Effective method of removing saltwater wedge for preserving agricultural water quality

Saltwater wedge intrusion often renders the quality of water from the Shinkawa River in the city of Niigata (Japan) unsuitable for agricultural use. To determine an effective and economic strategy to limit this intrusion and prevent irrigation water from contamination by seawater, we propose a countermeasure, pushing the saltwater layer outside the river through submerged flow generated by sluice gates near the river mouth. To demonstrate this effect, an in situ experiment was conducted. During that experiment, longitudinal and vertical distributions of the saltwater wedge were observed by an acoustic profiling system and electrical conductivity meters. As a result, elevation of the density interface between freshwater and saltwater was reduced by 1.04 m on average and the longitudinal extent of the saltwater wedge was withdrawn by 1.08 km seawards over about 4 h. With the intrusion extent shortened and the density interface lowered, three of five intake pumping stations could operate without risk of saltwater mixture into irrigation water, and the risk to two other intake pumping stations was reduced. We conclude that the use of the sluice gates to generate submerged flow underneath them is an effective countermeasure to limit the saltwater wedge intrusion along the Shinkawa River and preserve the agricultural water quality during irrigation season. This method may be used in other estuaries suffering from saltwater wedge intrusion.