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Retention of Inorganic Carbon-14 by Isotopic Exchange in Soils

Environmental Science Branch, AECL, Whiteshell Laboratories, Pinawa, MB, Canada R0E 1L0.

^* Corresponding author (sheppards{at}wl.aecl.ca).

ABSTRACT

Carbon-14 is present naturally and is a waste product of the nuclear industry. It is readily incorporated into biological materials and potential impacts have to be considered carefully. From underground waste repositories, much of any ¹⁴C released will be inorganic and will pass rapidly through most terrestrial environments because of gaseous transfer. However, there will be some ¹⁴C retained in soils, in both organic and inorganic forms. This study addresses retention of inorganic ¹⁴C through isotopic exchange with soil carbonates. An outdoor study with incubations lasting up to 24 mo was conducted. A carbonated sandy soil was used unamended, amended with additional carbonate, or amended with ground alfalfa (Medicago sativa L.). One set of soil containers was left open and another set gamma-sterilized and sealed. Volatilization of some of the ¹⁴C happened within minutes, with 98% lost in the alfalfa-amended soils and 56% lost in the carbonate-amended soil. However, the fractions remaining in all the soils had residence half-times of over 1000 d. Much of this retained ¹⁴C was still inorganic and had undergone isotopic exchange with the solid ¹²C minerals in the soil. A progressive extraction method was used to monitor the degree of isotopic exchange, and in certain treatments the exchange process was still measurably progressing after 12 mo of incubation. Clearly, assessments of the impacts of ¹⁴C releases to the terrestrial environment must account for these long-term reactions and retention of ¹⁴C in soils.