Changes in water quality condition at sequential monitoring stations based on IRWQI along the Little Zab River in Northwestern Iran.

Water quality monitoring is essential for understanding ecosystem health and guiding effective water management strategies. In particular, water quality indices (WQI) are crucial tools for assessing the status of surface water bodies, providing a simplified measure of water quality across various parameters along sequential monitoring stations along a river system. This study aims to assess the spatial and temporal variations in water quality along the Little Zab River in northwestern Iran, using the Iranian Water Quality Index (IRWQI). This study examines water quality at four sequential monitoring stations along the river using the IRWQI, incorporating critical water quality parameters. Data for the analysis were collected from 2015 to 2024 and analyzed using non-parametric tests, including Kruskal-Wallis, to detect significant variations in water quality across the monitoring stations. According to the results, water quality varies across the stations. Water quality varies across stations. The upstream Mirabad-Upland station has a low IRWQI (56.51) due to wastewater from Chaku village. It improves at Grzhal-Bridge (60.04) via self-purification but declines at Nalas (57.35) due to pollution from Vavan village and agriculture. Sardasht-Dam records the highest IRWQI (64.46), likely benefiting from self-purification and cleaner inflows. Mirabad-Upland has "Fairly Good" to "Moderate" water quality. Grzhal-Bridge improves slightly, with some "Good" and "Very Good" cases, but occasional "Bad" levels. Nalas declines to mostly "Bad" and "Fairly Bad," likely due to pollution. Sardasht-Dam shows partial recovery, though some "Bad" cases persist. Overall, water quality worsens downstream due to pollution and hydrological changes. The Little Zab River's water quality followed a seasonal pattern, improving in wet months and declining in dry months due to flow changes and pollutant levels, indicating the need for year-round monitoring. The results suggest that localized pollution sources, such as wastewater discharge impact water quality, particularly in upstream sections. These results indicate the need for improved pollution control.