Engineering Geology (CE 101)
CHAPTER 7: Groundwater.
Subsurface water: is water that is beneath the earth’s surface. Usually, it
is between soil and rock and is the supply of most water wells.
Groundwater: is water that basically comes from the ground surface. It
occurs naturally beneath the Earth's surface, between the saturated soil
and the rocks or formations such as cracks.
Underground water: may be ground water, or it may be in man-made
pipes laid underground.
Importance of Groundwater Studies.
1. Groundwater is an important source for drinking and irrigation.
2. Most groundwater is an important part of the water cycle in nature.
3. Because it has a strong Geological pursuant including deposition
beneath the surface of the land.
4. Of the role of this water in the erosion process, and for being a major
factor in the distribution of vegetation.
5. Because they affect the equilibrium installations.
Sources of Groundwater.
a) Meteoric water
It is one of the most important sources of ground water and includes that
part of the rainwater, surface water and water resulting from the melting
of snow, which implements and permeates the surface of the earth and
gather in rock layers.
b. Connate water
Water created and gathered at the time of deposition of rocks and
sediments which contain, then isolated and separated from the water
cycle. Those water are often having high salinity.
c. Magmatic water
It is unknown water source but found in volcanic regions which are of
high temperatures and contain distinctive salts have graduated to the
Earth's surface associated with some of the gases.
Its source could be the separation of water molecules associated with
some salts and chemical compounds.
Water Cycle.
As rain or snow falls to the earth's surface:
Some water runs off the land to rivers, lakes, streams and oceans
(surface water).
Water also can move into those bodies by percolation below ground.
Water entering the soil can infiltrate deeper to reach groundwater which
can discharge to surface water or return to the surface through wells,
springs and marshes. Here it becomes surface water again.
And, upon evaporation, it completes the cycle.
This movement of water between the earth and the atmosphere through
evaporation, precipitation, infiltration and runoff is continuous.
Existence of groundwater.
Water exists beneath the surface of the earth in the voids and gaps
interfaces. Have those blanks completely saturated with water or water
and air together this saturation may be permanent or seasonally.
Groundwater are formed in two zones: saturated zone and unsaturated
zone.
The presence of ground water under the influence of hydrostatic pressure
lower than atmospheric pressure in the unsaturated zone, and higher than
atmospheric pressure in the case of saturation of those voids completely
with water.
Important Definitions.
a) Porosity is the ratio of volume of voids to the total volume, and is
related to the void ratio.
b. Void ratio is the ratio of the volume of voids to the volume of solids.
c. Permeability: ability of rock to pass liquids through.
Factors affecting permeability:
1. How the porous and voids are connected
2. Particle size of the rocks
According to its permeability, rocks can be classified into:
Permeable rocks: allow water to flow through it under normal
conditions.
Impermeable rocks: Do not allow the path of water through it known as
opaque rocks.
Hydraulic gradient.
Horizontal hydraulic gradient is simply the slope of the water
table or potentiometric surface. It is the change in hydraulic head over
the change in distance between the two monitoring wells or dh/dl.
In mathematical terms, horizontal gradient is rise over run.
dh/dl=difference in head / horizontal distance between wells = (h2 - h1) /
L.
Vertical hydraulic gradient
dh/dl = difference in head / vertical distance between wells
= (h2 - h1) / (z2-z1)
Hydraulic conductivity, k and Darcy's Law.
Darcy’s law: The rate of water flow through a tube is proportional to the
difference in the height of the water between the two ends of the tube,
and inversely proportional to the length of the tube.
Darcy discovered that flow was proportional to a coefficient, K, which is
called hydraulic conductivity.
Darcy's Law is the resulting equation:
Q = KA[(hA - hB) / L] OR Q = KA(dh/ dl)
where:
Q = volume of water flow in ft3/day
K = hydraulic conductivity in ft/day
A = cross-sectional area in ft2
dh/dl = hydraulic gradient
Velocity of Groundwater Movement.
Based on Darcy's work, the velocity of water can be estimated or one
can study how fast the water is moving between points.
Velocity is calculated by using hydraulic conductivity, porosity, and
hydraulic gradient.
V=(K / n) (dh / dl),
where: n=porosity.
Example.
Two wells are located 100 feet apart in a sand aquifer with a hydraulic
conductivity of 0.04 feet per day and 35% porosity. The head of well 1
is 96 feet and the head of well 2 is 99 feet.
a. What is the horizontal hydraulic gradient between the wells?
[(head 2 - head 1) / L
[(99-96) / (100)]=3/100 or 0.03.
What is the velocity of water between the two wells?
V= (K / n) (dh / dl)
V = (0.04 ft/d / 0.35) (0.03)
= 0.0034 ft/d
Factors affecting groundwater movement:
1) Permeability of rocks
2) Hydraulic gradient
3) Water temperature
Water Table.
The water table is the surface where the water pressure head is equal to
the atmospheric pressure (where gauge pressure = 0).