Investigating the Source and Factors Affecting Sulfate Concentration and Isotopic (34S and 18O) Variations in Water Resources of Sarpol-e Zahab Region

Sulfate is one of the important groundwater pollutant sources in many parts of the world and it can enter into groundwater from various sources, such as lithology (dissolution of evaporative and pyrite oxidation), atmosphere (sea water spray), industrial (combustion of fossil fuels, sulfide- minerals production, and agricultural fertilizers), and etc. Identifying sources of soluble sulfate in water sources is important.  The sulfate in groundwater can be investigated using isotope and geochemistry techniques. Using isotope (34S and 18O) and hydro geochemical techniques, it can be possible to find out: sulfate origins and the effective processes/reactions on sulfate concentrations and hydrogen sulfide gas (H2S) production. In this paper, for the first time, the sulfate source in groundwater of Sarpol Zahab and the parameters affecting sulfate concentration and its isotopic compositions (d34S and d18O) in groundwater were studied. Sarpol-e Zahab is located in the catchment area of Alvand river in the west of Kermanshah province, west of Iran. The formations of the region, based on age from old to new, include the Ilam, Gurpi, Amiran, Telezang, Pabdeh, Asmari, Gachsaran, Aghajari, Bakhtiari and Quaternary alluvium formations. Asmari formation and Quaternary alluvial sediments form the largest area of the region. Gachsaran formation contains evaporative materials which is reducing the quality of groundwater in the region. Ilam formation is effective in providing the organic matter required for the bacterial sulfate reduction process.
13 water samples were taken from the water resources (wells, springs and river) of Sarpol-e Zahab region in two steps (December 2014 and September 2015). Measuring field parameters (T, pH, Eh, Ec, and TDS) and sampling of water resources were performed according to the instructions of Groundwater and Geothermal Research Center (GRC), Ferdowsi University of Mashhad. Field parameters were measured by VWR Handheld Multi parameter Research meter at the location of each water source. The concentration of cations and the anions were determined by the devices inductively coupled plasma elemental analyzer (ICP-EA) and ion chromatography (IC), respectively. Chemical and isotopic analyses of all water samples were performed in Ottawa university geochemistry laboratory and Waterloo university isotope laboratory, respectively. The measurement reference for isotopic sulfate and oxygen were VCDT and VSMOW, respectively, and the value isotopic are expressed as permil ((‰.
The sulfate concentrations in different water resources of the region varied from 5 to 950 mg L, however, it is very high in Gandab spring’ water, due to the association with hydrocarbons, and in Patagh Tunnel water, due to discharging of water from Gachsaran Formation (339.6 mg L and 950.1mg L, respectively). Chenarpiran spring has the lowest sulfate concentration because it is located in highlands and is discharged from Asmari formation with good water quality. The amount of  d18O varied between 5.8 to 13.1 VCDT ‰ and the amount of  d34S ranges from 9.5 to 31.8 VSMOW ‰. In Gandab spring’ water, due to sulfate reduction by microorganisms, in addition to the unpleasant smell of sulfide hydrogen (H2S), the d34S and d18O values were enriched than the isotopic composition of other water sources ( about 31.8 ‰ and 10.3 ‰, respectively). In other hand, the Gel va Darreh spring's water, due to the effect of upstream bath waste water (soap and detergent with a value of about 1 ‰ for d34S), indicated depleted isotope values.
 The concentration and isotopic composition of sulfate in the water resources of the Sarpol-e Zahab region are affected by the geological, atmospheric, human and hydrocarbon materials. The impact of human and hydrocarbon factors on the concentration and isotopes of sulfate is local, however, the lithology effects are on all water resources of the region. Since, Gachsaran formation forms the alluvial aquifer bedrock of Ghaleshahin plain, the dissolution of gypsum has an effective role in the hydro chemical evolution of the alluvial aquifer of this plain. The lithology of the area is often limestone and evaporation. Due to the high solubility of these materials in water, they have been able to have a significant effect on sulfate isotopic in water sources. The amount of measured isotopic values indicated that the origin of evaporators and atmospheric sulfate is in water. The dissolution of the evaporation formation has reduced the water quality. The association of bedrock in the Gandab spring has led to have a reduction environment and the occurrence of sulfate reduction and production of hydrogen sulfide gas (H2S). The human activities (by making bath upstream) caused oxidizing conditions, isotopic depletion and contamination of the Glodarreh spring.