Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 142

of precipitation, whereas Texas is at a lower elevation and
has only 15 in./year. This results in a longer growing season
of 240 days in Atlanta relative to 200 days in Lubbock.
Moreover, the western United States has many
microclimates that range from the dry deserts of Nevada
and southeastern California to the moist rainforests of
Washington. Therefore, the average growing season length
for a particular area must be used with care when assessing a
site for phytoremediation.
Potential evapotranspiration maps have been constructed
for the conterminous United States and also are useful for
site-assessment and characterization purposes for potential
phytoremediation projects (Fig. 6.4 ). This map indicates that
higher potential evapotranspiration, ET p , rates are related to
the south and western states where the growing season
length is longer. In general, higher ET p rates indicate greater
potential for hydrologic control by phytoremediation. More-
over, the greater the proportion of an area's water
budget allocated to ET relative to precipitation suggests an
even stronger potential for hydrologic control by
phytoremediation to occur (Fig. 6.5 ). For example, values
less than zero indicate water-limited conditions and a high
potential for groundwater control and values greater than
zero indicate energy-limited conditions (modified from
Healy et al. 2007).

Fig. 6.5 Relation of precipitation to potential evapotranspiration in
areas of the conterminous United States. Values less than zero indicate
water-limited conditions. Values greater than zero indicate energy-
limited conditions (Modified from Healy et al. 2007).

The magnitude of the VPD represents the difference
between the vapor pressure at a given air temperature at
saturated conditions and the actual vapor pressure at the
same temperature (Kucera 1954). Vapor-pressure deficit
can be determined by using common meteorological
parameters, such as air temperature and relative humidity.
Although the atmospheric pressure remains fairly constant at
any given location, the lack of water and high specific heat
characteristics lead to great fluctuations in air temperature
on a daily basis in arid areas. Also, the relative humidity of a
location changes on a daily basis and with respect to the
measurement location. The VPD is important for plant tran-
spiration and, therefore, phytoremediation because at a
constant air temperature, with no direct input of radiant
energy, water evaporates along vapor-pressure gradients,
until equilibrium conditions are reached.

6.4.5 Plant-Available Water

Most plants thrive in well-drained soils for a number of
reasons. First, loose soils permit root penetration. Second,
well-drained soils permit the infiltration of precipitation.
Third, well-drained soils permit atmospheric influx, includ-
ing oxygen needed for respiration and nitrogen needed for
fixation. Fourth, water from specific retention is held to the
soil with less tension in soils with smaller pores and, there-
fore, is more readily available to the plant roots. Soils with
too large pore spaces, however, drain quickly and retain little
water, even though root penetration and gas distribution can
be high.
The sediments in the subsurface consist of solids with
pore spaces between them as was described in Chap. 4.
The pore spaces can be filled with water, water and air, or
air. As pores that are totally filled with water

Fig. 6.4 Ranges in average annual potential evapotranspiration, in
millimeters, for the 30-year period 1961-1990 for the conterminous
United States (modified from Healy et al. 2007). One millimeter is
equivalent to 0.039 in.

6.4.4.4 Vapor-Pressure Deficit
Vapor pressure is the gas solubility of a specific compound
at a specific temperature and pressure, as described in
Chap. 2. The higher the vapor pressure, the more likely the
compound is to enter a gas phase. Because transpiration
involves water vaporization controlled by diffusion, the con-
cept of a vapor-pressure deficit, VPD , is used when describ-
ing plant and water interactions, both liquid and gas.

(fully