Predicting Ecological Toxicity: Background
(Note: the figures are not available)
UNFORESEEN EFFECTS OF PESTICIDES
Persistence and Bioaccumulation: DDD
Clear Lake is a popular tourist area as well as a major breeding ground for the
Western Grebe [ Aechmophorus occidentalis], a particularly graceful, large water bird. It
is also the home of the Clear Lake gnat, an insect that is quite harmless, but so abundant
that it is a major nuisance. Clear Lake was treated to control the gnats in 1949, 1954,
and in 1957 at progressively higher levels of DDD, an insecticidal analog of DDT. The
results provide an excellent example of the interactions among species in an ecosystem.
After the 1949 spraying, water levels of DDD reached 14.2 ppb. No adverse
effects were noted. DDD is a persistent insecticide, and gnat levels were suppressed for
some years, but gradually increased again at Clear Lake. In 1954, DDD was again
sprayed. Water levels of DDD reached 20 ppb, which, although higher than after the first
spraying, are still trivial in terms of animal toxicity or even of insecticidal activity. There
were grebe deaths noted after the 1954 spraying, but no illness was found to account for
the deaths, and no connection to the DDD spraying was made. In 1957 gnat levels had
again increased to the point that spraying was considered advisable (note the decreasing
intervals between sprayings, which suggest that the gnat population was developing
resistance to DDD). This third spraying led to massive grebe deaths. Before 1949, the
Clear Lake grebe population had consisted of approximately 1000 pairs of breeding birds;
by 1954 it had decreased to 30 pairs, and even these produced no young. Again, no
illnesses were found. When assays for DDD were carried out, water levels remained in
the range of parts per billion. Levels in organisms were startlingly higher. DDD levels
were 5 ppm in plankton, 40-300 ppm in plant-eating fish, and as high as 2500 ppm in the
fat of one carnivorous fish. Grebes had up to 1600 ppm DDD in their fat. These data
illustrate the concept of bioaccumulation: fat-soluble compounds (which are relatively
water-insoluble compounds) that are not readily degraded will accumulate in fat and are
found at increasing levels in organisms as one progresses up a food chain.
The saga of the Clear Lake grebes was repeated many times in the 1950s and
early 1960s. The insecticide was not always DDD or even its close analog DDT. Several
other organochlorine insecticides had been developed to compete with DDT, and each
was lauded for its long persistence in the field, which meant that farmers had to treat less
often, saving time and money. Aldrin, dieldrin, chlordane, and heptachlor were the most
heavily used of these compounds. In time, each was shown to bioaccumulate and,
eventually, each was banned, as described in the case history on the cyclodiene
insecticides [below]. Unfortunately, the lesson that was learned from the experience with
DDD was not that either undue persistence or bioaccumulation was hazardous, but only
that bioaccumulation was. Not until several organochlorine insecticides had been banned
were factors other than bioaccumulation recognized as undesirable. The following cases
illustrate several patterns of toxic effects observed as a result of the indiscriminate use of
persistent pesticides that marked the post-World War II decades.
Endrin is an organochlorine insecticide, belonging to the family of insecticides
designated the cyclodienes because they are generated by the Diels-Alder condensation
reaction. Endrin is very closely related to aldrin and dieldrin, and somewhat less closely
to chlordane and heptachlor. Endrin is both persistent and bioaccumulative; but - unlike
DDD - it is also very acutely toxic to vertebrates. It is so toxic to mice that it has been
used as a rodenticide. Its toxicity to fish is so great that numerous fish kills occurred
whenever endrin was used or produced near bodies of water. Endrin use east of the Mississippi was banned because it was considered impossible to keep its levels in waterways
low enough to prevent fish kills. To get that ban, however, EPA had to compromise:
endrin use west of the Mississippi was allowed, since it could be argued that there were
fewer waterways in the high plains and fishing was less important. In 1981, endrin was
used, west of the Mississippi, against a severe infestation of grasshoppers, which
threatened the wheat harvest of the Great Plains. After widespread spraying, endrin
bioaccumulated in waterfowl, leading to a ban on hunting of ducks or geese in Dakota,
as well as causing numerous bird deaths. Public health authorities calculated that a
person could be poisoned by eating normal portions of duck or goose meat from birds
that had eaten contaminated insects, even though the birds had not died.
Persistence without bioaccumulation : aldicarb
Aldicarb is a carbamate pesticide produced under the brand name Temik. It was
banned in Wisconsin potato fields after contaminating ground water.1 Compared to the
organochlorine insecticide DDD, aldicarb is quite water-soluble [water solubility >
1ppm]. It shows little bioaccumulation in laboratory studies, but it is highly toxic to
mammals as well as to insects. After it appeared in the water in Wisconsin, it was also
found in the groundwater of Long Island, where it was also used on potato fields. When
wells on Long Island were shown to contain aldicarb at levels as high as 0.5 ppm [which
is the LD50 to fish], aldicarb use on Long Island was banned. In Florida, aldicarb was
used in orange groves as a soil treatment against nematodes. Massive groundwater
contamination occurred after heavy use in 1983-1984. In all three states, aldicarb had
been used according to the label2 but had been applied to soils that were sandy and
1
McWilliams, L. 1984. Environment 26(4):25. Groundwater pollution in Wisconsin: a bumper
crop yields growing problems.
2
A pesticide label is a legal document that identifies permissible uses of a product. Using a
pesticide on other crops, or for other uses, than permitted on the label is illegal. Unfortunately,
even deliberate violation of the law is only a misdemeanor, which has led to flagrant abuses of
federal and state pesticide laws. In 1992, over 80 restaurants in Chicago and its suburbs were
sprayed with parathion-methyl by an unlicensed applicator. Parathion-methyl has never been
labelled for indoor use; the applicator had been warned at least once, and had then disappeared,
together with his 55 gallon drum of parathion-methyl. There was evidence that he had crossed
into Indiana and was spraying restaurants there. Lawyers for the City of Chicago thought that the
egregiousness of the violation might actually lead to a jail term. But the maximum term in jail for
leached readily. In 1985, aldicarb contaminated watermelons in California, leading to
illness for over 1000 people and to the destruction of a sizeable fraction of the state's
watermelon crop. This last incident was probably due to illegal use of aldicarb on watermelon fields, rather than to unintentional environmental contamination. It must be
stressed, however, that the contamination was not direct -- aldicarb was not sprayed on
the watermelons. Only the soil was treated, since aldicarb is a systemic insecticide,
which is picked up by plants and carried into many parts of the plant, including the fruit.
Aldicarb did not bioaccumulate in the plants, either: it was present in plant water at levels
no higher than its concentration in soil water. Nonetheless, 1,000 people became ill: - a
rather dramatic illustration of aldicarb's toxicity.
Aldicarb pollution results primarily from its persistence. Even though its
relatively high water solubility prevents bioaccumulation, its long persistence
underground provides ample opportunity to reach, and contaminate, ground water.
Because of its toxicity to vertebrates, only low levels of contamination suffice to cause
health effects.
Acute toxicity without undue persistence: fenthion
Fenthion is an organophosphate insecticide that is neither terribly persistent like
endrin nor as toxic to mammals as aldicarb. It does not bioaccumulate like mirex or
DDD, and is favored because it is not extremely toxic to mammals. It is, however, very
toxic to birds, and is licensed as an avicide. When fenthion was used to poison starlings
in central Illinois in 1986, owls, falcons, and possibly some predatory mammals died
from eating the carcasses of fenthion-poisoned birds. The problem with fenthion is that it
is extremely toxic both to the seed-eating pest species [starlings, blackbirds] at which it
was directed, and to the highly desirable predatory species [owls, hawks] that prey on the
pest species. Moreover, in avicidal applications fenthion is applied to roosting surfaces,
and absorbed through the skin of the feet. Thus it is possible for predators to be exposed
to the unabsorbed toxicant on the bird's feet, as well as to the absorbed toxicant. It is not
known whether unabsorbed material played a role in this episode of secondary poisoning.
Pesticides as hazardous wastes
Even when pesticides have supposedly been disposed of, they can still cause
problems. OF the many hazardous waste sites that have attracted media attention in the
past decade, most have involved waste from pesticide production. Like many of the
problems our society is now grappling with, this was also foreshadowed within a decade
of the first use of DDT. The Rocky Mountain Arsenal near Denver, Colorado, produced
chemical warfare agents till 1949; it was then sold to a chemical company, which
continued to manufacture toxicants on the site. In 1951 it became apparent that groundwater was contaminated for a radius of several miles. In 1959, tests showed arsenic,
phosphonates (nerve gas and/or pesticide derivatives), chlorides and chlorates in the
groundwater. The herbicide 2,4-D was supposedly not manufactured at this Arsenal, but
a misdemeanor is 364 days, even under federal law. Neither lawyers nor city health officials
thought the penalty would deter the applicator.
nevertheless appeared in the groundwater, apparently generated in situ. Contamination of
groundwater led to contamination of shallow wells used for irrigation, to illness in livestock, and to reductions in crop yields. The U.S. government filed suit against the
chemical company, claiming much of the pollution is due to pesticide production rather
than to chemical warfare agents. The company contested this, and a settlement was
eventually reached. Cleanup may have begun.
Summary
Since the Clear Lake study of 1960 the examples of bioaccumulation and of
persistence of chemicals have multiplied: DDD, PCBs, aldrin/dieldrin,
chlordane/heptachlor, dioxins, Kepone. Contamination by persistent chemicals must be
suspected not only in areas of use, but also where the chemicals were manufactured and
where the byproducts of their manufacture are dumped. Thus, the concern about mirex
contamination in Mississippi, where it was most heavily used as an insecticide, is
mirrored by the concern of Canadian health authorities, who still find measurable
residues of mirex in fish near the plants where mirex was synthesized as a flame
retardant. PCBs contaminate numerous waterways (see PCB case history) and seriously
damaged reproduction of mink on ranches along the Great Lakes. The closely related
PBBs (see PBB case history) were dispersed throughout Michigan via an accident that
was followed by the determined efforts of the responsible organizations to minimize their
liability.
The frequency and the irreversibility of such incidents of environmental pollution
make it clear that we cannot depend on post-marketing experience to discover the
environmental hazards associated with chemicals produced by the kiloton. A "quick test"
or short-term assay is as badly needed for environmental pollution as for carcinogenesis,
and for the same reason: the latency period between release of the chemical and
identification of the hazard is so long, and the cost of repairing the harm so high (if it
prove possible at all) that prevention is the only possible solution. Such a short-term
assay can be found in the microcosm.
PREDICTING ENVIRONMENTAL BEHAVIOR : THE METCALF MODEL
ECOSYSTEM
Microcosms, also called model ecosystems, can be designed to evaluate the
toxicity, transport, or persistence of chemicals within a simplified ecosystem. The
system can be terrestrial and/or aquatic; it may or may not measure volatility; and it may
be either static or dynamic. Its cost, obviously, varies with its complexity. The degree of
complexity, in turn, varies with the purpose of the microcosm. Model ecosystems in use
today sometimes consist of " mesocosms", which are often large outdoor pools or even
ponds, which are used to investigate the dynamics of ecological systems, and are
correspondingly complex. The first microcosms, in contrast, were designed to evaluate
the fate and transport of toxicants, especially of pesticides, in soil and water, and to do so
quickly, cheaply, and repeatably. This first microcosm was the Metcalf model
ecosystem, called the terrestrial aquatic laboratory model ecosystem by its originator,
R.L. Metcalf.
The Metcalf model ecosystem simulates a farm pond surrounded by fields under cultivation. It consists of a 10 gallon aquarium containing a sloping shelf of washed quartz
sand as its "field", and a "lake" of 7 liters of standard reference water. The water is
aerated, and provides mineral nutrition for plankton, algae, snails, mosquito larvae and
fish in the aquatic phase and for sorghum plants in the terrestrial (sand) phase. The entire
system is kept in an environmental growth chamber at 26.5oC with a 12-hour diurnal
cycle of 50000 foot candles of fluorescent light.
The chemical to be tested is preferably, but not necessarily, radiolabeled3 to simplify
monitoring. The chemical is applied to the sand or to the sorghum plants, using 1-5 mg
of test chemical per experiment. This is equivalent to a realistic level of 0.2-2.0 lb/acre
for pesticides, and can be altered when higher or lower levels of a compound are known
to be applied. Ten last instar salt marsh caterpillars [ Estigmene acrea] are introduced to
eat the treated sorghum. The caterpillars and their excretory products contaminate the
lake portion of the ecosystem. The radiolabeled products enter the several aquatic food
chains:
plankton ---> daphnia ---> fish
algae ---> snail
The movement of radiolabeled compounds from plants to water is measured by
counting the radioactivity of duplicate water samples at specified intervals. Twenty-six
days after treating the ecosystem, 300 mosquito larvae are added. Three days later, 50
larvae and 50 Daphnia [also called "water fleas"] are removed for analysis, and the food
chains are completed by adding 3 mosquito fish ( Gambusia affinis). Three days after the
addition of fish the system is terminated (day 32). All organisms are weighed,
homogenized, and extracted with an organic solvent to obtain the lipid-soluble
radiolabeled products. The water from the system is also extracted and analyzed.
Aliquots of all samples are counted for total radioactivity by scintillation. Then the
extracts are concentrated to a few ml and applied to thin layer chromatography (TLC)
plates. TLC is carried out with solvents appropriate for the chemical being tested, and
the chromatograms are placed against X-ray film and exposed for 1-2 months to
determine the regions containing the radiolabeled products. These areas of the TLC
plates are then scraped into scintillation vials and the amounts of the individual
degradation products determined by scintillation counting. It is possible to extend the
analysis to the identity of key degradation products if the work is warranted by their importance.
3
The radiolabel consists of incorporation of a radioactive isotope, either C14 or H3, into
the structure of the chemical being evaluated. The isotope emits particles that can be
counted in a scintillation counter in order to quantify the amount of the chemical present
in each component of the system. In the absence of radiolabeled compounds, gas
chromatography/mass spectrometry can be used to identify the chemical, but this is far
more tedious. If 14C-labeled compounds are used, at least 107 cpm should be present in
the system.
The ecological magnification (EM) and biodegradation index (BI) of the parent
compound (and of degradation products) can be calculated as:
concentration of parent compound in the system
---------------------------------------------------------concentration of parent compound in water
and the degradative index is:
concentration of polar metabolites
BI =
-----------------------------------------------concentration of non-polar metabolites
EM =
The Metcalf Model Ecosystem was not designed to identify the toxicity of
chemicals to particular species or particular food webs. Its purpose is to identify the
environmental fate, transport and persistence of environmental pollutants, particularly of
pesticides. The major weakness of the Metcalf model ecosystem is that it is static, and
so does not adequately model stream and other flow-through systems. For example, a fish
living in a DDT-treated stream is exposed not only to a fixed amount of DDT from spray
"fallout", but also to increments due to freshly contaminated water that flows
downstream. The total DDT uptake may be much higher than had a pond, with a fixed
amount of water and a fixed amount of DDT, been involved. The Metcalf Model
Ecosystem models the pond. Once the ecological characteristics of a chemical are
known, however, it is relatively easy to translate "pond" behavior into "stream" behavior.
Against this minor weakness are set the advantages of the Metcalf ecosystem: it is
quick, relatively cheap, and reproducible. Since its design in 1965, over 150 chemicals
have been assessed in this system. For many of these chemicals, real-world data on
persistence and biodegradation are available, often under both static and flow-through
conditions. Therefore, remarkably accurate
TABLE: Ecological magnification of selected organic chemicals in two organisms.
Chemical
carbaryl
mirex
kepone
DDT
aldicarb
PCBs:
trichloro (2,5,2')
tetrachloro (2,5,2',5')
pentachloro (2,5,2',4',5')
methidathion (insecticide)
EM (fish)
EM (snails)
0.32
219
118
13,550
42
83.40
3.5
1,597
637
5,725
-
753
12,152
51.30
1.94
[from: R.L. Metcalf and J.R. Sanborn, Ill. Nat. His. Survey Bull. 31:379436, 1975; R.L. Metcalf et al., Arch. Environ. Contam. Toxicol. 3:151165, 1975; B.M. Francis and R.L. Metcalf, IES Research Report #9, 1981]
predictions can be made with respect to the fate of new compounds by comparing data to
ecosystem and to field data for the other 150+ chemicals that have been evaluated.
Moreover, the system is reliable even when metabolites of parent compounds are evaluated. For example, desbromoleptophos, which is a photodegradation product of the
organophosphorus ester insecticide leptophos, had EM = 3,888 in fish when calculated in
the leptophos ecosystem, and EM = 3,477 when evaluated in its own system. Examples
of EM values for several chemicals are shown in Table 5.1.