Environmental Engineering Reference
In-Depth Information
This approach should be performed with the appropriate
calibration or null hypothesis controls. For example, ground-
water-level fluctuations can be measured at the site before
plants are added or measured in areas that are not planted
after installation. As a result, the effect of the vegetation on
groundwater can be observed.
scale. This analysis can be accomplished by comparing the
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p
to groundwater flux through a site, and the latter based
on basic groundwater-level data. In general, control of site
hydrology by phytoremediation begins with a decrease in
water-table elevation; all else being held equal, decreases
in water-table elevation can be attributed to the plant-
facilitated reduction in recharge or to increased transpiration
such that net recharge is low. Groundwater levels need to be
measured in monitoring wells within and outside the planted
area to ensure these changes are related to the plants and not
to other site wide phenomena. A case study where these
measurements are made and applied is given in Sect.
8.3
.
8.1.4 Tank Experiments
The experiments of Charles Lee used tanks, or basins, where
the quantity of water removed from tanks that contained
plants could be compared to the water removed from tanks
that did not contain plants. Lee's results were used by
Robinson (1958) to determine the relation between evapo-
transpiration,
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, and depth to groundwater and temperature
(see Fig. 2.9). Tank experiments to investigate
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from
riparian trees also were conducted and results reported by
Robinson (1970). Robinson (1970) reported
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as a volume
per foliage amount or quantity of water per foliage amount,
and this volume also was affected by depth to groundwater,
length of growing season, and nutrient toxicity. Tank
experiments are time and resource intensive, and the results
may not be transferable to field conditions, however.
8.2.1 Precipitation
For most sites being considered for phytoremediation of
contaminated groundwater, precipitation can be measured
at the site or obtained from an existing precipitation station
nearby. Measurement of precipitation is necessary for the
water-budget approach and provides the amount of water
that enters the site. Precipitation varies annually at most
locations across the United States. Even if the average
annual precipitation is fairly constant, the frequency, dura-
tion, and amount of precipitation can vary significantly at
one location over time.
Precipitation, one of the most readily quantifiable meteo-
rological properties, can be measured by devices ranging
from simple, inexpensive onsite precipitation gages to
more sophisticated automatic monitoring devices such as
tipping-bucket precipitation gages attached to a data logger.
Most states have some network of raingages, either run by
state, academic, or Federal programs that track precipitation
amounts. Sources of precipitation information are many and
include, but are not limited to, the following agencies:
USGS; National Weather Service; National Oceanic and
Atmospheric Administration; universities and state coopera-
tive extension services.
8.1.5 Foliage Volume
The concept of foliage volume was advanced by Bowie and
Kam (1968) during their investigation into the consumptive
use of groundwater by riparian vegetation in Arizona.
Because leaves are the predominant location of water-
vapor removal, an estimate of total leaf volume can provide
an estimate of the amount of water lost by trees. Bowie and
Kam (1968) considered a tree to be a cylinder, with tree
height analogous to the cylinder height and the tree crown,
or radial extent of leaves, analogous to the cylinder width.
The volume of leaves contained within each cylinder is
represented by an acre-foot, where 1 acre-ft (1,233 m
3
)of
foliage is a space 1 ft (0.3 m) deep and 1 acre (4,047 m
2
)in
area. The loss of water by
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from this cylinder of leaves
should be related to the total foliage volume. This method
was later used by Robinson (1970) to investigate
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by
phreatophytes in Nevada.
8.2.2 Recharge
In most places in the United States less than 10% of annual
precipitation becomes recharge. In South Carolina, for
example, average annual precipitation is about 50 in./year
(127 cm/year) with shallow groundwater recharge of about
an average of 5 in./year (12.7 cm/year). In contrast, in Long
Island, NY, where subsurface sediments consist of perme-
able glacial deposits, recharge is closer to about 50% of
annual precipitation.
A water table that is within a few feet of land surface is
more apt to be recharged during precipitation events than are
deeper water tables. It also presents a challenge to using a
8.2
Assessment of Potential
Evapotranspiration, Recharge,
and Groundwater Discharge for
Phytoremediation Effectiveness
The various approaches and methods above provide either
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p
or groundwater-level changes, but not necessarily an
analysis of the interaction between the two on a site wide
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