Environmental Engineering Reference
In-Depth Information
introduction to diffusion was osmosis, a special case of
diffusion through a membrane, such that water diffused
into a plant cell that contained a higher solute concentration.
With respect to solute diffusion in groundwater, the concen-
tration gradient is that of the solute molecules themselves.
In contrast, the mass of contaminants in groundwater can
be decreased by biodegradation processes by the production
of a less harmful intermediates or the complete mineraliza-
tion to CO
2
. The degree of this transfer is dependent upon
the contaminant type and concentration, and the presence of
the predominant electron acceptor in the contaminated
groundwater (Chapelle 1993).
14.2.3 Sorption
Sorption of a solute to an immobile solid phase relative to
groundwater flow removes the solute from the aqueous
phase. The solid phase is the aquifer material in most
cases. This solid phase also can consist of plant material.
This removal is either onto the surface of the immobile solid
phase or its interior. It can be irreversible or reversible.
Sorption is an important property to simulate with respect
to contaminant plume fate. Sorption does not determine the
maximum extent of the plume at steady state; rather, it
determines the time that it will take to reach steady-state
dimensions, with longer times for aquifers with higher sorp-
tion capacity (Landmeyer et al. 1996b).
14.2.6 Plant Processes
The various physical components of plants can interact with
contaminants above and below ground to the extent con-
trolled by the chemical and physical properties of the partic-
ular contaminant. Some of the more important processes are
described below.
14.2.6.1 Sorption
The presence of root material in a soil or sediment leads to an
increase in the organic matter content. This is the process that
leads to the formation of an O horizon in soil. The release of
plant organic material, either in the form of exudates such as
organic acids or as dead root material, acts to add labile and
refractory carbon to previously mineral soils. This organic
matter can act to absorb contaminants from solution according
to the degree that the compound is attracted to organic matter,
described by log
K
ow
. As was discussed in Chap. 12, the RCF
is an indication of this capacity for chemicals to be sorbed
onto organic root material from groundwater.
14.2.4 Volatilization
Many of the compounds released to groundwater have high
vapor pressures and low to moderate water solubilities. As
such, these xenobiotics have a tendency to be present as a
vapor phase. If released into groundwater as a pure liquid or
mixture, these compounds will establish equilibrium in the pore
spaces of the unsaturated zone with respect to the pure-phase
concentration. Because this equilibration with the air in the
vadose zone decreases the concentrationinthepurephasethat
can ultimately dissolve into groundwater, it can be considered
mass removal and, therefore, a component of natural attenuation.
The equilibration is governed by diffusive flux along a
concentration gradient. As was discussed in Chap. 12, vola-
tilization permits uptake into plant roots. These volatilized
compounds, once in the air of the pore spaces, also can
undergo dispersion and diffusion, as well as undergo
oxidization to support microbial growth (Lahvis et al. 1999).
14.2.6.2 Rhizospheric Processes
The rhizosphere can affect contaminants by the presence of
increased microbial and fungal populations, increased oxy-
gen delivery from the atmosphere to the subsurface, the
release of organic substrate, and release of contaminant-
specific or non-specific degradative enzymes.
14.2.6.3 Uptake
The root systems of plants are the pathway for water uptake
and delivery to the xylem but also act as the endpoint of the
diffusive transport of atmospheric oxygen to the subsurface
in the cortex of some plants. Hence, plants can take up
solutes dissolved in groundwater as well as those that vola-
tilize to the vadose zone.
14.2.5 Aerobic and Anaerobic Biodegradation
Processes
The processes discussed above can all act to decrease the
dissolved-phase concentration of a particular contaminant in
groundwater by physical dilution with uncontaminated or
less-contaminated groundwater, or by removal of a contam-
inant from the aqueous phase to a solid phase. In both cases,
however, the total contaminant mass remains unchanged and
remains in the subsurface, and the occurrence of reversible
sorption can lead to the long-term release to groundwater.
14.3
Existing Conceptual Models
14.3.1 PLANTX
The model PLANTX (Trapp et al. 1994) describes the
entrance of xenobiotics into plants. The compounds are
simulated to enter the plant
through soil, water, or the
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