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Geologic Controls on 137Cs Cycling by Terrestrial Vegetation in the Eastern U.S.
Chellman, Kathleen
Chellman, Kathleen
Abstract
137Cs is a radioactive trace metal (T1/2 = 30 y) that was dispersed globally by nuclear weapons testing in the 1950s-1960s. Prevailing winds and precipitation systems caused some areas far from the test sites to receive significant fallout, which is still easily measured in soils, sediments and even some vegetation in the Eastern United States. Recent work near Chernobyl and Fukushima indicates that trace levels of 137Cs can harm insects, pollination services, and other ecological functions. In areas with low soil potassium, 137Cs is cycled in vegetation; however, soil potassium alone doesn't consistently predict the 137Cs content of plants. I hypothesize that local geology more broadly influences the cycling of 137Cs by vegetation, which ultimately controls potassium, clay mineralogy, and landscape factors such as slope and relief. I collected foliage and soil samples from sites in Tennessee and Virginia to examine geologic controls affecting 137Cs uptake in vegetation. My sites span conglomerate, quartzite, dolostone, graywacke, sandstone, and unconsolidated Coastal Plain sediments. Across this gradient of soil parent material, foliage concentrations of 137Cs ranged from undetected (0.1 Bq/kg) to 58.9 Bq/kg. By comparing the ratio of 137Cs in plants to soil, I found quartzite bedrock showed almost no 137Cs uptake, and sandstone had the highest with a median of 40.9%. Plant available potassium plotted against vegetation uptake percentage gives r2 = 0.44. This explains some of the differences in vegetation 137Cs since there can be K in soils that is not accessible for plants to uptake. Foliage at each site had widely varying 137Cs, indicating potential species-specific variation in uptake. For example, the same West Virginia site on sandstone had a spruce with 7.7 Bq/kg 137Cs and rhododendron with 54.9 Bq/kg 137Cs. This research will help us understand how geology affects 137Cs and trace metal cycling and thus predict which plant or animal species could be at increased risk to radiation exposure.
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2022-04-01
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Geology