PRACTICAL CONSIDERATIONS FOR MEASURING HYDROGEN CONCENTRATIONS IN GROUNDWATER

Several practical considerations for measuring concentrations of disso lved molecular hydrogen (H-2) in groundwater, including (1) sampling m ethods, (2) pumping methods, and (3) effects of well casing materials were evaluated. Three different sampling methodologies (a downhole sam pler, a gas-stripping method, and a diffusion sampler) were compared. The downhole sampler and gas-stripping methods gave similar results wh en applied to the same wells. The diffusion sampler, on the other hand , appeared to overestimate H-2 concentrations relative to the downhole sampler. Of these methods, the gas-stripping method is better suited to field conditions because it is faster (similar to 30 min for a sing le analysis as opposed to 2 h for the downhole sampler or 8 h for the diffusion sampler), the analysis is easier (less sample manipulation i s required), and the data computations are more straightforward (H-2 c oncentrations need not be corrected for water sample volume). Measurem ent of H-2 using the gas-stripping method can be affected by different pumping equipment. Peristaltic, piston, and bladder pumps all gave si milar results when applied to water produced from the same well. It wa s observed, however, that peristaltic-pumped water (which draws water under a negative pressure) enhanced the gas-stripping process and equi librated slightly faster than either piston or bladder pumps (which pu sh water under a positive pressure). A direct current(dc) electrically driven submersible pump was observed to produce H-2 and was not suita ble for measuring H-2 in groundwater. Measurements from two field site s indicate that iron or steel well casings produce H-2, which masks H- 2 concentrations in groundwater. PVC-cased wells or wells cased with o ther materials that do not produce H-2 are necessary for measuring H-2 concentrations in groundwater.