Use of statistical analysis to formulate conceptual models of geochemical behavior: water chemical data from the Botucatu aquifer in Sao Paulo state, Brazil

Water chemical data from the Botucatu Sandstone aquifer in the Sao Paulo State Bart of the Parana Basin, Brazil,. was evaluated using geochemical methods and two statistical analyses: cluster analysis and factor analysis. The results were used to develop a conceptual geochemical model, in which three geochemical regions were identified, and their chemical behavior was modeled. The characteristic chemicals, changing from the recharge area to the center of the basin, are: SiO2-(HCO3- and Ca2+)-(Na+, CO32-, and SO42-). The distribution of the chemicals is interpreted as controlled by different water-rock interaction processes in the different regions. In the recharge area, dissolution of alkali-feldspar minerals in the sandstone is the main reaction observed; in the mid-section of the basin, calcite dissolution results in high calcium and bicarbonate concentration; in the center of the basin, leakage from underlying layers becomes the governing factor. (C) 2001 Elsevier Science B.V. All rights reserved.