Disponibilidade ambiental e risco ecológico de metais pesados em solos de diferentes posições fisiográficas de manguezal

Abstract

Mangrove soils are considered geochemical filters due to their great capacity to retain heavy metals. The level of contamination of these soils can be assessed by the environmentally available levels of heavy metals and the ecological risk. With this, the work aimed to evaluate the levels of heavy metals based on the 3051A analysis, comparing with regional and international reference levels, as well as estimating the ecological risk in areas of different physiographic positions of the Botafogo mangrove. Soil samples were collected (0-40 cm) along two transects (T1 and T2) at distances of 0, 60, 120 and 180 meters from the bank of the Botafogo River. In the field, pH and Eh were determined in sections 0 to 5, 5 to 10, 10 to 20, 20 to 30 and 30 to 40 cm of the collected soils. In the laboratory, the samples were prepared for granulometric, organic matter (MOS) and heavy metal analysis. Metal contents were determined in ICP-OES after 3051A acid digestion of soil samples. Descriptive statistics were adopted for all variables evaluated. The pH values of the evaluated areas indicated slightly acidic to neutral soils; while the values of Eh, suboxic to anoxic environments. There was a predominance of clayey soils and high levels of SOM in T1; sandy soils predominated in T2. In general, the average levels of heavy metals in T1 were higher than in T2, and exceeded regional backgrounds. Only Cr and Cu showed levels above the Threshold Effect Level (TEL), indicating possible adverse effects on the biota. The distribution of metals in the transects varied as a function of clay and SOM contents, as verified in the positive and significant correlations between metals and clay, and between metals and SOM. Ecological risk coefficients (Ei) were higher at T1 when compared to T2, but all Ei were below 40, indicating low risk. The Potential Ecological Risk (PER), considering the effect of the set of metals, was below 150, showing that the presence of metals in mangrove forests does not offer high ecological risks.