The ecological effects of mine effluents on the South Esk River (North East Tasmania)

The ecological effects of mine effluents on the South Esk River have been investigated over two years on a 130 km section of river below the pollution source. The data was obtained from bimonthly samples at eight sites (three above the pollution inflow and five below it) and three supplementary sites.Basic water characteristics, and trace metal concentrations in the sediments, non-filterable residue (N.F.R ) and solution as well as quantitative sampling of the benthic macroinvertebrate fauna have been measured. Highest rainfall is normally in the winter with high surface run-off during this period and high retention in the ground during the summer. Large differences in rainfall and subsequent discharge occurred between the successive years of the study. The concentrations of total dissolved solids of the South Esk were about 100 mg/l and the water was soft. The pH was slightly acid and stable indicating a well buffered natural water. The major cations generally increased in concentration downriver; it is considered that surface run-off followed by -evaporation and concentration were most likely to control cation concentrations and exert overall control of the water chemistry. Oxygen levels were near saturation and chloride, nitrate and phosphate.concentrations were all low. Mn, Fe, Cd, Zn, Cu and Pb concentrations were measured and the first two of these have been shown to be independent of the mine effluents. The natural background levels of Cd, Zn, Cu and Pb have been found to be very low or below detection limits. However, in the contaminated stretch of river the concentrations of these four metals in the sediments, N.F.R. and solution were all well above the natural background levels up to 130 km from the pollution inflow. The spatial and temporal variations in the concentrations of each trace metal in the three fractions have been examined. The concentrations of Cd, Zn, Cu and Pb in the sediments and Cd and Zn in solution show an inverse relationship with the distance from the source of contamination, whereas the concentrations of Cu and Pb in solution and all four metals associated with the N.F.R. show no significant spatial differences. Possible physical and chemical mechanisms controlling spatial and temporal variations, and the partitioning between the three different fractions are discussed. The effects of the influx of trace metals are apparent from examination of the absolute numbers of animals and the numbers of species. This examination indicates no recovery as far as 80 km downriver from the inflow of trace metals. Spatial and temporal variations in the numbers of individuals and the numbers of species are discussed in relation to physical and chemical features of the river. Several subjective and mathematical procedures have been employed to group the species collected on the basis of their distributions and abundances. The mathematical procedures included Principal Component Analysis, a hierarchical classification and a non-hierarchical classification. The application of these methods to studies such as this one has been discussed and evaluated. By using these grouping procedures the species collected have been separated into three groups; a) species which were relatively abundant at both contaminated and uncontaminated sites, b) species which were most abundant at sites above the polluted section and c) species whose numbers were highest at sites below the source of contamination. A leptocerid caddisfly and a baetid mayfly were the only two species which maintained high numbers at sites above and below the pollution inflow. Several species which were numerically dominant at the uncontaminated sites were almost, or completely eliminated at sites below the inflow of metals. These included two Mollusc species, four species of leptophlebiid mayflies and five species of caddisflies. Generally the molluscs and crustaceans were found to be sensitive to trace metal contamination and the dipterans insensitive. However, wide differences in the reaction of even closely related species to the metal contamination have been shown to exist. The factors which are likely to be important in determining the harmful effects of trace metal contamination in the field are discussed and as an adjunct to this, the relationship between the toxic concentrations of metals reported by other workers for laboratory studies and those found to produce harmful effects in the South Esk are also discussed. The Margalef and Shannon and Weaver diversity indices both demonstrated only the gross effects of metal contamination. Statistical correlations have been carried out between these indices and the trace metal concentrations in order to evaluate their usefulness for identifying the causes and effects of trace metal contamination. The Margalef index was found to be more efficient in this regard. However, a comparison between the information gained from the two indices and that from statistical examination of the numbers of individuals and the numbers of species, showed that these indices were of limited value for this purpose. The composition of the drifting invertebrate fauna in contaminated and uncontaminated sites has been investigated in both summer and winter. Changes in the composition of the drifting fauna and possible reasons for these are discussed. It is suggested that the species comprising the drift may react in different ways when they enter the contaminated zone and that this may result in faunal changes in the section of a river immediately upstream from the source of contamination. Finally, general criteria for the assessment of trace metal contamination of rivers have been discussed and factors which may be generally applicable to the study of pollution in rivers are presented.