I metalli pesanti nei suoli agrari della Provincia di Pavia

This study aims at: 1) defining baseline concentrations of several heavy metals (Cu, Ni, Pb, Zn, Mn) in cultivated soils from the Pavia province; 2) identifying anomalous contents and 3) evaluating their origin (natural vs. anthropogenic). The plain and foothills of the Pavia province are intensively cultivated with rice, corn, maize and vineyards (Tav. 1A). Heavy metals concentrations were determined on 377 samples representative of the top layer, by atomic absorption analysis of the aqua regia soluble content. In addition, 10 samples with high Ni content were selected for mineralogical analyses by XRPD to evaluate the relationship existing between heavy metals concentration and mineral content. Statistical data treatment allowed the determination of distribution patterns and the identification of anomalous contents. The relationship between concentration and geology or land use is evaluated with distribution maps coupled to available information layers through GIS software. Results show different data ranges for the plain and foothill sectors, where mean heavy metals concentrations are higher in the latter area. This difference is likely related to the substratum type (alluvial deposits in the plain, sedimentary rocks in the hills) and the soil thickness (greater in the plain). Distribution patterns (Fig.1) allow to statistically define the expected concentrations ranges for cultivated soils and hence baseline concentrations (Tab. 4). The correlation matrix between heavy metals (Tab. 5) indicates that all metals are well correlated with Mn. Nevertheless, two trends may be observed (Fig. 2): one (F1) relating Mn to the other metals, likely corresponding to a lithogenic control over concentrations, and a second trend (F2) showing a data scattering at lower Mn content, attributed to an anthropic input. The latter trend is especially observed in the hills. Copper represents an environmental concern in the Pavia province, showing a number of anomalous contents (16 samples over 377 exceed the mandatory value of 120 ppm, maximum content 434 ppm) and being mostly influenced by an anthropic input. The highest concentrations are indeed observed in vineyards and orchards (Tav. 1B) and can be attributed to the use of Cu-based pesticides. Nickel also shows high concentrations along the Po and Apennine-derived river courses and in the hill sector (Tav. 1B) (46 samples over 377 exceed the mandatory value of 120 ppm, maximum content 332 ppm). In both cases these are natural abundances. In the Oltrepo area, anomalous concentrations are observed in correspondence to the outcrops of the Arenarie di Ranzano formation, which includes fragments of serpentinites and ophiolitic rocks. Mineralogical determinations in soils sampled close to the Po river and other Apennine-derived rivers has evidenced the presence of serpentine, a Ni-rich mineral phase derived by erosion of ultramafic rocks outcropping in western Alps and in the Ligurian Apennine. Lead and zinc do not show anomalous contents, although lead concentrations are higher than the average concentration of the continental crust (average concentration in our soils is 30 ppm) and higher than soils sampled in the Piemonte region. Manganese shows an important “negative” anomaly caused by the presence of concentrations lower than the natural average concentration of rocks (155 samples over 377 have concentrations lower than 400 ppm). This depletion is mostly observed in correspondence to rice paddies (Tav. 1C): periodical flooding of these soils causes the establishment of reducing conditions and favours Fe and Mn mobility. In conclusion, two main factors affect heavy metals distribution in soils from the Pavia province: the natural concentration of the substratum, especially in the hill sector, and the anthropic influence. The latter consists of an input, especially for Cu, but also a depletion, especially for Mn in rice fields.