inert sieves and was subsequently subjected to geochemical analysis. Chemical composition was determined using inductively coupled plasma (ICP) atomic emission spectrometry in accordance with the standards applied at Activation Laboratories Ltd. (ActLabs, 2023).

The concentrations of Zn were determined using the ICP method following the complete dissolution of 0.25 g aliquots. Each sample aliquot was digested using a mixture of HClO₄, HNO₃, HCl, and HF at 200°C until fuming, and subsequently diluted with aqua regia (ActLabs, 2023). During ICP analysis, reagent blanks with and without the lithium borate flux were analyzed alongside the method reagent blank, with interference correction verification standards being subject to analysis as well. For calibration purposes, USGS and CANMET certified reference materials (2 standards for each group of 10 samples) were used in order to bracket sample groups. Moreover, internal standards were added to the sample solution, which was then subject to further dilution. When introducing the sample into the Perkin Elmer SCIEX ELAN 6000 mass spectrometer, a proprietary methodology was used. The analyses conducted exhibited the following precision and accuracy levels: (a) at the lower detection limit: ±100%; (b) at 10 times the lower detection limit: ±15%–25%; (c) at 100 times the lower detection limit: better than 10%.

Statistics

Several indicators were used to facilitate the interpretation of study results: zinc levels in the environment, the variation of its spatial concentration in sediments, and the degree of zinc contamination of bottom sediments. These include the geoaccumulation index (Equation 1) developed by Müller (1979), the contamination factor (Equation 2) developed by Håkanson (1980), the ratio of concentrations to the regional geochemical background (Equation 3), and the anthropogenic enrichment factor of bottom sediments (Equation 4) (Rzetala, 2015a; 2015b; 2015c).

The geoaccumulation index (Equation 1) is a popular indicator for assessing the degree of contamination of bottom sediments with metals, metalloids, and nonmetals, which is used worldwide in geochemical studies. The geoaccumulation index compares the concentration of a given substance in sediments to the geochemical background level as the level considered to be the natural concentration. Among the many results of sediment geochemical background tests, the values that were the most recent and most commonly applied to geochemical analyses of bottom sediments have been used. In order to determine the range of results obtained in geoaccumulation index (I_geo) calculations, the most current and most common geochemical background values for zinc applicable to geochemical analysis of bottom sediments have been used, for example, 62 mg/kg (Lis and Pasieczna, 1995a), 67 mg/kg (Taylor and McLennan, 1995; Li, 2000; Li and Schoonmaker, 2005), 71 mg/kg (Turekian and Wedepohl, 1961), 175 mg/kg (Håkanson, 1980), and 259 mg/kg (Lis and Pasieczna, 1995b). Geoaccumulation index values make it possible to classify sediments into 7 quality classes: class 0—practically uncontaminated (I_geo ≤ 0.0); class I—uncontaminated to moderately contaminated (0.0 < I_geo ≤ 1.0); class II—moderately contaminated (1.0 < I_geo ≤ 2.0); class III—moderately to heavily contaminated (2.0 < I_geo ≤ 3.0); class IV—heavily contaminated (3.0 < I_geo ≤ 4.0); class V—heavily to extremely contaminated (4.0 < I_geo ≤ 5.0); and class VI—extremely contaminated (5.0 < I_geo) (Müller, 1979; Förstner and Müller, 1981).

1 $I_{geo}=lo{g}_2\frac{C_n}{1.5B_n}$

where:

• I _geo—geoaccumulation index,

• C_n—the concentration of the element in question in bottom sediments,

• B_n—geochemical background for the element in question,

• 1.5—coefficient expressing natural variation in the content of the element in question in the environment (Müller, 1979).

The contamination factor ($C_f^i$) was developed by Håkanson (1980) and is also commonly used in geochemical studies of lake sediments (Equation 2). The essence of the contamination factor is relating the content of a given substance found in the surface layer of lake sediments to the preindustrial content of that substance in the sediments Håkanson (1980). Given the age of the water bodies studied, which is often no more than 40–50 years (in total or since the last dredging), the mixed sediment sample was considered representative of contemporary concentrations in the 0–1 cm layer. According to the procedure proposed by Håkanson (1980), the results of contamination factor calculations should be interpreted as follows: $C_f^i<1$—no contamination or low sediment contamination; $1\le C_f^i<3$—moderate contamination; $3\le C_f^i<6$—significant contamination; $C_f^i>6$—very heavy contamination.

2 $C_f^i=\frac{{\overline{C}}_{0-1}^i}{C_n^i}$

where:

• $C_f^i$ —the contamination factor,

• ${{\displaystyle \overline{C}}}_{0-1}^i$ —the mean content of the substance in question (i) from superficial sediments (0–1 cm) from accumulation areas,

• ${{\displaystyle C}}_n^i$ —the standard preindustrial reference level; determined from various European and American lakes to be Zn = 175.0 ppm (mg/kg) (Håkanson, 1980).

The ratio of element concentration to the regional geochemical background (I_RE) describes the multiple by which the natural concentration of the element in regional sediments is exceeded by the concentration of that element in the sediment sample (Equation 3) (Rzetala, 2015a; 2015b; 2015c). The ratio of the value measured to the regional geochemical background (I_RE) calculated in this manner exceeds unity if the concentration of the element in question is higher than the regional geochemical background (the higher the concentration the higher the ratio) and is below unity when this level is not reached. When determining the ratio of actual concentrations to the regional geochemical background (I_RE), the zinc concentrations found in the bottom sediment samples were compared to the regional zinc geochemical background value of 259 mg/kg, which was representative of studies of bottom sediments of water bodies on the Silesian Upland and in adjacent areas, as published in the regional geochemical atlas (Lis and Pasieczna, 1995b).

3 $I_{\text{RE}}=\frac{C_{\text{BS}}}{C_{\text{GB}}}$

where:

• I _RE—the ratio of the value measured to the regional geochemical background (dimensionless number),

• C _BS—the average concentration of the element in question in bottom sediments (mg/kg),

• C _GB—the regional geochemical background level for the element in question in bottom sediments (mg/kg) (Rzetala, 2014).

For the purpose of comparing substance concentrations in bottom sediments and in substrate sediments, the anthropogenic enrichment factor of bottom sediments was used, which is related to the extent to which the water body studied effectively accumulates matter (Equation 4) (Rzetala, 2015a; 2015b):

4 $I_{\text{AP}}=\frac{C_{\text{BS}}}{C_{\text{SR}}}$

where:

• I _AP—the anthropogenic enrichment factor for bottom sediments (dimensionless number),

• C _BS—the average concentration of the element in question in bottom sediments of the water body (mg/kg),

• C _SR—the average concentration of the element in question in substrate sediments and in the surroundings of the basin (mg/kg) (Rzetala, 2014; 2015a).

The anthropogenic enrichment factor of bottom sediments (I_AP) has a value below unity if the concentration of the element in sediments is lower than its concentration in the substrate and in the vicinity of the basin and above unity if the concentration of the element in bottom sediments is higher than that in the vicinity of the basin (the higher the ratio the higher the factor).

Results

Zinc was present in the bottom sediments of the water bodies studied in amounts ranging from 83.0 mg/kg to 38,400.0 mg/kg (supplemental file [Preliminary Data.xlsx]; Figure 3; Table 2). The lowest concentration within the above range was found in a sample from the Wielikąt water body, and the highest one was found in the Hubertus I water body. The lowest zinc concentration ranges were found in the bottom sediments of several water bodies, that is, the aforementioned Wielikąt one (83.0–148.0 mg/kg), Kradziejówka (138.0–198.0 mg/kg), Harmęże (160.0–194.0 mg/kg), and Pławniowice (165.0–199.0 mg/kg). In contrast, the highest concentrations of this element were measured in the bottom sediments accumulated in the following water bodies: the Brzeziny (37,200.0–38,200.0 mg/kg), Hubertus I (32,000.0–38,400.0 mg/kg), Przy Leśnej (28,100.0–28,900.0 mg/kg), Gliniak (16,300.0–28,900.0 mg/kg), and Morawa (13,800.0–35,300.0 mg/kg).

Figure 3.

Zinc content in the bottom sediments of the water bodies examined (water body numbering as in Figure 1).

Table 2.

Basic statistical characteristics of zinc concentrations in bottom sediments of water bodies on the Silesian Upland in southern Poland

Statistical Characteristics	Average Concentration (mg/kg)	Minimum Concentrations (mg/kg)	Maximum Concentrations (mg/kg)
Minimum	116.0	83.0	148.0
Maximum	37,700.0	37,200.0	38,400.0
Arithmetic mean	4,573.0	3,946.9	5,224.3
First quartile	742.7	454.5	874.0
Median	1,431.5	1,200.0	1,530.0
Third quartile	3,330.0	2,802.5	3,680.0
Standard deviation	8,829.3	7,956.8	9,939.9

The arithmetic mean of zinc concentrations for individual water bodies ranged from 116.0 mg/kg to 37,700.0 mg/kg. The arithmetic mean of zinc concentrations in the bottom sediments examined, calculated for 50 water bodies on the basis of their individual mean values, was 4,573.0 mg/kg with a median equal to 1,431.5 mg/kg (first quartile—742.7 mg/kg, third quartile—3,330.0 mg/kg) and a standard deviation of 8,829.3 mg/kg. The statistical characteristics calculated for minimum concentrations of Zn in the sediments of the water bodies examined are as follows: 83.0–37,200.0 mg/kg (range of variation), 1,200.0 mg/kg (median), 3,946.9 mg/kg (arithmetic mean), and 7,956.8 mg/kg (standard deviation). The corresponding parameters for maximum concentrations are as follows: 148.0–38,400.0 mg/kg (range of variation), 1,530.0 mg/kg (median), 5,224.3 mg/kg (arithmetic mean) and 9,939.9 mg/kg (standard deviation). The simple measures used to describe the statistical set indicate a highly variable spectrum of zinc concentrations in the bottom sediments of water bodies, suggesting the existence of conditions other than natural ones that affect the concentration of this element. In a sense, this also confirms the representative selection of anthropogenic water reservoirs for the study conducted, as these present peculiar and highly varied conditions for zinc accumulation in bottom sediments.

Based on the results of zinc determinations in bottom sediments, several geoecological indices were calculated, that is, the geoaccumulation index (Table 3), the sediment contamination factor (Table 4), the anthropogenic enrichment factor of sediments (Table 5), and the ratio of the concentrations found to the regional geochemical background (Table 5). These indices cover a wide spectrum—from those at the natural background level to those suggesting anthropogenic influence.

Table 3.

Geoaccumulation index values (I_geo) for the minimum and maximum zinc concentrations (Zn_min, Zn_max) in bottom sediments and different geochemical background values for zinc (B_Zn)

Water Body Name (Numbering as in Figure 1)		Geochemical Background (BZn
		62.0a	67.0b	71.0c	175.0d	259.0e	62.0a	67.0b	71.0c	175.0d	259.0e
		I geo (Cn = Znmin)					I geo (Cn = Znmax)
1	Dzierżno Duże	2.00	1.89	1.80	0.50	−0.06	3.89	3.78	3.70	2.39	1.83
2	Dzierżno Małe	1.63	1.52	1.44	0.13	−0.43	2.37	2.26	2.17	0.87	0.31
3	Pogoria I	1.32	1.21	1.12	−0.18	−0.74	4.65	4.54	4.46	3.15	2.59
4	Pogoria II	2.21	2.10	2.02	0.72	0.15	3.08	2.97	2.89	1.59	1.02
5	Pogoria III	0.61	0.50	0.42	−0.89	−1.45	3.71	3.60	3.52	2.22	1.65
6	Chechło	3.87	3.76	3.67	2.37	1.81	3.99	3.88	3.80	2.50	1.93
7	Stawiki	6.00	5.89	5.80	4.50	3.93	6.63	6.52	6.43	5.13	4.57
8	Morawa	7.21	7.10	7.02	5.72	5.15	8.57	8.46	8.37	7.07	6.51
9	Hubertus I	8.43	8.31	8.23	6.93	6.36	8.69	8.58	8.49	7.19	6.63
10	Gliniak	7.45	7.34	7.26	5.96	5.39	8.28	8.17	8.08	6.78	6.22
11	Hubertus II	4.67	4.55	4.47	3.17	2.60	4.81	4.70	4.62	3.31	2.75
12	Borki	3.11	3.00	2.92	1.62	1.05	5.37	5.26	5.18	3.87	3.31
13	Borki Małe	5.20	5.08	5.00	3.70	3.13	5.22	5.11	5.03	3.72	3.16
14	Sosina	4.13	4.02	3.94	2.63	2.07	4.18	4.07	3.99	2.69	2.12
15	Pławniowice	0.83	0.72	0.63	−0.67	−1.24	1.10	0.99	0.90	−0.40	−0.97
16	Mały Zalew	3.46	3.35	3.27	1.97	1.40	3.48	3.36	3.28	1.98	1.41
17	Rogoźnik III (E)	3.41	3.30	3.22	1.92	1.35	3.47	3.36	3.27	1.97	1.41
18	Rogoźnik II	3.03	2.92	2.84	1.54	0.97	3.17	3.06	2.98	1.67	1.11
19	Rogoźnik I (W)	2.17	2.06	1.97	0.67	0.11	2.27	2.16	2.08	0.78	0.21
20	Balaton	3.43	3.32	3.23	1.93	1.37	3.47	3.36	3.28	1.98	1.41
21	Czeladź Norwida	3.70	3.59	3.51	2.20	1.64	3.84	3.73	3.64	2.34	1.78
22	Kazimierz	3.68	3.57	3.48	2.18	1.61	3.82	3.70	3.62	2.32	1.75
23	Gliniok	1.51	1.39	1.31	0.01	−0.56	1.54	1.43	1.34	0.04	−0.52
24	Koparki	3.55	3.44	3.36	2.05	1.49	3.60	3.49	3.41	2.11	1.54
25	Amendy	3.99	3.88	3.80	2.50	1.93	4.02	3.91	3.83	2.52	1.96
26	Kozłowa Góra	3.03	2.92	2.84	1.54	0.97	4.86	4.75	4.66	3.36	2.80
27	Przeczyce	3.84	3.73	3.65	2.35	1.78	4.06	3.95	3.86	2.56	2.00
28	Paprocany	2.90	2.79	2.70	1.40	0.83	3.10	2.99	2.91	1.61	1.04
29	Wielikąt	−0.16	−0.28	−0.36	−1.66	−2.23	0.67	0.56	0.47	−0.83	−1.39
30	Łężczok	2.50	2.39	2.30	1.00	0.43	2.53	2.42	2.34	1.04	0.47
31	Kradziejówka	0.57	0.46	0.37	−0.93	−1.49	1.09	0.98	0.89	−0.41	−0.97
32	Harmęże	0.78	0.67	0.59	−0.71	−1.28	1.06	0.95	0.87	−0.44	−1.00
33	Sławków	4.97	4.86	4.77	3.47	2.91	5.67	5.55	5.47	4.17	3.60
34	Przeczyce—pond	3.37	3.26	3.17	1.87	1.31	4.88	4.77	4.69	3.38	2.82
35	Ostrożnica	4.15	4.04	3.95	2.65	2.09	4.22	4.11	4.02	2.72	2.15
36	Przetok	5.65	5.54	5.46	4.16	3.59	5.67	5.56	5.48	4.18	3.61
37	Żabie Doły	4.74	4.63	4.54	3.24	2.67	5.69	5.57	5.49	4.19	3.62
38	Bobrek—flooded	5.31	5.20	5.11	3.81	3.25	5.33	5.22	5.14	3.84	3.27
39	Pekin—basin E	4.56	4.45	4.36	3.06	2.49	4.70	4.58	4.50	3.20	2.63
40	Pekin—basin S	4.13	4.02	3.94	2.63	2.07	4.20	4.09	4.01	2.70	2.14
41	Pekin—basin W	3.70	3.59	3.51	2.20	1.64	3.77	3.66	3.58	2.27	1.71
42	Przy Leśnej	8.24	8.13	8.04	6.74	6.18	8.28	8.17	8.08	6.78	6.22
43	Makoszowy Niecka	3.39	3.27	3.19	1.89	1.32	3.40	3.29	3.20	1.90	1.34
44	Milicyjny	2.19	2.08	1.99	0.69	0.13	2.31	2.20	2.12	0.82	0.25
45	Mały	4.99	4.88	4.80	3.49	2.93	5.03	4.92	4.84	3.53	2.97
46	Kajakowy	5.34	5.23	5.15	3.85	3.28	5.41	5.29	5.21	3.91	3.34
47	Łąka	6.46	6.34	6.26	4.96	4.39	6.48	6.36	6.28	4.98	4.41
48	Ozdobny	4.09	3.97	3.89	2.59	2.02	4.23	4.11	4.03	2.73	2.16
49	Makoszowy Las	2.15	2.04	1.96	0.65	0.09	2.18	2.07	1.98	0.68	0.12
50	Brzeziny	8.64	8.53	8.45	7.15	6.58	8.68	8.57	8.49	7.19	6.62

^aAccording to Lis and Pasieczna (1995a).

^bAccording to Taylor and McLennan (1995), Li (2000), and Li and Schoonmaker (2005).

^cAccording to Turekian and Wedepohl (1961).

^dAccording to Håkanson (1980).

^eAccording to Lis and Pasieczna (1995b).

Practically uncontaminated (class 0: I_geo ≤ 0.0)

Uncontaminated to moderately contaminated (class I: 0.0 < I_geo ≤ 1.0)

Moderately contaminated (class II: 1.0 < I_geo ≤ 2.0)

Moderately to heavily contaminated (class III: 2.0 < I_geo ≤ 3.0)

Heavily contaminated (class IV: 3.0 < I_geo ≤ 4.0)

Heavily to extremely contaminated (class V: 4.0 < I_geo ≤ 5.0)

Extremely contaminated (class VI: I_geo > 5.0)

Table 4.

The contamination factor values for zinc in the bottom sediments of water bodies used for recreational purposes on the Silesian Upland and its periphery

Water Body Name (Numbering as in Figure 1)		$C_f^i$ ($C_{\mathbf{0}\mathbf{-}\mathbf{1}}^i={\text{Zn}}_{\text{minimum}}$)	$C_f^i$ ($C_{\mathbf{0}-\mathbf{1}}^i={\text{Zn}}_{\text{mean}}$)	$C_f^i$ ($C_{\mathbf{0}-\mathbf{1}}^i={\text{Zn}}_{\text{maximum}}$)
		$C_n^i=\mathbf{175.0}$ mg/kg
1	Dzierżno Duże	2.1	4.2	7.9
2	Dzierżno Małe	1.6	2.1	2.7
3	Pogoria I	1.3	7.3	13.4
4	Pogoria II	2.5	3.5	4.5
5	Pogoria III	0.8	2.7	7.0
6	Chechło	7.8	8.1	8.5
7	Stawiki	33.9	43.3	52.6
8	Morawa	78.9	140.3	201.7
9	Hubertus I	182.9	201.1	219.4
10	Gliniak	93.1	129.1	165.1
11	Hubertus II	13.5	14.2	14.9
12	Borki	4.6	13.3	22.0
13	Borki Małe	19.5	19.7	19.8
14	Sosina	9.3	9.5	9.7
15	Pławniowice	0.9	1.0	1.1
16	Mały Zalew	5.9	5.9	5.9
17	Rogoźnik III (E)	5.7	5.8	5.9
18	Rogoźnik II	4.4	4.6	4.8
19	Rogoźnik I (W)	2.4	2.5	2.6
20	Balaton	5.7	5.8	5.9
21	Czeladź Norwida	6.9	7.3	7.6
22	Kazimierz	6.8	7.1	7.5
23	Gliniok	1.5	1.5	1.5
24	Koparki	6.2	6.3	6.5
25	Amendy	8.5	8.6	8.6
26	Kozłowa Góra	4.3	9.8	15.4
27	Przeczyce	7.6	8.2	8.9
28	Paprocany	4.0	4.3	4.6
29	Wielikąt	0.5	0.7	0.8
30	Łężczok	3.0	3.0	3.1
31	Kradziejówka	0.8	0.9	1.1
32	Harmęże	0.9	1.0	1.1
33	Sławków	16.6	21.8	27.0
34	Przeczyce—pond	5.5	9.2	15.7
35	Ostrożnica	9.4	9.7	9.9
36	Przetok	26.7	26.9	27.1
37	Żabie Doły	14.2	20.6	27.4
38	Bobrek—rozlewisko	21.1	21.3	21.4
39	Pekin—misa E	12.5	13.1	13.8
40	Pekin—misa S	9.3	9.5	9.8
41	Pekin—misa W	6.9	7.1	7.3
42	Przy Leśnej	160.6	162.9	165.1
43	Makoszowy Niecka	5.6	5.6	5.6
44	Milicyjny	2.4	2.5	2.6
45	Mały	16.9	17.1	17.4
46	Kajakowy	21.6	22.1	22.5
47	Łąka	46.6	47.0	47.3
48	Ozdobny	9.0	9.5	9.9
49	Makoszowy Las	2.4	2.4	2.4
50	Brzeziny	212.6	215.4	218.3

Explanations:

$C_f^i<1$ —No contamination or low sediment contamination

$1\le C_f^i<3$ —Moderate contamination

$3\le C_f^i<6$ —Significant contamination

$C_f^i>6$ —Very heavy contamination

Table 5.

Anthropogenic enrichment factor values (I_AP) and the ratio of the concentrations found to the regional geochemical background for zinc (I_RE) in bottom sediments of water bodies used for recreational purposes on the Silesian Upland and its periphery

Water Body Name (numbering as in Figure 1)		I RE		I AP
		(CBS = Znminimum)	(CBS = Znmaximum)	(CBS = Znminimum)	(CBS = Znmaximum)
		C GB = 259.0 mg/kg		42.0 mg/kg < CSR ≤ 576.0 mg/kg
1	Dzierżno Duże	1.4	5.3	2.1	7.8
2	Dzierżno Małe	1.1	1.9	1.5	2.6
3	Pogoria I	0.9	9.0	3.3	33.4
4	Pogoria II	1.7	3.0	5.7	10.5
5	Pogoria III	0.5	4.7	2.0	16.9
6	Chechło	5.3	5.7	3.5	3.8
7	Stawiki	22.9	35.6	51.2	79.4
8	Morawa	53.3	136.3	132.7	339.4
9	Hubertus I	123.6	148.3	426.7	512.0
10	Gliniak	62.9	111.6	140.5	249.1
11	Hubertus II	9.1	10.1	20.7	22.9
12	Borki	3.1	14.9	7.7	36.7
13	Borki Małe	13.2	13.4	34.8	35.4
14	Sosina	6.3	6.5	21.4	22.2
15	Pławniowice	0.6	0.8	3.9	4.7
16	Mały Zalew	4.0	4.0	24.4	24.6
17	Rogoźnik III (E)	3.8	4.0	7.0	7.3
18	Rogoźnik II	2.9	3.2	5.1	5.6
19	Rogoźnik I (W)	1.6	1.7	2.8	3.0
20	Balaton	3.9	4.0	8.9	9.2
21	Czeladź Norwida	4.7	5.1	8.2	9.0
22	Kazimierz	4.6	5.1	9.9	10.9
23	Gliniok	1.0	1.0	1.5	1.6
24	Koparki	4.2	4.4	9.0	9.3
25	Amendy	5.7	5.8	10.0	10.2
26	Kozłowa Góra	2.9	10.4	4.9	17.4
27	Przeczyce	5.2	6.0	2.3	2.7
28	Paprocany	2.7	3.1	3.2	3.6
29	Wielikąt	0.3	0.6	1.2	2.1
30	Łężczok	2.0	2.1	7.5	7.7
31	Kradziejówka	0.5	0.8	1.8	2.6
32	Harmęże	0.6	0.7	1.9	2.3
33	Sławków	11.2	18.2	13.2	21.5
34	Przeczyce—pond	3.7	10.6	12.8	36.5
35	Ostrożnica	6.4	6.7	6.5	6.9
36	Przetok	18.1	18.3	58.5	59.4
37	Żabie Doły	9.6	18.5	13.2	25.5
38	Bobrek—rozlewisko	14.2	14.5	38.8	39.5
39	Pekin—basin E	8.5	9.3	23.1	25.4
40	Pekin—basin S	6.3	6.6	18.5	19.4
41	Pekin—basin W	4.7	4.9	14.2	14.9
42	Przy Leśnej	108.5	111.6	223.0	229.4
43	Makoszowy Niecka	3.8	3.8	8.1	8.2
44	Milicyjny	1.6	1.8	7.3	8.0
45	Mały	11.4	11.7	47.7	49.1
46	Kajakowy	14.6	15.2	65.1	68.0
47	Łąka	31.5	32.0	140.7	142.8
48	Ozdobny	6.1	6.7	25.5	28.1
49	Makoszowy Las	1.6	1.6	5.90	6.01
50	Brzeziny	143.6	147.5	128.28	131.72

Explanations:

0.0 < I_RE ≤ 1.0; 0.0 < I_AP ≤ 1.0

1.0 < I_RE ≤ 5.0; 1.0 < I_AP ≤ 5.0

5.0 < I_RE ≤ 10.0; 5.0 < I_AP ≤ 10.0

10.0 < I_RE ≤ 100.0; 10.0 < I_AP ≤ 100.0

I _RE > 100.0; I_AP > 100.0

Discussion

Variations in zinc concentration

Since bottom sediments have multiple origins, their chemical composition is largely dependent on catchment conditions—both the geochemical background and human pressure (Pirrone and Keeler, 1996; Cheung et al., 2003; Nguessan et al., 2009; Joshi and Balasubramanian, 2010). Thus, water bodies serve as sedimentary basins and the sediments within them register the phenomena and geomorphological processes occurring in the environment (Jernström et al., 2010). This provides an opportunity to use the characteristics of these sediments as contamination indicators.

Recently, so-called trace elements, especially toxic metals, have been commonly used as contamination indicators. These are subject to sorption in the aquatic environment by mineral suspended matter (e.g., silty minerals) as well as by living and dead organic matter (e.g., algae, humic substances), and a significant part of these micropollutants accumulate in bottom sediments (Loska and Wiechuła, 2003; Yang and Rose, 2005; Ghrefat and Yusuf, 2006; Skorbiłowicz and Skorbiłowicz, 2011). Zinc was selected as a measure of contamination of bottom sediments because of its very broad range of occurrence—from concentrations corresponding to geochemical background levels to those rarely found in lake and water reservoir sediment environments.

In general, the lowest zinc concentrations are found in the bottom sediments of water bodies that are effectively isolated from the mass influx of pollutants, for example, Wielikąt (83.0–148.0 mg/kg), Kradziejówka (138.0–198.0 mg/kg), Harmęże (160.0–194.0 mg/kg), and Pławniowice (165.0–199.0 mg/kg).

The highest concentrations, on the other hand, are found in the bottom sediments of water bodies whose basins come into contact with zinc and lead ore mining and smelting wastes. High zinc concentrations reflect the transformation of the natural environment in their vicinity, including, inter alia, soil contamination and the formation of so-called anthropogenic soils, which often exhibit many similarities to sediments of similar origin accumulated in the vicinity. Such land (possibly including anthropogenic soils) often underlies water body basins (Jaguś et al., 2013). One example is the Brzeziny water body whose bottom includes material displaced from an adjacent zinc and lead smelter waste heap and transformed into sediments through deposition processes. Within the basin of the Hubertus II water body, waste from a nearby nonferrous smelter was stored. In the case of the Stawiki, Morawa, Hubertus I, and Gliniak water bodies, high zinc concentrations have been conditioned by the contact of their basins with metallurgical waste from a nearby nonferrous smelter, which waste was used as material for the construction of levees and embankments within these water bodies as well as lining the former mineral workings in which these water bodies formed. Similarly, mining and processing waste underlies the basin of the Przy Leśnej water body.

Compared to the aforementioned extreme values, most of the water bodies studied are characterized by intermediate zinc concentration levels in bottom sediments. These water bodies are subject to fairly strong anthropogenic impact. Local conditions, including primarily land use and air pollution, play an important role with respect to zinc concentrations in bottom sediments (Rzetala, 2014). These conditions, as expressed by trace element concentrations, are matched by the different patterns of use of water body catchments (Rzetala et al., 2023). Nevertheless, attempts to demonstrate correlation between the type of land use and land cover of a catchment area on the one hand and zinc concentration levels on the other hand are ineffective due to possible spurious relationships. In many cases, high levels of zinc contamination of bottom sediments are caused by a point source of pollution located in a forested catchment or outside the water body catchment. Conversely, in catchments associated with high human pressure—urbanized and industrialized ones—zinc contamination of sediments is low.

Compared to zinc concentrations in the bottom sediments of the water bodies studied (ranging from 83.0 mg/kg to 38,400.0 mg/kg), concentrations of this element in precipitation are several orders of magnitude lower, amounting to a maximum of a few mg per liter, for example, the vicinity of the Dzierżno Duże Reservoir: 0.03–0.330 mg/l, the Dzierżno Małe Reservoir: 0.69–1.34 mg/l, the Pławniowice Reservoir: 0.1–5.91 mg/l (Kostecki, 2002); for water reservoirs in the vicinity of Katowice city: 0.026–0.263 mg/l in 2017, 0.032–0.463 mg/l in 2018 (WIOŚ, 2025). Similarly, its contents in surface waters are in most cases in the range of a few to a few dozen µg per liter, for example, the Dzierżno Duże Reservoir: <10–32 μg/l in 2016, the Kozłowa Góra Reservoir: 10–80 μg/l in 2016, the Przeczyce Reservoir: 13 μg/l in 2017, the Pławniowice Reservoir: 20 μg/l in 2017 (WIOŚ, 2025). The figures cited demonstrate the overriding impact of anthropogenic supply on zinc concentrations in bottom sediments (e.g., as a result of the use of sediments produced in nonferrous metal smelters in the construction of water body basins, the supply of contaminated debris by surface tributaries, dry deposition in highly urbanized and industrialized regions), and, above all, the accumulation of this element in water body bottom sediments.

Geochemical properties of sediments—Comparison with literature data

Nowadays, the circulation of zinc in the natural environment is essentially due to human activities. The natural geochemical background in Poland for this element is relatively low and ranges from 10.0 mg/kg to 120.0 mg/kg depending on the type of rock (Kabata-Pendias and Pendias, 1993). The industrial use of zinc in the study area dates back to the late 18th and early 19th centuries, when it was successfully smelted for the first time. It should be noted that zinc-containing calamine ores were widely used much earlier for other purposes, including as an ingredient in brass production (Warchulski et al., 2018).

As mentioned above, the studies carried out demonstrated a wide range of zinc concentrations in the bottom sediments of the region’s anthropogenic water bodies: from 83.0 mg/kg up to 38,400.0 mg/kg. In the vast majority of cases, these zinc concentrations are much higher than the geochemical background proposed by Klimek et al. (1995) for the alluvia of the Przemsza River in the preindustrial period, and also exceed the zinc levels reported by Ciszewski (1992; 1999) that are considered natural for fluvial sediments in the region. The zinc levels observed in the sediments examined are also much higher than the concentration of this element measured by Kocel (1997) in the bottom sediments of former ponds (36.0–160.0 mg/kg) in the Ruda River Valley, in the west of the study area. These large variations result directly from the fact that individual artificial water reservoir are situated in catchments with different human pressure characteristics. Above-average (and even record-beating) concentrations of zinc in the bottom sediments of the water bodies studied, when compared to analogous concentrations of this metal in the sediments of dozens of lakes worldwide, indicate the uniqueness of the Silesian Upland and adjacent areas in terms of zinc levels (Table 6).

Table 6.

Zinc content in bottom sediments of lakes and anthropogenic water bodies in the world—A synthetic summary of study results

Location of the Study Area	Water Body Labels and the Concentration of Zinc in the Bottom Sediments (mg/kg)
Silesian Upland together with the peripheries of adjacent areas (this study)	Water body on the Silesian Upland together with the peripheries of adjacent areas (this study): 83.0 mg/kg–38,400.0 mg/kg
Lakes and water reservoirs in other regions of Poland	Żywiec Reservoir: 83.0–177.0 mg/kg (Magiera et al., 2002); Lake Sunia: 21.2–79.9 mg/kg (Sidoruk and Potasznik, 2015); reservoir in the urban area of Rzeszów: 86.72–120.5 mg/kg (Tarnawski and Baran, 2018); Zalew Zemborzycki: 1.9–40.6 mg/kg, Brody Iłżeckie Reservoir: 3.0–352.0 mg/kg (Smal et al., 2022); Chańcza Reservoir: 61.50–212.00 mg/kg (Baran et al., 2011); Czerniakowskie Lake: 123.7–269.1 mg/kg (Wojtkowska, 2013); Straszyn Reservoir: 14.69–92.47 mg/kg (Kulbat and Sokołowska, 2019); Słup Reservoir: 44.52–164.61 mg/kg (Senze et al., 2017); 5 small water bodies located in rural northwestern Poland: 15.9–195.9 mg/kg (Siwek et al., 2012); 6 water bodies located in lowland areas in western Poland: 6.08–1,990.4 mg/kg (Sojka et al., 2019); Hańcza Lake: 6.0–118.0 mg/kg (Choiński et al., 1999)
Europe—other countries (without Poland)	Lake Balaton (Hungary): 13.0–150.0 mg/kg (Nguyen et al., 2005); Lake Seliger system (Russia): 5.28–170 mg/kg (Kosov et al., 2004); Koronia Lake (Greece): 72.12–99.60 mg/kg, Volvi Lake (Greece): 47.93–58.93 mg/kg (Gantidis et al., 2007); San Giuliano Reservoir (Italy): 11–81 mg/kg, Camastra Reservoir (Italy): 67.6–106.0 mg/kg (Martellotta et al., 2024). Castilseras Reservoir: 65.61–116.82 mg/kg (García-Ordiales et al., 2016); Dnieper Cascade (Ukraine): 35.2–186.5 mg/kg (Linnik and Zubenko, 2000). Žalieji Lake (Lithuania): 106.0 mg/kg, Drūkšiai Lake (Lithuania): 69 mg/kg (Tautkus et al., 2007). 49 lakes in Latvia: 15.21–78.43 mg/kg (Klavinš et al., 1995); 20 lakes in northwestern Russia: 17.0–1,327.0 mg/kg (Dauvalter, 1994); water bodies in Stockholm: 170.0–1,539.0 mg/kg (Lindström and Håkanson, 2001).
Asia	Seyhan Reservoir (Turkey): 32.88–43.34 mg/kg (Çevik et al., 2009); Atatürk Reservoir: 59.14–60.79 mg/kg (Karadede and Ünlü, 2000); Lake Uluabat (Turkey): 0.74–8.36 mg/kg (Barlas et al., 2005); Kafrain Reservoir (Jordan): 75.0–184.0 mg/kg (Ghrefat et al., 2011); Lake Sapance (Turkey): 39.02–75.31 mg/kg (Duman et al., 2007); Three Gorges Reservoir (China): 152.0–211.0 mg/kg (Bing et al., 2016); Tongjiqiao Dam Reservoir (China): 47.6–219.0 mg/kg (Bao et al, 2023); Tong Reservoir (China): 141.45–186.73 mg/kg; Yang Reservoir (China): 54.89–95.02 mg/kg (Khan et al., 2019); Jiangang Reservoir (China): 93.80–111.28 mg/kg (Liu et al., 2023); Qingshan Reservoir (China): 125.44–232.31 mg/kg (Yang et al., 2016); Taihu Lake (China): 71.6–370.3 mg/kg (Wang et al., 2004).
Africa	Upper Wadi El-Rayan Lake (Egypt): 66.80–179.39 mg/kg, Lower Wadi El-Rayan Lake (Egypt): 43.02–253.20 mg/kg (Khedr et al., 2023); Kenya’s Rift Valley lakes: 96.2–229.6 mg/kg (Ochieng et al., 2007).
Australia	Lake Macquarie (Australia): 14.0–1003.3 mg/kg (Roach, 2005); zbiorniki wodne w regionie Coffs Harbour w Nowej Południowej Walii (Australia): 16.5–111.5 mg/kg (Conrad et al., 2021); Emigrant Creek Dam (Australia): 40.0–280.0 mg/kg (Akhurst et al., 2012); Mulwala Lake (Australia): 31.0–140.0 mg/kg (Baldwin and Howitt, 2007).
North and South America	North Nashua River Cascade (Massachusetts, USA): 53.0–439.8 (average) mg/kg (Clark et al., 2024); Lake Hope Reservoir (Ohio, USA): 101.0–157.0 mg/kg (López et al., 2010); Lake Pontchartrain (Louisiana, USA): 15.2–29.5 mg/kg (Byrne and DeLeon, 1986); Tuskegee Lake (USA): 8.72 mg/kg (Ikem et al., 2003); Lake Texoma (USA): 33.0–242.0 mg/kg (An and Kampbell, 2003); 7 reservoirs in the state of Sao Paulo (Brazil): 39.8–138.59 mg/kg (Frascareli et al., 2018); Foz do Rio Claro reservoir (Brazil): 26.38–159.60 mg/kg (Cabral et al., 2021).

Concentrations of the analyzed trace element in the sediments of lakes in other regions of Poland are generally lower than those found in the bottom sediments of water bodies on the Silesian Upland and its periphery. This pattern is evidenced by a number of results from studies conducted in other regions of the country. Some results of such studies, which concern inland bodies of water that serve recreational functions to varying degrees, are worth listing here.

Low zinc concentrations were found in the Żywiec Reservoir (83.0–177.0 mg/kg), which is not far from the water bodies studied (Magiera et al., 2002). Another example is the Rybnik water body where zinc concentrations in sediments range from 51.0 mg/kg to 2,451.0 mg/kg. Zinc concentrations much lower than those in the study area are also present in the bottom sediments of the Boszkowo, Dominickie, and Wielkie Lakes in Wielkopolska (Greater Poland) (Szymanowska et al., 1999). In Masuria (one of the regions in Poland least transformed by human pressure), zinc was recorded in the sediments of Lake Sunia near Olsztyn in the range of just 21.2–79.9 mg/kg, with an average of 72.3 mg/kg (Sidoruk and Potasznik, 2015). Slightly higher concentrations, ranging from 86.72 mg/kg to 120.5 mg/kg, were found in the bottom sediments of a reservoir impounded by dam located in the urban area of Rzeszów in southeastern Poland (Tarnawski and Baran, 2018). Zinc content in the bottom sediments of water bodies used for recreational purposes in southeastern Poland ranged from 1.9 mg/kg to 40.6 mg/kg with an average value of 11.08 mg/kg in the case of the Zalew Zemborzycki water body and from 3.0 mg/kg to 352.0 mg/kg with an average value of 92.93 mg/kg in the sediments of the Brody Iłżeckie water body (Smal et al., 2022); in the Chańcza water body, the range was from 61.50 mg/kg to 212.00 mg/kg with an average value of 112.12 mg/kg (Baran et al., 2011). In the bottom sediments of the Czerniakowskie Lake in Warsaw, the capital of Poland, zinc concentration averaged 199.75 mg/kg, ranging from 123.7 mg/kg to 269.1 mg/kg (Wojtkowska, 2013). In the Straszyn water body in northern Poland, located near the southern periphery of Gdańsk, zinc concentrations in sediments ranged from 14.69 mg/kg to 92.47 mg/kg with an average value of 55.19 mg/kg (Kulbat and Sokołowska, 2019). In the southwestern part of Poland, the Słup Reservoir was constructed on the Nysa Szalona River. Zinc concentration tests in its bottom sediments demonstrated both spatial and seasonal variability. Depending on the season and sampling location, the concentrations varied from 44.52 mg/kg to 164.61 mg/kg and averaged in the range of 96.62–117.35 mg/kg (Senze et al., 2017). The average zinc content in the bottom sediments of 5 small water bodies located in rural northwestern Poland ranged from 25.8 mg/kg to 118.2 mg/kg, with a minimum of 15.9 mg/kg in one water body and a maximum of 195.9 mg/kg in another (Siwek et al., 2012). A study of 6 water bodies located in lowland areas in western Poland yielded an average zinc concentration of 452.2 mg/kg in bottom sediments, with a range from 23.1 mg/kg to 903.7 mg/kg. Maximum values ranged from 60.1 mg/kg to 1,990.4 mg/kg, and minimum ones from 6.08 mg/kg to 143.6 mg/kg (Sojka et al., 2019). No water bodies in other regions of Poland showed zinc concentration levels that would be even similar to those found in the most polluted artificial water reservoirs sampled. In general, studies carried out in other regions of Poland have shown significantly lower concentrations of this element in the bottom sediments of lakes and artificial water bodies. The high concentration of zinc in the sediments studied is particularly striking against concentrations of this element in the range of 6–118 mg/kg cited by Choiński et al. (1999) for bottom sediments of Lake Hańcza—the deepest Polish lake, which is a nature reserve that is effectively isolated from the inflow of pollutants, and thus is considered a kind of reference point in limnological research.

Much lower concentrations of zinc in bottom sediments are also mentioned in research reports from other parts of Europe. These include Hungary’s Lake Balaton (Nguyen et al., 2005), Russia’s Lake Seliger system (Kosov et al., 2004), and Greece’s Koronia and Volvi Lakes (Gantidis et al., 2007). Reservoirs impounded by dams located in the southern part of Italy are characterized by zinc concentrations in bottom sediments ranging from 11 mg/kg to 81 mg/kg with an average value of 45.31 mg/kg (the San Giuliano Reservoir), and from 67.6 mg/kg to 106.0 mg/kg with an average value of 83.93 mg/kg (the Camastra Reservoir) (Martellotta et al., 2024). In the Castilseras Reservoir in Spain, the results obtained ranged from 65.61 mg/kg to 116.82 mg/kg with an average value of 103.58 mg/kg (García-Ordiales et al., 2016). The reservoirs that make up the Dnieper River cascade in Ukraine (the Kyiv, Kremenchuk, Zaporizhzhia, and Kakhovka Reservoirs) are characterized by zinc concentrations (mg/kg) in their bottom sediments within the following ranges, respectively: 36.8–105.8, 35.2–89.8, 50.6–186.5, 41.5–142.0 (Linnik and Zubenko, 2000). In the Lithuanian Žalieji and Drūkšiai Lakes, zinc was found in bottom sediments at concentrations of 106 and 69 mg/kg, respectively (Tautkus et al., 2007). Tests conducted in 49 lakes in neighboring Latvia showed zinc concentrations in bottom sediments ranging from 15.21 mg/kg to 78.43 mg/kg (Klavinš et al., 1995). Bottom sediments of more than 20 lakes in northwestern Russia, close to the border with Norway, which are subject to very strong industrial anthropogenic pressure, exhibit much lower concentrations of zinc (17.0–1,327.0 mg/kg) (Dauvalter, 1994). Even sediments of inner-city water bodies in Stockholm are generally characterized by lower concentrations of this metal (170.0–1,539.0 mg/kg) (Lindström and Håkanson, 2001).

A similar situation occurs in lakes and other water bodies outside Europe. The bodies of water in question are used for recreation purposes to varying degrees.

In the Seyhan Reservoir in the southern part of Turkey, zinc concentrations ranged from 32.88 mg/kg to 43.34 mg/kg (Çevik et al., 2009). In the sediments accumulated in the basin of the Atatürk Reservoir, the levels were slightly higher, oscillating around 60 mg/kg (Karadede and Ünlü, 2000). Significantly lower values (0.74–8.36 mg/kg) were found in the sediments of Lake Uluabat in western Turkey (Barlas et al., 2005). The highest average concentration of 8.36 mg/kg was measured in September, with even lower concentrations in other months (Barlas et al., 2005). In the Kafrain Reservoir located in the desert area of Jordan (Wadi Kafrain), zinc concentrations in the surface layer of sediments varied from 75 mg/kg to 184 mg/kg depending on the sampling location (Ghrefat et al., 2011). The sediments of Lake Sapance in Turkey are free of zinc pollution despite its exposure to urbanization and industrialization in its catchment—39.02–75.31 mg/kg (Duman et al., 2007). Numerous studies that have been conducted over the years on natural environment pollution in China show that many areas in the country have been degraded (Zhang et al., 2007; He et al., 2017; Sarkodie et al., 2020). Despite the facts cited in these studies that reflect the negative impact of human pressure on many aspects of the environment, several lakes and other water bodies in this part of Asia show low zinc accumulation in bottom sediments. This includes, for instance, one of the world’s largest reservoirs—the Three Gorges Reservoir—where zinc concentrations in sediments range from 152.0 mg/kg to 211.0 mg/kg (Bing et al., 2016). In the surface layer of bottom sediments of the Tongjiqiao Dam Reservoir, which is a typical small- or medium-sized reservoir (about 4.5 km²) by Chinese standards, zinc concentrations ranged from 47.6 mg/kg to 219.0 mg/kg (Bao et al., 2023). Toxic metal content, including zinc, was measured in the sediments of 2 (Tong and Yang) reservoirs located in southeastern China. Tests showed zinc in the former reservoir in the range of 141.45–186.73 mg/kg, and in the latter the readings were much lower at 54.89–95.02 mg/kg (Khan et al., 2019). In the Jiangang Reservoir, which is an important source of water for China’s central region, average zinc concentrations fluctuated from 93.80 mg/kg in February 2018 to 111.28 mg/kg in August 2019 (Liu et al., 2023). The Qingshan Reservoir located in the eastern part of China is also a source of water supply. The average concentration of zinc in its sediments was slightly higher, ranging from 125.44 mg/kg to 232.31 mg/kg (Yang et al., 2016). The bottom sediments of Taihu Lake in China (Wang et al., 2004), which is subject to urban-industrial and agricultural pressure, contain zinc in incomparably lower amounts (71.6–370.3 mg/kg) than those found in water bodies on the Silesian Upland.

In the case of bottom sediments of the Upper Wadi El-Rayan Lake and Lower Wadi El-Rayan Lake located in equally dry areas of Egypt, zinc concentrations varied over slightly larger ranges of 66.80–179.39 mg/kg and 43.02–253.20 mg/kg, respectively (Khedr et al., 2023). Despite the significant impact of lithospheric factors and human pressure, the bottom sediments of Kenya’s Rift Valley lakes show average zinc concentrations of 96.2–229.6 mg/kg (Ochieng et al., 2007).

Incomparably lower concentrations of this metal have been found in the bottom sediments of the Australian coastal lagoon called Lake Macquarie, which is subject to human pressure related to land use within its catchment and along its coast (Roach, 2005). Still more examples of zinc concentrations in bottom sediments concern the following water bodies: reservoirs in the Coffs Harbour region of New South Wales (Australia)—16.5–111.5 mg/kg (Conrad et al., 2021), Emigrant Creek Dam—40.0–280.0 mg/kg (Akhurst et al., 2012), Lake Mulwala—31.0–140.0 mg/kg (Baldwin and Howitt, 2007).

Concentrations of zinc that would be higher than those in Silesian Upland reservoirs were also not found in more than a dozen reservoirs impounded by dams in the North Nashua River catchment in central Massachusetts, United States. The lower part of this catchment area has been highly urbanized and industrialized for many years. Despite the long period of human activity in the area, zinc concentrations in bottom sediments averaged between 53.0 and 439.8 mg/kg. Only in one reservoir did the maximum zinc concentrations exceed 1,000 mg/kg (Clark et al., 2024). Relatively low average zinc concentrations (in the range from 101 mg/kg to 157 mg/kg) were found in the sediments of the Lake Hope reservoir, which was constructed on a stream draining water from a coal mine in southeastern Ohio, United States (López et al., 2010). In bottom sediments of Lake Pontchartrain where part of the water from the Mississippi River is diverted during high water stages as part of New Orleans flood protection measures, zinc concentrations (15.2–29.5 mg/kg) are at least an order of magnitude smaller than those found on the Silesian Upland (Byrne and DeLeon, 1986). Zinc concentrations are even lower in the bottom sediments of water bodies which are not subject to significant human pressure—a prime example here is Lake Texoma in the United States (An and Kampbell, 2003). Other examples include Tuskegee Lake (USA)—average Zn concentration 8.72 mg/kg (Ikem et al., 2003), 7 reservoirs in the state of Sao Paulo (Brazil)—39.8–138.59 mg/kg (Frascareli et al., 2018), Foz do Rio Claro reservoir (Brazil)—26.38–159.60 mg/kg (Cabral et al., 2021).

The research conducted suggests that the presence of zinc in significant amounts in the bottom sediments of water bodies on the Silesian Upland and its periphery should be considered a spectacular or at least rare phenomenon, given the results of analogous studies in different parts of the world (Dauvalter, 1994; Szymanowska et al., 1999; Lindström and Håkanson, 2001; Magiera et al., 2002; An and Kampbell, 2003; Ikem et al., 2003; Kosov et al., 2004; Wang et al., 2004; Roach, 2005; Duman et al., 2007; Gantidis et al., 2007; Ochieng et al., 2007; Mutia et al., 2012; Hahladakis et al., 2013; Jaguś et al., 2013; Khan et al., 2019; Rzetala et al., 2023).

Interpretation of geochemical indicators

In studies on limnic environments, cases where zinc concentrations in bottom sediments are higher than those usually found in sedimentary rocks provide a primary indicator of human pressure (Rzetala, 2008). Taking into account the range of natural zinc levels in the sediments of surface waters on the Silesian Upland and its periphery, as determined by Lis and Pasieczna (1995b), it can be concluded that elevated zinc concentrations are only absent in a few cases. The fact that the prevalent concentration of zinc in bottom sediments exceeds the natural range of concentrations of this element is confirmed by not only the median but also the first quartile of concentrations in the samples being much higher than the geochemical background (Table 2). This is also evidenced by the values of the geoaccumulation index (I_geo) (Table 3). Occasionally, these confirm the absence of zinc contamination of bottom sediments. Overwhelmingly, however, the geoaccumulation index is typical of moderately contaminated sediments (0.0 < I_geo < 1.0 or 1.0 < I_geo < 2.0) or indicates strong (2.0 < I_geo < 3.0 or 3.0 < I_geo < 4.0) or even extreme contamination (4.0 < I_geo < 5.0 or 5.0 < I_geo). Such contamination levels are indicative of a geoecological problem present in a region with a high population density, where the aquatic environment is considered to offer significant potential for recreation and leisure (Dwucet et al., 2016).

A slightly more accurate spectrum of zinc contamination of water body sediments was obtained by analyzing the ratio of values measured to the regional geochemical background (I_RE). As mentioned earlier, this indicator is dimensionless and exceeds unity if the concentration of the element in question is higher than the geochemical background (the higher the concentration the higher the ratio) (Rzetala, 2015a; 2015b; 2016). It may also fall below unity if the concentration of a given element in sediments is less than the geochemical background. For the sediments studied, the ratio of the values measured to the geochemical background ranged from 0.3 to 148.3. While cases where geochemical background for zinc was not exceeded were rather rare (6 times for minimum concentrations and 4 times for average and maximum concentrations), it was not uncommon to find concentrations that exceeded it a few, a dozen, or even more than a hundred times in several cases.

The presence of bottom sediment contamination is highlighted by comparing modern concentrations of zinc in sediments to its concentrations from the preindustrial period. Apart from the very rare cases of no or low sediment contamination ($C_f^i<1$) and rare cases of moderate contamination ($1\le C_f^i<3$), significant ($3\le C_f^i<6$) and primarily very heavy contamination ($C_f^i>6$) of sediments with zinc is the most common finding. The highest contamination factor values found in several cases reflect concentrations characteristic of the preindustrial period being exceeded by more than 200 times.

Basically, all the indicators presented identify the geoecological status of bottom sediments in a similar manner, indicating various contamination levels, and the differences in classification result from slightly different levels being considered natural. In this context, the most useful solution is assessing the anthropogenic enrichment factor of bottom sediments with zinc. This value can be determined by comparing zinc concentration in bottom sediments to that found in the substrate and in the vicinity of the basin in which the bottom sediments have accumulated. In view of the fact that the age of the water bodies in question does not exceed 40–50 years in most cases, the anthropogenic enrichment factor of sediments is the most reliable indicator of their pollution. The values of the indicator in question range from slightly above one in a handful of cases to as much as several hundred. This demonstrates that bottom sediments of the water bodies studied accumulate zinc to a very high degree and also points to its very significant retention in this environment.

Zinc contamination, in varying degrees, of bottom sediments of water bodies used for recreational purposes leads to many geoecological problems as well as conflicts concerning the natural and socioeconomic roles and functions of these water bodies. Some water bodies are treated as multifunctional recreation areas used for , sailing, canoeing, fishing, scuba diving, water sports, sport and recreational swimming, and beach leisure, as well as nature conservation spaces, while others have specific uses, with the prevalent one being recreational and sport fishing. Zinc concentrations in the bottom sediments of the water bodies studied, which deviate from natural levels, are also often an obstacle to non-recreational functions of these water bodies, for example, as a source of water for domestic use, agriculture and industry. Therefore, the proposed interpretations of geoecological indicators of zinc presence in bottom sediments (I_geo, I_RE, $C_f^i$, I_AP) can be used to optimize the monitoring of the quantitative and qualitative status of reservoir retention. A ratio of concentrations to the regional geochemical background of I_RE ≤ 5.0 can be considered a reasonable threshold for decisions permitting recreational use of water bodies (Rzetala et al., 2023). For higher values of this indicator, excluding the water body from recreational use, or at least restricting its recreational use should be considered, especially for activities that involve the direct contact of living organisms with bottom sediments. These restrictions should be reflected in prohibiting the consumption of fish caught and other aquatic organisms harvested from these water bodies, diving, swimming, and using beaches that come into contact with contaminated sediments.

The values of the geoecological indicators calculated document the process of enrichment of bottom sediments with zinc. This process occurs after the formation of water bodies on a substrate that is free of contamination. This substrate is exposed either as a result of open-pit mining or in the process of the basin being prepared for flooding. The basin prepared for flooding in this manner is free of zinc contamination. Zinc is supplied to the bottom sediments together with water from tributaries as well as through atmospheric deposition, which results in its accumulation. Zinc is present in the bottom sediments of these reservoirs at concentrations ranging from 83.0 mg/kg to 38,400.0 mg/kg. On the other hand, zinc concentrations ranging from 42.0 mg/kg to 576.0 mg/kg, with an average of 129.1 mg/kg, were found in the sediments deposited in the vicinity of the water bodies studied (i.e., those considered to form the substrate of their basins). Higher zinc concentrations can be found in the bottom sediments of water bodies in comparison to the substrate (vicinity) of their basins. The anthropogenic enrichment factor (I_AP) of the sediments with zinc ranges from 1.2 to 512. This demonstrates that in bottom sediments, zinc is always present at higher concentrations than in substrate sediments. Thus, the water body acts as a natural settling tank with the characteristics of a sedimentation basin, often endorheic, in which pollutants in transit (of allochthonous origin) accumulate. The local concentration of zinc in bottom sediments can be considered a manifestation of water purification, and the water bodies themselves can be seen as retention and siltation facilities that act as natural treatment plants. A case in point here is the Dzierżno Duże Reservoir, which accumulates pollutants from remote urban-industrial areas in its sediments. The basin of the Dzierżno Duże reservoir retains from about half (Kostecki, 2003) to more than 80% of the zinc load delivered with surface water and precipitation (Rzetala, 2000).

An ecological problem occurs when zinc contamination of bottom sediments is transposed to basin sectors where such contamination was not previously present. Pollutant transposition may also take place outside the water body via the water drained from it. In this process, water bodies provide a source of zinc released during the period of intensive mixing which takes place in shallow waters. This may be exacerbated by zinc being released from sediments under conditions of low sediment and water pH. The transposition of zinc contamination is typical of the Gliniak, Hubertus, Morawa, and Stawiki water bodies (Machowski et al., 2019). Their basins are flooded former sand-mining pits that do not exhibit zinc levels in excess of the geochemical background; the sources of contamination distributed throughout these basins are largely levees and transport embankments constructed using nonferrous smelting waste. A similar transposition of zinc contamination to uncontaminated areas from metallurgical waste disposal sites takes place in the Brzeziny water body. Tailings deposited in a reservoir with the characteristics of a settling tank may be a source of zinc migrating into groundwater, as was found in the Żelazny Most Reservoir in southwestern Poland in 1988; as a result of the infiltration of 20.0 hm³ of tailings, about 2,500.0 kg of zinc entered the groundwater (Haber and Urbański, 1994). A similar situation occurs with reservoirs that serve as settling tanks for nearby production facilities, for example, a tailings pond in Romania (Rzymski et al., 2017).

Environmental acidity contributes to contaminant migration and zinc mobility. Zinc is an element whose solubility and desorption decrease in proportion to the increase in sediment pH, and thus it reaches its highest concentration in acidic environments. The high concentration of zinc in the bottom sediments of most of the water bodies studied is a particular ecological threat in the context of the intensifying acidification of the environment—an issue that has been highlighted for many years (Newell and Skjelkvale, 1997; Mannio, 2001; Hynynen and Merilainen, 2005; Kopacek et al., 2006; Stuchlík et al., 2006). Therefore, the potential increase in the acidity of limnic water in some water bodies is a real threat associated with the unhindered increase in the mobility of the metals currently accumulated in bottom sediments and their movement to other areas linked by hydrographic drainage axes. The real environmental threat posed by the transport of zinc pollution is underscored by findings that mobile forms of this element account for up to 60% of its presence in bottom sediments under conditions of oxygen deficit and low pH (Kostecki, 2003). Oxygen deficit and even cases of anoxia are quite common below the epilimnion in periods of normal (anothermal) stratification and during the presence of compact ice cover in periods of reverse (catothermal) stratification (Rzetala, 2000). Therefore, the bottom sediments of some water bodies are a potential source of zinc contamination. The geoecological indicators used allow the identification of ecological risks and can help in the selection of sites that require reclamation.

A high concentration of zinc in bottom sediments, as confirmed by the values of these indicators, simply means a deterioration in the biotope and biocenotic conditions. At concentrations above 100 μg·dm⁻³, zinc is estimated to have toxic effects on fish (Jaguś et al., 2013). Excessive ingestion of zinc results in anemia (which adversely affects the assimilation of other elements). This element is deposited in the kidneys and liver and is also considered a carcinogen. As concerns the water bodies studied, high tench mortality (e.g., in the Hubertus water body) or the disappearance of eel populations (e.g., in the Morawa water body) are probably related to high toxic metal pollution (Kostecki, 2000; Jankowski et al., 2002), with zinc concentrations reaching the highest values in the world. The contamination of certain aquatic ecosystems on the Silesian Upland and in adjacent areas already poses a hazard to human health; the concentrations recorded in phyto- and zooplankton, vascular plants and ichthyofauna point to serious pollution (Kostecki, 2007; Rzetala et al., 2011). The local elimination of plant and animal species observed in the water bodies and in general in contaminated areas justifies the use of the term “zinc desert.” This is exemplified by the water bodies located in the vicinity of areas associated with nonferrous smelting (e.g., the Hubertus I and II, Morawa, Stawiki, Gliniak, Brzeziny).

Study results also indicate that the arrangement of water bodies in cascades along waterways affects zinc concentrations in bottom sediments. Typically, sediments in the first water body in the cascade formed along a stream are the most contaminated with zinc and the last water body exhibits a lower concentration (e.g., in the Pogoria and Rogoźnik water body complexes). A different pattern, however, can be observed in the water bodies situated along the Potok Leśny stream. In this catchment, the first water body within the cascade is polluted with trace elements to the lowest degree, while each subsequent water body is characterized by higher concentrations. This can be explained by, inter alia, the supply of pollutants together with rainwater and meltwater from the old (and today fragmentary) road drainage system, and the migration of pollutants from areas where waste from nonferrous metal smelters is stored (Molenda, 2001; Jankowski et al., 2002). The patterns found in the spatial variation of zinc concentrations in the bottom sediments of cascading water bodies indicate the possibility of leveraging the self-purification process in order to make the water bodies suitable for recreational use or optimize their use in this regard.

Very high values of geoecological indicators for zinc in bottom sediments, especially the ratio of its concentrations to the regional geochemical background, provide a strong rationale for revitalization, reclamation, and protection measures with respect to water bodies that are used for recreational purposes or those planned for such use. These values point to the need for targeted measures to remove bottom sediments containing toxic metals, nonmetals, and metalloids as well as other pollutants by, for instance, conducting dredging and silting works or removing them after draining the basin (Rzetala et al., 2023), but the use of other methods (e.g., phytoremediation using genera such as Typha, Nuphar, Nymphaea and other aquatic plants that can sequester heavy metals) cannot be ruled out as well. The possible use of this material as soil for reclamation and other earthworks is subject to compliance with soil and land quality standards. In light of legal restrictions, sediments from the water bodies in question could only be used to a limited extent—which additionally depends on soil water permeability—in industrial areas, opencast mines, and transport areas. The reason for this limitation is the exceeded permissible concentrations of primarily zinc, cadmium, and lead, and also other elements in some cases. The fact that bottom sediments, as discussed above, are contaminated with heavy metals, makes them completely unsuitable for use in agricultural, conservation, and forest areas. Irrespective of the destination of the sediments removed from the water bodies, the ultimate effect of reclamation treatments should be the elimination of the threat to the environment and human health and life associated with the potential exposure to toxic metals, nonmetals and metalloids.

Conclusions

Studies of zinc content in bottom sediments carried out within 50 water bodies make it possible to present several conclusions in terms of their suitability for recreational use.

• Bottom sediments in water bodies used for recreational purposes on the Silesian Upland and its periphery vary in terms of zinc concentrations both within individual water bodies and between them. These differences may reach several orders of magnitude, since the concentrations found range from 83.0 mg/kg to 38,400.0 mg/kg.

• Zinc is present in the bottom sediments of water bodies used for recreational purposes on the Silesian Upland and its periphery in amounts that usually exceed concentrations found in other water bodies, and in some cases its levels are record high and unprecedented among bodies of water in the world (e.g., the Hubertus I—38,400.0 mg/kg, Brzeziny—38,200.0 mg/kg, Morawa—35,300.0 mg/kg).

• The bottom sediments of water bodies used for recreational purposes on the Silesian Upland and its periphery are contaminated with zinc to varying degrees, as evidenced by the values of geoecological indices, that is, the geoaccumulation index (–2.2 < I_geo < 8.7), the sediment contamination factor ($0.5\le C_f^i<219.4$), the anthropogenic enrichment factor (1.2 ≤ I_AP < 512.0), and the ratio of the concentrations found to the regional geochemical background (0.3 < I_RE < 148.3). Zinc contamination of bottom sediments has been caused by human activity, with only minimal influence of natural conditions, and in several cases (e.g., the Hubertus II) it is the highest in the world.

• The presence of zinc in bottom sediments is a very good geoecological indicator of the surrounding environment and should be taken into account when classifying water bodies as suitable for recreational use, independently of the hydrochemical indicators used to date when assessing such suitability. It is proposed that a ratio of the concentrations found to the regional geochemical background of I_RE ≤ 5.0 be adopted as the acceptable range for recreational use of water bodies involving direct contact of living organisms with zinc-containing bottom sediments.

• The water bodies on the Silesian Upland and its periphery do not meet the geoecological conditions for their safe use in terms of recreation and leisure activities. Due to the fact that regional geochemical background levels of zinc in their bottom sediments are exceeded multiple times, forms of their recreational use that directly affect the participants’ health (e.g., fishing and the consumption of fish and other aquatic organisms, the use of sediments in pelotherapy) should be abandoned.

• The levels of zinc contamination in bottom sediments clearly indicate the need to revitalize, reclaim, and protect the water bodies in question, which should involve the removal of bottom sediments. These treatments should focus, inter alia, on eliminating the threat to the environment and human health.

Data availability statement

All data generated or analyzed during this study are included in this published article.

Supplemental files

The supplemental files for this article can be found as follows:

Preliminary Data.xlsx

Acknowledgments

We would also like to thank the linguistic team for correcting the English language of the manuscript.

Funding

The scientific work in this research project was financed by research funds available in the years 2020–2024 (project no. University of Silesia—WNP/INOZ/ZB-25).

Competing interests

The authors declare no competing interests.

Author contributions

All authors (MAR, MS, DB, AP, RM, and MR) have conceived and planned the study; conducted field work and analyzed the results; and wrote the paper. All authors have collaborated on manuscript editing at all stages.

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