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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 Page 1 of 14 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 Page 2 of 14 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. Page 3 of 14 Figure 2: Geological development of the Waipara Basin (Finnemore & Pettinga 2004) Page 4 of 14 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 Page 5 of 14 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. Page 6 of 14 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.

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