Environmental Engineering Reference
In-Depth Information
with the data from the MW-, C-, RW-, and IW-wells. MW-1 to MW-3, which
were screened over a shallower interval and were outside the contaminated
plume, were not utilized in this exercise, nor was MW-508, which was
screened over a deeper interval but showed high PCE concentrations.
Changes in concentrations of the measured parameters give an indication
of the change in subsurface geochemistry and the stimulation of microbial
activity as a result of the SERB process.
Ethanol concentrations stayed at approximately 10,000 mg/l in the area
of the cosolvent extraction during the first year of groundwater monitoring
and then decreased to less than 2000 mg/l after the second year (Figure 5.10).
MW-505 and MW-509 were the only monitoring wells outside the injection/
extraction area to show significant concentrations of ethanol (>4000 mg/l)
following the cosolvent flush. These wells bound the injection/extraction
system on the north and west. Downgradient wells (MW-510, MW-512,
MW-513, and MW-514) showed low concentrations of ethanol (<500 mg/l)
after 1 year of monitoring. After 19 months the downgradient C-wells (C2,
C3) had concentrations of ethanol similar to the MWs. C1, which is located
closer to the injection/extraction area, had concentrations of ethanol of more
than 2000 mg/l after 19 months. C4 had no measurable ethanol concentra-
tions during the monitoring period. Differences in the concentrations in these
wells may be because the C-wells were screened from 8.3 to 9.1 m bgs,
whereas the MWs were screened over a larger interval, from 7.9 to 9.6 m
bgs. This difference in screened interval can impact measured concentrations,
especially in layered sediments of differing hydraulic conductivities.
The ethanol contour plots show the decreasing concentrations over time
and the movement of ethanol downgradient (Figure 5.11 to Figure 5.13).
These data also show that although the overall ethanol concentration
decreased over time, a significant amount of electron donor is still available
for subsurface microbial activity after 2
1
/
2
years.
PCE concentrations were somewhat variable in the IWs and RWs during
the first year of monitoring (Figure 5.14). In general, concentrations appeared
to decrease for the sampling event immediately following the cosolvent
flushing test and then rebounded to near-initial concentrations. After approx-
imately 1 year of monitoring, concentrations began decreasing in all IWs and
RWs. This trend is shown best in the more complete data sets from RW-3,
RW-4, RW-6, and RW-7 (Figure 5.14). Interestingly, MW-509, which showed
high ethanol concentrations, also showed a decrease in PCE concentration
in the first sampling event and a general trend of lower concentrations over
time. MW-505, which also contained ethanol immediately following the
cosolvent flushing test, had low initial concentrations of PCE and maintained
these low concentrations throughout the monitoring. In general, the other
downgradient MWs showed an increase in PCE (from 10 to 60 mg/l) for the
first 6 months following the cosolvent flush and then a decrease in concen-
trations to less than 5 mg/l after 2 years of monitoring (Figure 5.14).
C1 and C2 had initially high PCE concentrations (>90 mg/l), and it is
suspected that these wells were completed in a previously undetected source
