Defining end uses for pit lakes is seen as a critical step towards their successful closure. Possible end uses for pit lakes include, as aquatic ecosystems, for various forms of power generation, recreational activities and agricultural/aquaculture uses. In many cases, there is considerable interest by regulators and the community to ensure that pit lakes are turned to productive uses, especially as once mining ceases opportunities for income and jobs in the local area are likely to contract. In our review of end uses (Lund and Blanchette 2023) we concluded that closure as an aquatic ecosystem was the lowest risk option for all stakeholders. Successful closure of pit lakes is currently based around notions of safe, stable, non-polluting, and sustainable. What characteristics would be expected for a pit lake to have a sustainable aquatic ecosystem?

We reviewed data on biodiversity and water quality collected by our team since the early 1990s across former coal mine pit lakes across Australia (Queensland, New South Wales, and Western Australia). This data has largely been collected using nearly identical methods. We interrogate this data (published and unpublished) to draw conclusions about what can be expected in terms of trajectories for biodiversity and water quality over years to decades when pit lakes are not rehabilitated. Additionally, we have similar data for two pit lakes that have been rehabilitated in ways that from an ecologist’s perspective should lead to significant increases in biodiversity, including revegetation of the catchment, increased catchment size, establishment of riparian and littoral areas.

Our data analysis showed that rehabilitation only marginally increased richness and abundance of the taxa studied compared to other nearby sites, over the time frames studied. In the absence of rehabilitation, water quality (except for salinity) and biodiversity remained largely unchanged over decades. Most taxa collected were cosmopolitan and pollution tolerant across all pit lakes rehabilitated and non-rehabilitated sampled.

In this talk, we discuss what might be responsible for this failure of ecosystem development in the absence of rehabilitation. Although we had few rehabilitated pit lakes to base our conclusions on, it does appear that rehabilitation increases biodiversity and can improve water quality. The rate of improvement for these rehabilitated lakes appears slow (in the order of decades), creating challenges for regulators defining closure criteria (success) for aquatic ecosystems.

W

Water management, in particular, perpetual water treatment, whether caused by the mine waste or orebody geochemistry or required due to laws and regulations, has been a challenge for mines going into closure and for legacy sites. The treatment of water and the potential social, economic and environmental impacts after the mine has stopped operating is a challenge most mining companies face. In many cases, the challenges are created by bad closure planning and promises made to stakeholders during the operational life without thinking of the consequences of these promises post-closure. This paper presents case studies of the various challenges associated with water management during closure and how they were dealt with, good and bad outcomes, on sites across Africa, USA, Canada, Europe, and South America.

Closing a mine site is very rarely a walkaway scenario, especially when it comes to water treatment and the legacy issues it creates. Through various de-risking projects, work has been conducted on reviewing closure plans at sites approaching closure as well as legacy sites already in closure. Through these reviews, the highest costs and biggest headache associated with closure were typically, the commitment to perpetual water treatment, the treatment method selected, and the reasons that led to the decisions. In many cases, the reason for the perpetual water treatment requirements was due to social commitments made by the mine during operation without thinking of the consequences on closure. In other cases, as in the USA and Canada, the closure plan and perpetual water treatment were aligned to the various legislative guidelines and requirements without thinking out of the box or challenging the norm.

Steering away from active treatment as being the default option and out-of-the-box thinking could have solved most of the need for this expensive closure option. For example, using a combination of passive treatment options and the diversions and separation of streams at a site in Peru could potentially reduce the site closure liability by USD 150M.

In most of the case studies presented in this paper, sound planning and management of water with an eye on closure could have avoided the associated social and environmental impacts. In this paper, an outline is presented of the various challenges identified and how they were dealt with to reduce future liabilities and de-risk existing closure projects.

This study builds upon prior research on a pilot-scale passive treatment (PT) system for manganese (Mn) and zinc (Zn), operated at circumneutral pH (6.5-7.5) at a legacy mine in northern Japan. The pilot-scale PT successfully removed Mn and Zn with removal efficiencies of 97% and 89%, respectively with for hydraulic retention time (HRT) of 2 days, however the optimum HRT has not yet been finalized. Therefore, we aim to (1) determine the treatment and mechanisms of Mn and Zn in the pilot-scale PT, (2) employ geochemical modeling to comprehensively understand the role of HRT in the long-term implications of passive treatment.

Detailed analyses of sludge in the pilot-scale PT were conducted, including X-ray diffraction, X-ray fluorescence, X-ray absorption spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. We constructed an inverse kinetic model for the Mn and Zn reactions in the pilot-scale PT based on field measurements to determine kinetic rate constants (k₁), mass transfer coefficients (k_m), and distribution coefficients (K_d). The geochemical model was constructed using Phreeqc and the Wateq4f database coupled with parameter estimation (PEST).

The analytical data highlights the role of Mn removal mechanisms by Mn-oxidizing bacteria, which facilitate birnessite formation. For Zn removal, mechanisms include co-precipitation with MnO₂, forming woodruffite, and adsorption on the birnessite surface. In addition, the inverse model successfully obtained k₁, k_m, and K_d, which are suitable HRT optimization in the pilot study, without the need for additional experiments. The changes in HRT (2 days, 0.5 days, and 0.3 days) decrease Mn removal capacity, but Mn remains below the Japanese effluent limit. However, the shorter hydraulic HRT (0.3 days) was not sufficient to treat Zn in the pilot treatment. The geochemical model suggests that an HRT of 0.5 days is particularly applicable for achieving optimal removal rates for both Mn and Zn in the pilot-scale PT.

This research addresses the critical need for sustainable AMD treatment in legacy mines and provides valuable insights into efficient Mn and Zn removal through geochemical modeling. It offers essential information for the development of PT strategies and environmental management, particularly for real-scale implementation. The utilization of geochemical modeling contributes to a deeper understanding of these treatment mechanisms and HRT optimization.