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The AAPG/Datapages Combined Publications Database

Environmental Geosciences (DEG)



Factors affecting the variability of stray gas concentration and composition in groundwater

Anthony W. Gorody1

1Universal Geoscience Consulting, Inc., 1214 West Alabama Street, Houston, Texas; [email protected]


Dr. Gorody received his master's degree and Ph.D. as a Weiss Fellow at Rice University in 1980. He is the president of Universal Geoscience Consulting, Inc., Houston, Texas, and is licensed to practice geology in the states of Texas, Pennsylvania, and Wyoming. He has been principally involved in site-specific casework related to identifying and mitigating stray gas and gasoline range hydrocarbon sources in groundwater. His research interests are on forensic methods useful for evaluating the influence of naturally occurring bacteria on the fate of dissolved hydrocarbons in groundwater.


I thank the Colorado Oil and Gas Conservation Commission (COGCC), Encana Oil and Gas (USA), Inc., and BP America for providing data and for supporting the costs of data analysis and interpretation.


Identifying the source of stray gas in drinking water supplies principally relies on comparing the gas composition in affected water supplies with gas samples collected in shows while drilling, produced gases, casing head gases, pipeline gases, and other potential point sources. However, transport dynamics of free and dissolved gas migration in groundwater aquifers can modify both the concentration and the composition of point source stray gases flowing to aquifers and occurring in the groundwater environment. Accordingly, baseline and forensic investigations related to stray gas sources need to address the effects of mixing, dilution, and oxidation reactions in the context of regional and local hydrology. Understanding and interpreting such effects are best addressed by collecting and analyzing multiple samples from baseline groundwater investigations, potential point sources, and impacted water resources.

Several case studies presented here illustrate examples of the natural variability in gas composition and concentration data evident when multiple samples are collected from produced gases, casing head gases, and baseline groundwater investigations. Results show that analyses of single samples from either potential contaminant point sources or groundwater and surface water resources may not always be sufficient to document site-specific baseline conditions. Results also demonstrate the need to consistently sample and analyze a variety of baseline groundwater and gas composition screening parameters. A multidisciplinary approach is the best practice for differentiating among the effects of fluid and gas mixing, dilution, and natural attenuation.

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