Carbon-14 and tritium in plants of a wetland containing contaminated groundwater

Although plants obtain essentially all of their C from the atmosphere, they can also become contaminated with 14C in settings where the soil is contaminated with 14C. Attempts to characterize this transfer of 14C from soils to plants using the traditional concentration ratio have proven problematic because the soil-atmosphere-leaf pathway likely dominates in most hypothetical contamination scenarios. This study investigated plant concentrationsof14C in a wetland and that has been contaminated by groundwater 14C and 3H over the past 30 a. The accumulation of 3H in the plants is by the soil-root-leaf pathway, and variations in the 14C3H ratio inthe plants would suggest that another pathway is dominant for 14C accumulation. The vertical distribution of 3H in cedar (Thuja occidentalis) foliage was quite constant, 46.7 ± 4.8 Bq g−1 dry weight, while the 14C decreased with height, from 6.9 Bq g−1 at 2.2 m to 1.5 Bq g−1 at 11.2 m. Along a transect through the wetland, the 14C3H ratio in the pore water was uniform whereas both moss and white spruce (Picea glauca) vegetation showed marked variation. We conclude that the soil-atmosphere-leaf pathway was a major contributor to plant 14C, and speculate that this will be typical of releases from underground nuclear waste disposal.