Environmental Engineering Reference
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trees that grew above the groundwater in the former source
area and in areas downgradient of the spill above the
dissolved-phase plume. Although concentrations of these
compounds detected in the headspace of vials that contained
tree cores were provided in Landmeyer et al. (2000), they are
not repeated here because the relation between tree core
headspace concentrations and aqueous groundwater
concentrations is not well established, although Ma and
Burken (2002) indicate a definitive relation. Finally, no
MTBE (or BTEX) was detected in tree cores taken from a
tree that was growing above the plume in the downgradient
area, as the plume was below a lens of uncontaminated
recharge in only that area.
The trimethylbenzene (TMB) isomers 1,3,5-TMB and
1,2,4-TMB also were detected in the headspace of tree
cores that contained MTBE (Landmeyer et al. 2000).
These compounds are relatively recalcitrant and, therefore,
are used as conservative tracers to judge the extent of atten-
uation of other fuel compounds. These results of tree uptake
of TMBs indicate that any study that proposes to use TMBs
as conservative tracers may need to consider the effect of
plants on these compounds.
Hong et al. (2001) describe a study where about 2 acres
(8,094 m 2 ) of hybrid poplar trees (DN-34 and NE-19) were
planted at a site in Houston, Texas, in 1998. MTBE was
released to the water-table aquifer, where the water table is
about 10 ft (3 m) below ground surface, and is composed of
silty sands, thought to be an old river channel. Above these
sands is a clay-rich sediment extending to land surface. The
trees were planted in rows on 6-ft (1.8 m) centers, and the
rows were separated by 8 ft (2.4 m). As for the water budget,
precipitation is about 39 in./year (100 cm/year). The ET P as
determined using the Penman equation is about 59 in./year
(150 cm/year), which suggests that if plants that reach the
water table are used, ET P will lead to a lower water table.
The 10-ft (3 m) whips were planted in holes 1-ft (0.3 m) in
diameter drilled into the clay sediments to the water table.
Mulch (40%) and fertilizer-amended sand (60%) was used
as backfill.
Sap-flow measurements on the 1-year-old trees at
the time (1999) indicate that the water-uptake rate was
about 4 gal/day/tree (15 L/day/tree) (Hong et al. 2001).
Using this data and estimates of water input
Fig. 13.14 Oak-tree roots on a water-level pressure transducer
removed from a monitoring well at a phytoremediation site near
Beaufort, South Carolina. These roots had not grown within the well
casing and down to the water table; rather, the roots had grown in
the unsaturated zone and entered the well through the well screen
(Photograph by author).
trees. For example, Figure 13.14 shows root growth on an in-
situ sampling device that was placed 1 ft (0.3 m) below the
water table. These roots had not grown within the well
casing and down to the water table; rather, the roots had
grown in the unsaturated zone and entered the well through
the well screen.
Tree core material was collected from trees located
upgradient of the MTBE plume as well as from trees growing
above the plume delineated by an extensive monitoring well
network (Fig. 13.13 , two monitoring wells are shown;
Landmeyer et al. 1998a). Tree cores were collected about
1 ft (0.3 m) above ground surface on the northeast side of
each tree sampled. The cores were placed in 40-mLVOA glass
vials and sealed with a Teflon-lined cap. In the laboratory, the
40-mL vials were amended with 5 mL of pesticide-grade
methanol and rolled on a heated hot-dog roller for 24 h to
mix the methanol with the tree cores. The final volume in the
vial was brought to 25 mL with addition of organic-free
reagent water. A purge-and-trap method similar to EPA
8260B was then used to separate and identify the compounds.
MTBE, as well as other gasoline compounds detected in
the groundwater such as BTEX, were not detected in the vial
headspace in the trees that grew above uncontaminated
groundwater upgradient of the source. Conversely, MTBE
and BTEX were detected in the headspace of cores from
to the
site from precipitation and irrigation, a total
input of
10 6 L)for1998andestimatedoutput
293,000 gal (1.1
10 6 L) suggest that the trees potentially
removed 441,000 gal (1.6
734,000 gal (2.7
10 6 L) of water from the study
site. However, because no groundwater-level data were
shown, it is hard to determine if this increase in extraction
was met by a water-table decline or by increased ground-
water flow from upgradient areas or from deeper aquifers.
Since MTBE was banned in the United States as part of
the 2006 Energy Policy Act that removed the oxygenate
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