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Distribution of Plutonium in Trinity Soils after 28 Years¹

ABSTRACT

The soils of four intensive study areas located along the fallout pathway of Trinity, the first nuclear detonation, were sampled to determine soil plutonium concentrations as a function of distance from Ground Zero and soil depth.

About half of the ^239,240Pu in Trinity Site soils was found at the 5–20 cm depth in 1973 compared to total plutonium inventories found only in the upper 5 cm of soil about 20 years ago. Soil plutonium concentrations of samples collected at the same depth of each study area generally exhibited coefficients of variation > 1.2. Maximum penetration depths of ^239,240Pu into Trinity Site soils were related to the presence of subsoil horizons containing carbonate accumulations and the maximum extent of rainwater penetration into these soil profiles.

Increased amounts of plutonium were associated with < 100 µm fractions as distance from Ground Zero and soil depth increased. The < 100 µm fraction contained 1.2 and 89% of the plutonium in the topsoil at the study areas located 1.6 and 44 km from Ground Zero, respectively. Total amounts of ^239,240Pu in the < 100 µm soil size fractions also generally increased with depth; at 1.6 km from Ground Zero, this size fraction contained 1.2% and 13% of the ^239,240Pu found at the 0–2.5 and 5.0–10 cm depths, respectively.

The soil plutonium data collected at Trinity Site is discussed relative to the sizes of Trinity fallout particles, setting meaningful health standards for plutonium in soils, and the distribution of plutonium in the biota of Trinity Site ecosystems.

Key Words: ^239,240Pu • radioactive fallout • radionuclide distributions in soil • sampling variation • soil size fractions • temporal behavior of radionuclides • terrestrial ecosystems

¹ Research funded under Contract No. W-7405-Eng. 36 between the U. S. Energy Res. and Develop. Admin. and the Los Alamos Sci. Lab.

² Soil Scientist and Radiation Ecologist, respectively, Los Alamos Sci. Lab., Los Alamos, NM 87545; Soil Scientist, USDA, SCS, Las Cruces, NM 88001.

Received for publication December 29, 1975.