A Quantitative Environmental Risk Assessment for Microplastics in Sewage Sludge Applied to Land

The application of sewage sludge to land delivers high levels of microplastics (MPs), contributing to soil contamination and chronic effects on soil biota. Despite this, the quantitative chronic risk assessment of MPs in sludge-amended soils (SAS) to soil biota has not been thoroughly addressed, and none has been done directly on sludge samples. Here, we combined environmental exposure and species sensitivity distributions, built on published data, with two-dimensional Monte Carlo simulations to characterize the risk from MPs in sludge samples and in SAS, testing worst-case and realistic case scenarios of sludge behavior in soil. Contamination with MPs frequently exceeded the concentration likely to affect more than 5% of the species. Risk characterization showed that the worst-case MP scenario affects 39% of species, while the realistic scenario affects 15-18% of soil species, implying that the current state of sludge application fails to protect 95% of soil biota. The percentage of species affected by MPs approximately doubled in SAS. Our conservative results suggest that regulatory limits on MPs in sludge being applied to land should be urgently considered.