Two different meanings of “layer”
Geological or conceptual layers represent hydrogeologic units: sandy aquifers, confining units, aquitards, major hydraulic-property contrasts, or aquifer systems. Computational layers subdivide a conceptual unit to improve numerical accuracy or represent vertical dynamics.
Why conceptual layers are heavier
More conceptual layers usually mean more property sets, more parameters, more storage, more calibration burden, and more conceptual complexity. They should be added when geology changes, not merely because vertical resolution is desired.
Why computational layers are lighter
Computational layers can share the same hydrogeologic properties while increasing vertical resolution. A deep aquifer of similar material may remain one geological layer while being subdivided computationally to recover vertical gradients, head variation, and transport structure.
Transport needs vertical detail
Contaminant plumes usually do not fill an aquifer uniformly. Even in a sandy aquifer treated as one geological layer, concentrations can vary strongly with depth. A plume may enter near the water table, sink, stratify, or respond to pumping and recharge in three dimensions.
If thick cells are used, concentrations are averaged across large cell volumes. That creates numerical dilution horizontally and vertically and can smear plume structure. Transport modeling often needs many computational layers even when the geology is conceptually simple.
Practical rule
Add geological layers when geology changes. Add computational layers when numerical detail changes.