This study examines the environmental impact of a nuclear power plant’s (NPP) open-cycle cooling systems (CCSs), focusing on the Rivne NPP in Ukraine. Regulatory standards from the Water Code of Ukraine and the EU’s Water Framework Directive guide the analysis. Over four years (2019–2022), water quality indicators were monitored. Results show compliance with regulatory standards for makeup and cooling water, with pH levels within the range of 6.5–8.5. Total dissolved solids (TDS) remained below 500 mg/dm³, while total hardness (TH) did not exceed 200 mg/dm³ as CaCO₃. Salinity components, including Cl⁻, SO₄²⁻, Na⁺, and K⁺, were within acceptable limits. Biogenic compounds, such as PO₄³⁻ and N-NH₃, occasionally exceeded maximum permissible concentrations in the receiving Styr River. Chemical oxygen demand levels were consistently below 30 mg/dm³. Pearson’s correlation coefficient and principal component analysis revealed strong relationships between water quality indicators. TH and SO₄²⁻ were identified as dominant components in CCS water discharge, remaining below prescribed maximum permissible discharge limits. Overall, this study indicates that there is no negative non-radiological impact of water discharge of polluting chemicals with the effluent of the Rivne NPP CCS on the surface waters of the Styr River.