Physical and Chemical Hydrogeology of the Otway Basin, Southeast Australia

Physical and chemical hydrogeology of the Otway Basin, southeast Australia Angela L. Bush Submitted in total fulfilment of the requirements of the degree of Doctor of Philosophy December 2009 School of Earth Sciences The University of Melbourne ii Declaration This is to certify that (i) the thesis comprises only my original work towards the PhD; (ii) this work has benefited from a data exchange with my colleagues at SKM, who also completed a hydrogeological mapping project in Victoria in 2009; (iii) due acknowledgement has been made in the text to all other material used, and; (iv) the thesis is less than 100,000 words in length, exclusive of tables, maps, bibliogra- phies and appendices. Signed, Angela Louise Bush iii iv Abstract The Otway Basin of southeast Australia is the subject of this thesis, which incorporates pre-existing geological, hydraulic and major element hydrogeological data with new isotope hydrogeochemical investigations. The region is an Upper Cretaceous–Tertiary basin, filled with siliciclastic and calcareous aquifers and aquitards and characterised by late volcanic activity, pervasive faulting and karstification. As part of this study, an hydrogeological database is compiled for the Otway Basin region from existing distinct datasets from the states of Victoria and South Australia. Utilising this new resource, the data are reinterpreted into a 3D model of the hydros- tratigraphy for the basin in GoCAD, and interpolated surfaces of hydraulic head and electrical conductivity are created for 5 aquifers/aquitards. The Victorian hydraulic head data is analysed for long term declining or inclining trends and hydrograph trend maps are created for different aquifer systems. The data are also compiled into representative cross sections of flow and chemical composition, with one section located in each of the three major sub-basins. The records of groundwater chemistry from the Victorian section of the Otway Basin are used to plot the relative concentration of major cations and an- ions for the main aquifers. More than 120 groundwater samples were taken for analysis of major and minor ion concentration and/or oxygen, hydrogen, carbon, strontium and chlorine isotope composition. These data are used to characterise the hydrogeochemical evolution of the groundwater and to identify the processes that the groundwater drives or experiences in the system. The potentiometric maps and cross sections reveal the interconnected nature of the flow in all aquifers and the relationship between local and regional flow systems. Regional flow paths originate inland near basement highs or the basin margins. In the shallower aquifers they terminate at the coastline where the groundwater mixes with ocean water at a diffuse interface and density differences induce groundwater discharge at the land surface or the ocean floor. In the deeper confined aquifers, discharge is submarine via several possible mechanisms, which include: diffuse intergranular leakage to overlying units; flow along faults or volcanic conduits; and/or seepage directly to the ocean from exposed sections of the aquifer, e.g. in submarine canyons. These mechanisms may be operating up to v 50 km offshore but the interface is currently migrating landward, which will result in a shortening of that estimated distance. Local-scale flow lines are complex and may be oriented against the direction of regional coastward flow. Local hydraulic divides are often associated with volcanic eruption centres, which have elevated topography and relatively high hydraulic head, making them important recharge zones. These zones contain low salinity groundwater because infiltra- tion is relatively rapid. Conversely, basalt flows that have developed clay horizons through weathering reduce drainage and allow significant evapotranspiration which concentrates the cyclic salts in solution. Many local flow systems discharge mainly via evapotranspiration, which acts again to concentrate the cyclic salts in solution. Other local discharge zones are rivers, creeks and lakes or lagoons that receive baseflow and seeps and springs associated with geological contacts or boundaries and faults. Evaporitic concentration of solutes in surface water bodies and shallow groundwater affects the quality of water recharging the underlying aquifers and aquitards. This quality has changed over the last 50,000 years or so due to fluctuations in climate and hence variation of the precipita- tion/evaporation ratio. Stresses on the aquifers are climate fluctuations, sea level change, land use change and groundwater extraction. These stresses have resulted in the system being out of hydraulic equilibrium in many cases. Lags in response to these changes in boundary conditions are identified and/or hypothesised. In particular, the confined aquifer’s response to sea level change could be subject to a lag in the order of millennia. The stress on an aquifer is often transferred to its adjacent units, in some cases inducing cross-formational leakage, which is possibly supported by radiocarbon dating evidence. The area of the Otway Ranges appears to have escaped the effects of stress to date because of its stable microclimate, its distance from the ocean and from groundwater extraction. Increase in demand on groundwater resources, development of geothermal, sequestration and hydrocarbon indus- tries and future climate change may yet have a detrimental effect on the groundwater of the Otway Basin. Isotopic composition of the groundwater confirms its meteoric origin and chlorine isotopes from several samples of the deep groundwater indicate that accumulation of solutes along the flow path is not due to diffusion or dissolution of connate salt. Thus, the salinity of the water is sourced from cyclic salts and solutes from water-rock interaction, both of which may be concentrated by evapotranspiration. Water-rock interaction is dominated by dissolution of carbonates and weathering of silicates as a result of the surficial geology being dominated by calcarenite or limestone and young basalt. The volcanic activity has produced gas that has interacted with the groundwater, and continues to do so, fractionating oxygen, hydrogen and carbon isotopes vi and contributing fluorine, boron and sulphur to solution. The addition of volcanic CO2 creates an uncommon situation for water-rock interaction, where continued dissolution of carbonate and silicate minerals along the deeper flow paths is demonstrated by the silicon/chloride ratios and strontium isotopic composition of the groundwater. These water-rock interaction processes, with the addition of cation exchange, are responsible for + − the development of a relatively fresh Na HCO3 type water that is characteristic in parts of the deep aquifer. The study confirms the existing hydrogeological understanding of the Otway Basin and forms new conclusions regarding the history of the groundwater and the processes of flow and chemical evolution by integrating numerous lines of evidence. Significant con- tributions of this work which improve current scientific knowledge include these findings: the maps and cross sections of hydraulic head and electrical conductivity reveal the con- nected nature of flow systems within all the aquifers and aquitards; geological features can induce discharge, e.g. at contacts or faults, and recharge, e.g. volcanic eruption centres; the changes to the surface drainage system as a result of the eruption of basalt flows have affected the water chemistry and flow systems in all the underlying units; there is a lag in aquifers’ responses to sea level change and therefore future migration of the interface is expected regardless of further boundary changes; climate change has influenced surface water quality by changing the regional water balance, and therefore has affected groundwater quality; the discharge from the confined aquifer is submarine via various pathways, interaction between the groundwater and volcanic gas has occurred in the past and is ongoing, and consequently mineral dissolution persists at deep levels; the origin of high salinity of brackish groundwater in all Tertiary aquifers and aquitards is concentrated solutes from water rock interaction and cyclic deposition. vii viii Acknowledgements Completing this work would not have been possible without the help, guidance, support and advice of others. First and foremost, I would like to thank my supervisor, Dr. Charles Lawrence, for his constant support, advice and interest throughout this long-lived project and for passing on the wealth of his knowledge in hydrogeology and never being too busy to read my work. I would also like to acknowledge other academics and researchers who have shown interest in my studies and who have at times acted in a supervisory role for me by proof reading, discussing theories, ideas or methods. Thanks for this support goes to: Sue White, Tamie Weaver, John Webb, John Moreau, Malcolm Wallace, John Bye, Suse Hayes, Andy Love, Ian Cartwright, Pete Dahlhaus, Roger Blake, Darren Bennetts, Andy Shugg, John Middleton, Daniel Ierodiaconou, Jack McConchie, Attila Kovacs, Peter Cook, Frank Stagnitti, Terry Byrne, Guy Holdgate, Ian Simmons, Allison Hennig, Tim Rawling and Laurent Ailleres. This project was supported by two research grants, one from the Glenelg Hopkins Catchment Management Authority and another from the Corangamite Catchment Man- agement Authority. I would like to thank all those at the CMAs who made this support possible, especially Peter Swanston, Kevin Wood and Peter Codd. Acknowledgement is due to those

Load more