Occurrence of seawater intrusion overshoot

A number of numerical modeling studies of transient sea level rise (SLR) and seawater intrusion (SI) in flux-controlled aquifer systems have reported an overshoot phenomenon, whereby the freshwater-saltwater interface temporarily extends further inland than the eventual steady state position. Recently, physical sand-tank modeling has shown overshoot to be a physical process. In this paper, we have carried out numerical modeling of SLR-SI to demonstrate that overshoot can occur at the field scale within unconfined aquifers. This result is contrary to previous conclusions drawn from a restricted number of cases. In addition, we show that SI overshoot is plausible under scenarios of gradual sea level rise that are consistent with conditions predicted by the Intergovernmental Panel for Climate Change. Overshoot was found to be largest in flux-controlled unconfined aquifers characterized by low freshwater flux, high specific yield, and large inland extent. These conditions result in longer timeframes for the aquifer to reach new steady state conditions following SLR, and the extended period prior to reequilibration of the groundwater flow field produces more extensive overshoot.