GROUNDWATER GEOCHEMISTRY AND ISOTOPES NEAR MULLAMULLANG CAVE, NULLARBOR PLAIN, WESTERN AUSTRALIA

Mullamullang Cave is a long horizontal cave beneath the Nullarbor Plain, Western Australia, with approximately 11 km of surveyed passage. Average annual rainfall in the area is approximately 200 mm, and there are no known drips or seeps into the cave. There are no active calcite speleothems in the cave; one area is spectacularly decorated with halite speleothems. The cave intercepts the water table at a number of pools, which are dominated by sodium chloride with an average TDS of 9500 mg/L. In-cave pool samples were complemented by regional borehole groundwater samples spanning a 45-km transect. The regional aquifer is similar in chemistry to the cave pools, ranging in TDS from 8500 to 13,400 mg/L. All cave pool and groundwater samples showed higher chloride/bromide ratios than seawater, suggesting a terrestrial halite source for at least some of the observed chloride.

Stable isotope ratios for cave pool water and regional groundwater are tightly clustered to the right of the present-day meteoric water line, and are lighter overall than average modern precipitation. Together, these observations suggest that the samples reflect old water from a different climatic regime. Although Mullamullang Cave pools display a strong horizontal thermal gradient (increasing from 16.5° C to 21.0° C in 5 km), the uniform stable isotope ratios do not indicate strong evaporative convection cells. Tritium and chlorine-36 analyses are currently underway, and will be discussed in terms of salinity sources and groundwater ages.