3.3.3 Potential groundwater changes


The hydrological response variable, dmax, which is the maximum difference in drawdown, obtained by choosing the maximum difference between two future groundwater model runs is used to summarise potential impacts on groundwater. These dmax values are presented for the baseline (difference from a ‘no-development’ model run) and due to additional coal resource development (difference between the CRDP and the baseline runs).

Drawdown greater than 0.2 m, greater than 2 m and greater than 5 m are the reporting thresholds to summarise groundwater modelling results across all BAs. These thresholds represent meaningful changes in the management of groundwater in NSW and Queensland. Minimum impact threshold provisions apply in most aquifers where an activity results in drawdowns greater than 2 m in NSW. For high-priority GDEs and culturally significant sites in the Great Artesian Basin (GAB), however, minimum impact thresholds can apply if drawdowns exceed 0.2 m.

Figure 13 shows the variation in depth of drawdown due to additional coal resource development. The main panel shows the 50th percentile (median), while the two smaller panels show extents for the 5th and 95th percentiles. Together, they illustrate the variability in model predictions due to parameter uncertainty. The areas where the additional drawdown is greater than 0.2 m, greater than 2 m and greater than 5 m for the 5th, 50th and 95th percentiles are summarised in Table 6. The area associated with the 5th percentile (156 km2), for additional drawdown greater than 0.2 m, can be interpreted as representing the extent of drawdown when the model parameters reflect lower pumping rates and/or lower hydraulic conductivities. On the other hand, the area of drawdown associated with the 95th percentile (2299 km2) includes the predictions based on higher pumping rates and relatively larger conductivity of geological layers. The influences of the different parameters can be complex and produce a range of drawdown responses; the interpretation given above is for general guidance only. Groundwater drawdown predictions indicate that drawdowns of greater than 5 m are very likely (greater than 95% chance; shown as the 5th percentile) due to the additional coal resource developments close to the coal mines (Figure 13).

Figure 13

Figure 13 Drawdown due to additional coal resource development

Additional drawdown is the maximum difference in drawdown (dmax) between the coal resource development pathway (CRDP) and baseline, due to additional coal resource development. The mine extents in the CRDP are the sum of those in the baseline and the additional coal resource development (ACRD).

Data: Bioregional Assessment Programme (Dataset 1)

The spatial distribution of drawdown under the baseline is shown in Figure 14, providing a visual comparison to the potential groundwater drawdown due to additional coal resource development in Figure 13. Table 7 summarises the drawdown information in terms of area (km2) in the zone of potential hydrological change for each drawdown class in each reporting area. Under the baseline, the area with at least a 5% chance of drawdown greater than 0.2 m is 479 km2 and the area with at least a 95% chance of drawdown greater than 0.2 m is 18 km2.

Figure 14

Figure 14 Drawdown due to baseline coal resource development

Baseline drawdown is the maximum difference in drawdown (dmax) under the baseline relative to no coal resource development.

Data: Bioregional Assessment Programme (Dataset 1)

Table 6 Surface area (km2) and stream length (km) potentially exposed to varying levels of additional drawdown in the zone of potential hydrological change


Extent

Reporting area

Extent in zone of potential hydrological change

Extent with additional drawdown ≥0.2 m

Extent with additional drawdown ≥2 m

Extent with additional drawdown ≥5 m

Extent in mine pit exclusion zone

5th

50th

95th

5th

50th

95th

5th

50th

95th

Area

(km2)

Upper Namoi

998

52

205

442

42

108

234

36

78

165

34

Mid Namoi

1531

104

255

631

75

168

309

63

127

245

82

Pilliga and Pilliga Outwash

1589

68

1225

16

310

9

110

Lower Namoi

2896

1

Total

7014

156

528

2299

117

292

853

99

214

520

116

Stream length

(km)

Upper Namoi

568

15

56

123

8

32

74

8

18

45

9

Mid Namoi

1671

167

315

710

126

228

368

111

191

300

145

Pilliga and Pilliga Outwash

1268

58

811

15

264

8

96

Lower Namoi

2014

0

Total

5521

182

429

1644

134

276

707

119

217

440

154

The area potentially exposed to ≥0.2, ≥2 and ≥5 m additional drawdown for the 5th, 50th and 95th percentile estimates of the maximum difference in drawdown (dmax) between the coal resource development pathway (CRDP) and baseline, due to additional coal resource development. Drawdowns in the mine pit exclusion zones cannot be quantified with confidence.

‘–’ means ‘not applicable’.

Data: Bioregional Assessment Programme (Dataset 5)

Table 7 Surface area (km2) and stream length (km) potentially exposed to varying levels of baseline drawdown in the zone of potential hydrological change


Extent

Reporting area

Extent in zone of potential hydrological change

Extent with baseline

drawdown ≥0.2 m

Extent with baseline

drawdown ≥2 m

Extent with baseline

drawdown ≥5 m

Extent in mine pit exclusion zone

5th

50th

95th

5th

50th

95th

5th

50th

95th

Area

(km2)

Upper Namoi

998

34

Mid Namoi

1531

18

115

290

2

51

133

2

30

88

82

Pilliga and Pilliga Outwash

1589

30

189

20

72

-

14

49

Lower Namoi

2896

Total

7014

18

145

479

2

71

205

2

44

137

116

Stream length

(km)

Upper Namoi

568

9

Mid Namoi

1671

26

137

344

6

52

158

6

34

105

145

Pilliga and Pilliga Outwash

1268

19

169

12

60

-

6

37

Lower Namoi

2014

Total

5521

26

155

513

6

64

218

6

40

142

154

The area potentially exposed to ≥0.2, ≥2 and ≥5 m baseline drawdown for the 5th, 50th and 95th percentile estimates of the maximum difference in drawdown (dmax) under the baseline relative to no coal resource development. Drawdown in the mine pit exclusion zones cannot be quantified with confidence.

‘–’ means ‘not applicable’.

Data: Bioregional Assessment Programme (Dataset 5)

Table 8 shows the area of the groundwater management zones of the alluvium that are within the groundwater zone of potential hydrological change. The alluvium zones that have the greatest area potentially impacted are the Upper Namoi Alluvium zones 4, 7 and 8 while the Lower Namoi Alluvium has less than 1 km2 that is within the groundwater zone of potential hydrological change. An area of 287 km2 of alluvium has a 5% chance of greater than 0.2 m additional drawdown. This represents around 8% of the Upper Namoi Alluvium and 0.01% of the Lower Namoi Alluvium.

Table 8 Area of alluvium within the groundwater zone of potential hydrological change


Groundwater management zone

Area in GW zone

(km2)

Percent of groundwater management zone in GW zone

%

Lower Namoi Alluvium

0.87

0.01%

Upper Namoi Alluvium_03

13.76

2.48%

Upper Namoi Alluvium_04

135.75

15.97%

Upper Namoi Alluvium_05

1.70

0.59%

Upper Namoi Alluvium_06

0.06

0.01%

Upper Namoi Alluvium_07

90.65

64.39%

Upper Namoi Alluvium_08

44.35

14.08%

Upper Namoi Alluvium_11

0.26

0.15%

GW = groundwater

Last updated:
10 January 2019
Thumbnail of the Namoi subregion

Product Finalisation date

2018
PRODUCT CONTENTS

ASSESSMENT