From Snow to Groundwater

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Oliver Schilling (Université Laval and Centre for Hydrogeology and Geothermics) and his colleagues used a miniRUEDI for on-site quantification of dissolved He, Ar, Kr, N2, O2 and CO2 in groundwaters of a boreal catchment in Canada. The gas data allowed them to quantify the contribution of snowmelt to groundwater recharge, to analyze the temporal recharge dynamics, and to identify the primary recharge pathways. Furthermore, they observed a systematic depletion of N2 in groundwater, which provides insights into the biological N‐fixation in boreal forest soils.

Full paper: O. S. Schilling, A. Parajuli, C. Tremblay Otis, T. U. Müller, W. Antolinez Quijano, Y. Tremblay, M. S. Brennwald, D. F. Nadeau, S. Jutras, R. Kipfer, R. Therrien. Quantifying groundwater recharge dynamics and unsaturated zone processes in snow‐dominated catchments via on‐site dissolved gas analysis. Water Research, doi: 10.1029/2020WR028479

Lake Kivu will not explode anytime soon, says miniRUEDI

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Fabian Bärenbold and his coworkers studied the gases in Lake Kivu in East Africa, which is well known for its huge reservoir of CH4 and CO2 dissolved in the deep waters. In view of the ongoing and planned extraction of CH4 for energy production, Fabian Bärenbold and his colleagues used a miniRUEDI and other gas analysis techniques (gas chromatography, laser spectrometery, and a total dissolved gas pressure). The measurement results show good agreement within 5–10%. The CH4 and CO2 dioxide concentrations in Lake Kivu are very similar to earlier results observed during the past few decades, which indicates that the risk for a limnic gas erruption of Lake Kivu has not increased.

Full paper: Fabian Bärenbold, Bertram Boehrer, Roberto Grilli, Ange Mugisha, Wolf von Tümpling, Augusta Umutoni, Martin Schmid. No increasing risk of a limnic eruption at Lake Kivu: Intercomparison study reveals gas concentrations close to steady state. PLOS, doi: 10.1371/journal.pone.0237836

Deconvolution and compensation of MS interferences

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For many gas species (e.g., He, Ar, Kr, N2, O2, CO2) the partial pressures measured with the miniRUEDI can usually be calibrated by simple peak-height comparison relative to ambient air or gas standard with well known partial pressures. However, depending on the composition of the analysed gases, the ion currents measured at certain m/z ratios may result from overlapping ion currents of multiple species.

We developed a method that extends the miniRUEDI peak-height comparison in order to resolve such overlap interferences. The method uses spectral deconvolution and was incorporated in the ruediPy software toolbox. The deconvolution method substantially improves the analytical accuracy in situations where mass-spectrometric interferences cannot be avoided.

Full details are availalble in the original publication: M.S. Brennwald, Y. Tomonaga, R. Kipfer: Deconvolution and compensation of mass spectrometric overlap interferences with the miniRUEDI portable mass spectrometer, MethodsX, 2020, doi: 10.1016/j.mex.2020.101038

Some typical examples for such overlap interferences:

  • Methane (CH4): The main CH4 peaks occur at m/z = 15 and 16, which are affected by interference signals from 15N, 16O, and doubly ionised O2 molecules.
  • Neon (Ne): The peaks of the main Ne isotopes (at m/z = 20 and 22) overlap with those of isotopically heavy H2O and the peaks of doubly ionised Ar and CO2.
  • Propane (C3H8), ethane(C2H6), or similar hydrocarbons in air-like samples: some peaks of the hydrocarbon mass spectra tend to overlap those of N2 and CO2.
  • Hydrogen (H2): the the mass-spectrometric peak of the H2 in the sample gas may be masked by H2 produced in the ion source of the mass spectrometer. Ionisation of molecules containing hydrogen may “knock off” one or multiple H atoms, which then interfere with the analysis of the H2 in the sample gas. H2 analysis therefore works best if the concentration in the sample gas is high (about 1‰ vol/vol or higher), and the concentrations of H-containing gas species is low.

Running on Batteries

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We frequently get asked how to run the miniRUEDI on batteries for field work at remote locations with no mains power.

Here are the basics to run the miniRUEDI on batteries:

  • The miniRUEDI runs on a voltage of 24 V.
  • The miniRUEDI draws about 2 A current during normal operation, and up to about 5 A or slightly more during startup of the pumps.
  • You need a fuse. Batteries do not like short circuits or similar mishaps. A 10 A rating should be fine.
  • Unplug the 24 V connections of the main power supply unit in the miniRUEDI, and connect the batteries to the miniRUEDI instead.

There are many ways to set up a battery power supply, but here is a simple setup that uses two «12 V car batteries»:

The capacity of the batteries determines how long they will be able to supply power to the miniRUEDI. Two of the typical «12 V car batteries» with a capacity rating of 60 Ah may last up to a full day before they need to be recharged.

In-situ monitoring of noble gases in groundwater during rock fracking

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Clement Roques and his colleagues published a paper in Nature Scientific Reports, where they used a miniRUEDI to study changes of dissolved-gas concentrations in groundwater in response to hydraulic stimulation and fracturing of the reservoir rocks (Nature Scientific Reports, 10, 6949, 2020). The miniRUEDI was installed on-site to continuously analyse the dissolved-gas concentrations of the groundwater. The high-frequency He and Ar measurements indicate that trapped fluids were mobilized from the rocks in response to the fracking. The miniRUEDI revealed the nature and evolution of the fracture network and flow paths, and showed the effect of the fracking procedures on groundwater quality.

Full paper: “In situ observation of helium and argon release during fluid-pressure-triggered rock deformation.” Sci Rep 10, 6949 (2020). https://doi.org/10.1038/s41598-020-63458-x

Eawag News article: https://www.eawag.ch/en/news-agenda/news-portal/news-detail/observing-how-fissure-systems-are-formed-thanks-to-the-gas-sniffer/

miniRUEDI goes sailing

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Researchers from the Max Planck Institute for Chemistry (Germany) installed a miniRUEDI on the Eugen Seibold, the world’s greenest research vessel. Since May of 2019, the innovative yacht has been sailing the high seas, and the miniRUEDI is set up to continuously analyse the dissolved gas concnetration in the surface water. By gradually gathering data on the various marine provinces, the climate geochemists at the Max Planck Institute for Chemistry in Mainz will be able to chart a detailed description of the world’s oceans, characterising their current properties and even reconstructing how they change over time.

A New in Situ Method for Tracing Denitrification in Riparian Groundwater

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Andrea Popp and her colleagues used a miniRUEDI to study the dynamics of the biogeochemical N2 turnover in a river/groundwater system over a six-month period (Environ. Sci. Technol. 2020, 54). In addition to N2, they analysed He, Ar and Kr concentrations in the water in order to quantify the air-derived N2 component in the groundwater. These miniRUEDI data allowed them to rigorously quantify the N2 excess produced by denitrification, and to unravel the spatio-temporal dynamics of N2 denitrification in the riparian groundwater system. The results show that denitrification is highly variable in space and time, emphasizing the need for spatially and temporally resolved data to accurately account for denitrification dynamics in groundwater.

Full paper: “A New in Situ Method for Tracing Denitrification in Riparian Groundwater”, Environ. Sci. Technol. 2020, 54, 3, 1562-1572, DOI: 10.1021/acs.est.9b05393

Using a miniRUEDI to identify and quantify groundwater mixing

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In their recent paper “Integrating Bayesian Groundwater Mixing Modeling With On‐Site Helium Analysis to Identify Unknown Water Sources“, Andrea Popp and her Eawag colleagues studied a groundwater system used for drinking water production. The drinking water field needs to be protected from several potential sources of contamination by artificially controlling the groundwater flow. In view of these problems, the Eawag team identified the origins of the different groundwater components in the drinking water field and quantified their mixing ratios using a miniRUEDI by analysing He and other dissolved gases as natural tracers for the different groundwater components.