Abstract <p>Anthropogenic pressure alters the hydrochemical regime of surface water sources, thereby degrading water quality and limiting its suitability for aquaculture. This issue underscores the critical need for continuous monitoring and comprehensive environmental assessment of surface water quality in aquaculture contexts. The present study aims to evaluate seasonal fluctuations—during spring and autumn—in the water quality index (WQI) of surface water sources within the Mohyliv-Podilskyi district of Vinnytsia oblast. The analysis focuses on the Kotlubaivka River basin, a left-bank tributary of the Dniester River, and the Dniester River itself, both upstream and downstream of the municipal wastewater treatment facilities operated by the municipal enterprise “Vodokanal” utility (Mohyliv-Podilskyi). This region exemplifies areas vulnerable to surface water contamination due to intensive agricultural practices, affecting rivers and ponds that are otherwise suitable for fish farming. The WQI was determined based on analytical measurements of hydrochemical parameters, including dissolved oxygen, pH, total (bicarbonate) alkalinity, total mineralization, nitrate concentration, and ammonium nitrogen. During the spring season, water samples from all five sampling points exhibited WQI values corresponding to the Good category. In contrast, in the autumn season, the water sample from Kotlubaivka River fell under the Satisfactory category, while the remaining samples remained Good. These findings indicate that spatial and seasonal variability in hydrochemical parameters significantly constrains the suitability of aquatic environments for aquacultural use.</p>