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Lecture # 8
Engineering Geology and Seismology
Geology of Aquifers, Wells, Springs and Ground
Water Conditions
Instructor:
Prof. Dr. Attaullah Shah
Department of Civil Engineering
City University of Science and IT Peshawar
1
Subsurface (ground) water
• Behavior of water in rocks and soils
– Porosity, hydraulic conductivity and permeability
– Porosity: The property of rocks/soils by which it can hold water. It
is expressed in terms of total voids in given volume, which is often
identified by void ratio: The ratio of volume of voids to the
volume of solids
• In general, crystalline igneous and metamorphic rocks have low
porosities unless secondary voids such as joints are produced
by fracturing
– The relative ease of flow by the hydraulic conductivity
(K) (referred to as the coefficient of permeability,
described empirically by Darcy’s Law.
– The properties of the rock alone that affect ease of flow
are defined by its intrinsic permeability (k), (usually
shortened to permeability)
• Rocks to have permeability and hydraulic
conductivity to retain water, but the relationship
between porosity and permeability depends
mainly on the size of the voids rather than on
their frequency
Natural circulation of subsurface water
• Precipitation and its dispersal
• juvenile water: product of recent volcanic activity
and have separated from magma. Often heavily
contaminated with dissolved minerals and gases.
• Connate water, sealed in porous sediment by
surrounding impermeable rocks. If the sediments
are marine, the connate water would be saline
Juvenile and connate waters form only a minor part
of the water present in the uppermost kilometre of
the solid Earth.
Global Water Supply
Distribution
• 3% of earth’s water is
fresh - 97% oceans
• 1% of fresh water in
lakes, streams, rivers
• 29% of the world’s
fresh water exists in
aquifers and 70% in
glaciers
Water Supply and Circulation
• Precipitation is dispersed in three ways: (a) by evaporation and
by transpiration from plants, (b) by direct runoff in streams and
rivers, and (c) by infiltration into the ground.
• The factors affecting evaporation are the intensity of the rainfall,
the ground temperature, the humidity and the wind strength. It is
difficult to measure it directly with accuracy
• Surface water runoff can be measured by gauging streams and
rivers. Data are collected by the Water Authorities and by some
other bodies. The results from selected stations are published
annually
• The amount of runoff from a given area is related directly to its
steepness of slope, and indirectly to the vegetation on it.
• The amount of runoff relative to infiltration after a fall of rain
depends also, and significantly, on the amount and concentration
of the precipitation.
•
A globule of water precipitated as rain is dispersed by
evaporation, by direct runoff and by infiltration into the
ground. The division between runoff and infiltration is
determined by the relative resistance to flow along either
path.
Basin Hydrologic Cycle – GW/SW Interaction
Ground Water
• Ground Water lies beneath the ground surface, filling
pores in sediments and sedimentary rocks and fractures
in other rock types
• Represents 0.6% of the hydrosphere (35x the water in
all lakes and rivers combined)
– Resupplied by slow infiltration
of precipitation
– Generally cleaner than surface water
– Accessed by wells
Porosity and Permeability
• Porosity - the percentage of rock or sediment that
consists of voids or openings
– Measurement of a rock’s ability to hold water
– Loose sand has ~30-50% porosity
– Compacted sandstone may have only 10-20% porosity
• Permeability - the capacity of a rock to transmit fluid
through pores and fractures
– Interconnectedness of pore spaces
– Most sandstones and conglomerates are porous
and permeable
– Granites, schists, unfractured limestones are impermeable
The Water Table
• Subsurface zone in which all rock
openings are filled with water is the
phreatic, or saturated zone
• Top of the saturated zone is the water
table
– Water level at surface of most lakes and
rivers corresponds to local water table
• Above the water table is an unsaturated
region called the vadose zone
• A perched water table is above and
separated from main water table by an
unsaturated zone
– Commonly produced by thin lenses of
impermeable rock (e.g., shales or clays)
within permeable ones
Ground Water Movement
• Movement of ground water
through pores and fractures is
relatively slow (cms to
meters/day) compared to flow
of water in surface streams
– Flow velocities in cavernous
limestones can be much higher
(kms/day)
• Flow velocity depends upon:
– Slope of the water table
– Permeability of the rock or
sediment
Aquifers and Aquitards
• Aquifer - body of saturated rock or sediment through
which water can move easily
–
–
–
–
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Sandstone
Conglomerate
Well-jointed limestone
Sand and gravel
Highly fractured
volcanic rock
• Aquitard - rock/sediment that retards ground water
flow due to low porosity and/or permeability
– Shale, clay, unfractured crystalline rocks
Aquifers- Sierra Nevada Mtns
and foothills
• Aquifer
- Forms in fractured
igneous rock (granitic)
- Porosity and
permeability are low
- Wells are difficult to
locate, yield is low
- Often limits development
Example Layered Aquifer System
Bedient et al., 1999.
Unconfined vs. Confined Aquifers
• Unconfined Aquifer
– Has a water table, and is only
partly filled with water
– Rapidly recharged by
precipitation infiltrating down to the
saturated zone
• Confined Aquifer
– Completely filled with water under
pressure (hydrostatic head)
– Separated from surface by
impermeable confining
layer/aquitard
– Very slowly recharged
Wells
• Well - a deep hole dug or drilled
into the ground to obtain water
from an aquifer
– For wells in unconfined aquifers,
water level before pumping is the
water table
– Water table can be lowered by
pumping, a process known as
drawdown
– Water may rise to a level above
the top of a confined aquifer,
producing an artesian well
Springs
• Spring - a place where
water flows naturally
from rock or sediment
onto the ground surface
Ground Water Contamination
Infiltrating water may bring
contaminants down to the water table,
including (but not limited to):
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–
–
–
–
–
–
–
–
–
–
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Pharmaceuticals
Pesticides/herbicides
Fertilizers
Feed lots
Mercury and gold mining
Landfill pollutants
Heavy metals
Bacteria, viruses and parasites from sewage
Industrial chemicals (PCBs, TCE)
Acid mine drainage
Radioactive waste
Oil and gasoline
Ground Water Contamination
• Contaminated ground water
can be extremely difficult and
expensive to clean up
Assignment: due within a week
• Pick your favorite groundwater contaminant
– Write a 1 page (maximum) paper that describes the
following:
1) Name of the contaminant, chemical formula
2) How it affects people- when/where is it a problem to
the human body, what are the harmful effects?
3) Where it is found, how is it transported, how does it
get into the groundwater system?
4) What can be done to solve the problem?
Due date: beginning of class, next week
Balancing Withdrawal
and Recharge
• If ground water is withdrawn more
rapidly than it is recharged, the
water table will drop
– Dropping water table can lead to
ground subsidence
• surface of the ground drops as
buoyancy from ground water is
removed, allowing rock or sediment to
compact and sink
– Subsidence can crack foundations,
roads and pipelines
– Areas of extremely high ground water
pumping (such as for crop irrigation in
dry regions) have subsided 7-9 meters
Recharge
Natural
• Precipitation
• melting snow
• Infiltration by streams
and lakes
Artificial
• Recharge wells
• Water spread over land
in pits, furrows, ditches
• Small dams in stream
channels to detain and
deflect water
Caves, Sinkholes, and Karst
• Caves - naturally-formed underground
chambers
– Acidic ground water dissolves limestone
along joints and bedding planes
• Caves near the surface may collapse
and produce sinkholes
• Rolling hills, disappearing streams,
and sinkholes are common in areas
with karst topography
Hot Water Underground
• Hot springs - springs in which the water
is warmer than human body temperature
– Ground water heated by nearby magma
bodies or circulation to unusually deep (and
warm) levels within the crust
– Hot water is less dense than cool water and
thus rises back to the surface on its own
• Geysers - hot springs that periodically
erupt hot water and steam
– Minerals often precipitate around geysers as
hot water cools rapidly in the air
Geothermal Energy
• Geothermal energy is produced using
natural steam or superheated water
– No CO2 or acid rain are produced (clean
energy source)
– Some toxic gases given off (e.g., sulfur
compounds)
– Can be used directly to heat buildings
– Superheated water can be very corrosive to
pipes and equipment
Streams and
Groundwater
• Gaining streams - receive water
from the saturated zone
– Gaining stream surface is local water table
• Losing streams - lose water to the
saturated zone
– Stream beds lie above the water table
– Maximum infiltration occurs through
streambed, producing permanent “mound”
in the water table beneath dry channel
PAKISTAN’S GROUNDWATER RESERVOIR
AND ITS SUSTAINABILITY
Muhammad Amin
Member Water, WAPDA, 705-WAPDA-House Lahore,
Pakistan
http://www.watertech.cn/english/amin.pdf
Groundwater Potentials of Pakistan
• Groundwater Potential in Punjab
– Four hydrogeological zones, namely Potohar plateau & salt range, Piedmont
areas, Alluvial plains and Cholistan desert
– The total available groundwater resource of the Punjab Province was estimated
42.75 MAF.
• Groundwater Potential in Sindh
– The groundwater lies in the three hydrogeological zones namely Eastern (Thar)
desert, Western mountain and Indus valleys
– The total available resource of the Sindh Province was assessed to be 18 MAF.
• Groundwater Potential in KP
– Four broad geological units namely, metamorphic and igneous rocks of the
northern mountains, Mesozoic rocks of the southeastern part, Tertiary rocks of
the southeastern part and upper Tertiary.
– The total available resource of the KP Province was assessed to be 3.11 MAF
• Groundwater Potential in Balochistan
– The groundwater resources in six basins of the province namely Hamune, Lora,
Kachhi Plain, Nari, Pishin and Zhob have been assessed as 1.21 MAF and Total
of 2.31 MAF
Conclusions
1. Groundwater has become a major source of water supply in
Pakistan
2. Excessive use of groundwater is causing secondary salinisation
3. The situation has worsened in a number of sub-basins in
Balochistan Province due to groundwater mining
4. Continuous over-draft has resulted in excessive groundwater
abstraction, so that 5% area of Punjab Province and 15% of
Balochistan Province contain groundwater out of reach of poor
farmers
5. Up-coning of deep saline water has started in some parts of
Pakistan
6. There is a need to realistically estimate and manage the demand for
water
7. The flat-rate energy charge has encouraged farmers to exploit more
and more groundwater, which has resulted in an unsustainable
Recommendations
1. A groundwater regulatory framework should be introduced and
implemented for the sustainability of groundwater use
2. There is need to manage the demand of water
3. Low water delta crops should be preferred to high delta crops
4. Efficient irrigation methods should be used to irrigate crops
5. Recharge of groundwater should be increased by increasing canal
diversions, rainwater harvesting and check dams
6. Saline water can be used for saline agro-forestry or for alternative
agricultural crops
7. Energy charges should be proportional to the use of energy for
tubewells
8. Research on groundwater recharge is urgently required.