## 4 Discussion
### Physicochemical Status  
Research carried out on the hydropower plants on the Bystrzyca and Sleza rivers (tributaries of the Oda River) shows that the general physicochemical status did not change noticeably within the hydropower plants [62; 72]. In the case of the Sadowice, Skalka, and Sadowice hydropower plants on the Bystrzyca River, a very good physicochemical condition was noted in terms of temperature, which was good for EC, NH\({}_{4}\)-N, and TP. This status was below good for pH, NO\({}_{3}\)-N, NO\({}_{2}\)-N, DO, and BOD\({}_{5}\) (exception: the condition within the Marszowice hydropower plant was below good for EC; in this case, there are pressures related to the urbanization processes as this hydropower plant is located in the city of Wroclaw, while the others are located in forest-agricultural and rural areas) [62; 73]. Concerning the hydropower plant on the Sleza River located in Wroclaw, the physical and chemical condition was below good for DO, BOD\({}_{5}\), EC, pH, NH4-N, NO\({}_{3}\)-N, NO\({}_{2}\)-N, and TP, and very good for the water temperature. Attention was drawn to a large per \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline ParameterReference & [PERSON] et al., 2015 [59] & [PERSON] et al., 2019 [60] & [PERSON] et al., 2022 [61] & [PERSON], 2012 [57] & [PERSON] & [PERSON] and & This \\  & & & **2019**[60] & 2022 [61] & 2012 [57] & et al., 2015 [58] & [PERSON] 2021 [62] & Research \\ \hline TP & \(0.00\%\) & \(-26.9\%\) & \(366.7\%\) & \(-85.00\%\) & \(-28.00\%\) & \(-7.22\%\) & \(-5.56\%\) \\  & & & & \(0.0\%\) & & to-0.19\% & to 0.25\% \\ \hline TN & \(-2.08\%\) & \(15.49\%\) & \(-13.33\%\) & & & & \(-5.06\%\) & \(-9.82\%\) \\  & & & & & & to 3.15\% & to 23.61\% \\ \hline NO\({}_{3}\)-N & & & & & \(-14.00\%\) & \(-4.97\%\) & \(-0.83\%\) \\  & & & & & & \(0.55\%\) & to 27.82\% \\ \hline NH\({}_{4}\)-N & & & & \(-72.22\%\) to & & & \(-38.89\%\) to 22.86\% & \(-6.25\%\) \\  & & & & & \(8.33\%\) & & & to 0.00\% \\ \hline
**Installed capacity** & \(0.09\%\) & \(0.15\) & \(900\) MW & \(1800\) & \(11.82\) MW & \(210\) MW & \(0.06\) & \(1.1\) \\  & \(-0.6\) MW & \(-1.3\) MW & \(-300\) MW & \(11.82\) MW & \(210\) MW & \(0.06\) & \(-0.38\) MW & \(-4.83\) MW \\ \hline
**Damming height** & \(<\)5 m & 5–15 m & \(105\) m & \(71\)–161 m & \(70\) m & \(243\) m & \(1.8\)–\(3.75\) m & \(5.2\) m \\ \hline
**Type of hydropower** & & & & & & run-of-river & \\ \hline
**River** & Viryte, Venta, Obelis, & & & & & \\  & Susve, Varduva & & & & & & \\ \hline
**County** & Lithuania & China & Spain & Brazil & & Poland \\ \hline \hline \end{tabular}
\end{table} 
Table 14: _Cont._centage of exceedances of EC (95.8%) and NO\({}_{2}\)-N (45.8%) and their values (maximum by 204-220% and 865-923%). It was indicated that the EC exceedances result from the characteristics of the catchment area, geological structure, and anthropogenic interactions. The hydropower plant itself has little effect on changes in the value of this parameter (the median change was 0.25% compared with the hydropower plant's lower and upper positions) [72]. Increased concentrations of NO\({}_{2}\)-N occurred in April and May 2020 caused by heavy rainfall, which carried large loads of pollution from the upper part of the agricultural catchment (fertilizers and plant protection products). The impact of the hydropower plant was visible only in 1 out of 24 examined months and the reason was most likely the release of sediments accumulated upstream of the damming of the hydrotechnical structure and the resuspension of the sediments due to a hydraulic jump on the lower side and the subsequent release of accumulated NO\({}_{2}\)-N [74]. Similar cases occurred, for example, in the area of an impoundment hydropower plant in the French Alps [75] or on hydraulic models imitating individual technical elements of hydropower plants (e.g., pressure sand traps) [76]. A similar phenomenon also happened within the analyzed Wroclaw II hydropower plant in June 2018.