Introduction
Karst water sources in many areas can be used as a main source of fresh water to supply water to residents and the chemical and stable isotopes technique can be can used for understanding of karstic aquifers. Sarpol-e-zahab area is located in Kermanshah province, west of Iran. Alluvial deposit and Asmary formation cover the most outcrops in this area (Mohammadzadeh et al., 2016) and has an important karst water resources (Mohammadzadeh and amiri, 2019). The concentrations of inorganic and organic carbon (DIC and DOC) and variations of δ¹³C values can be investigated to determine carbon sources in water resources. In this research, the seasonal and spatial variations of dissolved carbon concentration and values of carbon isotope in water resources of Sarpol-e-zahab region have been evaluated.
 
Materials and Methods
 In order to evaluate the origin of dissolved carbon (DIC and DOC), samples were collected in two wet (January 2014) and dry (September 2015) seasons from surface and groundwater resources and field parameters were measured at the time of sampling. For measuring DIC and DOC concentration and variation of δ¹³C_DIC, all water samples were collected in colored glasses 40 cc and were sent to the G.G Hatch Stable isotope laboratory at the university of Ottawa and Environmental isotope laboratory at Waterloo university, Canada, for DIC, DOC and ¹³C isotope analysis.
 
Discussion
The mean DIC concentrations in wet and dry seasons were 71/9 and 80/9 mg/l ppmC, respectively. By increasing the DOC concentration (with average of 0/3 mg/l), the δ¹³C_DIC values were depleted. The average values of isotopic composition of carbon (δ¹³C_DIC) in water samples was about -11/7‰ and -11/3‰ in wet and dry season, respectively. Among the water samples, Gandab spring has high DIC concentration and due to presence of organic matter of hydrocarbon origin has depleted carbon isotope than other samples. Glodare sample has high amounts of DOC due to entry of organic detergent from upstream bath. Some samples have depleted isotopic composition due to agricultural activities and the use of pesticides. High P_CO2 values (with average 10^-1/8 and 10 ^-1/6 atm in wet and dry seasons, respectively), which is more than atmosphere CO₂ pressure (10 ^-3/5 atm), indicate weathering and dissolution of carbonates occur by soil CO₂ in study area. By increasing P_CO2, the DIC concentrations were increased, however, the δ¹³C_DIC values were depleted that can be caused by the decomposition of organic matter and root respiration of plants.
 
Conclusion
The results indicated that the main origin of dissolved carbon in the water samples of the Sarpol-e-zahab area is due to dissolution of carbonate rocks (lithology) and partially of it comes from organic matter and soil CO₂.