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 will 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 diagenesis 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 pH, 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 approaches support the general idea that benthic organisms respond 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 lighter 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., pH, 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 generalize 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, 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 metallo¬thionein-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 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 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), measuring 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.
