Microplastics as Emerging Cotracers in Groundwater Quality Assessments

This study evaluates the feasibility of using microplastics (MPs) as cotracers within multitracer hydrogeological frameworks, alongside established tools such as water-stable isotopes, major and trace elements, and chemicals of emerging concern. Given that plastic production expanded globally only after the 1950s, MPs can serve as event markers delimiting recent recharge episodes or, when detected in older groundwater, as indicators of mixing or artificial recharge. MP properties including particle size, polymer type, shape, and density carry information about particulate transport dynamics and anthropogenic influence that dissolved tracers cannot provide. The utility of MPs as cotracers is particularly relevant in young groundwater systems directly connected to surface environments. Key limitations include contamination risks, the influence of artificial recharge on signal interpretation, and the cost and complexity of MP characterization. Integrating MP with hydrochemical and isotopic data sets, developing context-specific sampling protocols, and prioritizing open data sharing would advance this emerging methodological approach.