Conceptual Frameworks for the Phytoremediation of Groundwater Contamination - Introduction to Phytoremediation of Contaminated Groundwater - page 341

Phytoremediation of Groundwater Contamination 14

Conceptual Frameworks for the

In general, the log transform of K ow is a useful physical
property to prioritize which xenobiotics will interact with
plants at a potential phytoremediation site. However, this
physical property does not account for all factors that affect
how groundwater contaminants will interact with plants,
such as the groundwater-flow rate, the rate of transpiration,
and the volume of groundwater contamination.
There are at least two different approaches, there-
fore, to evaluate the overall effectiveness of using
phytoremediation to address contaminant remediation in
groundwater. One is based on a mass-balance approach,
where the flux of contaminants through a planted area is
compared to the original contaminant mass. The other
approach employs a method based on the one-dimensional
solute-transport equation similar to that used to evaluate
monitored natural attenuation, with the inclusion of terms
to represent the effect of plants on contaminant fate.
This chapter provides an introduction to both approaches
and provides examples of their data requirements and
implementation.

14.1.1 The Conceptual Framework

To demonstrate the remediation of groundwater
contaminated by xenobiotic compounds, the levels of the
contaminant expressed as a concentration of mass per unit
volume need to decrease over time and space. In other
words, the concentrations must decrease and the plume
must shrink. In most cases, site-characterization data will
exist or can be collected in order to delineate the size of the
contamination and the areas that contain dissolved-phase or
free product, and the aquifer properties that determine the
rate of groundwater flow.
To assess the potential for plants to hydrologically and
geochemically remediate the contaminated groundwater,
contaminant mass data prior to planting or upgradient
of unplanted areas can be compared to contaminant mass
levels that exits the site or at a specific location over time.
If the contaminant mass decreases, then some level of
phytoremediation will have occurred and the contaminant
concentrations compared to general or site-specific remedi-
ation goals.
To accomplish this comparison, the measured, average
contaminant mass flux, as determined in wells located
upgradient of the planted area, M up , as a product of the
groundwater discharge, Q ,inL 3 /T, the area, A , through
which the contaminated groundwater flows, and the con-
centration, C , of the contaminant from wells, QACM up ,in
M/L 3 is compared to the measured average contaminant
mass flux across the downgradient area, QACM down after
mass losses, M loss , due to biodegradation and volatiliza-
tion are removed. This approach is an extension of the
groundwater flux approach presentedbyEbertseta .
(1999).
Under conditions prior to the installation of vegetation,
the average contaminant mass flux would be equal to the
average contaminant mass flux leaving the area, or

14.1 Contaminant Mass Reduction

Framework

As shown in previous sections in this topic, the uptake of
groundwater by phreatophytic vegetation can affect ground-
water levels, change horizontal and vertical groundwater-
flow directions, and reduce the flux of groundwater to
downgradient areas. As a result, such vegetation also can
be used at contaminated sites to reduce the mass flux of
aqueous-phase groundwater contaminants flowing through
or contained beneath a planted area. These plants also can be
used to decrease contaminant mass by the direct uptake and
translocation of dissolved-phase contaminants, enhance bio-
degradation processes in the rhizosphere, and detoxify
contaminants once in plant tissues.

QACM up ¼ QACM down þ M loss (14.1)