Using environmental tracers and geological modelling to understand the effect of aquifer composition on groundwater quality in the basalt plains of Western Victoria (Australia)

Raiber, M; Webb, JA; White, PA; Cendón, DI; Jacobsen, GE

Using environmental tracers and geological modelling to understand the effect of aquifer composition on groundwater quality in the basalt plains of Western Victoria (Australia)

Date

2009-12-03

Authors

Publisher

Australasian Environmental Isotope Conference

Abstract

In the basalt plains of south-western Victoria, Australia, good-quality groundwaters are found only in small areas whereas poor quality groundwater with salinities sometimes in excess of seawater concentrations typically dominates most aquifers. As a result, the limited groundwater resources that are useful for domestic and agricultural purposes are under considerable pressure, and the prevailing drought which has affected Australia and especially its southeast since 1997 has further reduced the availability of surface waters and increased the reliance on groundwaters as the principal source for water supplies. In the present study, groundwater ages from more than 70 radiocarbon and tritium measurements of groundwater from a shallow basalt aquifer and an underlying Tertiary Deep Lead System (Streatham Deep Lead System) were used in combination with the analysis of the groundwater major ion composition, stable isotopes (deuterium and δ18O) and 87Sr/86Sr ratios to understand groundwater processes in the basalt plains of western Victoria. A three-dimensional geological model was developed in order to interpret and visualise the geological relationship between the major aquifers; this 3D geological model was also used to model the distribution of groundwater properties such as groundwater age and salinity within the major aquifers using the minimum tension interpolation technique of the EarthVision software. Recharge rates derived from independent chemical and physical estimation techniques provided further evidence on the link between the aquifer composition, aquifer interactions and the quality of groundwater. The study shows that groundwater is typically young (i.e. less than 100 years BP) and fresh near the major volcanoes, not only in the shallow basalt aquifer but also in the underlying deep lead aquifer. With increasing distance along the flow path away from these volcanic centres, both the groundwater age and salinity progressively increase within the shallow and deeper aquifers, yielding maximum radiocarbon groundwater ages of up to approximately 35000 yrs. BP. The comparison of groundwater age, salinity, recharge rates and soil thickness indicates that the soil structure and thickness are the major controls on groundwater quality. The absence of any significant soil development on the volcanoes facilitates rapid recharge through fractures with recharge rates of up to ~30 mm/yr, whereas the thick, heavy soils developed on basalts away from the volcanoes allow no substantial recharge to the aquifers, resulting in estimated recharge rates typically less than 1-2 mm/yr and primarily old and saline groundwaters here. The analysis and visualisation of groundwater age and groundwater quality parameters in 3D using a three-dimensional geological model is a useful tool to achieve a better understanding of the link between geology and groundwater salinity in the basalt plains of western Victoria. The knowledge gained from this study will assist water managers for example with the selection of suitable locations for the drilling of drought relieve bores and it will also help to highlight areas particularly vulnerable to aquifer over-exploitation and facilitate the optimisation of management strategies.

Keywords

Tracer techniques, Simulation, Aquifers, Ground water, Victoria, Australia, Basalt, Seawater

Citation

Raiber, M., Webb, J. A., White, P. A., Cendon, D. I. & Jacobsen, G. E. (2009). Using environmental tracers and geological modelling to understand the effect of aquifer composition on groundwater quality in the basalt plains of Western Victoria (Australia). Paper presented to The 10th Australasian Environmental Isotope Conference and 3rd Australasian Hydrogeology Research Conference, Resources and Chemistry Precinct, Curtin University Perth, Western Australia 1st – 3rd December 2009, (pp. 35).