The Influence of Anthropogenic Pollution on the Physicochemical Conditions of the Waters of the Lower Section of the Sąpólna River

River pollution resulting from unregulated and improper water sewage management is a global issue of concern. The discharge of inadequately treated sewage into rivers and the sudden release of excessive quantities during heavy rainfall can result in significant fish mortality. This phenomenon has been observed repeatedly in the case of the Sąpólna River, NW Poland. Consequently, a decision was made to monitor the water quality at two key locations: the drainage channel that feeds into the river and downstream of the channel. Seventeen water quality indicators were measured, including temperature, pH, conductivity, total suspended solids (TSS), dissolved oxygen (DO), biochemical oxygen demand (BOD₅), chemical oxygen demand (COD_Cr), total organic carbon (TOC), alkalinity, total hardness, total reactive phosphorus (TRP), total phosphorus (TP), nitrite-nitrogen (NO₂⁻-N), nitrate-nitrogen (NO₃⁻-N), total ammonia nitrogen (NH₄-N), total organic nitrogen (TON), and total nitrogen (TN). It was determined that, at the location farthest from the drainage channel, water quality still falls short of meeting the specified standards. The primary factors leading to the degradation of water quality at this point were TSS, TRP, NO₂⁻-N, and TN. It was concluded that the primary localized source of water pollution in the studied section of the Sąpólna River is the discharge from sewage treatment plants in Nowogard. Consequently, actions should be taken to address sewage quality and reduce discharge quantities.