In-line Sampling and Fixed Gas Analyses Help to Evaluate Dissolved Hydrocarbon Concentrations

Criticisms regarding the validity of dissolved methane concentrations in groundwater often cite in situ ebullition in the domestic water well environment as a factor affecting results. Such gas losses make it more difficult to confirm declining concentration trends that arise following remedial actions conducted in response to stray gas investigations. In-line groundwater sampling devices capture both dissolved and liberated gases during sampling. Subsequent analysis of both atmospheric and hydrocarbon gases in headspace samples can help resolve environmental variables affecting gas concentration.

Sample data presented here are derived from water wells screened in the Laramie-Fox Hills (LFH) aquifer located within the southwest corner of Weld County, Colorado. Total dissolved gas pressure (TDGP) is mediated by contributions from air derived from the air-water interface, dissolved bacteriogenic CH₄, bacterial CO₂, and dissolved Ar and N₂ from the aquifer recharge zone. Among water well sites at high elevations, TDGP far exceeds saturation causing samples to effervesce. Results of this study show that dissolved hydrocarbon concentrations reported for in-line water sample data are consistently greater than RSK-175 data for samples collected with 40 ml VOA vials.

Elevated bacteriogenic dissolved methane concentrations in the LFH aquifer are highly variable. The dissolved argon concentration in these young aquifer fluids is just as soluble as methane but has a more restricted concentration range. There is a linear and positive correlation between the headspace methane:argon ratio and dissolved methane concentrations. Anomalously high C₁:Ar ratios record Ar stripping resulting from either free phase bacteriogenic or stray gas migration bubbling locally within or in close proximity to the water column in the well. Dissolved Ar and N₂ concentrations further reveal three dominant mechanisms affecting dissolved gas composition and concentration: well column degassing by gas stripping, re-equilibration at ambient temperatures, and mixing with water near the air-water interface.