Influence of tides on the effectiveness of artificial freshwater injection in mitigating seawater intrusion in an unconfined coastal aquifer

Managed aquifer recharge (MAR) using artificial freshwater recharge from infiltration basins or injection wells is an effective method to control seawater intrusion. The effect of oceanic tides, however, are usually not considered when dimensioning and operating freshwater injection installations. This study employed lab-scale physical experiments and groundwater numerical models to investigate salt transport and salinity distributions as driven by artificial freshwater injection under the influence of tides in an unconfined coastal aquifer. Results revealed that the effectiveness of artificial freshwater injection is significantly reduced under tidal conditions as compared to conditions without tides. The response of the upper saline plume (the tidal recirculation cell) to artificial freshwater injection in tidal conditions is nearly twice more rapid than that of the lower seawater wedge. The seaward displacement of the seawater wedge toe is higher for low seawater concentration isohalines (5%) than for higher concentration isohalines (50% to 95%). It was further found that an overshoot phenomenon (whereby the seawater toe first moves seaward before moving landward again) occurs and mainly affects the higher concentration isohalines. This further decreases the effectiveness of freshwater injection in controlling seawater intrusion, and the phenomenon is more pronounced with increased injection rates, increased distance between injection well and coastal line, increased tidal amplitude and decreased tidal periods. This study provides insights with direct implications for design of artificial freshwater injection wells to control seawater intrusion in tidally-influenced unconfined coastal aquifers.