The ionic composition of the streams of the mid-Murrumbidgee River: implications for the management of downstream salinity.

The Murrumbidgee River catchment is a major region of both dryland and irrigated agricultural production in eastern Australia. The salinity of water in the lower reaches of the river is the subject of concern; changing land management upstream is one option to minimise accessions of salt to the river but this must be done in a way that provides an adequate quantity of water for downstream users and the environment.We examined 30 years of sporadic data on the ionic composition of water for 7 subcatchments contributing to the mid-Murrumbidgee River and for 2 gauging stations on the river itself. Despite the common local presumption that salinity, measured as electrical conductivity (EC), is primarily due to NaCl from cyclic marine salt, we found that NaCl was the dominant salt in only some streams. The presence of HCO3-'s of Ca2+ and Mg2+ in all streams, and their dominance in 2 streams, indicates that mineral weathering is also a major contributor to the salt load of water in the catchment. However, Ca2+ and Mg2+ bicarbonates have limited solubility and so their concentrations will not become a cause of osmotic stress when the water is used for drinking or irrigation. Therefore in our efforts to prioritise lower order catchments of the Murrumbidgee River for changed land management, it will be necessary to examine the nature of the salts they discharge, not just EC. By distinguishing between Cl- / Na+ dominated streams and Ca2+, Mg2+ / HCO3- dominated streams we can refine our search for sources of osmotic stress which might potentially worsen with time. This will enable us to target particular land management units so as to obtain the maximum reduction in downstream salinity with a minimal decrease in flow volume and minimal area of land undergoing changed landuse.