Fourteen-Year Assessment of a Permeable Reactive Barrier for Treatment of Hexavalent Chromium and Trichloroethylene - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 105

At this site, groundwater flow velocities are comparatively high (5-6 m/day)
and influent concentrations of dissolved oxygen are elevated (3.5-5 mg/L).
Both of these factors are expected to limit treatment effectiveness.
6.4.2 Volatile Organic Compounds
Before discussing the performance characteristics of the PRB with respect to
the degradation of chlorinated ethenes, it is important to point out that the
Elizabeth City PRB was not originally designed to treat TCE. If it had, the
PRB would have ideally been wider in some regions in order to increase resi-
dence time and it would have ideally extended deeper into the subsurface.
Nevertheless, it is highly instructive to analyze the performance of the PRB
with a focus on TCE treatment. As pointed out previously, the groundwater
chemistry and hydrologic features of the site are ideally suited for the treat-
ment of hexavalent chromium using granular Fe. An interesting question is
whether this is also true for the treatment of chlorinated solvents.
Monitoring well data for TCE are shown in Figure 6.2 for MW48 (up gradi-
ent), MW50, and MW49 (down gradient). TCE concentration trends observed
in the up gradient and down gradient regions are variable with time, with an
overall trend of decreasing concentrations with time.
As shown in Figure 6.3 using a cumulative distribution diagram for data
collected from Transect 2, influent TCE concentrations have ranged from ~1
to 9050 ppb, with 50% of the influent TCE concentrations above 30 ppb. In
contrast to Cr, groundwater with maximum TCE values enters the PRB at
its base. The cumulative concentration data for TCE within and down gradi-
ent of the PRB show close agreement, again indicating that reduced concen-
trations observed down gradient of the reactive medium are a consequence
of groundwater transport and reaction within the PRB. About 75% of the
observations of TCE concentrations within and down gradient of the PRB are
below the maximum contaminant level (MCL) of 5 ppb. The remaining 25%
of the observations follow a trend that corresponds to about 10% of the influ-
ent TCE. The average treatment efficiency for TCE is estimated to be >90%
considering the difference of the median TCE concentrations in influent and
effluent groundwater.
Interestingly, the distribution functions for transformation products, cis -
DCE and VC, show different trends compared to TCE. Influent concentra-
tions of cis -DCE and VC are low, with 90% of the observations below MCLs
for these chemicals of 70 and 2 ppb, respectively. After treatment, that is, in
the PRB and down gradient of the PRB, the proportion of observations with
low cis -DCE and VC concentrations decreases, indicating that cis -DCE and
VC are indeed products created as a consequence of TCE degradation. For
cis -DCE, the proportion of effluent concentrations below the MCL increases
to about 95% and maximum concentrations in the effluent decrease relative
to maximum concentrations observed in the influent. Whereas, for VC, the
proportion of effluent concentrations below the MCL drops to about 80% and