Conceptual Frameworks for the Phytoremediation of Groundwater Contamination - Introduction to Phytoremediation of Contaminated Groundwater - page 342

assuming other processes such as sorption and dilution were
constant in the contaminated aquifer beneath the area, with
no changes in water storage. After plants are installed and
take up groundwater that contains dissolved-phase
contaminants, however, the average contaminant mass flux
leaving the planted area would decrease, such that QACM up
>

t ¼ ð ln M = M o Þ= k (14.5)

where t represents the time needed to reach a remedial action
level in the remaining groundwater, M is the mass allowed at
t (M), and M o is the initial contaminant mass (M).
The advantage of this simple contaminant mass-reduction
approach is that it is essentially contaminant independent,
unless, of course, the plants are negatively affected at toxic
concentration levels of the contaminant. Because it requires
the change in concentration over the length of a groundwater
flowpath, however, it may not be useful at those sites
characterized by slow groundwater-flow rates or where
plants are installed over contaminant source areas. The fol-
lowing section offers an alternative approach that can be
used as a framework under these conditions.

QACM down . A maximum uptake rate could be expected
during closed-canopy conditions, with the result being a
larger gap between influent and effluent mass flux.
What about the fate of groundwater contaminants? This
type of evaluation is essential if the actual mass loss of
contaminants from a site need to be determined, rather than
just a decrease in mass. A computationally simple model
exists that describes the fate of sequestered contaminants, as
outlined in Burken and Schnoor (1998) and Schnoor (1997)
and presented in previous chapters. This model relates the
contaminant fate to the transpiration rate, itself a flow that,
when combined with a contaminant concentration, becomes
a flux. The benefits derived from such computationally
simple models, however, are at the cost of the simplifying
assumptions that need to be made for their use. These
assumptions include constant groundwater-contaminant
concentrations, steady-state plume distribution, and no
microbial biodegradation.
To summarize, the uptake of organic contaminants
dissolved in water by plants can be described by Eq. 14.2

14.1.2 Case Study, Fort Worth, Texas

A phytoremediation project was initiated in 1996 at a site
near Air Force Plant 4, Fort Worth, Texas (see Chap. 8 for
more information on this site). At the site, hybrid poplar
trees ( Populus deltoides ) were installed in early 1996 in two
areas using different approaches. The groundwater flux and
the contaminant concentrations were used to calculate the
flux of TCE in the contaminated aquifer beneath the
plantings. The volumetric flux of groundwater was calcu-
lated using Darcy's Law, and this product was multiplied
by the average TCE concentration for the wells, located
in a row, that define the cross-sectional areas up and
downgradient of the planted area, perpendicular to ground-
water flow. The parameters included the hydraulic gradient,
i

U ¼ ð TSCF Þð T Þð C Þ

(14.2)

where U is the uptake rate of the contaminant (M/T), TSCF
is the transpiration stream concentration factor discussed in
Chap. 12 (dimensionless; Burken and Schnoor 1997), T is
the transpiration rate (L 3 /T), and C is the concentration of
the dissolved-phase contaminant (M/L 3 ).
From Eq. 14.2 , the time required for plants to take
up a sufficient amount of groundwater to render the
concentrations in the remaining groundwater at or under
remedial concentrations, can be estimated from first-order
degradation kinetics, such as

807 ft 2 (75 m 2 ), the ¼ 2.25%, the cross-sectional area, A ¼

aquifer thickness b ¼ 3.2 ft (1 m), the aquifer width ¼ 246 ft

(75 m), the effective porosity, n e , ¼ 23%, and the hydraulic

conductivity of the saturated zone, K ¼ 19.6 ft/day

(6 m/day).
This calculation was done for various times of the year in
order to reflect the seasonal differences in groundwater flux.
The groundwater flux was calculated to be about 2,675 gal/
day (10,125 L/day). This was multiplied by the average TCE
concentration in a row of wells downgradient of the planted
area. The researchers used this approach to estimate the
change in calculated groundwater contaminant flux due to
the removal of groundwater by trees for each year after
planting until 1999. The decrease in calculated contaminant
flux ranged from 2% to 12% of conditions before the trees
were planted. Higher contaminant flux decreases were not
observed, however, due to incomplete groundwater flow
capture.

k ¼ U = M o (14.3)

where k is the first-order uptake rate constant (per T), U is
the contaminant uptake rate from Eq. 14.2 , and M o is the
initial mass of contaminant present (M). At any time t during
remediation, the mass remaining in the aquifer can be deter-
mined by

M o e kt M ¼ (14.4)

where M is the mass remaining (M) at time t . Solving for
t yields