1. Introduction
Reservoirs play important roles in water supply, irrigation, power generation and flood control. However, dam construction changes the hydrological regime and sediment transport [
1,
2,
3]. The suspended sediments are stored in the reservoir and with them pollutants including heavy metals (HMs) [
4]. Bottom sediment plays an important role in monitoring the aquatic environment [
5], especially evaluating the contamination levels [
6,
7], and ecological risk assessment [
8].
HMs have natural, i.e., geogenic (weathering and erosion), and anthropogenic origin (urban domestic and industrial waste and the use of chemical fertilizers, roadside soil) [
5,
7,
9,
10,
11]. They are classified as serious environmental pollutants due to persistence, tendency to bioaccumulate and high toxicity [
12].
The spatial distribution in HM concentrations among reservoirs is associated with different anthropogenic activities [
13,
14]. The distribution depends on hydrodynamic conditions, type of sediment and metal sources [
15]. Moreover, shape and morphology of the reservoir [
16], reservoir operation [
17] and the biochemical processes [
18] modify the HM deposition. The highest concentrations have been detected generally in the vicinity of polluted water inflow [
10]. River-reservoir interaction might modify the impact of HM point sources [
19]. Intensified accumulation of sediments and pollutants has been observed especially in the vicinity of dams [
17,
20]. The concentrations of HMs in bottom sediments varied in different sampling locations and layers, but the concentrations in the surface layer are higher than those in the deeper layers [
21]. Generally, the concentration of HMs was higher in reservoir sediments than in river sediments [
16,
22]. However, Frémion et al. [
23] indicated that the HM concentrations are lower in the reservoir than upstream and downstream, which is caused by the high fresh organic matter inputs, diluting the contamination. Seasonal variations of metal concentrations depend on hydrological conditions, the impact of non-point sources and lithogenic sources [
24].
The distribution of HMs in the sediments was mainly connected with grain size and organic matter content [
25,
26]. Fine-grain sediments contain higher HM concentrations [
27], which is related to higher magnetic susceptibility [
8]. Farhat and Aly [
28] suggested that the organic matter was more important in controlling HM distribution. Organic matter was a key mediator for ecological risk [
29]. Metals released into reservoirs are generally bound to the sediment bottom [
30,
31]. However, HMs may be released into the water column via sediment resuspension, due to changing chemical and hydrological conditions [
23,
32,
33,
34,
35] and accumulate in plants and animals [
2,
26,
36,
37]. Higher concentrations of HMs can cause toxicity risk to the biota and human health [
38,
39,
40,
41]. Birch and Apostolatos [
42] showed that anthropogenic metals have higher mobility and bioavailability than metals from geogenic origin.
Flow regulation, local geomorphological characteristics and reservoir operations determine the redistribution of HMs [
39,
43]. In dam reservoirs the water-level fluctuation zone is important for the accumulation and redistribution of HMs [
29,
44]. Moreover, dredging operations, the emptying of reservoirs and flood events might lead to release of HMs to the water environment [
45,
46,
47,
48].
The transport and enrichment of HMs in reservoir bottom sediments have been intensively studied, including concentration, spatial distribution, source identification and pollution assessment [
13,
49,
50,
51], temporal variability [
52], bioavailability [
53], ecological risk and organism toxicity [
4,
54,
55]. Moreover, some studies relate to HM concentrations in reservoir tributaries [
16,
56]. Many methods have been used for the assessment of bottom sediment HM pollution and to determine the potential risk of heavy metal contamination [
29,
57,
58,
59,
60,
61,
62,
63,
64,
65,
66,
67,
68,
69,
70,
71].
However, for the assessment of spatial distribution, identification of pollution sources and factors affecting their content in bottom sediments, multivariate statistical have been used [
7,
26,
71,
72,
73,
74,
75,
76].
Despite the numerous research results available, there are some deficiencies that still need attention. The studies usually focus on a certain reservoir, and less information is available about the HMs’ variation in a group of reservoirs [
14]. There is little information available about heavy metal pollution in reservoirs and their changes after prolonged exploitation [
77]. Moreover, Wu et al. [
78] suggested that future climate change will aggravate the ecological risk of HMs in the water environment due to the release of HMs from sediments to the water environment.
Poland is characterized by high seasonal and spatial variability of water resources. Therefore, in order to increase the efficiency of water management and ensure flood protection, retention reservoirs have been constructed for over 50 years. The largest number of retention reservoirs were constructed in lowland areas of Poland. Their location in the agricultural landscape means that they are exposed to a high supply of nitrogen and phosphorus [
79,
80,
81]. In the reservoirs, seasonal algal blooms and an overgrowing process are observed [
82,
83,
84]. Studies conducted by Baran et al. [
85], Wiatkowski [
86] and Kasperek and Wiatkowski [
87] show that the heavy metal concentrations in reservoir bottom sediments vary across Poland. Concentrations of heavy metals in bottom sediments depend on the catchment land use and the occurrence of pollution point sources [
25,
88,
89]. Therefore, each reservoir should be considered individually in order to expose the factors affecting the bottom sediment pollution. In this study, six reservoirs located in the lowland area of western Poland were selected for analysis. Their selection was influenced by the time of their construction, different morphometric parameters, catchment land use and hydrological conditions. The objectives of this study are to: (1) analyze the concentrations of HMs in bottom sediments, (2) show the spatial variability of heavy metal concentrations, (3) analyze the bottom sediment contamination, (4) assess ecological risk, (5) identify potential sources and factors determining the content and spatial distribution of HMs in the reservoirs.
4. Discussion
The HM content in bottom sediments of retention reservoirs and relations between them result mainly from the impact of anthropogenic sources [
14] as well as sediment texture and organic matter content [
26]. Meteorological, hydrological and geological conditions of the catchment affect the content of sand, silt, clay and TOM in water. These factors influence the transport of HMs in the river-reservoir system. In the world, bottom sediments of retention reservoirs are characterized by relatively high variability of HM concentration. The analyzed reservoirs are located in agricultural catchment; therefore, HM concentrations in bottom sediments are at a lower level. Only Zn concentrations are at a higher level with respect to those noted by other authors in Polish reservoirs [
25]. Probably Zn content may result from potential supply with domestic wastewater. The results presented in this paper confirmed the results obtained by Haziak et al. [
114] which indicated that Zn values exceeding the geochemical background were associated with anthropogenic activities (run-off from farmland and domestic wastewater).
High values of the I
geo, EF, PLI and MPI indices at individual sampling sites of the reservoirs may suggest their origin of HMs from anthropogenic sources. High values at the inlet to the reservoir result from their inflow from point sources. Bing et al. [
43] suggested that local anthropogenic activities increase contamination of specific HMs in the bottom sediments, especially of upper regions. Saleem et al. [
55] observed relatively high content of HMs at sites which were adjacent to urban and semi-urban areas. The HMs may also come from the discharges of untreated urban/industrial wastes, agricultural runoffs and automobile emissions. High HM concentration near the dam is a consequence of changes in hydrological conditions in reservoirs during floods and high flow periods. HMs absorbed on silt and clay and TOM are transported and deposited near the dam. Also, Bing et al. [
43] and Frémion et al. [
17] reported relatively high values of HM concentrations close to the dam. Sojka et al. [
25] noted that near the outlet pipe from the bottom sediments there can be discharged silt, clay and TOM, which are responsible for the transport of HM downstream from the reservoir.
The results have shown that concentrations of Cd, Cr, Cu, Ni, and Pb were strongly positively correlated with the silt and TOM contents in bottom sediments. However, the relationship with silt was at a higher level. Palma et al. [
52] obtained slightly different results which showed stronger relationships between HMs and silt and clay content. However, the relationship between HMs and silt was stronger. Farhat and Aly [
28] suggest that organic matter was more critical than grain size in controlling HM distribution in sediment. Different results were obtained by Frémion et al. [
23], who found no relationships between trace elements and organic fraction.
The content in the bottom sediments of HMs involves the possibility of ecological risks. It was observed that the ecological risk in the reservoirs occurred at the inlet and near the dam, where the highest Q
m-PEC and TRI values were obtained. Slightly different results were obtained by Bing et al. [
43], who found that the potential ecological risk level increased towards the dam. Results obtained by Zhao et al. [
50] suggested that the concentration and potential ecological risk of HMs in sediments near the dam were higher compared to the upstream sampling sites. The probable reason for the very high HM content at the reservoir’s inlet was the date of sampling during the very low flow period. The obtained values of Q
m-PEC and TRI were the most strongly correlated with TOM. Our research confirmed the results obtained by Lin et al. [
29] that the organic matter-bound fraction of heavy metal was a key mediator for ecological risk. Saleem et al. [
55] demonstrated that organic matter may retain heavy elements in sediments and play an important role in bottom sediment quality assessment.
The present results showed that there is a strong correlation between Cd, Cr, Cu, Ni, and Pb. Suresh et al. [
8] and Wang et al. [
7] indicate that the HMs have common geochemical behaviors and originated from similar pollution sources when the correlation coefficient between them is higher. The absence of a correlation among the HMs suggests that the contents of these metals are not controlled by a single factor. Low concentrations of Cd, Cr, Cu, Ni, and Pb indicate their origin mainly from natural sources. In addition, during the research, the impact of the areas immediately adjacent to the reservoirs is marked; moreover, in the case of Ni the number of road and river crossings (RRC) plays an important role. Studies have shown that reservoirs with a shorter water retention time are more likely to supply HMs. In the case of Zn, it was shown that it may come from anthropogenic sources related to wastewater management in the reservoirs’ catchment.