In this study the processes affecting groundwater chemistry in the Eocene Brussels sands aquifer in Central Belgium are identified based on evaluation of ionic ratios of major solutes. Based on these results, in combination with mineralogical and hydrogeological information of the aquifer, a geochemical mass-balance model is created to quantify the contribution of each of the processes to the observed composition of groundwater. After a rigorous validation process, a dataset of 99 groundwater samples is obtained from observation and pumping wells in the Eocene Brussels sands aquifer, which is one of the main aquifers for drinking water production in Belgium. The aquifer consists of heterogeneous alteration of calcified and silicified coarse sands, with local presence of clay drapes and glauconite-rich zones (Laga et al. 2001). The entire aquifer is overlain by Quaternary eolian deposits, mainly consisting of loam with the exception of the north east, where the Quaternary deposits are sandy loam. The groundwater in this aquifer is of Ca-Mg-HCO3-type with locally elevated nitrate concentrations. Based on the evaluation of ionic ratios and the mineralogy of the aquifer, a conceptual geochemical model is developed for mass-balance modeling, including (1) concentration of precipitation by a factor 1 to 5 due to evaporation, (2) dissolution of a pure calcite phase and a calcite phase containing 25 % magnesium by both carbonic acid and sulfuric acid, (3) anthropogenic inputs for all major cations and anions except bicarbonate, (4) dissolution of glauconite, (5) cation exchange of sodium and potassium for calcium and magnesium. The two calcite phases can be thought of as end-members of a solid solution of magnesium in calcite. The mass-balance modeling consists of a mole-balance equation for each considered element according to: [Obs] = p[Prec] + p1[Phase 1] + . + pi[Phase i] + a [Anthropogenic] +/- c[Cation Exchange] This set of linear equations is additionally constrained by (1) defining a range for concentration ...