<p>Oil pollution in surface waters particularly in rivers that serve as drinking water sources—represents a persistent and complex environmental challenge due to its long-term ecological impacts (Asif et al., J Mar Sci Eng 10:762, 2022). Diesel fuel, with its partial solubility and high mobility, poses considerable contamination risks. The Karaj River, a crucial water supply for the cities of Karaj and Tehran, has experienced multiple diesel spill incidents in recent years. This study examines the fate and transport of diesel contamination through an integrated approach that combines numerical modeling (MIKE 11) with a scaled physical model—an approach seldom applied simultaneously in riverine oil spill assessments. A scenario-based experimental design was implemented to simulate both instantaneous and continuous discharge events under realistic hydraulic conditions. A 3-km segment of the river was numerically modeled, and Total Petroleum Hydrocarbon (TPH) concentrations were measured according to the ASTM D7065 standard method (Fingas, Elsevier, Oxford, UK, 2017). The results indicated substantially higher TPH concentrations in the instantaneous spill scenario (29.5 mg L⁻¹ at the release point and 10.2 mg L⁻¹ downstream) compared to the continuous discharge (3.1 mg L⁻¹ at the release point and ≤&#xa0;0.2 mg L⁻¹ downstream), reflecting differences in transport dynamics and the degree of natural attenuation. These findings underscore the pivotal influence of spill duration and hydrodynamic conditions on pollutant behavior and emphasize the necessity for adaptive monitoring and mitigation strategies to protect vulnerable drinking-water sources.</p>