Module 2 ch-2 ground water

GROUND WATER
HYDROLOGY
Prepared by:
Prof. Ankit N Patel

BITS Edu Campus Prof. Ankit Patel 2

GROUND WATER
• Ground water: the water that lies beneath the ground
surface, filling the pore space between grains in bodies
of sediment and and filling cracks in all types of rock
• Groundwater is water located beneath the earth's
surface in soil pore spaces and in the fractures of rock
formations.
• The depth at which soil pore spaces or fractures and
voids in rock become completely saturated with water is
called the water table

SUB SURFACE WATER
• Water in a soil mantle is called as sub
surface water.
• Water beneath the surface can
essentially be divided into two zones
– the unsaturated zone (also known as the "zone of
aeration") which includes soil water zone,
– The zone of saturation which includes ground water.

• Saturated zone is classified into 4
categories.
i) Aquifer
ii) Aquiclude
iii) Aquifuge
iv) Aquitard

AQUIFER
• An aquifer is a layer of porous substrate that contains
and transmits groundwater.
• An aquifer is an underground layer of water-bearing
permeable rock or unconsolidated materials (gravel,
sand, or silt) from which groundwater can be extracted
using a water well.
• Aquifers may occur at various depths.

TYPES OF AQUIFER
UNCONFINED AQUIFER:
• unconfined aquifer: a partially filed aquifer
exposed to the land surface and marked
by a rising and falling water table

TYPES OF AQUIFER
UNCONFINED AQUIFER:
• Unconfined aquifers are sometimes also called
water table or phreatic aquifers, because their
upper boundary is the water table
• When water can flow directly between the surface
and the saturated zone of an aquifer, the aquifer is
unconfined.
• The deeper parts of unconfined aquifers are
usually more saturated since gravity causes water
to flow downward.
– Has a water table, and is only partly filled with water
– Rapidly recharged by precipitation infiltrating down to the saturated
zone

CONFINED AQUIFER:
• confined aquifer (artesian aquifer): an
aquifer completely filled with pressurized
water and separated from the land surface
by a relatively impermeable confining bed,
such as shale
• A water-bearing subsurface stratum that is
bounded above and below by formations
of impermeable, or relatively impermeable
soil or rock.

CONFINED AQUIFER:
– Completely filled with water under pressure
(hydrostatic head)
– Separated from surface by impermeable
confining layer/aquitard
– Very slowly recharged

Confined Aquifers

AQUICLUDE
• It is a solid, impermeable area
underlying or overlying an
aquifer. If the impermeable
area overlies the aquifer
pressure could cause it to
become a confined aquifer.
• A solid, impermeable area
underlying or overlying an
aquifer.
• It can absorb water due to high
porosity but cannot transmit it
in significant amount. (eg.
Clay)

AQUIFUGE:
• An impermeable body
of rock which contains
no interconnected
openings or interstices
and therefore neither
absorbs nor transmits
water.
AQUITARD:
• A bed of low
permeability adjacent
to an aquifer; may
serve as a storage unit
for groundwater,
although it does not
yield water readily to
wells

PROPERTIES OF THE AQUIFER
i) Porosity
ii) Specific yield
iii) Specific retention
iv) Storage by efficiency ( field
capacity)
v) Permeability
vi) Transmissibility

POROSITY:
• Porosity or void fraction is a measure of the
void (i.e., "empty") spaces in a material, and
is a fraction of the volume of voids over the
total volume, between 0–1, or as a
percentage between 0–100%.
• Porosity of surface soil typically decreases as
particle size increases.

SPECIFIC YIELD
• If water is allowed to drain from a saturated
sample of an aquifer some water will drain
freely under gravity.
• The specific yield of an aquifer is the ratio of
the volume of water drained freely from the
material to the volume of soil sample.

SPECIFIC RETNTION:
• Some quantity of water will be retained by the soil sample
and will not be drained under gravity. This is due to
molecular attraction and surface tension. The specific
retention is defined as the ratio of the volume of water
retained in the material to the total volume of a material,
when a saturated material is dewatered.

PERMEABILITY
• Just as the porosity of a soil affects how
much water it can hold, it also affects how
quickly water can flow through the soil.
• The ability of water to flow through a soil is
referred to as the soil's permeability.

TRANSMISSIBILITY
• The transmissivity of an aquifer is also
known as transmissibility. It is the product
of the coefficient of permeability and the
thickness of the aquifer.
T = K X b
• The transmissibility of an unconfined
aquifer depends upon the depth of the
GWT.

perched water table (Aquifer)
The top of a body of
ground water separated
from the main water table
beneath it by a zone that is
not
saturated.

DARCY’S LAW
• The law states the rate of flow per unit
area of an aquifer is propotional to the
gradient of potential flow in direction of
flow.
• Darcy's law is only valid for slow, viscous
flow;
Q=KiA
i= hydraulic gradient
w= Total crossectional area of porous medium
T= co.eff of Permeability

• Darcy’s law is valid for laminar flow. Since,
reynolds number serves as a criterion to
distinguish between laminar and turbulent
flow, the same may be employed to establish
limit upto which darcy’s law hold good.
Re=(ρVd)/μ
• where ρ is the density of water (units of mass
per volume)
• v is the specific discharge (not the pore
velocity — with units of length per time)
• d= diameter or particle size
• μ is the viscosity of the fluid.

• Experiments shows that Darcy's law is
valid for Re< 1. most of the natural
groundwater flows occurs with Re< 1 and
hence Darcy’s law is applicable.
• It is not applicable in aquifers containing
large diameter coarse gravel, rock fill,
where the flow is no longer laminar due to
steep hydraulic gradient.

THANK YOU…

Module 2 ch-2 ground water

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Module 2 ch-2 ground water