© 2017 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/\ud This author accepted manuscript is made available following 24 month embargo from date of publication (Aug 2017) in accordance with the publisher’s archiving policyThe inland migration of seawater in coastal aquifers, known as seawater intrusion (SWI), can be categorised as passive or active, depending on whether the hydraulic gradient slopes downwards towards the sea or the land, respectively. Despite active SWI occurring in many locations, it has received considerably less attention than passive SWI. In this study, active SWI caused by an inland freshwater head decline (FHD) is characterised using numerical modelling of various idealised unconfined coastal aquifer settings. Relationships between key features of active SWI (e.g., interface characteristics and SWI response time-scales) and the parameters of the problem (e.g., inland FHD, freshwater-seawater density contrast, dispersivity, hydraulic conductivity, porosity and aquifer thickness) are explored for the first time. Sensitivity analyses show that the SWI response time-scales under active SWI situations are influenced by both the initial and final boundary head differences. The interface is found to be steeper under stronger advection (i.e., caused by the inland FHD), higher dispersivity and hydraulic conductivity, and lower aquifer thickness, seawater density and porosity. The interface movement is faster and the mixing zone is wider with larger hydraulic conductivity, seawater-freshwater density difference, and aquifer thickness, and with lower porosity. Dimensionless parameters (Peclet number and mixed convection ratio) from previous steady-state analyses offer only limited application to the controlling factors of passive SWI, and are not applicable to active SWI. The current study of active SWI highlights important functional relationships that improve the general understanding of SWI, which has otherwise been founded primarily on steady-state and passive SWI

[1] In recent years, a number of numerical modeling studies of transient sea-level rise (SLR) and seawater intrusion (SWI) in flux-controlled systems have reported an overshoot phenomenon, whereby the freshwater-saltwater interface temporarily extends further inland than the eventual steady state position. In this study, we have carried out physical sand tank modeling of SLR-SWI in a flux-controlled unconfined aquifer setting to test if SWI overshoot is a measurable physical process. Photographs of the physical SLR experiments show, for the first time, that an overshoot occurs under controlled laboratory conditions. A sea-level drop (SLD) experiment was also carried out, and overshoot was again observed, whereby the interface was temporarily closer to the coast than the eventual steady state position. This shows that an overshoot can occur for the case of a retreating interface. Numerical modeling corroborated the physical SLR and SLD experiments. The magnitude of the overshoot for SLR and SLD in the physical experiments was 24% of the change in steady state interface position, albeit the laboratory setting is designed to maximize overshoot extent by adopting high groundwater flow gradients and large and rapid sea-level changes. While the likelihood of overshoot at the field scale appears to be low, this work has shown that it can be observed under controlled laboratory conditions.