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.