Characterizing particle size in agricultural runoff to inform stormwater basin design

Abstract

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) D ₅₀ ≈ 30 µm (coarser classification) versus number‐weighted (count‐based) D ₅₀ ≈ 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 D ₅₀ ( p  < 0.001) while total nitrogen yield increased with D ₅₀ ( 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.