xv EPA
United States
Environmental
Protection
Agency
Office of
Solid Waste and
Emergency Response
Publication 9345.0-051
March 1994
ECO Update
Office of Emergency Remedial Response
Hazardous Site Evaluation Division (5204G)
Intermittent Bulletin
Volume 2 Number!
Using Toxicity Tests in Ecological
Risk Assessment
Toxicity tests are used to expose test organisms to a
medium—water, sediment, or soil—and evaluate the effects of
contamination on the survival, growth, reproduction, behavior
and/or other attributes of these organisms. These tests may
help to determine whether the contaminant concentrations in a
site's media are high enough to cause adverse effects in
organisms. Generally, toxicity tests involve collecting
samples of media from a site and sending them to a toxicity
laboratory, where the tests are performed. On occasion
investigators1 measure toxicity by exposing test organisms to
soil or water on site—these are known as in situ tests.
As the general guidelines at the end of this Bulletin
indicate, not all sites require toxicity tests. But where they are
used, toxicity tests can contribute to ecological risk
assessments in specific ways and at different stages in the
assessment.
1. Toxicity tests can demonstrate whether contaminants
are bioavailable.2 The presence of a contaminant does not
itself indicate a potential for adverse effects. A contaminant
can have toxic effects only if it occurs in a bioavailable form.
Sometimes the presence of abrasives, such as the talc in
pesticides, can damage an organism's body covering, thereby
increasing the bioavailability of certain contaminants for that
organism.
The term "investigator" refers to the individual charged with
responsibility for designing and/or carrying out any part of an ecological risk
assessment. Investigators can include government scientists, contractors, or
university scientists. However the site manager (remedial project manager or
on-scene coordinator) retains ultimate responsibility for the quality of the
ecological risk assessment.
2 Bioavailability is the presence of a substance in a form that organisms
can take up. (Note that specialized terms appear in boldface and are defined
either in the text or in accompanying footnotes.)
2. Toxicity tests can evaluate the aggregate toxic effects of
all contaminants in a medium. Many Superfund sites present
a complex array of contaminants, with a mixture of potentially
harmful substances present in the media. At such sites,
chemical data alone cannot accurately predict the toxicity of
the contaminants. Rather, toxicity tests measure the aggregate
effects of contaminated media on organisms. These effects
result from characteristics of the medium itself (such as
hardness and pH, in the case of water), interactions among
contaminants, and interactions between contaminants and
media. Consequently, observed toxicity test results may often
vary from those predicted by chemical data alone.
3. Toxicity tests can evaluate the toxicity of substances
whose biological effects may not have been well
characterized. The contaminants at a Superfund site might
include substances that have not been previously investigated
regarding their toxicity to wildlife or other organisms.
Consequently, the scientific literature contains no relevant
data concerning these substances. At such sites, toxicity tests
of media samples indicate the combined toxicity of all
contaminants, including those that have not been previously
tested.
IN THIS BULLETIN
Measurement Endpoints in Toxicity Testing 2
Elements in a Toxicity Assessment 3
General Guidelines for Choosing Toxicity Tests 7
ECO Update is a Bulletin series on ecological risk assessment of Superfund sites. These Bulletins serve as supplements to Risk Assessment Guidance for
Superfund, Volume II: Environmental Evaluation Manual (EPA/540-1-89/001). The information presented is intended to provide technical information to EPA and
other government employees. It does not constitute rulemaking by the Agency, and may not be relied on to create a substantive or procedural right enforceable by any
other person. The Government may take action that is at variance with these Bulletins
-------
4. Toxicity tests can characterize the nature of a toxic
effect. Investigators can use toxicity tests to learn whether
contaminant concentrations have lethal or sublethal effects.
Some examples of sublethal effects include reduced growth,
impaired reproduction, and behavioral changes.
5. Toxicity tests characterize the distribution of toxicity at
a site. An investigator can have toxicity tests performed on
samples from a variety of locations at the site. In some
instances toxicity tests may be a cost-effective way to
determine the spatial extent of toxicity and identify areas with
high levels of toxicity.
6. Toxicity tests can be used to develop remedial goals.
Acceptable levels of toxicity, as measured by toxicity tests,
can form criterion for remedial goals. For example, a goal
might be to reduce the toxicity of pond water over a stated
time period. The remedial goal would specify the level at
which toxicity should be reduced and the species in which
toxicity should be measured. The species should be
representative of the site and sensitive to its contaminants.
The species also should be related to the overall assessment
endpoints.3
7. Toxicity tests have a role in monitoring. Toxicity tests
can be used to monitor the remediation of a Superfund site.
Specifically, toxicity testing can indicate whether sources of
contamination have been contained and whether remedial
measures are reducing toxicity.
8. Toxicity tests have a role in determining a site's
postremediation potential to support a viable ecological
community. For example, if a stream or waterbody receives
contaminants from numerous sources, including a Superfund
site, upstream toxicity testing may help to determine what the
water's potential for supporting a viable ecological community
might be if the Superfund loadings are removed and the other
sources remain unchanged.
Toxicity tests include a broad spectrum of tests, differing
in the species and exposure media they use and the effects
they measure. In making decisions about whether to conduct
toxicity tests, which tests to choose, and how many to
perform, investigators are well advised to seek advice from
qualified experts, such as those serving on a Regional
Biological Technical Assistance Group (BTAG).4
This Bulletin first describes two major classes of toxicity
tests—acute and chronic—and then explores the elements that
an investigator needs to consider in planning toxicity tests.
Finally, the Bulletin offers general guidance on when to use
toxicity tests and how to select those appropriate to different
3 An assessment endpoint is an ecological characteristic that may be
adversely affected by site contamination and that, at a Superfund site, can help
to drive remedial decision making (U.S. EPA, 1992).
4 These groups are sometimes known by different names, depending on
the Region. Readers should check with the appropriate Superfund manager
for the name of the BTAG coordinator or other sources of technical assistance
in their Region. A more complete description of BTAG structure and function
is available in "The Role of BTAGs in Ecological Assessment" (ECO Update
Vol.1, No. 1).
sites. The companion document, "Catalogue of Standard
Toxicity Tests for Ecological Risk Assessment" (ECO Update
Vol. 2, No. 2), provides an annotated list of standardized tests
appropriate for use with different media.
Measurement Endpoints In Toxicity
Testing: Acute Vs. Chronic Tests
Toxicity tests can measure lethal and/or sublethal effects.
These effects are known as measurement endpoints: that is,
they are ecological attributes that may be adversely affected
by exposure to site contaminants and that are readily
measurable. In addition, each measurement endpoint is
closely related to an assessment endpoint. Because of this
close relationship, a measurement endpoint can approximate
or represent the assessment endpoint if the assessment
endpoint is not amenable to direct measurement (U.S. EPA,
1992).
Acute toxicity tests are short-term tests that measure the
effects of exposure to relatively high concentrations of
chemicals. The measurement endpoint generally reflects the
extent of lethality.
Chronic toxicity tests, on the other hand, generally are
longer-term tests that measure the effects of exposure to
relatively lower, less toxic concentrations. For a chronic
toxicity test, the measurement endpoint concerns a sublethal
effect (e.g., reproduction, growth) or both lethality and sub-
lethal effect.
Acute Toxicity Tests
A typical acute toxicity test exposes test organisms to a
series of dilutions of a site's medium and records deaths
occurring over a specified period of time, usually 24 to 96
hours. Results can be analyzed by comparing percent
mortality of organisms exposed to site media to percent
mortality of organisms exposed to uncontaminated media.
(See section below entitled "The Reference Site.")
Alternatively, results of an acute toxicity test can be analyzed
to estimate the dilution of the medium at which 50 percent of
the organisms died. This dilution (also referred to as a
concentration), called the LC50, is the median lethal
concentration. When an acute toxicity test reports an LC50, the
test results usually will specify the test duration, the test
species, and the life cycle stage of the test species (e.g., the
fathead minnow 96 hour LC50). Since LC50s are point
estimates, which are estimates of the effects from specific
concentrations of contaminants, coefficients of variation can
be calculated for them. (See section below entitled "Statistical
Analysis.")
With some test organisms, toxicologists find death difficult
to determine unequivocally. In tests using such organisms,
toxicologists evaluate another effect, such as immobility, that
correlates closely with death. As with death for a
measurement endpoint, results can be analyzed by comparing
percent effect for organisms exposed to site media and those
March 1994 • Vol. 2, No. 1
ECO Update
-------
exposed to uncontaminated media. Alternatively, data can be
analyzed to estimate the dilution at which 50 percent of the
organisms displayed the effect. This dilution (also referred to
as a concentration), called the EC50, is the median effective
concentration. When an acute toxicity test reports an EC50, the
test results will specify the effect, the test duration, the test
species, and the life cycle stage of the test species. Like the
LC50, the EC50 is a point estimate and a coefficient of variation
can be calculated for it.
In still other approaches to evaluating results, the
laboratory analyzes the data for the Lowest Observed Effect
Concentration (LOEC), which is the highest dilution causing
statistically significant toxic effects, or the No Observed
Effect Concentration (NOEC), which is the lowest dilution
at which no statistically significant toxic effects occurred.5
Statistically determined using hypothesis testing, LOECs and
NOECs are not point estimates and consequently coefficients
of variation cannot be calculated for them.
Acute toxicity tests are short-term
tests that measure the effects of
exposure to relatively high
concentrations of chemicals.
Chronic toxicity tests generally
are longer-term tests that measure
the effects of exposure to relatively
lower, less toxic concentrations.
Chronic Toxicity Tests
A chronic toxicity test exposes test organisms to a series
of dilutions of a site's medium and measures sub-lethal
effects, and in some cases lethal effects as well. Sublethal
effects may include growth reduction, reproductive
impairment, nerve function impairment, lack of motility,
behavioral changes, and the development of terata, which are
structural abnormalities. Results can be analyzed in several
As used in the Bulletin, LOEC is synonymous with Lowest Observed
Adverse Effect Concentration (LOAEC) and Lowest Observed Adverse Effect
Level (LOAEL), and NOEC with No Observed Adverse Effect Concentration
(NOAEC) and No Observed Adverse Effect Level (NOAEL).
ways. One is simply by a direct comparison between percent
effect occurring in organisms exposed to site media and those
exposed to uncontaminated media. Other approaches to
analysis determine the EC50 and the LOEC, or the NOEC.
Ecological Significance of Sublethal Effects
Although it would be an oversimplification to extrapolate
from the outcome of chronic toxicity tests to ecological
conditions at a Superfund site, site managers need to be aware
that the sublethal effects that chronic toxicity tests measure in
laboratories are ecologically significant effects when they
occur in the environment. For example, reduced growth can
lead to decreased production, smaller size, lower fecundity
(eggs or young per female), increased susceptibility to
predation, and other effects. Reproductive impairment can
reduce the population size and also bring about changes in a
population's age structure. Production of individuals with
terata can adversely affect a population because these
individuals have a lower growth rate, are generally unable to
reproduce, and have an increased susceptibility to predation.
A Comparison of Acute and Chronic
Toxicity Tests with Respect to Time, Cost,
and Resolution
In general, acute and chronic toxicity tests differ in the
amount of time required to perform them, their cost, and their
resolution.
• Because chronic tests extend through either a life
cycle or a critical developmental phase, they generally
require more time to perform than acute tests with the
same type of test organisms.
• Requiring more time to complete than acute tests,
chronic tests also can require more funds. A chronic
test also may require more resources and increased
numbers of laboratory analyses, further increasing the
cost of the test.
• Chronic tests have greater resolution than acute tests.
For example, consider a chronic test that exposes
invertebrates to site surface water and records the
number of young they produce. In a highly toxic
medium, the organisms will die. In a less toxic
medium, they may survive, but their reproductive
capacity may be impaired when compared with
controls maintained in an uncontaminated medium.
Elements in a Toxicity Assessment
The investigator needs to consider many elements when
planning toxicity assessment: the objective, the reference site,
the medium analyzed, the test organisms, the test
methodology, the level of effort, the test site, and quality
assurance/quality control (QA/QC) standards. By the choices
that he or she makes, the investigator can tailor the toxicity
March 1994 • Vol. 2, No. 1
ECO Update
-------
assessment to meet the needs of the site and its stage in the
Superfund process.
The Objective
As with any study, before planning a toxicity assessment
the investigator needs to set clear objectives. In particular, the
assessment's objectives need to include some that address the
medium of concern, the characteristics of the contaminants of
concern, and the potential ecological components.6 For
example, if the study asks whether soil on the site is toxic to
macroinvertebrates, then the study will need to analyze bulk
soil rather than an elutriate7 and will need to use an
appropriate test organism.
The objectives of a toxicity assessment should indicate
the level of effort appropriate to the assessment. For example,
determining whether a particular medium is toxic would
generally require a low level of effort. Such a study might
specify only two species of test organisms and undiluted
medium collected from a limited number of sampling
locations. If the objective of a toxicity assessment is to
determine the appropriate range of dilutions for conducting
further tests (if these prove necessary) at a highly
contaminated site, a higher level of effort would be necessary.
Such a study might specify using a series of tenfold dilutions
of media collected form locations known to have high
contaminant concentrations. A reasonably detailed characteri-
zation of a site's toxicity would imply a high level of effort.
This type of study might include test organisms at different
trophic levels8 (such as alga, a macroinvertebrate, and a fish),
several sampling locations (possibly based on a grid or
selected from upstream and downstream areas), and several
dilutions of medium.
The Reference Site
When planning a toxicity assessment, an investigator
selects a reference site that as closely as possible mirrors the
characteristics of the site medium being analyzed but is
unaffected by site contamination. Analyzing a sample from
the reference site allows the investigator to measure
background conditions. The investigator should try to locate
the reference site as close as possible to the Superfund site so
that the reference site will accurately reflect the site's
conditions. Yet the reference site should lie at a great enough
distance from the Superfund site to be unaffected by site
contamination. Provided that pollutant loading from other
sources does not occur upstream, an upstream location may
6 An ecological component is an individual organism, a population, a
community, a habitat, or an ecosystem that may suffer adverse effects as a
result of site contamination.
7 An elutriate (or eluate) is the solution obtained when water removes
substances adsorbed to sediment particles
8 A trophic level is a stage in the flow of food from one population to
another. For example, as primary producers (organisms that convert the
energy from sunlight to chemical energy) plants occupy the first trophic level,
and grazing organisms occupy the second trophic level.
provide an appropriate reference site for a Superfund site with
contaminated surface water. Soil type and texture, vegetation,
and slope are important considerations in selecting a reference
site with the appropriate terrestrial characteristics.
The Medium
Toxicity tests vary as to the media they analyze. Aquatic
tests evaluate freshwater, marine, or estuarine samples. A few
tests are designed specifically to analyze bulk sediment
samples, and a few are specific for bulk soil samples. Bulk
sediment or soil tests specifically address toxicity in the test
medium. Alternatively, laboratory technicians can prepare
elutriates of sediment or soil samples and analyze the
elutriates by means of aquatic tests. Toxicity tests using
elutriates give information about the transfer of contaminants
from sediment or soil to water. Such information is most
valuable when predicting effects of runoff or leaching from
soil or determining the advisability of remediating a site by
dredging contaminated sediments.
A toxicity test also should include measurements of the
appropriate physical and chemical parameters of the sample
medium. For water, these parameters might include alkalinity,
hardness, pH, temperature, dissolved oxygen, total dissolved
solids, and total organic carbon. For a sediment sample, grain
size, percent water, pH, total organic carbon, and/or other
parameters may prove important to know.
A toxicity test should include
measurements of the appropriate
physical and chemical parameters of
the sample medium.
In some cases the physical or chemical parameters of the
test medium require adjustment in order to meet the conditions
of a test protocol. Sediment or soil may require dewatering.
Water samples may need to have their pH, hardness, or
dissolved oxygen content adjusted. Such adjustments can
change the solubility, bioavailability, or toxic properties of
sample constituents and therefore should be avoided or
minimized wherever possible. If the test medium requires
adjustment, the investigator should allow a portion of it to
remain unadjusted. This unadjusted portion is used in a
parallel control that will indicate whether the adjustment
contributes to, masks, or has no effect on toxicity. In cases
where the test medium requires adjustment, the investigator
should evaluate the data quality objectives (DQOs)9 to
9 Data quality objectives (DQOs) are statements that define the level of
uncertainty that investigator is willing to accept in environmental data used to
March 1994 • Vol. 2, No. 1
ECO Update
-------
determine whether the adjustments would interfere with the
study's objectives.
For many toxicity tests investigators must dilute sample
media to determine LC50s, EC50s, LOECs or NOECs.
Protocols for aquatic tests generally specify using specially
treated laboratory water as a diluent, but natural water can be
used as well. Diluting material for soils or sediments can
consist of artificial soil prepared in the laboratory.
Test Organisms
Toxicologists have based their selection of test organisms
on several factors: sensitivity to variety of substances,
availability, representativeness of a variety of ecosystems, and
ease of maintenance and culture under laboratory conditions.
For aquatic tests, the frequently used test organisms are those
employed for toxicity testing for National Pollutant Discharge
Elimination System (NPDES) permits. Table 1 summarizes
information about the organisms used in the standardized tests,
while Figure 1 illustrates a few of these organisms.
When choosing from among the available standard test
organisms, the investigator should select a species that is
representative of resident organisms, sensitive to site
contaminants, relevant to the overall assessment endpoints,
and consistent with DQOs. In a toxicity test, the test
organisms serve as surrogates for organisms present on the
site. For instance, although fathead minnows (Pimehales
promelas), a common test organism, may not occur on the site,
they can serve as surrogates for other fish. Consequently an
LC50 for fathead minnows can serve as a measurement
endpoint for the assessment endpoint "survival of the minnow
populations in a specific stream that flows through the site."
In a broader context, fathead minnows might represent all
warmwater fish on a site, since research has shown that
organisms at the same taxonomic level (level of classification,
such as genus or family) often respond similarly to a
contaminant (Baker, 1989). When selecting test organisms,
the investigator should keep the study's DQOs in mind. If the
investigator's selection is not consistent with the DQOs, the
applicability of the test data to the site is questionable.
Although the existence of well-established protocols and
considerable historical data makes the standard test organisms
useful, in some cases investigators find that none of the
standard organisms is representative of a site's ecosystem. If
this situation occurs, the investigator must account for this
lack of representativeness when interpreting test results.
Alternatively, the investigator may decide to use a "non-
standard" or alternative species instead of the one specified in
the test protocol. The alternative species might better
represent resident organisms, show greater sensitivity to the
site's contaminants, or be more consistent with the study's
DQOs. State resource agencies can readily provide
information on resident species. Using resident species as an
alternative species has the potential of providing direct
support a remedial decision. DQOs address the purpose and use data, the
resource constraints on data collection, and any calculations based on the data.
information about the toxic effects to site species. Several
criteria must be specified for the use of alternative test species,
including the source for the test organism, the age range
suitable for the test, a means for eliminating variability in the
organism's condition, and conditions suitable for the test. In
addition, if the organism must be collected rather than
purchased the investigator will have to establish standards for
ensuring accurate identification and also should meet all local,
state, and federal requirements concerning the collection of
organisms.
When choosing form among
the available standard test
organisms, the investigator should
select a species that is
representative of resident
organisms, sensitive to site
contaminants, relevant to the
overall assessment endpoints, and
consistent with DOOs.
Generally, using alternative species increases the cost of
conducting toxicity tests, especially when the investigator
needs to determine optimal conditions for conducting the test.
However, the investigator, in consultation with the BTAG,
may decide that the added usefulness of the results justifies the
extra expense. In such a case, the investigator may be able to
reduce the added expense by employing a laboratory
experienced in the use of the species selected for the study.
Test Method
Standard toxicity tests can employ a variety of methods
for collecting samples and for exposing test organisms to
media. Designing a toxicity assessment for a site requires the
investigator to select the most appropriate methods for
studying the issues for that site.
Field biologists can collect media samples for testing
either by the grab or the composite method. As the name
implies, a grab sample is a single sample, usually entailing
little time and minimal equipment to collect. When the
investigator expects the site's contaminant picture to change
little over time, a single grab sample per location may
adequately represent contamination. A composite sample, on
the other hand, is a mixed sample, which may be collected at a
March 1994 • Vol. 2, No. 1
ECO Update
-------
Table 1. Plant and Animal Species Used in Standard Toxicity Tests*
Medium Test Organisms Test Temp (°C) Life Stage
F
R
E
S
w
A
T
E
R
MARINE
and
ESTUARINE
WATERS
FRESHWATER
SEDIMENT
MARINE
SEDIMENT
SOIL
VERTEBRATES
Brook trout (Salvelinm fontinalis)
Rainbow trout (Oncorhynchus mykiss)
Fathead minnow (Pimephales promelas)
INVERTEBRATES
Amphipod (Hyaletta)
Waterflea (Daphnia magna, Daphnia pulex, Ceriodaphnia)
Mayfly (Hexagenia limbata, Hexagenia bilineata)
Midge (Chironomm)
12
12
20-25
20 or 25
20 or 25
17, 20-22
20 or 25
30-60 days
15-30 days
1-14 days
7-14 days
1-24 hours
Young nymph
First to second instar
ALGA
Selenastrum capricomutum
25
4-7 day stock culture
VERTEBRATES
Sheepshead minnow (Cyprinodon variegatus)
Silverside (Menidia species)
20 or 25
20 or 25
1-14 days
9-14 days
INVERTEBRATES
Sea urchin (Arbacia punctulata)
Mysid shrimp (Mysidopsis)
20
20
<1 hour old
1-5 days
ALGA
Champiaparvula
Amphipod (Hyaletta azteca)
Midge (Chironomus tentans and Chironomus riparim)
Amphipod (Rhepoxynius abronim)
Amphipod (Eohaiistorius estuarius)
Amphipod (Ampelisca abdita)
Amphipod (Grandidier Uajaponica)
Earthworm (Eiseniafoetida)
Lettuce (Latuca saliva)
23
20-25
20 or 25
15
15
20
15-19
22
24
Sexually mature
7-14 days
First to second instar
Mature 3-5 mm, mixed sex
Mature 3-5 mm, mixed sex
Immature or mature females only
Immature 3-6 mm, no females w/
embryos
300-600 mg adult
Seed
*compiled from ASTM, 1992c; Green et al, 1989; US Army Corps of Engineers, 1993; Weber et al, 1988; Weber et al, 1989; Weber et al, 1991.
March 1994 • Vol. 2, No. 1
ECO Update
-------
Figure 1. Test Organisms Commonly Used in Toxicity Tests
The organisms shown in these figures are the adult life
stages of the test organisms, not necessarily those life
stages used in toxicity tests.
Fathead minnow (Pimephales promelas)
50 mm.
Waterflea (Daphnia)
up to 3.5 mm for Daphnia pulex
Purple sea urchin
(Stronglyocentrotus purpuratus)
6 to 12cm
Macroalga (Champia)
Actual size of branch tip 25 mm.
Mysid shrimp (Mysidopsis)
4.4 to 9.4 mm for Mysidopsis bahia
Marine amphipods
Species used in toxicity tests usually
3-6 mm.
Reproduced by courtesy of Aquatic Research Organisms,
P.O. Box 1271, Hampton, NH 03842.
March 1994 • Vol. 2, No. 1
ECO Update
-------
single location over a specified period of time or at multiple
locations at one time. When sampling a stream with a highly
variable flow rate, the investigator can specify the collection
of a flow weighted composite sample. The BTAG can advise
the investigator as to the preferred collection method for a
particular site.
Toxicity tests analyzing water or elutriates of soil or
sediment can expose test organisms using the same sample
medium throughout the test or arranging for limited
replacement of medium. Those using the same sample
medium throughout are called static tests, while static-
renewal tests are those that replace all or part of the sample
medium at specified times during the test. Since this approach
requires little space, manpower, and equipment, static tests are
comparatively simple and inexpensive to perform. In
addition, static tests require only small sample volumes of one
to twenty liters.
On the other hand, static tests, particularly those without
renewal of media, do have certain limitations. Over the course
of a non-renewal test, test organisms can deplete the dissolved
oxygen in the sample and surfer adverse effects unrelated to
toxicity. Alternatively, contaminants can break down,
volatilize, or adhere to the walls of the container. As a result,
the test might not accurately reflect the medium's toxicity.
Finally, as organisms metabolize they release substances, such
as carbon dioxide and wastes, called metabolites. As these
metabolites accumulate, they can prove toxic to the test
organisms. Alternatively, they may interact with contaminants
and alter the medium's apparent toxicity. The static-renewal
design overcomes to a certain extent the disadvantages of the
non-renewal design.
In place of the static design, aquatic toxicity tests may use
a flow-through method, continuously pumping fresh sample
medium through test chambers. With this method, dissolved
oxygen remains relatively high and metabolites are flushed
away. The flow-through approach also has the advantage of
minimizing loss of toxics to degradation, adsorption, and
volatilization. When conducted on-site, flow-through tests can
provide information about fluctuations in toxicity.
Disadvantages of the flow-through method include its
expense and inconvenience. This approach uses complex
equipment that requires more maintenance than the equipment
used for static tests. Large volumes of sample and diluent are
needed for flow-through tests. As a result, these tests are more
expensive than equivalent static tests.
Level of Effort
The quantity and nature of information provided by toxicity
tests vary considerably with the level of effort mandated by
the study's objective. One factor determining the level of
effort is the number of species used as test organisms. When
the investigator selects a test species that is quite sensitive to
the site's contaminants relative to other types of organisms in
the community, then a single-species test can suggest the
community's maximum susceptibility to the site's
contaminants. At a higher level of effort, employing more
than one test species may indicate whether single-species tests
have underestimated or overestimated the site's toxicity.
However, when evaluating water, toxicity tests should always
include at least two species, a fish and an invertebrate, unless
the site has only one contaminant and either fish or
invertebrates are known to be insensitive to that contaminant.
The level of effort also varies with the range of media
concentrations analyzed and the duration of the tests.
• Screening tests, which may be either acute or chronic
tests, evaluate only the undiluted sample. Positive values
on screening tests may indicate the need to proceed to
definitive tests.
• Range Finding tests are abbreviated static acute tests that
expose test organisms to a broad range of media dilutions
for 8 to 20 hours. Such tests identify the dilutions to use
for definitive tests.
• Definitive tests provide a dose-response curve and
reduced variability. Both acute and chronic tests can be
conducted as definitive tests. Investigators can use these
three types of tests to translate "level of effort" objectives
into appropriate toxicity assessments.
Test Site: Laboratory or in situ
In general, toxicity tests are conducted by collecting
samples from a site and sending them to a laboratory for
testing. Laboratory tests offer the advantage of standardized
protocols. In addition, laboratories experienced in performing
these tests are generally available.
Alternatively, toxicity tests can occur in situ, which means
"in place." That is, the investigator exposes test organisms to
soil or water on the site. In situ tests give organisms
continuous exposure to the site media under actual
environmental conditions such as temperature, stream flow,
and light.10 Consequently, data from in situ tests provide a
more realistic assessment of toxicity than do data from
laboratory tests. In situ tests also provide a more direct means
of comparing toxicity data with estimates of exposure derived
from field data. Ultimately, such comparisons help to
characterize the site's ecological risk.
On the other hand, in situ tests have certain disadvantages.
In particular, the investigator lacks control over the conditions
under which an in situ test occurs. For example, temperature
may vary considerably over the course of an in situ test,
whereas in the laboratory temperature would be carefully
regulated. When interpreting data from in situ tests, the
investigator should consider how the varying conditions of the
test could affect results. Another issue associated with in situ
tests concerns logistics, which can prove difficult in adverse
weather conditions.
At the present time, in situ tests generally consist of
standard laboratory tests adapted for on-site use. Some
In situ tests should not be confused with tests performed in a mobile
laboratory brought to the site. A mobile laboratory performs toxicity tests
under standard laboratory conditions, not site conditions.
March 1994 • Vol. 2, No. 1
ECO Update
-------
commonly used tests employ earthworms or lettuce seeds as
standard test organisms for soil toxicity tests. For in situ
aquatic toxicity tests, fish, clams, and oysters are often used.
Tests may instead use alternative or resident species. In
practical terms, to conduct in situ tests, test vegetation is
maintained in plots and test animals are held in containers on
site. The earthworm test, in particular, has been successfully
adapted for in situ use. In fact, some field biologists routinely
perform this test in situ rather than having personnel conduct it
as a laboratory test.
Statistical Analysis
The statistical analysis of toxicity test results depends upon
the measurement endpoint. As mentioned earlier, LC50s and
EC50s are point estimates; that is, they are estimates of effects
from specific dilutions of contaminants. To calculate point
estimates, test data are analyzed using regression models that
assume the less dilute the sample, the greater will be the
effects. Coefficients of variation can be calculated for point
estimates.
LOECs and NOECs compare results at test dilutions with
controls to determine whether the results are significantly
different. LOECs and NOECs are calculated by means of a
statistical method called hypothesis testing. Coefficients of
variation cannot be calculated for values determined using
hypothesis testing. Note too that the values obtained for
LOECs and NOECs can vary considerably depending on the
specific series of dilutions used in the test.
For detailed information concerning statistical analysis of
toxicity test data, investigators can consult Weber et al. (1988,
1989, 1991).
Quality Assurance/Quality Control (QA/QC)
Standards
The investigator must specify standards for quality
assurance and quality control (QA/QC) to ensure that toxicity
tests produce reliable results. QA/QC standards describe
appropriate sample handling and collection. Such matters as
the container type, storage temperature, and maximum
permissible storage time affect the reliability of data. For
example, elapsed time and exposure to air can alter sample
properties or result in the loss of volatile chemicals.
Generally, aquatic samples should be stored at 4°C until used.
Although holding time limitations for sediments vary with
individual sediment properties and contaminant
characteristics, the American Society for Testing and
Materials (ASTM) recommends storing sediments at 4°C and
using them within two weeks of collection.
Test parameters should specify which controls to perform
and what values to accept for controls. For example, in an
acute toxicity test with Daphnia pulex, a control consists of
test organisms exposed to dilution water only. For test results
to be considered valid, at least 90% of the animals in the
control must survive the test.
Finally, QA/QC measures need to ensure that the data
collected can support the appropriate statistical analysis.
The BTAG can advise the investigator as to whether the
proposed QA/QC standards are adequate.
General Guidelines for Choosing
Toxicity Tests
In an ecological risk assessment of a Superfund site, the
investigator must decide whether toxicity testing will
contribute to the assessment and, if so, which and how many
tests to perform. The great differences among Superfund
sites—differences in size, terrain, and contaminant profile, to
name a few—make a rigidly standardized approach to toxicity
studies unworkable. However, a few widely accepted general
guidelines do exist:
• Do not perform toxicity studies at a site where the
contaminants of concern do not cause effects
measured by toxicity tests. For example,
polychlorinated biphenyls (PCBs) bring about
reproductive effects that many toxicity tests do not
detect. The investigator should consult the BTAG to
find out whether toxicity tests can detect the type of
effects caused by the site's contaminants.
• At a site where several substances have contaminated
surface water, aquatic toxicity testing generally
should include both a fish and an invertebrate species.
At some sites, it may prove advisable to include
additional test organisms, as well.
• Select test organisms that are sensitive to the site
contaminants. For example, among standard
freshwater test organisms, water fleas of the genus
Ceriodaphnia and the embryos and larvae of fathead
minnows are sensitive to a broad spectrum of
contaminants. Of the two species of midges used to
conduct standard tests of freshwater sediments,
Chironomus riparius is the choice when metals are the
contaminants of concern. When conducting initial
tests of saltwater species, it is usually appropriate to
use a grass shrimp, penaeid shrimp, or mysid, because
these invertebrates often are more sensitive than fish.
As a final example, algae should be used for initial
tests when herbicides and materials with suspected
phytotoxicity are detected in fresh or salt water.
These are only a few examples of how test organisms
differ in their sensitivity to contaminants. Again,
investigators will want to consult the BTAG for
assistance in selecting appropriate test organisms.
• When testing water, select a test organism that can
tolerate the water's condition. For example, some
organisms, such as the waterflea Ceriodaphnia, are
extremely sensitive to water hardness. Also, certain
inland waters have high enough salinity to make the
use of freshwater test organisms inadvisable.
March 1994 • Vol. 2, No. 1
ECO Update
-------
To extend the limited guidelines offered above, several
EPA scientists were interviewed to learn how they design
toxicity assessments. Each scientist offered a somewhat
different outline and set of priorities. These differences reflect
differences in site characteristics and geography, such as
degree of urbanization and amount of rainfall. In spite of the
great variety in hazardous waste sites, three general designs
for toxicity assessments emerged from these interviews. The
following section presents the designs. Where a design refers
to specific toxicity tests, these are described in the companion
Bulletin, "Catalogue of Standard Toxicity Tests for Ecological
Risk Assessment" (ECO Update Vol. 2, No. 2).
Design 1
Design 1 is based on the premise that designing a
meaningful toxicity assessment requires considering a
contaminant's mode of action, the level of site contamination,
the sensitivity of the test type (acute or chronic), and the
sensitivity of the test organism. In particular, sites are first
reviewed to determine whether contaminants are expected to
cause effects detectable by toxicity tests that meet the stated
DQOs for that site. The investigator then matches
contaminant levels with test type. For example, in a heavily
industrialized site with high levels of contaminants, many of
the samples in a chronic test may give highly positive results.
Consequently, the study would provide little information
about differences in toxicity at various locations on the site.
At such a site acute tests might distinguish the varying toxicity
of different locations, thereby identifying the more highly
impacted areas and helping to establish priorities for remedial
decisions.
Similar concerns are addressed when choosing test
organisms: too sensitive a test organism can give negative
results that are misleading. Also, Design 1 uses screening
tests sparingly because these tests may suffer from over-
interpretation of results.
Design 2
Design 2 focuses on planning toxicity assessments for sites
where chemical data do not clearly indicate whether the site
contaminants represent an ecological problem requiring
further action. The premise is that sites with intermediate
levels of contamination will most likely require toxicity
testing. Where little contamination has occurred, the chemical
data will generally indicate that the site is not an ecological
problem and requires no further ecological risk assessment. A
heavily contaminated site will almost certainly require further
action.
With this design, the investigator decides on ecological
components and endpoints as early as possible, making use of
all the information available about the site and its contaminant
history. The ecological components and endpoints then serve
to guide choices of both toxicity tests and field studies.
If the chemical data indicate the need for toxicity tests, as
reflected by the literature, the investigator next decides which
media to test. Contamination may have migrated from the
initial contaminated medium into other site-associated media.
Toxicity test selection occurs next, with the choice between
acute and chronic tests depending on the objectives. In the
long run, gauging toxicity based on the sub-lethal effects
measured by chronic tests proves more protective. In addition,
chronic tests are considered more appropriate if the organism
spends most of its time on-site. Conversely, acute tests better
mimic conditions for organisms that spend a limited amount of
time on-site, such as migratory animals or those with a large
home range. Acute tests also prove useful when the
investigator has concerns about significant differences in
toxicity at different locations on the site. If initial tests show
no toxic "hot spots," then further testing may not be necessary.
Design 2 puts toxicity testing in perspective by viewing the
ecological risk assessment of a Superfund site as a triad
consisting of chemical testing, toxicity testing, and field study:
• Chemical testing indicates the presence of
contamination.
• Toxicity tests then explore whether biological effects
are possible.
• Field studies investigate whether actual harm
occurred at the site.
At a highly contaminated site, each leg of the triad will
most likely give evidence of impact. At many sites, however,
results are not clear-cut. For example, toxicity results might
not correlate well with chemical data. Alternatively, field
studies might not demonstrate adverse ecological effects.
Such sites require careful professional judgement to make a
decision regarding ecological effects and the need for
remediation.
Design 3
Design 3 begins with a site reconnaissance visit, followed
by a "desktop assessment" to determine whether a site requires
an ecological risk assessment. The desktop assessment
considers the site's background from scoping and also its
contaminants, their environmental concentrations, their
physical and chemical properties, and the nature of the
surrounding area. A site located in an urban industrial area,
for example, may not require an ecological risk assessment:
the site itself may have no ecological components of concern,
and the contaminants may not have a means of migrating to
areas having potential ecological components. On the other
hand, the existence of a conduit—a stream, a drainage ditch,
ground-water gradient, or land grade—that could carry
contaminants from the site to surface water, wetlands, or
terrestrial habitats would indicate the need for an ecological
risk assessment. In this design, toxicity studies are viewed as
useful tools and an effective use of Superfund resources at
sites requiring ecological risk assessments.
Selection of sampling locations is one of the first tasks
undertaken when designing a toxicity study using this design.
Especially at large sites, the investigator avoids random
sampling, which would generate an unmanageably large
number of samples to analyze. Instead, site terrain and
March 1994 • Vol. 2, No. 1
10
ECO Update
-------
contaminant history are carefully studied in order to place
sampling points in areas that will extend the knowledge of the
site. As a general rule, at a site with contaminated surface
water, the sampling plan should specify sufficient samples to
characterize fully the potential ecological effects.
When testing samples of surface water, the investigator
selects an initial battery of screening tests from among the
standardized acute tests used in the NPDES permitting
program. These initial tests usually include organisms at three
trophic levels (e.g., an alga, an invertebrate, and a fish). If
screening level tests show that surface water is toxic, further
tests include surface water dilutions and additional test species
to characterize the site's toxicity more fully.
This design favors performing initial soil or sediment
evaluations at the screening level using either bulk samples,
laboratory-prepared elutriates, or pore water, the water
located between particles and obtained by either centrifugation
of filtration. Like surface water samples, elutriates and pore
water samples are analyzed using screening level NPDES
aquatic tests. If these tests give positive results, the
investigator reviews the chemical data and contaminant
history for the site before making the serious commitment of
resources that testing of bulk soil or sediment entails.
Conclusion
The differences in these designs largely reflect the fact that
an ecological risk assessment of a Superfund site needs to
address the site's characteristics and contaminant history. As
scientists gain more experience in conducting toxicity
assessments, the several designs existing today may evolve
toward a common blueprint that will readily accommodate site
differences. In addition to reflecting site differences, the
designs also reflect strategic differences in the deployment of
resources, time, and personnel. Despite these differences,
however, the designs have the same end: to assess the toxicity
of contaminated media from Superfund sites.
To learn about other designs for toxicity assessments,
investigators may wish to consult the scientists in the EPA
Environmental Services Division (BSD) in their Region. In
making decisions about toxicity testing at a specific site, an
investigator should consult with the Regional BTAG. The
BTAG will be able to tell the investigator whether the Region
has a standard design for toxicity assessments. If the Region
does not, the BTAG can advise the investigator whether one of
the above designs, one offered by the BSD, or a modification
of any of these will further the ecological risk assessment of a
particular site. Alternatively, the BTAG may suggest another
design that is better suited to a particular site.
References
American Society for Testing and Materials (ASTM). 1992a.
Annual Book of ASTM Standards: Water and
Environmental Technology, Vol. 11.04. American
Society for Testing and Materials, Philadelphia, PA.
American Society for Testing and Materials (ASTM). 1992b.
Standard Guide for Conducting Sediment Toxicity Tests
with Freshwater Invertebrates. American Society for
Testing and Materials, Philadelphia, PA.
American Society for Testing and Materials (ASTM). 1992c.
Standard Guide for Conducting 10-day Static Sediment
Toxicity Tests with Marine and Estuarine Amphipods.
American Society for Testing and Materials, Philadelphia,
PA.
Baker, J.P. 1989. "Assessment Strategies and Approaches" in
Warren-Hicks, W., B.R. Parkhurst, and S.S. Baker Jr.,
eds. Ecological Assessment of Hazardous Waste Sites: A
Field and Laboratory Reference. EPA/600/3-89/013.
Environmental Research Laboratory, Office of Research
and Development, U.S. Environmental Protection
Agency, Corvallis, OR.
Bitton, G., B.J. Dutka, and C.W. Hendricks. 1989. "Microbial
Toxicity Tests" in Warren-Hicks, W., B.R. Parkhurst, and
S.S. Baker Jr., eds. Ecological Assessment of Hazardous
Waste Sites: A Field and Laboratory Reference.
EPA/600/3-89/013. Environmental Research Laboratory,
Office of Research and Development, U.S. Environmental
Protection Agency, Corvallis, OR.
Greene, J.C., C.L. Bartels, W.J. Warren-Hicks, B.R.
Parkhurst, G.L. Linder, S.A. Peterson, and W.E. Miller.
1989. Protocols for Short-term Toxicity Screening of
Hazardous Waste Sites. EPA/600/3-88/029.
Environmental Research Laboratory, Office of Research
and Development, U.S. Environmental Protection
Agency, Corvallis, OR.
Linder, G. etal. 1992. Evaluation of Terrestrial Indicators for
Use in Ecological Assessments at Hazardous Waste Sites.
EPA/600/R-92/183. Environmental Research Laboratory,
Office of Research and Development, U.S. Environmental
Protection Agency, Corvallis, OR.
U.S. Army Corps of Engineers. 1993. Evaluation of Dredged
Material Proposed for Discharge in Inland and Near
Coastal Waters—Testing Manual (Draft). Office of
Water, Washington, DC.
U.S. Environmental Protection Agency. 1992. Framework for
Ecological Risk Assessment. EPA/630/R-92/001. Risk
Assessment Forum, Washington, DC.
Weber, C.I. 1991. Methods for Measuring the Acute Toxicity
of Effluents and Receiving Waters to Freshwater and
Marine Organisms. 4th edition. EPA/600/4-90/027.
Environmental Monitoring Systems Laboratory, Office of
Research and Development, U.S. Environmental
Protection Agency, Cincinnati, OH.
Weber, C.L, W.I. Horning, D.J. Claim, T.W. Neiheisel, P.A.
Lewis, E.L. Robinson, J. Menkedick, and F. Kessler.
1988. Short-term Methods for Estimating the Chronic
Toxicity of Effluents and Receiving Waters to Marine and
Estuarine Organisms. EPA/600/4-87/028. Environmental
Monitoring Systems Laboratory, Office of Research and
March 1994 • Vol. 2, No. 1
11
ECO Update
-------
Development, U.S. Environmental Protection Agency,
Cincinnati, OH.
Weber, C.I., W.H. Peltier, T.J. Norberg-King, W.B. Horning
II, F. Kessler, and J. Menkedick. 1989. Short-term
Methods for Estimating the Chronic Toxicity of Effluents
and Receiving Waters to Freshwater Organisms. 2nd
edition. EPA/600/4-89/001. Environmental Monitoring
Systems Laboratory, Office of Research and
Development, U.S. Environmental Protection Agency,
Cincinnati, OH.
March 1994 • Vol. 2, No. 1 12 ECO Update
------- |