AN EVALUATION OF STANDARD AND NEW SAMPLING METHODOLOGIES FOR SAMPLING HYDROCARBON AND OTHER DISSOLVED GAS CONCENTRATIONS IN GROUNDWATER SAMPLES

The potential for drinking-water contamination is central to the debate about the environmental impacts of hydraulic fracturing in unconventional hydrocarbon reservoirs. The environmental implications for the presence of elevated methane and aliphatic hydrocarbons (ethane, propane, etc.) in drinking-water remain highly controversial and require geochemical methods that can distinguish between naturally occurring and anthropogenic sources. In addition to the ongoing contention regarding the environmental implications of methane and other light hydrocarbon gases in shallow aquifers, there is growing confusion about accepted methodologies and best practices for sampling hydrocarbon and other dissolved gas concentrations (e.g., N₂, noble gases) in drinking-water. The importance of reliable and reproducible sampling techniques, specifically for hydrocarbon concentrations, is essential to both baseline studies and investigations that occur after drilling and petroleum extraction has occurred. Here, I discuss and quantitatively compare various established and emerging sampling methodologies for sampling hydrocarbon and other dissolved gas concentrations (e.g., N₂, noble gases) in drinking-water. Sampling techniques include the standard USGS protocol (i.e., copper tubes), the bottle-fill/headspace method published by the EPA (Kampbell and Vandegrift, 1998), diffusion samplers, open- and closed-system VOA vials, and IsoFlask® (Molofsky et al., 2016). Preliminary data indicates that measured methane levels are highly susceptible to methane concentrations in known solutions and each sampling methodology experiences variable amounts of atmospheric contamination.

Kampbell, D.H. and Vandegrift, S.A. (1998) Analysis of dissolved methane, ethane, and ethylene in ground water by a standard gas chromatographic technique. Journal of Chromatographic Science. 36: 253-256.

Molofsky, L.J. et. al. (2016) Effect of different sampling methodologies on measured methane concentrations in groundwater samples. Groundwater 9-10 (669-680).