Salinity of atmospheric precipitation in the Murray-Darling drainage division

Rain and dry deposition were sampled together for 28-day periods throughout 1974-75 at 27 stations, all but three being in the Murray-Darling Drainage Division (1.06 x 106 km2). Samples were analysed for calcium, magnesium, potassium, sodium, chloride and sulfate. Similar procedures continued during 1975-77 for six stations used previously and a new one. Rainfall records and the mean ionic concentrations, including estimates of HCO3 + NO3, indicate that the mean aggregate concentration for eight ions in 1974-75 ranges from 3 to 35 mgL-1 for 24 stations in the drainage division and from 14 to 23 mgL-1 for three stations near the South Australian coast. Higher concentrations in the drainage division refer principally to stations in a semi-arid district and are associated with relative abundance of calcium and HCO3 derived from the calcareous soils there. Terrestrial and oceanic sources of the ions are considered. Oceanic input of chloride is estimated using mean annual concentrations to give a maximum value and concentrations for the most rainy periods to give a minimum value. These estimates are compared with discharge of chloride by the Murray and Darling Rivers, with the conclusion that in 1974-75 the oceanic input of chloride to the Darling catchment was mostly retained there, but amounts equal to the accessions to the Murray catchment were probably all flushed out. Details are given in an appendix of NO3 concentrations determined by others at two stations in frequent samples of rain.