Environmental Engineering Reference
In-Depth Information
14.5.3 Groundwater Flow and Solute-Transport
Models
zone remains saturated. As such, the tree roots either follow
the water table as it is lowered or remain suberized when the
water table is higher. Capillarity may also explain this
observation, where the roots are always above even the
higher water table elevations but evaporative demands
cause water to move upward by capillary action. Moreover,
the increase in groundwater level in June was caused by
recharge due to precipitation, and the removal of this
recharge by the
ET
from the plants.
14.5.3.1 FACT
The Flow and Contaminant Transport (FACT) model is a
three-dimensional, variably saturated finite-element model
of groundwater flow and solute transport (Hamm et al.
1997b). It is a potentially useful model for phytoremediation
purposes because it includes the removal of water from the
unsaturated zone by plant transpiration.
The FACT model was used to simulate plant and ground-
water interactions at the Savannah River Site (SRS) in South
Carolina. The area simulated was a pilot-scale test facility
where wastewater that contained chlorinated solvents was
discharged to unlined basins. Subsequent installation of
monitoring wells in the area and downgradient toward the
discharge area of the Savannah River indicated the
chlorinated solvents were in the shallow groundwater
downgradient and in the wetland flood plain of the Savannah
River (Vroblesky et al. 1999b).
Researchers conducted a comprehensive investigation
into the processes that affect the natural attenuation of the
chlorinated solvents in groundwater. After the collection of
the necessary groundwater-flow and solute-transport data,
such as absorption and biodegradation data, as well as
plant interaction data from laboratory experiments (see
Neitch et al. 1999), the FACT code was used to develop a
model to integrate the influence of these processes on con-
taminant movement (Hamm et al. 1997a). In the model, a
TCE plume in groundwater was generated and allowed to
migrate to the Savannah River. Different simulations were
performed to account for the impact on the plume by sorp-
tion and biodegradation, although this was set to zero based
on laboratory experiments with TCE and aquifer sediment.
Another simulation was used to estimate the influence of
ET
on the groundwater and plume distribution. The extinction
depth was set to 30 in. below the water table, and recharge
was set at 47 in./year (119 cm/year). All simulations
indicated that natural attenuation and
ET
processes did not
affect the discharge of TCE to the river.
Some interesting observations were made regarding the
influence of plants in the flood plain on groundwater levels
as part of this modeling study. Continuous measurements of
groundwater-level and surface-water-level fluctuations were
made with pressure transducers and the data recorded on
data loggers. Depth of shallow groundwater in the flood
plain sediments extends to 8 ft (2.4 m) below land surface
during periods of low groundwater level during the summer
(June). During this time, diel groundwater-level fluctuations
were observed in a well.
This observation is significant, in that the water-table
elevation was at its lowest during this time, and a fluctuation
was still observed. During periods of higher water table, this
14.5.3.2 SUTRA
SUTRA is a code to solve for saturated-unsaturated variable
density groundwater flow and solute transport (Voss 1984).
It is a finite-element code. Although it does not directly
simulate the uptake of water by plants, it does simulate the
fate of density-dependent contaminants and, therefore, may
have some application for phytoremediation.
14.5.3.3 SEAM3D with PUP
The numerical code SEAM3D was modified to account for
solute transport coupled to plant processes such as sorption
of solute to roots, translocation into plant vascular tissue,
and
ET
. The extent of such interactions is based on the
physical and chemical properties of the contaminant, such
as
K
ow
, using the
RCF
and
TSCF
. The modification to
SEAM3D entails the Plant-Uptake-Package, or SEAM3D-
PUP (Widdowson et al. 2005b).
The model has been used to evaluate the effectiveness of
phytoremediation on the proposed dimensions of the planting,
the effect of plant density onmaximum expected
ET
, the effect
of plants on through-site contaminant flux, and the time of
dissolved-phase contaminant mass removal after source mate-
rial extraction. Simulation results indicated that (1) the width
of a proposed phytoremediation system has a limited effect on
solute-mass removal (2) having a higher density of planting
near a source area has a greater impact on contaminant mass
removal relative to a uniform planting over the entire plume
area (3) if the
TSCF
of the contaminant is low, such that little
is taken up by plants, plant roots will exclude uptake and
concentrations in groundwater near the roots will increase
(4) the dimensions of a plume under steady-state conditions
are controlled by groundwater flow, which can be affected
by plant-water uptake (5) splitting the phytoremediation sys-
tem into two halves is less effective than one large mass
planting, and (6) after source removal, the contaminant con-
centration in the groundwater near trees increased, but
decreased in downgradient areas (Widdowson et al. 2005b).
14.5.4 Unsaturated-Zone Models
The fate of xenobiotics in the unsaturated zone is important
to the
success of phytoremediation of
contaminated
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