The annual cycle of plutonium in the water column of a warm, monomictic reservoir

Abstract

An annual cycle occurs in the 239,240Pu inventories of the water column of Pond B, an 87-ha warm monomictic reservoir on the US Department of Energy's Savannah River Site in Barnwell Co., South Carolina. The pond has elevated concentrations of 238Pu and 239,240Pu in sediments due to releases from former reactor operations and continues to receive additional Pu input from atmospheric deposition. The 239,240Pu inventories in the water column display annual cycles that differ between surface waters (i.e. 0–6 m deep) and deeper waters ( i. e. > 6 m deep) that are anoxic in summer months. For surface waters, the 239,240Pu inventory increases following turnover in November to a maximum in March followed by a decline until later summer when minimum inventories occur. For deeper waters, the 239,240Pu inventories increase rapidly following turnover and reach maximum values in March. The inventories in deeper waters remain large from March until turnover. Maximum inventories for the entire water column occur in March with minimum inventories at turnover in October and November. Turnover results in a redistribution of Pu across water depth but no measurable Pu loss from the water column. Ratios of 238Pu: 239,240Pu indicate that the cycle involves primarily Pu from sediment sources with little influence from atmospheric sources. Thus, the cycle represents net remobilization of 239,240Pu from the sediments to the water column during the oxic, holomictic portion of the year followed by a net loss of Pu from the water column once stratification occurs.