Insights into groundwater hydrochemistry and origin of the hydrocarbon contaminated karst aquifers, SW Iran

Khuzestan, Ilam, Lorestan, and Chaharmahal and Bakhtiari provinces in southwest of Iran. In this study, conventional geochemical, stable isotopes of H 2 O (δ 2 H, δ 18 O), SO 4 (δ 34 S, δ 18 O), and dissolved inorganic carbon (δ 13 C DIC ) were employed on the samples collected in February 2021, August 2021, and February 2022 from SW Iran. By the use of hydrogeochemical diagrams, sulfuric samples (SS), non-sulfuric samples (NSS), and oil field water (OFW) samples in karst aquifers have been distinguished. The findings reveal that: (1) the major chemical composition of the SS, NSS, and OFW samples consist of CaSO 4 , CaHCO 3 , and NaCl types, respectively; (2) the δ 2 H H2O values range between −17.37 and −15.86 ‰ V-SMOW and the δ 18 O H2O values range between −4.49 and −4.18 ‰ V-SMOW for the SS and NSS, indicating that they originated from local rainwater; (3) the δ 34 S SO4 and δ 18 O SO4 values of dissolved sulfate of SS, NSS, and OFW indicate that the sulfate resulted from interactions between water and rocks, particularly evaporitic rocks such as gypsum and anhydrite; and (4) the δ 13 C DIC values vary extensively, ranging from −16 ‰ to + 7 ‰ V-PDB. Higher δ 13 C DIC signatures are consistent with biogeochemical processes (i.e., methanogenesis) or exchange with marine carbonates. (5) The concentration of total petroleum hydrocarbons (TPH) ranged from 26.3 to 19,670 μg/L in the groundwater samples.Toluene was the most abundant species of BTEX in all samples. • The concentration of total petroleum hydrocarbons (TPH) ranged from 26.3 to 19,670 μg/L in the groundwater samples. • Hydrogeochemistry and multiple isotopes uses to study karst waters in SW Iran. • Hydrochemical characteristics of karst groundwaters are different. • Halite and gypsum dissolution are the main salinization processes. • Methanogenesis is supported by the δ 13 C DIC in NSS.