Evolution of 131I from freshwater and its partitioning in simple aquatic microcosms

Our investigation quantifies iodine evolution from freshwater to the atmosphere. A factorial experimental design was employed in which the distribution of 131I was determined after 7 days in microcosms containing either (1) water (untreated Winnipeg River water), (2) Chlorella in a nutrient solution, (3) water and sediment (aerobic), or (4) water, sediment and N2 gas (anaerobic). The 131I released from the microcosms to the atmosphere was retained by triethylenediamine (TEDA) impregnated charcoal traps. Evolution accounted for < 0.5% of the total 131I inventory and was in the order Chlorella > water > water and sediment > water, sediment and N2 gas. Combining all treatments, the evolution coefficient had a geometric mean (G.M.) of 4.5 × 10−5 day−1 with a geometric standard deviation (G.S.D.) of 3.7. In the water treatment, 97% of the 131I remained in solution, whereas 22% was algal-bound in the Chlorella treatment, and 80% and 7% were sediment-bound under aerobic and anaerobic conditions, respectively. A partition coefficient (Kd) value of 163 l · kg−1 dry weight (dry wt.) was obtained for the aerobic organic sediment, whereas a value of 3 l · kg−1 dry wt. was obtained for the anaerobic microcosms. On the basis of our Kd values, the water to sediment transfer rate for iodine in Canadian Shield lakes would have a G.M. of 0.002 day−1 and G.S.D. of 7.8. We conclude that evolution of iodine generally constitutes only a minor component of losses from freshwater. Under aerobic conditions iodine is lost to the sediments, but under low oxygen conditions most of it remains in solution.