An integrated framework for ecological risk assessment of microplastics in aquatic sediments

Microplastics and their associated toxic chemical compounds pose significant risks to both terrestrial and aquatic ecosystems. Risk evaluation commonly relies on three indices: the Pollution Load Index (PLI), Polymer Hazard Index (PHI), and Potential Ecological Risk Index (PERI), with each providing valuable yet isolated insight into different aspects of microplastics risk. The study areas included three urban creeks located in Southeast Queensland, Australia. Based on data obtained from individual sampling sites and multiple sampling rounds, PLI, PHI and PERI hotspot locations were identified. PHI and PERI values indicated a strong influence of hazardous polymer composition on toxicity-weighted ecological risk. Site-level Spearman correlation analysis showed that industrial, commercial, and residential land uses are positively associated with PLI, PHI, and PERI, whereas natural land use showed a negative correlation. A risk-zoning framework was developed based on elbow and silhouette analyses, which identified k = 3 as the optimal number of clusters, defining low, medium, and high-risk groups. Eight risk categories were then derived from these groups for finer operational risk classifications. These were expressed using a consistent hexadecimal colour scale derived from the 40 possible index-combination permutations. The resulting two-dimensional (2D) risk-zoning plots visualise how sediment microplastics are distributed across the combined risk space. Film and foam were identified as the most dominant shape compositions across all risk categories. The proposed innovative methodology offers a practical approach for identifying microplastic hotspots, interpreting risk-driving mechanisms, and supporting decision-making and policy formulation for targeted microplastics management and ecological protection strategies.