Metal accumulation in soils derived from volcano-sedimentary rocks, Rio Itapicuru Greenstone Belt, northeastern Brazil

Laíse Milena Ribeiro Dos Santos; Thomas Vincent Gloaguen; Francisco de Souza Fadigas; Joselisa Maria Chaves; Tamires Moraes Oliveira Martins

doi:10.1016/j.scitotenv.2017.06.035

Metal accumulation in soils derived from volcano-sedimentary rocks, Rio Itapicuru Greenstone Belt, northeastern Brazil

Sci Total Environ. 2017 Dec 1:601-602:1762-1774. doi: 10.1016/j.scitotenv.2017.06.035. Epub 2017 Jun 11.

Authors

Laíse Milena Ribeiro Dos Santos¹, Thomas Vincent Gloaguen², Francisco de Souza Fadigas¹, Joselisa Maria Chaves³, Tamires Moraes Oliveira Martins³

Affiliations

¹ Federal University of Reconcavo of Bahia, Cruz das Almas, Brazil.
² Federal University of Reconcavo of Bahia, Cruz das Almas, Brazil. Electronic address: thomasgloaguen@ufrb.edu.br.
³ State University of Feira de Santana, Feira de Santana, Brazil.

PMID: 28618665
DOI: 10.1016/j.scitotenv.2017.06.035

Abstract

Many countries and some Brazilian regions have defined the guideline values for metals in soils. However, the local geological features may be so heterogeneous that global or even regional guideline values cannot be applied. The Greenstone Belts are worldwide geological formations of vast extension, containing mineralization of various metals (e.g., Au, Cr, Ni, and Ag). Natural concentrations of soils must be known to correctly assess the impact of mining. We studied the soils of the Rio Itapicuru Greenstone Belt (RIGB), of Paleoproterozoic age, sampling at 24 sites (0-0.20m) in the areas not or minimally human impacted, equally distributed in the three units of the RIGB: Volcanic Mafic Unit (VMU), Volcanic Felsic Unit (VFU), and Volcano-clastic Sedimentary Unit (SU). The natural pseudo-total concentrations of Cr, Ni, Cu, Zn, Pb, Fe, and Mn were obtained by acid digestion (EPA3050b) both in the soil and the particle-size fractions (sand and clay+silt). The concentrations of metals in RIGB soils, especially Cr and Ni, are generally higher than those reported for other regions of Brazil or other countries. Even the sedimentary soils have relatively high metal values, naturally contaminated by the VMU of the RIGB; a potential impact on Mesozoic and Cenozoic sedimentary rocks located near the study region is highly expected. Metals are concentrated (80%) in the fine particle-size fraction, implying an easy availability through surface transport (wind and runoff). We introduced a new index, called the Fe-independent accumulation factor - AF_-Fe, which reveals that 90-98% of the dynamics of the trace metals is associated with the iron geochemical cycle. We primarily conclude that determining the guideline values for different soil classes in variable geological/geochemical environment and under semiarid climate is meaningless: the concentration of metals in soils is clearly more related to the source material than to the pedogenesis processes.

Keywords: Accumulation factor; Mafic rocks; Metals; Semiarid climate.