Enhancing Ecological Values of Coal Pit Lakes With Simple Nutrient Additions and Bankside Vegetation

This project builds upon the previous ACARP project (C19018) undertaken by the Mine Water and Environment Research Centre (MiWER) at study sites in Collie (Western Australia). Project C19018 identified that nutrients were limiting algal productivity, water quality improvements, and the development of ecosystem values in coal pit lakes. We also found that simple additions of nutrients (carbon, nitrogen and phosphorus), resulted in significant increases in algae. Algae form the base of natural lake aquatic food chains as food for macroinvertebrates (such as aquatic insects and zooplankton), with subsequent nutrient transfer to higher consumers such as fish and water birds. Algae also offer a number of 'phytoremediation' processes that may improve pit lake water quality impacted by acid and metalliferous drainage (AMD) that impedes mine closure.

Developing environmental values could be a valid alternative for pit lake closure criteria, rather than simply meeting (often difficult) water quality guideline closure criteria as the principle focus for relinquishment. Pit lakes showing particular environmental values could also provide opportunities to improve regional biodiversity. In turn, these opportunities will reduce environmental and social risk of these landforms at closure, facilitating a faster and easier relinquishment back to the state.

This ACARP project (C21038) focusses on the importance of the pit lake catchment at mine closure in supplying nutrients and organic matter to pit lakes. Essentially, we aim to determine how maximising use of this catchment material can help the coal industry meet biodiversity objectives at closure.

Experimental nutrient and bulk organic matter additions were evaluated to examine the potential of these materials to improve biodiversity in coal pit lakes across a range of water qualities; from acidic to neutral. Specifically, the project sought to:

· Determine which nutrient(s) is/are limiting in at least two contrasting coal pit lake types (acidic to neutral), and identify nutrient concentration thresholds for increased algal growth;

· Test whether additions of C, N and P prior to closure can significantly improve ecosystem values in pit lakes of differing acidities; and

· Quantify the role of bankside revegetation in providing nutrient inputs and aquatic habitat post‐closure for increasing pit lake environmental values (e.g., aquatic biodiversity).

We conducted a survey across a range of Collie pit lakes of differing acidities to determine the importance of catchment organic material to lake food webs. This survey allows us to assess how historic absence of rehabilitation versus current revegetation strategies and catchment designs provide opportunities for pit lake aquatic ecological restoration through organic matter accumulation in the lakes.

A survey of the macroinvertebrate communities of five Collie pit lakes has been completed, using a very high sampling intensity to ensure a robust estimate of total species present in each lake. The study relates the presence and abundance of these key aquatic ecosystem indicators to habitat and water quality variables. Of particular interest, is the apparent differences between taxa found in litter bags versus those caught around the edge.

Key recommendations from the project relate to maximising the value of the catchment to provide long term improvements in pit lake water quality. This approach contrasts with current practices which seek to minimise pit lake catchment areas. Catchments supply pit lakes with nutrients, particularly bulk organic carbon (leaves and branches). Our research suggests that enhancement of lakes with this type of material will increase biodiversity and abundance of macroinvertebrates regardless of the water quality. We believe that these positive changes will occur until the water quality becomes the predominant factor controlling the populations. We therefore recommend:

· Pit lakes are planted with riparian and aquatic plant species as part of revegetation;

· Littoral margins are redesigned to be better habitats for aquatic biota; and

· Organic materials are added to the lakes, when available, both pre- and post-filling.

Our research provides mining companies with opportunities to broaden the closure conversation with regulators beyond water quality to include ecological values, when water quality remediation is difficult. These opportunities are best implemented prior to filling but could be retrofitted to older pit lakes for relatively low cost.