Groundwater is the word used to describe water that saturates the ground, filling all the available spaces. By far the most abundant type of groundwater is meteoric water; this is the groundwater that circulates as part of the water cycle. Ordinary meteoric water is water that has soaked into the ground from the surface, from precipitation (rain and snow) and from lakes and streams. There it remains, sometimes for long periods, before emerging at the surface again. At first thought it seems incredible that there can be enough space in the "solid" ground underfoot to hold all this water.
The necessary space is there, however, in many forms. The commonest spaces are those among the particles-sand grains and tiny pebbles-of loose, unconsolidated sand and gravel. Beds of this material, out of sight beneath the soil, are common. They are found wherever fast rivers carrying loads of coarse sediment once flowed. For example, as the great ice sheets that covered North America during the last ice age steadily melted away, huge volumes of water flowed from them. The water was always laden with pebbles, gravel, and sand, known as glacial outwash, that was deposited as the flow slowed down.
The same thing happens to this day, though on a smaller scale, wherever a sediment-laden river or stream emerges from a mountain valley onto relatively flat land, dropping its load as the current slows: the water usually spreads out fanwise, depositing the sediment in the form of a smooth, fan-shaped slope. Sediments are also dropped where a river slows on entering a lake or the sea, the deposited sediments are on a lake floor or the seafloor at first, but will be located inland at some future date, when the sea level falls or the land rises; such beds are sometimes thousands of meters thick.
In lowland country almost any spot on the ground may overlie what was once the bed of a river that has since become buried by soil; if they are now below the water's upper surface (the water table), the gravels and sands of the former riverbed, and its sandbars, will be saturated with groundwater.
So much for unconsolidated sediments. Consolidated (or cemented) sediments, too, contain millions of minute water-holding pores. This is because the gaps among the original grains are often not totally plugged with cementing chemicals; also, parts of the original grains may become dissolved by percolating groundwater, either while consolidation is taking place or at any time afterwards. The result is that sandstone, for example, can be as porous as the loose sand from which it was formed.
Thus a proportion of the total volume of any sediment, loose or cemented, consists of empty space. Most crystalline rocks are much more solid; a common exception is basalt, a form of solidified volcanic lava, which is sometimes full of tiny bubbles that make it very porous.
The proportion of empty space in a rock is known as its porosity. But note that porosity is not the same as permeability, which measures the ease with which water can flow through a material; this depends on the sizes of the individual cavities and the crevices linking them.
Much of the water in a sample of water-saturated sediment or rock will drain from it if the sample is put in a suitable dry place. But some will remain, clinging to all solid surfaces. It is held there by the force of surface tension without which water would drain instantly from any wet surface, leaving it totally dry. The total volume of water in the saturated sample must therefore be thought of as consisting of water that can, and water that cannot, drain away.
The relative amount of these two kinds of water varies greatly from one kind of rock or sediment to another, even though their porosities may be the same. What happens depends on pore size. If the pores are large, the water in them will exist as drops too heavy for surface tension to hold, and it will drain away; but if the pores are small enough, the water in them will exist as thin films, too light to overcome the force of surface tension holding them in place; then the water will be firmly held.
Groundwater is the word
used
to
describe
water
that saturates the
ground
, filling all the available
spaces
. By far the most abundant type of groundwater is meteoric
water
; this is the groundwater that circulates as part of the
water
cycle. Ordinary meteoric
water
is
water
that has soaked into the
ground
from the
surface
, from precipitation (rain and snow) and from lakes and streams. There it remains,
sometimes
for long periods,
before
emerging at the
surface
again. At
first
thought
it seems incredible that there can be
enough
space
in the
"
solid
"
ground
underfoot to hold all this water.
The necessary
space
is there,
however
, in
many
forms. The commonest
spaces
are those among the particles-sand grains and tiny pebbles-of loose, unconsolidated
sand
and gravel. Beds of this material, out of sight beneath the soil, are common. They
are found
wherever
fast
rivers
carrying loads of coarse
sediment
once flowed.
For example
, as the great ice sheets that covered North America during the last ice age
steadily
melted away, huge volumes of
water
flowed from them. The
water
was always laden with pebbles, gravel, and
sand
, known as glacial
outwash
, that
was deposited
as the flow slowed down.
The same thing happens to this day, though on a smaller scale, wherever a sediment-laden
river
or stream emerges from a mountain valley onto
relatively
flat land, dropping its load as the
current
slows: the
water
usually
spreads out
fanwise
, depositing the
sediment
in the form of a smooth, fan-shaped slope.
Sediments
are
also
dropped where a
river
slows on entering a lake or the sea, the deposited
sediments
are on a lake floor or the seafloor at
first
,
but
will
be located
inland at
some
future date, when the sea level falls or the land rises; such beds are
sometimes
thousands of meters thick.
In lowland country almost any spot on the
ground
may overlie what was once the bed of a
river
that has since become buried by soil; if they are
now
below the water's upper
surface
(the
water
table), the gravels and
sands
of the former riverbed, and its sandbars, will
be saturated
with groundwater.
So
much for unconsolidated
sediments
. Consolidated (or cemented)
sediments
, too, contain millions of minute water-holding pores. This is
because
the gaps among the original grains are
often
not
totally
plugged with cementing chemicals;
also
, parts of the original grains may become dissolved by percolating groundwater, either while consolidation is taking place or at any time afterwards. The result is that sandstone,
for example
, can be as porous as the loose
sand
from which it
was formed
.
Thus
a proportion of the total volume of any
sediment
, loose or cemented, consists of empty
space
. Most crystalline
rocks
are much more solid; a common exception is basalt, a form of solidified volcanic lava, which is
sometimes
full of tiny bubbles that
make
it
very
porous.
The proportion of empty
space
in a
rock
is known
as its porosity.
But
note that porosity is not the same as permeability, which measures the
ease
with which
water
can flow through a material; this depends on the sizes of the individual cavities and the crevices linking them.
Much of the
water
in a sample of water-saturated
sediment
or
rock
will
drain
from it if the sample
is put
in a suitable dry place.
But
some
will remain, clinging to all solid
surfaces
. It
is held
there by the force of
surface
tension without which
water
would
drain
instantly
from any wet
surface
, leaving it
totally
dry. The total volume of
water
in the saturated sample
must
therefore
be
thought
of as consisting of
water
that can, and
water
that cannot,
drain
away.
The relative amount of these two kinds of
water
varies
greatly
from one kind of
rock
or
sediment
to another,
even though
their
porosities
may be the same. What happens depends on pore size. If the pores are large, the
water
in them will exist as drops too heavy for
surface
tension to hold, and it will
drain
away;
but
if the pores are
small
enough
, the
water
in them will exist as thin films, too light to overcome the force of
surface
tension holding them in place; then the
water
will be
firmly
held.