Environmental Engineering Reference
In-Depth Information
TCE to cis -DCE beneath an established cottonwood tree at
the site. The tree was located in an area where the dissolved
phase plume of TCE had been transported and oxygen
conditions were lowered, presumably by the release of
organic compounds by the tree (Godsy et al. 2003). The
more oxic, uncontaminated aquifer by comparison was
populated by aerobes, fermenters, and denitrifying bacteria,
low numbers of iron and sulfate reducers, and no
methanogens. DOC concentrations in the phytoremediation
area ranged from 0.8 to 1.8 mg/L, enough to support
methanogenesis, but not enough to decrease ambient nitrate
and sulfate levels to levels below detection, suggesting that
the aquifer is carbon limited, with respect to reduced carbon.
Iron reduction also produced dissolved iron, which was
detected in groundwater in the phytoremediation area.
At many sites, dissolved oxygen is rapidly depressed
following the release of the high BOD characteristic of
reduced petroleum hydrocarbons, such as BTEX and
PAHs. Rentz et al. (2003) investigated the possibility of
increasing the DO content around roots in high-BOD
contaminated sediments and its effect on the growth of
plants that might be planted at such sites to remediate the
contaminated soils. This is significant, because many of the
plants that could be planted at contaminated sites, such as
poplars, may not have the gas-transport structures (aeren-
chyma) to transport the O 2 that aquatic macrophytes or other
phreatophytes have. Additionally, as was shown at the site in
Texas, poplar trees can decrease the DO by their release of
labile organic matter. So, poplars can be a source or sink for
DO in contaminated aquifers and vadose zones.
At their study site, Rentz et al. (2003) report the rapid
disappearance of O 2 in soil gas with depth. Less than 1 mwas
required for complete removal, and the concomitant appear-
ance of CO 2 and CH 4 . As was reported above, CO 2 can be
toxic to plants at high concentrations. The DO was depressed
because the total petroleum hydrocarbon (TPH) concentra-
tion ranged from 820 to 11,000 mg/kg. Even at these
concentrations, no toxic effects were observed in laboratory
column incubation studies using poplar cuttings ( Populus
deltoides
over the control. No nutrients were added during this exper-
iment, so the researchers concluded that the observed
increase in biomass was due to increased oxygen content in
ORC
treatment. No oxygen content was measured, how-
ever. Also, ORC
®
contains high concentrations of PO 4 ,so
the possibility exists that the release of this nutrient was
partially responsible for the increased biomass in the control
pots. Moreover, ORC ® was originally manufactured in
England to serve the needs of gardeners' attempts to increase
the oxygen and phosphate contents of their characteristically
clay-rich garden soils.
®
12.3.4 Water-Table Fluctuations
and Contaminant Fate
The position of the water table affects and is affected by the
local sources and sinks of water and also can affect contam-
inant fate. The water table can rarely be described by a single
location in space or time. As was discussed in Chap. 4, it
changes in response to the balance between the sources and
sinks of water, such as recharge and ET .
Water-table fluctuation changes the composition of air and
water in the soil pores of a sediment profile. Sediments
located above the mean water table will contain air and
water under tension. As the water table rises, the air will be
displaced as the pore spaces fill with water. As the water table
falls, some of the water drains from the pores by gravity,
causing air to reenter. This change in gas composition will
impact the fate of contaminant compounds in the soil, water,
and gas phases near this water table and unsaturated zone
interface. Contaminants such as benzene or MTBE in the
dissolved phase during higher water-table elevations may
diffuse into the air space between pores when the water
table drops. This accelerates removal from groundwater and
will increase the potential that these gas-phase contaminants
will be biodegraded aerobically (Lahvis et al. 1999), volatilize
to the surface, or be taken up by plant roots. If the water table
is lowered by plant uptake of groundwater, this may increase
biodegradation of contaminants such as naphthalene
(Anderson et al. 2008), or BTEX (J. Burken, University of
Missouri, oral communication, 2007).
nigra DN34). The researchers compared various
inexpensive methods to deliver oxygen to the contaminated
unsaturated zone in column studies in the laboratory, relative
to unamended ambient conditions. These methods included
aeration by insertion of a perforated tube into the smear zone,
addition of high porosity gravel, use of proprietary oxygen
releasing compounds in filters, as well as increased drainage.
The effect of these various methods of O 2 delivery was
compared using net biomass—the biomass at the end of
experiment relative to the biomass at the beginning. Net
biomass was higher at the end of the experiment in the
columns where a proprietary oxygen releasing compound
(ORC
12.4
Plant Biochemical Processes
for Groundwater Contaminant
Degradation and Detoxification
We saw in Chap. 11 that plants produce allelopathic
compounds as an offensive act to survive in a competitive
world or render themselves inhospitable to herbivory. Plants
also must act defensively, however,
) was added in a filter in the smear zone, relative to
the control and other treatments, for a 2.46-fold increase
®
in order to protect
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