Spatial and temporal variability of ground water recharge in central Australia: A tracer approach

Two environmental tracer methods are applied to the Ti-Tree Basin in central Australia to shed light on the importance of recharge from floodouts of ephemeral rivers in this and environment. Ground water carbon-14 concentrations from boreholes are used to estimate the average recharge rate over the interval between where the gound water sample first entered the saturated zone and the bore, Environmental chloride concentrations in ground water samples provide estimates of the recharge rate at the exact point in the landscape where the sample entered the saturated zone. The results of the two tracer approaches indicate that recharge rates around one of the rivers and an extensive flood-plain are generally higher than rates of diffuse recharge that occurs in areas of lower topographic relief. Ground water H-2/H-1 and O-18/O-16 compositions are all depleted in the heavier isotopes (delta(2)H = -67parts per thousand to -50parts per thousand: delta(18)O = -9.2parts per thousand to -53parts per thousand) compared with the long-term, amount-weighted mean isotopic composition of rainfall in the area (delta(2)H = -33.8parts per thousand; delta(18)O = -6.3parts per thousand). This indicates that recharge throughout the basin occurs only after intense rainfall events of at least 150 to 200 mm/month. Finally, a recharge map is developed to highlight the spatial extent of the two recharge mechanisms. Floodout recharge to the freshest ground water (TDS < 1000 mg/L) is similar to1,9 mm/year compared with a mean recharge rate of 0.2 mm/year to the remainder of the basin. These findings have important implications for management of the ground water resource.