Effect of Different Sampling Methodologies on Measured Methane Concentrations in Groundwater Samples

Authors: L. Molofsky, S. Richardson, A. Gorody, F. Baldassare, J. Black, T. McHugh, J. Connor
Published: March 2016 in Ground Water.


Analysis of dissolved light hydrocarbon gas concentrations (primarily methane and ethane) in water supply wells is commonly
used to establish conditions before and after drilling in areas of shale gas and oil extraction. Several methods are currently used
to collect samples for dissolved gas analysis from water supply wells; however, the reliability of results obtained from these
methods has not been quantified. This study compares dissolved methane and ethane concentrations measured in groundwater
samples collected using three sampling methods employed in pre- and post-drill sampling programs in the Appalachian Basin. These
include an open-system collection method where 40 mL volatile organic analysis (VOA) vials are filled directly while in contact
with the atmosphere (Direct-Fill VOA) and two alternative methods: (1) a semi-closed system method whereby 40 mL VOA vials
are filled while inverted under a head of water (Inverted VOA) and (2) a relatively new (2013) closed system method in which the
sample is collected without direct contact with purge water or the atmosphere (IsoFlask®). This study reveals that, in the absence
of effervescence, the difference in methane concentrations between the three sampling methods was relatively small. However,
when methane concentrations equaled or exceeded 20 mg/L (the approximate concentration at which effervescence occurs in the
study area), IsoFlask® (closed system) samples yielded significantly higher methane concentrations than Direct-Fill VOA (open
system) samples, and Inverted VOA (semi-closed system) samples yielded lower concentrations. These results suggest that open and
semi-closed system sample collection methods are adequate for non-effervescing samples. However, the use of a closed system
collection method provides the most accurate means for the measurement of dissolved hydrocarbon gases under all conditions.