Managed aquifer recharge (MAR) is gaining robust interest from the mining sector as a method to control surplus water volumes while reducing the groundwater impacts of dewatering.
This methodology has already been embraced by at least two iron ore producers in Western Australia’s Pilbara.
Most miners currently using MAR are in arid or semi-arid regions, and are implementing it through infiltration basins or bore injection in order to manage excess water, preserve aquifers for environmental or human benefit and, when necessary, to adhere to licensing which require zero surface discharge.
According to research by the National Groundwater Association (NGWA), MAR is particularly applicable in arid and semi-arid regions where underground storage is useful in mitigating against future water scarcity or seasonal fluctuations in supply.
Upfront planning, however, is key when it comes to maximising its benefits as a truly effective and sustainable mine water management tool. “Groundwater mounding, well clogging, and interaction between adjacent mines are common challenges,” the NGWA researchers explained.
“Mitigation strategies include predictive groundwater modelling, extensive monitoring programs, rotation of infiltration or injection facilities, physical and chemical treatments for clogging, and careful location for MAR facilities in relation to adjacent operations.”
With increasingly strict environmental regulations and controls in place for groundwater control, miners have developed an interest in MAR to address mine water challenges.
MAR is generally applied by routing surplus water to injection wells, where water is injected straight into theManaged aquifer recharge (MAR) is gaining robust interest from the mining sector as a method to control surplus water volumes while reducing the groundwater impacts of dewatering. target aquifer, or to infiltration structures such as ponds, trenches, dams or galleries.
It has also been used to maintain groundwater levels for sensitive ecosystems or third party users that may be impacted by reduced water levels following mine dewatering.
More recently, injecting surplus water at a target distance from the pit during mining operations has been considered a suitable technique for accelerating groundwater recovery following mine closure.
According to the NGWA researchers, mine closure may require that pre-mining hydrological conditions are re-established ‒ including the recovery of groundwater levels.
Depending on climate and hydrogeological conditions, however, a full recovery which relies on the naturally occurring rainfall and recharge conditions may take tens to hundreds of years – if it occurs at all.
Through the controlled strategic injection of excess mine water into suitable sections of the subsurface during the operating phase of the mine, post-closure groundwater level recovery can be accelerated.
“Water needs to be injected distant enough to prevent excessive re-circulation back to the pit, but close enough that water is stored within the drawdown cone to facilitate groundwater recovery once mining has ceased,” the NGWA paper explained.
“More rapid groundwater recovery not only benefits groundwater dependent ecosystems and aquifer users, it also reduces exposure of pit walls within the mine workings and therefore the risks of acid mine drainage.”
Despite the growing use of MAR in mining, very little information is made publicly available regarding site-specific schemes.
This lack of accessible data on successful MAR schemes has resulted in decision makers and water resource managers to often view it as a costly and risky option.
“To improve the communication and shared knowledge on managed aquifer recharge in general, global reviews have been conducted,” the NGWA researchers noted.
“However, none of these focus on the use of MAR for mining.” According to the NGWA, miners generally implement MAR schemes for one or more of the following reasons:
surplus water disposal, environmental protection, aquifer preservation for groundwater users, and adherence to zero surface discharge licensing.
For the minerals sector, groundwater mounding and expression at the surface is a common challenge associated with MAR, particularly where it can impact sensitive ecosystems.
Predictive groundwater flow models are often used to not only estimate drawdown, but also for mounding around recharge facilities.
Water levels around recharge infrastructure must also be regularly monitored during infiltration and injection to ensure the water table remains within an acceptable range.
BHP has embraced MAR at its Mining Area C iron ore project in the Pilbara, which has a complex mine dewatering and surplus water network, including 34 operational dewatering bores, 16 operational reinjection bores and a large number of monitoring bores.
Dewatering volumes at the project are significantly larger than site water demand, generating a large surplus. In 2023 alone, collective abstraction was 45 megalitres per day, with almost half of this being surplus to site requirements.
Returning this to nearby groundwater aquifers benefits the environment by mitigating drawdown from nearby mines and storing water. Increasing the return to ground capacity has also allowed BHP’s reliance on creek discharge to be progressively reduced.
BHP’s WA iron ore projects will become increasingly reliant on mining below water table, with approximately 40 per cent of ore planned to be accessed from below the aquifer level by 2040.
This translates to approximately a four-fold increase in dewatering volumes to those in 2022 and an even bigger increase in surplus water.
Its current schemes consist of the Juna Downs MAR, South Flank MAR, Runaway Valley infiltration ponds, as well as Western and Central sediment basins, which use surplus water for beneficial use where water is returned to the environment to improve the sustainability of the regional groundwater resource.
The miner expects to expand the use of water management schemes, including additional aquifer reinjections, bores and infiltration ponds as the surplus volumes increase, so that the majority of surplus water is returned to ground for beneficial use.