Experimental Study Of The Interface Between Fresh Water And Salt Water In Coastal Zones And Numerical Simulation

Since the reform and opening-up, the economy of coastal zones has entered a new stage of rapid development, and the marketization and industrialization’s degree were enhanced unceasingly. With the population increase, the demand of water supply in agricultural, industrial and municipal usage keeps growing. Therefore, the scientific evaluation and reasonable development of groundwater in the coastal aquifers become an important subject.Since the 1960 s, because of the unreasonable exploitation of groundwater resources in coastal areas, the groundwater levels droped. When the groundwater level becomes lower than the sea water level, the interface between fresh-water and salt-water moves towards inland. It is called seawater intrusion that the sea water migrates inland under the influence of groundwater exploitation. Seawater intrusion first appeared in the city of Dalian in China, and followed by Qingdao, Laizhou and Beihai. Nowadays, the total area of seawater intrusion in China is 900 km2.The morphological characteristics and migration regularity of the fresh water-salt water interface is an important content in the study of seawater intrusion. In this paper, we establish an unconfined aquifer in a lab to study the interface between fresh water and salt water. By changing the density and water level of the salt water, the dynamic and balance characteristics of the interface are examined, and the relationship between the farthest distance of interface extending inland and the density or water level of salt water is analysed. The result shows the increasing density and head of the salt water will cause the farthest distance of the interface extending inland, and the morphology of the interface is mainly controlled by the density of salt water. As the density difference become smaller, the slope of the interface comes down. We also establish a numerical model with SEAWAT to simulate the experiment data, the result shows that when the water level of salt water is 45 cm and the density increases from 1.060 g/cm3 to 1.100 g/cm3, the fresh water discharge length decreases from 28 cm to 17 cm, the farthest distance of the interface extending inland increases from 21.5 cm to 50.5 cm, and the slope of the interface between the fresh water and salt water decreases. It is consistent to the experiment results that the density difference beween fresh water and salt water is the major reason controlling the morphology of the interface.The seawater intrusion distance caused by pumping is the major problem in the study of seawater intrusion. We derived a formula to calculate the height of the salt water up-coning and a formula to calculate the distance of seawater intrusion, they are verified by the experimental results and the formula can accurately reflect the actual situation. Because of the location of the observation well, the flow rate can not be as accurate as expected, leading to a little error of the calculation results. The numerical model results show that the water level of salt water is the major reason controling the distance of seawater intrusion. When the density of salt water is 1.050 g/cm3 and the pumping rate is 1 m3/d, the water level increases from 45 cm to 46 cm and the distance of seawater intrusion increases from 13 cm to 27 cm.