Ground Water
Ground water is water found beneath the ground surface. It occupies the pore space between grains in
bodies of sediment and sedimentary rock, and fills cracks and crevices in all types of rock. Ground water can also
be written as groundwater or ground-water.
I
Porosity and Permeability
A.
: The percentage of an Earth material's volume that has openings.
A porous rock or Earth material can hold a significant amount of water.
1.
Factors affecting porosity:
a)
________________: Generally, if the particles are rounded and sorted, particle size
does not affect the porosity.
As particles become unsorted the porosity decreases due to the smaller particles
filling in the pore spaces (See Diagram 1).
b)
____________________: Loosely packed soils have a lower porosity than more
compact soils. As packing increases, the pore spaces become smaller. (See diagram
2 below.)
c)
____________________: As the angularity of particles increases, porosity decreases.
This is because the particles will fit together more effectively.
DIAGRAM 1
B.
DIAGRAM 2
DIAGRAM 3
____________________: The ability of an Earth material to transit a fluid through pores and/or
fractures. This is often expressed as a rate (ie. cm/sec.).
1.
Factors Affecting Permeability:
a)
Particle ____________: As particle size increases, permeability increases.
b)
Particle _________: As particle shape becomes more angular, permeability decreases
.
Particle ______________: As particle packing decreases, permeability increases.
c)
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Ground Water
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2.
_______________ Earth materials:
A material that is impermeable will not allow fluids to
move through it.
3.
______________________Permeability:
A rock that is otherwise impermeable may
permit fluids to move through cracks that have developed due to weathering (ie. limestone,
granite). As a result not all permeable rocks have high porosity.
4.
_________________Infiltration Depth
a)
The depth to which fluids can move through permeable material is 3,000 m (3 km).
b)
At this depth confining pressure compresses the rock and closes pores and cracks.
5.
Porous Earth Material can be Impermeable
Not all porous Earth materials are permeable. For example, clay has high porosity but
because of its very small particles size, its permeability is so low that it is considered to
be impermeable.
II.
Aquifers
Any body of saturated rock or sediment through which water can move easily is referred
to as an aquifer.
“
A.
Aquifers are porous and permeable.
Subsurface Zones
1.
__________________________: The region in which all pore spaces are filled with water.
2.
______________________: The upper limit (“top”) of the zone of saturation. It follows the
surface topography.
3.
_________________________: The zone above the water table in which air occupies the
pore spaces. Water is present, on the surface of the soil or rock particles.
4.
__________________________: This is the zone above the water table in which water moves
upwards from particle to particle due to adhesion and cohesion.
a)
b)
5.
Capillarity increases as particle size decreases.
Generally, the capillary fringe is not more than one meter thick.
______________Water Table:
a)
The top of a body of ground water separated from the main water table beneath it
by a zone that is not saturated.
b)
It may form as ground water collects above a lens of realtively imermeable shale
with a more permeable rock, such as sandstone.
B.
_____________________Aquifers
1.
2.
3.
Recharged rapidly by precipitation
Partly filled with water
Gravity wells yield water by pumping
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Ground Water
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C.
__________________Aquifers (Artesian Aquifers)
1.
2.
3.
4.
5.
c)
Separated from the surface by an __________________ confining bed (often shale or clay)
called an ________________.
Completely filled with water under pressure
Recharged slow through the confining bed.
May have no response to seasonal changes in precipitation.
______________________
a)
Drilled into confined aquifers and have water that rises under its own pressure
b)
Common in South Dakota but also present on Long Island.
The name is derived from the Arois region of France (artesien in old French) where these
wells have been used for centuries.
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Ground Water
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D.
__________________
1.
2.
3.
E.
III.
Water flows natural onto the land surface.
Can occur where the water table intersects the land surface.
Can also occur in caverns, along fractures, along faults, or rock contacts that intersect
the land surface
Climate influences the relationship between stream flow and the water table.
1.
_________________Streams
a)
Rainy climates
b)
The stream’s surface coincides with the water table.
c)
Water from the zone of saturation flows through the streambed and banks that are
below the water table.
2.
_________________Streams
a)
Drier climates
b)
Water is lost to the zone of saturation.
c)
The stream channel lies above the water table.
d)
Water lost from the stream can cause a rise in the water table. As a result, in a
desert the nearest water source may be below a dry streambed.
Effects of Ground Water Action
A.
Caves, Sinkholes, and Karst Topography
1.
2.
3.
4.
Regions characterized by sinks, lost rivers, underground drainage, caves and caverns.
Limestone bedrock
Named after the Kars region of Yugoslavia
Also found in United States (Cumberland Plateau of Kentucky and Tennessee,
Shenandoah Valley of Virginia, and Florida).
KARST TOPOGRAPHY
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Ground Water
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B.
IV
Hot Water Underground
1.
___________________
a)
Circulation near a magma chamber
b)
Circulation near hot, cooling igneous rock.
2.
______________________: Hots springs that periodically erupt hot water and steam.
3.
Associated Features
a)
Precipitation of minerals forming travertine (calcite) and sinter (from silica and
called geyserite).
b)
Mudpots:
%Hot springs containing thick, boiling mud
%From intense chemical weathering of bedrock from strongly acidic solutions.
4.
___________________ Energy: Steam is harnessed to generate electricity.
Long Island’s Groundwater
The sediment deposits beneath Long Island are divided into several aquifers and confining layers.
They are all part of a system that is interconnected by the groundwater that moves through and between
them. The aquifer formations are:
‘
the Upper Glacial Aquifer:
On the average this aquifer has the greater rates of vertical flow due to its composition of
outwash deposits, such as fine to coarse sand and gravel. The high porosity and
permeability of these deposits readily permits water movement throughout.
‘
the Magothy Aquifer
Vertical movement of water is impeded by horizontal lenses of clay and silt. Discontinuity
(breaks) In these horizontal layers of impermeable sediment does allow water to move
vertically.
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Ground Water
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‘
the Lloyd Aquifer
Recharge is from water moving downward through the Upper Glacial, the Magothy, and
the
Raritan Clay. While the Raritan Clay that lies above the Lloyd sands confines the
aquifer, it does not completely restrict water flow from one aquifer to another. Sections of
the Raritan are more permeable than others. Beds of sand are common and thin beds of
gravel occur locally. The Raritan clay layers are not continuous throughout the formation
due, mst likely, to sporadic deposition and erosion after their formation.
Confining layers are another type of formation found in Long Island’s hydrologic system. The Raritan
Clay, located above the Lloyd Aquifer is characteristically impermeable. Beneath the Lloyd Aquifer is
crystalline bedrock. As a result, the Lloyd Aquifer is a confined aquifer, overlain by the aquitard of the
Raritan Clay and underlain by the impermeable bedrock.
Depending on its location, the groundwater in Long Island’s aquifers is stored under two distinct
conditions: (1) unconfined (water table) and (2) confined (artesian). The unconfined conditions are found
in the Upper Glacial aquifer. In the Magothy unconfined conditions and confined conditions are found,
depending upon the location of lenses of relatively impermeable sediments. Groundwater in the Lloyd
Aquifer moves under confined conditions.
Rainwater that travels through the soil and reaches the groundwater is called recharge. Water is
constantly leaving the groundwater system by natural outflow or by human use. Recharging water
replenishes the water that leaves the system. Recharage basins (sumps) are excavated in developed
regions to facilitate natural recharge. These artificial recharge areas are needed because building
construction and paving have disturbed natural rainwater infiltration patterns and increased runoff.
Groundwater moves downward vertically towards the underlying bedrock along the central eastwest region of Long Island. It then arcs back upwards flowing northward and southward. The region
where the groundwater is moving vertically is referred to as the groundwater divide and is marked by the
Ronkonkoma Moraine. Groundwater moves primarily horizontally as it moves away from the divide. As
a result of this shallow horizontal movement, water recharging in areas near the coast has a shorter
residence time in the aquifer system.
The porous soils of the Long Island Pine Barrens makes these landforms an excellent natural recharge
area. Fire climax ecosystems, ecological systems that maintain their dominance by encouraging and
adapting to fire developed in these areas. These have become associated with Long Island’s aquifer
system.
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Ground Water
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Vertical Exaggeration is approximately 50 X
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Ground Water
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