Modelling the chemical evolution of porewater in the Palfris Marl, Wellenberg, central Switzerland

Abstract

The chemical evolution of groundwater in the Palfris marl at Wellenberg has been simulated using a reactive transport model. The results were tested against the chemical and stable carbon isotopic composition of water samples from exploratory boreholes. Water chemistry is constrained by mineral and cation exchange equilibria. To reproduce measured CO2(tot) and delta(13)C values requires H+ ion exchange. Matching measured ratios between Cl- and other dissolved constituents constrains the relative amounts of reacting water and rock to porosities between 1 and 10%. NaHCO3 waters sampled from the Palfris are formed by replacement of the initial Na-Cl water by one to five pore volumes of infiltrating Ca-HCO3 recharge water. To entirely exhaust the exchange capacity of the formation so that Ca-HCO3 water persists requires several hundred to several thousand pore volumes of flow. The agreement between model results and measured water chemistry demonstrates a quantitative understanding of the geochemical processes controlling the chemistry of water naturally present in the Palfris mad. These processes will also determine the behaviour of material that might emanate from a repository. In addition, the modelling provides water flow information of use in testing groundwater flow models