Environmental Engineering Reference
In-Depth Information
gasoline-contaminated sites where anoxic groundwater that
contains BTEX and MTBE discharges to surface water
(Landmeyer et al. 2001). For example, the oxic and anoxic
interface in the one foot-thick stream hyporheic zone led to
the formation of a microbial system that mineralized MTBE
in groundwater prior to discharge. The contaminant does not
enter the wetland by overland flow, but rather the wetland
processes occur vertically as groundwater discharge brings
contaminants to the wetland redox interface. This also has
been shown to be the case for chlorinated solvents (Lorah
and Olsen 1999; Dinicola et al. 2002).
A constructed wetland can emphasize redox interfaces in
areas where organic matter accumulates at the bottom of the
surface-water column. This accumulation of organic matter
promotes anoxia and denitrification. This process of nitrate
removal is preferred over the uptake of nitrate by the wetland
plants, because the removal of nitrate by plants would lead to
huge increases in biomass that would have to be managed.
Phosphorus, on the other hand, is taken up by plants, but it
can be released back into the system when the plant dies.
Relative to the use of plants at a phytoremediation site to
control a subsurface plume in groundwater, the rate of sur-
face-water movement through a constructed wetland is fast.
However, wetlands are a surface expression, almost like
ex-
situ
treatment, and the risks associated to exposure potential
are higher than with the same contaminants in groundwater.
For example, flood conditions may increase the risk of
exposure to downstream areas.
A wetland system was used to examine the biological
effect of plants and associated rhizospheric microbes on
concentrations of nitrate and the xenobiotic perchlorate.
Perchlorate is a chlorinated hydrocarbon used as an explo-
sive, propellant, and pyrotechnic. It also has natural sources.
The rocket fuel propulsion industry has been implicated as
the source for perchlorate being detected in public water-
supply wells in California. Even though no MCL currently
exists for perchlorate, the potential health effects have pro-
moted research for remediation tools to remove perchlorate
from contaminated groundwater.
Krauter (2001) investigated the use of a bioreactor that
contained wetland plants to remove not only perchlorate but
the high concentrations of nitrate associated with perchlorate
detection. Each of four bioreactors built were filled with
coarse aquarium gravel, presumably to simulate the flow of
groundwater through a shallow aquifer. Wetland plants
placed in the bioreactors included bulrush (
Scripus spp.
),
sedges (
Cyperus spp.
), and cattails (
Typha spp.
) collected
from areas in California near Livermore. To deliver perchlo-
rate- and nitrate-contaminated groundwater to each bioreac-
tor, a 55-gal (207 L) drum was filled with groundwater
obtained from a well known to be contaminated with per-
chlorate (4.5
was used to inoculate the system and create microbial biofilms
acclimated to perchlorate prior to adding clean water to which
a known concentration of perchlorate was added.
Nitrate concentrations decreased from 80 to 4 mg/L
within the first day of the test. Perchlorate decreased from
44 to less than 4
g/L within 4 days (Krauter 2001). This
follows the usage of preferred electron acceptors; oxygen,
nitrate, and then perchlorate. In the absence of oxygen,
nitrate undergoes assimilative and dissimilative nitrate
reduction to nitrogen gas. Chlorate can be transformed to
chlorite in the presence of nitrate reduction. Microbes can
reduce the oxidized perchlorate to chlorate ion and then to
chlorite. The wetland plants are key to this process, because
they release organic matter to stimulate nitrate reduction,
and can maintain these reactions over time.
One of the first manmade wetlands constructed specifi-
cally to treat stormwater runoff was in the early 1980s in
Florida. In 1983, the Northwest Florida Water Management
District, and the USEPA and the Florida Department of
Environmental Regulation, were concerned that Lake
Jackson, located near Tallahassee, FL, was being negatively
affected by the accumulation of stormwater runoff resulting
from rapid urbanization. As with many lakes that consist of
deep water and shallow coves, the coves are the first to
experience changes in water quality as they are nearer the
source of runoff from the banks and have less water volume
for dilution. At Lake Jackson, one of these coves is called
Megginnis Arm, and it was here that the manmade treatment
system was constructed. It consisted of a retention pond to
capture the volume of flow generated by storms. Next, the
water flowed through a sand filter and then to the 9-acre
marsh. The marsh was constructed to contain three treatment
areas; one containing primarily sawgrass, one containing
primarily rushes, and the third containing broadleaf peren-
nial herbs. The most efficient removal of nutrients from the
stormwater runoff occurred in these three treatment areas.
In the southeastern United States, there is a large
constructed wetland that has been used to treat wastewater
prior to discharge since 1997. This wetland treatment area is
north of Augusta, SC, and is an extension of the naturally
occurring Phinizy swamp. Prior to 1968, untreated wastewa-
ter from Augusta was discharged to a local ditch that made
its way to a local creek that then discharged to the Savannah
River. In 1968, a wastewater-treatment plant was built.
In 1997, the Phinizy swamp was extended by the addition
of 12 constructed “cells” used to slow the path of the treated
wastewater as it makes its 2-day trip through the swamp. The
plants in these cells, which essentially are defined by berms,
contain cattails and rushes, which can handle the high nitrate
and phosphate concentrations in the wastewaters. The design
of the constructed wetlands attempts to mimic the flow
of flood waters through flood-plain vegetation sometimes
observed in natural flood-plain riverine ecosystems.
m
g/L) and nitrate (68 mg/L); it was allowed to
flow by gravity through the attached bioreactors. The flow
m
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