Reactive transport modelling of groundwater chemistry in a chalk aquifer at the watershed scale

This study investigates thermodynamics and kinetics of water–rock interactions in a carbonate aquifer at the watershed scale. A reactive transport model is applied to the unconfined chalk aquifer of the Champagne Mounts (France), by considering both the chalk matrix and the interconnected fracture network. Major element concentrations and main chemical parameters calculated in groundwater and their evolution along flow lines are in fair agreement with field data. A relative homogeneity of the aquifer baseline chemistry is rapidly reached in terms of pH, alkalinity and Ca concentration since calcite equilibrium is achieved over the first metres of the vadose zone. However, incongruent chalk dissolution slowly releases Ba, Mg and Sr in groundwater. Introducing dilution effect by rainwater infiltration and a local occurrence of dolomite improves the agreement between modelling and field data. The dissolution of illite and opal-CT, controlling K and SiO2 concentrations in the model, can be approximately tackled by classical kinetic rate laws, but not the incongruent chalk dissolution. An apparent kinetic rate has therefore been fitted on field data by inverse modelling: 1.5 × 10− 5 molchalk L − 1water year − 1. Sensitivity analysis indicates that the CO2 partial pressure of the unsaturated zone is a critical parameter for modelling the baseline chemistry over the whole chalk aquifer.

► 2D reactive transport modelling considering chalk matrix and fracture network.
► At the watershed scale, with a large set of field data, for further assessment of aquifer pollution.
► Homogeneity of chemistry is reached, plus an incongruent slow dissolution of chalk matrix.
► Requiring an inverse modelling.
► Sensitivity analysis vs. pCO2 in the unsaturated zone and rainwater infiltration.