Characterising alluvial aquifers in a remote ephemeral catchment (Flinders River, Queensland) using a direct push tracer approach

The availability of reliable water supplies is a key factor limiting development in northern Australia. However, characterising groundwater resources in this remote part of Australia is challenging due to a lack of existing infrastructure and data. Here, direct push technology (DPT) was used to characterise shallow alluvial aquifers at two locations in the semiarid Flinders River catchment. DPT was used to evaluate the saturated thickness of the aquifer and estimate recharge rates by sampling for environmental tracers in groundwater (major ions, 2H, 18O, 3H and 14C). The alluvium at Fifteen Mile Reserve and Glendalough Station consisted of a mixture of permeable coarse sandy and gravely sediments and less permeable clays and silts. The alluvium was relatively thin (i.e. < 20 m) and, at the time of the investigation, was only partially saturated. Tritium (3H) concentrations in groundwater was ∼1 Tritium Unit (TU), corresponding to a mean residence time for groundwater of about 12 years. The lack of an evaporation signal for the 2H and 18O of groundwater suggests rapid localised recharge from overbank flood events as the primary recharge mechanism. Using the chloride mass balance technique (CMB) and lumped parameter models to interpret patterns in 3H in the aquifer, the mean annual recharge rate varied between 21 and 240 mm/yr. Whilst this recharge rate is relatively high for a semiarid climate, the alluvium is thin and heterogeneous hosting numerous alluvial aquifers with varied connectivity and limited storage capacity. Combining DPT and environmental tracers is a cost-effective strategy to characterise shallow groundwater resources in unconsolidated sedimentary aquifers in remote data sparse areas.