The mobility of arsenic in sediments in Lake Ohakuri, a hydroelectric storage lake on the Waikato River in New Zealand, has been monitored between 1980 and 1982 and the release of arsenic to the overlying water related to seasonal changes in both lake water and sediment. In shallow areas of the lake the release of arsenic contributes to the continuous seasonal variation in the arsenic concentration in the lake water. In areas that become stratified in summer the arsenic released from the sediments accumulates in the hypolimnion until turnover when it is mixed with epilimnetic water. It has been estimated that the turnover effect results in a temporary increase in the arsenic concentration of between 10 and 20%. Important chemical transformations have been identified, and a model for the system has been compared with previous theoretical models for lacustrine systems. © 1985, American Chemical Society. All rights reserved.