Environmental Engineering Reference
In-Depth Information
VOCs tended to migrate downward through the soil column and contaminate groundwa-
ter without adsorbing to soil grains.
The remedial method chosen for groundwater was air sparging and soil vapor extraction
since the contaminants in groundwater were VOC compounds and did not extend past the
mid-portion of the saturated zone. Air sparging involved the injection of air beneath the
impacted groundwater and then letting the air rise naturally through the saturated zone.
As the air migrated upward through the saturated zone, it volatilized the contaminants,
and the vapors containing the VOCs were removed using a soil vapor extraction system
in the vadose zone. The vapors were removed from the air by passing them through a
granular activated carbon tank.
The VOCs contaminating groundwater at this site were DNAPL compounds having a
specific gravity slightly greater than water. Therefore, when present at sufficient concen-
trations, DNAPL compounds may sink through the water column and contaminate lower
portions of an aquifer. This sinking action did not occur at this site because its geology and
hydrogeology—in effect a stratigraphic control—prevented the VOCs from migrating to
the bottom of the aquifer. As listed and described in Table 6.5, the composition of the aqui-
fer gradually became finer grained with depth, and the hydraulic conductivity decreased
proportionately. This reduction of hydraulic conductivity, combined with low contami-
nant mass in groundwater, resulted in restricting contaminants to the upper portion of the
aquifer. With the VOCs restricted to the upper portion of the aquifer, air sparging became
the most practical remediation technique.
The time duration between the Phase I environmental site assessment and the receipt
of the closure letter was approximately 14.5 years. The dollar costs for remediating this
site stacked up this way: the PCB-contaminated soil, including investigation, was $1.1
million; $0.1 million for the VOC-contaminated soil; VOC-contaminated groundwater,
including investigation, was $6.6 million. The total cost for investigation and remedia-
tion of this site was $7.8 million, which translates into a cost of $27,180/kg ($60,000/lb) of
contaminant.
Remediation was verified by the regulatory authority through the collection and analy-
sis of the soil samples from each area remediated. In addition, four iterations of groundwa-
ter samples over a period of 1 year were made until cleanup levels were achieved. Closure
was then granted and a “No Further Action Required” letter was issued for the site. A risk
assessment was also conducted for the site and was designed to determine if the residual
contamination at the site would pose an ongoing and unacceptable risk to human health
and the environment. Locations included in this evaluation were the on-site areas with
persistent soil contamination, and the on-site and off-site areas with contaminated surface
water and groundwater.
The results of the risk assessment indicated that a deed restriction was appropriate for
the site property and banned the use of groundwater for any purpose. A cap consisting of
asphalt pavement was also required for portions of the property where some soil contami-
nation remained in place, and if any soil were to become disturbed or exposed, another
evaluation must be conducted to evaluate the need for further remedial actions. Land use
for the property was restricted to industrial use.
6.5.3 Site Comparison Analysis
A profound difference between these two examples is seen in the cost per kilogram to
remediate the contaminants. Despite the smaller acreage, shorter time of industrial
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