Environmental Engineering Reference
In-Depth Information
suitable for the PRBs to be keyed into (Figure 7.9). Therefore, the preliminary
design was for all panels to extend to a depth of approximately 10 m or until
this upper portion of the Leederville Formation was encountered.
The preliminary design drawings, together with key performance indica-
tors (KPIs), were included in the tender for the final design and construction
contract.
The KPIs included the following criteria:
• Location—the top of the PRBs must be above the 100-year flood level
and aligned as per the design specifications.
• Keyed into the silty clay at the top of the Leederville Formation
• Approximately 5 m distance between the denitrification and ZVI PRBs
• Wall thickness—continuous thickness greater than 30 cm, with no
holes or gaps
• Monitoring wells—installation of three multilevel monitoring wells
in the denitrification PRB and three multilevel monitoring wells in
the ZVI PRB
• Appropriate hydraulic conductivity of both PRBs—five times
greater than the geometric mean hydraulic conductivity along the
PRB alignments
• Sawdust/woodchips and sand mixture—not less than 2:1 sawdust-
to-sand ratio
• Specifications for the ZVI and sand mixtures for the central panel
and side panels
• Effluent TCE concentrations downgradient of ZVI PRB to be less
than 330 μg/L
• Capture contaminant plume—absence of underflow to be demon-
strated by capture zone analysis using water level measurements
and reference to downgradient concentrations
The successful tender was submitted by Menard Bachy and Geosolutions
(the contractor) who proposed to install the sequenced PRB system using a
biopolymer guar gum slurry. This method is a modified slurry trench tech-
nique that temporarily supports the trench excavation below the watertable
using a biodegradable polymer instead of bentonite slurry. The slurry is
made from a guar bean derivative comprising a mixture of polysaccharides
(sugars, i.e., a long-chain carbon polymer), which is relatively stable in solu-
tion over a pH range of 5-7. To degrade the guar gum at the end of the PRB
installation process, a slurry breaking agent is recirculated by air lifting in
temporary wells.
The final design of the contract varied from the preliminary design in the
following ways: the PRBs were moved away from the original alignment by
approximately 4 m to the southeast (for operational reasons), the thickness
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