8. • Groundwater is stored in, and moves slowly through,
moderately to highly permeable rocks called aquifers.
• Aquifers literally carry water underground. An aquifer
may be a layer of gravel or sand, a layer of sandstone
or cavernous limestone, a rubbly top or base of lava
flows, or even a large body of massive rock, such as
fractured granite, that has sizable cracks and fissures.
• In terms of storage at any one instant in time,
groundwater is the largest single supply of fresh water
available for use by humans.
9.
10. An estimated one million cubic miles of the world's
groundwater is stored within one-half mile of the
land surface.
Only a fraction of this groundwater, however, can
be practicably tapped and made available on a
perennial basis through wells and springs.
The amount of groundwater in storage is more than
30 times greater than the nearly 30,000 cubic-
miles volume in all the fresh-water lakes and
more than the 300 cubic miles of water in all the
world's streams at any given time.
13. • Groundwater is replenished by precipitation
and, depending on the local climate and
geology, is unevenly distributed in both
quantity and quality.
• When rain falls or snow melts, some of the
water evaporates, some is transpired by
plants, some flows overland and collects in
streams, and some infiltrates into the pores or
cracks of the soil and rocks.
14. • The first water that enters the soil replaces
water that has been evaporated or used by
plants during a preceding dry period.
• Between the land surface and the aquifer
water is the unsaturated zone. In this
unsaturated zone, there usually is at least a
little water, mostly in smaller openings of the
soil and rock; the larger openings usually
contain air instead of water.
15. • After a significant rain, the zone may be
almost saturated; after a long dry spell, it may
be almost dry. Some water is held in the
unsaturated zone by molecular attraction, and
it will not flow toward or enter a well.
16.
17. All geologic material beneath the earth’s surface
is either a potential aquifer or a confining bed.
An aquifer is also a saturated geologic formation
that will yield a usable quantity of water to a
well or spring. A confining bed is a geologic unit
which is relatively impermeable and does not
yield usable quantities of water. They restrict the
movement of ground water into and out of
adjacent aquifer.
18.
19. Recharge is the process by which ground water is
replenished. A recharge area is where water from
precipitation is transmitted downward to an
aquifer.
Most areas, unless composed of solid rock or
covered by development, allow a certain
percentage of total precipitation to reach the
water table. However, in some areas more
precipitation will infiltrate than in others. Areas
which transmit the most precipitation are often
referred to as "high" or "critical" recharge areas.
20. How much water infiltrates depends on vegetation
cover, slope, soil composition, depth to the water
table, the presence or absence of confining beds
and other factors. Recharge is promoted by natural
vegetation cover, flat topography, permeable soils,
a deep water table and the absence of confining
beds.
21. AQUIFER
Confined Aquifer Unconfined Aquifer
- Overlain by a confining bed -Has no confining bed
- Are recharged through - Open to infiltration from
cracks in impermeable layer surface
22. The water level in a confined aquifer does not rise
and fall freely because it is bounded by the
confining bed. Being bounded causes the water to
become pressurized. In some cases, the pressure
in a confined aquifer is sufficient for a well to
spout water several
feet above the ground.
Such wells are called
flowing artesian wells.
Confined aquifers are also
sometimes called artesian
aquifers.
23.
24. Discharge areas are the opposite of recharge
areas. They are the locations at which ground
water leaves the aquifer and flows to the surface.
Ground water discharge occurs where the water
table or potentiometric surface intersects the
land surface, or when we apply mechanical work
to pump out the water (wells). Where this
happens, springs or seeps are found. Springs and
seeps may flow into fresh water bodies, such as
lakes or streams, or they may flow into saltwater
bodies.
25.
26.
27. A water well is an excavation or structure
created in the ground, by digging, or drilling
to access water in underground aquifers.
28. The four most important parts of a water well
Casing is used to maintain an Screen keeps sand and gravel
open access in the earth while out of the well while allowing
not allowing any entrance or groundwater and water from
leakage into the well from the formations to enter into the
surrounding formations. The well. The most popular screen
most popular materials used for used are the stainless steel and
casing are black steel, slotted PVC pipe. Screen is
galvanized steel, PVC pipe and used when wells are drilled
concrete pipe. into unconsolidated materials.
Grout is a sealant that is used to fill in Gravel pack is placed
the spaces around the outside of the around the outside of the
well. It protects the well against the screen to prevent sand
intrusion of contaminants. A grout from entering the well or
mixture can be made of cement, or clogging the screen and to
concrete. stabilize the well assembly.
29. When water is withdrawn from a well, its water level
drops. When the water level falls below the water
level of the surrounding aquifer, ground water flows
into the well. The rate of inflow increases until it
equals the rate of withdrawal.
The movement of water from an aquifer into a well
alters the surface of the aquifer around the well. It
forms what is called a cone of depression. A cone of
depression is a funnel-shaped drop in the aquifer's
surface. The well itself penetrates the bottom of the
cone. Within a cone of depression, all ground water
flows to the well.
30.
31. As ground water flows downwards in an aquifer, its upper
surface slopes in the direction of flow. This slope is known
as the hydraulic gradient and is determined by measuring
the water elevation in wells tapping the aquifer.
For confined aquifers, the hydraulic gradient is the slope of
the potentiometric surface. For unconfined aquifers, it is
the slope of the water table.
32. A well can easily be contaminated if it is not
properly constructed or if toxic materials are
released into the well. Toxic material spilled or
dumped near a well can leach into the aquifer
and contaminate the groundwater drawn from
that well.
Contaminated wells used for drinking water are
especially dangerous. Wells can be tested to
see what chemicals, pathogens and other
contaminants may be in the well and if they are
present in dangerous quantities.
33. Things you can do to protect your groundwater and water well:
1. Maintain your well and test the water quality annually.
2. Keep household chemicals, paint and motor oil away from your well
and dispose of them properly by taking them to a recycling center or
household hazardous waste collection site.
3. Limit your use of pesticides and fertilizers.
4. Install a well cap and keep it clear of leaves, mulch, dirt, snow and
other materials.
5. Use caution when mowing around your well so you don't damage the
well casing.
6. Practice water conservation measures in your home and install low
water use appliances.
7. Learn more about well testing.
37. Natural Attenuation
• Natural attenuation refers to the strategy of
allowing natural processes to reduce
contaminant concentrations to acceptable
levels. Natural attenuation involves physical,
chemical and biological processes which act to
reduce the mass, toxicity, and mobility of
subsurface contamination These processes are
always occurring and in many cases may
reduce risk to human health and the
environment to acceptable levels.
