I. Fundamentals of groundwater in siliciclastic (clastic sedimentary)
rocks -- Groundwater is water that seeps into the ground and can
flow in the subsurface. It is one component of the hydrologic cycle,
mentioned previously. Groundwater seeps through the ground by way
of spaces between sediments in clastic (also siliciclastic) rock, and by
way of cracks in all types of rock, but especially crystalline rocks (igneous
and metamorphic) and carbonates (limestone).
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A. There are two main properties that describe the behavior of groundwater.
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1. Porosity is the amount of space (volume) that is openings which
are available to hold water. Porosity is made up of pores between
sediment grains (clastic sedimentary rocks), and cracks (igneous, metamorphic,
and limestone). In comparing two samples of sediments, each with
a different, but uniform sediment size, the porosity differs little (refer
to the lecture example). Porosity can be measured as a percentage
of the total volume of rock that is open space available to water or petroleum.
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2. Permeability refers to the ease of flow of a fluid through
rock. Permeability is an indication of how interconnected and how
big the pores and fractures are. Rocks with little or no porosity,
or with small sediment size will have no to little permeability.
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B. There are two main regions that can be identified with respect
groundwater.
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1. In the saturated zone, water completely fills all the pores/fractures.
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2. In the unsaturated zone, the pores/fractures are dry or
only partially filled with water. An unsaturated zone is usually
found above the saturated zone.
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C. The boundary between the saturated and unsaturated zones is called
the water table.
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1. The water table rises or falls depending on how much water has
recently seeped into the ground or how much groundwater is pumped out of
wells.
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2. Generally the water table surface follows the trend of the land
surface. Groundwater flows generally from regions where the water
table is higher to regions where it is lower.
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3. When the water table intersects the ground surface, standing or
flowing bodies of water -- such as lakes, ponds, streams, rivers -- may
appear.
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D. Aquifers
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1. Saturated sediment or rocks through which water may move easily is called
an aquifer. Sands, sandstones, gravels, and conglomerates
are good examples of aquifers.
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2. A sediment or rock in which rock tends to move slowly is an aquitard.
Shales, clay, and many crystalline rocks are good examples of aquitards.
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3. A confined aquifer is one which is sandwiched between two
aquitards. A confined aquifer is only very slowly recharged because
of the impermeable upper confining layer.
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4. An unconfined aquifer is one which is not separated from
the surface by a confining layer. An unconfined aquifer may be recharged
very quickly.
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5. Certain confined aquifers are under enough pressure that when
wells are drilled into them, water will rise to the ground surface under
its own pressure. Such wells are called artesian wells.
Such aquifers are called artesian aquifers. (You should study
the diagrams in your book to see how enough pressure is generated)
If these aquifers are drained o enough water, then there is no longer enough
pressure to spontaneously discharge water at the ground surface.
II. Groundwater in carbonate rocks (limestone)
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A. Groundwater typically flows through limestones by way of cracks.
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B. Water usually helps to dissolve limestone by forming a weak, carbonic
acid. Groundwater is slightly acidic because a small amount of
atmospheric carbon dioxide (CO2).
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C. This dissolution widens cracks into caves or caverns.
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D. Seeping groundwater dripping from the ceilings of limestone caves
can produce limestone column deposits such as stalactites or stalagmites.
Stalactites
are pillar-like structures that hang from the ceiling.
Stalagmites
build from the ground of the cave under a stalactite.
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E. A limestone cavern may grow large enough that the roof of the
cave may collapse, producing a sinkhole that is exposed to the ground
surface.
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F. Sinkholes are a main feature of karst topography. Karst
topography is landscape that forms where carbonate bedrock are common,
and there is substantial precipitation. Karst topography, therefore,
is common in temperate or tropical regions of the world -- Puerto Rico,
Florida, Southern China. Other features of karst topography include:
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1. Disappearing streams
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2. Underwater streams
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3. Karst towers
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4. Karst valleys
III. Groundwater management and contamination
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A. Pesticides, herbicides, radioactive waste, our own garbage, animal waste,
untreated sewage and old gasoline tanks are all sources of material that
can seep in and contaminate the groundwater.
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B. Groundwater often buoys up the ground, much in the same way that
a wet sponge is thicker than a dried out sponge. By drawing too much
ground water out of the ground, ground subsidence may result (refer to
the caption in your book.
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C. If groundwater flows through hot rock, then the water will quickly
expand and erupt as a geyser. A geyser can be capture and harnessed
to run geothermal plants.
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