The Araçatuba Formation (Upper Cretaceous) as an aquitard in the basalt window: Hydrostratigraphic implications for the Bauru and Guarani Aquifer Systems (SÃO Paulo, Brazil)

The Bauru Aquifer System (BAS), the hydrostratigraphic equivalent of the Bauru Group (Upper Cretaceous), serves as the primary source of private groundwater supply for the municipality of Bauru (State of São Paulo, Brazil). The BAS is widely contaminated by nitrate, while the Guarani Aquifer System (GAS) is the primary source of public water supply for the city. Usually, basalts from the Serra Geral Formation hydraulically isolate the BAS from the GAS. However, a local stratigraphic peculiarity—the "basalt window"—results in the absence of such basalts, raising concern for possible hydraulic connectivity, especially given the strategic importance of the GAS in the state of São Paulo. This study identified the Araçatuba Formation, a new geological unit in the area, which functions as an aquitard and can confine the GAS even where basalts are absent. The new lithostratigraphy was proposed based on (i) interpretation of geophysical logs from 46 tubular wells, (ii) drilling of a 130 m borehole with geophysical logging (gamma ray, spontaneous potential, resistivity, and sonic) and core description, and (iii) implicit 3D geological modeling using Leapfrog Works®. These combined methods enabled the delineation of the Bauru Group’s geological framework (Marília, Adamantina, and Araçatuba formations from top to base). The Araçatuba aquitard, with thicknesses ranging from 8 to 26 m in the study area, provides confirmed confinement for the GAS, even within the "basalt window" and under downward hydraulic gradients. Distinct geophysical signatures were defined for each mapped stratigraphic unit, providing a guide for future interpretations in other areas.