SEPA
United States
Environmental Protection
Agency
Off ice of
Solid Waste and
Emergency Response
Superfund Publication:
9380.3-06FS
November 1991
A Guide to Principal Threat and
Low Level Threat Wastes
Office of Emergency and Remedial Response
Hazardous Site Control Division OS-220W
Quick Reference Fact Sheet
The National Oil and Hazardous Substances Pollution Contingency Plan (NCP) promulgated on March 8,1990 states that EPA expects
to use "treatment to address the principal threats posed by a site, wherever practicable" and "engineering controls, such as containment,
for waste that poses a relatively low long-term threat." (40 CFR Section 300.430(aXlXiii)-) These expectations, derived from the
mandates of CERCLA § 121 and based on previous Superfund experience, were developed as guidelines to communicate the types of
remedies that the EPA generally anticipates to find appropriate for specific types of wastes. Although remedy selection decisions are
ultimately site-specific determinations based on an analysis of remedial alternatives using the nine evaluation criteria, these
expectations help to streamline and focus the remedial investigation/feasibility study (RI/FS) on appropriate waste management
options. This guide explains considerations that should be taken into account in categorizing waste for which treatment or
containment generally will be suitable and provides definitions, examples, and ROD documentation requirements related to
waste that constitute a principal or low level threat. EPA makes this categorization of waste as principal or low level threat waste
after deciding whether to take remedial action at a site. The "Interim Final Guidance on Preparing Superfund Decision Documents,"
(EPA/624/1-87/90, October 1990) and "A Guide to Developing Superfund Records of Decision" (Publication 9335.3-02FS-1, May
1990) provide additional information on ROD documentation.
NCP Expectations
EPA established general expectations in the NCP (40 CFR
300.430(a)(l)(iii)) to inform the public of the types of remedies
that EPA has found to be appropriate for certain types of waste
in the past and anticipates selecting in the future. These
expectations (see Highlight 1) provide a means of sharing
collected experience to guide the development of cleanup
options. They reflect EPA's belief that certain source materials
are addressed best through treatment because of technical
limitations to the long-term reliability of containment
technologies, or the serious consequences of exposure should
a release occur. Conversely, these expectations also reflect the
fact that other source materials can be safely contained and that
treatment for all waste will not be appropriate or necessary to
ensure protection of human health and the environment, nor
cost effective.
Identifying Principal and Low Level
Threat Wastes
The concept of principal threat waste and low level threat waste
as developed by EPA in the NCP is to be applied on a site-
specific basis when characterizing source material. "Source
material" is defined as material that includes or contains
hazardous substances, pollutants or contaminants that act as a
reservoir for migration of contamination to ground water, to
surface water, to air, or acts as a source for direct exposure.
HIGHLIGHT 1: NCP Expectations
Involving Principal and Low Level
Threat'Wastes
EPA expects to:
1. Use treatment to address the principal threats
posed by a site, wherever practicable.
2. Use engineering controls, such as containment,
for wastes that pose a relatively low long-term
threat or where treatment is impracticable.
3. Use a combination of methods, as appropriate, to
achieve protection of human health and the
environment. In appropriate site situations,
treatment of principal threats posed by a site,
with priority placed on treating waste that is
liquid, highly toxic or highly mobile, will be
combined with engineering controls (such as
containment) and institutional controls, as
appropriate, for treatment residuals and untreated
waste.
4. Use institutional controls such as water use and
deed restrictions to supplement engineering
controls as appropriate for short- and long-term
management to prevent or limit exposure to
hazardous substances.
Printed on Recycled Paper
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Contaminated ground water generally is not considered to be a
source material although non-aqueous phase liquids (NAPLs)
may be viewed as source materials. The NCP establishes a
different expectation for remediating contaminated ground
water (i.e., to return usable ground waters to their beneficial
uses in a time frame that is reasonable given the particular
circumstances of the site). Examples of source and non-source
materials are provided in Highlight 2.
HIGHLIGHT 2: Examples of Source
and Non-Source Materials
Source Materials
• Drummed wastes
• Contaminated soil and debris
• "Pools" of dense non-aqueous phase liquids
(NAPLs) submerged beneath ground water or
in fractured bedrock
• NAPLs floating on ground water
Contaminated sediments and sludges
Non-Source Materials
• Ground water
• Surface water
• Residuals resulting from treatment of site
materials.
Principal threat wastes are those source materials considered to
be highly toxic or highly mobile that generally cannot be
reliably contained or would present a significant risk to human
health or the environment should exposure occur. They include
liquids and other highly mobile materials (e.g., solvents) or
materials having high concentrations of toxic compounds. No
"threshold level" of toxicity/risk has been established to equate
to "principal threat" However, where toxicity and mobility of
source material combine to poseapotential risk of lO^'or greater,
generally treatment alternatives should be evaluated.
Low level threat wastes are those source materials that generally
can be reliably contained and that would present only a low risk
in the event of release. They include source materials that
exhibit low toxicity, low mobility in the environment, or are
near health-based levels.
Determinations as to whether a source material is a principal or
low level threat waste should be based on the inherent toxicity
as well as a consideration of die physical state of the material
(e.g., liquid), thepotential mobility of the wastes in the particular
environmental setting, and the lability and degradation products
of the material. However, this concept of principal and low
level threat waste should not necessarily be equated with the
risks posed by site contaminants via various exposure path ways.
Although the characterization of some material as principal or
low level threats takes into account toxicity (and is thus related
to degreeofrisk posed assuming exposure occurs),characterizing
a waste as a principal threat does not mean that the waste poses
the primary risk at the site. For example, buried drums leaking
solvents into ground water would be considered a principal
threat waste, yet the primary risk at the site (assuming little en-
no direct contact threat) could be ingestion of contaminated
ground water, which as discussed above is not considered to be
a source material, and thus would not be categorized as a
principal threat
The identification of principal and low level threats is made on
a site-specific basis. In some situations site wastes will not be
readily classifiable as either a principal or low level threat
waste, and thus no general expectations on how best to manage
these source materials of moderate toxicity and mobility will
necessarily apply. [NOTE: In these situations wastes do not
haveto be characterized as eitherone or theother. The principal
threat/low level threat waste concept and the NCP expectations
were established to help streamline and focus the remedy
selection process, not as a mandatory waste classification
requirement]
HIGHLIGHT 3: Examples of Pn~ipal
and Low Level Threat Wastes
Wastes that generally will be considered to constitute
principal threats include, but are not limited to:
• Liquids - waste contained in drums, lagoons or
tanks, free product(NAPLs) floating on or under
ground water (generally excluding ground water)
containing contaminants of concern.
• Mobile source material - surface soil or
subsurface soil containing high concentrations
ofcontaminants of concern that are (or potentially
are) mobile due to wind entrainment,
volatilization (e.g., VOCs), surface runoff, or
sub-surface transport
• Highlv-toxic source material-buried drummed
non-liquid wastes, buried tanks containing non-
liquid wastes, or soils containing significant
concentrations of highly toxic materials.
Waste that generally will be considered to constitute low
level threat wastes include, but are not limited to:
• Non-mobile con'^inated source material of
low to moderate toxicitv - Surface soil containing
contaminants of concern that generally are
relatively immobile in air or ground water (i.e.,
non-liquid, low volatility, low teachability
contaminants such as high molecular weight
compounds) in the specific environmental
setting.
• Low toxicitv source material - soil and subsurface
soil concentrations not greatly above reference
dose levels or that present an excess cancer risk
near the acceptable risk range.
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Examples of principal and low level threat wastes are provided
in Highlight 3.
Risk Management Decisions for
Principal and Low Level Threat
Wastes
The categorization of source material as a principal threat
or low level threat waste, and the expectations regarding
the use of treatment and containment technologies follows
the fundamental decision as to whether any remedial action
is required at a site. These determinations, and the application
of the expectations, serve as general guidelines and do not
dictate the selection of a particular remedial alternative. For
example, EPA's experience has demonstrated that highly mobile
wastes (e.g., liquids) are difficult to reliably contain and thus
generally need to be treated. As such, EPA expects alternatives
developed to address highly mobile material to focus on
treatment options rather that containment approaches.
However, as stated in the preamble to the NCP (55 FRat 8703,
March 8,1990),theremaybesiluations where wastes identified
as constituting a principal threat may be contained rather than
treated due to difficulties in treating the wastes. Specific
situations that may limit the use of treatment include:
i
• Treatment technologies are not technically feasible
or are not available within a reasonable time frame;
• The extraordinary volume of materials or
complexity of the site make implementation of
treatment technologies impracticable;
• Implementation of a treatment-based remedy would
result in greater overall risk to human health and
the environment due to risks posed to workers or
the surrounding community during implementation;
or
• Severe effects across environmental media
resulting from implementation would occur.
Conversely, there may be situations where treatment will be
selected for both principal threat wastes and low level threat
wastes. For example, once a decision has been made to treat
some wastes (e.g., in an onsite incinerator) economies of
scale may make it cost effective to treat all materials
including low level threat wastes to alleviate or minimize the
need for engineering/institutional controls.
While these expectations may guide the development of
appropriate alternatives, the fact that a remedy is consistent
with the expectations does not constitute sufficient ground^ for
the selection of that remedial alternative. The selection of an
appropriate waste management strategy is determined solely
through the remedy selection process outlined in the NCP (i.e.,
all remedy selection decisions are site-specific and must be
based on a comparative analysis of the alternatives using the
nine criteria in accordance with the NCP). Independent of the
expectations, selected remedies must be protective, ARAR-
compliant, cost-effective, and use permanent solutions or
treatment to the maximum extent practicable. Once the final
remedy is selected, consistency with the NCP expectations
should be discussed as part of the documented rationale for the
decision.
ROD Documentation
Declaration
The "Description of the Selected Remedy" section should note
whether the remedy is addressing any source materials that
constitute "principal" or "low level" threat wastes, or both.
The "Statutory Determinations" section should discuss how the
selected remedy satisfies the statutory preference stated in
CERCLA §121 to select remedial actions "in which treatment
which permanently and significantly reduces the volume,
toxicity or mobility of the hazardous substances, pollutants,
and contaminants is a principal element." In evaluating this
statutory preference, the site manager needs to decide whether
treatment selected in the ROD constitutes treatment as a major
componentof the remedy for that site. Remedies which involve
treatment of principal threat wastes likely will satisfy the
statutory preference for treatment as a principal element,
although this will not necessarily be true in all cases (e.g., when
principal threat wastes that are treated represent only a small
fraction of the wastes managed through containment). Ground
water treatment remedies also may satisfy the statutory
preference, even though contaminated ground water is not
considered a principal threat waste and even though principal
threat source material may not be treated.
Decision Summary
The "Decision Summary" of the ROD should identify those
source materials that have been identified as principal threat
and/or low level threat wastes, and the basis for these
designations. These designations should be provided in the
"Summary 3)f Site Characteristics" section as part of the
discussion focusing on these source materials that pose or
potentially pose a risk to human health and the environment In
addition, the "Description of Alternatives" and the "Selection
of Remedy" sections should briefly note how principal and/or
low level threat wastes that may have been identified are being
managed.
The "Statutory Determinations" section of the ROD should
includeadiscussion of how thestatutory preference for treatment
as a principal element is satisfied or explain why it is not
satisfied, stating reasons in terms of the nine evaluation criteria.
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NOTICE: The policies set out in this memorandum are intended solely as guidance. They are not intended, nor can they be
relied upon, to create any rights enforceable by any party in litigation with the United States. EPA officials may decide to
follow the guidance provided in this memorandum, or to act at variance with the guidance, based on an analysis of specific
site circumstances. The Agency also reserves the right to change this guidance at any time without public notice.
&EPA
United States
Environmental Protection
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