Bambra Fault 2016

Analysis of drawdown: Bambra Fault 2016

Jacobs SKM

Barwon Downs Hydrogeological Conceptual Model

Analysis of drawdown to refine the conceptual model at the Bambra Fault

FINAL 1

4th February 2016

mdGroundwater

author: Mike Dudding

ABN 75 940 815 281

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Analysis of drawdown: Bambra Fault 2016

Contents 1 Introduction ...... 1 1.1 Background ...... 1 1.2 Obectives ...... 1 2 Observation bores ...... 3 3 Response to Pumping ...... 4 3.1 General Aquifer Response ...... 4 3.2 Distance Drawdown ...... 8 3.3 Response Adjacent to the Fault ...... 10 4 Aquifer Analysis...... 14 4.1 Confined Homogeneous Aquifer ...... 14 4.2 Semi-Confined Homogeneous Aquifer ...... 16 4.3 Heterogeneous Aquifer ...... 18 4.4 Partial Barrier Boundary ...... 19 5 Other observations ...... 22 6 Conclusions ...... 24 7 Recommendations ...... 25 8 References ...... 26

mdGroundwater J0011 ii

Analysis of drawdown: Bambra Fault 2016

1 Introduction

1.1 Background The Barwon Downs bore field is operated under licence from Southern Rural Water. This licence was granted in 2004 after an extensive review process involving an expert advisory panel which considered potential impacts and conditions required for the new licence. This licence is due to expire in June, 2019.

A review of flora and groundwater levels completed under licence conditions(SKM and EA, 2008-09) recommended that a long term vegetation and hydrogeological monitoring program be designed and implemented to better understand a range of factors (such as groundwater extraction, drought and land use changes) that may be contributing to the drying of the catchment.

The Barwon Downs licence renewal process comprises four stages, with Stage 1 currently in progress. The Desk Top Study (Stage 1, Task B) identified potential gaps in the hydrogeological conceptual model and made recommendations on how these gaps should be addressed.

One of those recommendations was to use existing data to evaluate whether the conceptual model at the south-west model boundary (i.e. the boundary between the Barwon Downs and Gellibrand groundwater sub-basins, the north east model boundary (i.e. the combined Birregurra and Colac faults), and the Bambra Fault are sufficiently well defined. The conceptualisation of the SW and NE model boundaries was evaluated in MDG (2014). The Bambra Fault review forms the basis for this report. Note: in this report the NE model boundary is referred to as the Colac Fault.

1.2 Obectives The objectives of the review are as follows:

1. Identify the most likely hydrogeological conceptualisation for the Bambra Fault as indicated by the observed drawdown data 2. Identify any unusual or unexplained characteristics of the drawdown that may require further investigation and/or refinement of the conceptual model (as indicated by the drawdown data)

NOTE:  It is not the objective of this report to assess aquifer hydraulic properties, even though hydraulic values are produced using the methods applied in this assessment  Drawdown is calculated from the start of first main period of pumping which commenced in March 1987. Drawdown during short periods of pumping during the construction of the bore field prior to 1987 would have fully recovered before the March 1987 and, hence, not impact on this analysis.  It has been assumed that the Dilwyn, Mepunga, Pebble Point formations form a single aquifer (ie there are no aquitards between these units) because all three units are intersected and pumped by the bore field.

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Analysis of drawdown: Bambra Fault 2016

 Due to the long periods of pumping used in this analysis, groundwater flow within the aquifers will be predominantly horizontal (ie parallel to the aquifer bedding). As a result, the aquifer vertical hydraulic conductivity will have limited influence on groundwater flow so has assumed to be the same as the horizontal hydraulic conductivity (ie Kh/Kv ratio for the aquifer has been assumed to be 1).  Drawdown used in this analysis has not been adjusted for any long term groundwater trends due to non-pumping effects, such as drought. However, drawdown due to climatic effects (i.e. drought) is expected to small (Jacobs, 2015) and, hence, have negligible impact on the data used in this analysis.

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Analysis of drawdown: Bambra Fault 2016

2 Observation bores A total twenty seven observation bores monitoring the Lower Tertiary Aquifer on either side of the Bambra Fault were selected for this analysis (Figure 1). These bores were selected due to their relatively long monitoring record that commenced prior to or at the same time bore field operation commenced, and their location on either side of the Bambra Fault. The water levels from these observation bores have been converted to drawdown after commencement of pumping. Some of the bores are the same as those used in the analysis of the South-West model boundary and the Colac Fault (MDG, 2014).

The outcrop trace of the Bambra Fault is based on the 250,000 scale Colac Geology Map. The location of the fault shown in the various figures of this report are approximate only and should only be used as general guide. To the north-west of bore 108915 the fault trace bifurcates which indicates that the fault in this area comprises of at least two (and possibly more) up thrown blocks (Figure 1).

Bores identified in this analysis as being located to the east of the Bambra Fault are those that are to the east (or up thrown side) of the western most fault trace.

Figure 1: Location of Lower Tertiary Aquifer observation bores used in this report

5755000 Approx location of the Birregurra -Colac Fault

102868 107720 109114 109135 5750000

109134 47775 47774 82844 62578 109133 102869 47771 5745000 82841 109113 82842

82843 82846 64240 64230 82848 82845 5740000 64229 82851 82852 Approx location of 64236 82847 64237 the Bambra Fault 48249 108915 64241

5735000 West of Bambra Fault (excluding bores adjacent to fault) West of Bambra Fault (low T zone NW of bore field) West of Bambra Fault (adjacent to fault) East of Bambra Fault borefield 5730000 730000 735000 740000 745000 750000 755000

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Analysis of drawdown: Bambra Fault 2016

3 Response to Pumping

3.1 General Aquifer Response Drawdown has been calculated for each observation bore for the period between March 1987 and June 2015 (Figure 2 and Figure 3). The drawdown at the cessation of the three main periods of pumping, 1987 to 1990, 1997 to 2001, and 2006 to 2010 are presented in Table 1.

All bores respond to the three periods of pumping although the response is more subdued east of Bambra Fault (Figure 2 and Figure 3). Only one bore on the east side of the fault, 82845, has a drawdown that is similar to bores on the western side of the fault.The areal distribution of drawdown within the analysis area shows drawdown reaches and extends past the Bambra Fault by the end of the 1st period of pumping (Figure 4) and continues to extend further east across the fault at the end of the 2nd and 3rd periods of pumping (Figure 5 and Figure 6).

Figure 2: Drawdown for observation bores within 6 km of the bore field (Bores east of the fault, 48249, 64241, 82847, are plotted as lines without symbols).

55 Pumping Pumping Pumping 48249 1987 to 1990 1997 to 2001 2006 to 2010 50 64236 64229 64230 64236 45 64229 64240 64241 40 82841 82843

35 82847 82848 82851 30 82852 109113

25 Drawdown(m) 82843 20 109113 64240 64230 15 82841

82852 10 82848 82847 48249 5 64241

0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

M Dudding J0011: dd data_barwon downs.xlsx Time since start of pumping in March 1987 (day)

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Analysis of drawdown: Bambra Fault 2016

Figure 3: Drawdown for observation bores greater than 6 km from the bore field west of the Bambra Fault (Bores east of the fault, 82845, 82846, are plotted as lines without symbols).

Pumping 47771 50 2006 to 2010 47774 Pumping 1987 to 1990 47775 45 Pumping 64237 1997 to 2001 82844 64237 40 82845 82846 35 102868 102869

30 107720 108915

25 109135 109114 Drawdown(m) 109135 82844 20 102869 109114 102868 15

47775 82845 10 47771 107720

5 108915 47774 82846 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

M Dudding J0011: dd data_barwon downs.xlsx Time since start of pumping in March 1987 (day)

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Analysis of drawdown: Bambra Fault 2016

Table 1: Drawdown in each observation bore at the end of the 1st, 2nd, and 3rd periods of pumping

Bore Distance Drawdown at end of 1st Drawdown at end of 2nd Drawdown at end of 3rd ID from bore pumping period 1987 to pumping period 1997 to pumping period 2006 to field (km)1 1990(m) 2001 (m) 2010 (m) West of Bambra Fault 64230 0.7 56.0 48.0 56.0 64229 1.7 39.4 43.0 Not monitored 82851 3.3 39.2 Not monitored Not monitored 82852 3.3 39.4 Not monitored Not monitored 82848 3.4 42.4 Not monitored Not monitored 109113 3.7 28.9 26.6 33.5 82841 4.1 24.3 27.6 37 64240 4.6 15.1 20.4 33.1 64236 4.7 35.3 38.6 45.0 82843 5.0 25.9 Not monitored Water level not reliable 82844 6.4 10.3 14.3 Not monitored 64237 6.4 33.0 35.4 47.6 108915 7.7 0.8 1.9 5.3 82842 7.9 Water level not reliable Water level not reliable Not monitored 102869 8.8 6.8 10.6 17.5 109135 8.8 9.1 13.1 22 109114 9.2 7.9 12.1 20.5 102868 12.0 9.7 14.0 22.0 47771 12.6 1.9 4.7 9.2 47775 13.3 4.9 8.2 12.6 47774 16.9 0.9 3.8 6.0 107720 18.2 1.2 4.6 6.5 East of Bambra Fault 48249 4.6 1.1 2.7 6.0 82847 5.6 1.3 3.7 8.3 64241 5.7 1.4 2.8 7.7 82845 7.4 4.6 9.0 15.0 82846 10.2 0.2 1.1 2.8 1. Distance is from production bore GW4

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Analysis of drawdown: Bambra Fault 2016

Figure 4: Drawdown at the end of the 1st period of pumping

5755000 Approx location of the ~ 1 Birregurra -Colac Fault

9.5

7.7 9.0 5750000 3.8 4.9

10.3 5.0 6.8 1 5 10 1.9 5745000 28.7 24.3

40 0.2 25.9 Approx location of the Bambra Fault 57.1 15.1 4.6 39.4 5740000 20 1.3 42.4 40 33.0 35.3 borefield obs bores 20 10 1.1 0.8 5 1.4 1

5735000 732500 737500 742500 747500

Figure 5: Drawdown at the end of the 2nd period of pumping

5755000 Approx location of the ~ 1 Birregurra -Colac Fault

14.0

12.1 13.1 5750000 8.2 7.0

14.3 1 8.5 10.6 5 10 4.7 5745000 20 26.6 27.6

Approx location of the 40 no data 1.1 Bambra Fault 47.0 20.4 9.0 20 43.0 5740000 3.7 no data 35.4 38.6 20 2.7 10 5 2.8 1.9 borefield obs bores 1 5735000 732500 737500 742500 747500

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Analysis of drawdown: Bambra Fault 2016

Figure 6: Drawdown at the end of the 3rd period of pumping

5755000 Approx location of the ~ 1 Birregurra -Colac Fault

22.0

20.5 22.0 5750000 no data 12.6

no data 1 14.0 17.5 5 9.2 5745000 10 20 31.5 37.0

Approx location of the 2.8 no data Bambra Fault 15.0 33.1 43.0

no data 5740000 8.3 no data 40 47.6 45.0 6.0 40 20 5.3 10 7.7 borefield obs bores 5 1 5735000 732500 737500 742500 747500

3.2 Distance Drawdown Over most of the aquifer between the Bambra and Colac Faults excluding a small low transmissivity zone north-west of the bore field and the region adjacent to the Bambra Fault drawdown tends to decrease as expected along a log trend line at increasing distance from the bore field (Figure 7, Figure 8, and Figure 9). This ‘distance-drawdown’ relationship is maintained throughout the assessment period (although the total drawdown increases at the end of each pumping period) which indicates there is no major change in aquifer hydraulic properties in the region occupied by the observation bores (excluding the low transmissivity region NW of the bore field). Note: the scatter of drawdown values around the trend line indicates that the aquifer properties are not completely uniform (ie there is some variability in the aquifer properties).

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Analysis of drawdown: Bambra Fault 2016

Figure 7: Drawdown at the end of the 1st period of pumping for bores West of the Bambra Fault but not adjacent to the Bambra.

Drawdown at the end of the 1st period of pumping (1987 to 1990) 60

64230 55 West of Bambra Fault (excluding bores adjacent to the fault)

50 West of Bambra Fault (low T zone NW of borefield)

40 Best fit log curve for bores 64229 not immediately adjacent to 35 the Bambra Fault

30 109113

25 82841

15 64240

Drawdown at end of pumping periodbeteen 1997 2001 to (m) 82844 102868 10 Bores NW of borefield 109135 representing a small region of 102869 47775 low transmissivity near the Colac 109114 5 Fault have been excluded from 109133 107720 the log curve best fit to simplify 109134 47774 the Bambra Fault analysis 62578 0 0 2 4 6 8 10 12 14 16 18 20 Distance from Production Bore GW4 (km) M. Dudding J0011: bore_location.xlsx

Figure 8: Drawdown at the end of the 2nd period of pumping for bores West of the Bambra Fault but not adjacent to the Bambra Fault.

Drawdown at the end of the 2nd period of pumping (1997 to 2001) 50 64230

45 West of Bambra Fault (excluding bores adjacent to the fault) 64229 West of Bambra Fault (low T zone NW of bore field) 40

30 82841 Best fit log curve for bores not immediately adjacent to the Bambra Fault 25 109113

20 64240

82844 15 102868 109135 109114 Bores NW of borefield

Drawdown at end of pumping period beteen 1997 to 2001 end at Drawdown2001 beteen1997 period(m)pumping of to representing a small region of 10 102869 47775 low transmissivity near the 109133 Colac Fault have been 107720 5 excluded from the log curve 109134 best fit to simplify the 47774 Bambra Fault analysis 62578 0 0 2 4 6 8 10 12 14 16 18 20 Distance from Production Bore GW4 (km) M. Dudding J0011: bore_location.xlsx mdGroundwater J0011 9

Analysis of drawdown: Bambra Fault 2016

Figure 9: Drawdown at the end of the 3rd period of pumping for bores West of the Bambra Fault but not adjacent to the Bambra Fault

Drawdown at the end of the 3rd period of pumping (2006 to 2010) 60 64230 55 West of Bambra Fault (excluding bores adjacent to the fault)

West of Bambra Fault (low T zone NW of bore field) 50

40 82841

Best fit log curve for bores not 35 immediately adjacent to the 64240 Bambra Fault 109113 30

25 109135

20 109114 102868

102869 15 Bores NW of borefield representing a small region of Drawdown at end of pumping periodbeteen 1997 2001 to (m) low transmissivity near the 109133 47775 10 Colac Fault have been excluded from the log curve 5 best fit to simplify the 107720 Bambra Fault analysis 62578 47774

0 0 2 4 6 8 10 12 14 16 18 20 Distance from Production Bore GW4 (km) M. Dudding J0011: bore_location.xlsx

3.3 Response Adjacent to the Fault Drawdown in bores adjacent to the Bambra fault and east of the fault diverge significantly from the ‘distance drawdown’ trend line which indicates a significant change in aquifer hydraulic properties in the vicinity of the Bambra Fault (Figure 10, Figure 11 and Figure 12).

On the east or up-thrown side of the fault drawdown is significantly less than that expected and this is maintained at the end the three main periods of pumping indicating a significant drop in aquifer transmissivity across the fault. By the end of the 2nd and 3rd periods of pumping drawdown on the east side of the fault tends to fit a log ‘distance drawdown’ curve indicating that the drop in transmissivity across the fault is relatively uniform. An exception to this is bore 82845 which has significantly more drawdown than expected for its distance from the bore field indicating greater hydraulic connection across the fault at that location. The greater hydraulic connection is most likely to be localised around bore 82845 and could be due to greater hydraulic conductivity along the fault (probably due to less displacement on the fault in this area) or the existence or transform faults orientated normal to the main fault providing connection between the up-thrown and down-thrown sections of the aquifer.

Bores located on the downthrown (western) side of the fault but in close proximity to the fault plane tend to exhibit significantly greater drawdown compared to most other bores in the region, except bores 82843 and 47771 which exhibit drawdown similar to most other bores (Figure 10, Figure 11 and Figure 12). Bore 108915 exhibits significantly lower drawdown than expected, and is consistent with distance drawdown for bores on the east side of the fault, which may indicate the bore is mdGroundwater J0011 10

Analysis of drawdown: Bambra Fault 2016

actually located on an up-thrown section of the fault (that does not have a surface expression) or in a region of low transmissivity on the down-thrown side of the fault (as indicated in the SW model analysis, MDG 2014). Either way, bore 108915 appears to be in a region of low transmissivity due to a change in aquifer lithology or the presence of a fault.

The bores which exhibit significantly larger drawdown (64236, 64237, 82848, 82851, 82852) indicate a region of high transmissivity between the bore field and the Bambra Fault (Figure 13).

To the south-west of bore 108915 there is very limited observation bore data so the potential for leakage across the fault cannot be directly evaluated. Only one bore in this area, 114151, has any significant drawdown data but only for the 2nd period of pumping and the start of the 3rd period of pumping. Drawdown in bore 114151 is very subdued in response to the 2nd period of pumping (Figure 14) which tends to suggest that it is located on the eastern or up thrown side of the fault, or in a low transmissivity zone as indicated in MDG (2014). Mapping indicates bore 114151 is located directly on the fault, so it is unclear if leakage is occurring across the fault. Given that this section of the fault is adjacent to the low transmissivity zone identified in the analysis of the south-western model boundary (MDG, 2014), this is likely to reduce the sensitivity of model calibration to leakage rates across the fault in this location. In addition, the aquifer is very thin or unsaturated along the up-thrown (eastern) side of this section of the fault which is likely to result in a reduction in transmissivity on the up-thrown side that is greater than that on the section north-east of bore 108915.

Figure 10: Drawdown at the end of the 1st period of pumping for all bores

Drawdown at the end of the 1st period of pumping (1987 to 1990) 60

64230 West of Bambra Fault (excluding bores adjacent to Bambra Fault) 55 West of Bambra Fault (bores adjacent to the Bambra Fault) East of Bambra Fault 50

45 82848

40 82851 Log curve best fit to bores used in 64229 82852 Colac Fault analysis (MDG , 2014) 35 64236 64237 30 109113

25 82843 82841

15 64240

Drawdown at end of pumping periodbeteen 1997 2001 to (m) 82844 102868 10 109135 102869 47775 109114 5 64241 82847 107720 48249 82845 47774 108915 82846 47771 0 0 2 4 6 8 10 12 14 16 18 20 Distance from Production Bore GW4 (km) M. Dudding J0011: bore_location.xlsx

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Analysis of drawdown: Bambra Fault 2016

Figure 11: Drawdown at the end of the 2nd period of pumping for all bores

Drawdown at the end of the 2nd period of pumping (1997 to 2001) 50

64230 West of Bambra Fault (bores adjacent to the Bambra Fault) 45 West of Bambra Fault (excluding bores adjacent to Bambra Fault) 64229 East of Bambra Fault

40 64236

35 64237

30 82841 Best fit log curve for bores not immediately adjacent to the 25 109113 Bambra Fault

20 64240 Log curve best fit to bores eastof the 82844 15 Fault (excluding 109135 109114 102868 bore 82845)

Drawdown at end of pumping periodbeteen 1997 2001 to (m) 10 82845 102869 47775 64241 107720 5 82847 108915 82846 47771 47774 48249 0 0 2 4 6 8 10 12 14 16 18 20 Distance from Production Bore GW4 (km) M. Dudding J0011: bore_location.xlsx

Figure 12: Drawdown at the end of the 3rd period of pumping for all bores

Drawdown at the end of the 3rd period of pumping (2006 to 2010) 60

West of Bambra Fault (excluding bores adjacent to Bambra Fault) 55 64230 West of Bambra Fault (bores adjacent to the Bambra Fault)

50 64237 East of Bambra Fault 64236 45

40 82841 Best fit log curve for Colac Fault analysis bores, and rapid response SW Bounday 35 analysis bores (MDG, 2014) 64240 109113 30

25 109135 Log curve best fit to bores east of 20 the Fault (excluding bore 82845) 109114 102868

82845 102869 15

Drawdown at end of pumping periodbeteen 1997 2001 to (m) 64241 47775 10 82847 108915 47771 107720 5 82846 48249 47774

0 0 2 4 6 8 10 12 14 16 18 20 Distance from Production Bore GW4 (km) M. Dudding J0011: bore_location.xlsx

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Analysis of drawdown: Bambra Fault 2016

Figure 13: Zones of significantly higher and lower transmissivity as indicated by the distance drawdown analysis

5755000 Approx location of the Birregurra -Colac Fault

102868 107720 109114 109135 5750000 109134 47774 Low T 47775 109133 82844 62578 102869 47771 5745000 82841 109113 82842

82843 82846 64240 64230 82848 82845 5740000 64229 82851 82852 Approx location of 64237 64236 High T 82847 the Bambra Fault 48249 64241 108915 Low T 5735000

obs bores borefield

5730000 730000 735000 740000 745000 750000 755000

Figure 14: Drawdown at bore 114151 during the 2nd pumping period and start of 3rd pumping period compared to bores at similar distance from the bore field on the east (82846) and west (109114) sides of the Bambra Fault

1800 15 Barwon Downs pumping 14 82846 (east of fault) 1600 13 109114 12 1400 114151 11

1200 10

9 1000 8

800 Drawdown(m) 6

600 5 BarwonDownsPumping (Ml/month)

4 400 3

2 200 1

0 0 19/07/1997 19/07/1999 18/07/2001 18/07/2003 17/07/2005

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Analysis of drawdown: Bambra Fault 2016

4 Aquifer Analysis

4.1 Confined Homogeneous Aquifer In a homogeneous aquifer drawdown should propagate uniformly in all directions, and decrease in a uniform manner at increasing distance from the site of pumping. The Theis model is used to predict drawdown for this type of conceptual model.

As expected there is a moderately good match between the observed drawdown and that predicted using the Theis solution for the majority of bores west of the fault (ie excluding the bores adjacent to the fault and in the low transmissivity zone NW of the bore field) except for a relatively poor match to the recovery period after the 3rd period of pumping (Figure 15). This is very similar to the result for the Colac Fault (MDG, 2014) but not surprising given the similar set of bores used in both analyses (although a greater number of bores have been used for the Bambra Fault analysis).

Adding no-flow boundaries representing the Bambra and Colac Faults also produces a similar result to the Colac Fault analysis with an improved match to the recovery period after the 3rd period of pumping but a degraded match to drawdown for the 1st and 2nd periods of pumping (Figure 16 and Figure 17). This indicates that these faults may only be acting as partial barriers to drawdown, as identified by the distance drawdown analysis and drawdown plots in Section 3. Other factors such as semi-confined conditions may also have a significant impact on drawdown.

Figure 15: Theis analysis for bores west of the Bambra Fault excluding bores adjacent to the fault

40. Obs. Wells 109113 (r=3.7km) 82841 (r=4.1km) 64240(r=4.6km) 102869 (r=8.8km) 109135 (r=8.8km) 30. 109114 (9.2km) 47775 (r=13.3km) 47774(r=16.9km) 107720(r=18.2km) Aquifer Model Confined 20. Solution Theis

Displacement (m) Displacement Parameters T = 250.4 m2/day S = 0.002836 Kz/Kr = 1. 10. b = 150. m

0. 0. 2.0E+3 4.0E+3 6.0E+3 8.0E+3 1.0E+4 Time (day)

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Analysis of drawdown: Bambra Fault 2016

Figure 16: Theis analysis with no-flow boundary representing the Bambra Fault for bores west of the Bambra Fault excluding bores adjacent to the fault

Displacement (m) Displacement Parameters T = 407.5 m2/day S = 0.003792 Kz/Kr = 1. 10. b = 150. m

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day)

Figure 17: Theis analysis with two no-flow boundaries representing the Bambra and Colac Faults for bores west of the Bambra Fault excluding bores adjacent to the fault

Displacement (m) Displacement Parameters T = 500. m2/day S = 0.007647 Kz/Kr = 1. 10. b = 150. m

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day)

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Analysis of drawdown: Bambra Fault 2016

4.2 Semi-Confined Homogeneous Aquifer The semi-confined model without boundaries produces a similar but improved result, particularly for the bores at the greater distances from the bore field, to the confined model (Figure 18). This results in an aquitard Kv of 10-4 m/day (assuming an aquitard thickness of 400 m).

Introducing two aquifer boundaries significantly improves the match to the observed data (Figure 19). This also results in an increase in the aquitard Kv to 5 x 10-4 m/day, and a reduction of the storage co-efficient to a more realistic 3 x 10-4 compared to 2 x 10-3 for the no boundaries case.

Although there is a very good match to the observed data with the bounded leaky aquifer model, we do know that the aquifer includes regions that are unconfined, and that these regions may also contribute to the slow recovery rate seen after the 2nd and 3rd pumping periods (as indicated in the analysis for the South West Boundary in MDG 2014).

To examine the potential for unconfined regions to influence drawdown (and hence, be more significant than aquifer boundaries) an analysis using the Butler model was also undertaken.

Figure 18: Semi-confined analysis for bores west of the Bambra Fault excluding bores adjacent to the fault

Displacement (m) Displacement Parameters T = 188.6 m2/day S = 0.001843 1/B' = 4.174E-5 m-1 10. ß'/r = 1.834E-5 m-1 1/B" = 0. m-1 ß"/r = 0. m-1

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day)

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Analysis of drawdown: Bambra Fault 2016

Figure 19: Semi-confined analysis with two no flow boundaries representing the Bambra and Colac Faults for bores west of the Bambra Fault excluding bores adjacent to the fault

Displacement (m) Displacement Parameters T = 178.4 m2/day S = 0.0003119 1/B' = 8.664E-5 m-1 10. ß'/r = 0.0001478 m-1 1/B" = 0. m-1 ß"/r = 0. m-1

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day)

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Analysis of drawdown: Bambra Fault 2016

4.3 Heterogeneous Aquifer The Butler 1988 non-uniform aquifer model represents a confined aquifer comprising a concentric zone with variable radius centred on the bore field surrounded by an outer zone. Both zones can comprise different values of transmissivity and storage co-efficient (Note: a more detailed description of this model is presented in MDG, 2014). The radial distance to the boundary between the inner and outer zones is determined as part of the calibration process.

The results of the Butler analysis (Figure 20) are similar to the semi-confined aquifer analysis without aquifer boundaries (Figure 18). This suggests that a decrease in transmissivity and increase in storage co-efficient in an outer zone located 18 km from the bore field may also be a contributing factor to the observed drawdown. However while the analytical solution suggests the boundary of the outer zone could be located approximately 18 km from the bore field, given the analytical solution is an approximation of the real world, the actual location and orientation of the outer zone with lower transmissivity and higher storage coefficient is uncertain. For example, the Barongarook High may also be contributing to the lower transmissivity and higher storage co-efficient although it is significantly closer to the bore field than 18 km. The nature and location of this zonal boundary could be assessed using the updated numerical model. The result does however provide anecdotal support for a conceptual model where there is leakage across the Bambra and Colac Faults, compared to the Faults acting as no-flow boundaries).

Figure 20: Butler analysis for bores west of the Bambra Fault excluding bores adjacent to the fault

Displacement (m) Displacement Parameters T1 = 251. m2/day S1 = 0.0028 T2 = 39.15 m2/day 10. S2 = 0.06026 R = 1.82E+4 m

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day)

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Analysis of drawdown: Bambra Fault 2016

4.4 Partial Barrier Boundary The distance drawdown analysis shows that drawdown is occurring on the east side of the Bambra Fault. This indicates that there is some groundwater flow across the fault (ie the fault is not acting as a no-flow boundary). The Butler analysis also indicates that leakage into the aquifer from zones of lower transmissivity may also be occurring.

However, the best fit to the data is obtained for a semi-confined aquifer with no-flow boundaries located at both the Bambra and Colac Faults (ie zero transmissivity on the up-thrown side of both faults). To assess the degree to which the transmissivity can be increased on the east side of the Bambra Fault without significantly degrading the time drawdown match to the semi-confined aquifer (Hantush) model with no-flow boundaries an image well analysis was undertaken to simulate a partial barrier. An image well representing the Bambra Fault was inserted into the semi-confined aquifer model using the method described by Walton, 2006. The Colac Fault was represented as a no-flow boundary.

The image well analysis indicates that a transmissivity on the east side of the Bambra Fault could be up to 5% of the transmissivity on the west side of the fault (Figure 21, Figure 22 and Figure 23). Higher transmissivities are potentially feasible but this is likely to be for localised areas only, such as bore 82845 which exhibits significantly more drawdown than other bores east of the fault.

Note: simulating partial barrier boundaries for both the Bambra and Colac Faults requires a significantly more complex image well analysis than that for a single partial barrier, so has not been included in this assessment. However, the analysis undertaken for the Colac Fault (MDG, 2014) indicates that the drop in transmissivity on the up-thrown (northern) side of this fault would be similar to that for the Bambra Fault.

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Analysis of drawdown: Bambra Fault 2016

Figure 21: Semi-confined aquifer with 95% reduced transmissivity on east side of Bambra fault and no-flow boundary at Colac Fault (note: 100% reduced transmissivity is shown in Figure 19).

Displacement (m) Displacement Parameters T = 178.4 m2/day S = 0.0003119 1/B' = 8.664E-5 m-1 10. ß'/r = 0.0001478 m-1 1/B" = 0. m-1 ß"/r = 0. m-1

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day)

Figure 22: Semi-confined aquifer with 90% reduced transmissivity on east side of Bambra fault and no-flow boundary at Colac Fault

Displacement (m) Displacement Parameters T = 178.4 m2/day S = 0.0003119 1/B' = 8.664E-5 m-1 10. ß'/r = 0.0001478 m-1 1/B" = 0. m-1 ß"/r = 0. m-1

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day)

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Analysis of drawdown: Bambra Fault 2016

Figure 23: Semi-confined aquifer with 75% reduced transmissivity on east side of Bambra fault and no-flow boundary at Colac Fault

Displacement (m) Displacement Parameters T = 178.4 m2/day S = 0.0003119 1/B' = 8.664E-5 m-1 10. ß'/r = 0.0001478 m-1 1/B" = 0. m-1 ß"/r = 0. m-1

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day)

Figure 24: Semi-confined aquifer with 50% reduced transmissivity on the east side of Bambra fault and no-flow boundary at Colac Fault

Displacement (m) Displacement Parameters T = 178.4 m2/day S = 0.0003119 1/B' = 8.664E-5 m-1 10. ß'/r = 0.0001478 m-1 1/B" = 0. m-1 ß"/r = 0. m-1

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day) mdGroundwater J0011 21

Analysis of drawdown: Bambra Fault 2016

5 Other observations During this assessment it was noted that water levels after the cessation of the 3rd pumping period tend to recover at a very slow rate compared to the 1st and 2nd period of pumping. There are a number of possible causes including:

 a significant reduction in recharge  depletion of aquitard storage  reduction in transmissivity (eg drawdown reducing the saturated thickness if the aquifer or the water level dropping below a bedrock high creating a barrier to groundwater flow

One or more of these factors would have to have occurred at some time during the 3rd period of pumping.

A reduction in recharge or the reduction in aquifer transmissivity is most likely to occur in the Barongarook High area because this area is unconfined (i.e. is the main recharge site) and the aquifer has a relatively small saturated thickness and significant variable bedrock elevation that could form barriers to groundwater flow after significant drawdown has occurred. Depletion of water from an aquitard could occur over much of the project, however this is unlikely where the aquifer is very deep (i.e. >200 m deep) due to the 400 m thickness of the aquitard (unless there is a significant decrease in vertical hydraulic conductivity of the aquitard). However, the aquitard is relatively thin (5 m to <30 m thick) on the Barongarook High so could easily become depleted after long periods of pumping (Figure 25).

These possible causes of the slow water level recovery can be simulated in an analytical model as a partial barrier to groundwater flow using an image well analysis. An initial assessment was undertaken by simulating a linear partial barrier in the Barongarook High located at bore 114166 and orientated in a NE-SW direction. The barrier was simulated to form at the end of the 3rd period of pumping and results in a significant improvement to drawdown after the cessation of the 3rd pumping period. The partial barrier that provided the best fit to the slow recovery represented a 65% reduction in transmissivity and commenced 150 days after the start of the 3rd period of pumping.

Although this assessment is preliminary only it strongly suggests that the slow recovery of water levels is due to one or more of these processes occurring on the Barongarook high sometime during the 3rd period of pumping. The partial barrier could also represent a region of reduced transmissivity that was identified in the region between the bore field and the south-western boundary of the numerical model (Gellibrand area, MDG 2014).

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Analysis of drawdown: Bambra Fault 2016

Figure 25: East-West Section across the Barongarook High showing confining layer (boundary layer) that separates the aquifer into a shallow unconfined and deeper confined section.

Figure 26: Semi-confined analysis with two no flow boundaries representing the Bambra and Colac Faults and a partial barrier on the Barongarook High

Displacement (m) Displacement Parameters T = 178.4 m2/day S = 0.0003119 1/B' = 8.664E-5 m-1 10. ß'/r = 0.0001478 m-1 1/B" = 0. m-1 ß"/r = 0. m-1

0. 0. 2.1E+3 4.2E+3 6.3E+3 8.4E+3 1.05E+4 Time (day) mdGroundwater J0011 23

Analysis of drawdown: Bambra Fault 2016

6 Conclusions The best conceptual model that fits the observed drawdown in the area between the Bambra and Colac faults is a semi-confined model with the Colac Fault represented as a no-flow boundary and the Bambra Fault with partial barrier representing a 95% reduction in transmissivity. However, it is unlikely that the Colac Fault acts as a complete no-flow boundary (MDG 2014), but rather has a similar (but probably lower) reduction in transmissivity across the fault.

An exception is the section of the Bambra Fault near bore 82845 which shows a high degree of connection between the east and western side of the fault. This exception is likely to be isolated in the vicinity of bore 82845 and therefore is not representative of the general hydraulic connection within the aquifer across the Bambra Fault.

The section of the Bambra fault located south-west of bore 108915 is likely to have a larger drop in transmissivity than the section of fault to the north-east of bore 108915 due to the aquifer being truncated against bedrock and/or thin/unsaturated aquifer, and could potentially be represented (at least during initial numerical model calibration) as a no-flow boundary.

The very slow recovery of groundwater levels after the cessation of the 3rd period of pumping does not fit with the semi-confined conceptual model with partial flow barriers at the Bambra and Colac Faults. This is a significant divergence between the conceptual model and observed data which could have significant implications for model calibration if the mechanism is not correctly identified (particularly since the most sensitive sites are located on the Barongarook High). An initial assessment indicates that a transient partial flow barrier on the Barongarook High forming during the 3rd period may be the cause of the slow water level recovery. An alternative location for the partial barrier may also be the area of lower transmissivity identified near the south-western model boundary, however, this would not be a transient barrier (MDG, 2014). Possible causes for the transient barrier include depletion of an aquitard, reduction in aquifer transmissivity, or reduction in recharge.

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Analysis of drawdown: Bambra Fault 2016

7 Recommendations

1. The Bambra Fault north-east of bore 108915 be represented as a partial barrier to flow with model calibration starting with 95% reduction in aquifer transmissivity on the up- thrown side of the fault 2. The Bambra Fault south-west of bore 108915 be represented during initial numerical model calibration as a no-flow boundary 3. Numerical model calibration allow for an increased flux across the Bambra Fault in the area around bore 82845 4. Further development of the conceptual model be undertaken in the Barongarook High area to better conceptualise the cause of the slow water level recovery after the 3rd period of pumping, and to integrate the conceptual model developed for the south- western and northern model boundaries and the Bambra Fault.

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Analysis of drawdown: Bambra Fault 2016

8 References Butler, J.J., Jr., (1988): Pumping tests in nonuniform aquifers—the radially symmetric case, Journal of Hydrology, vol. 101, pp. 15-30

Hantush, M.S., (1960): Modification of the theory of leaky aquifers, Jour. of Geophys. Res., vol. 65, no. 11, pp. 3713-3725

Jacobs (2015): Barwon Downs Monitoring Program - Review of Conceptual Model at Numerical Model Boundaries. 28 August 2015

Jiao and Zheng (1997): The Different Characteristics of Aquifer Parameters and their Implications on Pumping-Test Analysis. Ground Water Vol. 35, No. I, p25 – 29

MDG (2014): Barwon Downs Hydrogeological Conceptual Model. Analysis of drawdown to refine conceptual model at the SW and NE Barwon Downs numerical model boundaries. Report by mdGroundwater, ref J0011 November 2014

Theis, C.V., (1935): The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using groundwater storage, Am. Geophys. Union Trans., vol. 16, pp. 519-524

Walton, W. C. (2006): Aquifer Test Modelling. CRC Press (November 15, 2006). ISBN-10: 1420042920

Witebsky, S., Jayatilaka, C., and Shugg, A. (1995): Groundwater Development Options and Environmental Impacts. Barwon Downs Graben, South Western Victoria. DNRE

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