SECTION NINE
Observations and Lessons Learned
PERFORMANCE
During the operation of the treatment system in SVE-only mode, TCE concentrations were
reduced from initial concentrations of 235 ppb to 110 ppb. The addition of AS to the system
reduced TCE concentrations in the soil gas from initial concentrations of 110 ppb to 56 ppb.
The AS/SVE system reduced TCE concentrations in groundwater. At the three wells located near
suspected hot spots of contamination, TCE concentrations were reduced from 310 ppb to
170 ppb (DOS-1), from 220 ppb to 170 ppb (DOS-2), and from 140 ppb to 23 ppb (MW8A).
However, the concentrations remained above the cleanup goal of 5 ppb for TCE.
The results of Mn sampling before and after sparging indicated that Mn levels decreased in six of
the eleven wells samples, but increased in five of the wells.
The following observations were made in a technical memorandum summarizing the system's
operation.
With respect to optimal air extraction rate, an extraction rate of 110 scfm is likely to capture
all volatilized contaminants within about 200 feet of each extraction well.
The radius of influence of an air injection well is about 20-30 feet.
A pressure of approximately 8 psi was required to overcome resistance in the injection well.
However, at injection pressures above 8 psi, air bubbles would be more likely to occur. At
8 psi, the air injection rate into the aquifer was about 45 scfm. The 45 scfm (8 psi pressure)
was determined to be the optimal flow rate, reflecting site and conditions of injections 12 feet
below static water level. The vendor noted that changes in depth of the injection well will
affect the injection pressure and radius of influence.
The major problem encountered during the pilot test was that the SVE vacuum pump did not
produce a vacuum sufficient to be detected by the automated sensors. Because of schedule
constraints, a larger blower could not be obtained. However, according to the vendor,
adequate data was obtained from the pilot test to design the full-scale system.
While overall TCE concentrations decreased in the groundwater, there were several instances
when TCE concentrations increased during operation. These increases may be attributed to the
new source material (from contaminated soil) infiltrating into the groundwater.
COST
The total cost for the pilot study of the AS/SVE system at LF5 was 1,000. This amount is
not included in the amounts shown in Table 8-1.
Differences between the actual costs and the adjusted ROD estimates are largely attributable to
the installation of passive injections wells, a SVE system capable of sustaining a 600 scfm
average volumetric airflow rate, and additional groundwater monitoring wells, and the increased
groundwater sampling costs associated with the additional wells. However, as shown in
9-1
Ft. Griffey Landfill 5 OU Interim RA Report
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