Thirlmere Lakes Research Newsletter - Spring 2019

The continuing drought has been challenging on many fronts, but researchers continue to generate results and communicate with the local community.

In association with the University of New South Wales (UNSW), GeoQuEST Research Centre, the University of Wollongong (UOW), the Australian Government and the Australian Nuclear Science and Technology Organisation (ANSTO).

Research efforts intensify at Thirlmere Lakes

The sampling program at Thirlmere Lakes has intensified as the lakes dry out. There is now a total of 20 shallow groundwater bores installed by the Connected Waters team from UNSW at depths between 2.5 and 8.5m measuring water levels and water temperature to get a more accurate picture of shallow groundwater movements. However, as water tables have fallen steeply in the last few months, these bores may be left high and dry.

Geomorphology team

The UOW geomorphology team have been drilling boreholes up to 14 and 15m in Lakes Werri Berri and Baraba and while they have yet to find bedrock, they have found that lake sedimentation to be highly variable between depths. This indicates that there were several differing climate regimes operating on the lakes in the last 100,000 years. The lake sediments may be used in the future as a useful regional environmental record.

Fang Bian, a UNSW researcher installing a piezometer at Lake Couridjah
Lake Werri Berri sediment core, 14m below ground level

Geology team

The UNSW geology team have been hard at work analysing rock cores from the deep bores at Lake Nerrigorang. They have found that the Bald Hill Claystone at 102 m below ground level, previously thought to be an impermeable layer between upper and lower sandstone aquifers, is not nearly as impermeable at this location as expected.

Water balance budget team

The UNSW water balance budget team has been developing an historical model of lake levels dating back to the 1900s. They’ve found that it was likely the lakes were also very dry, during the World War 2 drought in the 1940s. Using water chemistry and isotope analysis the ANSTO team have found that the water in the peat below and surrounding the lakes becomes a more important water source to the lakes as they dry. During small rainfall events, the groundwater that enters the lakes is not part of the actual rainfall event but is groundwater stored in the sediment. This is especially true for Lake Baraba.

Long term lake level variability showing the 1940’s low lake levels and present-day similarities

Hidden bank storage providing water to lakes

 

The Lake Baraba conundrum

Lake Baraba is, at the time of writing, almost dry. This lake has been the most resilient of lakes but even it has succumbed to the dry weather. The hypothesis that we will be testing over the next month or so is that Baraba’s considerable peat stores and bank sediments are a large water store, providing this lake with water in dry times. We’ll be taking sediment cores in several locations on the lake bed to calculate peat volumes and water holding capacity. We’ll keep you posted on the findings.

Surface water in Lake Baraba March 2019
A dry Lake Baraba in September 2019