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 (aquiclude or aquifuge).
• Recharge occurs locally through rainfall, infiltrating and percolating through interconnected pores.
• The aquifer system is open to the surface, allowing direct contact between air and water.

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.

Confined Aquifer

• Definition: A confined aquifer is sandwiched between impervious layers both above and below.
• Recharge: Does not directly receive local recharge from rainfall but gets recharged through lateral flow from adjacent exposed areas.
• Pressure Dynamics: Groundwater pressure is greater than atmospheric pressure, creating a piezometric surface.
• Characteristics: The top of the saturation zone aligns with the top of the confined aquifer.
• Overexploitation Effects: Overexploitation causes piezometric surface decline, resembling unconfined aquifer pressure dynamics.

Illustrative Examples

• A confined aquifer as a sponge layer between two impermeable layers of plastic.
• Visualize a balloon squeezed between two boards, with internal pressure higher than the external pressure.

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Aquifer Properties: Understanding Confined and Semi-Confined Aquifers

Artesian Wells and Recharge Areas

• Artesian wells flow naturally due to pressure in a confined aquifer, requiring no pumping.
• The recharge area is where water enters the aquifer, replenishing groundwater supplies.

Confined Aquifers

• Water in confined aquifers is under pressure, leading to artesian wells.
• The piezometric surface represents the level to which water rises in a confined aquifer well.

Semi-Confined Aquifers

• Partially bounded by semi-pervious layers, allowing some water movement.

• Referred to as leaky confined aquifers due to semi-pervious materials.

• Water movement occurs primarily through the semi-pervious top layers.

Illustrative Examples

• A sponge between two plastic sheets represents a semi-confined aquifer.
• A water balloon with small holes at the top simulates semi-confined aquifer characteristics.

Aquifer Properties

• Definition of Aquifer: An aquifer is a geological formation capable of storing and transmitting groundwater in significant quantities.
• Hydrogeology and Engineering Geology: Aquifer properties are crucial in hydrogeology and engineering geology, focusing on the characteristics and behavior of aquifers.
• Storage and Transmission: Aquifers store and transmit water, ensuring a sustainable groundwater supply.
• Quantitative Considerations: Aquifer properties are essential for quantifying water storage and transmission.
• Geological Significance: Understanding aquifer properties is key for assessing geological formations as reliable groundwater sources.
• Example: An aquifer as a large underground sponge that holds and releases water for human consumption and agriculture.

Perched Aquifer

• Definition: A perched aquifer is a localized saturation above the main water table, caused by clay layers hindering vertical water movement.
• Formation: Results from clay layers creating barriers that trap water at shallow depths.
• Utilization: Shallow tube wells access water, though tapping below the primary water table is preferable for sustainability.
• Illustration: A schematic drawing depicting 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 measures the void spaces in a formation.
• Expressed as the ratio of the volume of voids to the total volume of the rock.
• Usually expressed as a percentage and is dimensionless.

Primary Porosity

• Formed during the rock's genesis (e.g., porosity of loose sand).

Secondary Porosity

• Formed after the rock's genesis (e.g., porosity due to fractures in sandstone or limestone).

Examples of Porosity Types

• Porosity of sand and clay represents primary porosity.
• Fracture-induced porosity in sandstone and limestone represents secondary porosity.

Conclusion
Understanding porosity is essential for evaluating water storage and flow characteristics of aquifers.

Primary and Secondary Porosity

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

Effective Porosity

• Effective porosity is the portion of porosity available for fluid flow in a formation.
• Calculated as the ratio of interconnected pore space to the total volume of the rock.

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.

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Hydrogeology and Aquifer Properties

Specific Yield and Specific Retention

• Specific Yield: Volume of water an aquifer releases under gravity.
• Specific Retention: Volume of water retained by a rock formation against gravity.
• Porosity and Total Voids: Specific yield + specific retention equals porosity.

Storage Coefficient and Storativity

• Storage Coefficient: Assesses an aquifer's storage capacity. Quantifies water absorption/release per unit area per hydraulic head change.

Aquifer Properties

Storage Coefficient (Sc):
Formula:
Sc = Vw * A * ∆h

• Sc = Storage coefficient, Vw = volume of water, A = surface area, ∆h = change in hydraulic head.
• Storage coefficient is dimensionless.

Specific Storage (Ss):

Formula:
Ss = Vw * ∆h

• Ss = Specific storage, Vw = volume of water, ∆h = change in hydraulic head.

Relationship between Storage Coefficient and Specific Storage
Formula:
S = Ss * b (b = saturated thickness of aquifer).

• Example
  Given: Surface area = 100 m², Volume change = 50 m³, Hydraulic head change = 10 m.
  Solution: Sc = 50 * 100 * 10 = 50,000.
  Storage coefficient = 50,000.

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Understanding Hydraulic Conductivity and Intrinsic Permeability

Hydraulic Conductivity (K)

• Refers to the aquifer's ability to transmit water.
• Formula:
  K = Q / (A * ∆h)
  • Q = Volume rate of water, A = Cross-sectional area, ∆h = Hydraulic gradient.

Intrinsic Permeability (k)

• A fundamental property of the aquifer determining its ability to transmit any fluid.
• Formula:
  k = C x d² (C = constant, d = grain diameter).

Key Concepts

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

Transmissivity

• Definition: Refers to an aquifer's ability to transmit water.
• Formula:
  T = k × b
  • T = Transmissivity, k = Hydraulic conductivity, b = Aquifer thickness.
• Unit: m²/day.

Relationship between Hydraulic Conductivity and Transmissivity

• Hydraulic conductivity and transmissivity are fundamental aquifer parameters related by specific equations.

Key Concepts in Thermodynamics

• Kinetic Energy (K): Energy due to motion.
• Temperature (T): Measure of average kinetic energy of particles.
• Heat (Q): Energy transferred due to temperature differences.
• Enthalpy (Ah): Total heat content at constant pressure.
• Entropy (S): Measure of disorder.
• Gibbs Free Energy (G): Maximum potential energy available for useful work.

Examples

• Kinetic Energy (K): A moving car possesses kinetic energy due to its motion.
• Temperature (T): Water temperature increases due to energy input.
• Heat (Q): Cooking food involves transferring heat.
• Enthalpy (Ah): Ice melting into water involves enthalpy.
• Entropy (S): Mixing gases increases entropy.
• Gibbs Free Energy (G): A battery converts chemical energy into electrical energy.