Integration of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 27

2

Integration of Plant and Groundwater Interactions

When he first came here with his divining rod,
he saw a thin vapor rising from the sward,
the hazel pointed steadily downward,
and he concluded to dig a well

growing where water was located very near the land surface.
Hence, even folkloric evidence of an interaction between
plants and groundwater has existed since the advent of water
witching. Fortunately, a more scientifically defensible pro-
cess that unifies the fields of plant physiology and hydroge-
ology is found in one of the most fundamental concepts of
hydrology—the hydrologic cycle.

Walden (H.D.Thoreau 1854)

The early investigations by plant physiologists who exam-
ined the interaction between plants and water were not
concerned about the source of the water. Hydrogeologists,
on the other hand, examined the effects that various demands
had on groundwater resources in the water-poor region of the
United States and reported the relation between depth to
groundwater and plant distribution. For the most part, these
investigations were carried out independent of each other
over time and space (refer to Table 1.3). The lack of integra-
tion between the fields of plant physiology and hydrogeol-
ogy continued throughout most of the twentieth century.
Evidence, however, of an early beginning of integration
can be traced back to, of all things, the ancient act of
dowsing for water using a divining rod. Commonly referred
to as water witching, the diving rod conjures up images of a
person under intense concentration pacing the ground hold-
ing a forked tree branch and waiting for the cut end to point
downward under its own energy, thus indicating the pres-
ence of groundwater (Roberts 1951). Although water witch-
ing dates back to the sixteenth century, it was discredited in
USGS Water-Supply Paper 416 because it was not consid-
ered to be scientifically defensible (Ellis 1917).
Whether or not one believes that water dowsers can find
groundwater, it is interesting in terms of the history of plant
and groundwater interactions to note the choices of wood
used by dowsers. For example, the most common woods
used are willow and witch hazel. Witch is from the Anglo-
Saxon word wicen meaning to bend. This aptly describes the
pliable nature of the wood of the witch hazel. Both witch
hazel and willow often can be found where the depth of
groundwater is shallow; these plants, therefore, can be clas-
sified as being facultative phreatophytes. Early dowsers
likely chose the willow and witch hazel for use as a divining
rod principally because the dowsers found these trees

2.1 Historical Observations of Water

Movement

Of the total volume of water on earth of about 330 million
cubic miles or about 360 quintillion gallons (1,360 quintil-
lion L), 97% is in oceans and too salty for direct use by
humans. The remaining 3% is freshwater, but most (2%) is
ice. About 0.6% is groundwater, and less than 0.001% is
surface water. Perhaps an easier way to envision the avail-
ability of freshwater is by analogy: if all of the earth's water
were equal to a gallon, the amount of freshwater available
for use would equate to a full tablespoon.
Under the influence of less than half of the sun's energy
that reaches the earth, water is continually vaporized from
the oceans and other surface-water bodies into the atmo-
sphere where the water vapor condenses as precipitation
and returns to the earth as freshwater. Because little new
water has been created over time, the water in the oceans,
rivers, lakes, atmosphere, and underground are continually
exchanged back and forth between compartments. Some
water is slowly returned into the cycle, as occurs when
inland surface water evaporates and returns as precipitation
in another basin. Groundwater discharge takes the longest
amount of time to complete the cycle. Total exchange of
global water occurs about every 3,000 years.
The exchange of a fixed, finite source of water between
compartments is defined as the hydrologic cycle. The hydro-
logic cycle can be viewed as an account, at any given time,
of the status of water in the various compartments of the