Environmental Engineering Reference
In-Depth Information
where
T is the
amplitude of the tide across a 12-h period from high to low
tide. In general,
D
W is the groundwater-level change and
D
on an adjacent stream. This holds because the removal of
groundwater by phreatophytes can be envisioned as being
analogous to that of a pumped well. This assumption is
especially valid for riparian phreatophytes that produce a
measurable decrease in river flow, as described in Chap. 5.
Mower et al. (1964) also tried the transpiration-well method
first developed by White (1932) to determine groundwater
use by phreatophytes. As was previously discussed, this
method is based on the assumption and observations by
White (1932) that water from the capillary fringe is used
by transpiring plants during the day, when the stomata are
open. A decline is observed in nearby wells because the rate
of depletion of the capillary water is greater than the rate that
it can be resupplied by hydrostatic pressures. The reverse is
true when the plants are not transpiring. It is important to
note that since the advent of water-potential measurements
that came after White, the capillary fringe can supply water
to plant roots up to but not beyond tensions that represent the
wilting point, or about
a b +
a t ¼
1, in areas during conditions of no
recharge or pumping.
To monitor anticipated small changes in groundwater
levels, pressure transducers need to be used that can resolve
up to a 0.01 ft (0.30 cm) or greater change in groundwater
level. The manual tape downs often used at most sites during
monitoring events are accurate to about 0.1 ft (3 cm) and are
not useful in detecting typical plant drawdowns. Groundwa-
ter-level fluctuations also should be made over a period of at
least a few days, to determine the presence or absence of a
diurnal pattern. Many pressure transducers also meet this
requirement of long-term deployment and come equipped
with internal data loggers or can transmit the data to an
external logger. Data also can be retrieved remotely to
permit a near “real-time” examination of groundwater-
level changes. The use of pressure transducers to measure
groundwater levels is particularly applicable in gravel
or sandy aquifers, because these highly permeable sediments
often result in smaller groundwater-level fluctuations com-
pared to changes observed in fine silt or clay. Moreover,
pressure transducers that have been constructed and
calibrated only to measure changes in barometric pressures
need to be installed above ground at sites where pressure
transducers are installed in wells.
An example of using the pressure-transducer method
to monitor groundwater levels is given for the phyto-
remediation site at the former MGP sites near Charleston,
South Carolina, discussed previously. There, groundwater-
level fluctuations have been measured before trees were
planted in 1998 in wells in the shallow silty aquifer. For
example, groundwater-level changes were monitored
between June and July 2000, 2 years after planting, using a
pressure transducer
1.5 MPa.
Davis and DeWiest (1966) using basic groundwater-level
fluctuation data presented a method to calculate the volume
of groundwater transpired by plants. The amount of ground-
water moving into a root zone per unit area, q , can be
solved by
q
¼
n e V H
(9.8)
where n e is the effective porosity of the unit area and V H is
the velocity of the rise, or high, in groundwater level, also
known as recovery, over time, normalized by any longer-
term decreases in groundwater levels caused by changes in
storage between consecutive daily lows. Changes in ground-
water storage include that caused by recharge from precipi-
tation infiltration or from nearby streams after a flood, or by
discharge through springs. The solution to Eq. 9.8 is the
volume of groundwater moving into area around the tree
roots. This value can then be multiplied by the assumed area
of root influence for either a single tree or a stand of trees in a
phytoremediation application to obtain a volumetric rate of
removal of groundwater per unit time.
Even though measurement of groundwater levels can
provide the most direct evidence of plant and groundwater
interaction, there are some precautions that need to be stated
when examining the data. First, it is possible that roots may
have grown into the well being monitored. In fact, root mass
is typically found in many monitoring wells at many sites
around the country even where phytoremediation plants
have not been added (please refer to Chap. 13, Fig. 13.14).
This condition is a consequence of hydrotropism, in which
plant roots multiply quickly in areas where water supplies
are the most readily available.
Second, the diurnal groundwater-level decrease being
measured in a well often will be lower than that occurring
) placed in
a temporary, 2-in. (5 cm) diameter well screened in the
upper 0.5 ft (0.15 m) of the water-table surface. Precipitation
data recorded at an on-site weather station revealed the
monitoring period had both dry and wet conditions. Prior
to precipitation, groundwater levels remained low with no
diurnal fluctuations. After a 1-ft (0.3 m) rise in the ground-
water level was observed in wells after an intense 4-h precip-
itation event, however, a maximum groundwater-level
fluctuation of 0.07 ft (2.1 cm) was recorded and coincided
with solar radiation also measured at the on-site weather
station. This recharge event increased the elevation of the
water table to be nearer the roots of the 2-year-old trees.
Similar diurnal changes for cottonwood trees in Ohio have
been reported by Gatliff (1994).
To determine the removal of groundwater by phreato-
phytes planted at a site, a method similar to that presented
in Mower et al. (1964) can be used. Mower et al. (1964) used
the Theis method for computing the effect of a pumping well
(Hydrolab Minisonde
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