Cadmium-113m in the water, sediments, and adjacent soils of Lake Michigan

Abstract

Cadmium-113m entered the environment as a consequence of testing of nuclear weapons in the atmosphere. Earlier we demonstrated that 113mCd can be determined accurately in water, soils, and sediments. In Lake Michigan, the present concentrations of 113mCd are 350 ± 150 μBq l −1 ( n = 16) in the water, and 10.0 ± 5.5 mBq g −1 ( n = 12) in the surficial sediments. The specific activities in water and sediment are statistically indistinguishable from one another and average 9.9 kBq g −1 Cd. The inferred K D is thus 29 ± 20 l g −1. In undisturbed soils near Lake Michigan, the mean 113mCd/ 137Cs activity ratio is 2.0 ± 1.4 × 10 −2 ( n = 4), implying a total 113mCd input to the lake of 2.7 ± 1.3 TBq (corrected for decay to 1984). Using the K D for 113mCd, and taking K D for 137Cs to be 90 ± 6 l g −1, the predicted 113mCd activities are 1.1 ± 1.1 mBq g −1 in sediment, and 120 ± 125 μBq l −1 in water. These values are comparable to the observed activities. Application of a simple one-box model to the observed activities leads to a residence time for Cd of 40 ± 30 years, or about 0.4 times the hydraulic residence time. Resuspension may account for the difference between the 113mCd-based value and an earlier estimate based on a mass balance for stable Cd.