EVIDENCE FOR DIFFUSION-CONTROLLED SOLUTE TRANSPORT ACROSS A MAJOR REGIONAL AQUITARD, CENTRAL MURRAY BASIN, AUSTRALIA

The Murray Basin is the most important agricultural region in Australia. It is also the site of the most severe environmental problem on that continent: ground water salinization. The Geera Clay is a marine shale in the center of the basin that plays a major role in basin hydrogeology. Analysis of pore fluids squeezed from sediment cores obtained during a drilling operation in the central Murray basin provide information which can be used to deduce the role of the Geera in the salt balance of the basin. Pore water salinities increase downward from land surface to a maximum of 90 g/L at the top of the Geera. They then decrease with depth in a nearly linear fashion over 100 m to 15 g/L at the base of the unit. Salinities of 3 g/L are found in the underlying Renmark sands. Water levels in wells screened below the Geera are typically 6 m higher than water levels in wells screened above. It has been thought on the basis of these differences in water levels that water must be moving up through the Geera flushing out marine salts which contribute to the salinization problem. However, it can be shown that the differences in water levels can be quantitatively accounted for simply by the fact that a column of dense salty water overlies less dense fresher water. Rayleigh analysis shows the estimated permeability of the Geera is sufficient to inhibit density-driven convective overturn. This is consistent with the observed nearly linear decrease in salinity downward through the Geera, which reflects diffusion-controlled solute transport. Numerical simulations of diffusive transport indicate that it has taken from 10^5 to 10^6 years to develop the observed salinity profile. The Geera has acted as a long-term sink for subaerially concentrated solutes and not as a source of salt.