5.3 Predicted ecological impact
The impact of non-acid-forming coal mines on streams will be dependent on metal loads, and in some cases, sediment loads or turbidity.
Impacts on aquatic ecosystems arising from high turbidity are largely physical in nature such as smothering of benthic organisms, and reduction in light penetration. The impacts arising from high turbidity can be considered as direct and indirect impacts. Direct effects occur as a result of a direct effect on an organism and include smothering of benthic organisms and eggs of some species, and clogging of the gills of fish. In contrast, indirect effects include reduction in primary production (algal growth) due to decreased light penetration, and changes in predator–prey relationships due to prey species being hidden from predators.
Excluding the impacts from turbidity, two main outcomes are possible (Figure 22).
Outcome 1
Streams with water around neutral pH clearly are not affected by acidity. These waters, if downstream from mines, may have high concentrations of metals such as Zn or Cu, which can be toxic to fish and invertebrates. Furthermore, Fe and Al hydroxide precipitates, if present in high amounts, may limit the habitat or food supply of fish and invertebrates. Finally, sedimentation from mining areas could cause ecological effects in streams from high turbidity (from TSS) and loss of habitat quality (from deposited sediment).
Outcome 2
Waters of neutral pH and very low metal content should support a full diversity and abundance of aquatic life for the area. Natural features of the catchments could affect some biota, such as waterfalls blocking migratory fish species. Mining still could affect stream habitat if turbidity and sedimentation (from mining operations) were present. Otherwise, species and food webs should be comparable with those in pristine streams in the area.
5.4 Operational management and treatment
Total suspended solids are expected to be the only water quality issue requiring management and treatment. Best management practices to prevent or reduce high TSS will be more cost effective than ongoing treatment of mine drainage. In particular, waste-rock and tailings management techniques can help to minimise the formation of TSS in mine drainage, although further treatment may also be required. Management of TSS is typically undertaken using sediment traps, wetlands or active treatment using flocculants and thickeners. Refer to Appendix B for a detailed discussion on operational management and treatment techniques for the management of TSS. When mining operations commence, suspended solid loads, in particular that present in the discharge from the treatment system, should be monitored to assess the effectiveness of treatment systems.
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