xvEPA
United States
Environmental Protection
Agency
Office of Radiation
and Indoor Air
Office of Solid Waste and
Emergency Response
Washington, DC 20460
9355.4-16A
EPA/540-R-00-007
PB2000 963307
October 2000
ORIA/Superfund
Soil Screening
for Radionuclides: User's
Guide
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EPA/540-R-00-007
October 2000
Soil Screening Guidance
for Radionuclides:
User's Guide
Office of Radiation and Indoor Air
Office of Emergency and Remedial Response
U.S. Environmental Protection Agency
Washington, DC 20460
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ACKNOWLEDGMENTS
The development of this guidance was a team effort led by the staff of the Office of Radiation and Indoor Air (ORIA), and the
Office of Emergency and Remedial Response (OERR). Phil Newkirk and Ron Wilhelm of ORIA, and Stuart Walker of OERR,
are the principal EPA authors of the document, with significant contributions from Ken Lovelace and Janine Dinan of OERR.
Early drafts of this document and the Technical Background Document were prepared by Sandy Cohen and Associates (SC&A)
under EPA Contract 68D70073. John Mauro of SC&A led their team effort.
In addition, the authors would like to thank all EPA reviewers whose careful review and thoughtful comment greatly contributed
to the quality of this document.
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DISCLAIMER
Notice: The Soil Screening Guidance is based on policies set out in the Preamble to the Final Rule of the National Oil and
Hazardous Substances Pollution Contingency Plan (NCP), which was published onMarch 8,1990 (55 Federal Register 8666).
This guidance document sets forth recommended approaches based on EP A's best thinking to date with respect to soil screening
for radionuclides. This document does not establish binding rules. Alternative approaches for screening radionuclides in soil
may be found to be more appropriate at specific sites (e.g., where site circumstances do not match the underlying assumptions,
conditions and models of the guidance). The decision whether to use an alternative approach and a description of any such
approach should be placed in the Administrative Record for the site. Accordingly, if comments are received at individual sites
questioning the use of the approaches recommended in this guidance, the comments should be considered and an explanation
provided for the selected approach. The Soil Screening Guidance for Radionuclides: Technical Background Document (TBD)
may be helpful in responding to such comments.
The policies set out in both the Soil Screening Guidance for Radionuclides: User's Guide and the supporting TBD are intended
solely as guidance to the U.S. Environmental Protection Agency (EPA) personnel; they are not final EPA actions and do not
constitute rulemaking. These policies are not intended, nor can they be relied upon, to create any rights enforceable by any party
in litigation with the United States government. EPA officials may decide to follow the guidance provided in this document, or
to act at variance with the guidance, based on an analysis of specific site circumstances. EPA also reserves the right to change
the guidance at any time without public notice.
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TABLE OF CONTENTS
1.0 INTRODUCTION 1-1
1.1 Purpose 1-1
1.2 Role of Soil Screening Levels 1-2
1.3 Scope of Soil Screening Guidance for Radionuclides 1-6
2.0 SOIL SCREENING PROCESS 2-1
2.1 Step 1: Developing a Conceptual Site Model 2-1
2.1.1 Collect Existing Site Data 2-1
2.1.2 Organize and Analyze Existing Site Data 2-1
2.1.3 Construct a Preliminary Diagram of the CSM 2-3
2.1.4 Perform Site Reconnaissance 2-3
2.2 Step 2: Comparing CSM to SSL Scenario 2-3
2.2.1 Identify Pathways Present at the Site Addressed by Guidance 2-3
2.2.2 Identify Additional Pathways Present at the Site Not Addressed by Guidance 2-4
2.2.3 Compare Available Data to Background 2-4
2.3 Step 3: Defining Data Collection Needs for Soils 2-5
2.3.1 Stratify the Site Based on Existing Data 2-5
2.3.2 Identify Exposure Areas 2-6
2.3.3 Develop Sampling and Analysis Plan for Surface Soil 2-6
2.3.4 Develop Sampling and Analysis Plan for Subsurface Soils 2-11
2.3.5 Develop Sampling and Analysis Plan to Determine Soil Characteristics 2-13
2.3.6 Determine Analytical Methods and Establish QA/QC Protocols 2-14
2.4 Step 4: Sampling and Analyzing Site Soils & DQA 2-15
2.4.1 Delineate Area and Depth of Source 2-15
2.4.2 Perform DQA Using Sample Results 2-15
2.4.3 Revise the CSM 2-18
2.5 Step 5: Calculating Site-specific SSLs 2-18
2.5.1 SSL Equations-Surface Soils 2-19
2.5.2 SSL Equations-Subsurface Soils 2-24
2.5.3 Address Exposure to Multiple Radionuclides 2-28
2.6 Step 6: Comparing Site Soil Radionuclide Concentrations to Calculated SSLs 2-29
2.6.1 Evaluation of Data for Surface Soils 2-29
2.6.2 Evaluation of Data for Subsurface Soils 2-30
2.7 Step 7: Addressing Areas Identified for Further Study 2-30
REFERENCES R-l
in
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TABLE OF CONTENTS (continued)
ATTACHMENTS
A. Conceptual Site Model Summary A-l
B. Soil Screening DQOs for Surface Soils and Subsurface Soils
for Radionuclides Not Present in Background B-l
C. Radiological Properties for SSL Development C-l
D. Regulatory and Human Health Radiological Benchmarks Used for SSL Development D-l
LIST OF EXHIBITS
Exhibit 1 Conceptual Risk Management Spectrum for Contaminated Soil 1-2
Exhibit 2 Exposure Pathways Addressed by SSLs for Radionuclides 1-7
Exhibit 3 Key Attributes of the User's Guide 1-7
Exhibit 4 Soil Screening Process for Radionuclides 2-2
Exhibit 5 Data Quality Objectives Process 2-8
Exhibit 6 Defining the Study Boundaries 2-9
Exhibit 7 Designing a Sampling and Analysis Plan for Surface Soils 2-12
Exhibit 8 Designing a Sampling and Analysis Plan for Subsurface Soils 2-16
Exhibit 9 U.S. Department of Agriculture Soil Texture Classification 2-17
Exhibit 10 Q/C Values by Source Area, City, and Climatic Zone 2-21
Exhibit 11 Site-Specific Parameters for Calculating Subsurface SSLs 2-25
Exhibit 12 Simplifying Assumptions for the SSL Migration to Ground Water Pathway 2-26
IV
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LIST OF ACRONYMS
ARAR
ASTM
CERCLA
CLP
CSM
CV
DAF
DQA
DQO
EA
EPA
HEAST
HELP
HHEM
IRIS
ISC2
MARSSIM
MCL
MDC
NPL
NTIS
OERR
ORIA
PA/SI
PEF
PRO
QAPP
Q/C
QA/QC
RAGS
RCRA
RfD
RI
RI/FS
RME
ROD
SAB
SAP
SSL
TBD
USDA
WRS
Applicable or Relevant and Appropriate Requirement
American Society for Testing and Materials
Comprehensive Environmental Response, Compensation and Liability Act
Contract Laboratory Program
Conceptual Site Model
Coefficient of Variation
Dilution Attenuation Factor
Data Quality Assessment
Data Quality Objective
Exposure Area
Environmental Protection Agency
Health Effects Assessment Summary Table
Hydrological Evaluation of Landfill Performance
Human Health Evaluation Manual
Integrated Risk Information System
Industrial Source Complex Model
Multi-Agency Radiation Survey and Site Investigation Manual
Maximum Contaminant Level
Minimum Detectable Concentration
National Priorities List
National Technical Information Service
Office of Emergency and Remedial Response
Office of Radiation and Indoor Air
Preliminary Assessment/Site Inspection
Paniculate Emission Factor
Preliminary Remediation Goal
Quality Assurance Project
Site-Specific Dispersion Model
Quality Assurance/Quality Control
Risk Assessment Guidance for Superfund
Resource Conservation and Recovery Act
Reference Dose
Remedial Investigation
Remedial Investigation/Feasibility Study
Reasonable Maximum Exposure
Record of Decision
Science Advisory Board
Sampling and Analysis Plan
Soil Screening Level
Technical Background Document
U.S. Department of Agriculture
Wilcoxon Rank Sum
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1.0 INTRODUCTION
1.1 Purpose
The Soil Screening Guidance for Radionuclides is a tool
that the U.S. Environmental Protection Agency (EPA)
developed to help standardize and accelerate the
evaluation and cleanup of soils contaminated with
radioactive materials at sites on the National Priorities
List (NPL) with future residential land use.1 This
guidance provides a methodology for environmental
science/engineering professionals with a background in
radiological risk assessmentto calculate risk-based, site-
specific, soil screening levels (SSLs) for radionuclides
in soil that may be used to identify areas needing further
investigation at NPL sites. :
SSLs are not national cleanup standards. SSLs alone
do not trigger the need for response actions or define
"unacceptable" levels of radionuclides in soil. In this
guidance, "screening" refers to the process of
identifying and defining areas, radionuclides, and
conditions, at a particular site that do not require further
Federal attention. Generally, at sites where radionuclide
concentrations fall below SSLs, no further action or
study is warranted under the Comprehensive
Environmental Response, Compensation and Liability
Act (CERCLA). Generally, where radionuclide
concentrations equal or exceed SSLs, further study or
investigation, but not necessarily cleanup, is warranted.
This radionuclide SSL guidance is a continuation of
other EPA documents related to SSL for chemicals.
These include EPA's Soil Screening Guidance: User's
Guide (U.S. EPA, 1996a) and the Soil Screening
Guidance: Technical Background'Document(U.S. EPA,
1996b) that apply the SSL framework to NPL sites with
hazardous organic and inorganic soil contaminants.
They do not address sites with radioactive contaminants.
These documents provide standardized exposure
equations for deriving generic and site-specific SSLs for
chemicals under a residential land use setting, assuming
three soil exposure pathwayssoil ingestion, inhalation
Note that the Superfimd program defines "soil" as having a
particle size under 2 mm, while the RCRA program allows for particles
under 9 mm in size.
of volatiles and fugitive dusts, and ingestion of
contaminated ground water. Chemical- specific SSLs
are based on a target risk of one-in-a-million (10~6) for
carcinogens, a hazard quotient of 1 for noncarcinogens,
or, for the ground water migration pathway, a nonzero
maximum contaminant level goal (MCLG), maximum
contaminant level (MCL), or a risk-based level. For
each contaminant, the lowest pathway-specific SSL is
selected as the appropriate screening level.
An overview of a comparison between the key features
of the soil screening frameworks for chemicals and
radionuclides is provided in Table 1 below. Much of the
guidance for radionuclides is based on or cites
information presented in the chemical Soil Screening
Guidance documents. Users are therefore strongly
encouraged to become familiar with these documents.
This guidance elaborates a framework developed for soil
screening levels for radionuclides that is consistent and
compatible with the SSL framework for chemicals.
Radionuclide SSLs are risk-based concentrations, in
activity units of picocuries per gram of soil (pCi/g),
derived from equations combining exposure information
assumptions with EPA radiotoxicity data. This 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 and is part
of a larger framework that includes both generic and
more detailed approaches to calculating screening levels.
The Soil Screening Guidance for Radionuclides:
Technical Background Document (TBD) (U.S. EPA,
2000), provides detailed information about these other
approaches. Generic SSLs for the most common
radionuclides found at NPL sites are included in the
TBD. Generic SSLs are calculated from the same
equations presented in this guidance, 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
conservative 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 framework presented in 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
1-1
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emphasized in this guidance. More detailed approaches
may be appropriate when site conditions (e.g., very
deep water table, very thick uncontaminated unsaturated
zone, soils underlain by karst or fractured rock aquifers)
are different from those assumed in the simple site-
specific methodology presented here. The technical
details supporting the methodology used in this guidance
are provided in the TBD.
SSLs developed in accordance with this guidance are
based on future residential land use assumptions and
related exposure pathways. 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.2 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.
1.2 Role of Soil Screening Levels
In identifying and managing risks at sites, EPA
considers a spectrum of radionuclide concentrations.
The level of concern associated with those
concentrations depends on the likelihood of exposure to
radioactive soil contamination at levels of potential
concern to human health.
Exhibit 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 warrant no further study
under CERCLA. Screening levels identify the lower
bound of the spectrumlevels below which EPA
believes no further study is warranted 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.
No further study
warranted under
CERCLA
Site-specific
cleanup
goal/level
Response
action clearly
warranted
"Zero"
concentration
Screening
level
Response
level
Very high
concentration
Exhibit 1. Conceptual Risk Management Spectrum
for Contaminated Soil
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 1996).
EPA anticipates the use of SSLs as a tool to facilitate
prompt identification of radionuclides 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. If
comments are received at individual sites questioning
the use of the approaches recommended in this
guidance, the comments should be considered and an
explanation provided as part of the site's Record of
Decision (ROD). 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 for
Radionuclides 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) or risk
assessment, but use of screening levels can focus the RI
and risk assessment on aspects of the site that are more
likely to be a concern under CERCLA. By screening out
1-2
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areas of sites, potential radionuclides of concern, or
exposure pathways from further investigation, site
managers and technical experts can limit the scope of
the remedial 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 process presented in this guidance to develop and
apply simple, site-specific soil screening levels is 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 those other uses.
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 radionuclide levels 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.
An important part of this guidance is a recommended
sampling approach that balances the need for more data
to reduce uncertainty with the need to limit data
collection costs.
Knowledge of background radionuclide concentrations
at the site is critical when screening site soils, since
facility operations may have contaminated site soils with
some of the same radionuclides that are found naturally-
occurring in background soil. In many cases, the
concentration of the radionuclide of concern in
background soil, and the variability of the background
soil concentration, may be much greater than the
screening level. In these situations, the site manager
should not exclude the radionuclide of potential concern
from being evaluated in the risk assessment, as the
contamination from the facility may pose a threat to
human health and the environment. Risk management
options for the radionuclides of concern will be
evaluated in the CERCLA remedy selection process.
This guidance provides the information needed to
calculate SSLs for 60 radionuclides (See Attachment C
for list of radionuclides). Sufficient information may
not be available to develop soil screening levels for
additional radionuclides. These radionuclides 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
radionuclides, 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.
SSLs are concentrations of radionuclides 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 radionuclides in
soil. For each pathway, radiotoxicity criteria are used to
define an acceptable level of contamination in soil,
based on a one-in-a-million (10~6) individual excess
lifetime cancer risk. SSLs are backcalculated for the
migration to ground water pathway using ground water
concentration limits [maximum contaminant levels
(MCLs)].
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Table 1. Comparison of Soil Screening Frameworks for Chemicals and Radionuclides
Guidance
Chemicals
Radionuclides
Comments
Applicable land use expo-
Residential only
Same as chemicals
sure scenanos
EPA may include additional
guidance for other land uses
(e.g., commercial/ industrial,
suburban, playground, and
hunter/fisher) in future up-
dates.
Target receptor
RME individual
Same as chemicals
Ecological receptors are not
addressed
Standardized equations
for deriving SSLs for soil
exposure pathways
Soil ingestion
Inhalation of volatiles
and fugitive dusts
Ingestion of potable
ground water contain-
ing chemicals leached
from soil
Identifies dermal absorp-
tion, plant uptake, and
migration of volatiles into
basement pathways but
does not calculate SSLs
for these pathways
Soil ingestion
Inhalation fugitive
dusts
Ingestion of potable
ground water contain-
ing radionuclides
leached from soil
Direct external radia-
tion exposure
Ingestion of home
grown fruits and vege-
tables
Chemical-specific SSLs are
expressed in mass concen-
tration units of milligrams of
contaminant per kilogram of
soil (mg/kg). Radionu-clide-
specific SSLs are expressed
in activity concentration
units of picocuries per gram
of soil (pCi/g). Additional
equations are required for
radionuclides to account for
other significant soil expo-
sure pathways while some
chemical pathways are not
applicable to radionuclides.
Basis for SSLs
Target risk limit of
10"6 for carcinogens
Hazard quotient of 1
for noncarcinogens
Nonzero MCLGs or
MCLs (whichever is
most protective), or if
neither were available
risk-based limits, for
the ground water mi-
gration pathway
Uses same target risk
limit as chemicals
Uses MCLs, proposed
MCLs (for uranium),
or risk-based limits for
the ground water mi-
gration pathway for
radionuclides
EPA classifies all radionu-
clides as known human
(Group A) carcinogens. For
noncarcinogenic chemicals,
nonzero MCLGs are consid-
ered (if available). MCLs
exist for almost every
radionuclide.
Default values for the
age-adjusted soil inges-
tion factor
IF,
soil/adj
114 mg-yr/kg-day
T soil/adj
120 mg-yr/day
The radionuclide slope fac-
tors for soil ingestion use a
biokinetic model that
accounts for the age and sex
weighted mass of the
affected organs. Therefore,
it is not necessary to include
the mass of the receptor in
the default IFsofl/adj for
radionuclides.
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Table 1. Comparison of Soil Screening Frameworks for Chemicals and Radionuclides
Guidance
Chemicals
Radionuclides
Comments
Default values for the
dilution/attenuation factor
(DAF) and the paniculate
emission factor (PEF)
DAF = 20
PEF=1.32E+9m3/kg
Same as chemicals
The default PEF is the same
as for chemicals. A key as-
sumption in the derivation of
the PEF is that the 1/2 acre
lot has only 50% vegetative
cover. Although the inges-
tion of homegrown produce
is not quantitatively evalu-
ated in the SSG for chemi-
cals, the assumption of 50%
vegetative cover allows for
the presence of a family gar-
den.
Soil measurement/
verification of guidelines
Measured average soil
contaminant concentra-
tions in exposure areas
of concern
Exposure area (EA) for
averaging concentra-
tions: 0.5 acres (resi-
dential lot)
Averaging depth for
surface soils: 0-2 cm
Evaluation depth for
subsurface soil con-
tamination: surface to
the limit of detectable
contamination or to the
top of the saturated
zone
Number of surface soil
samples required:
Based on site-specific
conditions or a default
value of 6 randomly-
selected specimens
composited into 4 sam-
ples for analyses.
Number of subsurface
soil samples required:
For each source area,
takes 2 or 3 soil bor-
ings in areas suspected
of having the highest
contaminant concentra-
tions.
Measures same param-
eter as for chemicals
Uses same exposure
area (EA) as chemicals
Averaging depth for
surface soils: 0-15 cm
Uses same evaluation
depth for subsurface
soil contamination as
for chemicals
Uses same number of
surface soil samples as
for chemicals.
Uses same number of
subsurface soil sam-
ples as for chemicals
Conducts surface scans
for small areas of
elevated activity
See Step 3, Defining Data
Collection Needs for Soils
for more detailed guidance.
1-5
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One exception to the above approach is uranium, which
presents both chemical and radiological hazards. SSLs
for uranium must consider both of these types of
hazards. As a general rule, the radiological hazard
dominates inhalation of insoluble forms of uranium,
while the chemical toxicity is the major hazard from
intake of soluble forms of uranium. Chemical toxicity
of uranium in the kidney has been a concern in
establishing health protection standards for workers and
the general public for many years. EPA developed for
its rulemaking addressing radionuclide MCLs an
updated oral RfD for uranium of 0.6 ^g/kg/day (U.S.
EPA, 1998c). SSLs for uranium should be calculated
using both the radiological guidelines presented in this
document and the approach provided in the Soil
Screening Guidance for non-carcinogenic chemicals.
Since the SSL is a numerical concentration, it should be
based on the most protective health quantity whether it
be kidney toxicity or radiological risk.
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, 1991c). The models, equations, and
assumptions presented in the Soil Screening
Guidance for Radionuclides supersede those
described in RAGS HHEM, Part B, for residential
soils. In addition, this guidance presents
methodologies to address the leaching of
contaminants through soil to an underlying potable
aquifer. This pathway should be addressed in the
development of PRGs.
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 Superfund Remedy
Selection Decisions (U.S. EPA, 1991d) 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 the other criteria, including cost. Note that
potential soil ARARs exist for several of the more
common naturally-occurring radionuclide s (226Ra, 228Ra,
230Th, 232Th, 235U, and 238U) under 40 CFR Part
192.12(a), Part 192.32(b)(2), and Part 192.41, and 10
CFR Part 40 Appendix A, I, Criterion 6(6). For further
guidance on using these ARARs, see OSWER Directive
9200.4-25 (U.S. EPA, 1998b), dated February 12, 1998
and OSWER Directive 9200.4-35P (U.S. EPA, 2000a),
dated April 11, 2000. The equations presented in this
document supersede those described in RAGS HHEM,
Part B, and should be used to determine PRGs and RGs.
1.3 Scope of Soil Screening Guidance for
Radionuclides
In a residential setting, potential pathways of exposure
to radionuclides in soil included in this guidance are as
follows (see Exhibit 2):
Direct ingestion of soil
Inhalation of fugitive dusts
Ingestion of contaminated ground water caused by
migration of radionuclides through soil to an
underlying potable aquifer
External radiation exposure from photon-emitting
radionuclides in soil
Ingestion of homegrown produce that has been
contaminated via plant uptake
1-6
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Produce IngesUon
xxxx:
son .III . Radlau
y^O^l
Groundwater
Exhibit 2: Exposure Pathways Addressed by
SSLs for Radionuclides
The Soil Screening Guidance for Radionuclides
addresses each of these pathways to the greatest extent
practical. The mode of exposure to radionuclides is
different than that of chemicals. This renders some
chemical pathways inapplicable to radionuclides (e.g.,
inhalation of volatiles, dermal absorption) while adding
other pathways unique to radiation (e.g., external
exposure to photons emitted by radionuclides). The
radiological pathways listed above represent the most
likely exposure mechanisms for individuals in a
residential setting. The external exposure pathway is,
for most radionuclides, the dominant exposure and
typically represents the most significant risk. For some
radionuclides, the ingestion of contaminated produce
and drinking water constitute the most likely exposure
pathways provided that these items are obtained from
onsite sources. The inhalation of fugitive dust pathway
is included in the analysis; however, it is of significance
for only a very few radionuclides. All of these pathways
have generally accepted radiological risk methods,
models, and assumptions that lend themselves to a
standardized approach.
The Soil Screening Guidance for Radionuclides
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 in 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 for meat or milk consumption,
fugitive dusts caused by heavy truck traffic on
unpaved roads) should be considered in the RI/FS to
determine whether SSLs are adequately protective.
The Soil Screening Guidance for Radionuclides
should not be used for screening out areas with
chemical contaminants.
Exhibit 3 provides key attributes of the Soil Screening
Guidance for Radionuclides: User's Guide.
Exhibit 3: Key Attributes of the
User's Guide
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 parameter values are provided to
calculate generic SSLs when site-specific
information is not available.
SSLs forthe migration to ground water pathway
are based on maximum contaminant levels
(MCLs), while SSLs for all other pathways are
based on a 10"6 lifetime cancer risk to an
individual.
Radiation risk coefficients used to calculate
SSLs represent the average risk per unit
exposure to members of a population exposed
throughout life to a constant concentration of a
radionuclide in a specific environmental
medium. They assume no radioactive decay.
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2.0 SOIL SCREENING PROCESS
The soil screening process (Exhibit 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
Calculating site-specific SSLs
Comparing site soil radionuclide concentrations to
calculated SSLs
Determining which areas of the site require further
study.
It is important to follow this process to implement the
Soil Screening Guidance for Radionuclides properly.
The remainder of this guidance discusses each activity
in detail.
2.1 Step 1: Developing a
Conceptual Site Model
The CSM is a three-dimensional "picture" of site
conditions that illustrates radionuclide 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 radionuclide release
and migration to potential receptors. Developing an
accurate CSM is critical to proper implementation of the
Soil Screening Guidance for Radionuclides.
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.
Developing the CSM involves several steps, discussed
in the following subsections.
2.1.1 Collect Existing Site Data. The initial
design of the CSM is based on existing site data
compiled during previous studies. These data may
include site sampling data, historical records, aerial
photographs, maps, and State soil surveys, as well as
information on local and regional conditions re levant to
radionuclide migration and potential receptors. Data
sources include Superfund site assessment documents
(i.e., the PA/SI), documentation of removal actions, and
records of other site characterizations or actions.
Published information on local and regional climate,
soils, hydrogeology, and ecology may be useful. In
addition, information on the population and land use at
and surrounding the site will be important to identify
potential exposure pathways and receptors. The RI/FS
guidance (U.S. EPA, 1989c) discusses collection of
existing data during RI scoping, including an extensive
list of potential data sources. The Multi-Agency
Radiation Survey and Site Investigation Manual
(MARSSIM) (U.S. EPA, 1997b) [Section 3.4] discusses
the collection of existing data specific to sites
contaminated with radioactive materials.
2.1.2 Organize and Analyze Existing Site
Data. One of the most important aspects of the CSM
development process is to identify and characterize all
potential exposure pathways and receptors at the site by
considering site conditions, relevant exposure scenarios,
and the properties of radionuclides present in site soils.
Attachment A, the Conceptual Site Model Summary,
provides four forms for organizing site data for soil
screening purposes. The CSM summary organizes site
data according to general site information, soil
radionuclide source characteristics, exposure pathways
and receptors.
Note: If a CSM has already been developed for the site
in question, use the summary forms in Attachment A to
ensure that it is adequate.
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Exhibit 4
Soil Screening Process for Radionuclides
Step One: Develop Conceptual Site Model
Collect existing site data (historical records such as previous surveys and sampling data, site operating 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 and potential contaminants
Identify potentially contaminated areas and 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
Stratify the site based on existing data
Identify exposure areas
Develop sampling and analysis plan for determining mean soil radionuclide concentrations
Determine appropriate survey instruments and techniques and establish QA/QC protocols
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, porosity, soil texture, pH)
Determine appropriate field methods and establish QA/QC protocols
Step Four: Sample and Analyze Soils at Site
Identify radionuclides
Delineate area and depth of sources and identify non-impacted areas as appropriate
Determine soil characteristics
Conduct preliminary data review
Revise CSM, as appropriate
Step Five: Derive Site-specific SSLs, if needed
Identify SSL equations for relevant pathways
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
Select appropriate statistical tests and verify test assumptions
For surface soils, screen out exposure areas where all composite samples do not exceed SSLs by a factor of two
For subsurface soils, screen out source areas where the highest average soil core concentration does not exceed
the SSLs
Step Seven: Decide How to Address Areas Identified for Further Study
Review and confirm the data that led to the decision
Consider likelihood that additional areas can be screened out by collecting additional 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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2.1.3
Construct a Preliminary
Diagram Of the CSM. Once the existing site data
have been organized and a basic understanding of the
site has been attained, draw a preliminary "sketch" of
the site conditions, highlighting source areas, potential
exposure pathways, and receptors. Ultimately, when site
investigations are complete, this sketch will be refined
into a three-dimensional diagram that summarizes the
data. Also, a brief summary of the contamination
problem should accompany the CSM. Attachment A
provides an example of a complete CSM summary.
2.1.4 Perform Site Reconnaissance. At
this point, a site visit would be useful because conditions
at the site may have changed since the PA/SI was
performed (e.g., removal actions may have been taken).
During site reconnaissance, update site
sketches/topographic maps with the locations of
buildings, source areas, wells, and sensitive
environments. Anecdotal information from nearby
residents or site workers may reveal undocumented
disposal practices and thus previously unknown areas of
contamination that may affect the current CSM
interpretation.
Based on the new information gained from site
reconnaissance, update the CSM as appropriate.
Identify any remaining data gaps in the CSM so that
these data needs can be incorporated into the Sampling
and Analysis Plan (SAP).
2.2 Step 2: Comparing CSM to
SSL Scenario
The Soil Screening Guidance for Radionuclides is likely
to be appropriate for sites where residential land use is
reasonably anticipated. However, 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
radionuclides. 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.
2.2.1 Identify Pathways Present at the
Site Addressed by Guidance. The following are
potential pathways of exposure to radioactive soil
contaminants in a residential setting and are addressed
by this guidance document:
Direct ingestion of soil
Inhalation of fugitive dusts
Ingestion of contaminated ground water caused by
migration of radionuclides through soil to an under-
lying potable aquifer
External radiation exposure from radionuclides in soil
Ingestion of homegrown produce that has been
contaminated via plant uptake
This guidance quantitatively addresses each of these
pathways. Whether some or all of the pathways are
relevant at the site depends upon the radionuclides and
conditions at the site.
For surface soils under the residential land use
assumption, the external exposure pathway will typically
be the dominant exposure pathway for most
radionuclides (e.g., 54Mn, 60Co, 137Cs, etc.). For some
radionuclides (e.g., 3H, "Tc, 129I, etc.), the ground water
pathway often dominates, although not to the extent that
the external exposure pathway does. The plant ingestion
pathway and soil ingestion pathway also play a
dominant role for a few radionuclides of interest (for
plant ingestion - 14C, 63Ni, 90Sr, etc.; for soil ingestion -
241Am, 244Cm, 230Th, 232Th, etc.). In the majority of
cases, the inhalation of fugitive dust pathway plays an
insignificant role.
For subsurface soils, risks from migration of
radionuclides to an underlying aquifer is the only
potential concern for this scenario. Volatilization is not
included as a pathway since it is a concern for only a
very limited number of radionuclides (such as 3H and
14C). The majority of all radionuclides are present in
soil as nonvolatile ionic species or inorganic compounds
(i.e., Henry's law constant is zero). Thus, volatilization
and subsequent inhalation has not been included. If 3H
or 14C volatilization is a concern, an approach similar to
that in the Soil Screening Guidance for chemicals can be
used to model the exposure. Consideration of the
ground water pathway may be eliminated if ground
water beneath or adjacent to the site is not a potential
source of drinking water. Coordinate this decision on a
site-specific basis with State or local authorities
responsible for ground water use and classification. The
20
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rationale for excluding this exposure pathway should be
consistent with EPA ground water policy (U.S. EPA,
1988a, 1990a, 1992a, 1992c, and 1993b).
In addition to the more common pathways of exposure
in a residential setting, concerns have been raised
regarding the potential for migration of radon from
subsurface soils into basements. The dominant factor in
indoor radon levels is home construction practices and
the extent to which these practices employ radon-
resistant techniques. Homes built atop soil with
identical levels of radium can have orders of magnitude
differences in indoor radon levels depending on the
extent to which radon-resistant techniques are used. As
NORM, radium is present in all soils. Reducing the
radium content in the soil may not result in any
reduction in indoor radon levels. However, taking
simple and inexpensive steps in home construction will
ensure that radon levels in homes are kept below ARAR
levels. For existing homes with elevated levels of radon,
a variety of methods can be used to reduce radon
concentrations to ARAR levels. Discussion of radon
mitigation standards may be found in several EPA
publications, including Radon Mitigation Standards,
EPA 402-R-93-078. Also note that potential soil
Applicable or Relevant and Appropriate Requirements
(ARARs) exist for radon under 192.12(b)(l) and
192.41(b). For further guidance on using these ARARs,
see the August 1997 memorandum from Stephen Luftig
(OERR) and Larry Weinstock (ORIA) titled
"Establishment of cleanup levels for CERCLA sites with
radioactive contamination," OSWER Directive 9200.4-
18, (U.S. EPA, 1997c).
2.2.2 Identify Additional Pathways
Present at the Site Not Addressed by
Guidance. The presence of additional pathways does
not preclude the use of SSLs in site areas that are
currently residential or likely to be residential in the
future. However, the risks associated with these
additional pathways should also be considered in the
RI/FS to determine whether SSLs are adequately pro-
tective. Where the following conditions exist, a more
detailed site-specific study should be performed:
The site is adjacent to bodies of surface water
where the potential for contamination of surface
water by overland flow or release of contaminated
ground water into surface water through seeps should
be considered.
There are potential terrestrial or aquatic ecological
concerns.
There are other likely human exposure pathways
that were not considered in development of the SSLs
(e.g., local fish consumption; raising of beef, dairy, or
other livestock; recreational activities such as
playground activities, hunting and fishing,
construction activities).
There are unusual site conditions such as large areas
of contamination, unusually high fugitive dust levels
due to soil being tilled for agricultural use, or heavy
traffic on unpaved roads.
There are certain subsurface site conditions such as
karst, fractured rock aquifers, or contamination
extending below the water table, that result in the
screening models not being sufficiently conservative.
There is the probability of prolonged skin contact
with high levels of high energy beta-emitting
contaminants for periods of time (several years), and
all other pathways show a very low risk. The skin
contact exposure pathway is normally several orders
of magnitude lower than either the inhalation,
ingestion, or external exposure pathway (depending
on the radionuclide, see Section 2.2.1) due to very
low risk coefficients and normal hygiene practices
(washing skin routinely).
2.2.3 Compare Available Data to
Background. EPA may be concerned with two types
of radioactivity background at sites: naturally-occurring
and anthropogenic. Naturally-occurring background
radiation is much more ubiquitous in the environment
than naturally-occurring background chemicals.
Natural background radiation includes terrestrial
radionuclides, cosmic radiation and cosmogenic
radionuclide s. Anthropogenic background consists of
manmade isotopes which are distributed in the
environment due primarily to releases from nuclear
weapons testing and to the very small, but measurable
releases from nuclear facilities.
A comparison of available data (e.g., State soil surveys
or other sources of soil radioactivity analyses) on local
background concentrations with generic SSLs may
indicate whether background concentrations at the site
are elevated. Generally, EPA does not cleanup below
natural background levels; however, where
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anthropogenic background levels exceed SSLs and EPA
has determined that a response action is necessary and
feasible, EPA's goal will be to develop a comprehensive
response to address area soils. This will often require
coordination with different authorities that have
jurisdiction over other sources of contamination in the
area. This will help avoid response actions that create
"clean islands" amid widespread contamination.
Knowledge of background radionuclide concentrations
at the site is critical when screening site soils, since
facility operations may have contaminated site soils with
some of the same radionuclide s that are found naturally-
occurring in background soil. In many cases, the
concentration of the radionuclide of concern in
background soil, and the variability of the background
soil concentration, may be much greater than the
screening level. In these situations, the site manager
should not exclude the radionuclide of potential concern
from being evaluated in the risk assessment, as the
contamination from the facility may pose a threat to
human health and the environment. Risk management
options for the radionuclides of concern will be
evaluated in the CERCLA remedy selection process.
Note that potential soil ARARs exist for several of the
more common naturally-occurring radionuclides (226Ra,
228Ra, 230Th, 232Th, 235U, and 238U) under 40 CFR Part
192.12(a), Part 192.32(b)(2), and Part 192.41, and 10
CFR Part 40 Appendix A, I, Criterion 6(6). For further
guidance on using these ARARs, see OSWER Directive
9200.4-25 (U.S. EPA, 1998b), dated February 12, 1998
and OSWER Directive 9200.4-35P (U.S. EPA, 2000a),
dated April 11,2000.
2.3 Step 3: Defining Data
Collection Needs for
Soils
Once the CSM has been developed and the site manager
has determined that the Soil Screening Guidance for
Radionuclides is appropriate to use at a site, an
Sampling and Analysis Plan (SAP) should be developed.
Attachment A, the Conceptual Site Model Summary,
lists the data needed to apply the Soil Screening
Guidance for Radionuclides. The summary will help
identify data gaps in the CSM that require collection of
site-specific data. The soil SAP is likely to contain
different sampling strategies that address:
Surface soil
Subsurface soil
Soil characteristics
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, health physicists,
chemists and hydrogeologists. These experts can assist
the site manager to use the Data Quality Objectives
(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. Using the
DQO Process ensures that the type, quantity, and quality
of environmental data used in decision making will be
appropriate for the intended medium. 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, 1994b). In addition,
MARSSIM provides extensive discussions of the DQO
Process as it is applied to conducting radiation site
surveys.
Most key elements of the DQO process have already
been incorporated as part of this Soil Screening
Guidance for Radionuclides. Exhibit 5 shows the
general components of the DQO process as it is applied
to environmental data analysis. Detailed DQOs for the
soil screening process are provided in Attachment B.
Exhibit 6 expands upon step 4 of the DQO process, and
provides additional guidance to define site study
boundaries The remaining elements involve identifying
the site-specific information needed to calculate SSLs.
The following sections present an overview of the
sampling strategies needed to use the Soil Screening
Guidance for Radionuclides. For a more detailed
discussion, see the supporting Soil Screening Guidance
for Radionuclides: Technical Background Document
(TBD).
2.3.1 Stratify the Site Based on Existing
Data. At this point in the soil screening process,
existing data can be used to stratify the site into three
types of areas requiring different levels of investigation:
Areas unlikely to be contaminated
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Areas known to be highly contaminated
Areas that may be contaminated and cannot be
ruled out.
Areas that are unlikely to be contaminated generally will
not require further investigation if historical site use
information or other site data, which are reasonably
complete and accurate, confirm this assumption. These
may be areas of the site that were completely
undisturbed by activities at the facility.
A crude estimate of the degree of soil contamination can
be made for other areas of the site by comparing site
concentrations to the generic SSLs in Appendix A of the
TBD. Generic SSLs have been calculated for 60
radionuclides using default values in the SSL equations,
resulting in conservative values that will be protective
for the majority of site conditions.
The pathway-specific generic SSLs can be compared
with available concentration data from previous site
investigations or removal actions to help divide the site
into areas with similar levels of soil contamination and
develop appropriate sampling strategies.
The surface soil sampling strategy discussed in this
document is most appropriate for those areas that may
be contaminated and can not be designated as
uncontaminated. Areas which are known to be
contaminated (based on existing data) will be
investigated and characterized in the RI/FS.
2.3.2 Identify Exposure Areas. An exposure
area (EA) is a physical area of a specified size and
shape for which a separate decision will be made as to
whether or not the area exceeds the screening criteria.
To facilitate survey design and ensure that the number of
survey data points for a specific site are relatively
uniformly distributed among areas of similar
contamination potential, the site is divided into EAs that
share a common history or other characteristics, or are
naturally distinguishable from other portions of the site
(see Exhibit 6).
An EA should not include areas that have potentially
different levels of contamination. The EA's
characteristics should be generally consistent with the
SSL exposure pathway modeling. EAs should be
limited in size based on classification, exposure pathway
modeling assumptions, and site-specific conditions.
This guidance suggests an upper bound for the size of an
EA is 2,000 m2 (0.5-acre).
This limitation on EA size is intended to ensure that
each area is assigned an adequate number of data points.
Because the number of samples is independent of the
EA size, limiting the size of an EA ensures that the
default sample density does not exceed 333 m2 per
sample. This also serves to limit the sample spacing.
The statistical basis for the default sample number is
provided in Section 3.3.3.
2.3.3 Develop Sampling and Analysis
Plan for Surface Soil. The surface soil sampling
strategy is designed to collect the data needed to
evaluate exposures via direct ingestion of soil, inhalation
of fugitive dusts, external radiation exposure, ingestion
of homegrown produce pathways, as well as migration
of contaminants to groundwater.
The SAP developed for surface soils should specify
sampling and analytical procedures as well as the
development of QA/QC procedures. To identify the
appropriate analytical procedures, the screening levels
must be known. If data are not available to calculate
site-specific SSLs (Section 2.5.1), then the generic SSLs
in Appendix A of the TBD should be used.
The depth over which surface soils are sampled should
reflect the CSM and the pathway assumptions that form
the basis for the SSL determination. The residential
setting used to develop the SSLs for each pathway
assumes that: 1) there is no clean cover of soil; 2) the
top few centimeters of soil are available for
re suspension in air; 3) the top 15 cm of contaminated
soil are homogenized by agricultural activities (e.g.,
plowing); 4) there is a sufficiently large area and depth
of contamination to approximate an infinite slab source
for external exposure purposes; 5) there is enough land
for the residential garden to supply one-half of the
residents' annual produce consumption; and, 6) while
the plant root system grows to a depth of 1 meter, most
plant nutrients are obtained from within the upper 20 cm
of soil. Further discussion of the basis for these
assumptions is provided in the appropriate pathway
discussions in Section 2.5.1.
Note that the size, shape, and orientation of sampling
volume (i.e., "support") for heterogenous media have a
significant effect on reported measurement values.
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Sample characteristics such as sample depth, volume,
area, moisture level, and composition, as well as sample
preparation techniques which may alter the sample, are
important planning considerations for Data Quality
Objectives. Comparison of data from methods that are
based on different supports can be difficult. Defining
the sampling support is important in the early stages of
site characterization. This maybe accomplished through
the DQO process with existing knowledge of the site,
contamination, and identification of the exposure
pathways that need to be characterized. Refer to
Preparation of Soil Sampling Protocols: Sampling
Techniques and Strategies (U.S. EPA, 1992e) for more
information about soil sampling support.
As explained in the Supplemental Guidance to RAGS:
Calculating the Concentration Term (U.S. EPA, 1992d),
an individual is assumed to move randomly across an
exposure area (EA) over time, spending equivalent
amounts of time in each location. Thus, the
concentration contacted overtime is best represented by
the spatially averaged concentration over the EA.
Ideally, the surface soil sampling strategy would
determine the true population mean of radionuclide soil
concentrations in an EA. Because determination of the
"true" mean would require extensive sampling at high
costs, the maximum radionuclide concentration from
composite samples is used as a conservative estimate of
the mean.
The number of samples required to satisfy the DQOs for
the survey is then based on the selection of a statistical
test, which in turn is based on whether or not the
radionuclide of concern is present in background. For
guidance when the radionuclide of concern is present in
background, refer to the TBD.
Radionuclide Not Present in Background. For
those radionuclides that are not generally present in
background, measurement of background soil
concentration is not necessary and radionuclide
concentrations are compared directly with the screening
level. With only a single set of EA samples, the
statistical test used here is called a one-sample test. The
one-sample test may also be used for those radionuclides
that are present in background but are found only at a
small fraction of the SSL. In this case, the background
contribution is included in the radioactivity in the
samples for the EA. Thus, the total concentration is
compared to the screening level. This option should
only be used if one expects that ignoring the background
concentration will not affect the outcome of the
statistical test. The advantage of ignoring a small
background contribution is that a background reference
area is not required and no background sampling is
needed. This may simplify the soil screening process
considerably.
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Exhibit 5: Data Quality Objectives Process
1. State the Problem
Summarize the contamination problem that will require new environmental
data, and identify the resources available to resolve the problem.
2. Identify the Decision
Identify the decision that requires new environmental
data to address the contamination problem.
3. Identify Inputs to the Decision
Identify the information needed to support the decision, and
specify which inputs require new environmental measurements.
4. Define the Study Boundaries
Specify the spatial and temporal aspects of the environmental
media that the data m ust represent to s upport the decision.
5. Develop a Decision Rule
Develop a logical "if... then ..." statement that defines the conditions that
would cause the decision maker to choose among alternative actions.
6. Specify Limits on Decision Errors
Specify the decision maker's acceptable limits on decision errors, which are
used to establish performance goals for limiting uncertainty in the data.
7. Optimize the Design for Obtaining Data
Identify the most resource-effective sampling and analysis design
fora data that are ex pected to satisfy the DQOs.
-------
Exhibit 6: Defining the Study Boundaries
Study Boundaries
1. Define Geographic Area
of the Investigation
2. Define Population
of Interest
Subsurface Soil.
Surface Soil (usually top 15 centimeters)
\
3. Stratify the Site
Area of Suspected
Area Unlikely to be Contamination
Contaminated
Water Table
(Saturated Zone)
Area of Known
Contamination
(possible source)
4. Define Scale of Decision Making for Surface or Subsurface Soils
SURFACE SOILS SUBSURFACE SOILS
0.5-acre exposure areas (EAs)
Contaminant Source
Back to Exhibit 5 Step 5, "Develop a Decision Rule"
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The Max test, which is used when the radionuclide of
concern is not generally present in background, is a
simple decision rule comparing the maximum
radionuclide concentrations of composite samples with
soil screening levels. Another, more complex strategy
called the Sign test is presented in Part 6 of the TBD.
The User's Guide for Radionuclides uses the Max test
rather than the Sign test to maintain consistency with the
approach taken in the SSG for chemicals. While the
Sign test is a more complex statistical method than the
Max test, it is based on the same statistical null
hypothesis (i.e., the EA requires further investigation).
Some EAs that cannot be screened out with the Max test
could be screened out with the Sign test since it uses a
less conservative estimate of the mean concentration
than does the Max test.
In addition to determining the mean concentration of a
radionuclide in an EA, it is important to identify the
presence of small areas of elevated activity. This is
done by the performance of scanning surveys. The
sensitivity of scanning surveys will be insufficient to
detect small areas of elevated activity for most
radionuclides with levels of contamination as low as
those of the SSLs calculated for large areas of uniform
contamination. However, standard scanning survey
techniques may be able to detect SSLs calculated for
smaller areas of contamination. Scan surveys are
intended to provide a degree of confidence that any
significant areas of elevated activity are identified.
Therefore, scanning surveys should be performed for all
EAs prior to sampling. The extent of the survey
coverage should be dictated by the potential for small
areas of elevated activity in the EA. EAs with a high
potential for small areas of elevated activity should
receive 100% coverage. In EAs with a very low
potential for small areas of elevated activity, scanning
surveys should be performed in at least 10% of the area.
In such cases, the areas selected for scan should be those
with highest potential based on professional judgement.
Due to the limited sensitivity of scan surveys, any small
areas of elevated activity found during the survey should
be identified for further investigation (i.e., not screened
out).
Exhibit 7 provides a summary of SAP design
considerations for EAs when the radionuclides of
concern for surface soils are not present in background.
The following strategy can be used for surface soils to
estimate the mean concentration of radionuclides in an
EA when the radionuclide of concern is not present in
background.
Divide areas to be sampled in the screening process
into 0.5-acre exposure areas, the size of a suburban
residential lot. If the site is currently residential, the
exposure area should be the actual residential lot
size. The exposure areas should not be laid out in
such a way that they unnecessarily combine areas of
high and low levels of contamination. The
orientation and exact location of the EA, relative to
the distribution of the contaminant in the soil, can
lead to instances where sampling the EA may have
contaminant concentration results above the mean,
and in other instances, results below the mean.
Composite surface soil samples. Because the
objective of surface soil screening is to estimate the
mean contaminant concentration, the physical
"averaging" that occurs during compositing is
consistent with the intended use of the data.
Compositing allows sampling of a larger number of
locations while controlling analytical costs, since
several individual samples are physically mixed
(homogenized) and one or more subsamples are
drawn from the mixture and submitted for analysis.
Strive to achieve a Type I (false negative) error rate
of 5 percent (i.e., in only 5 percent of the cases, soil
contamination is assumed to be below the screening
level when it is really above the screening level).
EPA also strives to achieve a 20 percent Type II
(false positive) error rate (i.e., in only 20 percent of
the cases, soil contamination is assumed to be above
the screening level when it is really below the
screening level). These error rate goals influence
the number of samples to be collected in each
exposure area. For this guidance, EPA has defined
the "gray region" as one-half to 2 times the SSL.
Thus, the width of the gray region, also known as
the shift, A, is equal to 1.5 times the SSL. Refer to
Section 2.6 for further discussion.
The default sample size chosen for this guidance
(see Exhibit 7) provides adequate coverage for a
coefficient of variation (CV) based upon 250
percent variability in contaminant values (CV=2.5).
(If a CV larger than 2.5 is expected, use an
appropriate sample size from the table in Exhibit 7
of the User's Guide, or tables in the TBD.)
2-10
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Take six composite samples for each exposure area
with each composite sample made up of four
individual samples. Exhibit 7 shows other sample
sizes needed to achieve the decision error rates for
other CVs. Collect the composites randomly across
the EA and through the top 15 centimeters of soil,
which are of greatest concern for the external
exposure and consumption of homegrown produce
pathways.
Analyze the six samples per exposure area to
determine the radionuclides present and their
concentrations.
For further information on compositing across or within
EA sectors, developing a random sampling strategy, and
determining sample sizes that control decision error
rates, refer to the TBD.
Note that the Max test requires a Data Quality
Assessment (DQA) test following sampling and analysis
(Section 2.4.2) to ensure that the DQOs (i.e., decision
error rate goals) are achieved. If DQOs are not met,
additional sampling may be required.
2.3.4 Develop Sampling and
Analysis Plan for Subsurface Soils. The
subsurface and surface soil sampling strategies differ
because the exposure mechanisms differ. Exposure to
surface contaminants occurs as individuals move around
a residential lot. The surface soil sampling strategy
reflects this type of exposure.
In general, exposure to subsurface contamination occurs
when radionuclides migrate down to an underlying
aquifer. Thus, subsurface sampling focuses on
collecting the data required for modeling the migration
to ground water pathway. Measurements of soil
characteristics and estimates of the area and depth of
contamination and the average contaminant
concentration in each source area are needed to supply
the data necessary to calculate the migration to ground
water SSLs.
Source areas are the decision units for subsurface soils.
A source area is defined by the horizontal extent, and
vertical extent or depth of contamination. Sites with
multiple sources should develop separate SSLs for
each source.
The SAP developed for subsurface soils should specify
sampling and analytical procedures as well as the
development of QA/QC procedures. To identify the
appropriate procedures, the SSLs must be known. If
data are not available to calculate site-specific SSLs
(Section 2.5.2), the generic SSLs in Appendix A of the
TBD should be used.
The primary goal of the subsurface sampling strategy is
to estimate the mean radionuclide concentration and
average soil characteristics within the source area. As
with the surface soil sampling strategy, the subsurface
soil sampling strategy follows the DQO process (see
Exhibits 5, 6, and 8). Exhibit 8 provides a summary of
SAP design considerations for subsurface soils. If the
radionuclide of concern is not present in background, the
decision rule is based on comparing the mean
radionuclide concentration within each contaminant
source with source-specific SSLs.
Current investigative techniques and statistical methods
cannot accurately determine the mean concentration of
subsurface soils within a contaminated source without a
costly and intensive sampling program that is well
beyond the level of effort generally appropriate for
screening. Thus, conservative assumptions should be
used to develop hypotheses on likely contaminant
distributions.
2-11
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Exhibit 7: Designing a Sampling and Analysis Plan for Surface Soils
Radionuclide Not Present in Background
1. Subdivide Site
Into EAs
A7
2. Divide EA
Into a Grid
3. Organize
Surface
Sampling
Program for
EA
O
05 O2
1
o
O6
O4
o
Q
60
For surface soils, the
individual unit for
decision making is an
"EA," or exposure area.
It measures 0.5 acre in
area or less.
This step defines the
number of specimens
(N) that will make up one
composite sample.
a. Placement of sample locations
on the grid was developed
using a default sample size of
6 (which is based on
acceptable error rates fora CV
of 2.5) and a stratified random
sampling pattern.
b. Potential for small areas of
elevated activity determines
degree of scan coverage.
If the EA CV is suspected to be greater than 2.5, use the table below to select an
adequate sample size or refer to the TBD for other sample design options.
Probability of Decision Error at 0.5 SSL and 2 SSL Using Max Test
Sample Size"
6
7
8
9
CV=2.5a
E0.5°
C d
E2.0
CV=3.0
E0.5
E2.0
CV=3.5
E0.5
E2.0
CV=4.0
E0.5
E2.0
C = 4 specimens per composite e.
0.21
0.25
0.25
0.28
0.08
0.05
0.04
0.03
0.28
0.31
0.36
0.36
0.11
0.08
0.05
0.04
0.31
0.36
0.42
0.44
0.11
0.09
0.07
0.07
0.35
0.41
0.41
0.48
0.16
0.15
0.09
0.08
a The CV is the coefficient of variation for individual, uncomposited measurements across the entire EA,
including measurement error.
b Sample size (N) = number of composite samples
c EQ 5 = Probability of requiring further investigation when the EA mean is 0.5 SSL
d £2 Q = Probability of not requiring further investigation when the EA mean is 2.0 SSL
e C = number of specimens per composite sample, when each composite consists of points from a stratified
random or systemic grid sample from across the entire EA.
NOTE: All decision error rates are based on 1,000 simulations that assume that each composite is representative
of the entire EA, half the EAhas concentrations below the limit of detection, and half the EAhas concentrations
that follow a gamma distribution (a conservative distributional assumption).
2-12
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This guidance bases the decision to investigate a source
area further on the highest mean soil boring contaminant
concentration within the source, reflecting the
conservative assumption that the highest mean
subsurface soil boring concentration among a set of
borings taken from the source area represents the mean
of the entire source area. Similarly, estimates of
contaminant depths should be conservative. The
investigation should include the maximum depth of
contamination encountered within the source without
going below the water table.
For each source, the guidance recommends taking 2 or
3 soil borings located in the areas suspected of having
the highest contaminant concentrations within the
source. These subsurface soil sampling locations are
based primarily on knowledge of likely surface soil
contamination patterns (see Exhibit 6) and subsurface
conditions. However, buried sources may not be
discernible at the surface. Information on past practices
at the site included in the CSM can help identify
subsurface source areas.
Take soil cores from the soil boring using either split
spoon sampling or other appropriate sampling methods.
Description and Sampling of Contaminated Soils: A
Field Pocket Guide (U.S. EPA, 1991f), and Subsurface
Characterization and Monitoring Techniques: A Desk
Reference Guide, Vol. I&II(U.S. EPA, 1993e), can be
consulted for information on appropriate subsurface
sampling methods. For radioactive contaminants, core
samples may also be obtained and monitored intact in
the field to determine if layers of radioactivity are
present. In addition, the use of a subsurface sampling
technique, which results in a borehole or soil face, may
be "logged" using a gamma scintillation detector. This
enables scanning of the exposed soil surface to identify
radioactive contamination within small fractions of hole
depth, thus facilitating the identification of the presence
and depth distribution of subsurface radioactivity. This
information may be used to direct further core sampling
and laboratory analysis as warranted.
Sampling should begin at the ground surface and
continue until either no contamination is encountered or
the water table is reached. Subsurface sampling
intervals can be adjusted at a site to accommodate
site-specific information on subsurface contaminant
distributions and geological conditions (e.g., very
deep water table, very thick uncontaminated unsaturated
zone, user well far beyond edge of site, soils underlain
by karst or fractured rock aquifers). Sample splits and
subsampling should be performed according to
Preparation of Soil Sampling Protocols: Sampling
Techniques and Strategies (U.S. EPA, 1992e).
If each subsurface soil core segment represents the same
subsurface soil interval (e.g., 2 feet), the average
concentration from the surface to the depth of
contamination is the simple arithmetic average of
contaminant concentrations measured for core samples
representative of each of the 2-foot segments from the
surface to the depth of contamination. However, if the
sample intervals are not all of the same length (e.g.,
some are 2 feet while others are 1 foot or 6 inches), the
calculation of the average concentration in the total core
must account for the different lengths of the segments.
If Cj is the concentration measure in a core sample,
representative of a core interval or segment of length li;
and the n-th segment is considered to be the last segment
sampled in the core (i.e., the n-th segment is at the depth
of contamination), the average concentration in the core
from the surface to the depth of contamination should be
calculated as the following depth-weighted average ( c )
Alternatively, the average boring concentration can be
determined by adding the total contaminant activities
together (from the sample results) for all sample
segments to get the total contaminant activity for the
boring. The total contaminant activity is then divided
by the total dry weight of the core (as determined by the
dry bulk density measurements) to estimate average soil
boring concentration.
Finally, the soil investigation for the migration to ground
water pathway should not be conducted independently
of ground water investigations. Contaminated ground
water may indicate the presence of a nearby source area
that would leach contaminants from soil into aquifer
systems.
2.3.5 Develop Sampling and Analysis Plan to
Determine Soil Characteristics. The soil
parameters necessary for SSL calculations are soil
texture, dry bulk density, and pH. Although laboratory
measurements of these parameters cannot be obtained
under Superfund's Contract Laboratory Program (CLP),
independent soil testing laboratories across the country
can perform these tests at a relatively low cost.
2-13
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To appropriately apply the migration-to-ground water
models, average or typical soil properties should be used
for a source in the SSL equations (see Step 5). Take
samples for measuring soil parameters with samples for
measuring contaminant concentrations. If possible,
consider splitting single samples for contaminant and
soil parameter measurements. A number of soil testing
laboratories can handle and test radioactive samples.
However, if testing contaminated samples for soil
parameters is a problem, samples may be obtained from
clean areas of the site as long as they represent the same
soil texture and are taken from approximately the same
depth as the contaminant concentration samples.
Soil Texture. Soil texture class (e.g., loam, sand, silt
loam) is necessary to estimate average soil moisture
conditions and to apply the Hydrological Evaluation of
Landfill Performance (HELP) model to estimate
infiltration rates (see Attachment A). The appropriate
texture classification is determined by a particle size
analysis and the U.S. Department of Agriculture
(USDA) soil textural triangle shown in Exhibit 9. This
classification system is based on the USDA soil particle
size classification.
The particle size analysis method in Gee and Bauder
(1986) can provide this particle size distribution. Other
methods are appropriate as long as they provide the
same particle size breakpoints for sand/silt (0.05 mm)
and silt/clay (0.002 mm). Field methods are an
alternative for determining soil textural class. Exhibit 9
presents an example from Brady (1990).
Dry Bulk Density. Dry soil bulk density (pb) is used
to calculate total soil porosity and can be determined for
any soil horizon by weighing a thin-walled tube soil
sample (e.g., Shelby tube) of known volume and
subtracting the tube weight [American Society for
Testing and Materials (ASTM) D 2937]. Determine
moisture content (ASTM 2216) on a subsample of the
tube sample to adjust field bulk density to dry bulk
density. The other methods (e.g., ASTM D 1556, D
2167, D 2922) are generally applicable only to surface
soil horizons and are not appropriate for subsurface
characterization. ASTM soil testing methods are readily
available in the Annual Book of ASTM Standards,
Volume 4.08, Soil and Rock; Building Stones, available
from ASTM, 100 Barr Harbor Drive, West
Conshohocken, PA, 19428.
pH. Soil pH is used to select site-specific partition
coefficients. This simple measurement is made with a
pH meter in a soil/water slurry (McLean, 1982) and may
be measured in the field using a portable pH meter.
2.3.6 Determine Analytical Methods and
Establish QA/QC Protocols. Assemble a list of
feasible sampling and analytical survey methods during
this step.
Routinely, radiological soil surveys are conducted using
a mix of three types of radiation measurement methods:
1) scans, 2) direct measurements, and 3) sampling and
laboratory analysis. Based on the potential radionuclide
contaminants and their associated radiations, the
detection sensitivities of various instruments and
techniques are determined and documented. Methods
must not only be chosen based on their reliability and
suitability to the physical and environmental conditions
at the site, but they must be capable of detecting the
radionuclides of concern to the appropriate minimum
detectable concentration (MDC). During survey design,
it is generally considered good practice to select a
measurement system with an MDC between 10-50% of
the SSL.
For soil screening purposes, most SSLs for
radionuclides are too low to be detected using scans and
direct measurements. Therefore, sampling and
laboratory analysis must be the primary means of soil
screening for the majority of radionuclides. Once the
survey design and sampling methods are selected,
appropriate standard operating procedures (SOPs)
should be developed and documented. Both sample
depth and area are considerations in determining
appropriate sample volume, and sample volume is a key
consideration for determining the laboratory MDC. The
depth should also correlate with the CSM developed for
the site.
Field methods will be useful in defining the study
boundaries (i.e., areaand depth of contamination) during
both site reconnaissance and sampling. The design and
capabilities of field portable instrumentation are rapidly
evolving. Documents describing the standard operating
procedures for field instruments are available though the
National Technical Information Service (NTIS).
Additionally, MARSSIM provides further information
on field (Chapter 6) and laboratory (Chapter 7)
2-14
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measurement methods and instrumentation for
radionuclides. Appendix H of MARSSIM describes
typical field and laboratory equipment plus associated
cost and instrument sensitivities. MARSSIM also
discusses the concept of detection sensitivity and
provides guidance on determining sensitivities and
selecting appropriate measurement methods.
SAP quality control may be thought of in three parts: 1)
determining the type of QC samples needed to detect
precision or bias; 2) determining the number of samples
as part of the survey design; and 3) scheduling sample
collections throughout the survey process to identify and
control sources of error and uncertainties.
Because a great amount of variability and bias can exist
in the collection, subsampling, and analysis of soil
samples, some effort should be made to characterize this
variability and bias. A Rationale for the Assessment of
Errors in the Sampling of Soils (U.S. EPA, 1990c)
outlines an approach that advocates the use of a suite of
QA/QC samples to assess variability and bias. Field
duplicates and splits are some of the best indicators of
overall variability in the sampling and analytical
processes. At least 10 percent of both the discrete
samples and the composites should be split and sent to
a laboratory for confirmatory analysis. (Quality
Assurance for Superfund Environmental Data Collection
Activities, U.S. EPA, 1993c).
Regardless of whether surface or subsurface soils are
sampled, the Superfund quality assurance program
guidance (U.S. EPA, 1993c) should be consulted. In
addition, Specifications and Guidelines for Quality
Systems for Environmental Data Collection and
Environmental Technology Program (ANSI/ASCQ,
1994) describes a basic set of specifications and
guidelines by which a quality system for programs
involving environmental data collection and
environmental technology can be planned, implemented,
and assessed. Standard limits on the precision and bias
of sampling and analytical operations conducted during
sampling do apply and should be followed to give
consistent and defensible results.
2.4 Step 4: Sampling and Analyzing
Site Soils & DQA
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. Results of the analyses should identify the
concentrations of potential radionuclides of concern for
which site-specific SSLs will be calculated.
2.4.1 Delineate Area and Depth of Source.
Both spatial area and depth data, as well as soil
characteristic data, are needed to calculate site-specific
SSLs for the external exposure and migration to ground
water pathways. Site information from the CSM or prior
surveys can be used to estimate the areal extent of the
sources.
2.4.2 Perform DQA Using Sample
Results. Data Quality Assessment (DQA) is a
scientific and statistical evaluation that determines if the
data are of the right type, quality, and quantity to
support their intended use. The nature of the DQA is
dependent upon whether the radionuclide of concern is
present in background. For guidance for performing
DQA when the radionuclide of concern is present in
background, refer to the TBD. The following is a
discussion of DQA for radionuclides not present in
background.
Radionuclide Not Present in Background. After
sampling has been completed, a DQA should be
conducted if all composite samples are less than 2 times
the SSL. This is necessary to determine if the original
CV estimate (2.5), and hence the number of samples
collected (6), was adequate for screening surface soils.
2-15
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Exhibit 8: Designing a Sampling and Analysis Plan for Subsurface Soils
Radionuclide Not Present in Background
1. Delineate Source Area
2. Choose
Subsurface
Soil Sampling
Locations
3. Design Subsurface
Sampling and Analysis
Plan
Lab/Field
Analysis for soil
parameters
Soil Boring
(depth below grou nd surface in feet)
For screening purposes, EPA
recommendsdrilling 2 to 3
boringspersourcearea and
an equivalent number in a
backg-ound reference aiea
(when radionucide ispresent
in background) in a re as of
highestsuspected
concentraions. Soil sampling
should not extend pastv\ater
table or saturated zone.
Lab Analysis for soil
contaminants in source
area and background
reference area (when
radionuclide is present
in background)
10
Pictire depictsa contiruous borirg with 2foot segments. Forinformatbn on other methodssuch asinterval sampling and
depthweighted analyss, pleaserefer b 2.3.3 cf the User's Guice or 4.2of the TBD.
b Soil Texture, Dry Bulk Density , Soil Orgaric Carbon Moisture Content, pH. Retain samples for possibe discrete contamhant
samplhg.
2-16
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Exhibit 9: U.S. Department of Agriculture soil texture classification
100
Percent Clay
Percent Silt
\ 70
100
100
90
80
70
60 50 40
Percent Sand
30
20
10
Criteria Used with the Field Method for Determining Soil Texture Classes
(Source: Brady, 1990)
Criterion
Sand
Sandy loam
Loam
Silt loam
Clay loam
Clay
1. Individual grains Yes Yes Some Few
visible to eye
2. Stability of dry Do not form Do not form Easily Moderately
clods
No
Hard and
broken easily broken stable
3. Stability of wet Unstable Slightly stable Moderately Stable
clods
stable
Very stable
No
Very hard
and stable
Very stable
4. Stability of Does not Does not form Does not formBroken appearance Thin, will break Very long,
"ribbon" when form
wet soil rubbed
between thumb
and fingers
Particle Size,
mm
0.002 0.05 0.10 0.25
U.S.
Department C
of Agriculture
ay Silt
Very Fine Fine M
0.5 1.0
ed. Coarse Very Co
Sand
flexible
2.0
arse
Gravel
Source: USDA.
2-17
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To conduct the DQA for a composite sample whose
mean is below 2 SSL, first calculate the sample CV for
the EA in question from the sample mean ( ^), the
number of specimens per composite sample (C), and
sample standard deviation(s) as follows:
* s
Use the sample size table in Exhibit 7 to check, for this
CV, whether the sample size is adequate to meet the
DQOs for the sampling effort. If sampling DQOs are
not met, supplementary sampling may be needed to
achieve DQOs.
However, for EAs with small sample means (e.g., all
composites are less than the SSL), the sample CV
calculated using the equation above may not be a
reliable estimate of the population CV (i.e., as %
approaches zero, the sample CV will approach infinity).
To protect against unnecessary additional sampling in
such cases, compare all composites against the formula
SSL^-y'C. If the maximum composite sample
concentration is below the value given by the equation,
then the sample size may be assumed to be adequate and
no further DQA is necessary. In other words, EPA
believes that the default sample size will adequately
support walk-away decisions when all composites are
well below the SSL. The TBD describes the
development of this formula and provides additional
information on implementing the DQA process.
2.4.3 Revise the CSM. Because these analyses
reveal new information about the site, update the CSM
accordingly. This revision could include identification
of site areas that exceed the generic SSLs.
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 this
section. For a description of how these equations were
developed, as well as background on their assumptions
and limitations, consult the TBD.
In the SSG for chemicals, SSLs are expressed in mass
units of mg/kg (i.e., mg of chemical per kg of soil). The
concentrations of radioactive material in soil could also
be expressed in units of mass. Instead, they are
expressed in the traditional radiological units of pCi/g
(i.e., picograms of activity per gram of soil). These
units reflect the number of atoms of the isotope
undergoing radioactive transformation (referred to as
radioactive decay) per unit time. For more information
concerning activity and mass, refer to appendix B of the
TBD.
All SSL equations were developed to be consistent with
RME in 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, Exposure Factors Handbook,
U.S. EPA, 1997a). 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.
Equations for calculating SSLs are presented for surface
and subsurface soils in the following sections. For each
equation, site-specific input parameters are
highlighted in bold and default values are provided
for use when site-specific data are not available.
Although these defaults are not worst case, they are
conservative. At most sites, higher, but still protective
SSLs can be calculated using site-specific data. The
TBD describes development of these default values and
presents generic SSLs calculated using the default
values.
Attachment D provides toxicity criteria for 60
radionuclides commonly found at NPL sites. These
criteria were obtained from the Health Effects
Assessment Summary Tables (HEAST), which is
regularly updated. Prior to calculating SSLs at a site,
check all relevant - radionuclide-specific values in
Attachment D against values from HEAST at the
following internet webpage
http://www.epa.gov/superfund/programs/risk/calctool.
htm. Only the most current values should be used to
calculate SSLs.
2-18
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Where toxicity values have been updated, the generic
SSLs should also be recalculated with current toxicity
information.
2.5.1 SSL Equations-Surface Soils.
Exposure pathways addressed in the process for
screening surface soils include direct ingestion of soil,
inhalation of fugitive dusts, ingestion of contaminated
ground water, external radiation exposure, and ingestion
of homegrown produce.
Direct Ingestion of Soil. The Soil Screening
Guidance for Radionuclides addresses chronic exposure
to radionuclides through direct ingestion of
contaminated soil in a residential setting.
A number of studies have shown that inadvertent
ingestion of soil is common among children age 6 and
younger (Calabrese et al., 1989; Davis et al., 1990: Van
Wijnen et al., 1990). In some cases, children may ingest
large amounts of soil (i.e., 3 to 5 grams) in a single
event. This behavior, known as pica, may result in
relatively high short-term exposures to radionuclides in
soil.
Default values are used for all input parameters in the
direct ingestion equations. The amount of data required
to derive site-specific values for these parameters (e.g.,
soil ingestion rates, chemical-specific bioavailability)
makes their collection and use impracticable for
screening. Therefore, site-specific data are not generally
available for this exposure route. The generic ingestion
SSLs presented in Appendix A of the TBD are
recommended for all NPL sites.
However, for radionuclides, both the magnitude and
duration of exposure are important. Duration is critical
because the toxicity criteria are based on "lifetime
average daily dose." Therefore, the total intake,
whether it be over 5 years or 50 years, is averaged over
a lifetime of 70 years. To be protective of exposures to
radionuclides in the residential setting, Superfund
focuses on exposures to individuals who may live in the
same residence for a high-end period of time (e.g., 30
years) because exposure to soil is higher during
childhood and decreases with age. Equation 1 uses a
time-weighted average soil ingestion rate for children
and adults. The derivation of this time-weighted average
is presented in U.S. EPA, 1991c.
Equation 1: Screening Level Equation for Ingestion of
Radionuclides in Residential Soil
cc; _ TR
^^L soil ing 3
Parameter/Definition (units)
TR/target cancer risk (unitless)
SFs/soil ingestion slope factor
(pci)-1
IRs/soil ingestion rate (mg/d)
1x10"3/conversion factor (g/mg)
EF/exposure frequency (d/yr)
ED/exposure duration (yr)
Default
ID'6
See Attachment D
120 (age-averaged)
350
30
Inhalation of Fugitive Dusts. Inhalation of fugitive
dusts is a consideration in surface soils.
Equation 2 is used to calculate fugitive dust SSLs for
radionuclides. This equation requires calculation of a
particulate emission factor (PEF, Equation 3) that relates
the concentration of contaminant in soil to the
concentration of dust particles in air. This PEF
represents an annual average emission rate based on
wind erosion that should be compared with chronic
health criteria. It is not appropriate for evaluating the
potential for more acute exposures.
Both the emissions portion and the dispersion portion
(Q/C) of the PEF equation have been updated since the
first publication of RAGS HHEM, Part B, in 1991. As
in Part B, the emissions part of the PEF equation is
based on the "unlimited reservoir" model developed to
estimate particulate emissions due to wind erosion
(Cowherd et al., 1985). The box model in RAGS
HHEM, Part B has been replaced with a Q/C term
derived from the modeling exercise using the AREA-ST
model incorporated into EPA's Industrial Source
Complex Model (ISC2) platform. The AREA-ST model
was run with a full year of meteorological data for 29
U.S. locations selected to be representative of a range of
meteorologic conditions across the nation (EQ, 1993).
The results of these modeling runs are presented in
Exhibit 10 for square area sources of 0.5 to 30 acres in
size.
When developing a site-specific PEF for the inhalation
pathway, place the site into a climatic zone (see
Attachment B). Then select a Q/C value from Exhibit
10 that best represents a site's size and meteorological
2-19
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conditions.
Additional information on the update of the PEF
equation is provided in the TBD. Cowherd etal. (1985)
present methods for site-specific measurement of the
parameters necessary to calculate a PEF.
The default PEF for radionuclides presented in Equation
2 is the same as the one given in the SSG for chemicals.
The default parameter values shown in Equation 3 have
been chosen using the guidance of Cowherd et al.
(1985), based upon the assumption of a family garden.
The calculated PEF thus accounts for the increase in the
fugitive dust concentration anticipated with an area of
tilled soils.
Equation 2: Screening Level Equation for Inhalation
of Radioactive Fugitive Dusts from
Residential Soil
SSL,,
TR
SF -
)xlx;o*3x£7?x£T)x[£T + (£T x DP)]
} L ° ^ ' '"
Parameter/Definition (units)
TR/target cancer risk (unitless)
SF,/inhalation slope factor (pCi~1)
IR,/inhalation rate (m3/d)
PEF/particulate emission
factor (m3/kg)
1x10+3/conversion factor (g/kg)
EF/exposure frequency (d/yr)
ED/exposure duration (yr)
ETo/exposure time fraction,
outdoor (unitless)
ET/exposure time fraction,
indoor (unitless)
DF/dilution factor for indoor
inhalation, (unitless)
Default
10-6
See Attachment D
20
1.32x10+9
(Equation 3)
350
30
0.073
0.683
0.4
Equation 3: Derivation of the Particulate Emission
Factor
PEF = QIC
3600
0.036 x (1 - V) x (UJUf x p(x)
Parameter/Definition (units)
PEF/particulate emission factor
(m3/kg)
Q/C/inverse of mean cone, at
center of a 0.5-acre-square
source (g/m2-s per kg/m3)
V/fraction of vegetative cover
(unitless)
Um/mean annual windspeed (mis)
11,/equivalent threshold value of
windspeed at 7 m (mis)
F(x)/function dependent on Um/U,
derived using Cowherd et al.
(1985) (unitless)
Default
1.32x10+g
90.80
0.5 (50%)
4.69
11.32
0.194
2-20
-------
Exhibit 10 . QIC Values by Source Area, City, and Climatic Zone
Zone I
Seattle
Salem
Zone II
Fresno
Los Angeles
San Francisco
Zone III
Las Vegas
Phoenix
Albuquerque
Zone IV
Boise
Winnemucca
Salt Lake City
Casper
Denver
Zone V
Bismark
Minneapolis
Lincoln
Zone VI
Little Rock
Houston
Atlanta
Charleston
Raleigh-Durham
Zone VII
Chicago
Cleveland
Huntington
Harrisburg
Zone VIM
Portland
Hartford
Philadelphia
Zone IX
Miami
0.5 Acre
82.72
73.44
62.00
68.81
89.51
95.55
64.04
84.18
69.41
69.23
78.09
100.13
75.59
83.39
90.80
81.64
73.63
79.25
77.08
74.89
77.26
97.78
83.22
53.89
81.90
74.23
71.35
90.24
85.61
1 Acre
72.62
64.42
54.37
60.24
78.51
83.87
56.07
73.82
60.88
60.67
68.47
87.87
66.27
73.07
79.68
71.47
64.51
69.47
67.56
65.65
67.75
85.81
73.06
47.24
71.87
65.01
62.55
79.14
74.97
QIC (g/m2-s
2 Acre
64.38
57.09
48.16
53.30
69.55
74.38
49.59
65.40
53.94
53.72
60.66
77.91
58.68
64.71
70.64
63.22
57.10
61.53
59.83
58.13
60.01
76.08
64.78
41.83
63.72
57.52
55.40
70.14
66.33
per kg/m3)
5 Acre
55.66
49.33
41.57
45.93
60.03
64.32
42.72
56.47
46.57
46.35
52.37
67.34
50.64
55.82
61.03
54.47
49.23
53.11
51.62
50.17
51.78
65.75
55.99
36.10
55.07
49.57
47.83
60.59
57.17
10 Acre
50.09
44.37
37.36
41.24
53.95
57.90
38.35
50.77
41.87
41.65
47.08
60.59
45.52
50.16
54.90
48.89
44.19
47.74
46.37
45.08
46.51
59.16
50.38
32.43
49.56
44.49
43.00
54.50
51.33
30 Acre
42.86
37.94
31.90
35.15
46.03
49.56
32.68
43.37
35.75
35.55
40.20
51.80
38.87
42.79
46.92
41.65
37.64
40.76
39.54
38.48
39.64
50.60
43.08
27.67
42.40
37.88
36.73
46.59
43.74
2-21
-------
External Exposure to Radionuclides in Soil. An
individual residing on a contaminated site will be
exposed to radiation emitted by radionuclides present in
the soil. In modeling external exposure to contaminated
soil, the RAGS/HHEM Part B model (U.S. EPA, 1991c)
does not account for the following processes:
radioactive decay and progeny (i.e., radioactive
daughters) ingrowth;
correction factors for the geometry of the
contaminated soil;
depletion of the contaminated soil horizon by
environmental processes, such as leaching, erosion,
or plant uptake; and
corrections for shielding by clean cover material.
The RAGS/HHEM Part B model does not provide any
corrections for radioactive decay. When ingrowth of
progeny is expected to be of importance, the progeny are
included at the outset of the SSL calculations.
The RAGS/HHEM Part B model assumes that an
individual is exposed to a source geometry that is
effectively an infinite slab. The concept of an "infinite
slab" means that the thickness of the contaminated zone
and its aerial extent are so large that it behaves as if it
were infinite in its physical dimensions. In practice, soil
contaminated to a depth greater than about 15 cm and
with an aerial extent greater than about 1,000 m2 will
create a radiation field comparable to that of an infinite
slab.
To accommodate the fact that in most residential
settings the assumption of an infinite slab source will
result in overly conservative SSLs, an adjustment for
source area is considered to be an important
modification to the RAGS/HHEM Part B model. Thus,
an area correction factor, ACF, has been added to the
calculation of SSLs.
No soil depletion processes are assumed to take place.
Accordingly, the SSL model assumes that the
contaminated zone is a constant, non-depleting source
of radioactivity. This assumption provides an upper
bound estimate of exposure to radionuclides in soil.
For the purposes of this report, adjustments for clean
cover are not needed since, in all cases, it is assumed
that the contaminated soil extends to the surface. The
SSL model provides adjustments for indoor occupancy
and associated shielding effects by the simple
application of a gamma shielding factor and indoor
occupancy time adjustment.
Equation 4: Screening Level Equation for External
Exposure to Radionuclides in Soil
ssr - TR
EXT T^TJ
SFe*()*ED* ACF x [ETo + (ETi x GSF)]
Parameter/Definition (units)
TR/target cancer risk (unitless)
SFe/external exposure slope factor
(g/pCi/yr)
EF/exposure frequency (d/yr)
ED/exposure duration (yr)
ACF/area correction factor
ET0/exposure time fraction, outdoor
(unitless)
ET/exposure time fraction, indoor
(unitless)
GSF/gamma shielding factor
Default
1Q-6
See Attachment D
350
30
0.9
0.073
0.683
0.4
With the exception of the area correction factor, default
values are used for all input parameters in Equation 4 to
calculate external exposure SSLs. The amount of data
required to derive site-specific values for these
parameters makes their collection and use impracticable
for calculation of simple site-specific SSLs. Therefore,
site-specific data are generally not available for this
exposure pathway. Alternative area correction factors,
for use when site-specific data are available, are
discussed in the TBD. The generic SSLs presented in
Appendix A of the TBD are recommended for all sites.
Ingestion of Homegrown Produce. Persons living
on a contaminated site may ingest radioactive material
by consumption of plants grown in a family garden. In
this model, the fruits and vegetables primarily become
contaminated by root uptake of radionuclides contained
in the pore water of the soil in which the plants are
growing.
The following factors have been added/changed for
exposure through this pathway for radionuclides as
compared to chemicals:
soil-plant transfer factors used to estimate root
uptake from soil assume that the roots are wholly
exposed to contaminated soil;
2-22
-------
air deposition, rain splash, and irrigation are not
included;
environmental equilibria assumed to exist for
estimating concentrations of 3H and 14C in plants;
and
no more than 50% of produce is assumed to be
homegrown (i.e., contaminated plant fraction < 0.5),
with adjustment for small site areas (i.e., <2,000
m2).
The model accounts for that uptake with a simple soil-
to-plant transfer factor. These soil-to-plant transfer
factors have been developed based upon the assumption
that the entire plant root system is wholly exposed to
contaminated soil.
If the plant roots extend to a depth of 100 cm but the
radionuclide contaminants are confined to the upper 15
cm, an initial assumption may be that only 15% of the
root system is active in accumulating contaminants and
that the reported soil-to-plant transfer factors should be
reduced by a correction factor of 0.15. However, the
equation for calculation of SSLs for this pathway does
not apply any reduction to the soil-to-plant transfer
factors. The basis for this assumption is as follows.
Most plant root systems are in fact very active in the
upper soil horizon, especially in the upper 15 cm of soil.
This point is illustrated in a number of ways: 1) by
illustrations of root morphology and growth habit, 2)
positive physiological factors including the availability
of water, oxygen and nutrients near the soil surface, 3)
negative physiological or agronomic factorsincluding
subsurface soil compaction, subsurface zones of acidity,
perched water tables, hypoxia, etc., 4) interactions with
soil microbeswith a special focus on mychorrizal
fungi, and 5) split root studies. Thus, roots commonly
proliferate in the upper layers of soil. If one assumes
that a plant is actively growing, then ion uptake
characteristics and lateral root growth strongly suggest
that simply attributing 15% of root uptake activity to the
upper 15 cm of the soil is not a sound approach.
Environmental forces may influence root growth to one
or more meters in depth, but more so for obtaining water
than nutrients. In reality, the upper 15 cm of soil may
include 50% or more of the root systemand thus 50%
or more of the ion uptake (SC&A, 1994).
The decision to not include air deposition or rain-splash
does not affect any radionuclides because the increase in
concentration from this route is not significant or is
markedly reduced when peeling, washing, cooking, and
other food preparation processes are taken into
consideration (U.S. EPA, 1994d). The decision to not
include the irrigation pathway is only an issue when
there is medium to heavy irrigation using contaminated
water for a radionuclide with a long half-life, a low Kd
value, and an insignificant contribution from external
exposure. The model also makes a conservative
assumption to ignore the decay between harvest and
ingestion and any removal during food processing.
The model does not include any special calculations for
estimating concentrations of 3H and 14C in plants. Such
calculations assume that a state of equilibrium exists
among the concentrations of 3H and 14C in all
environmental mediaair, water, food products, and
body tissues. This assumption may be overly
conservative for a radioactively contaminated site with
a finite area, but may be appropriate for an individual
pathway, such as soil-to-plant pathway. For these
calculations, the 3H concentration in the plant is
assumed to be the same as that in the contaminated
water to which the plant is exposed. Similarly, the
specific activity of 14C in the plant (i.e., pCi/g of 14C per
gram of carbon in the plant) is the same as that of the
ambient CO2.
The model provides a factor, the Contaminated Plant
Fraction (CPF), to adjust for the fraction of fruits and
vegetables obtained from the contaminated site
(assuming that the family living on the site obtains a
portion of their fruits and vegetables from
uncontaminated sources). The ingestion rate used in the
calculation thus represents a total ingestion rate, which,
when multiplied by the CPF, gives the ingestion rate of
contaminated fruits and vegetables.
The CPF is dependent upon the surface area of the
contaminated zone in m2, As, and is calculated using the
following equation.
CPF = A, / 4,000 0 < A, < 2,000 m2
CPF = 0.5
A, > 2,000 m2
For an area greater than 2,000 m2 (i.e., the default
contaminated site surface area), the CPF is set at an
upper bound of 0.5 (i.e., site residents acquire no more
than one-half of their fruits and vegetables from onsite).
2-23
-------
The factor decreases linearly as the size of the
contaminated area decreases below 2,000 m2 (one-half
acre).
Equation 5: Screening Level Equation for Ingestion
of Radionuclides in Homegrown
Produce
SSL =
TR
SF x (m + JR. ) x 1x10+3 x CPF x-TF x-ED
p ^ vf lv> p
Parameter/Definition (units)
TR/target cancer risk (unitless)
SFp/produce ingestion slope factor
(pCi)-1
I Rvf/vegetable and fruit ingestion
rate (kg/yr)
IR,v/leafy vegetables ingestion rate
(kg/yr)
1x10+3/conversion factor (g/kg)
CPF/contaminated plant fraction
from the site (unitless)
TFp/soil-to-plant transfer factor
(pCi/g plant per pCi/g soil)
ED/exposure duration (yr)
Default
1Q-6
See Attachment D
42.7
4.66
0.5
See Attachment C
30
pathway when the size (i.e., area and depth) of the
contaminated soil source is known or can be
estimated with confidence.
Attachment D provides the toxicity criteria and
regulatory benchmarks for 60 radionuclides commonly
found at NPL sites. These criteria were obtained from
HEAST (U.S. EPA, 1995a), and Drinking Water
Regulations and Health Advisories (U.S. EPA, 1995c),
which are regularly updated. Prior to calculating SSLs
at a site, all relevant radionuclide-specific values in
Attachment D should be checked against the most
recent version of their sources to ensure that they are
up to date.
Site-specific parameters necessary to calculate SSLs for
subsurface soils are listed on Exhibit 11, along with
recommended sources and measurement methods. In
addition to the soil parameters described in Step 3, other
site-specific input parameters include soil moisture,
infiltration rate, and aquifer parameters. Guidance for
collecting or estimating these other parameters at a site
is provided on Exhibit 11 and in Attachment A.
Default values are used for all input parameters in
Equation 5 to calculate SSLs for this pathway. With the
exception of the contaminated site surface area, As, the
amount of data required to derive site-specific values for
these parameters makes their collection and use
impracticable for calculation of simple site-specific
SSLs. Therefore, site-specific data are generally not
available for this exposure pathway. The generic SSLs
presented in the TBD are recommended for all sites,
except for very small sites with As< 2,000m2 (i.e., <0.5
acre).
2.5.2 SSL Equations-Subsurface
Soils. The Soil Screening Guidance for Radionuclides
addresses only one exposure pathway for subsurface
soils: 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 this
2-24
-------
Exhibit 11. Site-specific Parameters for Calculating Subsurface SSLs
SSL Pathway
Migration to
Parameter ground water
Source Characteristics
Source area (A)
Source length (L)
Source depth
Soil Characteristics
Soil texture O
Dry soil bulk density (pb) O
Soil moisture content (w) O
Soil pH 00
Moisture retention exponent O
(b)
Saturated hydraulic O
conductivity (Ks)
Avg. soil moisture content (6W) O
Meteorological Data
Air dispersion factor (Q/C)
Hydrogeologic Characteristics (DAF)
Hydrogeologic setting O
Infiltration/recharge (I)
Hydraulic conductivity (K)
Hydraulic gradient (i)
Aquifer thickness (d)
Data source
Sampling data
Sampling data
Sampling data
Lab measurement
Field measurement
Lab measurement
Field measurement
Look-up
Look-up
Calculated
Q/C table (Table 5)
Conceptual site
model
HELP model;
Regional estimates
Field measurement;
Regional estimates
Field measurement;
Regional estimates
Field measurement;
Regional estimates
Method
Measure total area of contaminated soil
Measure length of source parallel to ground water flow
Measure depth of contamination or use conservative
assumption
Particle size analysis (Gee & Bauder, 1986) and USDA
classification; used to estimate 6W & I
All soils: ASTM D 2937; shallow soils: ASTM D 1556,
ASTM D 2167, ASTM D 2922
ASTM D 2216; used to estimate dry soil bulk density
McLean (1982); used to select pH-specific K,, (metals)
Attachment A; used to calculate 6W
Attachment A; used to calculate 6W
Attachment A
Select value corresponding to source area, climatic
zone, and city with conditions similar to site
Place site in hydrogeologic setting from Aller et al.
(1987) for estimation of parameters below (see
Attachment A)
HELP (Schroeder et al., 1984) may be used for site-
specific infiltration estimates; recharge estimates also
may be taken from Aller et al. (1987) or may be
estimated from knowledge of local meteorologic and
hydrogeologic conditions
Aquifer tests (i.e., pump tests, slug tests) preferred;
estimates also may be taken from Aller et al. (1987) or
Newell et al. (1990) or may be estimated from
knowledge of local hydrogeologic conditions
Measured on map of site's water table (preferred);
estimates also may be taken from Newell et al. (1990)
or may be estimated from knowledge of local
hydrogeologic conditions
Site-specific measurement (i.e., from soil boring logs)
preferred; estimates also may be taken from Newell et
al. (1990) or may be estimated from knowledge of local
hydrogeologic conditions
Indicates parameters used in the SSL equations.
O Indicates parameters/assumptions needed to estimate SSL equation parameters.
2-25
-------
Migration to Ground Water SSLs. The Soil
Screening Guidance for Radionuclides 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 (Exhibit 12). These
assumptions are inherent in the SSL equations and
should be reviewed for consistency with the conceptual
site model (see Step 2) to determine the applicability of
SSLs to the migration to ground water pathway.
To calculate SSLs for the migration to ground water
pathway, multiply the acceptable ground water
concentration by the dilution factor to obtain a target
soil leachate concentration. For example, if the dilution
factor is 10 and the acceptable ground water concen-
tration is 10 pCi/L, the target soil/water leachate concen-
tration would be lOOpCi/L. Next, the partition equation
is used to calculate the total soil concentration (i.e.,
SSL) corresponding to this soil leachate concentration.
Alternatively, if a leach test is used, compare the target
soil leachate concentration to extract concentrations
from the leach tests.
SoilAA/ater Partition Equation. The soil/water
partition equation (Equation 6) relates concentrations of
contaminants adsorbed to soil organic carbon to soil
leachate concentrations in the zone of contamination. It
calculates SSLs corresponding to target soil leachate
contaminant concentrations (Cw). An adjustment has
been added to the equation to relate sorbed
concentration in soil to the measured total soil
concentration. This adjustment assumes that soil-water
and solids are conserved during sampling.
Exhibit 12: Simplifying Assumptions for the SSL
Migration to Ground Water Pathway
The source is infinite (i.e., steady-state
concentrations will be maintained in ground
water over the exposure period of interest)
Contaminants are uniformly distributed
throughout the zone of contamination
Soil contamination extends from the surface to
the water table (i.e., adsorption sites are filled in
the unsaturated zone beneath the area of
contamination
There is no chemical or biological degradation in
the unsaturated zone
Equations in this document do not account for
decay, however an electronic version of these
equations will account for decay in the
unsaturated zone
Equilibrium soil/water partitioning is
instantaneous and linear in the contaminated
soil
The receptor well is at the edge of the source
(i.e., there is no dilution from recharge
downgradient of the site) and is screened within
the plume
The aquifer is unconsolidated and unconfined
(surficial)
Aquifer properties are homogenous and
isotropic
Chelating or complexing agents not present
No facilitated transport (e.g., colloidal transport
of inorganic contaminants in aquifer
The use of the soil/water partition equation to calculate
SSLs assumes an infinite source (steady-state) of
contaminants that extend to the water table. More
detailed models may be used to calculate higher SSLs
that are still protective in some situations. For example,
contaminants at sites with shallow sources, thick
unsaturated zones, degradable contaminants, or
unsaturated zone characteristics (e.g., clay layers) may
attenuate before they reach ground water. Part 3 of the
TBD provides information on the use of unsaturated
zone models for soil screening. The decision to use such
models should be based on balancing the additional
investigative and modeling costs required to apply the
more complex models against the cost savings that will
result from higher SSLs.
2-26
-------
Equation 6 : Soil Screening Level Partitioning
Equation for Migration to Ground Water
SSL = C
\xlO -
( K + )
Pi
Parameter/Definition (units)
SSL/ Screening Level in Soil (pCi/g)
Cw/target soil leachate concentration
( pCi/L)
1x10~3/conversion factor (kg/g)
Kd/soil-water partition coefficient
(L/kg)
e^water-filled soil porosity
C-wate/'-soil)
n/soil porosity (Lpore/Lsoil)
pb/dry soil bulk density (kg/L)
Ps/soil particle density (kg/L)
Default
MCL ax dilution
factor
chemical specific
0.3
1.5
2.65
a Radionuclide -specific (see Attachment D).
b See Attachment C.
Leach Test. A leach test may be used instead of the
soil/water partition equation. If a leach test is used,
compare the target soil leachate concentration (MCL x
Dilution Factor) to extract concentrations from the leach
tests. In some instances, a leach test may be more useful
than the partitioning method, depending on the
constituents of concern and the possible presence of
RCRA wastes. If this option is chosen, soil parameters
are not needed for this pathway. However, a dilution
factor must still be calculated. This guidance suggests
using the EPA Synthetic Precipitation Leaching
Procedure (SPLP, EPA SW-846 Method 1312, U.S.
EPA, 1994e). The SPLP was developed to model an
acid rain leaching environment and is generally
appropriate for a contaminated soil scenario. Like most
leach tests, the SPLP may not be appropriate for all
situations (e.g., soils contaminated with oily constituents
may not yield suitable results). Therefore, apply the
SPLP with discretion.
EPA is aware that many leach tests are available for
application at hazardous waste sites, some of which may
be appropriate in specific situations (e.g., the Toxicity
Characteristic Leaching Procedure (TCLP) models
leaching in a municipal landfill environment). It is
beyond the scope of this document to discuss in detail
leaching procedures and the appropriateness of their use.
Stabilization/Solidification of CERCLA and RCRA
Wastes (U.S. EPA, 1989b) and the EPA SAB's review
of leaching tests (U.S. EPA, 1991c) discuss the
application of various leach tests to various waste
disposal scenarios. Consult these documents for further
information.
See Step 3 for guidance on collecting subsurface soil
samples that can be used for leach tests. To ensure
adequate precision of leach test results, leach tests
should be conducted in triplicate.
Dilution Factor Model. As soil leachate moves through
soil and ground water, contaminant concentrations are
attenuated by adsorption and degradation. In the
aquifer, dilution by clean ground water further reduces
concentrations before contaminants reach receptor
points (i.e., drinking water wells). This reduction in
concentration can be expressed by a dilution attenuation
factor (DAF), defined as the ratio of soil leachate
concentration to receptor point concentration. The
lowest possible DAF is 1, corresponding to the situation
where there is no dilution or attenuation of a
contaminant (i.e., when the concentration in the receptor
well is equal to the soil leachate concentration). On the
other hand, high DAF values correspond to a large
reduction in contaminant concentration from the
contaminated soil to the receptor well.
The Soil Screening Guidance for Radionuclides
addresses only one of these dilution-attenuation
processes: contaminant dilution in ground water. A
simple mixing zone equation derived from a water-
balance relationship (Equation 7) is used to calculate a
site-specific dilution factor. Mixing-zone depth is
estimated from Equation 8, which relates it to aquifer
thickness along with the other parameters from Equation
7. Mixing zone depth should not exceed aquifer
thickness (i.e., use aquifer thickness as the upper limit
for mixing zone depth).
Because of the uncertainty resulting from the wide
variability in subsurface conditions that affect
contaminant migration in ground water, defaults are not
provided for the dilution model equations. Instead, a
default DAF of 20 has been selected as protective for
contaminated soil sources up to 0.5 acre in size.
Analyses using the mass-limit models described in the
SSG for chemicals suggest that a DAF of 20 may be
protective of larger sources as well; however, this
2-27
-------
hypothesis should be evaluated on a site-specific basis.
A discussion of the basis for the default DAF and a
description of the mass-limit analysis is found in Part
2.6.4 of the TBD. However, since migration to ground
water SSLs are most sensitive to the DAF, site-specific
dilution factors should be calculated.
Equation 7: Derivation of Dilution Factor
_ Kx/xd
w /XL
Parameter/Definition (units)
DF^/dilution factor (unitless)
K/aquifer hydraulic conductivity
(m/yr)
i/hydraulic gradient (m/m)
I/infiltration rate (m/yr)
d/mixing zone depth (m)
L/source length parallel to ground
water flow (m)
Default
20 (0.5-acre
source)
Equation 8: Estimation of Mixing Zone Depth
d = (0.0112Z,2)0-5 + dax [1 exp(
Lxl
Parameter/Definition (units)
d/mixing zone depth (m)
L/source length parallel to ground water flow (m)
I/infiltration rate (m/yr)
K/aquifer hydraulic conductivity (m/yr)
i/hydraulic gradient (m/m)
da/aquifer thickness (m)
Mass-Limit SSLs. Use of infinite source models to
estimate migration to ground water can violate mass
balance considerations, especially for small sources. To
address this concern, the Soil Screening Guidance
includes models for calculating mass-limit SSLs for this
pathways (Equation 9) that provide a lower limit to
SSLs when the area and depth (i.e., volume) of the
source are known or can be estimated reliably.
A mass-limit SSL represents the level of radionuclide in
the subsurface that is still protective when the entire
volume of contamination leaches over the 30 year
exposure duration and the level of radionuclide at the
receptor does not exceed the health-based limit.
To use mass-limit SSLs, determine the area and depth of
the source, calculate both standard and mass-limit SSLs,
compare them for each radionuclide of concern and
select the higher of the two values.
Note that Equation 9 requires a site-specific
determination of the average depth of contamination in
the source. Step 3 provides guidance for conducting
subsurface sampling to determine source depth. Where
the actual average depth of contamination is uncertain,
a conservative estimate should be used (e.g., the
maximum possible depth in the unsaturated zone). At
many sites, the average water table depth may be used
unless there is reason to believe that contamination
extends below the water table. In this case SSLs do not
apply and further investigation of the source in question
is needed.
Equation 9: Mass-Limit Soil Screening Level for
Migration to Ground Water
C x/xEDx \xiQ-3
SSL - w
Pbxds
Parameter/Definition (units)
SSL/ Soil Screening Level in Soil
(pCi/g)
Cw/target soil leachate concentration
(pCi/L)
I/infiltration rate (m/yr)
ED/exposure duration (yr)
1x10~3/conversion factor (kg/g)
pb/dry soil bulk density (kg/L)
dj/depth of source (m)
Default
( MCL, f *
dilution factor
site-specific
70
-
1.5
site-specific
a Radionuclide -specific, see Attachment D.
2.5.3 Address Exposure to Multiple
Radionuclides . The SSLs generally correspond to
a 10~6 lifetime cancer risk level. The potential for
additive effects has not been "built in" to the SSLs
through apportionment. While the pathways included in
the analysis are considered to represent those a
residential setting, SSLs are not calculated for a specific
scenario (i.e., SSLs are not summed over a set of
pathways). For radionuclides, EPA believes that setting
a 10~6 risk level for individual radionuclides and
pathways generally will lead to cumulative site risks
within the 10~4 to 10~6 risk range for the combinations of
radionuclides typically found at NPL sites.
2-28
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SSLs and the Use of Surrogate Measurements.
For sites with multiple radionuclides, it may be possible
to measure just one of the radionuclides and still be able
to demonstrate compliance (with the target risk level of
10~6) for the radionuclides present through the use of
surrogate measurements. Both time and resources can
be saved if the analysis of one radionuclide is simpler
than the analysis of the other. For example, using the
measured 137Cs concentration as a surrogate for 90Sr
reduces the analytical costs because the wet chemistry
separations do not have to be performed for 90Sr on
every sample. In using one radionuclide to estimate the
presence of others, a sufficient number of
measurements, spatially separated throughout the EA,
should be made to establish a consistent ratio. The
number of measurements needed to determine the ratio
is selected using the DQO process and based on the
chemical, physical, and radiological characteristics of
the nuclides and the site.
The potential for shifts or variations in the radionuclide
ratios means that the surrogate method should be used
with caution. Physical or chemical differences between
the radionuclides may produce different migration rates,
causing the radionuclides to separate and changing the
radionuclide ratios. Remediation activities have a
reasonable potential to alter the surrogate ratio
established prior to remediation. When the ratio is
established prior to remediation, additional post-
remediation samples should be collected to ensure that
the data used to establish the ratio are still appropriate
and representative of the existing site condition. If these
additional post-remediation samples are not consistent
with the pre-remediation data, surrogate ratios should be
re-established.
2.6 Step 6: Comparing Site Soil
Radionuclide
Concentrations to
Calculated SSLs
Nowthatthe site-specific SSLs have been calculated for
the potential radionuclides of concern, compare them
with the site radionuclide 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 for Radionuclides.
In theory, an exposure area would be screened from
further investigation when the true mean of the
population of radionuclide concentrations falls below
the established screening level. However, EPA
recognizes that data obtained from sampling and
analysis are never perfectly representative and accurate,
and that the cost of trying to achieve perfect results
would be quite high. Consequently, EPA acknowledges
that some uncertainty in data must be tolerated, and
focuses on controlling the uncertainty which affects
decisions based on those data. Thus, in the Soil
Screening Guidance for Radionuclides, EPA has
developed an approach for surface soils to minimize the
chance of incorrectly deciding to:
Screen out areas when the correct decision would
be to investigate further (Type I error); or
Decide to investigate further when the correct
decision would be to screen out the area (Type II
error).
The approach sets limits on the probabilities of making
such decision errors, and acknowledges that there is a
range (i.e., gray region) of radionuclide concentrations
around the screening level where the variability in the
data will make it difficult to determine whether the
exposure area average concentration is actually above or
below the screening level. The Type I and Type II
decision error rates have been set at 5 percent and 20
percent, respectively, and the gray region has been set
between one-half and two times the SSL. By specifying
the upper edge of the gray region as twice the SSL, it is
possible that exposure areas with mean radionuclide
concentration values slightly above the SSL may be
screened from further study.
2.6.1 Evaluation of Data for Surface Soils.
Thus, for surface soils, the radionuclide concentrations
in each composite sample from an exposure area are
compared to two times the SSL. Under the Soil
Screening Guidance DQOs, areas are screened out from
further study when radionuclide concentrations in all of
the composite samples are less than two times the SSLs.
Use of this decision rule (comparing radionuclide
concentrations to twice the SSL) is appropriate only
when the quantity and quality of data are comparable to
the levels discussed in this guidance.
2-29
-------
For existing data sets that may be more limited than
those discussed in this guidance, the 95 percent upper-
confidence limit on the arithmetic mean of the
radionuclide concentrations in surface soils (i.e., the
Land method as described in the Supplemental Guidance
to RAGS: Calculating the Concentration Term (U.S.
EPA, 1992c) should be compared to the SSL. If the 95
percentile on the arithmetic mean is less than the SSL,
the exposure area may be screened out.
The TBD discusses the strengths and weaknesses of
using the Land method for making screening decisions.
As an alternative to the Max test, the TBD provides
guidance on performing the Sign test when the
contaminant is not present in background.
2.6.2 Evaluation of Data for Subsurface
Soils. In this guidance, fewer samples are collected
for subsurface soils than for surface soils; therefore,
different decision rules apply.
Since subsurface soils are not characterized as well 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
radionuclide concentrations from each soil boring taken
in a source area are compared with the calculated SSLs.
Source areas with any mean soil boring radionuclide
concentration greater than the SSLs generally warrant
further consideration. On the other hand, where the
mean soil boring radionuclide 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
The radionuclides, exposure pathways, and areas that
have been identified for further study become a subject
of the RI/FS. 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,
199Id), remedial action at NPL sites is generally
warranted where cumulative risks for current or future
land use exceed 1x10"4. 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.
Once the decision has been made to initiate remedial
action, the SSLs can then serve as preliminary
remediation goals. This process is referenced in Section
1.2 of this document.
FOR FURTHER INFORMATION
More detailed discussions of the technical background
and assumptions supporting the development of the Soil
Screening Guidance are presented in the Soil Screening
Guidance for Radionuclides: Technical Background
Document (U.S. EPA, 1999). For additional copies of
this guidance document, the Technical Background
Document, or other EPA documents, call the National
Technical Information Service (NTIS) at (703) 605-6000
or 1-800-553-NTIS (6847). Copies may also be
downloaded from the internet at:
http: //www. epa. gov/superfund/re source s/radiation/rad
risk.htm.
2-30
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Attachment A
Conceptual Site Model Summary
-------
Attachment A
Conceptual Site Model Summary
Step 1 of the Soil Screening Guidance for Radionuclides: User's Guide describes the development of a
conceptual site model (CSM) to support the application of soil screening levels (SSLs) at a site. The CSM
summary forms at the end of this attachment contain the information necessary to:
Determine the applicability of SSLs to the site
Calculate SSLs.
By identifying data gaps, these summary forms will help focus data collection and evaluation on the site-
specific development and application of SSLs. The site investigator should use the summary forms during
the SSL sampling effort to collect site-specific data and continually update the CSM with new information
as appropriate.
The CSM summary forms indicate the information required for determining the applicability of the soil
screening process to the site. Forms addressing source characteristics may be photocopied if more than one
source is present at a site.
A site map showing contaminated soil sources and exposure areas (EAs) should be attached to the summary.
If available, additional pages of other maps, summaries of analytical results, or more detailed descriptions
of the site may be attached to the summary.
Form 1. General Site Information
The information included in this form is identical to the first page of the Site Inspection (SI) Data Summary
form (page B-3 in Guidance for Performing Site Inspections Under CERCLA, U.S. EPA, 1992). However,
the form should be updated to reflect any site activities conducted since the SI was completed.
Form 2. Site Characteristics
Form 2 indicates the information necessary to address the migration to ground water pathway and identify
subsurface conditions that may limit the applicability of SSLs.
A hydrogeologic setting is defined as a unit with common hydrogeologic characteristics and therefore
common vulnerability to contamination. Each setting provides a composite description of the hydrogeologic
factors that control ground water movement and recharge. These factors can be used to make generalizations
in the CSM about ground water conditions.
After placing the site into one of Heath's ground water regions (Heath, 1984), consider geologic and
geomorphic features of the site and select a generic hydrogeologic setting from Aller et al. (1987) that is
most similar to the site. If existing site information is not sufficient to definitively place the site in a setting,
it should be possible to narrow the choice to two or three settings that will reduce the range of values
necessary to develop SSLs. A copy of the setting diagram from Aller et al. (1987) should be attached to the
CSM checklist to provide a general picture of subsurface site conditions.
Ground Water Flow Direction. The direction of ground water flow in the uppermost aquifer underlying
each source is needed to determine source length parallel to that flow. If ground water flow direction is
unknown or uncertain, assume it is parallel to the longest source dimension.
A-l
-------
Aquifer Parameters. Aquifer parameters needed to estimate a site-specific dilution factor include
hydraulic conductivity (K), hydraulic gradient (i), and aquifer thickness (da). Site-measured values for these
parameters are the preferred alternative. Existing site documentation should be reviewed for in situ
measurements of aquifer conductivity (i.e., from pump test data), water table maps that can be used to
estimate hydraulic gradient, and boring logs that indicate the thickness of the uppermost aquifer. Detailed
information on conducting and interpreting aquifer tests can be found in Nielsen (1991).
If site-measured values are not available, hydrogeologic knowledge of regional geologic conditions or
measured values in the literature may be sources of reasonable estimates. Values from a similar site in the
same region and hydrogeologic setting also may be used, but must be carefully reviewed to ensure that the
subsurface conceptual models for the two sites show reasonable agreement. For all of these options, it is
critical that the estimates and sources be reviewed by an experienced hydrogeologist knowledgeable of
regional hydrogeologic conditions.
A third option is to obtain parameter estimates for the site's hydrogeologic setting from Aller et al. (1987)
or from the American Petroleum Institute's (API's) hydrogeologic database (HGDB) (Newell et al., 1989,
1990). Aller et al. (1987) present ranges of values for K and i by hydrogeologic setting. The HGDB contains
measured values for these parameters and aquifer depth for a number of sites in each hydrogeologic setting.
If HGDB data are used, the median value presented for each setting should be used unless site-specific
conditions indicate otherwise. Aquifer parameter values from these sources also can serve as a check of the
validity of site-measured values or estimates obtained from other sources.
If outside sources such as Aller et al. (1987) are used to characterize site hydrogeologic conditions, the
appropriate references and diagrams should be attached to the CSM checklist.
Infiltration Rate. Infiltration rate is used to calculate SSLs for subsurface soils (see Step 5). The simplest
way to estimate infiltration rate (I) is to assume that infiltration is equal to recharge and obtain recharge
estimates for the site's hydrogeologic setting from Aller et al. (1987). When using the Aller et al. (1987)
estimates the user should recognize that these are estimates of average recharge conditions throughout the
setting and site-specific values may differ to some extent. For example, areas within the setting with steeper
than average slopes will tend to have lower infiltration rates and areas with flatter than average slopes will
tend to have higher infiltration than average. An alternative is to use infiltration rates determined for a
better-characterized site in the same hydrogeologic setting and with similar meteorological conditions as the
site in question.
A third alternative is use the HELP model. Although HELP was originally written for hydrologic evaluation
of landfills (Schroeder et al., 1984), inputs to the HELP program can be modified to estimate infiltration in
undisturbed soils in natural settings. The most recent version of HELP and the most recent user's guide and
documentation can be obtained by sending an address and two double-sided, high-density, DOS-formatted
disks to:
attn. Eunice Burk
U.S. EPA
5 995 Center Hill Ave.
Cincinnati, OH 45224
(513)569-7871.
Meteorologic Parameters. Select a site-specific Q/C value from in the guidance for the particulate
emission factor (PEF) equation to place the site in a climatic zone (Figure A-l).
A-2
-------
Several site-specific parameters are required to calculate a PEF if fugitive dusts are of concern at the site (see
Step 5 for surface soils). The threshold windspeed at 7 meters above ground surface (Ut 7) is calculated from
source area roughness height and the mode soil aggregate size as described in Cowherd et al. (1985). Mode
soil aggregate size refers to the mode diameter of aggregated soil particles measured under field conditions.
Other site-specific variables necessary for calculating the PEF include fraction vegetative cover (V) and the
mean annual windspeed (Um). Fraction vegetative cover is estimated by visual observations of the surface
of known or suspected source areas at the site. Mean annual windspeed can be obtained from the National
Weather Service surface station nearest to the site.
Form 3. Exposure Pathways and Receptors
Form 3 includes information necessary to determine the applicability of the Soil Screening Guidance for
Radionuclides to a site (see Step 2 of the User's Guide). This form summarizes the site information
necessary to identify and characterize potential exposure pathways and receptors at the site, such as site
conditions, relevant exposure scenarios, and the properties of soil contaminants listed on Form 4. Table A-l
provides an example of exposure pathways that are not addressed by the guidance, but have relevance to
CSM development.
Table A-1. Example Identification of Radiological Exposure Pathways Not Addressed by SSLs
Receptors/
Exposure Pathways
Contaminant
Characteristics
Site Conditions
Human / Direct Pathways
inhalation - radon
inhalation -volatile
radionuclides
chronic health effects
chronic health effects
elevated levels of radium in soils
radionuclides bound chemically to
volatile organic compounds or "special
case" radionuclides (e.g., 3H, 14C,
222Rn)
Human / Indirect Pathways
consumption of meat or dairy
products
fish consumption
bioaccumulation,
biomagnification
biomagnification
nearby meat or dairy production
nearby surface waters with
recreational or subsistence fishing
Ecological Pathways
aquatic
terrestrial
aquatic toxicity
toxicity to terrestrial organisms
(e.g., DDT, Hg)
nearby surface waters or wetlands
sensitive species on or near site
A-3
-------
Figure A-l. U.S. climatic zones
A-4
-------
Form 4. Soil Contaminant Source Characteristics
This form prompts the investigator to provide information on source characteristics, including soil contaminant levels
and the physical and chemical parameters of site soils needed to calculate SSLs. One form should be completed for
each contaminated soil source. Initially, the form should be filled out to the greatest extent possible with existing
site information collected during CSM development (see Step 1 of the User's Guide). The forms should be updated
after the SSL sampling effort is complete.
Measurement of contaminant levels and the soil parameters listed on this form is described in Step 3 of this guidance.
Average soil moisture content (6J defines the fraction of total soil porosity that is filled by water and air.
These parameters are necessary to apply the soil/water partition equation. It is important that the moisture content
used to calculate these parameters represent the annual average soil moisture conditions. Moisture content
measurements on discrete soil samples should not be used because they are affected by preceding rainfall events and
thus may not represent average conditions. Volumetric average soil water content may be estimated by the following
relationship developed by Clapp and Hornberger (1978) and presented in the Superfund Exposure Assessment Manual
(U.S. EPA, 1988):
where
n = total soil porosity (Lpore/Lsoll)
I = infiltration rate (m/yr)
Ks = saturated hydraulic conductivity (m/yr)
b = soil-specific exponential parameter (unitless).
Total soil porosity (n) is estimated from dry soil bulk density (pb) as follows:
n = 1 - (Pb/Ps)
where
ps = soil particle density = 2.65 kg/L.
Site-specific values for infiltration rate (I) may be estimated using the HELP model or may be assumed to be
equivalent to recharge (see Form 2).
Values for Ks and the exponential term l/(2b+3) are shown in Table A-2 by soil texture class. Soil texture class can
be determined using a particle size analysis and the U.S. Department of Agriculture (USDA) soil textural triangle
shown as Exhibit 9 in the User's Guide. The particle size analysis method described in Gee and Bauder (Gee, G.W.,
and J.W. Bauder, Particle size analysis, A. Clute (ed.), Methods of Soil Analysis. Part 1. Physical andMineralogical
Methods. 2nd Edition, 9(1):383-411, American Society of Agronomy, Madison, WI, 1986) can provide the
appropriate particle size distribution. Other methods are appropriate as long as they provide the same particle
breakpoints for sand/silt (0.05 mm) and silt/clay (0.002 mm). Field methods are an alternative for determining soil
textural class. Table A.3 Presents an example from Brady (Brady, N.C., The Nature and Properties of Soils,
Macmillan Publishing Company, New York, NY, 1990).
A-5
-------
Table A-2. Parameter Estimates for Calculating Average Soil
Moisture Content (6J
Soil texture
Ks(m/yr)
1/(2b+3)
Sand
Loamy sand
Sandy loam
Silt loam
Loam
Sandy clay loam
Silt clay loam
Clay loam
Sandy clay
Silt clay
Clay
1,830
540
230
120
60
40
13
20
10
8
5
0.090
0.085
0.080
0.074
0.073
0.058
0.054
0.050
0.042
0.042
0.039
Source: U.S. EPA, 1988.
Table A.3 Criteria Used with the Field Method for Determining Soil Texture Classes
Criterion
Individual grains
visible to eye
Stability of dry clods
Stability of wet clods
Stability of "ribbon"
when wet soil rubbed
between thumb and
fingers
Sand
Yes
Do not form
Unstable
Does not
form
Sandy Loam
Yes
Do not form
Slightly
stable
Does not
form
Loam
Some
Easily
broken
Moderately
stable
Does not
form
Silt Loam
Few
Moderately
easily
broken
Stable
Broken
appearance
Clay Loam
No
Hard and
stable
Very stable
Thin, will
break
Clay
No
Very hard
and stable
Very stable
Very long,
flexible
Source: Brady, 1990.
Worksheets
The worksheets following Forms 1 through 4 provide a convenient means of assembling radionuclide-specific
parameters necessary to calculate SSLs for the contaminants of concern (Worksheet 1), existing site data on
contaminant concentrations collected during CSM development or the SSL sampling effort (Worksheet 2), and SSLs
calculated for EAs (Worksheet 3) or contaminant sources (Worksheet 4) of concern at the site.
CSM Diagram
The CSM diagram is a product of CSM development that represents the linkages among contaminant sources, release
mechanisms, exposure pathways and routes, and receptors to summarize the current understanding of the soil
contamination problem (see Step 1 of the guidance). An example SSL CSM diagram, Figure A-2 (U.S. EPA, 1989),
and a site sketch, Figure A-3 (U.S. EPA, 1987) are provided following the Worksheets.
A-6
-------
References
Aller, L., T. Bennett, J.H. Lehr, R.J. Petty, and G. Hackett. 1987. DRASTIC: A Standardized System for Evaluating
Ground Water Pollution Potential Using Hydrogeologic Settings. Prepared for U.S. EPA Office of Research
and Development, Ada, OK. National Water Well Association, Dublin, OH. EPA-600/2-87-035.
Brady, N.C. 1990. The Nature and Properties of Soils. Macmillan Publishing Company, New York, NY.
Clapp, R.B., and G.M. Hornberger. 1978. Empirical equations for some soil hydraulic properties. Water Resources
Research, 14:601-604.
Cowherd, C., G. Muleski, P. Engelhart, and D. Gillette. 1985. Rapid Assessment of Exposure to Paniculate
Emissions from Surface Contamination. Prepared for Office of Health and Environmental Assessment, U.S.
EPA, Washington, DC. NTIS PB85-192219 7AS. EPA/600/8-85/002.
Gee, G.W., and J.W. Bauder, 1986. Particle size analysis. A. Klute (ed.). Methods of Soil Analysis, Parti, Physical
and Mineralogical Methods, 2nd Edition, 9(1):383-411, American Society of Agronomy, Madison, WI.
Heath, RC. 1984. Ground-Water Regions of the United States. USGS Water Supply Paper 2242. U.S. Geological
Survey, Reston, VA.
Newell, C.J., L.P. Hopkins, and P.B. Bedient. 1989. Hydrogeologic Database for Ground Water Modeling. API
Publication No. 4476. American Petroleum Institute, Washington, DC.
Newell, C.J., L.P. Hopkins, and P.B. Bedient. 1990. A hydrogeologic database for ground water- modeling. Ground
Water, 28(5):703-714.
Nielsen, D.M. (ed.). 1991. Practical Handbook of Ground-Water Monitoring. Lewis Publishers, Chelsea, MI.
Schroeder, P.R., A.C. Gibson, and M.D. Smolen. 1984. Hydrological Evaluation of Landfill Performance (HELP)
Model; Volume 2: Documentation for Version 1. NTIS PB85-100832. Office of Research and Development,
U.S. EPA, Cincinnati, OH. EPA/530-SW-84-010.
U.S. EPA. 1987. Data Quality Objectives for Remedial Response Activities. Example Scenario: RI/FS Activities at
a Site with Contaminated Soil and Groundwater. Office of Emergency and Remedial Response,
Washington, DC. NTIS PB88-13188.
U.S. EPA 1988. Superfund Exposure Assessment Manual. OSWER Directive 9285.5-1. Office of Emergency and
Remedial Response, Washington, DC. EPA/540/1-88/001. NTIS PB89-135859.
U.S. EPA. 1989. Guidance for Conducting Remedial Investigations and Feasibility Studies under CERCLA.
EPA/540/G-89/004. OSWER Directive 9355.3-01. Office of Emergency and Remedial Response,
Washington, DC. NTIS PB89-184626.
U.S. EPA. 1992. Guidance for Performing Site Inspections Under CERCLA. EPA/540-R-92-0021. Office of
Emergency and Remedial Response, Washington, DC. NTIS PB92-963375.
A-7
-------
Soil Screening Guidance for Radionuclides
Conceptual Site Model Summary Forms
Form 1: General Site Information Site Name
EPA Region Date
Contractor Name and Address:
State Contact:
1. CERCLIS ID No
Address City
County State Zip Code Congressional District
2. Owner Name Operator Name
Owner Address Operator Address
City State City State
3. Type of ownership (check all that apply):
D Private D Federal Agency D State D County D Municipal
Other Ref
4. Approximate size of property acres Ref.
5. Latitude ° I " Longitude .... o ... | " Ref.
6. Site status D Active D Inactive D Unknown Ref.
7. Years of operation From To D Unknown Ref.
8. Previous investigations
Type Agency/State/Contractor Date
Ref.
Ref.
Ref.
Ref.
Ref.
Ref.
Ref. = reference(s) on information source
-------
Soil Screening Guidance for Radionuclides
Conceptual Site Model Summary Forms
Form 2: Site Characteristics Site Name
Hydroqeoloqic Characteristics (migration to ground water pathway)
Is ground water of concern at the site? Dyes D no (if no, move to Infiltration Rate below).
Heath region Hydrogeologic setting
(attach setting diagram)
Check setting characteristics that apply: D karst D fractured rock D solution limestone
Describe the stratigraphy and hydrogeologic characteristics of the site. (Attach available maps and cross-sections.
Ref
Identify and describe nearby sites in similar settings that have already been characterized.
Ref
Aquifer Parameters Unit Typical Min. Max. Reference or Source
hydraulic conductivity (K) m/y
hydraulic gradient (i) m/m
thickness (da) m
General direction of ground water flow across the site (e.g., NNE, SW):
(attach map.) Ref
Infiltration rate (I) m/yr Method
Meteorological Characteristics (inhalation pathway)
climatological zone: (zone#, city) Q/C (g/m2-s per kg/m3
fract. vegetative cover (V) (unitless) Reference
mean annual windspeed (Um) m/s Reference
equivalent threshold value of windspeed at 7 m (Ut) m/s
fraction dependent on Um/U, (unitless)
Comments:
A-9
-------
Soil Screening Guidance for Radionuclides
Conceptual Site Model Summary Forms
Form 3: Exposure Pathways and Receptors
Land Use Conditions
Site Name
Current site use:
residential
industrial
commercial
agricultural
recreational
other
Surrounding land use:
residential
industrial
commercial
agricultural
recreational
other
Future land use:
residential
industrial
commercial
agricultural
recreational
other
Size of exposure areas (in acres)
Contaminant Release Mechanisms (check all that apply):
Source # D leaching D volatilization D fugitive dusts D erosion/runoff D uptake by plants D direct exposure
Source # D leaching D volatilization D fugitive dusts D erosion/runoff D uptake by plants D direct exposure
Source # D leaching D volatilization D fugitive dusts D erosion/runoff D uptake by plants D direct exposure
(describe rationale for not including any of the above release mechanisms)
Media affected (or potentially affected) by soil contamination.
Source # D air D ground water D surface water D sediments D wetlands Dsubsurface
Source # D air D ground water D surface water D sediments D wetlands Dsubsurface
Source # D air D ground water D surface water D sediments D wetlands Dsubsurface
Check if present on-site or on surrounding land (attach map showing locations)
D wetlands D surface water D subsistence fishing D recreational fishing D dairy/beef production D elevated indoor radon
Check SSL exposure pathways applicable at site; describe basis for not including any pathway
D ingestion of soil D inhalation D migration to ground water D produce ingestion
D external exposure
Check if there is a potential for:
D Acute Effects (describe)
D Other Human Exposure Pathways (describe)
D Ecological concerns (describe)
D Small areas of elevated activity (describe)
A-10
-------
Soil Screening Guidance for Radionuclides
Conceptual Site Model Summary Forms
Form 4: Soil Contaminant Source Characteristics Site Name
Source No.:
Name: (e.g., drum storage area)
Type: (e.g., spill, dump, wood treater)
Location: (site map)
Waste type: (e.g., solvents, waste oil, tailings)
Description (describe history of contamination, other information)
Describe past/current remedial or removal actions
Source depth: m (D measures D estimated) Ref
Source area: acres m2 (D measures D estimated) Ref
Source length parallel to ground water flow: m (if uncertain, use longest source dimension)
Contaminant types (check all that apply): D volatile organics D other organics D metals D other inorganics
D radionuclides
Soil Contaminants Present (list):
(attach Worksheet #1)
Describe previous soil analyses, (attach available results and map showing sample locations)
(attach Worksheet #2)
Are NAPLs suspected? D Yes D No Reason
Average Soil Characteristics
average water content (6J (L wate/Lsoil) Ref.
dry bulk density (pb) (kg/L) Ref.
pH Ref.
A-ll
-------
Worksheet 1. Contaminant-specific properties
Regulatory and Human Health Benchmarks1
Site Name
Radionuclide
CASRN
MCL
(pCi/L)
Slope factors
Ingestion
- soil
(pCi)-1
Inhalation
(pCi)-1
Ingestion
- water
(pCi)-1
External
exposure
(kg/pCi-s)
Ingestion
- produce
(pCi)-1
Chemical Properties2
Contaminant
CAS#
Sources
(no.)
Koc3
(L/kg)
Kd4
(L/kg)
H5
Dia5
(cm2/s)
D 5
^IW
(cm2/s)
S5
(mg/L)
1. Attachment D
2. Attachment C
3. For organic compounds
4. For metals and inorganic compounds
5. Not applicable to metals except mercury
A-12
-------
Worksheet 2. Contaminant concentrations by source
Source #
Site Name
Contaminant
CAS#
average
standard
deviation
number of
samples
minimum
maximum
variance
Source #
Contaminant
CAS#
average
standard
deviation
number of
samples
minimum
maximum
variance
A-13
-------
Worksheet 3. Surface SSLs by Exposure Area (EA)
EA #: SSL type: n site-specific
Site Name
u generic (default) u detailed approach
Radionuclide
CASRN
Soil Screening Level (pCi/g)
Ingestion -
soil
Inhalation
Ingestion -
water
External
exposure
Ingestion -
produce
EA #: SSL type: D site-specific D generic (default) Ddetailed approach
Radionuclide
CASRN
Soil Screening Level (pCi/g)
Ingestion -
soil
Inhalation
Ingestion -
water
External
exposure
Ingestion -
produce
A-14
-------
Worksheet 4. Subsurface SSLs by source Site Name
Source #: SSL type: D site-specific D generic (default) Ddetailed approach
Radionuclide
CASRN
Soil Screening Level (pCi/g)
migration to ground water
Source #:
SSL type: D site-specific D generic (default) Ddetailed approach
Radionuclide
CASRN
Soil Screening Level (pCi/g)
migration to ground water
A-15
-------
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WOODED ARIA
GEOLOGIC CROSS SECTION
WOODED AREA
FILL MATERIAL
DEPRESSION
(DIRECT CONTACT^
'. {GROUND WATER)
\CUSTRINE
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Figure A-3. Example Site Sketch (adapted from U.S. EPA, 1987)
A-17
-------
Attachment B
Soil Screening DQOs for Surface Soils and Subsurface Soils
-------
Table B.1
Soil Screening DQOs for Surface Soils Using the Max Test
DQO Process Steps
Soil Screening Inputs/Outputs
State the Problem
Identify scoping team
Develop conceptual site model (CSM)
Define exposure scenarios
Specify available resources
Write brief summary of contamination
problem
Site manager and technical experts (e.g., health physicists, risk assessors,
statisticians)
CSM development (described in Step 1)
Direct ingestion of soil, inhalation of fugitive dusts, external radiation exposure, and
ingestion of homegrown produce in a residential setting;
Sampling and analysis budget, scheduling constraints, and available personnel
Summary of the surface soil contamination problem to be investigated at the site
Identify the Decision
Identify decision
Identify alternative actions
Do mean soil concentrations for particular radionuclides (e.g., radionuclides of
potential concern) exceed appropriate screening levels?
Eliminate area from further study under CERCLA
or
Plan and conduct further investigation
Identify Inputs to the Decision
Identify inputs
Define basis for screening
Identify analytical methods
SSLs for each pathway for specified radionuclides
Measurements of surface soil radionuclide concentration
Soil Screening Guidance for Radionuclides
Feasible analytical methods (both field and laboratory) consistent with program-
level requirements
Define the Study Boundaries
Define geographic areas of field
investigation
Define population of interest
Divide site into strata
Define scale of decision making
Define temporal boundaries of study
Identify practical constraints
The entire NPL site, (which may include areas beyond facility boundaries), except
for any areas with clear evidence that no contamination has occurred
Surface soils (usually the top 15 centimeters )
Strata may be defined so that radionuclide concentrations are likely to be relatively
homogeneous within each stratum based on the CSM and field measurements
Exposure areas (EAs) no larger than 0.5 acre each (based on residential land use)
Temporal constraints on scheduling field visits
Potential impediments to sample collection, such as access, health, and safety
issues
Develop a Decision Rule
Specify parameter of interest
Specify screening level
Specify "if..., then..." decision rule
"True mean" (u) individual radionuclide concentration in each EA. However, since
the determination of the "true mean" would require the collection and analysis of
many samples, another sample statistic, the maximum composite concentration,
or "Max Test" is used.
Screening levels calculated using available parameters and site data (or generic
SSLs if site data are unavailable)
Ideally, if the "true mean" EA concentration exceeds the screening level, then
investigate the EA further. If the "true mean" is less than the screening level,
then no further investigation of the EA is required under CERCLA.
B-l
-------
Table B.1
Soil Screening DQOs for Surface Soils Using the Max Test (continued)
DQO Process Steps
Soil Screening Inputs/Outputs
Specify Limits on Decision Errors*
Define baseline condition (null
hypothesis)
Define the gray region**
Define Type I and Type II decision errors
Identify consequences
Assign acceptable probabilities of Type I
and Type II decision errors
Define QA/QC goals
The EA needs further investigation
From 0.5 SSL to 2 SSL
Type I error: Do not investigate further ("walk away from") an EA whose "true
mean" exceeds the screening level of 2 SSL
Type II error: Investigate further when an EA "true mean" falls below the
screening level of 0.5 SSL
Type I error: potential public health consequences
Type II error: unnecessary expenditure of resources to investigate further
Goals:
Type I: 0.05 (5%) probability of not investigating further when "true mean" of
the EA is 2 SSL
Type II: 0.20 (20%) probability of investigating further when "true mean" of the
EA is 0.5 SSL
Analytical laboratory precision and bias requirements
10% laboratory analyses for field methods
Optimize the Design
Determine how to best estimate "true
mean"
Determine expected variability of EA
surface soil radionuclide concentrations
Design sampling strategy by evaluating
costs and performance of alternatives
Samples composited across the EA as physical estimates of EA mean (x).
Use maximum composite concentration as a conservative estimate of the true
EA mean.
A conservatively large expected coefficient of variation (CV) from prior data for
the site, field measurements, or data from other comparable sites and expert
judgment. A minimum default CV of 2.5 should be used when information is
insufficient to estimate the CV.
Lowest cost sampling design option (i.e., compositing scheme and number of
composites) that will achieve acceptable decision error rates
Develop planning documents for the field
investigation
Sampling and Analysis Plan (SAP)
Quality Assurance Project Plan (QAPP)
Since the DQO process controls the degree to which uncertainty in data affects the outcome of decisions that are
based on that data, specifying limits on decision errors will allow the decision maker to control the probability of
making an incorrect decision when using the DQOs.
The gray region represents the area where the consequences of decision errors are minor, (and uncertainty in
sampling data makes decisions too close to call).
B-2
-------
Table B.2
Soil Screening DQOs for Subsurface Soils
DQO Process Steps
Soil Screening Inputs/Outputs
State the Problem
Identify scoping team
Develop conceptual site model (CSM)
Define exposure scenarios
Specify available resources
Write brief summary of contamination
problem
Site manager and technical experts (e.g., health physicists, risk assessors,
hydrogeologists, statisticians).
CSM development (described in Step 1).
Migration of radionuclides from soil to potable ground water.
Sampling and analysis budget, scheduling constraints, and available
personnel.
Summary of the subsurface soil contamination problem to be investigated at
the site.
Identify the Decision
Identify decision
Identify alternative actions
Do mean soil concentrations for particular radionuclides (e.g., radionuclides
of potential concern) exceed appropriate SSLs?
Eliminate area from further action or study under CERCLA
or
Plan and conduct further investigation.
Identify Inputs to the Decision
Identify decision
Define basis for screening
Identify analytical methods
Migration to ground water SSLs for specified radionuclides
Measurements of subsurface soil radionuclide concentration
Soil Screening Guidance for Radionuclides
Feasible analytical methods (both field and laboratory) consistent with
program-level requirements.
Specify the Study Boundaries
Define geographic areas of field
investigation
Define population of interest
Define scale of decision making
Subdivide site into decision units
Define temporal boundaries of study
Identify (list) practical constraints
The entire NPL site (which may include areas beyond facility boundaries),
except for any areas with clear evidence that no contamination has
occurred.
Subsurface soils
Sources (areas of contiguous soil contamination, defined by the area and
depth of contamination or to the water table, whichever is more shallow).
Individual sources delineated (area and depth) using existing information or
field measurements (several nearby sources may be combined into a single
source).
Temporal constraints on scheduling field visits.
Potential impediments to sample collection, such as access, health, and
safety issues.
B-3
-------
Table B.2
Soil Screening DQOs for Subsurface Soils(continued)
Develop a Decision Rule
Specify parameter of interest
Specify screening level
Specify "if..., then..." decision rule
Mean soil radionuclide concentration in a source (i.e., discrete radionuclide
concentrations averaged within each boring).
SSLs calculated using available parameters and site data (or generic SSLs if
site data are unavailable).
If the mean soil concentration exceeds the SSL, then investigate the source
further. If mean soil concentration in a source is less than the SSL, then no
further investigation is required under CERCLA.
Specify Limits on Decision Errors
Define QA/QC goals
Analytical laboratory precision and bias requirements
10% laboratory analyses for field methods
Optimize the Design
Determine how to estimate mean
concentration in a source
Define subsurface sampling strategy by
evaluating costs and site-specific
conditions
Develop planning documents for the field
investigation
For each source, the highest mean soil boring concentration (i.e., depth-
weighted average of discrete radionuclide concentrations within a boring).
Number of soil borings per source area; number of sampling intervals with
depth.
Sampling and Analysis Plan (SAP)
Quality Assurance Project Plan (QAPP)
B-4
-------
Attachment C
Radiological Properties for SSL Development
-------
Attachment C
Radiological Properties for SSL Development
C.1 Radionuclides Included in Generic Soil Screening Analysis
Principal radionuclides are radionuclides with half-lives greater than six months. The decay products of any
principal radionuclide down to, but not including, the next principal radionuclide in its decay chain are called
associated radionuclides and consist of radionuclides with half-lives less than six months. It is assumed that
a principal radionuclide is in secular equilibrium with its associated radionuclides at the point of exposure.
This assumption is reasonable because it usually takes about three years or longer to clean up a site.
Principal and associated radionuclides for which generic Soil Screening Levels have been calculated are
listed in Table C.I. Associated decay chains are indicated, as well as principal radionuclide half-life and the
terminal nuclide or radionuclide (i.e., the principal radionuclide or stable nuclide that terminates an
associated decay chain).
C-l
-------
Table C.1 Radionuclides Included in Generic Soil Screening Analysis
Principal Radionuclide3
Nuclide
Ac-227+D
Ag-108m
Ag-110m
Am-241
Am-243+D
Bi-207
C-14
Cd-109
Ce-144+D
CI-36
Cm-243
Cm-244
Co-57
Co-60
Cs-134
Cs-135
Cs-137+D
Eu-152
Eu-154
Eu-155
Fe-55
Gd-153
H-3
1-129
K-40
Half-life (yr)
22
127
0.7
432
7400
38
5730
1.3
0.8
300000
28
18
0.7
5
2
3000000
30
13
8
5
3
0.7
12
16000000
1300000000
Associated Decay Chainb
[Th-227 (98.6%, 19 d)
Fr-223(1.4%, 22 min)]
Ra-223(11 d)
Rn-219(4s)
Po-215(2ms)
Pb-211 (36 min)
Bi-211 (2 min)
[TI-207 (99.7%, 5 min)
Po-211 (0.3%, 0.5s)]
-
Np-239 (2 d)
-
-
-
[Pr-244(9%, 17 min)
Pr-244m (2%, 7 min)]
-
-
-
-
-
-
-
Ba-137m(95%, 3 min)
-
-
-
-
-
-
-
-
Terminal Nuclide or
Radionuclidec
Nuclide
Pb-207
Pd-108(91%)
[Cd-108(98%}
Ag-108(9%)
Pd-108(2%)]
Cs-110(99%)
[Cd-11 0(99.7%)
Ag-110(1%)
Pd-110(0.3%)]
Np-237
Pu-239
Pb-207
N-14
Ag-109
Nd-144
S-36
Am-243 (0.2%)e
Pu-240
Fe-57
Ni-60
Ba-134(~100%)
Ba-135
Ba-137
Sm-152(72%)
Gd-152(28%)
Gd-154(~100%)
Gd-155
Mn-55
Eu-153
He-3
Xe-129
Ca-40 (89%)
Ar-40(11%)
Half-life (yr)
stable
stable
stable
2 min
stable
stable
stable
25s
stable
21000001
24000
stable
stable
stable
stable
stable
7400
6600
stable
stable
stable
stable
stable
stable
1.1E+14
stable
stable
stable
stable
stable
stable
stable
Note:2.1E+6 = 2.1xlO+'
C-2
-------
Table C.1 Radionuclides Included in Generic Soil Screening Analysis
Principal Radionuclide3
Nuclide
Mn-54
Na-22
Nb-94
Ni-59
Ni-63
Np-237+D
Pa-231
Pb-210+D
Pm-147
Pu-238
Pu-239
Pu-240
Pu-241
Pu-242
Pu-244+D
Ra-226+D
Ra-228+D
Ru-106+D
Sb-125+D
Sm-147
Sm-151
Sr-90+D
Tc-99
Th-228+D
Th-229+D
Th-230
Half-life (yr)
0.9
3
20000
75000
100
2100000
33000
22
3
88
24000
6500
14
380000
93000000
1600
8
1
3
110000000000
90
29
210000
2
7300
77000
Associated Decay Chainb
-
-
-
-
-
Pa-233 (27 d)
-
Bi-210(5d)
Po-210(138d)
-
-
-
-
-
-
U-240-100%, 14)
Np-240
Rn-222 (4 d)
Po-218(3min)
Pb-214(~100%, 27min)
Bi-214(20min)
Po-214(~100%, 1 min)
Ac-228 (6 h)
Rh-106(30s)
Te-125m(23%, 58 d)
-
-
Y-90 (64 h)
-
Ra-224 (4 d)
Rn-220 (56 s)
Po-216(0.2s)
Pb-212(11h)
Bi-212(61 min)
[Po-212(64%, 0.3 |js)
TI-208 (36%, 3 min)]
Ra-225(15d)
Ac-225(10d)
Fr-221 (5 min)
At-217(32ms)
Bi-213(46min)
[Po-213(98%, 4 |js)
TI-209 (2%, 2 min)]
Pd-209 (3 h)
-
Terminal Nuclide or
Radionuclidec
Nuclide
Cr-54
Ne-22
Mo-94
Co-59
Cu-53
U-233
Ac-227
Pb-206
Sm-147
U-234
U-235
U-236
Am-241
U-238
Pu-240
Pb-210
Th-228
Pd-106
Te-125
Nd-143
Eu-151
Zr-90
Ru-99
Pb-208
Bi-209
Ra-226
Half-life (yr)
stable
stable
stable
stable
stable
160000
22
stable
1.10000e+11
240000
700000000
2300000
432 y
4500000000
6500
22
2
stable
stable
stable
stable
stable
stable
stable
stable
1600
C-3
-------
Table C.1 Radionuclides Included in Generic Soil Screening Analysis
Principal Radionuclide3
Nuclide
Th-232
TI-204
U-232
U-233
U-234
U-235+D
U-236
U-238+D
Zn-65
Half-life (yr)
14000000000
4
72
160000
240000
700000000
2300000
4500000000
0.7
Associated Decay Chainb
-
-
-
-
-
Th-231 (26 h)
-
Th-234 (24 d)
[Pa-234m (99.8%, 1 min)
Pa-234 (0.2%, 7 h)]
-
Terminal Nuclide or
Radionuclidec
Nuclide
Ra-228
Pb-204 (97%)
Hg-204 (3%)
Th-228
Th-229
Th-230
Pa-231
Th-232
U-234
Cu-65
Half-life (yr)
6
stable
stable
2
7300
80000
34000
14000000000
240000
stable
Radionuclides with half-lives greater than six months. "+D" designates principal radionuclides with associated decay chains.
The chain of decay products of a principal radionuclide extending to (but not including) the next principal radionuclide or a stable nuclide. Half-
lives are given in parentheses. Branches are indicated by square brackets with branching ratios in parentheses.
The principal radionuclide or stable nuclide that terminates an associated decay chain.
A hyphen indicates that there are no associated decay products.
The branching decay for Pu-241 and Cm-243 involves multiple principal radionuclides and associated radionuclides.
C-4
-------
C.2 Soil-water Partition Coefficients for Radionuclides
As with organic chemicals, development of SSLs for inorganics (including radionuclides) requires a soil-
water partition coefficient (Kd) for each constituent. However, the simple relationship between soil organic
carbon content and sorption observed for organic chemicals does not apply to inorganics (including
radionuclides). The soil-water distribution coefficient (Kd) for inorganics (including radionuclides) is
affected by numerous geochemical parameters and processes, including pH; sorption to clays, organic matter,
iron oxides, and other soil constituents; oxidation/reduction conditions; major ion chemistry; and the
chemical form of the radionuclide. The number of significant influencing parameters, their variability in the
field, and differences in experimental methods result in as much as seven orders of magnitude variability in
measured metal Kd values reported in the literature (see Table 43 in the Soil Screening Guidance: Technical
Background Document (EPA 1996b)). This variability makes it much more difficult to derive generic Kd
values for metals (including radionuclides) than for organics. Therefore, it is recommended that Kd values
be measured for site-specific conditions. If the Kd is not measured site-specifically, then a conservative Kd
should be used in calculating SSLs.
Tables C.2a and C.2b list the default Kd values for each element. Table C.2a is derived from the EPA Office
of Radiation and Indoor Air's 1999 "Understanding Variation In Partition Coefficient, Kd, Values, Volume
1: The Kd Model of Measurement, And Application Of Chemical Reaction Codes, & Volume 2: Review Of
Geochemistry And Available Kd Values For Cadmium, Cesium, Chromium, Lead , Plutonium, Radon,
Strontium, Thorium, Tritium, And Uranium". The Kd values in Table C.2a are the most conservative values
provided for each element in (EPA 1999). Each of these values are based on the chemical behavior that was
considered to provide the most conservative Kd value for that element. Users that have measured pH values
at their site that differ from the range given in this report, may want to consult Tables 5.4 to 5.9 in the TBD
for alternative Kds that are still conservative.
The Kd values in Table C.2b are the most conservative values provided by Sheppard and Thibault (Sheppard,
1990) for the remaining elements not addressed in (EPA 1999), that are not based on soil-to-plant transfer.
EPA recommends that Kds based on soil-to-plant uptake data should not be used when estimating migration
of contaminants from soil to groundwater.
When estimating migration of contaminants from soil to groundwater for a contaminant which is not
represented with a default Kd value in either Table C.2a and C.2b, site decision-makers should develop a site-
specific Kd. Site decision-makers also may measure a site-specific Kds to more accurately estimate
contaminant migration rather than using the default values in either Tables C.2a or C.2b or Tables 5.4 to 5.9
in the TBD.
C-5
-------
Table C.2a Default KH Values for Selected Elements
Element
Cs
H
Pu
Rn
Kd value
10
0
5
0
Element
Sr
Th
U
Kd value
1
20
0.4
Source: EPA, 1999
Table C.2b Sheppard and Thibault's Default Kd Values for Selected Elements
Element
Ac
Ag
Am
Bi
C
Cd
Ce
Cl
Cm
Co
Kd value
NDA
2.7
8.2
NDA
0.8
2.7
35
NDA
86
0.1
Element
Eu
Fe
Gd
I
K
Mn
Na
Nb
Ni
Np
Kd value
NDA
3.1
NDA
0.03
NDA
4.9
NDA
NDA
34
0.1
Element
Pa
Pb
Pm
Ra
Ru
Sb
Sm
Tc
Tl
Zn
Kd value
NDA
6
NDA
3
5
NDA
NDA
0.007
NDA
0.1
Source: Sheppard, 1990
NDA: No Default Kd Available. A Kd for this element must be developed on a site-specific basis to
evaluate the potential for fate and transport of this contaminant from the soil to groundwater.
C-6
-------
C.3 Soil-to-Plant Transfer Factors
The soil-to-plant transfer factor is defined as the ratio of the concentration of the principal radionuclide in
plant in pCi/g to the concentration of the radionuclide in soil in pCi/g. This factor is also known as the plant
root uptake factor. The soil-to-plant or soil-to-vegetation transfer factor, for a given type of plant and for
a given radionuclide can vary considerably from site to site with season and time after contamination. These
variations depend on such factors as the physical and chemical properties of the soil, environmental
conditions, and chemical form of the radionuclide in the soil. Furthermore, soil management practices such
as ploughing, liming, fertilizing and irrigation can also effect the uptake of radionuclides by vegetation.
Readers are referred to the TBD for a discussion of the variability of this parameter. This is a
chemical/radionuclide specific parameter. The default values for different radionuclides are presented in
Table C.3.
C-7
-------
Table C.3 Default Soil-to-Plant Transfer Factors
Elem
H
Be
C
N
F
Na
Al
P
S
Cl
Ar
K
Ca
Sc
Cr
Mn
Fe
Co
Ni
TFP
4.8
0.004
5.5
7.5
0.02
0.05
0.004
1
0.6
20
0
0.3
0.5
0.002
0
0.3
0.001
0.08
0.05
Elem
Cu
Zn
Ge
As
Se
Br
Kr
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
TFP
0.13
0.4
0.4
0.08
0.1
0.76
0
0.13
0.3
0.0025
0.001
0.01
0.13
5
0.03
0.13
0.1
0.15
0.3
Elem
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Ho
Ta
TFP
0.003
0.0025
0.01
0.6
0.02
0
0.04
0.005
0.0025
0.002
0.0025
0.0024
0.0025
0.0025
0.0025
0.0025
0.0026
0.0026
0.02
Elem
W
Ir
Au
Hg
Tl
Pb
Bi
Po
Rn
Ra
Ac
Th
Pa
U
Np
Pu
Am
Cm
Cf
TFP
0.018
0.03
0.1
0.38
0.2
0.01
0.1
0.001
0
0.04
0.0025
0.001
0.01
0.0025
0.02
0.001
0.001
0.001
0.001
Source: ANL, 1993.
C-8
-------
Attachment D
Regulatory and Human Health Benchmarks
Used for Radionuclide SSL Development
-------
Attachment D
Regulatory and Human Health Benchmarks
Used for Radionuclide SSL Development
D.1 Current Radionuclide Slope Factors
The slope factors listed in Table D. 1 are taken from the Health Effects Assessment Summary Tables (HEAST) which
may be found on the internet at the following address: http://www.epa.gov/superfund/programs/risk/calctool.htm.
The slope factors are derived primarily fmmtfealth RisksfromLow-Level Environmental Exposure to Radionuclides,
Federal Guidance Report No. 13, Parti - , U.S. EPA, 1999 (also known as FGR13). Table D.I lists cancer slope
factors for each route of intake for principal radionuclides in units of picocuries (pCi).1 Radionuclides are presented
alphabetically by element.
Selected radionuclides and radioactive decay chain products are designated with the suffix "+D" (e.g., U-238+D, Ra-
226+D, Cs-137+D) to indicate that cancer risk estimates for these radionuclides include the contributions from their
short-lived decay products, assuming equal activity concentrations (i.e., secular equilibrium) with the principal or
parent nuclide in the environment. Decay chains are identified in Attachment C, Table C. 1.
In most cases, site-specific analytical data should be used to establish the actual degree of equilibrium between each
parent radionuclide and its decay products in each media sampled. However, in the absence of empirical data, the
"+D" values for radionuclides should be used unless there are compelling reasons not to.
Note that there may be circumstances, such as long disposal times or technologically enhanced concentrations of
naturally occurring radionuclides, that may necessitate the combination of the risks of a parent radionuclide and its
decay products over several contiguous subchains. For example, Ra-226 soil analyses at a site might show that all
radium decay products are present in secular equilibrium down to stable Pb-206. In this case, Ra-226 risk
calculations should be based on the ingestion, inhalation and external exposure slope factors for the Ra-226+D
subchain, plus the ingestion, inhalation and external exposure factors for the Pb-210+D subchain. For actual sites,
users should consult with a health physicist or radiochemist (1) to evaluate the site-specific analytical data to
determine the degree of equilibrium between parent radionuclides and decay members of contiguous decay chains
and (2) to assist in the combination of appropriate slope factor values.
1 Slope factors are reported in the customary units of picocuries (1 pCi = 10"12 curies (Ci) = 3.7xlO"2 nuclear
transformations per second) for consistency with the system used for radionuclides in the IRIS database. If required,
slope factors in Table 4 can be converted into the International System (SI) units of becquerels (1 Bq = 1 nuclear
transformation per second) by multiplying each inhalation, ingestion, or external exposure value by 27.03. Users can
calculate cancer risks using slope factors expressed in either customary units or SI units with equivalent results, provided
that they also use air, water and soil concentration values in the same system of units.
D-l
-------
Table D.1 Radionuclide Cancer Morbidity - Slope Factors (1)
Radionuclide
Ac-227+D
Ag-108m+D
Ag-110m+D
Am-241
Am-243+D
Bi-207
C-14
Cd-109
Ce-144+D
CI-36
Cm-243
Cm-244
Co-57
Co-60
Cs-134
Cs-135
Cs-137+D
Eu-152
Eu-154
Eu-155
Fe-55
Gd-153
H-3
1-129
K-40
Mn-54
Na-22
Nb-94
Ni-59
Ni-63
Np-237+D
Pa-231
Pb-210+D
Pm-147
Pu-238
Pu-239
Pu-240
Pu-241
Pu-242
Pu-244+D
Ra-226+D
Ra-228+D
Ru-106+D
Sb-125+D
Sm-147
Sm-151
Sr-90+D
Tc-99
Th-228+D
Th-229+D
Th-230
Th-232
TI-204
U-232
U-233
U-234
U-235+D
U-236
U-238+D
Slope Factor (Morbidity Risk Coefficient)
Lifetime Excess Cancer Risk per Unit Exposure
Water
Ingestion
(risk/pCi)
4.86E-10
8.14E-12
9.88E-12
1.04E-10
1.08E-10
5.66E-12
1.55E-12
5.00E-12
3.53E-11
3.30E-12
9.47E-11
8.36E-11
1.04E-12
1.57E-11
4.22E-11
4.74E-12
3.04E-11
6.07E-12
1.03E-11
1.90E-12
8.62E-13
1.52E-12
5.07E-14
1.48E-10
2.47E-11
2.28E-12
9.62E-12
7.77E-12
2.74E-13
6.70E-13
6.74E-11
1.73E-10
1.27E-09
1.69E-12
1.31E-10
1.35E-10
1.35E-10
1.76E-12
1.28E-10
1.44E-10
3.86E-10
1.04E-09
4.22E-11
5.13E-12
3.74E-11
5.55E-13
7.40E-11
2.75E-12
3.00E-10
5.28E-10
9.10E-11
1.01E-10
5.85E-12
2.92E-10
7.18E-11
7.07E-11
7.18E-11
6.70E-11
8.71E-11
Food
Ingestion
(risk/pCi)
6.53E-10
1.12E-11
1.37E-11
1.34E-10
1.42E-10
8.14E-12
2.00E-12
6.70E-12
5.19E-11
4.44E-12
1.23E-10
1.08E-10
1.49E-12
2.23E-11
5.14E-11
5.88E-12
3.74E-11
8.70E-12
1.49E-11
2.77E-12
1.16E-12
2.22E-12
6.51E-14
3.22E-10
3.43E-11
3.11E-12
1.26E-11
1.11E-11
3.89E-13
9.51E-13
9.10E-11
2.26E-10
3.44E-09
2.48E-12
1.69E-10
1.74E-10
1.74E-10
2.28E-12
1.65E-10
1.90E-10
5.15E-10
1.43E-09
6.11E-11
7.21E-12
4.77E-11
8.07E-13
9.53E-11
4.00E-12
4.22E-10
7.16E-10
1.19E-10
1.33E-10
8.25E-12
3.85E-10
9.69E-11
9.55E-11
9.76E-11
9.03E-11
1.21E-10
Soil
Ingestion
(risk/pCi)
1.16E-09
1.92E-11
2.37E-11
2.17E-10
2.32E-10
1.49E-11
2.79E-12
1.14E-11
1.02E-10
7.66E-12
2.05E-10
1.81E-10
2.78E-12
4.03E-11
5.81E-11
7.18E-12
4.33E-11
1.62E-11
2.85E-11
5.40E-12
2.09E-12
4.26E-12
9.25E-14
2.71E-10
6.18E-11
5.14E-12
1.97E-11
2.05E-11
7.33E-13
1.79E-12
1.62E-10
3.74E-10
2.66E-09
4.88E-12
2.72E-10
2.76E-10
2.77E-10
3.29E-12
2.63E-10
3.14E-10
7.30E-10
2.29E-09
1.19E-10
1.32E-11
7.59E-11
1.59E-12
1.44E-10
7.66E-12
8.09E-10
1.29E-09
2.02E-10
2.31E-10
1.54E-11
5.74E-10
1.60E-10
1.58E-10
1.63E-11
1.49E-10
2.10E-10
Inhalation
(risk/pCi)
2.09E-07
2.67E-11
2.83E-11
2.81 E-08
2.70E-08
2.10E-11
7.07E-12
2.19E-11
1.10E-10
2.50E-11
2.69E-08
2.53E-08
2.09E-12
3.58E-11
1.65E-11
1.86E-12
1.19E-11
9.10E-11
1.15E-10
1.48E-11
7.99E-13
6.55E-12
5.62E-14
6.07E-11
1.03E-11
5.88E-12
3.89E-12
3.77E-11
4.66E-13
1.64E-12
1.77E-08
4.55E-08
1.39E-08
1.61E-11
3.36E-08
3.33E-08
3.33E-08
3.34E-10
3.13E-08
2.93E-08
1.16E-08
5.23E-09
1.02E-10
1.93E-11
6.88E-09
4.88E-12
1.13E-10
1.41E-11
1.43E-07
2.25E-07
2.85E-08
4.33E-08
2.45E-12
1.95E-08
1.16E-08
1.14E-08
1.01 E-08
1.05E-08
9.35E-09
External Exposure
(risk/yr per
PCi/g soil)
1.47E-06
7.19E-06
1.30E-05
2.76E-08
6.36E-07
7.08E-06
7.83E-12
8.73E-09
2.44E-07
1.74E-09
4.19E-07
4.85E-11
3.55E-07
1.24E-05
7.10E-06
2.36E-11
2.55E-06
5.30E-06
5.83E-06
1.24E-07
0
1.62E-07
0
6.10E-09
7.97E-07
3.89E-06
1.03E-05
7.29E-06
0
0
7.97E-07
1.39E-07
4.21 E-09
3.21 E-11
7.22E-11
2.00E-10
6.98E-11
4.11E-12
6.25E-11
1.51E-06
8.49E-06
4.53E-06
9.66E-07
1.81E-06
0
3.60E-13
1.96E-08
8.14E-11
7.76E-06
1.17E-06
8.19E-10
3.42E-10
2.76E-09
5.98E-10
9.82E-10
2.52E-10
5.43E-07
1.25E-10
1.14E-07
Notes
2
2
2
2
3
2
2
4
5
2
2
2
2
2
2
2
2
2
2
2
2
D-2
-------
Radionuclide
Zn-65
Water
Ingestion
(risk/pCi)
1.17E-11
Food
Ingestion
(risk/pCi)
1.54E-11
Soil
Ingestion
(risk/pCi)
2.45E-11
Inhalation
(risk/pCi)
5.81E-12
External Exposure
(risk/yr per
PCi/g soil)
2.81 E-06
Notes
Notes:
1. A curie (Ci), the customary unit of activity, is equal to 3.7x 1010 nuclear transformations per second. 1 picocurie (pCi) = 10~12
Ci. If required, slope factors in Table D.1 can be converted into the International System (SI) units of becquerels(1 Bq = 1 nuclear
transformation per second) by multiplying each inhalation, ingestion, or external exposure value by 27.03. Users can calculate
cancer risks using slope factors expressed in either customary units or SI units with equivalent results, provided that they also
use air, water, food and soil concentration values in the same system of units.
2. For each radionuclide listed, slope factors correspond to the risks per unit intake or exposure for that radionuclide only, except
when marked with a "+D". In these cases, the risks from associated short-lived radioactive decay products (i.e., those decay
products with radioactive half-lives less than or equal to 6 months) are also included, based on an assumption of secular
equilibrium. These decay chains are identified in Table C.1 of Attachment C.
3. The inhalation slope factor listed represents inhalation of C-14 as a particulate. Alternative values for inhalation of C-14 as a
gas are 3.36E-15 risk/pCi for carbon monoxide and 1.99E-14 risk/pCi for carbon dioxide.
4. The inhalation slope factor for H-3 represents inhalation of titiated water vapor, which is considered the most likely form in the
environment. Alternative values of inhalation of H-3 include 1.99E-13 risk/pCi for particulates, 5.62E-18 risk/pCi for elemental
hydrogen gas, and 1.28E-13 risk/pCi for organic forms. Similarly, the ingestion slope factor values for H-3 represent ingestion
of tritiated water, which is considered the most likely form in the environment. Alternative values for ingestion of organically bound
forms of H-3 in water, food, and soil are 1.12E-13 risk/pCi, 1.44E-13 risk/pCi, and 2.02E-13 risk/pCi, respectively.
5. The food ingestion slope factor for 1-129 represents ingestion of milk. For ingestion of non-dairy foodstuffs, a lower value of
1.93E-10 risk/pCi ingested would apply. The inhalation slope factor for 1-129 represents inhalation of particulates; alternative
values for inhalation of 1-129 vapor are 1.24E-10 for inhalation of methyl iodide and 1.60E-10 for inhalation of other compounds
in vapor form.
D-3
-------
D.2 MCLs for Radionuclides in Drinking Water
Current MCLs for radionuclides are set at 4 mrem/yr for the sum of the doses from beta particles and photon emitters,
15 pCi/L for gross alpha particle activity (including Ra-226, but excluding uranium and radon), and 5 pCi/L
combined for Ra-226 and Ra-228. The current MCLs for beta emitters specify that MCLs are to be calculated based
upon an annual dose equivalent of 4 mrem to the total body or any internal organ. It is further specified that the
calculation is to be performed on the basis of a 2 liter per day drinking water intake using the 168 hours data listed
in "Maximum Permissible Body Burdens andMaximum Permissible Concentrations of Radionuclides in Air or Water
for Occupational Exposure, " NBS Handbook 69 as amended August 1963, U.S. Department of Commerce (U.S.
DOC, 1963). These calculations have been done for most beta emitters and published as part of the EPA Office of
Water Supply's National Interim Primary Drinking Water Regulations, Report EPA-570/9-76-003 (U.S, EPA, 1976).
The calculated MCLs are included in Table D.2. For those beta emitters not included in EPA-570/9-76-00, MCLs
have been calculated, for purposes of this guidance, using the existing MCL methodology, and are also included in
Table D.2.
In July 1991, EPA proposed to revise the MCLs for Ra-226 and Ra-228 to 20 pCi/L for each, change the methodology
used for determining a 4 mrem/yr dose for the sum of the doses from beta particles and photon emitters, alter the
definition of alpha particle activity to exclude Ra-226, and establishing new MCLs of 300 pCi/L for Rn-222 and 20
(ig/L (30 pCi/L) for uranium (56 FR 33050). EPA is under Court Order to either finalize the 1991 proposal for
radionuclides (except for radon), or to ratify existing standards by November 2000. On April 21,2000 EPA solicited
comment in a Notice of Data Availability (NODA) on three options for a uranium MCL: 1) 20 /j,g/l and 20 pCi/1 as
a preferred option, 2) 40 ,ug/l and 40 pCi/1, and 3) 80 ,ug/l and 80 pCi/1 (65 FR 21576). In this NODA, EPA
indicated that changes would not be made to the existing MCLs for radium, alpha particle activity, and beta particles
and photon emitters. The 1996 Amendments to the Safe Drinking Water Act (SOWA) require EPA to propose a
MCL for radon by August 1999, and to finalize the MCL by August 2000. To comply with the requirements of the
amended SDWA, on August 6,1997, EPA withdrew its 1991 proposal for Rn-222 (62 FR 42221). EPA issued a new
proposal for Rn-222 (65 FR 21576). EPA proposed an MCL of 300 pCi/1 with an alternative MCL of 4,000 pCi/1
if a state or local indoor radon mitigation program was established.
D-4
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Table D.2 Radionuclide Drinking Water MCLs
Radionuclide
Ac-227
Ag-108m
Ag-110m
Am-241
Am-243
Bi-207
C-14
Cd-109
Ce-144
CI-36
Cm-243
Cm-244
Cm-248
Co-57
Co-60
Cs-134
Cs-135
Cs-137
Eu-152
Eu-154
Eu-155
Fe-55
Gd-153
H-3
1-129
K-40
Mn-54
Na-22
Nb-94
Ni-59
Ni-63
Np-237
Pa-231
Pb-210
Pm-147
Pu-238
Current MCLa b
(pCi/L)
90
15
15
200
2,000
600
30
700
15
15
15
1,000
100
80
900
200
200
60
600
2,000
600
20,000
1
300
400
300
50
15
15
587
15
Proposed MCL
(pCi/L)
Risk Base Limit
(RBL)e
(pCi/L)
0.24
5.8
1.9
6.1
0.054
Mass Equivto MCL,
Proposed MCL, or
RBL (mg/L)
3.3E-12
2.2E-10
1.9E-11
4.4E-09
7.5E-08
4.4E-09
4.5E-07
2.3E-10
9.1E-12
2.1E-05
2.9E-10
1.9E-10
3.5E-06
1.2E-10
8.9E-11
6.2E-11
7.8E-04
2.3E-09
1.1E-09
2.3E-10
1.3E-09
8.3E-10
1.7E-10
2.1E-09
5.7E-06
2.7E-4
3.9E-11
6.4E-11
3.3E-8
3.7E-06
8.5E-10
2.1E-05
3.2E-07
7.1E-13
6.3E-10
8.8E-10
D-5
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Table D.2 Radionuclide Drinking Water MCLs
Radionuclide
Pu-239
Pu-240
Pu-241
Pu-242
Pu-244
Ra-226
Ra-228
Ru-106
Sb-125
Sm-147
Sm-151
Sr-90
Tc-99
Th-228
Th-229
Th-230
Th-232
TI-204
U-232
U-232
U-233
U-233
U-234
U-234
U-235
U-235
U-236
U-236
U-238
U-238
Zn-65
Current MCLa b
(pCi/L)
15
15
15
15
5C
5C
30
300
15
1,000
8
900
15
15
15
15
300
300
Proposed MCL
(pCi/L)
20d
(20 /.g/l)d
20d
(20 /4)/l)d
20d
(20 /.g/l)d
20d
(20 /4)/l)d
20d
(20 /.g/l)d
20d
(20 /4)/l)d
Risk Base Limit
(RBL)e
(pCi/L)
27
Mass Equivto MCL,
Proposed MCL, or
RBL (mg/L)
2.4E-07
6.6E-08
2.6E-10
3.8E-06
8.5E-04
5.1E-09
1.8E-11
9.0E-12
2.9E-10
6.5E-01
3.8E-08
5.9E-11
5.3E-05
1.8E-11
7.1E-08
7.4E-07
1.4E-01
6.5E-10
9.4E-10
2.0E-02
2.1E-06
2.0E-02
3.2E-06
2.0E-02
9.3E-03
2.0E-02
3.1E-04
2.0E-02
6.0E-02
2.0E-02
3.6E-11
Notes:
a Existing MCL is 4 mrem/yrto the whole body or an organ, combined from all beta and photon emitters.
b Existing MCL is 15 pCi/L, with the concentration level combined for all alpha emitters, except radon and uranium.
c Existing MCL is 5 pCi/L combined for Ra-226 and Ra-228.
d Preferred EPA proposed MCL standard is 20 ,ug/l and 20 pCi/l for uranium, with EPA soliciting comments on options of
40 /^g/l and 40 pCi/l, and 80 /^g/l and 80 pCi/l. The preferred proposed MCL standard for uranium of 20 /^g/l and 20 pCi/l
is represented in this table.
D-6
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Risk Based Limits calculated for 30-year exposure duration and 10~6 risk. These were calculated using equation 11' in
Risk Assessment Guidance for Superfund (RAGS): Volume i: Human Health Evaluation Manual (Part B, Development
of Risk-based Preliminary Remediation Goals), (page 37). The equations were adjusted to account for radioactive decay.
D-7
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