2.1.6.12 Mine footprints

To quantify the hydrological changes of mine developments, the location and spatial extent of the mine footprint over time is needed. Footprint polygons are used in the modelling to identify which cells in the models need to be modified to reflect impacts of mine development in the baseline and coal resource development pathway (CRDP).

2.1.6.12.1 Extraction of mine footprints from environmental impact statements and other sources

The main source of mine footprint is the respective environmental impact statements (EIS) for each mine. Figures depicting proposed mine plans for different years were used as the basis for determining how the mining footprint propagates over the mining period. Each figure was digitised and georeferenced using one of three methods:

The preferred method was to use maps or plans with coordinates already on them.
If there were no coordinates, then three point locations were matched with points on Google Earth and the latitude and longitude from Google Earth were used to georeference the image.
If there were not three clearly identifiable point locations in the image, then supplementary points were found by matching contour information to the Shuttle Radar Topography Mission Smoothed Digital Elevation Model (SRTM DEM-S) grid (Geoscience Australia, Dataset 3).

The runoff contributing areas were determined as a time series over the life of the mine. Since the contributing area was not provided for all years, the footprint areas were calculated through linear interpolation. Any area upslope of a water storage or dirty or contaminated water area that was not diverted around the mine was included in the surface water mine footprint area. The surface water footprints were exported as shapefiles (.shp) for modelling.

2.1.6.12.2 NSW Department of Trade and Investment historical data (2000 to 2012)

For Boggabri and Tarrawonga baseline mines for which EIS were not available, the NSW Department of Trade and Investment’s (DTI) historical mine footprint data were used.

2.1.6.12.3 Google Earth imagery

On-ground evidence of mines with planned 2015 or earlier start years was verified using Google Earth imagery. The Google Earth images were also used to verify the extent of the surface water mine footprint obtained from the DTI data. For Tarrawonga the baseline mine footprint data from DTI only covered the mine pit area, therefore adjustments were made on the footprint area based on Google Earth imagery. The image was also used to decide the start year of the Tarrawonga expansion as 2015.

Table 32 lists the assumptions made in generating the time series of footprint areas. It also includes the assumptions made for representing hydrological changes due to mines.

Table 32 Assumptions made in surface water modelling for representing hydrological changes of mines and generation of time series data

Characteristic	Assumption for modelling	Basis for assumption
Start date for baseline mines	As specified in development consent that was current at December 2012	The Assessment team does not have the information to model impacts of all historical mines. For modelling, the Assessment team use the timelines of the operation that were current at the date selected for defining baseline coal resource developments.
Start date for additional coal resource development	as specified in development consent or as indicated by a mining company representative, if different from development consent or assumed 2018 if not commenced and uncertain commencement date	Start dates have been determined from current mine reports or mine company contacts; mining activities may not always commence on intended schedule due to delays in the approval process, or to market related reasons. A start date was estimated for mines that were approved to start at a time but had since not started.
End date for baseline mines	As originally specified in development consent or revised (includes a change in end date from an approved modification that does not involve an expansion of mining area). Does not include extensions in time to accommodate expansion of mining areas that commenced after December 2012, which are included in the CRDP as additional coal resource development.	End dates have been determined from current mine reports or mine company contacts.
End date for additional coal resource development	End date obtained from development consent	End dates have been determined from current mine reports or mine company contacts. For mines that were approved to start at a time but had since not started, the end dates were estimated by adding the proposed mine life to the estimated start date.
Time series of footprints	Mine footprints expand and contract over time based on individual mining operational plans	Rehabilitation is typically undertaken progressively. As new areas of the mine excavation are commenced, the depleted pit and other mine-affected areas are rehabilitated.
Post-mining longwall footprints	Sustained at maximum footprint to 2102	Subsidence is permanent.
Post-mining open-cut footprints	use information available in the EIS in absence of relevant information, sustained at maximum footprint for 10 years scaled back to final void area (if known) or to 0.16 times of maximum footprint area for a further 10 years	Absence of any other information on this for some of the mines. Anecdotal evidence from environmental officers at Glencore in the companion Hunter subregion suggested 5–10 years for return to undisturbed conditions.
Final void areas	use information from EIS if final voids not known, then use 0.16 of maximum footprint area	In the companion Hunter subregion, for mines for which data were available, final void area was compared to the maximum footprint area. The median of the ratio of final void area to the maximum footprint area was 16%, with a range of 4% to 32%.
Longwall mine	Assume a permanent 5% reduction in surface water runoff from the affected area	Subsidence is not modelled, but is inevitable where longwall mining occurs. Impacts on surface runoff can vary from very little to more than 50% interception, although given that efforts would be undertaken to rehabilitate mine area as close to the pre-mining condition as possible, the latter is unlikely. Therefore the impact is likely to be smaller, so we conservatively assume a 5% reduction in runoff here. The Assessment team does not have any basis for varying this by mine location, longwall panel depth or other factors.
Open-cut and longwall mine site facilities	Treat the same as mining disturbed areas	Assume that site facilities are not abandoned immediately when mining ceases, allowing for rehabilitation of site. Rehabilitation is assumed to return disturbed areas to pre-disturbance conditions over 10 years, following completion of rehabilitation.

CRDP = coal resource development pathway; EIS = environmental impact statement

2.1.6.12.4 Mine footprints time series

A number of assumptions need to be made in the surface water modelling to represent the hydrological impacts of mining developments on water-dependent assets. These assumptions are consistent with the policy and legislative framework governing the operation of mines (Table 32). This section discusses the approach for defining surface water footprint time series and characterising their hydrological responses pre- and post-development. The time series data are used in surface water modelling to estimate impacts of the additional coal resource development mines on hydrological response variables. The hydrological impacts are reported in companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018).

It is important, therefore, to determine the areas where surface runoff will be intercepted. This area is termed the surface water footprint of the mine, and it can differ from the groundwater footprint. For the purposes of bioregional assessments, surface water footprint covers the entire area disturbed by coal mine operations, including pits, road, spoil dumps, water storages and infrastructure. It may also include otherwise undisturbed parts of the landscape from which natural runoff is retained in reservoirs within mining complex. The footprint does not include established rehabilitated areas from which surface runoff can enter the stream network. Nor does it include catchment areas upstream of drainage channels that divert water around a mine site and do not retain it.

For an underground mine, surface subsidence associated with the collapse of the longwall panels is expected to lead to increased ponding on the surface. This increased ponding is likely to result in a decrease in natural flow to the streams. As discussed in Table 32, a 5% reduction in runoff in areas covered by the underground mine footprint is conservatively (i.e. impact is likely to be smaller) assumed, which factors in regulatory requirements on mining companies to minimise the impacts from mine subsidence through such steps as appropriate longwall orientation and drainage management.

Mine footprint areas change over the lifetime of a mine’s operations. As new parts of the lease are opened up for active use, the footprint increases. As mined parts of the lease are rehabilitated and their runoff returned to natural drainage, the footprint decreases although not necessarily to pre-mining condition. As well as the area of any final voids, the final mine footprint may also include the area covered by any infrastructure (e.g. dams, levee banks, roads) that is intended to remain on the site after final rehabilitation.

Time series of mine footprints for baseline and CRDP mines were compiled from spatial data supplied by mining companies and the NSW Department of Trade and Investment, or extracted by the Assessment team from environmental impact statements and related documents, Landsat TM and Google Earth imagery.

Figure 47 to Figure 57 show temporal variations of mine footprint areas for Namoi coal resource development. Two of the projects have footprints shown for both the baseline and CRDP (Boggabri Coal Mine – Figure 47, and Tarrawonga Mine – Figure 54). Figures for the other projects show mine footprints either under baseline or CRDP.

Boggabri Coal Mine started operating in 2006. The surface water footprint of the Boggabri Coal Mine and Boggabri Coal Expansion Project spans two surface water modelling catchments.

Figure 47 shows the growth of mine footprint areas for both the baseline and CRDP from 2006 to the end of assessment year 2102. The baseline footprint reaches a maximum area of 5.5 km² in 2012, while the total CRDP footprint reaches its maximum area of 18.6 km² in 2033.