Application of Geophysical Logging and Straddle Packers for the Investigation of a Fractured Aquifer in a Contaminated Area by Chlorinated Solvents in Sao Paulo State, Brazil

Abstract

The investigation of contaminated areas in fractured aquifers represents a great technical challenge, due to the frequent conditions of heterogeneity and anisotropy of these environments, which often make it difficult to identify and predict pathways of subsurface contamination. This work aims to contribute to the development of this subject, through the presentation of the results of an investigation in which geophysical logging tools and straddle packers were used, which allowed the development of a more suitable hydrogeological conceptual model of the study area. Two boreholes were drilled and geophysical logging (gamma, caliper and high resolution acoustic televiewer) were used for the geological-structural characterization of the aquifer. Heatpulse flowmeter and straddle packers were then used to obtain data on hydraulic potentials and flows and for the collection of discrete water samples for chemical analysis. Two types of gneiss rocks were identified below the weathering zone (thickness >30 m), one with predominance of mafic bands, more weathered and with a higher fracture density, up to 65 m, and another deeper one, with the predominance of felsic bands. Seven groups of fractures were defined, being those of Group 1, with low dip angles, the most frequent and important for flow until 65 m, and Group 2 (N to NE-SW with high dips to W and NW) more frequent in deeper felsic gneisses. Downward hydraulic potentials were identified down to 65 m and upward potentials from the bottom up to 65 m. A hydraulic test allowed identifying the occurrence of hydraulic connection between the shallow weathering zone and the underlying fractured aquifer.