The city of Hanford, California, relies on ground water for its municipal water supply. Arsenic concentrations in local wells frequently exceed the current drinking water standard (50 μg/L), although some dilution is achieved within the distribution system. Water samples were collected from municipal water supply wells and storage tanks in July and December 1996. Total arsenic concentrations ranged from 9 to 75 μg/L. Arsenic was found to occur predominantly in the +III oxidation state. Lower contributions of As(III) to total arsenic concentrations were found in three wells and one storage tank. In all other samples, the percent As(III) was 89 ± 6%. The very low values of %As(III) in one storage tank (1% in July and 14% in December) indicate that As(III) oxidation occurs within the distribution and storage system. | The city of Hanford, California, relies on ground water for its municipal water supply. Arsenic concentrations in local wells frequently exceed the current drinking water standard (50 μg/L), although some dilution is achieved within the distribution system. Water samples were collected from municipal water supply wells and storage tanks in July and December 1996. Total arsenic concentrations ranged from 9 to 75 μg/L. Arsenic was found to occur predominantly in the +III oxidation state. Lower contributions of As(III) to total arsenic concentrations were found in three wells and one storage tank. In all other samples, the percent As(III) was 89 ± 6%. The very low values of %As(III) in one storage tank (1% in July and 14% in December) indicate that As(III) oxidation occurs within the distribution and storage system.