Open Cut » Environment
Background
The physical and chemical properties of waters can be modified as it flows over and through a mine site. Changes to the physico-chemical properties of mine waters have the potential to modify off-site aquatic ecosystems, by affecting the structure of aquatic habitats, or by directly affecting aquatic flora and fauna species. Although there is a relatively large body of information on Australian aquatic ecosystem responses to water quality changes, there is comparatively little information on the effect of specific pollutant types associated with coal mining activities.
This report, together with a companion volume published in 2002, examines the scale and types of impacts on aquatic ecosystems associated with coal mining activities in the Bowen Basin region of central Queensland. The project was conducted by WBM Pty Ltd and funded by the Australian Coal Association Research Program (ACARP Project C11055).
The broad aim of the study was to provide an improved understanding of the impact of variations in water quality on the ecology of the catchments within the Bowen Basin . To fulfil this aim, a fieldsampling program was undertaken to examine changes in stream condition indicators in space and time. The selected indicators included water quality, habitat condition, aquatic macroinvertebrates (animals =0.25 mm without backbones) and fish. Each set of indicators was examined for potential differences between areas unaffected by mining (including relatively undisturbed reference sites and control sites affected by grazing), and areas positioned at varying distances downstream of mining activities. Based on these results, the appropriateness of using various biological measures as indicators of mining impacts was assessed.
Three mine sites with different contaminant issues and site histories were examined in this study. Collinsville Mine, situated in the Bowen-Broken River catchment of the Burdekin Basin, was known a priori to have acid mine drainage issues within two of the main streams that drain the site (Strathmore and Pelican Creeks). Goonyella-Riverside and Moranbah North Mines have adjoining mine lease boundaries on the Isaac River (Fitzroy catchment), allowing individual and cumulative impacts of the two mines to be considered.
Industry Implications
The results of the present study provide a case study into the effects of mining on several stream environments, which are representative of conditions experienced in the central Queensland coalmining district. In addition the study identified several key issues and that need to be considered by mine operators.
The results show that ephemeral streams are naturally harsh and dynamic environments with biological assemblages showing marked variation in time and space. Superimposed on ?natural' background variation in water quality and biological assemblages are catchment-wide impacts of vegetation clearing and ongoing grazing pressures, which have resulted in irreversible changes to aquatic ecosystems on a regional scale. This has important implications from a mine management perspective, as follows:
1. Risk of Mining Impact to Natural Waterbodies As semi-permanent or permanent waterbodies are now an uncommon resource regionally, there is a need for mine operators to carefully consider impacts of operations both water quality and quantity (i.e. environmental flows). The scarcity of permanent in-stream pool refugia in the study area, together with ongoing impacts by cattle, are likely to have a major impact on aquatic communities on a regional scale. It would be prudent for mine management to consider (i) the provision of environmental flows, (ii) development of permanent in-stream habitats and (iii) potential off-site impacts to fish refugia, during all phases of mine planning.
2. Opportunities for Environmental Offsets The creation of on-site waterbodies may present opportunities for the development of environmental assets (i.e. aquatic habitat) during the operational stages and following mine closure. Such storages include mine pits and dams, on-stream reservoirs and off-stream farm dams. The value of these storages as aquatic habitat refugia would vary depending on a number of factors, including connectivity to stream habitats, surface water permanency, water quality and micro-habitat structure. Several coalmines in the region are now investigating opportunities, constraints and management implications associated with developing water storages as viable aquatic habitat features. While management of on-site water storages is a mine-specific issue, the implications should be assessed at broad sub-catchment or stream reach scales, possibly requiring coordination of management activities across numerous mine sites.
3. Biological Monitoring In the absence of an appropriately designed monitoring program, it is difficult to distinguish mine related impacts from background variation. The high degree of variation in assemblages and environmental conditions has two important implications in terms of designing a program to identifying impacts:
- There is a need to include multiple control and impact sites in the sampling design. Samples also need to be replicated within sites to take into account the high degree of within-site variability in micro-habitat structure and macroinvertebrate assemblages; and
- There is a need to undertake multiple surveys to adequately assess variability in environmental conditions and biological assemblages that are likely to be operating over a range of temporal scales (within and among seasons, inter-annual variation).
Based on the results on the present study, the ?best' statistical test appeared to depend upon the nature of impact and environmental processes controlling habitat/water quality. It is recommended that a wide variety of statistical tests be used in monitoring programs. Further studies are required to test and refine some macroinvertebrate-based analyses techniques that could prove useful for distinguishing impacts caused by mining activities.