A review of the geology and origin of CO2 in mineral water springs in east Belgium

Abstract

Naturally CO2-rich mineral water springs (pouhons) in east Belgium occur in the context of the Rhenohercynian domain of the Variscan fold-and-thrust belt, mostly within the Cambro-Ordovician Stavelot-Venn Massif. The origin of the CO2 is still unclear, although different hypotheses exist. In this review study, we show pouhon waters are of the calcium bicarbonate type (~310 mg/l HCO3- on average), with notable Fe (~15 mg/l) and some Ca (~43 mg/l). Pouhon waters are primarily meteoric waters, as evidenced by H and O isotopic signature. The δ13C of CO2 varies from -7.8 to +0.8‰ and contains up to ~15% He from magmatic origin, reflecting a combination of carbonate rocks and mantle as CO2 sources at depth. Dinantian and Middle Devonian carbonates at 2–6 km depth could be potential sources, with CO2 generated by dissolution. However, carbonates below the Stavelot-Venn Massif are only predicted by structural models that assume in-sequence thrusting, not by the more generally accepted out-of-sequence thrust models. The mantle CO2 might originate from degassing of the Eifel magmatic plume or an unknown shallower magmatic reservoir. Deep rooted faults are thought to act as preferential pathways. Overall low temperatures of pouhons (~10 °C) and short estimated residence times (up to 60 years) suggest magmatic CO2 is transported upwards to meet infiltrating groundwater at shallower depths, with partial to full isotopic exchange with carbonate rocks along its path, resulting in mixed magmatic-carbonate signature. Although the precise role and interaction of the involved subsurface processes remains debatable, this review study provides a baseline for future investigations.

Mineralogical and hydrogeological study of "pouhons" in the lower Palaeozoic formations of the Stavelot-Venn Massif, Belgium

“…Pouhons result from complex underground processes involving sequentially water infiltration, interactions with rocks, CO 2 assimilation, upwelling and finally water emergence (Barros et al, 2021). However, their spatial distribution shows that they are not randomly located but that their location seems to be controlled by geological and geomorphological factors.…”

“…4A-B). Isotopic signature suggests that pouhons are primarily fed by meteoric water (Barros et al, 2021). The origin of CO 2 in pouhons is still unknown and seems relatively complex.…”

“…The origin of CO 2 in pouhons is still unknown and seems relatively complex. According to Jobé (2014) and Barros et al (2021), CO 2 could result from a combination of various processes. Two hypotheses are generally advanced: (1) CO 2 would be produced by dissolution of marine limestones and/or carbonate nodules according to this reaction: CaCO 3 + 2H + → Ca 2+ + CO 2 (g).…”

“…However, no carbonate rocks were found in the SVM. They may occur at depth below the Midi-Eifel overthrust suggesting that meteoric water should infiltrate at least 2 km (Barros et al, 2021). ( 2) CO 2 would be of mantellic origin and produced by volcanic degassing related to the neighbouring Eifel area.…”

“…( 2) CO 2 would be of mantellic origin and produced by volcanic degassing related to the neighbouring Eifel area. The extension of the Eifelian mantle plume at depth or the presence of an unknown shallow magmatic reservoir under the SVM may then both constitute a potential source of CO 2 (Barros et al, 2021).…”