Numerical simulations of karst aquifer evolution
|Doushko Romanov, FU Berlin, Germany|
|Thomas Hiller, FU Berlin, Germany|
|Institut für geologische Wissenschaften, FU Berlin, Germany|
|Equilibrium chemistry; fluid dynamics; groundwater modelling; finite elements|
Weathering and evolution of soluble carbonate landscapes (limestones, dolomite, gypsum) is termed karstification. This process is significantly different to the evolution of non-soluble landscapes (e.g. sandstones). In a karst landscape, water seeps through the soil into the bedrock and on its way down it is enriched with carbon dioxide. The resulting weak carbonic acid is able to dissolve limestone, and the dissolution process enlarges initially small fissures and bedding partings in the bedrock. With time, the permeability of the carbonate bedrock increases significantly, and more and more water disappears from the surface and is channeled through enlarged fissures and caves the underground. The water often re-emerges in large karst springs.
The evolution of a karst aquifer is characterised by a transition of flow: In the beginning, diffuse flow with low flow velocities carries water through initially small fissures and the bedrock matrix towards the base level. Upon enlargement of fissures and bedding partings by chemical dissolution, a large secondary porosity developes in the karst aquifer. Flow becomes concentrated to the enlarged fissures and bedding partings, which finally might become cave systems penetrable by humans. The now concentrated flow through large voids is characterised by rapid flow velocities, and spring response becomes flashy.
In the framework of the project, numerical models are developed, which describe the processes responsible for karstification. Surface karst denudation is modelled as an important additional process for karst landscape evolution. The evolution of subsurface flowis simulated by a coupled flow and reactive transport model, describing the development of secondary porosity through the dissolution of limestone.