Hydrogeology, hydrochemistry and isotope hydrology of Palm Valley, Central Australia

Abstract

The Palm Valley oasis in arid central Australia is characterised by stands of palm trees (Livistona mariae). How these unique plants, separated by nearly a 1000 kilometres of rid country from their nearest relatives persist, has long fascinated visitors. Defining the hydrogeology of the Hermannsburg Sandstone, a regionally extensive and thick Devonian sequence of the Amadeus Basin that underlies Palm Valley, is the major thrust of investigation. Appraisal of drilling data shows this aquifer to be a dual porosity fractured rock aquifer which, on a regional scale, behaves as a low permeability, hydraulically continuous resource. Groundwater is low salinity (TDS &lt1000 mg/L) and bicarbonate rich. Slight variations in cation chemistry indicate different flow paths with separate geochemical histories have been sampled. Stable isotope (δ²H, δ¹⁸O) results from Palm Valley show groundwater to have a uniform composition that plots on or near a local meteoric water line. Radiocarbon results are observed to vary from effectively dead (&lt 4%) to 87 % modern carbon. To resolve groundwater age beyond the radiocarbon window the long lived radioisotope 36Cl was also used. Ratios of 36Cl/Cl range from 130 to 290 x 10-15. In this region atmospheric 36Cl/Cl ratio is around 300 x 10-15. Thus an age range of around 300 ka is indicated if, as is apparent, radioactive decay is the only significant cause of 36Cl/Cl variation within the aquifer. A review of previous, often controversial, 36Cl decay studies shows results are usually ambiguous due to lack of certainty when factoring subsurface Cl- addition into decay calculations. Apparently, due to the thickness of the Hermannsburg Sandstone, no subsurface sources of Cl- such as aquitards or halites, are encountered along groundwater flow paths, hence the clear 36Cl decay trend seen. The classic homogenous aquifer with varying surface topography, the &quotToth&quot flow model, is the simplest conceptual model that need be invoked to explain these isotope data. Complexities, associated with local topography flow cells superimposed on the regional gradient, signify groundwater with markedly different flow path lengths has been sampled. The long travel times (&gt 100 ka) indicate groundwater discharge would endure through arid phases associated with Quaternary climate oscillations. Such a flow system can explain the persistence of this arid zone groundwater-dependent ecosystem and highlight the possibility that Palm Valley has acted as a flora refuge since at least the mid-Pleistocene.