The water balance assessment presents a quantitative water balance for the Maranoa-Balonne-Condamine subregion for both the baseline and the CRDP. It was conducted under the guidance of companion submethodology M07 (as listed in Table 1) for groundwater modelling (Crosbie et al., 2016). The purpose of the water balance assessment is to quantify the effects of regional water pressure and water level changes in aquifers within the Maranoa-Balonne-Condamine subregion in response to depressurisation of the coal seams for CSG production and mine pit dewatering. The water balance is reported for a defined control volume in BA that includes all hydrologically connected changes predicted by the numerical models. The water balance components (e.g. recharge, evapotranspiration, baseflow (discharge to stream), licensed extractions, upward flow from deeper groundwater and change in storage) are compared with estimates of fluxes described in the regional-scale conceptual model and localised groundwater models to provide confidence in model predictions. Modelled losses are CSG water production, coal mine dewatering, licensed extractions, baseflow (discharge to streams) and negative change in storage fluxes. Modelled gains are recharge and positive change in storage fluxes.
The focus on the deep regional aquifers targeted by CSG development means that the OGIA model
‘includes a relatively simple representation of shallow groundwater systems, which is considered to be consistent with the regional scale and the overall aims of the model. In particular losses from near surface evapotranspiration and groundwater extraction are not represented in the model. Current estimates of modelled discharge to river and drain boundary cells exclude these losses and should therefore not be taken as estimates of baseflow to surface watercourses in the area. Where evapotranspiration and groundwater extraction from these shallow systems were included then a potentially significant proportion of the 93% of watertable recharge that is rejected from the current model would be subsequently lost via groundwater extractions and/or evapotranspiration from near surface systems’ (GHD, 2012, p. 109).
Spatial and temporal extent of the water balance
Coal resource development in the Maranoa-Balonne-Condamine subregion occurs predominantly in the Condamine river basin. The five baseline coal mines are located in the eastern part of the Condamine river basin, outside of the Condamine Alluvium. The five baseline CSG operations cover a large part of the Condamine river basin, the southern edge of the Fitzroy river basin, central parts of the Moonie river basin and north-eastern edge of the Border Rivers river basin. Geological units near the five baseline coal mines includes the Condamine Alluvium, Quaternary alluvia, Main Range Volcanics, Bungil Formation and Mooga Sandstone, Gubberamunda Sandstone and Walloon Coal Measures outcrop areas. The two open-cut coal mine project proposals are located on Walloon Coal Measures outcrop areas. The Range coal mine has adjoining Springbok Sandstone outcrop areas and Quaternary alluvium, and the New Acland Coal Mine has adjoining Main Range Volcanics and Quaternary alluvium. Companion product 2.3 (conceptual modelling) for the Maranoa-Balonne-Condamine subregion (Holland et al., 2016) includes cross-sections and maps of surface geology in the subregion.
Water balance reporting areas encompass all model grid cells where additional drawdown is greater than the minimum detectable difference (0.02 m) for the groundwater model. The locations of the two water balance areas are shown in Figure 14. The Range water balance area covers 13,497 km2 and the New Acland Coal Mine water balance area covers 6,688 km2. Water balances are reported for three 30-year periods (2013 to 2042; 2043 to 2072; and 2073 to 2102) to be consistent with the reported time periods for other BA bioregions. In the other bioregions, a 30-year historical climate sequence is modified and repeated to generate future climate input data for surface water and groundwater models. This 30-year period is repeated to ensure that the effect of droughts and floods does not confound the comparison between time periods. While the same time periods are reported, groundwater modelling in the Maranoa-Balonne-Condamine subregion uses long-term mean recharge rates instead of time-varying climate input data. The focus on the difference between two possible futures in BA means that climate change assumptions will not affect model predictions.
Figure 14 Location of water balance reporting areas in the Maranoa-Balonne-Condamine subregion
APLNG Project = Australia Pacific LNG Project, CSG = coal seam gas, GLNG Project = Santos Gladstone LNG Project + GLNG Gas Field Development Project, LNG = Liquefied Natural Gas, QCLNG Project = Queensland Curtis LNG Project, QGC = Queensland Gas Company
Data: Bioregional Assessments Programme (Dataset 2, Dataset 3)
Water balance uncertainty
The components of the water balance are calculated from a single run of the calibrated groundwater model, represent a different set of model outputs to the hydrological response variables generated at receptor locations and are therefore not included in the uncertainty analysis reported in Section 2.6.2.8. Drain cell conductance, which represents mine pit dewatering and has a major effect on modelled drawdown and water balance, is not considered in the formal uncertainty analysis. Drain cells are assigned a high conductance value (2500 m3/day) in the OGIA model, which means that ‘modelled impact predictions are considered unlikely to be sensitive’ to the assigned conductance value (GHD, 2012, p. 58).
The approach used to calculate water balance terms is described in Table 8.
Table 8 Calculation of water balance terms for the Maranoa-Balonne-Condamine subregion
CSG = coal seam gas, OGIA = Office of Groundwater Impact Assessment, CRDP = coal resource development pathway
The Range water balances
Table 9 shows The Range water balances for three 30-year periods. Most of the water that enters the model as recharge (126.36 GL/year) is discharged via the drain cells (104.53 to 106.09 GL/year), which is consistent with using drain cells to establish topographic control of modelled groundwater levels. Modelled CSG water production in the water balance area falls from 21.94 GL/year (17% of modelled losses) to zero over successive 30-year periods. This is consistent with the modelled cessation date for CSG production of 2065 in the Surat CMA (OGIA, 2014).
Table 9 Mean annual water balances for The Range water balance area for three 30-year periods
Data: Bioregional Assessments Programme (Dataset 1)
The drain cells used to represent coal mine dewatering remove water from modelled storage, that is, the drain and change in storage components of the water balance are equivalent during modelled coal mine operation, which is consistent with localised drawdown near the coal mines. Mean annual coal mine dewatering (equal to the difference in the drain component between the CRDP and baseline futures) is 0.54 GL/year in the first 30-year period, becoming positive after modelled mine closure in 2041 when the drain cells are deactivated in the regional model and water returns to storage. Coal mine dewatering accounts for 2% of modelled losses in the first 30‑year period and zero thereafter in The Range water balance area.
Licensed extractions increase slightly from 4.38 GL/year in the first 30-year period (2013 to 2042) to 4.39 GL/year in the third 30-year period (2073 to 2102). However, its relative importance in each 30-year period increases from 16% of modelled losses (CSG water production, coal mine dewatering, licensed extractions and baseflow) in 2013 to 2042, when CSG water production and the coal mines are operational, to 91% of modelled losses in 2073 to 2102, when water extraction for CSG and coal mining has ceased.
Baseflow (discharge to streams) is negative, meaning that water is leaving the model via river cells within the water balance area, which is consistent with the conceptualisation of surface water – groundwater interactions in the regional model. Baseflow fluxes are identical under the baseline and CRDP for each 30-year period, but decrease through time. This may be an artefact of the regional model construction, as the OGIA model assumes that all surface watercourses act as groundwater discharge boundaries (QWC, 2012). A net lateral flux into The Range water balance area is observed in each 30-year period.
New Acland Coal Mine water balances
Table 10 shows the New Acland Coal Mine water balances for three 30-year periods. About a third of the water that enters the model as recharge (96.32 GL/year) is discharged via the drain cells, with the remainder discharged as CSG water production, licensed extractions and baseflow (discharge to stream). CSG water production is projected to fall from 3.72 GL/year (3% of modelled losses) to zero over successive 30-year periods following modelled cessation of CSG production in 2065 in the Surat CMA (OGIA, 2014).
Coal mine dewatering (equal to the difference in the drain component between the CRDP and baseline futures) averages 0.09 GL/year over the first 30-year period (Table 10). Model predictions are negative (–0.85 GL/year) during mine operation (2012 to 2029) and positive (0.64 GL/year) after modelled mine closure in 2029 when the drain cells are deactivated in the regional model and water returns to storage. These values are consistent with the mine dewatering volume reported in the New Acland Coal Mine Stage 3 environmental impact assessment documents (maximum of 1.4 GL/year; SKM, 2013).
Licensed extractions are 77.07 GL/year in each 30-year period and account for 86% of modelled losses in 2013 to 2042 when CSG water production and the coal mines are operational to 92% of modelled losses in 2073 to 2102 when CSG water production and the coal mines have ceased operation. This is consistent with the high density of licensed bores in the Condamine Alluvium within the New Acland Coal Mine water balance area.
Baseflow (discharge to streams) decreases from 8.85 GL/year in 2013 to 2029 to 6.61 GL/year in 2073 to 2102 and is identical under the baseline and CRDP for each 30-year period. This means that coal resource development does not affect modelled baseflow to the river cells. A net lateral flux out of the New Acland Coal Mine water balance area is observed in each 30-year period.
Table 10 Mean annual water balance for New Acland Coal Mine water balance area for three 30-year periods
*The net impact of the additional coal resource development (ACRD) is defined as the difference between results for the coal resource development pathway (CRDP) and the baseline.
Data: Bioregional Assessments Programme (Dataset 1)
