Soil-water interaction in soils of the Po River Plain (Ferrara, Northern Italy): insights on heavy-metal mobility and phytoavailability

The soils of the Po River Plain, developed on the alluvial sediments, are often characterized by high concentrations of heavy metals, in particular chromium and nickel. These geochemical anomalies are geogenic, i.e. related to the nature of the rocks outcropping in the basin that typically include mafic and ultramafic lithologies. The elevated heavy metal backgrounds of soils potentially represent an effective geochemical risk considering the toxicity of these elements. In order to delineate soil quality thresholds and to provide guidelines for human activities (e.g. agriculture) the current legislation takes into consideration the “pseudototal” metal concentration obtained with aqua regia extraction tests. However, only a fraction of this chemical budget is available for plant and human uptake. Soil leaching tests with deionized water plausibly provide a better analogue to simulate soil-water interactions, in order to predict the behavior of metals in the environment. In particular, in this paper we investigate with water leaching tests agricultural soils sampled in the surrounding of Ferrara (eastern part of the Po River Plain) that were previously characterized by XRF bulk analyses and aqua regia extractions. The approach gives insights on the specific transport parameters of distinct elements, giving clues for a) the possible contamination of natural waters and b) the soil-to-plant uptake processes and phytoavailability. The results, expressed as solid-water partition coefficients, highlight that nickel and arsenic are mobile and bioavailable and should be monitored in the local agricultural products to avoid its possible transfer and bioaccumulation in the food chain.