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Alexandre Lisboa Lago, CPRM- Geological Survey of Brazil (Brazil)
Vagner Roberto Elis, University of São Paulo (Brazil)
Welitom Rodrigues Borges, SIGEO-Soluções Integradas em Geotecnologias (Brazil)
Giovanni Chaves Penner, Instituto de Ensino Superior Colégio Oswaldo Cruz (Brazil)
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Geophysics has been shown to be effective in identifying areas contaminated by waste disposal, contributing to the greater efficiency of soundings programs and installation of monitoring wells. In this context, the purpose of this work is to evaluate the potential of the resistivity and GPR methods in characterizing the contamination of the lubricant oil waste disposal area in Ribeirão Preto - SP, Brazil, situated on the geological domain of the basalt spills of the Serra Geral Formation and the sandstones of the Botucatu Formation. The study site received wastes generated by an oil company from the mid-1970's to 1995. The waste was disposed in 4 trenches with a total volume of about 25.000 m3. These pits were almost completely filled with waste consisting of re-refined lubricating oil for 25 years. No protection liners were used in the bottom or the laterals of these disposal trenches. The technique used in this work was electrical profiling. 20 dipole-dipole electrical profiling lines (called C1 to C20) were laid out with 10 meters between electrodes. GPR data was acquired in 7 lines in the common offset mode, (Lines 1 to 7). In the GPR method the acquisition parameters were: 0.2 meters trace spacing, 1640 MHz sampling frequency, 32 traces stacking. The shoots and trace records were registered continuously with the use of a calibrated wheel. The results have shown great potential for the application of the combined resistivity and GPR methods in an environmental characterization of the lubricant oil waste disposal area, making it possible to identify processes of contaminant percolation below the trenches, and also to define the geometry of the trenches.
The results indicate that electrical resistivity is influenced by the presence of lubricant oil residues disposed of in the ground. The GPR shows efficiency in mapping the trenches' geometry and in identifying their limits and bases, as evidenced in Line 4. The GPR data enable the identification of objects (drums and/or tanks) inside the waste. The results show that there is attenuation of the electromagnetic signal due to the enhancement of the electrical conductivity underneath the trenches, possibly caused by the bacteriological activity inside the contaminant. The direct analysis confirms the existence of contamination and suggests the existence of bacteriological activity as evidenced in the analysis of the soil samples and water from the monitoring wells. Anomalous concentrations of heavy metals and altered values of some physical-chemical parameters were observed in the locations indicated by the geophysical results, evidencing the importance of the use of geophysics in environmental characterization programs.
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