Migration and risks of potentially toxic elements from sewage sludge applied to acid forest soil

The application of sewage sludge (SS) to forested lands may lead to the downward migration of potentially toxic elements (PTEs) through rainfall and thus pose risk to the subsoil and groundwater. Batch column experiments were conducted using leaching water equivalent to the rainfall amount in the study area over 3 years to investigate changes in concentrations of PTEs, including copper (Cu), zinc (Zn), lead (Pb), cadmium (Cd), and nickel (Ni) in the leachate from the acidic forest soil. Water quality index of leachate, potential ecological risk and human health risk in soil at different leaching stages were compared. Sewage sludge was applied at SS/soil mass ratios of 0:100 (controls), 15:85 (T1), 30:70 (T2), 45:55 (T3), 60:40 (T4), and 75:25 (T5). All treatments resulted in increased PTEs concentration in the upper 20 cm soil, T3-T5 increased potential ecological risk from "low" (control) to "moderate" or "considerable". During first year leaching, PTEs concentration increased with increasing SS/soil ratios, but the water quality index of T1-T3 was "excellent" or "good". Pb, Cu, Cd, and Ni in the 20-40 cm soil depth, and Zn in the 60-80 cm soil depth were also enriched, but potential ecological risk was "low". In subsequent leaching, PTEs concentration of leachate gradually returned to the background value and water quality index was "excellent". There were no significant changes in PTEs and ecological risk observed. During the monitoring process, the health risk caused by the migration of PTEs to the human body was always within the acceptable range. Overall, this study provides a reference for the management of risks from the application of SS on forestlands, i.e., SS/soil ratios < 45:55 is recommended on forestlands, and special attention should be given to early leaching risk. In addition, it also provides an important assessment method for the risk of PTEs leaching and migration in forested land application.