Evaluation of Quality of Bottom Sediments of Water Reservoir Poraj by Applying Sediment Quality Guidelines and Spiatal Analysis

Accurate assessment of the quality of the aquatic environment is related to the analysis of contamination of bottom sediments, as they reveal the essential features of aquatic ecosystems. Bottom sediments constitute a habitat of many organisms, they participate in the biogeochemical process of elements circulation and they are the place of deposition and chemical changes of many compounds entering the waters. Due to the structure of sediments they form a natural geosorbent, in which pollutants introduced into the aquatic environment are accumulated [1, 2].

The chemical composition of the matter lying on the bottom of water reservoirs is conditioned by both natural and anthropogenic factors. An important role is played, among others, by the geological structure of the basin, the type of soil cover, terrain and climatic conditions. Additionally, the chemical analysis of bottom sediments is a source of information on human activity in a given region [3]. The most frequently observed forms of anthropogenic impact affecting the quality of sediments are discharges of municipal and industrial sewage, atmospheric and gaseous pollution of the atmosphere as well as leaks from landfills. In agricultural areas, additionally, contaminants introduced into the basin of the reservoir (fertilisers, plant protection chemicals), as well as sewage sludge, which is used to fertilize the arable land [3, 4, 5]. As a result of the total effect of the presented factors, the material deposited on the bottom of water reservoirs may contain significant loads of organic (e.g. PAH, pesticides) and inorganic (e.g. trace elements) pollutants, which accumulate in sediments and may subsequently pose a threat to water quality due to remobilisation. A feature of these compounds is that they can remain active for a long period of time and constitute a secondary source of water contamination [6].

Modern studies on trace elements in bottom sediments are often related to ecological risk, toxicity evaluation, health risk caused by trace elements and geochemical cycling [7, 8, 9, 10]. Sediment quality guidelines (SQGs) and potential ecological risk index (PERI) are group of useful methods which allows assessment of pollution risk of an aquatic system by integrating total trace element content [9, 10, 11, 12, 13].

The main aim of this study was to evaluate the quality of bottom sediments of water reservoir Poraj by applying a set of sediment quality guidelines, potential ecological risk and spatial analysis. Toxicity effect and total trace element content were described in previous studies [2, 14].

Statistical analysis for total trace element content in bottom sediments are presented in Table 2. Data is characterized by significant difference between min and max value, high value of standard deviation compared to mean and high skewness and kurtosis (except Ni). Those factors indicate that total trace element content in bottom sediments was not normally distributed. Distribution is characterized by strong rightsided asymmetry which is typical for environmental studies. Similar results were obtained for PECQ, PERI and TEC values (Table 3). Normal distribution can be crucial for some of geostatistical methods.

Considering PEC and TEC value we can distinguish three primary groups:

Concentrations of three trace elements were above PEC value: Zn (22% of samples), Pb (17%) and Cd (17%). Based on TEC value, 76% of bottom sediments samples were predicted to be non-toxic.

Mean PECQ value provides a basis for assessing the potential effect of sediment-associated contaminants when they occur in a complex mixture [10, 23]. Values of PECQ ranged from 0.04 to 2.08 with mean of 0.38. PERI values ranged from 4.36 to 323.62 with mean of 55.35. PERI of less than 150 indicates low potential ecological risk, and PERI above 150 indicates moderate potential risk [10, 20, 21, 24, 25]. Both mean values of PERI and PECQ indicate low potential risk, but it should be mentioned that PERI above 150 was observed in 17% of collected samples. By considering mean PECQ value, 17% of samples were predicted to be toxic. The Coefficient of determination was determined between studied factors: PERI and PECQ (R ² = 0.98), PECQ and TEC (R ² = 0.99), PERI and TEC (R ² = 0.98), PERI and PE (R ² = 0.06), PECQ and PE (R ² = 0.07). High correlation between PERI, TEC and PECQ indicates that both factors are characterized by similar results and they can be used interchangeably in terms of assessing quality of bottom sediments (same results were obtained by other authors [10]). Low correlation between toxic effect and PERI or PECQ indicated that there are other factors that contribute to the toxicity of the sediments. [2, 10]. Additionally, the BPI value of 38 indicates that water reservoir Poraj is an eutrophic basin [27]. In eutrophic basins trace elements are often characterized by low bioavailability, which can directly influence the response of Alivibro fischeri bacteria [27]. Also other studies showed that specific bacteria can have different responses to various factors [28].

Values of mean PECQs can be classified to four classes [29]: below 0.1, 0.1 to 0.5, 0.5 to 1.0 and above 1.0. Those classes indicate probability that respectively 10, 17, 56 and 97% of samples are toxic. On this basis, classification of obtained spatial distribution was generated (Fig. 2. and Fig. 3.).

Figure 3.

Spatial distributions of PECQ, PERI and TEC were generated (Figure 2, 3 and 4). The area is characterized by significant diversity in terms of localization of contaminants. Highest values of PECQ and PERI occurred in points 1, 2, 3, 8, 12, 17, 26 and 45 which are mainly localized in the northern part of water reservoir Poraj. This part of the reservoir is also the deepest [14]. The lowest values were observed in central and east region. Maximum values of PERI and PECQ were observed in point number 45 (PECQ 2.08 and PERI 323.62).

Figure 4.

The distribution of the indicators to a large extent coincides with the dominant current in the reservoir what can be an indication of the mobility of the analysed material. The increased content of the trace elements in the bottom sediment of the water reservoir Poraj can stem from the fact that the analysed reservoir and its basin are exposed to the harmful influence of anthropogenic activities.

Additionally, the increased content of trace elements in the tested object can also be related to the surface runoff, which introduces pollution from agricultural areas (fertilizers, pesticides) as well as sewage sludge, with which fields are fertilized [2, 14, 16, 30]. Coefficient of determination was calculated between PECQ, PERI, TEC and studied trace elements (Table 4). The highest values were observed between Zn, Cd, Pb, Cu and calculated indexes, the lowest concentration was found between Cr and analysed factors. It can lead to an assumption that in terms of water reservoir Poraj Cr was not directly correlated with PERI and PECQ. Thus, Cr should be considered separately. Other trace elements are strongly correlated with applied rules of evaluation of quality of bottom sediments.

Despite the dominating low ecological risk, the Poraj water reservoir, due to its function (including recreational aspects), should be constantly monitored due to the threat to the health of people resting there [2, 14]. In particular due to high concentrations of trace elementss as well as locally occurring toxicity effect. The drawn maps of spatial distribution (Figures 2 and 3) can form the basis for undertaking activities aimed at planning prevention, protection and processes aimed at improving the current state [2, 14].

On the basis of conducted studies, following conclusions were established: