Fracture analysis of the Ribeirao Preto basalts, SP: application for developing a conceptual hydrogeological model

Abstract

The study of the physical geology and geochemistry of the Ribeirão Preto basalts was part of a hydrogeological research, which aimed to investigate the recharge of the Guarani Aquifer System (SAG) through the basalts of the Serra Geral Aquifer (ASG), a project shortly named FRATASG. In addition to the hydrogeological research, a detailed geological investigation was conducted to develop a conceptual model of groundwater flow in complex aquifers, as is the case of the fractured basalts of the ASG. Therefore, this study included a thorough structural survey and analysis of horizontal and subvertical fractures, which resulted in the identification of four tectonic events. It was concluded that the vertical groundwater flow is important up to 10 m in depth and, secondarily, up to 25 m. Horizontal flow, more evident up to the depth of 50 m, predominates and occurs along sub-horizontal fractures, which occur mainly at the contact between basalts 2 and 3 and in its vicinity. Because the great majority of subvertical fractures do not propagate into the vesicular layers, which occur at the basalt contacts, it is suggested that these layers act as regional hydraulic barriers, and greatly hamper the recharge of the SAG through the ASG. As a consequence, groundwater flow in the basalt stack is of stratabound type, as it occurs mainly along the basalt contacts. Based on diagnostic features of the flow, it is proposed in this study that the vertical flow, which crosses the basalt stack reaching the SAG, is local and probably occurs along NE structures.