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, Terre.maize@spotless.co.nz

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

3

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.