『Abstract
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 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℃)
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℃. 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.
Keywords: Submarine groundwater discharge; Numerical modeling;
Satellite imagery; Southern Perth Basin; Australia』
1. Introduction
1.1. Background
1.2. Purpose and scope
1.3. Methodology
2. Description of the study area
2.1. Onshore study area
2.2. Climate
2.3. Physical oceanography
3. Geological and hydrogelogical overview
4. Hydrogeological assessment of submarine groundwater discharge
4.1. Superficial Aquifer
4.2. Leederville Aquifer
4.3. South West Yarragadee Aquifer
5. Submarine groundwater discharge derived from the SWAMS model
5.1. Superficial Aquifer
5.2. Leederville Aquifer
5.3. South West Yarragadee Aquifer
6. Satellite thermal infrared images of Geographe Bay
7. Seabed bathymetric analysis
8. Discussions
9. Conclusions and recommendations
Acknowledgments
References