Excess Air Correction of SF₆ and Other Dissolved Gases in Groundwater Impacted by Compressed Air From Drilling or Well Development

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 and 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.