Effect of increasing grain size on aquifer properties and well yield When the representative grain size of a granular soil increases (with similar packing), how do surface area, specific retention, specific yield, and expected well yield change?

Introduction / Context:
Grain-size characteristics strongly influence groundwater storage and transmission. Coarser soils like sands and gravels typically offer higher permeability and different storage components than fine-textured silts and clays. Understanding these trends helps in siting wells and predicting aquifer performance.

Given Data / Assumptions:

• Grains are assumed mineralogically similar; only size changes.
• Packing and sorting are broadly comparable between the two soils.
• Hydraulic gradient and boundary conditions for yield comparison are similar.

Concept / Approach:

As grain size increases, the specific surface area per unit volume decreases, reducing adsorptive water held by capillarity. Consequently, specific retention (water retained against gravity) decreases, while specific yield (drainable water under gravity) increases. Larger pores in coarse media also raise hydraulic conductivity, which supports higher well discharge for a given drawdown.

Step-by-Step Solution:

Verification / Alternative check:

Empirical relations (e.g., Hazen, Kozeny–Carman) show K increases with effective grain size and decreases with tortuosity and specific surface, consistent with the stated trends.

Why Other Options Are Wrong:

• Options (a)–(d) each state a correct partial effect; omitting any would be incomplete. The comprehensive answer is therefore “All the above.”

Common Pitfalls:

• Confusing porosity (which may not change dramatically with size) with permeability (which is highly size-dependent).
• Assuming clayey soils yield more water because of higher total water content; much of it is retained and not drainable.

Final Answer:

All the above.