Potential for mine water sharing to reduce unregulated discharge

Journal of Cleaner Production 131 (2016) 133e144

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Potential for mine water sharing to reduce unregulated discharge Lei Gao a, *, Caihong Hou a, b, Yun Chen c, Damian Barrett d, Dirk Mallants a, Wanggen Li e, Rui Liu c, f a

CSIRO Land and Water, Private Mail Bag 2, Glen Osmond, SA 5064, Australia Department of Customs Management, Shanghai Customs College, Shanghai 200240, China c CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia d CSIRO Energy, GPO Box 1666, Canberra, ACT 2601, Australia e School of Mathematics and Computer Science, Anhui Normal University, Anhui 241003, China f Key Laboratory of Geographic Information Science (Ministry of Education), East China Normal University, Shanghai 200241, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 August 2015 Received in revised form 29 April 2016 Accepted 11 May 2016 Available online 20 May 2016

Australia's mining sector periodically suffered huge losses both directly and indirectly from mismanagement of mine water during extreme climatic events. Mine water managers still lack cost-effective tools and strategies to manage both climate-influenced drought and flooding challenges. This paper aims to answer a fundamental question in mine water management: how much can a water sharing approach do to reduce unpermitted (unregulated) mine-affected water to overflow to the environment on a regional scale? To this end, we built a climate-driven hierarchical systems model (C-HSM) of sixteen coal mines in the Bowen Basin of Queensland, Australia. The C-HSM simulated the dynamics of the mine water systems, which was then the basis to assess the potential of mine water sharing to reduce unregulated discharge. We found that mine water sharing could greatly cut down regional unregulated discharge during the 2010e2011 wet season (which included an extreme flooding event). The cost of building such a regional-scale sharing infrastructure for redistributing water was found to be competitive to the lost revenue due to reduced coal production. The capital cost could potentially be further reduced by using existing water pipelines e such as those used for coal seam gas water management e or considering cheap transporting options. Once the sharing infrastructure has been installed, it can prevent periodic suffering of mines from climate extremes. Combined with an actively regulated discharge strategy, the water sharing approach could almost completely eliminate the unregulated discharge and maintain mine water storages at a secure level. The capital and operational cost for the combined approach can be considerably reduced and the shared water could be stored at other sites (mines, water holding ponds from coal seam gas industry) and used for water-limited periods. This is the first work that explores the theoretical potential of mine water sharing to mitigate the risk of unregulated discharge. The work presented here reveals that mine water sharing could help mine water managers respond to a sudden change in extremes such as switching from extreme ‘dry’ to extreme ‘wet’ conditions. A combined mine water sharing strategy would be worth considering by mine water managers, due to its advantages in regards to safety, relatively lower implementation costs, its effectiveness of reducing unregulated discharge that may cause environmental harm and conserving excess water for future water-limited periods. The ultimate value of water savings through a sharing scheme may be considerable as it influences mining industry's social license-to-operate in the long term, especially under conditions where there is a water security issue of decreasing availability and increasing competition. The work also highlights the benefits of systems modeling in supporting mine water managers with strategic decision making. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Mine water management Water sharing Water trading Climate extremes Systems modeling Coal seam gas water management

1. Introduction * Corresponding author. CSIRO Land and Water, Private Mail Bag 2, Waite Road, Glen Osmond, SA 5064, Australia. Tel.: þ61 8 8273 8109; fax: þ61 8 8303 8750. E-mail address: [email protected] (L. Gao). http://dx.doi.org/10.1016/j.jclepro.2016.05.061 0959-6526/© 2016 Elsevier Ltd. All rights reserved.

The mining industry consumes massive volumes of water (Prosser, 2011; Gunson et al., 2012). The consumption can severely influence local supplies if mining activities occur in water-stressed

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areas, such as arid/semi-arid areas or where there is significant competition with municipal, agricultural and other industrial demands (e.g., Freitas and Magrini, 2013; Gao et al., 2013b; Kirby et al., 2014, 2015; Wang et al., 2014). In addition, it has been found that a rapid increase in production and significant climate variability have brought on a pressure of securing water within the mining industry ^ te et al., 2010; Loechel et al., 2013). (Co In the face of these challenges occurring over decades, some mines have explored their potential to recycle mine-affected/ contaminated water (worked water) by retaining water within a closed cycle or reserving water in a storage facility. Especially after a decade of drought, most mines in Australia have been designed with raw water and worked water storage facilities to fully utilise water and catch as much on-site runoff as possible. Nevertheless, during periods of excess water supply, this may result in a range of environmental, social, and economic risks (DRET, 2008; Kemp et al., 2010), which are expressed through unregulated discharge of worked water (unregulated discharge here refers to unpermitted discharge of worked water into the environment, which is usually caused by significant rainfall events). The negative impacts were witnessed during recent flooding periods in Queensland, Australia. For instance, during 2010e2011, the discharge from flooded coal mines in the Bowen Basin in Queensland led to a much higher mortality rate of marine species and brought potential health issues to the local communities (Queensland Floods Commission of Inquiry, 2012). The mines were inundated by overflows from their own reservoirs and incoming floods, and the resulting loss of coal exports was estimated as A$5.7 billion (Queensland Government, 2011). Traditional measures such as developing water storage infrastructure, are unable to entirely address the spill-over of worked ^te et al., 2010). water caused by heavy and prolonged rainfall (Co Innovative approaches are required to remove or mitigate the risk of unregulated discharge (Liu et al., 2011), which may pour into pits, leak into underground areas, and flow into nearby rivers. Gao et al. (2013a, 2014a) explored the strategy of actively regulated discharge in advance to avoid the spill-over during the wet season. The strategy was evaluated at a coal mine in the Bowen Basin, and the results showed that the risk of unregulated discharge could be reduced. However, the action of actively regulating discharge is strongly restricted by the characteristics of the receiving water, the water quality of the discharge, and the weather predictions. In addition, coal mines need to store some water for water-limited periods, rather than releasing it to a receiving water body. This is especially important in Australia, where competition of water use with other sectors is increasingly intensifying, surface water and groundwater are already highly allocated, and water resource use is projected to increase with national commitment on limiting greenhouse gas emissions (Connor et al., 2015; Hatfield-Dodds et al., 2015; Bryan et al., 2016). Barrett et al. (2010) considered a water market that allowed trading of worked water from wetter mines to drier mines. The potential for worked water sharing was evaluated by defining a risk indicator of being not “too wet” or “too dry” in a statistical sense. The evaluation method considered controlling worked water levels during both water-limited periods and water-excess periods. However, more potential of a water sharing strategy should be exploited. For example, during prolonged rainfall periods, existing storage space can be better utilised to accommodate incoming water to avoid unregulated discharge. The method Barrett et al. (2010) proposed was based on a long-term statistical analysis and was not supported by simulation modeling on a daily time scale. Short-term high-intensity rain events can result in extreme flooding in a matter of days, which can only be captured by using simulation models with highly discretised time stepping.

In this paper, we consider water sharing among mines as an approach to eliminate or diminish unregulated discharge. The water sharing approach has been widely applied in water resources management and positive impacts of water markets have been demonstrated on improving water allocation efficiency, allocating scarce water resources between competing users, and managing over-allocated water resources (Dudley and Musgrave, 1988; Dudley, 1999; Dlamini et al., 2007; Hadjigeorgalis, 2009; Gao et al., 2013b; Truong and Drynan, 2013). The approach would reallocate worked water among mines (by water transportation facilities, such as pipelines) without losing water that was stored for drier periods. Current work aimed to answer a fundamental question regarding the applicability of this approach: under extreme climate conditions, how much can water sharing do to reduce overflows from worked water storages by moving surplus water from mines with excess water to mines with available storage space. For this ^ te et al., 2008; Zhang et al., purpose, we selected 16 coal mines (Co 2014) in the Bowen Basin, Queensland, Australia as a case study and applied a newly-built climate-driven hierarchical systems model (C-HSM) into the theoretical evaluation of worked water sharing to mitigate the risk of unregulated discharge during the 2010e2011 wet season. The case study region has experienced intermittent floods since 2000, and extreme flooding events occurred during periods of 2007e08, 2011e12, and 2013 (Sharma et al., 2013). Because extreme flooding events may occur intermittently within extended periods of drought, it is vital to evaluate an approach that can deal with both climate-influenced drought and discharge challenges. Designed to comply with the water accounting framework for the minerals industry (Sustainable Minerals Institute, 2012; Danoucaras et al., 2014), the C-HSM is capable of simulating water use processes in mining at both site and regional levels (Barrett et al., 2014; Gao et al., 2014b). The model depicts quantity and quality dynamics among all water objects (for example, water stores, water tasks, and processing plants), and takes mine operation details as inputs to the simulation that represents all stores, flows, tasks, and plants in a consistent and logical way. We believe that the work reported in this paper is the first to explore the theoretical feasibility and potential of applying water sharing into the mitigation of unregulated discharge. In the next section, we briefly review the study region, the C-HSM, and how the C-HSM was applied to the case study area. Next, we present methods of evaluating the feasibility and potential of mine water sharing. Results and discussion from the case study are presented in Sections 3 and 4, respectively. The paper concludes in Section 5.

2. Methods 2.1. Study region The study region is located at the Bowen Basin of central Queensland, Australia, and covers an area of more than 60,000 km2 (Fig. 1). The basin is one of Australia's primary coal mining areas, playing a key role in the economic prosperity of Queensland by producing and exporting high quality coal. The region contains the largest coal reserves in Australia with an ongoing plan to be transitioned to the Coal Hub for Queensland. Average annual precipitation in the area is approximately 830 mm/year. The mean annual discharge from the gauge station at the catchment outlet of the case study area (130,401A) is approximately 1930 GL (gigalitre). The region also experiences one of the most highly variable climates in the world (Sharma and Franks, 2013). The combination of prolonged flooding and extended drought conditions, with continuing coal developments, a long-suffering agricultural sector and a new

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used in modeling a mine water system embedded within an optimisation scheme that is capable of examining the feasibility of optimal pathways for managing water on sites. The C-HSM provides an integrated and strategic approach to incorporate interactions between site processes, water supply and mine water management. In this section, we describe the conceptual architecture of the C-HSM, and how the simulation model was built and calibrated. 2.2.1. Architecture The simulation component of the C-HSM (another component deals with optimisation) is constructed in four steps:  familiarisation and understanding of mine water infrastructure (the engineering model)  identification and classification of model objects  aggregation of objects as either ‘input’ or ‘output’ fluxes, stores or tasks  representation of the mine water system of model objects. The simulation component consists of five basic model objects described in the water accounting framework for the minerals industry (Sustainable Minerals Institute, 2012; Danoucaras et al., 2014):

Fig. 1. Study region and locations of 16 mines.

regulatory regime for managing water has placed the issue at the top of the public agenda (Chen et al., 2015; Liu et al., in press). In 2010e2011, following a long-term drought during 1995e2010 (this period also included intermittent floods), this region experienced the highest rainfall on record. The resulting inundation brought major disruption to mining activities and there was approximately 500 GL of excess saline water on more than 30 mine sites in this region (Queensland Floods Commission of Inquiry, 2012). The subsequent release of mine-affected water into waterways has caused considerable public concerns on cumulative impacts of mining developments on environmental health (Eberhard et al., 2013). In this paper, sixteen mines from the Bowen Basin were selected in the assessment of the water sharing strategy (Fig. 1). These mines are referred to as mines 2e17 in our previous study ^ te et al., 2008; Barrett et al., 2010; Zhang et al., 2014). (Co

 water store e a water storage in a mine water system. All stores on-site are aggregated to two stores: the raw water store and the worked water store. A raw store only receives and stores raw water by definition. A worked water store receives worked water from tasks, raw water in the form of precipitation and runoff, and/or raw water from the raw water store.  water task e a type of activity that consumes water for a particular purpose; water tasks import and export water of varying and potentially constrained quality  treatment plant e an on-site treatment facility for improving water quality  water input e water from external sources into the mine water system  water output e water from the mine water system to external destinations.

2.2. Climate-driven hierarchical systems model (C-HSM)

A sufficient level of discretisation of a mine water system is necessary to enable the model to capture the dynamics of water flows under non-equilibrium conditions (e.g. rapidly changing climate). The design of the C-HSM follows an object-oriented paradigm for facilitating the reuse of existing model blocks. The model can present a mine water system at both the site and the regional scale by building a network of several system components as model blocks. An example of a mine water system is shown in Fig. 2. The configuration of the model objects in a mine water system is used to determine the coupled water balance.

The mine water circuits are complex systems with many feedbacks (Kunz et al., 2013a, 2013b). The modeling technique that is commonly used by mine water managers is based on simulation of the site layout, detailed facilities, operations, and processes within a mine water circuit. Examples of such engineering models include GoldSim (GoldSim Technology Group, 2005) and OPSIM (Water Solutions Pty Ltd, 2012). Provided that engineering models are not readily adaptable to capture the dynamics of such complex ^te et al., 2010; Keir and Woodley, systems, some systems models (Co 2013; Woodley et al., 2013; Gao et al., 2014b) were built to assess effectiveness of water management strategies. As one of newly built systems models, the C-HSM describes the essence of the water system in mines without full detail of the site configurations. It is

2.2.2. Implementation and calibration The water and salt balance system model for each mine is simplified (as mine a in Fig. 2), with two water stores (one for raw water and one for worked water), three water tasks (coal handling and preparation plant, dust suppression, as well as underground demand) and one treatment plant, representing a desalination plant. Each element is associated with a water flow object, which is a simplified pair of input and output fluxes, and the concentration of solutes (salt concentration in this work) associated with each flux. A multi-objective optimisation framework (Durillo and Nebro, 2011) was integrated in the C-HSM and used to calibrate the 16 mine models against a number of calibration objectives. The

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2.3. Data sources

Fig. 2. Example of a mine water system constituted by mines a and b, showing two raw water stores (SR,a and SR,b), two worked water stores (SW,a and SW,b), two treatment P P plants (Pa and Pb), along with two combined water tasks ð Ta and Tb Þ, a combined task includes three tasks d coal handling and preparation plant, dust suppression, as well as underground demand). Symbols are: I, input flux (e.g., pipeline, rainfall, and runoff); O, output flux (e.g., evaporation, seepage, and discharge); L, water loss.

regional model was calibrated site by site with rainfall and evaporation data from 1 July 1977 to 30 June 2007. The calibration data was limited to this time series because some indicators required for ^te et al., calibration were only reported during that period (Co 2008). We assumed that each mine site was able to offer a secure water supply (i.e. its supply failure rate equals 0) for production during this period. Assuming normal coal production, the calibration ensured that the water demand for each water task, the wet indicator (percentage of time above 90% full) and the dry indicator (percentage of time below 20% full) in the worked water store, and the discharge per unit of coal production were close to the corresponding reported indicators. We assumed that there were no regulated discharges, so only unregulated discharges occurred when calibrating the simulation models. The parameters adjusted against calibration objectives include:  Rainfall-runoff factors of the undisturbed area (where the runoff does not come into contact with the by-products of the mine site) for the raw water store in four season/month groups (JanuaryeFebruaryeMarch; AprileMayeJune; JulyeAugusteSeptember; OctobereNovembereDecember)  Rainfall-runoff factors for disturbed areas in the catchment of the worked water store in four season groups  A factor representing a threshold of daily rainfall above which no stored water is used for dust suppression  Percentages of initial stocks in the raw and worked water store. The calibration performances of the 16 mine sites are shown in Supplementary material A (Tables A1eA16). The adjusted model parameters for the 16 mines are presented in Table B1.

Some data for building the simulation model of the 16 coal ^ te et al. (2008). The annual production, mines were sourced from Co raw water store and worked water store capacities, demands and losses of water tasks, wet indicator, dry indicator and pipeline allocation data for each mine site are shown in Table C1 (Supplementary material). The catchment area, surface area, and proportion of disturbed area in the catchment for both raw and worked water stores in a mine site came from WaterMiner (Silvester and Callaghan, 2008). All system-level data was combined and are provided in the summary table in Supplement C (Supplementary material). The average discharge data per unit of ^te coal production came from simulation results of SiteMiser (Co et al., 2006). The other parameters are set as in Appendix Table B1 in Gao et al. (2014b). The mine water sharing distances between mine sites were obtained by conducting a cost distance analysis in ArcGIS (ESRI, 2011). The resultant least accumulative cost distances (in kilometer), or the shortest accumulated travel cost distances, over the elevation cost surface, are shown in Table D1 (Supplementary material). Spatial maps showing least costly routes were also generated. An example (Mine 13) is shown in Fig. D1 (Supplementary material). A climate database for the study area was established by integrating rainfall and evaporation data for selected weather stations downloaded from the SILO website (http://www.longpaddock.qld. gov.au/silo/datadrill/index.php), which provides an enhanced climate databank hosted by the Science Delivery Division of the Queensland Department of Science, Information Technology, Innovation and the Arts. The stream discharge data at the nearby gauging station in the receiving river was obtained from Water Monitoring Data Portal (http://watermonitoring.derm.qld.gov.au/ host.htm). The daily time series rainfall and evaporation dynamics from 1 July 1977 to 30 June 2007 for the 16 coal mine sites were used to calibrate the model and the time series from 1 July 2007 to 30 June 2011 were used to test the feasibility and potential of the water sharing method. 2.4. Methods of testing water sharing feasibility and potential The main purpose of this work is to evaluate the potential and feasibility of mine water sharing to reduce unregulated discharge rather than design a practical sharing strategy. This goal is achieved by simulating each mine's water use and comparing all available accommodation space with accumulated unregulated discharge of all worked water stores. We first evaluated a base sharing scenario: under current mine settings, whether water sharing can eliminate unregulated discharge through comparing all available space of the worked water stores with accumulated unregulated discharge in the 16 mines during 01/07/2010e30/06/2011, which includes a significant flooding period (October 2010eMarch 2011). In this work, water systems in the 16 mine sites were simulated without water transfers between sites. Then, we assessed the potential of water sharing under a sharing & regulated discharge scenario by combining a management means of regulated discharge with water sharing. Further, we estimated the optimal routes and cost of mine water sharing under the two scenarios. Some assumptions and exclusions were made in this study: (1) All 16 mines participated in sharing activities; (2) the means of actively releasing worked water to receiving waters/streams (referred as regulated discharge) (Gao et al., 2013a) was not limited to pumping capacities, but limited to times of natural flow events, permitted release rates, and discharge water quality criteria, and (3) operation costs (labor costs) and maintenance costs of water sharing infrastructure were not considered.

L. Gao et al. / Journal of Cleaner Production 131 (2016) 133e144

The volume of water in worked store s is updated by an aggregation of inputs and outputs: rainfall and runoff supply, pipeline supply, supply from the other water stores, seepage loss, evaporation loss, discharge, exit flux to the other water stores, loss in treatment plants as brine, and the sum of flow volumes to all water tasks, as presented below.

wiþ1;s  wi;s ¼ wi;rfro/s þ wi;pl/s þ 

X

X s0

wi;s0 /s þ

X

 wi;s/discharge 

X

The overall available space vi,S (in ML) at simulation time step i is represented as:

vi;S ¼

S X

vi;s

(4)

s¼1

where vi,S is the overall available capacity in all S worked water stores (S ¼ 16 in our study). The accumulated unregulated discharge of worked water over the 16 mine sites at the simulation step i wi;S/discharge unreg (in ML units) is calculated as the sum of unregulated discharge over the 16 sites, accumulated from the 1st simulation step to the current step i, as shown below.

wi;t/s

t

wi;s/s0  wi;s/evap  wi;s/seep

s0

137

wi;s/t

t

(1) where wiþ1,s and wi,s represent the volumes of water in store s at simulation steps (i þ 1) and i, respectively; wi,rfro/s represents the amount of water brought by a rainfall event into water store s at step i; wi,pl/s is the pipeline water supply to water store s at step i; wi;s0 /s is the volume of water supplied from water store s' to s; wi,t/s is the volume of returned water from water task t to water store s; wi;s/s0 is exit flux from water store s to s0 ; wi,s/evap is the amount of water lost by evaporation in water store s; wi,s/seep is a seepage loss in water store s; wi,s/discharge includes regulated (wi,s/discharge_reg) and unregulated discharge (wi,s/discharge_unreg) from water store s; and wi,s/t represents the volume of water supplied by water store s to water task t. All variables are defined in megaliter (ML) units. The salt (dissolved) balance of water stores for each simulation time period is shown in Eq. (2). The difference in salt (dissolved) mass between input and output fluxes should equal the change of salt (dissolved) mass in a water store.

X   X4 wiþ1;s $ciþ1;s ¼ wi;s $ci;s 4 x wi;x/s $ci;x/s  wi;s/y $ci;s/y

wi;S/discharge

unreg

¼

i X S X

wj;s/discharge

unreg

(5)

j¼1 s¼1

The symbol ai (in ML units) is used to indicate whether unregulated discharge can be eliminated by the means of water sharing at simulation step i, as shown in Eq. (6). The negative value of ai means the accumulated unregulated discharge from the beginning of the simulation to step i cannot be completely removed through worked water sharing among the 16 mines.

ai ¼ vi;S  wi;S/discharge

unreg

(6)

Under the sharing & regulated discharge scenario, we assessed the potential for this combined water sharing strategy to reduce unregulated discharge by minimising the initial water volume wi¼0,s (volume of water on 01/07/2010 in the case study) in worked water store s through a measure of actively regulated discharge (from 01/07/2007 to 30/06/2010). The regulated discharge is limited to times of natural flow events, permitted release rates, and discharge water quality criteria (DEHP, 2013). The objective function is shown below:

y

(2) where ci,s is the salt concentration in water store s at simulation step i; wi,x/s represents water quantity of input flux from model entity x to water store s; ci,x/s means water quality (salt concentration) of input flux from C-HSM model entity x (for example a water task) to water store s; wi,s/y is water quantity of output flux from water store s to model entity y; ci,s/y is water quality of output flux from water store s to model entity y; and the symbol 4 represents the mixing of different mine water where chemical reactions could occur (for instance, Naþ may be involved in cation exchange reactions). The above water quality variables are defined in mg/L units and water quantity variables are defined in ML units. We refer the reader to Gao et al. (2014b) for calculation details of the above inputs and outputs for water and dissolved salt balance. Next, the available capacity vi,s in worked water store s at simulation step i can be calculated as below.

 vi;s ¼

bs *fs  wi;s ; bs *fs  wi;s 0; bs *fs < wi;s

(3)

where bs is called the safety coefficient, which is an indicator for the maximum reference water level of worked water store s (a full or very high storage level can easily lead to unregulated discharge. Practically, the mine water manager keeps the storage level below a threshold to create a buffer space for accommodating incoming water caused by heavy rainfall events); fs (in ML units) is the full capacity of the worked water store s; and wi,s (in ML units) is the volume of water in store s at simulation step i.

min wi¼0;s

(7)

subject to Eqs. (8)e(10):

 g1 wi0 ;s/discharge

reg

 g2 wi0 ;s/discharge

reg

  g3 wi¼0;s ¼ 0

 

¼0

(8)

¼0

(9) (10)

where g1 ðwi0 ;s/discharge reg Þ represents the failure rate that the water store s is not able to offer acceptable supply to water tasks at time step i0 (before time step i ¼ 0) with the regulated discharge wi0 ;s/discharge reg . g2 ðwi0 ;s/discharge reg Þ represents the failure rate that the regulated discharge wi0 ;s/discharge reg does not meet times of natural flow events, permitted release rates, or discharge water quality criteria at time step i0 (before time step i ¼ 0) with the regulated discharge wi0 ;s/discharge reg . The conditions for regulated discharge (such as minimum receiving water flows for discharge, maximum release rates, and electrical conductivity limits) are reported in Barrett et al. (2014). g3(wi¼0,s) represents the failure rate that the water store s is not able to offer acceptable supply to water tasks with the minimised initial water volume wi¼0,s. The mine water system on each site is then simulated with the minimal worked water storage wi¼0,s, and the value of ai in Eq. (6) was recalculated to indicate whether unregulated discharge could be eliminated by means of the combined water sharing strategy.

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The costs of building sharing infrastructure and transporting water across distances can be minimised under both sharing scenarios as:

min

OUT IN  XX

ccapital $bk/l $dk;l þ ctrans $wk/l $dk;l



(11)

k¼1 l¼1

in mines 2, 4, 5, 7, 10, 13, 14, 15, 16, and 17 (Table 1 and Figs. E1eE16). In the mines where no unregulated discharge occurred, there was a good proportion of average available space per day (see Table 1, here the safety coefficient b ¼ 1 as we wanted to explore the maximum potential of available storage space in this experiment), which can be considered to accommodate the excess water from other mines.

subject to

 bk/l ¼ OUT X

1; 0;

wk/l > 0 wk/l ¼ 0

(12)

wk/l  vl

(13)

k¼1 IN X

wk/l ¼ wk/discharge

unreg

(14)

l¼1

where wk/l  0 is a decision variable that represents the volume of worked water that is moved from store k to store l, dk,l is the cost distance between k and l, ccapital is the capital cost for building aunit-distance sharing infrastructure, ctrans is the average cost for transporting a-unit-volume worked water per unit distance, bk/l is assigned a value of 1 if wk/l > 0 and a value of zero otherwise with Eq. (12), OUT is the total number of stores that need to move out their unregulated discharge, IN is the total number of stores that have available space for accommodating incoming worked water, vl in Eq. (13) is the available space in store l, and wk/discharge_unreg in Eq. (14) is the accumulated unregulated discharge of store k. We assumed that the capital cost ccapital was A$1 million per kilometer (km) based on a water pipeline project in this region (Hegarty and Truscott, 2010) and the average cost for transporting 1 ML worked water to 1 km distance was A$0.25 (DSEWPC, 2010). 3. Results With the calibrated simulation model on water use in the 16 mine sites, we first examined the dynamics of each mine's water system. Then, we compared the available storage space with accumulated unregulated discharge over the 16 sites at each simulation step to examine whether redistributing the water into other mines can eliminate the unregulated discharge, under both the base sharing scenario and the sharing & regulated discharge scenario. Further, we optimised the sharing routes and estimated the sharing costs under the two scenarios. We only reported simulation results during 01/07/2010e30/06/2011 (the period is a water accounting year and includes the 2010e11 flooding event). 3.1. Simulated storage dynamics Rainfall is an important factor to affect mine water storage. As shown in Figs. E1eE16 (Supplementary material), the effect of rainfall was distinct in the time series of worked water storage and unregulated discharge: the volume in each worked water store decreased when there were relatively fewer large rainfall events and increased after each major rainfall event. For example, during the large rainfall events between 18 November and 16 December in 2010, the volume of water in worked water stores increased in all mine sites and, in some mines, water exceeded the worked store capacity and resulted in heavy, unregulated discharge (Figs. E1eE16). During 01/07/2010e30/06/2011, as a result of the high risk of filling in the worked water stores, unregulated discharge occurred

3.2. Comparison between daily available storage space and accumulated unregulated discharge Large volumes of unregulated discharge (more than 1 GL) began in early August and became frequent events between November and March (the upper panel of Fig. 3). Correspondingly, the overall available storage space progressively decreased during AugusteOctober and the reduced space was measured approximately as 3 GL. The overall available space declined sharply during NovembereDecember e about 33 GL across the 16 mines (the upper panel of Fig. 3). The gap ai between the available storage space and the accumulated unregulated discharge diminished from August. After 20 March, the accumulated unregulated discharge could not be completely eliminated by means of worked water sharing among the 16 mines (the lower panel of Fig. 3). 3.3. Potential of water sharing by including regulated discharge Under the sharing & regulated discharge scenario, we tested the theoretical potential of water sharing on diminishing unregulated discharge by decreasing initial water levels in the 16 mine sites as far as possible through regulated discharge. Following Gao et al. (2014a), we set bs as 0.88 for each worked store s. This value means that the maximum reference water level for each store is 88% of the store's full storage capacity. We then examined whether the combined strategy (water sharing and regulated discharge) was able to steer the water level of each store below the maximum reference water level during the reporting period. By lowering the initial water levels before the reporting period through regulated discharge, the unregulated discharge dramatically decreased even without mine water sharing (see Table 2 and the upper panel of Fig. 4), compared to that under the strategy without regulated discharge. By means of minimising the initial water storages, unregulated discharge only occurred in mines 2, 5, 10, 14, and 15 (Table 2). Under this combined strategy, the available storage space (here the available storage space is calculated as the maximum 88% of the full storage capacity minus the current storage level) was able to accommodate the accumulated unregulated discharge and the extra water over the reference level (the lower panel of Fig. 4). Table 1 Accumulated unregulated discharge and average available space for 16 mines during 01/07/2010e30/06/2011. Mine ID Accumulated unregulated discharge (ML)

Average available space (ML/ day)

Mine ID

Accumulated unregulated discharge (ML)

Average available space (ML/ day)

2 3 4 5 6 7 8 9

200 46,453 305 24 24,556 190 3119 19,090

10 11 12 13 14 15 16 17 Total

24,458 0 0 2572 12,312 4826 228 8133 125,270

15 27,529 12,490 944 14 17 1298 172 136,416

6231 0 359 1814 0 64,337 0 0

L. Gao et al. / Journal of Cleaner Production 131 (2016) 133e144

139 300 unregulated discharge available storage space

10 8

200

6

150

4

100

2

50

0 01 Available storage space - accumulated unregulated discharge (GL)

250

l-J u

Available storage space (GL)

Unregulated discharge (GL)

12

0 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 0 0 1 1 0 10 l-10 l-1 ug-1 ug-1 ep-1 ep-1 ct-1 ct-1 ov-1 ov-1 ec-1 ec-1 ec-1 an-1 an-1 eb-1 eb-1 ar-1 ar-1 pr-1 pr-1 ay-1 ay-1 un-1 un-1 un-1 u u -J -J -A - O 4 -N - A 5 -M 9 - M -O -J -J -J -J -J -M 4 - M -F -F -A -D -S -S -A -D -N -D 29 15 07 21 21 07 16 27 30 13 02 10 2 10 24 26 09 12 23 16 30 0 18 02 1 0

300 250 200 150 100 50 0 -50 01

-J u

1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 0 0 1 1 0 0 0 r -1 r- 1 t-1 t -1 r- 1 y -1 r -1 y -1 l -1 l -1 l-1 v- 1 c -1 v -1 c-1 c -1 g -1 g -1 p -1 p- 1 b-1 b -1 n -1 n- 1 n -1 n -1 n -1 -Ju 9-Ju 2-Au 6-Au 9-Se 3-Se 7-Oc 1-Oc 4-No 8-No 2-De 6-De 0-De 3-Ja 7-Ja 0-Fe 4-Fe 0-Ma 4-Ma 7-Ap 1-Ap 5-Ma 9-Ma 2-Ju 6-Ju 0-Ju 2 15 0 2 0 2 3 0 1 1 2 2 2 1 1 0 0 1 1 3 2 1 2 0 1 0

Fig. 3. Daily unregulated discharge and available storage space of the 16 mines.

Table 2 Accumulated unregulated discharge and average available space for 16 mines under the sharing & regulated discharge scenario during 01/07/2010e30/06/2011. Mine ID Accumulated unregulated discharge (ML)

Average available space (ML/ day)

Mine ID

Accumulated unregulated discharge (ML)

Average available space (ML/ day)

2 3 4 5 6 7 8 9

1159 49,756 888 213 27,904 25,782 3606 18,035

10 11 12 13 14 15 16 17 Total

21,853 0 0 0 3977 4165 0 0 34,763

240 31,233 13,168 11,553 2660 80 3265 9826 199,367

3684 0 0 1084 0 0 0 0

3.4. Optimal routes and estimated costs of mine water sharing under the two scenarios Table 1 shows simulated accumulated unregulated discharge and average available space among the 16 mine sites under the base sharing scenario. Based on the data and formulae (9)e(12), we optimised worked water sharing routes and volumes during the period of 01/07/2010e30/06/2011 (Fig. 5), and estimated the cost for building sharing infrastructure and moving worked water from stores with high risk of unregulated discharge (stores 2, 4, 5, 7, 10, 13, 14, 15, 16, and 17, see Table 1) to the other stores (stores 3, 6, 8, 9, 11, and 12). The cost was estimated at about A$1.5 billion in total. Under the sharing & regulated discharge scenario, unregulated discharge from worked stores 2, 5, 10, 14, and 15 needed moving to the other stores with available storage. The optimised sharing 300 unregulated discharge available storage space space over reference level

10

200

6

150

4

100

2

50

0 -J 01

Available storage space - space over reference level - accumulated unregulated discharge (GL)

250

8

u l-

10

-J 15

ul-

10

-J 29

ul -

Available storage space (GL)

Unregulated discharge (GL)

12

0 11 10 -1 1 - 1 1 n - 1 1 n- 1 1 n - 1 1 -10 g-10 p-10 p-10 ct-10 ct-10 v-10 v-10 c-10 c-10 c-10 n-11 n-11 b-11 b-11 ar-11 ar-11 r-11 p r- M a y M a y p e e e o o ug u e e e e a u a u u A A O O J J J J J F F M M D D A A S N S D N 21 07 07 21 30 02 13 16 27 24 10 10 24 30 16 09 04 26 12 02 18 23 19 05

300 250 200 150 100 50 0 -50 01

- Ju

l -1

0 15

-J u

1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 0 0 1 1 0 0 r -1 r -1 t-1 t-1 r- 1 y-1 r- 1 y -1 l -1 l-1 v -1 v-1 c- 1 c -1 c -1 p -1 p- 1 g -1 g -1 b-1 b -1 n-1 n -1 n- 1 n -1 n -1 -Ju 2-Au 6-Au 9-Se 3-Se 7-Oc 1-Oc 4-No 8-No 2-De 6-De 0-De 3-Ja 7-Ja 0-Fe 4-Fe 0-Ma 4-Ma 7-Ap 1-Ap 5-Ma 9-Ma 2-Ju 6-Ju 0-Ju 29 2 0 2 0 1 0 1 2 3 1 1 2 2 1 0 0 3 1 2 2 1 0 0 1

Fig. 4. Daily unregulated discharge, available storage space, and storage space over reference level of the 16 mines under the sharing & regulated discharge scenario.

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Fig. 5. Optimised mine water sharing routes and volumes under the base sharing scenario (unit: ML). The yellow lines on the map represent the sharing routes (the routes do not represent geographical paths of mine water sharing) from stores with high risk of unregulated discharge (represented by blue dots) to the other stores (represented by pink dots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

routes and volumes during 01/07/2010e30/06/2011 are shown in Fig. 6. The total cost under the sharing & regulated discharge scenario was estimated at A$189 milliondabout 13% of that under the base sharing scenario. 4. Discussion 4.1. Effects of mine water sharing scenarios on reducing unregulated discharge Mine water sharing could significantly reduce unregulated discharge, but could not thoroughly eliminate discharge in this case study, even if all spare capacity of the 16 worked water stores were fully used. During the reporting period, most mine sites (10 out of 16) experienced unregulated discharge and the accumulated discharge of these sites amounted to approximately 125 GL (Table 1). We compared the accumulated unregulated discharge with the available storage space of all worked water stores under the base sharing scenario, and estimated that the unregulated discharge could be controlled below 15 GL (Fig. 3) by fully utilising the capacities of all worked water stores (here the safety coefficient b ¼ 1). However, the unregulated discharge could not be completely removed by means of mine water sharing (see the lower panel of Fig. 3, after 20/03/2010, the available storage space was less than the accumulated unregulated discharge). It should be recognised that, in reality, mine water managers tend to keep worked water storage levels under a threshold to avoid risking penalties associated with unregulated discharge.

Fig. 6. Optimised mine water sharing routes and volumes under the sharing & regulated discharge scenario (unit: ML).

By combining mine water sharing with a management option of actively releasing worked water into the receiving water body before the reporting period (01/07/2010e30/06/2011), we assessed the potential of this combined strategy, and found out that the unregulated discharge could be entirely removed (see the lower panel of Fig. 4) and the water storage of each store could be kept at a safe level (below 88% of each store's full capacity). Nevertheless, the risk of unregulated discharge was reduced at the cost of losing the benefits of released water, which can be used during water-limited periods and has considerable value in mining industry's social license-to-operate (Barrett et al., 2010). The release of mine water to the environment is regulated by an environmental authority, which includes a set of standard conditions created for coal mines in this region (DEHP, 2013). The conditions for regulated discharge are usually determined by the quantity and quality of the mine water to be discharged, and by stream flow rate (measured by cubic meters per second) and water quality (measured by its electrical conductivity (EC)). Mine water can only be released from designated release points, and its impact will be monitored at a gauge station downstream of the river. Only mine water with an EC less than a given threshold could be discharged, and its impact on the receiving water quality must also be controlled. Two mechanisms control the amount of mine water that could be discharged (the discharge rate). One occurs at the end-of-the-pipe points and specifies the discharge rate threshold with corresponding stream flow rate of the receiving water. The other occurs at the receiving water; it does not specify the discharge rate but controls the quality of water. In other words, the mine water can be released at any rate as long as it maintains the water quality to a specified threshold at the receiving water.

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4.2. Feasibility of mine water sharing based on cost estimation The estimated cost under the base sharing scenario was about A$1.5 billion, which was high because of the unbalanced distribution of excess water and spare capacity. Thus, some mines had to move their excess water to distant stores and this led to a high sharing cost. For example, part of excess water from store 7 needed moving to stores 11 (188 km apart) and 12 (375 km apart). However, we think mine water sharing is still economically feasible based on the following six considerations. 1 The sharing cost is still competitive compared to the loss of coal exports due to excess water flooding the mines. During the 2010e11 wet season, worked water stores in Bowen Basin became full which triggered mine managers to pump excess water into pits putting 85% of coal mines in the region into restricted production or shut down mode. The lost production time that impacted royalty revenues to Queensland's gross state product was estimated at A$5.7 billion (Queensland Floods Commission of Inquiry, 2012; Sharma and Franks, 2013). This does not include the cost for mine reconstruction and rehabilitation or costs due to impacts on the surrounding socioeconomic and ecological landscapes. Another similar weather event in this region (from 2007 to 2008) costed the Ensham mine A$270e300 million, besides the direct loss of revenue from coal sales (Sharma and Franks, 2013). 2 The shared water was estimated at 112,000 ML in the base sharing scenario. The monetary value of the shared water was assessed as A$32 billion based on revenue for coking coal and Moran's calculation method (2006) at a selling price of A$60 per tonne. The shared water can also be valued as A$4e37 million if the water is traded for irrigation products (Moran et al., 2008). 3 Once installed, the sharing infrastructure can contribute to dealing with climate extremes, which seem to become more intense in Australia. Largely as a result of Australia's oceanographic characteristics and susceptibility to seasonal changes to natural weather patterns (mainly El Nino and La Nina cycles), the Bowen Basin periodically suffered from extreme climatic events, such as droughts (e.g., 2002e2008), floods (e.g., 2007e08, 2010e11, and 2013), and cyclones (e.g., 2006 and 2011) in the past decade. Given that the extreme climatic events occurred more frequently and suddenly switched with greater intensity, climatic disasters are expected to happen again and losses will continue to increase if the industry does not have the tools to manage the situation. 4 The sharing cost could be significantly reduced by incorporating the means of regulated discharge e the cost was only 13% of that under the base sharing scenario and 3% of the loss of coal exports resulted in the 2010e11 flooding period. The distribution of excess water and spare capacity was more balanced under the sharing & regulated discharge scenario, where the excess water from stores 2, 5, and 10 could be transferred to adjacent stores 3 and 4, as well as stores 14 and 15 could move water to store 17. The total distance for mine water sharing between mines under the sharing & regulated discharge scenario was only 167 km. Furthermore, all stores under this scenario were controlled below the maximum reference water level (88% of each store's storage capacity). However, it should be noted that the reduction in the sharing cost under the sharing & regulated discharge scenario was obtained in return for releasing part of excess water into the receiving water body, rather than to store this water for water-limited periods. 5 Coal and coal seam gas (CSG) production areas are often in similar regions which opens the possibility to use CSG water pipelines to transfer and store mine water as a means to reduce

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the sharing cost. CSG production needs to extract groundwater in coal seams to depressurise coal measures for allowing natural gas to flow and to be extracted. The produced CSG water must be gathered and transported via a network of pipelines, pumps and ponds to water treatment plants. Under which circumstances these existing pipeline networks and water holding ponds can be shared for mine water management is worth exploring. 6 On the basis of several water pipeline/canal projects (e.g., Harvey Water, 2009; Hegarty and Truscott, 2010; Eurobodalla Shire Council, 2012), the capital cost was estimated at $0.13e1.71 million per km. The cost may be decreased if other transporting options are considered, for example transporting mine water by tankers (DSEWPC, 2010). Overall, the excess water in mines resulted in huge economic losses and environmental damages. The costebenefit analysis results under the base sharing scenario has shown that the cost of building a regional scale sharing infrastructure and enabling water sharing in the case study is appealing, when compared to the lost revenue caused by reduced coal production. The case study region periodically suffered from extreme climatic events and the losses are likely to be reproduced if no management measures (such as mine water sharing) are taken. The sharing cost can be significantly reduced through actively releasing water to be compliant with discharge regulations, utilizing/sharing water management infrastructure of the CSG industry, and/or considering cheaper transporting options. 4.3. Implications for mine water management A key reason for the severe impacts of extreme ‘wet’ conditions on coal production and revenue generation in 2007e08 and 2010e11 was the lack of a region-wide preparedness for a sudden change over a relatively short time frame (no more than three months) (Sharma et al., 2013). No effective means were available to help mine water managers conserve water during the dry season and deal with excess water during the subsequent wet period. Even impacted mine sites were allowed to release excess mine water into waterways under transitional environmental program licenses during the 2010e11 wet season, causing concerns by environmental groups that the discharge had caused environmental harm (Queensland Floods Commission of Inquiry, 2012; Sharma et al., 2013). We believe that mine water sharing can work as an effective means to respond to the sudden condition switch between extreme ‘dry’ and ‘wet’ in the Bowen Basin, where there are over 50 operational open-cut and underground coal mines with on-site worked water storages. The benefits are at least fourfold. First of all, with an operational sharing infrastructure, the sharing process can be completed within a short period. Secondly, these mitigation measures conserve excess water regionally for future water-limited periods, thus adding significant revenue and improving regional water productivity for coal mining. Thirdly, discharge of mineaffected water into the environment can be largely reduced, or even eliminated. Finally, the value of saved water through a sharing scheme may be considerable as it influences mining industry's social license-to-operate in the long term, especially under conditions where there is a water security issue of decreasing availability and increasing competition (Barrett et al., 2010). From an implementation feasibility and cost perspective, a mine water sharing scheme would be more appropriate in a region where mines are in close proximity with both excess water capacity for dry conditions and storage capacity for wet conditions. However, some distant mines may have a higher priority to share their excess water than others, due to greater losses if unregulated

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discharge occurs. A typical example is the unregulated discharge that flows into, and damages, a sensitive and iconic ecosystem. Therefore, the successful development of mine water sharing strategies requires a good understanding of the effects of sharing strategies on regional water supply, storage, and discharge, as well as risks associated with water quantity and quality under dynamical climates. Scientific tools can play a key role in the planning, analysis, and evaluation of mine water sharing strategies. Models similar to the C-HSM allow mine water managers to make better informed decisions in response to a sudden dry-to-wet change. 4.4. Alternative management options In addition to the water sharing strategy discussed above, there are a number of alternative management options that may also be effective in reducing unregulated discharge of mine water: (a) enhancing the evaporation of worked water, for example, through large-scale evaporation dams/ponds; (b) performing regulated discharge to surface waters; (c) applying levee banks to reduce runoff into worked water stores; (d) using worked water for largescale irrigation of selected trees and crops using drip irrigation and center-pivot irrigation techniques; (e) transferring/sharing worked water between mines and other industrial users; (f) injecting worked water into underlying formations or coal seams; and (g) treating worked water for discharge, injection, or beneficial uses (such as public recreation, irrigation, urban water supply, and industrial use). Management options (a)e(c) can gradually and effectively reduce worked water storage in mine sites. Evaporation dams usually have large surface areas and are used to store worked water for the purposes of passive treatment via evaporation. Due to the potential of evaporation dams for significant impact on local properties and land, they have been recently banned by the Queensland government (DEHP, 2012). The regulated discharge of mine-affected water is limited to rigorous release conditions and still has possible effects on receiving waters. Levee banks are manmade embankments to reduce or prevent overland flows and are also thought to have possible impacts on local properties and catchment (DNRM, 2014). The three options are able to decrease the risk of unregulated discharge by cutting down on worked water storage, which can be actually used for water-limited periods. These options have some possible impacts on the environment and are not able to deal with the sudden change between extreme ‘dry’ and ‘wet’ conditions, but they can be used as valuable additions to mine water sharing that can decrease sharing costs. Management options (d)e(g) can store the excess water for future use during water-limited periods or for other industrial users. Given the typical quality of worked water, using it for irrigation is not a sustainable option (Biggs et al., 2013). Sharing worked water with other industrial users may induce a much higher transferring costs than sharing water between mines in close proximity. The injection of worked water into underlying formations or coal seams may result in contamination of target formation or connected formations and high injection costs. Option (g) has low risk against regulatory guidance, but may involve considerable costs for building and operating a treatment plant. Management options (d) and (g) cannot provide a quick response to the sudden dry-to-wet change. All the options are subject to regulatory, environmental, technical, and/or socio-economic risks. Options (b), (c), (e), and (g) have the potential to reduce unregulated discharge and need detailed appraisal. In addition, a combination of mine water management options was thought significantly attractive in exploring the potential of mine water storage (Gunson et al., 2012). The value of

mine water sharing is maximised when combined with other management options, such as regulated discharge. However, seeking an effective combination of management options is not straightforward, even when a prediction tool such as C-HSM is available for assessing the potential impacts of combined management options. To deal with this problem, a global sensitivity analysis can be used to identify critical model factors and quantify the impact of interactions between these factors on a model output (e.g., accumulated unregulated discharge) (Saltelli et al., 2008; Gao and Bryan, 2016; Gao et al., 2016). Then mine water managers can consider those management options that are able to control/change critical model factors as candidates to work with the mine water sharing strategy.

4.5. Future directions There are three possibilities for further study with respect to implementing mine water sharing in the region. (1) The water sharing process was not simulated in the study. The practical sharing scope and mechanisms (for example, which mine sites share how much water with which, and at what time and cost) are worth exploring. The detail of how a mine water sharing strategy can be implemented, including key externalities, as well as implementation and transaction costs (Kunz and Moran, 2014) need to be further investigated and included in future simulations. (2) Based on historical climate data, this work has shown that combining water sharing with the measure of lowering worked water storage much earlier was effective in reducing unregulated discharge under extreme climatic events. When practically implementing water sharing strategies, for example, one needs to determine which mines have higher priority to share or when to perform regulated discharge to decrease the levels of mine water storages as required. Furthermore, more efforts are needed to integrate the predictions of rainfall and stream flow (levels, duration and quality) and to make operational decisions that are robust to prediction errors (Gao et al., 2013c; Zhou et al., 2013; Barrett et al., 2014; Gao et al., 2014a, 2014c; Kunz and Moran, 2016). Adaptive management frameworks and tools, such as robust decision-making (Lempert and Collins, 2007; Bryant and Lempert, 2010) and reinforcement learning (Sutton and Barto, 1998; Gao et al., 2006; Gao and Hailu, 2010), can all contribute to dealing with risks and uncertainties associated with mine water sharing decisions. In addition, regulated discharge in the current case study region is subject to local license conditions (DEHP, 2013). The release of mine water is restricted to times of natural flow events, release rates, and discharge water quality criteria to minimise impacts on receiving waters. A pretreatment (such as desalination or dilution) may be required if release water quality is not accepted by a receiving water body. (3) Further efforts are required to investigate and assess alternative (or integrated) management options with lower costs, for instance, options (b), (c), (e), and (g) in Section 4.4. The emerging CSG water management options and infrastructure may possibly be applied to mine water sharing. Beneficial use of CSG water is encouraged to minimise potential environmental harm and maximise its productive use as a valuable resource (DEHP, 2012). Some applications of CSG water sharing for local agriculture, mining, industry and communities are being tested (Khan and Kordek, 2014).

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5. Conclusions We evaluated the theoretical potential of mine water sharing to reduce or eliminate unregulated discharge by simulating the dynamics of a group of mine water systems based on the climatedriven hierarchical systems model (C-HSM). This involved comparing accumulated unregulated discharge with available accommodation space of all mine sites, and by estimating the costs of building infrastructure and transferring water for optimised water sharing networks under two scenarios. For a demonstration region, we found that mine water sharing strategies could significantly reduce unregulated discharge of mine-affected water under the 2010e11 wet season climate conditions. The cost of building such a regional scale sharing infrastructure and the actual redistribution of water was high, but competitive to the lost revenue due to reduced coal production. The sharing cost can be further reduced, for example, through using water management infrastructure from other industries. By combining mine water sharing with other management approaches, such as actively regulated discharge, the unregulated discharge could be completely eliminated and mine water storages could be maintained at a secure level. Under the sharing & regulated discharge scenario, the cost could be significantly decreased, and shared water could be stored regionally and used for water-limited periods. As the demonstration region periodically suffered from extreme climatic events, similar losses may reoccur if no specific mitigation measures (such as mine water sharing) are taken. Currently, mine water managers lack tools and strategies to manage both climateinfluenced drought and discharge challenges. The work presented here reveals that a combined mine water sharing strategy is an effective (in reducing unregulated discharge), secure (in maintaining safe levels of worked water stores), and economic (in relatively lower implementation costs and conserving excess water for future use) tool for responding to a sudden change from extreme dry to extreme wet. The ultimate value of mine water sharing may be considerable in the long term by impacting mining industry's social license-to-operate. To further develop a practical mine water sharing scheme, the costs, benefits, risks, opportunities, and uncertainties need to be assessed, and robustness and adaptation to uncertain future conditions also need to be incorporated into the strategy development of mine water sharing. The value of models that allow resource managers to evaluate effectiveness of potential management options (such as mine water sharing in this paper) cannot be overstated. Acknowledgments This work is supported by the CSIRO Land and Water. Dr. Jane Hodgkinson and Ms. Sue Cuddy of CSIRO made helpful comments on an earlier version of the paper. We also appreciate the comments of two anonymous reviewers which have greatly improved this manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jclepro.2016.05.061. References Barrett, D., Chen, Y., Gao, L., Zhou, M., Renzullo, L., Liu, R., Emelyanova, I., 2014. Managing Mine Water under Extreme Climate Variability. ACARP report C21037. http://www.acarp.com.au/abstracts.aspx?repId¼C21037. ^te, C., 2010. A method for estimating the potential trading Barrett, D., Moran, C., Co of worked water among multiple mines. Mine Water Environ. 29 (2), 92e98.

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