• Deslandes, Alec
• Gerber, Christoph
• Wilske, Cornelia
• Crane, Punjehl
• Suckow, Axel

Abstract

In semi-arid and arid regions, groundwater is often the only reliable source of water for human livelihood and economic activities. In those climates, recharge of groundwater can often be a highly episodic process with recharge dominated by occasional large rainfall events every few years. Understanding such groundwater systems to improve their management and protection is challenging, but the challenge is even more difficult if the host formation is karstified or fractured. Many of the methods traditionally used for studying karst – such as active tracer tests and time series of discharge and hydrochemistry - may not work well in such an environment because aquifers in arid regions tend to function across larger areas or the next recharge event may not occur for multiple years. Environmental tracers are a set of tools that can still provide useful information to better conceptualize and characterize such groundwater systems. Environmental tracers are compounds or isotopes that are naturally present in the environment and allow tracing certain processes, such as groundwater recharge and flow. Over several years, a very diverse set of environmental tracers (including ¹⁸O and ²H of water, tritium, ¹⁴C, CFCs and SF₆, stable noble gases, radioactive noble gases (⁸⁵Kr and ³⁹Ar ), ⁸⁷Sr/⁸⁶Sr, sulphate isotopes, rare earth elements, and basic hydrochemistry) has been applied to the Cambrian Limestone Aquifer (CLA) in northern Australia. The karstified CLA extends over several hundred kilometres from a dry savanna climate to a monsoon-dominated climate and is the key groundwater resource in a region which holds one of Australia’s largest prospective shale gas resources. The CLA feeds culturally significant springs in the Mataranka region and provides baseflow to the Roper River. The CLA is partly confined and has an unsaturated zone which varies in thickness from a few metres to >100 m. This presentation is intended as a high-level overview of different processes that were investigated with environmental tracers, together with some of the surprises along the way: a) developing a consistent conceptual model to quantify recharge when different tracers seem to indicate very different results; b) understanding recharge processes and pathways to provide measurement-based evidence for testing existing conceptual ideas; c) investigating contributions of different water sources to springs; and d) identifying potential upward leakage of groundwater from deeper aquifers to investigate fluid pathways between the CLA and the shale gas resource.These results served to establish baseline data and underpin environmental risk assessments and have implications for managing the aquifer sustainably and investigating and minimizing the risk from potential oil and gas extraction activities.For example, there was evidence in the tracers for recharge through sinkholes quickly bypassing the unsaturated zone, which has implications for the groundwater contamination risk in case of a surface spill. The relative importance of shallow and fast local flow paths and slower regional paths to springs as quantified based on tracers informs the risk level of different springs to pollution and changes in hydrological boundary conditions.

Topics

• impedance spectroscopy
• surface
• compound
• extraction
• rare earth metal