Seawater intrusion in heterogeneous coastal aquifers under flooding events

Coastal flooding due to storm surge and sea-level rise (SLR) affects seawater intrusion (SWI) in aquifers. Under coastal flooding events, SWI from both sea boundary and land-surface is expected to increase. SWI in coastal aquifers is also strongly influenced by geological heterogeneity characteristics. In this study, the combined impacts of fully geological heterogeneity and coastal flooding on SWI is quantified in a two-dimensional unconfined coastal aquifer. Three indices including the seawater toe location (x(toe)), the mixing volume (v(mv)) and the SWI volume (v(swiv)) are used to assess the vulnerability to the intrusion of the seawater. A series of 50 log-normally distributed random hydraulic conductivity with a spherical correlation function is generated using the GCOSIM3D code. Then, numerical simulations of transient, saturated-unsaturated variable-density flow and solute transport, SUTRA, are performed in a stochastic Monte Carlo framework to understand how coastal flooding and geological heterogeneity effects can change the groundwater salinity. During storm surge events, seawater penetrates into the aquifer and leads to reduce temporary of the freshwater quality. The results indicate that the storm surge has a short-term impact on fresh groundwater. The aquifer is significantly influenced by SLR as a long-term effect while it causes a larger x(toe) in comparison with pre-flooding events' conditions. Also, the results obtained under gradual and instantaneous assumption of SLR show that the gradual assumption leads to a smaller x(toe) which is more realistic. Comparison of the homogeneous and heterogeneous scenarios indicates the heterogeneity affects SWI status both in the presence and the absence of coastal flooding events. It causes reduction in x(toe) and v(swtv) as well as increase in v(mv). This study shows the significant value of considering a wide range of variations due to heterogeneity, in order to better manage the coastal fresh groundwater aquifers.