\
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
MEMORANDUM
SUBJECT:
FROM:
Determining Groundwater Exposure Point Concentrations, Supplemental
Guidance
Dana Stalcup, Acting Director ^H^^ I
Assessment and Remediation Division
Office of Superfund Remediation and Technology Innovation
Superfund National Policy Managers, Regions 1-10
Purpose
The mission of the Superfund program is to protect human health and the environment consistent
with the Comprehensive Environmental Response, Compensation and Liability Act, as amended,
and as implemented by the National Oil and Hazardous Substances Pollution Contingency Plan.
This memorandum transmits Determining Groundwater Exposure Point Concentrations, which
is attached, and is to be used in the remedial investigation and feasibility study process (e.g.,
assessing baseline health risks, evaluating risks of remedial alternatives) and five-year reviews of
selected remedies.
Background
During the October 2011 to February 2013 period, a workgroup comprised of members of two
EPA forums, the OSWER Human Health Regional Risk Assessors Forum (OHHRRAF) and the
Ground Water Forum (GWF), deliberated about how to determine groundwater exposure
concentrations. As a result of a consensus-driven process, the attached guidance document was
prepared, vetted, and finalized.
Objective
The attached guidance has been developed to reduce unwarranted variability in the exposure
assumptions used by Regional Superfund staff to characterize exposures to human populations in
the baseline risk assessment. Other cleanup programs in the Office of Solid Waste and
Internet Address (URL) • http://www.epa.gov
Recycled/Recyclable . Printed with Vegetable Oil Based Inks on Recycled Paper (Minimum 20% Postconsumer)
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Emergency Response (OSWER) are welcome and encouraged to adopt this guidance, much as
they have historically adopted other aspects of the Risk Assessment Guidance for Superfund
(RAGS).
Implementation
The attached guidance is based on: the Supplemental Guidance to RAGS: Calculating the
and the updates provided in Q/
Confidence Limits for Exposure Point Concentrations at Hazardous Waste Sites (Publication.
928S.6ilO). The attached guidance supplements these documents by adding a recommended
approach for calculating the ground-water exposure point concentration. Procedures
recommended in this directive are consistent with the intent of these previous guidance
documents on the subject.
The guidance can be found at www.epa.gov/oswer/riskassessment/superfund_hh_exposure.htrn
Please contact Richard Kapuscinski at (703) 305-741 1 if you have questions or concerns.
Attachment
cc: Mathy Stanislaus, OSWER
Barry Breen, OSWER
James Woolford, OSWER/OSRTI
Larry Stanton, OSWER/OEM
Barnes Johnson, OSWER/ORCR
David Lloyd, OSWER/OBLR
Reggie Cheatham, OSWER/FFRRO
Carolyn Hoskinson, OSWER/OUST
Rafael Deleon, OECA/OSRE
Dave Kling, OECA/FFEO
John Michaud, OGC/SEWRLO
OSRTI Managers
Regional Superfund Branch Chiefs, Regions 1-10
Lisa Price, Superfund Lead Region Coordinator, Region 6
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v>EPA
United States Environmental Protection Agency
Office of Solid Waste and Emergency Response
QSWER Directive 9283,1-42, Februarv 2014
Determining Groundwater Exposure Point
Concentrations
Disclaimer: This document presents current technical and policy recommendations of the U.S.
Environmental Protection Agency (EPA). This guidance document does not impose any requirements or
obligations on the U.S. Environmental Protection Agency (EPA), the states, or the regulated community.
Rather, the sources of authority and requirements for addressing groundwater contamination are the
relevant statutes and regulations. Decisions regarding a particular situation should he made based upon
statutory and regulatory authority. EPA decision-makers retain the discretion to adopt or approve
approaches on a case-hy-case basis that differ from this guidance document, where appropriate, as long
as the administrative record supporting its decision provides an adequate basis and reasoned explanation
for doing so.
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OSWER Directive 9283.1-42
GW EPC Guidance
Table of Contents
Contents Page Number
Work Group Members iii
List of Acronyms iv
Definitions v
Introduction 1
Information Needed to Calculate an Appropriate GW EPC 2
Well Types 4
Data Quality 5
Method to Develop GW EPCs 6
Summary 8
References and Citations 9
Figure 10
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OSWER Directive 9283.1-42
GW EPC Guidance
Prepared Jointly By the
OSWER Human Health Regional Risk Assessment Forum & the
Ground Water Forum
Work Group Members
(* indicates chair)
Region 1 - Mary Ballew
Region 2 - Lora Smith*, Rob Alvey
Region 3 - Dawn loven, Kathy Davies
Region 4 - Glenn Adams*, Bill O'Steen
Region 5 - Keith Fusinski, Luanne Vanderpool
Region 6 - Alethea Tsui-Bowen, Gregory Lyssy
Region 7 - Todd Phillips, Dan Nicoski
Region 8 - Wendy O'Brien, Andrew Schmidt
Region 9 - Herb Levine*
Region 10 - Marcia Bailey, Bernie Zavala
HQ Representative: Rich Kapuscinski
Additional Reviewers
Region 1 - Chau Vu, Margaret McDonough
Region 2 — Marian Olsen
Region 3 — Jennifer Hubbard
Region 6 - Vince Malott, Chris Villarreal
Region 10 - Marcia Knadle
HQ: Helen Dawson (retired), Linda Gaines, Zubair Saleem (retired)
HI
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List of Acronyms
ARARs: Applicable and/or Relevant and Appropriate Requirements
CERCLA: Comprehensive Environmental Response, Compensation, and Liability Act
DQO: Data Quality Objective
EPA: U.S. Environmental Protection Agency
EPC: Exposure Point Concentration
GW CSM: Groundwater Conceptual Site Model
GWF: Groundwater Forum
MCL: Maximum Contaminant Level
MDL: Method Detection Limit
NAPL: Non-aqueous phase liquid
OHHRRAF: OSWER Human Health Regional Risk Assessment Forum
OSWER: Office of Solid Waste and Emergency Response
QAPP: Quality Assurance Project Plan
QL: Quantitation Limit
RAGS: Risk Assessment Guidance for Superfund
RCRA: Resource Conservation and Recovery Act
RL: Reporting Limit
RSL: Regional Screening Level (http://www.epa.gov/region9/superftind/prg/)
SAP: Sampling and Analysis Plan
UGL: Upper confidence limit
VOC: Volatile organic contaminant
IV
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OSWER Directive 9283.1-42
GW EPC Guidance
Definitions1
Commingled Plume: A commingled plume exists where groundwater contaminant plumes
from two or more discrete releases have mixed or encroached upon one another.
Comprehensive Environmental Response, Compensation, and Liability Act: The
Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA),
commonly known as the Superfund law, was enacted by Congress on December 11, 1980 and
amended by the Superfund Amendments and Reauthorization Act (SARA) on October 17, 1986,
It is a United States federal law designed to clean up sites contaminated with hazardous
substances. (hJtjT^ywwjv^a_,g.oy/superfund/policy/ccrcla.htm)
Contaminant Plume: A groundwater contaminant plume is a three-dimensional, dynamic (i.e.,
may vary temporally), potentially irregular distribution of contaminants dissolved or suspended
in groundwater. The shape and size of a plume depends on the geologic framework, ground-
water flow system, type and concentration of contaminants, and variations in the contaminants'
release history. For the purposes of groundwater exposure point concentration (GW EPC)
calculations, a groundwater contaminant plume is defined as the volume of groundwater with
contaminant concentrations exceeding risk-based tapwater Regional Screening Levels (RSLs)
and/or Maximum Contaminant Levels (MCLs) or other applicable criteria.
Contaminant Source: A contaminant source (source area) is a three-dimensional zone of high
contaminant concentrations resulting from a release of contaminants to the environment and
from which contaminants may migrate.
Core/center of the plume: The three-dimensional core/center of the plume is defined as the
zone of highest concentrations of each contaminant within a delineated groundwater plume. See
Figure 1.
Exposure Point Concentration (EPC): As defined generally for EPA's cleanup programs, the
EPC is intended to be a conservative estimate of the average chemical concentration in an
environmental medium (EPA 2002). For the purposes of this document, the environmental
medium is groundwater.
Groundwater Conceptual Site Model: A groundwater conceptual site model (GW CSM) is a
multi-dimensional qualitative and quantitative representation of the groundwater flow and solute
transport system. The GW CSM conveys what is known or suspected about contaminants of
potential concern, locations of probable contamination sources, release mechanisms and timing,
potential migration pathways, and potential (current and future) receptors. The GW CSM uses a
concise combination of written and graphical work products (e.g., maps, cross sections,
diagrams) to provide a site-specific description of the migration and fate of contaminants with
respect to possible receptors and the geologic, hydrologic, biologic, geochemical, and
anthropogenic factors that control contaminant distribution. A robust GW CSM is a
comprehensive, clear, internally consistent, multi-dimensional understanding of site conditions
and processes, including the temporal variability of conditions and processes at the site. Like
1 Definitions are provided for the purposes of this guidance.
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OSWER Directive 9283.1-42
GW EPC Guidance
CSMs utilized in other arenas (e.g., risk assessment), the GW CSM is a dynamic tool and is
reevaluated, refined, and revised as new site information is collected.
Human Health Risk Assessment: A liMIEMlit^hji^isiesjmiTii (HHRA) is the process to
estimate the nature and probability of adverse health effects in humans who may be exposed to
chemicals in contaminated media, now or in the future. EPA begins the HHRA with a
and scoping phase which is then followed by the four steps: !]azMdJdcnii£^ dose:rcsj}onse
.iiM>--:rKM!', «p; NUT ..i-v^ -n.t •.:, and li^diajiictajzaiion. Risk Assessment Guidance for
Superfund, Part A ('•', \< > ^ ! ' •- ; \) provides guidance on the human health evaluation activities
that are conducted during the baseline risk assessment at Superfund sites (EPA Human Health
Risk Assessment website: http://www.epa.gov/risk_assessment/health-risk.htm).
Method Detection Limit: The method detection limit (MDL) is a laboratory/method/instrument
capability value and varies by laboratory and instrument. It is the minimum concentration of an
analyte that can be identified, measured and reported with 99% confidence that the analyte
concentration is greater than zero. This is a statistical determination of precision; accurate
quantitation is not expected at this level. As a result, this value is not usually specified for a
specific project, so it is usually not reported in the data.
Non-Aqueous Phase Liquids: Non-aqueous phase liquids (NAPLs) are liquids that, like oil, are
immiscible in water. There are two classes: light NAPLs (LNAPLs), such as gasoline, are less
dense than water and will tend to float on the water table; dense NAPLs (DNAPLs), such as the
common solvent trichloroethylene, are denser than water and tend to sink once they reach the
water table.
Quality Assurance Project Plan: A QiMil3LAssLiraji(^,Pr^ecl£ia|i (QAPP) is developed to
document the planning, implementation, and assessment procedures for a particular site, as well
as any specific quality assurance and quality control activities that are needed. The data are
evaluated to determine whether the objectives in the QAPP have been met and the data is of
adequate quality for further evaluation. The establishment of data quality objectives (DQOs) is a
critical component of the process and assuring that data is collected in a manner that will allow
its use in the calculation of the GW EPCs.
Quantitation Limit: The quantitation limit (QL) is the minimum concentration that can be
reported as a quantitative value for an analyte in a sample, typically a reference sample. This
concentration can be no lower than the concentration of the lowest calibration standard for that
analyte. It generally is specified in advance for a specific project. EPA generally establishes the
Project QL prior to sample analysis through the identification of Data Quality Objectives in the
Quality Assurance Project Plan (QAPP). The Project QL ideally is 3-10 times lower than the
Screening Level, when technically feasible. Laboratories with a laboratory QL at or below the
Project QL generally are selected to perform the analyses. The Laboratory QLs for the Contract
Laboratory Program are found at httjj^^/wjA^ejjaj^^^ and are
called "Contract Required Quantitation Limits" in that reference.
Piezometer: A type of well whose primary purpose is to measure the elevation of the water table
or the groundwater pressure head at a point in the subsurface (i.e. the potentiometric surface).
Generally piezometers have a relatively small (less than I inch) diameter and are not designed to
obtain groundwater samples for chemical analysis. Monitoring wells differ from piezometers in
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OSWER Directive 9283. 1-42
GW EPC Guidance
that they are designed so that ground-water samples can be obtained and are larger than
piezometers (typically larger than 1.5 inches in diameter), although sampling devices have been
developed that allow groundwater samples to be obtained from smaller diameter wells.
Potable: Water designated as a drinking water source.
Remedial Investigation: A rcjj^m^ - --„-."' ' (RO serves as the mechanism for collecting
data to characterize site conditions, determine the nature of the waste, assess risk to human
health and the environment and conduct treatability testing to evaluate the potential performance
and cost of the treatment technologies that are being considered.
http://www.epa.gov/superfund/cleanup/rifs.htm
Reporting Limit: A reporting limit (RL) is a sample-specific quantitation limit that has been
adjusted for dilutions, moisture content, or other sample-specific factors. This value is the
quantitation limit actually achieved in the analysis. The RL may be the same as the quantitation
limit that was set as the goal for project planning. Often, however, it will be higher than the
quantitation limit for samples with high concentrations of contaminants or a matrix that
interferes with the analysis. This is the value that normally appears on the data sheet for data
reporting.
Sampling and Analysis Plan: A Sampling and Analysis Plan (SAP) is a project proposal that
describes how the assessor will address a specific situation. Typically, it details the project goals
and purpose, sample locations, sample frequency, parameters measured, field and laboratory,
protocols, etc. The SAP must meet the DQOs outlined under the QAPP. SAPS also describe
how the data will be managed and used and how data quality will be evaluated SAPs may refer
back to the programmatic QAPP for the QA/QC protocols.
Screening Level(s): Screening Levels are chemical-specific concentrations for individual
environmental contaminants may warrant further investigation or site cleanup. Screening Levels
for establishing data quality objectives for groundwater sampling and for delineating plume
extent include: M .• :• 11 n> < ". :,iU!,na.-,. i c\i ii- - \!' " ) as established by the Safe Drinking
Water Act program by EPA's Office of Groundwater and Drinking Water in the Office of Water
and 'C'^Mui!.'. v <*,.i.ii-:j - ,»,' for residential tapwater. Other applicable criteria (e.g.,
promulgated State Drinking Water Standards) are considered on a site-specific basis,
Site Team: For purposes of this document, a site team is typically composed of the Project
Manager, Hydrogeologist, Ecological Risk Assessor, Human Health Risk Assessor, and other
scientists as needed.
Superfund: Superfund is the name given to the environmental program established to address
abandoned hazardous waste sites. It is also the name of the fund established by the
Comprehensive Environmental Response, Compensation and Liability Act of 1980, as amended
(CERCLA) (EPA Superfund website: http: v\ \v\VArria .uo\ 'super! iintl 'about, htn], August 21 ,
2013),
Superfund Sites: Sites included in the Superfund Program, (http://www.epa.gov/superfund/)
Vll
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1.0 Introduction
Human health risk assessments conducted at sites investigated in accordance with the
Comprehensive Environmental Response, Compensation and Liability Act of 1980, as amended
(CERCLA) and the Resource Conservation and Recovery Act, as amended (RCRA) require
estimates of the contaminant concentrations in various media to which humans are currently
exposed or are reasonably expected to be exposed in the future. Where contaminated
groundwater at a site is designated as potable, currently or in the future, an estimate of the
exposure point concentration for groundwater (GW EPC) is needed. As defined here, a GW
EPC is a conservative estimate of the average chemical concentration in groundwater at a
potential location and point in time. This guidance outlines a recommended approach for
estimating a GW EPC for use in evaluating risk posed by reasonable maximum exposure
conditions at sites with contaminated groundwater and is intended to improve the quality and
consistency of calculating EPCs for groundwater in risk assessments performed at EPA's
Superfund and RCRA corrective action sites. This recommended approach is based on the
itil Jjiiitluncc 10 RA(.iS: ( 'ulculafing ihe (Joncenii'aiion Term (Publication ^285.7-081)
and updates provided in CiihwLinngJJ^
Co)iwniraii< >m at Hazardous Wuxie Silcs (Pub! ication 9285 .6- 1 Oj, but adds a recommended
approach for calculating the GW EPC, Procedures recommended in this guidance are consistent
with the intent of these previous guidance documents on the subject.
The OSWER Human Health Regional Risk Assessment Forum (OHHRRAF) and the US EPA
Ground Water Forum (GWF) joined together to prepare this document. The OHHRRAF consists
of US EPA human health risk assessors who develop and promote scientifically defensible and
nationally consistent methods for conducting human health risk assessments and who provide
risk managers at CERCLA and RCRA corrective action sites with information needed to make
and communicate transparent, reasonable, and protective decisions. The GWF is a group of EPA
groundwater technical specialists representing US EPA's Regional Superfund and RCRA
offices, responsible for the identification and resolution of groundwater issues impacting the
remediation of sites. These two forums are referred to as "the Forums" throughout the remainder
of this guidance.
This guidance presents current technical and policy recommendations of the EPA's Office of
Solid Waste and Emergency Response (OSWER). The recommendations herein are not intended
to apply or establish a precedent for any other purpose, including the purpose of evaluating
completion of groundwater restoration.
The intended audience for this guidance is CERCLA and RCRA risk assessors, hydrogeologists
and site project managers. This document does not address vapor intrusion or non-aqueous
phase liquids (NAPLs) and is not intended to determine the attainment of Applicable and/or
Relevant and Appropriate Requirements (ARARs) and/or cleanup goals. For more information
on the vapor intrusion issue, please refer to the following EPA website:
hup,:// www x'rja-gojkVojiw^ryjjrjp^^
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OSWER Directive 9283.1-42
GW EPC Guidance
2.0 Information Needed to Calculate an Appropriate
Groundwater Exposure Point Concentration
Prior to the development of a GW EPC, it is assumed that a site team has been assembled, the
Remedial Investigation and human health risk assessment (HHRA) are underway, and a robust
GW CSM that clearly defines the plume(s) has been developed. Decisions about the data to be
used in EPC calculations ideally are based on the groundwater conceptual site model (GW
CSM). In addition, information about well construction, including screened interval, is generally
useful for supporting decisions about data sets to use for EPC calculation. Decisions about the
data to be used in EPC calculations ideally are based on the GW CSM.
Data used in EPC calculations are most informative if recent and from the core of the plume. It
is typically not appropriate to use modeled concentrations in GW EPC calculations; actual
sampling data are preferred.
If the GW CSM has identified a seasonal or other temporal influence (e.g. drought patterns, tidal
cycles, or changes in patterns of groundwater withdrawal or irrigation) on contaminant
concentrations, OSWER recommends using data collected during times of higher detected
concentrations in the calculation. Regional hydrogeologists can be consulted to determine if
seasonality or other temporal influences are an issue at the site and if so, determine the
appropriate sampling and dataset for that site.
If seasonality or other temporal influences are not a site issue, then OSWER recommends using
data collected from the latest two rounds of sampling for each selected well. Generally it is
recommended to use data collected within the last year so that the data will be representative of
current conditions. (Depending on the GW CSM, the amount of time required to be
representative of current conditions could be more or less than one year).
Factors to consider when evaluating whether or not data are representative of current conditions
include the following:
> Movement - OSWER recommends that groundwater flow rates be considered
when determining which data are representative of current conditions (the faster
the How rates, the less representative older data will be) and to evaluate future
risks.
> Fate and transport - OSWER recommends that attenuation processes be
considered when determining which data are representative of current conditions
(the higher the attenuation rates, the less representative older data may be) and to
evaluate future risks, For more details on this and other fate and transport issues,
consult with a regional hydrogeologist.
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OSWER Directive 9283.1-42
GW EPC Guidance
Generally, when there is more than one aquifer at a site, OSWER recommends that the aquifers
be considered separately when calculating an EPC (i.e., EPC values for each aquifer).
When the monitoring network provides sample concentrations from multiple sample depths at a
given location within a plume (e.g., nested, paired, and/or multiport monitoring wells in the same
aquifer), OSWER recommends using the highest detected concentration from such samples at
each location to calculate the GW EPC for each aquifer (e.g., if there are two samples from
different depths in a two-port well in the same aquifer at a given location within the plume
footprint, it is recommended that the higher of the two sample concentrations be used in the EPC
calculation, along with concentrations from other wells in the aquifer). When there are multiple
groundwater contaminants with concentrations that are not proportional, this recommended
approach could result in different samples (e.g., sampling depths) being used to characterize
exposure concentrations for a given plume.
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OSWER Directive 9283.1-42
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3.0 Well Types
Several different types of wells may be present on a site. Typically, sampling results from
monitoring wells are the only data acceptable for use in the GW EPC calculation. If sampling
results from a well type other than a monitoring well are being considered, coordination with the
site team is important to assure appropriate use of the data in the calculation of the GW EPC.
a. Monitoring wells: Monitoring wells in the core of the plume are the preferred source for
data used in GW EPC calculations for purposes of characterizing a reasonable maximum
exposure condition. At any given location, there may be a single well that provides groundwater
samples at one depth or there may be groundwater samples from multiple depths (e.g., two
paired wells, multiple clustered or nested groups of monitoring wells, or multi-port wells with
multiple sampling depths located in a single hole). It is recommended that the monitoring wells
used have documentation that they have been properly constructed and maintained.
b. Private (residential) drinking water wells: Groundwater samples from residential wells
provide valuable information about current exposure conditions at individual locations; however,
reliable information about the construction and/or depths of residential wells is often limited, if
available at all. It is recommended that each residential well be evaluated on a well-by-well basis
for risk assessment purposes to inform the risk management decision for each individual
property. However, residential well data are not included with monitoring well data in a GW
EPC calculation for evaluating a reasonable maximum exposure condition.
c. Temporary wells (e.g., hydro-punch): Generally, data from these types of wells are not
recommended for use in a GW EPC calculation because the results are not reproducible. There
may be some exceptions to this based on site-specific conditions (e.g., absence of any other type
of well in the core of the plume). OSWER recommends that use of data from temporary wells be
determined on case-by-case basis. If data from temporary wells are used, consultation with the
site team is recommended to assure the approach meets project goals.
d. Piezometers: Depending on the details of their construction, data from piezometers may
or may not be acceptable for use hi GW EPC calculations. Consultation with the site team is
recommended prior to using any data from piezometers.
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GW EPC Guidance
4.0 Data Quality and Other Issues to be Addressed when. Calculating jhe
Groundwater.ExposurePointConcentration
In addition to well type, which is addressed in Section 3.0, the following are some factors to
consider when evaluating data for inclusion in a data set for GW EPC development:
a. Detection/reporting limits - OSWER recommends that Quality Assurance Protection
Plans (QAPPs) be reviewed to ensure that the laboratory can achieve detection/reporting limits
that are below Maximum Contaminant Levels (MCLs) and/or tapwater Regional Screening
Levels (RSLs). OSWER also recommends that sample quantitation limits be reviewed to ensure
that they achieved these data quality goals. If the detection limit is elevated due to interference
for a hazardous substance that is known to be site-related, then OSWER recommends that re-
analyzing or re-sampling be considered.
b. Sampling methods - Sampling methods should be assessed to ensure they meet the data
quality objectives (DQOs) identified in the site-specific Sampling & Analysis Plan (SAP),
c. Turbidity - OSWER recommends that turbidity levels be stable and be as low as possible
(generally less than 5-10 NTUs) prior to sampling. If turbidity levels cannot be stabilized or
adequately reduced by longer purging time and/or lowered pumping rates during purging and
sampling, OSWER recommends that additional well development (and potentially well
replacement) be considered/undertaken before collecting a sample.
d. Filtered vs. unfiltered samples - Unfiltered data are recommended for use in EPC
calculations. From L^LKIld JWiiiMt^lM»L£zlilifen£& Jil^L;.! .......... (!M£i> "If unfiltered water is of
potable quality, data from unfiltered water samples should be used to estimate exposure." In the
rare exception that use of filtered data is needed, it must be agreed upon by the site team.
e. Type of contaminant - Some additional considerations should be made based on the type
of contaminant including fate and transport processes for volatile organic compound (VOC)
breakdown products, NAPL, metals, the potential presence of contaminants of emerging concern
(e.g., 1 ,4-dioxane), etc.
f. Non-detects - Non-detects are frequently an issue at sites. Consultation with the site team
and use of software like ProUCT should be used to address this issue.
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5.0 Method to Develop GW EPCs for Use in the Risk Assessment
There often is a dichotomy between the data needs for site characterization, which focuses on the
nature and the extent of the contamination, and the data needs for a GW EPC calculation, which
focuses on the core (or center) of the contamination plume. For the GW EPC, the assessors need
adequate characterization of the entire plurne to be able to identify the core of the plume
(distinguished by higher concentration levels compared to the lower levels of the plume fringe).
Multiple discrete plumes may be present at a site due to releases from individual sources.
OSWER recommends that each plume be evaluated individually for a unique EPC. For sites that
have commingled plumes from multiple sources and/or separated and distinct plumes that may
commingle under stress conditions, the aggregate risk needs to be evaluated based on the
consideration of the combined effects, when appropriate, from each contaminant present.2 The
risk assessment needs to include information regarding the analysis and calculations.
There are various approaches available to calculate a GW EPC. This guidance provides one
approach to calculating GW EPCs that is expected to be appropriate for a majority of sites.
There may be cases where regional policies and/or site-specific conditions require certain
methods be used that differ from the default method described here. Before selecting the
approach for a site-specific situation, consultation within the regional site team is recommended
to assure that project goals are met.
OSWER generally recommends that monitoring wells within the core/center of the plume be
used to calculate the GW EPC for each contaminant. Data from a minimum of 3 wells in the core
of the plume is generally recommended for this calculation. OSWER recommends that the GW
EPC be calculated as the 95% Upper Confidence Limit (95% UCL) of the arithmetic mean
concentration for each contaminant addressed in the risk assessment. A statistical software
2 When a single contaminating chemical is present in groundwater, the noncancer health risk can
be characterized by calculating the noncancer hazard quotient (HQ). When multiple chemicals
are present in groundwater, the HQ estimates for each chemical are aggregated (as a simple
sum), based upon the assumption that each chemical acts independently (i.e., there are no
synergistic or antagonistic toxicity interactions among the chemicals), after segregating the
chemicals by toxic effect to derive separate hazard index (HI) values for each effect.
The carcinogenic risks can be characterized by calculating the excess cancer risk over a lifetime
(LCR) and, if multiple chemicals are present, aggregating the LCR estimates for each carcinogen
(as a simple sum), based upon the assumption that each chemical acts independently.
3 Sometimes the project team will be interested in quantifying risks in contaminated areas that
are beyond the center of the plume. This may occur when, e.g., there are overlapping plumes,
there are actual receptors located in certain parts of the plume, or there are other site-specific
considerations. Therefore, in such cases, it may be necessary to evaluate risks in other parts of
the plume, in addition to the center of the plume,"
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package such as ProUCL can be used to calculate a 95% UCL. It is generally desirable to use at
least 10 data points for each contaminant (e.g., 5 wells and 2 rounds of data representative of
current conditions equate to 10 data points) to compute a 95% UCL, If the 95% UCL is greater
than the maximum detected concentration, OSWER recommends that the GW EPC default to the
maximum detected concentration for that contaminant. If less than 3 wells are within the core of
the plume, OSWER recommends that maximum detections be used as the EPC for that
contaminant. It is recommended that the uncertainty of using so few data points be discussed in
the Risk Characterization portion of the risk assessment, specifically the uncertainty section of
the risk assessment.
The recommended averaging of concentration data from multiple monitoring wells is intended to
apply solely for purposes of the baseline risk assessment. The recommendations herein are not
intended to apply or establish a precedent for any other purpose, including the purpose of
evaluating completion of groundwater restoration.
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6.0 Summary
This guidance recommends an approach for calculating the GW EPC at Superfund and RCRA
corrective action sites for use in human health risk assessments. The recommended approach is
to calculate a 95% UCL on the arithmetic mean based on data from the core of a contaminant
plume and to use that value (or the maximum value if the 95% UCL exceeds the maximum
value) to represent the GW EPC for potentially exposed individuals. This approach is expected
to be appropriate for a majority of sites,
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7.0 References and Citations
U.S. Environmental Protection Agency (EPA). 2013. Human Health Risk Assessment website.
Available: http://www.epa.gov/risk_assessment/health-risk.htm.
U.S. Environmental Protection Agency (EPA). 2013. Remedial Investigation/Feasibility Study
website. Available: http://www.epa.gov/superfund/eleanup/rifs.htm
U.S. Environmental Protection Agency (EPA). 2013. Superfund website. Available:
http://www.epa.gov/superfund/about.htm.
U.S. Environmental Protection Agency (EPA). 2002. Supplemental Guidance to RAGS:
Calculating Upper Confidence Limits for Exposure Point Concentrations at Hazardous Waste
Sites. Publication Number 9285.6-10. Office of Solid Waste and Emergency Response.
December.
U.S. Environmental Protection Agency (EPA). 1992. Supplemental Guidance to RAGS:
Calculating the Concentration Term. Publication Number 9285.7-081. Office of Solid Waste and
Emergency Response. May.
U.S. Environmental Protection Agency (EPA). 1989. Risk Assessment Guidance for Superfund
(RAGS) Volume I: Human Health Evaluation Manual (Part A). Publication Number EPA/540/1-
89/002. December.
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OSWER Directive 9283.1-42
GW EPC Guidance
FIGURE 1
/• — i-- - "~—~:
i 'r ' ',-
\^j ~T—
1
Source
Area
High
Concentration
Plume Core
Up — TmmHH^B=i==
- -- - :-:;_. ^™
- "" ' tj i i ' ' '
/
Low
Concentration
Plume Fringe
Non-Hazardous
Degradation
Products & Other
Geochemical Indicators
Ground-Water Flow <=*^£3Es-
Idealized Plan View of a Groundwater Contaminant Plume for Purpose of Distinguishing the
"Core" from Fringe Areas
10
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