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Summary Chapter 19 A hidden reserve: Groundwater
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Summary Chapter 19 A hidden reserve: Groundwater. Marshak: Earth portrait of a planet
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Chapter 19: A hidden reserve: Groundwater19.1 Introduction
Groundwater: the liquid water that resides in sediment or rock under the surface of the Earth, had
gradually dissolved the limestone over tme, carving open rooms underground. Owen’s backyard
began to collapse, forming a circular depression called a sinkhole. We can easily see Earth’s surface
water and atmospheric water (clouds and rain), but we can’t see groundwater, lies hidden beneath
the surface.
19.2 Where does groundwater reside
A small part of the water from the hydrologic cycle, sinks or percolates downward, by a process
called infltraton, into the ground. Can take place because most materials (sediment and rocks) are
not perfectly solid, but rather contain some open spaces.
Porosity: The open space in rock and regolith
The existence of water underground requires the resistance of open space underground. Small part is
provided by caves. But most groundwater resides in tny open spaces or voids within sediment and
within seemingly rock. Pore: any open space within a volume of sediment, or within a body of rock.
Porosity: the total amount of open space within a material, specifed as a percentage.
Primary porosity: develops during sediment depositon and during rock formaton. The space that
remains between solid grains or crystals immediately afer sediment accumulates or rock forms. By
badly-sorted sediments, there are less pores because small grains fll those up. Also depends on
amount of compacton and cementaton. Tends to decrease with increasing burial depth. In chemical
and biochemical sedimentary rocks, it develops because mineral crystals don’t grow against each
other. In crystalline igneous/ metamorphic rock it can happen because grains don’t interlock. And in
fne-grained or glassy igneous rocks, it may consist of vesicles, relicts of air bubbles that were
trapped during cooling. 1% crystalline igneous and metamorphic to 30% in well-sorted poorly
cemented sandstone.
Secondary porosity: new pore space in rocks, created some tme afer a rock frst forms. When water
dissolves minerals or cement, creatng small soluton cavites. Also forms from fracture. The space
between breccia fragments can also serve as secondary pore space.
Permeability: the ease of fow
Permeability: the ability of a material to allow fuids to pass through an interconnected network of
pores. fows easy through a permeable material and fows slowly or not at all through an
impermeable material. (high, low, no). depends on several factors:
¤ Number of available conduits: more conduits is higher permeability
¤ Size of conduits: wider conduits is a larger volume, they can travel faster because of less
fricton with the wall.
¤ Straightness of the conduits: water fows faster through straight conduits.
A material whose pores are isolated from each other can have high porosity but low permeability. So
it is not the same thing.
Aquifers and aquitards
Aquifers: sediment or rocks that hold a lot of groundwater and transmit it easily because they have
high porosity and permeability. Coarse gravels, poorly cemented sandstone, highly fractured and
partally dissolved limestone.
1
, Aquitards: sediment or rocks that have low permeability and do not transmit groundwater easily.
Shales, evaporates and very well-cemented sandstones.
Water can infltrate directly into an unconfned aquifer. Water can’t migrate directly down into a
confned aquifer, for an aquitard isolates the water in a confned aquifer from the Earth’s surface.
An unconfned aquifer is one that connects to the ground surface, while a confned aquifer is one
that lies beneath a aquitard. There are many diferent kinds of aquifers, each with distnct geologic
characteristcs.
Examples read them.
19.3 characteristics of the water table
Unsaturated zone (vadose zone), is the region of the subsurface in which water only partally flls
pores. If the top of the unsaturated zone includes a soil, we can also refer to the water wetng the
surface of grains and organic material making up the soil as soil moisture. This water tends to
evaporate, seep into stream, or get sucked up by roots and transpire back into the atmosphere.
Under the unsaturated zone we fnd the saturated zone (phreatc zone). Water flls all pores. Water
table: is the horizon that separates the unsaturated zone above from the unsaturated zone below.
Capillary fringe: the thin subsurface layer in which water molecules seep up from the water table by
capillary acton to fell pores. (5-30 cm)
The depth of the water table varies greatly over diferent regions. Rainfall rates afect the water table
depth. Groundwater only occurs in the upper crust (12-20km)
Topography of the water table
In hilly regions, if the subsurface has low to moderate permeability. The water table’s shape mimics,
in a subdued way, the shape of the overlying topography. Water table lies higher under a relief. The
relief of the water table is not as great as that of the overlying substrate. Water table is smoother.
The elevaton of the water table varies because groundwater moves so slowly through rock and
sediment that it cannot quickly assume a horizontal surface. When it rains water table rises slightly.
When it doesn’t rain it will likely fall again, but so slowly that rain fll likely fall again, making the
water table rise again, before it has tme to sink very far.
perched water tables
a quantty of groundwater that lies above the regional water table because an underlying lens of
impermeable rock or sediment prevents the water from sinking down to the regional water table.
Where the water table intersects valleys or depressions, streams and lakes form.
19.4 groundwater fow
Groundwater fow paths
In the unsaturated zone water is just sinking down under the pull of gravity. Groundwater fow paths:
the overall trajectory that groundwater follows over tme, we must frst understand the origin of
pressure in groundwater (drijfracht behind groundwater fow). right no unconfned aquifer.
Pressure is caused by the weight of all the overlying water from that point up to the water table. (not
of overlying rock!). if the water table is horizontal, the pressure is equal on a horizontal axis. If it is
not horizontal, it is not equal on a horizontal axis.
Both the elevaton of a volume of groundwater and the pressure within the water provide energy
that, will cause water to fow. (provides potental energy). Hydraulic head: the potental energy
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