Environmental Engineering Reference
In-Depth Information
that consisted of taking a solid piece of Plexiglas about 5 cm
wide and cutting out small cells from the pipe that could hold
about 20 mL of deionized water added prior to installation.
These holes were then covered with a semipermeable mem-
brane. This membrane allowed contaminants in the pore
water to diffuse from the contaminated media to the cells.
The sampler was advanced inside the hollow rod of a direct-
push rig. The samplers were installed in the root zone of a test
tree in the planted area (about 1 ft away from the trunk) and
left for 2 weeks to equilibrate.
Upon removal and analysis of the dialysis samplers, the
depth profile of contaminant concentrations such as
cis
-1,2-
DCE and TCE was variable in the planted area, both for
parent compound and daughter-product formation, relative
to a control sampler placed in contaminated sediment not
planted with poplars. Whereas the control had more
cis
-1,2-
DCE, the planted area had more TCE. Some dialysis cells
had no water upon retrieval, which the authors suggest was a
result of direct uptake through the membrane by adjacent
roots. Redox- sensitive dissolved gases such as methane
were lower in cells near the roots, suggesting that atmo-
spheric oxygen may have diffused through the root cortex
into the rhizosphere. Such dialysis sampling may be valid at
sites where the majority of contamination resides in the
vadose zone as a long-term source to groundwater.
results often are not directly transferable to field situations.
In the field, however, rates of degradation of contaminants
can be calculated using a variety of methods. One of these
methods is called a push-pull test.
Push-pull tests are field-scale controlled tests similar to
that performed in the laboratory. A single monitoring well is
used to inject, or push, a solution that contains a contaminant
of interest that is expected to behave non-conservatively,
along with a tracer that will behave conservatively. The
same well is later used to extract, or pull, water out,
while sampling for the presence of the tracer and the reactive
solute. Comparison of the concentrations of these compounds
during the push-and-pull cycles of the test are used to deter-
mine degradation rates. This approach may not be hydrogeo-
logically feasible at sites that have aquifers with low
hydraulic conductivity, or may be uneconomical at other
sites due to disposal of extracted and possibly contaminated
groundwater. However, these tests are useful in providing
a gross loss of contaminants such as metal, chlorinated
solvents, and petroleum hydrocarbons. The primary concern
with any fluid-injection-based technology, however, is the
degree to which mixing with the voids in the aquifer
sediments occurs outside of the well bore.
Push-pull tests were attempted at a PAH-contaminated
shallow aquifer in Oneida, Tennessee, where more than
1,100 poplar trees were planted in 1997, as described in
Chap. 13. Widdowson et al. (2005a) reported the decrease
in total PAH concentrations and naphthalene at shallow
depths below the water table relative to deeper depths near
DNAPL. They performed a series of push-pull tests to deter-
mine if there were any differences in biodegradation of
naphthalene in the areas planted relative to unplanted
contaminated areas (Pitterle et al. 2005). They injected the
conservative tracer bromide at concentrations near 750 mg/L,
dissolved oxygen, and naphthalene near 2 mg/L in the push-
pull wells. The tests consisted of injecting 9.2 gal (35 L)
of this solution at a rate of 0.1 gal/min (0.5 L/min).
Hydrogeologic conditions at the site indicated that the radius
of travel of the injectate from the well into the aquifer was
about 11.8-15.7 in. (30-40 cm). Once injection was stopped,
extraction was started and continued until at least three
volumes of the injectate were recovered or after DO
stabilized back to pretest conditions (Fig.
15.9
).
Little can be said about the difference in push-pull tests
between planted and unplanted areas because the tests were
done at different times of the year; the planted test was done
in June, whereas the unplanted test was done in February.
Also, differences in temperature will affect the rate of aero-
bic microbial respiration, which may be the simplest process
to explain the observed differences, such that the cooler
groundwater temperatures would decrease mineralization,
thus producing less DO consumption. It is possible that the
biodegradation rates, k, presented from the push-pull tests
15.3
Integrative Methods
The successful application of phytoremediation for ground-
water restoration will warrant that a combination of methods
be used to unequivocally determine that plant-groundwater-
contaminant interactions are occurring at a particular site.
This section discusses some additional approaches that can
be used.
15.3.1 Root Zone Models
As was discussed in Chap. 12, much work was done on the
fate of pesticides and herbicides applied to plants such as
commercial crops. Models were developed to understand the
fate of these compounds after application and include PRZM
and PRZM-2, the Pesticide Root Zone Model (Carsel et al.
1984) available from the USEPA. Whether or not these
models are applicable to those contaminants that commonly
are encountered in groundwater is a subject for future
investigation.
15.3.2 Push-Pull Tests
The fate of groundwater contaminants can be evaluated from
laboratory and field tests. In many cases, the laboratory
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