Environmental Engineering Reference
In-Depth Information
Table 8.1
The parameters and range of values
for an ideal
phytoremediation site for hydrologic control.
Parameter
Range
Comment
Aquifer
Unconfined.
Depth to water table less than
15 ft (4.5 m) from land surface.
Water quality
Fresh.
Chloride less than 250 mg/L.
Water
fluctuations
None.
Located away from tidal bodies.
Porosity, total
20-50%.
Soil organic
matter
None.
Redox
condition
Oxic.
Dissolved oxygen greater than
1.0 mg/L.
Flow path
Discharge area.
Growing
season length
12 months.
Fig. 8.1
Relation between the size of a planted area and time needed to
achieve hydrologic capture, where the larger planted area achieves it
faster than a smaller area (Modified from Matthews et al. 2003). One
square meter is equivalent to 2.47
Relative
humidity
Less than 20%.
Precipitation
Less than 4 in./year
(10 cm/year).
10
4
acres.
Wind speed
Greater than 5 mph
(8 kph).
trees, the need for installation of an extra number of smaller
trees, the need to purchase additional property, and the cost
of increased time to successful phytoremediation (Fig.
8.1
).
The ideal site conditions for phytoremediation to have an
observed effect on the water table need to meet the following
criteria (Table
8.1
). Concerning the groundwater hydrogeol-
ogy, the aquifer would be unconfined, with a shallow to
moderate depth to a water table, primarily less than 15 ft
(4.5 m) below land surface. Then, little effort would be
required to plant even the small trees where roots would
immediately be interacting with the water table. The ground-
water quality would be fresh (low total dissolved solids) and
not be influenced by saline surface-water bodies. The water-
table surface would have little natural fluctuation, such that
any fluctuation observed could be attributed readily to plant
uptake. The aquifer would have no to low amounts of natural
organic matter, contaminants present would be in the
dissolved phase at concentrations much lower than solubil-
ity, and the aquifer would contain dissolved oxygen. The
aquifer would be characterized by high porosity (greater
than 30%) but with groundwater-flow rates slower than the
water removal rate by transpiration. The aquifer would con-
tain enough silt or fine sand to increase the thickness of the
capillary fringe. The site would be located in an area of
groundwater discharge where deeper groundwater flowlines
converge upward toward land surface and plant roots. No
surface water would be present to recharge the aquifer.
Concerning the plants, the growing season would be year
round, with abundant solar radiation. There would be little to
no precipitation, and dry, low relative humidity conditions,
with strong winds. No demographic obstructions would exist
to prohibit the planting of the entire area above the aquifer,
and deed restrictions would not convey. There would be no
overhead restrictions such as powerlines, and there would be
Source area of
contamination
Removed.
no insect pests, molds, diseases, beavers, deer, livestock, or
voles. With respect to contamination, the source area would
be completely removed, and any contamination remaining
would be dissolved at concentrations less than solubility, and
the physical and chemical properties of the contaminant
would permit passive uptake into the transpiration stream.
As can be imagined, all these conditions are rarely found
at one site. In fact, the conditions that make up the ideal site
can be considered to be mutually exclusive, or at least not
hydrologically defensible. For example, according to the
above list, the meteorological conditions that would lead to
the greatest amount of groundwater extraction by plants
would be similar to those of the arid parts of the southwest-
ern United States, in general, or of southeastern California in
particular, but the hydrogeologic conditions there are
characterized by a depth to water table far below land sur-
face. The conditions that lead to the presence of a shallow
water table more ideal for phytoremediation are typical of
the humid parts of the eastern United States. There, however,
abundant precipitation during the summer and fall can result
in precipitation as the primary source of water to plants at a
phytoremediation site. However, most of the above criteria
are met in riparian zones adjacent to surface-water bodies
characterized by shallow water-table conditions. These
riparian zones are the ecological niches that support the
phreatophytes found throughout the United States in arid
and humid areas.
If a site has many or even a few of the parameters listed
in Table
8.1
seeming to favor
the implementation of
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