A hydrogeochemical approach to coastal groundwater-dependent ecosystem conservation: The case of Cooloola Sand Mass, Australia

Site-focused hydrogeological studies are often touted as a fundamental requirement for the effective conservation of groundwater-dependent ecosystems (GDEs) as they unveil important detail about processes that threaten groundwater and connected ecosystems. We used a hydrogeochemical and isotopic approach to characterise groundwater source, movement and recharge in a highly complex coastal dune system, the Cooloola Sand Mass, in south-east Queensland, Australia. For a relatively small area (620 km²), Cooloola contains a variety of GDEs including wetlands, fens, lakes, and streams. GDEs within Cooloola are exposed to a myriad of threats including over-abstraction, saltwater intrusion and climatic drying. Conservation approaches are broadly applied to these valuable habitats, where greater hydrological understanding could inform management actions. Major ions, trace elements, stable water isotopes (δ¹⁸O and δ²H) from 83 sites, as well as isotope tracers (³H and ¹⁴C) from 12 sites were used to characterise groundwater sources, movement, recharge processes and estimate mean residence time (MRT). Estimated MRTs ranged from 12 to 100 years, with recharge of the regional aquifer reliant on high monthly rainfall totals (>150-200 mm) associated with La Niña phases across eastern Australia. Results highlight the complex nature of the groundwater system with GDEs depending solely on the regional aquifer, shallow perched aquifers, or a mix of both sources. Each of these sources are exposed to different threats depending on recharge mechanisms, coastal proximity and surrounding land-use.