The mine closed in 1997 at the Western No. The site is now known as Lake Kepwari, a recreational lake that supports activities such as swimming, boating and camping and reaches a maximum depth of about 65 meters. 5B, or WO5B, pit in Collie, Western Australia, leaving behind a large open mine void. A case study published in the AusIMM Bulletin describes it as Western Australia’s first successfully relinquished pit lake.
The case study also reported that the lake could improve some aspects of downstream water quality relative to the water entering it, particularly through the trapping of sediment and certain phosphorus compounds. At WO5B, most reactive overburden and exposed coal seams were covered with waste rock. The first results were disappointing. Acid continued to form as exposed and reactive materials interacted with air and water, while acid mine drainage also left the lake short of phosphorus, limiting algal growth. One visual benefit was the water’s unusually high clarity, which resulted from the lack of phytoplankton. Nature then forced a change in approach. The event indicated that maintaining the lake as a flow-through system could provide a longer-term closure strategy. The results showed that downstream river water quality was not significantly degraded by the flow-through system. The largest volumes of water leaving the lake generally occurred when the river was already flowing strongly, allowing greater dilution. The monitoring also indicated that the lake could remove sediment and some nutrient forms from the water as it passed through. Seasonal river flow now helps neutralize acidity generated by acid mine drainage in the backfilled waste rock and mine walls. Monitoring after the flow-through system was implemented found that lake pH was higher than historic levels and was trending upward. This did not mean that the river itself was pristine: the case study notes that the historical river channel was already degraded, which was one reason the flow-through approach posed a relatively low additional risk to downstream water resources. In another lake with acidic, metal-rich water, animal communities appeared to be associated with leaf litter and other organic material. The researchers suggested that plant material could provide a food source for organisms living in pit lakes despite poor water chemistry.
Collie lies about 160 kilometers south-southeast of Perth. More than 100 years of coal mining in the area has produced at least 13 pit lakes, all affected by acidic and metalliferous drainage. Parts of the mine-void edge were backfilled and graded to create beaches and an island, while more than 60 species of native vegetation were established through direct seeding. River water initially pushed the lake’s pH above 7, but the pH later fell below 4 after the river inflows stopped. In August 2011, heavy rain caused the Collie River South Branch to overtop the diversion channel and the embankment separating it from Lake Kepwari. About 2 billion liters of river water flowed through the lake and substantially improved its water quality and ecosystem conditions. A trial conducted from 2012 to 2014 tested that approach. The permanent flow-through arrangement was approved in November 2018, and the civil works were completed in June 2019. A related 2019 study in Mine Water and the Environment provides a glimpse below the surface. Researchers sampled three pit lakes in the Collie area in April 2013 and found that Kepwari had the greatest diversity of small aquatic organisms among the lakes examined, with an average of about seven different taxa per sample compared with roughly three at Lake Stockton.
Because of low rainfall, quarterly monitoring of the lake and river, and annual reporting to regulators and the community, it included more than two years of flow, a year without flow.
