Environmental Engineering Reference
In-Depth Information
TABLE 7.2
Estimated ZVI Thickness Required for 15-Year Treatment Lifetime
Median
TCE450
μ
g/L
95% Percentile
TCE 1700
μ
g/L
Median Nitrate
7.5 mg/L
95% Percentile
Nitrate 14 mg/L
Connelly
0.017 m
0.06 m
0.78 m
2.88 m
QMP
0.007 m
0.026 m
0.45 m
1.62 m
laboratory results combined with the interpreted groundwater flow veloc-
ity of 0.16 m/day across the alluvial aquifer. Similarly, the thickness of ZVI
required to allow for potential nitrate passivation over 15 years was assessed
from the laboratory-derived passivation rates. The values for the two types
of ZVI are presented in Table 7.2. The total ZVI thickness needed for an
effective PRB is determined by summing the TCE degradation value with a
nitrate passivation value (and adding an appropriate margin of safety).
The above analysis indicated that nitrate passivation accounts for approxi-
mately 98% of the required ZVI thickness within the PRB, while TCE treat-
ment accounts for less than 2% (based on the QMP ZVI). Given that the ZVI
material is likely to represent a significant portion of the overall project cost,
it was clear that efforts to reduce nitrate influent concentrations could greatly
reduce the required volume of ZVI and/or extend the longevity of the PRB.
7.4 Field Investigation
A detailed delineation monitoring program was undertaken in January, May,
and September 2009 in order to characterize the contaminants of interest, to
determine their respective concentrations along the proposed PRB location,
and to better characterize the local hydrogeological conditions. This pro-
gram included the installation of four multilevel monitoring wells (MWG87,
88, 89, and 90) directly upgradient of the Damplands Pond and two rounds of
groundwater monitoring for volatile organic compounds (VOCs) and nitrate.
The January 2009 TCE and nitrate results are presented (in cross-section)
in Figures 7.3 and 7.4, respectively. The results indicate that the TCE and
nitrate plumes overlap and are relatively consistent in shape over the year.
The concentrations of TCE vary over time, with the maximum concentration
in January 2009 being 2 mg/L in comparison with 0.82 mg/L in September
2009. Nitrate concentrations were found to be less variable with the maxi-
mum concentrations ranging from 14 mg/L in January 2009 to 19 mg/L in
September 2009. It should be noted that nitrate contamination is not associ-
ated with the WCS fire and is instead likely to be the result of upgradient
septic tank usage or possibly an upgradient livestock sale yard.
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