Estimating hydraulic properties from tidal propagation in circular islands

Observations of tidal propagation in coastal aquifers allow for rapid, low-cost quantification of aquifer parameters that are integrated over significant distances. Previous methods for aquifer property estimation from tidal propagation have focussed on continental aquifers and assumed straight, infinite shorelines. A recent investigation has proposed a correction to the Ferris analytical solution for straight shorelines (‘Ferris solution’) to be applied under radial flow conditions. In this paper, an existing analytical solution as a function of radial flow conditions (‘circular solution’) and the corrected Ferris solution are applied for the first time to obtain aquifer diffusivity in orbiculate-shaped islands. Inversion of the circular solution (to obtain diffusivity) requires iterative methods, which are applied to synthetic island aquifers with known diffusivity values ranging between 150,000 m2/d and 500,000 m2/d. The circular solution was tested for both confined and unconfined conditions and for tidal periods ranging from around 8 to 24 hours. Known aquifer diffusivity values were reproduced within 2.5% of known values, even where randomised noise was added to synthetic groundwater tidal signals, and for both confined and unconfined aquifer conditions. Application of the corrected Ferris solution was found to be reliable for both confined and unconfined conditions, with discrepancies in aquifer diffusivity < 1% and < 2.5%, respectively, including with noise added to the groundwater tidal signals. The methodology presented in this work to estimate aquifer diffusivity in orbiculate-shaped islands is an improvement over classic approaches (e.g., the Ferris solution). In particular, the radial-flow correction to the Ferris solution provides for simple calculations of aquifer diffusivity of similar accuracy to the more complex radial analytical solution.