Could managed aquifer recharge be an answer to mine water management challenges?
Water management strategies are becoming increasingly important in mine planning as greater burdens are placed on supplies. A lack of process water may be a major hindrance to site activities, whereas surplus water produced as a by-product needs to be managed appropriately to minimise the impact on the receiving environment. Managed aquifer recharge (MAR) is one method that can be used to accomplish this. MAR involves the controlled transfer of water to replenish aquifers for subsequent recovery or to achieve environment benefits. MAR provides a means to generate future water supply from water that may otherwise be wasted as mining by-products. It may be used to assist in securing water supplies, and to protect aquifers and groundwater-dependent ecosystems in the face of climate change and growing water demands. While MAR is growing in popularity globally and in Australia, the benefits as a water management tool in the mining sector have yet to be realised.
How does managed aquifer recharge work?
For MAR to be viable, the following site criteria must be fulfilled:
- the presence of a geological formation capable of receiving, storing and transmitting significant quantities of water with additional storage capacity
- a suitable source of water for recharge/injection
- an ongoing local demand or clearly defined environmental benefit.
As outlined by Dillon et al (2009), the seven common components of MAR are:
- collection of water to be used for MAR
- pre-treatment – the treatment of MAR water prior to injection and possibly secondary processes such as filtration that may be required prior to injection
- recharge, ie the injection strategy – for example deep injection bores or infiltration surface basins or galleries
- subsurface storage – the aquifer within which water is being injected
- recovery – the method of access to injected water at a later time
- post-treatment – additional treatment that may be required following recovery and prior to end use
- end use – the future intended use of MAR water.
A variety of water sources may be used for recharge, including treated mine water or wastewater, water abstracted through dewatering schemes, or collected stormwater. Generally, recharge water for MAR must have water quality that is similar to, or better than, the receiving aquifer water quality. Typical sources of water, methods of capture, treatment and end use are given in Figure 1.
Two options for aquifer replenishment schemes are generally employed:
- the use of injection wells where water is directly injected into suitable aquifers
- the use of infiltration structures such as ponds, basins, galleries and trenches where water slowly soaks into the ground (Figure 2).
The benefits and opportunities of managed aquifer recharge
MAR can provide several benefits and opportunities in managing mine water in a manner that may have a low impact, or preferably have direct benefit, to the environment; it can also be used to sustain or buffer water supplies, especially in the drier climates of Australia. These, and other benefits, are explored further below.
Reinjection of surplus water
Often mine dewatering results in surplus volumes of water. Typical mine water management techniques for this surplus might include disposal to a surface water system, which can often result in environmental issues. Alternatively, the use of MAR and the injection of surplus water into an underlying aquifer may provide an alternative disposal option that results in environmental benefits and the potential for the reuse of water that may have otherwise been wasted.
Sustaining water supply
Mines are often in remote locations with a scarce or fluctuating water supply. MAR can be used to store water during periods of water surplus in order to meet future water demand in a deficit scenario. As an underground storage system, MAR is not strongly affected by the seasons and a sustainable water supply can be created by injecting excess water in times of surplus for subsequent abstraction during periods of drought. This system can mitigate the fluctuations between water surplus and water deficit, providing a dependable source of water for the whole year and increasing water availability for commercial and environmental uses.
Often the mining process may result in unwanted environmental impacts to ecosystems that are dependent on water. MAR can be used as a tool to maintain environmental water flows such as groundwater levels to sustain ecosystems such as wetlands. MAR may also be used as an alternative to surface water discharge, which can often have large environmental impacts.
MAR can be considered in mine closure. The injection of water into the mine void can accelerate recovery of groundwater levels. This results in submergence of the mine workings as soon as practically possible, which can reduce oxidation in the workings and fill material and subsequently minimise the risk of acid mine drainage development.
The benefits of using MAR in mine water management include:
- provision of emergency and strategic reserves
- improved reliability of supply through dry periods
- the application of environmental benefits such as sustaining environmental flows and reducing the environmental impact from the mine
- no loss of valuable land at surface
- no evaporation losses, algae or mosquitoes as water is stored beneath the ground instead of at surface
- the potential for additional water quality treatment of stored water due to aquifer characteristics
- opportunities to locate storages close to locations of demand.
Managed aquifer recharge schemes in mining
The longest serving MAR operation in Australia was established on the Burdekin Delta, Queensland, in the mid-1960s using infiltration basins. Since the 1990s there has been considerable progress in MAR in Australia. MAR schemes have been investigated or in operation in all states and territories (Figure 3).
In mining, MAR schemes using both injection and infiltration have been applied at mine sites both in Australia and overseas.
Cloudbreak mine in Western Australia is currently considered to have one of the largest MAR schemes in Australia and has been in operation since 2008. It returns approximately 73 per cent of extracted water from dewatering back into the aquifer via injection bores, greatly reducing their effect on groundwater levels and quality (Willis-Jones and Brandes de Roos, 2013). Aquifer injection has been adopted as a primary water management tool and to date has proven to be practical and effective. The benefits of the system are conservation of brackish water for redraw over the life of the mine, minimisation of the drawdown footprint from the dewatering operation, and limiting the environmental and cultural concerns associated with the surface discharge of excess water to the nearby marsh.
An example of a MAR scheme using infiltration basins is Ophthalmia Dam in Western Australia. Constructed on the Fortescue River approximately 5 km upstream of the Ethel Gorge, this scheme started operation in 1982. It was constructed to enhance recharge and augment groundwater resources in the Ethel Gorge area and is important in supporting the eco hydrology of the gorge. Surplus water from the dewatering process is discharged and stored in Ophthalmia Dam; the dam is designed to retard the flow of some surface water and enable passive infiltration into the shallow alluvial aquifer (Douglas and Pickard, undated).
Cobre Las Cruces is an open-pit copper mine located in south-west Spain. The mine has a complex drainage and re-injection system consisting of 32 peripheral dewatering wells connected to a ring of 28 injection wells. Abstracted water is treated by reverse osmosis to remove metals and reinjected into wells located 0.7 to 2.5 km from the mine pit. The scheme has been operating without interruption since 2006 and balances the needs of mine dewatering versus maintaining the equilibrium in the aquifer. The system maintains the water balance in the aquifer and reduces the extent of the cone of depression from the open-pit (Baquero et al, 2016).
Managed aquifer recharge is being adopted by several coal seam gas operators in the Surat Basin in Queensland for coal seam water management. During the coal seam gas process, water is abstracted from coal seams to reduce the pressure in the seam and allow the release of gas to the surface.
This coal seam water, often referred to as ‘produced water’, is treated and re-injected into aquifers that may have suffered from depletion due to over-abstraction by the local community or has been impacted by depressurisation of the underlying coal seams. The objective here is to maintain or restore pressure in affected aquifers and produce environmental benefits to local communities.
Despite the potential for MAR in mining, there are possible issues with its use. None of the known issues are show-stoppers, but must be considered when developing and operating a MAR scheme. Some of the issues relevant to the application in mining are discussed further below.
Re-circulation of injected water back into the dewatered body
Where water is injected into the ground, there is always the risk that this water will re-enter a mining facility and have implications for the efficiency of dewatering operations. The distance of the injection bore to the mine workings needs to be great enough to reduce the occurrence of re-circulation. MAR schemes such as the Cobre Las Cruces (mentioned above) are evidence that water can be reinjected without implications on the running of the mine
if designed correctly.
Water quality issues and the potential for clogging
MAR mixes treated process water (which is generally higher in oxygen) with aquifer water (which generally contains less oxygen), and this may have an impact on aquifer water quality. Changes in water chemistry are accompanied by changes in physical conditions between the well head and the injection depth (eg pressure and temperature); these may cause precipitation of minerals within the aquifer, along the injection pipework and within the pumping facilities (Wanner et al, 2017), and may result in greater maintenance and disruptions in service. The presence of bacteria may exacerbate the issue, with biofouling often observed in injection wells. Clogging can be overcome by well rehabilitation, including back-flushing, chlorine disinfection or acidification. The oldest aquifer storage and recovery (ASR) facility in the US, Wildwood, has suffered from clogging due to iron precipitate and deploys acidisation of the injection wells every three months to combat clogging (Bloetscher et al, 2014). Filtration of injectate can be used to remove suspended solids from recharge water at the well head. Deoxygenation of injectate is another technique employed to mitigate adverse effects and studies are currently underway at Klohn Crippen Berger to investigate the costs and benefits of this method.
Injection wells need to withstand high pressures and be constructed of the correct material and with proper completion. They must be designed to enable rehabilitation from clogging. In drilling, to reduce the risk of clogging, the use of bentonite muds must be avoided and the wells must be fully developed.
The key impediment to MAR implementation is the inherently high level of initial uncertainty in the technology compared with other water management solutions. With further development of guidelines, support from state and federal governments and an increase in knowledge, it is likely that MAR will shift from being a niche technology to a standard water management method in mining. There is no doubt that MAR offers a useful tool in mine water management, offering a low-impact technology with environmental benefits suited to the unpredictable Australian climate.
For further information on MAR, contact the authors via email@example.com.
Baquero J, Jose de los Reyes M, Custodio E, Scheiber L and Vazquez-Sune E, 2016. Groundwater Management in Mining: The Drainage and Reinjection System in Cobre Las Cruces, Spain, Modern Environmental Science and Engineering, 2(10): 631-646.
Bloetscher F, Sham C H, Danko J and Ratick S, 2014. Lessons Learned from Aquifer Storage and Recovery (ASR) Systems in the United States, Journal of Water Resource and Protection, 6(17).
CSIRO. Using MAR in Australia [online]. Available from: https://research.csiro.au/mar/using-managed-aquifer-recharge/. Accessed August 2017.
Dillon P, Pavelic P, Page D, Beringen H and Ward J, 2009. Managed Aquifer Recharge: An Introduction, Waterlines Report No 13. National Water Commission.
Douglas B and Pickard S, undated. Eastern Pilbara Water Resource Management Plan. Draft for Consultation. Western Australia Iron Ore, BHP Billiton.
NRMMC-EPHC-NHMRC, 2009. Australian Guidelines for Water Recycling, Managed Aquifer Recharge, National Water Quality Management Strategy, Document No 24.
Wanner C, Eichinger F, Jahrfeld T and Diamond L W, 2017. Causes of abundant calcite scaling in geothermal wells in the Bavarian Molasse Basin, Southern Germany, Geothermics 70:324-338.
Willis-Jones and Brandes de Roos, 2013. Application of Large Scale Managed Aquifer Recharge in Mine Water Management, Cloudbreak Mine, Western Australia, in Clogging Issues Associated with Managed Aquifer Recharge Methods (ed: R Martin), pp 156-162 (IAH Commission on Managing Aquifer Recharge, Australia).