Chloride refers to chlorine ions in water. The concentration of Chloride in natural fresh water is low. The main sources are seawater intrusion, salt bed exudation, and industrial wastewater. High chloride concentration is corrosive and can retard crop growth. Chlorine ions are essential to maintain cell permeability in the human body. Within normal concentrations, they do no harm to the body. However, high concentrations would affect patients with kidney disease. Chloride can be used as an important index to assess the influence of seawater intrusion caused by groundwater over-pumping in coastal areas.