Experimental and numerical investigations on density-driven dissolution of CO2 and related carbonate dissolution in karst water

• Applicants: Professor Dr.-Ing. Holger Class; Dr. Martin Zimmer
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 508470891

The role of density-driven CO2 dissolution in karst systems is not well understood to date. It is known that CO2 dissolved in water drives the karstification process, and it is likewise generally understood that its major source is biogenic, i.e. produced by microorganisms in the soil or by root respiration. Dissolution of carbonates takes place primarily near the ground surface, where percolating meteoric water is enriched with CO2 and leads to so-called denudation, which is, in essence, a wearing-down of the terrestrial surface area. But why do cavities also grow deep inside the rock? This is currently explained through two different prevailing mechanisms: (1) Mixing corrosion refers to dissolutional potential created when two water streams mix, since the mixed water is always calcite-aggressive. (2) Non-linear dissolution kinetics are assumed on the basis that water may retain a certain residual amount of its dissolutional potential until deep into the rock. Our new claim is that a third important and, so far, underestimated mechanism for replenishing CO2 in water bodies - we refer to it as density-driven dissolution - may also play a role in cavity development at depth. We demonstrated in a recent article that density-driven dissolution of CO2 in water can replenish stagnant water bodies with CO2 and, thus, the dissolution potential on a time-scale of weeks to months. What has not been investigated to date, due to its particularly high complexity, is the interaction of density-driven CO2 dissolution with limestone, i.e. the reactive-transport system. One can assume that density-driven dissolution of CO2 takes place within a fracture of a certain aperture. Density-induced flow then depends on the actual fracture opening, which may increase as a result of carbonate dissolution, thus, triggering a self-enhancing process.This project's overall aim is to contribute to a better understanding of the role of density-driven CO2 dissolution in relation to already known mechanisms, such as mixing corrosion and non-linear dissolution kinetics. To understand the interaction between different karstification mechanisms on geologic time scales, the only appropriate tool is modelling, validated by sophisticated and well-controlled laboratory and field experiments. The numerical model solves Navier-Stokes equations with density dependent on the concentrations of dissolved components. The validation is aimed at including a coupling of reactive flow due to density-driven dissolution in calco-carbonic systems and induced morphological changes on the surface of a limestone.In summary, we propose to- improve numerical modeling capabilities by systematic validation of our numerical simulator DuMux with data from well-controlled experiments.- quantify CO2 entry rates into karst waters resulting from density-driven dissolution and reaction at limestone surfaces.- quantify corresponding dissolution rates of carbonates and modification of the limestone surface.

• DFG Programme: Research Grants