<p>Groundwater is a vital source of freshwater, and its contamination by microplastics (MPs) and potentially toxic elements (PTEs) is a growing concern. This study explores the occurrence and associated ecological and health risks of MP-PTE co-contamination in groundwater in the mid-Brahmaputra Valley, India. A total of 169 MPs, derived from polyamides, polyacetonitrile, polyethylene, polymethacrylates, polypropylene and polyvinylchlorides, were identified in 21 samples. Polypropylene MPs (41%) were the most abundant while polymethacrylate MPs (8%) were found to be the most hazardous, both ranking in hazard category V. The samples also contained elevated levels of Fe, Mn and Pb, with 26% of samples exceeding permissible limits. Spectroscopic analysis confirmed PTE adsorption onto MP surfaces suggesting a synergistic contamination mechanism. Risk assessment based on incremental lifetime cancer risk showed that 33% of samples posed potential risks to children and none for adults. Non-carcinogenic risks via oral intake were observed in 76% of samples for children and 24% for adults. Sites were categorized into four pollution-based clusters using hierarchical clustering. The PTE index rated 70% of samples as excellent, 25% as poor to very poor and 5% as unsuitable for drinking. These findings underscore the importance of integrated pollution control, reduced plastic usage and improved waste management strategies. Further research is needed to explore the long-term fate of MPs and their role in contaminant transportation under varying environmental conditions.</p> Graphical Abstract <p></p>