IN THE MATTER OF The Resource Management Act 1991
AND
IN THE MATTER OF The Proposed Waipara Catchment Environmental Flow and Water Allocation Regional Plan 2010.
S42A Report of John Weeber
Introduction
1. My full name is John Howard Weeber. I am a hydrogeologist with Canterbury Regional Council (the Council), based in the Christchurch office. I have held this position for 36 years. I have the qualification of a BSc degree in Geology from the University of Canterbury. I am a member of the New Zealand Hydrological Society and the Geoscience Society of New Zealand.
2. Throughout my time with the Council and its predecessor the North Canterbury Catchment Board, I have been involved with groundwater resource investigation work in North Canterbury.
3. This work includes the establishment of groundwater level monitoring networks and preparation of water level (piezometric) contour maps, establishment of a computer based database for wells, collection and interpretation of driller‟s geological well logs (e.g. cross sections showing stratigraphy including thickness and spatial extent of the major geological units identified, developing three-dimensional geological model of Christchurch), assisting with the preparation of a variety of publications relating to groundwater in Canterbury and the provision of groundwater related information/advice to internal and external customers (e.g. Institute of Geological & Nuclear Sciences).
4. I acknowledge that I have read the code of conduct for expert witnesses contained in the Environment Court‟s Practice Note dated 25 June 2009. I have complied with it when preparing this report.
5. I have been asked to comment on the hydrogeological nature of “deep” groundwater in the Waipara Basin in response to a submission on this issue. This evidence briefly describes the stratigraphy, geological structure, and aquifer definition of the Waipara
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Basin, and defines “deep” groundwater in relation to this geological context. I have also outlined the current knowledge of this resource.
6. My assessment and conclusions are that the hydrogeology of the Waipara Basin is complex and that there are uncertainties regarding the relationships between the various sources and their relative contributions to the groundwater resource.
Geology
7. It is important to understand the geological structure of the Waipara Basin to provide a context for the sources and movement of groundwater.
Stratigraphy
Figure 1: Simplified geological map of the Waipara Basin, North Canterbury after Rattenbury et al. (2006) and Forsyth et al. (2008)
8. A simplified geological map based on Rattenbury et al. (2006) and Forsyth et al. (2008) is displayed in Figure 1. To show the main geological features, I have grouped
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the original detailed geological units into larger stratigraphic units. For the purposes of this assessment seven stratigraphic units have been identified:
i. The Amberley swamp resulting from impeded drainage created by the „Broomfield Fault‟ (Dodson 2009) across the basin;
ii. Quaternary alluvial gravels (locally referred to as Canterbury and Teviotdale gravels (Wilson 1963)) occupying the basin floor;
iii. Quaternary marine deposits located along the coast. These deposits have no bearing on the groundwater resources in the Waipara Basin;
iv. Quaternary fan deposits located discontinuously around the basin margin;
v. Pliocene Kowai Formation deposits, marine at the base passing into alluvial gravels;
vi. Tertiary sediments including sandstone, mudstone limestone; and
vii. Basement greywacke (sandstone/mudstone) underlying the stratigraphic sequence described above.
Geological structure
9. The geological structure of the Waipara Basin is complex owing to the tectonic activity in the area since the Late Pliocene. This geological history has been summarised in a diagram from Finnemore & Pettinga (2004) and is displayed on Figure 2. In North Canterbury, the Kowai Formation and Tertiary cover sediments have been „wrinkled‟ into fold structures creating discrete basins which have preserved these sediments. In many places these sediments have been „ruptured‟ and displaced by faults. The known distribution of folds and faults is shown on Figure 1. The Omihi Fault is a major feature along the eastern margin of the basin causing significant displacement of the geological units. The most recent (Quaternary) basin infill is sourced from the upper Waipara River, Weka Creek, Home Creek and Omihi Stream sub-catchments (Appendix 1, Map 1, Environment Canterbury 2010). Fan deposits derived from erosion within the Waipara Basin occur discontinuously around the basin margin and are interbedded with the recent alluvial infill.
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Figure 2: Geological development of the Waipara Basin (Finnemore & Pettinga 2004)
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Aquifer Definition
10. Aquifers are not easy to define with regard to their spatial extent and thickness. Their definition requires an understanding of the depositional, erosional, and structural history of an area, as well as a conceptual understanding of the stratigraphy that is derived from an interpretation of many drillers‟ well logs. A good example of a well- defined aquifer system is the Christchurch Artesian System (Brown and Weeber 1992). Continuous aquifers and aquitards have been mapped in Christchurch with the guidance of hundreds of well logs. The artesian system in that example, can be traced from Christchurch city south to Te Waihora/Lake Ellesmere and north along the coast to Leithfield. The generally flat-lying, continuous nature of those aquifers and aquitards simplifies the process of stratigraphic correlation.
11. The physiography of the Waipara Basin is different to the Canterbury Plains in that it is surrounded by hills in a „horseshoe‟ configuration and is structurally far more complex. The hydrogeological complexity of the alluvial deposits in the Waipara Basin has been described by Lois (2000), Lloyd (2002), Dodson (2009) and Finnemore & Pettinga (2004). Well log evidence to date indicates that the alluvial deposits are dominated by low permeability clay/silty gravels and are interbedded with more permeable meandering river channel deposits. The aquifers are of limited thickness (generally <10 m), are not laterally extensive, do not transmit water very fast and when pumped many of the wells in the area experience large self-induced drawdowns. Palaeo- channel features were identified as potential aquifers in the Omihi valley by Finnemore & Pettinga (2004) using seismic reflection techniques. Aquifer tests are necessary to predict the interference connection between wells. Wells abstracting from the same channel deposits are likely to affect each other whereas wells at the same depth beyond the channel or at shallower or deeper levels may not be affected.
12. The accuracy and reliability of well logs is highly variable depending on the driller‟s logging skills and the drilling method used i.e. cable tool or rotary. Sediments recovered during the drilling process tend to be highly mixed. Most wells are now drilled with rotary machines and well logs are may be less accurate than wells drilled by cable tool unless particular care is taken.
13. An examination of bore log data show that a number of discrete fine sediment layers dominated by grey/blue clays occur within the eastern Waipara Basin on either side of the Waipara River, where it enters its lower gorge. These layers may represent deposition in a lacustrine (lake) environment that was created by landslide blockage in the lower Waipara River gorge as a result of an earthquake or high rainfall event.
14. The late Quaternary basin fill thickness is varaible but generally thickens to the south. The precise thickness is difficult to establish without direct borehole evidence deep enough to penetrate the Tertiary sediments. The maximum thickness in the central
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Omihi valley was estimated by Finnemore & Pettinga (2004) to be at least 130 m, and it probably thins out to 10 m or less around the basin margin. The thickness is poorly constrained in the southern part of the basin. The Kowai Formation is also likely to be variable in thickness and may be up to 100 m thick in the upper Waipara Basin and at least 240 m thick to the south (Wilson 1963). Only one well within the Waipara Basin is known to have penetrated pre-Kowai Tertiary sediments: Well M34/5573 is located on the western margin at “The Deans” where there are fault complexities. Some wells, especially those near the basin margins are deep enough to penetrate the Kowai Formation.
Groundwater
15. Drilled well depths are shown on Figure 3. I have grouped well depths on this map into the following depth ranges: 1–15 m (red symbol), 15– 50 m (green symbol), 50-100 m (blue symbol), 100-175 m (purple symbol) and greater than 175 metres (back symbol). Throughout much of the basin, groundwater well depths are shallower than 50 m depth. Additionally, it can be observed that there are five wells that have been drilled to greater than 175 m depth; four are located near the Waipara River and one is located northwest of Omihi. Well M34/5540 is the deepest well drilled to 225 m. Shallow wells less than 15 m deep are widespread south of the Waipara River.
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Figure 3: Drilled well depths in the Waipara Basin: 1-15 m (red), 15-50 m (green), 50- 100 m (blue), 100-175 m (purple) and >175 m (black). The Waipara groundwater allocation zone and the geology are shown for reference
16. Most of the Tertiary sediments in the Waipara Basin are too fine-grained and resistant to groundwater flow to be considered regional aquifers. However, localised coarser lithologies, as well as bedding planes and fractures that dip into the basin may transmit some groundwater to deeper levels.
Definition of “deep” groundwater within the Waipara Basin
17. The term “deep” is a relative and subjective term that may be interpreted differently between various experts or members of the community. As noted in paragraph 12 above, the Quaternary gravels that fill the floor of the basin are variable in thickness, and are generally thinnest around the basin margin and are thickest to the south. It is often difficult to distinguish between Kowai Formation deposits and Quaternary gravels in borelogs. For this reason, they are not logged as separate units. The combined thickness of these two units is between about 230 m at Omihi to at least 450 m thick in
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the southern Waipara Basin. The depth to the groundwater in these deposits is also variable with discrete aquifers present throughout the depth range. It was first suggested by Loris (2000) that the deeper part of the Quaternary infill and the Kowai Formation may be receiving groundwater recharge from the surrounding and underlying Tertiary sediments via structural features such as faults, joints and fractures. While I agree that this process is likely to occur, the volume of recharge from this source is unknown.
18. Four possible sources of groundwater recharge can be identified:
i. Direct rainfall infiltration within the Waipara Basin as defined by the groundwater allocation zone (Weeber 2006);
ii. Potential rainfall infiltration into the outcropping Kowai Formation on the western margin of the basin;
iii. River/stream losses from water derived from within and outside the basin; and
iv. Deep sources from within the Tertiary sediments that comprise the ranges bordering the basin.
The relative contribution of the recharge from the Kowai Formation and the Tertiary sediments is poorly understood.
19. Structural features (faults and folds) within and bordering the Waipara Basin are likely to impact on groundwater flow by forming either barriers or preferred flow paths. Such structures are likely to divide the Waipara Basin into discrete flow compartments. For example, the “Broomfield Fault” (Figure 2) may be a natural barrier for groundwater flow southward out of the Waipara Basin (Dodson 2009). This fault is close to the current Waipara groundwater allocation zone boundary. The water table flow is impeded and results in the formation of the Amberley Swamp (Figure 1). Other evidence for basin compartments is supported from an interpretation of groundwater age, chemistry data, and water level data (Dodson 2009).
20. There are two pieces of evidence which may impact on the long-term sustainability of the groundwater resource: groundwater age and groundwater levels.
Groundwater age
21. Investigations by Loris (2000) showed that a significant proportion of the groundwater was older than 50 years. These sites were undifferentiated with regards to depth of aquifer. Recent radiocarbon (Carbon-14) dating of water from six wells has revealed that there is a component of groundwater ranging in age from about 1300 to 23,500
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years old (Dodson 2009). The oldest water dated (last glaciation) is from Omihi and indicates that the deep groundwater in this area is not part of an active flow system. The more recent ages (~1300 - ~9000 years) indicate slow groundwater flow velocities, long flow paths, and also slow natural discharge. It is possible that heavy pumping of these deeper aquifers may induce groundwater flow from adjacent aquifers, poorly connected aquifers or from surface sources. Although without an understanding of the connectivity of these units and the deeper recharge processes it is difficult to determine whether effects will occur. The location of the six wells sampled and their depths are shown on Figure 4.
Figure 4: Map showing carbon-14 ages determined from groundwater sites in the Waipara Basin (Dodson 2009)
Groundwater levels
22. The groundwater level monitoring record is relatively short for both the shallow and deeper groundwater systems within the Waipara Basin. The records generally cover the last 11 years for some wells and provide a limited understanding of seasonal variability in groundwater storage. The competing trends of groundwater abstraction versus natural recharge is difficult to determine from such a short monitoring period. In comparison, more than 50 years of monitoring data has been recorded for some wells
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on the Canterbury Plains. The observed trends for some of the “deep” wells show that the winter recovery levels have progressively declined since 2001, despite the recent wet winters e.g. wells M34/5573 (90.5 m deep) and N33/0282 (170.6 m deep), displayed on Figure 5 and Figure 6, respectively. The locations of these two wells are shown on Figure 8. The lack of recovery to known winter recharge periods does highlight the structural complexity of these deeper units and the uncertainty regarding their recharge mechanisms. This is in contrast to other wells that have shown a recovery during 2008-10 close to the groundwater levels recorded in 2000/01 e.g. well N34/0060 (33.5 m deep) (Figure 7).
Groundwater Level M34_5573
16
14
12
10
8
6
Level Level (m) from GL 4
2
0
4/05/2002 4/11/2002 4/05/2003 4/11/2003 4/05/2004 4/11/2004 4/05/2005 4/11/2005 4/05/2006 4/11/2006 4/05/2007 4/11/2007 4/05/2008 4/11/2008 4/05/2009 4/11/2009 4/05/2010
Date
Figure 5: Hydrograph for well M34/5573
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Groundwater Level N33_0282
-11
-12
-13
-14
-15
Level Level (m) from GL -16
-17
15/11/2000 15/05/2001 15/11/2001 15/05/2002 15/11/2002 15/05/2003 15/11/2003 15/05/2004 15/11/2004 15/05/2005 15/11/2005 15/05/2006 15/11/2006 15/05/2007 15/11/2007 15/05/2008 15/11/2008 15/05/2009 15/11/2009 15/05/2010 Date
Figure 6: Hydrograph for well N33/0282
Groundwater Level N34_0060
-20 -20.5 -21 -21.5 -22 -22.5 -23 -23.5
Level Level (m) from GL -24 -24.5
-25
5/09/1999 5/09/2000 5/09/2001 5/09/2002 5/09/2003 5/09/2004 5/09/2005 5/09/2006 5/09/2007 5/09/2008 5/09/2009 5/09/2010
Date
Figure 7: Hydrograph for well N34/0060
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Figure 8: Map showing locations of wells N33/0282, N34/0060, M34/5573, Amberley swamp and “Broomfield Fault” (Dodson 2009)
Concluding comments
23. The geological structure of the Waipara Basin is complex and that appears to have resulted in partitioning of the basin with poor groundwater flow connections between partitions. This feature is reflected in the variable groundwater age distribution and the inconsistent recovery of groundwater to 2000/1 levels.
24. There is much uncertainty regarding the nature of the groundwater flow system relating to the deeper connection between the Tertiary sediments, Kowai Formation gravels and the more recent Quaternary infill within the Waipara Basin. There is potential for “deeper” groundwater abstraction to affect groundwater resources in the shallow Quaternary aquifers, thereby reducing reliability of supply for any users of the Quaternary aquifer.
25. For continued groundwater development and management in the Waipara Basin, the question of renewability of this resource is most important. The exploitation of a non-
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renewable resource can have serious implications especially where the groundwater is old and the water levels in some deeper wells in the Waipara Basin are showing declining winter highs. This may be due to pumping (mining), reduced winter recharge or a combination of these factors.
Signed:
John Weeber Hydrogeologist
REFERENCES
Brown, L.J.; Weeber, J.H. 1992: Geology of the Christchurch Urban Area, Geological map 1:Institute of Geological and Nuclear Sciences, Lower Hutt New Zealand, scale 1:25 000, 1 sheet.
Dodson, D.D. 2009: Active tectonics, geomorphology and groundwater recharge to the Waipara-Kowai Zone, north Canterbury. MSc thesis, University of Canterbury.
Environment Canterbury 2010: Proposed Waipara Catchment Environmental Flow and Water Allocation Regional Plan.
Finnemore, M.; Pettinga, J. 2004: Seismic Reflection Study of Omihi Valley in North Canterbury. Final report submitted to Omihi Irrigation Society. University of Canterbury, Christchurch, New Zealand.
Forsyth, P.J.; Barrell, D.J.A.; Jongens, R. (compilers) 2008: Geology of the Christchurch area. . Institute of Geological & Nuclear Sciences 1:250 000 geological map 16. GNS Science, Lower Hutt, New Zealand.
Lloyd, I. 2002: The Water Resources of the Waipara Catchment and their Management. MSc thesis, University of Canterbury.
Loris, P. 2000: Hydrogeology of the Waipara Alluvial Basin, North Canterbury, New Zealand. MSc Thesis, University of Canterbury.
Rattenbury, M.S.; Townsend, D.B.; Johnston, M.R. (compilers) 2006: Geology of the Kaikoura Area. Institute of Geological & Nuclear Sciences 1:250 000 geological map 13. GNS Science, Lower Hutt, New Zealand.
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Weeber, J.H. 2006: Waipara Groundwater Zone Boundary. File IN6C/00039-05.
Wilson, D.D., 1963: Geology of the Waipara Subdivision. New Zealand Geological Survey Bulletin n.s. 64.
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