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

Abstract

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.

Assessing Methane in Shallow Groundwater in Unconventional Oil and Gas Play Areas, Eastern Kentucky

“…Potassium concentrations did not significantly correlate with methane. These relationships are consistent with our findings that methane predominantly occurs in higher concentrations in sodium dominated water types, and inversely, in lower concentrations in calcium dominated waters (Siegel et al ; Molofsky et al ). In addition, similar to in groundwater in northeastern Pennsylvania (Molofsky et al ), methane is associated with more reducing conditions (i.e., water that exhibits lower sulfate concentrations [Figure ] and low oxidation‐reduction potential [Table ]).…”

Biogeochemical and physical controls on methane fluxes from two ferruginous meromictic lakes

“…No significant difference in dissolved CH 4 concentration was observed between 2017 and 2018 samples. The field observation of exsolution in waters retrieved from depth was consistent with gas concentrations within the range of CH 4 saturation (e.g., Molofsky et al, ) as displayed in our reported values and also consistent with previous reports of a negligible impact of filtering on dissolved CH 4 concentrations (e.g., Alberto et al, ). Dissolved inorganic carbon (DIC) concentrations and isotopes were filtered (0.45 µm PES) and injected into exetainers that were He‐flushed and contained 1 ml of concentrated phosphoric acid.…”

Methane Occurrences in Aquifers Overlying the Barnett Shale Play with a Focus on Parker County, Texas

“…A dissolved methane concentration of 10 mg/L is generally accepted as the action level at which a water well should be vented (NGWA ). Note that the amount of methane for samples >20 mg/L is only semiquantitative because of the sampling methodology at approximately atmospheric pressure (Molofsky et al ). Several authors (Wardrop ; Christian et al ; Loomer et al ) have documented seasonal and other temporal variations of dissolved methane but the variations rarely show large swings from high methane to none.…”