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.