The characteristics of the Waiwhetu artesian aquifer beneath Wellington Harbour including the spatial distribution and causes of submarine spring discharge

Abstract

This thesis is a study of the sub-harbour Waiwhetu Artesian Aquifer, and in particular the nature and characteristics of artesian leakage from submarine springs. This aquifer is a sheet of gravel and other coarse sediments which continues from the Lower Hutt Valley and extends beneath Wellington Harbour where it varies in thickness from approximately 70m against the Wellington Fault scarp to just over 20m thick against the eastern harbour margin. The water it contains is a valuable resource - supplying approximately one third of Wellington's municipal water consumption. At present, there are plans to utilise this artesian water source to a greater extent in the future, to support a greater burden of the region's water requirements. However concerns over possible salt-water intrusion and contamination of the aquifer led to an interest in developing a better understanding of the characteristics of this artesian system, particularly beneath the harbour floor. Harbour floor depressions were selected as likely sources of artesian water leakage based on the presumption that they had been formed by the action of leaking artesian water from beneath. Eleven depression 'zones' were investigated by recording the salinity of the water within the depressions using a portable conductivity/temperature meter. SCUBA diver's observations and bathymetric mapping revealed that depressions ranged in width from 53m to 369m (at the harbour floor) and 12m to 69m (at the depression base), with depths ranging from 13.3m to 31.3m below sea level. Only a few depressions were found to be actively discharging significant amounts of artesian water. SCUBA diver investigations found this leakage to be typically concentrated at a number of small and discrete spring 'vents' located on the base of the active depression. Typical salinities recorded ranged from 28 - 33 ppt within a few centimetres of the discharge vents. Deployment of an S4 current meter in two depressions showed that spring vent discharges vary with the pattern of abstraction from the pumping stations in the Lower Hutt Valley and as a consequence of the tidal cycle. High tides generate a greater load on the underlying aquifer, which in turn compresses the aquifer structure to a greater extent than at low tides, thus 'squeezing' out more water. Almost all of the recorded leakage was found to occur from a cluster of submarine springs within one of the depression zones, roughly 1100 metres from the Hutt River mouth. One other notable area of leakage was found close to Seaview Wharf. No significant leakage was observed or recorded from the two deep depressions south of Somes Island, which had previously been considered to be the major outlet of artesian leakage in the harbour. Lower than normal salinity values were also recorded in the harbour entrance. In this region the aquiclude is hypothesised to peter out, allowing artesian water to escape from the aquifer through a large area of the seabed in the form of widespread leakage as opposed to the concentrated, or discrete, form associated with spring vent discharge. Seismic profiles were used to map the extent of the sub-harbour Waiwhetu Artesian Aquifer and its upper confining aquiclude, the Petone Marine Beds. This work showed that the aquifer gravels extend across the entire harbour area. However, the water-bearing capacity of these deposits was found to be inconsistent. Preferential pathways, present as paleochannels (relic river channels), can be mapped within the aquifer. They concentrate the flow of groundwater through the aquifer and as such supply the submarine spring regions with much higher rates of water flow than the inter-paleochannel areas. The harbour floor depressions are thought to have formed as a consequence of the deformation of aquifer and aquiclude deposits during intense shaking associated with earthquakes leading to the removal of portions of the confining aquiclude. This typically occurs from a combination of raised piezometric pressures as a result of consolidation of the aquifer material during shaking, and from failure of the aquiclude by liquefaction. This rupture of the aquiclude results in the release of large volumes of artesian water through the aquiclude and to the sea. As this flow of water moves upward through the aquiclude, it erodes and transports away the fine sediment that forms this member. The features left behind are the characteristic harbour floor depressions we associate with submarine spring discharge. The abstraction of water from the Waiwhetu Artesian Aquifer (for the Wellington municipal water supply) lowers the piezometric pressure within the aquifer close to the abstraction zone. As such, the relocation of the abstraction focus (during 1980) to three kilometres inland from Petone Foreshore has greatly improved the 'health' of the sub-harbour aquifer and has similarly reduced the threat of salt-water intrusion. Data gathered during this study implies that while the two deep depressions south of Somes Island are unlikely to be a threat with regard to salt-water intrusion, the cluster of depressions off the Hutt River mouth could be a site of salt-water entry if piezometric pressures in the aquifer beneath them dropped low enough. Spring discharge velocities collected over one spring vent indicate that the presently set minimum piezometric Petone Foreshore level (below which abstraction must cease) needs to be revised.