<p>This study revisits and extends the analytical model presented by Maas (J Hydrol 347:223–228, 2007), which describes simultaneous downward infiltration of fresh water and upward seepage of fresh or saline groundwater beneath an elongated island. Under steady-state conditions and using the sharp interface approximation, the interface between the two types of water forms a semi-elliptical shape. Notably, the unusual y-linearity of the flow potential provides an analytical advantage, enabling a direct and elegant extension from a one-phase to a two-phase system. For transient conditions, Maas (J Hydrol 347:223–228, 2007) employed a successive steady-state approach based on elliptical shapes. We demonstrate that this approach is physically inconsistent, as the tip of the lens must possess a finite derivative to allow for movement. Instead, we introduce an interface motion equation to track this front dynamically. Additionally, we identify potentially unstable configurations during outward lens migration, where denser salt water temporarily overlays fresh water near the lens tip. To address this, we incorporate diffusion, moving beyond the sharp interface assumption. Finally, we assess the applicability of Maas’s elliptical approximation for interface motion and find it remains robust as long as the approximation that lens thickness is negligible compared to its width is used with caution. These findings broaden the analytical understanding of fresh water lens dynamics in island aquifers and refine the conditions under which simplified models remain valid.</p>