How size-dependent settling can both bias and inform microplastics observations

TY - JOUR

T1 - How size-dependent settling can both bias and inform microplastics observations

AU - Hoffman, Matthew J.

AU - DiBenedetto, Michelle H.

N1 - Publisher Copyright: © 2026 The Author(s). Published by IOP Publishing Ltd. Original content from this work may be used under the terms of the https://creativecommons.org/licenses/by/4.0/. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

PY - 2026/6

Y1 - 2026/6

N2 - Particle settling and rise velocities play a central role in modeling microplastic (MP) transport, residence times, and distributions in aquatic systems. Recent studies have focused on deriving accurate models for MP settling velocities, but the detailed particle-shape information required is usually unavailable in real-world data. Here, we investigate the relative importance of settling velocity model choice versus particle size and shape information using a previously published laboratory dataset of measured settling velocities for irregular MP fragments. We find that uncertainty in particle geometry dominates model error, while differences between commonly used settling velocity models are comparatively small. Settling and rise velocities vary widely across realistic MP sizes and shapes, which leads to orders-of-magnitude differences in residence times. We show how this differential settling can alter observed particle size distributions: for example, even when the underlying fragmentation spectrum follows a power law, we find separate power-law exponents for large and small particles due to differential vertical transport. Our results imply that improving geometric characterization of particles is more critical for modeling vertical transport than further refinement of settling formulas, and that commonly used power-law extrapolations across size classes may be physically inconsistent. These findings have direct implications for interpreting MP observations and for the harmonization and risk-assessment frameworks that rely on size-distribution corrections.

AB - Particle settling and rise velocities play a central role in modeling microplastic (MP) transport, residence times, and distributions in aquatic systems. Recent studies have focused on deriving accurate models for MP settling velocities, but the detailed particle-shape information required is usually unavailable in real-world data. Here, we investigate the relative importance of settling velocity model choice versus particle size and shape information using a previously published laboratory dataset of measured settling velocities for irregular MP fragments. We find that uncertainty in particle geometry dominates model error, while differences between commonly used settling velocity models are comparatively small. Settling and rise velocities vary widely across realistic MP sizes and shapes, which leads to orders-of-magnitude differences in residence times. We show how this differential settling can alter observed particle size distributions: for example, even when the underlying fragmentation spectrum follows a power law, we find separate power-law exponents for large and small particles due to differential vertical transport. Our results imply that improving geometric characterization of particles is more critical for modeling vertical transport than further refinement of settling formulas, and that commonly used power-law extrapolations across size classes may be physically inconsistent. These findings have direct implications for interpreting MP observations and for the harmonization and risk-assessment frameworks that rely on size-distribution corrections.

KW - particle size distributions

KW - plastic pollution

KW - rise velocity

KW - risk assessment

KW - settling velocity

UR - https://www.scopus.com/pages/publications/105043518522

UR - https://www.scopus.com/pages/publications/105043518522#tab=citedBy

U2 - 10.1088/2515-7620/ae76df

DO - 10.1088/2515-7620/ae76df

M3 - Article

SN - 2515-7620

VL - 8

JO - Environmental Research Communications

JF - Environmental Research Communications

IS - 6

M1 - 065061

ER -