Several practical considerations for measuring concentra tions of dissolved molecular hydrogen (H2) in groundwater, including (1) sampling methods, (2) pumping methods, and (3) effects of well casing materials were evaluated. Three different sampling methodologies (a downhole sampler, a gas-stripping method, and a diffusion sampler) were compared. The downhole sampler and gas-stripping methods gave similar results when applied to the same wells. The diffusion sampler, on the other hand, appeared to overestimate H2 concentrations relative to the downhole sampler. Of these methods, the gas-stripping method is better suited to field conditions because it is faster (?30 min for a single analysis as opposed to 2 h for the downhole sampler or 8 h for the diffusion sampler), the analysis is easier (less sample manipulation is required), and the data computations are more straightforward (H2 concentrations need not be corrected for water sample volume). Measurement of H2 using the gas-stripping method can be affected by different pumping equipment. Peristaltic, piston, and bladder pumps all gave similar results when applied to water produced from the same well. It was observed, however, that peristaltic-pumped water (which draws water under a negative pressure) enhanced the gas-stripping process and equilibrated slightly faster than either piston or bladder pumps (which push water under a positive pressure). A direct current (dc) electrically driven submersible pump was observed to produce H2 and was not suitable for measuring H2 in groundwater. Measurements from two field sites indicate that iron or steel well casings produce H2, which masks H2 concentrations in groundwater. PVC-cased wells or wells cased with other materials that do not produce H2 are necessary for measuring H2 concentra tions in groundwater.
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