Does In-Situ Remediation Really Work?
Lessons learned from hydrocarbon remediation
Presented by Terre Maize
Former ARCO 5310
Former ARCO 5310
Located on the Las Vegas Strip
Release from northwestern dispenser discovered in 1993
Limited soil excavation as facility needed to remain operational
Former ARCO 5310
Store
Monitoring wells N
Former ARCO 5310
Soil vapour and groundwater pump and treat tests were successful, so remediation started in 1994
Estimated time to remediate was two years
In 1999, vapour and groundwater concentrations of hydrocarbon still well above remedial action goals
Former ARCO 5310
In late 1999, additional wells were installed, the vapour extraction and pump and treat system was shut off, and hydrogen peroxide injections began
Groundwater concentrations immediately rebounded following system shut-down
Former ARCO 5310
In 2004, additional injection wells added and quantity of peroxide injections increased from 3700 L per week to 10,000 L per week
Former ARCO 5310 Excavation
The Excavation
The excavation continues
And continues…
Final excavation area
Former ARCO 5310 Results
Soil sampling results indicated that in-situ remediation had worked as soil concentrations were below action levels and only heavy end hydrocarbons were left
Groundwater contamination was found to be initiating upgradient at site of old tank pit
Former DeLuca Liquors Site
Former DeLuca Site
UST found to be leaking upon removal
Tank was underlain by 2m thick layer of caliche based on data from two wells installed
Groundwater pump and treat and soil vapour extraction initiated in 1993
Air sparging initiated in 1998
Former DeLuca Site
Groundwater plume continued to migrate
Assumption was that remedial technology was not working
Hydrofracing of caliche and hydrogen peroxide injection initiated in 2001
Concentrations in groundwater remained high
Former DeLuca Excavation
Excavation revealed tank parts, piping, and contaminated fill had been left in the tank pit, acting as a constant source
In-situ remediation had been effective –
Caliche was highly fractured
Soil results were below action levels and only heavy end hydrocarbons were left
What went wrong?
At the time, it was not known if in-situ remediation was viable for sites with caliche and high clay content
Some progress was shown, so it was assumed that the technology was the reason sites were not reaching remedial action goals
Lessons learned
Rather than re-thinking the site was a whole, only the technology was examined
The primary lesson learned was that the site conceptual model was wrong – not the technology
If the model had been re-evaluated, and a minimal amount of additional site characterisation done, the technology would have been shown to work
Thank you!
Any questions?
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DOES IN-SITU REMEDIATION REALLY WORK? LESSONS LEARNED FROM HYDROCARBON REMEDIATION
Terre Maize
Energy & Sustainability Engineer, Spotless Co. NZ Ltd 85 The Esplanade, Petone, Wellington, New Zealand
+64 210 613 983, [email protected]
1. Introduction
In-situ remediation of hydrocarbon impacted soil and groundwater has been used in the
United States since the late 1980s. A wide variety of techniques has been used,
including bioremediation, air sparging, soil vapour extraction, and chemical treatment.
At two sites in Las Vegas, Nevada, a variety of in-situ techniques were used for soil and
groundwater treatment following hydrocarbon releases. In both instances, after more
than 10 years of in-situ treatment, groundwater contamination levels remained above
the standard of 5 micrograms per litre (µg/L) of benzene.
This paper describes the remedial actions taken at the sites and what caused the in-situ
remediation to appear to fail.
2. Former ARCO 5310
The former ARCO 5310 petrol station was located on Las Vegas Boulevard, also know
as “The Las Vegas Strip.” It had been in operation for several years, with underground
storage tanks (USTs) present since at least the 1950s. The tanks had been replaced
over the years, with the most recent replacement in the late 1980, when it was reported
that contamination was removed from the tank pit. At that time, heavy duty-plastic was
installed as a pit liner under the new fibreglass tanks. The site was believed to be
relatively free of contamination.
2.1 Background
US regulations required upgrades of underground storage tank (UST), piping, and
dispenser systems in the early 1990s. During the upgrade, a pipe was not properly
connected, resulting in a large petrol release. Figure 1 shows the relevant site features.
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Figure 1 – Former ARCO 5310 Site Features
It was the only petrol station toward the northern end of The Strip and had two islands
and a mini-mart. The site comprises approximately 0.2 ha. Because it was an
operating site, it was not cost effective to shut the site down and excavate the
contaminated soil.
The soil was comprised of silty sand from the surface to approximately 2 metres below
ground surface (bgs). The silty sand was underlain by clayey sand, with fine-grained
sand and some gravels. Groundwater was present at depths of approximately 3 to 4
metres bgs.
2.2 In-situ remediation
Soil venting and groundwater pumping tests were conducted and shown to be viable. It
was estimated that two years of soil venting and groundwater pump and treat would be
required to treat the plume to regulatory levels.
USTs
Dispensers Store
MW4
MW5
MW1
MW2
MW3
MW6
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Five groundwater monitoring wells, a soil vapour extraction well, and two pumping wells
were installed and treatment began in 1995. In monitoring well MW-1, benzene
concentrations were at 4,900 µg/L in 1995. In December 1998, concentrations had only
decreased to 2,800 µg/L. Two additional vapour extraction wells were installed, and
treatment continued until 1999. However, results indicated that contamination was still
present, with the MW-1 benzene concentration at 330 µg/L. The soil vapour extraction
and groundwater pump and treat system was taken out of commission in December
1999, and benzene concentrations immediately rebounded to 2,000 µg/L.
Hydrogen peroxide injections began in early 2000. An additional downgradient
monitoring well (MW-6) was added, as was an injection trench. The initial benzene
concentration in MW-6 was 2,000 µg/L. Hydrogen peroxide at a concentration of 3%
was injected at a rate of 3,785 litres per week into the three vapour extraction wells, an
injection trench, MW-1, and MW-3. Three injection wells were added in 2004, and up to
11,000 litres of hydrogen peroxide was added through a slow-release system. By the
end of 2005, the benzene concentration had fallen to below laboratory detection levels
in MW-1, and to 190 µg/L in MW-6. However, MW-5 concentrations had gone from
below detection levels to 210 µg/L, indicating that the plume was moving. In MW-3,
benzene was also detected for the first time, at a concentration of 7.9 µg/L.
2.3 Excavation
In early 2006, the site was purchased and the new owner requested clean closure at the
earliest possible date. The petrol station was closed, the building demolished, and the
tanks, piping, and dispensers were removed.
A remedial action plan to excavate the site was developed and excavation commenced
in June 2006. The plan was to begin excavation near the site of the piping release,
near the northwestern-most dispenser, and move downgradient to the east. It was
estimated that about 1,300 cubic metres of soil would require removal. The estimated
volume of soil requiring removal was based on a site conceptual model developed for
the excavation. The model was based on monitoring data and assumed that the soil
conditions had prevented the remediation systems from performing well.
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3. Former DeLuca Liquors site
The former DeLuca Liquors site had a petrol UST and dispensers for fueling company
vehicles. When the steel tank was removed in 1993, it was found to have leaked and
contamination was present. Figure 2 shows the site.
Figure 2 – Former DeLuca Liquors Site
The site was challenging in that the UST had been installed above a layer of caliche, a
hardened deposit of calcium chloride. The caliche was estimated to be nearly 2 metres
thick, based on boring logs from two wells: MW-1 and MW-3. Because of the fractured
nature of the caliche, the groundwater had been impacted. Caliche is difficult and
expensive to remove; therefore, in-situ remediation was selected as the appropriate
technology, despite the less than ideal ground conditions.
3.1 In-situ remediation
Groundwater pump and treat, air sparging, and soil vapour extraction were initially used.
However, after more than five years of treatment, concentrations of benzene in the
Injection
trenches
USTs
MW1
MW3
5
groundwater remained high. In MW-1, installed in the tank pit, initial benzene
concentrations were on the order of 3,300 µg/L. Five years later, the concentration had
fallen to 1,300 µg/L, still well above the site cleanup value of 5 µg/L. In addition, the
plume continued to travel downgradient.
Since the initial remedial action technologies did not appear to be working, alternative
techniques were tried. Hydrofracing, used in the oil fields, was used to try to break up
the caliche. Hydrogen peroxide injections commenced, and the concentration in
groundwater fell to 800 µg/L. However, the plume continued to travel downgradient,
with the easternmost well concentration going from below laboratory detection limits to
over 300 µg/L of benzene.
3.2 Excavation
In late 2005, the site was purchased and, as with the former ARCO station, the owner
wanted clean closure as quickly as possible. Since the in-situ remedial action did not
appear to working, excavation commenced in February 2006. There were numerous
impediments to excavation, including a building, a high-pressure gas line, a high-
pressure water line, a high-voltage electrical line, and the property boundary.
It was estimated that approximately 400 cubic metres of soil would be excavated.
Groundwater direction was present at approximately 3 metres bgs and flowed to the
east. The conceptual site model developed for excavation assumed that the in-situ
remedial action had not been effective, largely because of soil conditions
4. Findings during excavation
During and following excavation at each of the sites, it became obvious that that the in-
situ remediation had actually worked as planned. However, neither site had reached its
remedial action goals. The findings during excavation are described below..
4.1 Former ARCO 5310
For health and safety purposes, the excavation needed to be benched. Excavation
began at the approximate location of the 1993 piping release and then continued to the
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west to satisfy requirements for benching. As the excavation moved further west, which
was upgradient of the release, unexpected contamination was discovered. A thick layer
of black soil with a strong petroleum hydrocarbon odour was discovered approximately
5 metres upgradient of the 1993 release. Excavation continued to the west, and
graphite rods were discovered, along with heavily discoloured soil. At this point, the
excavation extended well to the west of the foot print of the former building.
The excavation was also extended to the south, cross-gradient of the 1993 release.
Again, unexpected contamination was discovered, along with a large concrete vault.
Upon further investigation, the vault was identified as an oil water separator, which had
been situated beneath the building.
Instead of a single release from a pipe in 1993, the site had been subjected to
numerous releases. Information obtained during excavation activities indicated that the
site had been a petrol station and workshop in the 1950s and 1960s. The oil/water
separator was part of this installation. In the 1970s, new tanks were installed with
cathodic protection. These tanks were situated west of the building that was
demolished in 2006. The tanks were apparently removed in the 1970s; however, the
contamination was left in place, along with piping and graphite rods. Another release
beneath the south-eastern dispenser also became apparent during the excavation.
Soil samples collected from the vicinity of the 1993 release, where in-situ remediation
had taken place, showed that only heavy end hydrocarbons remained. The soil
concentrations in 1993 were on the order of 6,500 milligrams per kilogram (mg/kg), with
mainly light-end hydrocarbons present. In 2006, all but two samples were less than the
regulatory limit of 100 mg/kg, and only heavy-end hydrocarbons remained.
At the end of the excavation project, nearly 5,800 cubic metres of soil was removed,
along with 408,000 litres of contaminated water generated during excavation
dewatering. A layer of oxygen release compound was placed in the bottom of the
excavation. The excavation was then backfilled and paved.
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4.2 Former DeLuca Liquors Site
The problem became obvious at the former DeLuca Liquors site after the pavement and
upper layer of fill was removed. When the UST had been removed, the piping, turbines,
pumps, and other equipment had been left in the excavation. In addition, the
contaminated fill from around the original UST had been replaced in the excavation.
Because there was a relatively active source of contamination still present, the in-situ
remediation had appeared to fail. Upon observing the excavation, the caliche fracturing
was apparent, so the hydrofracing technique had worked. As only heavy-end
hydrocarbons were present in the areas which had undergone remediation, it was also
apparent that the in-situ remediation had a beneficial effect.
Since there was limited room for excavation, the initial excavation volume estimates
were accurate. The equipment and contaminated gravel from the original UST pit was
removed, along with impacted soil along the sidewalls of the excavation. While
contamination was still evident from both observation of soil staining and soil sampling
results, the excavation could not be extended. Instead, the excavation was allowed to
fill and the water was removed. This was repeated several times in an attempt to
remove impacted groundwater. Approximately 200,000 litres of groundwater was
removed. A layer of oxygen release compound was placed in the bottom of the
excavation at groundwater level, with trenches excavated perpendicular to flow and
filled with oxygen release compound. The excavation was backfilled with gravel and
paved.
Approximately two years later, the site was finally clean enough to allow for regulatory
site closure, with benzene levels in groundwater dropping to near the regulatory limit.
5. Lessons learned
In both instances, it was assumed that due to challenging soil conditions that the sites
were not suitable for in-situ remediation. This assumption was made, in part, because
in-situ remediation had been shown to be effective at other sites in Las Vegas where
soils had a lower clay content and where caliche was not present. The assumption was
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also made because after years of in-situ remediation, the sites were still showing
relatively high levels of contamination present.
The assumptions were erroneous in both cases because there were previously
unidentified sources of contamination, which resulted in an erroneous site conceptual
model.
Before excavation was conducted, the Phase 1 assessments and past reports were
reviewed, historic aerials were reviewed, and the consultants involved were interviewed
when possible. However, the documentation reviewed was not adequate to show that
additional contamination sources might be present.
A lessons-learned evaluation was conducted following project completion. The primary
finding of the lessons learned evaluation was if the system doesn’t appear to be
working, then maybe the model is wrong and more investigation should be conducted.
As the excavation extended to the east at the former ARCO 5310 site, discoloured soil
was shown to be present around MW-3 and MW-5, with the leading edge of the plume
just encroaching on the wells. The clayey nature of the soil meant that the plume took
preferential pathways and just happened to go around both wells. This explained the
benzene detections in late 2005. Had MW-3 been installed about 150 mm to the west or
north, it may have detected the second dispenser plume . MW-4 was less than 0.3
metres south of the plume from the westernmost tanks.
At the former DeLuca Liquors site, had another boring been advanced that intersected
the historic tank pit, the contaminated gravel would have been evident. In addition,
while the boring logs for MW-1 and MW-3 showed a thick layer of caliche, both wells
happened to penetrate a single thick lens. The thickness was less than 0.5 metres
within the tank pit and to the west and north of MW-1 and MW-3, respectively. This
meant that excavation would have been practical and cost effective early on.
To the credit of the firms that were responsible for remediating these sites, there was
scant documentation available regarding past site conditions and in-situ remediation
had not been tried in these soil conditions before. The in-situ remediation helped
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minimise downgradient contamination. However, with hindsight being 20/20, if the site
conceptual model had been questioned, along with the technology, a great deal of time
and money could have been saved. More than $2 million was spent on remedial action
at each site.
The primary lessons learned were that in-situ remediation does work, even in
challenging soil conditions, and that if the system doesn’t appear to be working, it may
be because of an error in the site conceptual model. It is important to not just revisit the
technology, but also question whether assumptions made about the conditions at the
site are correct, identify possible data gaps, and conduct additional characterisation to
address the data gaps.
6. References
Broadbent and Associates, 2006. Fourth Quarter 2005 Groundwater Monitoring Report,
ARCO 5310, Las Vegas, Nevada.
Harding Lawson Associates, 2005. Groundwater Monitoring Report, DeLuca Liquors
Site, Las Vegas, Nevada.
TRC Companies, Inc., 2007a. Formal Site Closure Report, Former ARCO 5310, Las
Vegas, Nevada.
TRC Companies, Inc., 2007b. Remedial Action Report, Former DeLuca Liquors Site,
Las Vegas, Nevada.