Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 183

Phytoremediation can occur under these conditions by using
engineered means to have the roots reach the deeper con-
fined aquifers. Boreholes can be advanced and planted with
large cuttings, or poles, while the holes are backfilled with
porous material. A case study where this process occurred is
discussed in Chap. 13. For groundwater under semiconfined
conditions, such a plant design and installation encourages
upward flow to the roots by capillary action.

geochemical data from areas that had phreatophytes,
existing wells, or boreholes that could be easily dug. Some
of the conclusions he drew from the groundwater geochemi-
cal and plant-distribution data suggested that phreatophytes
could exist if the mineral composition in the upper part of the
water table ranged from low (TDS < 1,000 ppm) to high

(TDS > 40,000 ppm) (Meinzer 1927).

7.3.9 Plant Ecological Conditions

7.3.8 Groundwater Geochemistry

Plants that rely on the water table for moisture are the ones
that were successful in reaching the water table and in
reproducing. These are the hardiest of each species, because
before any growth from interacting with groundwater can
occur, the seedlings must endure a time following deposition
in which the water table is beyond the reach of the rootlet—
this is natural selection at its most obvious. Also, because the
air is dry in the arid western United States, the plants must
not only reach the water table rapidly but also have the
capability to use large quantities along the prevailing humid-
ity gradient.
It is a common observation that poplar trees are hard to
kill by even cutting the trunk at ground level. This is because
within a few weeks during the growing season, numerous
saplings will sprout up from the cambium all around the
circumference of the cut tree. A unique feature of poplar
trees is that they can produce new aboveground growth even
after complete trunk removal to ground surface, or coppice.
Upon reflection, this manner of survival makes sense for
phreatophytes, because it allows the tree to continue life
without having to expend energy to make and release seeds
whose roots, following germination, may not reach the water
table.
The relation of tree health to the ideal spacing interval
between plants at a potential phytoremediation site raises
many concerns. Close spacing typically is encouraged for
the maximum removal of groundwater or prevention of
recharge while maintaining individual tree health. The ulti-
mate control variable on the minimal plant spacing to use
should be the factor that limits plant transpiration. At most
sites, this is the amount of solar energy input to an area that
can be used to evaporate water. If water is not limiting, as
can be assumed to be the case once trees reach the capillary
fringe, then the factors that maximize evaporation should be
enhanced. It has been shown that water use on a per-tree
basis is higher in open forests relative to trees in a dense
forest (Stewart 1984). Plants grown too close together have
to compete for limited soil moisture and nutrients and often
appear stunted. The upper Coastal Plain forests along the
eastern seaboard of the United States provide an example of
this scenario. The point is that larger trees have more leaves

The ambient geochemistry of groundwater, in terms of the
concentrations of dissolved solutes acquired along the flow
path by interactions with the porous media, can influence the
determination of the types of phreatophytes that grow in an
area. This is important to consider during site-assessment
and characterization, because simply determining the pres-
ence and depth of groundwater does not necessarily infer
that plant growth can be sustained. For example, cottonwood
and willow typically are not found in areas characterized
by high concentrations of dissolved salts. However,
certain hybrid poplar trees, such as the OP-367, can live
in soils where the pore water is characterized by high
salinities. Some native plants that can tolerate high salinity
groundwater include greasewood, saltcedar, and pickleweed
( Allenrolfia ocidentalis ), although these plants rarely are
used in phytoremediation applications.
Iron probably is just as important an element to measure
in groundwater as are sodium and chloride. Although iron is
a micronutrient for plants, at high concentrations it can be
toxic. Iron concentrations in plant tissues can accumulate to
between 400 and 1,000 mg Fe/kg plant tissue, but these
higher concentrations can decrease plant health. Many
aquifers that contain either naturally high concentrations of
labile organic matter, such as aquifers that underlie swamps
or peat lands, or aquifers that contain high concentrations of
petroleum hydrocarbons from gasoline releases often have
high dissolved iron concentrations from iron reduction under
the prevailing anoxic subsurface conditions. Therefore,
these indicators of higher iron concentrations should be
assessed as part of site-assessment and characterization—
many test kits that provide iron concentrations in the field are
commercially available.
The classifications of plant type and groundwater geo-
chemistry indicates that on some level interspecies competi-
tion for a limited resource occurs relative to subsurface
moisture and groundwater, which produces a generalized
differentiation of plants that use groundwater of different
geochemistry. O.E. Meinzer's study of the three basins of
the southwestern United States (described in Chap. 1)
provided the opportunity to collect and analyze groundwater