Sequenced Permeable Reactive Barrier for the Pretreatment of Nitrate and Remediation of Trichloroethene - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 124

of the PRBs was increased from 30 to 60 cm, and a clay cap was added to the
top of the ZVI PRB.
Each PRB took approximately 7 days to install. The ZVI and sand mixture
was mixed on surface, wetted and then placed in the trench using a trem-
mie system. The panels were installed through the use of a removable end
stop. The mixture for the denitrification PRB was installed using a conven-
tional progressive displacement technique since using the woodchips had
the potential to clog the tremmie.
Some sections of the final PRBs were larger than the final design width of
60 cm. This was a result of sections of the trench collapsing due to guar gum
stability issues as well as the removal of a number of unanticipated large
boulders (large concretions). The geotechnical investigation that was under-
taken along the original alignment did not encounter any such obstructions.
The additional width resulted in additional costs associated with extra mate-
rial requirements and manpower time. However, a benefit of the extra width
is that the resulting retention times would likely be longer than designed.
In one instance, a boulder was encountered at the base of the denitrification
PRB. This boulder was left in situ as it was not possible to remove it and the
sawdust and sand mixture emplaced around it.
7.7 Results
A long-term monitoring network was installed, following the completion of
the PRBs. The monitoring network consisted of 48 sampling location (Figure
7.10). The network included a number of multilevel monitoring wells that were
designated with the letters A, B, C, and D and which approximately repre-
sented 1 m screens at 3, 6, 9, and 12 m below ground level, respectively. The D
series monitoring wells were all located in the upper portion of the Leederville
Formation for the purpose of monitoring potential underflow of the system.
Monitoring wells with 3 m screens across the watertable were installed along
the ends of the system for the purpose of monitoring potential flow bypass.
Following the installation of the PRBs, eight quarterly monitoring rounds
have been undertaken. The first occurred 3 months after the installation was
completed. At each monitoring location, the following field measurements
were obtained:
• Water level (depth to groundwater)
• Redox potential
• Dissolved oxygen
• pH
• Electrical conductivity