Hydrogeochemical and isotopic constraints on the origins of dryland salinity, Murray Basin, Victoria, Australia

Combined hydrogeological and hydrogeochemical data allow flow systems and the origins of solutes in the Honeysuckle Creek area of the southeastern Murray Basin, which is an area affected by dryland salinity, to be constrained. Recharge occurs both on the uplands that are composed of fractured Violet Town Volcanic rocks and the Riverine Plain that comprises sediments of the Shepparton and Coonambidgal Formations. Groundwater from the Violet Town Volcanics has low salinity (<20 mmol/L Cl) and major ion geochemistry that is controlled largely by dissolution of silicate minerals. Low Cl/Br ratios (as low as 281 molar) suggest that this groundwater has not dissolved halite. Groundwater that recharged through the Riverine Plain sediments has higher Cl/Br ratios (up to 1146) and Cl concentrations of <20 mmol/L, consistent with it dissolving minor halite. Higher salinity (>20 mmol/L) groundwater has intermediate Cl/Br ratios (600–1000), which indicate that the high salinities do not simply result from halite dissolution. Rather, mixing of groundwater homogenises Cl/Br ratios, and evaporation as a consequence of a shallow water table is the dominant process that increases salinity. Oxygen and H isotopes also indicate that mixing and evaporation have occurred. These results indicate that land use over the whole region, not just the uplands, needs to be considered in any salinity management plans. Additionally future development of salinity is controlled by depth to the water table on the plains and the efficiency of recharge rather than by salt stores (halite or brines) in the unsaturated zone.