Shotgun plastic wads as persistent pollutants and a vector for lead transport in aquatic environments: Evidence from Lake Geneva

Plastic wads (PWs) from shotgun cartridges are an overlooked yet persistent source of environmental contamination. We combine environmental monitoring with machine learning to assess the occurrence and distribution of PWs around Lake Geneva (Switzerland/France). Using data from four shoreline litter surveys (2015-2022), we applied supervised machine learning models-Random Forests, Logistic Regression, Naive Bayes, gradient-boosted decision trees-to estimate the probability of encountering PWs based on region and presence thresholds. These classification models were assessed using both temporal and random data partitions to evaluate their predictive and explanatory capabilities. Model performance ranged from AUC (area under the receiver operating characteristic curve) 0.57-0.80 depending on encounter metric and temporal partition. Random Forests achieved the best discrimination under temporal validation, while Logistic Regression performed best under random splits, indicating that regional structure dominates while temporal drift limits predictive transfer. Results show a widespread presence of PWs, with predicted encounter probabilities ranging from 44 % to 50 %, and the highest density hotspots found in the Haut lac. Among 138 PWs sampled from lake beaches, 62 % contained lead detected by XRF (X-ray fluorescence) spectrometry, with concentrations up to 344 mg kg^-1. Samples located near shooting installations consistently contained lead, even exceeding 1000 mg kg^-1 in some samples. SEM-EDXS (scanning electron microscopy and energy dispersive X-ray spectroscopy) analysis revealed lead embedded in the plastic surface, indicating its persistence and the potential for wide distribution, while imaging also indicated that microbial colonisation on some PWs appeared to accumulate lead. Overall, PWs act as persistent macroplastics and vectors for lead contamination, functioning as a long-lived lead-source in aquatic environments.