The mystery of Thirlmere Lakes

Thirlmere Lakes National Park, located south-west of Sydney in an ancient river meander, contains five lakes – Lake Gandangarra, Lake Werri Berri, Lake Couridjah, Lake Baraba, and Lake Nerrigorang (Figure 1).

Two WRL research teams (EcoEng, opens in a new window and Connected Waters, opens in a new window) have investigated the water balance budget and surface-groundwater interaction in Thirlmere Lakes. These investigations were supported by coordinated research projects with ANSTO, opens in a new window, University of Wollongong, opens in a new window, and the NSW Department of Planning, Industry and the Environment, opens in a new window (DPIE).

In collaboration with these groups, WRL engineers undertook extensive fieldwork between 2017 and 2020 to monitor the site, including remote sensing bathymetry surveys, deploying micro-meteorological stations for measuring evapotranspiration, and installing a piezometer network for groundwater investigations.

In February 2020, the WRL research teams presented their findings at the Annual Science Day meeting in Picton, NSW. Hosted by the NSW DPIE, interesting outcomes presented to the community included:

• The lakes are more sensitive to climate than other anthropogenic activities. When evaporation is greater than rainfall, the lakes lose water and eventually dry out. This balance drives the lake water levels.
• The recent drought is not unprecedented. Evidence from the water balance calculations, which are supported by satellite data, indicate that the lakes have dried out previously. Previous dry periods include the World War II drought and other earlier periods (Figure 2).
• All lakes are set within a complex fabric of geological layers deposited over hundred thousands of years (Figure 3). Some layers have a high hydraulic conductivity e.g. sand, which potentially allows groundwater to move, while silt and clay layers with a low hydraulic conductivity inhibit the movement of groundwater. 
• Peats found in the lake beds have a variable but generally low hydraulic conductivity. At low water levels the surface water is perched on this peat.
• During the recent drought the differences between surface water and groundwater levels indicated that water flows from the lakes towards the groundwater.
• The low hydraulic conductivity of the peat, silt and clay layers however prevent substantial downward movement of water to the regional aquifer (Figure 4), which suggest it may take years to decades for water to move through the sediments.
• However, every lake has a different degree of groundwater-surface water connection depending on its local geology.

Investigation into the hydrogeologic processes has revealed that the rainfall and evapotranspiration has mostly driven the recent lake level declines.