Water content and metal pollution dynamics in the surface layer of urban soils: first results of the PROFILES project

This work presents the first results obtained in the context of the PROFILES project. The main objective of PROFILES concerns the identification of a possible correlation between water content dynamics and the distribution of metal pollutants in the surface layer of urban soils.

The research activity combined experimental, analytical and numerical approaches. Field activity was conducted in Bagnolo Mella, (Brescia, Northern Italy) where a ferroalloy industry operated for more than forty years (1974-2015). Four sites within the study area, at different radial distances and up/downwind with respect to the plant, have been considered. At these sites, the distribution of As, Cu, Fe, Mn, Ni, Pb, Zn concentrations in the surface soil was inferred by means of a portable X-Ray Fluorescence device. A tension infiltrometer allowed to estimate the local value of hydraulic conductivity at saturation, K_s. Physico-chemical properties evaluated on soil samples via laboratory analysis were found to be rather homogeneous. However, metal concentrations were remarkably different at the four sites, larger values being detected up-wind and closer to the production plant, within the top layer (≈ 20 cm) of the soil column. In particular, sequential chemical extraction processes and X-ray Absorption Near Edge Spectroscopy showed that Mn exceeded considerably typical background levels and was present in a hybrid form of magnetite (Fe, Mn)₃O₄, resistant to acid dissolution. Considering that it is difficult to form Mn-substituted magnetite in surface layers at low temperatures, its presence indicates this pollutant as a by-product of ferroalloy production transported by water along the soil column. Numerical simulations with the HYDRUS 1D software have been performed to model water dynamics along the uppermost 6 meters of soil at the investigated sites, over a time range of 4 years (2013–2016). A homogeneous domain, with a constant K_s value measured in the field for the top layer, has been compared against a heterogeneous case, in which the distribution of lithological categories has been determined via indicator kriging, based on available stratigraphic data. Surface recharge and evapotranspiration have been estimated from meteorological data (temperature, relative humidity, precipitation, global solar radiation and wind speed) available on an hourly basis. Numerical results allowed to characterize the time evolution of the zero-flux-plane (ZFP) depth, defined as the plane separating zones with upward and downward water flux in a thoroughly wetted soil, when evaporation and drainage are simultaneously occurring. Key findings are: (i) for the whole simulated period, the ZFP oscillates between the ground surface and a maximum depth of about 20 cm, consistent with the vertical range where peak concentrations of heavy metals were found; (ii) simulations in the homogeneous and heterogeneous cases provided analogous results, highlighting the importance of the characterization of the top surface layer. 

Acknowledgments: The project PROFILES was awarded the 2019 edition of the Florisa Melone Award, promoted by the Italian Hydrological Society (SII). The authors thank the SII for the support. Part of the research was carried out within the ISEIA project of the University of Brescia (grant UNBSCLE 9015).