Geometric fracture model and brittle tectonic analysis applied to the study of the crystalline aquifer flow, São Paulo (SP)

Abstract

Detailed structural analysis is still incipiently applied to fractured aquifers, although it provides relevant data for the elaboration of more realistic groundwater flow conceptual models and is complementary to hydraulic data obtained from boreholes. This article presents an analysis of data collected in quarries of the Embu Complex aiming at elaborating a geometrical fracture model for Precambrian granites and gneisses, in order to provide a basis for characterizing the aquifer contamination by chlorinated solvents in the Jurubatuba urban and industrial area, located in the São Paulo municipality. The data collected along <i>scanlines</i>, to which an orientation bias correction was applied, and in isolated spots, permitted to characterize fractures sets, as well as their respective stress fields and age relationships. The main subvertical sets that occur in the quarries are likely to be present also in the Jurubatuba area, and complement the fracture data obtained through vertical well profiling.
The relative transmissivity of the fracture sets is indicated by the weathering intensity, as well as presence of vegetation and water seepage. The quarry constituted by gneiss is the best analog of the geology of the industrial area. The gneiss foliation draws an open and asymmetric cylindrical fold and dips in a low angle to SE or NW, being intensively reactivated as continuous low-dip and transmissive fractures. Abundant subvertical NW fractures are probably the main vertical flow path. Subvertical NE fractures are transmissive, however are largely spaced and, as a result, are considered less important for water flow, as are the quite frequent subvertical EW fractures, due to their relatively low transmissivity.