Question

Measurement of specific yield of an aquifer in field is done by

• A. Slug tests
• B. Grain size analysis
• C. Drain tests
• D. Pumping tests

Hint:

• Pumping tests are field procedures where a well is pumped and water level changes (drawdown) are monitored.
• Analysis of pumping test data allows for the estimation of aquifer properties like transmissivity and storativity.
• For unconfined aquifers, the storativity determined from a pumping test (especially after delayed yield effects) is taken as the specific yield.
• Slug tests are more for local hydraulic conductivity. Grain size analysis is a lab method for estimation.

Answer 1

The Correct Option is D

Specific yield (S$_y$) is the ratio of the volume of water that drains by gravity from a saturated unconfined aquifer to the total volume of the dewatered aquifer material. It is a key parameter for estimating groundwater availability. Field methods for determining specific yield often involve observing water level changes in response to known volumes of water extraction or recharge.

• Pumping Tests (option d): This is a common and effective field method. A well is pumped at a known constant rate for a period, and the drawdown (decline in water level) is observed in the pumping well and/or nearby observation wells.
  • By analyzing the time-drawdown data using appropriate analytical solutions (e.g., Theis equation for confined aquifers, Hantush-Jacob for leaky-confined, or solutions for unconfined aquifers like Neuman or Boulton, which account for delayed yield), aquifer parameters including transmissivity (T) and storativity (S) can be estimated.
  • For unconfined aquifers, the storativity (S) determined from a pumping test over a sufficiently long duration (after delayed yield effects subside) is essentially equal to the specific yield (S$_y$).
  • The volume of the cone of depression created by pumping can also be related to the volume of water pumped and the specific yield.
• Slug Tests (option a): A slug test involves rapidly changing the water level in a single well (by adding or removing a known volume or "slug" of water) and observing the rate at which the water level returns to its initial position. Slug tests are primarily used to determine the hydraulic conductivity (K) or transmissivity (T) in the immediate vicinity of the well. They are less commonly used for directly determining specific yield over a large aquifer volume, although some methods exist for unconfined aquifers.
• Grain Size Analysis (option b): This is a laboratory method where the particle size distribution of an aquifer sample is determined. Empirical relationships exist to estimate porosity and, less directly, specific yield from grain size data, but this is an indirect estimation, not a direct field measurement of how the aquifer behaves.
• Drain Tests (option c): This term is less standard in this specific context. Field drainage experiments or lysimeter studies could give insights into drainable porosity, but "pumping tests" is the more established field method for aquifer-scale specific yield. If it refers to monitoring drainage from a dewatered area, it's conceptually similar to observing the effects of pumping.

Therefore, pumping tests are a primary field method for determining the specific yield of an unconfined aquifer. \[ \boxed{\text{Pumping tests}} \]