Estimation of submarine groundwater discharge into Geographe Bay, Bunbury, Western Australia

A study of the submarine groundwater discharge (SGD) into Geographe Bay between Bunbury and Dunsborough in the south west of Western Australia has been carried out under the CSIRO Wealth from Oceans Flagship program. The study focused on establishing a conceptual understanding of SGD and its indirect quantification using hydrogeological (hydraulic) and modeling techniques. Satellite infrared images and near shore bathymetry were used to obtain evidence of the possible geographic distribution of significant SGD. The study area forms an offshore part of the Southern Perth Basin. Past studies have shown that there is substantial groundwater flow within the Superficial, Leederville and South West Yarragadee aquifers of the Southern Perth Basin in the offshore direction. The groundwater from these aquifers must ultimately discharge offshore at the seabed into Geographe Bay in the north and the Southern Ocean in the south. Some discharge also takes place onshore via rivers and drains.The results of the study show that groundwater discharge into Geographe Bay from the Superficial Aquifer between Bunbury and Busselton is 240–284 m³/day/km of the 67 km long coastline. The offshore groundwater flow from the Leederville Aquifer along the 55 km length of the coast where the aquifer occurs is estimated at 270 m3/d/km of the coastline. The offshore flow from the South West Yarragadee Aquifer is at the rate of 3360 m3/d/km of the Geographe Bay coastline. The large flow rates in the South West Yarragadee Aquifer are due to substantial downward leakage from the overlying Leederville Aquifer as well as significant recharge from rainfall in the outcropping areas. The total offshore groundwater discharge to Geographe Bay from all aquifers is estimated to be around 80 million m3/year on the basis of hydraulic assessment and the modeling results. Remotely sensed infrared images were analysed to detect evidence of any submarine groundwater discharge. The seasonal temperature contrast between the groundwater discharge at the seabed and the seawater makes it theoretically possible to detect areas of significant offshore groundwater seepage using suitable thermal images. ASTER thermal-infrared satellite images were acquired and analysed for this study. The ASTER images showed a low-temperature plume (21 °C) in the southwest of the study area near Dunsborough Fault, possibly groundwater discharge, extending in an easterly direction and being surrounded by seawater of about 23 °C. Similar cooler zones were observed along the coastline indicating groundwater discharge from the Superficial Aquifer. Following a hypothesis that former onshore coastal groundwater discharge features are likely to continue to be SGD focus areas following marine transgression, the near shore bathymetry for Geographe Bay revealed a series of ridges and troughs running sub-parallel to the present-day beach. The hillshade rendition shows a sub-parallel trough feature located about 4–5 km offshore.The bathymetric features identified offshore and the low-temperature plume as observed in the satellite images provide indicators and target areas for more detailed investigation and actual measurement of SGD fluxes, for example by CTD (Conductivity, Temperature and Depth) and SSS (side-scan sonar) probing around target bathymetric features. In the interim, this study has provided valuable information about the rate of potential groundwater seepage from the Southern Perth Basin aquifers into Geographe Bay indicating the scale of possible offshore groundwater resource availability based on SGD recovery and management.