Coal pit lake closure by river flow through: risks and opportunities

Mark Lund, Melanie Blanchette, Colm Harkin & Paul Irving

Project background

This project (C23025) builds upon the previous ACARP project (C21038) undertaken by the Mine Water and Environment Research (MiWER) Centre in Collie (Western Australia). In project C21038 we identified that nutrients were limiting algal productivity, water quality improvements, and the development of ecosystem values in coal pit lakes. Small catchments commonly associated with pit lakes appeared to limit natural inputs of nutrients – particularly carbon. Terrestrial leaf litter and other coarse organic material stimulated macroinvertebrate biodiversity. There were increases in taxa abundance and richness and algal productivity, in the pit lakes, despite no improvement in overall water quality. The Collie pit lakes are acidic (pH down to 2), with high concentrations of some metals (such as aluminium) and range from fresh to brackish, yet contain little sulphate. The outcomes of the previous ACARP project (C21038) suggest that developing environmental values (e.g., increasing aquatic biodiversity) could be a valid alternative to meeting (often difficult) water quality guidelines for pit lake closure criteria and subsequent relinquishment.

Connecting a pit lake to natural drainage lines could increase the effective catchment size of the lake. The South Branch of the Collie River was diverted around the pit that would eventually form Lake Kepwari. In 2011, the diversion around the lake failed during storm flows, allowing river water to pass through the lake before returning to the river downstream. Downstream water quality parameters were within ANZECC/ARMCANZ (2000) guidelines for the protection of 80% of ecosystem values. Additionally, the flow-through event appeared to have improved the water quality (increased pH) and environmental values (macroinvertebrate biodiversity) of Lake Kepwari. Following the 2011 breech, a three-year trial allowing the lake to be deliberately connected to the seasonal Collie River was approved by Department of Water (WA).

Project objectives

ACARP project C23025 will use this unique trial to assess the impacts of connecting a river to a pit lake, particularly on downstream aquatic ecosystems. This project is also a trial of the concept that increasing effective catchment size has a positive effect on lake ecology.

The seasonal Collie River is degraded by secondary salinization, resulting in occasional highly saline flows. In ACARP project C23025, we will also assess the effects of saline river water on Lake Kepwari.

The main objective of this project is to determine the risks and opportunities associated with diverting a river through a mine pit lake. Specifically, we will:

  1. Determine the downstream effects of pit-lake decant, with a particular focus on environmental and amenity values.
  2. Determine the effects river of inflow on environmental values and water quality within the pit lake. (Essentially a field-scale demonstration of a key finding from C21038 that larger catchments should enhance pit lake water and environmental quality).
    1. Understand the impact of variably saline river water on mixing within a moderately saline pit lake.
  3. Develop a national standard protocol for seasonal river monitoring that could be applied by the coal industry to manage river flow-throughs (either accidental or planned), as a part of mine closure strategy.

Current activities

To commence this project, we have focused on site selection for monitoring the Collie River South. Sites have to be readily accessible, representative of the aquatic habitats of interest, and reflective of the overall nature of the catchment. We have also identified another local flow- through system for inclusion in the monitoring program. This new system is a small stream – topped up by dewatering flows from Griffin Coal operations–that flows through Stockton pit lake. Increasing replication (i.e., 21 sites across two flow-through systems) will enhance our ability to detect the impacts of river flow-through on river and pit-lake systems. In the process we have identified an additional 30 riverine potential sites in the Collie basin that could be useful for future research.

Regular monitoring of Lake Kepwari (as part of the trial conditions) occurs quarterly and we have added a similar monitoring program for Stockton Lake. Currently we have sampled Lake Kepwari five times and Stockton three. Preliminary data from Lake Kepwari indicates that the lake is stratified continually by salinity, enhanced by temperature stratification. Conductivity of the bottom waters is highest in March and June, possibly due to saline groundwater inflows. River inflow between August and October appears to slightly dilute the bottom waters (although the exact mechanism is not currently understood). Importantly, bottom pH is >6 during October, but then appears to return to 4.5 by June probably due to incoming acidity from groundwater. The installation of continuous monitoring gear in both lakes should help clarify the processes responsible for these water quality changes. We have used off-the-shelf monitoring gear that provides detailed insight into physical (stratification) and chemical (light, temperature, conductivity and dissolved oxygen) changes in a very economical package that could be used in any pit lake.

Value-adding

We have also value-added to the ACARP project with Edith Cowan University- funded support for an assessment of the impacts of catchment activities (mining, agriculture) on aquatic microbes in the Collie catchment. In November 2014, we hosted colleagues from Montana State University (USA) with whom we are collaborating on the microbial work. The microbial work is likely to prove highly beneficial to the mining industry by providing an economic way of understanding microbially-mediated environmental processes as well as developing microbes as tools for environmental assessment. In April 2015, we visited our colleagues at Montana State University to discuss and test how methodological differences might influence the microbial analysis.

Knowledge transfer

A paper on approaches to pit lake closure, based on ACARP projects C21038 and C23025 was presented at the International Mine Water Association (IMWA) Conference in Xuzhou, China in 2014. A copy of the paper can be obtained for free from http://imwa.info/docs/imwa_2014/IMWA2014_Lund_720.pdf. Abstracts based on work conducted in C21038 and a poster on our microbial work have been presented at ICARD/IMWA 2015 in Chile. Presentations on previous and current ACARP projects were made to the Hunter Coal Environment Group (NSW) in February 2015.

Figure 1. Section of Melaleuca- dominated river typical of SW Western Australia (Collie River South flowing into Lake Kepwari). Figure 2. Creek flowing into Lake Stockton.

Geochemical Modelling of Pit Lake Water Chemistry to Support Management Decisions

Mike Mueller (Hydrocomputing, Germany), Katja Eulitz (Hydrocomputing, Germany), Clint McCullough (MiWER), Mark Lund (MiWER)

How does pit lake water quality and depth change under different management scenarios?

Stage 1. Selection of appropriate model

This model will maximise the use of the currently available data for model creation and validation. The current preference is not to focus on a single pit lake and model in detail but develop a simpler model that can be easily applied to cohorts of Collie pit lakes identified by the inventory collection and conceptual modelling. This general model would be less detailed but more suited to the low input knowledge environment of the Collie groundwater region and would support important pit lake and ground water management decisions.

Stage 2: Model parameterisation and testing.

There is no option for detailed validation of the model at this stage, other than through use of existing historic and collection of new data sets arising from Task 1.

Stage 3: Scenario testing.

A series of different scenarios will be run to demonstrate model outputs and to test alternative pit lake management and environmental strategies for the different pit lake cohorts.

 

    

Figure: Important processes in pit lakes


Model: Schematic of model coupling in MODGLUE

Modelling Tools

This project is using the model PITLAKQ which is a coupled model that combines the groundwater model PCGEOFIM, the lake hydrodynamic and water quality model CE-QUAL-W2 and the hydro-geo-chemical model PHREEQC.

PITLAKQ is capable of modelling all processes that are important to pit lake water quality. Fig. 2 shows these processes that include weather induced hydrodynamics with thermal layering, heath and gas exchange with the atmosphere as well as flow, transport and chemical changes in the subsurface. In addition, a wide variety of water quality processes in the lake water such as biological processes including algae growth and nutrient dynamics as well as equilibrium and kinetic chemical reactions can be modelled. Pit lake specific chemical reactions may be defined by means of an extendable hydro-geo-chemical database and rate limited reaction paths.

PITLAKQ has already been applied to a variety of different mining pit lakes under different scenarios, producing results to guide surface and groundwater management.


Video: Model of Lake Kepwari during filling showing temperature changes


Video: Model of Lake Kepwari showing pH changes during filling

Outputs

Müller, M.; Eulitz, K.; McCullough, C. D. & Lund, M. A. (2010). Mine Voids Management Strategy (V): Water Quality Modelling of Collie Basin Pit Lakes. Department of Water Project Report MiWER/Centre for Ecosystem Management Report 2010-10, Edith Cowan University, Perth, Australia. 95pp. Unpublished report to Department of Water. link

McCullough, C. D.; Müller, M.; Eulitz, K. & Lund, M. A. (2011). Modelling a pit lake district to plan for abstraction regime changes. Mine Closure 2011: Proceedings of the Sixth International Conference on Mine Closure. Lake Louise, Canada. Fourie, A. B.; Tibbett, M. & Beersing, A. (eds.), Australian Centre for Geomechanics (ACG), Perth, Australia, 581-592pp. PDF

Müller, M.; Eulitz, K.; McCullough, C. D. & Lund, M. A. (2011). Model-based investigations of acidity sinks and sources of a pit lake in Western Australia. Proceedings of the International Mine Water Association (IMWA) Congress. Aachen, Germany. 41-45pp. PDF

 

Ecotoxicity limitations following liming and nutrient enrichment to remediate acid mine lakes

Clint McCullough (MiWER), Luke Neil (Curtin), Mark Lund (MiWER), Jess Sackmann (CWR, UWA), Dr. Anas Ghadouani (CWR, UWA), Dr. Yuri Tsvetnenko (Curtin), Dr. Jim Ranville (DCG-Colorado School of Mines), Prof. Louis Evans (Curtin)

Is liming and enhanced primary production able to reduce ecological toxicity and increase biodiversity of Collie lakes?

Twelve 1,200 L mesocosms at ECU have been filled with a 40 mm layer of lake sediment from the bottom of the fast river-filled Lake Kepwari. This representative sediment layer has then been covered with Lake Kepwari water. Treatments have been allocated in a randomised two-way factorial design to test the effects of liming, phosphorus enrichment and combined liming and phosphorus amendment on different aspects of the AMD water chemistry, ecotoxicity and ecology.

                             Not limed    Limed

No nutrients    U U U    U U U

Nutrients            U U U    U U U

Figure: Experimental Design

Photo: Collecting sediment from Lake Kepwari


Photo: The mesocosms back established at ECU

This was a collaborative multidisciplinary project with co-supervised students at Curtin University of Technology and University of Western Australia. The Edith Cowan University team examined water chemistry, sediment and periphyton dynamics, including the effect of liming and enhanced primary production upon dissolved heavy metal and nutrient concentrations, alkalinity and pH. Jess Sackmann examined correlations between phytoplankton community water quality, and Luke Neil examined the effect of different treatments on aquatic ecotoxicity between each other and over time.

Funding:

Part of L. Neil’s Ph.D. project and J. Sackmann’s Honours Project. Partially funded by Curtin, Edith Cowan, UWA and the Centre for Sustainable Mine Lakes.

Outputs:

Neil, L. L. (2008). Bioassay assessment of mine pit lake water for aquaculture and biodiversity conservation, Ph.D. thesis, Curtin University of Technology, Perth, Australia. 298pp. link

Neil, L.; McCullough, C. D.; Lund, M.A.; Tsvetnenko, Y. & Evans, L. (2009). Bioassay toxicity assessment of mining pit lake water remediated with limestone and phosphorus. Ecotoxicology and Environmental Safety. 72: 2,046-2,057.link (Highlighted article).

Neil, L.; McCullough, C. D.; Tsvetnenko, Y. & Evans, L. (2006). Toxicity assessment of limed and phosphorus amended mine pit lake water. RACI/ASE Interact 2006 conference. Perth, Australia 24-28 September. PDF

Sackmann, J. (2006). The effect of experimental liming and nutrient addition on phytoplankton of an acidic mine lake, B.E. (hons) thesis, University of Western Australia, Perth, Australia. 50pp.

Influence of phosphorus and organic carbon on benthic productivity and ecological diversity in coal mine lakes.

Mark Lund (MiWER), Naresh Radhakrishnan (MiWER),  Clint McCullough (MiWER), Lorraine Wyse, Digby Short (Premier Coal)

Can amendments of organic matter and nutrient improve ecological values of abundance and biodiversity in coal mine lakes?

The objective of this project is to examine whether pit lake ecosystem values rather than water quality could be considered by regulators as criteria for accepting pit lake closure and relinquishment back to the state. Specifically, the project seeks to:

  • determine which nutrient is limiting in each of the two lake acidity types and what, if any, thresholds exist for the amount of nutrient that needs to be added to increase algal growth.
  • test whether additions of simple nutrients (N and P) can encourage significant improvement of ecosystem values in pit lake types  with different acidity;
  • examine the role that bankside vegetation may play in providing inputs of nutrients and habitat for increasing aquatic biodiversity environmental values.

 

Photo: Riparian vegetation around pit lakes is often sparse; but may be very important to lake ecosystem function.


Photo: Benthic chambers are used to measure benthic primary productivity.

 

Photo: Water quality data are collected by submersible logging sondes.

 

Outputs

Lund, M. A.; Van Etten, E. J. B. & McCullough, C. D. (2013).Importance of catchment vegetation and design to long-term rehabilitation of acidic pit lakes. Proceedings of the International Mine Water Association (IMWA) Congress. Bunbury, Australia. Brown, A.; Figueroa, L. & Wolkersdorfer, C. (eds.), International Mine Water Association (IMWA), 1029-1034pp.