Chemical-mineralogical characterisation of clay sediments around Ferrara (Italy): a tool for an environmental analysis

The content of heavy metals in water and soil is a key parameter for evaluating the geochemical vulnerability of an ecosystem. These elements display a limited solubility and are easily trapped and adsorbed by phyllosilicate minerals; they are thus preferentially partitioned in the fine fraction of sediments. In this light, an analysis of recent river sediments gives information on possible water pollution, and more in general on the related ecosystem. We therefore investigated the chemical-mineralogical features of clay sediments outcropping around the town of Ferrara, paying particular attention to their fine fraction (grain size<2 m).X-ray fluorescence (XRF) analyses indicate that the abundance of transition trace elements, such as Cr and Ni, is positively correlated with MgO wt.%, and discriminates two well-delineated populations of samples, respectively characterised by high (Cr>180 ppm; Ni>100 ppm) and low (Cr<180 ppm; Ni<100 ppm) contents of these elements. The mineralogical composition of the fine fraction (<2 m) was investigated through X-ray powder diffraction (XRPD) integrated with differential thermal (DTA) and thermogravimetric analyses (DTG), showing that: low-Cr samples are characterised by a higher proportion of clay minerals in which smectite+mixed layers are more abundant than chlorite (Sm+ML/Chl>1); on the other hand, the high-Cr samples have a coarser grain size, and a lower abundance of clay minerals in which chlorite (Mg-rich chlorite in this group of samples) predominates over smectite+mixed layers (Sm+ML/Chl<1). These two distinct groups of samples are ascribed to different sources: high-Cr lithologies are related to the sedimentary contribution of the Po river, whereas low-Cr sediments plausibly derive from small rivers of Apennine origin (e.g. the Reno river). Within the high-Cr group, concentrations of Ni and Cr tend to be higher than those indicated by the current environmental Italian legislation. However, in the study-case presented here, the detected high heavy-metal concentrations are not related to urban-industrial-agricultural activities, but instead appear to be typical of the original lithologies.An integration of similar scientific contributions would be useful to set up a geochemical-mineralogical database as a first step toward the preparation of more complete thematic maps. These would provide information relative to the behaviour (e.g. distribution and abundance) of chemical elements within the different geochemical spheres, and would be useful for recognising and interpreting possible geochemical anomalies induced by pollution processes.