Conceptual Frameworks for Phytoremediation to Achieve Hydrologic Goals - Introduction to Phytoremediation of Contaminated Groundwater - page 190

average precipitation for an area. For example, the free-
water surface evaporation rate in arid southeastern California
is between 60 and 85 in./year (152-216 cm/year), or
5-7 ft/year (1.5-2 m/year) (Lines and Bilhorn 1996).
In order to have an effect on groundwater the water
removed needs to be from infiltration prior to recharge or
from the water table. Even if the water table is deep, and the
soil moisture levels low, the assessment of free-surface
water evaporation will provide an indication of the potential
for water to be removed by transpiration, a special case of
plant-controlled evaporation. The free-surface water evapo-
ration rate can be quantified at a site using indirect and direct
methods, as described in Chap. 2.

is important when the goal of a phytoremediation application
is recharge reduction or hydrologic control of groundwater
flow. To differentiate between the amounts of groundwater
removed by plants relative to other water sources, the
transpiration-well approach can be used. Existing phreato-
phytes can be used or planted trees that have had the chance
to grow and reach the water table can be used.
As described in Chap. 1, W.N. White took field
observations of groundwater-level changes in wells installed
in forests of phreatophytes and developed an empirical
equation that could be used to determine the total amount
of groundwater used by transpiration during the previous
24-h period. Equation 1.1 is re-listed here

Q ¼ y 24 r ð þ s Þ (8.1)

8.1.2 Micrometeorological Data

where Q is the depth to the groundwater; y is the specific
yield of the soil; r is the hourly rise in water table (from
midnight to 4 a.m., the time of assumed zero transpiration
when groundwater levels recover by induced local conver-
gent flow), and; s is the net fall, or rise, in groundwater
during the same 24-h period. The variables r and s are
derived from the water-table fluctuation data recorded in a
monitoring well located near the plants. The nighttime rise
in the groundwater level during periods of no precipitation is
due to the movement of groundwater toward the plants in
response to lowered water levels—the nighttime rise in
groundwater level is not derived from the plant releasing
water taken up during the day, because water in the plant is
under tension and prohibits reverse flow by gravity.
This equation of White's provides a fundamental base
that connects the hydrogeologic characteristics of the sub-
surface with plant use of groundwater and provides a metric
to assess this interaction, i.e., the change in groundwater
level. As roots take up groundwater from the capillary
fringe, a gradient in water potential is established. This
gradient causes groundwater to move upward to the capil-
lary fringe. If the uptake of water by plants from the capil-
lary fringe and replenishment by the upward movement of
groundwater is faster than the rate of recharge of the aquifer
by hydrostatic pressure, artesian flow, change in storage, or
lateral flow from upgradient areas, then the water-table level
will decline. When the plants are not removing water from
the capillary fringe, for instance at night when transpiration
is lower or ceases altogether, the water table rises to reach
equilibrium. This is an important consideration to keep in
mind when monitoring a phytoremediation site for hydro-
logic control, because the absence of groundwater-level
fluctuation does not necessarily indicate a lack of plant
and groundwater interaction. It could simply indicate that
the groundwater-flow rate is faster than the plant uptake
rate.

Whereas the free-surface water evaporation rate will pro-
vide a general rate of evaporation of exposed water, the
effect of various meteorological properties on how plants
control evaporation to their advantage through transpiration
can be assessed using site-specific meteorological data. The
total removal of water by transpiration calculated from
meteorological data is a part of potential evapotranspiration,
ET p . The ET p of a site can be estimated using site atmo-
spheric data, as described in Chap. 2. Monthly precipitation
data can be collected onsite or from the nearest existing rain
gage.
Various methods have been developed to estimate ET p
although perhaps the simplest are based on certain meteoro-
logical parameters. These methods include those offered by
Thornthwaite and Holzman (1939); Penman (1948); Van
Bavel (1966); Monteith (1965); and the Bowen ratio and
Blaney-Criddle methods (Kramer 1983). These methods all
are based primarily on an energy budget concept, in which
inflows and outflows of energy are balanced. The newer
method of growing degree days also can be used to estimate
ET p . As defined in Lorenz and Delin (2007), growing degree
days (GDD) is the annual sum of the average temperature
each day minus a base temperature for a particular area. The
GDD method rather than ET p was used by Lorenz and Delin
(2007) to estimate the amount of net precipitation remaining
per year that potentially may become recharge.

8.1.3 Transpiration Well

The above two approaches can be used to estimate the total
amount of water that could be evaporated from a particular
site. These approaches do not differentiate, however,
between the sources of the water removed. This distinction