TRANSPORT OF SOLUBLE SALTS IN A LARGE SEMIARID BASIN - RIVER MURRAY, AUSTRALIA

Median annual concentrations of Cl, SO4, HCO3, Na, K, Mg and Ca increase greatly downstream.in the River Murray along its 2500 km route to the sea. Chloride increases from less than 2 mg l-1 near the headwaters to about 170 mg l-1 near the downstream end of the main channel. Water near the head of the river has HCO3 balanced by similar equivalents of Na, Ca and Mg, while higher salinity water in the downstream half of the river is dominated by Na and Cl. Sources of these dissolved ions include: (a) sea-derived airborne salts, contributing essentially all of the Cl, Br and 80% of the Na, and (b) rock-derived ions from weathering of sedimentary rock and soil minerals, contributing most of the observed HCO3, 80% of the Ca, and 20-25% of the Mg and SO,. Much of the change in chemistry along the axis of the Murray is due to influx of saline groundwater which has a major ion composition similar to sea water. Inflows of irrigation drainage water also enhance downstream transport of salts, especially HCO3. The stable isotope composition of the River Murray (deltaH-2 and deltaO-18) begins with relatively depleted heavy isotope abundances (- 47 parts per thousand, - 8 parts per thousand) in the uplands of the southeastern margin of the Murray-Darling basin, increasing steadily by + 30 parts per thousand and + 6 parts per thousand, respectively, due to evaporative enrichment from river surfaces and influx of drainage returned from irrigation diversions. Despite the similarity of groundwater major ion chemistry to that of sea water, stable isotope compositions of groundwaters clearly demonstrate that water molecules from past marine incursions have been completely flushed from the system. Similarly, the Cl/Br ratios of groundwaters are not appreciably different from seawater values, indicating that dissolution of fossil evaporites does not contribute significant salt to the groundwaters or the river.