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

We report Cl-36 determinations for groundwaters in the major confined aquif er, the Mereenie Sandstone, in the Amadeus Basin, in arid central Australia . Groundwaters at the synclinal margins of the Amadeus Basin have Cl-36 rat ios, Cl-36/Cl, averaging about 176 x 10(-15). These groundwaters have radio carbon contents ranging from 6 to 39% modem carbon, suggesting that they mi ght "average out" episodic recharge processes over the last 30,000 years. D eep within the Basin, groundwaters in the Mereenie Sandstone at Palm Valley sampled from petroleum exploration drilling, have Cl-36 ratios ranging fro m 8 x 10(-15) to 99 x 10(-15). The average chlorinity of the deep groundwat er samples is up to double that of the synclinal margin waters, indicating some addition of chloride in the aquifer. Even so, the groundwater residenc e times are implied to be about four hundred thousand years. These Mereenie Sandstone groundwaters are sourced several hundred metres below ground lev el, and their minimum flow path is 40-50 km from the recharge zone at the m argin of the Basin. This gives an average flow rate of 0.1 m/a, in accord w ith previous hydraulic flow modelling which indicated that groundwater trav el times in the Mereenie Sandstone are slow and uneven. (C) 1999 Elsevier S cience B.V. All rights reserved.