Water resources in Rayong groundwater basin are still limited, since water demand tends to increase following the increase of various activities, such as consumption of local people, agricultural and industrial areas. As a result, groundwater resources will significantly play an important role for basic needs and further development in this area. Generally, there are 2 main species of arsenic in groundwater, which are arsenite (As3+) and arsenate (As5+), depending upon their oxidation and reduction in aquifer. The objectives of this study were to investigate the species of arsenic contaminated in Rayong groundwater basin and to establish the hydrogeological cross-sections, which may further use to explain arsenic contamination. The arsenic concentrations in groundwater ranged from 0.0002 – 0.1598 mg/l with an average of 0.0188 mg/l (exceeding the drinking water standard: 0.01mg/l). The results showed that arsenic mainly exists in the form of arsenite (As3+) under reducing condition (ORP<0) corresponding to the deeper confined aquifer (Granite aquifer).
