Origins of salinity and salinisation processes in the Wybong Creek catchment, New South Wales, Australia

This work was supported by ARC Linkage grant number LP05060743. Scholarship fundingwas provided by The Australian National University Faculty of Science and ResearchSchool of Earth Sciences, with project funding and support also provided by HunterCentral Rivers Catchment Management Authority and the New South Wales Office ofWater.

The Wybong Creek catchment is located in the upper Hunter Valley of New South Wales, Australia, and contains award winning beef and wine producing operations. Solute concentrations in Wybong Creek are often too high for irrigation use, however, with previous research showing that the saline and Na-Cl dominated water discharged from Wybong Creek decreases water quality in both the Goulburn and Hunter Rivers into which it flows. This study therefore aimed at identifying the source of solutes to the Wybong Creek catchment and the processes which cause salinisation of surface water, soil (regolith) and groundwater. Surface water was sampled at ten sites along Wybong Creek over three years, while groundwater was sampled from most of the bores and piezometers occurring in the Wybong Creek valley. Surface and groundwater in the upper catchment were dominated by Na-Mg-HCO3. Ratios of 87Sr/86Sr and cation/HCO3 indicated these facies were due to silicate weathering of the Liverpool Ranges, with localised groundwater bodies recharging in the Liverpool Ranges and discharging in the upper Wybong Creek valley. Wybong Creek became saline, and Na-Mg-Cl dominated in the mid-catchment area, with salinity doubling between the 55 and 60 km sample sites on some dates. Changes in surface water chemistry occurred independently of surface water input from tributaries, with abrupt salinity increases within a pool between these sites attributed to groundwater input via fractures beneath the Creek. One of two salt scalds in the Wybong Creek catchment also occurs adjacent to this stretch of river. A field site was established at the mid-catchment locality of Manobalai, therefore, in order to constrain the relationship between surface water, regolith and groundwater salinity. Ten piezometers were established at Manobalai, including three piezometer nests. Most regolith at Manobalai was found to be non-saline, including that within the salt scald, with the most saline and Na-Cl dominated regolith samples occurring in some of the most moist and coarse sandy/gravel layers. Groundwater sampled from piezometers installed in the holes drilled for regolith samples had salinities up to 20 times higher than the regolith on a per weight basis, and were similarly dominated by Na-Cl. A lack of carbonate and sulfate minerals within the soils and no indication of Ca-Mg/HCO3- SO4 dominated facies within alluvial soil solutions indicated groundwater did not evolve from rainwater to Na-Cl dominated facies while infiltrating the regolith. Groundwater samples from Manobalai were instead found to be amongst the most fresh and the most saline within the Wybong Creek catchment, and changed salinity abruptly down-gradient along a transect. Groundwater flow occurred through fractures in the Narrabeen Group sandstones and conglomerates, with vertical groundwater flow via fractures causing abrupt changes in salinity. Ratios of Na/Cl, Cl/Br and 87Sr/86Sr indicated saline groundwater at Manobalai and in the lower catchment was influenced by a marine endmember and halite dissolution. A poor relationship between salinity and d18O indicated this marine endmember was not evapoconcentrated rainwater. The occurrence of saline surface and groundwater in the Wybong Creek catchment was instead attributed to discharge from the regional groundwater system occurring in the Wittingham Coal Measures, with the abrupt increases in salinity at Manobalai indicating mixing between local, intermediate and/or regional groundwater systems. Salinity is likely to function similarly to this in the rest of the Hunter Valley also. The occurrence of salinity in both the Hunter River and Wybong Creek catchments is a naturally occurring phenomenon with salinity mitigation difficult due to the regional extent of the saline groundwater systems. Living with salt strategies are therefore recommended, such as limiting irrigation using both saline and fresh water and continuing with restrictions on saline discharge from coal mines.