Understanding the salinisation process for groundwater in an area of south-eastern Australia, using hydrochemical and isotopic evidence

Increases in groundwater salinity down gradient are a common occurrence in catchments throughout Australia, exacerbating the effects of dryland salinisation in discharge areas. Groundwater hydrochemical and isotopic data from the Willaura catchment in south-eastern Australia, which is affected by both primary (natural) and secondary (human-induced) salinity, allowed the factors that cause the down gradient trend in salinisation within this catchment to be determined. The overall salinity of the groundwater (0.8–70 mS/cm) is controlled by evapotranspiration, which concentrates cyclic salts in the unsaturated zone, greatly increasing the salinity of the soil–water to 3700–6400 mg/L Cl−. The progressive addition of this saline soil–water to fresher groundwaters recharged on the catchment margins causes a gradual increase in salinity down gradient. The groundwater stable isotope compositions show that most recharge occurs in winter and spring. Groundwater salinities increase around the saline lakes of the discharge zone, due to direct evaporation from shallow watertables and minor dissolution of halite (as indicated by Cl−/Br− ratios). Mineral–water interactions within the aquifers cause a slight overall reduction in salinity; conversion of kaolinite to smectite and illite, and cation exchange of Na+ for Ca2+ on smectites, cause Na+/Cl−, Mg2+/Cl−, K+/Cl−, Si/Cl− and HCO3−/Cl− ratios to progressively decline and pH to gradually increase down gradient. Oxidation of organic matter causes a strong decrease in the redox potential of the groundwaters down gradient. The groundwater ages increase from modern (containing tritium) on the catchment margins to ∼8000 years (based on 14C dating) at the end of the ∼25 km flow path. The older groundwaters are isotopically heavier than the younger waters, probably due to recharge during a wetter climate in the early Holocene. The groundwater ages show clearly that the progressive addition of saline soil–water, causing groundwaters to become more saline down gradient, has occurred throughout the Holocene and may explain the occurrence of primary salinisation within many Australian catchments.