The completion of the high-resolution (ca. 1 sample per 1.5 km2) geochemical survey of Wales and part of the West Midlands by the British Geological Survey has created a unique data set containing over 13,500 site-specific hydrochemical and stream-sediment analyses for a typical suite of over 21 major and trace components. The collection of such a wide range of data over an area with contrasting geology, soils, geomorphology and land use has enabled the delineation and study of hydrochemical processes on a regional basis. Methods are presented describing transformations performed from basic concentration-based data (pH, conductivity and 21 major and trace elements) into hydrochemically relevant trilinear and solubility diagrams predicting aqueous speciation and ionic domination. These data are then displayed and interpreted using a series of thematic composites. The utility of coupling speciation diagrams, based upon pure ideal systems, with Geographical Information Systems for describing processes controlling hydrogeochemical evolution at a regional scale is described. The validity of using solubility diagrams to describe large regional hydrochemical data sets is tested using geographically linked representations of aqueous saturation and chemical speciation generated from the individual modelling of each sample using PHREEQC. Comparison of the pure and real (PHREEQC) systems for calcite and gypsum indicated that there was a small over-prediction in the pure system; the greatest differences occurred in samples of the highest ionic strength.\ud \u
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