Environmental Engineering Reference
In-Depth Information
cuttings are installed in trenches also have irrigation pipes
installed in the trench before backfilling.
An irrigation system similar to the type installed in many
residential areas could be adequate for a phytoremediation
system, with a water-supply line going through a manifold
that then supplies individual drip-irrigation lines. The whole
process can be automated such that a control box turns
separate solenoids on and off in the manifold that controls
the water flow to individual pipes. Filters also should be
installed before the first series of drip-irrigation emitters, to
prevent small particles from clogging the lines. Although
this can help to reduce the chance that the emitters will
become clogged, there is a possibility for the emitters to
become clogged by algal growth or other bacterial growth,
especially if concentrations of dissolved iron in the supply
water are high. These problems can be dealt with by a
periodic flushing with a low concentration of bleach, but
this will need to be done for the life of the project or until
the source of dissolved iron is removed.
Low soil-moisture conditions, however, do not always
require the installation of an irrigation system to achieve
positive results. A phytoremediation system of poplar
cuttings planted in November 1998 in Charleston, South
Carolina did not have an irrigation system, which was a
concern as the planting coincided with the beginning of a
5-year drought in the study area. A deep root system was
established, however, because the 6-ft (1.8 m) cuttings
planted had been deeply installed near the water table at
3-4 ft (0.9-1.2 m) below land. Under natural conditions in
a riparian area in Arizona, McQueen and Miller (1972)
reported that saltcedar and willow thrived during drought
conditions because the roots tapped the water table.
Soil moisture levels that are high are not always detri-
mental to plant health. Certain plants, albeit not ones used
for phytoremediation such as cultivated rice, require contin-
ual flooded conditions. Other plants, such as those that
characterize swampy areas or parts of flood plains that are
inundated most of the season, also require water-saturated
conditions. A key point is that the water remains flowing and
does not become stagnant.
The type of soil present in the root zone of plants has a
major influence on the relation between total soil moisture
and total water potential, or the difference between the
presence of water and its potential for movement into
plant roots. For example, the wilting point, defined as a
soil-water potential of
2005), as these microorganisms also require water for
survival.
Soil moisture often plays a critical role in the root density
of many plants. Typically, as the depth below land surface
increases, root density decreases. This holds not only for
most plants but for obligate and facultative phreatophytes
as well. Lower root density is compensated, however, by
fewer deep roots characterized by higher root-hydraulic
conductivities. This explains how water can be allocated to
plants to support transpiration even when water potentials in
shallow soils are more negative than the wilting point
(Teuling et al. 2006).
7.3.5 Soil Topography
As described in Chap. 3, the topographic character of a site
is a factor that can control soil-moisture conditions and
should be evaluated as part of site-assessment and character-
ization activities. In general, water tends to collect at or
near land surface in lower elevations. A site that has too
steep of a land-surface gradient may not be a candidate for
phytoremediation. This caution has less to do with plants not
being able to grow but that either the depth to water table
will be too great or the hydraulic gradient too steep and,
therefore, rate of groundwater flow too high for measurable
hydrologic control by plants.
7.3.6 Depth to Water Table
Not surprisingly, the depth to the water table is perhaps the
most important factor in determining the
a priori
success of
a phytoremediation system designed to control contaminated
groundwater, as has been discussed. Although the term
depth indicates some constant level where the water table
is encountered, the position of the water table is not a
constant but fluctuates around an average value (Holzer
2010). Many sites that need to be remediated are
contaminated because a shallow depth to water table resulted
in a lack of contaminant attenuation or because the source of
the contamination was only slightly above or at the water
table. For example, some USTs or pipelines are installed
only slightly above the water table and are inundated during
seasonal increases in the water table, thereby providing a
direct conduit for contamination of groundwater if leaks are
present.
The depth to water table and its effect on plant growth
have been studied by agronomists for some time. Many
shallow-rooted crops can be grown without irrigation or
with supplemental irrigation all over the world in areas
where the water table is shallow. Such crops include cotton,
alfalfa, and barley. As the depth to the water table increases,
1.5MPa,occursinclaysata
higher moisture content, about 15%, and at a lower mois-
ture content of 2.5% for more permeable sands. This is
because more air can penetrate sand than clay because of
the higher degree of interconnected pores even though clay
has a higher porosity than sand. The soil-moisture content
also affects the viability of rhizosphere microbes (Cho et al.
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