Long-term Water-rock Interaction in the Bohemian Cretaceous Basin (Central Europe): Field Evidence and Modelling
Two models of reversible and irreversible water – mineral interaction are developed to simulate geochemical evolution of old groundwater in the Cenomanian and Turonian sandstone aquifers in the Bohemian Cretaceous Basin in Central Europe. The kinetic constants of dissolution and equilibrium constants of reversible reactions are results of laboratory experiments published in literature. Reactive surface areas of minerals are calculated. The model simulates the evolution of the chemical composition of groundwater whose age was derived from isotopic data. The results are compared to the chemical composition of groundwater along the trajectory of the prevailing flow. The model represents the interaction between groundwater and sandstones containing quartz, amorphous silica, a mixture of clay minerals, calcite and traces of residual feldspars. The three processes of water rock interaction are the irreversible dissolution of feldspars and carbonate, aluminium and silica equilibrium. The composition of groundwater in the Cenomanian aquifer with a confined water table is a result of geochemical evolution during the last 26,000 years, while the groundwater in the Turonian aquifer with a free water table is a mixture of old groundwater and recent water infiltrated from the surface. The proposed models are applicable to confined aquifers with residual rock matrix derived from granitic rocks decomposed by weathering without mixing of waters of different ages.