T37 Gas Bubbles and Dissolved Gases in Groundwater (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

A MEMBRANE INLET MASS SPECTROMETRY (MIMS) SYSTEM TO MEASURE DISSOLVED NOBLE GASES IN WATER

Paper number: T37-4
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Noble gases provide valuable insight into the recharge conditions (temperature, water table fluctuations) and flow patterns (travel times) of groundwater. Because of low abundances and the volatility of the (lighter) noble gases, analysis involves specialized sampling in clamped copper tubes and laboratory separation of the noble gases from the abundant atmospheric gases present in groundwater, followed by mass spectrometry. Membrane Inlet Mass Spectrometry (MIMS) systems provide real-time dissolved gas analysis of water flowing across a membrane. The small size and portability of MIMS systems would offer great advantages for fieldwork involving in situ noble gas studies and long-term tracer monitoring. However, with the exception of argon, noble gases have not been measured at atmospheric equilibrium levels in groundwater on a MIMS system so far.

We have developed a MIMS capable of measuring the concentrations of noble gases and noble gas isotopes in water samples. The NG-MIMS consists of a membrane inlet, a dry ice water trap, two getters, a turbo pump and a quadrupole mass spectrometer with an electron multiplier. All components are commercially available. The dimensions of the NG-MIMS are 100x60x40 cm and it weighs about 40 kg. The noble gases are measured at mass-over-charge (m/z) ratios 4 (helium), 22 (neon), 38 (argon), 84 (krypton) and 132 (xenon). The measurement precision of the MIMS at atmospheric equilibrium concentrations is below 2% for helium, argon and krypton and below 3% for neon and xenon. A measurement takes 5 minutes and requires 2.5 ml of water. Various methods of sampling and sample introduction have been studied. Glass vials appear to be sufficient for the containment of the heavier (argon, krypton and xenon) noble gases.

Potential applications for the NG-MIMS include active tracer studies in groundwater, fast detection and detailed profiling of radiogenic helium, water column and down-well depth profiling, and the study of excess air formation and noble gas transport at high temporal resolution. In addition, the NG-MIMS provides a much less expensive and less complex alternative for noble gas studies where the high precision of traditional noble gas mass spectrometry systems is not needed.

This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.