Concentration and variability of soil trace elements in an agricultural area in a semiarid region of the Irecê Plateau, Bahia, Brazil

Abstract

In a previous study, a high concentration of Pb, Zn, Cr, and Mn in soil profiles located in the semiarid region of Brazil were found in areas used for decades by small family farmers for the production of the crops and raising small animals. This question motivated the current study to evaluate the soil variability spatial dependence on trace elements to quantify and map its occurrence in this area, comparing their concentration with reference values set by the Brazilian environmental agencies. Soil samples were collected at 0-0.20 and 0.20-0.40 m depths and 25, 50, 100, 200, and 400 m from the center and to both sides of five transects spaced from 1,000 m, forming a grid. The concentration of the trace elements As, Cd, Pb, Cu, Cr, Fe, Mn, Ni, Ti, and Zn was obtained in SW-846 3050B USEPA soil extract by ICP-OES. Spatial dependence for all evaluated elements allowed the elaboration of dispersion maps in the area, except for Cd. Trace elements concentrations were below the investigation values (limit value of a trace element in the soil with risk potential to humans and the environment) of the Brazilian environmental legislation (Conama). Pb and As concentrations were above the prevention values (limit value of a trace element in the soil that can sustain its primary functions), but, still, they presented low dispersion in the area. Zn concentration was above the prevention value, and dispersion was higher, regardless sampling point, which requires a major concern. Cr, Cu, Ni, Fe, Mn, and Ti concentrations were below the Conama reference values, and the dispersion was also low. Fortunately, except for Fe, it was observed that trace elements were concentrated around the central point of the "gossan", instead of distributed throughout in the agricultural area around it.