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.