Hydrogeochemical controls of variable regolith materials on the distribution, mobility and age of salts: Barmedman Creek Catchment, NSW

INTRODUCTION The widespread occurrence of dryland salinity and its ramifications on land management practices are well documented throughout Australia, yet the origin, nature, and mobility of salts are less well understood. Numerous studies (e.g., Macumber 1969, Gunn & Richardson 1979, Johnston et al. 1980, Johnston & McArthur 1981, Mazor & George 1992) associate the occurrence of salt affected areas with low-lying, highly weathered landscapes in Australia. In the majority of these areas, the salts tend to be concentrated in low permeability clay-rich units and are considered relatively immobile (e.g., Macumber 1969, Johnston et al. 1983, Johnston 1987). However, considering the large timeframes available for accumulation of salts in the landscape, far greater concentrations would be expected if these salts were not continually being exported from basins over time. Simpson & Herczeg (1991) note that Cl is being exported from the Murray Darling Basin at rates 2.6-4.1 times greater than that contributed from atmospheric precipitation. This imbalance is attributed to the mobilization of unsaturated zone salts by infiltrating irrigation water and rising groundwater tables following land clearance. Less attention has been focused on the mixing of saturated zone salt stores residing in low permeable clay units with fresher aquifer water. Ortega-Guerrero et al. (1997), Timms et al. (2001) and Peck & Hatton (2003) consider isotopic and hydrochemical properties of both aquitard pore fluids and aquifer waters. These results suggest that appreciable degrees of mixing occur between the two. Recent studies in other countries have begun to identify the significance of mixing between aquitard pore fluids and aquifer groundwater in relation to groundwater salinity. Hendry et al. (2000) investigated mixing between pore fluids and groundwater within a till aquifer system in Saskatchewan, Canada and noted that saline groundwater is diffusing into near surface freshwater bearing till aquitards. Ortega-Guerrero et al. (1997) studied the interaction between saline pore fluids in a clay-rich lacustrine aquitard and a freshwater alluvial aquifer supplying Mexico City, Mexico. The authors concluded that under natural conditions the hydraulic gradient is sufficiently high enough for fresh groundwater to flow upward through the saline aquitard. However, in areas where over-pumping of the aquifer has occurred, a downward vertical hydraulic gradient has formed and saline pore fluids are leaking into the aquifer, thereby compromising the potable water supply for 20 million people. In Australia, alteration of the hydrologic budget due to clearing of native vegetation has caused groundwater levels to rise and mobilize salts formerly residing in the unsaturated zone (e.g., Salama et al. 1999). This process, known as secondary salinization, has been attributed to the widespread degradation of soil and water quality throughout Australia. Starr (1999) estimates that up to 1,500,000 hectares of land within the Murray-Darling Basin will be affected by secondary salinization by 2010. The widespread presence of significant salt stores in low permeability, highly weathered regolith materials may have a greater impact than previously assumed on the quality of surface water and groundwater throughout the Murray Darling Basin. A detailed assessment of pore fluid and groundwater properties is therefore fundamental for addressing the salt balances within catchments. This study is part of an evaluation of the origins, nature and mobility of salt stores in the Barmedman Creek catchment of the Bland Basin and will be useful for understanding the nature of other salt affected landscapes in the Murray Darling Basin.

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