Assessment of Bitumen-Influenced Aquifers in Southwestern Iran Based on Groundwater Geochemistry and Stable Isotopes Characteristics

Iran is situated in a region with natural bitumen deposits and bitumen mines. This study compared the hydrogeochemical and isotopic composition of groundwater contaminated by abandoned bitumen mines (GCBM) with the deep formation water of oil reservoirs (FWOR). The GCBM was found to be dominated by Ca²⁺, Na⁺, SO₄²⁻, and Cl⁻, and is typically characterized by Ca–SO₄ type water, in contrast to the Na–Cl type in the FWOR. The δ²H_H2O and δ¹⁸O_H2O isotopes were valuable tools to distinguish the groundwater sources, as the isotope signatures of GCBM and FWOR samples are markedly different. Ionic ratio diagrams, such as Na²⁺ vs. Cl⁻, Ca²⁺ vs. Cl⁻, and SO₄²⁻ vs. Cl⁻, indicate that the groundwater chemistry in the study area is mainly influenced by gypsum and carbonate dissolution due to mining. The concentration of total petroleum hydrocarbons (TPH) ranged from 26.3 to 19,670 µg/L in the GCBM samples. This study confirmed unacceptable levels of groundwater contamination by TPH caused by seepage from abandoned bitumen mines and established a framework for future groundwater remediation efforts in the study area.