A multifaceted approach to investigating hydrogeological complexities in the Koolpinyah Dolostone Aquifer, Howard East, Northern Territory

Groundwater is an essential part of Darwin’s water supply mix, and is sourced from Howard East Borefield (HEB) and McMinns Borefield in the Koolpinyah Dolostone Aquifer (KDA), east of Darwin. Previous work suggested that electrical conductivity anomalies observed in airborne electromagnetic (AEM) data within 8 km of HEB may be caused by saline groundwater within the KDA that is separated from HEB by geological features that effectively compartmentalise the aquifer. Nevertheless, concerns grew that increased groundwater use may result in migration of saline groundwater towards HEB, which could compromise the groundwater resource. We collected hydrochemistry, including isotopes, time-series groundwater salinity and AEM data to better understand the complexities of the KDA. These data are presented here, along with a hydrodynamic analysis undertaken by the Northern Territory Department of Environment and Natural Resources, which shows that drawdown is occurring more rapidly from the NE of HEB and that dykes ~8 km NE of HEB act as barriers to groundwater flow. We show that groundwater sampled on the NE side of these dykes has a seawater composition. We use new AEM data to map the elevation of the top of unweathered dyke material and to characterise AEM conductors proximal to HEB. Our mapping reveals that the top of the unweathered portion of these dykes is commonly below sea level. We also show that AEM conductors proximal to HEB are more likely mineralised clays than saline groundwater within the aquifer. Drilling is required to confirm these results. Our findings contribute to building a robust conceptual understanding of the KDA and will inform future modelling of the groundwater system. Citation: Haiblen, A.M., Symington, N.J., Woltmann, M.J., Ray, A., Gow, L.J., Leplastrier, A. and McGrath, E.S.B., 2020. A multifaceted approach to investigating hydrogeological complexities in the Koolpinyah Dolostone Aquifer, Howard East, Northern Territory. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.