Water is the most vital resource for all kinds of life and it is adversely affected both qualitatively and quantitatively by all kinds of human activity on land, in air and in water. It is the prime medium in which physical and chemical transformations, particularly those of biological significance ,take place. Physico -chemical quality of any water body is the major deciding factor of the pattern of aquatic biota as well as primary and secondary productivity. Water as environment medium maintains the integrity of the entire ecosystem. The present paper describes the results of survey of the physico-chemical characteristics of ground water of eight villages at Saharsa District, Bihar over a period of one year. It can be concluded from this study that the concentrations of chlorides, fluorides are within the limits as prescribed by WHO. On the other hand, nitrate-nitrogen, alkalinity and hardness are much higher. Entry of pollutants to the ground water is quite complex and several processes affect its movement.