Intermittently Closed and Open Lakes and/or Lagoons (ICOLLs) as groundwater-dominated coastal systems: evidence from seasonal radon observations

Intermittently Closed and Open Lakes or Lagoons (ICOLLs) are dynamic coastal systems that may be vulnerable to changes in catchment hydrology. However, little is known regarding the role of groundwater on the hydrological cycles of ICOLLs. Groundwater discharge in two ICOLLs (Welsby and Mermaid) and a nearby wetland (South Welsby Lagoon) located on Bribie Island (Australia) was quantified using radon (²²²Rn, a natural geochemical groundwater tracer) during four seasonal surveys. The distribution of radon revealed temporal and spatial changes over the study period with higher surface water radon concentrations found in winter for Welsby ICOLL and in autumn for Mermaid ICOLL. The average estimated groundwater discharge rates from a radon mass balance were 3.4 ± 2.1, 7.3 ± 8.9 and 2.6 ± 1.1 cm d⁻¹ in Welsby, South Welsby and Mermaid Lagoons, respectively. These values are at least 8-fold greater than the average annual precipitation that falls directly over the ICOLLs (1420 mm per year, or 0.4 cm d⁻¹), which, coupled with minimal surface water runoff due to the permeable sandy soils, demonstrates that these systems are groundwater-dominated. Overall, groundwater discharge rates in these ICOLLs were much larger than has been reported in other lake systems which is most likely due to the high permeability of regional sandy soils and their large shoreline to volume ratio.