Environmental Engineering Reference
In-Depth Information
groundwater. Many contaminants that enter groundwater
were released at the surface or in the unsaturated zone and
will be transported to groundwater by density or infiltration
and leaching. Residual contamination in the unsaturated zone
represents a long-term continuous source of dissolved-phase
contaminants to groundwater. The distribution of plant root
systems, including for phreatophytes, is greater in the unsatu-
rated zone and, therefore, plays a role in determining the
concentration and structure of contaminants. The combination
of enhanced plant processes and contaminant presence are
why an understanding of such interactions is imperative to
include in phytoremediation studies.
The models HYDRUS, RZWQM, and VS2D solve for
water flow in the unsaturated zone using the Richards equa-
tion. Also, the uptake of water by plants is directly
simulated. For solutes, the number of contaminants that
can be simulated ranges from 1 to 5, and processes that
affect these contaminants include sorption, volatilization
(except for VS2D), dispersion, degradation, uptake by
plants, and effect of various physical parameters on degra-
dation such as soil moisture (Nolan et al. 2005).
HYDRUS (Simunek et al. 2005) is finite-element model
that simulates the movement of water in the saturated zone
as well as being used predominantly for simulating water
movement in the unsaturated zone, under one and two
dimensions (HYDRUS-2D). It solves for flow in the unsatu-
rated zone using the Richards equation. It also solves for the
uptake of water by plant roots, which can be selected from a
database of values, and simulates the growth of roots using a
logistic growth function. Also,
ET
values can be added and
simulated. Contaminant fate of up to five solutes is simulated
as being affected by sorption, volatilization, dispersion, deg-
radation, and uptake by plants.
A model that can be used to examine the relation between
plants, xenobiotics, and hydrology in the unsaturated zone is
the Root Zone Water Quality Model (RZWQM). The
RZWQM is a one-dimensional model developed by
researchers with the U.S. Department of Agriculture
(Ahuja et al. 2000). In an investigation of various such
models, it was determined that RZWQM simulated pesticide
fate and transport with the smallest error of all models
examined (Nolan et al. 2005). Inputs required are extensive
but similar to most unsaturated-zone models and include
values for soil organic content, microbial populations, deg-
radation-rate coefficients, among others; the effect of
dispersivity on contaminant fate is not, however, simulated.
Water flow is simulated using the Richards equation. Up to
three contaminants can be simulated simultaneously.
VS2D stands for Variably Saturated 2-Dimensional
Transport. It is a finite-difference code that simulates the
flow and transport of solute in variably saturated porous
media (Lapalla et al. 1983). Unlike HYDRUS, the root
growth distribution can be set as a function of time. Unfor-
tunately, only one solute can be tracked over time.
14.5.5 Guidelines for Model Evaluation
Groundwater-flow models are applications of numerical
codes that approximate the flow of groundwater through
porous media. The application of such codes to solve field
problems is subject to user bias and the way that the concep-
tual model has been conceived. As such, in order to properly
evaluate the application of any form of model to examine a
problem, the objectives of the use of the model need to be
stated clearly. Only then can it be determined if the model
was used correctly or used within its set of limitations
(Reilly and Harbaugh 2004).
14.6
Summary
Models are simulated approximations of physical phenom-
ena, such as groundwater flow and solute transport. Simple
models include those that describe the interaction of a
chemical species with water or organic matter, such as
the log transform of
K
ow
or the interaction between the
sediment characteristics or an aquifer and groundwater
flow as in Darcy's Law. Because the processes of ground-
water flow and solute transport are complex, the complex-
ity of models necessarily increases to more sophisticated
analytical and numerical models. Some of the existing
models incorporate plant and water relations, from either
the vadose zone or the saturated aquifer. Recently,
attempts have been made to simulate plant and water
interactions along with plant-water-solute reactions. Such
models will undoubtedly become more useful as they are
applied to phytoremediation field trials before, during, and
after installation.
Why is this information important to the phyto-
remediation of contaminated groundwater?
Both simple
and complex models provide the ability to test hypotheses
at phytoremediation sites in a relatively rapid manner. Also,
models can be (cautiously!) used in a predictive manner
to estimate how long a phytoremediation system will need
to remain active in order to reach remedial goals. In terms
of the perceived cost effectiveness of phytoremediation,
such information provided by models is invaluable.
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