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Guide
to Reading 
Chapter 16 continues the study of the compound that
makes Earth unique in our solar system, liquid water. As explained
in the previous two chapters, the greatest amount of Earth’s
water is salty and is found in oceans (Chapter 15), and the obvious
freshwater supply flows on the surface in streams and lakes (Chapter
14). A much larger quantity of freshwater, hidden below the surface
and rarely seen, is the subject of this chapter.
Your author begins by reminding you that water moves
between various reservoirs on, above, and under Earth’s surface
as part of a phenomenon known as the hydrologic cycle. Most groundwater
begins as moisture that falls on land and infiltrates the ground.
Some remains close to the surface as soil moisture; more percolates
downward through narrow, crooked channels in the bedrock. The reason
water can exist and travel through “solid rock” involves
a discussion of pore spaces, porosity and permeability. A rock layer
is classified as an aquifer (confined or unconfined) or as an aquiclude
(or aquitard) on the basis of its porosity and permeability (that
is, its ability to hold and conduct water).
Freshwater is a vital natural resource. To successfully
and efficiently tap a groundwater supply, hydrologists study geochemistry
and subsurface structure in the potential resource area. They are
concerned with the ion content of the water (precipitates and hard
water). They try to discern where the unsaturated zone (or zone of
aeration), capillary fringe, water table, and saturated zone lie.
An ordinary well must be drilled down past the first three layers
and into the fourth if it is to become a dependable, long-term water
producer and not just a seasonal well or a dry well. They identify
special features like perched water tables and artesian wells (flowing
and nonflowing) and special regions like recharge and discharge areas
so they can be used advantageously and not create problems. They
consider the head of the water table, the hydraulic gradient and
hydraulic conductivity of a region, and the effects of local drawdown
and cones of depression when deciding how much groundwater an area
will yield. They use the formula called Darcy’s law to calculate
the discharge figure. Sometimes hydrologists don’t have to
search for groundwater; it reaches the surface on its own. You read
about several types of geologic settings that yield springs and about
the special setting needed to produce an artesian spring.
Geothermal areas offer unique groundwater-caused
surface features including hot springs, mud pots, and geysers that
serve sometimes as tourist or recreational attractions and sometimes
as energy sources.
There are problems associated with groundwater usage,
like water depletion. Are we carelessly exhausting a nonrenewable
resource and ruining land areas in the process? Your author uses
several examples in discussing this complex issue. In many locations
it is true that
- Mining of groundwater is occurring (withdrawal
faster than natural recharge), so the water table is being lowered
and the flow direction of groundwater is being reversed.
- Saline intrusion is causing wells to yield useless
salty water.
- Irreversible pore collapse and land subsidence
are occurring.
Groundwater contamination is another problem, one
that’s especially serious because it’s so hard to clean
up once it has occurred. Society is growing increasingly concerned
about putting any wastes in the ground that might pose a threat to
local groundwater. You read about categories of contaminants, injection
wells, contaminant plumes, and bioremediation.
The chapter concludes on a lighter note, with a
discussion of unusual karst landscapes (with sinkholes, natural bridges,
disappearing streams, and tower karst), and the fascinating world
spelunkers explore, caves. Since groundwater is naturally slightly
acidic, it dissolves rock (chiefly limestone) to produce cave networks
of passages and chambers that sometimes, due to the precipitation
of limestone out of solution, get decorated with dripstone speleothems
(soda straws, stalactites, stalagmites, and limestone columns) and
flowstone and populated by unusual life forms that have adapted to
life away from light.
In review, Earth’s subsurface region is, like
its surface region, a world of water, but in a subtle way. There
are few streams that disappear into the ground and flow there, but
there is a great quantity of water moving slowly underground through
connecting microscopic pores, transporting materials in solution,
indirectly sculpting the land, and providing fresh water for human
needs when we’re wise enough to figure out where and how to
tap into the supply.
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