DOI: 10.4225/03/5897b67c1382a
title: Understanding connectivity within groundwater systems and between groundwater and rivers

description: Sustainable use of water resources has become increasingly important in the past decades. The water resources that we use influence almost the entire hydrological cycle, in particular shallow and deeper aquifers, as well as rivers and lakes. The interactions between aquifers in a sedimentary basin and the connectivity of these aquifers with rivers are complex. To understand these interactions in relation to climate, geology and anthropogenic use, a sound hydrogeological framework is required. The Gippsland Basin in southeast Australia is a good example of the complexity of hydrological and hydrogeological inter-connectivity. The basin contains valuable energy resources including brown coal, gas and oil. Furthermore, it is an important area for agriculture in Australia. Extractions of groundwater to drain the open pit mines and for irrigation are suspected to have led to a decline in water levels in many aquifers across the basin. While the energy reserves have been well studied, information on the hydrogeology of the basin is scarce and understanding the complexity of the aquifer system is essential for a sustainable water management in the area. This study focused on three particular aspects of inter-aquifer and aquifer-river connectivity; (i), deeper aquifer systems, (ii) the connectivity of major rivers with the shallow aquifers in the area and (iii) the role of short to medium-term reservoirs. A methodological study on the use of radionuclides from the U/Th-series in surface water groundwater interactions studies is also included as this method was developed in the process of investigating the three main aims. Hydrogeochemical data were collected from the main aquifers in the Latrobe Valley in the western part of the Gippsland Basin to understand inter-aquifer mixing in deeper aquifer units in the basin. Major ion chemistry and environmental isotopes were used to discuss a geochemical and chronological framework of the most abundant aquifers. Major ion chemistry is heterogeneously distributed within and across aquifers, indicating that the major aquifer units are hydraulically connected and that mixing between the units occurs. 14C and 3H data was used to discuss a chronological structure and carbon ages range from ∼36000 years to modern. Ages are also heterogeneously distributed and there is no significant trend along the major flow path in the basin, supporting the hypothesis of intensive inter-aquifer mixing. Surface water groundwater interactions were studied along the Avon and the Mitchell Rivers in the centre of the Gippsland Basin, using hydrogeochemistry and in particular radon (222Rn) as a tracer. Radon as a short-lived radioactive gas can be used to determine short- to medium- term reservoirs contributions, such as bank storage in the direct proximity of rivers, to the total discharge of a river. The study has shown that groundwater discharge and discharge from the riverbanks to the streams is temporally and spatially variable. It was concluded that most rivers have gaining and losing sections and these may invert depending on the flow conditions of the river. The knowledge gained from this study lead to the investigation of other tracers for surface water groundwater interactions (e.g. U/Th-series) and the development of a continuous radon-monitoring device.