A story of water and bubbles

As a follow-up on the hairdresser’s blog entry, we remain in the beauty department. For – literally – centuries, people from all over the European continent have been traveling to a little town in the southeast of Belgium to enjoy the goodness of bathing in and drinking its healing spring water. Because next to being famous for chocolate, waffles and beer, Belgium is also home to the spa of all spas, called “Spa”.
For those of you still wondering about the definition of a geomanifestation, the Spa sources are a perfect example. On the plateau of the High Fens (Hautes Fagnes), rain infiltrates the soil and is purified by peat, sand and gravel layers. Downhill, the first water to resurface is exactly this very pure water, lean in nearly all minerals (Spa Reine for the connoisseurs). While traveling further through shales and fractured quarzitic rocks, a still largely unknown source of CO2 is encountered. It is this CO2 which produces the most famous bubbly water (Spa Marie-Henriette). The CO2 is thought to come from yet-to-be-discovered carbonate rocks, dissolved by the low-pH water to produce CO2, or possibly even from the volcanism of the Eifel region in Germany.
The occurrence of the carbonate rocks at depth, which would be much younger that the Cambrian rocks at the surface, is not yet confirmed and can possibly redraw the structural history of the wider region. To say it in GeoConnect³d project terms: here you have a very clear link between a geomanifestation and the structural interpretation of the regional geology. Moreover, studying this unique setting where natural CO2 travels towards the surface through fractured rocks can teach us a lot regarding the geological storage of anthropogenic CO2 for climate purposes, and it provides a natural laboratory for testing for example surface monitoring equipment.
A first attempt to quantify the emission of natural CO2 from the underground in this region was already made in the past years. The picture on the left shows one of the many small sources of iron and CO2 rich water near the Delcor stream in Spa. On the right, a thermal image of the same view is shown. Two observations can be made immediately. Firstly, there is a clear difference between the warmer spring water from the colder surface water. Secondly, the spring water occurrences are very localized, and therefore probably linked to fractures in the subsurface.

The occurrence of these spring waters is thus a clear manifestation of local and regional geology, with water travelling through and interacting with the surrounding rocks, emerging at the surface as a world-renowned beverage. Future research efforts, also within GeoConnect³d, will be directed towards high-precision temperature monitoring as a proxy for CO2 flux.

Kris Welkenhuysen
GSB, Belgium


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