Passive Grab Sampling for Dissolved Methane in Deep Groundwater

Groundwater sampling for methane in deep wells is challenging due to difficulties related to high hydraulic pressure, degassing, and physical retrieval of representative samples. At a site in Mississippi, where groundwater is saturated with natural gas at depths of up to 700 feet below ground surface and under hydraulic pressure of up to 180 psi, two passive groundwater sampling methods were used to collect water samples for methane analyses: the Snap Sampler and the Solinst Discrete Interval Sampler.   

Several rounds of groundwater samples were collected from site wells to assess the passive groundwater samplers and to characterize the deep groundwater. Methane concentrations measured for samples collected using the Snap Sampler and the Solinst Discrete Interval Sampler method closely approximated calculated methane-saturated concentrations at depth. Degassing within the Snap Sampler vials did occur upon retrieval because the vials are not pressure sealed; however, because degassed vapor and fluid is contained within the vessel, the methane concentrations measured in the Snap vials appear to be representative of downhole concentrations. Split samples of Snap Sampler vials indicated inter-laboratory variability, which was partially attributed to inconsistency in laboratory protocols. Based on experimentation, we found that several approaches were effective in obtaining deep groundwater samples for groundwater with dissolved methane at concentrations of tens to hundreds of mg/L.