Evaluation of the contamination of the subsurface and groundwater by monoaromatic hydrocarbons in an eastern Amazonian town in northern Brazil

Abstract

The present study evaluated the contamination by monoaromatic hydrocarbons (BTEX: benzene, toluene, ethylbenzene, and xylene) of the subsurface of gas stations and adjacent residences in the Amazonian town of Bragança, in northern Brazil. We combined the ground penetrating radar (GPR) geophysical tool, to detect possible plumes of contamination by BTEX, with gas chromatography (GC) and mass spectrometry (MS), which were used to diagnose the presence of BTEX in water samples obtained from boreholes located within the study area. The GPR identified low-amplitude reflections in the vadose zone of two gas stations, whose attenuation near the surface was due to the low permittivity recorded in the radargrams, which may have been caused by the reduced permittivity of the hydrocarbons (gas vapor) in comparison with water. The GC–MS identified benzene, toluene, ethylbenzene and/or xylene in 13 water samples (19.7% of the samples collected). Only three of the samples contained two contaminants, invariably toluene and xylene, whereas all the others contained only a single BTEX. Toluene and xylene were the BTEX compounds found most frequently in the samples. However, none of the samples collected in the study exceeded the threshold established by Brazilian legislation for BTEXs in drinking water. The integrated application of GPR and CG-MS produced satisfactory results for the diagnosis of the contamination of the subsurface of the gas stations surveyed in the present study.