Abstract:
Previous groundwater studies have indicated that up to 195��000 ha of the Liverpool Plains catchment, south of Gunnedah, New South Wales, Australia, are at risk from shallow saline watertables. Replacement of hydraulically stable, native perennial grasslands with more ?leaky? annual cropping systems since the 1950s, particularly long fallow wheat?sorghum rotations, has been held responsible for an apparent increased frequency of shallow watertables and saline discharge. If so, then it follows that shallow groundwater in the alluvium will be recent (less than about 30 years old) and the solution to the problem is a straightforward reduction in deep drainage under farming systems via increased evapotranspiration. However, in this study, we have found levels of bomb pulse tritium in shallow groundwaters that indicate that about half of the shallow groundwaters in the Mooki subcatchment pre-date current agricultural practices. A hypothesis for this unexpected outcome suggests that the problem is complex and that solutions need to be site-specific.