Trace Hydrocarbon Signatures in West Virginia Groundwater
Abstract
Dissolved methane (CH4) is commonly present in anoxic groundwater. It is an important water quality measurement in areas where oil and gas development is considered a risk to groundwater availability. CH4 is introduced into groundwater by bacteria under sulfate reducing conditions, or when water interacts with thermogenic natural gas. Its presence can indicate the redox conditions in an aquifer, and variation in concentration over time can signal changes in subsurface process and flow paths. As there are multiple possible sources of CH4, additional information is needed to determine its origin, transport, and fate. Common indicators include stable hydrogen and carbon isotopes and concentration ratios of dissolved CH4 and associated hydrocarbons. Most analytical approaches for measuring dissolved CH4 and C2-C6 hydrocarbons have a limit of detection of around 1 μg (10-6 g)/L. This limit of detection is too high to quantify hydrocarbons in many samples where methane is detectable, and thus many datasets have non-detects for C2-C6 hydrocarbons for samples with biogenic methane or low methane concentrations.
To increase our understanding of the hydrogeochemical processes governing these compounds in the environment, we employ a high sensitivity technique capable of measuring C2-C6 hydrocarbons into the picogram (10-12 g) per liter range. At this level, non-detect frequencies are greatly reduced, allowing patterns to be resolved in samples ranging from oxic water to water with milligram per liter levels of dissolved hydrocarbons. Over 60 groundwater wells were sampled in West Virginia for trace hydrocarbons between 2014 and 2018. In these samples, the ratios of alkane isomers are highly fractionated compared to values observed in the free atmosphere and in thermogenic gases. The ratio of isobutane/n-butane was as high as 3.0 in samples with more than 1 mg/L of CH4, and the ratio of isopentane/n-pentane was 2.5 in the same samples, indicating an enrichment process for substituted alkanes, which usually have ratios between 1.5 and 0.5. This finding may be a useful indicator for biogenic methane, and it may provide more clarity in determining the exact sources of dissolved CH4 in other studies if it is indicative of specific aquifer processes.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H14B..08H
- Keywords:
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- 1041 Stable isotope geochemistry;
- GEOCHEMISTRYDE: 1065 Major and trace element geochemistry;
- GEOCHEMISTRYDE: 1829 Groundwater hydrology;
- HYDROLOGYDE: 1834 Human impacts;
- HYDROLOGY