Distribution of Plutonium in Soil Particle Size Fractions of Liquid Effluent‐Receiving Areas at Los Alamos

Abstract The alluvial soils of three liquid effluent‐receiving areas at Los Alamos, New Mexico were sampled to determine the distribution of 238Pu and 239,240Pu in soil size fractions as a function of soil depth, distance from the waste outfall, and soil physical‐chemical properties. Although the plutonium concentrations in the <53 µm soil size fraction were 10 times higher than plutonium levels in the 2–23 mm fraction, the largest portion of the plutonium inventory in the average soil sample was found in size fractions > 105 µm. Plutonium concentrations decreased curvilinearly with distance from the waste outfall and with soil depth in the soils of discharge areas currently receiving treated liquid wastes. The < 53 µm size fractions contained 7‐fold larger surface areas and 3‐fold larger cation exchange capacities than the 2–23 mm size fractions; plutonium concentrations in the size fractions were correlated with surface area in all three discharge areas. Significant differences in the distribution of 238Pu and 239,240Pu were found in the soil size fractions from Mortandad canyon. Most soil separates from this canyon exhibited an approximate doubling of the 239,240Pu/238Pu ratio with increased soil depth and within 40 m from the waste outfall. The <53 µm soil size fraction had a 2‐fold higher average 239,240Pu/238Pu ratio for each sampling station than the 2–23 mm size fraction. The distribution of plutonium in the soil size fractions is discussed as a function of changing isotopic composition of the liquid wastes added to the canyons, movement of soil and plutonium in these intermittent streams, and potential isotopic differences in the behavior of 238Pu and 239,240Pu.