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

Abstract

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 (P critical ) was overestimated. The atmospheric sampling method is recommended for use where gas-saturated groundwater can be collected only ex situ under atmospheric conditions.

Improvements in Drill-Core Headspace Gas Analysis for Samples from Microbially Active Depths

“…The δ 13 C CH4 values for IsoJar™ samples (Figure 4(b)) increase markedly with time regardless of additives and fluctuate by more than 30‰ after 80 d; separate trends for different depths are not evident. The concentration of CH 4 remaining in the groundwater at atmospheric pressure after sampling is relatively low (~3 mmol kg −1 ; [47]), and δ 13 C CH4 values for the IsoJar™ samples mainly represent gases adsorbed on cores. Therefore, while the relatively large amounts of dissolved CO 2 reduced the effects of isotopic fractionation on δ 13 C CO2 values, δ 13 C CH4 values were strongly affected.…”

“…The two trends suggest that the δ 13 C CO2 ratios represent dissolved gases from different depths. The concentration of CO 2 in the groundwater, as HCO 3 − , is relatively high even when degassed at atmospheric pressure (30-50 mmol kg −1 ; [47]), compared with that of adsorbed CO 2 , so carbon isotopic values should be largely dependent on dissolved CO 2 . The δ 13 C CO2 values of samples with 50 m groundwater (dashed line in Figure 4(a)) are distinct from those of samples with 350 m groundwater, indicating that the former samples are strongly contaminated.…”

Characterization of sediment and granite hosted deep underground research laboratories reveals diverse microbiome functions, limited temporal variation and substantial genomic conservation

“…Temperatures range from 13 - 25 °C, pH ranges from 6.4 to 7.2, and Eh values range from −315 to −204 mV. Unlike at Mizunami, the Horonobe groundwater is saturated with both CH 4 and CO 2 (Miyakawa et al, 2017; Tamamura et al, 2018), which degas during sample recovery.…”