The transition zone between a river and the groundwater underlying it, the hyporheic zone, is an important habitat for microbes that reduce nutrients and substrates filtrated into the groundwater. O₂ depletion directly reflects the activity of those microbes and is linked to the availability of organic matter and water quality.

However, biogeochemically induced O₂ is hard to determine because the total input of O₂ into the groundwater is unknown. Lars Mächler and his team, Swiss Federal Institute of Aquatic Science and Technology, Drübendorf, Switzerland, developed a membrane inlet instrument to measure concentrations of dissolved O₂ and noble-gases quasi-continuously on-site with a resolution of 12 minutes per measure.

Their gas equilibrium membrane inlet mass spectrometer (GE-MIMS) system enables measurements with a standard error of 4 %. The system takes in water and automatically analyses the dissolved gases with a mass spectrometer by calibrating itself with ambient air. Taking all measured gas concentrations into account they were able to estimate the real consumption rates of O₂. The team noted that the GE-MIMS can easily be adapted to the use with other gases like CO₂ or N₂.

• Membrane Inlet Mass Spectrometer for the Quasi-Continuous On-Site Analysis of Dissolved Gases in Groundwater,
  Lars Mächler, Matthias S. Brennwald, Rolf Kipfer,
  Environ. Sci. Technol. 2012.
  DOI: 10.1021/es3004409