Naturally CO2-rich water springs in Belgium evidencing complex subsurface interactions

<p>Numerous naturally CO<sub>2</sub>-rich mineral water springs, locally called ‘pouhons’, occur in southeast Belgium. These are oversaturated in CO<sub>2</sub> (up to 4g/L) and have attracted economic, touristic and scientific interest for centuries. Water sources occur within Palaeozoic rocks of the Rhenohercynian deformation zone, a fold-and-thrust belt at the north of the Variscan orogeny in central Europe. Many occurrences are concentrated in the Cambro-Ordivician Stavelot-Venn massif. A widely accepted model, supported by H-O isotopic signatures, is that sources are primarily fed by meteoric water, which infiltrates through Quaternary sediments, then reaching Lower Palaeozoic rocks to meet the mineral and CO<sub>2</sub> source at unknown depth.</p><p>Different ideas for the origin of CO<sub>2</sub> are grouped in two main hypotheses: a) generation by dissolution of carbonate rocks and/or nodules, and b) volcanic degassing related to the neighbouring Eifel area in Germany. These well-known interpretations are mostly based on geochemical studies that are dispersed and poorly accessible. These have now been gathered in the light of new sampling campaigns, allowing to revisit and compare the views of earlier authors. We also for the first time include the geotectonic setting of the region.</p><p>Carbonate rocks in the region are represented by Lower Carboniferous and Middle Devonian limestones. Depending on the assumed structural evolution for this foreland fold-an-thrust belt, these may occur at >2 km depth below the Stavelot-Venn massif. Carbonate nodules are present in other formations, but their limited volume is unlikely to originate high and long-lived quantities of CO<sub>2</sub>. Springs enriched in CO<sub>2</sub> are also common in the volcanic Eifel area, with presence of mantle CO<sub>2</sub> well established. The supposed extension of the Eifel plume would allow for a magmatic CO<sub>2</sub> source below the Stavelot-Venn massif from degassing of the plume (>50 km deep), or of an unknown shallower magmatic reservoir. Available stable and noble isotopes point to a mixed carbonate-magmatic origin.</p><p>If considering the presence of limestones at depth, meteoric water should infiltrate at least 2 km. Known deep-rooted faults are thought to act as preferential groundwater pathways. However, such deep circulation is incompatible with the low temperatures of springs (~10<sup>o</sup>C), unless the ascent is slow enough to fully dissipate heat prior to resurfacing. Another possibility is that meteoric water does not infiltrate as deep, with CO<sub>2</sub> being transported upwards to meet groundwaters at shallower depths. The presence of CO<sub>2</sub> surface leaks, locally called ‘mofettes’, could be evidence of such relatively shallow availability of CO<sub>2</sub>.</p><p>The evaluation of existing hypotheses highlights complex subsurface processes that involve water infiltration, CO<sub>2</sub> assimilation and water resurfacing in southeast Belgium. As such, this review is an important guide for the newly launched sampling campaigns.</p><p>This work is part of two research projects: GeoConnect³d-GeoERA that has received funding by the European Union’s Horizon 2020 research and innovation programme under grant agreement number 731166, and ROSEAU project, as part of the Walloon program «Doctorat en Entreprise», co-funded by the SPW Région Wallonne of Belgium and the company Bru-Chevron S.A. (Spadel group), under grant number 7984.</p>