3.5 Groundwater as a component of the mine site water balance
Most mining operations rely upon a site-wide water balance to guide their water management strategy. The water balance is usually constructed in the format of a spreadsheet model that quantifies the water transfers illustrated schematically in Figure 7. The model tracks water intake to the operation from precipitation, groundwater withdrawals during pit dewatering and any make-up water for the mill obtained from external water sources. Outflows of water from the site include water consumption, water retained in the tailings pore space, and water releases. Groundwater withdrawals from an open pit or from underground workings are in some instances released directly to the environment if no water quality constraints preclude this option and this water is not required for ore processing at the mill. If there are limitations on the release of the groundwater due to water quality constraints, groundwater withdrawals might be transferred to the tailings storage facility for temporary storage before treatment and release. The inflow and outflow terms balance the change in the volume of water retained in storage. Water balance models can also be run in a dynamic mode, for example, tracking changes in storage on a monthly basis. Commercial software is in common use for this purpose.
It is common to develop a water balance model specific to a tailings storage facility as an aid in planning the schedule for dam raises, to ensure a water supply sufficient to support continuous operation of the mine during dry periods, and to routinely update projections of the required containment volume to safely store the design flood for the tailings impoundment. Groundwater outflows from a tailings facility are a relatively small but nonetheless important component of the water balance, especially in semi-arid to arid climates. Closing the water balance (effectively a check that mass conservation is achieved) provides some assurance the facility is performing according to the design base assumed for the hydrologic system.
The Minerals Council of Australia (2014) has published a helpful accounting tool that guides the development of a mine site water balance. It incorporates groundwater as one of the key elements to be considered in tracking water surpluses and deficits.