Watertable waves in unconfined aquifers: sloping boundary effects

Watertable waves generated by forcing from oceanic oscillations play an important role in the availability and quality of coastal groundwater resources. Here we present results from laboratory experiments which examine the influence of a boundary slope on the mass transfer from an oscillating clear water reservoir into a homogeneous, unconfined aquifer. The experimental forcing period is sufficiently short (T = 348sec) to expose limitations of theories based on the simplifying assumptions of a shallow aquifer (hydrostatic pressure) free of any influence from the capillary fringe above the watertable. A new, small amplitude perturbation solution to the linearised Boussinesq equation is derived, taking into account a sloping boundary to an aquifer finite in depth and influenced by capillarity. However, discrepancies between the experimental observations and theory still exist, suggesting other contributing mechanisms. The influence of a truncated capillary fringe is suggested as a possible mechanism and warrants further investigation.