Bomb ¹³⁷Cs in modern honey reveals a regional soil control on pollutant cycling by plants

Mid-20th-century atmospheric nuclear weapons testing dispersed numerous radioactive fission products globally, including ¹³⁷Cs, which has a 30-year half-life. While the majority of radiation from these tests decayed quickly, ¹³⁷Cs remains a significant environmental concern due to its long half-life and its ability to mimic potassium in biological systems. This study focuses on the biogeochemical cycling of ¹³⁷Cs, specifically its uptake by plants and subsequent transfer to honey via pollinators. Previous research highlighted the global ubiquity of bomb spike radionuclides and their use as chronological markers. However, the long-term biogeochemical fate and ecological consequences in ecosystems distant from testing sites remain uncertain. Although cesium is not required for plant growth, its chemical similarity to potassium allows its absorption through potassium-specific membrane transporters. This pathway from soil to plants and into the food chain was predicted decades ago, but research on the presence of ¹³⁷Cs in eastern U.S. plants and the food supply since 1988 has been lacking. This study addresses this gap by measuring ¹³⁷Cs in honey from the eastern U.S. and correlates it with soil potassium concentrations to reveal regional patterns in ¹³⁷Cs cycling. Honey is an ideal indicator due to its concentration of environmental contaminants from nectar.

Several studies have documented the presence of ¹³⁷Cs in honey and pollen following the Chernobyl disaster, primarily focusing on human consumption safety and identifying pollution hotspots. However, these studies were geographically limited and lacked comprehensive geospatial data beyond flower type or soil ¹³⁷Cs burden. Furthermore, the ¹³⁷Cs source in these studies was a mixture of Chernobyl fallout and pre-existing contamination. This research differs by focusing on the eastern U.S., where ¹³⁷Cs contamination originates primarily from mid-20th-century weapons testing, providing a unique opportunity to study the long-term biogeochemical cycling of a legacy pollutant.

The study involved the collection of 122 honey samples from North America, with a focus on 110 samples from the eastern U.S. for which county-level soil data was available. Low background gamma spectrometry was used to measure ¹³⁷Cs concentrations in the honey samples. The researchers identified hive locations at the U.S. county level, allowing them to correlate ¹³⁷Cs levels in honey with average soil potassium concentrations and ¹³⁷Cs deposition rates from existing datasets. These datasets included county-scale information on 20th-century ¹³⁷Cs deposition and mean soil potassium concentrations determined from airborne radiometric surveys. The researchers also utilized a smaller dataset of ammonium acetate-extractable K to represent plant-available K. They performed statistical analyses to determine the relationship between soil potassium, ¹³⁷Cs deposition, and ¹³⁷Cs concentrations in honey. Undetected ¹³⁷Cs samples were assigned a value of half the detection limit for analysis. To further understand the long-term trend of ¹³⁷Cs in the eastern U.S., the researchers analyzed vegetation archives from the Hubbard Brook Experimental Forest and more recent plant collections. Additionally, they compared their honey data to the historical data from the U.S. Pasteurized Milk Network (PMN) to assess the long-term trends in ¹³⁷Cs in the food supply. Finally, they calculated the ionizing radiation exposure rates to honeybees from measured ¹³⁷Cs and ⁴⁰K concentrations using external dose coefficients published by the International Commission on Radiological Protection.

Detectable ¹³⁷Cs (≥0.03 Bq ¹³⁷Cs kg⁻¹) was found in 68 of the 122 honey samples. Contrary to expectations based on ¹³⁷Cs deposition patterns, honey from the southeastern U.S. showed significantly higher ¹³⁷Cs concentrations than the northeastern U.S. A strong inverse relationship was found between the transfer of ¹³⁷Cs from soil to honey and mean total soil potassium concentrations (p < 0.001). Honey from agricultural areas tended to have lower ¹³⁷Cs levels, likely due to potassium fertilization. Analysis of vegetation archives revealed a significant decline in ¹³⁷Cs concentrations in native plants over the past 50 years, although peak levels in the 1960s-1980s were extremely high. Comparison with PMN data showed that ¹³⁷Cs concentrations in milk from Florida historically mirrored the pattern observed in modern honey, indicating persistent regional differences in ¹³⁷Cs uptake. In some honey samples, ¹³⁷Cs contamination more than doubled the ionizing radiation from background ⁴⁰K, highlighting the significant contribution of ¹³⁷Cs to radiation exposure in low-potassium soil regions.

The findings demonstrate that soil potassium levels exert a primary regional control on ¹³⁷Cs uptake by plants and subsequent transfer to honey. The southeastern U.S., characterized by old, weathered soils low in potassium, shows disproportionately high ¹³⁷Cs concentrations in honey, even decades after atmospheric deposition. The inverse relationship between soil potassium and ¹³⁷Cs uptake is consistent with existing research on cesium biogeochemistry. The high ¹³⁷Cs concentrations in honey, particularly in the southeastern U.S., raise concerns about the ecological impacts of this legacy pollutant on pollinators, although current levels are below established dietary thresholds for human consumption. The decline in ¹³⁷Cs concentrations observed in vegetation archives suggests that radioactive decay and soil migration are reducing bioavailability over time, but significant regional differences remain.

This study reveals the persistent environmental legacy of ¹³⁷Cs from nuclear weapons testing, highlighting the crucial role of soil potassium in controlling its uptake by plants and transfer to the food chain. The regional patterns observed in honey ¹³⁷Cs concentrations underscore the need for continued monitoring and further research into the ecological impacts of legacy nuclear contamination on pollinators and ecosystems. Future research could explore the biogeochemical cycling of other fission products and the development of predictive models for regional pollutant uptake.

The study relied on existing datasets for soil potassium and ¹³⁷Cs deposition, which may introduce uncertainties due to spatial variability and data limitations. The use of total soil potassium as a proxy for plant-available potassium is a simplification, although the results using ammonium acetate-extractable potassium showed a similar trend. While the study analyzed a large number of honey samples, it may not fully represent the diversity of honey sources and plant species across the study area. Further research is needed to investigate the threshold levels of ¹³⁷Cs that negatively impact pollinators.