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Two-dimensional
ground-water flow modeling was used to
predict the hydraulic performance of passive
and actively controlled, straight or winged
funnel-and-gate configurations over a range
of hydrogeologic and ambient groundwater
flow conditions.
The results of these analyses were
used to construct generic correlation
diagrams that relate expected upstream
capture zone or gate through-put to barrier,
gate, aquifer, and (for active systems)
control well parameters. These diagrams serve as convenient screening tools that may
be used to (1) quantitatively estimate the
capture zone of predetermined
funnel-and-gate configurations, or (2)
develop preliminary funnel-and-gate designs
that will yield a desired capture zone,
independent of aquifer characteristics.
The
effects of variations in aquifer hydraulic
conductivity, saturated thickness, and
hydraulic gradient for all straight and
winged funnel-and-gate configurations were
considered during this study. Sensitivity analyses revealed that the width of the upstream
flow channel, or capture zone, intercepted
by a given funnel-and-gate configuration and
passing through the treatment zone is
predominantly determined by the physical
geometry and dimensions of the barrier walls
and gate, and is effectively independent of
both aquifer transmissivity and the
magnitude of the ambient hydraulic gradient.
The
implications of these observations for
effective treatment gate design are
significant.
It must be emphasized that, because
the width of the capture zone is fixed by
funnel-and-gate geometry for a given angle
of incident groundwater flow, the volumetric
flow rate through the gate represented by
this zone will vary with aquifer properties
and the ambient hydraulic gradient in direct
proportion to Darcy's law.
For example, if the saturated
thickness or
hydraulic gradient of a particular
unconfined aquifer doubles, flow through the
treatment zone would increase by a factor of
two over base conditions. If both saturated thickness and
the ambient hydraulic gradient were doubled,
flow through the treatment gate would
increase four-fold.
In
an attempt to characterize and quantify the
effects of heterogeneity on funnel and gate
system performance, a numerical modeling
study of 15 simulated heterogeneous flow
domains was conducted.
Each realization was tested to
determine if the predicted capture width met
the capture width expected for a homogeneous
flow domain with the same bulk properties.
These numerical experiments revealed
that the capture width of the funnel and
gate system varied significantly with the
level of heterogeneity of the aquifer.
Two
possible remedies were investigated for
bringing systems with less than acceptable
capture widths to acceptable levels of
performance.
First, it was determined that
enlarging the funnel and gate via a factor
of safety applied to the design capture
width could compensate for the capture width
variation in the heterogeneous flow domains.
In addition, it was shown that the
use of a pumping well downstream of the
funnel and gate could compensate for the
effects of aquifer heterogeneity on the
funnel and gate capture width.
However, if a pumping well is placed
downstream of the funnel and gate to control
the hydraulic gradient through the gate,
consideration should be given to the gate
residence time in relation to the
geochemistry of the contaminant removal or
destruction process in the gate.
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