Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 176

water molecules to soil particles, and if these attractions are
inhibited by the presence of organic matter, the water droplet
remains intact as a continuous film on top of the soil particles
and does not infiltrate (Dekker 1998). Moreover, retardation
of xenobiotics onto the soil organic-matter fraction tends to
decrease contaminant bioavailability. In fact, residual
weathered petroleum-hydrocarbon contamination has been
shown over time to become integrated into the soil O horizon
(Guthrie et al. 1999).
These factors of bulk density, porosity, and organic-matter
content represent part of the soil physical composition.
A variety of options are available to understand the soil
physical composition at a site that is a candidate for
phytoremediation and to determine if any pre-existing con-
dition needs to be addressed. Hard copies or digital files of
county soil maps can provide an initial generalized descrip-
tion of the surficial soils at a site. These may be less useful,
however, in terrains where great heterogeneity exists over
only a short vertical distance, such as in glaciated terrain. At
the site, soil samples can be collected in various areas,
mixed, and analyzed as a composite sample for properties
or characteristics such as bulk density, porosity, organic
matter, and grain-size analysis. To determine the ease at
which water will infiltrate through these sediments, samples
can be sent to the laboratory for falling-head permeameter
tests. In the field, infiltrometer tests can be done to determine
infiltration potential.
The presence of undesirable physical soil characteristics
can be dealt with by using a variety of approaches. Poor soils
can be removed and replaced by higher quality soils, or
removed and amended with more desirable soils, or left in
place for the plants to be installed with a more desirable,
loamy material as backfill. Tight soils that cannot be removed
can be tilled or ripped with mechanical equipment. An
increase in soil permeability before planting is essential,
even if deep-planting methods have to be used, to ensure
contact with the water table at depth. Over time, however,
plants naturally increase soil porosity and permeability even
in tight soils, as roots grow, die-back, and slough off as they
explore the subsurface. Moreover, this growth over time helps
to establish a rhizosphere where perhaps none before existed.
In combination with soil porosity and bulk density, soil
water potentials determine whether or not water is available
for plants to remove. In clay-rich sediments, for example,
there may be a higher amount of void space than in sand, and
porosities can approach 50-70%. However, clay particles
have a larger surface area than do sand grains of similar
porosity and, therefore, hold water more tightly under higher
tensions, so the water potential is more negative than that for
sand. In other words, clay soils may contain more water than
sandy soils, but it may not be bioavailable.
Individual root hairs seek water almost on the molecular
level and, therefore, will follow the path of least resistance,

i.e., through the most permeable sediments. As water enters
the root hairs by diffusion and osmosis and is depleted in the
soil, additional water must be acquired. The root hairs either
grow toward a new source of water; or water will be supplied
by infiltration, groundwater flow, or capillary movement; or
the root hairs will die. This indicates the relation between the
presence of roots and in zones of higher hydraulic conduc-
tivity. This further emphasizes the importance of the multi-
disciplinary approach to site assessment and characterization
that was advocated in Chap. 6.

7.3.3 Soil Chemical Composition

Ashes to ashes, dust to dust.
The Book of Common Prayer (1979)
There was an old belief that in the embers of all things their
primordial form exists, and cunning alchemists could re-create
the rose with all its members from its own ashes...
H.W. Longfellow, Flower-de-Luce, Palingenesis
(McClatchy 2000)

Nutrients that are locked up in the non-living inorganic
material or previously living organic material of soils are
made available to plants by soil microorganisms. Plants
require almost 20 trace elements and nutrients to ensure
successful growth. This coupling between soil and plant
composition was revealed, in part, by the combustion of
plant matter and the analysis of the material that remained
or the gases emitted. The gases released reveal that the
biomass we recognize as being part of plants, such as
wood, bark, leaves, stems, etc., are almost entirely derived
from the inorganic gas CO 2 (this is the missing information
that led J.B. van Helmont, as described in Chap. 1, to state
that only water was needed to make plant biomass). The
gaseous composition makes sense, because photosynthesis
requires CO 2 along with water—plants, after all, make their
own food, so there should be little else that they require,
correct? The chemical composition of the ash tells a differ-
ent story, however, such that more than CO 2 and water are
necessary to sustain plant life. And this includes plants that
will be used for phytoremediation of

contaminated

groundwater.
As stated, the analysis of the ash leftover from plant
combustion can reveal much about what else plants need to
survive. At least 13 elements are considered essential to
plant health; nitrogen (in the form of the oxidized anion
NO 3 - or the reduced cation NH 4 + ), phosphate (as the anions
H 2 PO 4 - , HPO 4 2- ,orPO 4 3- ), and potassium (K + ). These
elements are the big 3 that compose most lawn and garden
fertilizers. With the introduction of the Haber process in the
early 1900s, nitrogen from air could be combined with
hydrogen from coal oxidation to produce ammonia (NH 2 ).
This process made access to this essential plant nutrient