Environmental Engineering Reference
In-Depth Information
Investigation at this site began with an assessment of a subsurface sump used to temporar-
ily store waste plating fluids. Results of soil samples collected near the sump indicated the
presence of chromium VI at elevated levels, and this finding triggered further investigation.
Subsequent Phase II investigations revealed groundwater had been impacted at a depth of 21 m
(70 ft) beneath the ground surface, and an additional chromium VI source was located upgra-
dient and adjacent to the facility. This establishment was also performing chrome plating.
Several phases of investigation had been conducted to evaluate other potential sources of
contamination and whether other contaminants were present. Contaminants detected in
soil included cadmium, chromium VI, lead, mercury, and nickel. Contaminants detected
in groundwater from site operations were restricted to chromium VI. The chromium VI
contamination in groundwater extends more than 3.2 km (2 mi) downgradient from the
source. Figure 14.20 shows the location of soil borings and monitoring wells and the pre-
liminary extent of contaminant impacts requiring remediation.
The geology of the site consists of an interbedding of sand, silt, and small clay lenses
of groundwater encountered at a depth of approximately 21 m beneath the surface.
Groundwater is present in a complex network of aquifers that are used as a potable source
of water. The aquifers are also used to store billions of gallons of water for potential future
use. Groundwater beneath the site is flowing west toward a river located less than 4 km
(2.5 mi) from the site.
With its vulnerability rating of 65, this site is prone to groundwater contamination. As
demonstrated here, even though groundwater exists at a relatively deeper depth than the
other sites, contaminants can easily impact deep groundwater under favorable conditions
and reach surface water discharge locations far downgradient. Figure 14.22 shows the
interpolated chromium VI groundwater plume from this site reaching the riverbed over
3.2 km (2 mi) away. If a geologic map for this urban area existed, it would have simplified
the interpolation of the contaminant plume.
The technologies selected to remediate soil were excavation of the near-surface soil
and in situ chemical treatment of subsurface soil. Excavation of surface soil was selected
because in situ chemical treatment and other technologies were not feasible due to the
presence of multiple heavy metals requiring remediation. In situ treatment was selected
for subsurface soil at depths between 6.7 m (22 ft) and 21 m (70 ft) because chromium VI
was the only contaminant requiring remediation. The in situ treatment of the deeper soil
consisted of injecting a chemical reactant (ferrous sulfate) that reduced the chromium VI
to chromium III. This chemical reaction immobilized the chromium so it could no longer
migrate and impact groundwater. Figure 14.21 shows a geologic cross section of this site
and the areas selected for soil excavation and in situ chemical treatment.
Since the remedial alternative selected for the shallow soil was soil excavation, demolition
of the building was necessary to gain unobstructed access to impacted areas. Demolition
of the building and removal of the footings had the following added benefits:
• Enhanced redevelopment potential
• Lowered carrying cost
• Minimized trespass liabilities
• Assisted in confirmation of contaminant sources
• Lowered overall remediation cost by providing unobstructed access to all poten-
tial AOCs
• Minimized the potential for missing—or not properly investigating—all potential
contaminant sources
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