Abstract
The aquifer of South Dunedin, set within low-lying coastal sediments that infill a paleovalley, needs to be understood better to evaluate how the hydrogeological system responds to large influxes of surface water from the valley’s catchment. The purpose of this study is to determine the hydrogeological conditions and characteristics of the South Dunedin aquifer, based on the geological basement configuration and properties of the infilling sedimentary units, which will be estimated using gravity measurements in South Dunedin. Understanding the groundwater systems in this coastal area is crucial because the region is highly vulnerable to surface ponding and has experienced extensive flooding after heavy rain. Although initial investigations have led to the development of a regional groundwater model, more field data and other parameter approaches are required to provide sufficient vertical and horizontal resolution for detailed future planning in the area. Existing gravity data supplemented by a new coastal gravity survey will be used to develop a new detailed geological model that can be used to better consider the effects of physical processes (waves, tides, storms) and forecast future conditions on the South Dunedin aquifer. The purpose of aquifer model is generally to better describe the system in order to analyse various assumptions about its nature and dynamics. In addition to providing a description of the current South Dunedin subsurface, this gravity modelling based through ties to borehole data will enable assessment of lithologies through the region and thereby aid in the development of infrastructure and drainage plans in the future.
2D and 3D stratigraphic models of the subsurface have been generated in Oasis Montaj using two distinct methods: fence diagrams and 3D inversion. The bedrock structure of South Dunedin depicts a paleovalley connecting Otago Harbour to the Pacific Ocean through the present-day area of South Dunedin. The interpretations suggest that the silt deposits, which are dominant through the centre of South Dunedin, correspond to a region of lower permeability, which increases the flooding hazard. In contrast, shallow regions around the periphery of South Dunedin are dominated by sandier sediments with higher permeability and lower flooding hazard.