United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid
Waste and
Emergency
Response
EPA/540/5-96/500
December 1995
&EPA Engineering Forum Issue
DETERMINATION OF BACKGROUND
CONCENTRATIONS OF INORGANICS IN
SOILS AND SEDIMENTS AT HAZARDOUS
WASTE SITES
R. P. Breckenridge1 and A. B. Crockett1
INTRODUCTION
The National Engineering Forum is a
group of U.S. Environmental Protection
Agency (EPA) professionals representing
EPA Regional Offices, committed to the
identification and resolution of engineering
issues affecting the remediation of
Superfund sites. The forum has identified
the need to provide remedial project
managers (RPMs) and other state or private
personnel working with hazardous waste
sites a thought-provoking, technical-issue
paper on how to determine background
concentrations of inorganics in soils and
sediments at hazardous waste sites. Mr.
Frank Vavra and Mr. Bob Stamnes,
Engineering Forum members, provided
technical guidance and direction in the
development of this Issue paper.
This paper was prepared by R. P.
Breckenridge and A. B. Crockett. Support
for this project was provided by the
National Exposure Research Laboratory's
Characterization Research Division with
the assistance of the Superfund Technical
Support Project's Technology Support
Center for Monitoring and Site
Characterization. For further information,
contact Ken Brown, Technology Support
Center Director, at (702) 798-2270, or R. P.
Breckenridge at (208) 526-0757, U.S.
Department of Energy, Idaho National
Engineering Laboratory.
U.S. Department of Energy, Idaho National Engineering Laboratory.
Technology Support Center for
Monitoring and Site Characterization,
Characterization Research Division
Las Vegas, NV 89193-3478
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
Technology Innovation Office
Office of Solid Waste and Emergency Response,
U.S. EPA, Washington, D.C.
Walter W. Kovalick, Jr., Ph.D., Director
Printed on Recycled Paper
764asb95
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PURPOSE AND SCOPE
The purpose of this paper is to provide RPMs and
others investigating hazardous waste sites a
summary of the technical issues that need to be
considered when determining if a site (i.e.,
hazardous waste site/area of concern) has elevated
levels of inorganics relative to the local background
concentrations. This issue paper is narrowly focused
and is for educational use only by project
managers. It is not meant to be a formal guidance
document or "cookbook" on determination of
background concentrations of inorganics at
hazardous waste sites. This issue paper provides the
investigator with information needed to determine
whether activities conducted at a site have resulted
in elevated concentrations of inorganic contaminants
in soils or sediments compared with naturally
occurring and off-site anthropogenic concentrations
of the same contaminant.
The first portion of this paper provides a definition
for and discusses factors that influence background
concentrations. The second portion is separated into
Part A, "Comparing the Concentrations of
Inorganics in Soils and Sediments at Hazardous
Waste versus Background Sites," and Part B,
"Guidance for Addressing High Background
Concentrations of Inorganics at CERCLA
(Comprehensive Environmental Response,
Compensation, and Liability Act) Sites." Part A is
a modification of the State of Michigan guidance on
conducting soil surveys (Michigan 199la, 1991b)
and discusses issues that need to be considered by
investigators attempting to establish background
concentrations for hazardous waste sites. It can be
used to provide potentially responsible parties a
summary of issues they need to consider when
determining whether a hazardous waste site has
elevated concentrations of inorganics compared to a
background site. Part B presents a summary of a
draft issue paper titled, "Options for Addressing
High Background Levels of Hazardous Substances
at CERCLA Sites" (EPA 1992a) and includes
updated information and approaches.
This paper addresses technical issues for scientists
and engineers faced with how to determine
background concentrations. It is not intended to
address agency policy-related decisions on how to
use background data to achieve cleanup levels or
achieve applicable or relevant and appropriate
requirements (ARARs). Technical issues discussed
here include selection of background sampling
locations, considerations in the selection of sampling
procedures, and statistical analyses for determining
whether contaminant levels are significantly
different on a potential waste site and a background
site. How to statistically define background for
purposes of remediating a hazardous waste site to
background levels is not addressed.
This paper focuses on inorganics and, in
particular, metals. Radionuclides are not specifically
addressed; however, metals with radioactive isotopes
(e.g., cobalt-60) that may be encountered at
hazardous waste sites are included. This paper does
not specifically address background concentrations
of organics at a site, but the approach would be very
similar in many respects (except for partitioning),
and some unique aspects regarding organics are
noted.
Statistics play a major role in establishing
background concentration levels, and methods vary
widely in their degree of complexity. No specific
recommendations regarding statistical techniques are
provided because they could be misused or have
policy implications. However, some general
guidance is presented to acquaint the reader with
issues that should be discussed with a statistician
early in the design of a study. Statistics should be
used throughout the development of a sampling plan
in the same manner as quality assurance. Sampling
objectives, design, data analysis, and reporting can
all be influenced by statistical considerations.
To provide recommendations that can be used at a
variety of sites, information was gleaned from
several different approaches to the background issue.
The approach employed by the State of Michigan
(Michigan 1990, 1991b) provides one of the most
straightforward and scientifically sound strategies
that, in combination with EPA documents (EPA
1989a, 1989b) and scientific literature (Underwood
1970; Kabata-Pendias and Pendias 1984), form the
basis for this issue paper. This paper discusses the
generic issues from various strategies that should be
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considered when addressing the background issue.
However, information presented here may need to be
modified to meet site-specific soil and sediment of
data-quality objective concerns.
STATEMENT OF ISSUE
Hazardous waste sites may pose a threat to human
health and the environment when toxic substances
have been released. The hazardous substances at a
site may originate from either "on-site" (i.e.,
resulting from releases attributable to site-specific
activities) or "off-site" (i.e., resulting from sources
not on-site). These "off-site" substances may result
either from natural sources (e.g., erosion of naturally
occurring mineral deposits) or anthropogenic sources
(e.g., widespread lead contamination from auto-
mobile exhaust in urban areas) (EPA 1992a). To
determine the appropriate action to take at a hazard-
ous waste site, EPA must distinguish between
substances directly attributable to the hazardous
waste site (i.e., "site" contaminants) and those
attributable to "natural background" concentrations.
Definitions
Soils and sediments for this issue paper are
defined as all mineral and naturally occurring
organic material located at a site and will mostly be
related to the material <2 mm in size because it is
usually the finer material that has a greater affinity
for inorganic contaminants. The U.S. Department of
Agriculture and the International Soil Science
Society use the 2-mm breakpoint to differentiate
between soils or sediment (consisting of sands, silts,
and clays) and gravel (Breckenridge et al. 1991;
Lewis et al. 1991). When establishing background
concentration levels, it is usually more cost effective
to focus on the finer materials; however, some bias
is introduced. Large particles can be rinsed and the
rinsate analyzed if necessary. Soils and sediments
are heterogeneous and contain a wide range of sizes
from fine clays to larger gravel and coarse fragments
(Soil Science Society of America 1978).
In the soils literature, the term "background"
usually refers to areas in which the concentrations
of chemicals have not been elevated by site
activities. In the sediment literature, terms such as
"background sediment" (in the Code of Federal
Regulations (CFR)-^0 CFR 131.35-91) and
"reference sediment" (ASTM 1990) are used in
similar manners and are often interchangeable.
To minimize confusion, the term "background
concentration" is defined in this document as the
concentration of inorganics found in soils or
sediments surrounding a waste site, but which
are not influenced by site activities or releases. A
"background site" should be a site that is
geologically similar and has similar biological,
physical, and chemical characteristics (e.g., particle
size, percent organic carbon, pH) as the
contaminated site (ASTM 1990) but also should be
upstream, upgradient, or upwind of the site.
Samples taken from a site to determine background
concentrations will be referred to as background
samples.
Almost anyone involved with hazardous waste site
evaluations will at some time be involved in
determining background concentrations of
inorganics at a site. There are two issues to be
considered when addressing background. The first
is whether the site and local area have a high natural
variability in concentrations of inorganics. The
second is to differentiate between natural and
anthropogenic sources at a site with high
background concentrations (e.g., lead in soil due to
automobile emissions). The broad range in
concentrations of naturally occurring inorganics may
lead to the erroneous conclusion that an area has
been contaminated with inorganics. Establishment
of background concentrations based on adequate
site-specific sampling data and comparison to
normal background ranges for a specific area and
land use can help resolve the confusion.
EPA in its Risk Assessment Guidance for Super-
fund: Volume 1, Human Health Evaluation Manual
(Part A.) (often referred to as RAGS) (EPA 1989c)
discusses two categories of background:
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1. Naturally occurring-substances present in the
environment in forms that have not been influenced
by human activity.
2. Anthropogenic-natural and man-made sub-
stances present in the environment as a result of
human activities not specifically related to the
CERCLA site.
Figure 1 shows the relationship between the on-
site-related and off-site-related "populations" of
substances that contribute to concentrations at a site.
In some locations, the background concentrations
resulting from naturally occurring or anthropogenic
sources may exceed contaminant-specific standards
promulgated to protect human health (EPA 1992a).
The background concentration defined in this
document includes both the naturally occurring and
local/regional anthropogenic contributions (see
Figure 1).
Background concentrations are needed when
deciding whether a site is contaminated. Knowledge
of background concentrations helps address issues
such as (a) the effects of past land use practices on
levels of inorganics in soil and sediment, and
(b) establishing lower limits when conducting risk
assessments for soil and sediment contamination.
Figure 2 illustrates a process for determining
whether contaminant concentrations in soil and
sediments at a hazardous waste site are elevated
relative to background concentrations.
Determining the effect of past land use practices
on levels of inorganics in soils and sediments is an
important initial step towards quantifying the
potential threat to human health and the
environment. Information obtained from this step
can provide the first indication that background
concentrations may be elevated. Preliminary site
investigations should be carefully planned so that
high-quality data can be gathered to gain an
understanding of the nature and degree of threat
posed by a site and to determine whether immediate
response is required.
Usually, remedial action is taken only on sites
that exceed a 10"4 incremental cancer risk or exceed
a hazard index of 1.0 for systemic effects.
Superfund cleanups are generally conducted to 10"6
incremental cancer risk or to a safe hazard index.
However, many states have developed statutes
(ARARs) that require more stringent cleanup levels
than risk-based levels and sometimes require
cleanup to natural background concentrations.
It is often best to compare mean concentrations
between groups of similar samples from the
hazardous waste and background sites. Mean values
can be developed for a soil series or an operable
unit. The operable unit is usually the smallest area
that would be considered under a remediation plan
(e.g., 10 m x 10 m if a bulldozer is used to remove
the top 6 inches of soil). However, there may be
cases when it is important to know if a single sample
has a high probability of exceeding background. In
this case, the single value can be compared to the
background maximum limit (mean background
concentration plus three standard deviations), which
is discussed later.
Background Concentration
Numerous natural and anthropogenic sources
influence background concentrations and need to be
accounted for during an initial hazardous waste site
investigation. Proper accounting of these sources is
important when establishing cleanup standards and
are critical if discussions about ARARs develop.
It is not feasible to establish a single universal
background concentration for soils or sediments; it
is more useful to discuss the range of background
concentrations for a contaminant. Single values are
hard to establish because concentrations vary
depending on how physical, chemical, and biological
processes, and anthropogenic contributions have
affected parent geological material at a site. If a site
has various soil or sediment textures (e.g., sands,
loams), a range in inorganic concentrations should
be developed for different soil series or textural
groupings. Thus, physical and chemical parameters
need to be identified when investigating a site to
ensure that soils or sediments with similar
parameters are compared. This is important because
there are often different soil types at a site, and
sediments differ depending on where (e.g., in a pool
or main channel) and when samples are collected.
The following parameters should be similar when
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ON-SITE
Total
On-site
Concentrations
CONCENTRATION
FROM
SITE-RELATED
ACTIVITIES
ON-SITE
BACKGROUND CONCENTRATIONS
ANTHROPOGENIC
SOURCES
(Local, Regional
or Global)
NATURALLY
OCCURRING
SOURCES
BACKGROUND
AREA
Background
Concentrations
OFF-SITE
BACKGROUND CONCENTRATIONS
ANTHROPOGENIC
SOURCES
(Local, Regional
or Global)
NATURALLY
OCCURRING
SOURCES
Figure 1. Relationship between on- and off-site concentration groupings when defining background concentrations
for hazardous waste sites.
comparing paired hazardous waste site samples to
background samples:
• pH/Eh
• salinity
• cation exchange capacity (CEC)
• percent organic carbon
• particle size and distribution
• thickness of horizon (soil)
• soil type, structure (soil)
sample design
depth of sampling
sampling equipment and compositing regime (if
applicable)
number of samples
digestion/analytical method
acid volatile sulfide concentrations (sediment)
simultaneously extracted metal concentrations
(for determining sediment toxicity)
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At times some of these soil parameters such as
percent organic carbon, pH and salinity may be
altered by hazardous waste site activities. These
changes in soil chemistry could falsely imply that the
hazardous waste site and background site
soil/sediment matrices are totally very comparable.
For example, if oil were released at a hazardous
waste site where mercury is of a concern, the percent
organic carbon values could be much higher than at
the background site. This could lead to an incorrect
conclusion that the sites are not similar for
comparison of inorganic concentrations.
Many of these soil parameters can be obtained by
contacting the local Natural Resources Conservation
Service (NRCS) Office and requesting a soil survey
report for the county (usually free of charge) where
the site is located. Most soils on private lands in the
U.S. have been mapped by the NRCS. By using a
soil survey report, the field personnel can evaluate
how the soils were originally classified and gain
access to average values for the soil series located at
the site. By consulting with a soil scientist and
comparing current site soils to those previously
mapped, an assessment can be made of the amount
of change and disturbance that has occurred to the
soil profile. Aerial photographs used to map soils
are also helpful in evaluating past land use, locating
stream channels, determining parent material for
sediment loading, and determining site factors that
affect movement of contaminants (e.g., low
percolation rate). More detail on how and why to
characterize soils at hazardous waste sites can be
found in a companion issue paper (Breckenridge et
al. 1991).
A special case occurs for hazardous waste sites
that contain fill. "Fill areas" may be present around
construction or disposal areas and should be sus-
pected if the site is located in areas frequently in-
undated with water. Sites where dredge material
(e.g., sediments from shipping areas) is suspected to
have been used as fill should be given additional
attention because the dredge material may have
elevated levels of contaminants. A soil scientist can
usually identify fill locations and areas disturbed by
construction because of the disturbed nature of the
soil profile.
Natural and Regional Anthropogenic
Contributions to Background Concentrations
Table 1 presents concentration ranges and mean
values of inorganics in selected surface soils of the
United States. Most of this contribution is due to
natural and regional/global anthropogenic sources.
The soil types presented are general, but cover many
of the major categories found in the United States.
There is one omission from the table and that is for
cadmium, since cadmium mobility is strongly
dependent on soil pH and percent organic carbon.
The mean global content of cadmium in soils is
between 0.07 and 1.1 ppm (ppm-dry weight - mg/kg
for SI units); for the United States, values range
from 0.41 to 0.57 ppm, but values of up to 1.5 ppm
have been documented in some forest soils (Kabata-
Pendias and Pendias 1984). In all cases, the higher
cadmium values reflect anthropogenic contributions
(from local and regional sources) to topsoils.
Table 2 provides average, range, and no-effect
levels for selected inorganics in sediment and soils
that can be used to compare to background concen-
trations for a site. The no-effect levels are the metal
concentrations in sediment that have a low proba-
bility of causing a measurable impact on benthic
populations. The control values for soils and sedi-
ments approximate the average concentrations of
metals contributed by natural and anthropogenic
(local and global) values (Lee et al. 1989; EPA
1992b). These values should not be used as back-
ground concentrations but can be used to guide
investigators in determining whether elevated levels
of contaminants may be present at a hazardous waste
site.
Local Anthropogenic Sources that Influence
Background Concentrations
Note: Some of the activities discussed here may
not be waste handling or disposal activities;
however, they could qualify as releases under
Superfund (e.g., mining activities may result in
releases that can be addressed under Superfund).
Numerous local anthropogenic activities can
contribute to the inorganic concentrations at a
hazardous waste site yet are not directly related to
site activities. Local soils and sediments may be
contaminated by ore deposits or mining, by
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agricultural application of pesticides or sewage
sludge, and by emissions from motor vehicles. In
urban areas, sites may become contaminated by air
emissions from home heating, automobiles, and
industry. Table 3 provides ranges of contaminant
levels in surface soils from some local anthropogenic
sources of inorganics that could contribute to
background concentrations at or near a hazardous
waste site. Table 4 provides a review of some of the
more common agricultural sources of inorganics
associated with practices such as sludge, pesticides,
and fertilizer applications.
These tables identify elements that could be
associated with different land uses at or around a
hazardous waste site. For example, if the site is
located in an area with high agricultural chemical
usage, elevated background concentrations of
arsenic, bromine, lead, vanadium, and zinc could be
expected. To determine what background concen-
trations might be without the agricultural
contribution, the investigator needs to rely on some
investigative skills. These skills are detailed in Part
A of this document.
Accessing Data and Methods for Establishing
Background Concentrations
The previous discussion presented information
on ranges of background concentrations that could
be expected for inorganics of greatest concern at
hazardous waste sites. Additional information
sources that should be consulted include soil
scientists from the NRCS and county extension
agents who may have conducted soil surveys that
describe the natural soils' physical, chemical, and
biological status. However, many of these surveys
were conducted for purposes such as mineral
development, farming, and soil conservation; the
data focus on properties of soils. The NRCS
maintains the SOILS-5 data base that provides
attributes of soils (e.g., texture, pH, CEC, salinity,
clay content) that can be accessed at the local NRCS
Office or through the NRCS Office of Technology,
Cartography and Geographic Information System
Division at (202) 447-5421. Many data sets are
available on World Wide Web (WWW). The U.S.
Geological Survey (USGS) Global Land Information
(GLI) system is another source for most land-based
data and can be located on WWW at http://
edcwww.cr.usgs.gov/glis/glis.html. The Agricultural
Stabilization and Conservation Service can also be
a good source of aerial photographs for hazardous
waste site assessment. Information on soil surveys,
aerial photographs, and other sources that may be
useful for identifying soil types and land use in the
United States is presented in Table 5.
Several large projects have been conducted to
address the issue of characterizing background soil
concentrations. For example, the Oak Ridge
Reservation (a U.S. Department of Energy facility)
conducted a background soil characterization project
to establish a database, to recommend how to use the
data for contaminated site assessments, and to
provide estimates of the potential health and
environmental risks associated with the background
level concentrations of potentially hazardous
constituents (see Table 5, ORNL 1993). This source
provides a detailed approach for those faced with
conducting a detailed background investigation.
Approach for Establishment of Background
Reference Values
The approaches described in this document, for
the most part, combine discussion of issues generic
to soils and sediments. However, when warranted,
attributes unique to the two different media are
discussed in separate sections. One such section
discusses sediments that require sampling through
overlying water.
PART A
COMPARING THE CONCENTRATIONS OF
INORGANICS IN SOILS AND SEDIMENTS
AT HAZARDOUS WASTE VERSUS
BACKGROUND SITES
The objective of Part A is to determine whether
hazardous waste site-related activities have caused
an increase in the levels of inorganic contaminants in
soils and sediments compared to background
concentrations.
SOILS
Step 1—Evaluation of Land Use History and
Existing Data
Purpose: This effort is designed to identify land
use history both on and near the hazardous waste site
(i.e., within the air and watershed connected to
11
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TABLE 3. INORGANIC CONTAMINATION OF SURFACE SOILS, AVERAGE VALUES
FROM VARIOUS ANTHROPOGENIC SOURCES IN THE UNITED STATES
(PPM-DW)1 (SOURCE: KABATA-PENDIAS AND PENDIAS 1984).
Element
Site and pollution source
Mean or range of content
Arsenic (As)
Cadmium (Cd)
Cobalt (Co)
Copper (Cu)
Lead (Pb)
Mercury (Hg)
Zinc (Zn)
Metal-processing industry
Application of arsenal pesticides
Metal-processing industry
Urban garden
Vicinity of highways
Mining or ore deposit
Metal-processing industry
Roadside or airport area
Urban gardens, orchards, and parks
Sludged farmland
Metal processing industry
Urban garden and urban vicinity
Roadside soil
Non-ferric metal mining
Hg mining or ore deposit
Urban garden, orchard, and parks
Non-ferric metal mining
Metal processing industry
Urban gardens and orchards
10-380
31-625
26-160
0.02-13.6
1-10
13-85
42-154
7.9
3-140
90
500-6,500
218-10,900
960-7,000
15-13,000
0.1-40
0.6
500-53,000
155-12,400
20-1,200
a. Equivalent to mg/kg-DW.
or in proximity to the site) to determine what
contribution the anthropogenic activities from
previous land use at or near the hazardous waste site
have had on background concentrations.
Approach: Early in a hazardous waste site
investigation, site history should be determined by
examining available records and by interviewing
personnel familiar with the site. This information
can be used to assess the types of contaminants
associated with past operations that may be of
concern and may be compared to Appendix DC,
Superfund and Priority Pollutant Compounds, lists
which identify the inorganic contaminants of
concern. Evaluation of site history can provide
important data when determining those compounds
for which background concentrations need to be
established. For example, if releases of cadmium
and lead are suspected at a hazardous waste site and
the site is located in a heavily industrialized area,
there is high potential that metals like mercury, lead,
and cadmium may be present and elevated in soils
and sediment from off-site contributions. An initial
evaluation of on-site data should be sensitive to the
issue of elevated background so that off-site
contributions can be properly accounted for.
Another advantage of evaluating existing
hazardous waste site data is to determine if pre-site
operation values are available for inorganics in
sediments or soil. These data can be obtained from
site records or other existing sources discussed later
in this paper. NRCS soil surveys should be checked
both for aerial photographs that show previous land
use on or near the site and for average physical and
chemical properties for soils at and around the
hazardous waste site. Local county agricultural
12
-------
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TABLE 5. SOURCES OF INFORMATION FOR IDENTIFYING SOIL TYPES, LAND USE,
AND DETERMINING BACKGROUND LEVELS OF INORGANICS IN SOILS
AND SOME SEDIMENTS IN THE U.S.
Source
Supporting background
information
Locations
Contact point
Bureau of Land
Management—BLM
National Park Service
(NFS)
Provides data on areas in the country
that have naturally occurring
substances that pose a hazard to
humans or the environment.
Inventory and monitoring of trace
levels of inorganics in soils in natural
areas.
Mostly BLM Service Center
western U.S. Denver Federal Center
Lakewood, CO 80225
(303) 236-0142
Nationwide Local NPS Headquarters
U.S. Geological
Survey
U.S. Geological
Survey
USDA-Agricultural
Stabilization and
Conservation Service
(ASCS/SCS)
National Ocean
Service (NOS)
Several reports on the concentration of
inorganics in the environment;
Background geochemistry of some
rocks, soils, plant, and vegetables in
the conterminous U.S. "Geological
Survey," professional paper 574-F,
1975—Summary of determination
between natural and anthropogenic
contributions—shows natural values
vary widely and are highly site specific
and regionally dependant.
An accounting of pesticides in soils
and ground water in the Iowa River
Basin, 1985-88 (IA 86-055).
Aerial photographs of sites.
Aerial photographs of current and
previous land use and soils types
including erosion potential.
Coastal and Geodetic Surveys
including aerial photographs.
Nationwide
Nationwide
Midwest
Nationwide
Nationwide
Coastal areas
Water Resources Information Center:
(703)648-6818
National Technical Information Service
(NTIS)
U.S. Department of Communication:
(703) 487-4650
NTIS
USGS, Salt Lake City ESIC
8105 Federal Bldg.
125 South State St.
Salt Lake City, UT 84138-1177
(801) 524-5652/Fax: (801) 524-6500
ASCS/SCS
Aerial Photography Field Office,
P.O. Box 30010
Salt Lake City, UT 84130-0010
(801) 975-3503/Fax: (801) 975-3532
National Ocean Service
Coast and Geodetic Survey Support
Sec. N/CG236
SSMC#3, Rm. 5212
1315 East-West Highway
Silver Spring, MD 20910
(301)713-2692/Fax: (301) 713-0445
14
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TABLES. (CONTINUED)
Source
Supporting background
information
Locations
Contact point
National Archives
Research Administra-
tion/National Air
Survey
(NARA/NASC)
Series of infrared Landsat photographs
for mid-1970s by state.
Nationwide NARA/NASC
National Air Survey
4321 Baltimore Ave.
Bladensburg, MD20710
(301)927-7180/Fax: (301) 927-5013
U.S. Forest Service
(FS)
U.S. Department of
Energy (DOE)
Often have data on trace elements as
pan of soils inventory and monitoring
program.
Collects and publishes data on trace
metals and radionuclide concentrations
around DOE facilities and for
reference sites.
Nationwide Nearest FS experiment station
Nationwide Nearest DOE office, Environmental
Monitoring Division
Oak Ridge National
Laboratory
The background soil characterization Local -
project provides background levels of Roane
selected metals, organic compounds, County, TN
and radionuclides in soils from
uncontaminated sites at the Oak Ridge Approach
Reservation. Also a good approach for useful
evaluating background for use in nationwide
baseline risk assessments
D.R. Watkins
Oak Ridge Reservation
Environmental Restoration Div.
P.O. Box 2003
Oak Ridge, TN 37831-7298
(615)576-9931
See ref.ORNL 1993.
National Climatic Provide data on wind roses and climate Nationwide
Data Center parameters for most areas of the
country.
EPA Most complete source of data that
includes EPA and other Agency
information on Hazard ID, Dose-
Response, and Risk Characterization.
STORET (physical and chemical
parameters in soils and sediments).
Journal Articles Can provide local, regional, or national Local to
background concentration values. Can National
be accessed via literature searches, but
usually need to be searched by element
or media.
User Service Branch
Asheville, NC
(704) 259-0682
Nationwide EPA/540/1-86/061 (EPA 1986)
EPA Office of Water and Hazardous
Material
(202) 382-7220
Commercial product—more user
friendly, Earthlnfo: (303) 938-1788
Selected references: (1) Metals in
Determining Natural Background
Concentrations in Mineralized Areas,
1992 (Runnells et al. 1992), and (2)
Sediment Quality and Aquatic Life
Assessment (Adams et al 1992).
15
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agents and state, county, and federal environmental
quality officials are also sources of information on
local emissions or previous sampling data that may
be used to establish background concentrations.
EPA or state regulators of chemical storage, use, and
emission data bases of local industries may be a
good source of chemicals used or stored in the local
area. EPA's STORE! data base should be checked
Step 2—Establishment of Data Quality
Objectives
Purpose: The purpose of Step 2 is to establish
data quality objectives (DQOs) (EPA 1993) for the
decision-making process.
Approach: The DQO process is described in
"Standard Practice for Generation of Environmental
Data Related to Waste Management Activities:
Development of Data Quality Objectives" (ASTM
1995) and is summarized in a companion issue paper
titled, "Characterizing Soils for Hazardous Waste
Site Assessments" (Breckenridge et al. 1991). The
companion issue paper explains how to classify soils
when faced with different classification systems and
what soils data need to be considered when
establishing DQOs. EPA's external working draft,
"Guidance for Data Quality Assessment" (EPA
1995), is helpful in discussing the role of statistics in
the DQO process. This document has a companion
PC-based software program to help support the
document. Since this is designed as a "living
document," contact the Quality Assurance Division
[Fax number (202) 260-4346] in the Office of
Research and Development (401 M Street, S.W.,
Washington, D.C. 20460) to obtain the latest
version.
Step 3—Determining Sample Location and
Numbers to Collect
Purpose: The purpose is to design a statistically
valid approach that yields representative samples
from areas of concern and from background areas
and to factor judgement (bias sampling) into
selecting sampling locations to maximize the
possibility of detecting elevated levels of
contaminants on-site.
Approach: There are a number of options in
sampling design that determine where to collect
samples from a hazardous waste site to compare
against a background site. The investigator needs to
discuss the DQOs with a statistician to select the
appropriate design. Numerous design options are
available. One option is for those areas where the
sites' soil and sediment matrix and distribution of
suspected contaminants appear to be homo-geneous.
Establishing a consistent grid (i.e., systematic
sampling grid) across the entire site and sampling at
set locations should provide a reasonable
characterization of the contamination values across
the site. A second option may apply if certain parts
of a site are suspected of being contaminated due to
historical use. In this case, bias sampling or
intensifying the grid in highly suspected areas could
be considered. This approach maximizes the
possibility of determining whether contaminant
concentrations at a site are above background and
minimizes the risk of not taking action at a
hazardous waste site.
There is a wealth of guidance on soil sampling.
One document that is useful because of its coverage
of soil sampling methods and design for reducing
various sources of sampling error is titled,
"Preparation of Soil Sampling Protocols: Sampling
Techniques and Strategies" (EPA 1992c). This
document also provides information for those
uncertain about sampling design options and
composite collection techniques.
The following discussion points should be
considered when selecting and designing the
sampling plan.
Point A—For a given site, there may be several
areas of concern based on known or suspected past
site activities. Once these areas are identified, a
sampling plan can be developed. Historical data
should be identified and evaluated early in the
process to determine their use in identifying areas of
concern or if the entire site needs to be sampled.
Historical and land use information identified from
Step 1 plays a key role in determining the degree of
bias in the sampling plan. Factors such as location
of tanks, piping, staging areas, disposal ponds, and
drainage areas (e.g., sumps) should be considered
when designing a sampling plan. Several soil
properties or processes that govern the mobility of
contaminants can also bias sample location:
Soil pH: A quick check using a field test kit can
identify if the pH of the soil is in a range to mobilize
contaminants. In acid soils (pH <6.5), inorganics
16
-------
such as zinc, manganese, copper, iron, cobalt, and
boron are easily leached. However, if soil pH is
above 7.0, these inorganics form stable compounds.
Other inorganics, such as molybdenum and
selenium, are mobilized in alkaline soils, whereas in
acid soils they become almost insoluble. Thus, pH
and contaminants of concern need to be factored into
the selection of sampling depth in soils.
Soil Texture: Most soils are a combination of
the following grain sizes:
Medium to large grain size material has moderate
to high porosity (15 to 40 percent) and low capacity
for adsorbing inorganics. These soils have low
capacity to hold contaminants in the grain interstices
due to low cation exchange capacity and low
capillary action. Investigators should look for
surface staining and consider sampling at deeper
depths.
Fine sands to silt materials have a stronger
capillary action, and silts are capable of sorbing
inorganics. Special attention should be given to
sampling at the interface between fine material
layers and larger grains, or where fine sand lenses
are mixed in clay soils (these often form conduits for
contaminant movement).
Clays are fine particles and possess a net negative
charge, and most have high cation exchange
capacities. This may cause heavy metal cations
(e.g., Cr4*, Cd+2, Pb+2) to adsorb to the clay surface.
Clays also form large cracks and fractures due to
shrink/swell and freeze/thaw effects. Investigators
should look at the profile in clay soils to determine
if inorganics (e.g., iron and manganese) have
oxidized or been reduced in fractures causing a color
change (e.g., under oxidation, iron changes to a
red/yellow/brownish color compared to the natural
blue/gray color). The sample design should consider
these factors by collecting samples from fractures
and especially from areas that show signs of
oxidation.
Soil Organic Carbon Content: Organic carbon
content plays a key role in the sorption of
contaminants. Special attention should be given to
sampling layers that have excessive organic carbon
(e.g., darker soils, upper soil layers, peat).
Cation Exchange Capacity (CEC): The ability
of the solid soil phase to exchange cations is one of
the most important soil properties governing
movement of inorganics in soils. In general, the
CEC is related to the surface area of the soils and
sediments (Kabata-Pendias and Pendias 1984). Soils
and sediments that have larger surface areas (e.g.,
clays) have a greater CEC, while those with smaller
surface areas (e.g., sands) have a lower CEC.
Transport: The transport of dissolved or
colloidal inorganics takes place through the soil
solution (diffusion) and with the moving soil
solution (leaching) (Kabata-Pendias and Pendias
1984). Investigators should be aware that in cool,
humid climates, inorganics generally leach
downward through the profile; in warm, dry climates
and hot, humid areas, the movement is often upward.
However, specific soil properties, mainly the soil's
CEC and moisture availability, control the rate of
migration of inorganics in a soil profile.
Point B—A grid system can be used to establish
the locations to be sampled. A grid will also help
define the total population from which a subset may
be selected using a statistical approach (e.g.,
systematic random, random, or stratified random) to
identify the specific sample population. If the site
has excavations or steep depressions, sample points
along both sidewalls and the base of any excavations
should be included in the grid. If samples are
collected from excavations, similar soils (i.e., same
depth, type, and horizon) should be sampled from
the background site for evaluation. However, soils
are heterogenous and spatial patterns do exist. Some
soil types exhibit spatial correlations that should be
considered by the project's statistician. The area
represented by each grid point should be
proportional to the size of the area for equal
weighting and be equal to or greater than the
operable unit (discussed earlier). One of the
following equations may be used to determine grid
intervals for three different size categories (Michigan
1991b):
= GI
Small site
(0 to 0.25
acre)
(1)
17
-------
= GI
Medium site
(0.25 to 3
acre)
Large site
(>3.0 acre)
(2)
Intensified grid
(3)
where:
GI = grid interval
A = area to be gridded (in square feet)
GL = length (or longest side) of area to be
gridded.
For example, for a 1.5-acre site (the longest side
being 280 feet), and given that 1 acre = 43,560
square feet, substituting values in Equation (2)
above, we have:
v/65.340/3.14 =
4
between
points
Grid systems are useful but have limitations. An
option is to select a sampling area that is equal to an
operable unit (i.e., the size of the smallest remedial
action unit) and divide the site into equal units.
Samples can then be collected, following a statistical
design that represents the unit.
Point C—After the grid point interval is
determined, a scaled grid overlay can be made and
superimposed on a map of both the hazardous waste
and background sites. Some specified point (e.g.,
the southwest comer) should be designated as the
(0,0) coordinate. The grid can then be oriented to
maximize sampling coverage. Some grid orientation
may be necessary for unusually shaped areas. Also,
the site can be subdivided with different calculated
grid intervals so that proportional sampling can be
intensified for suspect areas, such as sumps or sinks
or low-lying drainage areas where contaminants
have a higher probability of concentrating. The
following is an example of a grid:
• •••••
• • •
• • • • •
• • •
* * *
•#••*••#••#•
(0.0) •
Point D—Several options exist for collecting
samples: (a) collect a sample at all (or a minimum of
four) grid points as discussed under Point B, (b) use
the systematic random sampling approach referenced
in SW-846, Third Edition, Section 9.1.1.3.3, or
(c) use a stratified random design with an intensified
grid for suspected problem areas. The selected
number of sample locations are determined by
sampling objectives, number of analytes to be
evaluated per sample, the analytical techniques to be
used, and budget constraints.
Point E—The determination of depth sampling
increments are dependent on DQOs and the capacity
of different soil layers to hold (sorb) metals.
Recommended depth sampling increments are for
the following: clay and organic soils on-site, 0.25 to
0.5 feet or by major horizon; silts and loams, 1.0 to
2.5-foot intervals or by master horizons: and sands,
1.0 to 5.0 feet. The selection of depth sampling
increments also depends on the suspected amount of
contamination released, mobility of contaminant,
amount of water or liquid available for transport
(e.g., ponding), and funding. Samples collected
from specified depths can be either single or in
multiple replicates, depending on the statistical
method used for background data comparison (see
Step 5). At locations where soil type is the same,
compositing can be considered to save costs and
more precisely estimate the mean value. However,
compositing may be a concern if the data are used
for future enforcement purposes.
Point F—For a background site, a minimum of
four samples collected from the same soil type are
needed to establish "background" concentration for
a soil type (Michigan 199la). These sample
numbers will help account for natural constituent
occurrences and inherent variability (i.e., range)
18
-------
within each distinctive soil type. When determining
if contaminants have moved into a profile (i.e., by
depth), samples should be taken at comparable
depths from similar soil types for both the
background site and the hazardous waste site. If site
environmental conditions have resulted in leaching
of contaminants into the soil profile, the major soil
horizons (i.e., O, A, and B) for a soil type may need
to be sampled at both the contaminated and
background site. If the site is sampled by major
horizon, a minimum of four samples should be
collected from each horizon) (see Figure 3). Sample
size (e.g., weight) at all locations at the hazardous
waste and background sites should be the same.
GROUND SURFACE
1st major horizon — Brown
medium-coarse SAND
2nd major horizon — Lt. brown
silty fine SAND
3rd major horizon — Gray silty
CLAY w/trace of fine-medium
sand
4 samples
4 samples
4 samples
Figure 3. Approach for sampling sites where compar-
ison is needed between major soil horizons
or layers within a soil type.
Point G—Background samples should be taken
from areas unaffected by site activities. If similar
soils cannot be found in areas unaffected by site
activities, possible locations for determining back-
ground are areas on-site, such as under stationary
objects like storage sheds or porches, large flag-
stones, and old trees.
Point H—Wind rose data can be used to identify
background sample locations in the predominant
upwind direction from the hazardous waste site.
Wind rose data usually provide monthly averages
(based on hourly observations) on the percentage of
time the wind blew from the 16 compass points or
was calm. Wind rose data can be obtained for the
area from the local weather station or the National
Oceanic and Atmospheric Administration (NOAA).
Generally, if airborne deposition is the primary
method of contaminant release from sites in arid
environments, the investigator should focus on
sampling the soils near the surface. However,
consideration must be given to the mobility of the
contaminant and texture of the soil.
Step 4—Sample Collection, Preservation,
Handling, Analysis, and Data Reporting
Purpose: The purpose is to ensure that all samples
are handled in a manner that protects their integrity
and are analyzed using comparable, standard
methods.
Approach: The following is the recommended
approach:
1. All sample collection, preservation, preparation,
handling, and analytical methods should follow
standard methods [e.g., U.S. EPA SW-846, "Test
Methods for Evaluating Solid Waste,
Physical/Chemical Methods" (EPA 1986)]. It is
important that all samples are handled using
comparable methods when preparing for and during
analysis. For example, using different digestion
methods can change results significantly.
2. For inorganics, it is recommended to use a total
metals procedure with results reported in mg/kg (or
percent for iron) on a dry weight basis. This
minimizes additional sources of variation, since
these constituents are often naturally occurring. To
assess the bioavailability of metals in anoxic
sediments, acid volatile sulfide and simultaneously
extracted metals should be determined (Da Toro et
al. 1990, 1992).
Step 5—Statistical Comparison of Hazardous
Waste and Background Sites
Purpose: The objective is to determine if
concentrations of inorganics from a hazardous waste
site are elevated compared to those from a back-
ground site.
Approach: The following discussion outlines
some basic statistical concepts in the context of
background data evaluation. A general statistics
textbook such as Statistical Methods for
Environmental Pollution Monitoring (Gilbert 1987)
should be consulted for additional detail. Also, the
following list of published statistical guidance may
be useful (Figure 4). There are numerous statistical
approaches that are applicable when collecting,
assessing, and analyzing background data. The
approaches presented here have been adopted by the
State of Michigan (Michigan 1990, 1991b) and
modified based on the authors' experience. They are
readily understandable and easy to use. However, it
is recommended that investigators consult a
19
-------
statistician to assist in the design or review of a
sampling plan prior to collecting samples.
1. Careful consideration must be given to the
selection of a statistical procedure based on site-
specific factors. These include the size of the
background data base and the number of samples
available for comparison, variability of soil type, and
coefficient of variation of data. The following are
some statistical methods that can be used if data
from the site follow a normal distribution. Some
environmental sample sets are normally distributed.
However, the majority of environmental contamin-
ation data sets are not normally distributed. Some of
the more commonly used tests of normality are
presented in Table 6. Tests should be conducted on
all data to determine if the data meet the assumption
of normality. If the data are not normally
distributed, log or other types of transformations
should be conducted to approximate normality prior
to using the data sets in statistical comparisons, such
as t-tests or analysis of variance procedures
(ANOVA).
If the data cannot be normalized, additional
attention needs to be given to selecting appropriate
statistical tests, and the situation needs to be
discussed with a statistician. Special statistical
consideration may be warranted if samples are
composited and the data are needed to support
regulatory requirements discussed in Part B.
Statistical Methods Guidance3
Basic
Statistical Methods for Environmental Pollution Monitoring, Van Nostrand Reinhold Company (Gilbert 1987).
Guidance for Data Quality Assessment (EPA 1995).
Soils Sampling Quality Assurance Guide (EPA 1989d).
Guidance for Conducting Remedial Investigations and Feasibility Studies under CERCLA (EPA 1988).
EPA's Guidance Manual: Bedded Sediment Bioaccumulation Tests, pp. 82-91 (Lee et al. 1989).
Statistical Guidance for Ecology Site Managers, Washington State Department of Ecology (WDOE 1992).
Risk Assessment Guidance for Superfund, Volume 1: Human Health Evaluation Manual (Pan A), pp. 4-5 to 4-10
(EPA 1989c).
Advanced
Estimation of Background Levels of Contaminants (Singh and Singh 1993).
Statistical Analysis of Ground-Water Monitoring Data at RCRA Facilities (EPA 1992d).
Background and Cleanup Standards
Methods for Evaluating the Attainment of Cleanup Standards, Volume 1: Soils and Solid Media (EPA 1989b)
(detailed statistical discussion).
"If time and resources are limited, Gilbert (1987), Hardin and Gilbert (1993), and EPA (1995) provide some of
the most relevant statistical information.
Figure 4. Statistical Methods Guidance.
20
-------
following procedures could be used with any of the
preceding statistical methods:
a. For any
-------
metals, toxic potential is reduced as particle size
decreases or organic carbon increases. If the particle
size or organic carbon content of the background and
contaminated site sediments differ significantly, it is
not appropriate to directly compare contaminant
residue levels without normalizing the data. Some
organic contaminants can be normalized to organic
carbon by dividing by the fraction of organic carbon
(Adams et al. 1992); the same approach has been
used for divalent cationic metals—lead, nickel,
copper, cadmium, and zinc. Metals data can be
normalized to acid volatile sulfide levels (Di Toro
et al. 1990, 1992), a key element such as aluminum
(Schropp and Windom 1988; Daskalakis and
O'Conner 1995), or particle size (NOAA 1988).
EPA is also refining its equilibrium partitioning
approach, which could be used to normalize
contaminant levels among different sediments
(Adams et al. 1992; EPA 1992b). Further
discussion of these procedures is beyond the scope
of this paper, and expert assistance should be
obtained.
Step 2—Comparison of On-site Data to Sediment
Quality Criteria
Simply comparing the level of metals in bulk
sediments, deposited under similar conditions,
upstream and downstream from a suspected facility
can provide an indication that a facility may have
contaminated the downstream site. These data,
however, provide no indication of bioavailability
that may justify remediation. Indeed, bulk sediment
contamination is only poorly correlated with adverse
impacts. For metals, the key parameter in
determining toxicity is the pore water concentration
of a metal. In situations where no background site
data exist, yet contaminated site data do, it may be
useful to compare sediment contamination levels to
various sediment quality criteria.
EPA is developing sediment quality criteria for
metals, but the factors determining bioavailability
are complex, and an approach has yet to be selected
(Ankley et al. 1994). Research has shown that in
anaerobic sediments, toxic impacts due to cadmium,
copper, lead, nickel, and zinc are not present when
the sum of the molar concentrations of these metals
divided by the AVS concentration is less than one
(Di Toro et al. 1990, 1992). Essentially, when
excess sulfide is present, the metals are complexed
and metal concentrations in pore water are low.
While bulk sediment contamination levels do
not correlate well with toxic effects, a number of
statistically based, sediment-quality indices have
been developed. If resources are limited, and useful
background site data do not exist, concentrations at
the contaminated site can be compared with metal
concentrations in bulk sediment known to have a
low probability of causing adverse impacts on
benthic organisms. Several "no-effect" levels are
presented in Table 2. NOAA developed effects-
based guidelines including no-effects, possible
effects, and probable-effects levels (Long and
Morgan 1990) based on National Status and Trends
Program data, and MacDonald (1992) used the same
approach with additional data to develop similar
marine guidelines. The guidance for marine
sediments was further updated by Long et al. (1995).
The most recent guidance on metals is presented in
Interim Sediment Quality Assessment Values
(Environment Canada 1994), in which NOAA
National Status and Trends Program data and
spiked-sediment toxicity tests were used to develop
threshold effects values for fresh and marine waters,
presented in Table 2. If the contaminated site levels
are below the no-effects levels, further investigation
may not be required even if the site has been
contaminated. If the no-observed-effects levels are
exceeded, further investigation may be justified.
Bulk sediment guidelines have also been developed
by EPA Region V (U.S. Army Corps of Engineers
1977) for classifying sediments of Great Lakes
harbors. The Ontario Ministry of Environment
(Persaud et al. 1989) and other guidelines are
summarized in EPA's Guidance Manual: Bedded
Sediment Bioaccumulation Tests (Lee et al. 1989).
In addition, these and other guidelines have been
reviewed and summarized by Giesy and Hoke
(1990).
In areas where an entire watershed has been
impacted, such as from mining activities, it may not
be possible to select a suitable background site. In
such situations, historical data (Runnells et al. 1992)
or archived samples may be the best source of data.
Another approach would be to sample surficial
sediments and compare them to deeper
uncontaminated sediments (those laid down prior to
the watershed being impacted).
No specific rules can be provided regarding the
use of existing background reference data for
sediments or soils. Judgement should be made with
full knowledge of the specific objectives of the
investigation, data limitations, and qualifications and
with the help of appropriate experts (e.g., chemists,
24
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TABLE 6. TESTS FOR EVALUATING NORMALITY OF DATA SETS. SOURCE: EPA (1995).
Test* Sample Size (N) Notes on Use Reference
Shapiro Wilk W Test
Filiben's Statistic
Studentized Range Test
Lilliefors Kolmogorov-
SmirnoffTest
Coefficients of Skewness
and Kurtosis Tests
Geary's Tests
Coefficient of Variation
Test
Chi-Square Test
z 50 Highly recommended
z 100 Highly recommended
z 1000 Highly recommended
> 50 Useful when tables for other tests
are not available
> 50 Useful for large sample sizes
> 50 Useful when tables for other tests
are not available.
^ 50 Use only to discard assumption of
normality quickly.
Large6 Useful for group data and when
the comparison distribution is
known.
Gilbert (1987)
EPA(1992c)
EPA (1995)
EPA (1995)
Madansky(1988)
EPA (1995)
EPA (1995)
EPA (1995)
Introductory Statistics
Books
"By order of Recommendation.
"The necessary sample size depends on the number of groups formed when implementing this test
Each group should contain at least five observations.
When comparing a contaminated site with a
background site, a null hypothesis should be
developed. For example, a null hypothesis could be:
There is no difference between the mean
contaminant concentration of the hazardous waste
site and background site. The alternate hypothesis
would be: The mean concentration for the
contaminated site is different from that of the
background site. If parametric statistics are used for
this assessment, such as a t-test or ANOVA, the data
can be normalized to selected parameters (e.g.,
organic carbon, particle size). Many parametric and
non-parametric statistical procedures exist to
compare a background site with one or more
hazardous waste sites. A variety of such procedures
are reviewed in Lee et al. (1989), EPA's Risk
Assessment Guidance for Superfimd, Volume 1
(EPA 1989c), and EPA's Guidance for Data Quality
Assessment (EPA 1995). The latter source provides
examples and a discussion of most of die tests
needed to conduct comparisons between data sets.
a. Empirical Rule. (Note: Many of the following
calculations can be performed using calculators that
are Dreprogrammed.) Use mean (xb) and variance
(Sb) of background concentrations to establish an
"upper limiffor delineating significant
concentrations, such as:
1) Calculate the background mean (x b) by dividing
the sum of the total background readings by the total
number of background readings for each element of
concern:
n
2) Calculate the background variance (Sb) by
taking the sum of the squares of the difference
between each reading and the mean, and dividing
by the degrees of freedom (the total number of
background samples minus one):
s,2 =
n-1
3) Calculate the background standard deviation (Sb)
by taking the square root of the variance:
21
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4) The Coefficient of Variation Test (CV) where
CV = s, / *, is used to evaluate data distribution. The
background data should have a CV of less than 0.5
for sandy soils, less than 0.75 for finer soils, or an
explanation accounting for higher CV values. The
maximum recommended CV is 1.00. If the data
distribution exceeds a CV of 1.00, then a thorough
evaluation should be made to account for this
variability (e.g., laboratory QA/QC, soil
classification, sample location, outlier classification,
and sample location), and the outlier data addressed
(see EPA 1989c). Additional samples may need to
be analyzed to ensure that a sufficient data base
population (n) is achieved.
There are several classical procedures (Gilbert
1987; EPA 1995) and robust outlier tests (Singh and
Nocerino 1994) available in the statistical literature.
Consult Outliers in Statistical Data by Barnett and
Lewis (1994) for a full account of this issue.
Outliers often distort statistical estimation, and
resulting inferences and can lead to incorrect
conclusions. The solution is to consult a statistician
who understands outliers and knows how to use
robust procedures to identify multiple outliers.
If an outlier is found, an option is to take a
substitute sample, have it analyzed, and repeat the
statistical process. (To avoid costly delays, it is
recommended to collect extra samples for laboratory
analysis.)
For example, four background samples are
collected from a site for lead analysis. The lead
values from the laboratory analysis were 56, 25, 18,
and 35 mg/kg. The investigator wants to examine
the data set to determine if the 56-mg/kg sample is
an outlier. The summary statistics for these samples
are:
xb (mean) =
56 1- 25 + 18 + 35
= 33.5
Sb (variance) - I (56 - 33.5)2 * (25 - 33.5)2 + (18 - 33.5)2
* (35 - 33.5)2 } /3 = ^|1 = 273.67
Sb (standard deviation) =
CV (coefficient of variation) = -^ =
x
——
33.5
16.5
0.49
The test for a single outlier in a normal sample
with the population mean and variance unknown
(Barnett and Lewis 1994, p. 218-222) is appropriate
for the above identified sample. The test statistic is:
xm»- * 56-33.5
16.5
1.36
The theoretical cut-off point (Barnett and Lewis
(1994), Table XIH, p. 485) for « = 0.05 is 1.46.
Since the calculated value of the test statistic (1.36)
is less than the theoretical cut-off point (1.46), the 56
mg/kg sample is not an outlier.
Background concentrations should be determined
for major soil types at the hazardous waste site. If
this is not feasible, then a mean background
concentration should be determined on the soil type
at the hazardous waste and background sites with the
lower absorption capacity (usually the sandiest soil)
and those with the higher absorption capacity
(usually silts and clays). The finer-texture soils (silts
and clays) will usually sorb the most contaminants
and provide a good value for comparison. If the
mean concentration from the fine textured soils from
the hazardous waste site is above similar values for
the background site, there is a high probability that
the site is contaminated.
Once a mean background concentration is
established, similar statistical tests should be
conducted on the data from the hazardous waste site.
After the data sets have met the assumptions for
normality or have been corrected (see Figure 2), then
statistical comparisons can be made between the site
and background data sets.
b. t-Test. Any t-test should be discussed with a
statistician prior to use since there are a number of
variations and assumptions that can apply. The
Gosset Student T-test has good application when
comparing background sites to potentially
contaminated sites (Michigan 199la).
c. Cochran's Approximation to the Behrens-Fisher
Student's t-test. This test is also available for
evaluating background variance versus exceedances
(i.e., contamination) as referenced in 40 CFR
Part 264, Appendix IV. Note that this statistical
comparison method does require that two or more
discrete samples be taken at each sampling location.
d. In some cases, it may be of interest to establish an
upper limit of background for the site. This would
be useful if the investigator wanted to compare
single values for a soil type from the hazardous
waste site with the background population for a
similar soil. The mean background concentration
(xb) plus three standard deviations (3Sb) comprises
a reasonable maximum allowable or upper limit.
2. Procedures for non-detect values. If more than
50 percent of the background analytical values are
below the detection limit (DL), either of the
22
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hydrologists, statisticians, benthic ecologists, and
sampling experts).
Step 3—Selection of Sites for Collection of
Background Sediment
The ideal background site data for comparison
with contaminated site data are obtained from
samples:
1. Collected immediately upcurrent of the
contaminated site in an area not impacted by the
suspected contaminant source.
2. Collected at the same time the contaminated
sediment is sampled.
3. Having very similar particle size and organic
carbon content.
4. Collected using identical sampling equipment.
5. Collected using the same statistically based
sampling design (i.e., numbers and configuration)
and compositing handling procedures (if any).
6. Analyzed using identical analytical methods.
When comparing data sets, it is important that
everything about the sampling and analytical
procedures for the background and contaminated site
sediments be as similar as practical. If both fine and
coarse sediments are available for sampling at the
contaminated site and the background site, the finer
sediments are preferred because they have a greater
affinity for metals.
The ideal situation is seldom achieved, and
compromises may have to be made. For streams, it
may be practical to sample directly opposite from the
contaminant or contaminant source or even
downstream as long as the background site is not
impacted by the plume of concern. Contaminated
and background site sediment characteristics will be
most alike where the currents are similar, with fines
being deposited in areas of low currents and coarser
material being associated with faster currents. A
significant complication of sampling streams is the
potential for severe erosion, which may remove
massive amounts of sediments during flood
conditions. Thus, even the contamination observed
in the sediments directly downstream of a point
source may not be attributable to the existing source.
The surface sediments after a flood may represent
contamination deposited from upstream sources or
historical contamination from another facility
previously operating at the hazardous waste site.
Knowledge of a suspected contaminated source's
effluent, the processes generating the effluent, and
historical stream flow information can be helpful to
link the suspected source and the contaminated site's
metal concentration. Since it is difficult to establish
a background reference site after a major flood event
has occurred, previous data or deep sediments may
be the best source for background reference data.
For estuarine and marine sites, selection of a
similar but unimpacted background site also can be
difficult. The process may involve the use of
hydrologic models to simulate tides and currents to
avoid areas impacted by the contaminated plume. It
may be necessary to select distant background sites
when currents are highly variable. The same
considerations need to be addressed in lakes with
wind-driven currents. When questions arise about
site selection, it is best to consult with an expert who
is familiar with the hydrology of the area.
When it is not possible to obtain background
sediment with the same particle size and organic
carbon content, normalization procedures, discussed
previously, should be considered. While it is beyond
the scope of this document to recommend specific
sampling and assessment methods, excellent
comprehensive references for such information are
Procedures for the Assessment of Contaminated
Sediment Problems in the Great Lakes (UC 1988),
Assessment and Remediation of Contaminated
Sediments (ARCS) Program, Assessment Guidance
Document (EPA 1994b), and Manual of Aquatic
Sediment Sampling (Mudroch and Azcue 1995).
Additionally, EPA's Office of Science and
Technology within the Office of Water is developing
a methods manual that will cover all aspects of
sediment monitoring, from sample collection to
analytical methods to assessment techniques (EPA
1994c).
PARTB
APPROACHES FOR DETERMINING
BACKGROUND LEVELS OF INORGANICS
THAT CAN BE COMPARED WITH
CONCENTRATIONS OF INORGANICS AT
CERCLA SITES
Part B was developed to address issues that
need to be considered when the establishment of
background under CERCLA is required. Part B also
provides a summary of technical issues rather than
an in-depth statistical evaluation of the topic. Those
needing an in-depth level of statistical evaluation
25
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should seek the expertise of a statistician and may
benefit from a paper that addresses estimation of
background concentrations of contaminants, for
example Singh and Singh (1993) or Hardin and
Gilbert (1993).
Step 1—Conduct On- and Off-site
Reconnaissance as Preliminary Assessment
Phase of CERCLA
Purpose: The initial reconnaissance is
performed to identify concerns associated with on-
site or off-site activities that may have resulted in
contributing to enhanced inorganic background
concentrations.
Approach: See Step 1 under Part A (a similar
approach should be considered).
Step 2—Collect Preliminary Information and
Samples About Background Levels of Concern
Purpose: During the preliminary assessment/
site investigation (PA/SI) stage, existing soil and
sediment analytical data for contaminated sites and
background sites can provide initial information to
identify problems that might be encountered with
establishing background values.
Approach: The PA/SI stage is not designed to
evaluate all concerns at the site. However, an initial
site visit can be advantageous to evaluate site con-
dition, assess analytical data, or collect samples of
equal number (i.e., number from contaminated site
= number from background site) from media that
have similar physical (e.g., texture) and chemical
(e.g., pH, percent organic carbon, CEC) properties.
During the SI phase, sufficient information is needed
to support the hazard ranking score (MRS) to
identify if a contaminated site should be nominated
for inclusion to the National Priority List (NPL) due
to high threat to humans or the environment (HRS
scores 2 28.5 are usually nominated for inclusion to
the NPL). The SI stage may present the first
opportunity to actually measure background con-
centrations for assessing if observed releases have
occurred. Section G of the preamble to the HRS
final rule (55 FR 5/546) on "Observed Releases"
states that an observed release is established when a
sample measurement that equals or exceeds the
sample quantitation limit is at least three times the
background level (EPA 1992a, p. 2-3).
Step 3—Determine Potential Magnitude of a
Problem by Combining Information from
Steps 1 and 2
Purpose: Combining information on past land
use and site operations with data from local,
regional, and global contributions can help alert
investigators that the issue of background
concentration might require more attention when
developing a sampling and analysis plan during the
remedial investigation (RI) process.
Approach: Information sources other than
chemical analysis (e.g., information or data obtained
from other sources or Steps 1 and 2) may be used for
characterizing the background concentrations for a
site. A multi-tiered approach is often helpful to
lump background concentrations, such as:
1. Global contributions—mostly atmospheric
contributions from wet and dry deposition.
2. Regional contributions—influence of
geological formations (e.g., increased selenium in
western regions).
3. Local contributions—contributions due to land
use (e.g., high arsenic, lead, and mercury values due
to pesticide use in fruit production), local air
emission sources, and nearly all industrial activities.
Step 4—Establish a Clear Statement of the
Problems at the Site and Develop RI Sampling
Strategy
Purpose: The RI process is the time to conduct
detailed measurements of background concentrations
at CERCLA sites. Section 300.430(b)(8) of the
National Contingency Plan requires that a sampling
and analysis plan (SAP) be developed during the
scoping phase of the RI process.
Approach: A number of EPA guidance docu-
ments discuss the need to characterize background
concentrations as part of the SAP formulation step.
For example, according to the Guidance for Data
Useability in Risk Assessment (Data Useability
Guidance) (EPA 1989a), the SAP should be
developed to resolve four fundamental risk assess-
ment decisions, one of which is to determine
"whether site concentrations are sufficiently different
from background." Similarly, the Guidance for
Conducting Remedial Investigations and Feasibility
Studies under CERCLA (RI/FS Guidance) (EPA
1988) states that when determining the nature and
extent of contamination at a site, background
sampling should be conducted to help identify the
26
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respective areas of both site-related contamination
and the background concentrations.
Step 4a—Confidence Interval Determination
Purpose: The purpose is to develop confidence
intervals for determining mean metals values and
problem statements for a contaminated site.
Approach: Literature sources such as those
presented in Table 5 can be useful to determine if a
site has a potential contamination problem.
However, literature values should be used only to
support or help evaluate data from contaminated and
background site samples. Site variability must be
accounted for when conducting a characterization.
Some sites have fairly homogenous soils, sediments,
and areas impacted by emissions. More often, a site
has a high degree of variability, and the problem
statement and SAP need to reflect this. For many
sites, a 95 percent confidence interval of the mean
metals concentrations would be reasonable (i.e., if
the mean for lead is 20 ppm-dry weight ±4, the 95
percent confident background value would be
between 16 and 24 ppm-dry weight). However, if
the site is complex due to different soils/sediments
and areas of concern, a 90 percent confidence
interval may be more acceptable due to an increased
number of samples and cost. Once a confidence
interval is developed, a problem statement for the
site can be formulated to guide further effort. An
example of a problem statement could be: "Are
on-site concentrations of mercury, lead, arsenic, and
zinc statistically different from off-site background
concentrations?"
Step 4b—Develop Hypotheses for Testing
Purpose: EPA's RAGS "provides guidance on
developing hypotheses to frame a problem in a
manner that can be tested."
Approach: There are two types of hypothesis
most often used: null hypothesis (a) — the site-
related concentration is less than or equal to the
background concentration, and null hypothesis (b) —
the site-related contaminant concentration is greater
than or equal to the background concentration.
Additional guidance on selecting hypotheses is
presented in the Risk Assessment Guidance for
Superfund, Volume I (EPA 1989c).
Step 4c—Determine Level of Precision
Purpose: RI/FS guidance is that the level of
precision be determined before sampling and
analysis strategies are developed. This will guide
the number and location of samples.
Approach: Determining the level of precision
early in the development of a SAP will minimize
many future problems. Two types of statistical
errors are encountered when testing hypotheses
about differences between on-site and off-site
concentrations. The following definitions are true
only if the null hypothesis (a) is used, but not with
(b).
1. Type I error (a) (false positive): "Rejecting the
null hypothesis when it is true." Because of the
uncertainty related to sampling variability, an
individual could falsely conclude that the site-related
contaminant concentration is greater than
background concentration when it actually is not. In
this case, the null hypothesis is rejected, and the site-
related concentration is considered to be statistically
different from the background concentration.
2. Type n error (P) (false negative): "Accepting
the null hypothesis when it is false." Alternatively,
an individual might accept the null hypothesis that
the contaminated site-related concentration is less
than or equal to background concentration when it
actually is not. In this case, the null hypothesis is
accepted and the site-related concentration is
considered to be no different from the background
concentration.
A decision based on a Type I error could result
in unnecessary remediation, while a Type n error
could result in the failure to clean up the
contaminated site when remediation is necessary.
The Greek letter alpha (a) is used to represent the
probability of a false positive, and beta (P) is used to
represent the probability of a false negative decision.
Precision associated with hypothesis testing is
defined by the parameters of confidence level and
power. These are defined in EPA (1992a) and EPA
(1989a)as:
1. Confidence level (100 percent - a)—One
hundred percent minus the confidence level is the
percent probability of concluding that the
contaminated site-related concentration is greater
than background when it is not (Type I error or
"false positive"). As the confidence level is lowered
(or alternatively, as a is increased), the likelihood of
committing a Type I error increases.
27
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2. Power (100 percent - P)—One hundred percent
minus the power is the percent probability of
concluding that the site-related concentration is less
than or equal to background when it is not (Type n
error or "false negative"). As the power is lowered
(or alternatively, as P is increased), the likelihood of
committing a Type n error increases.
Although a range of values can be selected for
these two parameters, as the demand for precision
increases, the number of samples and cost will
generally also increase. The Data Useability
Guidance states that for risk assessment purposes,
the minimum recommended performance measures
are: Confidence, 80 percent (a = 20 percent) and
Power, 90 percent (P = 10 percent). These values
can be interpreted to mean the following:
1. Confidence level = 80 percent—In 80 out of
100 cases, contaminated site-related concentrations
would be correctly identified as being no different
(statistically) from background concentrations, while
in 20 out of 100 cases, site-related concentrations
could be incorrectly identified as being greater than
background concentrations.
2. Power = 90 percent—In 90 out of 100 cases,
site-related contaminants would be correctly
identified as being greater than background
concentrations, while in 10 out of 100 cases, site-
related concentrations would be incorrectly
identified as being less than or equal to background
concentrations.
If the site situation requires a higher level of
precision to reduce the probability of committing a
Type I or n error, it can only be accomplished by
increasing the number of samples and overall cost
[see guidance in EPA (1995)]. These decisions need
to be made on a site-specific basis and are primarily
related to remediation and risk reduction goals.
Step 5—Develop a Sampling Approach That Will
Answer the Problem Statement and Meet the
Established Level of Precision
Purpose: The environmental scientist can
develop a range of costs and options for different
ranges of probability values of committing either a
Type I or n error. The guidance developed in Step
1 of conducting a preliminary background evaluation
can be expanded, with the assistance of a statistician,
to determine the number of samples, location of
samples, and statistical test to employ.
Approach: Developing a full-scale SAP
directed at determining if there is a difference
between contaminated site and background values
requires knowledge about how the inorganics of
concern move in the environment, site variability,
and level of precision. The environmental scientist
should initially seek the support and guidance of
scientists and a statistician familiar with the issues.
A SAP can then be devised to evaluate questions
about background where off-site concentrations are
elevated due to off-site anthropogenic contributions.
Guidance for reaching decisions in these cases can
be obtained from the Draft Issue Paper (EPA 1992a).
CONCLUSIONS AND
RECOMMENDATIONS
The issue of establishing background concen-
trations for inorganic metals for comparison to levels
at a potentially contaminated site can be complicated
by natural and anthropogenic contributions to total
background concentrations. The issues presented in
this paper are designed to provide investigators with
sufficient knowledge to assess whether concen-
trations of inorganics at a hazardous waste site are
statistically above background concentrations. There
are also discussions on how to approach background
determinations at a hazardous waste site if there is a
high potential for regulatory enforcement action.
There are a wide variety of methods that are
available in the literature and in various EPA
documents for evaluating background. Each method
is slightly different, but there are a number of
common issues that are presented in this paper. The
most important factor to consider when determining
background concentrations is to ensure that the
physical, chemical, and biological aspects of the
media to be sampled at both the contaminated site
and the background site are as similar as possible.
There are references and data included in this paper
that provide average concentrations and reference
values for selected soils and sediments in the United
States. Most of the values in the literature are for
concentrations that include natural and global
anthropogenic contributions. These should be
considered but should not take the place of
conducting a thorough site-specific investigation to
determine the previous land use both on and in the
vicinity of the hazardous waste site to determine
local anthropogenic contributions. The time spent
using well-documented investigative skills to
identify unaffected background sites that are similar
geologically to the contaminated site will be of great
value when establishing background concentrations.
28
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This paper presents the issues that are important to
consider when comparing if inorganics at a
hazardous waste site are statistically different from
those found at a background site areas.
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