Comparison of selected methods used in the assessment of contamination with heavy metals in littoral sediments of lakes

Heavy metals are elements whose density is greater than 4.5 g cm⁻³ (Streit 1994). They are divided into two groups. The first group consists of elements necessary for the proper functioning of the body in trace amounts, while at higher concentrations they usually exhibit toxic effects. These include iron, manganese, zinc, molybdenum, chromium and nickel. The second group includes metals of unknown metabolic functions, which have toxic properties even in small quantities, such as cadmium, lead, mercury, silver and platinum (Zamorska & Papciak 2010).

The presence of heavy metals in aquatic ecosystems is a result of natural processes and anthropogenic activities (Shaikh 2013). Rock weathering is one of the natural sources of metals. However, the content of elements of this origin is usually low and is referred to as “geochemical background”, which is specific for a given region. On the other hand, human activity has contributed to a significant increase in the concentration of heavy metals in the aquatic environment (Wei & Wen 2012). Currently, metals pose a serious threat to organisms because of their resistance to degradation and the capacity for accumulation, including bioaccumulation and biomagnification (Tao et al. 2012). This particularly applies to cadmium, copper, nickel, lead, zinc, mercury and chromium. Even small concentrations of heavy metals may adversely affect the proper functioning of living organisms and cause mutagenicity, teratogenicity and carcinogenicity (Ociepa-Kubicka & Ociepa 2012; Sardar et al. 2013).

Heavy metals in natural waters undergo various chemical transformations, adsorption and complexation followed by sedimentation and accumulation in sediments (Allen 1995). Depending on the properties of individual metals, as well as environmental and hydrological factors related to the channel morphology, the speed and character of water flow, they become a more-or-less permanent deposit in sediments (Rabajczyk & Jóźwiak 2008). The littoral zone usually has the highest biological diversity within a lake due to good oxygenation, a large amount of incoming light as well as the presence of shady locations and varied terrain structure. It is also the most sensitive area of environmental and hydrological variability, interacting directly with the land (Kajak 2001). Therefore, an assessment of the contamination level in water bodies with heavy metals should also include analysis of their content in littoral sediments.

The objective of the study was to assess the pollution of littoral sediments with heavy metals (Zn, Cu, Pb, Ni and Cd) in lakes of the Borecka Forest, using different methods based on the geochemical criteria for Poland and the Earth’s crust. The Borecka Forest, located in north-eastern Poland, has a high density of lakes, which makes it one of the most important ecological habitats and an attractive place for tourists. Although the lakes differ in the degree of anthropo-pressure and morphometric and direct catchment parameters, there is no information on the sediments of these lakes, especially in terms of heavy metal content.

The content of TOC and TIC in sediments can affect their ability to bind and accumulate heavy metals. In the present study, the sediments are characterized by varying concentrations of organic and inorganic carbon (Table 4). The average content of total carbon (TC) in dry mass of sediments amounted to 60.21 g C kg⁻¹. The highest concentrations of TC were observed in lakes Kacze and Dubinek, 494.68 and 60.72 g C kg⁻¹ respectively, while the lowest in sediments from lakes Krzywa Kuta and Ciche, 6.04 and 8.67 g C kg⁻¹ respectively. The highest content of TOC was recorded in lakes Kacze and Dubinek, while the lowest in lakes Krzywa Kuta and Wolisko. The mean concentration of organic carbon in the analyzed sediments amounted to 56.60 g kg⁻¹, which accounted for 94.0% of TC. The maximum concentrations of TIC were recorded in sediments from lakes Łaźno and Dubinek and the minimum concentrations in sediments from lakes Szwałk Mały and Krzywa Kuta. The average content of TIC in the dry mass of sediments from these lakes amounted to 3.61 g kg⁻¹ (6.0% of TC) (Table 4). Clearly, the dominant form of carbon in the sediments of most lakes was TOC. The content of TOC in sediments from eight lakes (Kacze, Szwałk Mały, Szwałk Wielki, Smolak, Krzywa Kuta, Biała Kuta, Ciche and Litygajno) ranged from 97 to 100% of TC. On the hand, the TIC dominated only in sediments from lakes Łaźno and Wolisko (about 60% of TC) (Fig. 2).

Figure 2

Statistical analysis revealed strong correlations between the concentrations of individual metals in the examined sediments: zinc and copper (r = 0.85), lead and zinc (r = 0.82) and copper and lead (r = 0.73). A high significant correlation was also found between TOC and Pb content (r = 0.97), TOC and Zn (r = 0.78), and TOC and Cu (r = 0.69) (Table 10).

In the present study, sediments from different lakes differed in the total concentration of individual metals. Varied metal concentrations in the sediments are determined by catchment factors and the degree of anthropopressure as well as environmental factors, i.e. the nature and structure of sediments, the type and the development stage of the vegetation in the lake (Szafran 2003). Bojakowska and Krasuska (2014) analyzed the content of heavy metals in sediments from 14 lakes located in the Gniezno Lake District (Poland), sediments from 9 lakes outside the power plant cooling system and sediments from 5 lakes within the power plant cooling system. Sediments from lakes within the power plant cooling system are characterized by a considerably higher content of Ba, Zn, Cu, Ni, Hg compared to sediments from the other lakes. Szafran (2003) showed that the lowest concentrations of heavy metals in sediments from lakes Sumin and Rotcze were found in sandy sediments within the area of emergent plants (on average 4.64 mg Zn kg⁻¹, 2.77 mg Pb kg⁻¹, 1.15 mg Cu kg⁻¹, 0.04 mg Cd kg⁻¹), and the highest in sediments in the middle part of the lake (on average 147.25 mg Zn kg⁻¹, 55.39 mg Pb kg⁻¹, 8.08 mg Cu kg⁻¹, 1.77 mg Cd kg⁻¹). Lower metal concentrations in the littoral sediments compared to the sediments in the middle part of the lake may be due to the bioaccumulation effect. Sediments from the deeper zones of the lake show a higher sorption capacity toward metals. This results from a higher content of organic matter and clay fraction as compared to the littoral sediments. Mielnik et al. (2011) confirmed that heavy metal content in sediments from selected lobelia lakes was spatially differentiated. In most cases, the content of Zn, Cu and Mn was significantly higher in the profundal zone of each of the investigated lakes compared to the littoral zone.

Sediments are generally rich in organic matter. In the present study, the average concentration of TOC in sediments was 56.60 g kg⁻¹, which accounted for 94.0% of TC. The content of TOC in sediments can vary within very wide limits and reach a value of even 460 g kg⁻¹ (Cieślewicz 2005; Gälman et al. 2008). In the sediments from lakes located on the Wałeckie Lake District, Chodzieskie Lake District, Iławskie Lake District, Drawska Plain and Tucholska Plain (Poland), Cieślewicz & Różański (2010) found the highest concentration of organic carbon in the following lakes: Arkońskie (386 g kg⁻¹), Hanki (339 g kg⁻¹) and Piaseczno Małe (315 g kg⁻¹). The highest concentration of inorganic carbon was determined in the sediments from Lake Wdzydze (65.4 g kg⁻¹).

In the present study, the highest I_geo and CF values among the analyzed metals in lake sediments were obtained for Cd. It was found that due to the increased content of Cd, the largest contamination occurred in sediments from Lake Szwałk Mały. One possible reason for this may be the surface runoff from agricultural land, which occupies almost 40% of the direct catchment area of the lake. The amount of Cd which is introduced into the soils with the fertilizers and herbicides is undoubtedly much lower than that introduced with the industrial pollutants. The one cause of water pollution with cadmium is using phosphate and calcium-magnesium fertilizers (Bojakowska & Sokołowska 1996). Despite this, their regular and prolonged use may cause accumulation of this metal in the surface soil layers, which then reaches the lake with the runoff waters, melt waters or wind erosion. One of the potential sources of cadmium may also be the effluent from leaking cesspools, located in the area of farms and recreational plots in the vicinity of the lake (Bąk et al. 2013). In addition, Lake Szwałk Mały is very popular in Poland as a carp fishing area. In this type of lakes, large amounts of groundbait are used. Therefore, it can be assumed that a certain load of cadmium gets into the lake with the bait, mainly to the littoral zone where carp has feeding areas. Additionally, a drainage ditch flows into the lake (about 0.7 m long) from the village of Szwałk, which receives part of the waters from fields and the asphalt road that runs along the shoreline of the lake.

The littoral sediments of Lake Kacze were also characterized by an increased content of cadmium. However, it should be noted that the catchment area of this lake is completely covered by forests and the lake has no surface flow. This small, shallow lake is different from the others by the highest content of organic carbon in the sediment. Therefore, it should be noted that the primary source of cadmium and other heavy metals in the sediments of Lake Kacze could be the weathering processes and bonding of metals, which occurs mainly with a considerable amount of allochthonous organic matter flowing from the catchment (Kabata-Pendias & Pendias 1999; Potarzycki & Maciejewska 2005). Cieślewicz & Różański (2010) found that the cadmium concentration was high (3.40 mg kg⁻¹) in the sediments of Slim Lake, which is located in the catchment dominated by deciduous forest.

In the present study, a strong and very strong correlation between TOC and Zn, Cu and Pb may indicate that these metals were bound to organic matter in all analyzed sediments. In addition, these metals were correlated with each other. Although cadmium generally did not correlate with organic carbon (r = 0.34 for all sediments) (Table 10), sediments from Lake Kacze were characterized by a strong correlation between Cd and TOC (r = 0.86 at p<0.05) (full data not shown). Cieślewicz & Różański (2010) also found a strong correlation (r = 0.87 at p<0.05) between TOC and cadmium in lake sediments. The concentration of organic carbon was high (89-386 g kg⁻¹) in these sediments and the total cadmium concentration varied between 0.73 and 3.04 mg kg⁻¹. Nickel correlated neither with TOC nor TIC in the sediments.

The correlations between heavy metals and TOC or between metal pairs in sediments were also indicated by other authors. Bojakowska & Krasuska (2014) showed that Pb concentration in sediments coming from the following lake districts: Great Poland, Pomeranian and Masurian was relatively well correlated with the total organic carbon (r = 0.59), aluminum (r = 0.45) and sulfur (r = 0.47) as compared to other analyzed chemical parameters. In the sediments from 13 lakes, Cieślewicz & Różański (2010) showed that the concentration of zinc, copper and lead was also positively correlated (p<0.05) with the organic carbon content. Such correlation was not observed for nickel. Similar relationships between the organic carbon content and zinc, copper, lead, and nickel were determined in the present study. If the correlation coefficient between the heavy metal factors is positive, these factors may have a common source and identical behavior during transport (Saeedi and Salmanzadeh, 2012).

Due to the lack of any legal regulations regarding the permissible levels of heavy metals in the lake sediments in Poland, the commonly used method of assessing the sediment pollution is to compare the total metal concentrations with the geochemical criteria. A method using the ecotoxicological criteria (PEL TEL) or pollution indices, i.e. the geoaccumulation index (I_geo), the contamination factor (CF) and the contamination degree (C_d) could be more sensitive. A considerable effect on the final quality of sediments has the assumed background value for a given metal (GBP or GBE). As the GBP has been developed specifically for the Polish conditions, it seems to be more appropriate for assessing the purity of lake sediments in Poland. On the other hand, taking into account the very valuable region of Borecka Forest, the GBE can also be appropriate, especially in relation to the most toxic metals such as Cd.

In the present study, the lowest quality was determined for the littoral sediments from two lakes, Szwałk Mały and Kacze, based on the ecotoxicological criteria and pollution indices. The sediments from the other lakes of Borecka Forest were basically unpolluted, regardless of the method used. From the practical point of view, the most useful tool for assessing the quality of lake sediments seems to be the degree of sediment contamination (C_d). This index refers to the combined effect of all metals on the sediments. Thus, the overall sediment quality can be estimated and not the quality related to individual metals.