Use of variance analysis (ANOVA) in the distinction of sedimentary aquifers in the Recôncavo Norte Region-Bahia

Abstract

The purpose of this report is to compare the existence of hydrogeochemical similarity between the Barreiras, Marizal and São Sebastião Formations located in the Recôncavo Norte region of the State of Bahia. Geochemical data was obtained from the databases of the Companhia de Pesquisa e Recursos Minerais (CPRM) and the Companhia de Engenharia Ambiental e Recursos Hídricos da Bahia (CERB). The results showed that the waters of the three Formations are predominantly of a chlorinated-sodic nature, with primarily rCl > rHCO₃> rSO₄ and secondarily rHCO₃> rCl> rSO₄, ionic ratios the latter of which was the most common relationship in the São Sebastião Formation, where there is a higher frequency of sodium bicarbonate composition. Comparative ANOVA tests indicated a difference among the aforementioned formations only for the presence of sodium and potassium. In the case of sodium, there was a difference between the Barreiras and São Sebastião Formation and in the case of potassium; there were differences between the Barreiras Formations and the Marizal Formation, and between de Marizal Formation and the São Sebastião Formations respectively. The explanation for this phenomenon is the exchange of cations between sodium and calcium, regulated by pH due to the greater or lesser presence of the bicarbonate ion. This was strongly correlated to sodium (r = 0.70) in the São Sebastião Formation. Finally, the model used in this study showed that there is more evidence of ion exchange between the clay levels of the São Sebastião Formation and groundwater, than in relation to the other formations studied. This was to be expected, since the study area is a multi-layered aquifer consisting of several levels of clay.