Integrated trend and risk assessment of hydrochemical dynamics and discharge-pollution interactions in the Chirchik...

Water quality degradation in semi-arid, flow-regulated transboundary rivers is shaped by interactions between anthropogenic pollution and hydrological variability; however, long-term assessments of discharge-pollutant dynamics remain scarce. This study aims to provide the first multi-decadal (1991-2021) integrated assessment of hydrochemical dynamics, hydrological controls, and ecological risk zoning for the Chirchik River Basin, Uzbekistan. Discharge records and water quality data from six monitoring sites were analyzed using trend tests, spatial mapping, and an exceedance frequency-severity risk index. Results revealed a critical regime shift: declines in BOD and iron concentrations indicate progress in controlling organic effluents, yet significant increasing trends in ammonium and nitrite at downstream stations signal the emergence of a nitrogen-dominated pollution regime with progressive eutrophication risks in the Syr Darya. Nitrite emerged as the most flow-sensitive parameter, systematically elevated during low-flow years and reaching up to nine times the allowable limit, while BOD and iron peaked under normal-flow conditions, suggesting that mobilization rather than dilution governs their concentrations. The risk index identified the Chirchik City industrial zone as a high-risk hotspot and downstream sites as moderate-risk zones. Chronic nitrite and copper exceedances indicate that no monitored river stretch reliably meets drinking water standards. Targeted industrial pre-treatment at identified pollution hotspots may provide substantial basin-wide water quality improvements. In addition, maintaining river discharge above the 25th percentile during dry years could potentially lower pollutant concentrations by approximately 20-30%; however, this management option should be considered carefully because it may reduce water availability for irrigation.