资源环境科技发展态势分析平台

Atmospheric gases that dissolve in groundwater at the time of recharge are valuable tracers of groundwater residence time, but corrections are sometimes required. Low solubility gases like SF₆ are particularly susceptible to the effects of excess air, which is generally assumed to occur at the time of recharge. If after excess air correction SF₆ is still elevated above the expected concentration for water in equilibrium with the atmosphere this is often attributed to a terrigenic source. We propose that compressed air used during and after well installation is a potential source of excess air, which can sometimes explain elevated SF₆ concentrations from wells drilled in the last few decades. This concept is demonstrated by correcting elevated SF₆ and CFC‐12 measurements from 55 wells at field sites in the Pilbara region of Western Australia with up to 10 mL (STP)/kg of atmospheric air from the time of drilling (2006‐2016). The resulting SF₆ and CFC‐12 concentrations are consistent with recharge between 1950‐1970 for most wells. Excess air in recharge from that period could not have contained enough SF₆ to explain the measured concentrations. This decoupling of excess air from recharge is potentially also significant for other gaseous age tracers with limited solubility including SF₅CF₃, Halon 1301, ³⁹Ar, ⁸⁵Kr, and ⁸¹Kr.