Mining sustains modern living. The materials for building solar panels, wind farms, sustainable homes and computers are obtained through large-scale mining, with thousands of open pit mines created across the globe.
Pit lakes are therefore inevitable by-products when abandoned open-cut mines are flooded by rain and groundwater. Some of these lakes are toxic, posing risks to adjacent communities and ecosystems.
There is a call for an ‘integrated’ approach to pit lake management from design to closure. Current in situ treatment options such liming of acid waters, pumping out water from pit lakes and actively treating the water with chemicals are both expensive and unsustainable.
Strategies that are integrated and sustainable include engaging ecologists in pit lake design and prioritising ecological progress such as planting vegetation in and around the water to facilitate passive treatment of water and create habitat for biodiversity. This type of mine closure planning can empower communities with post-mining options.
Kathleen Southway spoke to Associate Professor Mark Lund, Principal, Mine Water and Environment Research Centre (MiWER), Edith Cowan University, about his research into pit lakes.
Q. Tell us about the work you do through the Mine Water and Environment Research Centre.
A. I am the Principal of the Mine Water and Environment Research (MiWER) Centre at Edith Cowan University. To my knowledge, MiWER is the only research group interested in the environmental and ecological aspects of mine-affected waters, including pit lakes, discharges to natural waterbodies and river diversions.
We have a focus on the rehabilitation and remediation of mine-affected waters post mining.
Over the years, we have worked with the Australian Coal Association Research Program (ACARP) to develop approaches to treat acid mine drainage across the country, especially in pit lakes, using low-cost biological approaches.
More recently, we turned our focus towards understanding the biology of saline pit lakes for closure.
Q. How does water in Australian pit lakes become saline?
A. When open-cut mining occurs below the natural water table, a combination of rainwater, surface runoff and groundwater fills the open-cut pit, creating a pit lake when mining activity ceases.
In many parts of Australia, the nature of surrounding geologies mean water quality within these pit lakes are generally good, meaning they are low in potentially toxic metals and metalloids.
However, in areas where evaporation exceeds rainfall, these lakes are likely to slowly become concentrated over time, resulting in increased salinity.
The issue of increasing salinity is made worse when the lakes are typically groundwater sinks, with no surface discharge, which means water goes in but only leaves via evaporation.
Sometimes, this is a natural reflection of the local area but it is often a design feature to prevent possible discharge into downstream groundwater or surface waters.
Salt consists of sodium and chloride, however it is the chloride that is the main issue as it is a conservative ion. It does not react readily with other elements or biologically, meaning that there are few natural removal pathways.
As chloride is not easily removed, small amounts that enter the lake build up over time, causing rising salinity.
Most of Australia’s pit lakes are likely to be saline. There is currently little empirical data available to show how salty, and how quickly these pit lakes become salty, hence the predictions are still largely being done through modelling.
Our research group is getting out into the field to collect real data on the chemistry and ecology of these saline pit lakes.
Q. Could you explain more about the backfilling process?
A. As part of the original mine approval, many mines were not required to plan for the filling-in of open-cut pits. There has been a lost opportunity to incorporate the backfilling as part of mine planning.
‘Backfilling as you mine’ is generally the most cost-effective approach.
At many sites, where the mine plans do not cover backfilling, considerable effort and expense are required to fill in pit lakes, not to mention the dust and greenhouse emissions from machinery used to undertake the backfill.
There are also risks associated with the creation of new groundwater aquifers and groundwater drainage lines through crushed porous backfill, which could result in discharges from the site years after closure.
Water quality associated with the newly created aquifer could be problematic.
A compromise would be to partially backfill the pit to reduce its size and allow better sculpting of the sides of the pit to make it more ‘lake-like”.
Q. What studies is MiWER undertaking with existing pit lakes?
A. Australia has a large number of naturally saline lakes ranging from brackish to hypersaline, which is saltier than the ocean. Most of these lakes are shallow.
Ecologists have found a large diversity of organisms living in these natural lakes, making them valuable ecosystems.
As salinity increases, the diversity of most species tends to decrease, especially at levels approaching seawater.
At more intermediate levels, Australia has a range of natural salt-tolerant species.
Hypersaline lakes occur naturally in Australia, meaning there could be potential ecological value to having hypersaline pit lakes.
MiWER is currently evaluating what biodiversity naturally occurs in unrehabilitated pit lakes, with a view to determining what rehabilitation could be undertaken to enhance that biodiversity.
MiWER is also collecting valuable data on the long-term changes in water quality that might be occurring in these pit lakes to better inform model predictions.
At MiWER we believe that saline pit lakes are a potential closure option that creates valuable habitat, as well as opportunities for future uses of these lakes such as tourism and recreation.
I am keen to focus on ecological values of pit lakes as this minimises the risks when the rehabilitated pit lake is sold or returned to the state.
While there could be opportunities for pit lakes to be used as pumped hydropower storages, however these are likely to be the exception rather than the rule.
Q. Outline some of the challenges to having saline pit lakes.
A. The challenges for saline pit lakes are low community acceptance in some regions, low levels of ecological data on existing saline pit lakes, and almost no research on how to enhance the ecological values of pit lakes.
If the industry is keen to explore the potential of saline pit lakes as a closure option, regulators and the public will need to be convinced through long-term data sets showing the ecological benefits and risks posed by these lakes, how water quality changes long-term and if these lakes are a safe, stable and non-polluting option.
