Question:

Which of the following is not an assumption of Theis equation?

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Theis assumes a fully penetrating well — partial penetration introduces vertical flow, which the model doesn’t account for.
Updated On: Jun 12, 2025
  • constant discharge of water
  • homogeneous and isotropic soil
  • partially penetrating well
  • water released immediately from aquifer storage
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The Correct Option is C

Solution and Explanation

To understand which option violates the assumptions of the Theis equation, let's first examine what the Theis equation represents and its fundamental assumptions:

1. What is the Theis Equation?
The Theis equation is a mathematical model used to analyze groundwater flow to wells in confined aquifers. It helps predict how water levels change when we pump water from wells.

2. Key Assumptions of the Theis Equation:
The equation makes several important assumptions about the aquifer and pumping conditions:

  • Confined aquifer: The aquifer is bounded above and below by impermeable layers
  • Homogeneous and isotropic: The aquifer material has uniform properties in all directions
  • Infinite areal extent: The aquifer extends infinitely in all horizontal directions
  • Fully penetrating well: The well screen extends through the entire aquifer thickness
  • Constant pumping rate: The well is pumped at a steady, unchanging rate
  • Instantaneous release: Water comes from aquifer storage immediately when pressure drops

3. Analyzing the Options:
Now let's examine each option to see which one contradicts these assumptions:

Option A: Constant discharge of water
- This is actually required by the Theis equation - The equation assumes pumping at a steady, unchanging rate

Option B: Homogeneous and isotropic soil
- This is a core assumption of the equation - The aquifer must have uniform properties throughout

Option C: Partially penetrating well
- This violates the Theis assumptions - The equation requires wells to fully penetrate the aquifer - Partial penetration creates vertical flow components the equation can't account for

Option D: Water released immediately from aquifer storage
- This is actually assumed by the equation - Theis model considers instantaneous release when head drops

4. Why Partially Penetrating Well is the Correct Answer:
The Theis solution was specifically developed for fully penetrating wells. When a well only partially penetrates the aquifer: - Flow lines become curved near the well - Vertical flow components develop - The simple radial flow pattern assumed by Theis no longer applies - Special modifications or different equations are needed

Final Answer:
The assumption that is not part of the Theis equation is: partially penetrating well.

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