using risk assessment to achieve cost-effective property transfers and site closures for former ust...

Using Risk Assessment To Achieve Cos t-Effective Property Transfers and Site Closures for Former UST Sites

Teri L. Copeland Richard Pesin Lisa Barfield

Teri L Copeland is a Principal Toxicologist and manager of tbe Soutbern California Risk Assessment and Toxicol- ogy services for Harding Lawson Associates, Santa Ana omce. Sbe bas over 16years of experi- ence In tbefields of

-ntaLsc&nce and toxicology and bas conducted risk assessments In support of numerous regulatory programs, personal injury and property damage litigation, and property transactions. Ricbard Pesin is a Senior Toxicologist for Harding Lawson Associ- ates, Santa Ana omce. He specializes in buman bealtb risk assessment,‘ environmental fate and transport modeling, and litigation support sewices. Lisa Barfleld is a Senior Toxicologist for Harding Lawson Associ- ates, Santa Ana omce. Her expertise includes litigation support services, buman bealtb risk assessment, and regulatory analysis.

R&k assessment has been increasingly applied as a tool in making risk management decisions that affect cleanup of contaminated sites, property transactions, and liability issues. As a site-specific evaluation, risk assessment takes into account the unique characteristics and intended future uses for site property in evaluating chemical concentrations which may remain in place without risk to public health and the environment. The results of a risk assessment can be used to determine reuse options for a property, facilitate site closure, and reduce liabilities (Copeland and Robles, 1994; Copeland et al., l993a).

7bis article describes the risk assessment process, the role of risk assessment in determining the need for remedial action and identifying site-specific cleanup goals, and the cost effectiveness of applying risk assessment in remedial decisions. Because of the prevalence of former UST sitesthroughout the Unitedstates, thisarticle focuseson riskassessment and remediation of USTsites. However, theprocess can be applied at sites where other chemicals have been released. Three case studies are presented to illustrate the application of risk assessment in achieving cost-effective site closure at sites containing leaking underground storage tanks.

Risk assessment is defined by the National Research Council (1983) as “the use of the factual base to define the health effects of exposure of individuals or populations to hazardous materials and situations.” In the last decade, risk assessment of contaminated sites has developed from an immature science into what is now recognized as a sophisticated scientific methodology that is used as a basis for determining environmental cleanup goals. Growing out of a need to evaluate the potential for health and environmental effects related to sites such as Love Canal and Times Beach, risk assessment protocols for contaminated sites were formally introduced by EPA in 1989 (USEPA, 1989a). In addition to the numerous federal programs that employ risk assessment (e.g., CERCLA, RCRA, Clean Air Act), many state and local agencies have developed risk assessment programs and/or rely on risk assessment as part of the regulatory decision-making process.

ccc I 051 -565a/951060ioi-i 7 0 1995 John Wiley & Sons, Inc.

1

TERI L. COPELAND RICHARD PESIN LISA BARF~ELD

I l l Baseline risk aseeesment evaluates potential risk8 to human health and the environment that could occur i f n~ remedial actions are taken.

As of June 1995, there have been over 295,000 confirmed releases from underground storage tanks (USTs) at sites nationwide (Josh Baylson, Office of USTs, personal communication). USTs are therefore a frequent characteristic of sites considered for remediation where risk assessment can serve to guide remedial activities. The remainder of this paper provides an overview of the risk assessment process and its application to the evaluation of human health and environmental risks. Key issues relevant to the assessment of potential risks at UST sites are highlighted.

THE RISK ASSESSMENT PROCESS Risk assessment of contaminated sites includes the evaluation of

human health risk as well as environmental risk. The characterization of human health risk includes the evaluation of cancer risk as well as the potential for noncancer toxic effects. The characterization of environmental risk includes the evaluation of the potential for chemical migration to groundwater or other environmental media (e.g., surface water) and the potential effects on biota. The necessity of an environmental risk assessment is determined by chemical-specific characteristics and site location. The vast majority of industrial sites are located within urban areas with limited biota of specific interest (e.g., endangered species, commercially desirable species). Accordingly, assessment of these areas is often limited to the evaluation of chemical impact to groundwater, and this article only addresses the chemical impact to groundwater under the environmental risk category.

Risk assessment can be applied at many stages of the remedial investigation/feasibility study (RI/FS) process tc help guide Most commonly, risk assessment is used to provide:

an analysis of baseline risks (i.e., risks associated with the site as is) and determine the need for action at a site; a basis for determining levels of chemicals that can remain at the site without posing a risk to public health or the environment; and a basis for comparing potential risks associated with various remedial alternatives.

The first two applications are discussed in subsequent sections of this article. The role of risk assessment in the evaluation of remedial alternatives is reviewed by EPA (USEPA, 1991a).

BASELINE RISK ASSESSMENT Baseline risk assessment evaluates potential risks to human health and

the environment (current and future) that could occur if no remedial actions are taken (“baseline” risk). Results of the baseline risk assessment are used to determine if remedial action is necessary and, if so, which areas of the site and which chemicals must be addressed. If the results of the baseline risk assessment indicate that no significant risks are posed to human health or the environment under “baseline” conditions, then the baseline risk assessment is used to support a “no further action” alternative

2 REMEDIATION~~INTER 1995/96

USING RISK ASSESSMENT To ACHIEVE COST-EFFECTIVE PROPERTY TRANSFERS AND SITE CLOSURES

for the site. If the results of the baseline risk assessment indicate that acceptable risk levels may be exceeded, then the baseline risk assessment provides the foundation for development of risk-based cleanup levels (RBCLs). The key steps of the baseline risk assessment include site history and data review, development of a conceptual site model, hazard identification, toxicity assessment, exposure assessment, and risk characterization (Exhibit 1). These steps are discussed in detail below.

As an initial step, a comprehensive review ofsite btktoryandoperations, site characterization, and site setting is performed. Information is com-

Exhibit 1. The Risk Assessment Process: Baseline Risk Assessment

Review Site HistorylOperations Site Characterization Data

Site Settin

I Develop Conceptual Site Model

Conduct Hazard Identification

Conduct Toxicity Assessment

Conduct Exposure Assessment

Conduct Risk characterization and Uncertainty Analysis

Develop Risk-Based Cleanup Levels No Further Action

REMEDIATION/WINTER 1995/96 3

TEN L. COPELAND RICHARD PESIN LISA BARFIELD

piled to help guide the identification of source areas and chemicals of potential concern and the development of a conceptual site model.

Following the review of site history and site characterization data, a conceptualsite model(CSM) is developed to guide the risk assessment. The CSM characterizes the relationships between chemical releases, retention/ transport media, and receptor exposures. The CSM provides a basis for the identification of exposure pathways to be evaluated and defines specific data needs for the risk assessment. Development of a CSM requires a conceptual formulation of the potential intermedia transport processes at a site. For most UST sites, the intermedia transport processes are volatilization to air and leaching to groundwater from soil (Exhibit 2). Because leaking USTs result in contamination in the subsurface only, there is not a potential for direct contact exposure (e.g., dermal contact, inadvertent soil ingestion) with contaminated soil in situ. The potential complete exposure pathway at UST sites is typically the inhalation pathway, as a result of vapor emissions from subsurface soils (Exhibit 3). Although chemicals may leach to or already be present in groundwater, exposure to groundwater is usually an incomplete pathway at UST sites.

In the hazard identification step, potentially toxic substances present at the site are identified as chemicals of concern. This step entails identification of the most toxic/mobile chemicals, and comparison of site concentrations with background (non-site related) concentrations.

-bit 2. Contaminant Fate and Transport Conceptual Model

GROUNDWATER GROUNDWATER FLOW

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4 REMEDIATION/WINTER 1995/96


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TEN L. COPELAND RICHARD PESJN LISA BARFIELD

The toxicity assessment considers the relationship between the magnitude of exposure and adverse effects.

Distinguishing between site-related chemical releases and naturally occurring or off-site chemical sources establishes (1) the proportion of site risk attributable to background levels and (2) the proportion, if any, of off-site chemical concentrations related to on-site chemical releases and subsequent migration.

For UST sites, selection of chemicals of concern follows a consistent process under most regulatory programs. Although commercial petroleum products consist of blends of hundreds of hydrocarbons, as well as trace amounts of inorganics, the chemicals that are selected for risk assessment include only the most toxic and mobile constituents. Total petroleum hydrocarbons (TPH), as measured by EPA Methods 418.1 and 8015 Modified, are not relevant indicators of toxicity and therefore are not evaluated in a risk assessment. However, detection of TPH is useful as a screening tool for defining the extent of petroleum product releases at the site and may be required by the regulatory agency. Specific petroleum- related chemicals that are evaluated in a risk assessment include benzene, toluene, ethylbenzene, and xylene (BTEX); polycyclic aromatic hydrocarbons (PAHs); and lead. Because BTEX concentrations are approximately 60 to 360 times lower in diesel fuel than in gasoline, these chemicals are frequently not observed in diesel releases. However, PAHs are present in significantly higher concentrations in diesel fuel than in gasoline and may be observed more frequently with a diesel fuel release. Therefore, indicator chemicals may differ according with the source of release.

The next step of the risk assessment is the toxicity assessment. In this step, the relationship between the concentration of a chemical and a specific health effect is characterized. The toxicity assessment considers (1) the types of adverse health effects associated with chemical exposure, (2) the relationship between the magnitude of exposure and adverse effects, and (3) related uncertainties (such as the weight of evidence for a particular chemical to cause cancer in humans). When avaiiable, human exposure data are considered; however, toxicity assessments are frequently based on data collected from animal studies and theoretical assumptions about what might occur in humans. When animal data are considered, mathematical models are used to estimate the possible response in humans at exposure levels far below those tested in animals. The models contain conservative assumptions which should be considered when the results of the risk assessment are evaluated. Conservatism arises from the use of animal models because of the uncertainty in extrapolating results obtained in animal research to humans and from the shortcomings of extrapolating responses obtained from high-dose studies to estimate responses at very low doses. For example, humans are typically exposed to environmental contaminants at levels that are over 1,000 times lower than the lowest dose tested in animals. Such doses may be easily handled by the myriad of biological protective mechanisms in humans (Ames et al., 1987). There- fore, the conservative methods used in the toxicity assessment allow for a very high margin of safety in the risk estimates.

Regulatory agencies have used dose-response data reported in the literature to establish reference doses (“acceptable” levels of human


USING &SK ASSESSMENT To ACHIEVE COST-EFFECTIVE PROPERTY TRANSFERS AND S m CLOSURES

Toluene, ethylbenzene, xylene, and specific PAHs are evaluated a8 noncarcinogens.

exposure to chemicals) for chemicals causing noncancer toxicity, and cancer potency factors (to estimate probability of cancer) for carcinogenic chemicals. For the chemicals of concern associated with leaking underground fuel tanks, benzene and specific PAHs are evaluated as carcinogens. Toluene, ethylbenzene, xylene, and specific PAHs are evaluated as noncarcinogens. Lead, if present, is evaluated for potential neurotoxicity using a biokinetic model (e.g., EPA's Integrated Exposure Uptake Biokinetic Model for Lead in Children, or CalEPA's LEADSPREAD Model (1992)).

Exposure assessment is the process of estimating the frequency and duration of human exposure to a chemical currently present in the environment and/or estimating hypothetical exposure that might result from future site conditions. Conducting an exposure assessment involves identifying potentially exposed populations (e.g., adult and child resi- dents, workers, visitors) and all potential pathways of exposure (e.g., ingestion, inhalation, skin contact), estimating exposure concentrations based on sampling data and/or predictive chemical modeling, and estimating chemical intake (dose) for each exposure pathway. A chemical dose is estimated by the risk assessor to evaluate the potential for adverse health effects in humans or animals. Consistent with EPA guidance (1989a), the following general dose equation is used to assess chemical uptake for each exposure pathway considered in the assessment:

where, ADD C

IR

EF ED FC B BW AT

ADD = C X IR X EFX ED X FC x B BW x AT

Average daily dose (mg/kg-day) Chemical concentration in environmental medium (mgkg) Intake rate (inhalation, ingestion, dermal contact) (mdday) Exposure frequency (daydyear) Exposure duration (years) Fraction of contaminated area (unitless) Bioavailability (unitless) Body weight (kg) Averaging time (days) For noncarcinogenic effects, AT = Exposure duration X

365 daydyear. For carcinogenic effects, AT = 70 years x 365 daydyear.

It is well-known that rates and duration of contact with environmental media can differ significantly with age. Accordingly, age-specific estimates of exposure are used to evaluate doses. Standard values for exposure parameters such as inhalation rates, body weights, soil ingestion rates, and skin surface area are provided in EPA guidance documents (USEPA, 1985; 1990; 199lb; 1992a).


TERI L. COPELAND Rrc- PESIN 6 LISA BARFIELD

Volatile organic compounds (VOCs) in soils may be released as vapors into the ambient air (Exhibit 3). In order to evaluate potential exposure to vapors released from soil, emissions models are used in the exposure assessment. There are numerous models that quantitatively predict the emissions rates of VOCs from soils, many of which are discussed in EPA guidance documents (1989b, 1992b, 1992~). Two of the most popular emissions models used in risk assessment are the Hwang-Falco model (USEPA, 1992~) and the Jury Behavior Assessment model (Jury et al., 1983). Both of these models allow for the evaluation of time-dependent emissions of VOCs from soil. With both of these models, vapor emissions rates from soils are highly dependent on the chemical concentration in soil, chemical properties (e.g., Henry’s constant, organic carbon partition coefficient), and soil properties (e.g., organic carbon content, porosity). The results of vapor emissions modeling must be incorporated in an air dispersion model in order to estimate an exposure concentration over the area of interest. The simplest model that can be used to estimate air dispersion is a box model. A box model is a basic mass-balance equation that uses the concept of a theoretically enclosed space, or box, over the area of interest. The model assumes the emission of compounds into a box, with the dilution of the compounds based on wind speed.

Risk characterization is the final step in the risk assessment process. In this step, the likelihood of an adverse effect is estimated based on the results of the toxicity and exposure assessments. Risk characterization includes the evaluation of noncancer hazard as well as incremental lifetime cancer risk (increased risk above the background cancer incidence of one in three). Noncancer hazard is estimated by first dividing the site-specific chemical intake (daily dose) by the reference dose (established safe dose) to determine the hazard quotient. Hazard quotients for all chemicals having a noncancer toxicity are then summed to determine the hazard index. A hazard index of one or less indicates that it is unlikely that adverse noncancer health effects would be associated with exposure to chemicals at the site, even for the most sensitive individuals.

Cancer risks are characterized as probabilities. The probability of cancer associated with site-related exposures is estimated by multiplying the lifetime average daily dose by the chemical-specific cancer potency factor (which assigns a probability of cancer per unit dose). For a risk assessment evaluating potential Future residential use, cancer risks for each carcinogenic chemical are summed for child and adult exposures to estimate the lifetime cancer risk for each chemical. Total site-related cancer risk is estimated by summing liFetime cancer risk for all carcinogens.

The hazard index and cancer risk equations are shown below:

Cancer risks are characterized as probabilities.

Hazard Index (HI) = Sum of Chemical-Specific Hazard

where: Quotients (HQ)

HQ - - Site-Related Daily Dose (my/ky-day) Reference (Acceptable) Dose (mg/kg-day)

~~


USING R ~ S K ASSESSMENT To ACHIEVE COST-EFFECTIVE PROPERTY TRANSFERS AND S m CLOSURES

The traditional regulatory approach to risk assessment is to offset uncertainty in the risk estimates by “erring on the side of conservatism.”

Incremental Cancer Risk = Sum of Chemical-Specific Cancer Risks

where:

Chemical-Specific Cancer Risk = Lifetime Average Daily Dose (mg/kg-day) x Cancer Potency Factor (mg/kg-day)

A key component of the risk assessment is an analysisof uncertainties. The traditional regulatory approach to risk assessment is to offset uncertainty in the risk estimates by “erring on the side of conservatism.” EPA requires that a “reasonable maximum risk” be evaluated by using upper- bound (conservative) values for many of the exposure parameters. Coupling upper-bound exposure parameters with the extremely conservative chemical toxicity criteria may result in overprediction of risks ranging from 10 to 1,000-fold or greater (U.S. Office of Management and Budget, 1991; Copeland et al., 1993b; Finley et al., 1993; Copeland et al., 1994). Recently, specific mathematical techniques (Monte Carlo analysis, Latin hypercube sampling) have been applied in the development of computer models that use all data within a given data distribution and the probability of each value occurring within the human population. Probability distributions for exposure parameters, recently developed and published in the scientific literature, can be input into these models in order to generate a distribution of all potential risks and the probabilities associated with each risk value. The probabilistic approach to risk assessment incorporates all available data points, including all sampling concentration values, rather than upper-bound “representative” chemical concentrations (which invari- ably overestimates risk). When used in risk management decisions, probabilistic risk assessment provides risk characterization information in an optimal format for use by risk managers. Although regulatory agencies have been slow to accept probabilistic analyses of risk, recent EPA policy endorses the use of a probabilistic approach to risk assessment where appropriate data are available (USEPA, 1992d). Additionally, both Califor- nia and Arizona are accepting probabilistic analysis of risk at the state level.

POTENTIAL IMPACT TO GROUNDWATER QUALITY Although risk assessments may evaluate environmental impacts to

groundwater quality and biota, this discussion focuses on the assessment of chemical impact to groundwater quality, as this is the predominant environmental concern for urbadindustrial sites. The principal mechanisms by which chemicals in soil may migrate downward to groundwater are as follows:

1. Chemicals in soil may dissolve in rainwater or irrigation water and subsequently migrate vertically with infiltrating water (i.e., advection).

2. For VOCs such as BTEX, chemical vapors may move from areas in soil of high vapor concentration to areas of lower concentration (i.e., concentration-driven vapor-phase diffusion). Even if vapor-

REMEDIATION~~INTER 1995/96 9

TEN L. COPELAND R I c m PESIN LISA BARFIELD

3.

In

phase diffusion may result in loss of chemical mass in soil by volatilization to ambient air, some mass may also be transported downward to the groundwater table. At high concentrations in soil, certain chemicals may exist as nonaqueous phase liquids (NAPLs). In this form, concentrations of liquid chemicals in soil exceed the combined capacity of soil moisture to dissolve them and soil particles to bind them. NAPLs migrate through the subsurface by gravitational forces.

order to estimate the potential impact to groundwater from chemicals released to the vadose zone, vadose zone transport models are used. Numerous vadose zone models have been developed by EPA and others. Nearly all of the models estimate the rate of vertical contaminant transport and the time-related mass impact of contaminants to groundwater, provided the chemicals do not exist as NAPLs in soil. A conceptual model of these transport mechanisms is presented in Exhibit 3. The vadose zone transport model VLEACH (CH2M Hill, 19!20), originally developed for EPA, has been accepted by various regulatory agencies for use in risk- based site closures. Another vadose zone model, SESOIL (Seasonable Soil Compartment Model; Bonazountas and Wagner, 1984) is also widely accepted by regulatory agencies as a risk assessment model.

The VLEACH model was developed to evaluate contaminant transport associated with vapor phase diffusion and liquid transport (i.e., advection) of dissolved contaminants to the groundwater. Chemical concentration intro duced in the model is partitioned among three phases: a liquid phase in soil moisture, a vapor phase in the air-filed pore space, and a sorbed phase on soil particles. A chemical in the liquid phase is subject to downward advection; a chemical in the vapor phase is subject to concentrationdriven gas diffusion. A number of numerical values associated with chemical properties and soil characteristics are input for the model run.

The vadose zone transport model, SESOIL, differs significantly from the VLEACH model in a number of respects. Whereas the soil column in the VLEACH model is assumed to have uniform properties from the surface to the water-bearing zone, SESOIL allows for different soil characteristics among the various layers in the soil column. Additionally, SESOIL can model contaminant transport associated with surface runoff, chemical or biodegradation of contaminants, and metal transport through the subsurface. The number of input parameters required to run the SESOIL model is much larger than required for the VLEACH model, and obtaining numerical values associated with some of these parameters can be an intensive process. These models differ significantly in the evaluation of transport of VOCs. Although both models can incorporate volatilization losses to air, only VLEACH allows movement of VOCs to groundwater through downward diffusion in the vapor phase. As such, SESOIL does not account for a potential transport pathway to groundwater for VOCs.

As noted earlier, petroleum products such as gasoline and diesel fuel are blends of hundreds of hydrocarbons. A health risk assessment selects a limited number of these chemicals to serve as indicator chemicals. Indicator chemicals

Obtaining numerical values associated with some of these parameters can be an intensive process.

10 REMEDIATION/WXNTER 1995/96


I t is not possible to run the models in reverse.

that require modeling are benzene, toluene, ethylbenzene, and xylene. Additionally, the most mobile PAHs (e.g., naphthalene) may be included in the evaluation of potential impact to groundwater. Regardless of the model used, if a risk assessment for the site may be conducted in the future, it may be advisable to obtain data for soil characteristics during the course of a site investigation. Specific soil parameters used in both the VLEACH and SESOIL models are organic carbon content, dry bulk density, effective porosity, and volumetric water content. Laboratory tests for evaluating these properties are relatively inexpensive.

Predicted potential groundwater impact of indicator chemicals derived from the fate and transport evaluation are compared to EPA or state water quality criteria (e.g., Maximum Contaminant Levels (MCLs)). Certain states and regulatory agencies allow for elevated levels of chemicals in groundwater above listed water quality criteria when it can be demonstrated that the aquifer is not of beneficial use.

RISK-BASED CLEANUP LEVELS Risk-based cleanup levels (RBCLs) for soils can be established by

identifying concentrations of chemicals that (1) do not exceed acceptable risks (health-based cleanup levels) and (2) would not pose a threat to groundwater underlying the site (environmental-based cleanup levels). Development of site-specific RBCLs is accomplished by conducting a “reverse” assessment that uses the baseline risk equation shown above to solve for media concentrations that would not exceed an acceptable cancer risk or a noncancer hazard.

Identification of soil cleanup levels that would not pose a threat to groundwater is accomplished by using vadose zone transport models in a manner similar to their application in the baseline risk assessment. While the health risk equations can be “reversed” to readily calculate a cleanup level, it is not possible to run the models in reverse. Accordingly, model iterations are conducted to determine the concentration of soil contaminants that may remain in place and not pose a threat to groundwater, as defined by applicable water quality criteria.

The general methodology for developing site-specific cleanup levels is outlined by EPA (USEPA, 1989a, 1991~) and is summarized in Exhibit 4. The conceptual structure of the equations used is as follows:

For Carcinogens Health-Based Cleanup Level = Acceptab le Risk (safe media concentration) (Cancer Slope Factor x

Exposure Factors)

For Noncarcinogens Health-Based Cleanup Level = Reference Dose (safe media concentration) Exposure Factors

The acceptable cancer risk is usually targeted to fall within the range of lo4 to 10“ (1 in 10,000 to 1 in 1,000,000) and an acceptable hazard index

REMEDIATION/”INTER 1995/96 11

TEN L. COPELAND RICHARD PESIN LISA BARFIELD

-bit 4. The Risk Assessment Process: Development of Risk-Based Cleanup Levels

Identify Exposure Parameters Relevant to

I Select Key Chemicals

Identify Acceptable Risk Levels

Allowable Risk for Each

Back-Calculate a Concentration for Each Chemical in Each Media So That Total Risk s

I

Com m u nicate Res u Its to Remedial Engineer

Feasiblity Study/Selection of Remedial Alternative

Determine Treatment Area and Treatment Levels

Design Confirmatory Sampling Program

Confirm That Treatment Meets RBCLs and/or Conduct 1 Confirmatow RA

12 REMED~ATION/WINTER 1995/96

USING RISK ASSESSMENT To ACHIEVE COST-EFFECTIVE PROPERTY TRANSFERS AND S m CLOSURES

of 1 (USEPA, 1991d). However, when factors such as technical and economical feasibility and the size of the potentially exposed population are considered, the acceptable risk may be set in exceedence of lo4 (USEPA, 1991d; Travis et al., 1987). It should be noted that the total incremental cancer risk and hazard index (i.e., the sum for all chemicals) cannot exceed the acceptable risk criteria.

Site-specific RBCLs, once derived, are communicated to the remedial engineer for use in consideration of remedial alternatives. Following remediation, confirmatory sampling and analysis is conducted to ensure that RBCLs have been met.

CASE STUDIES To illustrate the application of risk assessment in achieving cost-

effective site closures, three risk assessment case studies are presented which involve the release of petroleum-related chemicals. These case studies represent actual sites in California and Arizona where the results of a baseline risk assessment were used to substantiate a “no further action” alternative (Case Studies I and 111, and where RBCLs were used to establish a remediation program employing soil vapor extraction (Case Study 111). Each of the case studies is described below, followed by a discussion of costs and benefits associated with each risk assessment. A summary of the three case studies is provided in Exhibit 5.

site-specific RBcLs are communicated to the remedial engineer for use in

remedial alternatives. Case Study 1 I I I consideration of

In this case study, the owner of a former distribution facility used a baseline risk assessment to determine whether remediation by vapor extraction (estimated cost: $120,000) was needed to protect human health or groundwater quality. The site contained a gasoline UST and a diesel UST to fuel delivery trucks. When site operations ceased, the tanks and surrounding soils with visual staining were removed. Site characterization data (EPA Method 8015M) revealed the presence of TPH in localized areas (maximum concentration: 2,000 mg/kg), identified as gasoline-range hydrocarbons. Analyses for benzene, toluene, ethylbenzene, and xylene were conducted within the areas where TPH was detected (EPA Method 8020). Benzene was not detected in any of the samples analyzed; ethylbenzene, toluene, and mixed xylene were detected at up to 500 mg/ kg. The proximity of impacted soils to an industrial building precluded complete excavation as a remedial option.

Vapor flux and air dispersion modeling were conducted to evaluate potential exposure to vapors emitted from toluene, ethylbenzene, and xylene at the site. Based on dose estimates for a residential receptor, there was not a potential for adverse health effects associated with exposure to soil contaminants by the inhalation pathway (the only complete exposure pathway). Potential impact to groundwater underlying the site was evaluated using the VLEACH model and a groundwater mixing model to estimate maximum concentrations in the shallowest aquifer. The results of the model predicted that groundwater underlying the site would not be significantly affected by the indicator chemicals detected in soil. Based on


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the results of the risk assessment, a “no further action” alternative was accepted for the site. The cost associated with the risk assessment was $11,000, representing a cost savings of approximately $110,000 over estimated remediation by vapor extraction.

Case Study II The second case study is of a former gas station. Four USTs from the

station were removed in 1985. Subsequently, the station was razed and replaced with an underground parking garage underlying a commercial building. The owner of the building wished to sell the property and had a preliminary site assessment conducted. This was followed by a Phase I1 environmental assessment of the property. In the Phase I1 assessment, borings were drilled to 20 feet below the floor of the lowest level of the parking garage, and samples were collected and analyzed for BTEX, TPH, lead, and semivolatiles (including PAHs). Groundwater was present at approximately 62 feet below surface grade at the site, or approximately 32 feet below the third-floor garage level of the building.

Resultsof the site characterization indicated that the activities of the former gasoline station had apparently affected the subsurface soils in one area beneath the subject site and indicated elevated levels of BTEX and TPH. In order to determine if there was a potential for a significant impact to groundwater or an adverse human health effect, a human health risk assessment was conducted. BTEX were identified as the chemicals of concern. The exposure pathway considered was inhalation of vapors emitted from soil. Potential impact to groundwater was evaluated using the VLEACH model. The findings of the risk assessment indicated that BTEX present in soil would not result in a significant impact to groundwater and would not be associated with adverse human health effects or a significant cancer risk.

The findings of the risk assessment were submitted to a city department of environmental health. Based on a review of the risk assessment, the city department concurred that the presence of elevated levels of petroleum hydrocarbons did not present a significant potential to adversely affect groundwater underlying the site or result in adverse health effects associated with vapor emissions into the garage and building. Therefore, no further action was required at the site. The estimated cost of remediation at the site using soil vapor extraction was $500,000. The cost of conducting a risk assessment for the site was approximately $15,000; conducting a risk assessment provided an estimated savings of $485,000.

Case Study IIl In the third case study, a risk assessment was conducted to assist a

grade school in making a decision regarding the purchase of adjacent property during a campus expansion program. The property under consideration was formerly used by commercial and light industrial facilities. The objective of the risk assessment was to provide a legally defensible document to be used as the basis for reducing potential liabilities associated with the presence of low levels of chemicals in soils at the property under consideration. Additionally, risk-based remediation


TERI L. COPELAND RICHARD Pmm LISA BARFIELD

goals were established for a soil vapor extraction system installed in a limited hot spot area where three USTs were located prior to removal.

Existing data characterizing site soil revealed the presence of low levels of chlorinated VOCs, BTEX, and TPH in discrete areas of the property. The area immediately surrounding the former USTs contained chemical concentrations that were significantly elevated relative to the rest of the site. Following a review of existing site characterization data, supplemental sampling and analysis were implemented to alleviate data gaps and provide a sufficient database for the risk assessment. This included further characterization of chlorinated VOCs and BTEX, and the analysis of surface soils for PAHs.

Based on analysis of all site characterization data, the chemicals of concern evaluated in the risk assessment were tetrachloroethene, trichloroethene, 1,1,l-trichloroethane, methylene chloride, and xylene. Receptors evaluated in the exposure scenarios were students, adult faculty, and campus maintenance workers. Potential risks associated with inhalation and direct soil contact pathways were below the conservative benchmark risk levels of one in one million (1 x 10“) cancer risk and 1.0 noncancer hazard index for all areas excluding the former UST area. For this area, a soil vapor extraction system was installed and operated until concentrations of chemicals of concern were below risk-based concentrations identified using the process outlined in Exhibit 4. Based on the results of the risk assessment and successful remediation using soil vapor extraction, the property was purchased and developed for use by the school.

There icr growing regulatory agency acceptance of thie approach.

CONCLUSIONS Site-specific risk assessment can characterize soil cleanup levels that

are protective of human health and groundwater quality. There is growing regulatory agency acceptance of this approach as it allows realistic site cleanup goals to be achieved which are protective of human health and the environment and reduces potential liabilities associated with non- health-based closure approaches. Site-specific risk assessments can also be employed to guide land use planning and may serve as an important component of a cost-benefit analysis of alternative remedial actions at a contaminated site.

REFERENCES

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Bonazountas, M., and J. Wagner, 1984, SESOIL: A Seasonal Sol1 Compartment Model, Cambridge, MA: Arthur D. Little, prepared for EPA, Office of Toxic Substances, C-85875.

California Environmental Protection Agency, 1332, Supplemental Guidance for Human Health Multimedia Risk Assessments of Hazardous Waste Sites and Permitted Facilities, Chapter 7, Assessment of Health Risks from Inorganic Lead in Soil, Department of Toxic Substances Control.

CH2M Hill, Inc., 1990, “VLEACH-A One Dimensional Finite Difference Vadose Zone Leaching Model ,” prepared for EPA.

Copeland, T., B. Kerger, and M. Bono, 1333a, “The Use of Health Risk Assessment in Toxic Tort Litigation: A Case Study Evaluating Off-Site Impact of Dioxin-Contaminated Soils,”


USING RISK ASSESSMENT To ACHIEVE COST-EFFECTIVE PROPER^ TRANSFERS AND SITE CLOSURES

presented at the 32ndAnnual Meeting of the Society ofToxicology, New Orleans, Louisiana, March.

Copeland, T.L., D.J. Paustenbach, M.A. Harris, and J. Otani, 1993b, “Comparing the Results of a Monte-Carlo Analysis with EPAs Reasonable Maximum Exposed Individual (RMEI): A Case Study of a Former Wood Treatment Site,” Regul. Toxicol. Pbarmacol. 16: 275.

Copeland, T., A. Holbrow, J. Otani, K. Connor, and D. Paustenbach, 1994, “Use of Probabilistic Methods To Understand the Conservatism in California’s Approach to Assessing Health Risks Posed by Air Contaminants,” Journal of Air E. Waste Management Association, Vol. 44, December.

Copeland, T., and H. Robles, 1994, “Risk-Based Cleanup Goals Based on Practical Future Land Uses,” invited paper, Annual Hatemacon Conference, San Jose, California, March.

Finley, B.L., P. Scott, and D.J. Paustenbach, 1993, “Evaluating the Adequacy of Maximum Contaminant Levels (MCLs) as Health-Protective Cleanup Goals: An Analysis Based on Monte-Carlo Techniques,” Regul. Toxicol. Pharmacol. 18: 438.

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National Research Council, 1983, RiskAssessment in theFederal Government: Managingthe Process, Washington, DC, National Academy Press.

Travis, C., S. Richter, C. Crouch, R. Wilson, and E. Klema, 1987, “Cancer Risk Management,” Environ. Sci. Technol. 21: 415-420.

United States Environmental Protection Agency (USEPA), 1985, Development of Statistical Distributions or Ranges of Standard Factors Used in lhposure Assessments, EPA/b00/8-89/ 043, Office of Health and Environmental Assessment.

USEPA, 1989a, Risk Assessment Guidance for Superfund. Volume I. Human Health Evaluation Manual, Part A, Interim Final.

USEPA, 1989b, Air/Superfund National Technical Guidance Study Series, Volume 11: Estimation of Baseline Emissions at Superfund Sites.

USEPA, 1990, Exposure Factors Handbook, EPA/@O/8-89/043, Office of Health and Environmental Assessment.

USEPA, IYiIla, Risk Assessment Guidance for Superfund. Volume I. Human Health Evaluation Manual, Part C, Rhk Evaluation of Remedial Alternatives, Interim, December.

USEPA, 1%1b, Risk A.wsment Guidance for Superfund. Volume I. Human H d t h iiiduatkm Manual. S u # p h m t a l Guidance ‘Standard Default Exposure Factors, ” Interim Final, March 25.

USEPA, 1331c, Risk Assessment Guidance for Superfund. Volume I. Human Health Evaluation Manual, Part B, Development of Risk-Based Preliminary Remediation Goak, Interim Final, December.

USEPA, 1991d, “Role of the Baseline Risk Assessment in Superfund Remedy Selection Decisions,” Don R. Clay, Assistant Administrator, Office of Solid Waste and Emergency Response, OSWER Directive 9355.0-30, April.

USEPA, 1992a, Dermal Exposure Assessment: Princtples and Application, EPN@O/8-91/ OllB, Office of Research and Development.

USEPA, 1992b, AWSuperfund National Technical Guidance Study Series, Assessing Poten- tial Indoor Air Impacts for Superfund Sites.

USEPA, 1992c, Guideline for Predictive Baseline Emissions Estimation Procedures for Superfund Sites.

USEPA, 1992d, “Guidance on Risk Characterization for Risk Managers and Risk Assessors,” F. Henry Habicht, Deputy Administrator, Office of the Administrator, February 26.

United States Office of Management and Budget, 1991, Regulatory Program of the United States Government, April 1, 1990-March 31, 1991.


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