Water Quality Assessment of the Jerma River: Application of the Water Pollution Index (WPI) Within a Torrential Catchment

This study evaluates long-term (2014–2023) and seasonal water quality in the Jerma River, a transboundary watershed between Serbia and Bulgaria, using annual mean physicochemical and microbiological parameters at two monitoring sites as well as Water Pollution Index (WPI) classifications. Seasonal datasets (spring, summer, autumn and winter) were applied to assess intra-annual variability. The water quality reflects the influence of geomorphology, hydrological seasonality, and anthropogenic pressures. The river reflects the influence of geomorphology, hydrological seasonality, and anthropogenic pressures typical of mountain catchments. Upstream conditions were favorable, characterized by stable oxygen regimes, low suspended solids, and low nutrient levels, with occasional phosphorus enrichment. Downstream sections showed stronger deviations, including elevated nutrients, departures from reference conditions, and variability in microbiological indicators. Seasonal analyses indicated stable oxygen conditions at both sites, while nutrient and microbial parameters varied during specific periods. Dissolved metals remained within Class I thresholds throughout the study. WPI values confirmed spatial and seasonal differences. Upstream water quality mainly corresponded to Class II, with occasional shifts toward Class III, whereas downstream values fluctuated between Classes II and IV, with short-term deterioration during seasons. Higher variability occurred in spring and summer, while winter conditions were more stable. Overall, the results indicate better upstream status and increased nutrient and microbial pressures downstream, highlighting the need for continued monitoring and management.