14 April 2015
Key post-mining landscape features known as final voids present a number of management challenges. In addition to safety issues, final voids can be a point source of saline and acidic waters, potentially resulting in impacts to groundwater systems and downstream receiving environments.
There are two commonly employed, main strategies applied to mine voids at mine closure: maintain as an open void, or backfill with waste material. There are many environmental, social, visual and economic issues to consider when selecting the most appropriate closure strategy. Backfilling can mitigate many social and environmental issues; however this can be prohibitively expensive and may lead to a loss of environmental values supported by the waterbody. Open voids are often seen as the most practical option, and can provide long term environmental benefits if appropriately managed.
Mine voids can support many environmental values. For example, some mine voids created by coal mining in Queensland’s Bowen Basin provide locally important dry season refugia for aquatic biota and waterbirds, as well as potential stock watering sites. BMT has found that many pit lakes, often with poor quality, can contain abundant fish and aquatic invertebrate communities, even when there is no apparent surface water connection to other water bodies. Mine voids from historical sand mining activities, such as those on North Stradbroke Island, can represent locally important recreational resources and support rich and abundant wetland ecosystems. Active treatment of pit lake waters can also produce water that is suitable for commercial uses such as aquaculture, tourism and agriculture.
Ultimately, the capacity for mine voids to support important ecosystem services and other environment values depends, to a large extent on the quantity and quality of water within and entering the void, and the degree of connectivity to natural stream systems. Management of flow paths is one tool that can be used to ensure that there is sufficient, good quality water available to maintain these values. For example, BMT used pollutant and hydrological modelling at a Bowen Basin mine to optimise flow paths such that sufficient, good quality water entered the lake pit to sustain healthy ecosystems, while more saline, nutrient enriched waters were diverted into another pit with lower environmental values.
In most cases, pit waters have poor water quality and are unlikely to support a wide range of environmental values. If back-filling is not a viable option, then it is important to understand whether there is potential for waters to enter groundwater and surface water systems and cause environmental harm. This requires a good understanding of the system’s hydrological and physio-chemical processes, based on modelling and data analysis.
Modelling of mine void water levels provides a means for predicting the likelihood of pit waters overtopping and entering downstream environments. In order to adequately assess the hydrologic and water quality variability in voids, BMT has found that modelling using long climatic sequences, coupled with detailed groundwater interactions are essential to properly understand the system.
Modelling of mine void water quality can be undertaken using a variety of approaches, depending on the issue under consideration. This can include, for example, modelling of long term changes in salinity, nutrients and metal concentrations; algae and nutrient dynamics; and stratification processes. Fundamentally, the modelling tools provide valuable information for assessment of the end uses of mining voids, such as recreational or agricultural uses, but also on issues such as typical final water levels, water level variations, sustainable water harvesting yields and similar issues. Such information provides a basis for assessing potential hazards, and possible environmental and social values supported in the long term.
Back-filling and associated rehabilitation of mine voids often requires de-watering of pit waters, which presents a range of other logistical, regulatory and environmental challenges. Potential disposal options include beneficial re-use, recycling, transfer to other storages, or discharges to natural systems. While the most appropriate disposal option will vary on a case-by-case basis, it is BMT's experience that it is critical that regulatory agencies are engaged throughout the process, particularly if off-site disposal is a preferred option.
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