Background

Microplastic (MP) pollution has garnered global attention due to its ubiquity in marine and freshwater systems, as well as its potential—though still uncertain—risks to human health. While MP concentrations in drinking water remain relatively low, safeguarding reservoir-based drinking water supplies against potential contamination remains a pressing concern. In this study, we applied a rigorously validated, two-dimensional hydrodynamic model (CE-QUAL-W2) to Germany’s largest drinking water reservoir, the Rappbode Reservoir, to examine MP retention under realistic inflow, meteorological, and operational conditions. Our primary aim was to quantify how varying particle settling velocities (0.1–1.0 m d⁻¹) influence MP transport, sedimentation, and breakthrough to the raw water outlet over a 2-year simulation period.

Results

We demonstrate that reservoir-scale retention efficiency rises sharply with increasing MP settling velocity, with near-complete retention (> 95%) achieved at settling velocities of 0.9 m d⁻¹ or higher. Conversely, slower-sinking particles (≤ 0.3 m d⁻¹) exhibit significant downstream export, indicating that weak sedimentation can negate the reservoir’s inherent trapping capacity even under long residence times (~ 1 year). Furthermore, episodic phenomena such as stratification breakdown or shortcut currents can rapidly redistribute or mobilise MP particles, bypassing much of the reservoir volume and potentially delivering MP particles directly to outflows. These findings highlight the critical roles of both hydrodynamics (stratification, mixing, and lateral transport) and particle-specific traits such as settling velocity in determining MP fate.

Conclusions

By integrating comprehensive field-derived meteorological inputs and a validated numerical framework, this study provides novel insights into MP retention in drinking water reservoirs and underscores the vulnerability of such systems to episodic transport events. Our approach offers a robust tool for reservoir managers and policy-makers to anticipate MP contaminant pathways, optimise withdrawal strategies, and develop early warning systems for drinking water preparation. This work thus advances both the scientific understanding of MP dynamics in lentic systems and supports more informed, adaptive water-resource management.