Tracing groundwater flow in granite landscapes using δ18O, δ2H, 3H and 14C in the Upper Wimmera catchment, western Victoria
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Australasian Environmental Isotope Conference
Previous work [1,2] has suggested that low relief, highly weathered granites in the Upper Wimmera in western Victoria are a significant source of salts exported to the catchment via groundwater, whereas high relief, less weathered granites are a source of fresh, good quality groundwater. In order to understand the processes of salt accumulation and movement in the catchment, groundwater recharge and flow has been investigated using the isotope ratios δ18O and δ2H, and the isotopes 3H and 14C. The Upper Wimmera forms part of the Murray Darling Basin [Figure 1], near the township of Ararat in western Victoria. The main geological units in the area are: the highly weathered and fractured basement of Ordovician aged metasediments, the Devonian aged granites that intruded the metasediments (with surrounding Quaternary-aged colluvial aprons), and the overlying Quaternary alluvial sediments of the Shepparton Formation. These three geological units also form the main aquifers of interest in the study area. Stable isotope ratios (δ18O and δ2H) suggest that the groundwater forms two groups; one that plots along the local meteoric water line and is relatively depleted in the heavy isotopes, and another that is more enriched in heavy isotopes, and plots on an evaporation trendline with a slope of 4.1. This latter group of groundwater is also much more saline on average (9.4 mS/cm compared to 2.6 mS/cm). However this is not due to open-water evaporation; the fractionation of isotopes seen in the evaporated water can only account for a loss of <10% of the original water body. However the high salinities observed require a loss of at least 99.5% of the original water body. Hence, the high salinity is likely due to transpiration by vegetation, as this process does not affect stable isotope ratios. Twenty three of these groundwater samples were analysed for 3H and 10 samples contained detectable tritium, suggesting that these samples are ‘modern’ (less than ~50 years old). All of these 10 samples are in the group of groundwater that has undergone evaporation, suggesting that this group is also younger on average. The radiocarbon data agrees with the tritium data; 38 samples were analysed for 14C and the samples in the group of evaporated groundwater are younger on average (78 pMC compared to 60 pMC). The radiocarbon data also shows that vertical flow is an important component of the flow system in weathered granite colluvium, and to a lesser extent in the Ordovician basement. However vertical flow does not appear to be a significant component in the Shepparton Formation.
Ground water, Granites, Stable isotopes, Carbon 14, Watersheds, Victoria, Australia, Salts, Weathering, Sampling
Hagerty, S., Webb, J., Dyson. P., Hocking, M., Jacobsen. G., Chisari. R., & Poulson, S. (2009). Tracing groundwater flow in granite landscapes using δ18O, δ2H, 3H and 14C in the Upper Wimmera catchment, western Victoria. 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. In Grice, K. & Trinajstic, K. (eds), The 10th Australasian Environmental Isotope Conference and 3rd Australasian Hydrogeology Research Conference abstract volume : Resources and Chemistry Precinct, Curtin University, Perth, Western Australia, 1st-3rd December 2009, (pp. 19).