Question

Which aquifer property is associated with specific yield of the aquifer?

• A. Porosity
• B. Permeability
• C. Storativity
• D. Transmissivity

Hint:

• Porosity ($n$): Total void space in a material.
• Specific Yield (S$_y$): Fraction of water in a saturated material that drains by gravity.
• Specific Retention (S$_r$): Fraction of water retained against gravity.
• Relationship: Porosity ($n$) = Specific Yield (S$_y$) + Specific Retention (S$_r$).
• Therefore, specific yield is directly related to and a component of porosity.
• Storativity of an unconfined aquifer is approximately equal to its specific yield.

Answer 1

The Correct Option is A

Aquifer properties describe its ability to store and transmit groundwater.

• Porosity ($n$): The ratio of the volume of voids (pore spaces) in a rock or soil to its total volume. It represents the total water storage capacity of the material when saturated.
• Specific Yield (S$_y$): For an unconfined aquifer, specific yield is the ratio of the volume of water that will drain by gravity from a saturated rock or soil to the total volume of the rock or soil. It represents the portion of water in the pores that can actually be extracted by pumping or drainage.
• Specific Retention (S$_r$): The ratio of the volume of water that a saturated rock or soil will retain against the pull of gravity (held by capillary forces) to its total volume.

The relationship between these three is: Porosity ($n$) = Specific Yield (S$_y$) + Specific Retention (S$_r$) This means that specific yield is a component of the total porosity. The water stored in the pores (porosity) is divided into water that can drain out (specific yield) and water that is retained (specific retention). Therefore, porosity is directly associated with specific yield. Other options:

• Permeability (Hydraulic Conductivity, K) (option b): A measure of the ability of a porous material to transmit fluids (water). While a material must be permeable for water to drain (and thus have a specific yield), specific yield itself is a measure of storage (drainable water volume), which is a fraction of porosity.
• Storativity (Storage Coefficient, S) (option c): The volume of water that an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in hydraulic head. For an unconfined aquifer, storativity is approximately equal to its specific yield. For a confined aquifer, it's much smaller and related to the compressibility of the aquifer material and water. So, S$_y$ is related to S for unconfined aquifers, but the fundamental property S$_y$ is derived from is porosity.
• Transmissivity (T) (option d): A measure of how much water can be transmitted horizontally by the full saturated thickness of an aquifer. T = K $\times$ b (where K is hydraulic conductivity and b is saturated thickness). It relates to flow, not directly storage in the sense of specific yield as a fraction of total void space.

Specific yield is fundamentally a part of the total porosity. \[ \boxed{\text{Porosity}} \]