Impact of Agricultural Runoff-Derived Fertilizer Contamination on the Physicochemical, Nutritional, and Biological Quality of Tigris River Water in the Salah al-Din Governorate, Iraq

Background: Agricultural intensification along the Tigris River corridor in Salah al-Din Governorate has substantially increased the volume of nitrogen- and phosphorus-enriched runoff entering the river, particularly during rainfall events. The seasonal mobilization of synthetic fertilizers from adjacent farmlands represents one of the most underexamined stressors on Mesopotamian water resources. Aim: This study aimed to assess the extent and seasonality of fertilizer-derived contamination in the Tigris River by quantifying physicochemical, nutrient, heavy metal, and biological parameters at sites situated upstream, adjacent to, and downstream of active agricultural zones. Methods: Water samples were collected bi-seasonally (spring and autumn of 2022–2023) from four river sites spanning approximately 140 km of the Tigris within Salah al-Din. Physicochemical parameters (pH, dissolved oxygen, electrical conductivity, total dissolved solids, turbidity, BOD, COD), dissolved inorganic nutrients (NO₃⁻-N, NO₂⁻-N, NH₄⁺-N, PO₄³⁻-P), trace metals (Fe, Mn, Zn, Cu, Pb, Cd, Cr), and biological indicators (total coliforms, fecal coliforms, chlorophyll-a, phytoplankton density, water quality index) were determined following standard APHA (2017) and ISO protocols. One-way ANOVA with Duncan's post-hoc test was used for inter-site comparisons. Results: Agricultural sites recorded significantly elevated concentrations of NO₃⁻-N (14.62 ± 1.43 mg/L in spring), PO₄³⁻-P (0.62 ± 0.07 mg/L), and total coliforms (1840 ± 182 CFU/100 mL) relative to the upstream reference site (p < 0.05). Turbidity, BOD, and COD at Site 2 exceeded Iraqi national standards by 3.7-, 1.9-, and 1.9-fold, respectively. The water quality index dropped from 88.4 ± 3.2 (good) at the upstream site to 41.6 ± 4.8 (poor) at the primary agricultural site. Chlorophyll-a and phytoplankton abundance were approximately 6.7- and 8.0-fold higher at agricultural sites, indicating incipient eutrophication. Partial recovery was observed at the downstream site, though several parameters remained above permissible limits. Conclusion: Rainfall-driven fertilizer runoff from adjacent agricultural lands constitutes a quantifiable and seasonally variable threat to Tigris River water quality in the Salah al-Din region. Targeted best management practices, including riparian buffer zones, controlled-release fertilizers, and drainage regulation, are urgently needed to mitigate non-point source nutrient loading.