Hydrogeochemical transport modeling of 24 years of Rhine water infiltration in the dunes of the Amsterdam Water Supply

Abstract

Water quality changes were modelled along a flowpath in a plume of artificially recharged, pretreated Rhine water in the dunes of the Amsterdam Water Supply, after 24 years of infiltration. The hydrogeochemical transport model PHREEQC was extended with dispersion/diffusion and kinetics for selected chemical reactions. In the model the following reactions were included: cation-exchange, calcite dissolution and precipitation, and kinetic oxygen consumption and denitrification by oxidation of organic matter. Monthly-averaged values were used for the infiltration water quality. Traveltimes from infiltration area to sampling points were determined with chloride and tritium, and used to place the 3D field-observations in the 1D column-model. Values for CEC were variable for seven layers in the model. Infiltration of pretreated Rhine water in the dune aquifer can be considered an intrusion of more saline water. It caused desorption of Ca 2+, in exchange for Na +, K + and Mg 2+ from Rhine water. Because of variations in total solute concentrations in infiltration water, local small scale freshening fronts (Ca 2+ sorption, Na + desorption) were created by seasonally decreasing salt concentrations. The undersaturation with respect to calcite in the infiltration water, and the CO 2 produced during consumption of oxygen, resulted in dissolution of calcite. Precipitation of calcite occurred in response to desorption of calcium from the exchanger in the downstream parts. Overall, a net dissolution of calcite was simulated. Good results were generally achieved for all components: sulfate, nitrate, chloride, alkalinity, calcium, magnesium, potassium, sodium, 3H and O 2. The contributions of the different geochemical reactions to the water quality are illustrated with computer simulations for the individual processes.