Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 316

The elevated concentrations of benzene, toluene, and
naphthalene measured during the fall

benzene, toluene, and naphthalene measured during the

winter months did
rebound over time but to progressively lower concentrations
(Fig. 13.5 ). Similar information regarding the effect of
phytoremediation on such plume control can be found in
Van Den Bos (2002).
As the contaminant concentrations decreased in these
wells, the level of dissolved oxygen in groundwater
increased from 0 to 3.6 mg/L (Fig. 13.5 ). This increase
may be due to (1) lowered demand on DO added to the
aquifer during precipitation events, which had been shown
at the site earlier to deliver up to 8 mg/L each event but
decreasing rapidly thereafter; (2) an increase in DO from
generation by poplar trees combined with the effects of (1);
(3) a decrease in the generation of reduced species that
consume DO, and; (4) an overall decrease in the amount of
DNAPL upgradient of the well.
A few wells in the planted area were characterized by
measurable levels of DNAPL at the base of the water-table
aquifer adjacent to or below the well. For example in a
monitoring well (CM-03A), where 1.12 ft (0.3 m) of
DNAPL was detected in September 2005, benzene, toluene,
and naphthalene decreased from their highest concentration
by 54%, 80%, and 55%, respectively (Fig. 13.5 ). For ben-
zene and naphthalene, this reduction was not as great as that
observed for wells in the area were DNAPL was not present.
Essentially, the levels of contaminant after 75 years and
installation of hybrid poplar trees are not greatly changed
from initial concentrations.
Even though the concentration decreases are not as great
as observed in earlier years, a seasonal response in
concentrations can still be seen, especially during 2001,
indicating a positive influence of the trees on groundwater
contaminant levels (Fig. 13.5 ). The influence of tree uptake
is overshadowed, however, by the continual generation of
dissolved-phase contaminants from the DNAPL.
In another monitoring well (USGS-A) located near the
upgradient part of the western part of the planted area,
contaminant concentrations are an order of magnitude
lower than those seen in other wells in the phytoremediation
area. Between 2000 and 2007, concentrations of benzene,
toluene, and naphthalene in USGS-A decreased from their
highest concentration measured by 99%, 100%, and 100%,
respectively (Fig. 13.6 ). During this time concentrations of
the compounds did not decrease steadily, but rather
fluctuated from low to high back to low concentrations—
an annual, seasonal variation in concentrations of these
dissolved-phase contaminants was observed. For example,
in 2001, 2002, and 2003, the lowest concentrations of
benzene, toluene, and naphthalene were observed during
the spring

fall winter months did rebound over time but to progres-

sively lower concentrations (Fig. 13.6 ). As the contaminant
concentrations decreased, the level of dissolved oxygen in
groundwater increased over time from 0 mg/L to a high of
4.2 mg/L (Fig. 13.6 ).
In a monitoring well (USGS-C), installed near the
downgradient area of the western part of the planted area,
concentrations of benzene, toluene, and naphthalene
decreased between 2000 and 2005 from their highest
concentration measured by 75%, 70%, and 48%, respec-
tively (Fig. 13.6 ). There also was a seasonal variation
in concentrations of these dissolved-phase contaminants
over time. For example, in 2001, 2002, 2003, and 2005, the
lowest concentrations were observed during spring summer

sampling events and the highest concentrations observed dur-
ing fall winter sampling events. Concentrations of DO in
groundwater increased from 0 mg/L to a high of 2.9 mg/L
(Fig. 13.6 ).
Another site where PAH contamination provided the
opportunity to investigate the interaction between poplar
trees, groundwater hydrology, and contaminant fate was
near Oneida, TN. The use of coal-tar to treat railroad ties
to inhibit water uptake occurred between 1950 and 1973 at a
site in the north-eastern part of Tennessee (Widdowson et al.
2005a). Even though it was stored in an AST and a holding
pond, the viscous properties of most coal-tars enable them to
be found in many locations at sites adjacent to where they
were used. At the site in Tennessee, creosote contaminants
were found to seep out at the bank of a local stream. The
stream is hydrologically connected to the shallow aquifer
comprising sands and clays to a depth of about 11.4 ft
(3.5 m). As is the initial remedy at many sites where either
DNAPL or LNAPL has been detected and is flowing toward
a surface-water body, an interceptor trench was constructed
in an attempt to cut off the movement of creosote to the
stream.
At the bottom of the aquifer a dense shale of low perme-
ability is encountered. This has acted to limit the downward
vertical migration of coal tar DNAPL, and there exists a pool
of DNAPL up to 11 in. (30 cm) thick. The rate of groundwa-
ter flow is about 0.06 ft/day (0.02 m/day). As is typical of
many shallow aquifers, recharge is by precipitation, which
averages about 59 in./year (152 cm/year). At the site, up to
1,026 hybrid poplar trees were planted in 1997; these trees
were between 2 and 3 years old. An additional 120 trees
were planted in 1998.
Monitoring wells screened at various depths within
the shallow aquifer indicated that the saturated thickness
decreased to less than 3.2 ft (1 m) during the summer months,
following establishment of the trees. The researchers
measured the depth of root penetration at some locations to
be 6.5 ft

summer sampling events and the highest
concentrations were observed during the fall winter sam-

pling months (Fig. 13.6 ). The elevated concentrations of

(2 m) below land surface. For total PAH