Chapter 16 – Groundwater
Water beneath the surface is known as groundwater. The amount of open space between soil
particles is porosity. The ability of water to pass through the soil particles is called
permeability. Just because the soil is porous, this does not mean it is permeable. Tiny clay
particles have space between them (they are porous) but the spaces are so small that it is
difficult for water to pass through them (they are not permeable). Well-sorted sediment, where
particles are separated by size, typically does not pack as tightly as poorly sorted sediment. This
is because the smaller particles fill in the spaces between the larger particles. A rock or
sediment layer that stores and allows water to pass through it is called an aquifer. Well-sorted
sand and gravel layers make the best aquifers because the spaces between the particles are
large. The area where the aquifer is on the surface is called the recharge zone.
The upper boundary of the ground water is called the water table. Below the water table, the
zone of saturation, the spaces between the sediment is filled with water. Above the water
table, the zone of aeration, the spaces are filled with air. Just above the water table, a process
called capillary action may occur as the water molecules are attracted to the sediment in the
zone of aeration.
The water table often, but not always, mirrors the topography. It depends on the geology
below the surface. If there is an impermeable layer, such as clay or bedrock, an artificially
shallow depth water table may form. This is called a perched water table. When a sloping
aquifer is sandwiched between two impermeable layers, the water may be forced to the
surface by gravity and flow freely on the surface without the need for a pump. This is called an
artesian well. The geologic structure described above is called an artesian formation.
Where the water table intersects the land surface at one point a spring is produced. A hole that
is drilled to a depth below the water table is called a well. Wells usually are drilled far below the
water table into a reservoir area so that the well will not run dry during seasonal fluctuations of
the water table. A well may also go dry as the result of a larger well drilled into the same
aquifer, causing the water table to drop in the immediate vicinity. When water is being pumped
from a well the water table near it drops in what is called a cone of depression. In coastal areas,
when fresh water is pumped out, the more dense saltwater may intrude and render the well
unusable.
Carbon dioxide from the air combines with precipitation to produce carbonic acid. The acidic
groundwater easily erodes the limestone to produce caves, caverns, sinkholes, and
disappearing streams. This type of topography is referred to as karst topography. When the
roof of a cavern collapses a sinkhole is produced. This often occurs when the drainage of an
area is changed. Underground caves and caverns that were once filled with water lose support
when the water is drained away and collapse. When two or more sinkholes are formed,
sometimes the rock between them is left as a natural bridge. The limestone and dolomite
features that form on the ceiling of caverns as the rock precipitates out of the groundwater are
called stalactites (c for ceiling). The features that form on the ground of the cavern are called
stalagmites (g for ground). Sometimes a stalactite will join with a stalagmite to form a column.
Water that contains relatively high amounts of minerals rich in calcium, magnesium, and iron is
called hard water. Water with low amounts of these minerals is referred to as soft water.
Groundwater pollution in the Shenandoah Valley is a concern. This is due to the abundance of
limestone bedrock that is easily dissolved by the groundwater. Pollution is transported
extremely fast and easily underground in these areas as the pollutants are not filtered by the
soil.
Magma interacting with groundwater produces a number of spectacular features such as hot
springs, geysers, and mud pots. The feature produced largely depends on the type of bedrock
or soil that the hot water rises through.