How water isotopes (¹⁸O, ²H, ³H) within an island freshwater lens respond to changes in rainfall

Coastal aquifers provide an important source of water globally. Understanding how groundwater responds to changes in rainfall recharge is important for sustainable development. To this end, we investigate how water isotopes (¹⁸O, ²H, ³H) and chloride (Cl) concentrations within an island freshwater lens respond under varying rainfall conditions in a region experiencing climate change. Uniquely, this study presents a three year dataset of groundwater collected seasonally between May 2013 and August 2016 from ten wells. Variation in all tracers was observed. The Cl and tritium (³H) show opposing seasonal variation in some sections of the lens, with higher Cl observed in the austral summer when less rainfall occurs and evapotranspiration is highest. The opposite occurs in the austral winter months when ³H increases from atmospheric input via rainfall recharge, and Cl is diluted. An overall decline in ³H values and enrichment in stable water isotopes over the study period was also observed. This study shows that understanding groundwater of freshwater lenses should not rely on a single sampling campaign because seasonal variability is large. The identification of a dual recharge regime, with contributions from both winter rainfall and episodic events, has important implications for understanding the future fate of the freshwater lens on Rottnest Island. The finding that episodic rainfall is a major contributor to groundwater recharge is important and can only be assessed with a multi-year isotope dataset for groundwater and rainfall.