Environmental Engineering Reference
In-Depth Information
denitrification PRB and MWG112 with MWG110 for the ZVI PRB). An exami-
nation of the data shows that concentrations of TCE are reduced and DCE
concentrations are increased in the denitrification PRB (Figures 7.13 and
7.14). However, Figure 7.16 indicates that the denitrification PRB is not reduc-
ing the overall mass of chlorinated solvents, suggesting that the predomi-
nant method of TCE reduction within the denitrification PRB was through
sequential dechlorination to DCE.
The detection of VC downgradient of the PRB remediation system may indi-
cate that β-elimination may not be the preferred dechlorination process through
the ZVI PRB. The most recent monitoring data (note scale change from Figures
7.13 and 7.14) indicate that concentrations of VC in MWG110B suggest that there
is an upward trend with VC production higher in the second year of operation.
7.11 Eh and pH
Oxidation reduction potential (ORP) and pH were measured in the field at
all monitoring locations during each monitoring round. The ORP was con-
verted to Eh by adding 200 mV and the transformed results plotted on the
Eh-pH stability diagram for iron (Figure 7.17). The figure indicates that the
conditions are favorable for the formation of potentially passivating iron pre-
cipitates such as ferric hydroxide (Fe(OH)). To date, no trend in the Eh-pH
relationship has been noted within the ZVI PRB.
7.1 2 Su m m a r y
The results from the monitoring rounds indicate that to date the remediation
system has been successful. The denitrification PRB is successfully removing
more than 99% of nitrates from the groundwater ensuring that the downgradi-
ent ZVI does not become passivated by nitrates. Measurements of TOC indicate
that the natural aquifer strip between the two PRBs is likely to have acted as
a buffer strip, minimizing the potential for ZVI to have been affected by TOC
emanating from the upgradient denitrification PRB. TOC concentrations sug-
gest that residual guar gum and its associated breakdown products remained
within both the denitrification and ZVI PRBs for up to a year after installation.
While results indicate that β-elimination may not be the preferred path-
way in the ZVI PRB, it is still degrading on average more than 60% of the
chlorinated solvent mass. This has resulted in downgradient TCE concentra-
tions at all monitoring locations to be below the RBC to protect the Helena
River. Some elevated concentrations of VC have been detected immediately
downgradient of the PRB system and further work has been undertaken to
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