Distribuzione dei metalli pesanti nei suoli agricoli ferraresi: analisi geochimica e cartografia tematica su base G.I.S.

The Ferrara province, as most of the River Po plain, is characterized for more than 70% of its territory by agricultural soil. An environmental concern is the elevated concentration of heavy metals in the agricultural soils that can determine a potential toxicity for the plants and for their consumers. In this framework, the Department of Sciences of the Earth of the University of Ferrara realized a study on the heavy metals concentration in the agricultural soils surrounding the town of Vigarano Mainarda in which the two principal sediment types characterizing the Ferrara alluvial plain (related to the Po River and Reno River respectively) are represented. The sampling has been done including all the principal geological characteristics representative of the area. Two different samples were collected at each sampling site: a) within depths of 90-100 cm to determine the “background values” of heavy metals in the soil and b) at depths of 20-30 cm to determine the Top Enrichment Factor of heavy metals in the soil. The chemical analysis by X-ray fluorescence (XRF) for the determination of bulk rock major (wt%) and trace elements, discriminates two well-delineated populations of samples, respectively characterized by high values of Cr (> 150 ppm) and Ni (> 100 ppm) representing the Po alluvial sediments and low values of Cr (< 150 ppm) and Ni (< 100 ppm) representing the Reno alluvial sediments. The statistic and graphic elaboration of the chemical data show a positive correlation among the distributions of Cr and Ni (R2 > 0,6). Cr and Ni are also positively correlated with MgO (wt.%), Al2O3 (wt.%) and V (ppm), as well with the clay content determined through grain size analysis . Specific thematic cartography highlighted relationships among the sedimentary facies of the area and the concentration of heavy metals. Cr and Ni show maximum concentration in the Po Floodplain organic clays, while lower concentration is found in in Reno channel-levee sands and silt. Coherently, the analysis of the fine-grained fraction of the samples (< 63 μ m) show higher (20-60%) concentration of Cr and Ni respect to the coarse fraction. The ratios Cr/V and Cr/Al2O3 are good geochemical indicators of sediment provenance of the Ferrara alluvial plain: Cr / V > 1,4 and Cr / Al2O3 > 10,7 correspond to the sediments of the Po river, Cr / V < 1,4 and Cr / Al2O3 < 10,7 correspond to the sediments of the Reno river. The values of the TEF (Top Enrichment Factor), determined by the ratio between the concentrations of the Cr and Ni of the superficial samples and those recorded at the depths of 90-100 cm, are nearly equal to 1 for Cr and smaller of 1 indicating absence of heavy metals pollution of the examined agricultural soils. The agricultural soils that are found in the alluvial sediments of the Po are characterized by elevated background values of Cr and Ni that represent a natural geochemical anomaly originated from a peculiar mineralogical composition of the fine-grained fraction, probably constituted by Cr-Ni-rich clay minerals such as chlorite and serpentine. This hypothesis was confirmed by “in situ” electron microprobe analyses of these minerals. Further investigation of these samples has been performed with leaching tests. These tests of heavy metals extraction, done with HNO3 and HCl 1:3 (according to the Italian official methods of analysis of soil) and analysed by ICP-MS, indicate the potential mobility of metals under the different chemical-physics conditions and therefore are useful to determine a potential geochemical risk to cultivations and natural waters. These tests highlighted for all the investigated samples concentration lower than the tolerance limits of the Italian legislation.