Open Cut » Environment
The management of mining impacts on rivers and other aquatic ecosystems at both a local and regional scale is a key priority for the mining industry in order to improve the industry's ability to manage environmental issues. The mining industry needs to be able to accurately monitor and assess the impacts mining has on water quality: both on site and the downstream effect.
Globally, assessments of river health or condition incorporate measurements of the chemical and physical properties and an element of the aquatic flora or fauna. Frequently, the biotic component is macroinvertebrates, and these are assessed through a comparison with index or reference condition values. In Australia, the ANZECC guidelines recommend that one of the approaches for assessing the impact of human activity on aquatic ecosystems recommended by the AUSRIVAS bio-assessment approach. AUSRIVAS models are predictive models, and the technique uses macroinvertebrates and specific environmental data from a test site to determine the condition of the test site compared to a reference condition from a regional database of minimally impacted sites (the predictive model). Most of the AUSRIVAS models in use in Australia were developed from data collected over a two-year period in the late 1990's.
In sub-tropical and semi-arid regions of Australia such as Central Queensland, the assessment of the quality or health of waterways can be difficult. These water bodies typically have a wet and a dry cycle that can vary significantly between years. However, the existing guidelines and predictive models for river health are typically determined from single samples taken in one or two years. In addition, the extensive man-made disturbances across the Central Queensland landscape have meant that few streams remain in a natural condition. This has meant that reference sites for water quality and aquatic health standards have mostly been taken from sites outside of the Fitzroy Basin.
Despite national recommendations, the AUSRIVAS approach has not been widely used for assessing the ecological condition of streams in Central Queensland. The database used to create the QLD AUSRIVAS reference condition includes reference sites from all across Queensland. Data to produce this model were collected in 1997 and 1998, which were wetter years than the decade that followed. If predictive models do not account for temporal variation, the false assessment of suspect sites may occur because communities may be responding to natural changes in environmental variables, and not to environmental impact. However, it is possible to create a region specific model, and account for seasonal differences by collecting samples from sites at different times of the year, and across different years. Once a model has been created, it provides a quick, easy, and relatively inexpensive tool for assessing various impacts of mining including river subsidence on the ecology of streams.
This project developed and tested a region-specific tool for assessing the condition of temporary streams on mine sites in Central Queensland. Macroinvertebrate communities were examined at the start and the end of the wet season to determine the effect of seasonal change on assessments of aquatic health, develop a predictive modelling tool for assessing river health. The two objectives of this project were:
· The creation of a Fitzroy Basin-specific model (the Fitzroy Model) for assessing mining impacts on stream ecology using macroinvertebrates as the indicator for river health. This model included samples that were collected over a 20-year period including wet and dry years. This model produced different assessments of some test sites compared with the nationally recognized AUSRIVS model for Queensland. In particular, sites that had previously been assessed as richer than reference in the standard AUSRIVAS Model were assessed as reference condition in the Fitzroy Model. A richer than reference assessment occurs when more taxa are found at the test site than were expected to occur. Most significantly, the Fitzroy Model was also able to distinguish the difference between reference sites and impacted sites.
· Macroinvertebrate index values for the Fitzroy Basin including indices recommended in the Queensland Water Quality Guidelines (2009) were created from the 20- year data set using only Fitzroy Basin reference sites. A comparison of values found that the range of values determined from the Fitzroy Basin reference sites were not significantly different to the range of values determining water quality objectives currently used to assess river health in the Fitzroy Basin.
Using macroinvertebrates to assess the impacts of land-use including mining on waterways is a useful tool and a regionally specific macroinvertebrate model can provide a finer resolution of site condition. Macroinvertebrates in Central Queensland are generally highly tolerant due to the relatively short wet periods, and the ubiquitous damage caused by livestock across the catchment. The regional Fitzroy Model accounts for this by including reference sites that are unaffected by mining from each of the catchments, and that have been sampled in years with different rainfall patterns.
The Fitzroy Model would be a useful addition to a suite of tools to assess the condition of streams affected by mine water discharge. It adds value by not only providing an assessment of whether or not the site has been affected, but it gives an assessment of how heavily a site has been affected. This is achieved through the interpretation of the banding scheme.
The Fitzroy Model would need to be developed into an online model that could be accessed remotely if it were to be of practical use. In addition, the usefulness of this model would be greatly enhanced through the testing of sites that are impaired along a gradient of potential impacts including salinity, pH, and heavy metals.
The index values developed in this project were statistically similar to the existing water quality objectives. Given these results, the current water quality objectives for macroinvertebrate indices for the Fitzroy Basin catchments provide a realistic and achievable reference range for assessing stream condition in the Fitzroy Basin and there would be little benefit in updating these values at present. Further analysis is required to determine the value of using the Plecoptera Ephemeroptera Trichoptera (PET) index in central Queensland due to the general absence of Plecoptera family from edge samples, the habitat most likely to be encountered at the majority of test sites.