Geochemical distribution of selected heavy metals in stream sediments affected by tannery activities

This study describes the geochemical distribution and potential mobility of selected heavy metals in sediments affected by tanneries established in the Cadeia-Feitoria River basin, South Brazil. Surficial samples were taken at ten sites, in January 2000, during dry season. Metal contents were determined according to their association with main sediment phases by applying a sequential extraction scheme proposed by the Community Bureau of Reference' of the European Commission. In order to compare the results, total and partial extractions were performed, following analytical methods used by US EPA and US Geological Survey. Metals were analyzed in the silt/clay fraction, using inductively coupled plasma - optical emission spectrometry. The results indicate the following order of metal mobility: Ti (1%) < Al (5%) < Ni (6%) < Fe (10%) < Cu (20%) < Zn (28%) < Cr (49%) < Mn (76%). Manganese was highly mobile along the sampling sites, and mainly bound to exchangeable and reducible phases. Total Ti, Al, Fe, Cu, and Mn exceeded two or three times the average shale metal concentration, but generally reflected background conditions in the basin, where basalt rocks predominate. Ni and Zn were homogeneously distributed along the rivers, with concentrations comparable to the average shale and local concentration levels. Sediments affected by tanneries were enriched with chromium (geoaccumulation index = 2 and 3), preferentially associated to the oxidizable phase. The potential release of this metal and conversion to toxic forms seems closely related to the prevailing redox conditions. The relationship between data obtained from sequential extraction and from partial and total extractions are briefly discussed.