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

Currell, Matthew J.; Dahlhaus, Peter; Ii, Hiroyuki

Stable isotopes as indicators of water and salinity sources in a southeast Australian coastal wetland: identifying relict marine water, and implications for future change | Utilisation des isotopes stables comme indicateurs de l’origine de l’eau et de la salinité dans une zone humide côtière du Sud-Est Australien: identification des eaux marines résiduelles et implications pour les changements à venir Los isótopos estables como indicadores de fuentes de agua y salinidad en un humedal costero del sudeste de Australia: identificación de agua marina relicta, e implicancias para cambios futuros 稳定同位素作为澳大利亚东南沿海湿地水源和盐分来源的指示物:确定残余海水及未来的变化 Isótopos estáveis como indicadores de origem da água e da salinidade numa zona húmida costeira no sudoeste da Austrália: identificação de água marinha fóssil e implicações para as alterações futuras

2015

Currell, Matthew J. | Dahlhaus, Peter | Ii, Hiroyuki

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.

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