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.
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.
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.
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.
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.
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.