Salt mapping in the Billabong Creek catchment
The Billabong Creek catchment is in southern New South Wales north of Albury. Part of this catchment was flown for the Murray-Darling Basin Commission to support the CSIRO Heartlands Program. The purpose was to demonstrate the usefulness of airborne geophysics for the management of salinity at a catchment scale, and to help develop sustainable farming systems for salt-affected land.
We knew that around 64,000 tonnes of salt a year was being exported from somewhere in the 300,000 hectare Upper Billabong catchment, and eventually into the Murrumbidgee River, but where was the salt coming from?
64,000 tonnes of salt per year is exported from 300,000 ha
in the Upper Billabong catchment.
Where does the salt come from?
Stream sampling of conductivity identifies the areas
which are the source of salinity "hot-spots" (yellow / red dots)
Systematic instream sampling of electrical conductivity showed that most of the salt was coming from the area west of Culcairn. This area was then flown to collect airborne electromagnetic (AEM) data. Analyses of these data, together with borehole information showed that salt stores are restricted to the alluvial terrace of Kangaroo Creek; a shallow clayey blanket in the Simmons Creek sub-catchment; and a deeper sump downstream of Walbundrie. Of these, only Simmons Creek contributes significant salt to Billabong Creek.
AEM survey showing the location of salt stores
Red = salt stores
Blue = no salt
The airborne geophysical techniques afford a three-dimensional perspective, providing new insights into the location of salt in the landscape and its delivery to streams and to the land surface. First, large parts of the landscape are salt-free. Secondly, salt is lodged in various facets of the landscape, and these may be unique to a particular landscape, related more to soil (or regolith) than to geology. Salt is stored in clays, whilst the conduits that carry groundwater (and salt) through the landscape are interconnected sands and gravels.
Mobilisation of this salt can now be addressed by hydrogeological modelling using the 3-dimensional framework. Management options can be identified with greater confidence, and the precise areas where this management is required can be specified.