Fast desalinization of residual saltwater using subsurface dams combined with saltwater discharge or freshwater recharge

•Physical process of saltwater intrusion and retreat is explored in experiments.•Timescale and removal mechanism of residual saltwater with synergistic countermeasures are revealed.•Combining subsurface dams and extraction-injection promotes saltwater removal efficiency.•Extraction-injection imposes more remarkable influence on desalinization than aquifer properties. Countermeasures against seawater intrusion (SWI) are critical to prevent coastal groundwater resources from deterioration. Subsurface dams have been recognized as one of effective approaches to prevent SWI, but most likely to produce residual saltwater at the landside of the dam. Understanding dynamic behaviors and removal mechanism of residual saltwater can facilitate aquifer desalinization, especially under combined effect of subsurface dams and hydraulic controls. Little attention has been paid to synergistic impact on residual saltwater removal. This study performed laboratory-scale experiments and numerical simulations to investigate residual saltwater trapped in the storage area upon construction of a subsurface dam along with saltwater discharge/freshwater recharge. Effects of subsurface dam designs, aquifer properties as well as discharge and recharge rates on the flow and salinity transport process were explored and compared in a coastal aquifer. Two indexes including a reduction rate of residual saltwater length (RRSL*) and removal rate of residual salt mass (RRSM*) were introduced to quantify effectiveness of residual saltwater removal. Experimental and numerical results found that the subsurface dam needs a minimum effective height to achieve complete removal of residual saltwater. Moreover, a shorter subsurface dam (e.g., 9 cm in the experiment) achieves a faster removal rate compared to a taller dam (e.g., 15 cm). Particularly, attenuation of relatively low-concentration mixing zone between 10% and 50% isolines was a quite slow process. Saltwater discharge or freshwater recharge near the dam exhibits more effectiveness in desalinization than far from it. Furthermore, extraction-injection imposed more remarkable influences on saltwater removal than aquifer properties. Findings from the study have significant implications in better understanding of residual saltwater removal mechanism and timescale for nearshore groundwater systems.