Surface water resources in the Australian interior are unreliable and scarce. However, they are vital in supporting the vegetation along and near river channels and in maintaining a delicate ecological balance along the waterholes “billabongs”.
Our research has identified and described shallow groundwater of low salinity forming freshwater lenses up to 1 km wide. These are located directly adjacent to major waterholes, and overlying the otherwise widespread saltier regional groundwater along the Cooper Creek floodplain near Ballera (south-west Queensland).
Why we study the Cooper Creek
Cooper Creek (Fig. 1), with a total catchment area of 306,000 km2 is the longest and probably most ecologically important dryland river in Australia [1]. It starts at the confluence of the Barcoo and Thomson Rivers and is fed largely by the Australian summer monsoon rainfalls along the north-western slopes of the Great Dividing Range.
Its middle to lower course is characterised by a multiple channel floodplain where interacting aeolian and fluvial sedimentation and gradually reducing river discharges over the Late Quaternary have resulted in a unique stratigraphic and geomorphic setting [2,3].
Local geomorphic features such as remnant dunes, mud-capped waterholes and floodplain, combined with large hydrological variability, result in huge uncertainties in recharge and groundwater evolution processes.
The Cooper Creek catchment provides a nondisturbed analogue to understand how other presently wetter regions, particularly those along the Murray-Darling Basin, could evolve in the future if their climate and hydrology becomes even more variable.
Furthermore, this research could be applied to understand the effects on dry-land rivers of artificially depleted natural flooding cycles and their impacts on shallow groundwater.
Defining fresh water lenses
Fresh water lenses were defined after piezometers had been installed and groundwater analysed for major-element chemistry, water stable isotopes (δ2H and δ18O) and dissolved sulfate stable isotopes (sulfate δ34S and δ18O) along a floodplain transect.
This research identified shallow groundwater of low salinity forming freshwater lenses, directly adjacent to major waterholes, and overlying the otherwise widespread saltier regional groundwater.
The groundwater is recharged through the base of the waterholes at times of flood scour, but not through the impermeable floodplain mud. Total dissolved solids rise with distance from the waterhole and stable water isotopes confirm that recharge is consistent with and dependant on, monsoonal flooding events.
Our findings at Cooper Creek have coincided with the recent description of similar lenses in the Murray River [4]. While the Murray lenses are still within a predominantly gaining stream (watertable above the base of the stream), the Cooper Creek lenses are below the base of the waterholes and is therefore a hydrologically losing stream.
The freshwater lenses we found
The freshwater lenses are asymmetrical and directly related to the size of the nearby waterhole. The lenses spread downstream with total-dissolved solids <5000 mg/L for distances of up to ~300 m from the waterholes, mixing progressively with saline regional groundwater and increasing total dissolved solids between 5000 and 15000 mg/L at 1000 m (Fig. 1b).
Higher total-dissolved solids concentrations in groundwater are found farther south in central areas of the floodplain.
Complete mixing with regional saline groundwater is not achieved in the Goonbabinna-Chookoo transect as shown by sulfate-δ34S and -δ18O of groundwater between waterholes, with generally more enriched isotopic values than regional groundwater (Fig 1c)
The hudrochemical information suggests that some degree of communication between waterholes and the underlying groundwater exists.
This is greatest during recharge episodes facilitated by channel scour and direct water contact with underlying sands and is minimised during no flow conditions due to clay settling from the receding flood waters.
All conservative major and minor ions analysed in groundwater show a progressive increase in concentrations farther from waterholes, regardless of the presence of sand dunes (Fig. 2, 3).
This trend would not be possible unless freshwater was being supplied from the waterholes and slowly mixing with the regional saline groundwater.
It also suggests there is little or no significant contribution of freshwater through the dunes, despite being comprised of porous sands and stratigraphically connected to the underlying fluvial sands.
Water stable isotopes indicate that the shallow groundwater within the Cooper Creek floodplain is recharged by water with a homogeneous isotopic signature, during major flooding events. Sulfate stable isotopes reveal a limited connection between the Chookoo waterhole and the regional shallow groundwater.
In the proximity of the waterhole sulfate -δ18O shifts towards enriched values, probably as the result of hyporheic redox processes within the waterhole sediments, and suggests diffuse leakage to the underlying watertable may occur during no flow conditions.
The limited extent of freshwater lenses surrounding the waterholes may have important controls on the floodplain vegetation ecology, despite the ability of these trees to tolerate highly saline ground and soil water.
The ground and sufrace water dynamic investigated may also rivers that are expected to experience more ephemeral flow regimes under changing climatic and anthropogenic conditions.
Future work
A new piezometer transect has been installed and sampled with four monitoring piezometers left to investigate seasonal fluctuations in groundwater level. The new samples are being analysed at ANSTO for other naturally occurring isotopes (14-Carbon, Tritium), these results will define the time scales involved in the formation of the fresh water lenses.
References
- Kingsford R.T., Curtin A.L. and Porter J., Biological Conservation, 88 (1999) 231 248.
- Nanson G.C., Price D.M., Jones B.G., Maroulis J.C., Coleman M., Bowman H., Cohen T.J., Pietsch T.J. and Larsen J.R. Geomorphology, 101 (2008) 109-129.
- Maroulis J.C., Nanson G.C., Price D.M. and Pietsch T, Quaternary Science Reviews, 26 (2007) 386-404.
Authors
Dioni l. Cendón1,2, Josh R. Larsen1,2, Brian G. Jones2, Gerald C. Nanson2 and
Stuart I. Hankin1
1ANSTO and 2University of Wollongong, Australia
Published: 15/09/2009