Voluntary Remediation Program
Risk Assessment Guidance
The regulatory basis for performing risk assessments under the Virginia Voluntary Remediation Program (VRP) is found in the Voluntary Remediation Regulations section, 9 VAC 20-160-70(A)(1)(a) of the Virginia Administrative Code. The risk assessment should be included in the site characterization report and should include an evaluation of the risks to human health and the environment posed by the release or threatened release of a contaminant into the environment. If the risk assessment shows that a removal or other remedial action is necessary, a proposed set of remediation levels as described in section 9 VAC 20-160-90 should also be included.
Risk assessments under the VRP generally follow the methodology described in Risk Assessment Guidance for Superfund (RAGS). The risk assessment process consists of four major steps. These steps are data collection and evaluation, exposure assessment, toxicity assessment, and risk characterization. These steps, as applied to the VRP, are described in the following sections. This guidance applies primarily to the human health risk assessment, although there may be some overlap with ecological risk assessment. VRP guidance for ecological risk assessment will be developed separately. The U.S. Environmental Protection Agency, however, has issued its own guidance in Ecological Risk Assessment Guidance for Superfund: Process for Designing and Conducting Ecological Risk Assessments. Also see the EPA Region 3 Ecological Risk Assessment website for screening values and other guidance for ecological risk assessments.
Throughout this guidance, links are provided to Microsoft Excel spreadsheets that the participant may use to present the risk assessment. The tables in the spreadsheets are intended to insure that the participant provides VRP staff with sufficient information to evaluate risks at the site. The participant may choose to use a different format to present the risk assessment information, however, all data requested on the VRP tables must be included. Note that the use of the VRP recommended format will expedite the review process significantly. Also note that the tables must be accompanied by an explanation of how the data were derived and a rationale for all site specific model inputs.
Two other types of links are also provided in this guidance. Links in bold type lead to the glossary, where the highlighted term is defined. Some links in normal type lead to Microsoft Excel spreadsheets the participant can use to help prepare the risk assessment; others, however, lead either to other sections of the report or to risk assessment information sources located elsewhere on the World Wide Web. Links in italic type also lead to other sources of risk assessment information.
2.0 Data Collection and Evaluation
During the data collection and evaluation step of the risk assessment, site data relevant to human health and ecological evaluation are gathered and analyzed. In addition, potential contaminants of concern are identified.
The sampling performed during the site characterization should be planned such that the resulting data will support a risk assessment. In addition to source sampling, samples should be collected at potential exposure points and at site boundaries. In some cases, off-site samples may be needed. The VRP regulations require that the site characterization contain a delineation of the nature and extent of releases to all media. All media (except air and biota) should be routinely sampled unless a sufficient rationale is presented that sampling is not required.
The Virginia Department of Environmental Quality (VDEQ) does not have guidance on sampling methodology. The following documents from the U.S. Environmental Protection Agency, however, may be helpful in preparing sampling plans for VRP sites.
- Soil Screening Guidance: Technical Background Document
- Soil Screening Guidance: User's Guide
- Guidance for Performing Site Inspections Under CERCLA
- Risk Assessment Guidance for Superfund, Volume I. Human Health Evaluation Manual (Part A), Chapter 4
- Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils, Appendix E
The following sections briefly describe some of the issues relating to sampling methodology that may be relevant to VRP sites.
Both surface and subsurface soil should be sampled. Unless land use controls are put in place that prevent intrusive soil activities, VRP considers subsurface as well as surface soil to be potentially accessible for risk assessment purposes. During construction or excavation activities, subsurface soil could be brought to the surface and become available for exposure. If subsurface contaminants are at greater depths than reasonably expected to be reached during a construction or utility project, those samples may be eliminated from the risk assessment. As a general default, VRP assumes that contamination down to 15 feet could be encountered and should be included in the risk assessment. In some cases, such as sites with anticipated high-rise construction or subsurface parking garages, contaminants at greater than 15 feet will need to be included. In addition, in some cases, samples deeper than 15 feet may be required in order to assess the potential for contaminant migration to groundwater.
Groundwater samples should be collected at the majority of VRP sites. A sufficient number of monitoring wells should be installed in order to delineate groundwater flow direction and both the horizontal and vertical extent of the contaminant plume. Any existing production wells or drinking water wells should also be sampled. At most sites, several rounds of groundwater monitoring will be needed in order to characterize the dynamics of the plume over time. In addition to contaminant sampling, hydrogeologic properties that may be needed to model contaminant migration should also be determined.
2.1.2.1 Filtered versus Unfiltered Samples
Groundwater samples collected for use in VRP risk assessments should be unfiltered. VRP Tier II screening levels are based on maximum contaminant levels (MCLs), which are based on unfiltered concentrations. (Note: "Tiers" are explained in Section 2.4.) In addition, site-specific groundwater exposure scenarios such as construction work are also based on total concentrations.
2.1.2.2 Direct Push versus Monitoring Wells
Direct push sampling is often helpful for determining the extent of contamination at a site or for locating monitoring wells. However, there are concerns regarding the reproducibility of this method. Monitoring wells should be installed to provide the data quality necessary for a quantitative risk assessment. Direct push samples should not be used in the quantitative risk assessment for groundwater. However, any direct push samples that were taken for site characterization purposes should be discussed qualitatively in the text of the risk assessment.
Water samples should be collected from any surface water body (including rivers, streams, lakes, ponds, impoundments and estuaries) potentially receiving surface or groundwater discharge from a site.
2.1.3.1 Filtered versus Unfiltered Samples for Inorganics
Ideally, both filtered and unfiltered samples should be taken. The Virginia Water Quality Standards for Surface Water are based on dissolved (filtered) concentrations. However total concentrations (unfiltered) are more appropriate for assessing dermal exposure to surface water. If the participant proposes taking one or the other, then unfiltered samples are recommended. Note that this will result in a conservative comparison to standards.
Sediment samples should be collected from any surface water body (including rivers, streams, lakes, ponds, impoundments and estuaries) potentially receiving surface or groundwater discharge from a site.
Air samples may be needed at some VRP sites. Of particular concern are sites that have volatile organic contamination in the soil or groundwater. However, there are many other potential sources of volatile organic contaminants in indoor air. Therefore, the VRP recommends that indoor air sampling be performed only if the subsurface characterization indicates the potential for migration to indoor air. In most cases, the subsurface investigation should include collection of subslab soil gas samples. If modeling predicts an unacceptable risk, the participant may opt to do air sampling to validate or refute the model. Acceptable models are discussed in Section 3.2.1.
There is considerable variability in the results of both soil vapor and indoor air sampling, depending on the ambient conditions at the time of sampling and the location of the sample. Therefore the VRP recommends that several rounds of sampling at several depths and locations be conducted for these media in order to capture the seasonal and spatial variability. Please see Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils, Appendices E and I for further information on sampling considerations for these media.
A limited number of VRP sites may require biota samples. If commercial or recreational fishing takes place on or adjacent to the site, sampling of edible aquatic organisms may be needed. However, the participant may prefer to model bioconcentration from surface water and/or sediment to estimate concentrations in aquatic organisms rather than sample initially. If modeling predicts an unacceptable risk, sampling may be needed. If the site is adjacent to an agricultural area or home gardens, sampling of produce may be needed.
The VRP regulations require the use of Test Methods for Evaluating Solid Waste, USEPA SW-846, revised December 1987. It is recommended that any applicable updates to SW-846 be used.
The analyses should be targeted to the types of contaminants that would be suspected at the site based on site history. The following Brownfields Technical Resources from the U.S. Environmental Protection Agency Region III office may be used to determine the types of analyses to select:
If the site contaminants are unknown, such as for some landfills, the analyses should include metals and cyanide, volatile organic compounds (VOCs), semivolatile organic compounds (SVOCs), and pesticides/PCBs. Methods providing only a total for a certain group of contaminant -- such as total petroleum hydrocarbons (TPH) -- cannot be used for risk assessment.
2.3 Quality Assurance/Quality Control
The participant is responsible for insuring that adequate field and laboratory quality assurance/quality control (QA/QC) procedures are followed. The quality assurance plan and QA/QC sample results must be made available upon request for review by the VRP staff. Participants should be aware that QA/QC could become a critical issue if the site were to become the subject of litigation. The EPA Region III Brownfields Quality Assurance Project Plan Template may be helpful for planning QA/QC for VRP projects.
Detection limit is a generic term that refers to the lowest amount that can be distinguished from the normal "noise" of an instrument or method. Before choosing a laboratory, the participant should review laboratory performance data, such as method detection limits (MDLs) and instrument detection limits (IDLs), to insure that they can routinely detect most contaminants at the VRP Tier II screening levels. (See Section 2.4.2 for a discussion of Tier II screening levels.) For VRP risk assessments, the participant should request that the laboratory report the limit of quantitation (LOQ) or the sample quantitation limit (SQL) Since LOQs and SQLs take into account sample characteristics, sample preparation, and analytical adjustments, they are the most appropriate limits for Tier II screening and quantitative risk assessment. More information on detection and quantitation limits can be found in the USEPA document, Guidance for Data Usability in Risk Assessment (Part A).
2.4 Screening for Chemicals of Potential Concern
The maximum detected concentration should be compared to the VRP Tier I and/or Tier II screening levels as described below. If contaminant concentrations exceed the screening levels, the participant may proceed to the site-specific quantitative risk assessment (Tier III). In some cases it may be more beneficial to the participant, however, to remediate the site to Tier I or Tier II levels. For example, if the participant is seeking unrestricted closure, a site-specific risk assessment would not necessarily result in less stringent remediation levels. If all contaminant concentrations in all media are less than the Tier I background levels and/or Tier II screening levels no further risk assessment is required. Documentation of the screening must be provided in tabular form.
Tier I screening is not required. Participants may choose to begin with Tier II screening without consulting the VRP project manager. If, however, participants seek to go directly to Tier III screening, they must first obtain the project manager's consent.
In Tier I screening, contaminant concentrations from the site for all media of concern are compared to those from background samples collected from nearby areas that have not been affected by the substances of concern. If concentrations from the affected area exceed background levels, the participant may choose to employ Tier II or Tier III screening methods.
Tier I screening should be based on site-specific samples from an unimpacted area of the site or nearby property. An upgradient location may not always be an appropriate background location because VRP sites must also consider contamination that enters the site from an upgradient source. In general, background screening is applicable only to inorganic contaminants that are also naturally occurring.
The Tier I screening should be presented in a table showing the background concentration(s) and the maximum site concentration. The table should also indicate whether the contaminant is retained for quantitative risk assessment or eliminated from consideration. A description of the methodology and locations used in obtaining background samples must also be submitted. Tier I screening tables for inorganics are given in tables 2.1 through 2.4. Once values for background and soil concentrations are provided, the spreadsheets will indicate whether a given contaminant is of potential concern and should be retained for further screening.
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In Tier II screening, contaminant concentrations from the site for all media of concern are compared to medium-specific values obtained from published sources such as the USEPA Region III Risk-Based Concentration Tables, the USEPA Soil Screening Guidance, maximum contaminant levels or other action levels established by the Safe Drinking Water Act and the National Primary Drinking Water Regulations.
Tier II screening should be reserved for unrestricted-use sites. Tier II levels for soil and groundwater are based on the assumption of residential exposure. However, sites such as schools, day care centers, hospitals, nursing homes, parks, and agricultural areas should be considered unrestricted for screening purposes.
Tier II levels for soil are based on the lower of the USEPA Region III Risk-Based Concentration Table or the values derived from the USEPA Soil Screening Level (SSL) guidance for transfer from soil to air or groundwater. For non-carcinogens the target hazard quotient has been adjusted to 0.1 to allow for potential additive toxicity of multiple contaminants. This is equivalent to dividing the risk-based concentration (RBC) by 10. Assuming there are ten or fewer non-carcinogenic contaminants, this adjustment will result in a hazard index (sum of hazard quotients) less than or equal to one. (See Section 5.2 for a discussion of the hazard quotient and hazard index.)
If there are fewer than 10 non-carcinogenic contaminants that exceed the Tier II level, the participant may choose to recalculate the Tier II screening level. In such a case, the RBC or SSL should be divided by the number of contaminants exceeding the Tier II screening level. This is equivalent to adjusting the target hazard quotient so that the target hazard index does not exceed one. If the Tier II screening level is recalculated, the participant should insure that the non-carcinogenic screening level does not exceed a screening level based on carcinogenic effects.
VRP staff used the EPA SSL calculation web site to calculate SSLs for Virginia. The SSL site provides climatic data for nearby cities in each of the three climate zones that cover Virginia -- Raleigh, N.C., in zone VI; Huntington,W.Va., in zone VII; and Philadelphia, Pa., in zone VIII. The climatic data resulting in the most conservative SSL were chosen. Climatic data from Huntington were used to calculate SSLs for inhalation of volatiles since these resulted in the most conservative (lowest) SSLs. Data from Philadelphia were used to calculate SSLs for inhalation of fugitive dust since these resulted in the most conservative SSL. SSLs were calculated for both carcinogenic and non-carcinogenic effects and the most conservative of the two values was chosen. Values based on non-carcinogenic effects were divided by 10. The lower of the SSL for volatiles or the SSL for fugitive dust was then chosen as the SSL for inhalation. For non-carcinogens, the SSL for migration to groundwater was divided by 10 only if the SSL was based on a health-based number and not on an MCL. The lower of the SSL for inhalation or the SSL for migration to groundwater was then chosen as the SSL value. The SSL was then compared to the Region III RBC value. The lower of the two values was then chosen as the Tier II screening level.
The SSL for migration to groundwater was divided by 10 to allow for potential additivity of multiple contaminants only if the SSL was based on a health based number and not on a maximum contaminant level. The lower of the SSL for inhalation or the SSL for migration to groundwater was then chosen as the SSL value. The SSL value was then compared to the Region III RBC table. The lower of the two values was then chosen as the Tier II screening value.
The Tier II screening values for soil are presented in Table 2.5. Note that a contaminant that exceeds only the SSL for transfer to groundwater may still be screened out under Tier II if groundwater results (based on adequate sampling) indicate that transfer is not occurring at levels of concern. Once values for soil concentrations are provided, the spreadsheet will indicate whether a given contaminant is of potential concern and should be retained for quantitative risk assessment.
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The Tier II screening values for groundwater are based on federal (MCLs) established by the Safe Drinking Water Act and the National Primary Drinking Water Regulations. Lead and copper do not have an MCL but they have a treatment technology action level that should be used for screening. For contaminants that do not have an MCL the EPA Region III Risk-Based Concentrations for Tap Water should be used. For non-carcinogens the target hazard quotient has been adjusted to 0.1 to allow for potential additive toxicity of multiple contaminants; the hazard index for multiple contaminants should not exceed 1. The Tier II screening values for groundwater are presented in Table 2.6. The MCLs may be found in Drinking Water Regulations and Health Advisories. Once values for groundwater concentrations are provided, the spreadsheet will indicate whether a given contaminant is of potential concern and should be retained for quantitative risk assessment.
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By regulation, the Tier II screening values for surface water are based on the Virginia Water Quality Standards (WQS). For contaminants that do not have a Virginia WQS, the Federal Water Quality Criteria (WQC) may be used if available. The Virginia WQS are designated for either protection of aquatic life or protection of human health. Although this guidance primarily addresses human health risk assessments, the aquatic life standards have been included in the screening tables since they would be applicable to any surface water body. The Federal water quality criteria for aquatic life included on these tables are based on the criterion continuous concentration (CCC). Three screening tables (Tables 2.7a, 2.7b and 2.7c) are provided for surface water depending on whether the water is designated as a public water supply and whether the water is fresh or marine. On each table, the lower of either the human health or the aquatic life WQS has been chosen as the Tier II screening level. Consult the River Basin Section Tables in the WQS 9 VAC 25-260-360 to determine whether a specific water body should be screened as a public water supply. See 9 VAC 25-260-140 to determine whether a specific surface water body should be screened as marine or fresh water. Once values for surface water concentrations are provided, the spreadsheet will indicate whether a given contaminant is of potential concern and should be retained for quantitative risk assessment.
If neither a Virginia WQS nor a Federal WQC are available for a particular contaminant detected in surface water, the participant should perform a literature search to determine if alternative screening values are available. If alternative values are not available, the detected contaminants should be carried through to a Tier III risk assessment.
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VRP Tier II screening values for sediment were obtained by multiplying USEPA Region III residential soil Risk-Based Concentration values by a factor of 10 to account for decreased exposure to sediments. For non-carcinogens, however, the target hazard quotient has been adjusted to 0.1 (the RBC has been divided by 10) so that additive toxicity will not result in a hazard index greater than one for multiple contaminants. Thus the Tier II screening concentration for sediments for non-carcinogens is equal to the unadjusted soil RBC, while the Tier II concentration for carcinogens is equal to the RBC times 10. Screening values for sediments in unrestricted areas are given in Table 2.8. Once values for sediment concentrations are provided, the spreadsheet will indicate whether a given contaminant is of potential concern and should be retained for quantitative risk assessment.
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Tier III screening is based upon site-specific analysis that weighs current and potential exposure scenarios for the population(s) of concern and characteristics of the affected media.
Tier III screening should be used for sites that are or will be formally restricted to a specified use. Tier III cannot be used for unrestricted-use sites. The participant should consult with the VRP project manager to determine whether Tier III screening is appropriate for a particular site.
The Tier III screening values for commercial/industrial soils (Table 2.9) should be used when soil concentrations exceed Tier II levels and appropriate restrictions are in place or formally proposed. Residential use of the site must be prohibited. Since the SSLs for migration to groundwater are not included as Tier III screening levels, the site must either have a restriction prohibiting the use of groundwater or have sufficient groundwater samples to demonstrate that groundwater has not been impacted and will not be impacted in the future. This should include a demonstration that groundwater has not and will not migrate off-site above Tier II screening levels.
The Tier III screening levels for commercial/industrial soils are the lower of either the EPA Region III industrial soil RBC or the SSL for migration to air. The SSLs for migration to air have been adjusted to commercial/industrial exposure factors. Once values for soil concentrations are provided, the spreadsheet will indicate whether a given contaminant is of potential concern and should be retained for quantitative risk assessment. The RBCs and SSLs for non-carcinogens have been adjusted to a hazard quotient of 0.1 (the RBC or SSL values have been divided by 10).
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For groundwater contaminants that exceed Tier II screening levels, the participant may choose to perform Tier III screening based on the receptors of concern and the proposed restrictions for the site. For sites where land use will not be restricted but the use of groundwater will be prohibited, the receptors of concern could be residents, commercial/industrial and construction/utility workers.
Residents would be a concern if the contaminants retained after Tier II screening could volatilize into enclosed spaces. The participant should enter the appropriate depth to groundwater (4, 8 or 12 feet) in Table 2.10 to compare the maximum contaminant concentration in groundwater to the Tier III groundwater concentration. VDEQ derived the Tier III concentration from the Screening Level Model for Groundwater Contamination spreadsheet of the Johnson and Ettinger model. Default values were used in the model with the following exceptions: Depth below grade to water table (Lwt) was changed to 243.8 cm (8 ft.); SCS soil type was changed to silt (SI) both directly above water tables and in the vadose zone; and average soil/groundwater temperature (Ts) was changed to 13.88 degrees C. The default values were changed in order to provide a conservative screening value.
Construction/utility workers would be receptors of concern at any site where intrusive activities are not expressly prohibited. Two scenarios are possible for evaluating construction worker exposure to groundwater. In the first scenario, the depth to groundwater is below the level of the construction trench. Therefore, the construction worker would not have direct contact with the groundwater by incidental ingestion or dermal exposure. However, the worker could inhale vapors that migrate from the groundwater through the soil and collect in the trench. In the second scenario, the construction trench reaches the groundwater, causing groundwater to pool at the bottom. The construction worker may then be exposed to groundwater by incidental ingestion, dermal contact and/or inhalation. The participant should choose the appropriate scenario from Table 2.12 to compare the maximum groundwater concentration to the Tier III groundwater concentration. The participant should also provide documentation on the depth to groundwater at the site to verify that the appropriate scenario was used. These Tier III values were derived from an emissions equation and a box model combined with the VDEQ default exposure factors for construction workers. See sections 3.2.2 and 6.1 for more detail.
For sites that have or will have a prohibition against residential development and a prohibition against groundwater use, both commercial/industrial workers and construction workers would be receptors of concern. For commercial/industrial workers, the participant should enter the appropriate depth to groundwater (4, 8 or 12 feet) in Table 2.11 to compare the maximum groundwater concentration to the Tier III screening concentration. These Tier III screening concentrations have been derived from the Johnson & Ettinger model as described above, but the exposure factors have been adjusted to a standard commercial/industrial scenario. The participant may choose either an 4-, 8- or 12-foot depth to water table with appropriate site-specific documentation. The 8- and 12-foot scenarios assume that the building has a basement while the 4-foot scenario assumes a slab on grade. Once values for soil concentrations are provided, the spreadsheet will indicate whether a given contaminant is of potential concern and should be retained for quantitative risk assessment.
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In some cases, the participant may choose to collect soil gas samples to demonstrate that volatilization to indoor air will not occur above levels of concern. For sites where land use will not be restricted but the use of groundwater will be prohibited and soil gas samples are available, the participant should use Table 2.13 to compare maximum soil gas concentrations to Tier III soil gas concentrations. These Tier III concentrations were derived from the Screening Level Model for Groundwater Contamination spreadsheet of the Johnson and Ettinger model. (See the Source Vapor Concentration on the "Intercalcs" worksheet of the Johnson and Ettinger model.)
For sites where construction work is not expressly prohibited and soil gas samples are available, the participant should use Table 2.14 to compare maximum soil gas concentrations to the Tier III soil gas concentrations. These Tier III values were derived from an emissions equation and a box model combined with the VDEQ default exposure factors for construction workers. The emissions equation was obtained from Appendix A of the Air Superfund National Technical Guidance Study Series, Assessing Potential Indoor Air Impacts for Superfund Sites. Once values for soil gas concentrations are provided, the spreadsheet will indicate whether a given contaminant is of potential concern and should be retained for quantitative risk assessment.
For sites with current buildings and the potential for vapor intrusion, the VRP recommends subslab soil vapor samples. The results of the subslab samples should be compared to the screening levels on Table 2.15. The screening levels were derived by applying a 0.1 attenuation factor to acceptable indoor air concentrations as recommended in the EPA Vapor Intrusion Guidance.
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| Table 2.15 Subslab Soil Gas | |
VDEQ's approach to assessing chromium concentrations has been to conservatively assume that total chromium is all hexavalent chromium. If chromium concentrations drive a cleanup decision, VDEQ recommends that chromium samples be analyzed for the specific valence states to verify or refute this assumption.
The current screening value for lead in soils is 400 mg/kg for unrestricted-use sites. This screening level is based on the Revised Interim Soil Lead Guidance for CERCLA Sites and RCRA Corrective Action Facilities (OSWER Directive 9355.4-12, July 14, 1994).
The current screening value for lead in soils is 800 mg/kg for restricted-use sites. Please see the Adult Lead Methodology (ALM) Frequently Asked Questions (FAQ) web site for further information on the basis for the commercial/industrial screening level. Also see Recommendations of the Technical Review Workgroup for Lead for an Approach to Assessing Risks Associated with Adult Exposures to Lead in Soil for more information on assessing risks from adult exposure to lead.
If TPH levels in soil samples are more than 100 ppm, then the samples must be analyzed for metals, VOC's and SVOC's.
There are no risk-based screening levels for some of the possible chemicals of concern. In these cases proxy screening levels based on substances with similar chemical structures are substituted.
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Currently, the VRP Tier II and Tier III screening tables contain chemicals on the Target Analyte List (TAL) and Target Compound List (TCL). The TAL and TCL were developed for chemicals that are routinely sampled at Superfund sites. Sampling for VRP sites may (and often should) include contaminants that are not included on these lists. For chemicals that are not included on the VRP screening tables, the participant should derive screening levels based on the methods and references described in the previous sections.
The goals of the exposure assessment step are to analyze contaminant releases; to identify exposed populations; to identify potential exposure pathways; and to estimate exposure concentrations and contaminant intakes for each pathway.
3.1 Selection of Exposure Pathways
Table 3.1a presents the on-site exposure pathways that routinely should be considered under VRP risk assessments. The participant should provide a rationale for eliminating or selecting a pathway for evaluation. It is anticipated that some of the receptors listed on the table will be ruled out. Note that a commitment to place an institutional control on the property may be used to rule out specific pathways and/or receptors on site. Anticipated engineering controls, personal protective equipment or remedial actions should not be used to eliminate pathways or receptors. Note that restrictions may be medium specific. For example, a site may require a restriction on groundwater use but may be found to be acceptable for residential development based on soil sampling.
Table 3.1b presents the off-site exposure pathways that should be routinely considered. The participant should provide a rationale for eliminating or selecting a pathway for evaluation. Unlike the on-site pathways, institutional controls may not be used to rule out off-site exposure pathways since a participant generally does not have control of off-site properties. The rationale for selecting or excluding an off-site pathway may be based on sampling, modeling, or knowledge of past site history.
Also note that additional pathways may need to be evaluated for some sites. For example, ingestion of contaminated fruits, vegetables, or meats may be a concern if the site is adjacent to an agricultural area.
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The exposure assessment should consider the potential for direct exposure to groundwater, soil, surface water and sediment for any contaminants that exceed Tier II screening values. It should also consider the potential for inter-media transfer and off-site migration.
For unrestricted sites, exposures to an adult and child residential receptor must be evaluated. Exposure of a child and adult recreational receptor must also be evaluated if surface water and sediment are media of concern.
For each receptor, exposure by ingestion (including inadvertent ingestion), dermal contact and inhalation must be evaluated. For sites that impact surface water that supports edible aquatic organisms, ingestion of aquatic organisms by both a child and adult must be evaluated.
The exposure assessment should consider the potential for direct exposure to groundwater, surface and subsurface soil, surface water and sediment for any contaminants that exceed Tier III screening values. It should also consider the potential for inter-media transfer and off-site migration. If the participant proposes to limit intrusive activities at the site to utility work, contaminated soils below the depth at which the utility work would take place may be excluded from the subsurface soil exposure assessment. In order to eliminate this exposure pathway, the participant must provide a detailed description of the utility lines in the area and assurance that new utilities will not be added in the future in contaminated areas.
For restricted sites, residential receptor exposure to soil may be ruled out if a land use control will be placed on the site prohibiting residential development and there is no indication of off-site migration.
For groundwater, residential exposure to drinking water may be ruled out if a use restriction will be placed on the site prohibiting potable use and there is no indication of off-site migration. Participants choosing to cite property-use restrictions as an exposure mitigation action must provide formal documentation in support. A site-specific use restriction restricting future uses of the groundwater will be required. This will ensure adequate notification to future property owners and allow for the immunity associated with successful program completion to run with the property and thereby facilitate future transfers of the property.
A well survey should be performed within at least a one-mile radius of the site. The survey should include contacting the local agency with authority for approving well installation to obtain records on existing wells in the area. The survey should also include a visual inspection of the surrounding properties to determine whether there are any undocumented wells. In addition, the potential for future use of the groundwater must be considered. Any restrictions on future groundwater use should be described.
If the participant desires to eliminate off-site use of groundwater from consideration in the risk assessment, documentation from the relevant locality must be provided to demonstrate prohibitions on the use of the impacted aquifer. This documentation must be provided in the form of formal correspondence from the appropriate local entity to VDEQ, and would include, at a minimum, either a copy of a local ordinance prohibiting or restricting groundwater use in the areas affected by the contaminated groundwater, a domestic well surveillance plan, or other comparable mechanism approved by the appropriate local agency with jurisdiction over groundwater well installations in the affected area. It is the participant's responsibility to facilitate the submission of this correspondence to VDEQ.
A construction/utility worker should be evaluated unless there will be a deed restriction prohibiting intrusive activities. A commercial/industrial worker should be evaluated unless the site is inactive and there will be a prohibition on future use of the site.
For sites that either are or may be zoned for recreational use (e.g. parks, playgrounds, etc.), exposures to adult and child recreational users should be evaluated. Exposures to adult and child trespassers should be evaluated on sites that are not or will not be specifically set aside for recreational use.
For each receptor, exposure by ingestion (including inadvertent ingestion), dermal contact and inhalation must be evaluated. For sites that impact surface water that supports edible aquatic organisms, ingestion of aquatic organisms by both a child and adult must be evaluated.
3.2 Determining Exposure Point Concentrations
For VRP risk assessments either the maximum contaminant concentration or the 95% upper confidence limit (UCL) on the arithmetic mean should be used as the exposure point concentration (EPC) for each medium. If the UCL is used, the calculation must be based on the appropriate data distribution. The data distribution should be checked using the Shapiro-Wilk method [Shapiro, S.S., and M.B. Wilk, 1965. An analysis of variance test for normality (complete samples), Biometrika 52:591-611]. If the 95% UCL is greater than the maximum concentration the maximum concentration should be used as the EPC. Consult Calculating Upper Confidence Limits for Exposure Point Concentrations at Hazardous Waste Sites for details on calculating UCL's. Also see http://epa.gov/esd/tsc/software.htm to download EPA's ProUCL software to calculate UCLs. For samples that are non-detect, substitute 1/2 of the SQL when calculating the UCL. For duplicate samples, the results should be averaged. Exposure point concentrations should be presented on Tables 3.2-3.5 for each medium along with a summary of the data on which the EPC's were based. The participant should record the results of exposure point concentration calculations in the appropriate table for the medium of concern.
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When the EPC is based on inter-media transfer (e.g., the exposure medium is different from the original medium) or off-site migration, modeling -- from groundwater to indoor air, for example -- may be used. For most exposure media, established models are available for estimating EPC's. Some examples of models that may be used are:
Soil to air particulates: Soil Screening Guidance (USEPA 1999c)
Soil to air volatiles: Soil Screening Guidance (USEPA 1999c)
Groundwater to shower air: Foster and Chrostowski (1987)
Groundwater to indoor air: Johnson and Ettinger (1991)
Groundwater to outdoor air: ASTM RBCA (ASTM 1998)
Groundwater to surface water: AT123D (Yeh 1981)
Soil to indoor air: Johnson and Ettinger (1991)
Soil to groundwater: SESOIL (Hetrick et al. 1989, Hetrick 1993)
Soil gas to indoor air: Johnson and Ettinger (1991)
Surface Water Migration: EXAMSII (USEPA 1990a)
The EPC resulting from modeling should be provided in Table 3.6. Documentation of the model inputs should also be provided along with a rationale for any site-specific parameters used. The participant should record the results of exposure point concentration calculations in the appropriate table for the medium of concern.
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Applicable spreadsheets: |
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Medium-Specific Exposure Point Concentration Summary: |
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3.2.2 Exposure of Workers to Volatiles in a Construction/Utility Trench
There are no well-established models available for estimating migration of volatiles from groundwater into a construction/utility trench. VDEQ recommends the following approach -- based upon a combination of a vadose zone model to estimate volatilization of gases from contaminated groundwater into a trench, and a box model to estimate dispersion of the contaminants from the air inside the trench into the above-ground atmosphere -- to estimate the EPC for air in a construction trench. Two different methods are used to estimate volatilization into the trench. The choice of method depends on the site-specific depth to groundwater.
Airborne concentration of a contaminant in a trench can be estimated using Equation 3-1:
|
Ctrench |
= |
CGW x VF |
(3-1) |
Where:
|
Ctrench |
= |
concentration of contaminant in the trench |
?g/m3 |
|
CGW |
= |
concentration of contaminant in groundwater |
?g/L |
|
VF |
= |
volatilization factor (see equations 3-2 and 3-4) |
L/m3 |
VDEQ assumes that a construction project could result in an excavation as deep as 15 feet. At some sites, there is a high probability that construction projects with deeper excavations may occur. Contact your VRP project manager to discuss the appropriate assumptions for your site.
If groundwater at the site is greater than 15 feet deep, assume that the worker would not have direct contact with the groundwater but could still be exposed to vapors released from the soil in the bottom of the trench. In that case, Equation 3-2 should be used to calculate VF.
If the depth to groundwater at a site is less than 15 feet, VDEQ assumes that a worker would encounter groundwater when digging an excavation or a trench. The worker would then have direct exposure to the groundwater. The worker would also be exposed to contaminants in the air inside the trench that would result from volatilization from the groundwater pooling at the bottom of the trench. Equation 3-4 should be used to calculate VF.
VDEQ assumes that the trench is 3 feet wide by 8 feet long. The dimensions may be too restrictive, however, depending on the nature of anticipated projects. Tables 3.7 and 3.8 allow the participant to assess exposure based on site-specific factors. Appropriate documentation should be provided if values other than the defaults are used.
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Applicable spreadsheet: |
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3.2.2.1 Groundwater Greater Than 15 Feet Deep
Table 3.7 presents the calculations and the default input values for the equations described in this section. Site-specific information may be used in place of default values where appropriate. If site-specific information is used, appropriate documentation should be provided.
|
VF |
= |
( Hi x Dair x ACvad3.33 x A x F x 10-3 x 104 x 3600 ) / |
(3-2) |
Where:
|
Hi |
= |
Henry's Law constant for contaminant (Table 3.7) |
atm-m3/mol |
|
Dair |
= |
diffusion coefficient in air (Table 3.7) |
cm2/s |
|
ACvad |
= |
volumetric air content in vadose zone soil |
cm3/cm3 |
|
A |
= |
area of trench |
m2 |
|
F |
= |
fraction of floor through which contaminant can enter |
Unitless |
|
R |
= |
ideal gas constant |
atm-m3/ |
|
T |
= |
average system absolute temperature |
?K |
|
Ld |
= |
distance between trench bottom and groundwater (Equation 3-3) |
Cm |
|
ACH |
= |
air changes per hour |
h-1 |
|
V |
= |
volume of trench |
m3 |
|
Porvad |
= |
total soil porosity in vadose zone |
cm3/cm3 |
|
10-3 |
= |
conversion factor |
L/cm3 |
|
104 |
= |
conversion factor |
cm2/m2 |
|
3600 |
= |
conversion factor |
s/hr |
The value for R is 8.2 x 10-5. A default value of 298?K may be used for the average system absolute temperature. For some locations, however, it may be best to use the average annual temperature for a representative weather station nearby. Thirty-year climate averages for Virginia weather stations can be obtained from the Virginia State Climatology Office.
Studies of urban canyons suggest that if the ratio of trench width -- relative to wind direction -- to trench depth is less than or equal to 1, a circulation cell or cells will be set up within the trench that limits the degree of gas exchange with the atmosphere. In consultation with USEPA Region III, VDEQ has assumed an ACH in this case of 2/hr -- based upon measured ventilation rates of buildings. If the ratio of trench width to trench depth is greater than one, air exchange between the trench and above-ground atmosphere is not restricted, thus ACH is assumed to be 360/hr -- based upon the ratio of trench depth to the average wind speed.
|
Ld |
= |
Lgw - Dtrench |
(3-3) |
Where:
|
Lgw |
= |
depth to groundwater |
Cm |
|
Dtrench |
= |
depth of trench |
Cm |
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Applicable spreadsheets: |
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For chemicals that are not included on Table 3.7, the participant should calculate exposure point concentrations for air in a construction trench according to the method described above. References should be provided for the chemical-specific parameters used.
3.2.2.2 Groundwater Less Than or Equal to 15 Feet Deep
VDEQ assumes that the trench would only intercept the groundwater for a few inches since a groundwater pool of more than a few inches would likely require dewatering. Therefore trench depth should be set to equal the actual depth to groundwater at the site. Table 3.8 presents the calculations and the default input values for the equations described in this section.
|
VF |
= |
( Ki x A x F x 10-3 x 104 x 3600 ) / ( ACH x V ) |
(3-4) |
Where:
|
Ki |
= |
overall mass transfer coefficient of contaminant (Equation 3-5) |
cm/s |
|
A |
= |
area of the trench |
m2 |
|
F |
= |
fraction of floor through which contaminant can enter |
Unitless |
|
ACH |
= |
air changes per hour |
h-1 |
|
V |
= |
volume of trench |
m3 |
|
10-3 |
= |
conversion factor |
L/cm3 |
|
104 |
= |
conversion factor |
cm2/m2 |
|
3600 |
= |
conversion factor |
s/hr |
Studies of urban canyons suggest that if the ratio of trench width -- relative to wind direction -- to trench depth is less than or equal to 1, a circulation cell or cells will be set up within the trench that limits the degree of gas exchange with the atmosphere. In consultation with USEPA Region III, VDEQ has assumed an ACH in this case of 2/hr -- based upon measured ventilation rates of buildings. If the ratio of trench width to trench depth is greater than one, air exchange between the trench and above-ground atmosphere is not restricted, thus ACH is assumed to be 360/hr -- based upon the ratio of trench depth to the average wind speed.
|
Ki |
= |
1 / {(1/kiL) + [(R T) / (Hi kiG)]} |
(3-5) |
Where:
|
kiL |
= |
liquid-phase mass transfer coefficient of i (Equation 3-6) |
cm/s |
|
R |
= |
ideal gas constant |
Atm-m3/mole-?K |
|
T |
= |
average system absolute temperature |
?K |
|
Hi |
= |
Henry's Law constant of I |
atm-m3/mol |
|
KiG |
= |
gas-phase mass transfer coefficient of i (Equation 3-7) |
cm/s |
The value for R is 8.2 x 10-5. A default value of 298?K may be used for the average system absolute temperature. For some locations, however, it may be best to use the average annual temperature for a representative weather station nearby. Thirty-year climate averages for Virginia weather stations can be obtained from the Virginia State Climatology Office.
|
kiL |
= |
(MWO2/MWi)0.5 x (T/298) x kL,O2 |
(3-6) |
Where:
|
kiL |
= |
liquid-phase mass transfer coefficient of component i |
cm/s |
|
MWO2 |
= |
molecular weight of O2 |
g/mol |
|
MWi |
= |
molecular weight of component i |
g/mol |
|
kL, O2 |
= |
liquid-phase mass transfer coefficient of oxygen at 25?C |
cm/s |
The value of kL, O2 is 0.002 cm/s.
|
kiG |
= |
(MWH2O/MWi)0.335 x (T/298)1.005 x kG, H2O |
(3-7) |
Where:
|
kiG |
= |
gas-phase mass transfer coefficient of component i |
cm/s |
|
MWH2O |
= |
molecular weight of water |
g/mol |
|
kG,H2O |
= |
gas-phase mass transfer coefficient of water vapor at 25?C |
cm/s |
The value of kG, H2O is 0.833 cm/s.
(Superfund Exposure Assessment Manual, U. S. EPA, Office of Remedial Response, April, 1988.)
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Applicable spreadsheets: |
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For chemicals that are not included on Table 3.8, the participant should calculate exposure point concentrations for air in a construction trench according to the method described above. References should be provided for the chemical-specific parameters used.
3.3 Determining Chemical Intakes
In this step of the exposure assessment, chemical-specific intake levels (exposure estimates) are calculated for each receptor group and exposure pathway. Exposure estimates are expressed as a mass of chemical per unit body weight per unit time (e.g., mg/kg-day.) Chemical intakes are estimated by multiplying exposure point concentrations by intake factors as described in the following sections.
Equations and recommended exposure factors for determining chronic daily intake levels for each medium, receptor, and exposure route are presented Sections 3.3.1 through 3.3.4 and in Tables 3.10 through 3.21. The references for each of the exposure factors are provided on the tables. Exposure scenarios or factors that may require further explanation are discussed briefly in the following sections. Note that these tables should only be included for the pathways selected for inclusion in Table 3.1a or 3.1b. If the participant proposes to use an exposure factor other than the VRP default, a reference and rationale should be provided.
NOTE: VRP has developed spreadsheets that simplify the calculation of chemical intakes, cancer risks and non-cancer hazards. If you have questions about the spreadsheets, contact appropriate VRP representatives listed on the "Contact us" page.
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Applicable spreadsheets: |
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Values Used for Chronic Daily Intake Calculations: |
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|
Current or Future Resident, Construction/Utility Worker |
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Table 3.10 Groundwater/Dermal |
Table 3.13 Soil/Dermal |
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Recreational or Trespasser Scenarios: |
|
|
Table 3.16 Surface Water/Dermal |
Table 3.19 Soil or Sediment/Dermal |
The following variables are common to the exposure equations:
|
ED |
= |
exposure duration |
Years |
|
EF |
= |
exposure frequency |
days/year |
|
ET |
= |
exposure time |
Hours/day or Hours/event |
|
EV |
= |
event frequency |
events/day |
|
BW |
= |
body weight |
Kg |
|
AT |
= |
averaging time |
Days |
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Applicable spreadsheet: |
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To estimate dermal exposures from groundwater, surface water, soil or sediment, two values need to be calculated: dermally absorbed dose and an intake factor. For dermal exposures from groundwater and surface water, absorbed dose per event must also be calculated.
3.3.1.1.1 Groundwater or Surface Water
To calculate dermally absorbed dose and intake factor, respectively, from contact with groundwater or surface water, use equations 3-8 and 3-9.
|
DAD |
= |
DAevent x IF |
(3-8) |
Where:
|
DAD |
= |
dermally absorbed dose |
mg/kg-day |
|
DAevent |
= |
absorbed dose per event (Equation 3-10, 3-11 or 3-12) |
mg/cm2-event |
|
IF |
= |
intake factor (Equation 3-9) |
event-cm2/ |
|
IF |
= |
EV x SA x EF x ED x 1/BW x 1/AT |
(3-9) |
Where:
|
SA |
= |
skin surface area available for contact |
cm2 |
Carcinogenic and non-carcinogenic intake factors using VRP default parameters are given in Table 3.10 for residential, construction/utility and commercial/industrial populations and in Table 3.16 for recreational/trespasser populations.
The method to calculate DAevent depends on whether the contaminant is an inorganic or an organic chemical. Calculating DAevent for inorganics is relatively straightforward with Equation 3-10:
|
DAevent |
= |
Kp x CW x ET |
(3-10) |
Where:
|
Kp |
= |
permeability coefficient (see Table 3.22) |
cm/hr |
|
CW |
= |
concentration of chemical in water |
mg/cm3 |
To calculate DAevent for organic chemicals, first look up the value of t* (time to reach steady-state) provided in Table 3.22. Next, select ET for the appropriate receptor. Values for ET are listed in Section 3.3.2. If ET is less than t*, Equation 3-11 should be used:
|
DAevent |
= |
2 x FA x Kp x CW x SQRT((6 x tau x ET)/pi) |
(3-11) |
Where:
|
FA |
= |
Fraction Absorbed |
Unitless |
|
tau |
= |
lag time (see Table 3.22) |
Hours/event |
|
t* |
= |
time to reach steady state (see Table 3.22) |
Hours |
|
SQRT |
= |
square root |
|
|
pi |
= |
pi, or approximately 3.14 |
If ET is greater than or equal to t*, then use Equation 3-12 to calculate DAevent:
|
DAevent |
= |
FA x Kp x CW x {(ET/(1+B)) + (2 x tau x [(1 + 3B+3 x B2)/(1 + B)2]} |
(3-12) |
Where:
|
B |
= |
Permeability ratio property (see Table 3.22) |
Unitless |
Chemical-specific dermal permeability constants for water are included on Table 3.22. For chemicals not included on Table 3.22, see Risk Assessment Guidance for Superfund, Volume 1, Part E for equations to calculate chemical-specific values.
To calculate dermally absorbed dose and intake factor, respectively, from contact with soil or sediment, use equations 3-13 and 3-14.
|
DAD |
= |
CS x ABS x IF |
(3-13) |
Where:
|
CS |
= |
chemical concentration in soil or sediment |
mg/kg |
|
ABS |
= |
absorption factor (see Table 3.23) |
Unitless |
|
IF |
= |
intake factor (Equation 3-14) |
Day |
|
IF |
= |
SA x CF x AF x EF x ED x 1/BW x 1/AT |
(3-14) |
Where:
|
SA |
= |
skin surface area available |
cm2/day |
|
CF |
= |
conversion factor |
kg/mg |
|
AF |
= |
soil to skin adherence factor |
mg/cm2 |
Carcinogenic and non-carcinogenic intake factors using VRP default parameters are given in Table 3.13 for residential, construction/utility and commercial/industrial populations and in Table 3.19 for recreational/trespasser populations.
Chemical-specific dermal absorption factors for soil are given in Table 3.23.
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Applicable spreadsheets: |
3.3.1.2 Ingestion of Contaminated Media
To estimate incidental ingestion from groundwater, surface water, soil or sediment, two values need to be calculated: chronic daily intake and an intake factor.
To calculate chronic daily intake and intake factor, respectively, from ingestion of contaminated groundwater, use equations 3-15 and 3-16.
|
CDI |
= |
CW x IF |
(3-15) |
Where:
|
CDI |
= |
chronic daily intake |
mg/kg-day |
|
CW |
= |
chemical concentration in water |
mg/L |
|
IF |
= |
intake factor (Equation 3-16) |
L/kg-day |
|
IF |
= |
IR-W x EF x ED x 1/BW x 1/AT |
(3-16) |
Where:
|
IR-W |
= |
ingestion rate of water |
L/day |