The hydrology and hydrochemistry of six small playas in the Yarra Yarra drainage system of Western Australia

The hydroperiod, filling frequency, local shallow groundwater movement and surface water and shallow groundwater chemistry of six small (< 3km2) playas from the Yarra Yarra drainage system of Western Australia were monitored from September 2002 to June 2004. The playas are morphometrically similar and adjacent and represent a hydrological continuum of ephemeral basins ranging from mostly wet (Mongers B) to mostly dry (Kadji A). Hydroperiod ranged from 19 to > 211 days and filling frequency from 1 to 3 cycles over the study period, reflecting rainfall and sub-catchment variability. The playas are net discharge points for groundwater. However, small local vertical head variations suggest that groundwater does not discharge at the same rate across the playa surfaces and that playas may have short-lived recharge phases. Chemically, the playas are typical of salt lakes in Australia. Surface waters showed an ionic dominance consistent with seawater with minor variations attributed to transitional phases in the geochemical evolution of the waters. Shallow ground waters showed a common and consistent pattern of ionic dominance: Na+ > Mg2+ > K+ > Ca2+: Cl- > SO42- > HCO3- > CO32-. A geochemical pathway of brine evolution is proposed. � Royal Society of Western Australia 2007.