Geophysical and physicochemical investigations of an area contaminated by tannery waste: a case study from southern Brazil

摘要

The process of tanning bovine leather produces several types of waste that, due to their high organic load, can deplete the dissolved oxygen in watercourses. The scope of this study is the investigation of the shallow subsurface by geophysical methods of electrical resistivity (ER) and induced polarization (IP) and physicochemical analyses of water samples in an area in southern Brazil in which a tannery industry is located. The main element used in this sector is chromium (Cr), and the concentration and mobility of this element are related to its speciation. To obtain the geophysical data, 12 resistivity survey lines were performed in a dipole-dipole array (AB = MN = 10 m) at six investigation levels ( approximately equal to 15 m deep). The geophysical results were represented in two- and three-dimensional models that characterized the contaminant plume by low resistivities ( 50 omega m) that coincide predominantly with low chargeabilities ( 3 mV/V) in contrast to elevated indices ( 50 omega m and 3 mV/V), which were interpreted as regoliths and underlying uncontaminated sandstones. The models made it possible to estimate the dimensions of the contaminant plume (50 x 60 m and 7 m deep) and to determine that its flow direction was preferentially downstream. Some of the analyzed physicochemical parameters, such as chromium, iron, and ammoniacal nitrogen concentrations; biochemical and chemical oxygen demand; and conductivity (sigma), exceeded the maximum values or concentrations recommended by Brazilian references. The occurrence of hexavalent chromium seems to be associated with slightly acidic and oxidizing environments. The results suggest that the contaminated area is restricted to the area near the pollution source. Studies using this approach could contribute to environmental management.