Ancient groundwaters in the Amadeus Basin, Central Australia: Evidence from the radio-isotope ³⁶Cl

We report ³⁶Cl determinations for groundwaters in the major confined aquifer, the Mereenie Sandstone, in the Amadeus Basin, in arid central Australia. Groundwaters at the synclinal margins of the Amadeus Basin have ³⁶Cl ratios, ³⁶Cl/Cl, averaging about 176 x 10^-15. These groundwaters have radiocarbon contents ranging from 6 to 39% modern carbon, suggesting that they might 'average out' episodic recharge processes over the last 30,000 years. Deep within the Basin, groundwaters in the Mereenie Sandstone at Palm Valley sampled from petroleum exploration drilling, have ³⁶Cl ratios ranging from 8 x 10^-15 to 99 x 10^-15. The average chlorinity of the deep groundwater samples is up to double that of the synclinal margin waters, indicating some addition of chloride in the aquifer. Even so, the groundwater residence times are implied to be about four hundred thousand years. These Mereenie Sandstone groundwaters are sourced several hundred metres below ground level, and their minimum flow path is 40-50 km from the recharge zone at the margin of the Basin. This gives an average flow rate of 0.1 m/a, in accord with previous hydraulic flow modelling which indicated that groundwater travel times in the Mereenie Sandstone are slow and uneven.