Oral Presentation NCGRT/IAH Australasian Groundwater Conference 2019

Seawater intrusion and aquifer compartmentalisation in the Darwin rural area (352)

Anna Haiblen 1 , Melissa Woltmann 2 , Larysa Halas 1 , Klara Steklova 1 , Anandaroop Ray 1 , Donna Cathro 1 , Ken Lawrie 1 , Desmond Yin Foo 2
  1. Geoscience Australia, Symonston, ACT, Australia
  2. Department of Environment and Natural Resources, Palmerston, NT, Australia

Groundwater is the primary source of water for residential and agricultural use in the Darwin rural area, and also supplies part of Darwin’s city water. In the Howard East area, groundwater bores are screened in the Koolpinyah Dolomite aquifer, a ‘spill and fill’ aquifer that responds to wet-dry seasonal cycles. The 2018-2019 wet season saw low levels of rainfall and groundwater recharge, resulting in significant water stress. The Koolpinyah Dolomite aquifer is geologically complex, and is intersected by faults and/or shear zones that may compartmentalise the aquifer or act as preferential flow paths for groundwater. This project was initiated to better understand the influence of these geological features on potential seawater intrusion.

The geology and hydrogeology of the Howard East area has been refined using airborne electromagnetic (AEM) and airborne magnetic data collected for this project as part of the Geoscience Australia’s Exploring for the Future Programme. This AEM dataset was compared with previous AEM and borehole data to investigate changes in groundwater quality and dynamics. Water level loggers were installed in existing monitoring bores close to production bores for town water supply and the fault/shear zones. Pumping data were supplied by the bore-field operator, and data were interrogated to capture background groundwater dynamics and drawdown responses to pumping. Drawdown responses from either side of different geological structures were compared to test if aquifer compartmentalisation is occurring. Groundwater samples were collected throughout the area to investigate groundwater age and quality, and to test if possible aquifer compartmentalisation is affecting groundwater composition.

Preliminary analysis suggests that geological faults/shear zones compartmentalise the aquifer. Work is ongoing to assess the role of structures in seawater intrusion. The findings from this project will be used to inform bore-field and seawater intrusion management.