# Change in Water Quality of the Vistula River During the Emergency Discharge of Untreated Wastewater
## 3 Results and Discussion  
The results of water quality tests at eight stations located on the river path are presented in Table 3. As a result of statistical analysis, statistically significant differences (Friedmann test, \(\alpha=0.05\)) between stations S3 and S4 and the remaining stations were found. A statistically significant increase in pollution was found in the case of TN, N-NO3\({}^{-}\), N-NH4\({}^{+}\), TP, P-PO4\({}^{3+}\), SS, EC, COD, and TOC. Since statistically significant value differences concerned 9 of the 16 analyzed parameters, the authors performed cluster analysis using the Ward method. The results of the classification are presented graphically in the form of a dendrogram (Figure 3). Based on the similarity measure between the analyzed parameters on the dendrogram, a mixing zone (marked green) and an urban area (marked violet) were distinguished. Stations S3 and S4 were placed in the mixing zone (MZ), while stations S1-S2 and S5-S8 were placed in the urban area (UA). Then, box plots were made for the two clusters that were separated. Box plots (Figure 4) were prepared for the clusters selected in the dendrogram. The box represents the first and third quartiles. The horizontal line across the center of the box represents the median. A filled red square designates the mean value of the data. The whiskers are drawn to the most extreme observations located no more than 1.5 times the interquartile range away from the box. Any observation not included between the whiskers is considered an outlier and is plotted with a filled circle. When there are no outliers, whiskers indicate the minimum and maximum values. The plot presents observed values of particular parameters, marked with black dots. In this article, the authors decided not to analyze the results for individual stations but only for the separated clusters. This method of interpretation makes it easier for the reader to understand the impact of sewage pollution on changes in water quality. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Parameter** & **Unit** & **Stat. ** & **S1** & **S2** & **S3** & **S4** & **S5** & **S6** & **S7** & **S8** \\ \hline TN & mg\(\cdot\)L\({}^{-1}\) & Mean & 3.050 \({}^{\text{  
In the case of TN, there was a twofold increase in pollution concerning the mixing zone. The TN concentration in the mixing zone was 6.43 mg\(\cdot\)L\({}^{-1}\) (Table 3). The TN concentration in the urban area was 3.1 mg\(\cdot\)L\({}^{-1}\) (Figure 4). Due to the concentration of TN, water samples from stations in the urban area meet the standards of good ecological status [37]. The rapid increase in pollution was related to the inflow of untreated municipal wastewater, with an average concentration of 50 mg\(\cdot\)L\({}^{-1}\) TN and 5 mg\(\cdot\)L\({}^{-1}\) TP [35]. In the mixing zone, the threshold value for TN of good ecological status of 4.0 mg\(\cdot\)L\({}^{-1}\) is significantly exceeded. The increasing load of nitrogen removed from the catchment is most often associated with the runoff of rainwater or meltwater. The literature emphasizes the importance of identifying nitrogen as the main factor contributing to the increase in primary production and environmental eutrophication [43]. Excessively high loads of nitrogen runoff cause an increase in oxygen demand, which may cause the formation of anaerobic conditions in water [44]. Studies of river pollution conducted in China confirm the influence of the inflow of untreated municipal sewage on the increase in the concentration of total nitrogen [45]. In Europe, as a result of the use of effective removal methods, nitrogen is effectively removed in sewage treatment processes [46]. A much  
Figure 3: Cluster analyses: S1–S8—station, mixing zone (marked green), and an urban area (marked violet); axis “x”—Euclidean measure. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Parameter** & **Unit** & **Stat.