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 particlessand grains and tiny pebblesof
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.