## 3 Results
### 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 [PERSON] [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 [PERSON] 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 \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Variable** & **BOD** & **COD** & **TOC** & **pH** & **EC** \\ \hline \multirow{3}{*}{BOD} & \multirow{3}{*}{1} & **PIEZOMETER 2A** & \multirow{3}{*}{0.649 *} \\  & & & 0.859 * & 0.601 * & \(-\)0.246 & 0.649 * \\  & & 1 & 0.696 * & \(-\)0.154 & 0.813 * \\  & & & 1 & \(-\)0.338 ** & 0.645 * \\  & & & 1 & \(-\)0.214 \\  & & & & 1 & \\ \hline \multirow{3}{*}{BOD} & \multirow{3}{*}{1} & **PIEZOMETER 7A** & \multirow{3}{*}{0.052} & 0.576 * \\  & & & 0.621 * & 0.378 ** & 0.052 & 0.576 * \\  & & 1 & 0.652 * & 0.268 & 0.679 * \\  & & & 1 & 0.054 & 0.354 ** \\  & & & & 1 & 0.246 \\  & & & & 1 \\ \hline \multirow{3}{*}{BOD} & \multirow{3}{*}{1} & **PIEZOMETER 9** & \multirow{3}{*}{0.018} & \multirow{3}{*}{\(-\)0.255} & 0.137 \\  & & & & 0.772 * & \(-\)0.004 & 0.815 * \\  & & & 1 & 0.063 & 0.826 * \\  & & & & 1 & \(-\)0.057 \\  & & & & & 1 \\ \hline \end{tabular}  
* correlation significant at \(p\) < 0.01 (two-tailed), ** correlation significant at \(p\) < 0.05 (two-tailed).  
\end{table} 
Table 4: Pearson’s correlation analysis between selected pollution indicators based on monitoring data from piezometers located within the landfill area. 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.