Berhe, D.T.; Kellett, R.L.; Herpe, M.; Sahoo, T.R. 2026 Aqua Intel Aotearoa Phase 3: Gisborne SkyTEM Interpretation of East Coast groundwater systems. GNS Science consultancy report 2025/146; Aqua Intel Aotearoa report 2025/08. 96 p. https://doi.org/10.21420/YSH2-Z453
Abstract
Aquifers are present in a series of small coastal basins in Hicks Bay, Te Araroa Bay, the Waiapu River valley and the ŪawaRiver valley, including Tolaga Bay. The SkyTEM resistivity models for the four surveys that cover these basins (Rawlinson et al. 2025) have been interpreted to provide a model of the groundwater systems. In all four areas, the interpretation included identifying characteristic resistivity contrasts between Quaternary sediments and bedrock (hydrogeological basement) units. These interpretations were constrained primarily using borehole lithological logs and geological maps. Hicks Bay and Te Araroa - In Hicks Bay, the Quaternary layer is thin along the coast, reaching a maximum thickness of 20 m at the northern end of the beach. A thicker Quaternary section is present in the hills behind the beach on uplifted terraces, but most of that section is unsaturated or dominated by fine-grained sediment, i.e. relatively low permeability. The thickest zone of potential aquifer material lies adjacent to the river in the Hicks Bay flats. Drilling in the Matakoao Formation volcanics on the ridge to the north identified a fractured basalt with aquifer potential. The leading edge of the overthrust volcanics has been mapped using the SkyTEM data. In Te Araroa, the Quaternary cover is thin across the entire plain behind the beach. There is an obvious paleo-channel that lies to the north of the Karakatuwhero River. The paleo-channel is incised up to 20 m into the Miocene mudstone hydrogeological basement and is filled with moderate resistivity sediment. The borehole drilled at the edge of the channel is characterised by sands and gravels, and the groundwater has a low salinity, making it a potential water source. A simple assessment of groundwater potential was completed by weighted overlay of the classified quaternary thickness map and classified mean SkyTEM resistivity above hydrogeological basement. Waiapu and Ūawa Tolaga Bay These two basins are larger and contain more borehole data than the small basins in Hicks Bay and Te Araroa. A machine-learnng approach to aquifer characterisation has been applied using the borehole lithologies and groundwater data. The modelled Quaternary thickness in Waiapu varies from 0 m to 62 m, and to about 102 m in the Tolaga Bay area. Groundwater potential maps were produced to delineate zones of varying potential. Higher potential zones coincide with coarse-grained channel sediments, representing the most favourable regions for groundwater development, whereas low-potential zones correspond to clay-silt-dominated aquitards limiting groundwater flow. In the Waiapu area, the high groundwater potential zones are concentrated along the central northeastern parts along the main river courses. In the Ūawa/ Tolaga Bay area, these zones are dominant in the northern and western parts of the basin. Overall, the groundwater systems in the coastal basins of the East Cape are small in areal extent and are restricted to a thin Quaternary layer. In some places, the Quaternary layer is greater than 50 m thick in paleo-channels. The lithologies are dominated by silt, but some sand and gravel
