A multidisciplinary study of groundwater conditions in sedimentary strata at Thirlmere Lakes (NSW)

Abstract

The Thirlmere Lakes include five natural wetlands within a Blue Mountains World Heritage listed national park, where there are concerns over an apparent possibility of a long term decline in water levels. Lake levels correlate with rainfall variability and are historically known to have dried several times during prolonged droughts. However, the effects of long term hydrological changes on the Lakes are unclear, as are uncertainties associated with extraction of water for local uses and the possible effects of nearby longwall coal mining. This study is part of a large multi-disciplinary research program, of which this part focuses on groundwater conditions insedimentary strata, and the possibilities of interactions with sediments below the Lakes. Surface geophysical techniques and mapping of geological structures will be combined with deep drilling, wireline logging, geological and hydrogeological investigations. Characterisation of sedimentary strata include permeability, bulk density, moisture content, porewater stable water isotopes and XRD mineral identification. New deep drillholes are planned to obtain information on hydraulic properties of formations. A staged geophysical survey program is designed to complement the geological investigation. Resistivity imaging, time-domain electromagnetics and ground penetrating radar (GPR) will be employed to define sedimentary structure within the unconsolidated alluvium (sand, clay and peat layers) and depth estimates to the underlying rock. A combination of these geophysical methods and contextural geological information, will be used to identify structural and sedimentological anomalies and their hydrogeological properties such as permeability and connection to deeper strata. The results of this work provide new data on groundwater conditions in structured rock that underlie the sediments of Thirlmere Lakes.