Environmental Engineering Reference
In-Depth Information
The initial subsurface investigation conducted involved drilling 12 soil borings in
areas with the highest likelihood of detecting contamination. This operation was per-
formed with similar objectives to those at Site 1: characterize the geology; identify the
contaminants; and analyze the worst case samples. The results confirmed the presence
of contamination at three of the four RECs identified in the Phase I environmental
site assessment. Contaminant concentrations in near-surface soil were also detected
at sufficient levels to require further investigation. The one location not pursued for
further investigation was near the former underground storage tank. Four soil borings
made at the location of this tank and the analysis of four soil samples taken from the
soil beneath the tank did not confirm the presence of contamination above detectable
concentrations.
Groundwater was encountered at a depth of 3.0-3.7 m (10-12 ft) beneath the surface of
the ground during the initial investigation. Temporary monitoring wells were installed
at select locations to evaluate the possible presence of groundwater impacts and to esti-
mate the direction of groundwater flow. The analytical results suggested the presence of
groundwater contamination likely originating from on-site sources. This finding was con-
firmed because there were levels of several contaminants exceeding applicable cleanup
criteria, and the same contaminants were detected in near-surface unsaturated soil at the
locations where the RECs were identified but were not detected in soil or groundwater at
upgradient locations.
Six additional investigation phases were conducted to define the nature and extent of
contamination. During these subsequent phases, a total of 132 additional soil borings were
drilled, with many of the soil borings having multiple samples analyzed to help character-
ize the vertical extent of contamination. A total of 80 monitoring wells were installed to
define the nature and extent of impacts to groundwater.
The general subsurface geology of the site immediately beneath the surface consisted
of a sand deposit originating from a Pleistocene age glacial lake that occupied the region
more than 12,000 years before the present. Specific geology beneath the site consisted of
sand from the surface to a depth of 7.9-9.8 m (26-32 ft). Beneath this glacial lacustrine beach
sand deposit was a ground moraine or lodgment till deposit extending to a depth of least
23 m (75 ft). Historical geological literature of the region indicated that the ground moraine
or lodgment till deposit extended to depths of approximately 60 m (200 ft) beneath the
ground in the area. Table 6.5 describes the geology between the ground surface and 23 m
beneath the site.
During the investigation, multiple groundwater monitoring wells were installed at the
same location but were screened at different depths within the saturated zone to evaluate
the vertical distribution of contaminants within the aquifer. Several samples of the ground
moraine deposit beneath the aquifer were also analyzed for the presence of contamina-
tion and for certain hydrologic parameters (such as hydraulic conductivity and grain size
analysis) to evaluate whether the ground moraine deposit was an effective confining layer,
preventing contaminant migration to deeper aquifers. In addition, three deep soil borings
were drilled to a depth of 23 m in nonimpacted areas of the site to verify the horizontal
distribution and thickness of the ground moraine deposit.
Other technical and geologically related information relevant to the site analysis
included the following:
• Storm sewers in the immediate vicinity did not intersect any of the contamination
• Surface water drainage was controlled by storm sewers
• No buried utilities intersected contaminated areas
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