Block 3 Resources

Groundwater (Subsurface Water)

Ground water is the water present below the earth’s surface in the pore spaces of soil and rocks. It is an important natural resource used for drinking, agriculture, and industry. Ground water is derived mainly from rainfall that infiltrates into the ground.

Objectives

After studying this unit, you should be able to:

Define groundwater and its occurrence

Explain the movement and storage of groundwater

Understand aquifers and water table

Describe the importance of groundwater

Definition of Groundwater

Groundwater is the water present below the Earth’s surface in pore spaces, cracks, and crevices of rocks. It is also called subsurface water or underground water. Only about 0.6% of the total water on Earth occurs as groundwater within the lithosphere.

Groundwater is extremely important because many regions depend on it for domestic, agricultural, and industrial water supply. It contributes about 15–20% of freshwater requirements in many countries. Groundwater also plays a role in maintaining stream flow and causing erosion, which leads to the formation of caves and caverns.

Sources of Groundwater

Meteoric Water

The main source of groundwater is precipitation, such as rainfall and snowfall. This water is called meteoric water.

After precipitation:

About half of the water returns to the atmosphere through evaporation and transpiration.

Around one-fourth flows as surface runoff into rivers, lakes, and oceans.

The remaining water infiltrates into the ground through pores, cracks, and fissures of rocks, becoming groundwater.

Factors that favor groundwater infiltration include:

Humid climate

Dense vegetation

High porosity and permeability of rocks

Presence of cracks and fractures in rocks

Distribution of Groundwater

Groundwater that enters rocks may follow different paths. Some of it returns to the surface through springs and seepages, while some evaporates after rising through capillary action. A portion is absorbed by plants and released through transpiration, and some is extracted through wells for human use. A small amount becomes chemically combined with minerals in rocks.

Connate Water

Another type of groundwater is connate water, which is trapped within sediments during their deposition in lakes or seas. When these sediments later form sedimentary rocks, the trapped water remains inside the rock layers. Connate water is often found along with petroleum in oil fields.

Juvenile (Magmatic) Water

Groundwater may also originate from deep-seated magma inside the Earth. This water is called juvenile or magmatic water. It is commonly associated with hot springs and geysers, such as those found in Yellowstone National Park.

Zones of Groundwater

Groundwater occurs in different zones beneath the Earth’s surface.

 

Zone of Aeration

This is the upper layer beneath the ground surface where pores in rocks contain both air and a small amount of water. This zone includes soil moisture used by plants.

Vadose Zone

The vadose zone lies within the zone of aeration and contains soil moisture used by plants. Water movement here is limited and mainly occurs during rainfall or snowmelt.

Zone of Saturation

Below the zone of aeration lies the zone of saturation, where all pores and spaces in rocks are completely filled with water. This zone stores the main supply of groundwater.

Water Table

The water table is the upper boundary of the zone of saturation. It represents the level below which all rock spaces are filled with water.

The water table is not perfectly flat; it usually follows the shape of the land surface. It tends to be higher beneath hills and lower beneath valleys. In areas with lakes or swamps, the water table may reach the ground surface.

The position of the water table is important for understanding:

Behavior of wells

Flow of springs and streams

Fluctuations in lake levels

Capillary Water

Above the water table, water may rise through capillary openings in rocks or soils. This water is called capillary water, and the upward movement is known as capillary rise.

Types of Streams Based on Groundwater Interaction

Effluent Streams

In humid regions, groundwater flows into streams and maintains their flow. Such streams are called effluent streams.

Influent Streams

In arid regions, the water table lies below the stream channel. As a result, streams lose water to the ground. These streams are called influent streams.

Perched Water Table

Sometimes groundwater accumulates above the main water table because of an impermeable layer such as clay. This creates a small, isolated saturated zone called a perched water table.

Porosity and Permeability

Porosity

Porosity is the percentage of pore spaces or voids present in a rock compared to the total volume of the rock. Rocks contain small openings such as pores, cracks, and interstices, and groundwater occupies these spaces. The amount of porosity varies depending on the nature of the rock material.

Sedimentary rocks generally have higher porosity than igneous or metamorphic rocks because they contain spaces between grains. Sediments may have 10–60% pore space, depending on the size, shape, sorting, and packing of grains. Clay has very high porosity (about 50%), while sands and gravels usually have about 20% pore space.

Igneous and metamorphic rocks consist of tightly interlocking crystals, so they normally have very little pore space. Their porosity mainly depends on fractures, joints, and fissures.

However, high porosity does not always mean that water can move easily. For example, clay holds a large amount of water but does not allow it to move freely. Such materials are called impervious rocks.

Permeability

Permeability is the ability of a rock or sediment to allow water to pass through it. It measures the rate at which water can flow through the pore spaces of a rock under pressure.

Groundwater moves through connected openings within rocks. Materials such as coarse sands and gravels are highly permeable because they have large and well-connected pore spaces. These materials allow water to move easily.

A rock may have high porosity but low permeability. For example, clay has high porosity but low permeability because its pore spaces are extremely small, and the particles swell when wet, preventing water movement. In contrast, sand allows water to flow freely because its pore spaces are larger.

Types of Aquifers

Aquifer

An aquifer is a rock or sediment layer that can store and transmit groundwater. Good aquifers usually consist of coarse sand, gravel, or sandstone because these materials have good porosity and permeability.

Some limestones also act as aquifers because solution enlarges fractures and bedding planes, forming underground channels for water movement.

Aquiclude

An aquiclude is a geological formation that may contain water but does not allow it to flow easily. Clay is a common example because its very small pore spaces restrict water movement.

Aquifuge

An aquifuge is a rock formation that neither stores nor transmits groundwater. These rocks have no significant pore spaces or fractures.

Aquitard

An aquitard is a formation that stores water but allows only very slow movement of groundwater. Water may move through it mainly through joints or cracks.

Depth of Groundwater Penetration

The depth to which surface water penetrates into the ground depends on the type and structure of rocks. In some areas groundwater may reach depths of several thousand meters, while in other places it may occur only a few hundred meters below the surface.

Springs and Wells

Springs

A spring is a natural outlet through which groundwater flows out onto the Earth’s surface. Groundwater moves downward until it reaches an impervious layer or the water table. When this underground water emerges naturally at the surface, it forms a spring.

Springs commonly occur on hillsides or valley sides where permeable rocks such as sandstone or gravel rest over impervious rocks like clay or shale. In such situations, water flows along the contact between the layers and appears at the surface as small seepage springs.

When an aquifer layer exposed at the surface releases water naturally, it is called an aquifer spring.

Wells

A well is an artificial opening dug or drilled into the zone of saturation to obtain groundwater. Wells act as reservoirs where groundwater collects and from which water can be pumped to the surface.

When water is withdrawn from a well, the water level around the well declines, forming a cone of depression. If large amounts of groundwater are pumped, the water table may drop significantly in the surrounding area.

Most wells in humid regions are dug a few meters below the water table, usually not deeper than 15 meters. Wells may also be bored using augers in loose sediments or drilled using rotary drilling methods in hard rocks.

Artesian Springs and Wells

Artesian Wells

An artesian well is formed where permeable rock layers (aquifers) are confined between two impermeable layers such as shale or clay. Water enters the aquifer at higher elevations and becomes confined under hydrostatic pressure.

When a well penetrates this confined aquifer, the water rises automatically in the well, sometimes reaching the ground surface. Such wells are called artesian wells.

The name artesian comes from the Artois region of France, where this type of well was first developed.

Examples in India include: Cuddalore sandstones (Tamil Nadu),      Rajahmundry sandstones (Andhra Pradesh)

Hot or Thermal Springs

Thermal springs are springs that discharge warm or hot water. Water is considered thermal when its temperature is 15–20°F higher than the average local temperature.

The heat usually comes from:

• Cooling magma beneath the Earth’s crust
• Deep groundwater circulation heated by geothermal gradient

One of the most famous regions with thermal springs is Yellowstone National Park (USA).

Geysers

A geyser is a special type of thermal spring that periodically ejects hot water and steam with great force. Geysers occur mainly in recent volcanic regions.

The name comes from “Geysir” in Iceland, where such springs were first studied.

Mineral Springs

Mineral springs are springs that contain dissolved minerals and gases such as calcite, gypsum, CO₂, and H₂S. These waters are often called hard water.

Mineral springs are sometimes believed to have therapeutic or medicinal properties and are commonly associated with spa resorts. In India, temples and cultural sites have often been built near such springs, for example Mahanandi Temple near Nandyal (Andhra Pradesh).

Geological Work of Groundwater

Solution Activity

Groundwater acts as an important agent of erosion, mainly by dissolving soluble rocks such as limestone. Rainwater absorbs carbon dioxide (CO₂) and forms carbonic acid, which reacts with calcium carbonate in limestone to form soluble calcium bicarbonate.

Caves and Caverns

Groundwater dissolves limestone along joints and bedding planes, gradually enlarging them into caves and caverns.

Famous examples include: Carlsbad Caverns (USA), Mammoth Caves (Kentucky, USA)

In India, examples include: Borra Caves (Andhra Pradesh), Kona Rameswaram Caves (Andhra Pradesh) Cave Deposits (Speleothems)

Stalactites

Stalactites are icicle-like deposits hanging from cave ceilings, formed by dripping water depositing calcium carbonate.

Stalagmites

Stalagmites grow upward from the cave floor, formed by mineral deposition from dripping water.

When stalactites and stalagmites join, they form pillars or columns.

Sinkholes

A sinkhole is a depression formed when the roof of a cave collapses or when limestone dissolves extensively. Sinkholes are common in limestone regions.

Karst Topography

Karst topography develops in regions where limestone dissolution by groundwater is intense. It is characterized by: Numerous sinkholes, Caves and underground drainage,Lack of surface streams.

The term Karst comes from a limestone region near the Adriatic Sea between Italy and Yugoslavia.

Deposition by Groundwater

Groundwater also causes mineral deposition when dissolved materials precipitate. Common deposits include:

Travertine – calcium carbonate deposits around springs

Kankar – lime deposits in semi-arid regions

Concretions – rounded mineral masses formed around a nucleus

Geodes – hollow rocks lined with crystals

Groundwater may also cause replacement of minerals, such as the formation of petrified wood, where original plant material is replaced by silica.

Uses of Groundwater

• Domestic water supply for cities and villages.
• Agriculture and irrigation in rural areas.
• Industrial water supply.
• Geothermal energy generation in some regions.

Considered a renewable natural resource because it is continuously recharged by rainfall.