Quantitative separation and determination of inorganic and organic forms of total carbon and radiocarbon in natural waters and application at a radioactive waste management site

A method for the quantitative separation and measurement of both total carbon and 14C in dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in moderate-volume (up to 1.5 l) samples is presented, accompanied by a systematic optimization of process and analytical variables. Inorganic carbon is removed by acidification and gas stripping, while DOC is wet-oxidized using a mixture of potassium persulphate, potassium dichromate, and perchloric acid and an oxygen sweep while boiling the sample. This treatment is shown to provide quantitative oxidation of even refractory compounds (humic acid) at concentrations above those observed in natural waters. In both cases, the CO2 is trapped in NaOH, precipitated as BaCO3, and subsequently re-converted to CO2 and transferred to Carbo-Sorb for liquid scintillation counting. The study analyzed the effects of CO2 loading, storage time, and Carbo-sorb/Permafluor E+ ratios on quenching, sensitivity and efficiency in 14C measurements. The high precision and accuracy (each ± 5%), and low detection limits (0.1 and 0.3 mg.l-1 for DIC and DOC, respectively and 0.06 Bqcdotg-1 of carbon for 14C) provided by the method make it suitable for radiocarbon speciation studies in natural waters. The method has been applied to measure radiocarbon concentrations in ground and surface waters downgradient of a low level radioactive waste management site at the Chalk River Laboratories. Radiocarbon releases from the waste to groundwater occur in both inorganic and organic forms. Approximately 20% of the total carbon and 10% of the total 14C in groundwater are present in organic species; less than 2% of both the DI14C and DO14C delivered to the wetland by groundwater discharge subsequently leave in surface waters. (orig.)