China's first sea-water intrusion observation network is in the Laizhou Bay region. The data obtained for four consecutive years are presented. There is usually a wide transition zone in the study area, but in some regions, a relatively distinct and sharp interface is formed. A detailed analysis has shown that there are at least two types of salt-water/fresh-water interface. The interface in the study area had moved about 140 m along the main observed section in a period of three years and three months due to the effect of pumping wells. Salt water may invade via the riverbed deposits but usually changes its invading direction due to the united effect of the pumping wells. Sea-water intrusion was caused by the excessive pumping of ground water from the coastal aquifer. The interface was generally formed on the inland side not far away from the powerful discharge pumping center, and sea-water intrusion stops there. The monitoring data suggested that if the powerful discharge pumping center moves landward, salt water will start advancing landward until a new equilibrium is reached, and even if a new pumping well, whose discharge is very small, is drilled, the equilibrium would be upset, thus causing the interface to advance inland slowly.
