Os valores de referência, assinatura isotópica de fatores de enriquecimento de metais Pb e pesados em solos que cercam uma baía do ambiente impactado no Brasil.

Abstract

This work was carried out to investigate the possible influence of human activities on heavy metal concentrations, by enrichment factor and isotopic signature of Pb and determine the natural levels of heavy metals in the vicinity Todos os Santos Bay-BA (BTS). For this, two sampling design of the soil around the BTS were realized. The first sampling 114 topsoil samples (0.0 - 0.2 m depth) were collected from 38 samplings sites and the concentrations of Pb, Zn, As, Cd, and Hg were determined by ICP-OES or AA-FIAS after microwave sample digestion. Results showed that mean values for heavy metal concentrations in soils (mg kg-1) followed the order Pb (39.45) > Zn (12.52) > As (1.95) > Cd (0.23) > Hg (0.09). The mean natural concentrations of heavy metals found in the soils of an important economic region of Brazil’s were generally lower than those reported in the national and international literature, with exception Pb concentration that was higher than all compared soils.The quality reference values (QRvs) calculated for the TSB based on the 75th percentile, were as follows (mg kg−1): Pb (54.37); Cr (36.55); Zn (10.40); Cu (7.10); Ni (4.05); Co (0.38); Cd (0.30). The second sampling six soil profiles were collected, the samples were collected from different horizons, totaling 44 samples. Soil pits were dug until 2 m or until the parent material (C horizon) was reached.The soil samples were microwave digested in the mixture of HNO3, H2O2 using the method described by Sah and Miller (1992). Concentrations of Pb and the isotopes 204Pb, 206Pb, 207Pb and 208Pb were measured on an inductively coupled plasma (quadrupole) mass spectrometry. Pb concentrations varied from 3.2 to 44.1 mg kg−1 with an average concentration of 13.65 mg kg−1. In general the samples feature a wide range of Pb isotope ratios, ranging from 36.71 to 47.38 for 208Pb/204Pb, 16.86 to 20.59 for 206Pb/204Pb, 1.10 to 1.31 for 206Pb/207Pb and a ranged slightly from 15.00 to 15.65 for 207Pb/204Pb. The PCA performed identified that Pb, Zn, and Cd were associated with the same factor (F1) and had chiefly anthropogenic origin whereas Pb and Zn have also contributions from both sources (natural, lithogenic and anthropogenic). The As and Hg concentrations (F2) were related to the natural component; the parent material underlying the soils (igneous-metamorphic rocks) seemingly confirm this hypothesis. The natural concentrations of heavy metals in soils of TSB had the following decreasing order: Pb > Cr > Zn > Cu > Ni > Co > Cd. Pb concentration in all soil profiles not seem have been influenced by the abandoned lead smelter plant located in the municipality of Santo Amaro da Purificação-BA. The distinct Pb isotopic compositions were clearly related to the different exposure of the sampling sites in relation to atmospheric deposition and geological parent material.