Pit Lakes in Western Australia

Pit Lake Formation

The extraction of mineral can occur via open cut and underground mining methods, depending on the mineral being targeted (American Geosciences Institute, 2024). For minerals near the surface or mineral with low grade and large volumes are typically extracted through open-cut or open-pit mine.  In these situations, the open-cut may interact with local groundwater sources or required the diversion of surface water around the mining area to dry out or dewatering the mining area (APT Water, 2018). When mining has finished, dewatering stops, groundwater, rainfall and catchment run-off can return to the void or open pit creating a new water feature ofter call a pit lake (Figure 1) (Kolkovski, 2011).
 

Depending on the local conditions such as  the rate of groundwater flow, and rock type a pit void can be describe as either a groundwater sink, groundwater throughflow or groundwater recharge (Figure 2) (Johnson & Wright, 2003). Groundwater recovers slowly in groundwater sink and throughflows but rapidly for groundwater recharge voids.
 

Groundwater sinks are commonly found in igneous and metamorphic rocks, and have a larger rate of evaporation compared to groundwater in flow (Grid Club, 2009; Johnson & Wright, 2003). Once sink void fills up, the water level will typically be lower than pre-mining levels.

Throughflow voids occur when a gradient is present on either side of the mine void, sometimes due to the low permeability of the surrounding rocks (Moser, Cook, Miller, Dogramaci, & Wallis, 2024). These voids can take decades to fill with water and also will have water level lower than pre-mining levels (Johnson & Wright, 2003).

Groundwater recharge voids are common when mining diverted a river or the void was created in a high rainfall area (Johnson & Wright, 2003). When dewatering stops, the void can fill within years to pre-mining levels as the rate of evaporation is lower than the rate of groundwater inflow.

Western Australia has approximately 2,000 voids, with over half expected to form permanent pit lakes (DEMIRS, 2023). However, this is expected to increase as more open-cut mines below the groundwater table (Blanchette & Lund, 2016).

Legislation and Guidance

The Department of Energy, Mines, Industry Regulation and Safety (DEMIRS) and the Environmental Protection Authority (EPA) recommend backfilling voids, where possible to reduce the creation of pit lakes (DEMIRS, 2023; Parker, 2022). The backfilling of a pit voided can occur with mine waste such as waste rock or tailings and typically occur progressively over the life of a mine (Glencore, 2019)(Johnson & Wright, 2003).  It should be noted that for a pit void to be backfilled, DEMIRS requires miners to demonstrate all economically recovered minerals have been removed.

Assessing potential impacts associated with pit lakes may occur prior to establishment of a mine through impact assessment or when completing a risk assessment undertaken as part of mine closure planning (EPA, 2013). The DEMIRS’ Mine Closure Plan Guidance (the Guidance) (2023) provides information on pit lakes and how to manage them.

The Guidance (DEMIRS, 2023) outlines the type of information to be included in a Mine Closure Plan as required under the Mining Act 1978 (WA), or used to support Part IV assessment under the Environmental Protection Act 1986 (WA). For pit lakes, DEMIRS recommends creating models on differing environmental aspects and completing a risk assessment. The models are created with baseline information on the void, such as surface water, groundwater, potential sources of contamination and climate, which provides a better understanding on potential risks and knowledge gaps. However, models shouldn’t be relied on as the sole source for assessing pit lakes as their accuracy is dependent on the quality of the input data and its appropriateness for the particular mining stage (DEMIRS, 2023). It is recommended the input data include appropriate field-based data for the model, with different scenarios modelled, and the data is updated as it is acquired and the mine progresses (Oldham, 2014). The model are typically simpler in the early stages of mine development as less information is available and become more compared as additional information is gain over the mine life (DEMIRS, 2023).

During closure, DEMIRS (2023) recommends validating the model to ensure its accuracy, and reduce remaining uncertainties where possible. For example, validating a model on groundwater with final groundwater levels from surrounding bores and rate of groundwater inflow. However, if a model can’t be validated, then verification with information from similar pit lakes is recommended (DEMIRS, 2023).

The Guidance also recommends conducting a risk assessment, with aid from models, to determine potential impacts to sensitive receptors, such as increased feral animal abundance, discharges to surrounding water features and collapse of the void wall (DEMIRS, 2023). During the risk assessment, water quality guidelines can be used to assess water quality risks but they need to be appropriate for the receptor (DEMIRS, 2023). For example, using a drinking water standard such as ANZECC & ARMCANZ’s (2000) Australian and New Zealand Guidelines for Fresh and Marine Water Quality when the pit lake water is likely to flow to a bore used for potable water, and there are no other exposed receptors.

Should the risk assessment determine the pit lake is highly polluted or there is a High or Critical level of risk to the environment, Western Australia’s Contaminated Sites Act 2003 (CS Act) should be adhered to. In this instance, the pit lake may be classified as a contaminated site under the CS Act, with people and companies responsible for managing and remediating it to reduce the environmental harm.

Environmental and Social Considerations

Water quality in a pit lake can evolve and change over time with the surrounding environment. For example, sites with high sulfide geologies can have acidic and metalliferous reactions occur in the exposed void walls (Parker, 2022). The resulting products can then be washed or leach into the pit lake and impact the water quality and surrounding environment (Figure 3). Large pit lakes can also impact a wider catchment area through groundwater and surface water outflows leading to substantial environmental impacts (Blanchette & Lund, 2016). However, outflows are more common in groundwater recharge voids where there is potential for the pit lake water levels to be above the groundwater level or to overfill the void (Parker, 2022).

As a result of the high evaporation rate in throughflows, salinity increases slowly with a saline plume forming and moving down-gradient of the void (Johnson & Wright, 2003). The environmental impact from the saline plume depends on the rate of the throughflow.

Similarly, the base of groundwater sinks can form salt crystallisation due to an increase in salinity from the high evaporation rate and lack of throughflow (Johnson & Wright, 2003). When the salinity in the pit lake water is higher than the surrounding groundwater, it can discharge into the environment.

Pit lakes also have social impacts and can be detrimental to a nearby community rather than a benefit. For example, the Black Diamond mine in the Allanson town site area within Western Australia’s Shire of Collie was mined from 1948-1953, with no rehabilitation work completed at the time (DEMIRS, 2016). When a pit lake formed (Figure 4) it became an unmanaged recreational area for the community, with a death and multiple injuries occurring due to safety issues.

In 2015, the Department of Mines and Petroleum (currently DEMIRS) created a Working Group to provide advice on rehabilitation works for the pit lake (DEMIRS, 2016). Funding was provided by the Mine Rehabilitation Fund, which wasn’t sufficient to rehabilitate all the safety risks identified by the Working Group; instead DEMIRS targeted efforts on rehabilitating aspects with the largest safety risks. In particular, DEMIRS reduced the angle on the southern void wall from 90 degrees to a safer 22-26 degrees, managed erosion on the northern and western ends and revegetated areas around the pit lake (DEMIRS, 2016). DEMIRS also collaborated with the University of Western Australia for students to undertake water sampling, soil sampling and vegetation surveys at the pit lake.

The pit lake is now a known tourist destination due to its blue water, with use restricted to day time only as sections can be very cold and a private property partly covers the pit lake (Collie River Valley, 2024). However, while people can swim in the pit lake, there are signs warning that the pH is low with it recorded at pH4.4-6.8 in 2010 (Collie River Valley, 2024; Hinwood, Heyworth, Tanner, & McCullough, 2012). The pit lake also has elevated levels of mercury above the ANZECC & ARMCANZ’s (2000) water quality guidelines (Hinwood, Heyworth, Tanner, & McCullough, 2012).

Pit Lake End Goals

There are multiple opportunities for post mining use of pit lakes, such as aquaculture, irrigation, water resources for remote communities, wildlife conservation and recreation (Parker, 2022). However, these end goals are not feasible for all pit lakes as they are dependent on water quality and potential safety concerns.

Lake Kepwari is a successful pit lake in Western Australia’s Collie area (Figure 5), which operated as a coal mine from the 1970’s until closure in 1997 (McCullough & Evans, 2024). The Collie River South Branch was diverted for the mine, which would be permanent as per the initial closure plan, to form a closed catchment lake (Yancoal, 2020). However, it was predicted the void would fill 100 years after closure, with an earlier closure requiring water from the Collie River South Branch (McCullough & Evans, 2024). A valve-regulated offtake was installed to divert water during high river flow periods, which worked and in 2007, the pit lake was filled and named Lake Kepwari out of respect for the Aboriginal people in the area. Water quality was acceptable with above neutral pH while the valve was in use, however, without the inflows, pH dropped to pH4 and some elevated metals were present (McCullough & Evans, 2024).

In 2011, substantial rainfall occurred in the area which caused the Collie River to breach the diversion channels to Lake Kepwari and the water to flow through (Yancoal, 2020). An analysis of the water quality from the breach event in 2012-2016 showed it was improved by becoming a throughflow pit lake and there were no adverseimpacts to the environment. As a result, the Collie River South Branch was diverted back through Lake Kepwari, with work finishing in 2019 (McCullough & Evans, 2024).

Another option for pit lake closure is to provide renewable energy such as hydropower as there isn’t a pre-existing ecosystem which may be impacted by the process (Lund & Blanchette, 2023). For example, the Kidston Gold Mine in far-north Queensland is partway through constructing a 250MW Pumped Hydro Energy Storage Plant to provide electricity to the northern grid, with its construction expected to finish in 2025 (Genex, 2024). The plant will utilise the existing pits (Wises and Eldridge as the upper and lower reservoirs respectively), along with the existing mine infrastructure (Figure 6).

Pit lake water can corrode infrastructure utilised for renewable energy which leads to costly repairs (Lund & Blanchette, 2023). Additionally, the void walls need to be structurally stable as hydropower involves frequently changing water levels. Despite this, with pre-existing voids and site infrastructure, the cost to transform a pit lake into a hydro plant is less compared to establishing a new one (Genex, 2024).

Active remediation (such as constantly treating or pumping pit lake water) is not an approved end goal option for pit lakes as there is a high cost for companies which are expected to invest in a closed mine (DEMIRS, 2023). Instead, the EPA and DEMIRS prefers companies to chose an end goal option which is feasible, has low environmental and safety risks, and low liability cost.

Integrate Sustainability understands the requirements of the Mining Act 1972 and Environmental Protection Act 1986, so if your company wants more information on your obligations for pit lakes, please call us on 08 9468 0338 or email us at enquiries@integratesustainability.com.au.

ISPL Insight – Pit Lakes in Western Australia

References

American Geosciences Institute. (2024). What are the main methods of mining? Retrieved from American Geosciences Institute: https://www.americangeosciences.org/critical-issues/faq/what-are-main-mining-methods

ANZECC & ARMCANZ. (2000). Australian and New Zealand guidelines for fresh and marine water quality.

APT Water. (2018). Dewatering techniques in mining: How pumps keep mines dry. Retrieved from APT Water: https://aptwater.com.au/dewatering-techniques-in-mining-how-pumps-keep-mines-dry/

Blanchette, M., & Lund, M. (2016). Pit lakes are a global legacy of mining: An integrated approach to achieving sustainable ecosystems and value for communities. Current Opinion in Environmental Sustainabiltiy, 23, 28-34. Retrieved from http://dx.doi.org/10.1016/j.cosust.2016.11.012

Bravus Mining and Resources. (2021). What is open cut mining? Retrieved from https://www.bravusmining.com.au/what-is-open-cut-mining/

Collie River Valley. (2024). Black Diamond Lake. Retrieved from Collie River Valley: https://collierivervalley.com.au/local-listings/black-diamond-lake/

DEMIRS. (2016). Case study: Derelict mine Western Australia Black Diamond rehabilitation project. Prepared by Department of Mines and Petroleum. Department of Energy, Mines, Industry Regulation and Safety.

DEMIRS. (2023). Mine Closure Plan Guidance: How to prepare in accordance with Part 1 of the Statutory Guidelines for Mine Closure Plans. DMIRS.

EPA. (2013). Environmental Protection Authority annual report 2012-2013. Environmental Protection Authority.

Genex. (2024). 250MW Kidston pumped storage hydro project. Retrieved from Genex Power: https://genexpower.com.au/250mw-kidston-pumped-storage-hydro-project/

Glencore. (2019). Fact sheet: Final voids. Glencore.

Grid Club. (2009). What is hard rock geology? Retrieved from Grid Club: https://gridclub.com/subscribers/info/fact_gadget_2009/1001/earth/rocks_and_minerals/113.html

Hinwood, A., Heyworth, J., Tanner, H., & McCullough, C. (2012). Recreational use of acidic pit lakes: Human health considerations for post closure planning. Journal of Water Resource and Protection, 4(4), 1061-1070. doi:10.4236/jwarp.2012.412122

Johnson, S., & Wright, A. (2003). Mine void water resource issues in Western Australia. Water and Rivers Commission.

Lund, M., & Blanchette, M. (2023). Closing pit lakes as aquatic ecosystems: Risk, reality, and future uses. WIREs Water, 10(4), 1-18. Retrieved from https://doi.org/10.1002/wat2.1648

McCullough, C., & Evans, E. (2024). Behind Western Australia’s first successful pit lake closure: Lake Kepwari. Retrieved from AusIMM: https://www.ausimm.com/bulletin/bulletin-articles/behind-western-australias-first-successful-pit-lake-closure-lake-kepwari/

McCullough, C., Schultze, M., & Vandenberg, J. (2020). Realising beneficial end uses from abandoned pit lakes. Minerals, 10(2). Retrieved from https://doi.org/10.3390/min10020133

Moser, B., Cook, P., Miller, A., Dogramaci, S., & Wallis, I. (2024). The hydraulic evolution of groundwater-fed pit lakes after mine closure. Groundwater, 62, 1-15. doi:10.1111/gwat.13419

Oldham, C. (2014). Environmental sampling and modelling for the predication of long-term water quality of mine pit lakes. UWA Publishing.

Parker, T. (2022). Pit lake planning critical to a sustainable mine site. Retrieved from https://www.australianresourcesandinvestment.com.au/2022/08/31/pit-lake-planning-critical-to-a-sustainable-mine-site/

South Western Times. (2019). Illegal use of Collie’s Lake Kepwari to be targeted. Retrieved from South Western Times: https://www.swtimes.com.au/news/south-western-times/illegal-use-of-collies-lake-kepwari-to-be-targeted-ng-b881069200z

Yancoal. (2020). The tale of Lake Kepwari: From open-cut mine to water sports landmark. Yancoal.