Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 221

understand diurnal impacts of plants on groundwater, then a
short time interval is needed. If the goal is to determine a
seasonal or annual site water budget, less frequent data
collection will suffice. In many cases, however, more atten-
tion is paid to the location of monitoring wells rather than to
monitoring frequency. Quinn and Johnson (2005) examined
the benefits of using automatic groundwater-level sensors
at their phytoremediation site near Chicago discussed in
Chap. 8. In that study, continuous groundwater-level
measurements were used to refine the site conceptual
model to account for a source of subsurface seepage of
stormwater runoff that previously had not been recognized.

groundwater discharge. The potential for vertical flow can
be evaluated by installing vertically spaced, or nested, wells.

9.2.4 Groundwater Volume Removed
by Plants

The amount of diurnal groundwater-level fluctuation caused
by phreatophytes not only reveals the occurrence of plant
and groundwater interaction but also the volume of ground-
water removed by plants. This determination can be made
because departure of the measured water level from static
conditions represents the withdrawal of a finite volume of
groundwater, initially S y as described earlier, from the aqui-
fer. For example, the maximum groundwater-level change,
D

9.2.2 Groundwater Flow

The uptake of groundwater by phreatophytes can affect not
only the groundwater level in shallow aquifers, as described
above, but also the horizontal direction of groundwater flow.
For example, Fig. 8.2 in Chap. 8 depicts a hypothetical
hydrologic scenario in a sandy, homogeneous, isotropic
water-table aquifer. Under these idealized conditions, the
vertical lines represent equal water potentials prior to the
installation of poplar trees. Groundwater flow crosses these
lines of equipotential at a right angle (Freeze and Cherry
1979; Fetter 1988), such that groundwater flow is from
higher to lower water potentials, or from left to right in
Fig. 8.2, and conditions are at steady state.
After roots reach the water table, groundwater levels can
decline locally. The initial volume of groundwater removed
to cause this decline is equivalent to that amount drained by
gravity, or specific yield, S y . This decrease in head can,
theoretically, lead to the reversal of the downgradient flow
of groundwater. More advanced analytical techniques can be
used to determine the areal extent of this decline, or capture
zone, using wells (Landmeyer 1994) or simulated trees
(Gorelick et al. 1993).

h , observed at the phytoremediation site near Charleston
was 0.07 ft (0.02 m). This decline occurred over an assumed
radius, r , of at least 1 ft away from a tree, and the effective
porosity, n e , was assumed to be 0.30, the groundwater vol-
ume removed can be estimated using V

( B )( r 2 )(

h )(0.30).
Therefore, a 0.07-ft (0.02 m) decline in the water table
represents about 0.50 gal (1.8 L) of groundwater removed
per day per tree. The 2-year-old trees actually transpired a
larger volume of water per day of around 5 gal/day/tree
(18.9 L/day/tree), the difference being supplied from soil
moisture.
Another method was presented by White (1932). In his
method, introduced in earlier chapters but repeated here
because of its importance, the quantity of groundwater with-
drawn by plants during a 24-h period is determined by:

¼ D

GW ¼ S y 24 r ð s Þ (9.9)

where GW is the amount of groundwater transpired (L/T), S y
is the specific yield (% by volume; usually 2%, 22%, or 40%
for clay, sand, or soil, respectively), r is the hourly rate of
groundwater inflow within 24 h (L/T, in h), and s is the net
rise (or fall) of the water table (equivalent to D h ) (see Freeze

and Cherry 1979). The 0.07-ft (0.02 m) water-table fluctua-
tion discussed above when evaluated using the White
method indicates a removal of near 0.14 gal/day/tree
(0.5 L/day/tree), similar to the result using the simple volu-
metric method described previously.
The clear advantage gained using either method is that an
estimate of the volumetric uptake of groundwater by plants
can be made without having to use intensive plant-physio-
logic methods. The White method is more applicable for
plants using groundwater in coarse-grained aquifers relative
to fine-grained aquifers. When the groundwater level drops
in fine-grained sediments, the specific yield is released more
slowly over time than for coarse-grained aquifers, especially
for sites where the depth to water table is shallow. A unit

9.2.3 Vertical Groundwater Flow

The plant-induced removal of groundwater also affects the
originally uniform distribution of equipotential lines and can
result in a vertical component of groundwater flow (Fig. 8.2)
in an originally horizontal flow regime. This is not the same
as plant-induced hydraulic lift, which affects the distribution
of water tension in the unsaturated zone. The reduction in
groundwater levels lowers water pressures beneath the trees
throughout the entire saturated thickness, not just near the
water-table surface. This induces a vertical flow component
at depths greater than the depth of maximum root penetra-
tion (Fig. 8.2). The result is that planted areas that are not
initially areas of groundwater discharge can become areas of