Subsurface gases composition and quantities are affected from various processes: equilibrium of meteoric water with atmospheric gases, partial to full dissolution of “excess air” during water aquifer recharge events, microbial respiration, water-rock interactions, leakages from oil and gas reservoirs and deep crustal or mantle thermochemical reactions. Therefore, the composition of subsurface gases can provide valuable insights into the processes that took place in hydrological and geological history.
To investigate these processes, gases were separated from groundwater production wells using a gas-permeable membrane. From each well, independently collected samples were analyzed for N2, O2, CO2, hydrocarbons (C1-C4), H2, H2S, He, Ne, Ar, Kr and Xe. Results from four distinct groundwater systems revealed unique patterns in gas compositions: (a) the Yarkon-Taninim carbonate aquifer showed decreasing O2 concentrations and increasing CO2 concentrations down the flow path, which indicates that the recharge in the Judea and Samaria anticlines was the last place the groundwater came in contact with the atmosphere. (b) the Hazeva Formation clastic aquifer in Karkom Graben showed elevated CO2 concentrations compared to relatively high O2 concentrations. Since recharge and storage both occur locally within the Graben, O2 continues to enter the system by younger meteoric water, while allowing CO2 to accumulate within the system and also dissolve carbonate rocks (c) the Kurnub Group (Nubian Sandstone) aquifer in the Southern Arava is enriched with He and CO2, which is indicative to a subsurface source. (d) the Kurnub Group aquifer in the Northeastern Negev, contained C1-C2 and H2S, which is likely contributed from organic material bearing formations. Since all the samples from this region (the Tzin Syncline) showed such behavior, it is suggested that this hydrocarbon system is limited to a consistent flow path. Overall, these results demonstrate the potential of subsurface gases as a geochemical tool to detect and quantify subsurface processes.