Use of statistical analysis to formulate conceptual models of geochemical behavior: water chemical data from the Botucatu aquifer in Sa Äo Paulo state, Brazil Susan X. Meng * , J. Barry Maynard Department of Geology, ML 0013, University of Cincinnati, Cincinnati, OH 45221, USA We dedicate this paper to Dr Rosa Beatriz Gouve Ãa da Silva, who was killed on June 25, 1986, in Peru in a terrorist attack, and whose work was never wholly published. Received 4 May 2000; revised 8 February 2001; accepted 25 April 2001 Abstract Water chemical data from the Botucatu Sandstone aquifer in the Sa Äo Paulo State part 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 identi®ed, and their chemical behavior was modeled. The characteristic chemicals, changing from the recharge area to the center of the basin, are: SiO 2 ÐHCO 2 3 and Ca 21 )ÐNa 1 , CO 22 3 , and SO 22 4 ). 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. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Botucatu Formation; Water±rock interaction; Ground-water chemistry; Cluster analysis; Factor analysis 1. Introduction The Parana  Basin of Brazil, South America is one of the largest intracratonic basins in the world. A major ground-water bearing formation, the Botucatu Sandstone Triassic±Jurassic age), has been widely used as a source of fresh water in the area for a long time. A large water supply system based exclusively on groundwater was installed and has been in use since the early 1940s Gilboa et al., 1976). Although much work has been done on the hydrology of the sandstone on a regional scale, the scattered data compared to the size of the basin makes detailed modeling of the aquifer dif®cult. Because of the limited understanding of the detailed hydrologic system in the area, interpretation of the geochemical system in the area is also incomplete. In this paper, statistical analysis, coupled with geochemical meth- ods, was used to develop and test a conceptual geochemical model for the Botucatu Sandstone aqui- fer in the Sa Äo Paulo state part of the Parana  Basin. The Botucatu Sandstone was chosen for this study because it produces freshwater over its entire extent, and at depths of up to 2000 m, making the aquifer a useful model for deeply penetrating meteoric water. Journal of Hydrology 250 2001) 78±97 www.elsevier.com/locate/jhydrol 0022-1694/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0022-169401)00423-1 * Corresponding author. Fax: 11-513-556-6931. E-mail address: susan_meng@yahoo.com S.X. Meng).