Groundwater hydraulics: In well hydraulics, the “radius of influence” is defined as which of the following?

Introduction / Context:
During pumping, a cone of depression develops in the potentiometric surface (or water table). The lateral extent of measurable drawdown defines the effective impact zone of the well, a key parameter in spacing wells and in analyzing interference between wells.

Given Data / Assumptions:

• Confined or unconfined aquifer with a pumped well at steady or quasi-steady conditions.
• Drawdown s(r) decreases radially and tends to zero at some large distance.
• Definition focuses purely on geometry of drawdown.

Concept / Approach:
The radius of influence R is the radial distance from the well centre to the point in the aquifer where drawdown becomes negligible (approaches zero). Analytical solutions (e.g., Thiem, Theis) and field tests (pumping tests) are used to estimate R for design and management decisions.

Step-by-Step Clarification:
Consider drawdown as a function of radius r from well centre.Find r where s(r) → 0 (practical cut-off based on field sensitivity).Define that r as the radius of influence.

Verification / Alternative check:
In design, R helps determine whether neighbouring wells will hydraulically interfere during simultaneous pumping and informs the size of the capture zone.

Why Other Options Are Wrong:

• Well wall distance (option b) misses the centre reference used in radial flow solutions.
• Casing radius and gravel-pack radius are construction dimensions, not drawdown reach.
• Saturated thickness is a vertical dimension, not lateral reach of drawdown.

Common Pitfalls:

• Confusing construction geometry with hydraulic influence radius.
• Assuming a fixed R; in reality, it varies with transmissivity, storativity, and pumping rate/time.

Final Answer:
The distance from the well centre to the point of zero drawdown