Hydrophysics

Hydrophysics

Investigation of Oil Pollution Trends in the Karaj River Using Numerical Modeling

Document Type : Original Article

Authors
Mohsen Asadi 1
Amir Hossein Javid 2
Sara Allahyari Beik 3
Mir Masoud Kheirkhah Zarkesh 4
1 Department of Environmental Engineering, SR.C., Islamic Azad University, Tehran, Iran.
2 Science and Research Branch, Islamic Azad University, Tehran, Iran
3 Department of Physics, Faculty of Convergent Sciences and Technologies SR.C, Islamic Azad University ,Tehran, Iran..
4 Soil Conservation and Watershed Management Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran.
Abstract
Oil pollution in surface waters and rivers is one of the most critical environmental challenges, with severe impacts on aquatic ecosystems and water quality. The Karaj River, as a major drinking water source for the city of Karaj and parts of Tehran, has experienced several diesel spill incidents in recent years. In this study, the spatial and temporal evolution of diesel‑induced oil pollution along a 3‑km reach of the Karaj River (Saroudaran–Bilqan section) is investigated using one‑dimensional numerical modeling in the MIKE 11 software. Flow is simulated by solving the continuity and momentum equations, while pollutant transport is modeled using the one‑dimensional advection–dispersion equation. The governing equations are solved numerically with the finite‑difference method and the implicit Abbott–Ionescu scheme. Two numerical scenarios are considered: an instantaneous diesel spill and a continuous spill with constant volume. In the instantaneous spill scenario, concentration changes exhibit a shock‑like behavior, with the pollutant concentration reaching a peak value of 29.5 mg/L about 10 seconds after the spill. Subsequently, under the combined effects of advection and decay (k = 0.0003 s⁻¹), the concentration decreases and reaches 12.6 mg/L at the downstream end of the study reach after approximately 1.5 minutes. In the continuous spill scenario, the upstream concentration is 0.2 mg/L; due to its more uniform temporal distribution and consistency with the mean flow velocity (U ≈ 3.3 m/s), the concentration approaches a quasi‑steady state more rapidly, attaining about 3.1 mg/L at the downstream boundary. Model evaluation shows that MIKE 11 achieves a good level of agreement with observations (R² = 0.88), predicting pollutant behavior with a relative error of less than 25%. These results indicate that numerical modeling is a reliable tool for risk assessment, water quality monitoring, and the design of emergency management systems for oil spill events in drinking‑water supply rivers in Iran.
Keywords
Karaj River
Oil pollution
Pollutant transport
Diesel spill
Numerical modeling
MIKE 11
Subjects
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  • Receive Date 10 February 2026
  • Revise Date 13 March 2026
  • Accept Date 01 April 2026