Clean-Up of VOC Spills

Evaluating vapour intrusion and mitigation strategies at chemical spill sites

by Steve Long

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Vapour intrusion is an emerging issue that has become increasingly important when assessing the health risks and determining the clean-up strategies for volatile organic compound (VOC) spills.

VOCs include a wide variety of common industrial degreasers and petroleum fuel compounds, and a number of these compounds are suspected or known carcinogens. While these chemicals are somewhat soluble in water; they exhibit high vapour pressures and Henry’s law constants, and therefore are apt to volatilise from contaminated soil or groundwater into the soil vapour. Vapour intrusion may occur when volatile chemicals that have seeped into the soil or groundwater from leaking gasoline tanks, chemical manufacturing facilities, degreasing operations, dry cleaners, or other sources migrate into nearby buildings through cracks and openings in foundations, creating indoor air problems and potential health risks. The contaminant source need not be on the same property as the impacted building. In fact, we have seen a number of large chlorinated solvent plumes that have impacted tens or hundreds of houses and commercial buildings that are downgradient of a chemical spill site.

Vapour intrusion guidance

Over the last several years in the U.S., we have seen the U.S. Environmental Protection Agency (USEPA) and many U.S. states issue vapour intrusion guidance. USEPA has made it clear that vapour intrusion must be considered when assessing VOC-impacted sites, and this exposure pathway must be addressed in the clean-up strategy. Many U.S. states have actually re-opened clean-up cases where a property was remediated to clean-up levels based on direct contact, but vapour intrusion was not considered at the time that the clean-up was conducted.

In Europe, vapour intrusion guidance and rules vary from country to country and are continuing to develop over time. Canada, Mexico, Australia, and New Zealand all have vapour intrusion guidance in place or in development, but vapour intrusion in the remainder of the world is not widely considered or regulated. Vapour intrusion investigation procedures in various countries tend to be similar to those employed in the United States. There are differences, however, from jurisdiction to jurisdiction on policy related questions such as acceptable risk level.

Indoor air quality

The potential for vapour intrusion to result in elevated indoor air concentrations is dependent upon a number of factors including the chemical properties of the spilled chemical, the geology at the site, and the construction characteristics of any buildings overlying the contaminated area. Some of the most significant factors and their impact on vapour intrusion potential are summarised below.

Summary of factors that impact vapour intrusion potential

Chemical volatility Compounds with higher vapour pressure and Henry’s law constant represent a higher risk
Chemical biodegradability Chemicals that are biodegradable with shorter half-lives result in decreased risk
Chemical toxicity Carcinogens generally represent higher risks than non-carcinogenic chemicals
Soil type Coarse-grained soils represent increased risk
Depth to water Deeper groundwater results in decreased risk
Building construction Older buildings with basements represent increased risks; residential structures typically represent a higher risk than commercial
Building size Larger buildings with high ceilings represent lower risks due to the presence of more dilution air in the indoor space
Building ventilation Buildings with high air exchange rates represent lower risk; buildings with HVAC systems that operate at negative pressure represent increased risk

Screening criteria

The U.S. has developed screening criteria to determine when vapour intrusion may be a potential risk that should be further evaluated. The most commonly used criteria is ASTM E2600-15 Standard Guide for Vapour Intrusion Screening. This guidance considers the chemical type, distance from the source, and presence/absence of nonaqueous phase liquids (NAPL; i.e., free phase product) as primary factors that influence site risk. The initial screening is non-invasive and results in a very conservative determination of whether additional study is warranted to evaluate vapour intrusion potential.

The American Petroleum Institute (API) has also issued vapour intrusion guidance for volatile petroleum compounds, such as benzene. The API guidance is less conservative because it attempts to account for the biodegradation of petroleum compounds. As petroleum compounds volatilise from a contaminated underground soil or groundwater source and migrate upward, they are rapidly degraded in the oxygen rich vadose zone close to the earth’s surface. API utilises the vertical separation distance (separation between building foundation and contaminant source) as an additional factor in the screening process. It should be noted, however, that aerobic degradation does not occur with most chlorinated solvents (e.g, trichloroethene) and this API guidance is applicable to petroleum only.

Field studies are commonly conducted when initial vapour intrusion screening indicates a potential for concern. Most studies are conducted in a phased approach, which generally includes:

  • A building reconnaissance and survey
  • Soil gas sampling
  • Indoor air sampling

The initial building survey is an important, but often overlooked component of the investigation. Components of a building survey include the following tasks:

  • Identification of potential background sources of volatile compounds
  • Assessment of the building construction characteristics (e.g., ventilation, slab construction, etc)
  • Identification of preferential pathways into a building (utility penetrations, slab cuts, sumps, etc)
  • Identification of possible sample locations

Screening instruments

The use of handheld field screening instruments, such as a photoionisation detector (PID) may be helpful during the building survey. These instruments can provide useful information for identifying stored chemical products as potential background sources, determining sampling locations and identifying preferential pathways into the building. Care should be taken to ensure that the instrument is capable of detecting the contaminants of concern. It should be noted that available hand-held instruments are capable of detecting gross levels of contaminants only, and they are not capable of detecting low concentrations of VOCs at, or near the indoor air action levels. Therefore, they are appropriate only for initial screening purposes, and not for soil gas sampling.

“as petroleum compounds volatilise from a contaminated underground soil or groundwater source and migrate upward, they are rapidly degraded in the oxygen rich vadose zone close to the earth’s surface”

All of the information collected during the building survey should be utilised to develop the soil gas sampling plan. Multiple types of soil gas sampling may be conducted including sub-slab, near slab, and exterior soil gas sampling.

Sampling variations

The distinction between sub-slab, near slab and exterior soil gas sampling is critical for the vapour intrusion investigation. Sub-slab soil gas (SSSG) samples are collected from inside a building, while near slab and exterior soil gas samples are collected outside the building. SSSG samples generally provide the most representative and useful data for determining whether a potential vapour intrusion pathway into the building may exist.

Volatile contaminant concentrations tend to be highest within the granular fill layer that is placed immediately below the building slab in most modern buildings. Therefore, near slab and exterior soil gas sampling is only recommended when specific technical issues make SSSG sampling impractical or access issues prevent sampling inside the building. The primary objective in collecting SSSG samples is to assess if there is a potential for a vapour intrusion pathway into the building.

Multiple SSGS samples are typically collected within the building and multiple sampling events may be required to address temporal variations in the soil gas VOC concentrations in order to provide a robust and defensible investigation. Recommendations on the number of samples per building may vary per jurisdiction. The table below shows recommendations from the New Jersey Department of Environmental Protection (NJDEP) as an example.

NJDEP recommended number of sub-slab soil gas samples for buildings

Square Footage of Building Footprint Number of SSSG Samples
Up to 1,500 2
1,501 to 5,000 3
5,001 to 10,000 4
10,001 to 20,000 5
20,001 to 50,000 6
50,001 to 250,000 8
250,001 to 1,000,000 10
>1,000,000 12+

The determination of the necessary number of sub-slab samples to characterise the impacts to a building from VI will vary from building to building due to various features and uses of the building. Evaluate the features and uses of a building based on professional judgment to determine the number of sub-slab samples.

“the primary objective in collecting SSSG samples is to assess if there is a potential for a vapour intrusion pathway into the building”

SSSG samples are collected from just below the floor slab on the lowest level of the building. A small corehole is drilled through the slab and a stainless steel screen is inserted into the borehole, just below the slab. The screen is attached to the surface by tubing, and the corehole is grouted to prevent leakage of air from the surface from entering the screen.

Tubing is used to connect the sub-slab probe and the collection container (usually a Summa canister). Inert, small diameter tubing, such as 1/8” or 1/4” diameter rigid wall nylon, stainless steel, or Teflon is preferred. Tygon, LDPE (low density polyethylene), vinyl and copper tubing should be avoided because these materials may adsorb the VOCs. A one-Liter stainless steel vacuum canister (aka/Summa Canister) is normally utilised to collect the sample. Six-liter canisters may also be employed where lower detection limits are required.

Leak detection tests

Special care is taken to ensure that leakage of air from the surface into the screen section does not occur. Leak tests should be conducted for every sub-slab vapour sample in order to establish air tightness. Fittings connect the tubing between the sub-slab screen and the collection container (usually a Summa canister). These fittings, along with the probe seal, must be airtight or ambient air can leak into the Summa canister and significantly bias the measured sub-slab vapour concentration results.

The leak detection tests normally include a shut-in test to evaluate the tightness of the sampling train fittings, and a helium shroud test to evaluate the tightness of the stainless steel probe seal. For the shut-in test, the valves at the sample port and the sample container are closed, the sample line is evacuated using a hand held vacuum pump, and the vacuum pressure is monitored for a short period of time to ensure that the vacuum is maintained. For the helium shroud test, a shroud is placed over the sample port and helium is pumped into the shroud under pressure. A hand-held pump and a helium detector are used to draw air from below the slab into the sample port and to detect any leakage from above the surface that enters the sample port. It is not uncommon to detect small leaks in the seal, and these leaks are corrected, and a follow-up leak test performed prior to sampling.

Soil gas samples

Soil gas samples may be collected instantaneously (over a period of a few minutes) or the vacuum canister may be equipped with a flow regulator to allow collection of a sample over a longer period of time (e.g., 8 hours, 24 hrs.). We have normally collected 8-hour samples to allow capture of some of the inherent variations in building pressure, barometric pressure, temperature, etc. that occur during a day, but there is no regulatory consensus on the appropriate amount of time. When sampling, the vacuum pressure in the canister is measured before and after collection of the sample to ensure that a vacuum was maintained throughout the sampling period.

Once collected, the samples are shipped to the laboratory for analysis. The SSSG samples can be analysed using USEPA Method TO-15 (or other appropriate certified methods). Sampling with automatic thermal desorption tubes and analysis by USEPA Method TO-17 is also acceptable. Laboratory results are most often reported in micrograms per cubic meter of air ( g/m3). Some laboratories may report the results using different units, so care should be taken to ensure that the units are appropriate for comparison to any regulatory guidance concentrations.

Evaluating gas concentrations

Two approaches have been commonly utilised to evaluate soil gas concentrations and the need for mitigation or further study. Both of these approaches attempt to estimate the concentration of vapours in the indoor air space that may result from the soil gas concentrations beneath the building that were determined through the soil gas study. One approach utilises numerical modelling to estimate indoor air concentrations. Examples of this approach include the US EPA Spreadsheet for Modelling Subsurface Vapour Intrusion (aka/Johnson and Ettinger model) and the VOLASOIL model from the Dutch National Institute of Public Health and the Environment (RIVM). These models take into account the building construction (e.g., basement or slab on grade), building size, ceiling height and other factors to estimate indoor air concentrations. Such models were initially very commonly utilised, but have more recently fallen out of favour in many jurisdictions because they are difficult to calibrate and may produce wide variations in indoor air concentration estimates.

The other approach utilises a simple empirical attenuation factor to account for the differential in concentrations that occurs across the building slab. This simplistic, empirical approach may result in overestimation of indoor air VOC concentrations, but is widely used by regulators because it is conservative and easy to implement.

For example, USEPA compiled a database of empirical attenuation factors through review of data from hundreds of buildings with indoor air concentrations paired with sub-slab soil gas, groundwater, exterior soil gas, or crawl space concentrations. 95th percentile empirical attenuation factors were utilised by EPA to derive the VISLs for health protection.

One common question is why direct measurements of indoor air quality are not made, in lieu of collecting soil gas measurements and then attempting to estimate indoor air concentrations through numerical or empirical calculations. While direct measurement of indoor air concentrations would be desirable, the widespread presence of VOCs in building products, such as mastics, carpets, and plastics, and in common cleaning products, cosmetics and other household products makes interpretation of indoor air results very difficult. In our experience on hundreds of sites, it is very uncommon for no VOCs to be detected above screening levels in the indoor air samples. Additionally, the ambient air quality outside the building may contain contaminants from automobile exhaust, and a wide variety of local and regional emissions sources.

“in our experience on hundreds of sites, it is very uncommon for no VOCs to be detected above screening levels in the indoor air samples”

We do commonly collect indoor air measurements if sub-slab soil gas studies indicate the presence of VOCs at concentrations of concern. However, we commonly restrict the laboratory analyte list to just those chemicals that are present in the sub-surface. Even still, estimating the relative contribution of common contaminants such as benzene in measured indoor air concentrations can be difficult. We usually collect multiple indoor air samples and background air samples to assist in this evaluation.

Seasonal factors

It should also be noted that indoor air concentrations may be significantly impacted by seasonal factors such as operation of heating or cooling systems, and use patterns (doors open or closed), so indoor air concentrations may vary significantly over time. Soil gas concentrations will also vary temporally, but to a lesser degree than indoor air measurements. We typically conduct multiple rounds of sampling prior to drawing conclusions regarding the need for vapour intrusion mitigation.

When vapour intrusion studies indicate a potential for health concerns, mitigation may be required. In some cases, mitigation may be as simple as re-balancing of the heating ventilation and air conditioning (HVAC) system to operate at positive pressure and sealing slab penetrations and openings that could serve as preferential pathways for vapour migration. A building survey is typically conducted to determine the presence of conduits that are most likely to result in vapour intrusion. Sumps, elevator shafts, pits, and other large openings are most likely to result in high VOC concentrations entering the building.

When more significant attenuation is necessary, a sub-slab depressurisation system (SSDS) may be required. An SSDS typically consists of a series of sub-slab extraction points attached via piping to a vacuum blower or to an exhaust vent. Negative pressure is maintained beneath the building slab and VOC vapours beneath the slab are discharged via an exhaust stack. Both passive and active systems have been utilised. In passive systems, the wind blowing over the top of an exhaust stack will create a small vacuum in the collection piping that is often sufficient to remove contaminants from beneath the slab before they can enter a building. Where a passive system is not sufficient, a vacuum blower may be added to increase the subsurface vacuum.

Buildings on contaminated lands

Vapour intrusion considerations may also be taken into account when designing and constructing new buildings on contaminated lands. In these scenarios, a chemical vapour barrier may be installed beneath a building. These vapour barriers are intended to create an impervious seal beneath the slab. Sealing around utility conduits is also required to ensure a positive seal. High density polyethylene (HDPE) sheeting has been utilised for this purpose, but sealing the sheeting at utility conduits, building foundations, and at joints is difficult. Multi-layer barriers that include a spray applied asphalt emulsion layer are commonly used in the U.S. The spray applied barriers are more expensive from a materials perspective, but are easier to implement in the field as the spray applied material easily seals around penetrations and the perimeter. Once the barrier is installed, a smoke test is commonly conducted to ensure that no leaks are present. In high risk cases, a passive or active SSDS may be installed beneath the vapour barrier.

Summary

Vapour intrusion guidance and regulations are still evolving and litigation has been on the rise. Therefore, when advising clients on vapour intrusion risks, it is important for engineers and consultants to consider the uncertainty in the regulatory landscape to ensure that today’s solution will be considered adequate in the foreseeable future. We have seen many of our clients taking a conservative approach in addressing vapour intrusion risks. In some cases, companies are installing vapour barriers as a protective measure on new construction when the initial screening indicates a potential concern.

Author Details

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Steve Long

Stephen Long, PE, PG, has more than 29 years’ experience assessing and formulating cleanup strategies for a wide variety of contaminated sites – ranging from petroleum bulk terminals, to hazardous waste disposal facilities, to silicon chip manufacturing facilities. As both a licensed professional engineer and professional geologist, he has a unique perspective on the collection of high-resolution site characterization data to inform effective remediation decisions. Stephen has designed and implemented cleanup activities on more than a hundred sites, including many innovative treatment technologies. He currently leads Intertek’s contaminated site assessment and remediation practice.