Combined effect of cutoff wall and tides on groundwater flow and salinity distribution in coastal unconfined aquifers

•Cutoff wall and tides jointly affect the groundwater flow and salinity distribution in coastal unconfined aquifers.•The flow and salinity distribution depend on the depth and location of cutoff walls.•Modification of cutoff walls on salinity distribution is not evident in a tidally-influenced aquifer. Cutoff walls are widely used to alter groundwater flow and mitigate seawater intrusion in coastal areas. While their behavior and performance in aquifers under static oceanic conditions are well established, how do they combine with tides to affect groundwater flow and salinity distribution in coastal aquifers remains unclear. Here we addressed this research question based on physical experiments and numerical simulations. Cutoff walls were showed to regulate the inland freshwater flow and thus form a sharp waterlevel change in the landside of the cutoff wall. As such, inland groundwater flow was concentrated in the lower aquifer that inhibited seawater intrusion in the unconfined aquifer subjected to a static sea level (under the non-tidal condition). This modification was enhanced as the cutoff wall went deeper or seaward. However, the tide pushed the saltwater wedge seaward in comparison with the case under the non-tidal condition. Under this tidal condition, the modification of the cutoff wall on saltwater intrusion was no longer evident. In both cases under non-tidal and tidal conditions, the cutoff wall was found to alter the path and travel time of the inland fresh groundwater and seawater, and total freshwater storage in the aquifer. These results offer significant implications for designing engineering measures to mitigate seawater intrusion and managing groundwater resources in coastal zones.