Nước dưới đất
và địa hình karst
Tuần hoàn nước (WATER CYCLE)
Condensation
Evaporation
Precipitation
Runoff
Nước dưới đất
Nước
có thể tồn tại và vận
động trong đất đá dưới dạng
lấp đầy lỗ hổng, khe nứt, dập
vỡ, khoảng trống
Nước bổ cập xuống hệ thống nước ngầm qua đường
thấm từ nước mưa, dòng chảy của sông, hồ..
Nước ngầm
Ground water that meets the land surface also helps keep
rivers, streams, lakes and wetlands filled with water.
% of lượng nước ngọt trên Trái
đất tồn tại ở dạng nước ngầm, lớn
hơn 30 lần lượng nước tồn tại ở sông
và hồ trên Trái đất.
 14
 Nước
thấm xuống đất và trở thành
một phần của nước dưới đất.
 Hệ
thống nước dưới đất cũng gần tương
tự như hệ thống nước sông (nước mặt)
 Khác
với hệ thống nước mặt là cần có
đường, kênh dẫn dòng trong việc lưu
thông dòng chảy trong lưu vực, nước dưới
đất được lưu thông qua qua hệ thống lỗ
rỗng, dập vỡ, khe nứt
Groundwater
Một số đặc điểm:
- Mặt nước ngầm (water table) thường có hình thái trùng với hình thái bề
mặt địa hình.
- Sông, suối, hồ phần lớn là điểm liên kết của mặt nước ngầm với bề mặt
địa hình
- Nước ngầm di chuyển, vận động theo hướng dốc của bề mặt nước
ngầm
- Hầu hết nước dưới đất được thoát ra hệ thống sông suối (hoặc ra biển)
Đới thông khí (zone of Aeration): khu
vực khảảng trống, lỗ rỗng.. không có nước.
Mặt nước ngầm (Water Table): ranh giới
giữa đới bão hòa và đới thông khí; lỗ khoan
cần được khoan sâu dưới mặt nước ngầm có
thể khai thác nước ngầm
Đới bão hòa (zone of Saturation): phần
đất đá mà lỗ rỗng được lấp đầy với nước
Lỗ rỗng (porosity) và độ thấm (permeanility)
Độ thấm: khả năng nước có thể di chuyển trong, qua tầng chứa,
vật liệu chứa nước
Độ rỗng (Porosity): tỷ lệ giữa
thể tích lỗ rỗng trên thể tích
của đất đá
Độ dẫn nước (Permeability)

Soil and rock are permeable if the pore
spaces are connected and water can pass
through them easily.
 If
a rock or sediment has few pore spaces,
or they are not well connected, then the
flow of groundwater is blocked.

These materials are impermeable, which
means that water cannot pass through them.
 Granite
and clay are impermeable;
sandstone is permeable.
 Groundwater
keeps going deeper into the
Earth’s crust until it reaches a layer of
impermeable rock.


At this point, the water stops moving
down.
Water begins filling up the pores in the
rocks above. The layer of permeable
rock that lets water move freely is an
aquifer.

The zone of aeration is the first layer of
soil that is permeable. Water and air pass
through this layer easily.

The area where all of the rocks are filled
with water is called the zone of saturation.
 The
upper surface of this zone is called
the water table.
GROUNDWATER FEATURES
Hệ thống nước ngầm
1. Earth’s Surface
2. Precipitation
3. Body of water
4. Zone of Aeration
5. Water Table
6. Zone of Saturation
 Nước
ngầm cung cấp cho con người
được khai thác qua lỗ khoan khoan trong
tầng bão hòa Many people get their water
from groundwater through wells that have
been drilled into the zone of saturation.
 The
supply of groundwater is limited.
During a drought the water table drops,
which is why you should conserve water.
 People
get the majority of their water from
wells. A good well extends deep into the
zone of saturation, where groundwater
flows into the well and a pump brings it to
the surface.


If too much water is pumped out, then
wells can go dry.
The land level can sink from the weight
of the sediments above the now-empty
pore spaces.
What does a normal household
well look like?

Water flows into a
well and then a pump
brings the water to
the surface.
 Wells can go dry if too
many are located in
one area, or if a
drought takes place.
ARTESIAN WELL

Water pumped from the ground-water system causes
the water table to lower and alters the direction of
ground-water movement. Some water that flowed to the
stream no longer does so and some water may be
drawn in from the stream into the ground-water system,
thereby reducing the amount of streamflow.
Dangers
Associated with
Groundwater
Effects of Over-Pumping a Well
Sinkholes from groundwater
overpumping, Antelope Valley, CA
Dust storm during Dust Bowl in
Kansas, when overpumping from
wells & a drought caused farmers
to ‘overspend’ the water budget
CONTAMINATED?!
CONES OF DEPRESSION
Shallow wells go dry due to
excessive pumping
 In
some places, the water table is so
close to Earth’s surface that water
flows out and forms a spring.

Springs are found on hillsides or
other places where the water table
meets a sloping surface.
SPRING FORMATION
HOT SPRINGS
Hot Spring
A
geyser is a hot spring that erupts
periodically, shooting water and steam into
the air.
 Groundwater is heated to high
temperatures from magma under the
Earth’s surface. As the water heats, it
expands, forcing water out of the ground
along with steam, as the remaining
water boils.
 Old Faithful in Yellowstone National Park
is a famous geyser.
GEYSERS: OLD FAITHFUL
http://www.yellowstone.net/russfinley/tour/video/oldfaith2.rm
Picture of a geyser in action!
Geyser
 Groundwater
mixes with carbon dioxide to
form carbonic acid.
 Limestone
is a rock that is easily dissolved
by carbonic acid.
 As
the acidic groundwater moves through
pores in limestone, the rock dissolves,
enlarging cracks until an underground
opening called a cave is formed.

Groundwater not only dissolves limestone
to make caves, but it also can make
deposits on the insides of caves.


Water dripping from the cave walls
contains calcium ions dissolved from the
limestone. If the water evaporates while
hanging from the ceiling, the calcium
carbonate is left behind and builds up to
form a stalactite.
Where water drops fall to the floor, a
stalagmite forms.
Cave
Development
CAVE ENTRANCE
Disappearing Stream
NATURAL CHIMNEYS
NATURAL BRIDGE
 If
underground rock is dissolved near the
surface a sinkhole may form.


A sinkhole is a depression on the surface
of the ground that forms when the roof of
a cave collapses, or when rock near the
surface dissolves.
These are common in areas that have lots
of limestone and enough rain water to
keep the groundwater system supplied
with water.
Sinkhole
SINKHOLE
 Karst
topography is common in the
Valley and Ridge Province of Virginia,
where limestone is abundant.
KARST TOPOGRAPHY FEATURES
Human Impact on Groundwater






Groundwater accounts for 20% of all water
used in the U.S.
Unwise use of this valuable resource can
damage or deplete groundwater reservoirs
Some of the problems related to groundwater
use include:
overpumping-lowering the water table
subsidence
contamination
Pollutants

Contaminants introduced at the land surface may
infiltrate to the water table and flow towards a point
of discharge, either the well or the stream.
A SOURCE OF GROUNDWATER
CONTAMINATION: SEPTIC TANKS
Contamination results in a decline
in water quality.
ANOTHER SOURCE OF POLLUTION:
STORAGE TANKS
YET ANOTHER SOURCE OF POLLUTION:
LANDFILLS
Groundwater Moves – Usually
Slowly