Coastal wetlands are dynamic ecosystems where freshwater and saltwater interactions are governed by both hydrological and oceanic processes. However, climate change and human interventions, such as lagoon breaching, increasingly disrupt these processes, posing challenges for ecosystem conservation. This study investigates the hydrogeological controls on water levels and salinity within a coastal wetland adjacent to Avoca Lagoon, an Intermittently Closed and Open Lake or Lagoon (ICOLL) in New South Wales, Australia. The wetland was specifically designed to provide breeding habitat for the endangered Green and Golden Bell Frog, which requires low-salinity conditions for breeding. To assess groundwater-surface water interactions and saltwater dynamics, we employed a multidisciplinary approach, integrating water level and salinity monitoring using surface water and groundwater piezometers at depths of 3.5 to 5.5 m, electrical resistivity tomography to characterise subsurface conditions, and bathymetric and salinity profiling of the lagoon. Results indicate strong hydraulic connectivity between the wetland and the lagoon, with groundwater discharge playing a critical role in sustaining freshwater conditions. While high lagoon levels can breach natural barriers and introduce brackish water into the wetland, low lagoon levels - resulting from controlled breaching - can cause wetland desiccation, reducing its suitability for frog breeding. These findings underscore the need for conservation strategies that balance flood management with habitat preservation. By advancing our understanding of ICOLL-influenced wetland hydrogeology, this research highlights the delicate interplay between hydrological processes, anthropogenic management, and frog habitat. Integrating groundwater dynamics into coastal wetland management will be crucial for sustaining ecological functions under future climate variability and sea level rise.

Estimation of groundwater recharge rates (GRRs) in space and time is notoriously challenging but underpins water resource management. GRRs derived from different techniques often differ as they estimate different components (i.e., potential, net, gross) and/or estimate different GRR mechanisms (i.e., diffuse or focused). We evaluate GRRs from daily groundwater level data from ~400 bores across Australia using the water table fluctuation (WTF) method. Specific yield values are estimated using lithological information linked to literature values. Comparisons were made between mean inter-annual GRRs from 224 bores and long-term GRRs derived from the chloride mass balance (CMB) method. Mean inter-annual WTF-based GRRs were 365.5 mm/y for humid, 248 mm/y for dry subhumid, 128.6 mm/y for semi-arid and 50.3 mm/y for arid zones. Inter-annual recharge variability is higher in arid and semi-arid climate zones relative to wetter climates. WTF and CMB-based GRR estimates exhibited low agreement in arid and semi-arid zones, where most WTF-derived GRRs exceeded CMB values by over an order of magnitude. While this can be explained by differing dominance of focused vs diffusive recharge, we show influence from other factors including the inability of the WTF method to quantify low GRRs, impacts of land use change, and non-ideal conditions like river-aquifer connections. Major differences between the WTF and CMB methods are attributed to CMB reflecting pre-land clearing GRRs in many instances. This study serves as a comparative framework for evaluating the appropriateness and differences between the WTF and CMB methods which can be applied to groundwater recharge studies globally.

Harmonic Earth tide components in well water levels have been used to estimate hydraulic and geomechanical subsurface properties. However, the validity of various methods based on analytical solutions has not been established. First, we review the theory and examine the latest analytical solution used to relate well water levels to Earth tides. Second, we develop and verify a novel numerical model coupling hydraulics and geomechanics to Earth tide strains. Third, we assess subsurface conditions over depth for a range of realistic properties. Fourth, we simulate the well water level response to Earth tide strains within a 2D poroelastic layered aquifer system confined by a 100 m thick aquitard. We find that the analytical solution matches two observations (amplitudes and phases) to multiple unknown parameters leading to non-unique results. We reveal that undrained and confined conditions are necessary for the validity of the analytical solution. This occurs for the dominant M_2 frequency at depths >50 m and requires specific storage at constant strain of Sε ≥ 10-6 m-1, in combination with hydraulic conductivity of the aquitard kl ≥ 5*10-5 ms-1 and aquifer ka ≥ 10-4 ms-1. We further illustrate that the analytical solution is valid in unconsolidated systems, whereas consolidated systems require additional consideration of the Biot modulus. Overall, a priori knowledge of the subsurface system supports interpretation of the groundwater response. Our results improve understanding of the effect of Earth tides on groundwater systems and its interpretation for subsurface properties.