Aspects of a conceptual groundwater flow model of the Serra Geral basalt aquifer (Sao Paulo, Brazil) from physical and structural geology data

摘要

A preliminary conceptual model of groundwater flow was developed for the Serra Geral fractured basalt aquifer in order to assess the recharge to the underlying sandstone Guarani Aquifer System, one of the main aquifer systems in Brazil, which supplies water to millions of people. Detailed geological investigations included macroscopic description of the basalt flow units and the underlying sandstone. Petrographic and chemical analyzes were conducted on rock samples from outcrops and from five drilled boreholes. Detailed fracture surveys were accomplished at outcrops to characterize fracture sets and their potential to transmit water in the current tectonic context. Four basalt flows were identified in the Ribeirao Preto area and were named B1, B2, B3 and B4 (from oldest to youngest). The cooling process in flow B3 led to the generation of large sub-horizontal fractures at the contacts B2/B3 and B3-C/B3-E, which are the most transmissive structures. Groundwater flow in the basalt appears to be of the stratabound type because fractures, in general, do not propagate through the basalt vesicular layers, which behave as a regional hydraulic barrier for the vertical groundwater flow. However, it is proposed that the localized, continuous and closely spaced subvertical tectonic fractures, the only features that have the potential to crosscut the vesicular layers and the intertrappe sediments, can vertically connect the sub-horizontal transmissive fractures. Weathering and water seepage, observed in rock exposures, indicate that subvertical NE-trending fractures would be the most transmissive in the Ribeirao Preto area.