aquifer; concentrations at this time were used as initial conditions in
subsequent remediation trials.
In all mass transport simulations, longitudinal dispersivity was 1.0 m,
transverse dispersivity was 0.1 m and the effective molecular diffusion
coefficient was 0.00001 m2/d. The contaminant plume boundary had a
concentration of 1 mg/L.
For each setting, remediation trials identified a minimum extraction
rate – to contain the contaminant plume onsite – for an extraction well
located on a north-south transect 5 m downgradient of the plume. Trials identified the lowest extraction rate for wells downgradient of the
plume’s leading tip, at the plume’s crossgradient midpoint and at the
All groundwater flow and mass transport simulations used the precon-
ditioned conjugate gradient and generalized conjugate gradient solvers,
respectively. Mass balance errors were less than 0.01%.
RESULTS AND DISCUSSION
In settings A-C, contaminant plumes traveled for 350, 430 and 520
days, respectively, before coming within 45 m of the eastern boundary
(Figures 1-3). The plumes had irregular shapes reflecting complex hydraulic conductivity distributions and groundwater flow fields. In each
setting, without an extraction well, dilution and hydrodynamic dispersion without pumping lowered concentrations, but did not contain the
contaminant plume onsite. Without active remediation, contaminant
plumes reached the eastern boundary in settings A-C after 420, 570 and
560 d, respectively.
Active remediation combined with dilution and hydrodynamic dis-
persion effectively contained the contaminant plume in each setting.
In setting A, the best overall well, located 4.0 m south of the plume’s
downgradient tip, extracted 3.0 m3/d (Figure 1). The downgradient
well pumped a minimum of 3. 9 m3/d – a rate 30% higher than the best
overall well – to contain the plume onsite. The crossgradient midpoint
well, located 2. 5 m south of the downgradient point, pumped a mini-
mum rate of 3. 4 m3/d to contain the plume onsite.
The best overall, downgradient and midpoint wells pumped for 1190,
1370 and 1170 d, respectively, before lowering contaminant concentrations at all nodes in the model to less than 1 mg/L. Although the best
crossgradient well pumped for less time than the best overall well, it
removed more water and contaminant mass, resulting in more costly
treatment. The best overall, downgradient and crossgradient wells
removed 3570, 5343 and 3978 m3 of groundwater and 12.237, 12.691
and 12.505 kg of contaminant mass, respectively, during the remediation period. Portions of the contaminant plume moved past each well,
but did not travel offsite (Figure 1).
In setting B, the best overall well, located 0.5 m south of the downgradient point, pumped at a minimum rate of 3. 5 m3/d to contain the contaminant plume onsite (Figure 2). The downgradient well also pumped
3. 5 m3/d, but the plume advanced 2.0 m farther than the plume for the
best overall well. In contrast, the crossgradient midpoint well, located
5. 5 m south of the downgradient point, had to pump 4. 8 m3/d to contain the plume onsite. The best overall, downgradient and crossgradient
wells pumped for 1220, 1390 and 1180 d; extracted 4270, 6672 and
4130 m3 of groundwater; and removed 12.698, 12.978 and 12.706 kg
of contaminant mass (respectively) during the remediation period. As
in setting A, the contaminant plumes moved past each well, but did not
reach the downgradient boundary in setting B (Figure 2).
Finally, in setting C, the best overall well coincided with the crossgra-
500 d with best-performing well in Setting B; contours in mg/L.
dient midpoint, located only 0.5 m south of the downgradient point,
and pumped at a minimum rate of 2. 6 m3/d to contain the contaminant
plume onsite (Figure 3). The downgradient well also pumped at a
minimum rate of 2. 6 m3/d to contain the plume onsite. The best overall
well pumped for 1510 d prior to remediation; this rate slightly exceeds
the pumping time (1470 d) for the downgradient well. Thus, the best
overall well removed slightly more water than the downgradient well –
3926 versus 3822 m3. However, the best overall well removed slightly
less contaminant mass requiring treatment ( 5.654 kg) than the
Figure 2. Map of initial (top) and residual (bottom) contaminant plume after
Figure 3. Map of initial (top) and residual (bottom) contaminant plume after
500 d with best-performing well in Setting C; contours in mg/L.