Environmental Engineering Reference
In-Depth Information
derived from shallow contaminated groundwater by a shal-
low pumping well at each planted area. Apparently no pre-
vious calculations to estimate the effect of the trees on the
site hydrology were reported. Dinicola et al. (2002) state that
the trees had no immediate effect on groundwater levels or
of contaminant concentrations.
10 and 15 ft (3-4.5 m) deep and were backfilled with fine
sand. Up to 3,300 cuttings were planted.
8.4.6 Other Conceptual Frameworks
Most of the approaches, or frameworks, previously discussed
are based on decreasing the flow of groundwater to offsite
areas by redirecting the groundwater to plants. An alterna-
tive approach presented here is to decrease the flow of
groundwater through a source area. This process is accom-
plished using a constant-head moat, called an ET moat. The
ET moat is created by a trench dug at a depth of the mean
water table and a closed-loop drain pipe is installed. The
trench is then backfilled. Due to the presence of the drain
pipe, the water-table elevation can rise no higher than the
elevation of the drain pipe, as long as recharge rates do not
exceed groundwater-flow rates. Trees are then planted
within the moat area of lower water table in order to remove
additional water by ET . The limitation for widespread appli-
cation is the high cost for the trench and pipe installation,
disposal cost for removed material, and potential for root
clogging of the drain pipes, which would decrease the effi-
ciency of the constant head over time.
8.4.5.2 Case Study: Portsmouth Plant, Ohio
The Portsmouth site is the location of a Department of
Energy (DOE) uranium enrichment facility. It is located in
central Ohio and was constructed in the early 1950s to enrich
uranium. These activities stopped in 2001. Processes
performed at the former oil-handling facility at the site's
Gaseous Diffusion Plant left a legacy of chlorinated solvents
in the water table and bedrock aquifers; at least five separate
plumes were detected beneath the site in the early 1990s. At
two of the plumes, concentrations of TCE ranged from 5 to
10,000
g/L. These plumes were selected for remediation by
phytoremediation as part of corrective action alternative
studies done in the late 1990s. The depth to the surficial
water table is about 10 ft (3 m). The depth to the
contaminated bedrock aquifer is about 25 ft (7.6 m).
A phytoremediation system was installed at one plume in
1999 (Ferro et al. 2000). Trees were planted in the source
area as well as downgradient from the plume. The trees were
installed using two approaches: (1) 2-ft (0.6 m) wide
trenches were dug to a depth of 10 ft (3 m), or to the top of
the water table, and (2) 2-ft (0.6 m) diameter boreholes were
made 10 ft (3 m) deep. Each was backfilled with fine sand.
As such, 240 cuttings were installed in the trenches, and 526
trees were installed in the 10-ft (3 m) deep borings. Also at
the site, 8-in. (20 cm) diameter borings were drilled 30 ft
(9 m) deep to the local aquifer contact with bedrock. These
boreholes were not planted but were filled with sand. The
fine sand was used to physically wick-up water by capillary
action to bring the contaminated groundwater in the deeper
bedrock up into the area where the tree roots were growing.
This method is essentially using a modification of shallow-
rooted planting methods to remediate deeper groundwater
contamination without using a proprietary method.
At another plume, the source of the chlorinated solvents
was believed to be either a landfill or an old paint shop. A
phytoremediation system was installed in 2002. A similar
planting method was used, such that numerous 2-ft (0.6 m)
wide trenches were dug to the top of the water table between
m
8.5
Summary
Naturally occurring plant interactions with groundwater
affect groundwater levels and flow direction. This interac-
tion provides the basis for various approaches (or
frameworks) to affect contaminated groundwater through a
decrease in recharge and increase in transpiration.
Why is this information important to the phyto-
remediation of contaminated groundwater? The effects of
plants on contaminated groundwater often are not readily
observed because the interaction occurs at depth. The
approaches described in this chapter also indicate that a
water-budget method used on its own is limited by the
inability to account for the amount of groundwater removed
by trees in the presence of multiple sources of water. Fortu-
nately, there are geochemical methods that can be used in
conjunction with the water-budget method to determine the
source of water to trees at phytoremediation sites and are
discussed in Chap. 9.
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