Conceptual Frameworks for Phytoremediation to Achieve Hydrologic Goals - Introduction to Phytoremediation of Contaminated Groundwater - page 192

groundwater-level change approach to monitor plant and
groundwater interactions. This is because even though the
shallow water table may be a source of water to plants, the
water table can rise after precipitation events. This occurs
because either the plants are facultative phreatophytes and
are using infiltrating water or the groundwater is recharged
at a rate faster than removal by trees.
Precipitation, recharge, and groundwater flow from other
areas upgradient are not the only inputs that need to be
considered as part of the water-balance approach to using
plants for hydrologic control. For example, septic systems,
storm drains, agricultural tile fields, or leaking water mains
that may be located near the site often can contribute an
unrecognized source of recharge to the water table. If this
recharge flows through a source area that contains
contaminants, it will create additional contaminated ground-
water. Conversely, it may act to dilute the contaminant
concentrations dissolved in groundwater at the site.

As stated earlier, the magnitude of the ET p at a site
provides an estimate of the atmospheric demand for water,
which can be supplied by surface water, soil moisture, cap-
illary water, and groundwater. The fraction of ET p that
actually will be derived from the capillary fringe or water
table will vary with each site and will be a function of the
depth to water table, plant coverage and type, and
hydrogeologic conditions. For most cases, a conservative
estimate of groundwater contribution to ET p can assumed
to be no more than 25% for a site that consists of a water
table of less than 10 ft in a sandy aquifer.
In general, if ET p is less than precipitation within a
common time period, be it weekly, seasonally, or annually,
then net recharge of water to the shallow aquifer in the study
area will occur. Conversely, if ET p is greater than precipita-
tion, then a net discharge of groundwater from the study area
will occur, assuming that the source of the water being
evaporated or transpired is groundwater. By strict definition,
the successful implementation of plants at a site to achieve
hydrologic containment or control would occur when
conditions favor the net discharge of groundwater, by ET ,
from the site such that no water is stored, and

8.2.3 Potential Evapotranspiration

S in Eq. 2.4 is
negative. This holds true when ET p is derived from soil
moisture, the capillary fringe, or from the saturated zone.
The balance between groundwater recharge from precip-
itation and groundwater discharge from ET p is not a constant
number but one that will change over time. The frequency of
precipitation is not constant over time at a site, whereas ET p
tends to be more constant for a given climate. For example,
in the humid southeastern United States, net recharge occurs
in the winter and spring, even though precipitation amounts
are higher in the summer. Groundwater recharge occurs
during precipitation events but is quickly removed by tran-
spiration. During winter, precipitation is lower, but ET is
much lower than the summer. It is important to remember
that for either case, groundwater recharge may not occur if a
high percentage of impermeable surfaces are present. In
these cases, water can still enter the groundwater system
by groundwater flow from upgradient areas.
The energy parameters that affect the ET p for an area can
be considered to be a given property that cannot easily be
modified. The hydrogeologic conditions at a particular
contaminated site being evaluated as a candidate for
phytoremediation also have an effect on the success of
phytoremediation because of how the aquifer sediments
determine the resistance to water bioavailability and flow
to plants. The presence of a thriving grove of trees does not
mean, however, that groundwater is bioavailable and being
used by the plants or that the amount removed from the
water table is sufficient enough in volume to affect site
groundwater flow. Moreover, a lack of evidence to support
hydrologic control may not become apparent at a planted site
until the plant community is 3-4 years old.

D

As was introduced in Chap. 2, the ET p of a particular area is
an estimation of the total amount of water from all sources
that can be removed as vapor under given weather
conditions. The inputs necessary to determine ET p are the
air temperature, air relative humidity, solar radiation, wind
speed, and precipitation amount (van Bavel and van Bavel
1990). These data can be taken from monthly or annual
averages, collected onsite using a weather station that has
sensors to measure these variables, or from reference mate-
rial. An amount for ET p in depth (length) per time (such as
millimeters of water per h or in. per month) will be calcu-
lated. The units of ET p refer to the rate that a unit thickness
of water in a unit area will exit the system as water vapor ET .
Because ET p usually is reported in units of length per time,
multiplication of ET p by the area of the site to be planted (in
length squared) gives ET p in units of volume (length cubed
per time). This volume per unit time can be converted
to gallons per unit time by the conversion factor of 1 ft 3 ¼

7.48 gal of water.
Knowledge of the magnitude of ET p in relation to precip-
itation for a particular area or region is invaluable when
phytoremediation to achieve hydrologic goals is being
assessed. In general, if ET p on a daily, monthly, or annual
average is greater than precipitation, then the potential exists
for a plant-based system to affect groundwater. If precipita-
tion is greater than ET p , the picture is not as clear—the
answer is then dependent upon the depth to water table. If
the water table is shallow, groundwater levels may rise even
though water is being removed, and if the water table is
deep, recharge will be decreased as plants take up infiltrating
water.