A water accounting framework for mining

Dr Claire Cote* and Professor Chris Moran**

In Australia, the water sector is experiencing reforms associated with the National Water Initiative (NWI) that will change the regulatory environment to modernise the water sector and encourage adoption of consistent approaches. One of the important goals of the NWI is to improve water resource accounting, which will underpin sustainable water resource management and efficient water markets.
The mining industry as a whole has adopted a formal approach to develop sustainable development principles through global accords that are reflected in company policies and reported annually (ICMM, 2003). This process has achieved the development of a sustainability framework comprising three elements: a set of ten principles, independent assurance, and public reporting. This reporting system also includes three core indicators related to water. Despite the use of the GRI Mining and Metals Sector Supplement to guide the calculation of the core water indicators across sites, there remains a gap between a site’s operational water balance and a GRI report card for water use, from philosophical, technical and stakeholder requirement’s perspectives. Additionally, since there is no agreed national standard for water use accounting, the possibility of changes to reporting requirements will make a long-term assessment of sustainability difficult.
There is a clear need for a water accounting framework that will provide information related to the minerals industry water use that can be reported to the NWI and other stakeholders in a consistent manner, but can also be used when reporting on GRI and other sustainability indicators. As reported at the SDIMI Conference, 6-8 July 2009, this paper proposes a framework for water accounting by: (1) defining key terms for water metrics that are consistent with the wider water sector; (2) creating a suite of metrics to represent key components of an operation’s water balance; (3) designing a systems approach to the calculation process and recommending a reporting protocol. This excerpt from this paper presents suggestions for a consistent terminology of water metrics to be used in water accounting, and a brief overview of our proposed calculation tool for a water accounting framework.
To test the proposed definitions and calculation methodology, values for water metrics were derived for eight test sites. These included mines of various commodities and a range of post-mining metal processing operations. It was concluded that this accounting framework provides a consistent language and metrics for quantifying and communicating water management, both at the intersection of the site with the surrounding landscape and within operational activities.
A water balance simulation model produces reports at two levels: at the whole of site level and at the task level. These reports list water inputs and outputs by both status and source. The site level accounts also provide an estimate of the change in store volume for each status and source tracked. The source and status accounts are represented as overall fluxes (ML/yr) and volumes per unit production of each commodity (ML/Mt) or ore processed (ML/Mt). Each task in the simulation model can be allocated to different commodities by the proportion of the task (and therefore water use) that is used for each.
This framework will provide a tool for industry to demonstrate leadership amongst water users in water resource stewardship. For example, the fact that the reused volume is often currently reported as a percentage of site inputs for a given year implies that the site inputs will control the amount of reuse that can be achieved. This is a misrepresentation of mine site water management. The factors that will influence reuse are the site’s water balance and the status of the site’s water stocks. The approach presented here is applicable to a wide variety of mining and processing activities, as well as biophysical environments, and can be adopted as the first step towards developing national water accounts for the Australian minerals industry.
A clear and consistently applied water accounting framework holds numerous benefits for industry, government and other stakeholders. It will reduce costs associated with developing water accounting in an ad-hoc manner, and will allow for the quantifying and documenting of water use within an operation and provide a platform for compiling and comparing minerals industry water use with other sectors. This in turn will increase the capability of markets and regulators to quantify water quality and therefore support water access pricing arrangements that reflect water quality and source.

A proposed minerals industry water accounting framework
For the minerals industry, water accounting is concerned with two key areas:
1.    The intersection of the site with the surrounding landscape, including interactions with the environment, community and other stakeholders. Reporting at this level addresses water source, water quality and outputs, i.e., “Where does the water come from and where does the water go?”; and
2.    Water use within the mine site, describing the typical actions that water is subjected to whilst in control of the minerals operation. This includes activities and operational tasks such as dewatering ore bodies or processing ore; water quality treatment such as cyanide destruction and physical settling; and the storing of water.
The accounting framework that is proposed aims at providing a consistent language and metrics for quantifying and communicating water use in each of these two areas. The first area (the intersection of the site with the surrounding landscape) is referred to as “Input-Output Model” and the second area as “Operational Model”.

Input-Output Model
Inputs and quality
Inputs to a site include flows from rainfall, runoff, engineered reservoirs such as water supply dams, lakes and rivers, seas and oceans, third-party entities, aquifers and water that is contained in extracted materials which will be processed by the site (“entrainment”).
To assess the quality of water we have adopted the generalised terminology of “high quality” and “low quality” water. Any site or company has the flexibility to define “high” and “low” quality water based on consideration of cost to the company or of value to the surrounding region. For example, cost may vary with the purchase of expensive potable water (high quality) vs. cheaper non-potable water (low-quality). Similarly, a quality measurement based on value may label water that can be used by other stakeholders for irrigation or livestock drinking as high-quality, whereas water that can only be used by the industry would be low-quality. For the purpose of testing the water accounting framework, we used the following assessment: High quality water is defined as water that (1) has a total salt concentration less than 2000 mg/L and (2) does not contain any constituent that could prevent its use in the site tasks. Low quality water is defined as water (1) that has a total salt concentration greater than 2000 mg/L or (2) that contains a constituent that could prevent its use in site’s tasks.

Sources of water
The sources of water are fundamentally interconnected. Obvious examples of this connection include the infiltration of rain that leads to the recharge of aquifer, or municipal water schemes that obtain water from surface and groundwater sources. However, it is useful to distinguish between the sources of water as it enables an assessment of the physical water reserve that is being depleted and a better understanding of a site’s environmental and community context.

Outputs
Outputs can contain water from a mixture of all sources. They include seepage, evaporation, discharge, environmental flows, entrainment, flows to third-party and volume entrained with the processed material (entrainment).

Operational Model
Use - Treat - Store
The “Use – Treat – Store” cycle refers to the operational management of water. “Use” indicates the tasks that are performed in an operation which involves water. The full set of tasks will depend on the operation. However, a number of tasks are common to many operations, e.g. dust suppression, potable water use for drinking and showering, equipment wash down, mineral concentrating/cleaning and cooling. “Treat” refers to any process that is employed on a site to improve the quality of water. In many cases, this improvement will make the treated water more similar to potable water for a subset of parameters. “Stores” are the facilities on the site that hold and/or capture water.

Diversions
The water accounting framework accounts for water that is moved around an operation but does not serve any consumptive purpose as “diverted water.” The operation must be able to account that the diverted water has arrived at the agreed destination. This definition of diversion is different from the Commonwealth Water Act (2007), which defines diversion as an extraction from waterways and storages for use by pumping or gravity channels. In the Water Act, “return flow” is water that has been diverted by industry but is returned after non-consumptive use. The terminology of diverted water used here roughly matches the Water Act terminology for return flow. However, the Water Act includes cooling water in return flows. This water accounting framework considers cooling as a task.
 
Status of water
Existing definitions of water reuse and recycling are confusing and lack clarity and robustness. A new terminology is required to remove circular definitions. The selected terminology is based on the history of water use within a site, referred to as its “status”: raw, worked or treated. The status of the water is not related to its source. Raw, worked, treated and diverted water can be any combination of surface water, groundwater, sea water and third-party water.
Raw water is water that is supplied or captured (from any of the four types of sources) and has not been previously used for any purpose within the site. 
Worked water is water that has been used for a purpose on site and is returned potentially for future use. This water can come from multiple site sources within the site boundary, as outflow from operational tasks (processing plants, industrial areas, wash down). Its quality is defined by its sources and application history. A volume of worked water that is returned to a store can be used again for a number of times. Therefore, an interesting parameter to track is the number of times the worked water has gone through a cycle of being an input to a task and then returned to a store. It is referred to as the worked water cycle number.
Treated water is raw and/or worked water that is treated on site to provide water of a more appropriate quality for on-site uses (and often, but not always, closer to the standards of potable water). The quality of treated water is dependent on the intended use.
In material waste management, reuse and recycling are differentiated. “Reuse” is when worked water is passed to a task without transformation and associated energy consumption. Recycling is when worked water is treated before it is used in a task. These definitions can account for some Australian States’ definition of recycling as a type of reuse where the water is treated before being tasked again.
It should be noted here that the volume of rainfall and runoff that is collected on site remains raw water until it has been used in a task. With such definitions, the on-site collection of rainfall and runoff is not included in the calculation of the volume of reused water and does not align with the current definition of GRI indicator EN10.
The relationships between inputs, outputs, sources, quality and status are summarised in Table 2. By definition, all site inputs are raw water and their quality will depend on their source. The characteristics of the output flows result from the mixing that has occurred within the use-treat-store cycle and combine the various sources, status and quality. Calculations are required to obtain the proportion of water from each source and status in the output flows, as detailed in the next section.

Proposed calculation methodology
The systems model is a simplified representation of a mine water system to assist with calculating the elements of the water account. It consists of: (1) two types of water stores, one for raw water and one for worked water; (2) a blending facility, which is a piece of ‘virtual’ infrastructure representing all water reticulation around a site; (3) several tasks, which import and export water of varying and potentially constrained qualities; and (4) a treatment facility.
All storages present on site are represented with a reduced number of stores, characterised by the status of the water that they contain (raw or worked). This follows from the important feature that the storage property at the scale of interest (overall water stock) arises from the interaction of the vast number of storages at the lower (engineering) level.
Water enters the system as a combination of inputs characterised by sub-sets of source and status flows. Specific water quality constituents, such as salt, can be added to characterise the water quality of these sub-sets. Feedback mechanisms related to water quantity are represented by means of rules for the stores. Feedback mechanisms related to water quality are represented through use of a tolerance to specific constituents that can be set at the water intake to any task. All water tasks are represented.
Simulations are driven by the climate information that is provided by the user, including long term sequence of daily rainfall and evaporation. Such information can be sourced from on-site measurements, if available, or from the bureau of meteorology (Silo Data Drill at http://www.bom.gov.au/silo/).
All inputs must be characterised by a status and a source. In the calculation methodology developed here, default status and source values for each type of input have been set and inputs can only be from a single source and a single status. Therefore mixed water is represented as multiple inputs so that the source and status are accurately represented.
During a simulation, the source and the status of water are tracked. While the source of water cannot change, water can undergo a change in status. The following rules apply:
. All water that leaves an operational task becomes worked water; and
. All water that leaves the treatment facility becomes treated.
The collection of on-site rainfall and runoff is a site input and is considered raw water.
When a water flow characterised as a list of quantities of sources and status enter a store, the distribution of sources and status within the store is updated accordingly, along with the worked water cycle number (the number of times the worked water has gone through a cycle of being an input to a task and then returned to a store). A raw water store however, should only accept raw inputs and its status distribution should not change. Sites with large site input flows and small return flows to worked water stores would be characterised by worked water with a high proportion of low cycle numbers. Conversely, sites with small site input flows and large return flows to worked water stores would be characterised by worked water with a high proportion of high cycle numbers.
The simulation model produces reports at two levels: at the site level and at the task level. These reports list water inputs and outputs by both status and source. The site level accounts also provide a value for the change in store volume for each status and source tracked. It was assumed the reporting period was the financial year.
The source and status accounts are represented as overall fluxes (ML/yr) and volumes per unit production of each commodity (ML/Mt) or ore processed (ML/Mt). Each task in the simulation model can be allocated to different commodities by the proportion of the task (and therefore water use) that is used for each.
The volume of reused water is calculated as sum of worked water flows entering the tasks. The volume of recycled water is calculated as the sum of treated worked water flows entering the tasks. Each time a volume of worked water is passed through a task, its cycle number is incremented by one.

Example accounts
The construct of the systems model, the account definitions and scorecard calculations were all tested across a range of commodities and biophysical environments found in the Australian minerals industry. Testing of the framework involved generating water accounts for eight sites, using the calculation methodology described above. A five-step procedure was used: site selection, data collection, system representation, calibration and account generation. For details of these ‘example accounts’ contact the authors directly.

* Dr Claire Cote is a senior research fellow at the Centre for Water in the Minerals Industry, Sustainable Minerals Institute, The University of Queensland – email: c.cote@smi.uq.edu.au

** Professor Chris Moran is director of the Centre for Water in the Minerals Industry, Sustainable Minerals Institute, The University of Queensland – email: chris.moran@uq.edu.au

To view “Table 1 – Sources of water” click here.

To view “Table 2 - General relationship between inputs, outputs, sources, quality and status” click here.

To view “Figure 1 – Systems model for calculation of water accounts” click here.

To view “Figure 2 – Example of worked water tracking” click here.

 

Figure 1- Systems model for calculation of water accounts