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 waterand shallow groundwater chemistry of six small (<3km2) playas from the Yarra Yarra drainagesystem of Western Australia were monitored from September 2002 to June 2004. The playas aremorphometrically similar and adjacent and represent a hydrological continuum of ephemeralbasins ranging from mostly wet (Mongers B) to mostly dry (Kadji A). Hydroperiod ranged from 19to > 211 days and filling frequency from 1 to 3 cycles over the study period, reflecting rainfall andsub-catchment variability. The playas are net discharge points for groundwater. However, smalllocal vertical head variations suggest that groundwater does not discharge at the same rate acrossthe playa surfaces and that playas may have short-lived recharge phases. Chemically, the playasare typical of salt lakes in Australia. Surface waters showed an ionic dominance consistent withseawater with minor variations attributed to transitional phases in the geochemical evolution ofthe 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 isproposed.