Fourteen-Year Assessment of a Permeable Reactive Barrier for Treatment of Hexavalent Chromium and Trichloroethylene - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 102

core of the Cr VI plume. Details of the groundwater sampling methodology
have been published in Paul et al. (2003).
6.4 Outcomes
6.4.1 Chromium
Long-term trends in Cr concentrations in monitoring wells MW13 and MW48
(up gradient) and MW46, MW49, and MW50 (down gradient) are shown in
Figure 6.2. After placement of the PRB, Cr concentrations above 3 ppb have
never been observed in wells located on the down gradient side of PRB. Also
note that beginning in 2001, Cr concentrations on the up gradient side of
the PRB began to drop precipitously, a trend likely linked to the source-area
dithionite treatment described above.
Concentration data collected from Transect 2 provide a snapshot series of
performance of the PRB by revealing influent, interior, and effluent values
of contaminant levels. The concentration data in Transect 2 over 14 years
are summarized on cumulative percent diagrams to give an overall picture
of performance through the lifetime of the PRB (Figure 6.3). For Cr, influent
concentrations have ranged from <0.1 to 4000 ppb, with about 50% of the
samples collected from the up gradient ML21 cluster above 50 ppb. The high-
est concentrations of chromium have been observed over the depth interval
from 4 to 5 m below ground surface. Cumulative concentration data for Cr
within and down gradient of the PRB show close agreement indicating that
treatment of the down gradient aquifer is a consequence of groundwater
transport and reaction through the reactive medium. Chromium concentra-
tions within and down gradient of the PRB (ML24 and ML25) have ranged
from <0.1 to 3 ppb; the average treatment efficiency over 14 years is >99.5%.
Influent concentrations of chromium to the PRB have decreased with time
(Figure 6.2), which is likely a result of the dithionite treatment of the source
area and natural attenuation in the aquifer between Hangar 79 and the PRB.
Chromium treatment by the Elizabeth City PRB has been excellent over
a sustained period of time. The reactive lifetime of the PRB has indeed out-
lasted the Cr plume. The removal mechanism of Cr VI has been explored in
a previous publication (Wilkin et al., 2005). The sustained performance of
the PRB can be linked to several key factors: (i) pH and redox conditions
within the barrier have been maintained at ideal levels for Cr reduction to
the trivalent state, (ii) influent groundwater chemistry is low in dissolved
solids, so mineral accumulation due to carbonate precipitation has not sig-
nificantly impacted reactivity and hydraulic conductivity, and (iii) the influ-
ent dissolved oxygen loading has been low, so iron corrosion reactions have
not significantly impacted reactivity or hydraulic conductivity.