7.3 Predicted ecological impact
The impact of alluvial gold mines on streams on the West Coast is usually related to fine sediments/turbidity, although impacts related to low pH and elevated metal concentrations (AMD) may also occur occasionally. However, in Southland, impacts of low pH and metals may be more common due to the presence of acid-forming gravel deposits.
The 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 on an organism 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 turbidity impacts, the following flow chart (Figure 27) illustrates the outcomes that may occur with an alluvial gold mine. As stated above, AMD issues are rare at alluvial gold mines; as such, the outcomes associated with neutral mine drainage (NMD Outcomes 5 and 6) are most likely to be observed (refer to section 4.3 for a description of Outcomes 1-4, and Appendix D for more details).
Mercury, when present in drainage from alluvial gold mines, typically occurs at concentrations < 10 ng/L (0.01 mg/L, e.g. Holley et al. in press). By way of contrast, ANZECC water guidelines sets trigger values of 0.06 and 0.6 mg/L for protection of 99% and 95% of species respectively present in an aquatic ecosystem (ANZECC and ARMCANZ 2000). As such, it is unlikely that Hg from mine drainage will cause impacts in aquatic systems (see also Appendix D).
Outcome 5
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 zinc or copper that are toxic to fish and invertebrates. Furthermore, metal precipitates, if present in high amounts, may limit the habitat or food supply of fish and invertebrates.
Outcome 6
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 could affect stream habitat if turbidity or sedimentation (from mining operations) were present. Otherwise, species and food webs should be comparable with those in pristine streams in the area.
7.4 Operational management and treatment
Sediment load in the mine discharge is the primary issue for mitigation of aquatic impacts. Techniques for the management of sediment loads are the same as that described for non-acid-forming coal mines and include options for management of the waste rock, tailings, and active and passive treatment techniques to manage TSS (see Appendix B).
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