Release risk assessment of trace metals in urban soils using in-situ DGT and DIFS model

Urbanization and urban construction lead to entensive environmental deterioration. Trace metals in urban soils pose a threat to urban water bodies and local populations. However, the release ability of labile metals and their release risk in urban soils remains unclear. Here, soils were collected from different functional zones in the Pingshan District (PSD) of Shenzhen. Based on results of soil properties, total contents of trace metals, geochemical index (I_geo), and risk assessment code (RAC), diffusive gradients in thin films (DGT) and DGT-induced fluxes in soil (DIFS) model were further used to assess the release risk of trace metals in urban soils. The results showed that the average total concentrations of trace metals (As, Cr, Cu, Pb, and V) were higher than the local soil background values, implying that trace metals accumulated in urban soils. However, the distributions of labile metals determined by DGT were not similar to those of total metal concentrations. Except for As, urban soils from PSD sites exhibited "uncontaminated to moderately contaminated" levels based on the average values of I_geo. Moreover, the pollution and migration of Cu in urban soils are problematic as evidenced by the I_geo and RAC assessments. Release ability of Cu was assessed using parameters of DIFS model (i.e., bioavailability concentrations (C_E), resupply ability (R), response time (T_c), desorption rate (k_-1), and sorption rate (k₁)). Residential areas showed high C_E values for Cu, while the resupply ability was low. Furthermore, considering the influences of R, T_c, k_-1, and k₁, membership function value was used to re-calculate the order of C_E in urban soils. The final results suggested that the agricultural zone exhibited the highest release risk among soils from various functional zones. Therefore, DGT and DIFS model should be effective tools to assess the release risk of trace metals in urban soils.