Metais pesados em águas e em solos cultivados com coqueiro no perímetro irrigado Curu-Pentecoste, Ceará

Abstract

Heavy metals are components of the earth crust, occur in soil primarily through weathering of rocks and can be toxic even in small concentrations. Researches related to heavy metals in ecosystems have indicated high levels at sites near areas under anthropogenic activity, promoting contamination. Therefore, metals may be present in surface waters, underground waters and soil as a result of natural processes and / or activities related to agriculture. This study has sought information about the heavy metal content in soils under coconut cultivation at Curu-Pentecoste Irrigated Perimeter, in the State of Ceará, Brazil. Three composite soil samples were collected from agricultural and native forest areas at different depths (0 – 0.20 m and 0.60 - 0.80 m depth). The quality of irrigation water and drain water was evaluated. It was performed physical, chemical and heavy metals analysis in soil (total and fractionated contents) and water (As, Cr, Cu, Fe, Mn, Ni, Pb and Zn). The analytical results were evaluated by descriptive statistics. Data were subjected to analysis of variance and the means compared by the ScottKnott test (P <0.05) using the Assistat 7.7 program. The analytical results were evaluated by Pearson correlation analysis (P <0.05), estimating the correlations between variables. Likewise, factor analysis was performed by the principal components analysis. Despite the possibility of the metals in agricultural areas being derived from agricultural inputs, it is not possible to prove it, and this may be related to other factors. The total content of As, Cr, Ni, Pb and Zn (except for Cu in some areas), were lower than those of CONAMA prevention values, indicating that these metals do not cause harmful changes to soil quality. Fractionation analysis indicated that most of the Cr, Cu, Fe, Mn and Zn are bonded to oxidic, organic and residual fractions, giving them lower availability and mobility. Pb and Ni are well-distributed between fractions, however, at exchangeable fraction, these metals are present in small concentrations. Water analysis indicated adequate levels of pH, Ca, Mg, K and P, but higher levels of HCO3, according international standards, indicating moderate restriction of use (due to EC, SAR, Na and Cl). The Ni and Fe contents were not detected, but the metals Cr, Pb and Zn indicated acceptable contents, showing, however, Cu (all agricultural areas) and Mn contamination (in drained water of the A4 area).