6. Groundwater
Dr. SaMeH Ahmed
Associate Prof. Mining and Environmental Engineering
Email: s.mohamed@mu.edu.sa
Engineering Geology CE101
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Contents
a) Basic definitions
b) Wells and Springs
c) Groundwater contamination
b)
1.
2.
3.
4.
5.
a) Basic definitions
Definition
Importance of Groundwater Studies
Sources of Groundwater
Water cycle
Groundwater movement
Water level
Engineering Geology CE101
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1. Definition
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.
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2. Importance of Groundwater
Studies
•
•
•
•
•
Groundwater is an important source for drinking and
irrigation.
Most groundwater is an important part of the water
cycle in nature.
Because it has a strong Geological pursuant including
deposition beneath the surface of the land.
Of the role of this water in the erosion process, and
for being a major factor in the distribution of
vegetation.
Because they affect the equilibrium installations.
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3. 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.
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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.
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4. 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.
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4. Water Cycle, cont.
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.
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5. The presence 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.
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5. The presence of groundwater
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.
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Intermediate zone
Capillary zone
W. T.
Saturated Z
Unsaturated Zone
Soil water zone
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‫نطاق الالتشبع‬
‫‪Aeration zone, unsaturated zone‬‬
‫في هذا النطاق تحوي الفراغات البينية ماء وهواء‬
‫وتتميز مياه هذا النطاق بوجود الماء بنسبة تقل‬
‫عن حالة التشبع‪.‬‬
‫يمكن تقسيم هذا النطاق إلى ثالثة أجزاء ‪:Sub-zones‬‬
‫نطاق ماء التربة أو رطوبة األرض ‪:Soil water zone‬‬
‫•‬
‫• نطاق مياه الحزام الوسطـى أو النطاق المتوسط‬
‫‪ :Intermediate zone‬وهو الذي تتأثر فيه حركة المياه‬
‫بالجاذبية األرضية وتكون حركتها إلى األسفل‪.‬‬
‫نطاق مياه الخاصية الشعرية ‪ :Capillary zone‬وهو‬
‫•‬
‫الذي يكون قريبا من سطح خزان الماء الجوفي ويكون‬
‫اتجاه حركة المياه نحو األعلى بتأثير الخاصية الشعرية‪.‬‬
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‫نطاق التشبع‬
‫‪Saturated zone‬‬
‫وهو النطاق الواقع أسفل نطاق الالتشبع‪.‬‬
‫ويتميز هذا النطاق بوجود الماء بنسبة كبيرة تصل‬
‫إلى حالة التشبع مكونا خزان المياه األرضية‬
‫(مكمن ماء أرضي ‪.)Aquifer‬‬
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‫‪6. Groundwater Movement‬‬
‫تنفذ مياه األمطار أو المياه السطحية خالل‬
‫المسام الصخرية الموجودة بين حبيبات‬
‫الصخور المكونة للقشرة األرضية أو في‬
‫الشقوق والكسور المفتوحة إلى أن‪..‬‬
‫تتقابل مع طبقات صماء غير منفذة ال تسمح‬
‫بمرور الماء خاللها‪.‬‬
‫عند ذلك تتجمع المياه لتكون خزان ماء جوفي‪.‬‬
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Important Definitions
1.
2.
3.
4.
5.
Porosity
Void ratio
Permeability
Hydraulic gradient
Darcy's law
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a) Porosity, n
is the ratio of volume of voids to the total volume,
and is related to the void ratio
b) Void ratio
Void ratio, e , is the ratio of the volume of voids to
the volume of solids
c)
Permeability
‫هي قابلية الصخر للسماح للسوائل (ماء أو نفط) بالمرور‬
.‫خالل مسامه‬
Factors affecting permeability:
1. How the porous and voids are connected
2. Particle size of the rocks
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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
:‫وعلى ذلك فإن‬
‫• كل صخر منفذ يكون مساميا بينما‬
.‫• ليس بالضرورة أن يكون كل صخر مسامي منفذا‬
‫ الطين ذو مسامية عالية نسبيا إال أنه غير‬:‫• مثال‬
.‫منفذ وذلك لدقة حبيباته ومسامه‬
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4. 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
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4. Hydraulic gradient
Vertical hydraulic gradient
dh/dl = difference in head / vertical distance
between wells
= (h2 - h1) / (z2-z1)
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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.
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Hydraulic conductivity, k and Darcy's Law
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
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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.
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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
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b. 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
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Factors affecting groundwater movement:
1) Permeability of rocks
2) Hydraulic gradient
3) Water temperature
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7. Water Table
The water table is the surface where the
water pressure head is equal to
the atmospheric pressure (where gauge
pressure = 0).
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‫ونتيجة لسقوط بعض المياه الموسمية فقد يرتفع‬
‫منسوب المياه األرضية مؤقتا‪.‬‬
‫‪Temporary surface‬‬
‫‪Groundwater surface‬‬
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