Groundwater discharge complexes, locally known as boinkas, are a distinctive feature of the Mallee region, the western sector of the Murray Basin (300,000 km²) in southeast Australia. The discharge complexes are nested, with modern playas (salinas) set in late Pleistocene playa sediments and associated aeolian deposits. Playa evolution started with the onset of aridity in the mid-Pleistocene. In the Murray Basin, groundwater discharges, and has discharged in the past, to topo-graphically low parts of the landscape,, including the Murray River itself. The fundamental control on the distribution of the discharge complexes is the presence of subsurface permeability barriers, which are tectonically or stratigraphically controlled, and which disrupt lateral groundwater flow. With the general absence of surface drainage in the Mallee region, the location of the topographic lows is probably influenced by differential compaction of sediments.

Within the modern salinas, brine is generated by evaporative concentration and resolution of salts. The structure of some of the investigated brine pools suggests that density instability leads to brine fingering, sinking, and mixing with regional groundwaters. This process is facilitated by more permeable sediments. “Fossil” brines generated by the late Pleistocene playas can be identified and have contributed to the salinity of the regional unconfined aquifer. Brine pools have migrated in time and space in response to climatic change. This has practical implications for the long-term containment of saline wastewaters, which is a serious problem in the Murray Basin.