Source-to-sink hydrochemical dynamics and contaminant risks in the transboundary Syr Darya River basin.

Abstract

The Syr Darya River Basin is a major transboundary freshwater system in Central Asia facing increasing pressure from climate change, agricultural intensification, mining, industrial development, and urbanization. This study provides a systematic review and quantitative hydrochemical synthesis of published literature addressing water and sediment quality across the basin between 2000 and 2026. A PRISMA-based search identified 1073 records, of which 24 studies containing primary quantitative datasets were retained. Following harmonization, 123 site-specific water observations from 19 studies were integrated into a basin-wide hydrochemical database and evaluated across four operational sectors: Upper Sector, Uzbek-Tajik Sector, Middle Sector, and Small Aral Sea. document a clear downstream hydrochemical evolution from low-mineralized Ca-HCO3 waters in the Upper Sector (TDS 281-302 mg L-1) to increasingly mineralized Ca-Mg-SO4-Cl and Na-SO4 waters in downstream reaches, where TDS frequently exceeds 1000 mg L-1. Salinization emerges as the dominant basin-scale water-quality issue, driven by irrigation return flows, collector-drainage water recirculation, shallow saline groundwater, and strong evaporative losses. Contaminant enrichment is concentrated within specific industrial and mining corridors, particularly in the Chirchik-Akhangaran system, the Angren-Almalyk district, the Shardara Reservoir, and the Small Aral Sea. Arsenic, cadmium, lead, and uranium are the most frequently reported inorganic contaminants of concern, while sediments function as long-term reservoirs for trace elements and persistent organic pollutants. The review highlights major monitoring gaps and supports the need for harmonized transboundary monitoring and integrated basin-scale water-quality management.

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