CHAPTER 6: GROUNDWATER

97%

Earth’s water - Ocean

3%

Earth’s water - Freshwater

69%

Earth’s water- Icecaps and Glaciers

30%

Earth’s water- Groundwater

87%

Earth’s water- Lakes

11%

Earth’s water - Swamps

2%

Earth’s water - Rivers

GROUNDWATER

is water that exists underground in saturated zones beneath the land surface.

GROUNDWATER

water that is found underground in the spaces and cracks of soil, sand , and rocks. It is a vital natural resource that is used for drinking, agriculture, industry

AQUIFER

A subsurface layer or layers of rock or other geological strata with sufficient porosity and permeability to allow either the flow of groundwater or the abstraction of significant quantities of groundwater.

POROSITY

The quality or state of being porous.

PERMEABILITY

The quality or state of being permeable.

• UNCONFINED AQUIFERS

• CONFINED AQUIFERS

TYPES OF AQUIFERS

UNCONFINED AQUIFERS

are where the rock is directly open at the surface of the ground and groundwater is directly recharged, for example, by rainfall or snowmelt.

CONFINED AQUIFERS

are where thick deposits overlie the aquifer and confine it from the Earth's surface or other rocks.

• POROSITY

• PERMEABILITY

• HYDRAULIC CONDUCTIVITY

• SPECIFIC YIELD AND RETENTION

• STORAGE COEFFICIENT

• TRANSMISSIVITY

• HYDRAULIC GRADIENT

AQUIFER PARAMETERS

POROSITY

Percentage of void spaces in the aquifer material

POROSITY

Primary (original voids) and secondary (due to fractures or dissolution).

POROSITY

It determines how much water an aquifer can store.

PERMEABILITY

Ability of material to transmit water

PERMEABILITY

Factors affecting _____: Grain size, sorting, and pore connectivity

• GRAIN SIZE

• SORTING

• PORE CONNECTIVITY

Factors affecting permeability

HYDRAULIC CONDUCTIVITY

Rate of water movement under a hydraulic gradient

HYDRAULIC CONDUCTIVITY

It is critical for groundwater flow modeling

SPECIFIC YIELD

Usable water released by gravity.

SPECIFIC RETENTION

Water retained against gravity.

STORAGE COEFFICIENT

Volume of water released per unit change in head.

STORAGE COEFFICIENT

Low in confined aquifers, higher in unconfined aquifers.

STORAGE COEFFICIENT

Applications: Groundwater resource evaluation.

TRANSMISSIVITY

Rate of water transmission through the aquifer thickness.

TRANSMISSIVITY

It is important in determining the aquifer productivity

HYDRAULIC GRADIENT

Slope of the water table or potentiometric surface.

HYDRAULIC GRADIENT

It is the key in determining flow direction

DARCY’S LAW

Equation the describes the flow of fluid through a porous medium. The law was formulated by Henry Darcy (1856) based on the results of an experiment on the flow of water through beds of sand.

DARCY’S LAW

“For laminar flow through saturated soil mass, the discharge per unit time is proportional to the hydraulic gradient”

WATER WELLS

Artificial structures or excavations designed to access groundwater from underground aquifers.

WATER WELLS

They are one of the oldest and most common methods of extracting groundwater.

• DUG WELLS OR OPEN WELLS

• TUBE WELLS

TWO BROAD CLASSES OF WELLS

OPEN WELLS

have been the major means of domestic water supply throughout the span of the recorded history of mankind.

OPEN WELLS

They are also used extensively in small-scale irrigation.

OPEN WELLS

are shallow and usually to tab water table aquifers

OPEN WELLS

Some characteristics are:

• have an exposed water surface

• typically have a large diameter (up to 10 meters)

• generally have a depth of up to 20 meters

• vulnerable to contamination from surface activities, animals, and weather events

• exposed water surface

• large diameter (up to 10 m)

• depth of up to 20 m

• vulnerable to contamination from surface activities, animals and weather events

OPEN WELLS characteristics

• UNLINED WELLS

• WELLS WITH PERVIOUS LINING

• WELLS WITH IMPERVIOUS LINING

• DUG-CUM-BORE WELLS

TYPE OF OPEN WELLS

UNLINED WELLS

No lining or casing

UNLINED WELLS

Dug directly into the water-bearing formation.

WELLS WITH PERVIOUS LINING

Lining allows water to seep in from surrounding formations.

WELLS WITH PERVIOUS LINING

Often made of porous materials such as stone or gravel.

WELLS WITH PERVIOUS LINING

can increase water yield but may also introduce contaminants.

WELLS WITH IMPERVIOUS LINING

Lined with materials such as concrete, bricks, or other impermeable materials that do not allow water to seep through.

WELLS WITH IMPERVIOUS LINING

This reduces the risk of contamination, as it prevents surface water or pollutants from entering.

DUG-CUM-BORE WELLS

combination of dug wells and bore wells

DUG-CUM-BORE WELLS

provides access to both shallow and deep groundwater

DUG-CUM-BORE WELLS

more reliable water source

TUBE WELLS

Narrow, cylindrical wells drilled into the ground to extract groundwater.

TUBE WELLS

They consist of a pipe that penetrates the aquifer and is equipped with a strainer to filter out sand and debris.

TUBE WELLS

A pump is used to draw water to the surface.

TUBE WELLS

Some characteristics are:

• cased and screened to prevent contamination, ensuring a stable water supply.

• drilled into the aquifer using a drill rig

• have a smaller diameter, typically 10 cm to 60 cm.

• can be drilled to greater depths, up to 500 meters or more.

• less vulnerable to contamination

• cased and screened

• drilled into the aquifer using a drill rig

• smaller diameter typically 10 cm to 60 cm

• drilled to greater depths up to 500 meters or more

• less vulnerable to contamination

Some characteristics of TUBE WELLS

• SCREEN WELLS

• CAVITY WELLS

Classification based on entry of water.

SCREEN WELLS

The screen permits water to enter the well from the saturated aquifer, while preventing sediment from entering.

CAVITY WELLS

It is the shallow tube well drilled in alluvial formation. A _____ tube

well draws water from bottom of the well and not from the sides

• PARTIAL CASING

• FULL CASING

Types of Well Construction

PARTIAL CASING

Only portion of a well is cased. Typically, the upper part to prevent collapse and contamination

PARTIAL CASING

This type of construction is commonly used in hard, fractured rock

FULL CASING

Entire length of the well is cased. This provides complete protection against collapse and contamination

FULL CASING

Commonly used in unconsolidated sediment, such as sand and gravel