Bubble extraction of dissolved gases from groundwater samples

A technique is described for measuring the concentration of dissolved metha ne in water samples. A 250 mu L bubble is injected into a 5 mL vial to crea te a headspace. The headspace gas is subsequently analysed by gas chromatog raph. The technique requires a headspace equilibrium time of at least 13 ho urs. The extraction efficiency of the method for methane saturated water wa s 101.9% +/- 1%. The technique is applicable to other dissolved gases in gr oundwater. The method was used during a comparative evaluation of groundwat er sampling devices at a field site. It was found that low flow rate sampli ng with either a Grundfos electric submersible pump or a peristaltic pump w ere more precise than using a bailer to measure dissolved methane.