Both carbonate (as the oil-gas reservoir) and evaporite rocks (as caprock) coexist in the Masjed Soleyman oil field. The Asmari karstic aquifer is formed within Oligo-Miocene carbonate rocks in the south of the oil reservoir. A mixing between fresh karstic groundwater and oil-field brines is to be expected because of underground migration of the brines toward the aquifer. This process can reduce the groundwater quality by both increasing the water salinity and by adding hydrocarbon and sulfur contaminants into the groundwater. Tembi river contains saline water that can affect groundwater resources. Leaking of these brines into the aquifer was distinguished using total dissolved solids, the relative concentration of major elements, bromide ion (as a trace element), total organic carbon, ion ratios, and mixing curve diagrams. The polluted zone was determined by tracking the hydrochemistry changes across the groundwater flow direction. The volume percentage of different water sources in mixed groundwater was calculated and validated using PHREEQC software. The results revealed that the contribution of the oil-field brine in the groundwater is much lower than the salty river brine, but even this low amount has a considerable impact on water quality by increasing water salinity and adding hydrocarbon and sulfur into the groundwater.