‘Age’, recharge rates and connectivity of groundwater in deeper aquifers of the Sydney Basin

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Geological Society of Australia
The Permo-Triassic Sydney Basin covers almost 50 000 km2 and extends from the outer continental shelf inland to the Great Diving Range, from Newcastle in the north to Batemans Bay in the south. Major lithological units broadly include the Permian Coal Measures, the Permo-Triassic Narrabeen Group, the Triassic Hawkesbury Sandstone and the Wianamatta Shale. The Hawkesbury Sandstone is generally made of very thick heavily compacted quartz sands, with minor discontinuous shale units. Its aquifer system is a complex, dual porosity, deep fractured system with three aquifers typically recognised. The shallow and intermediate aquifers contribute to spring and base stream flows as well as groundwater dependent ecosystems, and the deep regional aquifer system. It is this deeper system that is investigated in this study. Groundwater from the Sydney Basin, and in particular Hawkesbury Sandstone aquifers, forms part of emergency supply strategies for coping with future severe droughts, with >5 million people living in the region, in addition to large industrial development. Despite the significance of these resources there are still large gaps in our knowledge of these aquifers including aspects such as age recharge and mixing rates. Filling these knowledge gaps has become even more critical in order to understand impacts of existing and planned coal and coal seam gas (CSG) mining of the underlying Illawarra Coal Measures. Community concerns over risks associated with CSG extraction have reached fever pitch in recent years, and there is public demand for research into these aquifers. Understanding of these systems has been complicated by the poor quality of existing data – commonly relying solely on driller bore-logs, reporting only being carried out for specific mine or extraction activities, and therefore conducted over localised zones, and the lack of communication between companies and agencies with data. Additionally, large variations in hydraulic properties have been noted over localised areas. This study sampled bores along a loosely east–west transect across urban Sydney, targeting the deeper Hawkesbury Sandstone and Narrabeen group aquifers. Very high salinities are recorded by several samples, interpreted to relate to the influence of the overlying Wianamatta Group and Cumberland Basin sediments in those locations. Equally however, this signal may record the impact of interaction with coal seams. Results also show inconsistencies between tritium and radiocarbon groundwater ‘ages’ in multiple locations, suggesting that extensive mixing occurs between aquifers. A relationship between bicarbonate, depth and δ 13C isotopic ratios highlights the influence of methanogenesis for deeper samples and either interaction with localised organic matter or deeper inputs derived from the coal measures. These findings have implications for potential coals seam gas extraction in the region, demonstrating that impacts could be significant in areas of high fracturing and connectivity. This supports previous assessments of groundwater vulnerability and the need for further detailed research. © Geological Society of Australia Inc
Australia, New South Wales, Ground water, Groundwater recharge, Droughts, Coal mining
Kermode, S., Cendón, D., Hankin, S. & Russell, G. (2014). ‘Age’, recharge rates and connectivity of groundwater in deeper aquifers of the Sydney Basin. Presentation to the Australian Earth Sciences Convention 2014 (AESC 2014), 22nd Geological Convention, Newcastle NSW, 7-10 July 2014, (pp. 223). Retrieved from: http://aesc2014.gsa.org.au/assets/Various-reg-partner-opp-workshop-summ-/AESC-Abstract-Proceedings.pdf