For this assessment, we:
- installed 16 dedicated seawater intrusion monitoring bores
- completed six electrical resistivity imaging transects
- conducted electrical conductivity profiling on 13 deep bores every three months for a year
- analysed chemistry samples for 15 bores
- collaborated with EMM Consulting to analyse all data and:
- develop conceptual cross-sections for the four subareas in the groundwater area
- create a spatial map of the depth and extent of the seawater interface.
A summary of our findings follows. Details on the methods we used and the full results of the analysis are in the report HR443: Cockburn seawater interface – Priority area assessment.
To request a copy, send an email to groundwater.info@dwer.wa.gov.au.
Where is the seawater interface?
The seawater interface is in the Superficial aquifer. It extends down into the Leederville aquifer where the Leederville and Superficial aquifers are connected in the Valley and Wellard subareas. It does not extend into the Leederville aquifer in the Kogalup and Thompsons subareas, where the Kardinya Shale aquitard separates the two aquifers.
The Superficial aquifer in the Cockburn groundwater area consists of limestone and sands that extend along the coastline and out into the ocean. Where the dense salty water from the ocean meets the fresh water flowing out to the sea from the Superficial aquifer, the waters mix in a wedge-shaped zone of dispersion called the seawater interface. The ‘toe’ of the seawater interface refers to where the wedge flattens out to its farthest inland position along the base of the aquifer.
In the Cockburn groundwater area, the salinity in the zone of dispersion of the seawater interface begins at 10,000 ms/cm in electrical conductivity or 6,700 mg/l total dissolved solids. Salinity increases as you move towards the ocean until it reaches the same salinity as seawater which is > 35,000 mg/l total dissolved solids.
We developed a spatial layer to show the location of the seawater interface in the Superficial aquifer. The seawater interface follows the coastline, extends up to 1,000 m inland and can be nearly 30 m below sea level. We have developed cross-sections which show how the seawater interface works in each subarea. The cross-sections can be downloaded in the sub area fact sheets below and the depth to seawater interface spatial layer can be downloaded at www.data.wa.gov.au.
Depth and extent of the seawater interface in the Cockburn groundwater area
Where to get more details
The location and depth of the seawater interface in the Cockburn groundwater area are available as a spatial layer and can be downloaded via DataWA.
Download the representative cross-section and a brief summary for each subarea below:
Download the Cockburn seawater interface project drilling program – Bore completion report, HR408.
Request copies of the internal technical reports for this investigation by emailing groundwater.info@dwer.wa.gov.au.
These include:
- Cockburn seawater interface – Priority area assessment, HR443 (Department of Water and Environmental Regulation 2024)
- Electrical Resistivity Imaging for Coastal Hydrogeology; Cockburn Perth Western Australia (Curtin University 2017).
Go to our Water Information Reporting portal to access data from the monitoring bores installed during the investigation.
