Westerhoff, R.S.; Herpe, M.; Sahoo, T.R.; Santamaria Cerrutti, M.E.; Strogen, D. P.; Rawlinson, Z.J.; Narvaiza, M. 2025 Hydrogeological mapping of the Aupōuri Peninsula, Northland: interpretation of SkyTEM resistivity models. Aqua Intel Aotearoa report 2025/04; GNS Science consultancy report 2025/89. 93 p. https://doi.org/10.21420/B2ZY-TD33
Abstract
This report contributes to Aqua Intel Aotearoa investigations for the Aupōuri Peninsula, Northland. A SkyTEM survey, flown during November 2022, collected a total of 6471 km of data flown in parallel lines that covered most of the peninsula. Building on previous reports of data collection and the processing of these line data to profiles and a 3D volume of electrical resistivity, this report describes the hydrogeological interpretation of these resistivity data.The interpretation centres around four major hydrogeological features of interest in the study region: (1) location and geometry of the saline interface; (2) the depth to hydrogeological basement; (3) presence of aquifer and aquitards in the aquifer, including the presence of shell-rich sands, i.e. ‘shell beds’; and (4) near-surface features that can act as barrier to upward and downward movement of shallow groundwater, such as iron pans (hardened sand deposits) and peats. Our methods incorporate a combined approach of literature review, bore data and measurements of ground-based TEM and SkyTEM resistivity data. The application of a suite of classification techniques has translated the resistivity data to information suitable for future groundwater investigation, including groundwater modelling. The resulting 3D datasets can be viewed for depth slices or cross-sections of interest. Despite no large (but some small) areas of elevated salinity being identified, caution is advised on pumping directly above low-resistivity (likely Taipa mudstone) saline basement, as there is risk of inducing inflow of saline fossil water. This study has allowed us to more accurately delineate the groundwater system. The system consists of the following hydrogeological units (HU): • Shallow aquifer (HU1): Mostly Quaternary, and some older, yellow/brown fine sands, interbedded with silt, peat and clay layers. Iron pans are more prevalent in the older parts of this unit.; • Deep aquifer (HU2): Mostly Plio-Pleistocene, largely fluvial, tidal and estuary grey-blue sands that, in some instances, contain increased amounts of shell material – a key local aquifer.; • Basement (HU3), upon which the HU2 was deposited, is complex and consists of many different rock types, both allochthonous as well as in situ deposits. he groundwater system has been classified by ‘aquifer potential’, i.e. the relative potential to have hydraulic properties such that a structure can be identified as an aquifer. The near-surface geological QMAP delineation of Pleistocene to Pliocene dune-sand deposits map the presence of iron pans in the shallow subsurface better than SkyTEM data. Indirectly, the SkyTEM data enable mapping the locations of subsurface peat lake deposits, as the resistivity of these saturated zones show contrast with the surrounding unsaturated areas. SkyTEM data has provided a uniform, spatially consistent, 3D dataset over the survey area, including areas (forests, wetlands, areas with restricted access) that are inaccessible with ground-based methods. The limitations of the SkyTEM technique identified in this study are the decreasing vertical resolution with depth, compared to a high-quality bore log, and the inability to distinguish between layers of similar electrical resistivity. This study has made use of the combination of SkyTEM data with existing and new bore information and aquifer testing, allowing it to combine the strengths of the methods. (auths)
