2.1.6.1 Boggabri Coal Mine (baseline) and Boggabri Coal Expansion Project (ACRD)

The Boggabri Coal Mine is an open-cut coal mine targeting the Merriown Coal Member within the Maules Creek Formation (Hansen Bailey, 2010). The mine operations were approved in 1989, for a maximum production rate of 5 Mt/year. Operations commenced in 2006.

In 2012 the NSW Government granted approval for the Boggabri Coal Expansion Project covering an additional area of 658 ha adjacent to the original open-cut mine (NSW Government, 2012). The extension allows the proponent to extract coal at an increased rate of up to 7 Mt/year, up to December 2033.

Water management actions for both the Boggabri Coal Mine and the Boggabri Coal Expansion Project are addressed in the water management plan (Boggabri Coal Pty Ltd, 2014a). Unless otherwise stated, the information in this section has been derived from that document.

2.1.6.1.1 Mine water use

Water balance modelling was undertaken using 103 years of climate data to produce 103 climate simulations, developed by ‘stepping through’ the historical data (i.e. starting each simulation on a different year of the climate record, and cycling back through the data). A summary of the median (50th percentile) estimated water demands, site inflows and outflows is shown in Table 14, and indicates that the maximum demand will be constant from years 5 to 21 at 1297.9 ML/year.

The water demands for the Boggabri Coal Mine include: construction water; potable water (for drinking and amenities); dust suppression; vehicle washdown; and a coal handling and preparation plant (CHPP).

Table 14 Summary of estimated Boggabri Coal Mine water inflows and outflows representing median (50th percentile) modelled water balance for range of climate realisations modelled

^aThis rate is applied as a daily rate of 1.5 ML/day which is only applied for days with rainfall less than 5 mm.

^bThe water use reported for vehicle washdown reflects the volume lost from the system. Total water use for vehicle washdown will be significantly higher than this value, and the volume remaining after losses will be recycled.

^cTotal demand = sum of all outflow volumes

MWD = mine water dam, CHPP = coal handling and preparation plant

Data: Boggabri Coal Pty Ltd (2014b)

2.1.6.1.2 Surface water management

The Boggabri Coal Mine is situated within the catchment of an ephemeral drainage line locally referred to as Nagero Creek, which lies within the Namoi river basin. Clean water runoff from surrounding undisturbed catchments is diverted around the working area into Nagero Creek. If, during the mine life, increased mine footprint means that it is no longer possible to divert clean water into the creek, a highwall dam will be constructed upslope to intercept clean water runoff and provide temporary storage for later discharge to a suitable receiving creek system downstream. Should a diversion drain or highwall dam not be suitable due to advancing topsoil stripping and stockpiling, clean water will be allowed to enter the active mining area and dirty water diversion system through the use of an appropriate licence or harvestable right volume.

Dirty water runoff within the operational mine site will be captured in sediment dams. Runoff from large storm events will overtop the dams and be discharged to Nagero Creek if the water quality is suitable, otherwise it will be pumped to mine water dams for storage and reuse.

Contaminated water (including groundwater inflows) will be stored in contaminated water dams or the mining void, and usually will not be discharged to Nagero Creek. The contaminated water will be reused on-site for dust suppression and coal washing (excluding a raw water component required for coal washing). Should there be a surplus of contaminated water, temporary pit storages will be constructed. A one-off emergency discharge of up to 700 ML of mine water to Nagero Creek was allowed in February 2012 after heavy rainfall for pit dewatering, through a licence variation.

Mine water dams hold water of similar quality to the contaminated water dams; they may also store clean water ‘top-ups’ sourced from imported surface water and groundwater allocations during dry periods when the site is in water deficit.

2.1.6.1.3 Groundwater management

The expected maximum pit depth is to the base of the Merriown Coal Member, however no depth relative to a datum has been reported.

Estimated mean annual seepage rates of groundwater into the mining void were assessed by AGE Pty Ltd (2010) (and reported in Boggabri Coal Pty Ltd (2014b), shown in Table 15). Inflows to the pit are predicted to rise gradually as mining progresses and then stabilise between 365 to 438 ML/year from about year 13, with a peak rate of approximately 457 ML/year. The calculated groundwater inflow over the 2014 reporting period was 224 ML/year (Boggabri Coal Operations Pty Ltd, 2015).

Table 15 Estimated groundwater inflows to mining void at the Boggabri Coal Mine

Data: Boggabri Coal Pty Ltd (2014b)

Mining is predicted to result in the reduction in the rate of groundwater flow from the Permian coal seam aquifers to the base of the alluvial aquifer (see Table 3). Modelling indicates that the loss of water from the alluvial aquifer due to the Boggabri Coal Mine reaches a maximum at almost 73 ML/year at the end of mining when the zone of influence and depressurisation of the bedrock has expanded to the maximum extent (AGE Pty Ltd, 2010).

The expected final void depth, after partial backfilling, is 285 mAHD. Once mining operations cease, water levels in the open void will be allowed to rise, resulting in a slow recovery in groundwater levels in the area. This will take approximately 15 to 20 years to reach equilibrium (Hansen Bailey, 2010). Groundwater levels are predicted to stabilise at about 283 mAHD, consistent with pre-mining groundwater levels (Hansen Bailey, 2010).