Characterizing particle size in agricultural runoff to inform stormwater basin design.

Upper US Midwest sediment-basin design commonly relies on parent-soil texture classes and texture-based settling-velocity defaults (Natural Resources Conservation Service Conservation Practice Standard Code 350), yet conservation-managed fields can export fine particles that bypass gravity settling. Runoff from 13 edge-of-field watersheds in Wisconsin and Indiana showed systematic divergence between particle size distribution representations: volume-weighted (mass-based) D50 ≈ 30 µm (coarser classification) versus number-weighted (count-based) D50 ≈ 3.5 µm (clay-rich suspension). A mixed-effects framework identified seasonality and water chemistry as stronger predictors than agronomy, including winter coarsening (β ≈ +4.35, p < 0.001), consistent with low-density organic flocs inflating apparent size without proportional settling benefit. Nutrient partitioning diverged by fraction, with total phosphorus yield decreasing with D50 (p < 0.001) while total nitrogen yield increased with D50 (p = 0.014), supporting phosphorus coupling to fine mineral particles and nitrogen association with larger organic aggregates. Fine-particle dominance limits gravity settling in sedimentation basins, so substantial reductions in suspended sediment and turbidity for agricultural runoff typically require coagulation/flocculation in addition to hydraulic detention.