MEV 012: Unit 12 - Hydrogeology

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 UNIT 12: HYDROGEOLOGY

12.0 Introduction

Hydrogeology is the branch of geology that focuses on the distribution, movement, and quality of groundwater in the Earth's crust. It integrates geological, hydrological, and geochemical knowledge to understand how subsurface water behaves in various rock formations and terrains. Hydrogeology is vital for groundwater resource management, especially in water-scarce regions like India.

12.1 Objectives

After completing this unit, you will be able to:

  • Understand how geology controls groundwater occurrence
  • Explore geomorphological and lithological influences
  • Identify groundwater characteristics in various geological terrains of India
  • Learn about Darcy’s Law and its applications
  • Classify rocks based on their water-bearing capacity
  • Understand groundwater tracing techniques

12.2 Geological Control of Groundwater

Groundwater distribution and movement are profoundly influenced by geological properties.

12.2.1 Petrographic Control

  • Petrography refers to the mineral composition and texture of rocks.
  • Rocks with coarse grains (e.g., sandstone) generally have higher porosity and permeability.
  • Crystalline rocks (e.g., granite) are generally less porous but may store water in fractures.

12.2.2 Control of Porosity

  • Porosity is the percentage of void spaces in a rock that can hold water.
  • Primary porosity: Inherent in the rock’s structure (e.g., sedimentary rocks).
  • Secondary porosity: Due to fractures, joints, or solution cavities (e.g., in limestones or basalts).

12.2.3 Control of Permeability

  • Permeability is the ability of a rock to transmit water through its pores or fractures.
  • Rocks may be porous but impermeable if pores are not interconnected (e.g., clay).

12.2.4 Stratigraphic Control

  • Stratigraphy refers to the layering of rock units.
  • Aquifers often occur in specific stratigraphic horizons, separated by impermeable layers called aquitards or aquitards.
  • Example: Alternating layers of sandstone (aquifer) and shale (aquitard).

12.3 Geomorphological Control

  • Landforms affect groundwater recharge and discharge.
  • Alluvial plains and valleys have better recharge potential.
  • Hilly regions tend to have rapid runoff and less infiltration.
  • Karst topography (limestone terrain) allows water to infiltrate through sinkholes and caves.

12.4 Lithological Control

  • Lithology refers to the physical characteristics of rocks.
  • Sandstones: Good aquifers due to high porosity.
  • Granites and Gneisses: Poor aquifers unless fractured.
  • Limestones: Good water-bearing properties if karstified.
  • Clays and shales: Poor aquifers due to low permeability.

12.5 Mode of Occurrence of Groundwater in Different Geological Terrains of India

12.5.1 The Archaean Crystalline Rocks

  • Found in peninsular India (e.g., Karnataka, Tamil Nadu)
  • Low primary porosity but groundwater found in fractures and weathered zones
  • Bore wells are common in these hard rock areas

12.5.2 Cuddapah Super Group

  • Sedimentary rocks like quartzite, shale, and limestone
  • Groundwater stored in jointed and fractured zones
  • Limestones here show karst features

12.5.3 Vindhyan Super Group

  • Mainly composed of sandstone and shale
  • Sandstones are moderate aquifers
  • Fractures improve groundwater storage

12.5.4 Gondwana Super Group

  • Includes coal-bearing sandstones and shales
  • Found in central India
  • Sandstones act as good aquifers; shales restrict water flow

12.5.5 Deccan Traps

  • Volcanic basaltic rocks covering much of Maharashtra, MP, Gujarat
  • Water occurs in weathered, fractured, and vesicular zones
  • Aquifers are discontinuous but locally productive

12.6 The Tertiary Group (65–1.64 Million Years)

  • Found in the coastal and northeastern regions
  • Includes sandstone, siltstone, clay
  • Coastal aquifers may suffer from saline intrusion
  • Good aquifers in sandy and fractured units

12.7 Recent Super Group

  • Composed of alluvium, sand, silt, clay, and gravels
  • Found in river valleys, floodplains, deltas
  • Highly porous and permeable—excellent aquifers
  • Major source of irrigation and drinking water in northern India

12.8 Classification of Rocks with Reference to Their Water-Bearing Properties

12.8.1 Factors That Determine Water-Bearing Capacity of Rocks

  • Porosity
  • Permeability
  • Degree of weathering
  • Presence of fractures or joints
  • Lithological composition

12.8.2 Water-Bearing Capacity of Different Rock Types

Rock Type

Water-Bearing Character

Sandstone

Good aquifer if well-sorted and consolidated

Limestone (Karst)

Excellent in fractured/solution zones

Basalt

Moderate to good in vesicular and fractured zones

Granite/Gneiss

Poor to moderate (fracture-controlled)

Shale/Clay

Poor (impermeable)

Alluvium

Excellent (high porosity and permeability)

12.9 Darcy’s Law and Its Validity

Darcy’s Law describes the flow of groundwater through porous media:

Q = K × A × (dh/dl)

Where:

  • Q = Discharge (m³/s)
  • K = Hydraulic conductivity (m/s)
  • A = Cross-sectional area (m²)
  • dh/dl = Hydraulic gradient (m/m)

12.9.1 Limitations of Darcy’s Law

  • Applicable only for laminar flow (not turbulent)
  • Valid in homogeneous, isotropic media
  • Fails in fractured media or at very high/low flow velocities

12.9.2 Determination of Hydraulic Conductivity

  • Laboratory Methods: Constant head and falling head permeameters
  • Field Methods: Pumping tests, slug tests
  • Values vary with soil type and rock formation

12.10 Groundwater Tracers

Groundwater tracers are substances introduced or naturally present in water to trace its movement or origin:

  • Artificial Tracers: Fluorescent dyes, salts, radioactive isotopes
  • Natural Tracers: Temperature, stable isotopes, chloride, tritium
  • Applications:
    • Determining flow paths and velocities
    • Estimating recharge zones
    • Tracing contamination sources

12.11 Let Us Sum Up

Hydrogeology examines the interaction between water and geological formations. Groundwater distribution is controlled by lithology, structure, porosity, and permeability. Different geological formations in India—from ancient Archaean rocks to modern alluvium—exhibit diverse groundwater behavior. Tools like Darcy’s Law and groundwater tracers help in evaluating aquifer characteristics and water movement.

12.12 Keywords

·         Hydrogeology-Study of groundwater within geological contexts

·         Aquifer-Water-bearing rock formation capable of transmitting water

·         Porosity-Percentage of voids in a rock or sediment

·         Permeability-Ability of a material to allow fluids to pass through

·         Darcy’s Law-Fundamental law governing groundwater flow

·         Archaean Rocks-Very old crystalline rocks with poor primary porosity

·         Deccan Traps-Basaltic formations known for fractured aquifers

·         Alluvium-Loose sediments with high groundwater potential

·         Groundwater Tracer-A marker used to trace groundwater movement or origin

·         Hydraulic Conductivity-Rate at which water can move through a porous medium

·         Stratigraphy-Study of layered rock formations

·         Karst-Terrain formed from the dissolution of soluble rocks like limestone

 

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