Groundwater as a driver of salinity in the Wybong Creek catchment, New South Wales, Australia

Wybong Creek is a 90 km-long tributary of the Hunter River, in the upper Hunter Valley of New South Wales, Australia. Previous research showed Na ⁺-Cl ^- dominated water from Wybong Creek decreases the quality of the Hunter River. This study aimed to identify the origin of solutes in the Wybong Creek catchment, in order to constrain salinity mitigation measures. Surface water was sampled at 10 sites along Wybong Creek. Groundwaters were sampled from most bores occurring in the Wybong Valley. Major ion analyses indicated that saline, Na ⁺-Cl ^- dominated water rose abruptly in the mid-catchment area, with surface and groundwater in the upper catchment being dominated by fresher, Na ⁺-Mg ²⁺-HCO ₃ ^- type-water. Based on these findings, more intensive research including soil sampling and piezometer installation was conducted in the mid-catchment area. Results showed only slightly saline regolith, with up to 2185 mg TDS kg ^-1 soil occurring within a salt scald. Groundwater samples had up to 6960 mg TDS L ^-1. Ratios of Cl ^-/Br ^- and isotope ratios of ⁸⁷Sr/ ⁸⁶Sr indicated that this saline groundwater is influenced by both halite dissolution and a marine source, with a lack of δ ¹⁸O enrichment indicating evapoconcentration has limited affects on salinity. The only marine and halite containing source of solutes occurring in the catchment is the Wittingham Coal Measures. Salinity therefore occurs naturally, with the dilution of Wybong Creek by other fresher tributaries the only practical means of limiting the impacts of salinity on water users downstream.