Best Sampling Practices for Collecting and Measuring Dissolved Hydrocarbons in Groundwater

Dissolved hydrocarbon concentrations in groundwater, especially dissolved methane concentrations, have become a topic of increasing interest since the proliferation of shale gas and shale oil wells across the United States and Canada. While some sampling practices, such as purging times and bottle filling methods have been investigated, various groups, including consultants, regulatory agencies, and academic research groups still utilize different bottles types and/or holding times for samples of dissolved methane and higher chain hydrocarbons. This leads to potential problems in comparison of groundwater methane concentrations between groups or regions. Here we systematically compare measurements and holding times for VOA vials with Teflon-lined septa and serum bottles with butyl rubber septa. We statistically compare differences in methane concentrations between VOA vials and serum bottles for over 300 direct-fill field samples, where duplicates of VOA vials and serum vials were taken at each site. Field samples included a range of methane concentration from detection limit to over 40 mg/L. We also show results from a lab-controlled experiment in which VOA vials and serum bottles were held for known time periods and analyzed to calculate the leakage rate of methane from different bottle types in order to determine maximum shelf life. Leakage rates were determined for two different concentrations of dissolved methane. These results have implications for future study design of groundwater monitoring projects, and collection and storage of large quantities of samples.