The mean content of arsenic (As) in the unconfined groundwater of Jianghan plain is 91.5μg L−1, and the average content of manganese is 1.19 mg L−1. In the shallow confined groundwater (15–60 m), the content of arsenic is 40.34 μg L−1, the average content of manganese is 0.59 mg L−1. With the concentration of manganese rises, the concentration of As drops. In the monitoring field of Jianghan plain, we find that the concentration of manganese has seasonal change, and this change has a tendency to inhabit the reduction of arsenic. This can be explained that the reduction of manganese consumed the reducer of system.