Introduction:
Water quality plays a crucial role in sustaining aquatic ecosystems, human health, and socio-economic development. River ecosystems are particularly vulnerable to anthropogenic pressures such as urbanization, agricultural runoff, and industrial discharges, which disrupt their physicochemical balance and ecological integrity. Monitoring key water quality parameters—such as Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Nitrogen (TN), Total Phosphorus (TP), Total Suspended Solids (TSS), and pH—is essential for assessing the health of river systems. Seasonal variations, especially between dry and wet periods, can significantly influence pollutant concentrations due to changes in river discharge, dilution, and runoff intensity. In regions like Kurdistan Province, Iran, where rivers such as Gheshlagh and Gaveroud converge into the Sirvan River, the combined impacts of municipal effluents and agricultural return flows pose serious environmental challenges. This study aims to evaluate the spatial and temporal variations of major physicochemical parameters using the Inverse Distance Weighting (IDW) method within a GIS environment to identify pollution hotspots and provide insights for sustainable watershed management.

Materials and Methods: 
Water samples were collected at six selected stations along the Gheshlagh and Gaveroud tributaries and the main Sirvan River during two contrasting seasons—summer (low flow) and winter (high flow) of 2024. Station 1, located downstream of Gheshlagh Dam, served as a reference site with minimal anthropogenic influence, while stations 2–4 were situated upstream and downstream of the Sanandaj wastewater treatment plant to assess the effects of effluent discharge. Station 5 represented the Gavehroud tributary with relatively low pollution, and station 6 was located at the Sirvan outlet near Zhaveh Dam. Three replicate samples were collected per site at a depth of 20–30 cm using pre-cleaned polyethylene bottles, transported on ice at 4°C, and analyzed according to APHA (2017) and WHO (2017) standards. BOD was determined using the five-day incubation method at 20°C, COD by the dichromate oxidation method, TN through persulfate digestion followed by spectrophotometric detection, TP via ammonium molybdate–ascorbic acid reaction, and TSS by gravimetric filtration through 0.42 μm glass-fiber filters. pH was measured in situ using a calibrated WTW portable meter. Spatial interpolation of the measured parameters was conducted using the IDW technique in ArcGIS 10.8, with a power parameter of 2, a search radius including 8–12 neighboring points, and a raster cell size of 30 m. This method assigns weights inversely proportional to the distance between sampled and unsampled locations, enabling the continuous prediction across the study area.

Results and Discussions:
The spatial maps derived from the IDW model revealed distinct seasonal trends in water quality. During summer, BOD exceeded 6 mg/L near stations 2 and 3, indicating the impact of untreated municipal effluents from the Sanandaj wastewater treatment plant. In contrast, the Gaveroud tributary (station 5) exhibited the lowest BOD values. COD values surpassed 25 mg/L in summer and increased to over 50 mg/L in winter, likely due to enhanced surface runoff carrying refractory organic compounds. The lower BOD/COD ratio in winter confirmed the dominance of non-biodegradable organic matter. The persistently high TN values in both seasons indicated sustained nutrient loading from domestic sewage and agricultural fertilizers. TP levels in summer exceeded 3 mg/L at stations 3 and 4—several orders above global thresholds for eutrophication (0.05 mg/L)—and showed a slight decline. This sustained phosphorus enrichment represents a critical eutrophication risk to downstream reservoirs such as Zhaveh Dam. TSS concentrations reached up to 80 mg/L in summer but decreased to 20–60 mg/L in winter due to dilution. Elevated suspended solids reduced water transparency and may impair photosynthetic activity and fish habitats. The pH values remained stable (7.6–8.2) across stations and seasons, staying within the safe range for aquatic life. Overall, spatial interpolation using IDW effectively delineated pollution gradients, particularly downstream of the wastewater discharge, confirming the method’s reliability in riverine water quality assessment.

Conclusion:
This study demonstrated that the Gheshlagh, Gaveroud, and Sirvan River systems are significantly influenced by municipal wastewater and agricultural runoff, particularly during the dry season. The marked increases in BOD, COD, TN, and TP in summer indicate intensified organic and nutrient pollution, leading to potential eutrophication and ecological degradation. Although winter flows partially dilute pollutants, persistent pollutant loads remain a serious concern. The application of the IDW interpolation method in GIS effectively visualized spatial pollution patterns and identified critical hotspots, supporting its use as a practical tool for environmental monitoring and watershed management. To safeguard the ecological integrity of these rivers, stricter wastewater control measures, improved treatment plant efficiency, and the adoption of sustainable agricultural practices are urgently recommended.