Kinetics of incongruent dissolution of carbonates in a Chalk aquifer using reverse flow modelling

SummaryWater–rock interactions are major processes that control the solute transfer (from natural or anthropogenic sources) in aquifers. In carbonate aquifers, these water–rock interactions are mainly characterised by the incongruent dissolutions of carbonates. The in situ quantification of these reactions is few studied. In the present paper, we provide a new quantification of the kinetic rates of simultaneous incongruent dissolution of composite carbonate and dolomite in a Chalk aquifer. The approach is based on a mass-balance model used to calculate the rates of mineral dissolution and precipitation along a 500 m to 4 km flow path starting at the groundwater divide line. The groundwater transfer times between the groundwater divide line and the wells are calculated thanks to flow and transport modelling, used in a backward particle tracking mode. The uncertainties on porosity, dispersivity and transmissivity are taken into account.The rates are found ranging between 1.5 × 10−5 and 1.5 × 10−4 molC5 chalkLw-1yr-1 for the composite carbonate dissolution, between 1.0 × 10−7 and 3.0 × 10−6 moldolomiteLw-1yr-1 for the dolomite dissolution and between 1.0 × 10−5 and 1.5 × 10−4 molcalcite Lw-1yr-1 for the calcite precipitation. The results highlight the importance of physical and geological properties of the aquifer.

► Water–rock interactions in a Chalk aquifer.
► Kinetics of simultaneous incongruent dissolution of composite carbonates and dolomite.
► A mass-balance approach is used to calculate kinetic rates.
► The stoichiometries of composite carbonates are defined from Chalk geochemistry.
► Transfer times are obtained from a reverse flow and transport modelling.