Aquifiers: Types and Properties | Geology Optional Notes for UPSC PDF Download

Geology: Understanding Aquifer Properties

Introduction

• An aquifer is a geological formation capable of storing and transmitting groundwater economically.
• Good aquifers, like non-indurated sand and gravels, have interconnected pores allowing for water storage and transmission.
• Aquicludes, such as clay, can store water but lack interconnected pores for economic water flow.
• Aquifuges, like granite and diorite, have minimal interconnected pores, hindering water storage and transmission.
• Aquitards, such as clayey silt, have limited interconnected pores, restricting water transmission to specific directions.

Properties Affecting Water Storage

• Specific Yield: The amount of water an aquifer releases under the influence of gravity.
• Storage Coefficient: The volume of water released from storage per unit decline in hydraulic head.
• Specific Storage: The amount of water an aquifer releases from storage per unit volume of the aquifer.

Properties Affecting Water Transmission

• Hydraulic Conductivity: The ability of an aquifer to transmit water.
• Transmissivity: The rate at which water flows through a unit width under a unit hydraulic gradient.

Geological Formations and Water Bearing Capacities

• Not all geological formations possess the same water-bearing and transmitting capabilities.
• Regions typically consist of a variety of rocks and formations, making pure aquifer occurrences rare.
• For substantial thicknesses of geological formations, aquifers can exhibit properties of both storage and transmission of water.

Types of Aquifer

• On the basis of geological settings and distinct hydrological regime, we have mainly four types of aquifer: unconfined, confined, semi-confined, and perched aquifer.

Unconfined Aquifer

• An unconfined aquifer is characterized by a single aquifer body from the ground surface that is underlain by an impervious layer, which can be either an aquiclude or aquifuge.
• In this type of aquifer, recharge happens locally through rainfall. Rainwater infiltrates and percolates through interconnected pores, replenishing the groundwater.
• The aquifer system is open to the surface, allowing direct contact between air and water through interconnected pores reaching the water table.

Schematic Drawing of an Unconfined Aquifer

• A schematic drawing of an unconfined aquifer depicts a three-layer system, with the top two layers being the aquifer underlain by an impervious layer.
• The hydrostatic pressure indicated by the water table coincides with the top of the saturation zone.

Reference: CGWB (1982), Kruesman & De Ridder (1990).

Paper: Hydrogeology and Engineering Geology

Module: Aquifer Properties

Confined Aquifer

• Definition: A confined aquifer is a type of aquifer that is sandwiched between impervious layers both above and below.
• Recharge: This aquifer does not directly receive local recharge from rainfall but gets recharged through the lateral flow of groundwater from adjacent areas where the aquifer is exposed to the surface.
• Pressure Dynamics: In a confined aquifer, the groundwater pressure is greater than atmospheric pressure, leading to the formation of an imaginary piezometric surface that represents the hydrostatic pressure.
• Characteristics: The top of the saturation zone aligns with the top of the confined aquifer, and the piezometric surface typically lies above both the top of the aquifer and the bottom of the upper confining layer.
• Overexploitation Effects: Overexploitation of groundwater from a confined aquifer can cause the piezometric surface to decline below the top confining layer, resulting in pressure dynamics similar to that of an unconfined aquifer.

Illustrative Examples:

• Imagine a confined aquifer as a layer of sponge between two impermeable layers of plastic. When you pour water on the sponge, it does not absorb directly but gets soaked through the sides.
• Visualize a balloon squeezed tightly between two boards. The pressure inside the balloon is like the pressure in a confined aquifer, higher than the pressure outside.

Aquifer Properties: Understanding Confined and Semi-Confined Aquifers

Artesian Wells and Recharge Areas

• An artesian well is a type of well from which water naturally flows out due to pressure in a confined aquifer.
• Artesian wells are typically found in areas where the aquifer is confined by impervious layers, allowing water to rise to the surface without pumping.
• The area where water enters the aquifer is known as the recharge area, where water percolates through the ground to replenish the aquifer.

Confined Aquifers

• A confined aquifer is a geological formation that is bounded above and below by impervious layers, preventing water from easily entering or leaving.
• Water in confined aquifers is under pressure, leading to the creation of artesian wells that flow without the need for pumping.
• The piezometric surface represents the level to which water can rise in a well drilled into a confined aquifer.

Semi-Confined Aquifers

• A semi-confined aquifer is partially bounded by semi-pervious layers that allow for some water movement.
• These aquifers are sometimes referred to as leaky confined aquifers due to the presence of semi-pervious materials.
• Water movement in semi-confined aquifers occurs through the semi-pervious layers, which act as conduits for water flow.

Characteristics of Semi-Confined Aquifers

• In a three-layer system, the top layer of a semi-confined aquifer is semi-pervious, while the bottom layer is impervious.
• Water movement in semi-confined aquifers primarily occurs through the semi-pervious top layers, as illustrated in Figure 4.

Illustrative Examples

• Imagine a sponge sandwiched between two plastic sheets. The sponge represents the semi-confined aquifer, while the plastic sheets act as the semi-pervious and impervious layers.
• Visualize a water balloon with tiny holes at the top. The balloon represents the confined aquifer, and the holes simulate the semi-pervious nature of a semi-confined aquifer.

Aquifer Properties

• Definition of Aquifer: An aquifer is a geological formation capable of storing and transmitting groundwater in significant quantities, enabling the extraction of water.
• Hydrogeology and Engineering Geology: Aquifer properties are crucial topics covered in the study of hydrogeology and engineering geology, focusing on the characteristics and behavior of aquifers.
• Storage and Transmission: Aquifers have the dual capacity to store water and facilitate its movement, ensuring a sustainable supply of groundwater.
• Quantitative Considerations: Aquifer properties are essential for quantifying the volume of water that can be stored and transmitted through these geological formations.
• Geological Significance: Understanding aquifer properties is key to assessing the geological formations that can serve as reliable sources of groundwater.
• Example: Imagine an aquifer as a large, underground sponge that can hold and release water, providing a vital resource for human consumption and agricultural activities.

Perched Aquifer

• Definition: A perched aquifer is characterized by a localized saturation above the main water table, typically caused by clay layers hindering vertical water movement.
• Formation: Perched aquifers often result from clay layers creating barriers that trap water at shallow depths, distinct from the primary saturation zone.
• Utilization: Shallow tube wells are sometimes used to access water from perched aquifers, though it is preferable to tap groundwater below the primary water table for sustainability.
• Illustration: Fig. 5 depicts a schematic drawing of a perched aquifer, highlighting the concept of localized saturation in the vadose zone.

Aquifer Properties: Understanding Porosity

Introduction to Aquifer Properties

• An aquifer is a geological formation that can store and transmit groundwater.
• Properties of aquifers play a crucial role in the movement and availability of groundwater resources.

Porosity: The Key Property of Aquifers

• Porosity is a measure of the void spaces in a formation.
• It is expressed as the ratio of the volume of voids to the total volume of the rock or formation.
• Porosity is usually expressed as a percentage.
• Porosity is dimensionless, represented as the ratio of two volumes.

Primary Porosity

• Primary porosity is formed during the genesis of the rock.
• For example, the porosity of loose sand is a primary porosity type.

Secondary Porosity

• Secondary porosity is formed after the genesis of the rock.
• For instance, porosity induced by fractures in sandstone or limestone is a secondary porosity type.

Examples of Porosity Types

• Porosity of sand and clay represents primary porosity.
• Porosity resulting from fractures in rocks like sandstone and limestone demonstrates secondary porosity.

Conclusion

• Understanding porosity is essential in evaluating the water storage and flow characteristics of aquifers.
• Primary and secondary porosity types have distinct origins and implications for groundwater movement.

Primary and Secondary Porosity

• Primary porosity depends on factors like sorting, packing, grain shape, and grain fabric.
• Secondary porosity is influenced by fracturing intensity and rock dissolution.
• Porosity isn't determined by grain size if all grains are spherical.
• Increasing grain size without changing sorting and packing doesn't affect porosity.

Effective Porosity

• Effective porosity is the portion of porosity available for fluid flow in a formation.
• It is calculated as the ratio of interconnected pore space to the total volume of the rock.
• Expressed typically as a percentage.
• Equation-2 for effective porosity is dimensionless.

Specific Yield

• Specific yield is the volume of water drained under gravity divided by the total rock volume.
• Measured as the water volume yielded post-saturation under the influence of gravity.
• Equation-3 represents the calculation for specific yield.

Hydrogeology and Aquifer Properties

Specific Yield and Specific Retention

• Specific Yield: Specific yield is the volume of water that an aquifer releases under the influence of gravity. It is dimensionless and represents the water volume required to saturate an aquifer by flow under gravity.
• Specific Retention: Specific retention is the volume of water retained by a rock or formation against gravity. It is also dimensionless and indicates the retained water volume in relation to the total volume of the rock.
• Porosity and Total Voids: When specific yield is added to specific retention, it equals the porosity of the formation. This sum represents the total voids or pores in the rock or formation.

Storage Coefficient and Storativity

• Storage Coefficient: The storage coefficient assesses the storing capacity of an aquifer. It quantifies the volume of water either absorbed or released by the aquifer per unit surface area per unit change in hydraulic head.

Aquifer Properties

Storage Coefficient (Sc)

• Storage coefficient (Sc) is represented by the formula: Sc = Vw * A * ∆h.
• Here, Sc stands for the storage coefficient, Vw for the volume of water in or out of the aquifer, A for the surface area of the aquifer, and ∆h for the change in hydraulic head.
• Notably, the storage coefficient is dimensionless, meaning it does not have a unit.
• In confined aquifers, the storage coefficient is also known as Storativity.

Specific Storage (Ss)

• Specific storage (Ss) is defined as the volume of water released per unit volume of the aquifer with a unit decline in hydraulic head.
• The formula for specific storage is: Ss = Vw * ∆h.
• Here, Ss represents specific storage, Vw denotes the volume of water in or out of the aquifer, and ∆h indicates the change in hydraulic head.
• Specific storage has units of per meter and dimension L⁻¹.

Relationship between Storage Coefficient and Specific Storage

• For confined aquifers, the relationship between Storativity (S) and Specific Storage (Ss) is given by the equation: S = Ss * b.
• Here, S is the storage coefficient, Ss is the specific storage, and b represents the saturated thickness of the aquifer.

Example: Imagine an aquifer with a surface area of 100 square meters, a volume change of 50 cubic meters, and a hydraulic head change of 10 meters. Calculate the storage coefficient.

Solution: Using the formula Sc = Vw * A * ∆h, we get Sc = 50 * 100 * 10 = 50,000. Therefore, the storage coefficient for this aquifer is 50,000.

Understanding Hydraulic Conductivity and Intrinsic Permeability

Hydraulic Conductivity (K)

• Hydraulic conductivity refers to the aquifer's ability to transmit water.
• It is defined as the volume rate of water with a given kinematic viscosity moving through a unit cross-sectional area per unit hydraulic gradient.
• The cross-sectional area mentioned is perpendicular to the direction of groundwater flow.
• Equation-9: K = Q / (A * ∆h)
• K: Hydraulic conductivity
• Q: Volume rate of water
• A: Cross-sectional area
• ∆h: Hydraulic gradient

Intrinsic Permeability (k)

• Intrinsic permeability is a fundamental property of the aquifer that determines its ability to transmit any fluid through it.
• It is a function of the porous media and the fluid passing through it.
• Equation-9 shows that the unit of k is in m/day with dimensions of LT^-1.
• k = C x d^2
• C is a constant dependent on factors like the distribution of grain size and roundness of grains.
• d is the diameter of the grains.
• The dimension of intrinsic permeability is L^2, with the unit commonly used being "Darcy," where 1 Darcy ≈ 10^-8 cm^2.

Relationship between Hydraulic Conductivity and Intrinsic Permeability

• Equation-11 helps us understand the connection between hydraulic conductivity and intrinsic permeability.
• Hydraulic conductivity is a function of the porous media and the fluid, while intrinsic permeability is a property of the aquifer itself.

By grasping the concepts of hydraulic conductivity and intrinsic permeability, we can better understand how water moves through aquifers and the properties that influence this movement.

Key Concepts:

• Hydraulic Conductivity (K):
• Intrinsic Permeability (k):
• Density (ρ):
• Acceleration due to Gravity (g):
• Kinematic Viscosity (ν):

Transmissivity (T):

• Definition:
• Transmissivity refers to an aquifer's ability to transmit water.
• It is the volume rate of water conducted under the influence of a unit hydraulic gradient through a unit saturated width of the aquifer.
• Transmissivity Formula: T = k × b
• Unit of Transmissivity: m²/day with dimensions of L² T⁻¹.

Relationship between Hydraulic Conductivity and Transmissivity:

• Hydraulic conductivity and transmissivity are fundamental aquifer parameters related by specific equations.
• By dividing the equation for hydraulic conductivity by the equation for transmissivity, the relationship can be established.
• This relationship helps understand how groundwater is transmitted through an aquifer.

Key Concepts in Thermodynamics

• Kinetic Energy (K): Energy possessed by an object due to its motion.
• Temperature (T): Measure of the average kinetic energy of particles in a substance.
• Heat (Q): Energy transferred between substances due to temperature differences.
• Enthalpy (Ah): Total heat content of a system at constant pressure.
• Entropy (S): Measure of the disorder or randomness of a system.
• Gibbs Free Energy (G):
  • Represents the maximum potential energy of a system available to do useful work.

Examples

• Kinetic Energy (K):
  • Example: A moving car possesses kinetic energy due to its motion.
• Temperature (T):
  • Example: When water is heated, its temperature increases due to the energy input.
• Heat (Q):
  • Example: Cooking food on a stove involves the transfer of heat from the stove to the pan.
• Enthalpy (Ah):
  • Example: When ice melts into water, enthalpy is involved in the phase change process.
• Entropy (S):
  • Example: Mixing different gases together increases entropy due to increased disorder.
• Gibbs Free Energy (G):
  • Example: A battery converts chemical energy into electrical energy, demonstrating Gibbs free energy.