FALLOUT RADIONUCLIDE CONCENTRATIONS IN CENTRAL AND WESTERN CUBAN RIVER SEDIMENT

Using gamma spectroscopy, we evaluate if changes in Cuban agricultural practices in the early 1990s may have changed erosion rates and processes. We analyzed the activities of the fallout radionuclides excess Pb-210, Cs-137, and Be-7 in both coarse (250-850 micron) and fine (<63 micron) river sediment collected during two sampling campaigns in Cuba (26 sample sites from central Cuba in August, 2018 and 20 sample sites from western Cuba in August, 2019). Few (n = 7) samples have detectable Be-7 in either grain size fraction. Fine-grained sediments from central Cuba had excess Pb-210 activities ranging from 5.6 to 57.4 Bq/kg (mean 22.1+/-15.4 Bq/kg, 1 SD). These samples had Cs-137 activities ranging from 0.8 to 5.9 Bq/kg with a mean of 2.9+/-1.9 Bq/kg (1 SD). Coarse grained sediments (250- 850 μm) from central Cuba have excess Pb-210 activities of 1.7 to 31.0 Bq/kg, mean 8.8+/-7.6 (1 SD) and Cs-137 activities 0.6 to 5.0 Bq/kg, mean 1.56+/-1.33 (1 SD).

Western Cuba sediments show a marked difference between the nuclide concentration of fine and coarse grained sediments. While fine grained sediments from western Cuba had the highest mean activities for the samples studied, none of the coarse-grained samples had any detectable Be-7, and few of the coarse grained samples (2 out of 18) had detectable Cs-137. Fine-grained samples from western Cuba had excess Pb-210 activities ranging from 3.3 to 167.1 Bq/kg (mean 43.6+/-37.6, 1 SD) and Cs-137 activities ranging from 2.3 to 7.7 Bq/kg (mean 4.3+/-1.5, 1 SD). Coarse grained sediment from western Cuba had excess Pb-210 activities ranging from 0.2 to 37.8 Bq/kg (mean, 6.8+/-8.4, 1SD). Cs-137 activities ranged from 1.1 to 1.5 Bq/kg (mean 1.3+/-0.3, 1 SD). Thus far, we find no correlation (R2 < 0.45) between the activities of the fallout radionuclides and factors such as slope, average rainfall, elevation, and land use type.

It appears that erosion over the past decades, the time frame for which excess Pb-210 and Cs-137 are most sensitive, is more rapid and/or deeper in central than western Cuba. Higher frequency of detectable excess Pb-210 may indicate slowing of erosion over the past decades. Furthermore, low to undetectable concentrations of Cs-137 may suggest channel bank erosion. Our ongoing research investigates which, if any, environmental factors are related to this difference.