Salinisation processes in a sub-catchment of Wybong Creek, Hunter Valley, Australia

Salinisation in the Hunter Valley, New South Wales, Australia, is a significant environmental issue which affects the regional wine, horse and beef industries. The sub-catchment of Wybong Creek causes increased salinity, sodicity and chlorine concentrations in the Hunter River. Sampling was undertaken in the scalded area of Manobalai, located in the mid-catchment area of Wybong Creek, to establish whether salt stores within the regolith, or saline groundwater discharge from deeper formations, are sources of Na⁺ and Cl^- dominated water to Wybong Creek. Ten soil cores were collected from eight sites in the Manobalai area, with bores and piezometers installed to characterise groundwater chemistry and hydraulic head. The regolith within the Manobalai field site was non-saline, though Na⁺ and Cl^- were the dominant ions in most regolith layers. The most saline regolith samples have coarse textures and a high moisture content, and occur in the valley floor. Cores from the salt scald have salt concentrations ranging from 274 to 2089 mg/kg, with a maximum Na⁺ and Cl^- wt.% of 0.17. Saline groundwater in piezometers has total dissolved solid concentrations of up to 7277 mg/L. In this saline groundwater, Cl^-/Br^- ratios of up to 1767, Na⁺/Cl^- ratios of 0.6-1.2 and ⁸⁷Sr/⁸⁶Sr ratios up to 0.709446 indicate halite dissolution and a marine source of solutes to groundwater. The Wittingham Coal Measures, which previous studies have linked to salinity elsewhere in the Hunter Valley, contain halite efflorescences and were intruded by marine water in their geological past. Salinity is due to discharge of this regional groundwater body in the Wybong catchment and not, as commonly assumed within Australia, due to dryland salinity.