RADIOCARBON SPECIATION AND DISTRIBUTION IN AN AQUIFER PLUME AND GROUNDWATER DISCHARGE AREA, CHALK RIVER, ONTARIO

The storage of low level radioactive waste in trenches overlying an un confined groundwater how system in sands has generated a contaminant p lume (with chemical characteristics of dilute sanitary landfill leacha te) containing C-14 both as dissolved inorganic and organic C. In the groundwater, dissolved organic compounds account, on average, for 22% of the total C and 10% of the C-14. Approximately 300 m from the waste management site, the groundwater discharges to the surface in a wetla nd containing up to 3 m of peat and an extensive tree cover. Drainage from the wetland passes through a gauged stream. Radiocarbon input to the groundwater discharge area in 1991 was determined to be between 3. 3 and 4.2 GBq, based on data from a line of sampling wells along the g roundwater input boundary of the wetland, with control provided by wat er and tritium balance data. During the 1991 study year, only 1.5-2% o f both the inorganic and organic C-14 inputs left the wetland in surfa ce water drainage. Vegetation growth in the wetland during the study y ear contained 8-10% of the released radiocarbon. If the rate of C accu mulation in the peat has been constant, 7-9% of the annual radiocarbon input has been retained in the organic soil. Much of this soil accumu lation can be attributed to litter from standing vegetation, making di stribution coefficients an inappropriate model for C-14 partitioning b etween groundwater and soil. The plant/soil C-14 concentration ratio w as 24 to 33, but application of a concentration ratio to describe the transfer of radiocarbon to plants is also believed to be inappropriate . This study indicates that over 80% of the groundwater radiocarbon is rapidly lost to the atmosphere when the groundwater comes to surface, and we infer that most of the C-14 accumulation in vegetation occurs by CO2 transfer from the air to the plant. Crown copyright (C) 1998 Pu blished by Elsevier Science Ltd. All rights reserved.