Groundwater‐Surface Water Interactions in Intermittently Closed and Open Lake and Lagoon (ICOLL) Wetlands: Implications for Salinity and Ecological Functioning

ABSTRACT Coastal wetlands are dynamic ecosystems where freshwater‐saltwater interactions are governed by both hydrological and oceanic processes. However, climate change and human interventions such as lagoon breaching increasingly disrupt these interactions, posing challenges for ecosystem conservation. This study investigates the hydrogeological controls on water levels and salinity in a coastal freshwater wetland enclosed by Avoca Lagoon, an Intermittently Closed and Open Lake and Lagoon (ICOLL) in New South Wales, Australia. The wetland supports the endangered Green and Golden Bell Frog (GGBF, Litoria aurea ), which requires low salinity for breeding. Using a multidisciplinary approach—combining groundwater and surface water monitoring, electrical resistivity tomography and lagoon bathymetry and salinity profiling—the study identifies strong hydraulic connectivity between the wetland and the lagoon, mediated by both lateral and vertical saltwater intrusion pathways. We identify four distinct hydrological stages of the lagoon that influence groundwater‐surface water interactions within the wetland. Groundwater discharge plays a critical but often constrained buffering role, sustaining freshwater conditions while being limited by anthropogenic regulation of lagoon water levels. These findings highlight the narrow hydrological range supporting GGBF habitat and the vulnerability of the system to even minor water level fluctuations. They also underscore the need for adaptive management strategies—such as dynamic berm operation, managed aquifer recharge and protection of recharge zones—to balance flood mitigation with wetland conservation. This study advances understanding of groundwater‐surface water interactions that underpin ecological resilience and demonstrates that groundwater connectivity must be integrated into ICOLL management frameworks to enhance preparedness for future climate variability and sea level rise.