Groundwater assessment of Thirlmere Lakes

The NSW Department of Industry (department) has undertaken an assessment of the possible causes of water level decline reported for Thirlmere Lakes in recent times.

Various lines of evidence have been considered to establish the likely cause of the reported water level decline. These have included consideration of Tahmoor Colliery operations, long-term rainfall records, broader catchment river flow data and regional groundwater levels.

No hydrology data exists for the lakes themselves, however the local and region hydrology data that is available indicates that the most likely cause of the reported water level declines in Thirlmere Lakes is due primarily to the prevailing climatic conditions. Specifically the continuing declining trend in rainfall, established during the recent severe drought, appears to be affecting on surface water run-off and recharge to groundwater. This trend needs to be reversed by extensive and protracted rainfall events before recovery in flows and lake levels can be observed.

As a result of this investigation, there was no evidence to suggest that mine fracturing or subsidence has affected the water levels in Thirlmere Lakes in any substantial way.

Read the full report: Thirlmere Lakes groundwater assessment (PDF, 7467.23 KB).

The department has completed the drilling of four groundwater monitoring bores in the vicinity of three of the five Thirlmere Lakes at the Thirlmere Lakes National Park. Drilling was undertaken to address one of the data gaps identified by the department in a groundwater assessment report published in December 2010. The lack of a groundwater monitoring record in the vicinity of the lakes has been a significant missing element in understanding the Thirlmere Lakes hydrogeologic environment.

The results of the completed drilling project have been documented in the Thirlmere Lakes drilling report (PDF, 7845.76 KB). Please note this will take 19 minutes to download at 56 Kbps). The report details the drilling and construction of the monitoring bores completed during May and June 2011. About 190 linear metres of drilling was successfully completed, of which samples have been collected for future research projects. A copy of the report was provided to the Thirlmere Lakes Independent Inquiry Committee.

The drilling will not result in immediate answers to the problem of declining water levels in the lakes, but the monitoring bores will be used to collect long-term information for future interpretation.

Additional works relating to the monitoring of lake levels and rainfall within the lakes catchment will be needed to complete the picture. The monitoring bores will provide evidence of groundwater level trends over the long-term as a progressive record of automatic data is gathered.