Sea level rise effect on seawater intrusion into layered coastal aquifers (simulation using dispersive and sharp-interface approaches)

Abstract

Concerning about sea level rise (SLR) effect on seawater intrusion (SWI) has been grown up for the last decade and numerous studies have addressed the extents, rates and timescales associated with SWI induced by SLR mostly for homogeneous cases. In layered aquifers, vertical leakage through layers makes the intrusion mechanism different compared to homogeneous one. In this study, series of simulations using dispersive SEAWAT and a developed sharp-interface models have been carried out to investigate gradual and instantaneous SLR (respectively named GSLR and ISLR) effect on SWI into field-scale layered aquifer with constant freshwater inflow boundary condition. Two scenarios have been defined for this purpose, including an aquitard with two different hydraulic conductivities (i.e. K′ [L/T] = 0.01 and 0.0001 m/d) that is placed between two extensively higher permeable layers. By SEAWAT, it is found that for ISLR problem with higher aquitard K′ value, where freshwater can leak upward across the aquitard, seawater intrude more appreciable into bottom layer rather than the upper one. For the upper layer, seawater intrude toward land at early times but then naturally driven back to almost its original position (called as overshoot). At the second scenario with lower K′ where almost no freshwater can leak upward, the ambient freshwater inflow push the lower layer salt wedge back toward the sea. For the GSLR, gradual increment of seawater head lets the freshwater to be delivered into unsaturated part of the upper layer and hence seawater intrudes through the system but with low rate. At the lower layer of higher K′ scenario with GSLR, freshwater upward leakage causes SWI more obvious compare to the other scenario and horizontal freshwater flow prevents seawater to intrude through the lower layer. The sharp-interface model successfully predicted the same trend compare to dispersive SEAWAT model but over-predicted the interface line. It weaker matched with SEAWAT result for lower K′scenario where it cannot successfully predict the correct amount of freshwater upward leakage.