Anisotropy and hydraulic confinement of the Guarany Aquifer system in Ribeirão Preto (SP, Brazil)

Abstract

A constant-rate pumping test executed during 171 h in the Guarani Aquifer System (GAS) was used to determine its respective hydraulic parameters and to test the existence of a drainage through an 84-m basalt layer (Serra Geral Aquitard), and the underlying sediments of the Corumbataí Formation. The test was prformed in a pumping well with hydraulic load monitoring in two minucipalities of Ribeirão Preto (São Paulo State, Brazil), located 10 km south of it. Results show a lack of drainage, and the positioning of the piezometers, in two distinct directions relative to the pumping well, allowed to infer the existence of anisotropy of the hydraulic conductivity. The calculated hydraulic parameters, after data correction for the pumping well’s partial penetration and for the aquifer’s anisotropy, are a hydraulic conductivity of 5,3x10^-6 m/s and 8,1x10^‑6 m/s, and storativity of 1,6 × 10^-3 e 8,4 × 10^-4, respectively for piezometers PPE-1G and PPE-2G. Maximum transmissivity, T_x = 1,9 × 10^-3 m²/s, occurs along the N-S direction, and minimum transmissivity, T_y = 1,2 × 10^-3 m²/s, sits along the E-W direction. On the horizontal plane, the relation between maximum and minimum transmissivities is 1,55. Drawdown does not stabilize during pumping, indicating that leakage through the basalts and through Corumbataí Formation was not sufficient to compensate for the discharge, or is nonexistent.