Experimental and numerical assessment of saltwater recession in coastal aquifers by constructing check dams

Artificial freshwater recharge has been considered as a feasible and effective procedure to mitigate seawater intrusion in coastal regions. The efficiency of freshwater infiltration through a check dam reservoir on saltwater recession (SWR) is investigated using two physical models. The results demonstrate the apparent tendency of recharge freshwater to move horizontally toward the boundaries rather than flowing downward to influence saltwater wedge toe. Thereby, it would affect the saltwater wedge tip instead of its toe due to the new establishment of a positive hydraulic gradient from a dam reservoir to the boundaries. Moreover, numerical dispersive simulations have been carried out on a large-scale aquifer to find the optimum location of the dam as well as the aquifer characteristics impacts on SWR efficiency. The results show that the best location to construct a check dam is immediately above the saltwater wedge toe. It is found that when saltwater head declines, the steeper hydraulic gradient between boundaries is established and the efficiency of recharge performance will be improved. Moreover, the reduction of hydraulic conductivity in vertical direction improves SWR, while higher hydraulic conductivity in the homogeneous cases only accelerates the infiltration rate but has no meaningful effect in the long term. The considered recharge method also works better in scenarios with higher dispersivity. However, the construction of check dams on floodways might be a practical and low-cost solution but it can be concluded that as the dominant direction of the recharged freshwater is toward boundaries, it cannot promptly retreat saltwater around toe position.