Dynamics of seasonal CO2 concentrations above and below the karst-water table are influenced by density-driven transport: monitoring data from a cave in the Swabian Jura and interpretation with numerical simulation models

It is long known that dissolved aqueous CO₂ is the key driving force of chemical reactions leading to rock corrosion, which is denoted as karstification. Accordingly, it is evident that meteoric water percolating through the biologically active vadose zone leads to replenishment of CO₂ concentrations in karst water. We performed long-term measurements in a cave that show quick responses of gaseous CO₂ concentrations in cave air after rain events.

More importantly, however, our research aims at highlighting a so far by the literature totally ignored process in karst research, which is density-driven dissolution of CO₂ at the karst-water table. Our preliminary results indicate that this process can have high significance for hydraulic conditions where water is stagnant or at small convective base velocities (Class et al., 2021).

In our most recent work (Class et al., 2022, submitted), we monitored the influence of seasonally fluctuating gaseous CO₂ concentrations in a deep karst cave on aqueous CO₂ concentrations in different depths of a stagnant water column. The data indicate that density-driven enhanced dissolution at the karst-water table is the driving force for a fast increase of aqueous CO₂ during periods of high gaseous concentrations in the cave, while during periods of lower gaseous concentrations the decline of aqueous CO₂ is limited to shallow water depths in the order of 1m. Numerical simulations with a Navier-Stokes model and water density dependent on CO₂ concentration can be used to interpret the data and, perspectively, to extrapolate to geologically relevant time scales. This can also include the dissolution of CaCO₃, which is likely further increasing the relevance of density-driven dissolution at the karst-water table.

References:
H. Class, P. Bürkle, T. Sauerborn, O. Trötschler, B. Strauch, M. Zimmer: On the role of density-driven dissolution of CO₂ in phreatic karst systems, Water Resources Research 57(12), e2021WR030912, 2021, doi:10.1029/2021WR030912

H. Class, L. Keim, L. Schirmer, B. Strauch, K. Wendel, M. Zimmer: Seasonal dynamics of gaseous CO₂ concentrations in a karst cave correspond with aqueous concentrations in a stagnant water column, manuscript submitted, December 2022