Environmental Engineering Reference
In-Depth Information
Plants can indirectly affect natural attenuation even if not
used in concert for clean up. This occurs for at least two
reasons—soil-moisture fluctuations and introduction of root
organic matter. The first reason is because the removal of
soil moisture by plants can affect the soil bulk density by
decreasing the water content and increasing the air content.
As soils become more oxic they also have decreased water
content, but water is necessary to support microbial metabo-
lism. Secondly, the flow of carbon, nitrogen, and phosphorus
as described in this chapter are affected by the availability of
microbes and moisture.
that reaches streams and rivers; this, in turn, controls the
amount of primary production by algae, which decreases the
BOD on available DO in the water column.
An interesting study of the effect of trees, such as the
phreatophytes used to remediate contaminated groundwater,
on water flow and quality was reported by Kollin (2006). A
fire in San Diego, CA, in late 2003 (the Cedar Fire) affected
about 13% of the city boundary, or about 28,000 acres
(113
10
6
m
2
). Up to 50% of the tree canopy in this area
was lost. As a result, it was calculated that runoff during
storm events increased about 12
10
5
m
3
).
This same loss in canopy and stormwater retention can occur
without fire, however, as other factors can remove tree
canopy, such as rapid or mismanaged development.
Engineered riparian buffers have similar characteristics
to natural systems (Mayer et al. 2005). A gradational change
occurs in plant types related to water use and source from the
riparian zone to more upland plants that derive most water
from recent or stored precipitation. Flood water is retained
during times of high flow for subsequent slower recharge to
the soils and aquifers and movement through the rhizosphere
of the riparian vegetation. As long as the water and
contaminants can contact the plant rhizosphere, roots, or be
up taken, contaminant transformation by plants, the pro-
cesses which are the focus of Chap. 12, can occur.
10
6
ft
3
(3.3
11.4.4 Plants and Riparian Buffers
The area above ground or below ground connected with a
surface-water body generally is termed the riparian zone.
The surface water can be moving water, such as creeks,
streams, or rivers, regardless of whether they flow continu-
ally or not. The surface water also can be quiescent, such as
lakes or ponds. The extent away from the interface between
land and surface-water level, in either direction, is further
categorized based on ecological criteria.
The riparian zone also has a legal definition. An area
adjacent to the surface-water body can be used as a buffer
in which no environmental degradation is permitted to
occur, and if it occurs, it must be remediated. This not only
protects the riparian buffer as a resource, it is designed to
protect the surface water from degradation by contaminants,
wastes, or decreases in DO. The exact width of the zone is
dependent on the regulatory agency, intended use, the regu-
latory requirements, and the source of the impacts to the
water.
The processes occurring in the riparian zone that act to
protect the surface-water system include physical, chemical,
and biological processes. The presence of plants and their
root systems decreases the probability of sediment erosion
and decreases the velocity of surface-water runoff by
displacing the suspended-sediment load (and contaminants)
before entering the surface water. These sources of problems
fall into the category of nonpoint sources. Sediment input
can endanger fish survival and reproduction; carry attached
contaminants, such as bacteria and other fecal problems;
reduce clarity; and increase the cost of treatment for drinking
water downstream. Contaminated groundwater, however,
can act as point sources to surface-water systems. The
same can be said for plumes of nutrient-contaminated
groundwater, such as high-nitrate levels in groundwater
that flows beneath agricultural or livestock lands. This
input of excess nutrients can cause eutrophication, or blooms
of algae and other plants. Buffers that contain tall, woody
plants with associated high
LAI
control the amount of light
11.5
Summary
Plants trap solar energy and convert it to ATP to form
glucose, cellulose and lignins, allelopathic chemicals, and
to support respiration. Many xenobiotic compounds interact
with plants because they have physical and chemical
properties that make them readily dissolve in water. The
interaction between plants and their natural, even seemingly
inhospitable, geochemical environment provides the funda-
mental basis that phytoremediation can be used remediate
contaminated groundwater—this is not the case for purely
physical or chemical processes of groundwater remediation
such as air stripping or Fenton's oxidation.
Why is this information important to the phyto-
remediation of contaminated groundwater?
The notion
that plants are not simply at the mercy of the environment
and have, in fact, demonstrated the ability to produce
chemicals not only to sustain their growth but to protect
them from predators and to diminish the effect of resource
competition is in direct contrast to the notions spelled out at
the beginning of the environmental age in the early 1970s.
This encounter of plants with allelopathic compounds and
the early use to treat wastewater are perhaps the best evi-
dence to support the application of plants for the remediation
of xenobiotics in contaminated groundwater.
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