# Change in Water Quality of the Vistula River During the Emergency Discharge of Untreated Wastewater
## 3 Results and Discussion
Symptoms of chloride excess also include disorders of the neuromuscular system, potentially leading to the development of metabolic acidosis [68]. Drinking water with a Cl\({}^{-}\) concentration above 250 mg\(\cdot\)L\({}^{-1}\) is dangerous to human health [39]. In the case of WT, there are no significant differences between the MZ and the UA, which was 18.9 \({}^{\circ}\)C. The higher temperature of surface water running out of the city could be evidence of the impact of people's activities on climate change. The COD concentration in the mixing zone averaged 61 mg\(\cdot\)L\({}^{-1}\) and was twice as high as at the other points, where it was 32 mg\(\cdot\)L\({}^{-1}\) (Table 3). Typically, a high level of COD suggests a higher level of contamination, but this is not always the case, as COD can also come from natural sources [69]. High levels of COD can cause oxygen depletion from the water, regardless of the COD source. This can cause a massive extinction of aquatic organisms. Every year, a large number of fish die due to oxygen deficiency caused by pollution [67]. Despite the twofold increase in COD in the mixing zone, the decrease in DO was slight. The DO decreased from a value of 9.44 mg\(\cdot\)L\({}^{-1}\) in station 2 to a value of 9.09 mg\(\cdot\)L\({}^{-1}\) in station 3. The level of TOC pollution was 5.9 mg\(\cdot\)L\({}^{-1}\) in the urban area and 10.1 mg\(\cdot\)L\({}^{-1}\) in the mixing zone. The high TOC load in the river is due to rainfall and temperature changes. In terms of DO and TOC concentration, the water meets the standards of good ecological status [37]. The performed correlation tests, which measure the strength and direction of the relationship between two variables, showed a correlation significantly different from zero (indicating no relationship) for most of the pairs of the parameters under consideration. A high correlation was found between the concentrations of TN, N-NH\({}_{4}\)\({}^{+}\), and N-NO\({}_{3}\)\({}^{-}\) (r > 0.8), but no correlation was found in the case of N-NO\({}_{2}\)\({}^{-}\). The high correlation between these parameters results from the fact that the components of total nitrogen are its forms (ammonia, nitrate, nitrite, organic). Therefore, an increase in the concentration of nitrate and ammonium ions causes an increase in the concentration of total nitrogen. The obtained strong correlations between nitrogen forms are in line with the results of studies conducted in Lake Taihu [70]. A high positive correlation was also found between the concentrationsof the salinity parameters, including EC, SO\({}_{4}\)\({}^{2+}\), Cl\({}^{-}\), and TDS (r > 0.67). The value of electrical conductivity results from the presence of the concentration of dissolved salts, such as calcium, chlorine, and sulfate. The value of EC and other parameter salinity are correlated, which is consistent with the results of studies conducted in Estonia [71]. A strong correlation was also found between COD and nitrogen forms (r > 0.68, Table 4). A similar result was obtained in the Liuxi River, where a clear correlation between N-NH\({}_{4}\)\({}^{+}\) and COD was observed [72]. Studies on river pollution confirm a rapid deterioration of oxygen conditions with an increase in the concentration of ammonium nitrogen. The Vistula River has a very high capacity for self-purification, as evidenced by the lack of water pollution at 1200 m after the collector outlet in the middle of the river (Figure 5). A similar effect was observed in studies conducted in Zimbabwe [73]. In another study, the discharge of raw sewage from the city of Novi Sad caused pollution of the Danube River along a length of 7 km. Pollution from incompletely treated sewage has been a serious problem in Serbia [17]. The presence of large amounts of untreated domestic and industrial sewage in Bangladesh contributed to water pollution in the river. Therefore, a strategy for controlling the pollution of urban rivers in Dhaka was developed, which included the analysis of nine social, environmental, and governance factors [25].