Site selection for productive wells in groundwater engineering:\r A well is generally considered “good” (high-yielding and reliable) when it is sunk into which type of water-bearing formation?

Introduction / Context:
A “good” well in water resources engineering is one that delivers high discharge with limited drawdown and minimal problems such as turbidity, clogging, or excessive fine migration. The hydrogeologic nature of the aquifer material determines yield, water quality, and long-term performance. This item tests understanding of which sediment type provides the most favorable conditions for a high-yield well.

Given Data / Assumptions:

• The well is a properly designed and developed tube well or dug-cum-bore well.
• Formation types considered: clay, sand, coarse gravel, silt, and hard rock with joints.
• Objective is sustainable yield with reasonable drawdown and good water quality.

Concept / Approach:
Specific capacity of wells is controlled by hydraulic conductivity and effective grain size. Coarse gravels possess very high permeability and transmit water readily, leading to large yields for small drawdowns. Conversely, clays and silts have low permeability, causing poor yield and severe well-loss issues. Clean sands are acceptable but commonly yield less than well-sorted gravels of similar thickness.

Step-by-Step Solution:
Identify formation with highest hydraulic conductivity: coarse gravel.Relate high conductivity to high specific capacity (Q/s), enabling “good” well performance.Eliminate formations with fine grains (clay, silt) due to low permeability and clogging risk.Recognize sand as acceptable but generally lower-yield than well-sorted coarse gravel.

Verification / Alternative check:
Empirical well design practice favors screened intakes opposite clean, well-graded sands and gravels; development tests consistently show superior yields in coarse gravels at comparable heads.

Why Other Options Are Wrong:

• Clay and silt: very low permeability; wells are poor-yielding and prone to turbidity.
• Sand: better than clay/silt, but typically less productive than coarse gravel.
• Hard rock with joints: yields depend on fracture connectivity and are highly variable.

Common Pitfalls:

• Confusing aquifer thickness with productivity; grain size and sorting strongly control yield.
• Ignoring well development; even good formations need proper development to achieve design yield.

Final Answer:
Coarse gravel