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5.3.1. Results
With the regional gradient directed roughly parallel to the dominant band trend, the
resulting contaminant plume is narrower and longer than would be the case in the absence
of the CB array (Figure 7.12a) and extends some 60 m up-gradient from the leak location.
The long axis of the two plumes (with and without CBs) remains coincident, however,
and parallel to the band trend. Looking in detail, the plume has clearly channeled along
the band array and been constrained from spreading laterally (Figure 7.12b).
With the regional gradient directed obliquely to the dominant band trend, the shape
and orientation of the resulting plume departs substantially from that of the CB-free
control case, remaining constrained by and aligned with the band array (Figure 7.13a).
The plume is, however, wider and shorter than in the parallel scenario, extends 80 m
obliquely up-gradient from the leak (along the band trend), and, where gaps in the CB
pattern allow, turns into the regional down-gradient direction (Figure 7.13b).
With the regional gradient directed normal to the dominant band trend, the resulting
plume bears scant resemblance to the CB-free control case, remaining distinctly elongated
along the band array (Figure 7.14a). The plume is shorter and broader again than in the
oblique scenario, and turns strongly into the regional down-gradient direction at a major
gap in the band pattern (Figure 7.14b, upper left). Finally, the trailing edge of the plume is
essentially straight, having pushed uniformly almost 20 m in the regional up-gradient
direction.
5.3.2. Discussion
As with the reservoir-production simulations above, the plume evolution results
reveal a dominating influence on fluid transport by the CB pattern, regardless of the
regional gradient direction. Limited sensitivity testing also indicated that realistic
variations in the magnitudes of the regional gradient and/or contaminant leak rate do not
substantially affect the results. For the hydrogeologist designing a pump-and-treat
remediation system therefore, recognizing and accounting for the effects of the CBs
would prove crucial, particularly with 100% contaminant capture as the goal.
For example, the standard cleanup approach for a leaking underground storage tank is
to install the minimum number of wells that can be spaced immediately down gradient
from the plume front to provide complete capture, assuming radial drawdown at an
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