Metal pollution in fluvial deposits impacted by a mining dam failure in the Doce river watershed (MG): earthworms as bioindicators

Abstract

This paper presents the results related to the ecotoxicological assessment associated with metal pollution in fluvial deposits from a segment of the Gualaxo do Norte River, which was impacted by the collapse of the Fundão dam under the responsibility of the SAMARCO S.A. (a joint-venture related to VALE S.A e BHP Billiton), in Mariana (MG). To achieve these purposes, bottom sediment samples were collected in the following fluvial deposits: riverbed (RB), floodplain (FP) and fluvial terrace (FT). Determinations of texture, toxic metals, pH and total organic carbon (TOC) were performed in these sediment samples. Another sample was obtained in a technogenic deposit, which consisted of the mud collected in the urban centre of the Paracatu de Baixo district. Other sediment samples were also collected in an area not impacted by dam collapse, and these samples were used as a reference. Acute and avoidance bioassays with earthworms (Eisenia andrei) were applied to these samples to evaluate their toxicity levels to soil biota. The results indicated that the tailing mud induced the increase of clay contents (especially in RB) and reduction of TOC concentrations, while pH values ranged close to neutrality. In addition, the tailing increased metal concentrations (mainly in FP), and arsenic and cadmium concentrations (extremely toxic elements) exceeded the threshold limits defined by Brazilian law for soil quality. The bioassays suggest low earthworm mortality and low toxicity levels. Although the FP has showed the highest metal concentrations, the bioassays indicated that the FT was the most toxic compartment. Such observation may be due to more intense pedogenesis and weathering processes on the deposits from FT.