3.1. Pollution Level in Groundwater Due to Localization of Piezometers
Based on the monitoring studies, the most vulnerable to contamination were the areas closest to the landfill (0.5 km zone), mainly to the northwest and southwest. Monitoring results from piezometer P-2A, located at the outflow on the eastern slope of the landfill, indicate significant changes in the levels of pollution indicators, in particular caused by organic waste storage. Slight increase of the TOC level observed in this piezometer may be a result of its localization in a forest area. A similar trend in the TOC level, caused by forest surroundings, was also noted in piezometer P-4.
Monitoring results from piezometer P-6A, localized in a local depression with periodically stagnant water on the surface, show pollution concentrations exceeding the acceptable values for the third water quality class. Exceeded values primarily refer to the TOC level, which can be regarded as typical of a forest area.
For piezometer P-7A, the most visible changes concern the COD level. Exceeded values of this parameter result from its localization in an overgrown area.
A similar trend in the COD level changes is observed for the monitoring results from piezometer P-9, which is very much dependent on the putrefaction of vegetal remnants along the railway siding situated near to this piezometer. In the case of piezometer P-9, the fluctuations of some pollutant concentrations (for example the TOC level) also result from its localization. Particular influence on the concentration of selected pollution indicators has the nearest area where the production and service facilities are situated.
Higher concentration levels of contaminants in piezometer P-11A result from a local depression where the piezometer is located. Because of this, water from the railway siding and the composting area can flow down easily to that point and directly cause groundwater pollution in this piezometer.
Fluctuations in the concentrations of pollution indicators (especially TOC) and exceeded standards for the third class of groundwater quality were also observed in piezometer P-12. The main reason for this is runoff of contaminants from the composting plant area.
Groundwater in piezometer P-14A meets the standards for the third class of water quality. Only periodic fluctuations in TOC and COD concentrations may result from seasonal changes of groundwater level, which means that increased values of these parameters correspond to lower groundwater levels.
Apart from the sporadic excesses above reference values, the TOC and COD concentrations in groundwater from piezometer P-15 located in the area of the MBT installation comply with Polish standards. Seasonal fluctuations of these indicators can be attributed to runoff from the compostory plant and seasonal changes of groundwater level.
Seasonal changes in pollution indicator levels observed in groundwater from piezometer P-17 are determined mainly by runoff of contaminants from Estrady St. and the industrial facilities located nearby.
Based on the obtained results, it can be concluded that the concentrations of the analyzed parameters have decreased with the distance from the landfill, which can be linked to biodegradation and dilution.
A similar trend proving that concentrations of contaminants in groundwater decrease with increased distance from the pollution source was presented by Ling and Zhang [
23], Aderemi et al. [
36] and Mor et al. [
37].
Regarding the influence of precipitation on contaminant concentration we can also claim that the concentrations are linked to the groundwater level which is mainly supplied by rainfall. According to groundwater level monitoring data (not presented in this article) it is noted that groundwater level in all piezometers show seasonal pattern. It should be noted that in accordance to high precipitation and resulting from this high groundwater level, lower concentration of indicators are observed.
3.3. Statistical Analysis of the Monitoring Data
3.3.1. Pearson’s Correlation Analysis
Pearson’s correlation analysis between selected pollution indicators were conducted for the monitoring data from piezometers located within the landfill, the composting plant, and the area of the adjacent facilities. Out of a total of 30 correlations (10 for each piezometer) calculated between the selected pollution indicators for piezometers located within the landfill area, 13 were found to be significant at a level of p < 0.05 (six for piezometer P-2A, four for piezometer P-7A and three for piezometer P-9).
The most significant correlation (
r > 0.8) was detected between BOD and COD, and EC and COD for piezometer P-2A (
r = 0.859 and
r = 0.813, respectively). According to the classification of the correlation coefficient (
r) presented by Evans [
47], it can be stated that strong correlation exists between BOD and COD, COD and TOC, and COD and EC for groundwater samples taken from piezometer P-7A (
r = 0.621,
r = 0.652,
r = 0.679, respectively). High values of COD compared to BOD and no correlation between them observed in piezometer P-9 can indicate that the major part of organic compounds is not biodegradable. Strong correlation (
r = 0.772) was also observed between COD and TOC for piezometer P-9, and very strong correlation was observed in groundwater from this piezometer between COD and EC, and TOC and EC concentrations (
r = 0.815 and
r = 0.826, respectively) (
Table 4). Positive correlations between BOD versus COD (
r = 0.652), and TOC versus COD (
r = 0.743) were also obtained by Maitera et al. [
48], indicating that both are products of organic matter oxidation.
For piezometers located in the area of the composting plant and the Municipal Waste Treatment Installation (P-11A, P-12, and P-15), pollution indicators show weaker mutual correlation (
Table 5).
Moderate positive correlation was observed between BOD and COD (r = 0.599), and TOC and EC (r = 0.424) for piezometer P-11A. Moderate negative correlation was observed in this piezometer between BOD and TOC (r = −0.517), and BOD and EC (r = −0.410). In piezometer P-12, the strongest correlation was calculated between COD and TOC (r = 0.717). Moderate correlation exists between COD and EC (r = 0589), and TOC and EC (r = 0.413). Other parameters detected for this piezometer have revealed weak (r < 0.39) or very weak (r < 0.19) correlation. For piezometer P-15, only the relation between BOD and COD was found to be strong (r = 0.666). Moderate negative correlation was observed in piezometer P-15 between BOD and EC (r= −0.407). Weak or very weak correlation was calculated among the rest of the selected parameters.
Among 30 correlations calculated for parameters measured in the piezometers located in the area of the facilities adjacent to the landfill (
Table 6), only one pair of variables has revealed a very strong relation (
r = 0.878, measured between BOD and COD in piezometer P-14A). In piezometer P-10A, moderate correlation was observed between BOD and COD (
r = 0.462), and COD and TOC (
r = 0.468). Correlations between other selected indicators were found to be weak or very weak for this piezometer. With the exception of the correlation between BOD and EC (
r = 0.494), a similar tendency was observed for parameters analyzed in piezometer P-17.
3.3.2. Box-and-Whisker Plots
The box-and-whisker plots displayed in
Figure 15,
Figure 16 and
Figure 17 show the minimum, median and maximum tendency, and the extreme results of monitoring data from 1998 to 2016 collected four times a year, usually in March, June, September and November.
The results show that the minimum BOD levels are in the range of 0.5 mg O2/L for all piezometers but the maximum concentrations are observed in piezometers P-9, P-7A, P-11A and P-2A (800, 650, 600 and 260 mg O2/L, respectively). However, these values were obtained from the period before the closure of the vertical barrier.
A similar situation was observed in the case of COD concentrations in groundwater. During the 1998–2016 monitoring period, the maximum COD levels were observed in piezometers P-7A (1758 mg O2/L), P-9 (1157 mg O2/L), P-2A (705 mg O2/L) and P-11A (377 mg O2/L).
In the case of TOC concentrations in groundwater, the minimum levels were observed in piezometers P-10A, P-12, and P-15 (1 mg C/L), and the maximum levels in piezometers P-9, P-2A, and P-7A (437, 359 and 129 mg C/L, respectively). This confirms that the BOD, COD, and TOC concentrations in groundwater are determined by the location of these piezometers.