A FIELD DEPLOYABLE METHOD FOR MEASURING METHANE CONCENTRATION AND δ13C OF METHANE IN GROUND WATER USING CAVITY RING DOWN LASER SPECTROSCOPY

The measurement of methane in ground water has historically required special sampling methods and subsequent laboratory analysis using mass spectrometry. The presence of methane in ground water is a growing concern for those with wells in close proximity to oil and gas operations, which has increased the need for methane ground water monitoring, including determining its biogenic or thermogenic origins. Here we present a method that couples a Picarro Cavity Ring Down Spectrometer that measures methane concentration (±5 ppb) and δ¹³C of methane (±0.5‰) with a gas preparation system that utilizes membrane technology to extract gasses from groundwater in situ. A suite of water samples varying in size from 0.25 ml to 10 ml is collected in sample loops from water being fed to the instrument via a simple garden hose. Vehicle batteries are capable of powering the entire system. Methane-free water is used to push samples through the de-gassing system and the captured gases are swept into the analyzer. Resulting instrument responses are analyzed to determine the quantity of CH₄ dissolved in the water by precisely knowing liquid volumes, air-flow rates, and instrument throughput. Stable isotopes are calculated in post processing by using Miller-Tans slope or Keeling plots of concentration and isotopic data to determine isotopic signatures for each sample size. Instrument calibration is accomplished with comparison to continuous flow IRMS analyses of IsoMetric Standards, and the NOAA-INSTAAR global measurement of methane isotopes. Preliminary results suggest dissolved gas determination uncertainties of 1% of the reading, and isotopic characterization to ±0.8 ‰. The time required for an entire analysis is about 1 hour. This field deployable system allows for a number of wells to be quickly assessed, potentially eliminating the need for off-site analysis and can provide more rapid reconnaissance of regions of interest.