vvEPA
United States
Environmental Protection
Agency
Office of
Solid Waste and
Emergency Response
Publication 9355.4-14FSA
June 1996
Soil Screening Guidance:
Fact Sheet
Office of Emergency and Remedial Response
Quick Reference Fact Sheet
This fact sheet summarizes key aspects of the U.S. Environmental Protection Agency's
(EPA) Soil Screening Guidance. The Soil Screening Guidance is a tool developed by EPA to help
standardize and accelerate the evaluation and cleanup of contaminated soils at sites on the National
Priorities List (NPL) where future residential land use is anticipated. The User's Guide provides a
simple step-by-step methodology for environmental science/engineering professionals to calculate
risk-based, site-specific soil screening levels (SSLs) for contaminants in soil that may be used to
identify areas needing further investigation at NPL sites. The Technical Background Document
presents the analysis and modeling upon which this approach is based, as well as generic SSLs
calculated using conservative default values, and guidance for conducting more detailed analysis of
complex site conditions, where needed.
SSLs are not national cleanup standards.
SSLs alone do not trigger the need for response
actions or define "unacceptable" levels of
contaminants in soil. In this guidance,
"screening" refers to the process of identifying
and defining areas, contaminants, and
conditions, at a particular site that do not
require further Federal attention. Generally, at
sites where contaminant concentrations fall
below SSLs, no further action or study is
warranted under the Comprehensive
Environmental Response, Compensation and
Liability Act (CERCLA), commonly known as
"Superfund." (Some States have developed
screening numbers that are more stringent than
the generic SSLs presented here; therefore,
further study may be warranted under State
programs.) Where contaminant concentrations
equal or exceed SSLs, further study or
investigation, but not necessarily cleanup, is
warranted.
The decision to use the Soil Screening Guidance
at a site will be driven by the potential benefits
of eliminating areas, exposure pathways, or
contaminants from further investigation. By
identifying areas where concentrations of
contaminated soil are below levels of concern
under CERCLA, the guidance provides a means
to focus resources on exposure areas,
contaminants and exposure pathways of
concern.
SSLs are risk-based concentrations derived from
standardized equations combining exposure
information assumptions with EPA toxicity
data. Three options for developing screening
levels are included in the guidance, depending on
how the numbers will be used to screen at a site,
and the amount of site-specific information that
will be collected or is available. Details of these
approaches are presented in the User's Guide
(EPA, 1996a) and the Technical Background
Document (TBD) (EPA, 1996b). The three
options for using SSLs are:
Applying generic SSLs
Developing simple, site-specific SSLs
Developing site-specific SSLs based on
more detailed modeling
The progression from generic to simple site-
specific and more detailed site-specific SSLs
1
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usually involves an increase in investigation
costs and, generally a decrease in the stringency
of the screening levels because conservative
assumptions can be replaced with less
conservative site-specific information.
Generally, the decision of which method to use
involves balancing the increased investigation
costs with the potential savings associated with
higher (but protective) SSLs. The User's Guide
focuses on the application of a simple site-
specific approach by providing a step-by-step
methodology to calculate site-specific SSLs.
The TBD provides more information about the
other approaches.
Generic SSLs for the most common
contaminants found at NPL sites are included in
the TBD. Generic SSLs are calculated from the
same equations presented in the User's Guide, but
are based on a number of default assumptions
chosen to be protective of human health for
most site conditions. Generic SSLs can be used in
place of site-specific screening levels; however,
in general, they are expected to be more
stringent than site-specific levels. The site
manager should weigh the cost of collecting the
data necessary to develop site-specific SSLs with
the potential for deriving a higher SSL that
provides an appropriate level of protection.
The TBD also includes more detailed modeling
approaches for developing screening levels that
take into account more complex site conditions
than the simple site-specific methodology
emphasized in the User's Guide. More detailed
approaches may be appropriate when site
conditions (e.g., a thick vadose zone) are
different from those assumed in the simple site-
specific methodology presented here. The
technical details supporting the methodology
used in the User's Guide are provided in the
TBD. SSLs developed in accordance with the
User's Guide are based on future residential land
use assumptions and related exposure scenarios.
Using this guidance for sites where residential
land use assumptions do not apply could result in
overly conservative screening levels; however,
EPA recognizes that some parties responsible
for sites with non-residential land use might still
find benefit in using the SSLs as a tool to
conduct a conservative initial screening.
SSLs developed in accordance with this guidance
could also be used for Resource Conservation and
Recovery Act (RCRA) corrective action sites as
"action levels," since the RCRA corrective
action program currently views the role of
action levels as generally fulfilling the same
purpose as soil screening levels.1 In addition,
States may use this guidance in their voluntary
cleanup programs, to the extent they deem
appropriate. When applying SSLs to RCRA
corrective action sites or for sites under State
voluntary cleanup programs, users of this
guidance should recognize, as stated above, that
SSLs are based on residential land use
assumptions. Where these assumptions do not
apply, other approaches for determining the
need for further study might be more
appropriate.
No further study Site-specific
warranted under cleanup
CERCLA goal/level
Response
action clearly
warranted
"Zero" Screening Response Very high
concentration level level concentration
Highlight 1. Conceptual Risk
Management Spectrum for Contaminated
Soil
1.2 Role of Soil
Levels
Screening
In identifying and managing risks at
contaminated sites, EPA considers a spectrum of
contaminant concentrations. The level of
concern associated with those concentrations
depends on the likelihood of exposure to soil
contamination at levels of potential concern to
human health or to ecological receptors.
Highlight 1 illustrates the spectrum of soil
contamination encountered at Superfund sites
and the conceptual range of risk management
responses. At one end are levels of
contamination that clearly warrant a response
action; at the other end are levels that are below
regulatory concern. Screening levels identify the
lower bound of the spectrum—levels below
1 Further information on the role of action levels in the RCRA
corrective action program is available in an Advance Notice of
Proposed Rulemaking (signed April 12, 1996).
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which there is generally no concern under
CERCLA, provided conditions associated with
the SSLs are met. Appropriate cleanup goals for
a particular site may fall anywhere within this
range depending on site-specific conditions.
EPA anticipates the use of SSLs as a tool to
facilitate prompt identification of contaminants
and exposure areas of concern during both
remedial actions and some removal actions
under CERCLA. However, the application of
this or any screening methodology is not
mandatory at sites being addressed under
CERCLA or RCRA. The framework leaves
discretion to the site manager and technical
experts (e.g., risk assessors, hydrogeologists) to
determine whether a screening approach is
appropriate for the site and, if screening is to be
used, the proper method of implementation.
The decision to use a screening approach should
be made early in the process of investigation at
the site.
EPA developed the Soil Screening Guidance to
be consistent with and to enhance the current
Superfund investigation process and anticipates
its primary use during the early stages of a
remedial investigation (RI) at NPL sites. It does
not replace the Remedial Investigation/
Feasibility Study (RI/FS), including the risk
assessment portion of the RI, but the use of
screening levels can focus sampling and risk
assessment on aspects of the site that are likely
to be a concern under CERCLA. By screening
out areas of sites, potential chemicals of
concern, or exposure pathways from further
investigation, site managers and technical
experts can limit the scope of the field
investigation or risk assessment.
SSLs can save resources by helping to determine
which areas do not require additional Federal
attention early in the process. Furthermore, data
gathered during the soil screening process can be
used in later Superfund phases, such as the
baseline risk assessment, feasibility study,
treatability study, and remedial design. This
guidance may also be appropriate for use by the
removal program when demarcation of soils
above residential risk-based numbers coincides
with the purpose and scope of the removal
action.
The simple, site-specific soil screening levels are
likely to be most useful where it is difficult to
determine whether areas of soil are
contaminated to an extent that warrants further
investigation or response (e.g., whether areas of
soil at an NPL site require further investigation
under CERCLA through an RI/FS). As noted
above, the screening levels have been developed
assuming residential land use. Although some of
the models and methods presented in this
guidance could be modified to address exposures
under other land uses, EPA has not yet
standardized assumptions for exposure scenarios
related to those other uses.
This guidance provides the information needed
to calculate SSLs for 110 chemicals. Sufficient
information may not be available to develop soil
screening levels for additional chemicals. These
chemicals should not be screened out, but should
be addressed in the baseline risk assessment for
the site. The Risk Assessment Guidance for
Superfund (RAGS), Volume 1: Human Health
Evaluation Manual (HHEM), Part A, Interim
Final (U.S. EPA, 1989a) provides guidance on
conducting baseline risk assessments for NPL
sites. In addition, the baseline risk assessment
should address the chemicals, exposure
pathways, and areas at the site that are not
screened out.
Although SSLs are "risk-based," they do not
eliminate the need to conduct a site-specific risk
assessment for those areas identified as needing
further investigation. SSLs are concentrations of
contaminants in soil that are designed to be
protective of exposures in a residential setting.
A site-specific risk assessment is an evaluation
of the risk posed by exposure to site
contaminants in various media. To calculate
SSLs, the exposure equations and pathway
models are run in reverse to backcalculate an
"acceptable level" of a contaminant in soil. For
the ingestion, dermal, and inhalation pathways,
toxicity criteria are used to define an acceptable
level of contamination in soil, based on a one-
in-a-million (10-6) individual excess cancer risk
for carcinogens and a hazard quotient (HQ) of 1
for non-carcinogens. SSLs are backcalculated for
migration to ground water pathways using
ground water concentration limits [nonzero
maximum contaminant level goals (MCLGs),
maximum contaminant levels (MCLs), or
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health-based limits (HBLs) (10-6 cancer risk or a
HQ of 1) where MCLs are not available].
SSLs can be used as Preliminary Remediation
Goals (PRGs) provided appropriate conditions
are met (i.e., conditions found at a specific site
are similar to conditions assumed in developing
the SSLs). The concept of calculating risk-based
contaminant levels in soils for use as PRGs (or
"draft" cleanup levels) was introduced in the
RAGS HHEM, Part B, Development of Risk-
Based Preliminary Remediation Goals. (U.S.
EPA, 1991b).
PRGs may then be used as the basis for
developing final cleanup levels based on the
nine-criteria analysis described in the National
Contingency Plan [Section 300.430
(3)(2)(I)(A)]. The directive entitled Role of the
Baseline Risk Assessment in Superfimd Remedy
Selection Decisions (U.S. EPA, 1991c) discusses
the modification of PRGs to generate cleanup
levels. The SSLs should only be used as cleanup
levels when a site-specific nine-criteria
evaluation of the SSLs as PRGs for soils
indicates that a selected remedy achieving the
SSLs is protective, complies with Applicable or
Relevant and Appropriate Requirements
(ARARs), and appropriately balances tradeoffs
between cleanup options with respect to the
other criteria, including cost.
1.3 Scope of Soil Screening
Guidance
In a residential setting, potential pathways of
exposure to contaminants in soil are as follows
(see Highlight 2):
• Direct ingestion
• Inhalation of volatiles and fugitive dusts
• Ingestion of contaminated ground water
caused by migration of chemicals through soil
to an underlying potable aquifer
• Dermal absorption
• Ingestion of homegrown produce that has
been contaminated via plant uptake
• Migration of volatiles into basements.
Direct Ingestion
of Ground
Water and Soil
Blowing.
Dust
Volatilization
Also Addressed:
• Plant Uptake
• Dermal Absorption
Highlight 2. Exposure Pathways Addressed by
SSLs.
The Soil Screening Guidance addresses each of
these pathways to the greatest extent practical.
The first three pathways — direct ingestion,
inhalation of volatiles and fugitive dusts, and
ingestion of potable ground water — are the
most common routes of human exposure to
contaminants in the residential setting. These
pathways have generally accepted methods,
models, and assumptions that lend themselves to
a standardized approach. The additional
pathways of exposure to soil contaminants,
dermal absorption, plant uptake, and migration
of volatiles into basements, may also contribute
to the risk to human health from exposure to
specific contaminants in a residential setting.
The guidance addresses these pathways to a
limited extent based on available empirical data.
(See Step 5 and the TBD for further discussion).
The Soil Screening Guidance addresses the
human exposure pathways listed previously and
will be appropriate for most residential settings.
The presence of additional pathways or unusual
site conditions does not preclude the use of SSLs
hi areas of the site that are currently residential
or likely to be residential in the future. However,
the risks associated with additional pathways or
conditions (e.g., fish consumption, raising of
livestock, heavy truck traffic on unpaved roads)
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should be considered in the RI/FS to determine
whether SSLs are adequately protective.
An ecological assessment should also be
performed as part of the RI/FS to evaluate
potential risks to ecological receptors.
The Soil Screening Guidance should not be
used for areas with radioactive
contaminants.
Highlight 3 provides key attributes of the Soil
Screening Guidance: User's Guide.
Highlight 3: Key Attributes of the
User's Guide
1 Standardized equations are presented
to address human exposure pathways in
a residential setting consistent with
Superfund's concept of "Reasonable
Maximum Exposure" (RME).
Source size (area and depth) can be
considered on a site-specific basis using
mass-limit models.
Parameters are identified for which site-
specific information is needed to
develop SSLs.
Default values are provided to calculate
generic SSLs when site-specific
information is not available.
SSLs are based on a 10'6 excess risk for
carcinogens or a hazard quotient of 1 for
noncarcinogens. SSLs for migration to
ground water are based on (in order of
preference): nonzero maximum
contaminant level goals (MCLGs),
maximum contaminant levels (MCLs), or
the aforementioned risk-based targets.
2.0 SOIL SCREENING PROCESS
Applying site-specific screening levels involves
developing a conceptual site model (CSM),
collecting a few easily obtained site-specific soil
parameters (such as the dry bulk density and
percent moisture), and sampling to measure
contaminant concentrations in surface and
subsurface soils. Often, much of the information
needed to develop the CSM can be derived from
previous site investigations [e.g., the
Preliminary Assessment/Site Inspection (PA/SI)]
and, if properly planned, SSL sampling can be
accomplished in one mobilization. This fact
sheet provides a brief overview of the steps in
the process. A full discussion of the steps and
their implementation is available in the User's
Guide.
The soil screening process (outlined in
Highlight 4) is a step-by-step approach that
involves:
• Developing a conceptual site model
(CSM)
• Comparing the CSM to the SSL scenario
• Defining data collection needs
• Sampling and analyzing soils at site
• Deriving site-specific SSLs, as
appropriate
• Comparing site soil contaminant
concentrations to SSLs
• Determining which areas of the site
require further study.
The overall outline is fundamentally the same,
whether you are using the simple site-specific
approach, the generic levels, or a more detailed
approach. However, the details of any specific
application will be different. In particular,
developing the simple site specific SSLs is
obviously more involved than using the generic
screening levels available in the TBD.
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Highlight 4
Soil Screening Process
Step One: Develop Conceptual Site Model
• Collect existing site data (historical records, aerial photographs, maps, PA/SI data, available background
information, State soil surveys, etc.)
• Organize and analyze existing site data
Identify known sources of contamination
Identify affected media
Identify potential migration routes, exposure pathways, and receptors
• Construct a preliminary diagram of the CSM
• Perform site reconnaissance
Confirm and/or modify CSM
Identify remaining data gaps
Step Two: Compare Soil Component of CSM to Soil Screening Scenario
• Confirm that future residential land use is a reasonable assumption for the site
• Identify pathways present at the site that are addressed by the guidance
• Identify additional pathways present at the site not addressed by the guidance
• Compare pathway-specific generic SSLs with available concentration data
• Estimate whether background levels exceed generic SSLs
Step Three: Define Data Collection Needs for Soils to Determine Which Site Areas Exceed SSLs
• Develop hypothesis about distribution of soil contamination (i.e., which areas of the site have soil
contamination that exceed appropriate SSLs?)
• Develop sampling and analysis plan for determining soil contaminant concentrations
Sampling strategy for surface soils (includes defining study boundaries, developing a decision rule,
specifying limits on decision errors, and optimizing the design)
- Sampling strategy for subsurface soils (includes defining study boundaries, developing a decision
rule, specifying limits on decision errors, and optimizing the design)
- Sampling to measure soil characteristics (bulk density, moisture content, organic carbon content,
porosity, pH)
• Determine appropriate field methods and establish QA/QC protocols
Step Four: Sample and Analyze Soils at Site
• Identify contaminants
• Delineate area and depth of sources
• Determine soil characteristics
• Revise CSM, as appropriate
Step Five: Derive Site-specific SSLs, if needed
• Identify SSL equations for relevant pathways
• Identify chemical of concern for dermal exposure and plant uptake
• Obtain site-specific input parameters from CSM summary
• Replace variables in SSL equations with site-specific data gathered in Step 4
Calculate SSLs
Account for exposure to multiple contaminants
Step Six: Compare Site Soil Contaminant Concentrations to Calculated SSLs
• For surface soils, screen out exposure areas where all composite samples do not exceed SSLs by a
factor of 2
• For subsurface soils, screen out source areas where the highest average soil core concentration does not
exceed the SSLs
• Evaluate whether background levels exceed SSLs
Step Seven: Decide How to Address Areas Identified for Further Study
• Consider likelihood that additional areas can be screened out with more data
• Integrate soil data with other media in the baseline risk assessment to estimate cumulative risk at the site
• Determine the need for action
• Use SSLs as PRGs
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However, developing site specific levels may be
worthwhile given the less stringent but equally
protective levels that will generally result.
An important part of this guidance is a
recommended sampling approach that balances
the need for more data to reduce uncertainly
with the need to limit data collection costs.
Where data are limited such that use of the
"maximum test" (Max test) presented in the
User's Guide is not appropriate, the guidance also
provides direction on the use of other
conservative estimates of contaminant
concentrations for comparison with the SSLs.
2.1 Step 1:
Developing a
Conceptual Site
Model
The conceptual site model (CSM) is a three-
dimensional "picture" of site conditions that
illustrates contaminant distributions, release
mechanisms, exposure pathways and migration
routes, and potential receptors. The CSM
documents current site conditions and is
supported by maps, cross sections, and site
diagrams that illustrate human and
environmental exposure through contaminant
release and migration to potential receptors.
Developing an accurate CSM is critical to proper
implementation of the Soil Screening Guidance.
As a key component of the RI/FS and EPA's
Data Quality Objectives (DQO) process, the
CSM should be updated and revised as
investigations produce new information about a
site. Data Quality Objectives for Superfund:
Interim Final Guidance (U.S. EPA, 1993a) and
Guidance for Conducting Remedial
Investigations and Feasibility Studies under
CERCLA (U.S. EPA, 1989c) provide a general
discussion about the development and use of the
CSM during RIs.
2.2 Step 2:
Comparing the
CSM to SSL
Scenario
In this step, the conceptual site model for a
particular site is compared to the conceptual site
model assumed for the development of the Soil
Screening Guidance. This comparison should
determine whether the SSL scenario is
sufficiently similar to the CSM so that use of the
guidance is appropriate. The Soil Screening
Guidance was developed assuming residential land
use. The primary exposure pathways associated
with residential land use (given in section 1.3)
are (1) direct ingestion, (2) inhalation of
volatile and fugitive dusts, and (3) ingestion of
contaminated ground water caused by migration
of chemicals through soil to an underlying
potable aquifer. The residential exposure
assumptions associated with these pathways are
given in Highlight 5.
Highlight 5
Residential Exposure Assumptions
Exposure frequency 350 days/year
Exposure duration 30 years
For Noncarcinogens
Body weight 15 kg
Ingestion rate 200 mg/day
For Carcinogens
Body weight .. age adjusted from 15 -70 kg
Ingestion rate age adjusted from
200 - 100 mg/day
Drinking water ingestion rate 2 L/day
Inhalation rate 20 m3/day
The CSM may include other sources and
exposure pathways that are not covered by this
guidance. Compare the CSM with the
assumptions and limitations inherent in the SSLs
to determine whether additional or more detailed
assessments are needed for any exposure
pathways or chemicals. The Soil Screening
Guidance can be used to screen those sources and
exposures pathways that are covered by the
guidance. Early identification of areas or
conditions where SSLs are not applicable is
important so that other characterization and
response efforts can be considered when
planning the sampling strategy.
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Where the following conditions exist, a more
detailed site-specific investigation will be needed:
• site adjacent to surface water,
• potential terrestrial or aquatic ecological
concerns
• other human exposure pathways likely (e.g.
local fish consumption, homegrown dairy,
livestock or other agricultural use, or
• unusual site conditions (e.g., presence of non-
aqueous phase liquids, unusually high fugitive
dust levels from site activities.)
A consideration of background concentrations
should be made to determine whether SSLs are
likely to be useful, since the SSLs have much less
utility where background concentrations exceed
the SSLs. Background concentrations exceeding
generic SSLs do not necessarily indicate that a
health threat exists, but may suggest that
additional analysis is appropriate. For example,
it may be important to determine whether the
high background concentrations are
anthropogenic or naturally occuring. Generally,
EPA does not clean up below natural
background; however, where anthropogenic
background levels exceed SSLs, EPA may
determine that some type of comprehensive
response is necessary and feasible.
2.3 Step 3:
Defining
Collection
for Soils
Data
Needs
Once the CSM has been developed and the site
manager has determined that the Soil Screening
Guidance is appropriate to use at a site, a
Sampling and Analysis Plan (SAP) should be
developed. Highlight 4 outlines the general
strategy for developing sampling plans likely to
be needed to apply the Soil Screening Guidance.
A different sampling approach is used for the
surface and subsurface because different exposure
pathways are being addressed. Sampling should
also provide site characteristics data necessary
to develop site-specific SSLs. The User's Guide
provides information on the development of
SAPs for these three types of information.
To develop sampling strategies that will
properly assess site contamination, EPA
recommends that site managers consult with the
technical experts in their Region, including risk
assessors, toxicologists, chemists and
hydrogeologists, who can assist the site manager
to use the DQO process to satisfy Superfund
program objectives. The DQO process is a
systematic planning process developed by EPA
to ensure that sufficient data are collected to
support EPA decision making. A full discussion
of the DQO process is provided in Data Quality
Objectives for Superfund: Interim Final
Guidance (U.S. EPA, 1993a) and the Guidance
for the Data Quality Objectives Process (U.S.
EPA, 1994a). Many of the key elements have
been incorporated as part of the guidance.
One of the critical decisions to make before
developing the SAP is to define the specific area
to which the Soil Screening Guidance will be
applied. Existing data (e.g., preliminary
assessment, other site investigation data,
historical documents discussing site activities)
can be used to determine what level and type of
investigation may be appropriate. Areas known
to be important sources of ground water
contamination should be sampled for subsurface
contamination, but it often will not be necessary
to develop screening levels based on surface
contamination for these areas. Sampling in
known source areas will focus on developing
remedial alternatives with some sampling to
confirm expected problems, as necessary. Other
areas may have good historical information to
indicate that no waste handling activities
occurred there and it is expected that these areas
are unlikely to be contaminated. A few samples
may be taken to confirm this hypothesis. Much
of the sampling effort for soil screening is likely
to focus on areas of uncertain contamination
levels and history. The User's Guide provides
more information about the use of historical
information, the statistical basis for the
sampling strategy, and the soil characteristics
that are needed to develop site-specific
screening levels.
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2.4 Step 4:
Sampling and
Analyzing Site
Soils
Once the sampling strategies have been
developed and implemented, the samples should
be analyzed according to the analytical
laboratory and field methods specified in the
SAP. An important outcome of these analyses
is the estimation of the concentrations of
potential contaminants of concern which will be
compared to the SSLs. At this point, the
generic SSLs may be useful for comparison
purposes. Where estimated concentrations are
above the generic SSLs, site-specific SSLs can be
calculated to provide another, less stringent but
still conservative comparison.
Because these analyses reveal new information
about the site, update the CSM accordingly.
2.5 Step 5:
Calculating Site-
specific SSLs
With the soil properties data collected in Step 4
of the screening process, site-specific soil
screening levels can now be calculated using the
equations presented in the User's Guide. The
Soil Screening Guidance provides the equations
necessary to develop a simple site-specific soil
screening levels. For a description of how these
equations were developed, as well as background
on their assumptions and limitations, consult the
TBD. When generic SSLs are being used as for
comparison to site concentration, this step may
be omitted.
All SSL equations were developed to be
consistent with reasonable maximum exposure
(RME) for the residential setting. The Superfund
program estimates the RME for chronic
exposures on a site-specific basis by combining
an average exposure-point concentration with
reasonably conservative values for intake and
duration (U.S. EPA, 1989a; RAGS HHEM,
Supplemental Guidance: Standard Default
Exposure Factors, U.S. EPA, 1991a). Thus, all
site-specific parameters (soil, aquifer, and
meteorologic parameters) used to calculate SSLs
should reflect average or typical site conditions
in order to calculate average exposure
concentrations at the site.
Exposure pathways addressed in the process for
screening surface soils include direct ingestion,
dermal contact, and inhalation of fugitive dusts.
While the guidance provides all the relevant
toxicity from EPA sources necessary to
calculate site-specific SSLs, Integrated Risk
Information System (IRIS) (U.S. EPA, 1995a)
or Health Effects Assessment Summary Tables
(HEAST) (U.S. EPA, 1995b) should be checked
for current values. Only the most current values
should be used to calculate SSLs.
The Soil Screening Guidance addresses two
exposure pathways for subsurface soils:
inhalation of volatiles and ingestion of ground
water contaminated by the migration of
contaminants through soil to an underlying
potable aquifer. Because the equations developed
to calculate SSLs for these pathways assume an
infinite source, they can violate mass-balance
considerations, especially for small sources. To
address this concern, the guidance also includes
equations for calculating mass-limit SSLs for
each of these pathways when the size (i.e., area
and depth) of the contaminated soil source is
known or can be estimated with confidence.
The Soil Screening Guidance uses a simple linear
equilibrium soil/water partition equation or a
leach test to estimate contaminant release in
soil leachate. It also uses a simple water-balance
equation to calculate a dilution factor to account
for reduction of soil leachate concentration
from mixing in an aquifer.
The methodology for developing SSLs for the
migration to ground water pathway was designed
for use during the early stages of a site
evaluation when information about subsurface
conditions may be limited. Hence, the
methodology is based on rather conservative,
simplified assumptions about the release and
transport of contaminants in the subsurface
(Highlight 6). These assumptions are inherent in
the SSL equations and should be reviewed for
consistency with the conceptual site model (see
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Step 2) to determine the applicability of SSLs to
the migration to ground water pathway.
Highlight 6: Simplifying
Assumptions for the SSL Migration
to Ground Water Pathway
• Infinite source (i.e., steady-state
concentrations are maintained over the
exposure period)
• Uniformly distributed contamination from
the surface to the top of the aquifer
• No contaminant attenuation (i.e.,
adsorption, biodegradation, chemical
degradation) in soil
• Instantaneous and linear equilibrium
soil/water partitioning
• Unconfined, unconsolidated aquifer with
homogeneous and isotropic hydrologic
properties
• Receptor well at the downgradient edge
of the source and screened within the
plume
• No contaminant attenuation in the aquifer
• No NAPLs present (if NAPLs are
present, the SSLs do not apply).
Address Exposure to Multiple Chemicals.
The SSLs generally correspond to a 1O6 excess
risk level for carcinogens and a hazard quotient
of 1 for noncarcinogens. This "target" hazard
quotient is used to calculate a soil concentration
below which it is unlikely that sensitive
populations will experience adverse health
effects. The potential for additive effects has
not been "built in" to the SSLs through
apportionment. For carcinogens, EPA believes
that setting a 1O6 excess risk level for individual
chemicals and pathways generally will lead to
cumulative site risks within the 10-4 to 1O6 risk
range for the combinations of chemicals typ-
ically found at NPL sites.
For noncarcinogens, there is no widely accepted
risk range, and EPA recognizes that cumulative
risks from noncarcinogenic contaminants at a
site could exceed the target hazard quotient.
However, EPA also recognizes that noncancer
risks should be added only for those chemicals
with the same toxic endpoint or mechanism of
action.
If more than one chemical detected at a site
affects the same target organ (i.e., has the same
critical effect as defined by the RfD
methodology), an overall hazard index (HI) for
the source (or exposure area) can be calculated.
If this HI exceeds 1, further investigation is
needed. The guidance provides a list of target
organs for all chemicals with SSLs based on
noncarcinogenic effect.
2.6 Step 6: Comparing Site
Soil Contaminant
Concentrations to
Calculated SSLs
Now that the site-specific SSLs have been
calculated for the potential contaminants of
concern, compare them with the site
contaminant concentrations. At this point, it is
reasonable to review the CSM with the actual
site data to confirm its accuracy and the overall
applicability of the Soil Screening Guidance.
Thus, for surface soils, the contaminant
concentrations in each composite sample from
an exposure area are compared to 2 times the
SSL. (When SSL DQOs were developed, 2 times
the SSL was determined to a reasonable upper
limit for comparison that would still be
protective of human health. Use of this decision
rule is appropriate only when the quantity and
quality of data are comparable to the levels
discussed in the User's Guide. For a complete
discussion for the SSL DQOs, see the TBD.) If
any composite has concentrations that equal or
exceed 2 times the SSL, the area cannot be
screened out, and further study is needed.
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However, if all composite samples are below 2
times the SSLs, no further study is needed.
For data sets of lesser quality, the 95% upper
confidence level on the arithmetic mean of
contaminant soil concentration can be
compared directly to the SSLs. The TBD
discusses strengths and weaknesses of different
calculations of the mean and when they are
appropriate for making screening decisions.
Since subsurface soils are not characterized to
the same extent as surface soils, there is less
confidence that the concentrations measured are
representative of the entire source. Thus, a
more conservative approach to screening is
warranted. Because it may not be protective to
allow for comparison to values above the SSL,
mean contaminant concentrations from each
soil boring taken in a source area are compared
with the calculated SSLs. Source areas with any
mean soil boring contaminant concentration
greater than the SSLs generally warrant further
consideration. On the other hand, where the
mean soil boring contaminant concentrations
within a source are all less than the SSLs, that
source area is generally screened out.
2.7 Step 7: Addressing Areas
Identified for
Further Study
Areas that have been identified for further study
become a subject of the RI/FS (U.S. EPA,
1989c). The results of the baseline risk
assessment conducted as part of the RI/FS will
establish the basis for taking remedial action.
The threshold for taking action differs from the
criteria used for screening. As outlined in Role of
the Baseline Risk Assessment in Superfund
Remedy Selection Decisions (U.S. EPA, 1991c),
remedial action at NPL sites is generally
warranted where cumulative risks for current or
future land use exceed IxlO-4 for carcinogens or
an HI of 1 for noncarcinogens. The data
collected for soil screening are useful in the RI
and baseline risk assessment. However,
additional data will probably need to be collected
during future site investigations. This additional
data will better define the risks and threats at the
site and could conceivably indicate that no
action is required.
Once the decision has been made that remedial
action may be appropriate, the SSLs can then
serve as PRGs. This process is referenced in
Section 1.2 of this document.
FOR FURTHER INFORMATION
The technical details (e.g, equations and
assumptions necessary to implement the soil
screening guidance are available in the Soil
Screening Guidance: User's Guide (U.S. EPA,
1996a). More detailed discussions of the
technical background and assumptions
supporting the development of the Soil
Screening Guidance are presented in the Soil
Screening Guidance: Technical Background
Document (U.S. EPA, 1996b). The final portion
of the guidance package is the Soil Screening
Guidance: Response to Comments, (U.S. EPA,
1996c) which describes changes made to the
guidance following peer review and public
comment. For additional copies of this fact
sheet, the User's Guide, the Technical
Background Document, Response to Comments,
or other EPA documents, call the National
Technical Information Service (NTIS) at (703)
487-4650 or 1-800-553-NTIS (6847).
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REFERENCES
U.S. EPA. 1989a. Risk Assessment Guidance
for Superfund (RAGS): Volume 1: Human
Health Evaluation Manual (HHEM), Part
A, Interim Final. Office of Emergency and
Remedial Response, Washington, DC.
EPA/540/1-89/002. NTIS PB90-
155581/CCE.
U.S. EPA. 1989b. Guidance for Conducting
Remedial Investigations and Feasibility
Studies under CERCLA. Office of
Emergency and Remedial Response,
Washington, DC. EPA/540/G-89/004.
OSWER Directive 9355.3-01. NTIS PB89-
184626.
U.S. EPA. 1991a. Human Health Evaluation
Manual (HHEM), Supplemental Guidance:
Standard Default Exposure Factors. Office
of Emergency and Remedial Response,
Washington, DC. Publication 9285.6-03.
NTIS PB91-921314.
U.S. EPA. 1991b. Risk Assessment Guidance
for Superfund (RAGS), Volume 1: Human
Health Evaluation Manual (HHEM), Part
B, Development of Risk-Based Preliminary
Remediation Goals. Office of Emergency
and Remedial Response, Washington, DC.
Publication 9285.7-01B. NTIS PB92-
963333.
U.S. EPA. 1991c. Role of the Baseline Risk
Assessment in Superfund Remedy Selection
Decisions. Office of Emergency and
Remedial Response, Washington, DC.
Publication 9355.0-30. NTIS PB91-
921359/CCE.
U.S. EPA. 1993a. Data Quality Objectives for
Superfund: Interim Final Guidance.
Publication 9255.9-01. Office of
Emergency and Remedial Response,
Washington, DC. EPA 540-R-93-071.
NTIS PB94-963203.
U.S. EPA. 1993b. Quality Assurance for
Superfund Environmental Data Collection
Activities. Quick Reference Fact Sheet.
Office of Emergency and Remedial
Response, Washington, DC. NTIS PB93-
963273.
U.S. EPA. 1994a. Guidance for the Data
Quality Objectives Process. Quality
Assurance Management Staff, Office of
Research and Development, Washington,
DC. EPA QA/G-4.
U.S. EPA. 1995a. Integrated Risk Information
System (IRIS). Cincinnati, OH.
U.S. EPA. 1995b. Health Effects Assessment
Summary Tables (HEAST): Annual Update,
FY1993. Environmental Criteria and
Assessment Office, Office of Health and
Environmental Assessment, Office of
Research and Development, Cincinnati,
OH.
U.S. EPA. 1996a. Soil Screening Guidance:
User's Guide, Office of Emergency and
Remedial Response, Washington, DC.
EPA/540/R-96/018. NTIS PB96-963505.
U.S. EPA. 1996b. Soil Screening Guidance:
Technical Background Document. Office
of Emergency and Remedial Response,
Washington, DC. EPA/540/R-96/128.
NTIS PB96-963502.
U.S. EPA. 1996c. Soil Screening Guidance:
Response to Comments. Office of
Emergency and Remedial Response,
Washington, DC. EPA/540/R-96/019.
NTIS PB96-963506.
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