In Situ Sampling Method for Determining Dissolved Gas Content and Major Ion Composition of Groundwater

We investigated the deployment of diffusive samplers within wellbores for determining the dissolved gas content and major ion composition of groundwater. Unlike active sampling strategies, an in situ method does not require purging the well until equilibrium conditions are established, and hence does not perturb the hydraulic gradient near the well, nor the natural pattern of groundwater flow. We tested the method at five monitoring wells installed within the Mahomet aquifer in central Illinois. We lowered two samplers, one containing a gas phase and another holding an aqueous fluid to screen level within the well and allowed them to equilibrate with groundwater over a period of several days. The gas sampler was constructed by filling a 3 mL syringe with N₂ gas and attaching it to a short length of gas-permeable silicon tubing. After retrieving the sampler, the gas was injected into a sealed tube filled with helium for storage and transport. The mixture was then analyzed for H₂, CO₂, CH₄, and CO using gas chromatography. The fluid sampler was a 1-inch dialysis cassette containing distilled water. Upon retrieving the sampler, we withdrew fluid with a needle and syringe and measured major ion concentrations using ion chromatography. Repeated measurements made over several months yielded consistent results for dissolved gases as well as major ions. The samplers can be deployed again immediately after being retrieved, suggesting they may prove useful not only for increasing the ease and perhaps accuracy of groundwater sampling, but in applications requiring long-term monitoring of groundwater quality.