Stable isotopes as indicators of water and salinity sources in a southeast Australian coastal wetland: identifying relict marine water, and implications for future change

The Lake Connewarre Complex is an internationally protected wetland in southeast Australia, undergoing increasing environmental change due to urbanisation. Stable isotopes of water (δ¹⁸O and δ²H) and other geochemical indicators were used to assess sources of water and salinity in the shallow groundwater and surface-water systems, and to better understand groundwater/surface-water interactions. While much of the shallow groundwater is saline (from 1.27 to 50.3 g/L TDS) with overlapping salinities across water groups, stable isotopes allow clear delineation of two distinct sources of water and salinity: marine water with δ¹⁸O between −1.4 and +1.3 ‰ and ion ratios characteristic of seawater; and meteoric water with δ¹⁸O between −6.1 and −3.6 ‰ containing cyclic salts, probably concentrated by plant transpiration. Groundwater bodies in shallow sediments beneath the wetlands have salinities and stable isotopic compositions intermediate between fresh wetland surface water and a marine water end-member. This marine-type water is likely relict seawater emplaced when the wetlands were connected to the estuary, prior to modern river regulation. Freshwater input to underlying groundwater is a recent consequence of this regulation. Future predicted changes such as increased stormwater inflow, will increase rates of freshwater leakage to shallow groundwater, favouring the proliferation of exotic reed species.