Historical Geology - Wharton County Junior College

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Transcript Historical Geology - Wharton County Junior College

Chp 16: Ground Water
GROUND WATER
Definition= water that fills open spaces (pores=porosity) in rocks, soil, sediments beneath the
Earth’s surface
-soil, rocks, sediments act as filters removing impurities from water
Terms:
porosity- pore space between grains
permeability- measure of how connected these pores are with one another
both are a function of grain size, grain shape, and any cements
examples: detrital or clastic rocks have very good porosity and permeability
limestones, igneous and metamorphic rocks have poorer porosity
and permeability
porosity can be enhanced by weathering-particularly in carbonates
When first deposited, shales and siltstones have higher porosity than sandstones. Upon
compaction shales and siltstones lose porosity faster….
Permeability: sandstones have greater permeability than other lithologies
Test: pour water on sandstone, shale and siltstone-watch result….
Definition: aquifer=a permeable layer transporting groundwater
The best aquifers are well sorted, well rounded sands and gravels
Porosity (pore space) is function of size, shape and arrangement of grains.
a. Well sorted
sedimentary rock
has good porosity
b. Poorly sorted
rock has low porosity
c. In soluble rocks, like
limestone, porosity can
be enhanced by
dissolution.
d. Fracturing in
Igneous and metamorphic rocks can
make them porous.
Permeability= measure of how connected pores are…sands are more
permeable than shales.
Chp 16: Ground Water
Water Table: surface separating zone of aeration from zone of saturation
a. zone of aeration: pore spaces contain air, water percolates down throug
this zone
b. zone of saturation: pore spaces filled with water
water table surface tends to mimic topography
Gravity provides energy for downward movement of ground water. Rate
of movement in zone of aeration is very slow.
Spring: place where groundwater flows or seeps out of the ground.
Usually flows along an impermeable layer (shale!) to location where
meets surface.
Perched water table: local trap is present where water trapped in zone of
aeration; e.g. shale lens in sandstone
Subsurface divided into 2 zones: 1. aeration (water and air) 2. saturation
(water only)
Groundwater moves down through the zone of aeration to zone of
saturation-top of which is known as water table. Some groundwater
can become trapped above water table if encounters impermeable shale.
Springs form whenever laterally moving groundwater encounters
Earth’s surface: 1. groundwater percolates laterally atop local
impermeable barrier until comes to surface.
Chp 16: Ground Water
Water Wells: usually the result of drilling down to zone of saturation.
Some are free flowing, most need pumps to bring water to surface.
When groundwater is pumped from a well, water table in vicinity of well
is lowered; this is called a cone of depression…
Cone of depression forms because water withdrawal is faster than natural
rate of replenishment
Artesian Systems: system where groundwater is confined and builds up
high hydrostatic (fluid) pressure. This requires:
-aquifer must be bounded by non-porous layer (aquiclude) above and belo
-rocks usually tilted and exposed at surface.
-there is sufficient precipitation in recharge area to keep aquifer filled
Windmills used to pump water to surface from wells.
Electric pumps more commonly used today to pump water.
Over time, as water is withdrawn from a well the water
table changes locally around the well-it is depressed. Nearby
wells can go dry if the deeper well removes water faster than it
can be replenished naturally.
Artesian System must have a porous rock confined above and below
by less porous rocks; and it must be exposed at surface-usually in mountains- where
lots of rainfall can replenish water supply to porous layer. Recharge Area- where
water is supplied to aquifer. Note the water table dips from the recharge area to the
basin. Free-flowing wells are those above the projected water table.
Chp 16: Ground Water
Does Groundwater erode and deposit material?
-Groundwater reacts with rocks and minerals it comes in contact with-weathers them chemically,
especially true with carbonates.
-Carbonates can erode if carbonic acid (H2CO3) is present in groundwater. Most groundwater is
slightly acidic because of reactions with CO2 in air and organic matter in soils, so…..
-Groundwater dissolves carbonates, forming calcium bicarbonate, which is carried away in solution
in groundwater.
Effects of Carbonate Dissolution:
Sinkholes and Karst Topography:
a. Sinkholes: depressions where underlying soluble rock dissolves, leading to collapse of the surface
These form in one of two ways1. solution valleys- merging of adjacent sinkholes
2. collapse of cave roof
Karst topography: groundwater erosion is the cause….
Karst- surface characterized by numerous caves, springs, sinkholes, solution valleys, or
disappearing streams.
Common theme here is that thick, soluble rocks occur just beneath the soil-usually carbonates.
Distribution of major limestone and karst areas of world:
karst= irregular topography caused by groundwater erosion of
soluble rocks (i.e. limestone, etc)
Sinkhole formed in previously dissolved limestone as water table dropped.
Water filled sinkhole-water enters from spring.
Common features associated
with Karst topography
Stone Forest-125km south of Kunming, China. Formed by dissolution
of carbonate rocks.
Solution valleys, sinkholes and sinkhole lakes in Kentucky
Chp 16: Groundwater
Caves and Cave Deposits
Caves form as groundwater moves through or along fractures and
openings that are connected in subsurface.
Cavern= a very large cave or system of connected caves
Groundwater dissolves carbonates along fractures and bedding planes,
eventually forming caves.
1.stalactite: icicle shaped structures hanging from ceiling
2. stalagmite: upward growing features from cave floor
3. column: when stalactite and stalagmite meet…
Stalactites- icicle shaped features hanging from cave ceiling.
Stalagmites- upward pointing features on cave floor.
Columns form when they meet….
Formation of Caves:
A. as groundwater percolates thru
zone of aeration and flows thru
zone of saturation, it dissolves carbonate
rocks, forms system of
passageways.
B. Groundwater moves along
water table, carrying dissolved
material, which enters surface streams
thru springs.
C. as surface streams erode deeper,
the water table drops and exposes the
passageways as caves.
Geographic extent of the
High Plains Aquifer and
changes in water level from
pre-development thru
1993.
Human impact upon Groundwater Systems:
Use of groundwater for irrigation causes….
1. lowering of water table
2. loss of pressure, causing free flowing wells to require pumping
3. saltwater incursion in coastal areas
4. subsidence: unconsolidated sediments compact as water removed
5. contamination: from sewage, landfills, toxic waste disposal sites,
agriculture. Containment is very difficult once contaminated…..
Saltwater incursion into water table:
a. freshwater is less dense than salt
water, so it forms a lens shaped wedge
above the salt water.
B. If excessive pumping occurs, a
cone of depression can occur that is
deep enough to allow the salty ground
water to flow into the well.
C. Pumping water back into the
fresh groundwater can restore the water table
and lower the interface between the
fresh and salt groundwater.
Excessive withdrawal of Ground Water
beneath Mexico City-buildings tilt…
Leaning Tower of Pisa is partly
due to groundwater removal
Withdrawal of petroleum from oilfield beneath Long Beach, CA
resulted in up to 9m of ground subsidence because sediment compacted…
Contamination of the water
table:
a. usually oxidation,
bacterial degradation occur
within aerated zone, BUT
if septic system too close
to water table,
contamination can occur.
B. Landfill: unless there is
an impermeable layer
between the base of
landfill and water table,
pollutants can percolate
down into saturate zone
and contaminate water
table.
Hydrothermal Activity
Hydrothermal-meaning ‘hot water’
1.Hot Springs: any spring in which water temperature is greater than 37 deg C
2.Mud Pot: chemically altered rocks yield clays that bubble as hot water and steam rise through the
Heat comes from magma or cooling igneous rocks…
3. Geysers: hot springs that intermittently eject hot water and steam with tremendous force.
-these are surface expression of an extensive underground system of interconnected
fractures within hot igneous rocks.
a. Water at bottom of fracture system is heated to very near boiling point.
b. Eventually heated to where forms steam, blows out of the fracture system at the surface.
-These commonly contain high percentages of dissolved minerals because minerals dissolve
faster in very hot water.
-When the mineral-rich hot water cools at the surface, the minerals in solution precipitate
(i.e. they are deposited) as travertine or calcareous tufa.
Geothermal Energy
Energy produced from Earth’s internal heat…..
-hot water and steam are used as energy: a well is drilled into fractured areas, steam is piped to
electricity plants where it is converted to energy….
Hydrothermal activity: Hot spring in Yellowstone Nat’l Park.
Hydrothermal activity:
water in this Hot spring at
Bumpass Hell in Lassen
Volcanic Park in CA is
actually boiling naturally….
Mud pot at Sulphur Works in Lassen Nat’l Park:
Bathhouses in Bath, England:
built by Romans over hot
springs, after they conquered
England in AD 43
Geysers in Yellowstone Nat’l Park: small geyser
Eruption of a geyser:
a. groundwater percolates down through
openings in rocks, is heated by igneous
rocks.
B. When water is heated above boiling
point, or pressure decreases, water
changes to steam, which pushes water
above it up and out of ground as geyser!
Hot springs deposits in Yellowstone Nat’l Park: Minerva Terrace
formed when calcium carbonate rich hot spring water cooled, precipitated travertine.
Liberty Cap- geyserite
mound formed by repeated
geyser eruptions of a silicondioxide rich spring water
Chp 16: Groundwater Summary
Definition= water that fills open spaces (pores=porosity) in rocks, soil, sediments beneath the
Earth’s surface
-soil, rocks, sediments act as filters removing impurities from water
porosity- pore space between grains
permeability- measure of how connected these pores are with one another
both are a function of grain size, grain shape, and any cements
examples: detrital or clastic rocks have very good porosity and permeability
limestones, igneous and metamorphic rocks have poorer porosity
Definition: aquifer=a permeable layer transporting groundwater
The best aquifers are well sorted, well rounded sands and gravels
Water Table: surface separating zone of aeration from zone of saturation
a. zone of aeration: pore spaces contain air, water percolates down through
this zone
b. zone of saturation: pore spaces filled with water
water table surface tends to mimic topography
Gravity provides energy for downward movement of ground water. Rate
of movement in zone of aeration is very slow.
Chp 16: Groundwater Summary
Spring: place where groundwater flows or seeps out of the ground. Usually flows along an
impermeable layer (shale!) to location where meets surface.
Perched water table: local trap is present where water trapped in zone of
aeration; e.g. shale lens in sandstone
Water Wells: usually the result of drilling down to zone of saturation.
Some are free flowing, most need pumps to bring water to surface.
When groundwater is pumped from a well, water table in vicinity of well
is lowered; this is called a cone of depression…
Cone of depression forms because water withdrawal is faster than natural
rate of replenishment
Artesian Systems: system where groundwater is confined and builds up
high hydrostatic (fluid) pressure. This requires:
-aquifer must be bounded by non-porous layer (aquiclude) above and below.
-rocks usually tilted and exposed at surface.
-there is sufficient precipitation in recharge area to keep aquifer filled
Chp 16: Groundwater Summary
Does Groundwater erode and deposit material?
-Groundwater reacts with rocks and minerals it comes in contact with-weathers them chemically,
especially true with carbonates.
-Carbonates can erode if carbonic acid (H2CO3) is present in groundwater. Most groundwater is
slightly acidic because of reactions with CO2 in air and organic matter in soils, so…..
-Groundwater dissolves carbonates, forming calcium bicarbonate, which is carried away in solutio
in groundwater.
Effects of Carbonate Dissolution:
Sinkholes and Karst Topography:
a. Sinkholes: depressions where underlying soluble rock dissolves, leading to collapse of the surfac
These form in one of two ways1. solution valleys- merging of adjacent sinkholes
2. collapse of cave roof
Karst topography: groundwater erosion is the cause….
Karst- surface characterized by numerous caves, springs, sinkholes, solution valleys, or
disappearing streams.
Common theme here is that thick, soluble rocks occur just beneath the soil-usually carbonates.
Groundwater Summary
Caves and Cave Deposits
Caves form as groundwater moves through or along fractures and openings that are
connected in subsurface.
Cavern= a very large cave or system of connected caves
Groundwater dissolves carbonates along fractures and bedding planes,
eventually forming caves.
1.stalactite: icicle shaped structures hanging from ceiling
2. stalagmite: upward growing features from cave floor
3. column: when stalactite and stalagmite meet…
Hydrothermal Activity- Hydrothermal-meaning ‘hot water’
1.Hot Springs: any spring in which water temperature is greater than 37 deg C
2.Mud Pot: chemically altered rocks yield clays that bubble as hot water and steam rise through them
Heat comes from magma or cooling igneous rocks…
3. Geysers: hot springs that intermittently eject hot water and steam with tremendous force.
-these are surface expression of an extensive underground system of interconnected
fractures within hot igneous rocks.
Geothermal Energy
Energy produced from Earth’s internal heat…..
-hot water and steam are used as energy: a well is drilled into fractured areas, steam is piped to
electricity plants where it is converted to energy….
Chp 16: Ground Water
Chp 16: Ground Water