Environmental Engineering Reference
In-Depth Information
The compound 1,4-dioxane, C
4
H
8
O
2
, also known as
dioxane, is a peroxide. Dioxane is primarily used as a stabi-
lizer for chlorinated solvents such as PCE and TCE but also
is used in paints, lacquers, and varnishes. Dioxane detection
in groundwater is problematic as dioxane is classified as a
probable human carcinogen. Dioxane is miscible in water
and not readily absorbed by sediments, and plumes of diox-
ane in groundwater can be extensive (Tillman 2009). The
physical properties of dioxane indicate that it is conducive to
phytoremediation, even with a log
K
ow
less than 0.
Tritium,
3
H, is the radioactive isotope of hydrogen, with a
half-life of 12.32 years. Tritiated water is formed naturally in
the upper atmosphere. Elevated levels of tritium are pro-
duced for thermonuclear purposes and biochemical tracers.
Tritium enters the hydrologic cycle from these sources,
including as leachate from landfills designed to contain
low-level radioactive wastes in both humid (Vroblesky
et al. 2009) and arid (Garcia et al. 2009) locations. The
tritiated water can be present in liquid and vapor form at
low-level radioactive waste landfills.
loss of NDMA was primarily by the removal of water by
plant uptake, since the loss of NDMA varied linearly with
the volume of water taken up over time. This is an important
result if transferable to field studies of NDMA-contaminated
groundwater, because measurements of transpiration can be
used to estimate the potential for NDMA mass loss from
contaminated groundwater. Moreover, these results chal-
lenge the commonly held belief that phytoremediation
strategies are handicapped by a lack of contaminant removal
during the winter; although these are laboratory studies,
significant contaminant removal was observed during the
winter. Yifru and Nzengung (2006) used the loss of
NDMA over time, along with the loss of water as an indica-
tion of the transpiration rate, to calculate a
TSCF
of about
0.28.
If the NDMA and perchlorate were removed from solu-
tion during transpiration, did the chemicals simply volatilize
from the leaves? In the experimental setup, only about 52%
of the original
14
C-NDMA was recovered, the balance of
which was presumed to be transported to the leaves and
volatilized. Of this recovery, 46% was contained in plant
tissues, and distributed between leaves (19%), main cutting
stem (16%), branches (8%), and in the roots (4%). About 5%
was recovered as being that volatilized from the total tissues
above the growth solution. On the other hand, perchlorate
was removed from the solution to levels below detection in
50 days, or 20 days longer if NDMA also was present. The
distribution of the
14
C-perchlorate in the plant was as
follows: leaves (98%), stem (1.3%), and roots and branches
(less than 1%).
Aitchison et al. (2000) examined the fate of 1,4-dioxane
in poplar cuttings (DN-34) about 10 in. (25.4 cm) long in
both hydroponic and soil treatments. Within 9 days of expo-
sure, more than 50% of the 1,4-dioxane was removed from
the hydroponic solution. Most of that taken up, between 76%
and 85%, was transpired. A
TSCF
of 0.76 was calculated
from this mass balance. In soil treatments, cuttings were
exposed to
14
C-1,4-dioxane. At the end of this experiment,
only 19% of the label remained in the soil that contained the
cutting, relative to 72% of the label
13.6.1 Plant Interaction and Uptake Pathways
Many studies of the fate of nitroaromatics have been
performed; they involve investigations of both microbial
and plant processes. The fate of nitroaromatic compounds
is typically determined in soils, rather than groundwater.
Early studies focused on the interaction between plants and
TNT from the standpoint of human health exposure. Because
the log
K
ow
of TNT is 2.0, the translocation of TNT was
predicted. However, Burken and Schnoor (1998) did not find
TNT in the shoots and leaves.
The fate of perchlorate exposed to woody plants was
investigated by Nzengung et al. (1999). They report almost
100% removal of perchlorate by willows exposed to solution
of 10-100 mg/L perchlorate. Loss from solution was
attributed to uptake and transpiration and by rhizospheric
degradation. Loss of perchlorate in the rhizosphere
coincided with an increase in chloride concentration.
Yifru and Nzengung (2006) investigated the effect of
woody plants such as black willow (
Salix nigra
) and hybrid
poplar (
Populus deltoides
that remained in
sterilized soil controls.
Tritium, being essentially part of the water molecule,
readily enters plants from the vadose zone (Garcia et al.
2009) and water table (Vroblesky et al. 2009). At the
USGS Amargosa Desert Research Site (ADRS) in southern
Nevada, the arid environment promotes the upward move-
ment of any tritiated water in the subsurface following
leakage from tritium waste-disposal facilities (Garcia et al.
2009). Although evaporation removed three times more
tritiated water vapor from the contaminated subsurface at
the ADRS, the presence of vegetation led to decreased
infiltration of precipitation, which enhanced the upward
movement of tritiated wastes. This study is perhaps the
nigra
, DN34) on the fate of
NDMA and perchlorate in water. They created microcosms
of individual cuttings placed in 2-L bioreactors filled with a
dilute Hoagland solution to which was added either 1 mg/L
NDMA, 0.65 mg/L NDMA and 27 mg/L perchlorate, or just
perchlorate. The fate of these two contaminants was moni-
tored over time by sampling small aliquots from the
Hoagland solution until non-detect levels were reached.
After 80 days, about 98% of the added NDMA had been
removed from the growth solution by the cuttings during the
summer, and 81% was removed during the winter. This large
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