Long‑term decline of Chernobyl‑derived 137Cs in mosses and lichens from the Eastern Black Sea region of Türkiye

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Abstract
Following the Chernobyl accident, Ordu province in Türkiye’s Eastern Black Sea region received significant 137Cs deposition. Mosses and lichens have been used as bioindicators at 50 sampling points in 1997, 2007, and again in 2025. This study presents the third measurement (2025) and calculates ecological half‑lives for the 2007–2025 period. Samples were collected from the same locations and species, and 137Cs activity was measured by HPGe gamma spectrometry. Detectable 137Cs was found in 14 of 30 moss sites (mean 10.1 Bq kg⁻¹) and 3 of 20 lichen sites (mean 5.2 Bq kg⁻¹). Ecological half‑lives for 2007–2025 averaged 4.4 years for mosses and 4.2 years for lichens, similar to values from 1997–2007. Nearly 39 years after Chernobyl, 137Cs remains detectable at very low levels. The consistent short ecological half‑life confirms that environmental removal continues to dominate over radioactive decay. This 28‑year dataset provides a robust baseline for long‑term radiocaesium monitoring.