Hierarchical cluster analysis (HCA) and inverse modeling (PHREEQ-code) were simultaneously useful approaches in interpreting surface water hydrochemistry within Talkhab River in the Tang-Bijar Oilfield, Iran, where large uncertainties exist in the understanding of the water quality system. Q-mode hierarchical cluster analysis (HCA) applied to the data revealed three major surface water associations distinguished on the basis of the major causes of variation in the hydrochemistry. The three water groups were classified as upstream waters (Group 1: Ca–SO4), intermediate waters (Group 2: Ca–SO4-Cl), and downstream waters (Group 3: Na-Cl). Geochemical reaction models of selected water groups were constructed using PHREEQC, which used to establish the reactions associated with the different mineral phases through inverse modeling. An inverse model describing the evolution of Group 1–Group 2 can be written as: Ca-SO4+ Halite+ Kaolinite+ Illite + K-mica+ gibbsite+ Quartz → Ca-SO4 -Cl + Calcite + Dolomite + Gypsum + Strantianite And an inverse model explaining the water chemistry along the flow path and can be written as: Ca-SO4-Cl+ Halite+ Kaolinite+ Illite+ Calcite+ Dolomite + Albite → Na-Cl-Ca-SO4 + Gibbsite + Gypsum The hydrochemical compositions of the water groups and the mass-balance calculations indicate that the dominant processes and reactions responsible for the hydrochemical evolution in the system are: (1) dissolution of evaporite minerals (2) precipitation of carbonate minerals; (3) silicate weathering reactions, (4) limited mixing with saline water, and (5) ion exchange

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