Integration of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 34

magnitude of the surface tension of water can be defined as
the energy needed to increase this reduced surface area by a
unit amount. The energy required to overcome these inter-
molecular forces is equivalent to about 540 cal/g (calories
per gram) of water evaporated at 100 C, and up to 590 cal/g
at 15 C. This is double the heat required to vaporize
alcohols, such as methanol and ethanol, and four times the
heat required to vaporize the organic compound benzene.
Even a compound similar in structure to water, hydrogen
sulfide (H 2 S), is a gas at room temperature and has a boiling
point of 60.7 C.

Under ambient environmental conditions, energy neces-
sary to vaporize water is provided by the radiated or
advected energy of the sun. The energy is conserved,
because the temperature of the evaporated water does not
change. Water also can be evaporated along gradients in
vapor pressure, which will be discussed next.

2.3.2 Vapor Pressure

As liquid water molecules become energized and enter the
atmosphere as vapor, the vapor molecules exert pressure on
the remaining liquid water molecules. If this process is
constrained inside a fixed volume, such as inside a glass
test tube inverted into a dish of mercury that rises to a
level in the tube balanced by the weight of the atmosphere,
the pressure exerted by the vapor molecules can be
measured. At equilibrium, vapor pressure on the liquid
water molecules is 17.5 millimeters of mercury (mmHg)
(0.694 in.) at room temperature (Fig. 2.6 ). The vapor pres-
sure of water is not constant, however, for it decreases as
solutes are added—solutes essentially dilute the water, and
lower the vapor pressure. This explains why the solute
ethylene glycol is added to the water used in pressurized
cooling systems for internal combustion engines, because
the water-coolant solution can exceed temperatures greater
than the boiling point of pure water.
The rate of water evaporation from a free surface can be
defined as the net exchange of water molecules across the
water surface per unit time. Evaporation continues until the
air becomes saturated with water vapor, which is the abso-
lute humidity that can be held by a given quantity of air at a
given temperature. Warmer air can hold more moisture than
cooler air as long as adequate supplies of water are available.
This relation to water availability explains the dry heat that
characterizes arid areas.

Fig. 2.5 Strong intermolecular forces of the water molecule lead to
surface tension and control the shape of water droplets, such as precipi-
tation on a leaf falling into porous media. The relative strength of
surface tension is depicted by the length of the arrows .

raindrops rather than the teardrop shape commonly depicted.
The spherical shape represents the smallest surface area a
volume of liquid water can achieve while keeping tension to
a minimum. This attraction also explains why isolated drops
of water take a spherical shape; the surface tension pulls the
water inwards in order to occupy the smallest volume. In
plant and groundwater interactions, the surface tension of
water also explains the capillary action of water in porous
media as well as the cohesion theory of the ascent of sap, and
why phreatophytes can use both capillary and groundwater,
all which are described in Chaps. 3 and 4.
For this minimal volume of a sphere to be changed or
increased, energy must be expended, so the shape of water
droplets also has a thermodynamic explanation. As we will
see in Chap. 3, this also explains why plants encounter a
wilting point, where water present in soil cannot be removed
because it is held under too great a tension. As such, the

2.3.3 Evaporation

Evaporation is fundamental to understanding the interaction
between plants, water, and groundwater. Under equilibrium