Suspended solids can advect into an External pathway from other pathways containing suspended solids. Suspended solids can also diffuse into an External pathway from a Cell or Aquifer containing suspended solids. When suspended solids (carrying species mass) enter an External pathway from another pathway, conceptually, the particulates are assumed to immediately disintegrate upon entering the pathway (releasing their species mass to the fluid in the pathway).
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Advection of Particulates to/from Cells