Integrating QUAL2KW nitrate modeling with ambient grey-water dilution pressure and wheat-pathway exposure screening in a semi-arid basin

Nitrate pollution in semi-arid rivers creates coupled ecological stress and downstream human exposure, particularly where surface water is reused for irrigation. This study integrated QUAL2KW hydrodynamic–water quality modeling with wheat-pathway exposure screening, ecological screening using Risk Quotients (RQ), and ambient grey-water dilution-pressure analysis in the Zayandeh-Roud River, Iran. Campaign observations showed substantial nitrate variability (CV: 14.9–91.3%) without a uniform longitudinal field pattern, while downstream irrigation-reuse sites exhibited dissolved oxygen depletion and elevated chemical oxygen demand. Under representative quasi-steady modeled conditions, nitrate exceeded the ecological screening threshold (RQ > 1) throughout the modeled corridor, including downstream irrigation-withdrawal nodes, while net algal production remained low. The coefficient-based calculation yielded a wheat-grain NO₃–N concentration of 0.249 mg kg⁻¹ at both downstream irrigation-reuse stations, indicating potential transfer into edible biomass under the adopted assumptions. The corresponding adult Chronic Daily Intake was 1.07 × 10⁻³ mg kg⁻¹ day⁻¹, producing a Hazard Quotient of 6.67 × 10⁻⁴ and suggesting low non-carcinogenic concern under the adopted wheat-only screening scenario. The ambient grey-water dilution-pressure indicator was 1.29 × 10⁶ m³ day⁻¹ at the primary downstream reuse node, exceeding representative daily river flow (relative pressure ratio = 1.25). These findings demonstrate persistent modeled nutrient pressure and downstream irrigation-reuse exposure despite campaign-scale variability, supporting integrated nutrient management in semi-arid basins.