Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 270

quality by the release of oxygen by plants and the removal of
suspended particles and turbidity in these low-energy
environments. Plants help keep the turbidity low by anchor-
ing the sediment and preventing re-suspension. Their pres-
ence increases the number of microbes present in the
submerged rhizosphere of the plants, and reduced organic
carbon compounds can be oxidized by these microbes.
A potential liability of using wetlands to cleanup wastes
is that the system primarily is anoxic. Under oxic conditions,
the transformation of reduced organic contaminants, such as
oils and fuels to carbon dioxide, occurs more rapidly than
under anoxic conditions. Anoxic conditions promote denitri-
fication, however and, therefore, facilitate the biological
removal of elevated nitrate, a common nutrient in runoff
and wastewater. Plant roots and leaves can take up nitrate
and phosphate dissolved in the water column.
The linkage between wetlands and municipal wastewater
treatment at first seems like a non-sequitur. In the days prior
to environmental laws that regulated pollution levels in
water in the United States, raw sewage was dumped directly
untreated into rivers. This practice occurred with little nega-
tive consequence as long as the population of the area was
constrained and great distances separated those dumping
wastes upstream and the downstream users of the water for
potable purposes. It was not until the typhoid epidemics
occurred in the nineteenth century that the idea of using
chemicals to treat drinking water was discussed and finally
implemented in 1948 in the United States and amendments
to the Federal Water Pollution Control Act (FWPCA) in
1970 took this idea further by requiring the treatment
of raw wastewater prior to discharge. In the developing
world, however, raw sewage often is not treated and
water for drinking is often obtained from surface water
contaminated by sewage.
The FWPCA was amended in the early 1970s and gave
rise to the Clean Water Act (CWA). The Clean Water Act
addresses water pollution through encouraging reductions in
point and non-point sources of contaminant release. The
National Pollutant Discharge Elimination System (NPDES)
specifically addresses the amount of pollutant that can safely
be added to a surface-water body. One of the sources is
runoff from highways and parking lots, which has been
associated with increased bacterial levels in waterways,
and with the increase in concentrations of PAH particles in
stream-bed sediments (Mahler et al. 2005).
Stormwater discharge from roadways is considered under
the NPDES program to be a municipal separate storm-sewer
system, or MS4. To reduce the load of pollutants that could
reach a regulated surface-water body under MS4, various
best management practices (BMPs) can be used. Ironically,
one of the BMPs is the construction of a surface-water body
designed to receive runoff prior to release to the regulated
surface-water system. These appear as holding ponds at the

Fig. 11.12 An artificial wetland to treat runoff from a commercial
parking lot in Florida. The plantings primarily consist of cattails and
sedges, as seen in the middle of the water, with willows along the banks
(Photograph by author).

edge of mall parking lots, usually surrounded by chain-link
fencing (Fig. 11.12 ). Contaminants that enter the holding
ponds during runoff events are reportedly removed by sedi-
mentation, biodegradation, and plant-nutrient uptake.
Natural wetlands are not necessarily the most efficient
way to treat wastes or contaminants. This lack of efficiency
primarily is a result of preferential water flow through only a
limited part of the wetland. A constructed wetland removes
this liability to pollutant removal by increasing the residence
time and space for water flow through the wetland. More
control over the water flow also permits control over
the water level, which in turn helps to control the distribution
of plants. Constructed wetlands enhance water-flow contact
through these sediments by promoting a loose rather than
compacted bottom layer. The hydraulic conductivity in the
bottom sediment of a constructed wetland will be higher
than that of a natural wetland (Horne 2000). Beneath these
more permeable sediments, however, is placed a lower-
permeability liner, such as clay.
Constructed wetlands are more prevalent outside the
United States than inside, for there are roughly 300 in
North America compared to 500 in Great Britain alone.
The preferred plants for constructed wetlands include cattail
( Typha spp .) and bulrush ( Scripus spp .). However, many
others are available for use and are being evaluated (Lewis
and Wang 1997). The purpose of constructed wetlands is to
maintain the surface-water level at a condition at or near the
land surface to satisfy the needs of the plants that grow there.
An additional benefit of constructed wetlands is the max-
imization of areas where redox gradients are present. Zones
of anoxic and oxic conditions within a relatively short dis-
tance can harbor a wide diversity of microbes that contain
high pollutant degrading potential. This has been observed at