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.
Highlights
In situations of aquifer contamination, some fundamental questions that need to be addressed for the proper management of the environmental problem are: where is groundwater moving to? what are their velocities and flow rates? how are the contaminants being transported? The answers to these questions are usually obtained through the consolidation of geology, hydrogeology and contamination information and represented in a conceptual model of the contamination of the area of environmental interest
Borehole drilling with the recovery of coreholes, associated with the use of different geophysical logging techniques, discrete hydraulic tests with straddle packers and the installation of multi-level monitoring wells of different designs are techniques frequently used in such investigations
The data obtained by the caliper, gamma and High Resolution Acoustic Televiewer (HRAT) logging were imported by WellCAD v.5 software, handled and analyzed together with the filming and other data collected in the field stage
Summary
In situations of aquifer contamination, some fundamental questions that need to be addressed for the proper management of the environmental problem are: where is groundwater moving to? what are their velocities and flow rates? how are the contaminants being transported? The answers to these questions are usually obtained through the consolidation of geology, hydrogeology and contamination information and represented in a conceptual model of the contamination of the area of environmental interest. all terrains exhibit particular geological complexities, cases of contamination of fractured aquifers, especially when contaminants are chlorinated organic compounds, represent even greater challenges for the definition of the conceptual model, given its generally high heterogeneity and anisotropy, which provide high complexity and many uncertainties in defining the paths and rates of the groundwater flow.In general, the techniques and approaches used for investigation in fractured aquifers are more complex than those conventionally used for aquifers of primary porosity. The answers to these questions are usually obtained through the consolidation of geology, hydrogeology and contamination information and represented in a conceptual model of the contamination of the area of environmental interest. All terrains exhibit particular geological complexities, cases of contamination of fractured aquifers, especially when contaminants are chlorinated organic compounds, represent even greater challenges for the definition of the conceptual model, given its generally high heterogeneity and anisotropy, which provide high complexity and many uncertainties in defining the paths and rates of the groundwater flow. The costs associated with drilling and in-depth investigations, in addition to the complexity, heterogeneity and variability of fracture networks, are some of the factors that make it difficult to thoroughly characterize contamination in fractured aquifers
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