Oil pollution in surface waters and rivers remains one of the most serious environmental challenges, exerting severe adverse effects on ecosystems and water quality. The present study investigates the spatial and temporal patterns of diesel-induced oil pollution along a 3-kilometer reach of the Karaj River (Saroudaran–Bilqan section) using one-dimensional numerical modeling in theMIKE 11 software package.
Two spill scenarios were simulated: an instantaneous (sudden) release and a continuous release, both with the same total volume of diesel.
Results for the instantaneous spill scenario showed a shock-like concentration profile: the pollutant concentration peaked at C_max = 29.5 mg/L approximately 10 seconds after the spill, then declined under the combined effects of advection and decay (with a first-order decay coefficient k = 0.0003 1/s), reaching 12.6 mg/L at the downstream end of the study reach after roughly 1.5 minutes.
In contrast, the continuous release scenario exhibited a more uniform temporal distribution of the pollutant, coinciding with the mean flow velocity (U ≈ 3.3 m/s). Consequently, the concentration rapidly approached a near-steady-state condition, with a much lower value of about 0.27 mg/L observed at the downstream boundary.
Performance analysis indicated that the MIKE 11 model achieved a good fit to observed behavior (R² = 0.88) and predicted pollutant concentrations with relative errors generally below 25%. These findings demonstrate that numerical modeling can serve as a reliable tool for risk assessment, water quality monitoring, and the design of emergency management systems for oil spill incidents in Iranian rivers that supply drinking water.