Valores de referencia de elementos inorgânicos potencialmente tóxicos em solos do Estado de São Paulo
Abstract
The use of guiding values has been the practice in countries with tradition in
monitoring the quality of natural resources. The first step of a pollution monitoring
program is to establish the natural concentrations of potentially toxic elements (PTEs)
in uncontaminated soil to the development of background values. Mean values of total
PTE concentration in worldwide are very heterogeneous and resulting from different
soils, extraction methods and techniques of analytical determination. The background
values recommended by CETESB were obtained from samples of 13 soils, submitted
to USEPA SW 846 3050 protocol, whose PTE contents were determined by atomic
absorption spectrometry. The objective of this work was to contribute to the
improvement of background values of Al, Ba, Cd, Co, Cr, Cu, Fe, Mn, Ni, V and Zn in
tropical soils, using topsoil samples (0-20 cm) of 30 representative soils of State of São
Paulo, Brazil, collected in areas without human disturbance. The soil samples were
submitted to the USEPA SW 846 3051, 3051 A and 3052 protocols, with microwaveassisted
acid digestion. Natural contents of PTEs were determined by inductively
coupled plasma optical emission spectrometry (ICP OES). Extraction protocols were
evaluated using the certified soil sample NIST SRM 2709 San Joaquin. Arithmetic
means were adopted to describe the central tendency and the variation of results from
the different acid digestions and to set the background values into 75 and 90
percentiles. Regardless the extraction method, the average contents of EPT were in
the following range of variation (mg kg-1): Al - 27118-42865; Ba - 78-101; Fe - 6318-
35888; Cd - ND - 2.2; Co - 18 to 37; Cr - 39-59; Cu - 43-61; Mn - 176-623; Ni - ND - 61;
V - 111-193 and Zn - 63 to 68. The contents of Al, Co, Cu, Fe, Mn, V and Zn correlated
with clay, amorphous (Feox) and crystalline (FeDCB) iron oxides contents. The method
3052 resulted in PTEs recovery levels within the limits considered acceptable for
certified sample. Within recommended methods by CONAMA Resolution n.420 / 2009,
the 3051 A was the best correlated with the method 3052 and was considered the
protocol with high potential to improve soil background values. The new soil
background values were (mg kg-1): a) percentile 75: Al - 66,700; BA - 68; CD - 1.53; Co
- 16; Cr - 47; Cu - 20; Fe - 24,830; Mn - 333; Ni - 57; V - 66; Zn - 61; percentile 90: Al -
99 820; Ba - 113; CD - 1.78; Co - 30; Cr - 62; Cu - 32; Fe - 39 280; Mn - 480; Ni - 113;
V - 119; Zn - 65. Since new background values differed from those currently practiced
by CETESB, the use of ICP OES to determine natural PTE levels extracted by USEPA
3051 A, from a larger and more representative set of soils, supported the improvement
of background values and may contribute to the improvement of the environmental risk
assessment estimates for edaphoclimatical conditions of the State of São Paulo.