The paper presents fuzzy groundwater classification rule base derivation from groundwater quality variation maps, which are based on any three ions. These maps are obtained by geostatistical (Kriging) methodology and the results are presented in the form of equal ion concentration lines (contours). They help to visualize mutual relationships between three ions with linguistic interpretations. For this purpose each ion is separated into three linguistic categories, as 'low,' 'medium' and 'high,' which are characterized numerically by membership functions (MFs). MFs are transformers of linguistic expressions into numbers and vice versa. Accordingly, the linguistic fuzzy rules are presented for total dissolved solids' (TDS) occurrence based on two different ions. In general, it is possible to deduce fuzzy logical rules relating any ion to the other two ions.