LITERATURE
REVIEW REPORT: POSSIBLE MEANS OF EVALUATING THE BIOLOGICAL EFFECTS
OF
SUB-AQUEOUS DISPOSAL OF MINE TAILINGS
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Project 2.11.2a
March 1993
EXECUTIVE
SUMMARY
This review
identifies promising geochemical and ecotoxicological approaches
that might be used to monitor the biological effects of the sub-aqueous
disposal of reactive mine tailings.
Submerged
mine tailings, and their constituent metals, may affect aquatic
life in two ways: indirectly (i.e., by leaching of the metals
into the ambient water, followed by their assimilation from the
aqueous phase), and directly (e.g., in macrofauna, by ingestion
of the tailings and assimilation of the metals from the gut). Both
routes of metal exposure are considered. The metals that have been
considered are those that are commonly present in reactive mine
tailings, that are recognized as potentially toxic at low concentrations
to aquatic biota, and that exist in natural waters as dissolved
cations (e.g., Cd, Cu, Ni, Pb, Zn).
Geochemical
considerations
- To evaluate
indirect exposure, one needs to estimate metal concentrations
in sediment pore waters (i.e., [M]i). Such concentrations are
thought to reflect the metal's chemical potential at the sediment-water
interface. Changes in this chemical potential wig affect the metal's
bioavailability.
- Two approaches
can be used to estimate [M]i : one applies to oxic conditions
and assigns control of [M]i to sorption reactions on such sorbants
as Fe-, Mn - oxyhydroxides or sediment organic matter; the second
applies to anoxic conditions and assumes that [M]i is controlled
by precipitation-dissolution reactions with reactive amorphous
sulfides (Acid-Volatile-Sulfides, AVS).
- The two
approaches differ in their choice of reactions controlling metal
solubility in sediment pore waters. This divergence stems from
different concepts of what constitutes biologically important
sediments - fully oxidized, surficial sediments (where amorphous
sulphide levels should be vanishingly low and [M]i should be controlled
by sorption reactions) vs partially oxidized, sub-oxic sediments
(where significant AVS levels would be expected to persist, and
exchange reactions with amorphous sulphides would control metal
partitioning between dissolved and solid phases).
- Choosing
between the two approaches is not straightforward, even for natural
sediments. In the real world, due to small scale spatial heterogeneity,
distinction between oxic and anoxic sediments is often blurred.
Most aerobic benthic organisms survive in sediments that are underlain
or even surrounded by anaerobic material, which constitute a potential
source of AVS.
- Depending
on which geochemical approach proves more appropriate for deposited
mine tailings (i.e., fully or partially oxidized conditions at
the tailings-water interface), it should be possible to predict
[M]i based on the geochemistry of the tailings after disgenesis
and admixture of natural particulate material.
Interactions
between dissolved trace metals and aquatic organisms
- Qualitative
evidence suggests that the total aqueous concentration of a metal
is not a good predictor of its bioavailability. The metal's speciation
greatly affects its availability to aquatic organisms.
- A convincing
body of evidence supports the tenet that the biological response
elicited by a dissolved metal is usually a function of the free
metal ion concentration, Mz+.
- This Free-Ion
Activity Model (FIAM) should apply to aquatic organisms that do
not assimilate particulate material (e.g., rooted aquatic plants),
and to organisms that do assimilate particulate material but for
which the dissolved phase remains the primary vector for metal
uptake.
- However,
most experiments designed to test the FIAM have been performed
in the laboratory at fixed pK with divalent metals (Cu, Cd, Ni,
Pb, Zn), in artificial (inorganic) media or in filtered sea water,
and in the presence of known quantities of synthetic ligands.
The applicability of the FIAM in natural waters, in the
presence of natural dissolved organic matter (DOM ), is poorly
documented.
- Two types
of study have been carried out to test the applicability of the
FIAM in the presence of sediments: (1) laboratory bioassays on
spiked sediments or on natural sediments collected from known
contaminated sites; and (2) field surveys of indigenous benthic
organisms. Both bind to the free-metal ion concentration at
the sediment-water interface.
Interactions
between particulate trace metals and aquatic organisms
- Benthic
organisms that ingest particles tend to select the smaller and
fighter particles in their environment. This nutritional strategy
results in the ingestion of particles that tend to be enriched
in metals.
- Assimilation
of particle-bound metals will normally involve their conversion
from particulate to dissolved form in the gut, followed by their
facilitated diffusion across the intestinal membrane. Digestive
processes and chemical conditions prevailing within the intestinal
tract thus assume considerable importance (e.g., pK digestion
times, redox status).
- Based on
feeding experiments where metal uptake from different sediment
phases was monitored, the efficiency of uptake of a given metal
varied greatly among different model sediments, and relative
availability from a given sink varied from metal to metal. Thus,
one cannot generalize that metals are more available from phase
A (e.g., organic detritus) than from phase Z (e.g., Mn(IV) oxyhydroxide)
- while the sequence A > Z may be true for one metal, it will
not necessarily hold for the next.
- Differences
in metal availability tend to be inversely related to the strength
of metal binding to particulates. Sediments that exhibited the
highest affinity for a metal (i.e., that released the least
amount of metal back into solution) were also the substrates from
which metal bioavailability was the least.
- From these
studies it is clear that the physicochemical form of a sediment-bound
metal affects its availability in the digestive tract of the marine
deposit feeder Macoma balthica. However, it is premature
to attempt to generate these results to all deposit-feeders.
- The results
of the feeding experiments with M balthica are admittedly
qualitative. Nevertheless, there is an interesting parallel between
the original "affinity" concept (according to which
the availability of a particle-bound metal is inversely related
to the strength of the metal-particle association) and more recent
suggestions that the concentration of a metal in the interstitial
water can be taken as a measure of its chemical potential in surficial
sediments and thus its availability. This inference seems intuitively
reasonable, provided that the chemical environment within the
animal's digestive tract is similar to that in its immediate environment
( i.e., provided the chemical potential of a metal doesn't change
drastically on passing from outside environment into the digestive
system). In this context, better knowledge of the chemistry of
invertebrate digestion will be very useful.
Biochemical
indicators of metal-induced stress
- Traditionally,
attempts to define the impacts of contaminants on aquatic ecosystems
have involved laboratory experiments under defined conditions
(toxicity tests) and, to a lesser extent, as field observations
on impacted indigenous populations. An alternative and complementary
approach involves the use of biochemical indicators to monitor
the response of individual organisms to toxic chemicals and to
provide a measure of ecosystem health.
- For metals,
much of the attention in the area of biochemical indicators has
focused on metal-binding proteins, in particular on metallothionein
(MT) and metallothionein-like compounds.
- Given its
molecular properties, and present knowledge of its role in metal
uptake, transport, storage and excretion, metallothionein offers
considerable potential as a contaminant-specific biochemical indicator
of metal exposure and/or stress.
- Possible
approaches include: (i) measurement of metallothionein concentrations
as an indicator of prior exposure to toxic metals, and (ii) examination
of the relative distributions of toxic metals in cytosolic ligand
pools to evaluate metal stress at the biochemical level.
- Field evidence
in support of the first approach is slowly accumulating in the
eco-toxicological literature. In studies involving the sampling
of indigenous populations of aquatic animals from sites chosen
to represent a spatial (metal) contamination gradient, the concentrations
of metallothionein-like proteins were consistently higher at the
more contaminated sites.
- Regarding
the second possible use of MT, i.e. to evaluate metal stress at
the biochemical level, responses of aquatic organisms to excess
metal have proven more diverse than originally postulated. Thus,
toxic metal distributions in the cytosolic fraction cannot yet
be used to evaluate metal stress at the biochemical level.
Effects
of metals at the population and community levels
- Structural
components of ecosystems are often considered the most sensitive
indicator of disturbance. These components include communities
(the biotic components of an ecosystem) and populations (the members
of a species that occupy the same habitat and can possibly interbreed).
- Recent studies
also suggest that chronic exposure to low levels of contaminants
often results in severe effects on populations and communities
in aquatic environments. Thus, community and population data are
essential for the prediction and monitoring of these effects.
- However,
the effects of natural population cycles, climate and other environmental
factors not related to contamination must be accounted for if
the impacts of contaminants on populations and communities are
to be properly assessed. This can be accomplished by the use of
representative reference sites and collection of sufficient background
chemical and biological data.
- Certain
community indices are more useful than others in detecting
the impacts of contaminants such as metals or acids.
(i) Changes
in the algal community (number of species, diversity, species composition,
dominant species) are often observed. However, changes in dominance
may also be due to other factors, such as the availability of nutrients,
reduced grazing pressure, or the development metal tolerance by
normally sensitive taxa. The most useful measures appear to be species
composition and richness.
(ii) Few studies
have examined impacts of metals on lake communities of crustacean
zooplankton, despite their importance as a prey item. Decreases
in diversity and in total biomass, as well as changes in dominance
among crustacean zooplankton communities have been observed.
(iii) The
benthic community is in intimate contact with both water and sediment
phases and has been extensively used as an indicator of metal
pollution in streams. In some studies of heavily or impacted
lakes, the density of benthic organisms proved to be a useful indicator,
though this may not be the case in less severely contaminated habitats.
Measurements of diversity have proven of limited usefulness for
the detection of community changes. Proportions of tolerant and
sensitive taxa have also been used as gradational indices of metal
or stress. However, the classification of entire taxa as tolerant
or sensitive may ignore important differences which exist
within them. Improved methods for the taxonomic differentiation
of benthic animals are needed both to improve the relationship of
community composition with the degree of metal stress and to place
these effects in an ecological context.
(iv) Most
studies involving fish communities have involved listing the species
present (community composition), the species richness (the number
of species) or using associations of species, which are considered
to prefer certain habitats, as indicative of water quality.
- Fish populations
are not necessarily the most sensitive predictors of the future
state of a system. However, there are certain advantages to their
use, notably the relative facility of collecting the data necessary
for such studies (age of individuals, reproductive status, growth,
condition) and the existence of historical records for comparison.
Furthermore, population data are essential to relate chronic toxicity,
often observed in individuals in the field, to effects at the
level of the population. Different strategies (r vs.
k) may be observed in populations adapting to stress.
- Many natural
fish populations appear to be tolerant of waterborne metal concentrations
that are acutely toxic in laboratory studies. This apparent discrepancy
may be related to differences in metal speciation. Such explanations
remain speculative, however, since in most field studies the chemistry
of the exposure medium (sediments, water column) was not sufficiently
defined.
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