Impacts of natural CO2 leakage on groundwater chemistry of aquifers from the Hamadan Province, Iran

The effect of natural CO2 leakage through water wells on groundwater chemistry from alluvial aquifers of Hamadan, Iran, has been investigated through analysis of water samples from 5 springs and 19 wells. The average CO2 partial pressure in gas charged groundwater has increased about 32 times with respect to background groundwater, leading to an increase in alkalinity and in the concentration of all ions, except for SO4, and to a decrease in pH and DO. Due to a high pH buffering capacity, pH of gas charged groundwater has decreased only one unit. The increase in salinity of the gas charged groundwater cannot be attributed to in situ weathering of aquifer materials because of (1) the lack of correlation between DIC vs delta C-13(DIC) and TDS vs pH, (2) the high concentration of SiO2 and F and (3) the Sr-87/Sr-86 ratio in the range from 0.7085 to 0.7118. Instead, it can be attributed to saline CO2-rich waters from deep sources, which can dissolve a variety of minerals during their migration towards the surface. Although it is not clear the role of CH4 as electron donor, the association of delta O-18(SO4) and delta S-34(SO4) with SO4 concentration suggests that sulfate reduction could occur in the environment. The salinity of Mesozoic gas-rich springs, which present higher CO2 pressure and lower pH, is five times lower than that of Cenozoic ones because of the different degrees of metamorphism, which lead to an increase in grain size and slower reaction rate in Mesozoic than in Cenozoic carbonate rocks.