Open Cut » Environment
Extreme rainfall conditions in the Fitzroy Catchment over an approximately 5 year period between 2007-2012 resulted in accumulation of water on many mine sites. This research investigated hydrological /salinity relationships and ecological processes with the view to better characterise the processes controlling relationships and the implications for mine water releases.
The objectives of the work were:
· Develop new knowledge for determining the sustainable salt load for the river system;
· Quantify the impact of saline discharge on aquatic ecosystem processes by examining changes in hyporheic (below surface of river bed) microbial community structure and function and the dynamics of system flushing under highly-variable seasonal river-flow conditions;
· Use the results, to develop guidelines for flow and water quality conditions that will minimise environmental impacts of mine water releases.
The research focussed on the Isaac River Catchment because it is one of the catchments that receives a large proportion of mine water releases. Analysis of spatial and temporal dynamics of flow and salinity showed that the amount of time available for mine water releases is restricted due to the ephemeral nature of the streams. The Isaac River only flows for about 50% of the wet season at Deverill while further downstream at Yatton flow increases in duration and volume predominantly due to inflow from the Connors River Catchment.
Analysis of flow and salinity relationships showed that while there is a general log-log relationship between streamflow and salinity in both the Isaac River and Cherwell Creek, baseflow and high flow salinity is variable from year to year. The processes driving this dynamic are primarily controlled by changing surface water/groundwater interactions and long term groundwater salinity. McNeil and Cox (2007) showed that salinity and level of alluvial groundwater systems is driven by El Nino Southern Oscillation/Interdecadal Pacific Oscillation over long (at least decadal) periods. Salinity of the groundwater increases during dry periods due to evaporation and the level gradually decreases. In response to high rainfall periods the alluvial groundwater system recharges reconnecting the stream with high salinity water. This work has shown that the higher salinity expression of the groundwater not only increases salinity during the annual flow recession but also during high flows in the wet season. This dynamic will need to be included in catchment models in order to better predict the volume of mine water that can be released during events.
Characterisation of the microbial consortia in the sediments showed that highest abundances were found in surface sediments of pools and in the channel during hydrograph recession. There was a highly mixed consortia typical of environments which are high variability in conditions. Measurement of microbial processes showed that biofilms were important sites of metabolic activity in the system with little or no detectable metabolism in the sediments outside of biofilms. Salinity was not shown to affect either nitrification or denitrification. Methanogenesis, however, was shown to be inhibited at 0.2 mS/cm. These results emphasise the importance of these biofilms as hotspots of biological activity in the catchment. Any changes to conditions for mine water releases should consider possible impacts on these communities and the role they play in regulating water quality.
Guidelines for mine water releases were developed from the outcomes of this research and previous investigations of water management and salinity on mine sites and ecological impacts. The guidelines were categorised into four areas and are listed below.
Proactive Planning and Management:
· Mine site water balance models must be capable of assessing changes to the mine water balance under variable climate regimes to establish the risk and timing of over- and under- supply.
· Mines should rationalise their water balance on long timescales using long term rainfall data. These measures will enable a site to understand the context of their current and near future water balance and understand whether mine water releases are likely to be required as well as release volumes in the upcoming wet season.
Managing Water on Site to Minimise Water Excess:
· BOM seasonal rainfall outlooks should be incorporated into water balance models to enable seasonal mine water planning including freshwater supply, the need for releases and operations;
· Excess water should be released as soon as possible to avoid accumulating salts.
· Sites should have:
o automated real-time monitoring of flow and EC (and other important parameters) located upstream, in the dam(s) at the discharge points and downstream to aid computation of volume (and release rate) of water that can be released given its salinity,
o a mass balance model that can used to calculate with an appropriate margin of error the rate at which mine water can be released without exceeding the trigger. Ideally this model would be able to be automatically updated during the release to ensure compliance,
o release infrastructure of sufficient capacity,
o automated gates at release points to allow remote operation in poor weather conditions,
o remember to check other compliance limits,
· Establish the role that changes in groundwater salinity and exchange play in controlling surface water salinity in the waterways that receive mine water releases.
· Incorporate, as appropriate, groundwater dynamics into streamflow and salinity predictive models.
Consideration of ecological processes during mine water releases:
· Low flow releases should be based on the mine water salinity and stream flow to ensure adequate dilution and lower the risk of mine water accumulation in remnant pools;
· Mine water releases made into the tail of a hydrograph during the wet season presents a lower risk to remnant pool ecosystems owing to the high likelihood of follow-up rainfall event flushing the system;
· Mine water releases should be avoided during the tail of the hydrograph at the end of the wet season to avoid the possibility of transport of salt into remnant pools via drainage through the alluvium;
· The risk to different downstream ecosystems should be assessed each year.
An e-newsletter has also been published for this project, highlighting its significance for the industry.