Transformations in river water chemistry following wastewater treatment implementation in a mountain region of the Polish Carpathians.

Mountain catchments play a critical role in water supply for lowland regions, contributing significantly to national water resources despite their limited geographic coverage. This study investigates the impact of wastewater treatment plant (WWTP) construction on the hydrochemistry of the Stara River in the Carpathian Foothills (southern Poland). Long-term hydrochemical data (2001-2024) and spatial surveys were analyzed to assess temporal trends and spatial variability before and after the WWTP installation. Results indicate that the WWTP significantly altered river water chemistry, increasing concentrations of conservative ions (Na, Cl) and nutrients (NO3, PO4), particularly during the vegetative season. Principal Component Analysis (PCA) revealed a shift from diffuse pollution linked to high flow rates prior to 2011, to a dilution-dominated regime after infrastructure development. Seasonal analyses highlighted the winter accumulation of nitrites, likely due to inhibited nitrification under low temperatures. Spatial profiles showed elevated ion concentrations immediately downstream of the WWTP, with partial attenuation further downstream. These findings demonstrate that the implementation of wastewater treatment infrastructure, while improving certain aspects of sanitary safety, may also introduce persistent shifts in river hydrochemistry, particularly under conditions typical for mountain catchments (low buffering capacity, high hydrological variability). To mitigate these effects, we recommend the following: (i) enhanced monitoring of nutrient and ion loads downstream of WWTP outlets, (ii) optimization of treatment processes during low-temperature seasons, and (iii) integration of hydrological modeling into local wastewater management planning. Such measures are essential to maintain ecological resilience and long-term water quality in mountain-fed river systems.