Environmental Engineering Reference
In-Depth Information
3 and 6 pounds of 1,4-dioxane per month are recovered from the on-site and off-site SVE systems
(Cole-Cameron, 2006).
Prior to discovery and characterization of 1,4-dioxane at the SSI site, it was believed that the
extent of groundwater contamination had been dei ned. Discovery of 1,4-dioxane and THF led to an
investigation to characterize their downgradient extent, which was signii cantly greater than that of
the chlorinated solvents, as shown in Figure 8.3 . The additional characterization effort dei ned a
1,4-dioxane plume extending beyond the capture zone of the existing remediation systems. These
i ndings were met with regulatory directives to expand the monitoring network by installing addi-
tional monitoring wells and expanding the groundwater extraction system. The operational schedule
of the existing remedial systems was increased to 24 h/day, seven days/week (24/7) for on-site SVE.
An additional, 87-foot-long off-site groundwater extraction trench was installed to address 1,4-
dioxane presence beyond the capture zone of the existing system (Cameron-Cole, 2006). Although
the presence of i ne-grained deposits limited the VOC migration, 1,4-dioxane and THF were esti-
mated to migrate 41 and 35 ft/year, respectively.
The off-site occurrence of 1,4-dioxane beyond the groundwater extraction trench is being
addressed by MNA. Consultants for the SSI site acknowledge that 1,4-dioxane is not prone to natu-
ral in situ degradation. Therefore, the MNA approach relies only on dispersion, diffusion, and dilu-
tion from incidental recharge. Because the sources of 1,4-dioxane contamination are cut off by
extraction trenches, the consultants have submitted that concentrations should continue to decline,
and perimeter wells are not expected to produce samples with detectable concentrations of 1,4-diox-
ane. Declining concentrations of 1,4-dioxane in monitoring wells between the off-site extraction
trench and the perimeter monitoring wells support this assertion.
8.2.2 1,4-D IOXANE D ETECTION AND L ABORATORY C HALLENGES
In the i rst quarter of 1998, the laboratory inadvertently analyzed eight samples for an extended list
of VOCs that included 1,4-dioxane and THF. Detections of 1,4-dioxane and THF were at i rst uncer-
tain because 1,4-dioxane also appeared in trip blanks and in upgradient wells; nevertheless, the
consultant decided to reanalyze the full complement of 48 quarterly groundwater samples for
1,4-dioxane. 1,4-Dioxane was detected in samples from all but one monitoring well; the concentra-
tions ranged from 500 to 56,000 ppb. Subsequent sampling revealed a maximum 1,4-dioxane
concentration of 340,000 ppb (Todd Engineers, 1998; McCraven, 2006).
The distribution of 1,4-dioxane is shown in Figure 8.3. Delineation of the extent of 1,4-dioxane
required installation of at least a dozen additional monitoring wells. At the SSI site, 1,4-dioxane was
present at higher concentrations than most of the other VOCs at the time when 1,4-dioxane was
discovered, nearly a decade after remediation began and 15 years after contamination was i rst
reported. By 2006, 1,4-dioxane concentrations in three wells had increased as much as sixfold; the
cause is under investigation. Many of the shallow monitoring wells at the SSI site display large
swings in 1,4-dioxane concentration; the variation exceeds 100%.
1,4-Dioxane analysis was carried out with EPA Method 8260 SIM selected-ion mode. Vapor
samples are analyzed for standard-list VOCs by using Method TO14, which includes 1,4-dioxane.
Many samples with elevated 1,4-dioxane concentrations required a high dilution level to obtain
results within the calibration range of the analytical laboratory equipment. All analytical results for
duplicate samples were within acceptable limits established in the Quality Assurance Project Plan
except those listed in Table 8.1 (Cameron-Cole, 2006). The few duplicate results that failed are
included here as a reminder of the importance of running the full complement of duplicate samples.
Apparently, the accuracy of 1,4-dioxane analysis by EPA Method 8260 SIM can be compromised
by a challenging matrix with elevated concentrations of other organic compounds, including ketones,
aromatics, ethenes, and ethanes.
The Water Board requests that any analytical results exceeding the historical range of results at
a particular well be called out in a table of exceedances. Many recent results exceed the historical
Search WWH ::




Custom Search