Coal Pit Lake Closure by River Flow Through: Risks and Opportunities

In 2011, the river diversion channel around Lake Kepwari, a former open cut coal pit in Collie (Western Australia) was breached during a storm event. The effects of the breach showed potential for improving water quality in the pit lake with limited impact on the river downstream (according to the use of ANZECC/ARMCANZ (2000a) guidelines for livestock drinking, recreational and aesthetic and 90% protection of aquatic ecosystems (McCullough et al. 2013)). One of the potential closure land uses of Lake Kepwari was the development of the lake as a water ski park, however until the breach lake waters remained acidic. After the breach, Premier Coal Ltd (PCL) was allowed to trial a closure strategy that involved controlled flow-through of the lake by the Collie River South Branch (CRSB). This project was commenced to investigate the impacts of the trial and subsequent monitoring phase on water quality and biota in the lake and river between 2014 and 2016.

Stockton Lake, another pit lake in Collie was also chosen for study as it was connected to a small intermittently-flowing stream (EW1) and received water from Ewington Mine.

The key objectives of the project were to:

• Determine the impacts of river flow-through on the pit lakes (particularly Lake Kepwari);
• Determine the impacts of pit-lake decant on the downstream rivers;
• Investigate how salinity from secondary salinization in the CRSB might influence Lake Kepwari; and
• Evaluate our monitoring approach for use as a standard monitoring protocol of seasonal rivers.

Both lakes were sampled at three sites quarterly during 2014-2016 to generate vertical physico-chemical profiles; top and bottom water samples were collected for analysis of metals/metalloids and major ions. Phytoplankton was collected from surface waters and zooplankton was collected via vertical tows. Monitoring chains anchored to the bottom of each lake at the deepest point collected measurements of temperature, conductivity, dissolved oxygen and depth throughout the water column from sediment to surface at 15-60 minute intervals. Five sites around the edges of each lake were also sampled quarterly for macroinvertebrates and diatoms.

The rivers were sampled quarterly for one year between October 2014 and September 2015 (four sites above and below Stockton Lake, six sites above and below Lake Kepwari, with an additional site below the confluence of both rivers). At each site, a 50m reach of the river was sampled for water quality, physico-chemical parameters, riparian condition, macroinvertebrates within each major aquatic habitat, as well as composite samples for environmental DNA (microbes; Archaea and Bacteria) and sediment chemistry across all in-stream habitats (including dry riverbed). Water, when present, was collected for pelagic microbes.

A national standard protocol for seasonal river monitoring is presented. The nature and location of a river flow-through as a closure strategy will heavily influence the design of any monitoring program. This project has shown how measurement of key indicators of water and sediment quality as well as a suite of bioindicators such as fish, riparian condition, macroinvertebrates and microbial communities can provide a detailed picture of the impacts through multiple lines of evidence.