We used a combination of a grouping algorithm and a weathering algorithm to assess the contributions made by chemical weathering and anthropogenic inputs to the groundwater composition in a granitoid area. The first algorithm is based on the mathematical concept of equivalent relations between objects and is used to find groups of water samples. Using the grouping algorithm, we identified groups with similar chemistries in a set of data relating to the water composition in 160 springs and wells in the Fund¿o area (Portugal). The second algorithm is based on stoichiometries, mass, and charge balances in weathering reactions and is used to relate the water composition of each identified group to water-mineral interactions in the area. Background information on the petrology and mineralogy of the area allowed us to choose the most realistic water-mineral interactions. We also had information about the use and composition of fertilizers in this agricultural area. In applying the weathering algorithm we considered all dissolved silica and bicarbonate to be produced by chemical weathering and all dissolved chloride, sulphate, and nitrate to be derived from other sources, mainly from fertilizers. The anthropogenic contributions to the water chemistries in the area are high. Despite the high background concentrations derived from pollution, we were able to assess the contribution of weathering to the water chemistries. We obtained realistic results with the two algorithms, supported by the fact that the spatial distribution of the samples belonging to the various groups shows a good correlation with the geology and petrology of the area. ¿ American Geophysical Union 1996