A Proposed Method to Estimate In Situ Dissolved Gas Concentrations in Gas-Saturated Groundwater.

Gas-saturated groundwater forms bubbles when brought to atmospheric pressure, preventing precise determination of its in situ dissolved gas concentrations. To overcome this problem, a modeling approach called the atmospheric sampling method is suggested here to recover the in situ dissolved gas concentrations of groundwater collected ex situ under atmospheric conditions at the Horonobe Underground Research Laboratory, Japan. The results from this method were compared with results measured at the same locations using two special techniques, the sealed sampler and pre-evacuated vial methods, that have been developed to collect groundwater under its in situ conditions. In gas-saturated groundwater cases, dissolved methane and inorganic carbon concentrations derived using the atmospheric sampling method were mostly within ±4 and ±10%, respectively, of values from the sealed sampler and pre-evacuated vial methods. In gas-unsaturated groundwater, however, the atmospheric sampling method overestimated the in situ dissolved methane concentrations, because the groundwater pressure at which bubbles appear (Pcritical ) was overestimated. The atmospheric sampling method is recommended for use where gas-saturated groundwater can be collected only ex situ under atmospheric conditions.