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 unconfined groundwater flow system in sands has generated a contaminant plume (with chemical characteristics of dilute sanitary landfill leachate) containing 14C 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 14C. Approximately 300 m from the waste management site, the groundwater discharges to the surface in a wetland 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 groundwater input boundary of the wetland, with control provided by water and tritium balance data. During the 1991 study year, only 1.5–2% of both the inorganic and organic 14C inputs left the wetland in surface water drainage. Vegetation growth in the wetland during the study year contained 8–10% of the released radiocarbon. If the rate of 14C accumulation in the peat has been constant, 7–9% of the annual radiocarbon input has been retained in the organic soil. Much of this soil accumulation can be attributed to litter from standing vegetation, making distribution coefficients an inappropriate model for 14C partitioning between groundwater and soil. The plant/soil 14C concentration ratio was 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 14C accumulation in vegetation occurs by CO2 transfer from the air to the plant.