Environmental Engineering Reference
In-Depth Information
In many cases, vertical groundwater flow to tree roots
from groundwater flowlines beneath the water table surface
need to be simulated. This is especially important to deter-
mine if hydrologic control can be achieved, which is
discussed in Chap. 8. Analytical models that are idealized
and describe the removal of groundwater from wells that
fully penetrate the thickness of a particular aquifer cannot be
used in this situation. However, numerical models account
for variations in the spatial hydraulic conductivity, K ,inan
aquifer, using ratios of vertical-to-horizontal hydraulic con-
ductivity to account for anisotropy and the ability to create
multiple layers with model cells or nodes discretized to the
appropriate scale. It may be possible to ignore the effect of
anisotropy on groundwater flow to plant roots, but the results
will underestimate the effect of a potential plantation. Three-
dimensional numerical models can be used to understand
this phenomenon, because they account for groundwater
flow in the lateral and vertical directions.
Numerical groundwater models can be used to account
for all of the variables in a water budget for a particular site.
As was stated previously, evapotranspiration can be the
largest loss of water in a basin. In the groundwater-flow
model MODFLOW (McDonald and Harbaugh 1988), loss
of water from an aquifer is estimated by using a linear
relation between evapotranspiration and water-table depth.
Evapotranspiration occurs from the water table linearly until
a predetermined depth is reached and evapotranspiration is
assumed to be zero. An alternative approach was taken by
Matthews et al. (2003) in which they simulated evapotrans-
piration in MODFLOW by using the recharge module,
which specified recharge as a negative value. This approach
was an improvement because the magnitude of evapotrans-
piration was not related to the water-table depth. This alter-
native approach does not represent flow of water in the
unsaturated zone, however. Currently, there is no explicit
way to simulate interactions of groundwater with vegetation
that link root growth and groundwater use.
The MODFLOW groundwater model can be coupled to a
conservative tracer package called MODPATH (Pollock
1994), which can be used to visualize individual groundwa-
ter flow paths following the release of artificial particles of
water into the calibrated groundwater-flow model. This can
be done to evaluate the effect of anisotropy on the deflection
upward of groundwater flow lines in a water-table aquifer
toward tree roots at or near the water table and capillary
fringe. MODPATH also can be used to represent a plume of
contaminant released from a source area and to determine
the final size a phytoremediation planting must be to capture
all the contaminant mass and provide maximum hydrologic
control (Matthews et al. 2003); this is discussed further in
Chap. 15.
Models can be used to further understand groundwater-
flow systems. Models are more effective at revising concep-
tual models, rather than being actual representations of
nature. This is especially true as the uncertainty of the
parameters used and simulated time from calibrated
conditions increases.
4.12
Summary
Groundwater is not readily observable but is readily quanti-
fiable. Monitoring wells can be installed in unconfined or
confined aquifers to measure groundwater levels and this
information used to determine the volume and flow direction
of groundwater by using Darcy's Law. Moreover, monitor-
ing wells can be used to observe the effects of plants on
groundwater, and is more fully presented in Part II.
Why is this information important to the phytore-
mediation of contaminated groundwater? The fundamental
interaction that exists between plants and groundwater can
be observed using these same fundamental concepts of
groundwater hydrology, such as wells, measurements of
groundwater levels, and Darcy's Law.
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