Seasonal And Spatial Hydrochemistry of Agricultural Collector-Drainage Water in The Navoi–Bukhara Irrigation Zone (Uzbekistan): Salinity, Sodicity, Fluoride and Boron As Constraints on Reuse

Collector-drainage water (CDW) from irrigated agriculture is a large but under-used water source in the arid lower reaches of Central Asian river basins, yet its seasonal composition is rarely documented at the resolution needed for reuse planning. This study characterised the seasonal and spatial hydrochemistry of CDW in a collector of the Navoi–Bukhara irrigation zone (Uzbekistan). Samples were collected at three sites along the collector during four agronomic periods of 2025 (winter leaching, spring, summer vegetation irrigation and autumn) and analysed for more than 20 parameters. At the outlet site, total dissolved solids (TDS) ranged from 2.8 to 7.2 g L⁻¹ and specific conductance (κ₂₅) from 4.2 to 9.8 mS cm⁻¹. The water was of sulfate–chloride sodium type in all seasons (Na⁺ 62–68 % of cation equivalents; sodium adsorption ratio 9.2–18.2), with Na⁺/Cl⁻ equivalent ratios above unity. Salinity, hardness, fluoride (up to 4.8 mg L⁻¹) and boron (up to 3.5 mg L⁻¹) peaked during winter leaching, whereas summer irrigation diluted the salts but raised phosphate (4.5 mg L⁻¹) and chemical oxygen demand (COD, 110 mg O₂ L⁻¹). The nearly constant F⁻/TDS (0.67–0.75 mg g⁻¹) and B/TDS (0.43–0.49 mg g⁻¹) ratios point to a common geogenic origin of fluoride, boron and salts. Along the collector, salinity and fluoride increased downstream while phosphate, nitrate and COD decreased despite rising salinity, indicating in-channel attenuation. Heavy metals occurred at trace levels (≤ 0.045 mg L⁻¹). Salinity, chloride, sodium, fluoride and boron exceeded FAO irrigation guidelines in every season, and phosphate and COD exceeded national limits. κ₂₅ was linearly related to TDS (r = 0.999), supporting low-cost conductivity monitoring as a real-time proxy for the salinity-, fluoride- and boron-dominated leaching regime. The governing constraint on reuse thus shifts from geogenic contaminants in winter–spring to agrochemical contaminants in summer, which must be reflected in any treatment or reuse scheme.